FIELD OF THE INVENTION
The present invention generally relates to solar photovoltaics and semiconductor
devices. More particularly, the invention relates to a method for etching single
side of a diffused Si wafer without using any mask.
BACKGROUND OF THE INVENTION
Emitter diffusion is one of the basic process steps in the manufacturing of solar
cells. During this process step of emitter diffusion, which is carried out in a tube
furnace, n-type emitter is diffused on both sides of the p-type Si wafer up to a
depth of about 0.2 to 0.5 micron, because the Si wafers are surrounded by
POCI3 vapours at ~ 850°C in oxygen atmosphere. For a solar cell device, an
emitter is required only on one side. The diffused emitter on the other side of the
Si wafer is generally compensated during the back contact formation process
step. If the diffused emitter is removed from one side of the wafer immediately
after the emitter diffusion step, solar cell efficiency can be enhanced by using
other additional process steps such as surface passivation, selective emitter etc.
In addition removal from one side the diffused emitter immediately after the
emitter diffusion step, eliminates a step of edge isolation, which is an additional
process step carried out subsequent to completion of the fabrication process.
The diffused emitter can be selectively removed from one side using acid
solutions under controlled conditions by covering or protecting the other side
using acid resist paints or mechanical masks. However, these techniques do not
quite meet the requirement of high throughput and cannot be used on an
industrial scale where thousands of Si wafers have to be handled on a daily
basis.
US patent US 7,943,526B2, describes a method, where the Si wafer is
transported on rollers of acid proof material. The level of wafers including the

etchant being precisely controlled so that the etchant wets only the lower face of
the wafer.
OBJECTS OF THE INVENTION
It is therefore an object of the invention to propose a method for etching single
side of a diffused Si wafer without using any mask.
Another object of the invention is to propose a method for etching single side of
a diffused Si wafer without using any mask, in which a mixture of acids is used
under controlled conditions for selectively etching one side of the Si-wafer.
A further object of the invention is to propose a method for etching single side of
a diffused Si wafer without using any mask, which is up-scalable to meet high
throughout requirement.
SUMMARY OF THE INVENTION
Accordingly, there is provided a method for etching single side of a diffused Si
wafer without using any mask. According to the invention, the property of
liquids, rising in fine capillaries, has been used to develop the method for etching
only one side of the Si wafer. A set of acid proof plastic rollers has been
designed which have very fine slits along the length of the rollers. These fine slits
of about 0.9 mm width, act as capillaries. These rollers are arranged in a PVC
tray and are rotated at a very low rpm using plastic gears and an electric motor.
When the diluted mixture of acid is filled in the tray and the rollers are partly
submerged in it up to a predetermined height, the acid solution rises in the
capillaries and remains very close to the top of the roller surface. The wafer is
placed on the first roller and it gets wet along the slit. While moving the wafer on
the rollers, acid solution keeps clinging on to the lower surface of the wafer

wetting it uniformly and etching the lower diffused surface. The removal of the
diffused layer can be confirmed by measuring the sheet resistance of the etched
surface. A number of diffused Si wafers have been etched successfully using this
technique. The method is up scalable and is suitable to be used for high
throughout requirement.
As the present invention uses liquids rising in fine capillaries, it does not warrant
very precise alignment of the level of liquid and wafers. The gap between the
rollers and the etchant is about 2 mm, hence there is no chance that the etchant
can go to the other side of the wafer resulting in a damage of the top surface.
This leads to higher yield and lower maintenance.
Due to gravity, the acid solution does not go up and the top surface of the wafer
remains unaffected. The lower surface gets etched uniformly. The removal of the
diffused layer can be confirmed by measuring the sheet resistance of the etched
surface. A number of diffused Si wafers have been etched successfully using
technique.
BRIEF DESCRIPTION OF THE ACCOMPANYING DRAWINGS
Figure 1(a) Shows front and side view of a PVC roller used in the process of
the present invention.
Figure 1(b) Shows the PVC roller of Figure 1(a) with slits constructed on the
roller surface.
Figure 1(c) Shows arrangement of the slitted PVC rollers of figure 1(b)
alongwith Si-wafer in a tank maintaining an exemplary gap of 75.00
mm between the rollers.

Figure 1(d) Shows the arrangement of figure 1(c) with the tank now-filled with
acid solution.
Figure 2(a) Shows an enlarged view of liquid solution rising in fine capillaries.
Figure 2(b) Shows acid solution clinging on wafer surface during movement
between the PVC rollers.
DETAIL DESCRIPTION OF THE INVENTION
According to the invention, the diffused emitter of Si wafers is removed by
exposing the surface of the wafer to a diluted mixture of hydrofluoric and nitric
acid under controlled conditions. Selective removal of the emitter from single
side of the wafer requires that only one side of the wafer is exposed to the acid
solution. It is a well known phenomenon that liquids rise in fine capillaries due to
surface tension. Using this known property of liquids, a set of acid proof plastic
rollers having very fine slits along the length of the rollers are provided which act
as capillaries.
As shown in figure 1(a) the plastic rollers are fabricated from a rod of diameter
about 42 mm. The rod is reduced to 40 mm diameter on a lathe machine to have
a very fine polished and uniform surface. Using a cutting tool slits of about 0.9
mm wide are formed along the length of the rollers as shown in figure 1(b). The
rollers with slits are arranged in a PVC tank such that a gap between the rollers
is maintained around 75 mm (figure 1(c)). This gap has been selected so that an
exemplary 156mm long Si wafer does not topple while going from one roller to
the next. The PVC rollers are rotated with PVC gears and an electric motor at a
very low rpm to achieve a linear speed of about 150 mm per minute. The
movement mechanism is not shown in the schematic. The tank is filled with a
pre cooled (~ 20°C) mixture of HNO3 and HF (figure 1(d). The level of the acid in

