SAW DEVICES
1. INTRODUCTION
SAW means Surface Acoustic Wave. Electronic signal processing by means of select selectiv ivee manipu manipulat lation ion of surfa surface ce acoust acoustic ic wave wave on piezo piezoel elect ectric ric substr substrat atee was initiated in 1965 with the invention of thin film interdigital transducer (IDT) by White and Voltmeter at University of California at Berkeley. Acoustic waves are type of longitudinal waves that propagates by means of adiabatic compression and decompression. Longitudinal waves have same direction of vibration as their direction of propagation. SAW ( Surface Acoustic Wave ) devices have emerged as the most unique passive compon component ents, s, having having advant advantage agess of being being small small,, rugge rugged, d, light lightwei weight ght and easil easilyy reproducible. These advantages have made SAW devices a key component in RF communication communication systems with innovative applications. applications. Other emerging applications of SAW SAW reso resona nato tors rs incl includ udee gas gas sens sensor ors, s, bios biosen enso sors rs,, and and chem chemic ical al sens sensor ors. s. The The signi signifi fican cance ce of these these devic devices es in indus industr tryy is eviden evidenced ced by their their large large world worldwid widee production, as an example approximately 3 billion acoustic wave filters are used annu annual ally ly,, prim primar aril ilyy in mobi mobile le cell cell phon phones es and and stat statio ions ns.. Rece Recent nt fabr fabric icat atio ionn techno technolog logica icall devel developm opmen ents ts in the the comple complemen menta tary ry meta metal-o l-oxid xidee semico semicondu nduct ctor or (CMOS) technology have led to a marked improvement in the performance of these CMOS devices in the high frequency range. In addition recently, a wide variety of novel MEMS devices have been successfully implemented using CMOS fabrication techniques.
1
2. DEFINATIONS OF SAW
1. SAW SAW is an acou acoust stic ic wave wave whic whichh trav travel elss alon alongg the the surf surfac acee of mate materi rial al exhibiting exhibiting elasticity, elasticity, with amplitude that decays exponentially with depth into substrate. 2. Surface Surface Acousti Acousticc Wave is a wave propag propagati ating ng along the the surface surface of an elastic elastic substrate. 3. A Surface Surface Acoustic Acoustic Wave Wave is a type type of mechanica mechanicall wave motion motion which which travels travels along the surface of solid material. In 1985 Lord Rayleigh explained the surface acoustic mode of propagation. So, these waves are also known as Rayleigh waves. Rayleigh waves have longitudinal and vertical shear components that can couple with any media in contact with coupling strongly affects the amplitude and velocity of wave, allow SAW sensors to directly sense mass and mechanical properties. With reference to the basic structure sketched in fig.1, metal thin-film IDTs are fabricated on the surface of suitable piezoelectric substrate that would act as electrical input and output ports. SAW devices consist of two IDTs on piezoelectric substrate such as quartz. IDTs consist of interleaved metal electrodes which are used to launch and receive the waves, so that electrical signal is converted into acoustic wave and then back to electrical signal. Any changes to the characteristics of the propagation path affect the velocity and/or amplitude of the wave. Changes in velocity can be monitored by measuring the frequency or phase characteristics of the sensor and can then be correlated to the corresponding physical quantity being measured.
2
2. DEFINATIONS OF SAW
1. SAW SAW is an acou acoust stic ic wave wave whic whichh trav travel elss alon alongg the the surf surfac acee of mate materi rial al exhibiting exhibiting elasticity, elasticity, with amplitude that decays exponentially with depth into substrate. 2. Surface Surface Acousti Acousticc Wave is a wave propag propagati ating ng along the the surface surface of an elastic elastic substrate. 3. A Surface Surface Acoustic Acoustic Wave Wave is a type type of mechanica mechanicall wave motion motion which which travels travels along the surface of solid material. In 1985 Lord Rayleigh explained the surface acoustic mode of propagation. So, these waves are also known as Rayleigh waves. Rayleigh waves have longitudinal and vertical shear components that can couple with any media in contact with coupling strongly affects the amplitude and velocity of wave, allow SAW sensors to directly sense mass and mechanical properties. With reference to the basic structure sketched in fig.1, metal thin-film IDTs are fabricated on the surface of suitable piezoelectric substrate that would act as electrical input and output ports. SAW devices consist of two IDTs on piezoelectric substrate such as quartz. IDTs consist of interleaved metal electrodes which are used to launch and receive the waves, so that electrical signal is converted into acoustic wave and then back to electrical signal. Any changes to the characteristics of the propagation path affect the velocity and/or amplitude of the wave. Changes in velocity can be monitored by measuring the frequency or phase characteristics of the sensor and can then be correlated to the corresponding physical quantity being measured.
2
Figure 1: shows a basic diagram of SAW device
3. CONSTRUCTION
As far as construction is concerned, basic structure of SAW device consists of: 1. Piezoe Piezoele lectr ctric ic crysta crystall substr substrat atee 2. Interd Interdigi igital tal trans transduc ducer erss •
PIEZOELECTRIC PIEZOELECTRIC CRYSTAL SUBSTRATE: In piezoelectric piezoelectric material there is a mechanism which offers coupling between electrical and mechanical disturbances. Application of electric field setup mechanical stresses and strain.
3.1 Piezoelectric Substrate Materials
Among the piezoelectric substrate materials that can be used for acoustic wave sensors and devices, the most common are quartz (SiO 2), lithium tantalite (LiTaO 3), and, to a lesser degree, lithium niobate (LiNbO 3). Each has specific advantages and disad disadvan vantag tages, es, which which inclu include de cost, cost, tempe temperat rature ure depend dependenc ence, e, atte attenua nuatio tion, n, and propagation velocity. An interesting property of quartz is that it is possible to select the temperature dependence of the material by the cut angle and the wave propagation direction. With proper selection, the first order temperature effect can be minimized. An acoustic wave temperature sensor may be designed by maximizing this effect. This is not true of lithium niobate or lithium tantalite, where linear temperature dependence always exists for all material cuts and propagation directions. Other materials with commercial potential include gallium arsenide (GaAs), silicon carbide
3
(SiC), langasite (LGS), zinc oxide (ZnO), aluminum nitride (AlN), lead zirconium titanate (PZT), and polyvinylidene fluoride (PVdF). 3.1.1. REQUIREMENT FOR PIEZOELECTICITY: PIEZOELECTICITY:
a) Primary requirement for piezoelectricity is that material must be anisotropic, so that its properties depend on orientation relative to internal arrangement of atoms. Anis Anisot otro ropi picc mate materi rial al does does not not have have cons consta tant nt prop propag agat atio ionn in all all dire direct ctio ionn of propagation. When this direction changes, there will be more or less losses occurs, which thus lead to beam steering and that can degrade the response of band pass filter. b) As material is anisotropic, the SAW properties depend on the orientation at which the substrate has been cut from original material.
Fig.2 shows interdigital transducer on piezoelectric substrate
3.2 Interdigital Transducer:
It is made up of thin-film of metal and is fabricated on the piezoelectric substrate. It is having an interleaved structure as shown in fig.2, in which alternative electrodes are connected to two pads and one pad is connected to RF power supply and other pad is connected to ground. Two IDTs are fabricated fabricated on a piezoelectric piezoelectric substrate; one which is use to transmit and another is used to receive the waves. Transmitter IDT converts the electrical signal into mechanical signal and receiver IDT converts the mechanical signal into electrical signal. Metallization Metallization film thickness thickness for IDT fabrication fabrication normally is in range 500Å to 2000Å. If thickness is less than 500Å, this may lead to poor electrical conductivity in IDT and there will be more insertion losses.
