Optical Furnace offers improved semiconductor device processing capabilities

Technology Marketing SummaryManufacturers 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. DescriptionThe optical furnace uses the principles of photoluminescence 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. The optical furnace can also be 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.Benefits

• N/P junction formation
• Aluminum alloying for back surface field preparation
• Contact formation
• Hydrogen passivation
• Phosphorous diffusion
• Hihg Wafer screening
• Gettering
• SiO₂ deposition
• Thin Film Si

More Information

Optical Furnace Portfolio

Equipment Patents

NREL 92-56 US 5,577,157 Optical processing furnace with quartz muffle and diffuser plate

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

Device Optimization

NREL 91-05 US 5,304,509 Back-side Hydrogenation Technique for Defect Passivation in Silicon Solar Cells               

NREL 90-88 US 5,358,574 Dry Texturing of Solar Cells

NREL 93-32FWC US 5,639,520 Application of Optical Processing for Growth of Silicon Dioxide

NREL 93-55 US 5,426,061 Impurity Gettering in Semiconductors 

NREL 96-07 US 6,852,371 Metal processing for impurity gettering in silicon

Metallization                                   

NREL 90-87 US 5,429,985 Fabrication of Optically Reflecting Ohmic Contacts for Semiconductor Devices

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

Thin film Processing

NREL 94-26 US 5,897,331 High Efficiency Low Cost Thin Film Silicon Solar Cell Design and Method For Making

