FIELD OF INVENTION
The present invention generally relates to semiconductor processing. More
particularly, the invention relates to a system for extracting heat from acidic
solution in a texturing process for multi crystalline silicon (mc-Si) wafers. The
invention further relates to a method of texturing mc-Si wafers in semiconductor
processing.
BACKGROUND OF THE INVENTION
It is well known that the reduction of optical losses due to reduced reflection and
better light trapping in silicon solar cells (both c-Si and mc-Si) by surface
texturing improves solar cell efficiency. Therefore, development of an effective
texturing process for mc-Si wafers is becoming more and more important for
increasing the efficiency of mc-Si solar cells. Wet chemical acidic etching is a
widely used method for texturing mc-Si wafers in the photovoltaic industry. The
etchant mixture HF: HNO3:H2O is widely used for texturing mc-Si wafers. One of
the major problems in industrial implementation of the acid texturing process is
the strong exothermic nature of the reaction. Therefore, the etching process
requires a controlled environment with adequate heat extraction from the acidic,
solution.
OBJECTS OF THE INVENTION
It is therefore an object of the invention to propose a heat extraction system to
extract heat from an acidic solution used for texturing rnc- Si wafers used for
solar cell applications.
Another object of the invention is to propose a method of texturing mc-Si wafers
in semiconductor processing.
SUMMARY OF THE INVENTION
The inventors through various experimentation observed that the silicon wafers
used in solar cells typically reflect, depending upon its wavelength in the range
400-110nm, about 35-40% of the incident light, on which the photovoltaic
conversion efficiency depends, and that the reduction of optical losses in silicon
solar cells (both c-Si and mc-Si) by surface texturing improves solar cell
efficiency due to reduced reflection and better light trapping.
According to the invention, a system is designed and developed for heat
extraction from an acidic solution in a texturing process of mc-Si wafers, wherein
two PVDF tanks, namely a process tank and a buffer tank are provided. The
process tank has a double walled configuration constituting an inner tank and an
outer tank, and is provided with a diffuser at the bottom of the inner tank for
uniform flow of the cold acid from the buffer tank through a chemical pump. The
acid solution is cooled in the buffer tank through heat exchanger coils made with
PFA material which is chemically inert to acids. Chilled water at around 7°C is
circulated in these coils; thereby heat generated during exothermic reaction is
extracted from the solution. The heat extraction coils are placed at the centre of
the buffer tank for maximum cooling of the solution. The acid from the buffer
tank is pumped from the lower side of the buffer tank so

that only cooled acid is pushed into the process tank. The cold acid is pumped
into the inner tank of the process tank from the bottom and is allowed to over
flow into the outer tank. The acid collected in the outer tank is thus allowed to
flow into the buffer tank. During the texturing process, a wafer carrier loaded
with mc-Si wafers is dipped in the process tank filled with acidic solution (HF:
HNO3:H2SO4:H2O) for a desired duration to get the required texture on the
surface of the mc-Si wafers. During the etching/texturing process, the acidic
solution temperature tends to increase due to exothermic nature of the reaction.
The gentle upward flow of the cold acid solution from the buffer tank to the
process tank replaces the hot solution in the process tank and the temperature
of the solution is kept constant in the range 8-10°C with an accuracy of ±2°C.
BRIEF DESCRIPTION OF THE ACCOMPANYING DRAWING
Figure 1 - shows a schematic diagram of a heat extraction system from acidic
solution according to the invention.
DETAILED DESCRIPTION OF THE INVENTION
As shown in figure-1, in a batch process, typically 25 six inch square mc-Si
wafers (la) are loaded into a wafer carried (1) and dipped into the acidic
solution (HF: HNO3:H2SO4:H2O) and etched for a short duration to get the
desired texturing on the surface of mc-Si wafers (la). As the chemical reaction of
silicon with HNO3 (Nitric Acid) is exothermic, the reaction produces enormous
heat instantaneously and the temperature of the acidic solution (2) increases.
The increase in temperature of the acidic solution (2) can lead to inconsistent
etching. A lack of control on the temperature can lead: to a run away reaction.
Also, the process of acid texturing of Si wafers (la) involves removal of a very
thin layer (~5 to 10 micrometer) from the surface In a precisely controlled
manner. This is possible only with accurate control over the temperature of the
solution (2) and thereby on the reaction. Therefore, it is very important to
provide adequate heat extraction from the acidic solution (2) to maintain a
constant temperature of the acidic solution (2) during etching duration to get a
consistent texturing of the of the Si wafer surface. Heat extraction from an acid
solution (2) is quite challenging as the metal parts that are thermally good
conductors cannot be used. To overcome these problems, several innovativer
features have been incorporated in the present invention for effective heat
transfer from the acid texturing system.
Accordingly, the system comprises two PVDF tanks namely a process tank (3a,
3b) and a buffer tank (4. The volume of the buffer tank (4) has been kept at
about six times the volume of the process tank (3a, 3b). The large volume of the
buffer tank (4) helps to maintain the temperature constant due to its large
thermal inertia. The process tank (3a, 3b) has double walled configuration fitted
with a diffuser (5) at the bottom of the inner tank (3b) for uniform flow of
acid(2) from the buffer tank (4) through a chemical pump (6). The acid solutiorf
(2) is pumped from the bottom of the inner tank (3b) and is allowed to over flow
from the top of the inner process tank (3b) into the outer process tank (3a). The
acid solution (2) collected in the outer tank (3a) is thus allowed to flow into the
buffer tank (4) under the action of gravity. The acid solution (2) in the buffer
tank (4) is cooled through a heat exchanger coil (7) made with PFA material
which is chemically inert to acids. Chilled water at around 7°C temperature is
circulated through the heat exchanger (7) coils to extract heat from the solution
(2). The heat extraction coils (7) are placed in the centre of the buffer tank (4)
for effective heat extraction. The acid (2) from buffer tank (4) is pumped from
the lower side of the tank so that only cooled acid is pushed into the process
tank (3b). The acid solution (2) continuously keeps flowing from the buffer tank
(4) to the process tank (3b) with the help of the chemical pump (6). During the
texturing process, the wafer carrier (1) loaded with mc-Si wafers (la) is dipped
in the process tank (3b) for a pre determined duration (60 sec to 300 sec) to get
the desired texture on the surface of mc-SI wafers (la). During the
etching/texturing process, the acidic solution temperature tends to increase due
to exothermic nature of the reaction. The flow of cold acid solution (F