Page 248 - Chalcogenide Glasses for Infrared Optics

224 Cha pte r Ni ne

FIGURE 9.11 Photograph of AMI fl oat zone machine and fi nished “paveway
grade” silicon single crystal.

crystals. Figure 9.11 shows a front photograph of the machine including
the attached turbomolecular pump used to evacuate the chambers
down to 1 to 10 torr during the purification processing. The bottom
image in Fig. 9.11 shows a finished float zoned paveway-grade
silicon single crystal.
The purification of the silicon uses the same machine setup under
vacuum. No seed is involved. A rod of special high-grade, high-purity
polycrystalline silicon, such as produced by Hemlock Semicon-
ductor, is the starting point. As the crystal rotates, a narrow zone is
slowly swept through the crystal from the bottom to the top, carrying
the impurities in the zone to the top of the crystal. Travel time is 3 in/h