FIELD OF THE INVENTION
The present invention relates to a device to measure the diffused reflectance of
full size (6" x 6") textured and AR coated silicon wafers forming solar cell. The
invention further relates to a non-destructive method for evaluating the
uniformity of texturing and AR coating of full size (6" x 6") Si wafers.
SUMMARY OF THE INVENTION
The present inventors recognized that efficiency of the solar cells can be
enhanced by reducing the reflection losses of incident radiations. Solar cell is
known to be a large area (6" square) device. Therefore, the quality and
uniformity of texturing and anti-reflection coating is likely to enhance the cell
efficiency and throughput. An improved device is provided for quantifying the
quality of texturing and AR coating nondestructively by measuring the diffused
reflectance of a full size (6" square) Si wafer in few seconds. In this device a
collimated beam of a light is allowed to fall on the Si wafer which is placed on a
specially designed test table. The reflected light from the Si wafer is measured
using a computer controlled multimeter. The value measured is averaged and
converted directly into % reflectance using a software. The test table has
features such as the exposed surface of the table has been minimized to reduce
reflected stray light. The test table is placed inside a light proof drawer of a
specific height

so that the light reflected from the bottom of the drawer has minimum
interference with the actual light reflected from the Si surface. A special mask
has been designed to measure diffused reflectance of the textured and AR
coated Si wafer from selected areas to quantify the uniformity of texturing and
AR coating.
In the present invention, a 6 " square wafer is fully illuminated using a collimated
beam of halogen light source which has spectrum similar to solar radiation. This
is a non destructive technique of average diffused reflectance measurement of
full 6" square wafer. A test table with low reflectance material is developed to
have a minimum exposure to the light, when the Si wafer is placed, so that the
accuracy of the device can be increased by reducing the stray reflection from the
table. The device constructed with a view of easy placement and removal of thin
fragile Si wafers in an industrial environment. This complete test table is placed
inside an easily retractable drawer with limit switches to ensure the proper
positioning of a sample under the light beam. The height of this table is such
adjusted so as to leave sufficient depth below the sample to minimize the stray
reflection coming from the base of the drawer. A computer with a display
attached to the device allows to take 100 reading and average the readings
before displaying to ensure accurate reading in every measurement. This device
can also be used for determining the uniformity of texturing by placing a mask
made of plasma textured silicon wafer on top of the test sample. By rotating the
test sample below this mask, the uniformity of the texturing and AR coating on Si
wafer can be compared.

BACKGROUND OF THE INVENTION
According to the prior art, chemical texturing of Si wafers and deposition of anti
reflection coating are the major process steps in making crystalline Si solar cells.
However, a quick measurement method of evaluating the quality of texturing of
full size (6"square) Si wafers is still not known on a commercial scale. The quality
of texturing or of anti-reflection (AR) coating, according to the prior art, is either
quantified through destructive technique of measurement through a
spectrophotometer or by evaluating the full cell at the end of the production
process through a short circuit current measurement. The data provided by these
known methods are costly for a production setup to be competitive, where
thousands of Si wafers are processed on daily basis. Hence there is a need for
developing a method for quick measurement of diffused reflectance after
texturing and AR coating of Si wafer. Indian Patent Application No.211-KOL-2007
in the name of BHEL gives the reference value of diffused reflectance in an
exemplary unit.
Indian Patent Application NO.211-KOL-2007 in the name of BHEL entitled "A .
portable equipment for quick measurement of diffused reflectance of textured Si
wafer for solar cells"; disclosed a system and method for the measurement of
average diffused reflectance from a full size (6" square) Si wafer non-
destructively. The other known Lab-scale techniques, used for the measurement
of diffused reflectance, using spectrophotometer, which take more than 10-15
minutes to run one sample, are not suitable for large-scale production, where
thousands of Si wafers are processed on daily basis.

