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CMOS Processing. VLSI Digital Systems Design. Si Purification. Chemical purification of Si Zone refined Induction furnace Si ingot melted in localized zone Molten zone moved from one end to the other Impurities more soluble in melt than in solid Impurities swept to one end of ingot

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si purification
Si Purification
  • Chemical purification of Si
  • Zone refined
    • Induction furnace
    • Si ingot melted in localized zone
    • Molten zone moved from one end to the other
    • Impurities more soluble in melt than in solid
    • Impurities swept to one end of ingot
  • Pure Si = intrinsic Si (impurities < 1:109)

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czochralski technique for single crystal ingot growth melt
Czochralski Technique forSingle-Crystal Ingot Growth, Melt
  • Remelt pure Si
    • Si melting point = 1412 C
    • Quartz crucible with graphite liner
    • RF induction heats graphite
  • Dip small Si seed crystal into melt
    • Seed determines crystal orientation

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czochralski technique for single crystal ingot growth freeze
Czochralski Technique forSingle-Crystal Ingot Growth, Freeze
  • Withdraw seed slowly while rotating
    • Withdrawal and rotational rates determine ingot diameter
    • 30-180 mm/hour
    • Largest current wafers = 300 mm
  • Si crystal structure = diamond

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single crystal ingot to wafer
Single-Crystal Ingot to Wafer
  • Diamond saw cuts grown crystal into slices = wafers
    • 0.25-1.00 mm thick
  • Polish one side of wafer to mirror finish

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oxidation converts si to sio 2
Oxidation Converts Si to SiO2
  • Wet oxidation
    • Oxidizing atmosphere contains water vapor
    • 900-1000 C
    • Rapid
  • Dry oxidation
    • Oxidizing atmosphere pure oxygen
    • 1200 C
  • Volume of SiO2 = 2 x volume of Si
    • SiO2 layer grows above Si surface approximately as far as it extends below Si surface

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dopants
Dopants
  • Si is semiconductor:Rconductor < RintrinsicSi < Rinsulator
  • Dopants = impurity atoms
    • Can vary conductivity by orders of magnitude
  • Dopant atom displaces 14Si atom in crystal
  • Each 14Si atom shares 4 electronswith its 4 neighbors in the crystal lattice,to form chemical bond
    • Group (column) IV-A of Periodic Table

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donor atoms provide electrons
Donor Atoms Provide Electrons
  • Group V-A of Periodic Table
  • Phosphorus, 15P, and Arsenic, 33As
  • 5 electrons in outer shell, 1 more than needed
  • Excess electron not held in bond is free to drift
  • If concentration of donors > acceptors,n-type Si

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acceptor atoms remove electrons from nearby atoms
Acceptor Atoms Remove Electrons from Nearby Atoms
  • Group III-A of Periodic Table
  • Boron, 5B
  • 3 electrons in outer shell, 1 less than needed
  • Incomplete bond,accepting electron from nearby atom
  • Movement of electron is effective flow of positive current in opposite direction
  • If concentration of acceptors > donors,p-type Si

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epitaxy
Epitaxy
  • Greek for “arranged upon” or “upon-ordered”
  • Grow single-crystal layeron single-crystal substrate
  • Homoepitaxy
    • Layer and substrate are same material
  • Heteroepitaxy
    • Layer and substrate differ
  • Elevate temperature of Si wafer surface
  • Subject surface to source of dopant

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deposition and ion implantation
Deposition and Ion Implantation
  • Deposition
    • Evaporate dopant onto Si wafer surface
    • Thermal cycle
      • Drives dopant from Si wafer surface into the bulk
  • Ion Implantation
    • Energize dopant atoms
    • When they hit Si wafer surface,they travel below the surface

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diffusion
Diffusion
  • At temperature > 800 C
  • Dopant diffuses from area of high concentration to area of low
  • After applying dopant, keep temperature as low as possible in subsequent process steps

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common dopant mask materials
Common Dopant Mask Materials
  • Photoresist
  • Polysilicon (gate conductor)
  • SiO2 = Silicon dioxide (gate insulator)
  • SiN = Silicon nitride

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selective diffusion process
Selective Diffusion Process
  • Apply dopant mask materialto Si wafer surface
    • Dopant mask pattern includes windows
  • Apply dopant source
  • Remove dopant mask material

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positive resist example
Positive Resist Example
  • Apply SiO2
  • Apply photoresist
    • PR = acid resistant coating
  • Pass UV light through reticle
    • Polymerizes PR
  • Remove polymerized areas with organic solvent
    • Developer solution
  • Etch exposed SiO2 areas

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lithography pattern storage technique 1
Lithography Pattern Storage, Technique 1
  • Mask
    • Two methods for making
      • Electron beam exposure
      • Laser beam scanning
    • Parallel processing

