VLSI sytem Design Lecture 1: Introduction Outline Syllabus

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VLSIsytem DesignLecture 1 Introduction

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Outline

• Syllabus
• Logistics (time, place, instructor, website,
  textbook)
• Grading
• Topics
• Outcomes
• Introduction to VLSI
• A brief history
• MOS transistors
• CMOS logic gates

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Course Information (1)

• Time and Place
• Tue/Thu 710-825pm, ENGR 1.250
• Instructor
• Hasina Huq
• hhuq_at_utpa.edu
• ENGR 3.278, 384-5017
• Office hours TWR 10.30-12.30 pm or walk in or
  by appointment

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Course Information (2)

• Prerequisites
• Digital logic (ELEE2330) and Solid state devices
  (ELEE 4328), or equivalent
• I assume you know the following topics
• Boolean algebra, logic gates, etc.
• Undergraduate physics Ohms law, resistors,
  capacitors, etc.
• Undergraduate math calculus

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Course Information (3)

• Text Ken Martin, Digital Integrated Circuits
  design, Oxford, 1999
• Reference Class handouts
• Cadence manual set
• H.Craig Casey, Jr., Devices for Integrated
  Circuits, John-Wiley,
• Baker, Li, Boyce, CMOS Circuit Design, Layout,
  and Simulation, IEEE Press, 1998.

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Course Information (4)

• Grading
• 50 project
• 10 homework
• 10 mid-term exam
• 30 final exam
• Laboratory Based Projects (3) 50 (10, 10, 30)
• Final project include design, report and
  presentation
• Total 100

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Course Information (5)

• Topics
• NMOS,POMS CMOS
• logic gate
• fabrication and layout
• MOS transistor modeling
• Performance analysis for VLSI circuits
• Combinational circuits
• Sequential circuits
• Memory circuits
• Low power design
• Testing
• System on chip

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Course Information (6)

• Outcomes
• Use the Electric CAD tool to design a chip
  including
• Schematic entry
• Layout
• Transistor-level cell design
• Gate-level logic design
• Hierarchical design
• Switch-level simulation (IRSIM)
• Design rule checking (DRC)
• Electrical rule checking (ERC)
• HDL design (Verilog)
• Place and route
• Pad frame generation and routing

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Course Information (7)

• Outcomes
• Estimate and optimize combinational circuit delay
  using RC delay models and logical effort
• Design high speed and low power logic circuits
• Understand interconnect and reliability issues
• Design functional units including adders,
  multipliers, ROMs, SRAMs, and PLAs
• Beware of the VLSI trends and challenges

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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

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• 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

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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

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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

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nMOS Operation

• Body is usually 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

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nMOS Operation Cont.

• 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

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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

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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

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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

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Oxidation

• Grow SiO2 on top of Si wafer
• 900 1200 C with H2O or O2 in oxidation furnace

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Photoresist

• Spin on photoresist
• Photoresist is a light-sensitive organic polymer
• Softens where exposed to light

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Lithography

• Expose photoresist through n-well mask
• Strip off exposed photoresist

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Strip Photoresist

• Strip off remaining photoresist
• Use mixture of acids called piranah etch
• Necessary so resist doesnt melt in next step

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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

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Strip Oxide

• Strip off the remaining oxide using HF
• Back to bare wafer with n-well
• Subsequent steps involve similar series of steps

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Polysilicon

• Deposit very thin layer of gate oxide
• lt 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

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Polysilicon Patterning

• Use same lithography process to pattern
  polysilicon

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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

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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 doesnt melt during later
  processing

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N-diffusion cont.

• Historically dopants were diffused
• Usually ion implantation today
• But regions are still called diffusion

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N-diffusion cont.

• Strip off oxide to complete patterning step

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P-Diffusion

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

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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

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Metalization

• Sputter on aluminum over whole wafer
• Pattern to remove excess metal, leaving wires

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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

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Simplified Design Rules

• Conservative rules to get you started

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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

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Summary

• MOS transistors are stacks of gate, oxide,
  silicon
• Act as electrically controlled switches
• Build logic gates out of switches
• Draw masks to specify layout of transistors
• The basic to start designing schematics and
  layout for a simple chip!