OBJECTS OF THE INVENTION
It is therefore an object of The invention to propose a device to measure the
diffused reflectance of full size (6" x 6") textured and AR coated silicon wafers
forming solar cell.
Another object of the invention is to propose a non-destructive method for
evaluating the uniformity of texturing and AR coating of full size (6" x 6") Si
wafers.
BRIEF DESCRIPTION OF THE ACCOMPANYING DRAWINGS
Figure 1 - shows a Test Table of the device according to the invention.
Figure 2 - shows a retractable drawer of the inventive device in the test table of
Figure 1 being placed inside the drawer.
Figure 3 - shows a mask to be placed on the wafers for reflectance measurement
According to the process of the invention.
Figure 4 - is a graphical representation of variation of reflectance from two
textured Si-wafers.

DETAILED DESCRIPTION OF THE INVENTION
Figure 1 shows a test table of a device from non-destructive measurement of
diffused reflectance from the surface of textured and anti-reflection coated
silicon wafers of 6 sq.inch size. The collimated light beam of more than nine inch
diameters allowed to fall on the test table. The test table has been designed
using a black matt finish surface. When a six inch square Si wafer is placed in a
slot, hardly any surface of the table is left exposed to the incident light. This
helps in reducing the stray reflected light. The slot has been designed for quick
and easy placement and removal of thin fragile Si wafers. This test table is
placed inside an easily retractable drawer (Figure 2). The movement of the
drawer is detected with the help of a plurality of limit switches to ensure correct
positioning of a sample under the light beam. The height of this table is
adjusted and fixed so as to leave sufficient gap below the sample to minimize the
stray reflections from the base of the drawer. The computer attached to the
device enable to acquire 100 readings and averages it out displaying the diffused
reflectance value in % to ensure accurate measurement. The device is first
calibrated by placing two pre-measured Si wafers on the test table one by one.
Table 1 shows variation in the measured values of diffused reflectance of a
textured Si wafer. Ten values have been measured after initial warm time of ten
minutes of the device. The variation is within 0.4 %.

WE CLAIM :
1. A device to measure the diffused reflectance of full size (6" x 6")
textured and AR coated silicon wafers forming solar cell, comprising:
- a retractable drawer having at least one limit switch to control movement
of the drawer,
- a height-adjustable test table with black mat finish top surface having a
slot for positioning thereon a sample of textured and anti-reflection
(T&AR) silicon wafer of 6 inch square size for non-destructive
measurement of diffused reflectance of the wafer, the test table being
accommodated inside the drawer and positioned to maintain a vertical gap
from the sample;
- a mask made of black plasma textured mc-Si wafer disposable to partially
cover said sample;
- a source producing and focusing a collimated light beam of 9 inch
diameter on the test table, the mask allowing light incidence on 25% area
of the sample thereby the mask reflecting only a small amount of light,

wherein the device is operably connected to a computer with a display means
to acquire at least one hundred readings relating to the intensity of light
reflected from the exposed area of the sample, calculate the average value,
and display the diffused reflectance value.
2. The device as claimed in claim 1, wherein the sample when rotated
below the mask, the device is enabled to determine the uniformity of
the texturing and anti-reflection coating on the silicon wafers.

ABSTRACT

The invention relates to a device to measure the diffused reflectance of full size
(6" x 6") textured and AR coated silicon wafers forming solar cell comprising a
retractable drawer having at least one limit switch to control movement of the
drawer, a height-adjustable test table with black mat finish top surface having a
slot for positioning thereon a sample of textured and anti-reflection (T&AR)
silicon wafer of 6 inch square size for non-destructive measurement of diffused
reflectance of the wafer, the test table being accommodated inside the drawer
and positioned to maintain a vertical gap from the sample; a mask made of black
plasma textured mc-Si wafer disposable to partially cover said sample; a source
producing and focusing a collimated light beam of 9 inch diameter on the test
table, the mask allowing light incidence on 25% area of the sample thereby the
mask reflecting only a small amount of light, wherein the device is operably
connected to a computer with a display means to acquire one hundred readings
relating to the intensity of light reflected from the exposed area of the sample,
calculate the average value, and display the diffused reflectance value.