NREL 10-24 PCT/US11/24584 Thin Film Heterojunction Silicon Solar Cells

Patents and Patent Applications

ID Number	Title and Abstract	Primary Lab	Date
Patent 5,639,520	Application of optical processing for growth of silicon dioxide A process for producing a silicon dioxide film on a surface of a silicon substrate. The process comprises illuminating a silicon substrate in a substantially pure oxygen atmosphere with a broad spectrum of visible and infrared light at an optical power density of from about 3 watts/cm.sup.2 to about 6 watts/cm.sup.2 for a time period sufficient to produce a silicon dioxide film on the surface of the silicon substrate. An optimum optical power density is about 4 watts/cm.sup.2 for growth of a 100.ANG.-300.ANG. film at a resultant temperature of about 400.degree. C. Deep level transient spectroscopy analysis detects no measurable impurities introduced into the silicon substrate during silicon oxide production and shows the interface state density at the SiO.sub.2 /Si interface to be very low.	National Renewable Energy Laboratory	06/17/1997 Issued
Patent 5,358,574	Dry texturing of solar cells A textured backside of a semiconductor device for increasing light scattering and absorption in a semiconductor substrate is accomplished by applying infrared radiation to the front side of a semiconductor substrate that has a metal layer deposited on its backside in a time-energy profile that first produces pits in the backside surface and then produces a thin, highly reflective, low resistivity, epitaxial alloy layer over the entire area of the interface between the semiconductor substrate and a metal contact layer. The time-energy profile includes ramping up to a first energy level and holding for a period of time to create the desired pit size and density and then rapidly increasing the energy to a second level in which the entire interface area is melted and alloyed quickly. After holding the second energy level for a sufficient time to develop the thin alloy layer over the entire interface area, the energy is ramped down to allow epitaxial crystal growth in the alloy layer. The result is a textured backside an optically reflective, low resistivity alloy interface between the semiconductor substrate and the metal electrical contact layer.	National Renewable Energy Laboratory	10/25/1994 Issued
Patent 5,426,061	Impurity gettering in semiconductors A process for impurity gettering in a semiconductor substrate or device such as a silicon substrate or device. The process comprises hydrogenating the substrate or device at the back side thereof with sufficient intensity and for a time period sufficient to produce a damaged back side. Thereafter, the substrate or device is illuminated with electromagnetic radiation at an intensity and for a time period sufficient to cause the impurities to diffuse to the back side and alloy with a metal there present to form a contact and capture the impurities. The impurity gettering process also can function to simultaneously passivate defects within the substrate or device, with the defects likewise diffusing to the back side for simultaneous passivation. Simultaneously, substantially all hydrogen-induced damage on the back side of the substrate or device is likewise annihilated. Also taught is an alternate process comprising thermal treatment after hydrogenation of the substrate or device at a temperature of from about 500.degree. C. to about 700.degree. C. for a time period sufficient to cause the impurities to diffuse to the damaged back side thereof for subsequent capture by an alloying metal.	National Renewable Energy Laboratory	06/20/1995 Issued
Patent 5,577,157	Optical processing furnace with quartz muffle and diffuser plate An optical furnace for annealing a process wafer comprising a source of optical energy, a quartz muffle having a door to hold the wafer for processing, and a quartz diffuser plate to diffuse the light impinging on the quartz muffle; a feedback system with a light sensor located in the wall of the muffle is also provided for controlling the source of optical energy.	National Renewable Energy Laboratory	11/19/1996 Issued
Patent 5,897,331	High efficiency low cost thin film silicon solar cell design and method for making A semiconductor device having a substrate, a conductive intermediate layer deposited onto said substrate, wherein the intermediate layer serves as a back electrode, an optical reflector, and an interface for impurity gettering, and a semiconductor layer deposited onto said intermediate layer, wherein the semiconductor layer has a grain size at least as large as the layer thickness, and preferably about ten times the layer thickness. The device is formed by depositing a metal layer on a substrate, depositing a semiconductive material on the metal-coated substrate to produce a composite structure, and then optically processing the composite structure by illuminating it with infrared electromagnetic radiation according to a unique time-energy profile that first produces pits in the backside surface of the semiconductor material, then produces a thin, highly reflective, low resistivity alloy layer over the entire area of the interface between the semiconductor material and the metal layer, and finally produces a grain-enhanced semiconductor layer. The time-energy profile includes increasing the energy to a first energy level to initiate pit formation and create the desired pit size and density, then ramping up to a second energy level in which the entire device is heated to produce an interfacial melt, and finally reducing the energy to a third energy level and holding for a period of time to allow enhancement in the grain size of the semiconductor layer.	National Renewable Energy Laboratory	04/27/1999 Issued
Patent 5,304,509	Back-side hydrogenation technique for defect passivation in silicon solar cells A two-step back-side hydrogenation process includes the steps of first bombarding the back side of the silicon substrate with hydrogen ions with intensities and for a time sufficient to implant enough hydrogen atoms into the silicon substrate to potentially passivate substantially all of the defects and impurities in the silicon substrate, and then illuminating the silicon substrate with electromagnetic radiation to activate the implanted hydrogen, so that it can passivate the defects and impurities in the substrate. The illumination step also annihilates the hydrogen-induced defects. The illumination step is carried out according to a two-stage illumination schedule, the first or low-power stage of which subjects the substrate to electromagnetic radiation that has sufficient intensity to activate the implanted hydrogen, yet not drive the hydrogen from the substrate. The second or high-power illumination stage subjects the substrate to higher intensity electromagnetic radiation, which is sufficient to annihilate the hydrogen-induced defects and sinter/alloy the metal contacts.	National Renewable Energy Laboratory	04/19/1994 Issued
Patent 5,429,985	Fabrication of optically reflecting ohmic contacts for semiconductor devices A method is provided to produce a low-resistivity ohmic contact having high optical reflectivity on one side of a semiconductor device. The contact is formed by coating the semiconductor substrate with a thin metal film on the back reflecting side and then optically processing the wafer by illuminating it with electromagnetic radiation of a predetermined wavelength and energy level through the front side of the wafer for a predetermined period of time. This method produces a thin epitaxial alloy layer between the semiconductor substrate and the metal layer when a crystalline substrate is used. The alloy layer provides both a low-resistivity ohmic contact and high optical reflectance.	National Renewable Energy Laboratory	07/04/1995 Issued

Technology Status

Development Stage	Availability	Published	Last Updated
Production	Available	06/24/2011	06/24/2011