Page 61 - Sami Franssila Introduction to Microfabrication

40 Introduction to Microfabrication

(100) (110) (111)
Figure 4.4 Some important silicon crystal planes with their Miller indices

(a) (b) (c)
Figure 4.5 Silicon crystal viewed from different angles: (a) face view (100); (b) edge view (110); (c) vertex view (111).
Figure courtesy Ville Voipio, Helsinki University of Technology

are designated {100}. There are 12 (1, 1, 0) and 8 (1, transparency and gluing it together will result in a 26-
1, 1) planes. Wafers are sometimes cut to other index gon, which visualizes the crystal planes nicely. It will
planes, most notably (311) and (511). be indispensable when crystal-plane dependent etching
Fourfold symmetry of (100) and sixfold symmetry of of silicon will be discussed in Chapters 21 and 28.
(110) and (111) can be seen in Figure 4.5, and it will Wafers of two crystal orientations are widely used
become apparent in anisotropic wet etching of silicon in microfabrication: <100> and <111>. The former
(to be discussed in Chapter 21). is the main material for CMOS and bulk microme-
The angles between the planes can be calculated from chanics; the latter for bipolar transistors, power semi-
the scalar product of the normal vectors conductor devices and radiation detectors that rely on
epitaxial deposition.
a · b = |a||b| cos(a, b) (4.7)
4.4 SILICON WAFERING PROCESS
Visual examination shows that (100) and (110) planes
◦
meet at 45 and all the other angles can be calculated As listed in Table 4.3, silicon ingots are transformed into
easily, when the negative unit vectors are accounted wafers by a long process which includes mechanical,
for: 110 is (−1, 1, 0). The angle between (111) and thermal and chemical treatments and many cleaning and
√
(100) planes is calculated from 1 = 3 cos α, giving inspection steps.
α = 54.7 . The silicon-crystal orientation is determined by the
◦
In order to get familiar with the silicon crystal seed crystal. After the ingot has cooled down, it is cut
structure, the paper fold model shown in Figure 4.6 to ca. 50 cm stocks, which are measured for crystal
becomes handy. Copying the model on an overhead orientation by X-ray diffraction. A flat or a notch is