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VLSI Digital Systems Design
CMOS Processing cmpe222_03process_ppt.ppt
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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) cmpe222_03process_ppt.ppt
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Czochralski Technique for Single-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 cmpe222_03process_ppt.ppt
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Czochralski Technique for Single-Crystal Ingot Growth, Freeze
Withdraw seed slowly while rotating Withdrawal and rotational rates determine ingot diameter mm/hour Largest current wafers = 300 mm Si crystal structure = diamond cmpe222_03process_ppt.ppt
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Single-Crystal Ingot to Wafer
Diamond saw cuts grown crystal into slices = wafers mm thick Polish one side of wafer to mirror finish cmpe222_03process_ppt.ppt
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Oxidation Converts Si to SiO2
Wet oxidation Oxidizing atmosphere contains water vapor 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 cmpe222_03process_ppt.ppt
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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 electrons with its 4 neighbors in the crystal lattice, to form chemical bond Group (column) IV-A of Periodic Table cmpe222_03process_ppt.ppt
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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 cmpe222_03process_ppt.ppt
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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 cmpe222_03process_ppt.ppt
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Epitaxy Greek for “arranged upon” or “upon-ordered”
Grow single-crystal layer on 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 cmpe222_03process_ppt.ppt
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Deposition and Ion Implantation
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 cmpe222_03process_ppt.ppt
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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 cmpe222_03process_ppt.ppt
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Common Dopant Mask Materials
Photoresist Polysilicon (gate conductor) SiO2 = Silicon dioxide (gate insulator) SiN = Silicon nitride cmpe222_03process_ppt.ppt
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Selective Diffusion Process
Apply dopant mask material to Si wafer surface Dopant mask pattern includes windows Apply dopant source Remove dopant mask material cmpe222_03process_ppt.ppt
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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 cmpe222_03process_ppt.ppt
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Lithography Pattern Storage, Technique 1
Mask Two methods for making Electron beam exposure Laser beam scanning Parallel processing cmpe222_03process_ppt.ppt
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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 cmpe222_03process_ppt.ppt
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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 cmpe222_03process_ppt.ppt
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Lithographic Printing
Contact printing Proximity printing Projection printing Refraction projection printing Reflection projection printing Catadioptric projection printing cmpe222_03process_ppt.ppt
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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 cmpe222_03process_ppt.ppt
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Projection Printing Projection printing Numerical Aperture
Higher resolution than proximity printing Numerical Aperture It was once believed that a high NA is always better. If NA too low, can't achieve resolution If NA too high, can't achieve depth of field DOF = lambda/(2 NA2) cmpe222_03process_ppt.ppt
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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 cmpe222_03process_ppt.ppt
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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 cmpe222_03process_ppt.ppt
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Minimum Channel Length and Gate Insulator Thickness Improve 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 cmpe222_03process_ppt.ppt
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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 cmpe222_03process_ppt.ppt
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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 cmpe222_03process_ppt.ppt
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Silicon Gate Process, Steps 4 & 5
Exposed thin oxide, not covered by poly, etched away Wafer exposed to dopant source by deposition or ion-implantation Forms n-type region in p-type substrate or vice versa Source and drain created in shadow of gate Si gate process called self-aligned process Polysilicon doped, reducing its R cmpe222_03process_ppt.ppt
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Silicon Gate Process, Final Steps
SiO2 layer Contact holes etched Metal (Al, Cu) evaporated Interconnect etched Repeat for further interconnect layers cmpe222_03process_ppt.ppt
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Parasitic MOS transistors
Formed from Diffusion regions of unrelated transistors Act as parasitic source and drain Thick (tfox) field oxide between transistors overrun 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 cmpe222_03process_ppt.ppt
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Four Main CMOS Processes
n-well process p-well process Twin-tub process Silicon on insulator cmpe222_03process_ppt.ppt
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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 cmpe222_03process_ppt.ppt
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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 cmpe222_03process_ppt.ppt
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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 for channel-stop mask C and field oxide step D cmpe222_03process_ppt.ppt
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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+ cmpe222_03process_ppt.ppt
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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 cmpe222_03process_ppt.ppt
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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 cmpe222_03process_ppt.ppt
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n-well Process, Planarity
Field oxide higher than gate oxide Conductor thins or breaks Problem called step coverage To fix, pre-etch field oxide areas by 0.5 field oxide depth cmpe222_03process_ppt.ppt
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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 cmpe222_03process_ppt.ppt
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n-well Process, Poly Mask E
Mask E defines polysilicon Poly gate electrode acts as mask for source & drain regions Called self-aligned cmpe222_03process_ppt.ppt
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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 cmpe222_03process_ppt.ppt
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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 cmpe222_03process_ppt.ppt
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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 cmpe222_03process_ppt.ppt
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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 effects than nMOS LDD = Lightly Doped Drain cmpe222_03process_ppt.ppt
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n-well Process, Contact Mask I
Defines contact cuts in SiO2 layer Allows metal to contact Diffusion regions Poly gates cmpe222_03process_ppt.ppt
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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 cmpe222_03process_ppt.ppt
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p-Well Process Transistor in native substrate has better characteristics p-well process has better pMOS than n-well process nMOS have better gain (beta) than pMOS cmpe222_03process_ppt.ppt
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Twin-Tub Process Separately optimized wells
Balanced performance nMOS & pMOS Start with epitaxial layer Protects against latchup Form n-well and p-well tubs cmpe222_03process_ppt.ppt
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Silicon-on-Insulator Process
Uses n-islands and p-islands of silicon on an insulator Sapphire SiO2 No n-wells, no p-wells cmpe222_03process_ppt.ppt
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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 cmpe222_03process_ppt.ppt
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