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lithography pattern storage technique 2
Lithography Pattern Storage, Technique 2
  • Direct Write
    • Two writing schemes
      • Raster scan
      • Vector scan
    • Pro
      • No mask expense
      • No mask delay
      • Able to change pattern from die to die
    • Con
      • Slow
      • Expensive

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lithography pattern transmission
Lithography Pattern Transmission
  • Four types of radiation to convey pattern to resist
    • Light
      • Visible
      • Ultraviolet
    • Ion
    • X-ray (does not apply to direct write)
    • Electron

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lithographic printing
Lithographic Printing
  • Contact printing
  • Proximity printing
  • Projection printing
    • Refraction projection printing
    • Reflection projection printing
    • Catadioptric projection printing

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contact and proximity printing
Contact and Proximity Printing
  • Contact printing
    • 0.05 atm < pressure < 0.30 atm
  • Proximity printing
    • 20 μm < mask-wafer separation < 50 μm
    • Pro
      • Low cost
      • Mask lasts longer because no contact
    • Con
      • Inferior resolution

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projection printing
Projection Printing
  • Projection printing
    • Higher resolution than proximity printing
  • Numerical Aperture
    • It was once believed that a high NAis always better.
    • If NA too low, can\'t achieve resolution
    • If NA too high, can\'t achieve depth of field
      • DOF = lambda/(2 NA2)

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refraction projection printing
Refraction Projection Printing
  • High resolution
  • To transmit deep UV, optical components are
    • Fused silica
    • Crystalline fluorides
  • Lenses are fused silica
    • Chromatic
    • Source bandwidth must be narrow
      • KrF laser

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reflection and catadioptric projection printing
Reflection and Catadioptric Projection Printing
  • Reflection projection printing
    • Polychromatic, larger spectral bandwidth
  • Catadioptric projection printing
    • Combines reflecting and refracting components
    • Larger spectral bandwidth
    • More than one optical axis
      • Aligning optical elements can be very difficult

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minimum channel length and gate insulator thickness improve performance
Minimum Channel Lengthand Gate Insulator ThicknessImprove Performance
  • Ids = Beta(Vgs – Vt)2 / 2
  • Beta = MOS transistor gain factor = ( (mu)(epsilon) / tox )( W / L )
  • mu = channel carrier mobility
  • epsilon = gate insulator permittivity (SiO2)
  • tox = gate insulator thickness
  • W / L = channel dimensions

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silicon gate process steps 1 2
Silicon Gate Process, Steps 1 & 2
  • Initial patterning SiO2 layer
    • Called field oxide
    • Thick layer
    • Isolates individual transistors
  • Thin SiO2 layer
    • Called gate oxide
    • Also called thinox
    • 10 nm < thin oxide < 30 nm

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silicon gate process step 3
Silicon Gate Process, Step 3
  • Polysilicon layer
    • Polycrystalline = not single crystal
    • Formed when Si deposited
      • Has high R when undoped
      • Used as high-R resistor in static memory
    • Used as
      • Short interconnect
      • Gate electrode
        • Most important:allows precise definition of source and drain electrodes
        • Deposited undoped on gate insulator
        • Then doped at same time as source and drain regions

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silicon gate process steps 4 5
Silicon Gate Process, Steps 4 & 5
  • Exposed thin oxide, not covered by poly,etched away
  • Wafer exposed to dopant sourceby deposition or ion-implantation
    • Forms n-type region in p-type substrateor vice versa
      • Source and drain created in shadow of gate
      • Si gate process called self-aligned process
    • Polysilicon doped, reducing its R

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silicon gate process final steps
Silicon Gate Process, Final Steps
  • SiO2 layer
  • Contact holes etched
  • Metal (Al, Cu) evaporated
  • Interconnect etched
  • Repeat for further interconnect layers

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parasitic mos transistors
Parasitic MOS transistors
  • Formed from
    • Diffusion regions of unrelated transistors
      • Act as parasitic source and drain
    • Thick (tfox) field oxide between transistorsoverrun by metal or poly interconnect
      • Act as parasitic gate insulator and
      • parasitic gate electrode
  • Raise threshold voltage of parasitic transistor
    • Make tfox thick enough
    • Add “channel-stop” diffusion between transistors

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four main cmos processes
Four Main CMOS Processes
  • n-well process
  • p-well process
  • Twin-tub process
  • Silicon on insulator

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n well process n well mask a
n-well Process, n-Well Mask A
  • Mask A defines n-well
  • Also called n-tub
  • Ion implantation produces shallower wells than deposition
    • Deeper diffusion also spreads further laterally
    • Shallower diffusion better for more closely-spaced structures

