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Optical Furnace offers improved semiconductor device processing capabilities

Award winning solar manufacturing process

National Renewable Energy Laboratory

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	<em>The highly versatile optical furnace provides semiconductor manufacturers with energy efficient methods to process devices in a high throughput capacity. &nbsp;</em></p>

The highly versatile optical furnace provides semiconductor manufacturers with energy efficient methods to process devices in a high throughput capacity.  

Technology Marketing Summary

Manufacturers of semiconductor devices in the microelectronic and photovoltaic industries have long been plagued by the costs of wafer fabrication. Currently, process steps such as phosphorous diffusion, aluminum alloying, coating deposition, hydrogen passivation and contact formation must be completed at extremely high temperatures. In addition to the substantial cost of heating the system, high process temperatures can introduce impurities and reduce the overall quality of a device.  Currently, wafers are either processed in conventional electric or infrared furnaces. Conventional electric furnaces require the exposure of the device to high temperatures for extended periods of time and, as a result, are expensive, slow, and can result in impurity redistribution and distortion of the wafer. In contrast, infrared furnaces use a powerful optical source to quickly raise the temperature of the semiconductor device, offering a slight improvement in required energy and process time over conventional systems.  Unfortunately, the non-uniform light source in the infrared furnace creates temperature variations over the wafer which can result in defects, dislocations, and wafer breakage. 

Recently, improved optical furnaces have been created that can provide a uniform light source, but lack the speed and throughput required to be utilized on a high throughput production line.  Due to the inherent limitations of the current state of the art, there exists a need for an energy efficient method to process wafers at a high throughput level.  Using the concepts of an optical cavity furnace, scientists at the National Renewable Energy Lab have created a low cost method to process high quality semiconductor devices at speeds required by both the semiconductor and photovoltaic industries. 


The optical furnace uses the principles of photo-absorption and other photonic effects to provide manufactures of semiconductor devices with a low cost, high throughput alternative to current process methods. The optical furnace sits on top of a manufacturing line and is formed by placing optical sources within the reflecting walls of the furnace. This design ensures that all energy coming from the light sources is transferred to the wafer only, resulting in substantial energy savings over the traditional method. Once inside the furnace, the wafer is processed in a manner dictated by the control of the following process parameters.  First, the power and location of the optical source can be tailored to provide the desired optical flux distribution. Second, the transport rate controller can alter the speed with which the wafers are transported through the furnace. Third, a process substance controller is used to control the withdrawal or addition of substances such as processes gasses from the optical furnace.  

These process parameters can be adjusted to a device manufacturer’s process line to complete a wide variety of process steps in an energy efficient manner that minimizes impacts to the surrounding layers of the device.  For example, manufacturers looking to remove impurities from a semiconductor substrate or solar cell can use the furnace to first hydrogenate the backside of the substrate, and then illuminate it with electromagnetic radiation at intensity and for a time period sufficient to cause the impurities to diffuse to the back side. There, the impurities alloy with a metal already present in the device and form a contact that captures the impurities. This process is ideally suited for a solar manufacturer who seeks to simultaneously passivate defects within the device and form a contact, as the high intensity electromagnetic radiation is sufficient to sinter/alloy the metal contacts.  Additionally, a solar manufacturer can use the furnace to produce a thin SiO2 film on a silicon substrate to minimize surface recombination and improve cell efficiency. During dry oxidation, the silicon wafer is about 1000°C, the furnace walls at about 500°C, and the quartz chamber is at about 200°C. This means that the furnace is almost immune to the contamination from hot walls of the quartz chamber that conventional furnaces are plagued with.

The optical furnace also lends itself as a very efficient concentrator of light. This feature of the furnace has been used to develop an online wafer screening system that separates those wafers that have propensity to break during solar cell processing. The light generated by the concentrating furnace is used 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. The optical furnace 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.

