TITLE:
A method for enhancing the uniformity of plasma texturing of multicrystalline
Silicon wafers.
FIELD OF INVENTION:
This invention relates to the use of a method for enhancing the uniformity of
plasma texturing of multicrystalline Silicon wafers.
BACKGROUND OF THE INVENTION:
Plasma processes are becoming increasingly popular for various applications in
solar cells industry as they offer unique advantages of large area processes at
relatively lower substrate temperatures and use of process gases in very small
quantities leading to cost advantages. Texturing of Silicon wafers is one of the
process steps for the production of solar cells. This process enhances the
absorption of sunlight and increases the cell efficiency. In addition to chemical
texturing of Si wafers, plasma texturing is also being developed globally as it
offers unique advantage of single side texturing without using wet chemicals in
huge quantities. The major challenge in this technology is the requirement of
high throughput process equipment. This requires a large number of Si wafers to
be processed in one run with almost 100% yield. A similar equipment has been
designed for texturing fifteen 156mmx156mm large size Si wafers in one run.
During process runs it is detected that Si wafers which are kept at the centre of
the platen, are getting textured uniformly i.e. becoming black while the other
wafers are only partly textured. This problem has been solved by covering the
outermost area of the SS platen by dummy Si wafers. The technique has been
used successfully and thousands of Si wafers have been textured with a
throughput of one wafer a minute and with a yield of more than 99%.

US20120295446A1 2012-11-22 Method for single side texturing (en) claims a
method of plasma texturing on single side. JP2012521078A 2012-09-10 Plasma
texture method (en) claims a method of plasma texturing of Si wafers.
US8211323B2 2012-07-03 Method for the removal of doped surface layers on
the back faces of crystalline silicon solar wafers (en). This patent describes the
method of removal of Si from the top surface. US20120064659A1 2012-03-15
Method for manufacturing solar cell (en). This patent describes the method for
cell manufacturing with plasma texturing of Si wafers. US20120061022A1 2012-
03-15 Plasma texturing reaction apparatus (en). This patent is for a plasma
texturing technology for a solar cell, and more particularly, to a plasma texturing
reaction apparatus which can improve the manufacturing efficiency and quality of
a solar cell by increasing the density and uniformity of plasma ions and
appropriately controlling ion energy when conducting plasma texturing through
dry etching on a solar cell wafer to increase a light absorption amount of
available light to the inside of the solar cell by reducing a light reflection amount
on the surface of the solar cell. US20120060915A1 2012-03-15 Method for
plasma texturing (en) The disclosed technology relates to a method for plasma
texturing of a surface of a substrate (e.g., a silicon substrate), which may be used
in an industrial process for texturing a surface of silicon photovoltaic cells.
OBJECTS OF THE INVENTION:
An object of this invention is to propose a novel method for enhancing the
uniformity of plasma texturing of multicrystalline Silicon wafers.
Another object of this invention is to propose a method to modify the RF local
field by changing the dielectric medium placing silicone wafers on an SS platen.

Still another object of this invention is to propose a simple solution to make the
RF field uniform in the desired area of an SS platen in a large area plasma
reaction for uniform texturing of Si wafers.
Yet another object of this invention is to improve the plasma texturing yield in a
high throughput system.
BRIEF DESCRIPTION OF THE INVENTION:
According to this invention there is provided a method for enhancing the
uniformity of plasma texturing of multicrystalline silicon wafers comprising:
Subjecting silicon wafers to the step of texturing by exposing of the said silicon
wafers to SF6 +O2 plasma in a vacuum chamber.
In accordance with this invention there is provided an apparatus for enhancing
the uniformity of plasma texturing of multicrystalline silicon wafers comprising a
set of three vacuum chambers namely load lock, process and exit lock,
SS platen loaded with Si wafers are fed into the chamber through load lock,
mixture of SF6 and O2 gas is fed into the process chamber at about 200 mTorr
pressure using electronic controls, and radio frequency (RF) plasma is struck
with about 1500 W between the shower head electrode and SS platen which
remains at ground potential.
DETAILED DESCRIPTION OF THE INVENTION:
Texturing of Silicon wafers is one of the process steps for the production of solar
cells. This process enhances the absorption of sunlight and increases the cell
efficiency. Plasma texturing is being developed globally as it offers unique
advantage of single side texturing without using wet chemicals. The major
challenge in this technology is the requirement of high throughput process
equipment with high yield. An equipment has been designed for texturing fifteen