4
3.3 Shielding Layer:
In senso sensorr appli applica cati tions ons it is often often neces necessar saryy to cover cover the the senso sensorr surfac surfacee or the interacting area of the sensor with a shielding layer. We present a well reproducible and easy method to shield a sensor surface with an aromatic polyimide layer. The shielding capacity was tested with surface acoustic wave (SAW) devices which are commerci commerciall allyy availabl availablee and furtherm furthermore ore meet the requirem requirements ents for (bio) (bio) sensor sensor applications in aqueous media. All experiments described here were done with these devices. One major advantage of this technique was the prevention of corrosion processes on the sensor surface, especially the damage of interdigital transducers which often consist of aluminium, because this material is known to have the best acoustoelectric properties for SAW-devices. Besides, we show that a thin polyimide film enhance the typical sensor characteristics in terms ‘of sensitivity and stability’.
3.2 Absorber:
The reflection of the surface acoustic waves at the edges of a SAW substrate causes interference with the main signal and degrades the characteristics of the device. Therefore, the edges of SAW substrates are often provided with wax or another material which absorbs the surface waves. SAW devices are being fabricated by sophisticated photolithographic processes, while the deposition techniques for the absorber material are mainly screen printing or manual painting. The development of new materials in IC technology technology gives opportunities opportunities for new acoustic absorption absorption films. A very useful material is polyimide, a viscous organic material which maintains its absorptive acoustic acoustic properties after curing. Polyimide can be spin coated on the wafer and patterned by photolithographic techniques. Not only thin (0.2–1 μm) but also thick (up to 20 μm and more) polyimide layers can be fabricated, a feature which makes the material very attractive for use on SAW devices in the lower frequency range (80–200 MHz). The layers show good adhesion on the substrate and excellent absorbing properties.
5
4.OPERATION
When RF signal is applied to the transmitter IDT, then interdigital transducer convert it into the mechanical waves i.e., acoustic waves. And these waves travel along the surface of piezoelectric substrate. As the wave passes, each atom of material traces out an elliptical wave motion.
Fig.3 shows the path trace by particles in presence of SAW As shown in fig.3 practical traces an elliptical path as it is having the vibrations in both horizontal and vertical plane. The atoms move by smaller amount as one look farther into depth, away from the surface. Thus the wave guided along the surface. Absorber absorbs the vibrations which degrade the RF signal. It absorbs the unwanted vibration in various directions and it increases the efficiency. Piezoelectricity is a great help for transduction. If an electric field is applied to the surface, corresponding stresses are set up which travel away from the source in the form of SAWs. The easiest method is to use a set of interleaved electrodes alternately 6
connected to two bus bars, as in (Figure-2). The left transducer is launching the waves. When a voltage is applied, the gaps between electrodes have electric fields and, via the piezoelectric effect, mechanical stresses. The fields and stresses alternate in sign because of the alternating connections of the electrodes, and the stresses act as sources of surface waves. If the frequency is chosen such that the saw wavelength equals the transducer pitch, the waves generated by subsequent gaps are all in phase and therefore reinforce each other. For a given voltage, a longer transducer will give larger wave amplitude. The transducer on the right is the same structure but used to receive the waves, i.e. to give an output voltage in response to an incident wave. It operates in a reciprocal manner to the launching transducer, so a longer transducer will give a larger voltage for a given saw amplitude.
5.FREQUENCY RESPONSE OF SAW DEVICES
Fig.4hows the frequency response of SAW devices Surface acoustic waves travels on the elastic surface with frequency given by F=v/ λ Where v= velocity of SAW≈ 3100 m/s The basic advantage of these waves travels at very slow speed ≈ 3100 m/s that large delay can be obtained.
7
When stress- strain relations are applied to a non-piezoelectric dielectric elastic solid then application of an electric field to such solid would have no effect on its mechanical stress and strain characteristics. SAW velocity v will be dependent on the elasticity, density and piezoelectric properties of substrate used. These all parameters can change with temperature. Temperature variation will lead to the phase shift around the loop therefore, reducing stability of oscillator. For stability, temperature coefficient should be less and for narrow band width and delay line K ² should be low. K ² is electromechanical coupling coefficient. It is a measure of efficiency of given piezoelectric in converting an applied electrical signal into mechanical energy associated with a SAW. Here K ² and velocity v represents two most important practical material parameters used in SAW filter design .
6. ACOUSTIC WAVE PROPAGATION MODES
Acoustic wave devices are described by the mode of wave propagation through or on a piezoelectric substrate. Acoustic waves are distinguished primarily by their velocities and displacement directions; many combinations are possible, depending on the material and boundary conditions. The IDT of each sensor provides the electric field necessary to displace the substrate and thus form an acoustic wave. The wave propagates through the substrate, where it is converted back to an electric field at the IDT on the other side. Figure 2 shows the configuration of a typical acoustic wave device. Transverse, or shear, waves have particle displacements that are normal to the direction of wave propagation and which can be polarized so that the particle displacements are either parallel to or normal to the sensing surface. Shear horizontal wave motion signifies transverse displacements polarized parallel to the sensing surface; shear vertical motion indicates transverse displacements normal to the surface.
8
A wave propagating through the substrate is called a bulk wave. The most commonly used bulk acoustic wave (BAW) devices are the thickness shear mode (TSM) resonator and the shear-horizontal acoustic plate mode (SHAPM) sensor. If the wave propagates on the surface of the substrate, it is known as a surface wave. The most widely used surface wave
devices are
the surface acoustic wave sensor and the
Figure 5 Although it is the oldest
acoustic wave device, the thickness shear mode resonator is still used for measuring metal deposition rates.
shear-horizontal surface acoustic wave (SH-SAW) sensor, also known as the surface transverse wave (STW) sensor.
All acoustic wave devices are sensors in that they are sensitive to perturbations of many different physical parameters. Any change in the characteristics of the path over which the acoustic wave propagates will result in a change in output. All the sensors will function in gaseous or vacuum environments, but only a subset of them will operate efficiently when they are in contact with liquids. The TSM, SH-APM, and SH-SAW all generate waves that propagate primarily in the shear horizontal motion. The shear horizontal wave does not radiate appreciable energy into liquids, allowing liquid operation without excessive damping. Conversely, the SAW sensor has a substantial surface-normal displacement that radiates compression waves into the liquid, thus causing excessive damping. An exception to this rule occurs for devices using waves that propagate at a velocity lower than the sound velocity in the liquid. Regardless of the displacement components, such modes do not radiate coherently and are thus relatively undamped by liquids.
9
7. DELAY LINE
Delay line is basically a filter. Delay line is a SAW device without absorber between transmitter and receiver IDT. Construction of SAW materials is done in such a way to produce desired delay. Due to dispersive characteristics of SAW devices, lines offer wide bandwidths but have high attenuation. Used in RF microwave processing e.g. pulse compression, spread spectrum systems, etc. APODIZATION: multiplication of length to some mathematical function so that desired delay can be achieved. IF frequency was set at 2.45 GHz because, in that frequency range, many low-cost components are available, and also to lower the relative bandwidth of the SAW delay line because bandwidths up to 800 MHz, or even more, are needed. Using a standard SAW technique with uniform sampled transducers would result in high-insertion attenuation because the impedance of the interdigital transducers (IDTs) would obtain bad matching conditions. Furthermore, a lot of small taps would occur from the compensation procedure (leading to additional losses) because small taps are affected by strong diffraction effects. The insertion loss of filters with high relative bandwidth can be reduced using dispersive IDTs. We, therefore, decided to realize the SAW delay line with two chirped and weighted IDTs.
Fig 6. shows frequency response of delay line
10
Standard solutions result in filters consisting of an apodized and uniform transducer, with one or both transducers being dispersive. In this configuration, Fresnel ripple originating from the uniform transducer leads to problems, therefore, the apodized transducer has to compensate for this Fresnel ripple. To overcome this situation, it is advantageous to use two dispersive and amplitude weighted transducers. There are, however, some drawbacks. The linear design is difficult because the transfer function of the filter is not even in first order equal to the product of the transfer functions of both transducers. Compared to configurations using only one uniform transducer, second order effects, e.g., diffraction, become more severe. In order to compensate for these second-order effects, precise simulation tools for all relevant second-order effects, e.g., diffraction, The design and optimization algorithm for SAW linear phase delay lines consisting of two chirped and weighted IDTs will be discussed. For high-volume, low-cost, and high yield production of SAW structures with line widths down to 0.3 μm, technological improvements in the SAW production technique had to be achieved, especially in photolithography.
