Sherescan - instrument for mapping Sheet Resistance of emitter

To achieve high efficiency crystalline silicon solar cells, the uniformity and reproducibility of the emitter plays a major role. The knowledge of the emitter sheet resistance across the surface of a single wafer, and of the reproducibility from wafer to wafer is essential in optimizing the emitter diffusion process.
Sherescan instrument also offers other modes of measurement: P/N recognition, and metal conductivity.

Sherescan application examples:

Emitter sheet resistance mapping

Below is a typical example of emitter sheet resistance mapping using a 10x 10 measurement location matrix. This measurement takes about 9 minutes. The figure to the right represents a high resolution mapping with a 100 x 100 matrix. In the figure it is possible to see the belt imprint, meaning that through optimization of the settings of the diffusion IR furnace, a more uniform emitter can be obtained.

13 point cross mapping:

The typical cross mapping shows the sheet resistance values, with a high resistance area in the center part. In the histogram the bi-modal character can be observed. This way of mapping and representation, as used for semi-conductor applications, does, in general, not reveal enough for solar cell applications.

Sheet resistance measurement on a non-uniform emitter:

The ring shaped emitter, as depicted in a 3-D plot, was produced in a belt type IR diffusion furnace.

High resolution Sherescan:

With maximum mapping resolution of 100 x 100 points this sheet resistance plot clearly shows two thermal effects of the IR diffusion furnace. The emitter diffusion was done under "extreme" conditions, meaning that belt imprints on the result are visible. Also, the influence of quartz rod that supports the belt becomes visible.
The histogram data show a peak around the target 67 Ohm/sq, and also area of higher sheet resistance as a result of the cooling by the rod and belt.

P/N determination example

This is the resulting panel in case of a P/N Determination measurement. The result shows in the panel that the material is of N-type.

Metal resistance example

Below is the panel of the result of the Metal Resistance measurement. The data in the plot show the resistance across a silver busbar. From these data the Sheet Resistance, and also the Specific Resistivity of the metal are calculated and shown.

Exotic result phosphorous diffusion

The result shows sheet resistance mapping from a mono crystalline wafer in a tube furnace the wafer was exposed to P-type diffusion. Different values can be observed.

Exotic result of P- and N-type diffusion

The result shows sheet resistance mapping from a wafer in a tube furnace the wafer was partly exposed to P-type diffusion and N-type diffusion. It results in an areas with a varying sheet resistance from 45 Ohm/sq to 600 Ohm/sq. The high ohmic area is where both emitter are compensating each other.

Emitter sheet resistance mapping

Due to too narrow spacing in a tube furnace the non uniform gas distribution cause the non-uniform sheet resistance in the left picture. After optimization the uniformity improved, as seen in the right hand picture.

Emitter sheet resistance mapping

Poor wafer pre-treatment caused the non-uniform emitter as shown on the right hand side picture. Optimization improved the uniformity.

Emitter diffusion on mono crystalline wafer

Homogeneous sheet resistance values on a mono crystalline wafer.