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Deer Park Heights, Looking Down on Lake Wakatipu and Queenstown. Deer Park Heights, Looking at the Remarkables (Rohan). 332:479 Concepts in VLSI Design Lecture 3 The VLSI Transistor and CMOS Fabrication. David Harris and Michael Bushnell Harvey Mudd College and Rutgers University Spring 2004.

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Concepts in VLSI Des. Lec. 3

Deer Park Heights, Looking Down on Lake Wakatipu and Queenstown


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Concepts in VLSI Des. Lec. 3

Deer Park Heights, Looking at the Remarkables (Rohan)


332 479 concepts in vlsi design lecture 3 the vlsi transistor and cmos fabrication l.jpg

332:479 Concepts in VLSIDesignLecture 3The VLSI Transistor and CMOS Fabrication

David Harris and Michael Bushnell

Harvey Mudd College and Rutgers University

Spring 2004


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Outline

  • nMOS transistor

  • pMOS transistor

  • CMOS Inverter

  • Elementary Logic Gates

  • Chip fabrication

  • Design Rules

  • Layout

  • Summary

Material from: CMOS VLSI Design,

by Weste and Harris, Addison-Wesley, 2005

Concepts in VLSI Des. Lec. 3


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Transistor and IC History

  • MOSFET idea:

    • J. Lilienfeld 1925

    • O. Heil 1935

  • Experiments in early Field-Effect Transistor (FET) failed due to material problems

    • Led to invention of Bipolar transistor at Bell Telephone Laboratories in 1947 by Shockley, Brittain, and Bardeen – 1956 Nobel Prize in Physics

Concepts in VLSI Des. Lec. 3


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CMOS MOSFET Inventions

  • Key Patents:

    • P.K. Weimer – Radio Corporation of America (RCA) 1962 – CMOS Flip-Flop Implementation

    • Frank Wanlass – Fairchild 1963 – CMOS Logic Gates

  • Two types of MOS transistors

Concepts in VLSI Des. Lec. 3


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Introduction

  • Integrated circuits: many transistors on one chip.

  • Very Large Scale Integration (VLSI): very many

  • Complementary Metal Oxide Semiconductor

    • Fast, cheap, low power transistors

  • Today: How to build your own simple CMOS chip

    • CMOS transistors

    • Building logic gates from transistors

    • Transistor layout and fabrication

  • Rest of the course: How to build a good CMOS chip

Concepts in VLSI Des. Lec. 3


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Silicon Lattice

  • Transistors are built on a silicon substrate

  • Silicon is a Group IV material

  • Forms crystal lattice with bonds to four neighbors

Concepts in VLSI Des. Lec. 3


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Dopants

  • Silicon is a semiconductor

  • Pure silicon has no free carriers and conducts poorly

  • Adding dopants increases the conductivity

  • Group V: extra electron (n-type)

  • Group III: missing electron, called hole (p-type)

Concepts in VLSI Des. Lec. 3


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p-n Junctions

  • A junction between p-type and n-type semiconductor forms a diode.

  • Current flows only in one direction

Concepts in VLSI Des. Lec. 3


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nMOS Transistor

  • Four terminals: gate, source, drain, body

  • Gate – oxide – body stack looks like a capacitor

    • Gate and body are conductors

    • SiO2 (oxide) is a very good insulator

    • Called metal – oxide – semiconductor (MOS) capacitor

    • Even though gate is

      no longer made of metal

Concepts in VLSI Des. Lec. 3


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nMOS Operation

  • Body is commonly tied to ground (0 V)

  • When the gate is at a low voltage:

    • P-type body is at low voltage

    • Source-body and drain-body diodes are OFF

    • No current flows, transistor is OFF

Concepts in VLSI Des. Lec. 3


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nMOS Operation (cont’d.)

