Chapter 2 CMOS Processing/Layout Supplement (II)

● Twin-tub CMOS process 1. Provide separate optimization of the n-type and p-type transistors 2. Make it possible to optimize "Vt", "Body effect", and the "Gain" of n, p

devices, independently. 3. Steps:

A. Starting material: an n+ or p+ substrate with lightly doped -> "epitaxial" or "epi" layer -> to protect "latch up"

B. Epitaxy" a. Grow high-purity silicon layers of controlled thickness b. With accurately determined dopant concentrations c. Electrical properties are determined by the dopant and its

concentration in Si C. Process sequence

a. Tub formation b. Thin-Oxide construction c. Source & drain implantations d. Contact cut definition e. Metallization

● Balanced performance of n and p devices can be constructed.

(Substrate contacts are included in Fig.3.10)

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Form n-island (for p-device)

Form p-island (for n-device)

Anisotropic Etch

(7~8um)

青玉 or SiO2 (二氧化矽)

Etch Polysilicon

- Grow gate oxide through

thermal oxidation

- Deposit Doped Polysilicon

Step (h): n-implantation for

source & drain

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Step (i) p-implantation

Step (j)

- Grow phosphorus glass

- Etch glass to form contact cut

- Evaporating alumni

3.3 CMOS Process Enhancement (Interconnection) 3.3.1 Metal Interconnect CMOS circuit =

CMOS logic process + Signal/Power/Clock-routing layers

- Contact: Metal1 (M1) to Poly or Diffusions (n+ , p+)

- Second-layer of metal (VIA1=M1 to M2)

- Note: M1 must be involved in any contact to underlying areas

EtcIsolay

ForVIA

Contact

- such as “polysilicon” an

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h lation er m a

d “diffusion (n+, p+)

4

Layout Examples: (A) Inverters

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(1) Vertical

diffusion

(2) Horizontal

diffusion

* Other Routing methods

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* Increase Beta Value (W/L) - Increase W

- Place transistor back-to-back (2 beta) - Round transistor (Donut connection) (4 beta)

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Layout of Transmission Gate

S

S

A B

Layout of 2-to1 Multiplexor

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3.4 Layout Design Rules - Function: obtain a circuit with optimum yield in an area as well as

possible

- Performance ←→ yield

* Conservative design rules → Functional circuit

→ Good yield

* Aggressive design rules → Bad yield

→ Compact circuit/layout for low cost and high speed

(A) Line width/spacing

Small → open circuit

Close → short circuit

(B) Spacing between two independent layers

- In process:

(a) Geometric features for mask-making and lithographical

(b) Interactions between different layers (e.g., poly + diffusion)

- Rules:

a. Micro(μ)-based rules – Industry (submicron)

b. Lambda-based rules: e.g.,, 1λ=0.6um for 1.2 um CMOS process) for 4-1.2um Scalable CMOS process. 2λ is the minimum channel length (L).

- See Table 3.2 and figures (next four pages)

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ayout Design Rules: L

Contact Rules: There are several generally available contacts:

- Metal to p-active (p-diffusion)

- Metal to n-active (n-diffusion)

- Metal to Polysilicon

- VDD and VSS substrate contacts

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.4.5 Layer assignment (Table3.4)

- Split (Substrate contacts)

3- CIF: Caltech Intermediate Form

- GDSII Format

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3.5 Latchup - Latchup : Shorting of VDD and Vss lines → Chip breakdown

- Latchup Equivalent Circuit:

Vertical : pnp - p = source/drain of p device (Emitter) - n = n-well (Base) - p = p-substrate (Collector) Lateral : npn - n = source/drain of n device (Emitter) - p= p-substrate (Base) - n= n-well (Collector) Rsubstrate, Rwell - Parasitic devices and resistors

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Latchup triggering: Transient/Impulse current in start-up

A. Lateral triggering: current flows in the emitter of the lateral npn-transistor

→Trigger point : In,trigger =

- Vpnp,on = 0.7V

- αnpn = common base gain of the lateral npn device

- Rwell = well resistance

B. Vertical triggering: Sufficient current is injected into the emitter of the vertical pnp transistor

3 .5.3 Latchup prevention - Latchup occur *

βnpn˙βpnp > 1 +

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Where

Total supply current

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* Observation to prevent latchup: 1. Reduce the resistor values

2. Reduce the gain of the parasitic devices

- Approach:

1. Latchup-resistant CMOS process

2. Layout techniques (see Section 3.5.4,3.5.5 of Neil Weste Textbook, or the supplement in next 3 pages)

* Technology-related CAD tools - Design Rule Check (DRC): On-line DRC and Off-line (Dracula)

(3.6.1)

- Circuit extraction (Layout Parameter Extraction, LPE) (3.6.2)

- CMOS process simulator (Process Input Description Language (PIDL))(Sec.3.9) and SUPREME by Stanford University.

*Latch-up prevention (layout approach) Prevent the diode from being forward biased By adding “well ties” and “substrate Ties”

The more well tie-downs you place, the less chance there is for the PN diode to become forward biased. * Rule of thumb: There is no such thing as too many tie-downs.

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Rule of Thumb: wherever there is any spare space in an N well, put in a

well tie. Wherever there is any spare space in the substrate, put in a substrate tie.

All this effort helps stop the parasitic diode between the well and the substrate from turning on.

* Well tie schemes

This design places some FETs too far from the nearest well tie. Your center PN junctions might blow up.

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