Allen and Holberg - CMOS Analog Circuit Design

II.5 - PASSIVE COMPONENTS CAPACITORS

εoxA C = tox

Polysilicon-Oxide-Channel Capacitor and Polysilicon-Oxide-Polysilicon

Capacitor

p+ bottom-plate implant

p- substrate

(a)

Polysilicon top plate

Polysilicon bottom plate

FOX

Inter-poly SiO2

p- substrate

(b)

Figure 2.4-1 MOS capacitors. (a) Polysilicon-oxide-channel. (b) Polysilicon-oxide-polysilicon.

Metal-Metal and Metal-Metal-Poly Capacitors

M2

T

M1

B

Figure 2.4-2 Various ways to implement capacitors using available interconnect layers.

M1, M2, and M3 represent the first, second, and third metal layers respectively.

Top plate Cdesired parasitic

Bottom plate parasitic

Figure 2.4-3 A model for the integrated capacitors showing top and bottom plate parasitics.

PROPER LAYOUT OF CAPACITORS

•Use “unit” capacitors

•Use “common centroid” Want A=2*B

Case (a) fails Case (b) succeeds!

y

Figure 2.6-2 Components placed in the presence of a gradient, (a) without commoncentroid layout and (b) with common-centroid layout.

Random edge distortion

Large-scale distortion

Corner-rounding distortion

VICINITY EFFECT

C

A B

C

A B

Figure 2.6-1 (a)Illustration of how matching of A and B is disturbed by

the presence of C. (b) Improved matching achieved by matching surroundings of A and B

IMPROVED LAYOUT METHODS FOR CAPACITORS

Corner clipping:

Clip corners

Street-effect compensation:

ERRORS IN CAPACITOR RATIOS

Let C1 be defined as

C1 = C1A + C1P

and C2 be defined as

C2 = C2A + C2P

CXA is the bottom-plate capacitance

CXP is the fringe (peripheral) capacitance

CXA >> CXP

The ratio of C2 to C1 can be expressed as

Thus best matching is achieved when the area to periphery ratio remains constant.

CAPACITOR PARASITICS

Top Plate

Top plate

parasitic Desired

Capacitor

Parasitic is dependent upon how the capacitor is constructed.

Typical capacitor performance

(0.8 m Technology)

p- substrate

(a)

Metal

Polysilicon resistor

FOX

p- substrate

(b)

Metal

n+

n- well

p- substrate

(c)

Figure 2.4-4 Resistors. (a) Diffused (b) Polysilicon (c) N-well

PASSIVE COMPONENT SUMMARY

(0.8 m Technology)