Fig7shows SEM photograph of part of the IDT (0.6-_m periodicity and 0.4-_m Line width).
11
8. COMPARISION WITH L-C FILTER •
Conventional linear phase L-C filters all have some inherent degree of phase non-linearity .The degrees to which their linearity of phase response is achieved over a prescribed frequency range increases with order of filter (i-e, with the number of reactive components).The resultant overall size of passive LC filter together with its complexity and cost, may render it suitable for many application.
•
SAW filter with comparable performance are often tiny by comparison.
•
Frequency response is better than that of LC filter.
•
SAW devices can not be implement low pass filters but only as band pass one because of alternative polarities of fingers in SAW IDT.
•
SAW filters can also be designed to operate very efficiently in harmonic modes up to about ninth harmonic frequency. This can lead to a considerable simplification of costly microelectronic lithographic processes required for gigahertz device fabrication.
9. APPLICATIONS
SAW devices are used as filters, oscillators and transformers, devices that are based on the transduction of acoustic waves. The transduction from electric energy to mechanical energy (in the form of SAWs) is accomplished by the use of piezoelectric materials.Electronic devices employing SAWs normally use one or more interdigital transducers (IDTs) to convert acoustic waves to electrical signals and vice versa by exploiting the piezoelectric effect of certain materials (quartz, lithium niobate, lithium tantalate, lanthanum gallium silicate, etc.). [2] These devices are fabricated by photolithography , the process used in the manufacture of silicon integrated circuits. SAW filters are now used in mobile telephones, and provide significant advantages in performance, cost, and size over other filter technologies such as quartz crystals (based on bulk waves), LC filters, and waveguide filters. Much research has been done in the last 20 years in the area of surface acoustic wave sensors. Sensor applications
include
all
areas
of
sensing (such
as
chemical,
optical, 12
thermal, pressure, acceleration, torque and biological). SAW sensors have seen relatively modest commercial success to date, but are commonly commercially available for some applications such as touch screen displays. At one extreme, the high volume low cost TV component market, SAW filters to be competitive in price and performance with L-C filters employed in IF circuit stages. At opposite extreme, the low volume high cost components of RADAR signal processing, maximum emphasis was given to efficient implement of SAW pulse compression filters with large compression gain.
10. MERITS OF RAYLEIGH WAVE DEVICES
•
SAW devices can generally be designed to provide quite complex signal processing function within a single package that contains only a single piezoelectric substrate with superimposed thin film I/O IDT. E.g., band pass filters with outstanding response that would require several hundred inductors and capacitors in conventional L-C filters.
•
They can be mass produced using semiconductor micro fabrication technique. As a result, they can be made to be cost-competitive in mass-volume application with some products selling for less than $1.00.
•
They can have outstanding reproducibility in performance from device to device. This is especially desirable for design of radio frequency (RF) and IF filters for mobile phones, narrow band high resonator, as well as for channelized receivers for spectral analysis or other system requiring precision filtering.
•
Although Saw devices are analog variety, they can be employed in many digital communication systems. e.g., Nyquist filter in quadrature- AM (QAM) digital radio modem.
•
SAW filters can be made to operate in high harmonics mode. Some devices for 2GHz band are now being fabricated in this manner in order to meet demanding lithographic tolerances and constraints.
13
11. DEMERITS OF SAW DEVICES
By 1980, SAW technique had reached a plateau in research because of the following reasons: •
Inherently high insertion losses (>6db of existing SAW filter design).
•
Signal-to-noise limitations.
So, this field was abandoned by many companies and research lab’s around the world. The preceding slow growth scenario has changed most dramatically in recent years, with discovery and utilization of other types of SAW. These are pseudo-SAW which made possible by discovery of new piezoelectric substrate and/or crystal cuts. Pseudo-SAW further classified into: •
LSAW (leaky SAW)
•
SSBW (surface skimming bulk waves)
•
STW (surface transverse wave)
DEMERITS OF SAW DEVICES AND MERITS OF PSEUDO-SAW DEVICES
Pseudo-SAW and Rayleigh wave devices both may be visually indistinguishable from one another but pseudo-SAW devices posses a number of attractive features over their SAW counterparts.
Velocities of pseudo-SAW devices can be much higher than Rayleigh wave devices and they can be operated upto about 1.6 times higher frequencies than for Rayleigh wave counterpart with same lithographic geometry.
Pseudo-SAW piezoelectric crystal cuts have much higher values of electromechanical coupling efficiencies which corresponding increase in operational band width capabilities in conjunction with lower attainable insertion loss.
Some pseudo-SAW piezoelectric crystal cuts of quartz can have superior temperature stability coefficient of delay (TCD) over their SAW counterparts.
14
LSAW and SSBW propagation is beneath the piezoelectric surface, such devices can be significantly less sensitive to surface contamination than Rayleigh wave devices.
As pseudo-SAW penetrates farther into substrate than Rayleigh waves, corresponding acoustic power densities will be less. That means, pseudo-SAW devices are capable of handling large powers before onset of IDT degradation due to violent surface vibrations.
12. FABRICATION OF DELAY LINES:
In today’s world, we are going towards miniaturization of circuits so that more number of circuit elements such as transistors fit in a smaller amount of space. So, we need photons or electrons to resolve such small dimensions. Delay lines are fabricated on a wafer by using lithography technique. Lithography is a process based on the ability of photo resist to store the replica of photo mask that is used for subsequent processing steps. e.g., etching, deposition or implantation. Resist protects the oxide layer from etching.
12.1 Lithography Lithography originally used an image drawn (etched) into a coating of wax or an oily
substance applied to a plate of lithographic stone as the medium to transfer ink to the blank paper sheet, and so produce a printed page. In modern lithography, the image is made of a polymer coating applied to a flexible aluminum plate. To print an image lithographically, the flat surface of the stone plate is (slightly) roughened — etched — and divided into hydrophilic regions that accept a film of water, and thereby repel the greasy ink; and hydrophobic regions that repel water and accept ink because the surface tension is greater on the greasy image area, which remains dry. The image can be printed directly from the stone plate (the orientation of the image is reversed), or it can be offset, by transferring the image onto a flexible sheet (rubber) for printing and publication. As a printing technology, lithography is different from intaglio printing (gravure), wherein a plate is either engraved, etched, or stippled to score cavities to contain the
15
printing ink; and woodblock printing, and letterpress printing, wherein ink is applied to the raised surfaces of letters or images. Most types of books of high-volume text are printed with offset lithography, the most common form of printing technology. Etymologically,
the
word lithography also denotes photolithography ,
a micro
fabrication technique used to make integrated circuits and microelectromechanical systems, as such are more technologically akin to etching than lithography, printing from a stone plate. The principle of lithography is: Lithography uses simple chemical processes to
create an image. For instance, the positive part of an image is a hydrophobic, or "water hating" substance, while the negative image would be hydrophilic or "water loving". Thus, when the plate is introduced to a compatible printing ink and water mixture, the ink will adhere to the positive image and the water will clean the negative image. This allows a flat print plate to be used, enabling much longer and more detailed print runs than the older physical methods of printing (e.g., printing, Letterpress).
TYPES OF LITHOGREPHY: LITHOGRAPHY is of various types but usually we use the following techniques of lithography •
ELECTRON BEAM LITHOGRAPHY
•
PHOTOLITHOGRAPHY
•
X-RAY LITHOGRAPHY
12.1.1 ELECTRON BEAM LITHOGRAPHY
Many light-based nanotechnology measuring and fabricating tools are limited by the wavelength of light. However, the smaller the wavelength of light, the higher the energy of the light, which can subsequently cause unwanted side effects. One way scientists get around this is to use electrons instead of light.
