FIELD OF INVENTION
The present invention relates to a system and method of measuring sheet resistance of
large area diffused Si wafer using multiple four probe technique.
BACKGROUND OF INVENTION
The Si solar cell industry is growing at a very fast rate of more than 40% per year and
it has been well understood that the cost of PV can be reduced by enhancing solar cells
efficiencies. Crystalline Si solar cells have been the workhorse of the PV industry for the
last four decades and are likely to continue for next few decades. The cell efficiency of
Si solar cells produced by diffusion process can be enhanced by shallowing the emitter
depth and using suitable contact pastes. Shallowing the emitter is easy but making it
uniformly shallow is a challenge. Another challenge is measuring the shallow emitter
sheet resistance quickly and accurately. In a 100 MW Si solar cell industry, where more
than 50,000, 6” x 6” Si wafers are handled every day, measurement of emitter sheet
resistance even on batch basis is impossible if is done using a conventional single four
probe setup as it takes a few minutes to measure emitter sheet resistance at multiple
points on one wafer. Also it is required to analyze the data so as to give feed back to
the process team to take corrective action for optimizing the diffusion process. Now a
technique has been developed for using nine four probe heads mounted on a thick

aluminum plate which can be made to contact the Si wafer at predetermined set of nine
points and measure the sheet resistance instantaneously and non-destructively.
OBJECTS OF THE INVENTION
Therefore, it is an object of the invention to propose a system and method of
measuring sheet resistance of large area diffused Si wafer using multiple four probe
technique, which is capable of quick measurement of emitter sheet resistance for high
efficiency solar cell development.
BRIEF DESCRIPTION OF THE ACCOMPANYING DRAWING
Fig.1 - Shows a schematic view of probe head.
Fig.2 - Shows a probe gantry with nine probe heads.
DETAILED DESCRIPTION OF A PREFERRED EMBODIMENT OF THE
INVENTION
Sheet resistance of semiconducting thin layer can be measured using four probe tester.
In the present invention the four probe tester has been developed by using a hollow
plastic shell with a closed bottom for fixing the pogos. The probe head has threaded
slots so that it can be fixed firmly to a plate. Special spring loaded gold coated round
bottom pogo pins have been selected for making contact with delicate Si wafer. The

pogo pins are in two pieces and the spring loaded pin can be replaced easily as and
when required.
The nine of this four-probe heads or tester are mounted on an aluminum plate. The
location of the probe heads has been selected so that maximum area of the Si wafer is
inspected. The probe heads are moved up and down using a stepper motor for smooth
movement.
All the 4x9=36 contacts from the probe heads are taken using flexible wires to the relay
card and the software command is given to power only one probe head at a time. The
use of mechanical relay in place of solid state relays ensures accurate measurement of
sheet resistance due to absence of any undesirable voltages at the probe heads.
It is well known that sheet resistance of semiconducting thin layer can be measured
using four probe tester. In the present invention the four probe head has been
developed by using a hollow plastic shell with a closed bottom for fixing the pogos. The
probe head has threaded slots so that it can be fixed firmly to a plate. Special spring
loaded gold coated pogo pins have been selected for making contact with the Si wafer.
The schematic of the probe head is shown in Fig 1. The contact surface of the pin is a
round bottom so that it does not damage or puncture the Si surface while making
contact. The spring tension is sufficient to make firm contact and not very high for the
delicate Si wafer.The pogo pins are in two pieces. One piece is fixed to the probe head
bottom and soldered wire contactsare taken from all the four probes from top. The

other piece is the spring loaded round bottom pin. This pin can be inserted into the
other pin which makes firm electrical contact. While using the setup continuously, there
are chances that the some of the spring loaded pogo pin may become defective due to
continuous usage. In such a case only the defective pins can be replaced with the good
ones and there is no need to replace the full probe head. Another advantage is that
other pogo pins of different bottom shapes can be used if the round bottom is not
suitable for some application.
The nine probe heads (1) are mounted on a 10 mm thick aluminum plate with the help
of screws. The location of the probe heads (1) has been selected so that maximum area
of the Si wafer is inspected. Details are given in Fig 2. The gantrycomprising nine probe
heads (1), aluminum plate (2) is suspended from the center using a heavy block (3) of
Aluminum. The vertical movement is guided with the help of linear guide (not shown in
the diagram). Thefirm support and guided movement ensures that all the nine heads
move up and down smoothly and touch the Si wafer simultaneously without any
horizontal disturbance. This is very much essential in order to protect the delicate Si
wafer from mechanical damage while lowering the probe gantry to make physical
contacts with the Si wafer. The vertical guide is driven up and down with the help of a
stepper motor (4). This ensures that the probe gantry moves exactly same vertical
distance every time so that the pressure applied remains constant for all
measurements.

