Download ICS 199 (Spring 1997) Lect 1 Commodity Computing

COSMOS Summer 2008
Chips and Chip Making
Rajesh K. Gupta
Computer Science and Engineering
University of California, San Diego.
©2008 R. Gupta, UCSD
Roadmap
• Topic:
– Integrated circuit chips
• This lecture
– IC Chips, Chip making ingredients and steps.
• Reference
– “How chips are made” – Intel
Keywords:
http://www.intel.com/education/makingchips/index.htm
– “Microelectronics 101” – IBM
http://www-306.ibm.com/chips/technology/makechip
©2008 R. Gupta, UCSD
The Chip: A Packaged Part
Quad Flat Pack (QFP)
Ball Grid Array (BGA)
http://education.netpack-europe.org/chipp.php
The Die Under a Microscope
Intel 4004 (‘71)
Intel 8080
Intel 8286
Intel 8085
Intel 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 chipmaking 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