3. Organisation
• Design rules, Schematic to layout, vice versa,
• cross-sectional diagram, big layouts
• Matched componentsp
Over-etching errors
unit components designunit components design
design using non unit component
Boundary condition matching Boundary condition matching
Common centroid layout, parasitic cap estimation
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4.
5.
6. Scalable design rules-----same set can be used for next
tech generation by changing λ. Worst case values of
spacings, widths etc. are used , so can’t be an optimized
set. e. g. MOSIS design rulesg g
Absolute design rules----optimized set but same set can’t
be used for next tech gen. Entire new set is to be
created.
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16. Dense MOS layouts
metal1metal1
metal2
B t t i t
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Waffle transistor Bent transistor
Compute w/L?
17. Circuit And Layout
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Try more examples
18. How to reduce parasitic
capacitances?
Careful layout by junction sharing
24. Overetching –
MOS dimensions
(W/L)u = 8um/2um= 4 desired
MOS dimensions
After over etching ---
(W/L)u = 7um/1um= 7; 0.5um= ∆e
LL
∆e
Poly layer
w Over etched Poly layer
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25. Absolute dimension of MOS
Remedy---use Unit components w=L
(W/L)u = 10um/10um, RATIO=1
Remedy---use Unit components w=L
After fab. (W/L)u 8um/8um, RATIO=1
Conclusion—Abs. dimensions change, ratio does not
changechange
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26. Ratio of matched devices
• (W/L)1 = 2, (W/L)2 = 8, ratio= 4
Ratio of matched devices
( )1 , ( )2 ,
• We take unit device (W/L)u = 10um/10um
• After fab. (W/L)u 8um/8um( )u
(W/L)2 8(W/L)u
4
( )2 ( )u
(W/L)1 2(W/L)u
= =4
Thus, ratio remains same, if same unit
device is used
32. Let
C1/ C2 = 3.4 = 2+1.4
= [6/3] + [1.4/1][6/3] [1.4/1]
[6/3]---can be implemented by using unit[6/3]---can be implemented by using unit
capacitors
[1 4/1]---we require non unit capacitor[1.4/1]---we require non unit capacitor
Mismatch can occur due to second term
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33. No mismatch conditionNo mismatch condition
• We should design non unit cap Such thatWe should design non unit cap. Such that
ratio (1.4) remains constant even after
overetchingoveretching
H t d i ?• How to design?
• What is the condition?
40. What if unit devices change randomly?g y
Since one device is facing larger change in dimension,g g g
maintaining constant ratio would be difficult.
So We should have same change in all unit devices how?So, We should have same change in all unit devices. how?
Inter-digitization
58. Multi fingered
Common Centroid layoutCommon Centroid layout
Parasitic cap. calculation of
MOS device
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59. 20λ 6λ 6λ 6λ
5λ 5λ
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