FIELD OF INVENTION:
The invention in general relates to solar photovoltaics and semiconductor devices. In particular, the invention relates to an equipment for measuring the emitter sheet resistance at multiple points of silicon wafer.
BACKGROUND OF THE INVENTION:
In a conventional solar cell, n-type emitter is diffused on p-type wafer in order to form a p-n junction. The most common method of forming an n type layer on top of the p type wafer is phosphoryl chloride (POC13) vapour diffusion. The uniformity of this junction is integral to improve the yield and efficiency of solar cell.
Globally, the most widely used technique for measurement of sheet resistance of diffused Si wafer is four-point probe technique. In this technique, a four-probe head having four probes is used, a current is passed through the outer two probes and the voltage drop is measured across the two inner probes. In PV industry where numerous wafers are used daily, a quick, non-destructive and simple to use equipment is required. In the proposed setup, these features have been incorporated. This is disclosed in the Patent Application Number 601/KOL/2015 dated 29.05.2015.
A four-point probe technique measures the sheet resistance of a minute section. However, in order to determine the uniformity of the diffused emitter layer, the above measurement should be taken at several points over the surface of, in general, 156 mm by 156 mm wafer. In practice, this is not 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 wafer.
PRIOR ART:
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 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.

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.
None of the teachings disclose about nine four probe heads for determining the uniformity of diffused emitter.
OBJECTS OF THE INVENTION:
An object of the invention is to develop a sheet resistance measurement system to determine sheet resistance at nine different places over the surface of a diffused silicon (Si) wafer quickly and to assess the uniformity of diffused emitter.
SUMMARY OF THE INVENTION:
The invention provides a gantry of nine heads, each being gold coated and spring loaded mounted over a 10-mm thick aluminium sheet that moves downwards and makes contact with the surface of a Si wafer and measures the parameters with the help of electronic switching card and mechanical relays. The results are displayed in a graphical form (with the position at which a reading corresponds) as well as in tabular form. It is then determined whether the uniformity in the diffused layer is appropriate.
BRIEF DESCRIPTION OF THE ACCOMPANYING DRAWINGS:
It is to be noted, however, that the appended drawings illustrate only typical embodiments of the present subject matter and are therefore not to be considered for limiting of its scope, for the invention may admit to other equally effective embodiments. The detailed description is described with reference to the accompanying figures. Some embodiments of system or methods in accordance with embodiments of the present subject matter are now described, by way of example, and with reference to the accompanying figures, in which:

Fig.1 shows the magnified view of a single four probe head in accordance to the invention.
Fig.2 shows the cubical assembly, housing all the components of the sheet resistance measurement setup (SRMS) in accordance to the invention.
Fig 3 shows all illustration of the results showing typical values of sheet resistance of diffused emitter layer that will be displayed on the computer monitor in accordance to the invention.
DETAILED DESCRIPTION OF THE PREFFERED EMBODIMENTS:
Four-point probe based instruments use a technique to measure the sheet resistance of a thin layer or sheet by passing current through the outer two pins of the probe and measuring the voltage across the inner two pins. A four-point probe head that is used for this technique is shown in figure 1.
If the spacing between the probe points is constant, and the conducting film thickness is less than 40% of the spacing, and the edges of the film are more than four times the spacing distance from the measurement point, where the sheet resistance of the film is given by:
Rs = 4.53 x V/I
The thickness of the film (in cm) and its resistivity (in ohm cm) are related to Rs by:
Rs = resistivity/thickness
An instrument has been designed where nine four probe heads are used to quickly measure emitter sheet resistance quickly and can inform the operator about the uniformity. The operator in turn can take corrective measures to correct the diffusion process parameters to enhance yield and cell efficiency.
The working model of sheet resistance measurement setup (SRMS) is shown in figure 2. It shows a cubicle, housing all the components of the sheet resistance measurement setup. In the middle section of the panel there is a gantry (2) of motorized probe assembly that have all the nine heads mounted over a 10-mm thick aluminium plate and all of them move together (up and down only) together

with the help of a stepper motor (3). A retractable tray has been provided for sample placement. It moves back and forth and can be drawn out for placing and withdrawing silicon wafer on the chuck. A suitable teflon chuck that can take a wafer of size 156 mm by 156 mm is used for placing the wafer. The teflon chuck has guides on all sides so that the silicon wafer fits in exactly at same position each time. This is essential as numerous of silicon wafers are kept in the diffusion tube. While measuring the sheet resistance, it is important to know which part of Si wafer is showing non-uniformity.
Accordingly, process corrections can be applied. After placing the wafer on the chuck and closing the tray which is held tightly at the rear end using a magnetic interlock which ensures that the tray does not open while the test is being conducted. The tray is designed to avoid any leakage of ambient light into the chamber so that the test can be performed under total darkness. This is essential to ensure accuracy and repeatability.
After placing the wafer on the chuck and closing the tray, the test is performed by a single touch on the touch screen monitor using the test software. The probe head gantry (2) having all the nine heads moves downwards and makes contact with the wafer kept at the chuck.
Each pin of each four-probe head is gold coated so that the contact resistance is minimized. All the nine heads are activated one after another using compatible switching card and mechanical relays and the data is recorded within a couple of seconds. The results are shown on a monitor of the cubicle shown in figure 3.
The cubicle shown in figure 2 also houses all the electronic components including current source and meter, voltmeter, switching card, and all the cables. The cubicles have been designed to suit the operator’s convenience.

WE CLAIM:

1. A method to measure uniformity of diffused emitter of a silicon (Si) wafer, the
method comprising the steps of:
- a motorised four point nine headed probe assembly to move downwards to make contact with the Si wafer;
- recording the data at the point of contact of the probe on the Si wafer;
- monitoring the recorded data in a tabular form to analyse the non-uniformity of the Si wafer;
wherein the Si wafer is placed in a chuck and the recorded data is displayed on a monitor.
2. The method to measure uniformity of diffused emitter of a silicon (Si) wafer as claimed in claim 1, wherein the four-pin probe is gold coated to minimise contact resistance.
3. The method to measure uniformity of diffused emitter of a silicon (Si) wafer as claimed in claim 1, wherein the nine heads are activated sequentially by means of switching cards and relays.
4. The method to measure uniformity of diffused emitter of a silicon (Si) wafer as claimed in claim 1, wherein the nine heads are monitored on an aluminium plate and moves upwards and downwards by means of a stepper motor (3).
5. A method to measure uniformity of diffused emitter of a silicon (Si) wafer as substantially described and illustrated herein with reference to the accompanying drawings.