U.S. patent application number 10/090915 was filed with the patent office on 2002-06-27 for etching solution for wet chemical pyramidal texture etching of silicon surfaces.
Invention is credited to Holdermann, Konstantin.
Application Number | 20020079290 10/090915 |
Document ID | / |
Family ID | 7861399 |
Filed Date | 2002-06-27 |
United States Patent
Application |
20020079290 |
Kind Code |
A1 |
Holdermann, Konstantin |
June 27, 2002 |
Etching solution for wet chemical pyramidal texture etching of
silicon surfaces
Abstract
A new and improved etching solution and etching method provide
wet chemical pyramidal texture etching of (100) silicon surfaces. A
uniform and completely pyramidal texture etching of silicon
surfaces is achieved with an etching solution including water, an
alkaline reagent, and isopropanol together with an aqueous alkaline
ethylene glycol solution.
Inventors: |
Holdermann, Konstantin;
(Offingen, DE) |
Correspondence
Address: |
CHRISTIE, PARKER & HALE, LLP
P.O. BOX 7068
PASADENA
CA
91109-7068
US
|
Family ID: |
7861399 |
Appl. No.: |
10/090915 |
Filed: |
March 5, 2002 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
|
|
10090915 |
Mar 5, 2002 |
|
|
|
09272022 |
Mar 18, 1999 |
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Current U.S.
Class: |
216/99 ;
257/E21.223; 257/E31.13 |
Current CPC
Class: |
H01L 31/0236 20130101;
Y02E 10/50 20130101; H01L 31/02363 20130101 |
Class at
Publication: |
216/99 |
International
Class: |
C23F 001/00 |
Foreign Application Data
Date |
Code |
Application Number |
Mar 18, 1998 |
DE |
DE-198 11 878.3 |
Claims
What is claimed is:
1. A free-standing etching solution for the wet chemical pyramidal
texture etching of silicon surfaces comprising: water, an alkaline
reagent, isopropanol and an aqueous alkaline ethylene glycol
solution.
2. The free-standing etching solution as in claim 1, wherein the
etching solution further comprises silicate.
3. The free-standing etching solution as in claim 1, wherein the
etching solution contains 0.5 to 5 vol % isopropanol.
4. The free-standing etching solution as in claim 1, wherein the
proportion of isopropanol present in the etching solution is
greater than the proportion of ethylene glycol.
5. The free-standing etching solution as in claim 4, wherein the
ratio of isopropanol the ethylene glycol is at maximum 1:1.
6. The free-standing etching solution as in claim 1, wherein the
aqueous alkaline ethylene glycol solution is reacted with
oxygen.
7. A free-standing etching solution for the wet chemical pyramidal
texture etching of silicon surfaces comprising: water, sodium or
potassium hydroxide, isopropanol and an aqueous alkaline ethylene
glycol solution.
8. The free-standing etching solution as in claim 7, further
comprising silicate.
9. The free-standing etching solution as in claim 7, wherein the
etching solution contains 0.5 to 5 vol. % isopropanol.
10. The free-standing etching solution as in claim 7, wherein the
proportion of isopropanol present in the etching solution is
greater than the proportion of ethylene glycol.
11. The free-standing etching solution as in claim 10, wherein the
ratio of isopropanol to ethylene glycol is at maximum 1:1.
12. The free-standing etching solution as in claim 7, wherein the
aqueous alkaline ethylene glycol solution is reacted with oxygen.
Description
CROSS-REFERENCE TO RELATED APPLICATION
[0001] This application is a continuation of pending U.S.
application Ser. No. 09/272,022, filed on Mar. 18, 1999.
BACKGROUND OF THE INVENTION
[0002] The present invention relates to a method for the wet
chemical pyramidal texture etching of silicon surfaces.
[0003] A textured surface reduces the reflection of incident light
across wide bands, thereby increasing the absorbed light intensity.
When manufacturing crystalline silicon solar cells, a systematic
structuring of the silicon surface leads to an increase in solar
cell performance. Such a wet chemical structural etching is not,
however, restricted just to the processing and production of solar
cells, but is also suitable for optical and electrochemical
detectors/sensors, biodetectors/biosensors, catalysts, electrodes
and the like.