Enter E-beam
lithography. Basically, E-beam lithography consists of shooting a narrow, concentrated beam of electrons onto a resist coated substrate. Electrons can induce the deposition of substances onto a surface (additive), or etch away at the surface
16
(subtractive). E-beam lithography is particularly important in micro electronics, which require extremely precise placement of micro sized circuit elements. E-beam lithography allows scientists to design and place elements at the smallest possible scale. Also, electrons can be used to etch a “mask” whose patterns can be later transferred onto a substance using other techniques (think of a stencil you used in grade school). However, with such precision, components can only be made very slowly and only one at a time, greatly increasing the time and cost and prohibiting mass commercial acceptance. Also, because electrons are charged particles, it is necessary to perform E-beam lithography inside a vacuum, further complicating the required equipment and process. E-beam Components. The process of E-beam lithography is simple, however, the schematics and the parts required are quite complex. Instead of understanding the process of E-beam lithography, it is more efficient to understand some of the important components required for E-beam lithography to work successfully. Electron Gun: The centerpiece behind E-beam lithography is the electron gun. The specifics of an electron gun could stretch pages, so it is sufficient to know that the electron gun is an apparatus that is able to “shoot” a beam of electrons in a specific direction. Two common E-beam emitters are lanthanum hexaboride crystal and a zirconium oxide coated tungsten needle. The emitter is first heated to produce and excite electrons on the surface. Then, when a high voltage is applied, the excited electrons accelerate towards a structure called the anode. By varying this voltage, the trajectory and the focus of the beam can be manipulated. Electron Optical Column: The electron optical column is a system of lenses that, by a combination of electromagnetism and optics, has the ability to focus the electrons into a concentrated beam in a desired direction. Two parallel plates inside the column can be electrostatically charged to a precise degree; the resulting electric field is able to bend the beam in a desired direction. Surface: After the beam is directed and concentrated by the optical column, it is ready to be focused on the surface. As with most lithography techniques, a substance called a photo resist covers the surface. However, E-beam photo resist are not as specific as other types. Technically, high energy electron bombardment will cause bond breakage in any polymer. When the beam hits the surface, either an additive or subtractive reaction takes place. An additive writing method uses the electrons to induce a deposition of a compound on the surface. Subtractive writing 17
methods use the e-beam to remove the sections of the resist and surface. This method is common in creating masks for other lithographic techniques such as UV lithography.
1. Negative
2. The resist is
electron
developed
beam resist,
leaving
a
resist mask.
material
the
3. Reactive
Ion
4. The device is
Etching (RIE)
covered with a
is used to etch
cladding
the
oxide using for
mask
that becomes
pattern into the
example
insoluble in
top
Plasma
developing
layer, down to
Enhanced
solutions
the
Chemical
when
oxide layer.
exposed
silicon buried
of
Vapor
to
Deposition
radiation, is
(PECVD).
spun on a silicon-oninsulator (SOI) wafer. Photonic structures are defined by electron beam lithography.
Fig.7 shows electron beam lithography process 18
SCANNING METHODS Raster Scan: The e-beam is swept across the entire surface, pixel by pixel, with the
beam being turned on and off according to the desired pattern. This method is easy to design and calibrate, however, because the beam is scanned across the entire surface, sparse patterns take the same amount of time to write as dense patterns, making this method inefficient for certain types of patterns.
Fig. 8 shows a) RASTER SCANNING b) VECTOR SCANNING Vector Scan: The e-beam “jumps” from one patterned area to the next, skipping
unwanted areas. This makes the vector scan much faster than the raster scan for sparse pattern writing. Adjustments to the beam can also be made relatively easily. However, it takes longer for the beam to settle, making it more difficult to maintain accurate placing for the beam.
Disadvantages:
Electron Backscattering and Proximity effects: When electrons are subjected directly to a surface, they tend to “scatter” quickly. This phenomenon, known as electron backscattering, causes unwanted reactions to take place outside of the focused electron beam. As a result, the resolution of an E-beam is not limited to only the size of the focused beam. In addition to backscattering, the focused Ebeam hitting the surface produces secondary electrons, which can expose the resist as much as several micrometers away from the point of exposure. These proximity effects can cause 19
critical variations when dealing with surfaces that need to be exact on the sub-micro level. •
Scattering of electron beam.
•
Serial processing. i.e., slow and small area processing.
Advantages: •
Better resolution.
•
Direct writing, no mask needed.
•
Arbitrary size, shape and order.
Efficiency:
While E-beam lithography is perhaps the most accurate and precise of all the lithographic techniques, perfection comes at a high price. The complex equipment and slow exposure times makes E-beam lithography impractical as a mass production micro manufacturing method. Also, because electrons are charged particles, E-beam lithography must be performed in a vacuum. Steps are being taken however, in customizing tools such as scanning electron microscopes into having the ability to produce focused electron beams. Because of some limitations as mentioned above electron beam lithography is not preferred in mass fabrication. Hence, optical lithography is used for fabrication of mask of SAW devices such as delay lines.
20
12.1.2. PHOTOLITHOGRAPHY
OPTICAL LITHOGRAPHY: It consists of a photo mask, an optical system and photo resist spinned on the top of wafer. It is a process by which geometric shapes on a mask or wafer are transferred onto a substrate using photons when it is exposed to light. PHOTOMASK: Photo mask is a precision glass plate coated with chrome having very small features of electronic circuit. The substrate (transparent) type used may be quartz, low expansion glass or soda lime. And several opaque materials are used to block light and they may be chrome, emulsion, iron oxide.
Fig.9 shows photomasks STEPS TO GENERATE MASK •
First step in mask generation for IC fabrication is to draw large-scale composite of set of masks, typically 100x to 2000x the final stage.
•
Then composite layout is converted into a set of oversized artwork with a drawing for each masking level.
•
Then the artwork is reduced from 10x glass retical.
•
The final mask is made from 10x retical using another photo reduction system that reduces the image to 1x.
Masks are made from glass emulsion plates like kodac high resolution plate or glass covered with hard surface material. Emulsion masks are least expensive but are usually only used with feature size in 5 μm region. All the electron beam masks are made up with hard surface materials such as chromium, chromium oxide, iron oxide or silicon. These masks are more expensive than emulsion but feature size in 1 μm region can be defined on the surface of wafer.
21
So, often master mask is made on quartz; then the pattern is transferred to less expensive glass where it is step and repeated to create several dies. There are two polarities of mask which are commonly used: Light field: This type of field is mostly clear. Bright field: This type of field is mostly dark. Fields of photomask
Fig.9 shows fields of photomask PHOTO RESIST: A photo resist is a light-sensitive material used in several
industrial processes, such as photolithography and photoengraving to form a patterned coating on a surface. It changes is chemical composition when exposed to light. Photo resist are basically of two types: Types of photoresist
Fig.10 shows types of photoresist
22
POSITIVE PHOTO RESIST: For positive resists, the resist is exposed with UV
light wherever the underlying material is to be removed. In these resists, exposure to the UV light changes the chemical structure of the resist so that it becomes more soluble in the developer. The exposed resist is then washed away by the developer solution, leaving windows of the bare underlying material. In other words, "whatever shows, goes." The mask, therefore, contains an exact copy of the pattern which is to remain on the wafer. NEGETIVE PHOTO RESIST: Negative resists behave in just the opposite manner.
Exposure to the UV light causes the negative resist to become polymerized, and more difficult to dissolve. Therefore, the negative resist remains on the surface wherever it is exposed, and the developer solution removes only the unexposed portions. Masks used for negative photo resists, therefore, contain the inverse (or photographic "negative") of the pattern to be transferred. The figure below shows the pattern differences generated from the use of positive and negative resist .
Fig.11 shows processing of photoresist
23
DIFFERNCE
BETWEEN
POSITIVE
AND
NEGATIVE
PHOTORESIST POSITIVE PHOTO RESIST •
It becomes more soluble when exposed to light.
