The present invention relates to solar photovoltaics and semiconductor devices.
In particular, the present invention relates to a multiple four probe heads for
determination of the uniformity of diffused emitter in a Si wafer.
BACKGROUND OF THE INVENTION
In a conventional solar cell n-type emitter is diffused on p-type wafer in order to
form a n-p junction. The uniformity of this junction is very important to improve
the yield and efficiency of solar cell.
The most widely used technique all over the world for measurement of sheet
resistance of diffused Si wafer is four point probe technique. In this technique a
4 probe head having 4 probes is used and a current is passed through the outer
two probes and the voltage drop is measured across the two inner probes. Some
patents related to the present disclosure are illustrated below:
In patent number US 3,609,537: This invention relates to a standard and method
of calibration for probes used to measure semiconductor resistances. More
particularly, it relates to a standard and to a method of Calibration for four-point

probes used to measure the resistance, and hence, the level of impurity diffusion
carried out in semiconductor manufacturing processes.
In patent number US 3,735,254: The invention relates to the determination of
the sheet resistance (Ohms per square) of a layer which changes into a second
layer via a rectifying junction. In this determination, four or more electrodes are
arranged on the layer to be measured, for example on a straight line. In a first
step, a current is conveyed through two electrodes at a comparatively low
voltage and the voltage associated with said current is determined between two
other electrodes as is usual in a normal four point measurement.
SUMMARY OF THE INVENTION
Therefore a new concept has been conceived, where the sheet resistance of
large sized Silicon wafer can be examined at multiple points almost
instantaneously and such a setup can be very useful on a production floor where
thousands of wafers are processed on daily basis.
The four point probe technique measures the sheet resistance of a small area
(any area of 40 times the distance between probe space (s) gives a result with
better than 1% accuracy and probe space (s) is typically 1 to 1.5 mm). However

in order to determine the uniformity of the diffused layer, the above
measurement should be taken at several points over the surface of a 6 inch by 6
inch wafer. In the solar cell production line this practice is not very encouraged
as it takes several minutes to take 8-10 measurements over different places of a
wafer and then to conclude the uniformity of the emitter in the wafer. The
present invention will club 9 heads mounted over a block that will sit on the
surface of a Si wafer and will take the readings quickly with the help of electronic
circuitry which will be part of the whole instrument. The results will be shown
over the desktop of a computer monitor in a graphical form (with the position at
which a particular reading corresponds) as well as in tabular form. In less than a
minute it can be determined whether the uniformity in the diffused layer is good
or bad. This kind of setup ideally suits the production line where time is a limiting
factor.
OBJECTS OF THE INVENTION
The object of the present invention is to examine the uniformity of emitter over
large sized wafers.

Another object of the present invention is to measure the sheet resistance at
plurality of points over the surface of a large sized Si wafer.
BRIEF DESCRIPTION OF THE ACCOMPANYING DRAWINGS
Fig.l - Shows magnified view of a single four probe head
Fig.2 - Shows a conceptual view of all the 9 probe heads mounted over a
single block
Fig.3 - Shows a silicon wafer showing size comparison with the 9 heads
mounted over a block in Fig.2.
Fig.4 - Shows an illustration of the results showing typical values of sheet
resistance of diffused emitter layer that will be displayed on the
computer monitor.
Fig.5 - Shows an imagination of the conceptual setup.
DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT OF THE
INVENTION
A picture of a 4 point probe head is shown in Fig. 1. Using a single four probe
head, the sheet resistance of diffused emitter can be measured at smaller areas

but in order to examine the uniformity of emitter over 6 inch by 6 inch large size
wafer, use of single head may not suffice.
Fig. 2 shows 9 equally spaced measuring heads mounted over a block in the
form of a 3 by 3 matrix. All the 9 heads come in contact with the wafer (shown
in Fig. 3) instantaneously and with the help of sophisticated electronics the
readings at 9 points will be taken one by one within a minute and will be
displayed.
In the present disclosure invention consists of 9 four probe heads mounted over
a block in the form of a 3 by 3 matrix. All the 9 heads along with the block move
up and down simultaneously and make contact with the Si wafer simultaneously.
The instrument uses electronic card where by switching probes one after the
other readings of each head is captured. Reading from each head is saved in the
memory of the computer and when the whole test finishes, the results are
displayed on the computer monitor. •
In view of the requirement of a solar cell production line where quick
measurement is required of the sheet resistance of diffused emitter of silicon
wafer to determine its uniformity, a setup is designed which will take the sheet
resistance measurements at 9 different points over a 6 inch by 6 inch silicon
wafer within a minute and will show the results on a computer monitor. The

measurement will be taken by using 9 four probe heads of the conventional four
point probe sheet resistance measurement method.
A block having 9 equally spaced (distance between any two adjacent probe
heads along any row or along any column is 2 inches) probe heads moves up
and down. Each probe head contains spring loaded contacts which makes
contact with the silicon wafer when the probe head comes down. An interlock is
positioned to shows that all the heads are in contact with the Silicon wafer. The
electronics of the instrument is activated and each head will take reading
corresponding to its point of contact. The electronics is designed in such a way
that only one head will be powered at the time of measurement. This will ensure
the accuracy of the readings of the powered head. Simultaneous presence of
voltage and current through other heads may alter the sheet resistance
measurement. The measurement setup is designed in such a way that the
sample under measurement remains in dark as the photosensitivity of the
semiconductor may change the measured values. The set up requires only
electrical power and all the controls and movements is powered by electricity and
no pneumatic supply is required. The software design will ensure that
measurements are done with a single click without the need of any special
training to operator. The software is also interactive and the supervisor is able to

fix a go-no-go value of the uniformity of the sheet resistance of the emitter to
enable the operator to decide whether the diffusion is OK. The results are
displayed on the computer monitor. An example of expected results that will be
displayed over the computer monitor is shown in Fig. 4 and a conceptual drawing
of the setup is shown in Fig. 5.

WE CLAIM
1. A method of measuring the uniformity of the sheet resistance of a Si wafer by
mounting plurality of four probe heads over a block that accommodates on
the surface of a Si wafer wherein the probe heads are positioned
equidistantly over the block and the probes moves up and down to
instantaneously come in contact with the Si wafer and on operator demand
the electronic circuitry activates each head to monitor the reading
corresponding to its point of contact.
2. The method of measuring as claimed in claim 1, wherein the electronic
circuitry is designed in a way that only one head get powered at the time of
measurement.
3. The method of as claimed in claim 1, wherein the readings are displayed with
the help of computer screen.