[0004] Known techniques for the wet chemical pyramidal structural
etching of (100)-oriented silicon surfaces use alkaline media,
mainly with solutions of alkali hydroxides, alkali carbonates,
ammonia or choline. Solutions of hydrazine or ethylene
diamine/pyrocatechol can also be used, but they are disadvantageous
on account of their toxicity. The most common recipes comprise
water, sodium or potassium hydroxide and alcohol. The alcohol
component used is either ethylene glycol or isopropanol.
Nevertheless, the known methods for the wet chemical structural
etching of silicon merely relate, purely and simply, to the
generation of a pyramidal (tetragonal) texture.
[0005] In terms of fine line metalization and the application of
doping resists or photoresist, however, a texture that exhibits
small pyramids is desirable. A method based on an etching solution
that contains ethylene glycol is described in EP 0 477 424 A1. The
etching solution recipe used here contains water, potassium
hydroxide, ethylene glycol and silicon. Oxygen is also added as a
further component. After the silicon surface has undergone wet
chemical structural etching, reproducibly uniform pyramids are
obtained by aerating the etching solution with oxygen. The pyramid
height can be varied by the aerating duration of the etching
solution. Introducing oxygen over a longer period, i.e. greater
oxidation of the glycol, results in smaller pyramids. In this way,
pyramid sizes of .ltoreq.2 .mu.m can be produced even in terms of
manufacturing engineering. But the disadvantage is that the etching
solution based on ethylene glycol cannot be used ad hoc because it
requires a preceding dissolution of silicon. Studies have shown
that a subsequent rest phase of several hours proves to be
beneficial for formation of smooth (111) faces. A simple addition
of silicate solution does not lead to success.
[0006] Methods which use an etching solution based on isopropanol
are known for example from U.S. Pat. No. 3,998,659, from "Uniform
Pyramid Formation on Alkaline-etched Polished Monocrystalline (100)
Silicon Wafers", Bressers et al., Progress in Photovoltaics, Vol.
4, 435-438 (1996), and from "Experimental Optimization of an
Anisotropic Etching Process for Random Texturization of Silicon
Solar Cells", King et al., IEEE 1991, 303-308. In contrast to the
etching solution based on ethylene glycol, an IPA-based etching
solution can be used immediately for the purpose of texture
etching. This etching solution can be used with and without
silicate. This type of solution nevertheless suffers from the
drawback of a high rate of evaporation, on account of the
isopropanol's boiling point of just 82.degree. C. This means that
problems arise with regard to uniform etching and reproducibility
of the pyramidal surface. In contrast to the etching solution based
on ethylene glycol, the IPA-based etching solution tends toward
large pyramids. According to Bressers et al., small pyramids can be
achieved by using nitrogen to displace the dissolved oxygen during
the etching process. But there are reports of laboratory
experiments in which a 10 vol. % isopropanol etching solution, a
nitrogen flow of 25 l/min and a process temperature of 80.degree.
C. are used. An enormous consumption of isopropanol must be
expected for production application in tanks. This entails high
costs since the durability of the etching solution is low on
account of such a high volatilization of IPA. This simultaneously
leads to high waste-disposal costs for the spent chemicals.
[0007] As regards the industrial production of solar cells, it is
important to guarantee a constantly good electrical quality of the
solar cells produced in large quantities. Constant good quality in
terms of the shape of the pyramid structure and a uniform
structuring of the total silicon surface must be ensured. The
ability to vary the pyramid size within a certain range is also
advantageous.
[0008] Neither the etching solution based on ethylene glycol nor
the isopropanol-based solution are able, however, to meet these
requirements simultaneously to a satisfactory extent. As summarized
above, the aqueous alkaline etching solution to which ethylene
glycol is added is indeed able to produce pyramids with a height of
.ltoreq.2 .mu.m, but it cannot be used immediately. Yet precisely
this is a drawback as regards industrial use. Although the etching
solution that contains isopropanol can, on the other hand, be used
immediately for texture etching, it does tend toward large pyramids
and proves to be disadvantageous as a result of the isopropanol's
high rate of evaporation, since this impairs the reproducibility of
a uniform pyramidal texture.