•
Solubility in developer is finite even at zero exposure energy.
•
At some threshold it becomes completely soluble.
•
Longer exposure time and throughput is less.
NEGETIVE PHOTO RESIST •
It becomes less soluble when expose to light.
•
At low energies it remains completely soluble in the developer.
•
Exposure above threshold energy is increased, more the resist film remain after development.
•
Exposure time is less and high throughput.
This figure shows the graph for film thickness versus exposure dose for a) positive resist and b) negative resist. Contrast is defined as the slope of the linear portion of falling (or rising) section of curve. Curves are affected by: Initial resist thickness, spectral distribution of exposure radiation, prebake conditions, developer chemistry, developing time and so on.
Fig.12 shows graph of positive and negative photoresist
24
COMPONENTS OF PHOTO RESIST •
Matrix material (Novolac resin).
•
Sensitizer (or dissolution inhibitor) – diazoquinones.
•
Solvent ( n-butyl acetate, xylene)
MATRIX MATERIAL
The binder is a Novolac resin which has the following key properties: •
Inert to radiation.
•
Provides good adhesion to substrate.
•
Etch resistant in wet and dry “etchers”.
Fig.13 shows chemical composition of matrix material
25
SENSITIZERS
Sensitizers are photoactive compounds (PAC) – diazoquinones. •
Sensitizers absorb chemical reaction to dissolution properties in the resist.
•
The net result is differential (100:1) between areas that absorbed radiation.
•
Sensitizers and Developer resistant before they absorb radiation – typical resist developer and hydroxides- KOH, NaOH, TMAH, etc.
Fig. 14 shows SENSITIZER REACTIONS after UV Exposure
26
SOLVENT •
XYLENE o
Keeps photo resist in liquid state.
o
Allows spin coating of photo resist.
o
Solvent content determines viscosity and hence thickness.
Fig.15 shows chemical composition of solvent
PHOTORESIST METRICS •
Resolution
•
Contrast
•
Sensitivity
•
Spectral Response Curve
RESOLUTION: How a fine line the resist can produce from an areal image.
Resolution of resist is determined by o
Contrast, Thickness, Proximity effects
o
Swelling and contraction after development.
27
CONTRAST: It is an ability of resist to distinguish between light and dark
regions. o
Measured by exposing the resist of given thickness to varying radiation dose and measuring dissolution rate.
Fig.16 shows contrast curve
SENSITIVITY: o
Incident energy necessary to produce the photochemical reactions required for defining patterns.
o
Related to quantum yield = ( # of photon- induced events) # of photon absorbed o
Higher sensitivity required at shorter wavelength because of limited brightness of UV sources and optics efficiency.
o
Trade-off between exposure time and brightness.
28
SPECTRAL
RESPONSE: Many photo
resist types exist to suit a large
variety of applications, and the action spectra can vary from one to the next. It is critical to control the action spectra exposure settings, both the intensity of the light and the time of exposure, to avoid wasteful under or overexposure of the photo resist during processing and production. To maintain these settings, a light meter with a spectral response as close to the photo resist’s action spectrum as possible is required to get an accurate idea of how well the photo resist is being exposed. Further complicating matters is the processing equipment where photo resists are exposed are frequently very compact and have minimal room for taking these measurements. International Light Technologies is committed to providing a variety of unique, photo resist-specific instruments to assist our customers in making these often difficult measurements .
Fig.17 shows spectral response of photoresist The typical emission spectrum of a mask aligner or stepper with Hg light source and without optical selective mirrors/filters contains g- (wavelength 436 nm), h- (405 nm)
29
and i-line (365 nm), with an i-line intensity approx. 40 % of the total emission between 440 and 340 nm. Especially for exposure dose sensitive processes (image reversal-, thick resist processing, high resolution) a calibration of the illumination intensity (changing with bulb operating time) is strongly recommended. A measurement of the lateral intensity distribution should reveal less then 10 % deviation over the substrate size in order to allow a proper exposure dose for central and edge-near regions of the resist film. HOW PHOTOMASKS ARE MADE CIRCUIT DESIGN:
The customer designs the circuit and digitizes the information. The customer then sends us the digitized data containing the design for each layer. The data can be sent on a floppy disk, magnetic tape, cassette or via modem .
CUSTOMER DATA
FRACTURED DATA
Fig. 18 shows types of design
DATA PREPARATION:
Photronics takes the customer's data and formats it for the write (lithography) tools or systems. This includes fracturing the data, sizing the data if needed, rotating the data if needed, adding fiducials and internal reference marks, and
30
making a job deck (instructions for the placement of all the different patterns on the mask). Fracturing the data means translating the customer data into a language the write tool can understand. The write system uses rectangles and trapezoids - so the customer data is divided up (fractured) into these shapes. The job deck with the fractured data is put on a magnetic tape and sent to the write area or pulled directly to the machines using network software.
PROCESSING OF PHOTO RESIST Photo resist layers have two basic functions: 1) precise pattern formation; and 2) protection of the substrate from chemical attack during the etch process. Typical resists consist of three components: 1) the resin, which serves as the binder of the film; 2) the inhibitor or sensitizer, which is the photoactive ingredient; and 3) the solvent, which keeps the resist in liquid state until it is processed. The pattern formed by the resist layer actually results only after the unwanted portions of the erstwhile uniformly distributed resist have been removed, as explained in the following discussion on photo resist processing.
Photos resist processing , or simply resist processing, basically consists of six steps:
1) dehydration and priming; 2) resist coating; 3) soft baking; 4) exposure; 5) development; and 6) post-development inspection. Prior to the application of resist to a wafer, the wafer must be free of moisture and contaminants,
both
of
which
cause
a
multitude
of
resist
processing
problems. Dehydration baking is performed to eliminate any moisture adsorbed by substrate surfaces, since hydrated substrates result in adhesion failures. The bake is usually performed between 400 deg C to 800 deg C. Convection ovens may be used for baking up to 400 deg C, while furnace tubes are used for 800 deg C baking. After dehydration baking, the wafer is coated with a pre-resist priming layer designed to enhance the adhesion properties of the wafer even further. One of the most common primers used for this purpose is hexamethyldisilazane (HMDS). Resist coating must follow as soon as possible after priming (within an hour after priming).
31
Resist coating, or the process itself of producing a uniform, adherent, and defect-free resist film of correct thickness over the wafer, is usually performed by spincoating. Spin-coating consists of dispensing the resist solution over the wafer surface and rapidly spinning the wafer until it becomes dry. Most spin-coating processes are conducted at final spin speeds of 3000-7000 rpm for duration of 20-30 seconds. Resist coating is followed by a soft bake, which is done to: 1) drive away the solvent from the spun-on resist; 2) improve the adhesion of the resist to the wafer; and 3) anneal the shear stresses introduced during the spin-coating. Soft baking may be performed using one of several types of ovens (e.g., convection, IR, hot plate). Soft bake ovens must provide well-controlled and uniformly distributed temperatures and a bake environment that possesses a high degree of cleanliness. The recommended temperature range for soft baking is between 90-100 deg C, while the exposure time needs to be established based on the heating method used and the resulting properties of the soft-baked resist. After a wafer has been coated with photo resist and subjected to soft baking, it has to undergo exposure to some form of radiation that will produce the pattern image on the resist. The pattern is formed on the wafer using a mask, which defines which areas of the resist surface, will be exposed to radiation and those that will be covered. The chemical properties of the resist regions struck by radiation change in a manner that depends on the type of resist used. Irradiated regions of positive photo resists will become more soluble in the developer, so positive resists form a positive image of the mask on the wafer. Negative resists form a negative image of the mask on the wafer because the exposed regions become less soluble in the developer. Development, which is the process step that follows resist exposure, is done to leave behind the correct resist pattern on the wafer which will serve as the physical mask that covers areas on the wafer that need to be protected from chemical attack during subsequent etching, implantation, lift-off, and the like. The development process involves chemical reactions wherein unprotected parts of the resist get dissolved in the developer. A good development process has a short duration (less than a minute), results in minimum pattern distortion or swelling, keeps the original film thickness of protected areas intact, and recreates the intended pattern faithfully.
