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©2008 R. Gupta, UCSD
COSMOS Summer 2008
Chips and Chip Making
Rajesh K. Gupta
Computer Science and Engineering
University of California, San Diego.
Roadmap
• Topic:– Integrated circuit chips
• This lecture– IC Chips, Chip making ingredients and steps.
• Reference– “How chips are made” – Intel
http://www.intel.com/education/makingchips/index.htm
– “Microelectronics 101” – IBMhttp://www-306.ibm.com/chips/technology/makechip
©2008 R. Gupta, UCSD
Keywords:
The Chip: A Packaged Part
http://education.netpack-europe.org/chipp.php
Quad Flat Pack (QFP)
Ball Grid Array (BGA)
The Die Under a Microscope
Intel 4004 (‘71)Intel 4004 (‘71)Intel 8080Intel 8080 Intel 8085Intel 8085
Intel 8286Intel 8286 Intel 8486Intel 8486
A Gate Layout
Defines a set of “masking layers” for printing purposes.
The Ingredients
• Silicon Wafers cut from an ingot of pure silicon.
• Chemicals and gases are used throughout the chip-making process.
• Metals, such as aluminum and copper, are used to conduct the electricity throughout the microprocessor. Gold is also used to connect the actual chip to its package.
• Ultraviolet (UV) Light has very short wavelengths and is just beyond the violet end of the visible spectrum.
• Masks used in the chip-making process are like stencils. When used with UV light, masks create the various circuit patterns on each layer of the chip.
[Courtesy Intel. Adapted from http://www.intel.com/education/teachtech/learning/chips/preparation.htm]
Building Chip
• Start with a disk of silicon called wafer• 75 mm to 300 mm in diameter, < 1 mm thick• cut from ingots of single-crystal silicon
• pulled from a crucible of pure molten polycrystalline silicon using a seed crystal
• Different processing steps and techniques• Introduce dopants• Oxidation• Masking• Polysilicon
Introduce Dopants
• Pure silicon is a semiconductor• bulk electrical resistance in between that of a conductor
and insulator
• Conductivity of silicon can be varied several orders of magnitude by introducing impurity atoms
• called dopants
• acceptors: accept electrons to leave holes in silicon
• lead to p-type silicon (e.g. Boron)
• donors: provide electrons to silicon
• lead to n-type silicon (e.g. Arsenic, Phosphorous)
Introduce Dopants (2)
• Deposition through diffusion– evaporating dopant material into the
silicon surface– thermal cycle: impurities diffuse deeper
into material
• Ion Implantation– silicon surface subjected to highly
energized donor or acceptor atoms
• atoms impinge silicon surface, and drive below it to form regions of varying concentrations
Ion Implantation
Oxidation
• Method 1: Heating silicon wafers in an oxidizing atmosphere (O2 or H2O)
• Consumes Si
• Grows equally in both vertical directions
• Method 2: Deposition• Deposited on top of existing layers
Masking
• Masks act as barrier against e.g.• ion implantation
• dopant deposition before diffusion (dopants do not reach surface)
• oxidation (O2 or H2O does not reach surface)
• Commonly used mask materials• photoresist
• polysilicon
• silicon dioxide (SiO2)
• silicon nitride (SiN)
Example: oxide mask
a. bare silicon wafer
b. oxidize wafer
c. deposit layer of photoresist
d. expose the photoresist selectively to UV light
• The drawn mask pattern determines which part is exposed
• Resist polymerizes where exposed
e. unexposed resist is removed with solvent: negative resist
(positive resist: exposed resist is removed)
f. exposed oxide is etched
g. photoresist is washed off
h. the oxide can now be used as a masking layer for ion implantation
The Printing Challenge
UV lithography: line width limited by diffraction and alignment tolerances, but tricks are used Electron beam lithography has emerged: directly from digital data, but more costly and slow
Polysilicon
• Silicon also comes in a polycrystalline form• called polysilicon, or just poly
• high resistance
• normally doped at the same time as source/drain regions
• Used as• an interconnect in silicon ICs
• gate electrode in MOS transistors
• most important: acts as a mask to allow precise definition of source and drain extension under gate
• minimum gate to source/drain overlap improves circuit performance (why?)
• called self-aligned process
The Design Process
Packaging
Single die
Wafer
From http://www.amd.com