Chemistry Reference
In-Depth Information
Formation rate and thickness of PS at the OCP can be altered by depositing a
metal film on part of the exposed surface to form a galvanic cell.
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The dissolution
rate of silicon is increased because the reduction of oxidants has a higher rate on the
metal surface. For example, in the stain etching of
p
(100) silicon with an A
l
film on
the back side, the A
l
film reduces the incubation time for the start of the etching
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process.
8.5. PS FORMED UNDER SPECIAL CONDITIONS
Different cell and electrode designs can be used for generation of PS of varying dis-
tribution and uniformity on the surface. For example, instead of back-side contact
through a metallic coating which covers the entire wafer, the wafer sample can be
clipped on one end which avoids the need for metallizing the back and PS can be formed
on both sides. The lateral distribution of current is very uneven at the beginning of
anodization but becomes more even later. As a result, the PS formed is thicker near the
top than near the bottom of the wafer, and has a varying morphology from the top to
the bottom of the wafer following the direction of the current flow.
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Another method is the use of two half-cells, in which Pt electrodes are immersed
in each cell and a Si wafer is used to separate and isolate the cells.
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In this design
there is an electrolytic contact on the back and thus there is no need to metallize the
back side of the wafer. The PS is found to be more uniform than the sample using back-
side metal contact.
The different positions for the
i-V
curves of different silicon materials indicate
that if a surface has different regions with different doping types or concentr-
ations, selective PS formation can occur in the regions that are favorably doped.
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Thus, when some regions of highly doped
n
-type wafer are ion amorphosed, formation
of PS occurs only on the amorphosed regions and no PS forms on the crystalline
regions.
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Hydrogen bubbles sticking on the surface tend to prevent the formation of
homogeneous PS.
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The bubbles can be broken free and removed with an ultra-
sonic oscillator source. Also, surfactants can be added to the solution to reduce the
amount of hydrogen bubbles adsorbed on the Si surface.
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Water has a high surface
tension; in inorganic solutions cations are known to increase the surface tension of
solution and fluoride ions are rather effective in this process. During the formation of
PS hydrogen gas is released so that if the surface tension is high the hydrogen gas is
effectively trapped on the silicon surface in the form of bubbles which then mask
the underlying surface preventing further dissolution of the surface. The presence of
an organic material in an aqueous solution will result in a decrease of its surface
tension. The amount of reduction depends on a number of factors such as the solubil-
ity of the organic solvent and the tendency of the organic material to adsorb preferen-
tially at the water/air interface. Ethanol and acetic acid are known to decrease the
surface tension. Under similar conditions the PS formed in the solution containing
organic solvent such as ethanol appears to be more uniform and also less prone to side
pore formation.
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