FIELD OF INVENTION
This invention relates to a constant temperature bath for damage
removal etch process used to etch plasma textured ms-Si wafers in
photovoltaic industry to achieve desired smoothening of the textured surface
in order to make the Si surface suitable for solar cell fabrication and to
prepare it for further processing into solar cells.
BACKGROUND OF INVENTION
Development of effective texturing process for mc-Si wafers is
becoming more and more important from the point of view of increasing the
efficiency of mc- Si solar cells as these cells offer cost advantage over c-Si
cells. Acid texturing is widely used for mc-Si wafers in photovoltaic industry
and it has its own disadvantages such as use of expensive acids and their
safe disposal.
Plasma texturing of mc-Si wafers is more effective, and holds a good
promise for texturing of mc-Si wafers in photovoltaic industry as it is a dry,
stress free, single sided process and does not involve the use of hazardous
acids in large quantities as in the case of acid texturing technique.
The grass type of structure produced in plasma texturing is very good
from the point of view of light absorption but is not device worthy. 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. Plasma texturing of mc-Si is attracting lot of attention
in photovoltaic industry as it, being a single sided, stress-free process, is
especially suited to thinner wafers.
Development of effective texturing process for mc-Si wafers is
becoming increasingly important for enhancing the efficiency of mc-Si solar
cells as these offer cost advantage over c-Si solar cells. The sharp peaks of
the grass structure need to be smoothened to make the Si surface suitable
for solar cell fabrication.
The sharp, needle-like, grass type of structure produced in plasma
texturing of mc-Si wafers can easily break during post diffusion process and
short the device electrically. The smoothening process, using chemical
etching of nanometers of silicon, is known as Damage Removal Etch (DRE).
Removal of a few nanometers of Si layer is quite tricky and requires a
constant temperature bath at ~10 to 12 °C of acid mixture (50% water+48%
HNO3 + 2% HF). The reaction of Si with this mixture is exothermic and the
acid temperature rises very fast, if not properly controlled. Quick removal of
heat from the corrosive acid mixture is a challenge as no metal heat
exchanger can be used.
Considering the corrosive nature of the acid mixture, an innovative,
constant temperature controlled bath has been developed using Teflon tubes
in a coiled form of ¼" diameter, inserted in a circular PVC tank of specific
size to have a snug fit, so as to keep the volume of acid mixture used at
minimum for etching at a time ten pieces of 15.6cm x 15.6 cm square mc-Si
wafers, and extracting the heat effectively from the acid mixture in the
process.
This arrangement has been made to minimise the quantity of
hazardous acid used in every run. Chilled water at a temperature of about
8°C is pumped into the coil at a flow rate of 2 1pm (liters per minutes).
The two consecutive etching runs can be taken up at a gap of five
minutes. The rise in temperature of the acid mixture is continuously
monitored using an acid proof, K- type thermocouple. Within a process run
(for 10 wafers), the flow of the chilled water is adjusted such that the acid
solution temperature remains constant within ± 1°C.
Following the DRE, the diffused reflectance from the textured surface
of mc-Si wafers is measured and it has been found that it increases from 0.5
to 1 % leaving the textured surface device worthy. Hundreds of mc-Si solar
cells have been made using this process with cell efficiencies of > 15.0 % on
15.6 cm xl5.6 cm wafers.
Thus, the invention reported relates to a simple constant temperature
acid bath which is best suited for carrying out industrial R&D on large size
Si wafers for the DRE process following plasma texturing.
OBJECTS OF THE INVENTION
The main object of the present invention is to develop a constant
temperature bath of optimum size for acid mixture used for damage removal
etch process of plasma textured of mc- Si wafers.
Another objective of the invention is to devise a smoothing process so
that the surface of mc-Si wafers are smoothened appropriately to make it
suitable for further processes of solar cell fabrication with minimum
increase in diffused reflection.