  • When the gate is at a high voltage:

    • Positive charge on gate of MOS capacitor

    • Negative charge attracted to body

    • Inverts a channel under gate to n-type

    • Now current can flow through n-type silicon from source through channel to drain, transistor is ON

Concepts in VLSI Des. Lec. 3


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pMOS Transistor

  • Similar, but doping and voltages reversed

    • Body tied to high voltage (VDD)

    • Gate low: transistor ON

    • Gate high: transistor OFF

    • Bubble indicates inverted behavior

Concepts in VLSI Des. Lec. 3


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Power Supply Voltage

  • GND = 0 V

  • In 1980’s, VDD = 5V

  • VDD has decreased in modern processes

    • High VDD would damage modern tiny transistors

    • Lower VDD saves power

  • VDD = 3.3, 2.5, 1.8, 1.5, 1.2, 1.0, …

Concepts in VLSI Des. Lec. 3


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Transistors as Switches

  • We can view MOS transistors as electrically controlled switches

  • Voltage at gate controls path from source to drain

Concepts in VLSI Des. Lec. 3


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CMOS Inverter

Concepts in VLSI Des. Lec. 3


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CMOS Inverter

Concepts in VLSI Des. Lec. 3


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CMOS Inverter

Concepts in VLSI Des. Lec. 3


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CMOS NAND Gate

Concepts in VLSI Des. Lec. 3


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CMOS NAND Gate

Concepts in VLSI Des. Lec. 3


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CMOS NAND Gate

Concepts in VLSI Des. Lec. 3


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CMOS NAND Gate

Concepts in VLSI Des. Lec. 3


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CMOS NAND Gate

Concepts in VLSI Des. Lec. 3


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CMOS NAND Gate

Concepts in VLSI Des. Lec. 3


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CMOS NOR Gate

Concepts in VLSI Des. Lec. 3


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CMOS NOR Gate

Concepts in VLSI Des. Lec. 3


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3-input NAND Gate

  • Y pulls low if ALL inputs are 1

  • Y pulls high if ANY input is 0

Concepts in VLSI Des. Lec. 3


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3-input NAND Gate

  • Y pulls low if ALL inputs are 1

  • Y pulls high if ANY input is 0

Concepts in VLSI Des. Lec. 3


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CMOS Fabrication

  • CMOS transistors are fabricated on silicon wafer

  • Lithography process similar to printing press

  • On each step, different materials are deposited or etched

  • Easiest to understand by viewing both top and cross-section of wafer in a simplified manufacturing process

Concepts in VLSI Des. Lec. 3


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Inverter Cross-section

  • Typically use p-type substrate for nMOS transistors

  • Requires n-well for body of pMOS transistors

Concepts in VLSI Des. Lec. 3


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Well and Substrate Taps

  • Substrate must be tied to GND and n-well to VDD

  • Metal to lightly-doped semiconductor forms poor connection called Shottky Diode

  • Use heavily doped well and substrate contacts / taps

Concepts in VLSI Des. Lec. 3


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Inverter Mask Set

  • Transistors and wires are defined by masks

  • Cross-section taken along dashed line

Concepts in VLSI Des. Lec. 3


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Detailed Mask Views

  • Six masks

    • n-well

    • Polysilicon

    • n+ diffusion

    • p+ diffusion

    • Contact

    • Metal

Concepts in VLSI Des. Lec. 3


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Fabrication Steps

  • Start with blank wafer

  • Build inverter from the bottom up

  • First step will be to form the n-well

    • Cover wafer with protective layer of SiO2 (oxide)

    • Remove layer where n-well should be built

    • Implant or diffuse n dopants into exposed wafer

    • Strip off SiO2

Concepts in VLSI Des. Lec. 3


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Oxidation

  • Grow SiO2 on top of Si wafer

    • 900 – 1200 C with H2O or O2 in oxidation furnace

Concepts in VLSI Des. Lec. 3


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Photoresist

  • Spin on photoresist

    • Photoresist is a light-sensitive organic polymer

    • Softens where exposed to light

Concepts in VLSI Des. Lec. 3


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Lithography

  • Expose photoresist through n-well mask

  • Strip off exposed photoresist

Concepts in VLSI Des. Lec. 3


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Etch

  • Etch oxide with hydrofluoric acid (HF)

    • Seeps through skin and eats bone; nasty stuff!!!

