Misr International University Faculty of Engineering Electronics and Communications Department
LAYOUT AND MICROWIND TUTORIAL DIGITAL ELECTRONICS II COURSE
Prepared By
Eng. Waleed El - Halwagy
NMOS, PMOS and CMOS Construction As the CMOS is composed of NMOS and PMOS we should at first present their properties and construction.
NMOS Properties 1. 2. 3. 4.
5. 6.
The Substrate : P – Type. The Drain and Source : n+ diffusion. The Select Area : p+ select. The p + select is connected to ground. The current flows from D to S. The Drain voltage > The Source voltage
NMOS Construction
PMOS Properties 1. 2. 3. 4.
5. 6.
The Substrate : n – Type. The Drain and Source : p+ diffusion. The Select Area : n+ select. The n + select is connected to VDD. The current flows from S to D. The Source voltage > The Drain voltage
PMOS Properties
CMOS Inverter Construction Elevation View
CMOS Inverter Construction Top View
CMOS Inverter Construction 3D View
Fabrication Process of the CMOS Inverter Mask 1: N – well mask in the P – substrate
Mask 1: N-well mask in the P-substrate
Mask 1: N-well mask in the P-substrate
Mask 1: N-well mask in the P-substrate
Mask 1: N-well mask in the P-substrate
Fabrication Process of the CMOS Inverter Mask 2: Active Mask Creation ( n+ and p+ )
Mask 2 : Active Mask Creation (n+ and p+)
Mask 2 : Active Mask Creation (n+ and p+)
Mask 2 : Active Mask Creation (n+ and p+)
Mask 2 : Active Mask Creation (n+ and p+)
Fabrication Process of the CMOS Inverter Mask 3: Poly Silicon Mask Creation
Mask 3 : Poly silicon Mask Creation
Mask 3 : Poly silicon Mask Creation
Mask 3 : Poly silicon Mask Creation
Mask 3 : Poly silicon Mask Creation
Fabrication Process of the CMOS Inverter Mask 4: P+ region Mask Creation
Mask 4 : P+ region mask creation
Mask 4 : P+ region mask creation
Mask 4 : P+ region mask creation
Mask 4 : P+ region mask creation
Fabrication Process of the CMOS Inverter Mask 5: n+ region Mask Creation
Mask 5 : n+ region mask creation
Mask 5 : n+ region mask creation
Mask 5 : n+ region mask creation
Mask 5 : n+ region mask creation
Mask 5 : n+ region mask creation
Fabrication Process of the CMOS Inverter Mask 6: Contacts Mask Creation
Mask 6 : Contact mask creation
Mask 6 : Contact mask creation
Mask 6 : Contact mask creation
Mask 6 : Contact mask creation
Fabrication Process of the CMOS Inverter Mask 7: Metal Mask Creation
Mask 7 : Metal mask creation
Mask 7 : Metal mask creation
Mask 7 : Metal mask creation
Mask 7 : Metal mask creation
The Design Rules and Layout It provides a set of guidelines for constructing the various masks needed in the patterning process. Scalable Design Rules : all dimensions are given as a function of λ Micron Rules : all design rules are expressed in absolute dimensions
The Design Rules It provides a set of guidelines for constructing the various masks needed in the patterning process
The Design Rules acts as the interface or even the contract between the circuit designer and the process engineer. Circuit designers generally want tighter, smaller designs, which lead to higher performance and higher circuit density. The process engineer on the other hand, wants a reproducible and high – yield process. Consequently, design rules are a compromise that attempts to satisfy both sides. Design rules consists of: Minimum width requirements. Minimum spacing requirements. Minimum Surface requirements. Requirements between objects on the same or different layers.
The Design Rules Scalable Design Rules : all dimensions are given as a function of λ
Even for the same minimum dimension, design rules tend to differ from company to company, and from process to process. This makes porting an existing design between different processes a time consuming task. To address this issue we can use the scalable design rules, which defines all the design rules as a function of a single parameter “λ”. Scaling of the minimum dimension is accomplished by simply changing the value of “λ”. This results in a linear scaling of all dimensions. For a given process, λ is set to a specific value and all design dimensions are consequently translated into absolute numbers. Minimum Feature size = 2 λ
The Design Rules Disadvantages of the Scalable design rules approach
Linear scaling is only possible over a limited range of dimensions (for example between 0.25 μm and 0.18 μm). When scaling over large ranges, the relations between different layers tend to vary in a non-linear way that can not be converted by the linear scaling rules. Scalable design rules are conservative, they represent a cross section over different technologies, and they must represent the worst case rules for the whole set. This results in over dimensioned and less dense designs. For these and other reasons, scalable design rules normally are avoided by industry. (while not entirely accurate, the lambda rules are still useful to estimate the impact of a technology scale on the design area).
