Page 56 - Sami Franssila Introduction to Microfabrication

4

Silicon

Silicon transistors were first made in 1952, five years noise than germanium devices with a 0.67 eV gap.
after the first germanium-based transistors. The elec- Silicon source gases can be purified to extremely high
tron mobility in germanium was much higher, and ger- degrees of purity, meaning that a high resistivity material
manium crystal growth was more advanced. However, can be made. Taken together with the high solubility
21
silicon, with its 1.12 eV bandgap, was better suited to of dopants, up to 10 cm −3 for the common dopants
higher operating temperatures, and the reverse currents boron, phosphorus and arsenic, this translates to eight
were also smaller. The real breakthrough came by the orders of magnitude resistivity tailoring opportunities
end of 1950s when the beneficial role of silicon dioxide (Figure 4.1). Optical absorption in the visible makes
was recognized: silicon dioxide provided the passivation silicon suitable for photodetectors and solar cells, and its
of semiconductor surfaces, and it resulted in improved transparency in the infrared (above 1.1 µm) is utilized
transistor reliability. When it was further noticed that in IR microsystems (Table 4.1).
SiO 2 layer could act as a diffusion mask and as iso- Silicon is strong: its Young’s modulus can be as
lation for integrated metallization, the way was open high as 190 GPa (for <111> orientation). The excellent
for the invention of the integrated circuit. Oxide was a mechanical properties of silicon have been utilized
suitable isolation material and aluminium metallization since the 1960s in micromechanical pressure and force
could be patterned on top of the oxide. Neither GaAs sensors that rely on bending beams and diaphragms.
nor Ge form stable and water insoluble oxides. Piezoresistivity detection depends on doped regions
Silicon crystal growth rapidly caught up with germa- for the resistors, and capacitive detection relies on
nium, and the steady increase in wafer size has continued the ability to micromachine shallow air gaps of the
up to this day, with 300 mm diameter wafers now in order of 1 µm. Both are standard processes in silicon
production. For other substrates, smaller sizes are still microfabrication.
widely used, and when new materials such as silicon Stress, σ, and strain (elongation), ε, are correlated via
carbide (SiC) are introduced, the crystal growth and the
wafering yield are so low that only small ingots and σ = εE (4.1)
small wafers make sense.
Some 150 million silicon wafers, corresponding to 3 with a constant of proportionality E, the Young’s
2
to 4 km , are processed annually. The largest proportion modulus. Elongation ε can also be stated as L/L, and
of them are 150 mm and 200 mm diameter wafers, ca. stress as force per area, which gives the most familiar
50 million each, with some 20 million wafers of both expression of Hooke’s law: F/A = E L/L. When a
100 mm and 125 mm sizes. The latest 300 mm wafers piece of material is tensile- stressed, its elongation leads
accounted for some 10 million slices in 2003. also to a lateral shrinkage of its diameter, ε lateral =
D/D. Poisson ratio is defined as ν = −ε lateral /ε tensile .
Silicon Poisson ratio, 0.27, in silicon is among the lowest
4.1 SILICON MATERIAL PROPERTIES of all solids.
Silicon is as strong as steel, but this fact is
Silicon material properties are an excellent compromise disguised by two factors: first, most of us do not
between performance and stability. An energy gap of have experience with 0.5 mm-thick steel plates, and
1.12 eV makes silicon devices less prone to thermal second, silicon is brittle and the breakage pattern

Introduction to Microfabrication Sami Franssila
 2004 John Wiley & Sons, Ltd ISBNs: 0-470-85105-8 (HB); 0-470-85106-6 (PB)