Semiconductor wafers are used for a variety of purposes in microelectronics, solar cell, and other industries. Wafer quality often depends on variables such as thickness and surface characteristics. Poor quality wafers may have uneven thickness or uneven surface characteristics; whereas, higher quality wafers will have substantially more uniform thickness and substantially more uniform surface characteristics. Wafer quality can dramatically influence the mechanical and/or electronic yield of wafer-based semiconductors.
Additionally, fuel cells using proton exchange membranes (PEM) have gained considerable acceptance for automotive and portable power needs. It is expected that the fuel cell market will grow rapidly in the future. As this growth occurs, the fuel cell industry will require monitoring techniques that are fast, non-destructive, and capable of high throughput. Such monitoring will be beneficial for membranes, electrode coatings, gas diffusion layers, and other layers, surfaces or structures. The component properties that could be monitored include, but are not limited to, catalyst distribution and loading, electrode structure and porosity, thickness, membrane uniformity and defects, extent of curing, gas diffusion layer porosity, surface structure, and hydrophobicity.
Silicon wafers and fuel cell membranes are significantly different structures. Nonetheless, the monitoring of selected quality parameters in a continuous production environment would be advantageous for both wafer based device and fuel cell production.
A typical reflectometer system is implemented in a static manner and configured to measure one sample at a time. Current systems require that a test wafer or a sample from a fuel cell membrane to be placed into an off-production line measurement chamber and then held stationary during optical examination. Thus, reflectometer based systems are best suited for process control where process monitoring is done by periodic sampling rather than continuous monitoring.
Scientists at the National Renewable Energy Laboratory (NREL) have developed a system that enables the on-line monitoring of conveyor-belt driven solar wafer and roll-to-roll fuel cell membrane manufacturing using reflective spectroscopy. Wafers/membranes are illuminated at a shallow angle by a specially designed optical fiber having a narrow, rectangular output aperture. The narrow aperture of the fiber produces a large divergence beam and the reflectance image of the wafers/membranes is acquired by a line camera. This image is recorded and transformed into a parameter image. The parameters include:
· Material surface roughness
· Material texture etch quality
· Anti-reflection coating thickness
· Wafer thickness
· Metallization quality
· Material back reflectance back layer thickness
· Crystalline grain size and orientation
· Membrane surface roughness
· Thickness values for different layers of a multiple-layer membrane
· Thickness of a single layer membrane
· Variations in porosity of the electrode-coated membrane
· Presence of absorbing defects
· Presence of non-opaque defects
- Continuous in-line monitoring, providing 100% inspection
- High throughput exceeding 1200 wafers per hour or membrane production at tens of feet per minute
- Removes expensive touch labor from the monitoring process
Applications and Industries
- Semiconductor wafers
- Solar energy
- Fuel cells
- Proton exchange membranes (PEM)
- Films and coatings
- Portable power
- Monitoring techniques
- Quality control