Still another object of the invention to avoid the damage to the
textured Si surface during DRE process on account of acid mixture used in
the process.

Further object of the invention is to allow the increase in reflectance of
the textured Si surface during DRE process thereby making it suitable for
further solar cells processes.
An additional object of the invention is to avoid wastage of acids by
minimizing the volume of acid used as finding and replenishing the right
quantities of individual acid in the mixture is not practical.
The foregoing has outlined some of the pertinent objectives of the
invention. These objectives should not be construed to be merely illustrative
of some of the more prominent features and applications of the intended
invention. Many other beneficial results can be obtained by applying the
disclosed invention in a different manner or modifying the invention within
the scope of disclosure.
Accordingly, other objectives and a full understanding of the invention
and the detailed description of the preferred embodiment in addition to the
scope of invention are to be defined by the claims undertaken.
These and other objects and advantages of the invention will be
apparent from the ensuing description.
STATEMENT OF INVENTION
According to this invention, there is provided a constant temperature bath,
for damage removal etch process in plasma texturing of mc-Si wafers,
comprising a coil of W diameter Teflon tube inserted into a PVC tank having
internal diameter of 27 cm containing acid mixture consisting of
2%Hydrofluoric Acid(HF), 48%Nitric Acid(HNO3) and 50%water, extracting
heat effectively from the acid mixture during an exothermic reaction with
silicon and maintaining the acid mixture temperature at 11 °C ± 1°C
throughout its volume by flowing chilled water through the tube at a flow
rate of 2 1pm (liters per minutes) to ensure uniform etching in 80-100 sec
etching time across a wafer and from wafer to wafer in a batch.
BRIEF DESCRIPTION OF ACCOMPANYING DRAWINGS;
Further objects and advantages of this invention will be more
apparent from the ensuing description when read in conjunction with the
accompanying drawings wherein:
Figure 1.: Schematic showing surface topography of mc-Si wafer
before and after DRE process.
Figure 2.: Schematic showing the arrangements of the set-up for
DRE process.
While the invention is described in conjunction with the illustrated
embodiment, it is to be understood that it is not intended to limit the
invention to such embodiment. On the contrary, it is intended to cover all
alternatives, modifications and equivalents as may be included within the
spirit and scope of the invention disclosure as defined by the claims.
DETAILED DESCRIPTION OF THE INVENTION
At the outset of the description, which follows, it is to be understood
that the ensuing description only illustrates a particular form of this
invention. However, such a particular form is only an exemplary
embodiment, and without intending to imply any limitation on the scope of
this invention. Accordingly, the description is to be understood as an
exemplary embodiment and teachings of the invention are not intended to
be taken restrictively.
For the purpose of promoting an understanding of the principles of
the invention, reference is now to be made to the embodiment illustrated in
the drawings and specific language is used to describe the same. It is
nevertheless to be understood that no limitations of the scope of invention is
hereby intended, such alterations and further modifications in the
illustrated bag and such further applications of the principles of the
invention as illustrated therein being contemplated as would normally occur
to one skilled in the art to which the invention relates.
The present invention illustrates a constant temperature bath which
has been developed using Teflon tubes in a coiled form. The Teflon tube coil
is inserted in a circular PVC tank of suitable size to have a snug fit. This
arrangement has been made to minimise the quantity of hazardous acid
used in every run. Chilled water at a temperature of about 8°C is pumped
into the coil at a flow rate of 2 1pm to ensure uniform etching in 80-100 sec
etching time across a wafer and from wafer to wafer in a batch.
The Si wafers loaded into a Teflon carrier are immersed into the acid
mixture for about 80 to 100 seconds and removed. This step produces the
desired smoothening of the textured surface and prepares it for further
processing into solar cells. These wafers are thoroughly washed in de-
ionized water and dried. Hundreds of high efficiency mc-Si solar cells have
been fabricated successfully using these wafers.
