Chapter 22 Materials Selection and Design Considerations

22.15 Introduction • W119
and also with regard to other factors to include material degradability, flexibility,
grip, puncture resistance, etc.
Materials for Integrated Circuit Packages
22.15 INTRODUCTION
The microelectronic circuitry, including the integrated circuits that are used in our
modern computers, calculators, and other electronic devices, was briefly discussed
in Section 18.15. The heart of the integrated circuit (abbreviated IC) is the chip, a
small rectangular substrate of high-purity and single-crystal silicon (or more recently
gallium arsenide) onto which literally millions of circuit elements are imprinted.
Circuit elements (i.e., transistors, resistors, diodes, etc.) are created by selectively
adding controlled concentrations of specific impurities to extremely minute and
localized regions near the surface of the semiconducting material using involved
photolithographic techniques. The chips are small in size, with the largest on the
1
4
order of 6 mm ( in.) on each side and approximately 0.4 mm (0.015 in.) thick.
Photographs of a typical chip are shown in Figure 18.27.
Furthermore, chips are very fragile inasmuch as silicon is a relatively brittle material
and gallium arsenide is even more brittle. It is also necessary to fabricate conducting
circuit paths over the surface of the chip to facilitate the passage of current
from device to device; on silicon ICs the metal conductor used is aluminum or an aluminum–silicon
alloy (99 wt% Al, 1 wt% Si) that is metallized onto the chip surface to
form a very thin film. The chip design also calls for these circuit paths to terminate at
contact pads on the chip periphery, at which points electrical connections may be made
with the macroscopic world. It should be obvious that a functioning microelectronic
chip is a very sophisticated electronic entity, that materials requirements are very stringent,
and that elegant processing techniques are involved in its fabrication.
A large number of IC chips are fabricated onto a circular thin wafer of singlecrystal
Si, as shown in the photograph in Figure 22.28. Single crystals of Si having
diameters as large as 200 mm (8 in.) are routinely grown.The small rectangular ICs
Figure 22.28
Photograph of a 100mm-diameter(4-in.diameter)
silicon
wafer. Each of the
small rectangles
shown is an
individual IC chip or
die. (Photography by
S. Tanner.)