The Design Rules Micron Rules: all design rules are expressed in absolute dimensions
As circuit density is a prime goal in industrial designs, most semiconductor companies tend to use the micron rules, which express all design rules in absolute dimensions and thus can exploit the features of a given process to a maximum degree. Scaling and porting designs between technologies under these rules is more demanding and has to be performed either manually or by using advanced CAD tools.
What is a Layout ?
A layout consists of a combination of polygons, each of which is attached to a certain layer. The functionality of the circuit is determined by the choice of the layers, as well as the interplay between objects on different layers. A transistor A MOS transistor is formed by the cross section of the diffusion layer and the poly silicon layer.
W
L
The Objective Is to design the minimum size inverter in the 0.18 μm technology and develop its seven masks.
DESIGN AN 0.18 μm technology CMOS INVERTER ( 2 λ = 0.18 μm ) The technology is specified by its minimum line width (minimum feature size) which is usually taken as the channel length of the transistor and it is denoted by 2λ Design using minimum sized NMOS and take the W PMOS = 3 WNMOS and take the channel length of both transistors as 2λ
First : Select the Design Technology from the Microwind
To get the Design Rules of the chosen technology
First : The minimum sized NMOS Transistor Design
First: Design the minimum sized NMOS transistor.
What do we need to construct an NMOS transistor ? P
– substrate Poly silicon for the gate. n+ diffusion regions for the drain and source. P+ select region that is connected to ground. Contacts to connect the active area with the metal layer.
Remark: The
Microwind assumes the Silicon ignot used in fabrication is doped with Boron, that is its a P-type silicon ignot .
What are the design rules we need to know to be able to construct the NMOS ? The Minimum Poly Width = 2λ The Minimum extra Poly surrounding the n diffusion = 3λ
The Minimum Poly Area = 16 λ2 The Minimum extra n diffusion surrounding the poly = 4λ The Minimum n diffusion Width = 4λ
The Minimum extra n diffusion surrounding the contact = 2λ
The Minimum n diffusion Area = 16 λ2
The Minimum spacing between the contact and the Poly = 3λ
The Minimum Contact Width = 2λ
What are the design rules we need to know to be able to construct the NMOS ?
The Minimum p diffusion Width = 4λ
The Minimum spacing between contacts = 4λ
The Minimum p diffusion Area = 16 λ2
How to Check that the design Rules of the layout are satisfied ?
Design Rule Check ( DRC )
If any of these design rules are not satisfied, the Microwind will signify it.
If any of these design rules are not satisfied, the Microwind will signify it.
Now what is the minimum size and area of the NMOS Transistor in the 0.18 μm technology ?
L : is the length of the poly silicon. W : is the width of the n+ diffusion region. From the design rules: Minimum
poly silicon length = 2 λ. Minimum n+ diffusion width = 4 λ. Taking into consideration that the minimum extra poly surrounding the n+ diffusion is 3 λ. Taking into consideration that the minimum extra n+ diffusion surrounding the poly is 4 λ.
Now what is the minimum size of the NMOS Transistor in the 0.18 μm technology ? L=2λ W=4λ Area = 10 λ x 10 λ
The Minimum extra n diffusion surrounding the poly = 4λ
The Minimum extra Poly surrounding the n diffusion = 3λ
The Minimum Poly Width L = 2λ
The Minimum extra n diffusion surrounding the poly = 4λ
The Minimum n diffusion Width W= 4λ
The Minimum extra Poly surrounding the n diffusion = 3λ
Now lets add the contacts.
The contacts – Design Rules: Minimum contact width = 2λ Minimum extra diffusion surrounding the contact = 2λ. Minimum spacing between the poly and contact = 3λ.
As we can see that the last two design rules are not satisfied when LD = 4λ and W = 4λ , so we have to expand them as follows :
LLDD ==4λ 4λ W = 4λ λ
λ
2λ
2λ λ
L D = 3λ (poly – contact) + 2 λ (contact width)+ 2λ (contact – diff.) = 7 λ W = 2λ (contact – diff) + 2 λ (contact width)+ 2λ (contact – diff.) = 6 λ
λ
Now lets add the contacts. L = 2λ W=6λ Area = 16 λ x 12 λ
Extra Poly 3λ Contact – diff= 2λ
Contact width= 2λ
W = 6λ
Contact – diff= 2λ Contact – diff 2λ
Contact Poly – width contact 2λ 3λ
Poly Width L =2λ
Poly – contact 3λ
Contact width 2λ
Contact – diff 2λ
Extra Poly 3λ
Optimizing the Aspect ratio and area of the NMOS Transistor
From the above discussion we conclude that:
The Question now is can not we optimize this aspect ratio.