  • N/P junction formation
  • Aluminum alloying for back surface field preparation
  • Front Contact formation
  • Hydrogen passivation
  • Wafer screening
  • Gettering
  • SiO2 deposition
  • Thin Film Si
Applications and Industries
  • Solar cell manufacturing
  • Solar module manufacturing
  • Wafer manufacturing
  • Solar equipment manufacturers
More Information

The NREL Optical Cavity Furnace won a 2011 R&D 100 Award for its potential to revolutionize the solar cell manufacturing industry in the U.S. by producing higher quality and higher efficiency solar cells at a fraction of the cost of conventional, thermal ovens.

Optical Furnace Portfolio

Equipment Patents

NREL 06-23 US 12/919,433 An Optical Cavity Furnace for Solar Cell Fabrication by Optical Processing

NREL 10-31 PCT/US11/24584 A Cavity-based Furnace for Wafer Screening Machine


Process Patents

Process Yield Improvement                                        

NREL 04-07 11/722,981 Screening of Silicon Wafers Used in Photovoltaics

NREL 10-31 PCT/US11/24584 A Cavity-based Furnace for Wafer Screening Machine

Patents and Patent Applications
ID Number
Title and Abstract
Primary Lab
Patent 8,006,566
Screening of silicon wafers used in photovoltaics
A method for screening silicon-based wafers used in the photovoltaic industry is provided herewith.
National Renewable Energy Laboratory 08/30/2011
Patent 8,796,160
Optical cavity furnace for semiconductor wafer processing
An optical cavity furnace 10 having multiple optical energy sources 12 associated with an optical cavity 18 of the furnace. The multiple optical energy sources 12 may be lamps or other devices suitable for producing an appropriate level of optical energy. The optical cavity furnace 10 may also include one or more reflectors 14 and one or more walls 16 associated with the optical energy sources 12 such that the reflectors 14 and walls 16 define the optical cavity 18. The walls 16 may have any desired configuration or shape to enhance operation of the furnace as an optical cavity 18. The optical energy sources 12 may be positioned at any location with respect to the reflectors 14 and walls defining the optical cavity. The optical cavity furnace 10 may further include a semiconductor wafer transport system 22 for transporting one or more semiconductor wafers 20 through the optical cavity.
National Renewable Energy Laboratory 08/05/2014
Patent 8,780,343
Wafer screening device and methods for wafer screening
Wafer breakage is a serious problem in the photovoltaic industry because a large fraction of wafers (between 5 and 10%) break during solar cell/module fabrication. The major cause of this excessive wafer breakage is that these wafers have residual microcracks--microcracks that were not completely etched. Additional propensity for breakage is caused by texture etching and incomplete edge grinding. To eliminate the cost of processing the wafers that break, it is best to remove them prior to cell fabrication. Some attempts have been made to develop optical techniques to detect microcracks. Unfortunately, it is very difficult to detect microcracks that are embedded within the roughness/texture of the wafers. Furthermore, even if such detection is successful, it is not straightforward to relate them to wafer breakage. We believe that the best way to isolate the wafers with fatal microcracks is to apply a stress to wafers--a stress that mimics the highest stress during cell/module processing. If a wafer survives this stress, it has a high probability of surviving without breakage during cell/module fabrication. Based on this, we have developed a high throughput, noncontact method for applying a predetermined stress to a wafer. The wafers are carried on a belt through a chamber that illuminates the wafer with an intense light of a predetermined intensity distribution that can be varied by changing the power to the light source. As the wafers move under the light source, each wafer undergoes a dynamic temperature profile that produces a preset elastic stress. If this stress exceeds the wafer strength, the wafer will break. The broken wafers are separated early, eliminating cost of processing into cell/module. We will describe details of the system and show comparison of breakage statistics with the breakage on a production line.
National Renewable Energy Laboratory 07/15/2014
Technology Status
Technology IDDevelopment StageAvailabilityPublishedLast Updated
REL ROI 04-07, 06-23, 10-31, 04-07ProductionAvailable06/24/201107/31/2013

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To: Bill Hadley<>