Page 307 - Manufacturing Engineering and Technology - Kalpakjian, Serope : Schmid, Steven R.

28 Chapter 11 Metal-Casting Processes and Equipment

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FIGURE |.26 Two methods of crystal growing: (a) crystal pulling (Czochralski process) and
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(b) the floating-zone method. Crystal growing is especially important in the semiconductor
industry. (c) A single-crystal ingot produced by the Czochralski process. Source: Courtesy of
Intel Corp.

are more expensive than other types, the lack of grain boundaries makes them resist-
ant to creep and thermal shock, so they have a longer and more reliable service life.

Single-crystal Growing. Single-crystal growing is a major activity in the semicon-
ductor industry in the manufacture of the silicon wafers in microelectronic devices
(Chapter 28). There are two basic methods of crystal growing:
° In the crystal-pulling method, also known as the Czochralski (CZ) process
(Fig. 11.26a), a seed crystal is dipped into the molten metal and then pulled out
slowly (at a rate of about 10 /,tm/s) while being rotated. The liquid metal begins
to solidify on the seed, and the crystal structure of the seed is continued through-
out. Dopants (alloying elements) may be added to the liquid metal to impart
special electrical properties. Single crystals of silicon, germanium, and various
other elements are grown with this process. Single-crystal ingots up to 400 mm
in diameter and over 2 m in length have been produced by this technique, al-
though 200- and 300-mm ingots are common in the production of silicon wafers
for integrated circuit manufacture (Part V).
° The second technique for crystal growing is the floating-zone method (Fig. 11.26b).
Starting with a rod of polycrystalline silicon resting on a single crystal, an induction
coil heats these two pieces while the coil moves slowly upward. The single crystal
grows upward while maintaining its orientation. Thin wafers are then cut from
the rod, cleaned, and polished for use in microelectronic device fabrication. This
process is suitable for producing diameters under 150 mm, with very low levels of
impurities.

ll.6 Rapid Solidification

The properties of amorphous alloys (also known as metallic glasses) were described
in Section 6.14. The technique for making these alloys (called rapid solidwcation)
involves cooling the molten metal at rates as high as 106 K/s, so that it does not have