Minimum L = minimum poly width = 2λ. Minimum W = 2 ( minimum contact - diff) + minimum contact width = 6 λ. We can not decrease L because we can not implement a dimension that is less than 2λ. But from the design rules, the minimum diffusion width is 4λ. We were forced to implement it as 6λ due to the contact design rules constraints.
By a small trick we could let W = 4λ without altering the contact design rules constraints. This can be accomplished by reducing the diffusion width to 4λ at the poly silicon surface and widening it in the region surrounding the contact.
Optimizing the Aspect ratio and area of the NMOS Transistor 7λ
L =2λ
7λ Extra Poly 3λ
diffusion width at the poly W = 4λ
L = 2λ W=4λ Area = 16 λ x 10 λ
Extra Poly 3λ
Diffusion width at the contact 6λ
Adding the P-select region Taking into consideration the contact design rules restrictions Area of p – select = 6λ x 6λ L = 2λ W=4λ Area = 22 λ x 10 λ
Extra Poly 3λ
2λ Contact 2λ – diff
Diffusion width at the poly W = 4λ
Contact – Contact 4λ
Contact 2λ width
Contact 2λ – diff
Extra Poly 3λ Contact Contact – diff width 2λ 2λ
Poly – contact 3λ
Poly Width L =2λ
Poly – Contact Contact Contact Contact Contact contact width – diff – diff width – diff 2λ 2λ 3λ 2λ 2λ 2λ
2λ
Second : The PMOS Transistor Design LPMOS = 2λ and W PMOS = 3 WNMOS
Now Design The PMOS with LPMOS = 2λ and W PMOS = 3 WNMOS
What do we need to construct a PMOS transistor ? N – well Poly silicon for the gate. p+ diffusion regions for the drain and source. n+ select region that is connected to ground. Contacts to connect the active area with the metal layer.
There are some additional design rules that we will need to know concerning the N-well before we proceed in our design of the PMOS. The design rules concerning with the minimum width and area of the diffusion as well as the relation between the poly and the contacts with the diffusion are the same for both n-type and p-type.
Design Rules Concerning the N-well The minimum extra n-well surrounding the p diffusion = 2 λ
The minimum extra n-well surrounding the p diffusion = 6 λ The Minimum n-well Area = 144 λ2
The Minimum n-well Width = 10 λ
The Number of Contacts in the PMOS
As a rule: the more the number of contacts, the better the performance. This is because the resistance through which the current flows will decrease. The following equation gives a relation between the diffusion width and the number of contacts.
W 18 λ
W = 2 ( contact – diff.) + N ( contact width ) + ( N – 1 ) ( contact – contact ) Example : W = 18 λ 18 λ = 2 ( 2 λ )+ N ( 2 λ ) + ( N – 1 ) ( 4 λ ) ------- N = 3 contacts Example : W = 17 λ 17 λ = 2 ( 2 λ )+ N ( 2 λ ) + ( N – 1 ) ( 4 λ ) -------- N = 2.83, that is N = 2 contacts
2λ 2λ 2λ 2λ 2λ 2λ
3λ
L 2λ
3λ 2λ 2λ
6λ
2λ
2λ 2λ
4λ
W 12 λ
24 λ
2λ 2λ L = 2λ W = 12 λ Area = 30 λ x 24 λ
3λ 6λ 3λ 30 λ
The CMOS is Constructed by connecting the NMOS and PMOS transistors together
The CMOS is constructed by interconnecting both the NMOS and PMOS
There are some additional design rule that must be taken in consideration when interconnecting the NMOS and PMOS to construct the CMOS Inverter.
The minimum spacing between the n-well of the PMOS and the n+ diffusion (drain) of the NMOS = 6 λ.
The metal used in the connections
Minimum metal width = 6 λ Minimum metal surface = 16 λ Minimum Spacing between metal layers = 4 λ Minimum extra metal surrounding the contact = 2 λ
We need metal to
NMOS: Connect the p+ select with the source to ground PMOS: Connect the n+ select with the source to V Connect the NMOS and PMOS drains to the output. Connect the poly to the input.
NMOS : L = 2λ W = 4 λ PMOS : L = 2λ W = 12 λ CMOS Area = 58 λ x 24 λ
The minimum metal spacing = 4 λ
The minimum spacing between the n-well and n-diffusion = 6 λ
The minimum extra metal surrounding the contact = 2 λ
The Seven Masks Extraction After we have finished the CMOS layout design and computed its dimensions, we are ready to extract the seven inverter masks to send them to the Fab to be manufactured.
CMOS Inverter Layout
Mask 1 : N – well Mask
Mask 2 : Active Area Mask
Mask 3 : Poly Silicon Mask
Mask 4 : p + Region Mask
Mask 5 : n + Region Mask
Mask 6 : Contacts Mask
Mask 7 : Metal Mask