All the 4x9=36 contacts from the probe heads are taken using flexible wires to the relay
card and the software command is given to power only one probe head at a time. This
is done to avoid any interference from other probe heads. The output of the relay card
is fed into the computer for converting these signals into a useful data. There is a
specific reason for using mechanical relays in place of solid state relays. The solid state
relays are electronic switches and cannot offer infinite resistance in open state. While
mechanical relays have physically open contacts in normally open state and can be
considered as fully open in comparison to solid state relays. Here we are sensing very
small voltage signals for finding the emitter sheet resistance of Si wafer at nine points
instantaneously. During measurement cycle all the probe heads are touching the Si
wafer physically. If we uses solid state relays, even in open condition, some voltages
might be present on the probe of all the probe heads. These may interfere with the
signal voltages of the measuring probe head and affect the accuracy of measurement.
Important Features of the Invention
1. Design of probe head of plastic material suitable for fixing four gold coated pogo
pins at a constant fixed distance of 1.2 mm.
2. Use of replaceable spring loaded pogo pins for easy repair and replacement.
3. Use of mechanical relays instead of solid state relays to avoid any interference
from other probe head during measurement cycle.
4. Vertical movement using a stepper motor ensuring smooth landing of spring
loaded pins on to the delicate Si wafers.

WE CLAIM
1. A system of measuring sheet resistance of large area diffused Si wafer using
multiple four probe technique, the said system comprising:
four probe tester (5) made from hollow plastic shell with a closed bottom for
fixing a plurality of pogo pins (6). The said probe tester having threaded holes to
facilitate fixing on an aluminum plate (2);
a gantry comprising, nine of said four probe tester (1) mounted on the aluminum
plate (2) suspended from the center of a aluminum block (3);
a plurality of spring loaded gold coated two pieces pogo pins (6) disposed at the
bottom of four probe tester (5) for making contact with Si wafer;
one piece of pogo pin fixed to the probe head bottom;
the other piece being spring loaded pin inserted into the other pin making firm
electrical contact;
a plurality of mechanical relays connected to probe heads;
Characterized in that,
the nine four head tester or the gantry (1) moving up and down smoothly
touching Si wafer simultaneously without any horizontal disturbance protecting the
delicate Si wafer from mechanical damage, wherein a vertical guide is driven up and
down with a stepper motor (4) ensuring the probe gantry (1) to move exactly same
vertical distance every time with smooth landing of spring loaded pins (6) on to the

delicate Si-Wafers, resulting applied pressure remaining constant for all
measurements, wherein he output of the relay card is fed into the computer for
converting these signals into useful data.
2. The system as claimed in claim 1, wherein the defective pogo pins (6) are
replaceable with good ones without replacing probe head.
3. The system as claimed in claim 1, wherein nine probe heads (1) are mounted on a
10mm thick aluminum plate with screws.
4. The system as claimed in claim 1, wherein the pogo pins (6) are spring loaded gold
coated round bottom pins to avoid damage or puncture Si wafer surface while
making contact.
5. A method of measuring sheet resistance of large area diffused Si wafer for the
system claimed in claim 1, comprising:
arranging four probe heads/tester (5) in a hollow plastic cell with closed bottom;
arranging nine of four probe tester mounted on an aluminum plate;
suspending a gantry comprising nine probe heads (1) from centre of a heavy
aluminum block (3);
creating 4X9 = 36 contacts of probe heads (5);

disposing a plurality of mechanical relay cards to ensure accurate measurement
of sheet resistance due to absence of any undesirable voltages at the probe heads
(5) fixing pogo pins (6) at the bottom of probe head (5) to make contact with Si
wafer;
connecting probe heads with relay card with flexible wires;
given command to power only one probe head at a time to avoid any
interference from other probe heads;
wherein, all nine heads move up and down with the help of a stepper motor
smoothly to touch the Si wafer simultaneously without any horizontal disturbance
protecting the Si wafer from mechanical damage and ensure the probe gantry (1)
moving exactly same vertical distance every time making applied pressure remaining
constant for all measurements,
wherein, output of the relay card is fed into the computer for converting these
signals into useful data.