32
Development is carried out either by immersion developing, spray developing, or puddle developing. Regardless of method used, it should always be followed by thorough rinsing and drying to ensure that the development action will not continue after the developer has been removed from the wafer surface. Post-development inspection, as the name implies, is an inspection conducted after development to ensure that the resist processing steps conducted earlier have produced the desired results. This is typically done using an optical microscope, although SEM and laser-based systems are also used in some post-development inspection tasks. Items that this inspection step checks for include the following: 1) use of the correct mask; 2) resist film quality; 3) adequate image definition; 4) dimensions of critical features; 5) defects and their densities; and 6) Pattern registration.
HOW ARE PHOTOMASKS USED?
The steps in making (fabricating) devices like ICs include deposition, photolithography, and etching. During deposition, a layer of either electrically insulating or electrically conductive material (i.e. metal, polysilicon, oxide) is deposited on the surface of a silicon wafer. This material is then coated with a photosensitive resist. A photomask is then used much the same way a photographic negative is used to make a photo. Photolithography involves projecting the image on the photomask onto the wafer. (If the image on the photomask is projected several times side by side onto the wafer, this is known as stepping and the photomask is called a reticle. )
33
The wafer is developed and then etched to remove material from the areas exposed with the photomask image.
PARTS OF PHOTOMASK DIE
A die is a single complete device image. A primary die (also called the primary pattern) has the device design that will be used to make the circuit. The primary die or dies will make up most of the array or fields. A test die contains a simplified functional device (of the same process type as the primary die). The test die is used for process control and monitoring during wafer fabrication and sometimes to test new design ideas.
Fig.19.1 shows die of photomask
34
SCRIBE LINES
Scribe lines (scribes) are the lines forming a border around each die separating the dies from one another. There may be small patterns placed within these scribes,usually alignment marks or test patterns used in wafer fabrication.
Fig. 19.2 shows scribe lines ARRAY
The array is the area made up of the rows and columns of dies on 1X masters, 5X reticles and other reduction reticles.
Fig. 19.3 shows array of photomask
35
FIELD
Fields are the blocks of dies on a UT1X reticle. Primary fields contain primary dies. A test field(s) contains test dies and/or patterns and usually some primary dies. ROW
A row consist of a horizontal line of fields on a UT1X reticle
36
FIDUCIALS
Fiducials are patterns on reticles used for alignment on wafer steppers. Each brand and model of stepper has specific types of fiducials. The fiducials are located outside of the array or fields.
Fig. 19.4 shows fudicials in photomask KLA/KLARIS REFERENCE MARKS
KLA/KLARIS reference marks are internal marks placed in the corners of the mask. These marks are used as a points of reference when setting up inspections on an inspection system (such as a KLA system) and when repairing defects found in such inspections. The design and placement of these reference marks may vary from one manufacturing site to another. AUTO-INSPECTION MARKS (KLA MARKS)
Auto-Inspection Marks (KLA Marks) are used on UT1X reticles and located outside each field. These marks are crosses defining the area to be inspected on the KLA or other inspection systems.
37
CLOSURE CHECKS
Closure Checks are sets of patterns used to monitor the performance and accuracy of the lithography equipment. These marks are located outside the array. The closure check consists of several patterns; half of a pattern is written at the beginning of the write process and the second half of a pattern is written at the end of the write process. Comparing the two halves of the patterns can show any shifts or discrepancies that have occurred while the mask was being written. The actual patterns used in a closure check may vary from one manufacturing site to another. BLANKS and AUXILIARIES (AUX.'S)
Blanks and Auxiliaries (Aux.'s) are used on 1X masters and UT1X reticles. A blank is a pattern, containing no circuitry, used to clear out a specified window area. An auxiliary is a pattern, containing no circuitry, which surrounds a smaller pattern in order to clear out a specified window area and isolate the smaller pattern. TYPES OF PHOTOMASK Masters (used in MultiMEMS):
►Used on projection aligners . ►Patterns are projected: only once onto the wafer, with no reduction ( 1X). ►The repetitions of the device are on the photomask.
38
Reticles (not used in MultiMEMS):
►Used on steppers. ►Patterns are projected: many times onto the wafer, with reduction (2X, 4X, 5X, 10X). ►The repetitions of the device are done by stepping the projections
QUALITY INSPECTION ROUTINE
In order for the circuits being made from the photomask imagery to function, the photomasks must meet certain quality standards. The masks are tested for critical dimensions, defects, registration, and contamination before being sent to the wafer fab. •
Checking the Critical Dimensions (CD).
•
Defect Inspection.
•
Data Verification.
•
Registration.
•
Contamination.
CRITICAL DIMENSIONS
Customers have very specific requirements for geometry sizes and specify certain geometry on the mask to be used as a gauge. These geometries are called critical dimensions or CDs. The customer indicates the target size or spec of the CDs
in microns (a thousand microns equals a millimeter) and the acceptable variance from that target (the tolerance ).
39
The amount of time a mask is processed (developed and etched) after the mask is written is important because it affects the size of the geometry. The longer a mask is processed the smaller the chrome geometry will be and the larger the clear areas will be. CDs are measured on a microscope during the processing of a mask to determine process times and to make sure the CD size is acceptable. (Sometimes CDs will also be measured again on registration equipment).
Fig. 20.1 shows critical dimension geometry DEFECT INSPECTION
A defect is any flaw affecting the geometry. This includes chrome where it should not be (chrome spots, chrome extensions, chrome bridging between geometry) or unwanted clear areas (pin holes, clear extensions, clear breaks). A defect can cause the customer's circuit not to function. The customer will indicate the size of defects that will affect their process (defect spec). All defects that size and larger must be repaired, or if they can not be repaired the mask must be rejected and rewritten. There is also a class of defects known as cosmetic defects. These are defects that may not affect the circuitry geometry but still may not be acceptable to the customer. Cosmetic defects include scratches on the chrome outside the array, damaged or partially removed AR coating, contamination on the chrome, glass chips on the edge of the mask, etc.
Fig. 20.2 shows defects of photomask
40
A die-to-die inspection system (KLA) is used to inspect for defects. These machines use two objectives and transmitted (bottom) light to compare similar die patterns to one another. The image seen through the objectives is divided into pixels. The information in each pixel is digitized and compared to the information found in the other objective. If the pixels do not match the machine registers a discrepancy. This process is known as image processing. After the inspection is completed, an operator must view each discrepancy and determine which kind of defect was found and give the defect a code number or classification.
DATA VERIFICATION
A die-to-database (KLARIS or Orbot) inspection is similar to a die-to-die inspection, except instead of comparing a die to another die it is compared to a database. This inspection is used to insure that the geometry on the mask matches the customer's design. The image seen through the objective is compared to the digitized image on the database. If the images do not match the system registers a discrepancy. Again when the inspection is complete, an operator must review each discrepancy and classify it.