156mmx156mm, Si wafers in one run. During initial trials it is detected that the
yield is very poor due to non-uniform texturing. After studying the problem in
detail, it is assumed that at the edges of Si wafers the electric field is not uniform
due to sudden change in the dielectric medium. After placing Si wafers on the
SS platen, some area of the platen remains uncovered and this gives rise to
sudden change in dielectric medium resulting in non-uniform plasma. In the
present method, exposed area of the SS platen is covered with dummy Si wafers
and all the gaps between Si wafers are reduced to zero. This probably moved
the abrupt change in the dielectric medium at the edges of the Si wafers away
from the real Si samples and the real samples are textured uniformly. The
technique has been used successfully and thousands of Si wafers have been
textured with a throughput of one wafer a minute and with a yield of more than
99%.
Texturing of Si wafers with high throughput requires a large number of Si wafers
to be exposed to SF6+O2 plasma in a vacuum chamber. A large area plasma
reactor is designed comprising a set of three vacuum chambers namely entry
load lock, process and exit lock. The sizes of all vacuum chambers are big
enough to accommodate a large wafer carrier of (900mm x 700mm). This wafer
carrier can accommodate 15-20 Si wafers of 156mm x 156mm size. The SS
platen loaded with Si wafers is fed into the process chamber through the load
lock following a specific sequencing of operation of valves to evacuate chambers
to the desired pressure levels. The process chamber is evacuated to a pressure
of about 800 mTorr using special pumps. A mixture of SF6 and O2 gas is fed into
the process chamber and process pressure of about 200 mTorr is maintained
using electronic controls. Radio frequency (RF) plasma is struck with about 1500
W between the shower head electrode and SS platen which remains at ground
potential. The plasma is kept ON for 5 to 10 minutes. Initial runs reveal that
texturing, which can be observed by the extent of darkening of the Si wafers, is

highly non-uniform and it is observed that the wafers kept at the center are
textured fully and the wafers kept away from the centre show gradual decrease in
the texturing intensity in the form of reducing darkness (as shown in Figure 1). It
is assumed that either the gas distribution or the RF power is non-uniform.
Uniformity of the gas flow is confirmed by measuring the flow of gas from all the
orifices of the shower head electrode. One experiment was performed by placing
the Si wafers at different locations of the SS platen to confirm the uniformity of
RF power over the entire area of SS platen. It is observed that all the Si wafers
become dark at the centre and etching was gradually decreasing towards the
periphery of Si wafers (as shown in Fig 2). The probable reason can be as
follows. The RF plasma is struck between the powered shower head electrode
and the SS platen which is at ground potential. In the absence of Si wafers on
the SS platen, the RF field remains uniform. When Si wafers are placed, which
are not as conducting as SS, the RF field becomes non-uniform especially close
to the Si wafer edges as it sees an abrupt change in the dielectric medium
resulting in non-uniform texturing. To circumvent this problem, 15 Si wafers are
kept without leaving any gaps in between and were also surrounded by more
number of dummy Si wafers (as shown in Fig 3). The result is seen as a large
improvement in the quality of texturing. All the 15 Si wafers are uniformly
darkened while the dummy Si wafers kept at the periphery are non-uniformly
dark. After confirming this effect, the dummy wafers have been permanently
fixed with an adhesive on the SS platen. In every run the 15 new Si wafers are
loaded on the SS platen and texturing process is carried out. Thousands of Si
wafers have been textured using this technique with an yield of more than 99%.

WE CLAIM:
1. A method for enhancing the uniformity of plasma texturing of multicrystalline
silicon wafers comprising:
subjecting silicon wafers to the step of texturing by exposing of the said
silicon wafers to SF6 +O2 plasma in a vacuum chamber.
2. The method as claimed in claim 1 wherein the process pressure required for
texturing is about 200 mTorr.
3. The method as claimed in claim 1 wherein the plasma is kept on for 5 to 10
minutes.
4. An apparatus for enhancing the uniformity of plasma texturing of
multicrystalline silicon wafers comprising a set of three vacuum chambers
namely load lock, process and exit lock
SS platen loaded with Si wafers are fed into the chamber through load lock,
mixture of SF6 and O2 gas is fed into the process chamber of about 200
mTorr using electronic controls, and radio frequency (RF) plasma is struck
with about 1500 W between the shower head electrode and SS platen which
remains at ground potential.

ABSTRACT

A method for enhancing the uniformity of plasma texturing of multicrystalline
silicon wafers comprising: subjecting silicon wafers to the step of texturing by
exposing of the said silicon wafers to SF6 + O2 plasma in a vacuum chamber.