ECE 3060 VLSI and Advanced Digital Design Lecture 4 Layout Design & Tools
CMOS Layers • “Standard” n -Well Process • Active (Diffusion) • Polysilicon • Metal 1, Metal 2, Metal3 • Poly Cut (connects metal 1 to polysilicon) • Active Cut (connects metal 1 to active) • Via (connects metal 2 to metal 1) • Overglass Cut (facilitates off-chip connections) • n Well • n Select (used with active to create n -type diffusion) • p Select (used with active to create p -type diffusion) • p -well, twin tub, etc. use slightly different layers ECE 3060 Lecture 3–7
Well, Active, and Select Layout n-select p-select Active Active Layout field oxide p+ n+ n- p- (substrate) Cross section ECE 3060 Lecture 3–8
Transistor Layout n-select p-select Active Active n-fet p-fet Layout field oxide thin oxide p+ n+ n- p- (substrate) Cross section ECE 3060 Lecture 3–9
Wiring and Contact Layout ECE 3060 Lecture 3–10
Substrate and Well Contacts • Properties • Set Well and Substrate Voltages to Vdd and Gnd • Prevent Forward Biasing and Latch-Up • Must Be at Least One per Well • Should Be Placed Regularly ECE 3060 Lecture 3–12
Design Rules • Minimum Separation [A] • Intralayer (all layers) • Interlayer (active to poly/well/select) • From Transistor • Minimum Width (all layers) [B] • Minimum Overlap [C] • Past Transistor (poly, active) • Around Contact Cut (all contacted layers) • Around Active (well, select) • Exact Size (contact cuts) [D] ECE 3060 Lecture 3–13
Lambda Rules for TSMC .18u • Lambda = .09u • Cadence Virtuoso does NOT use units of lambda; instead, units of microns (10 -6 meters) are used
Width/Spacing Rules for TSMC .18u 6 2 1 18 3 2.5 2 3 12 4 3 3 6 3 6 3 metal 4 metal 1 metal 2 metal 3
Contact Design Rules for TSMC .18u 4 2 3 1 1 1 1 2 2 2 2 Contact 2x2
Cell Design Principles • Cell “Floorplanning” • Separate nFETs and pFETs • Use One Continuous Well when Possible • Set Pitch Using Power and Ground Lines • Run Busses with Metal 1 and Metal 2 Perpendicular • Route External Signals to Edges of Cell • DON’T FORGET • Substrate and Well Contacts • Select Around All Active ECE 3060 Lecture 4–2
Layout Example: CMOS Inverter • Set Pitch (place well and power/ground busses) ECE 3060 Lecture 4–3
Layout Example: CMOS Inverter • Add Transistors (active, select, and poly) ECE 3060 Lecture 4–4
Layout Example: CMOS Inverter • Make Connections (poly, metal, and cuts) ECE 3060 Lecture 4–5
Layout Example: CMOS Inverter • Add Substrate and Well Contacts ECE 3060 Lecture 4–6
Layout Example: CMOS Inverter • Add External Wiring and Resize ECE 3060 Lecture 4–7
Symbolic Layout • Stick diagrams capture spatial relationships, but abstract away design rules Vdd Out Gnd B A C • What gate is this? • Note: sticks are done slightly differently in text. ECE 3060 Lecture 4–8
Design Capture Tools • HDL & Schematic capture • Hardware Description Languages (such as VHDL & Verilog) capture a textual hierarchical description of design at abstraction ranging from gate or even transistor level up to a behavioral description (more later) • Schematic editors (such as Cadence Composer) capture a structural, hierarchical graphical representation of the design netlist. • Layout • Layout editors (such as Cadence Cell1 Virtuoso) capture a hierarchical view of the physical geometric aspect of a Cell2 Cell2 design. The units of hierarchy are Instance2 Instance1 called cells, and have physical extent (size). In general, good design requires that only one cell contain the design info for a particular area of the chip ECE 3060 Lecture 4–9
Rules Checking • Complex designs invariably suffer design and design entry errors. There are a number of tools and method- ologies to detect and correct. • Physical Design Rules Checking (DRC) checks for design rule violations such as minimum spacing etc. DRC checking is complicated by hierarchy and overlap between cells. • Electrical Rule Checking checks for violations such as shorts between Vdd and GND, opens, and so on. • Layout vs Schematic (LVS) checks for a one to one correspondence between transistor schematic and the layout. • Formal verification is used to show that the design satisfies a formal description of what it should do. • Simulation is used to show that the design is func- tional on some well selected set of input vectors. • Timing analysis is used to predict design performance ECE 3060 Lecture 4–10
Verification • Two types: • Simulation based verification • test vectors • Formal verification • mathematical properties • boolean logic • Trend: include formal verification in the design • invariants ECE 3060 Lecture 4–11
Circuit Extraction • Circuit extraction extracts a schematic representation of a layout, including transistors, wires, and possibly wire and device resistance and capacitance. • Circuit extraction is used for LVS, and for spice simu- lation of layouts ECE 3060 Lecture 4–12
Cell (datapath) Generators • Custom layout of a chip is very time consuming and is justifiable only in very high volume design with critical requirements, or in research. • A datapath generator is a program designed to para- metrically create a data path cell (say an ALU cell) which can meet size, pitch, and timing constraints. ECE 3060 Lecture 4–13
Standard Cell Approach • Use full custom process, but design to higher level abstraction • SSI (NAND, NOR, INV, XOR, Register,...). • MSI (Decoder, Adder, Comparator,...). • Datapath (ALU, Shifter, Register file). • Memory (RAM, ROM, CAM,...). • Cells are designed at multiples of std pitch • Cells may be parameterized for power, speed,.. • Design target is netlist of cells which are then placed and routed using automated tools ECE 3060 Lecture 4–14
Standard Cell Design • Cells are designed to abut horizontally. • Cells are placed and routed automatically • Wiring channels may have variable # of tracks. ECE 3060 Lecture 4–15
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