Database image
Right objective
Fig. 20.3 shows images for data verification 41
REGISTRATION
Registration equipment (LMS 2000 and Nikon 2i) is used to measure the positioning of patterns on a mask in relation to other layers in the device or to a design grid. This can include checking the size, placement, and rotation of dies, fields, and arrays. This equipment can also be used to measure the placement of fiducials and measure CDs. Fig. 20.4 shows types of registration inspections:
DIE FIT
a) comparing the size,
ARRAY REGISTRATION
b) comparing the size,
distortion and rotation
distortion and rotation
of the die on mask
of the die on the array
to the design grid
to the design grid
CONTAMINATION
Contamination on a mask will have the same ill effect as a chrome defect when projected onto the wafer. That is why photomasks are made and used in cleanroom environments. Before being shipped to the customer the photomask must be carefully cleaned and then inspected for contamination. Pellicles (metal frames with a protective membrane) are often attached to the masks to help keep them clean. Photronics uses four ways to inspect for contamination:
42
1) Gross Light Inspection - the mask is inspected under a high intensity light for gross contamination (contamination that can be seen with the naked eye). 2) Microscope Inspection - an operator manually inspects the mask on a microscope using a high power (50 to 200 times) magnification . 3) Q.C. Optics or KLA Starlight Inspection - an automated inspection on a Q.C. Optics or a Starlight. These machines inspect for contamination using deflected light to detect any particles on the surface of the mask or pellicle. 4) Post-pell die-to-die or die-to-database inspections - an automated inspection on a KLA or Orbot defect inspection system. Though the KLA and Orbot systems are used to detect defects they will also detect contamination in the clear areas on the mask. Post-pell (post-pellicle) inspections are performed after the mask has been cleaned
and has had a pellicle attached to it.
OTHER QUALITY INSPECTIONS
Some other quality inspections are performed by operators. These inspections include verifying that the mask has the correct titles, fiducials, field tone, data parity, array rotation and has no cosmetic defects. Manual microscope inspections are used to look for defects in any areas that could not be inspected on automated equipment, this includes test patterns, scribes, and fiducials. Sometimes a microscope is also used to take array and fiducial placement measurements.
43
13.PHOTOLITHOGRAPHY PROCESSING
Fig. 21 shows the different process used for PHOTO RESIST PROCESSING
CLEAN: Before the processing, the surface of resist is made clean in which
moisture and oxides are removed from the surface of photo resist. To drive off the moisture, it is baked at temperature between 150- 200 degree Celsius. SPIN COATING: Spin coating is the preferred method for application of thin,
uniform films to flat substrates. An excess amount of polymer solution is placed on the substrate. The substrate is then rotated at high speed in order to spread the fluid by centrifugal force. Rotation is continued for some time, with fluid being spun off the edges of the substrate, until the desired film thickness is achieved. The solvent is usually volatile, providing for its simultaneous evaporation. SOFT BAKE: After coating, the resist film has a thickness-dependant remaining
solvent concentration (e.g. PGMEA). Soft baking minimizes the solvent concentration in -
order avoid mask
contamination or
prevent bubbling or improve minimize
foaming
the resist the dark
to: sticking by
to
the
N2 during
adhesion to erosion during
the
mask, exposure, substrate,
development,
- prevent dissolving one resist layer by a following during multiple coating, and - prevent bubbling during subsequent thermal processes ( coating, dry etching).
44
After spin coating, the typical average remaining PGMEA concentration in the resist film is between 20% (thin films) and 40% (thick films). Soft baking reduces the remaining solvent concentration in the resist by thermally activated solvent diffusion in the resist bulk, and evaporation as a function of the temperature and the resist surface solvent concentration. A good starting point for a sufficient softbake is 100°C hotplate for 1 minute/μm resist film thickness. Insufficiently soft baked resist films reveal high dark erosion during development (resist thinning), with the remaining resist structures too small and less sharp than desired. Beside solvent reduction, a long/hot softbake thermally decomposes a part of the photo active compound (DNQ-sulfonate) thus reducing the development rate. Therefore, despite lower dark erosion, the longer necessary development rate may increase the total dark erosion. Compared to a hotplate, the much more distinct temperature hysteresis of an oven as well as the different heat transfer mechanism (convection in stead of heat conduction) causes - especially for short (few minutes) - baking steps (softbake, reversal bake, hard bake) or substrates with high heat capacity (thick glasses and ceramics) different effective temperatures in the photo resist and time intervals for the required final temperature. EXPOSE:
During exposure with matched UV-light, the photo active compound DiazoNaphtoQuinone- (DNQ-) sulfonate (left) releases a N 2 molecule and converts 45
into indene carboxylic acid hereby requiring a H 2O molecule. Compared to unexposed DNQ-sulfonate, the carboxylic acid yields a resist development rate (alkaline solubility) several orders of magnitude higher. In order to i) improve the DNQ solubility in the resist, ii) to increase the inhibitor property (dark erosion reduction), and iii) to improve the thermal stability, generally several DNQ-sulfonate molecules are bonded to a so-called backbone-molecule. The photo reaction quantum efficiency defines the number of above-mentioned reactions in relation to the photons absorbed in the resist film. Using a sufficiently transparent resin, suited photon energy (g, h, i-line with respect to the specific DNQ) and a sufficient H 2O-concentration in the resist (rehydration!), the quantum efficiency in DNQ-based positive-tone photo resists achieves values of typically 20-30%. If the resist lacks a minimum concentration of water, the ketone formed during the photoreaction may perform various side reactions (e.g. esterify with the resin or polymerize accompanied by CO2 separation). In both cases, the development rate solely increases by the reduction of the inhibitor (DNQ-sulfonate) concentration. POST EXPOSURE BAKES:
The post exposure bake PEB (performed after exposure, but before development) can be applied above the softening point of the resist without destroying the structures to be developed due to the still closed resist film. Possible reasons for a PEB (typically performed at 110°C for 1-2 min on a hotplate) are: In chemically amplified resists, the PEB catalytically performs and completes the photo reaction initiated during exposure. The AZ ® and TI resists distributed by Micro Chemicals ® do not belong to chemically amplified resists, and therefore do not require a PEB for this purpose. A PEB performed near the softening point of the photo resist reduces mechanical stress formed during softbake and exposure of especially thick resist films due to the
expanding nitrogen and therefore improves resist adhesion and reduces under etching in subsequent wet chemical etching. However, a certain delay between exposure and PEB is required to outgas N 2. Otherwise, during PEB the N 2 in the resist will expand and increase mechanical stress in the film! The PEB promotes the thermally activated diffusion of carboxylic acid formed during exposure from the photo active compound. This diffusion step smoothens the spatial periodic pattern of carboxylic acid having their origin in standing light waves during 46
monochromatic exposure especially in case of highly reflective substrates. These patterns otherwise would transfer to the resist profile thus e.g. reducing the spatial resolution of the resist and the desired aspect ratio. In many cases, for processes without high (<1 micron) resolutions required, the PEB is not necessary. However, for certain (negative tone) resists such as the AZ ® 2000 nLOF family, a PEB is inevitable for the crosslinking induced by the exposure. DEVELOP: The carboxylic acid formed during exposure moves from the
hydrophobic to the hydrophilic part of the cresol chain and promotes the deprotonisation of the OH-group increasing the resist solubility in aqueous alkaline developers. Beside the carboxylic acid, also acetic acid (formed by alkaline developers from the resist solvent PGMEA) increases the development rate of exposed and unexposed resist. This explains the higher dark erosion of resist with a remaining solvent concentration too high (e.g. in case of an insufficient softbake). HARD BAKE:
The hard bake sometimes performed after development intends to increase the thermal, chemical , and physical stability of developed resist structures for subsequent processes (e.g. electroplating, wet-chemical and dry-chemical etching). Hereby the following mechanisms have to be considered: Coated photo resists react with atmospheric oxygen and embrittle above approx. 130°C. The different thermal expansion coefficient of resist/substrate can lead to the cracking of the resist making it useless as mask for wet or drying-chemical etching or electroplating. If the hard bake cannot be waived, nor the hard bake temperature reduced, the cracking can be suppressed by a slow cooling (e.g. -3°C/min ramp, or by keeping the substrate in/on the switched-off oven/hotplate for soft cooling, if feasible). ETCH: ETCHING is done by two methods:
WET ETCHING: Inferior resist adhesion causes lifting of the resist starting from the resist edges, with subsequent under etching. In exothermic etching solutions, this resist lifting is promoted by local heating and may further be promoted by mechanical forces if e.g. 47
H2 evolution occurs. Resist adhesion can be improved by i) using a suited resist, ii) a proper substrate
pretreatment (e.g.
