Page 166 - Sami Franssila Introduction to Microfabrication

Thermal Oxidation 145

1400 different diffusion paths: through the bulk, and along
1200 1050 grain boundaries. Because grains grow during oxida-
tion, this introduces complications in the analysis. In
Thickness (nm) 800 1000 doped polysilicon, dopants precipitate at grain bound-
1000
950
aries. Boron doping leads to minor rate enhancement
600
900
and phosphorus-doping to clearly increased oxidation
400
200 850 rate via increased vacancy concentration, just as in the
case of the single-crystal material.
0 Silicides will generally oxidize to form SiO 2 , with
0 50 100 150 200 250 the exception of TiSi 2 , which will turn into TiO 2 .
Time (min)
Tungsten polycide gates (WSi 2 /poly) can be processed
(a) similarly to polysilicon. Making the silicide silicon-rich,
250 WSi 2.2 , will ensure proper oxidation. Silicon carbide,
SiC, can be oxidized to produce SiO 2 with standard
Thickness (nm) 150 1050 compared to silicon oxidation.
200
silicon oxidation processes but the rate is very low
1000
950
100
900
50 850 13.3 OXIDE STRUCTURE
0 Thermally grown silicon dioxide is glassy, and exhibits
0 50 100 150 200 250 only short-range order, in contrast to quartz, which is
Time (min) crystalline SiO 2 . The basic unit of silica structure is SiO 4
(b) (Figure 13.5).
In a perfect arrangement, such as crystalline quartz,
Figure 13.3 Oxidation of <100> silicon at temperatures all oxygen atoms bond to two silicon atoms (oxygen has
between 850 and 1050 C: wet and dry
◦
valence 2, silicon has valence 4) but in thermal oxide
some bonds are not made, leaving unbonded charged
and the oxide quality will be inferior to oxides grown oxygen atoms, making the oxide less stable than quartz.
on smooth surfaces. Polysilicon consists of grains of This is also reflected in their properties: quartz density
3
3
many orientations, which have different oxidation rates. is 2.65 g/cm , silicon oxide density 2.2 g/cm ; Young’s
Polysilicon texture is most often (110) and the oxi- modulus is 107 GPa for quartz and 87 GPa for oxide.
dation rate of undoped poly falls between (100) and When dopant atoms are incorporated into silicon
(111) rates. In polycrystalline materials, there are two dioxide network, they can take either substitutional or

1400
1200

1000 <111> Wet
Thickness (nm) 800 <100> Wet

<111> Dry
600
400 <100> Dry

200

0
800 850 900 950 1000 1050 1100
Temperature (°C)
Figure 13.4 Difference between <100> and <111> silicon oxidation (constant oxidation time 240 min)