Download Semiconductor Materials

Semiconductor Materials
Whereas polymers are highly visible engineering materials with a major impact on
contemporary society, semiconductors are relatively invisible but have a comparable
social impact. Technology has clearly revolutionized society, but solid-state electronics is
revolutionizing technology itself. A relatively small group of elements and compounds
has an important electrical property, semi-conduction , in which they are neither good
electrical conductors nor good electrical insulators. Instead, their ability to conduct
electricity is intermediate. These materials are called semiconductors, and in general,
they do not fit into any of the four structural materials categories based on atomic
bonding. Metals are inherently good electrical conductors. Ceramics and polymers (nonmetals) are generally poor conductors but good insulators. The three semiconducting
elements (Si, Ge, and Sn) from column IVA of the periodic table serve as a kind of
boundary between metallic and nonmetallic elements. Silicon (Si) and germanium (Ge),
widely used elemental semiconductors, are excellent examples of this class of materials.
[Another important semiconductor material is GaAs, a compound of the group IIIA
element Ga; and the group V element, As.]
Taken from Introduction to Materials Science for Engineers, James F. Shackelford,
Prentice Hall, Inc., New Jersey.
One area of materials
engineering is called "process
engineering". It is perhaps
best illustrated by the
production of integrated
circuit devices. An excellent
montage representing IC
processing is illustrated in the
following IBM gif. Behind the
Process Engineer, one see's a
blow-up of the threedimensional array of
metallization lines that
interconnect the various device regions on an integrated circuit. The gray substrate under
the pink, white, yellow and green metallization pattern, is the silicon wafer, which is a
semiconductor material. These interconnects are typically less than a micro-meter in
thickness! The reference source URL is IBM chip technology
The metallization lines in the above figure
connect regions of the semiconductor that have
been specifically "doped" to change electrical
performance. Doping involves substituting other
elements (like phosphorous and boron, for
example) for individual Si atoms in the Siwafer. When differently doped regions of the
semiconductor are properly positioned and
electrically connected, transistors (for example)
can be made.
The image to the left shows the first transistor,
discovered by Bardeen, Brattain and Shockley at Bell Labs. The triangular hunk is a
piece of Ge crystal. In 1947, when they switched to using tremendously pure materials, it
dawned on Bardeen, Brattain and Shockley that they could build the circuit in the picture.
It was an amplifier! For more about this important discovery, visit the source URL at the
PBS website. For more about transistor regions in today's IC's, link to the following web
page.
The complex, interconnected, metal lines on today's integrated circuits begin as a metallic
thin-film layer, deposited on the top of a silicon wafer. Photolithography [use a search
engine to find out what this is all about] is used to establish the architecture of the lines:
subsequent etching steps define them. To learn more about the deposition of thin-film
metals and to see some of the equipment used, go to AltProcess.htm . For cross-sectional
view and operational description of six different IC (Integrated Circuits) devices, visit
these International Rectifier web pages. To see detail of IC architecture, visit the
Materials Characterization web pages of the WofMatE. The pbs transistor web pages
offer further detail of typical IC device concepts and architecture.
The products of the process engineer in the microelectronics industry are represented in
this montage (left), also from IBM. A typical IC (full die shot of the HP PA-8000
microprocessor using oblique illumination with blue and red gels) is shown on the right.
For more photomicrographs of IC dies, visit these Florida State University web pages.
One step in the production of semiconductor devices involves the growth of a large (10 or
more inches in diameter!) single crystal of silicon by the Czochralski process. In this
process, a solid seed crystal is rotated and slowly extracted from a pool of molten Si. This
is shown above. The various steps in the production of semiconductor devices are clearly
illustrated on the web pages of Fullman-Kinetics, the source of the Czochralski imagesequence, above. [The Fullman-Kinetics web pages appear to no longer be available on
the Internet, and this is a shame. They were excellent!] However, please visit the chip
fabrication processing pages at this German website. You may also wish to visit this
University of Delaware link .
In another IC processing step, after wafers of silicon are sliced from the single crystal and
then polished, they are thermal-annealed in an oven. Below (left) we see Si wafers
entering an oven for an annealing treatment. This is taken from the Intel web pages.
In a vacuum chamber, fine, intricate patterns are etched into silicon wafers with an ion
discharge (above, right). This is another critical step in the production of integrated
circuits. The violet glow is emitted by the ion plasma. The plasma etching process makes
possible the small geometries need for very large scale integration in silicon chips. This
image is taken from the Lucent Technologies web pages.
Now the above images illustrate methods applicable to the fabrication of integrated
circuits (IC's) [also termed "chips"] used in the production of microelectronic devices.
The process engineer plays a key role in the production of IC's. But the process engineer
too is involved in the packaging of these microelectronic devices; and in the assembly of
packaged device on to the printed circuit board (PCB). The whole sequence is quite
elaborate and a comprehensive treatment is well outside the scope of these web pages.
However, I would like to introduce you to one facet of IC packaging and that is the
bonding of the IC chip to a suitable substrate. For this example, consider "flip chip"
technology. The respective images presented below have been extracted from the flip
chip, MEMS web pages of the MEMS Technology Application Center, now CRONOS
Integrated Microsystems, Inc. MEMS stands for MicroElectroMechanical Systems.
'Flip chip' is a general term that describes a method of attaching an integrated circuit (ie,
IC) to a substrate. The substrate can be another chip, a PC board, a special carrier or a
multichip module. More specifically, flip chip is a technology in which one chip is placed
face down (flip chipped) on to the substrate and attached through solder bumps that
provide both mechanical and electrical connection. Flip chip provides a much higher
interconnection density than traditional methods (i.e. wire bonding) because the
interconnections are distributed across the entire chip surface rather than being restricted
to the periphery of the chip.
This technology can be used to attach a MEMS chip to a
die, as in the example introduced above. For information
about MEMS and MEMS applications, visit the following
links developed by Danny Banks in the UK.
The MEMS chip shown in the figure must be aligned so that the appropriate pads on the
die and chip are in contact. Then the solder is heated to the reflow-temperature (a
temperature where the solder "bumps" reflow and solidify upon subsequent cooling), thus
providing both mechanical and electrical contact. Shown is a joined pair, chip to die, after
solder reflow. A cross-section through a flip chip illustrating a typical solder bump
appears on the left; and an actual array of re-flowed solder bumps (prior to assembly) is
shown below.
If you are really into learning more about semiconductors from a materials perspective,
visit the following web pages which offer a collection of really cool JAVA Applets!
Is it not easy to see that the materials scientist and materials engineer must necessarily
play a key role in the design, production and quality assurance of semiconductor devices,
microelectronics packages and MEM systems?