The Silicon wafers used in solar cells typically reflects about 35-40%
of the incident light, depending upon its wavelength in the range of 400-
1100nm, on which the photovoltaic conversion efficiency depends. 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. Development of effective texturing process for mc-Si
wafers is becoming very important from the point of view of increasing the
efficiency of mc- Si solar cells as mc-Si wafers are cheaper than c-Si wafers
and thereby hold a promise to reduce cost of Si solar cells.
The grass type of surface structure produced in plasma texturing of
mc-Si wafers gives a black surface and the reflectance can be reduced
almost to zero. Such type of surface is good from the point of view of light
absorption but is not good from the point of device fabrication.
The sharp peaks of the grass structure can easily break during the
post-diffusion process and create electrical shorts in the device. The surface
needs to be smoothened appropriately to make it suitable for further
processes of solar cell fabrication with minimum increase in diffused
reflection.
This process is known as Damage Removal Etch (DRE) and this effect
is depicted in Fig. 1.
The Si wafers loaded into a Teflon carrier are immersed into the acid
mixture for about 80 to 100 seconds and removed. This step produces the
desired smoothening of the textured surface and prepares it for further
processing into solar cells. These wafers are thoroughly washed in de ionized
water and dried. Hundreds of high efficiency mc-Si solar cells have been
fabricated successfully using these wafers.
Removal of a few nanometers of Si layer is quite tricky and requires a
constant temperature bath at ~10 to 12 °C of acid mixture (50% water+48%
HNO3 + 2% HF).
The reaction of Si with this mixture is exothermic and the acid
temperature rises very fast if not properly cooled. If not controlled, it
damages the textured Si surface, increases its reflectance and also makes it
unsuitable for further solar cells processes.
Another problem is of frequent disposal of acid mixture. After a
number of process runs the acid mixture Joses its strength and needs to be
disposed off as finding the respective strength of two acids and replenishing
the required acid in the right quantity is not easy and practical. This means
that the volume of acid needs to be minimum so as to avoid wastage.
Addressing these problems, an innovative constant temperature bath
has been developed where the heat from the acid mixture is extracted by
passing chilled water directly through a coil of ¼" diameter Teflon tube,
inserted into a circular PVC tank of a specific size so as to reduce the
volume of acid mixture used.
The sketch of the bath is shown in Fig. 2. the details of which are
given below:
• Outer diameter of the Teflon tube: 6.35mm
• Total length of the tube: 3090 cm
• Temperature of the input chilled water: 8 °C
• Flow rate of chilled water: 2 1pm
• Initial temperature of the acid mixture bath: 28 °C (Room
temperature)

• Stabilized temperature of the bath after passing chilled water for 3
hrs: 10.0°C
• Number of 15.6 cm xl5.6 cm sq Si wafers immersed into the acid
mixture: 10
• Etching time: 80-100 seconds
• Internal diameter of the PVC tank: 27 cm
• Outer diameter of the Teflon tube coil: 24 cm
• Volume of acid used: 13 liter
• Diffused reflectance of the textured Si wafer before DRE: 4-5%
(Average for 500 wafers)
• Diffused reflectance of the textured Si wafer after DRE: 5-6 % (Average
for 500 wafers)
The Teflon tube coil is inserted into a circular PVC tank (internal
diameter 27 cm) just suitable to accommodate the Teflon tube coil, the
internal diameter of which has been kept sufficient to accommodate ten
pieces of mc-Si wafers of 15.6 cm xl5.6 cm size in vertical position in a
Teflon carrier. A closed cycle circulating system for chilled water is used to
pass chilled water at temperature of about 8°C into the coil at a flow rate of
2 1pm.
