Biomedical Engineering Reference
In-Depth Information
4.2.3.4 Oxidation
After this cleaning process, a layer of thermal silicon oxide was grown on the surface of
the (100) single crystal silicon samples that served as the mask for subsequent anisotropic
etching. Thermal oxidation is a high-temperature process that can be accelerated by using
water vapor. The following reactions take place at the silicon surface:
Si+O
2
→
SiO
2
(4.5)
Si+2H
2
O
→
SiO
2
+
2H
2
(4.6)
Thermally grown SiO
2
adheres firmly to the silicon substrate without producing either
cracks or pores. A furnace (Model RCA from Thermco) was used for this wet oxidation
process. Although the main oxidation was mainly wet, short periods of dry oxidation were
used at the beginning and end of the process. For annealing and oxidation of these 4 inch
silicon wafers, as well as smaller silicon samples, three furnaces were used and stacked
horizontally. First of all, the furnace set point was increased to the desired temperature,
1100
◦
C. While the temperature rose, the bubbler was prepared. The bubbler was rinsed
three times with DI water and then filled to approximately two-thirds full with DI water
and 10 vol% HCl was then added in order to accelerate the oxidation process. Between
10 and15 minutes before loading the samples into the furnace, the temperature of the
bubbler was set to 95
◦
C and its heater switched on. The samples were put in a glass rack
and kept inside the furnace tube. The first sample is usually a dummy sample because,
since it faces the stream flow, the silicon oxide obtained might not be as uniform as
desired. In the warm-up period, N
2
was connected to the chamber in order to prevent
any unwanted reaction during the warm up. This flow also cleanses the chamber and
helps create a uniform heat distribution by creating circulation. After reaching the set
temperature, the oxidation process was started by disconnecting the N
2
and connecting
dry O
2
. The dry oxygen flow was continued for 5 minutes because dry oxidation creates
a uniform and very adhesive sublayer. Then the oxygen flow was redirected through the
bubbler which had reached boiling point and had begun to evaporate. To achieve 1000 A
oxide thicknesses, a 20 minute period of wet oxidation was sufficient. The bubbler was
then bypassed and dry oxygen was again connected directly into the chamber. After 5
minutes of dry oxidation, the furnace was turned off. In the cooling-down phase, oxygen
flow was replaced with N
2
. In order to prevent any thermal shock, samples were gradually
removed from the furnace at about 400
◦
C.
4.2.3.5 Lithography
Patterning on the silicon oxide layer was carried out through a standard lithography pro-
cedure, including photoresist spinning, soft baking, exposing the medium using ultraviolet
light, development, and finally hard baking. The designed layout of the mask that was
used for the patterning is shown in Figure 4.27.
Spin Coating
In order to apply a uniform positive photo resist layer with a thickness of about 1.6
μm required for this work, a conventional spinner (S 1813 from Shipley) was used at
4000 rpm (acceleration 450 rpm s
-1
)foradurationof30s.