TI
PRIME),
and
iii)
optimized processing (sufficient softbake, image reversal bake). On glass and ceramics, a hard bake is not generally recommended due to the different heat expansion coefficients of resist and substrate, which might cause crackles in the resist. In isotropic etch media, even optimized resist adhesion results in under etching. If convection and diffusion of the etched medium is not the etch rate bottleneck, the lateral under etching near the substrate/resist interface is comparable to the etched depth. DRY ETCHING: In plasma, UV radiation with spectral lines within the absorption range (g-, h-, and iline) of photo resists might expose the resist during etching causing N 2 at a high rate forming bubbles and foaming at elevated temperatures. In this case, a flood exposure (without mask, with subsequent delay to outgas N 2) or/and the usage of
image reversal resists in the reversal mode will help. Also a good alternative is the usage of resists with a high thermal stability and without gas formation during exposure like AZ® nLOF 2000 series, which can be developed in aqueous alkaline solutions, and removed in O-plasma or, alternatively, in organic solvents such as acetone or NMP. STRIPPING: Photo resist stripping, or simply 'resist stripping', is the removal of
unwanted photo resist layers from the wafer. Its objective is to eliminate the photo resist material from the wafer as quickly as possible, without allowing any surface materials under the resist to get attacked by the chemicals used. Resist stripping can be classified into: 1) organic stripping; 2) inorganic stripping; and 3) dry stripping.
Organic stripping employs organic strippers, which are chemicals that break down the structure of the resist layer. The most widely-used commercially available organic strippers used to be the phenol-based ones, but their short pot life and difficulties with phenol disposal made low-phenol or phenol-free organic strippers the more popular choice nowadays. Wet inorganic strippers , which are also known as oxidizing-type strippers, are used for inorganic stripping, usually to remove photo resist from non-metalized wafers, as well as post-baked and other hard-to-remove resists. Inorganic strippers are
48
solutions of sulfuric acid and an oxidant (such as ammonium persulfate), heated to about 125 deg C. Dry stripping pertains to the removal of photo resist by dry etching using plasma etching equipment. Its advantages over wet etching with organic or inorganic strippers include better safety, absence of metal ion contamination, decreased pollution issues, and less tendency to attach underlying substrate layers.
13.1 Mask Alignment and Exposure
One of the most important steps in the photolithography process is mask alignment. A mask or "photomask" is a square glass plate with a patterned emulsion of metal film on one side. The mask is aligned with the wafer, so that the pattern can be transferred onto the wafer surface. Each mask after the first one must be aligned to the previous pattern. Once the mask has been accurately aligned with the pattern on the wafer's surface, the photo resist is exposed through the pattern on the mask with a high intensity ultraviolet light. There are three primary exposure methods: contact, proximity, and projection. They are shown in the figure below.
49
Fig. 22 shows types of PHOTOLITHOGRAPHY Contact Printing
In contact printing, the resist-coated silicon wafer is brought into physical contact with the glass photomask. The wafer is held on a vacuum chuck, and the whole assembly rises until the wafer and mask contact each other. The photo resist is exposed with UV light while the wafer is in contact position with the mask. Because of the contact between the resist and mask, very high resolution is possible in contact printing (e.g. 1-micron features in 0.5 microns of positive resist). The problem with contact printing is that debris, trapped between the resist and the mask, can damage the mask and cause defects in the pattern. Proximity Printing
The proximity exposure method is similar to contact printing except that a small gap, 10 to 25 microns wide, is maintained between the wafer and the mask during exposure. This gap minimizes (but may not eliminate) mask damage. Approximately 2- to 4-micron resolution is possible with proximity printing.
50
Projection Printing
Projection printing, avoids mask damage entirely. An image of the patterns on the mask is projected onto the resist-coated wafer, which are many centimeters away. In order to achieve high resolution, only a small portion of the mask is imaged. This small image field is scanned or stepped over the surface of the wafer. Projection printers that step the mask image over the wafer surface are called step-and-repeat systems. Step-and-repeat projection printers are capable of approximately 1-micron resolution.
13.2 Exposure of Photoresists
When using a laser beam as light source for photoresist exposure in stead of the usually used Hg bulbs, one has to consider two main points: The light intensity is quite different: While in laser interference lithography, the light intensity is rather low, laser scribing causes intensities many orders of magnitude beyond the intensity of a mask aligner or stepper. The exposure wavelengths of the laser often differs from the 365, 405, or 435 nm Hg lines which are matched to the spectral sensitivity of photoresists. The spectral sensitivity of photoresists does not abruptly end at a certain wavelength, but smoothly drops to zero over few 10 nm. Therefore, using an adjusted exposure dose (e. g. laser scribing), exposure with wavelengths outside the sensitivity range given in the technical data sheet is also possible to a certain extent.
51
16. FABRICATION OF DELAY LINE: Fabrication of delay line can be done by using photolithography process. And which can be achieved by these following processes:
•
Wafer(LiTaO3 or LiNbO3, or SiO2)
•
Al deposit (sputtering) 0.15 to 1.5 micron.
•
Photo resist (PR coating)
•
Exposure
•
Develop
•
Al etching (wet etching)
•
PR removal
•
QC check + Probing
•
Sieving (Scribing)
•
QC check (chips, cracks)
•
Mounting Ag/ UV bond
•
Wire bounding
•
Seam sealing
•
Marking
•
Final test and inspections
52
Fig. 23 shows fabrication of SAW device
Optical Absorption and Spectral Sensitivity
The optical absorption of most unexposed photoresist ranges from the approx. 440 nm in the VIS to near UV. This spectral sensitivity is matched to the emission spectrum of Hg lamps (i-line = 365 nm, h-line = 405 nm, g-line = 435 nm) in mask aligners left-hand) and causes the typical reddish-brownish color of many photoresists.
53
18. CONCLUSION The fabrication of Saw delay line is done successfully on a piezoelectric elastic substrate. Mask was prepared and complete design was transferred on piezoelectric bulk. The structure can be used for calibration of SAW DELAY LINE.
54
Table of Contents SAW DEVICES......................................................................... 1 1. INTRODUCTION..................................................................................................... 1 2. DEFINATIONS OF SAW........................................................................................ 2 3. CONSTRUCTION.................................................................................................... 3 3.1 Piezoelectric Substrate Materials................................................................................3
4.OPERATION.............................................................................................................6 5.FREQUENCY RESPONSE OF SAW DEVICES .................................................... 7 6. ACOUSTIC WAVE PROPAGATION MODES...................................................... 8 7. DELAY LINE..........................................................................................................10 8. COMPARISION WITH L-C FILTER...............................................................12 9. APPLICATIONS.................................................................................................... 12 10. MERITS OF RAYLEIGH WAVE DEVICES ................................................. 13 11. DEMERITS OF SAW DEVICES..................................................................... 14 12. FABRICATION OF DELAY LINES:.................................................................15 12.1 Lithography...............................................................................................................15 TYPES OF LITHOGREPHY:......................................................................................... 16
12.1.1 ELECTRON BEAM LITHOGRAPHY...........................................................16 SCANNING METHODS ................................................................................................. 19
Disadvantages: ........................................................................................................... 19 Advantages:.............................................................................................................. 20 Efficiency: .............................................................................................................. ...20 12.1.2. PHOTOLITHOGRAPHY ............................................................................... 21 DIFFERNCE BETWEEN POSITIVE AND NEGATIVE PHOTORESIST...............24 COMPONENTS OF PHOTO RESIST...........................................................................25
Fig.15 shows chemical composition of solvent.......................................................... 27 PHOTORESIST METRICS............................................................................................27 HOW PHOTOMASKS ARE MADE..............................................................................30 CIRCUIT DESIGN:.........................................................................................................30 The customer designs the circuit and digitizes the information. The customer then sends us the digitized data containing the design for each layer. The data can be sent on a floppy disk, magnetic tape, cassette or via modem................................................30 Fig. 18 shows types of design............................................................................................30 DATA PREPARATION:.................................................................................................30
55