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n well process active mask b page 1
n-well Process, Active Mask B, Page 1
  • Mask B defines thin oxide
  • Called active mask, since includes
    • Area of gate electrode
    • Area of source and drain
  • Also called thinox
  • thin-oxide
  • island
  • mesa

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n well process active mask b page 2
n-well Process, Active Mask B, Page 2
  • Thin layer of SiO2 grown
  • Covered with SiN = Silicon Nitride
    • Relative permittivity of SiO2 = 3.9
    • Relative permittivity of Si3N4 = 7.5
    • Relative permittivity of comb. = 6.0
  • Used as mask for steps forchannel-stop mask C andfield oxide step D

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n well process channel stop mask c
n-well Process, Channel-Stop Mask C
  • Channel-stop implant
  • Raises threshold voltage of parasitic transistors
  • Uses p-well mask= complement of n-well Mask A
    • Where no nMOS, dope p-substrate to be p+

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n well process field oxide step d
n-well Process, Field Oxide Step D
  • Thick layer of SiO2 grown
  • Grows where no SiN
  • Grows where no mask B = no active mask
  • Called LOCOS = LOCal Oxidation of Silicon

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n well process bird s beak
n-well Process, Bird‘s Beak
  • Just as dopant diffuses laterally as well as vertically:
  • Field oxide also grows laterally,underneath SiN
  • Tapering shape called bird’s beak
  • Causes active area to be smaller
    • Reduces W
  • Some techniques limit this effect
    • SWAMI = SideWAll Masked Isolation

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n well process planarity
n-well Process, Planarity
  • Field oxide higher than gate oxide
  • Conductor thins or breaks
  • Problem called step coverage
  • To fix,pre-etch field oxide areasby 0.5 field oxide depth

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n well process v t adjust after field oxide step d
n-well Process, Vt Adjust,After Field Oxide Step D
  • Threshold voltage adjust
  • Optional
  • Uses n-well mask A
  • 0.5 v < Vtn < 0.7 v
  • -2.0 v < Vtp < -1.5 v
  • Add a negatively charged layer at Si-SiO2
  • Lowers channel
  • Called “buried channel” device

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n well process poly mask e
n-well Process, Poly Mask E
  • Mask E defines polysilicon
  • Poly gate electrodeacts as mask for source & drain regions
  • Called self-aligned

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n well process n mask f
n-well Process, n+ Mask F
  • n+ mask defines active areas to be doped n+
    • If in p-substrate,n+ becomes nMOS transistor
    • If in n-well,n+ becomes ohmic contact to n-well
  • Also called select mask

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n well process ldd step g
n-well Process, LDD Step G
  • LDD = Lightly Doped Drain
  • Shallow n-LDD implant
  • Grow spacer oxide over poly gate
  • Second, heavier n+ implant
    • Spaced from edge of poly gate
  • Remove spacer oxide from poly gate
  • More resistant to hot-electron effects

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n well process p mask h
n-well Process, p+ Mask H
  • p+ diffusion
  • Uses complement of n+ mask
  • p+ mask defines active areas to be doped p+
    • If in n-well,p+ becomes pMOS transistor
    • If in p-substrate,p+ becomes ohmic contact to p-substrate

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n well process sio 2 after p mask h
n-well Process, SiO2,After p+ Mask H
  • Entire chip covered with SiO2
  • No need for LDD for pMOS
    • pMOS less susceptible to to hot-electron effectsthan nMOS
  • LDD = Lightly Doped Drain

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n well process contact mask i
n-well Process, Contact Mask I
  • Defines contact cuts in SiO2 layer
  • Allows metal to contact
    • Diffusion regions
    • Poly gates

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n well process metal mask j
n-well Process, Metal Mask J
  • Wire it up!
  • n-well Process, Passivation Step
    • Protects chip from contaminants
      • Which can modify circuit behavior
    • Etch openings to bond pads for IOs

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p well process
p-Well Process
  • Transistor in native substratehas better characteristics
  • p-well process has better pMOS thann-well process
  • nMOS have better gain (beta) than pMOS

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twin tub process
Twin-Tub Process
  • Separately optimized wells
  • Balanced performance nMOS & pMOS
  • Start with epitaxial layer
    • Protects against latchup
  • Form n-well and p-well tubs

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silicon on insulator process
Silicon-on-Insulator Process
  • Uses n-islands and p-islands of siliconon an insulator
    • Sapphire
    • SiO2
  • No n-wells, no p-wells

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soi process advantages
SOI Process Advantages
  • No n-wells, no p-wells
    • Transistors can be closer together
    • Higher density
  • Lower parasitic substrate capacitance
    • Faster operation
  • No latchup
  • No body effect
  • Enhanced radiation tolerance

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