The Teflon tube coil is inserted in a circular PVC tank of suitable size
with a snug fit. This helped to minimize the quantity of acid mixture (about
13 liters) to be used in each process run. The chilled water at about 8°C is
pumped into the coil at a flow rate of 2 1pm. The Si wafers loaded in a Teflon
carrier are immersed in the cooled acid mixture for a predetermined time of
80-100 sec, removed and washed in De-mineralized wafer. The constant
temperature bath has been successfully used to etch plasma textured mc-Si
wafers and provided a high throughput of ~ 1 wafer per minute with full
control over the bath temperature best suited for industrial R&D
on large size Si wafers for DRE process.
It is well known that the warmer liquid rises up and colder liquid
moves down up in a container. Therefore the input of the chilled water has
been passed from top of the coil and taken out from the bottom of the coil.
This arrangement helps maintain the temperature of the acid solution
uniformly low throughout its volume.
Ten (10) pieces of plasma textured Si wafers of thickness 180-200
microns are loaded in a Teflon carrier and immersed into the acid mixture
for time duration ranging from 80 sec to 100 sec.
The rise in temperature of the acid mixture is continuously monitored
using an acid proof, K- type thermocouple. It is observed that the
temperature of the acid solution remains low within ± 1°C during a single
run of DRE process.
The next batch is immersed after a gap of 5 minutes. In this manner,
about 500 plasma textured mc-Si wafers have been processed for DRE
without changing the acid mixture. The diffused reflectance from the
textured surface is measured and it is seen that a device worthy surface is
obtained at the cost of an increase in the diffused reflectance of the wafer by
0.5 to 1.0 %.
The change in diffused reflectance at regular intervals of 20 wafers
has shown that the rate of etching rate did not change remarkably even after
500 wafers. Hundreds of 15.6 cm xl5.6cm textured mc-Si wafers have been
prepared with this process and converted to solar cells with an efficiency of
> 15.0 %.
To summarise, a simple constant temperature acid bath has been
developed which is best suited for carrying out industrial R&D on large size
Si wafers for DRE process.
WE CLAIM:
1 A Constant Temperature Bath, for damage removal etch process in
plasma texturing of mc-Si wafers, comprising a coil of ¼" diameter
Teflon tube inserted into a PVC tank having internal diameter of 27
cm containing acid mixture consisting of 2%Hydrofluoric Acid(HF),
48%Nitric Acid(HNO3) and 50%water, extracting heat effectively from
the acid mixture during an exothermic reaction with silicon and
maintaining the acid mixture temperature at 11 °C ± 1°C throughout
its volume by flowing chilled water through the tube at a flow rate of 2
1pm (liters per minutes), to ensure uniform etching in 80-100 sec
etching time across a wafer and from wafer to wafer in a batch.
1. A Constant Temperature Bath, as claimed in Claim 1, where the bath is
so compact to effectively ensure DRE process of a maximum of 10 (ten)
15.6 cm x15.6 cm sq cm mc-Si wafers per run.
2. A Constant Temperature Bath, as claimed in Claim 1, where the acid
mixture is used in the quantity of 13 liters to ensure uniform etching in
80-100 sec etching time across a wafer and from wafer to wafer in a
batch.
3. A Constant Temperature Bath, as claimed in proceeding claims, where
the bath provides a high throughput of ~1 wafer per minute for large
size Si wafers for DRE process with full control over the bath
temperature best suited for industrial R&D.

4. A Constant Temperature Bath, as claimed in proceeding claims,
substantially as herein described.

A Constant Temperature Bath, for damage removal etch process in plasma
texturing of mc-Si wafers, comprising a coil of ¼" diameter Teflon tube
inserted into a PVC tank having internal diameter of 27 cm containing acid
mixture consisting of 2%Hydrofluoric Acid(HF), 48%Nitric Acid(HNO3) and
50%water, extracting heat effectively from the acid mixture during an
exothermic reaction with silicon and maintaining the acid mixture
temperature at 11 °C ± 1°C throughout its volume by flowing chilled water
through the tube at a flow rate of 2 1pm (liters per minutes) to ensure
uniform etching in 80-100 sec etching time across a wafer and from wafer to
wafer in a batch.