[0009] The object of the present invention is therefore to provide
a method for the wet chemical pyramidal texture etching of silicon
surfaces which is able--analogous to the etching solution based on
ethylene glycol--to generate reproducibly small (.ltoreq.2 .mu.m)
pyramid sizes, with different sizes of pyramids being adjustable
within a certain range or scatter, while simultaneously ensuring
complete texturization of the silicon surface and at the same time
reducing costs.
SUMMARY OF THE INVENTION
[0010] In an embodiment, the present invention provides a new and
improved etching solution used for the pyramidal structuring of the
silicon surface which comprises both isopropanol and an aqueous
alkaline ethylene glycol solution. The aqueous alkaline ethylene
glycol solution is advantageously reacted with oxygen, as is known
for example from EP 0 477 424 A1. The greater the amount of oxygen
reacted, the smaller the amount of ethylene glycol solution used in
the etching solution. The object is surprisingly solved just by the
simultaneous use of isopropanol and ethylene glycol in the etching
solution, which also entails the following advantages. At the start
of etching, the use of water which is already preheated and is
hence low in oxygen--as also described by Bressers et al.--has a
supportive effect in terms of small pyramids, but is not absolutely
necessary because it can be compensated by a corresponding
correction in the ratio of glycol. Preheated isopropanol--also
described in Bressers et al.--is problematic for reasons of
production technology.
[0011] The etching solution according to the invention is capable
of texture immediately after production, i.e. an etching process
can be performed directly after the solution's production.
[0012] The etching result is also relatively insensitive to the
exact composition of the etching solution, i.e. precise
optimization of the ratio of the individual dissolved components is
not necessary, which is beneficial particularly in terms of
industrial application.
[0013] Another advantage is the good reproducibility of the etching
result. At the same time, the pyramid size is adjustable, while
simultaneously ensuring complete texture of the silicon
surface.
[0014] A further advantage is obtained in that the present
invention's etching solution does not suffer as much from the
disadvantage of a conventional etching solution based just on
isopropanol, i.e. a considerable rate of evaporation. This means
that the etching solution according to the invention can be used
for etching over a much longer period than a conventional solution
that is based on isopropanol alone. The etching solution therefore
has to be replaced less frequently. This lowers costs since
downtime is decreased, thus reducing the amount of waste chemicals
to be disposed of.
[0015] Other objects and advantages of the present invention will
become apparent from the following detailed description, taken in
conjunction with the Drawings, in which:
BRIEF DESCRIPTION OF THE DRAWINGS
[0016] FIG. 1 shows a scanning electron micrograph of the etched
surface of a first exemplary embodiment;
[0017] FIG. 2 shows a scanning electron micrograph of the etched
surface of a second exemplary embodiment;
[0018] FIG. 3 shows a scanning electron micrograph of the etched
surface of a third exemplary embodiment; and
[0019] FIG. 4 shows a scanning electron micrograph of the etched
surface of a fourth exemplary embodiment.
DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT
[0020] The solution according to the invention for etching a first
exemplary embodiment is composed of 15 l water, 300 ml 50% sodium
hydroxide solution, 400 ml dissolved silicate, 600 ml isopropanol
and 15 ml aqueous alkaline ethylene glycol solution. The
temperature of the etching solution was set to 78.degree. C. and
the silicon wafers were immersed into this solution for 20 minutes.
The silicon wafers used here have a size of up to 6 inches, are
etched in part down to a thinness of 120 .mu.m and have reflecting
surfaces before the etching process. A scanning electron micrograph
of the surface of the silicon etched with this solution is shown in
FIG. 1, depicting a uniform and complete pyramid structure some 6
.mu.m in height.
