Processes of Fabrication

N Transistor Structure Review

P Transistor Structure Review

Semiconductor Review Create by doping a pure silicon crystal Diffuse impurity into crystal lattice Changes the concentration of carriers Electrons Holes More doping -> more carriers available n-type semiconductor (n or n+) Majority carrier: electrons Typical impurity: Arsenic (Column V) p-type semiconductor (p or p+) Majority carrier: holes Typical impurity: Boron (Column III) n+ n p+ p

Other key working materials Insulator - Silicon Dioxide (SiO2) Used to insulate transistor gates (thin oxide) Used to insulate layers of wires (field oxide) Can be grown on Silicon or Chemically Deposited Polysilicon - polycrystalline silicon Key material for transistor gates Also used for short wires Added by chemical deposition Metal - Aluminum (…and more recently Copper) Used for wires Multiple layers common Added by vapor deposition or “sputtering”

CMOS Processing Wafer Processing Photolithography Oxide Growth & Removal Material Deposition & Removal Diffusion of Impurities Putting it all together

AMD’s Dresden Fab - Source: AMD Corporation www.amd.com A View of the Cleanroom AMD’s Dresden Fab - Source: AMD Corporation www.amd.com

Creating Wafers - Czochralski Method Crucible Molten Silicon Start with crucible of molten silicon (≈1425oC) Insert crystal seed in melt Slowly rotate / raise seed to form single crystal boule After cooling, slice boule into wafers & polish

Definitions Wafer – a thin circular silicon Each wafer holds hundreds of dies Transistors and wiring are made from many layers (usually 10 – 15) built on top of one another the first half-dozen or so layers define transistors the second define the metal wires between transistors Lambda () – the smallest resolvable feature size imprinted on the IC; it is roughly half the length of the smallest transistor 0.2m IC – the smallest transistors are approximately 0.2m in length (= 0.1m)

Wafer Structure Current production: 200mm (10”) Newest technology: 300mm (12”)

Processing Wafers All dice on wafer processed simultaneously Each mask has one image for each die The basic approach: Add & selectively remove materials Metal - wires Polysilicon - gates Oxide Selectively diffuse impurities Photolithography is the key

Fabrication processes IC built on silicon substrate: some structures diffused into substrate; other structures built on top of substrate. Substrate regions are doped with n-type and p-type impurities. (n+ = heavily doped) Wires made of polycrystalline silicon (poly), multiple layers of aluminum (metal). Silicon dioxide (SiO2) is insulator.

Photolithography Coat wafer with photoresist (PR) Shine UV light through mask to selectively expose PR Use acid to dissolve exposed PR Now use exposed areas for Selective doping Selective removal of material under exposed PR Mask UV Light Photoresist Wafer

Adding Materials Add materials on top of silicon Methods Polysilicon Metal Oxide (SiO2) - Insulator Methods Chemical deposition Sputtering (Metal ions) Oxidation

Oxide (Si02) - The Key Insulator Thin Oxide Add using chemical deposition Used to form gate insulator & block active areas Field Oxide (FOX) - formed by oxidation Wet (H20 at 900oC - 1000oC) or Dry (O2 at 1200oC) Used to insulate non-active areas SiO2 Thin Oxide FOX SiN / SiO2 Silicon Wafer

Patterning Materials using Photolithography Add material to wafer Coat with photoresist Selectively remove photoresist Remove exposed material Remove remaining PR

Diffusion Introduce dopant via epitaxy or ion implant e.g. Arsenic (N), Boron (P) Allow dopants to diffuse at high temperature Block diffusion in selective areas using oxide or PR Diffusion spreads both vertically, horizontally

CMOS Well Structures Need to accommodate both N, P transistors Must implement in separate regions - wellls (tubs) N-well P-well Alternate approach: Silicon on Insulator (SOI) n-well p substrate n well n substrate p well p-well

Detailed View - N-Well Process Overall chip doped as p substrate, tied to GND Selected well areas doped n, tied to VDD Gnd VDD

CMOS Processing - Creating an Inverter Substrate Well Active Areas Gates Diffusion Insulator Contacts Metal P substrate n well wafer

CMOS Mask Layers Determine placement of layout objects Color coding specifies layers Layout objects: Rectangles Polygons Arbitrary shapes Grid types Absolute (“micron”) Scaleable (“lambda”) P substrate n well wafer

Mask Generation Mask Design using Layout Editor Pattern Generator user specifies layout objects on different layers output: layout file Pattern Generator Reads layout file Generates enlarged master image of each mask layer Image printed on glass reticle Step & repeat camera Reduces & copies reticle image onto mask One copy for each die on wafer Note importance of mask alignment

Simple cross section SiO2 metal3 metal2 metal1 transistor via poly n+ substrate n+ p+ substrate

Transistor structure n-type transistor:

0.25 micron transistor (Bell Labs) gate oxide silicide source/drain poly

Fabrication and Layout Ion Implantation Focus Neutral beam and beam path gated Beam trap and gate plate Wafer in wafer process chamber X - axis scanner Y - axis Neutral beam trap and beam gate Process Conditions Flow Rate: 5 sccm Pressure: 10-5 Torr Accelerating Voltage: 5 to 200 keV Gases Ar AsH3 B11F3 * He N2 PH3 SiH4 SiF4 GeH4 Solids Ga In Sb Liquids Al(CH3)3 180 kV Resolving Aperture Ion Source Equipment Ground Acceleration Tube 90° Analyzing Magnet Terminal Ground 20 kV Fabrication and Layout

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

Fabrication and Layout 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 Fabrication and Layout

Polysilicon Patterning Use same lithography process to pattern polysilicon Fabrication and Layout

Fabrication and Layout 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 Fabrication and Layout

Fabrication and Layout 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 Fabrication and Layout

Fabrication and Layout N-diffusion Historically dopants were diffused Usually ion implantation today But regions are still called diffusion Fabrication and Layout

Fabrication and Layout N-diffusion Strip off oxide to complete patterning step Fabrication and Layout

Fabrication and Layout P-Diffusion Similar set of steps form p+ diffusion regions for pMOS source and drain and substrate contact Fabrication and Layout

Fabrication and Layout Contacts Now we need to wire together the devices Cover chip with thick field oxide Etch oxide where contact cuts are needed Fabrication and Layout

Fabrication and Layout Metallization Sputter on aluminum over whole wafer Pattern to remove excess metal, leaving wires Fabrication and Layout

Physical Vapor Deposition (PVD) Chambers Cluster Tool Configuration Process Conditions Pressure: < 5 mTorr Temperature: 200 degrees C. RF Power: Wafers Transfer Chamber Loadlock Barrier Metals SiH4 Ar N2 Ti PVD Targets * Reactive Gases PVD Chamber N S Cryo Pump Transfer Chamber e - + Wafer Argon & Nitrogen Backside He Cooling DC Power Supply (+) Fabrication and Layout

Schematic Diagram of Vacuum Evaporation System (PVD) Fabrication and Layout

Fabrication and 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 Fabrication and Layout

Simplified Design Rules Conservative rules to get you started Fabrication and Layout

Fabrication and Layout Inverter Layout Transistor dimensions specified as Width / Length Minimum size is 4l / 2l, sometimes called 1 unit For 0.6 mm process, W=1.2 mm, L=0.6 mm Fabrication and Layout

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