Optical Metrology for Quality Control of Silicon Substrates for Solar Cells

Silicon substrates for solar cell applications must satisfy exacting technical requirements. By adding surface texture, the efficiency of light-gathering is enhanced, but if there is too much texture, light is scattered and efficiency is considerably reduced. It is also important to use a high quality diamond wires to saw wafers in order to ensure superior quality of the finished wafers.

The Zeta-200 Automated 3D Metrology System enables quality control of critical properties that include the following:

• Roughness of substrates at multiple steps, from very rough to very smooth
• Height and periodicity of saw marks
• Wafer bow
• Edge cracks and other defects
• Height and density of diamond features on saw wires

Silicon Wafer Inspection

The different types of silicon wafer inspection include the following:

• Edge inspection
• Wafer bow
• Step height
• Surface roughness

Edge Inspection

The image shows a Zeta 2D image of wafer edge. Length markers are 37 µm vertically and 15 µm horizontally.

Wafer Bow

The image shows the shape of a 5 in. wafer based on 2D or 3D measurements at 25 sites.

Step Height

Zeta 2D and 3D images of an incoming substrate (left) show saw marks. Distance between cursors is 1215 µm, height difference between cursors is 3.4 µm and the depth of a single groove is 2.3 to 3.0 µm.

Surface Roughness

The results obtained from the analysis of surface roughness are that Ra is 0.95 µm total, or 0.57 µm with waviness correction.

Diamond Wire Inspection

The figure shows Zeta 3D image of a new diamond wire and automated analysis of diamond particles. Each diamond in the field of view is analyzed for area, volume, and height.

The Zeta-200 System

The Zeta 200 Automated 3D Measurement System provides true color imaging of complex surfaces in less than one minute per site.

The Zeta 3D Software analyzes 2D or 3D images for the following parameters:

• Step height
• Surface roughness
• Feature size, diameter, area, and volume
• Multi-surface analysis for transparent features
• 3D surface visualization
• Statistics

The Zeta-200 can be used for the following purposes:

• Multi-site measurements with statistics
• Recipe-based measurement definitions
• Automated data and image export

The features of the Zeta-200 include the following:

• High-brightness white LED light source
• XY stage with 200mm x 200mm travel
• 30mm total vertical travel
• Multiple FOV configurations are available

The system includes an Intel Core2 Duo processor with 3GB RAM, 320 GB disk, and widescreen LCD monitor.

About Zeta Instruments

Zeta Instruments is a leading provider of precision microstructure and surface measurement systems that enable manufacturers of green-technology and bio-medical products to substantially improve yields and quality control. Zeta’s advanced metrology solutions provide direct benefit to the production of high-brightness LEDs, solar cells, micro-fluidics/bio-technology and magnetic storage media. Our patented Z-Dot™ technology enables manufacturers to quickly and accurately perform 3D measurement of the micron-scale features of these applications, positioning it as a strategic supplier to these high-growth industries.

This information has been sourced, reviewed and adapted from materials provided by Zeta Instruments.

For more information on this source, please visit Zeta Instruments.