the tank is kept such that the gap between the acid level and top of the rollers is
less than 2mm. The acid mixture rises in the fine slits due to surface tension and
remains very close to the surface of the roller (figure 2(a)). This ensures that
when Si wafer is kept on the rollers, the gap between the bottom of the Si wafer
and the liquid in the capillaries is almost zero. This value has been determined by
performing a few experiments by varying the level of the acid solution and
checking the extent of uniform wetting of the lower surface of the Si wafer. The
Si wafer has a textured surface and is hydrophilic in nature. When the wafer is
placed on the first moving roller and it comes in contact with the slit filled with
solution, it gets wet along the length of the slit. Once wet at one location, the
wetting keeps spreading. As the Si wafer moves to other rollers, the other parts
of the wafer come in contact with the other slits and the wafer is wetted
uniformly on the entire area. The liquid keeps clinging on to the lower surface of
the Si wafer due to surface tension and cannot go to the other side of the wafer
from edges due to gravitational pull (figure 2(b)). For example, 18 slits on each
roller can be constructed i.e. after every ~ 6.3 mm of linear movement; the
wafer meets a new slit filled with fresh solution. While rotating, part of the acid
solution keeps clinging to the wafer while part sticks to the roller and goes back
into the solution. The slits are cut all through the length of the roller and
therefore fresh acid solution automatically keeps getting replaced in the slits
from the sides of the roller also. The wafer keeps moving from one roller to the
next receiving fresh supply of acid solution. Due to presence of number of slits
and continuous replacement of acid mixture, the Si wafer gets etched uniformly.
The etching time varies with temperature and concentration of the acid solution
which can be adjusted based on the requirement. Using a mixture of
HF:HNO3:H2O in the ratios of 1:5:10, at 20°C, a number of diffused Si wafers
have been etched successfully by varying etching time from 3 minutes to 10
minutes. The sheet resistance of the diffused layer has increased from an initial
value of 40 Ω/□ to more than 200 Ω/□ showing that the diffused junction is fully
removed.

WE CLAIM
1. A method for etching single side of a diffused Si wafer without using any
mask, comprising :-
providing a tank accommodating a plurality of shifted PVC rollers, each
roller disposed equidistantly maintaining a gap and rotatable at a low
linear speed by a drive mechanism;
locating at least one Si-wafer on an upper surface of at least one of said
rotatable slitted PVC roller;
filling the tank with an acid mixture; and
- causing the PVC rollers to rotate so as to make the wafer moving from
one roller to the next roller resulting in uniform wetting of the lower
surface;
wherein the tank is filled with the acid mixture pre-cooled upto 20°C such
that a gap of at least 2mm is retained between the acid level and the
upper surface of the rollers, wherein the slits formed on the rollers act as
capillaries to allow the acid mixture to rise in the slits due to surface
tension and gravity, wherein the acid mixture consists of HF:HNO3:H2O in
a ratio of 1:5:10, and wherein the etching time varies between 3 to 10
minutes.
2. The method as claimed in claim 1, wherein the PVC rollers are machined
to have a fine and polished uniform surface, and wherein a plurality of
slots of about 0.9 mm wide is formed along the length of the roller

considering an exemplary roller length of 180 mm, and an exemplary Si-
wafer length of 156 mm.
3. The method as claimed in claim 1, wherein said low linear speed is for
example 150 mm/minute.
4. The method as claimed in claim 1 or 2 wherein said gap between the
rollers is about 75 mm for said exemplary roller length and Si-wafer
length.
5. The method as claimed in any of the preceding claims wherein the gap
between the bottom of the Si -wafer and the acid mixture in the capillaries
is substantially zero.
6. The method as claimed in any of the preceding claims wherein the liquid
keeps clinging on the lower surface of the wafer due to surface tension
and achieve uniform wetting.

ABSTRACT

The invention relates to a method for etching single side of a diffused Si wafer
without using any mask, comprising providing a tank accommodating a plurality
of shifted PVC rollers, each roller disposed equidistantly maintaining a gap and
rotatable at a low linear speed by a drive mechanism; locating at least one Si-
wafer on an upper surface of at least one of said rotatable slitted PVC roller;
filling the tank with an acid mixture; and causing the PVC rollers to rotate so as
to make the wafer moving from one roller to the next roller resulting in uniform
wetting of the lower surface; wherein the tank is filled with the acid mixture pre-
cooled upto 20°C such that a gap of at least 2mm is retained between the acid
level and the upper surface of the rollers, wherein the slits formed on the rollers
act as capillaries to allow the acid mixture to rise in the slits due to surface
tension and gravity, wherein the acid mixture consists of HF:HNO3:H2O in a ratio
of 1:5:10, and wherein the etching time varies between 3 to 10 minutes.