Page 170 - Sami Franssila Introduction to Microfabrication

Thermal Oxidation 149

Thermal oxide
Device silicon
Buried oxide

Handle wafer
(a) (b)
Figure 13.10 Silicon nanowire process on SOI: (a) SOI-structure after plasma etching and (b) after low-temperature
thermal oxidation: unoxidized silicon remains. Redrawn from Heidemayer, H. et al. (2000), by permission of AIP

Original Simulation of oxide stresses of KOH-etched V-
Si surface SiO 2 grooves is pictured in Figure 13.12. This stress-induced
oxide thinning has been used to advantage in nanohole
fabrication as shown in Figure 13.13. Etching in HF will
Convex open the apex only, creating a hole with dimensions in
corner the sub-100 nm range.
Si

13.6.1 Oxidation sharpening
Concave
corner Sharp tips are used as AFM probes and as field emitters
in vacuum microelectronic devices, for high resolution
Figure 13.11 Cross section of an oxidized silicon step in the former application and for low operating voltage
with oxide thinning at both convex (top) and concave in the latter. Such tips can be fabricated by isotropic
(bottom) corner. Reproduced from Minh, P.N. & T. Ono
etching, but the final part of the tip release is difficult:
(1999), by permission of AIP
the mask will fall off. Thermal oxidation can help: after
initial isotropic (or KOH anisotropic) etching, the final
sharpening takes place during oxidation. Mask removal
deep trenches the conformality may not be adequate. is done by isotropic etching, but this is non-critical, non-
Sacrificial thermal oxidation can be used to smooth cor- patterning etch, Figure 13.14. Thermal oxidation process
ners. Second thermal oxidation then provides the actual control is also much tighter than shape control in an etch
thin dielectric film, which serves, for example, as a process. In Chapter 39, a process for AFM cantilever-tip
DRAM capacitor dielectric. device will be presented.

60
2.6 SiO 2 50 2.6 50
40
40
2.8 30 2.8 30
20 20
y (µm) 3.0 10 y (µm) 3.0 10
5
5
3.2 1 3.2 1
3.4 Si 3.4

3.6 3.8 4.0 4.2 4.4 3.6 3.8 4.0 4.2 4.4
x (µm) x (µm)
(a) (b)
Figure 13.12 Oxide-stress simulation at the apex of etched groove; unit: MPa. Reproduced from Vollkopf, A. et al.
(2001), by permission of Electrochemical Society Inc