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Cavity based furnace for wafer screening

National Renewable Energy Laboratory

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NREL Principal Engineer Bhushan Sopori has fired up an optical furnace he developed to efficiently fabricate solar cells.
NREL Principal Engineer Bhushan Sopori has fired up an optical furnace he developed to efficiently fabricate solar cells.

Technology Marketing Summary

The U.S. Department of Energy (DOE) estimates that a $1 per watt installed photovoltaic (PV) solar energy system - equivalent to 5-6¢/kilowatt hour (kWh) — would make non subsidized solar competitive with the wholesale rate of electricity, nearly everywhere in the United States.  In order to reach this goal, manufacturers in the highly competitive solar manufacturing industry have placed a greater focus on two important aspects of their processes - throughput and efficiency. While all sectors of the industry must focus on process improvements, traditional crystalline silicon based manufacturers stand to benefit the most through process improvements because almost one half of the installed cost of a silicon cell solar module is driven by the cost of the silicon wafer. Silicon wafers break during manufacturing due to a combination of large stress in handling and thermal processing, and the fragile nature of the silicon substrate.  Wafers that break during the manufacturing process decrease the effective yield and increase the manufactured cost per watt. Therefore, there is a significant cost savings in isolating wafers that would break during manufacturing. Current wafer screening techniques use either highly unreliable infrared imaging or extremely energy intensive method based on optically induced thermal stress.  Both these approaches  require a great deal of time to accurately test each individual wafer for potential defects. There exists a need in the industry for a wafer screening process with low power requirements that can offer the throughput needed by the photovoltaic industry (1200-2000 wafers/hr). Using the concepts of an optical cavity furnace, scientists at the National Renewable Energy Lab have created a low-power system with the capacity to operate at throughput levels required by high speed solar cell manufacturers.

Description

Wafer screening by optically-induced thermal stress  is typically accomplished through a process that contains several water-cooled lamps with parabolic reflectors. As the wafer is transported underneath the reflectors via conveyor belt, all light sources are directed into one small area to optically induce the maximum thermal stress the wafer would be subject to during manufacturing. If the wafer survives this test, it is likely to survive the cell fabrication process without breakage.  While this process allows solar manufacturers to screen potentially bad wafers, the water-cooled lamps and reflectors require a tremendous amount of energy.  Furthermore, the technique does not allow for high throughput processing.

The proposed new method of screening wafers takes advantage of the concepts of an optical cavity furnace to build a high throughput wafer screening machine with low power requirements.  In much the same manner as the traditional method, the wafer in the improved system is transported through the process via conveyor belt. However, in the improved system, an optical cavity is formed by placing optical sources within the reflecting walls of the furnace.   This design ensures that -energy coming from the light sources is transferred to the wafer only, resulting in substantial energy savings over the traditional method.  After the wafer exits the cavity furnace, it is then subjected to either a a jet of cold air or an ambient of a water-cooled camber to release the thermal energy.  This dynamic temperature profile of the wafer produces a predetermined time-dependent stress in the wafer, which corresponds to the highest stress the wafer can experience during solar cell processing. This process has the capability to reach the required throughput levels of approximately 2,000 wafers/hr in a modular system that sits quietly on top of any existing line and uses far less energy than the traditional system.
Benefits
  • Increased effective yield of crystalline silicon wafers
  • Reduced overall cost of finished modules
  • High Throughput of approximately 2,000 wafers/hour
  • Reduced energy consumption over existing wafer screening systems
Applications and Industries
  • This method is applicable to all traditional wafer solar cell manufacturers   
Technology Status
Development StageAvailabilityPublishedLast Updated
PrototypeAvailable03/04/201104/06/2011

Contact NREL About This Technology

To: Ty Ferretti<Ty.Ferretti@nrel.gov>