[0021] A second exemplary embodiment made use of an etching
solution which comprises 15 l water, 600 ml 50% sodium hydroxide
solution, 300 ml isopropanol and 30 ml aqueous alkaline ethylene
glycol solution. Silicate was not added. This solution was heated
to a temperature of 78.degree. C. and the silicon wafers were
immersed in this solution for 10 minutes. The surface of the
silicon wafer structured with this etching solution is shown in
FIG. 2. A uniform pyramidal silicon surface can be seen, the
pyramid having a height of approximately 3 .mu.m.
[0022] Another exemplary embodiment made use of an etching solution
which contains 15 l water, 50 ml 50% sodium hydroxide solution, 100
ml isopropanol and 50 ml aqueous alkaline ethylene glycol solution,
likewise without addition of silicate. This solution was also
heated to a temperature of 78.degree. C. and a subsequent etching
process was performed for 5 minutes. The silicon surface structured
with this solution is shown in the scanning electron micrograph
depicted in FIG. 3. A uniform and complete pyramidal structure can
also be identified here. The height of the pyramids is less than
1.5 .mu.m.
[0023] Further silicon samples were etched with an etching solution
which contains 15 l water, 400 ml 50% sodium hydroxide solution,
400 ml isopropanol and 25 ml aqueous alkaline ethylene glycol
solution (without addition of silicate). This solution was heated
to a temperature of 60.degree. C. and the silicon sample was then
etched in this solution for 10 minutes. The result can be seen in
FIG. 4. In this instance, the surface also exhibits a pyramidal
structure having a height of approximately 1.5 .mu.m.
[0024] The manner in which the pyramid size decreases as the ratio
of glycol increases is apparent from exemplary embodiments 1 to 3.
In the 4th exemplary embodiment, a more considerably aerated
aqueous alkaline ethylene glycol solution is taken as a basis, for
which reason it was possible to reduce the ratio here in order to
generate pyramid sizes of .ltoreq.1.5 .mu.m, as in Example 3.
[0025] Each etching solution used in the exemplary embodiments can
be used immediately after production, i.e. the etching solution is
immediately capable of texturizing. A lengthy dissolution of
silicon, as is necessary in the known solutions based on ethylene
glycol, is omitted.
[0026] Although it is not necessary to add silicate, such an
addition does not, however, have any disruptive effect either. This
is evident for instance from the first exemplary embodiment in
which 400 ml dissolved silicate is present in the etching solution.
Uniform and complete pyramidal structuring was also achieved in
this exemplary embodiment. The proposed etching solution contains
only a small amount of isopropanol of preferably about 2 to 5 vol.
% in order to minimize isopropanol consumption.
[0027] It is also apparent from the exemplary embodiments described
above that a precise optimization of the ratio of the individual
dissolved components is not important because different
concentrations of different ratios of dissolved components make
satisfactory textures of the silicon surface possible. In other
words, the process window of the etching solution according to the
invention has a considerable size, which is particularly beneficial
as regards an industrial structuring process. As is evident from
the scanning electron micrographs of the individual exemplary
embodiments, the pyramid size can also be varied by a different
solution composition while ensuring constant reliable and complete
structuring. The process temperature is set between 60.degree. C.
and 80.degree. C. and the etching time varies between 5 and 10
minutes depending on the desired size of pyramid.
[0028] It should also be mentioned that naturally grown oxide
formed when the silicon wafer to be etched has been standing for
several weeks has just as little disruptive effect as for example a
different silicon surface property (e.g. rough or polished).
[0029] To examine any possible influence of naturally grown oxide,
wafer which had been previously standing in ambient air for several
weeks were immersed into the etching solution in a dry state and
any wafers which had been standing in the water rinsing basin for
about 60 minutes before the etching step were dipped into the
etching solution in a wet state. In both instances, naturally grown
oxide does not interfere with the texture etching. A previous
hydrofluoric acid dip for the removal of this oxide is
unnecessary.
[0030] Any consumption of the etching solution can be compensated
by addition of the necessary media. Such media do not have to be
especially heated up in order to minimize, for example, any
dissolved oxygen.
[0031] The etching solution according to the invention is suitable
for various forms of media movement. Bubblers, stirring devices,
lifting apparatus or pump circulation of the solution are possible,
as is a combination of such equipment.
* * * * *