  • Only attacks oxide where resist has been exposed

Concepts in VLSI Des. Lec. 3


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Strip Photoresist

  • Strip off remaining photoresist

    • Use mixture of acids called piranah etch

  • Necessary so resist doesn’t melt in next step

Concepts in VLSI Des. Lec. 3


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n-Well

  • n-well is formed with diffusion or ion implantation

  • Diffusion

    • Place wafer in furnace with arsenic gas

    • Heat until As atoms diffuse into exposed Si

  • Ion Implanatation

    • Blast wafer with beam of As ions

    • Ions blocked by SiO2, only enter exposed Si

Concepts in VLSI Des. Lec. 3


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Strip Oxide

  • Strip off the remaining oxide using HF acid

  • Back to bare wafer with n-well

  • Subsequent steps involve similar series of steps

Concepts in VLSI Des. Lec. 3


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Polysilicon

  • Deposit very thin layer of gate oxide

    • < 20 Å (6-7 atomic layers)

  • Chemical Vapor Deposition (CVD) of silicon layer

    • Place wafer in furnace with Silane gas (SiH4)

    • Forms many small crystals called polysilicon

    • Heavily doped to be good conductor

Concepts in VLSI Des. Lec. 3


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Polysilicon Patterning

  • Use same lithography process to pattern polysilicon

Concepts in VLSI Des. Lec. 3


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Self-Aligned Process

  • Use oxide and masking to expose where n+ dopants should be diffused or implanted

  • N-diffusion forms nMOS source, drain, and n-well contact

Concepts in VLSI Des. Lec. 3


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n-Diffusion

  • Pattern oxide and form n+ regions

  • Self-aligned process where gate blocks diffusion

  • Polysilicon is better than metal for self-aligned gates because it doesn’t melt during later processing

Concepts in VLSI Des. Lec. 3


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n-Diffusion (cont’d.)

  • Historically dopants were diffused

  • Usually ion implantation today

  • But regions are still called diffusion

Concepts in VLSI Des. Lec. 3


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n-Diffusion (cont’d.)

  • Strip off oxide to complete patterning step

Concepts in VLSI Des. Lec. 3


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p-Diffusion

  • Similar set of steps form p+ diffusion regions for pMOS source and drain and substrate contact

Concepts in VLSI Des. Lec. 3


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Contacts

  • Now we need to wire together the devices

  • Cover chip with thick field oxide

  • Etch oxide where contact cuts are needed

Concepts in VLSI Des. Lec. 3


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Metallization

  • Sputter on aluminum over whole wafer

  • Pattern to remove excess metal, leaving wires

Concepts in VLSI Des. Lec. 3


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Layout

  • Chips are specified with set of masks

  • Minimum dimensions of masks determine transistor size (and hence speed, cost, and power)

  • Feature size f = distance between source and drain

    • Set by minimum width of polysilicon

  • Feature size improves 30% every 3 years or so

  • Normalize for feature size when describing design rules

  • Express rules in terms of l = f/2

    • E.g. l = 0.3 mm in 0.6 mm process

Concepts in VLSI Des. Lec. 3


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Simplified Design Rules

  • Conservative rules to get you started

Concepts in VLSI Des. Lec. 3


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Inverter Layout

  • Transistor dimensions specified as Width / Length

    • Minimum size is 4l / 2l, sometimes called 1 unit

    • In f = 0.6 mm process, this is 1.2 mm wide, 0.6 mm long

Concepts in VLSI Des. Lec. 3


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Summary

  • MOS Transistors are stack of gate, oxide, silicon

  • Can be viewed as electrically controlled switches

  • Build logic gates out of switches

  • Draw masks to specify layout of transistors

  • Follow processing steps to make transistors on chips

    • Requires use of specific geometric design rules

  • Now you know everything necessary to start designing schematics and layout for a simple chip!

Concepts in VLSI Des. Lec. 3


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