Layout of CMOS Inverter

Layout is the process of drawing different mask layers of an IC. Each layer is drawn using a specific color and follows a set of design rules that act as an interface between the designer and the process engineer. Typical considerations include spacing, minimum width, overlap, manufacturability, performance, IC size, and parasitic elements.

Fig.1. Layout Flow

Design Rules

Design rules are categorized as Lambda (λ) rules and Micron rules:

1. Lambda-based rules

• Scalable rules based on a constant parameter (λ).
• 2λ equals minimum channel length; allows process portability.
• Processes rarely shrink uniformly, so adjustments may be needed.

2. Micron-based rules

• Minimum sizes and spacing specified directly in microns.
• Not tied to λ multiples; can reduce area by ~50% compared to λ rules.

Types of design rules include:

• Intra-layer: Width, spacing
• Inter-layer: Enclosures, distances, overlaps
• Specific rules: Antenna, density, minimum area

Fig.2. CMOS Process Layers

Fig.3. Intra Layer Design Rules

The λ rules were introduced by Mead and Conway (1980) to allow scalable, portable designs. The MOSIS service provides standard λ-rule sets for academic and commercial designs.

Fig.4. MOSIS λ-Based Design Rules

Fig.5. Lambda-Based Design Rules

CMOS Inverter Layout

Fig.6. Layout of CMOS Inverter

The layout is drawn in L-Edit following λ rules and CMOS process layers. After layout generation, it must pass:

• Design Rule Check (DRC): Automatically verifies that layout polygons follow all design rules.
• Layout versus Schematic (LVS) check: Ensures the layout matches the original circuit schematic.

Fig.7. Structure of the Layout of CMOS Inverter

Cross-Sectional View

L-Edit allows visualizing the cross-section of the CMOS device, showing the fabrication steps. Observe the drains, sources, gates, and bulks of NMOS and PMOS transistors:

Key observations:

• Drain and Source are fabricated similarly.
• All NMOS bulks are connected together.
• All PMOS bulks in the same N-well are connected together.

SPICE Programming and Graphs

SPICE code can be extracted using T-Edit, and simulations with W-Edit produce the corresponding graphs for verification.

Conclusion

The CMOS Inverter layout drawn using L-Edit was successfully verified using DRC, LVS, and SPICE simulations. The process demonstrated proper adherence to design rules and layout correctness.

Further Reading

1. Introduction to Tanner EDA Tools
2. CMOS Logic Gates: Inverter, NAND, and NOR
3. EDA Tools and VHDL for VLSI Design
4. Standard Cell Design in VLSI
5. Design of CMOS XOR/XNOR Gates
6. Design of CMOS Full Adder
7. Design of CMOS Flip-Flops (SR, D, JK)
8. Design of 8-bit Synchronous Counter
9. Design of 8-bit Bi-Directional Register
10. Design of a 12-bit CPU with Basic Instructions
11. VHDL Logical Function & CMOS Inverter
12. CMOS Layout Color Codes - IC Design Guide