Patent Application
20130020582
Light emitting diodes (LEDs) are typically fabricated using standard semiconductor packaging techniques. Die are mounted using epoxy or solder onto a submount, interconnect is via wirebonding or flip chip methods, and then organic encapsulants are formed over the assembly. While this approach is useful in low intensity applications, the cost and performance levels are insufficient for general lighting applications. Optoelectronics, electronics, solar, and sensors applications can also benefit from lower cost higher performance devices. The need therefore exists for novel fabrication methods, which reduce cost and enable electronic, optical, and optoelectronic applications.
Presently rapid thermal annealing creates ohmic contacts in nitride based devices. In this process various metals are deposited on p and n doped layers of the devices. Temperatures in excess of 650 degrees C. are then rapidly applied to the devices in the presence of a variety of atmospheric conditions. The resulting formation of conductive oxides and/or diffusion effects creates the ohmic contact. This contact formation, however, forms a very thin diffusion based layer which is susceptible to aging and environmental effects. The need therefore exists for more robust ohmic contacts to doped nitride layers. The thinness of the diffusional layers also limits what type of subsequent metal contacts can be used. This is unlike the solar cell industry in which thick film silver paste can be used rather than expensive vapor deposited metals as required in the LED industry. Vertical devices are preferred like the solar cell industry. The need however exists in the LED industry and nitride industry in general for economical methods of forming vertical nitride devices.
It is important to differentiate the freestanding nitride veneers disclosed in this invention from template and bulk nitride wafers. Templates are typically nitride layers grown on a non-native substrate, such as sapphire, SiC, silicon, or other single crystal substrates, with a reasonable lattice match to nitrides. In this case, device growth is done on a bimorph structure of two dissimilar materials. The lattice mismatch and the thermal expansion coefficient mismatches dictate that significant bow exists either during room temperature processing or growth processing. Bow at room temperature adversely affects yield for contact formation and liftoff processes. Bow at growth temperature leads to non-uniform device growth. As an example, a 2 inch 30 micron HVPE grown GaN template on sapphire can exhibit greater than 200 microns of bow at growth temperature if it is substantial flat at room temperature. This effect is even pronounced at thicker layers or larger diameter wafers. Excessive bow can also lead to wafer cracking which can lead to reactor damage.
The bimorph nature of a template also limits ramping times for any processes due to the potential of cracking of the whole wafer or the nitride film. This leads to increased reactor process times and compromises on device structures. The typical MQW is only 10s of angstroms thick. The reactor must rapidly change both process gases and temperature in order for a useful structure to be made. These rapid temperature changes will crack even thin templates, especially for 3 and 4 inch wafers. As a simple example, a flexible freestanding nitride can be heated white hot using a butane torch in a matter of seconds. Heating rates of 1000 degrees C. per second have been demonstrated using flexible freestanding nitride foils. If the same thing is done to a template, the template will shatter violently. The use of freestanding nitride veneers eliminates all these issues because they are substantially homogeneous, provide a lattice match and are flexible in nature.
Alternately, bulk nitride wafers are extremely expensive and must be surface polished which introduces surface defects. As disclosed by Dmitriev in US Pending Patent Application No. 20060280668, bulk wafers can be grown using a multiple step process that includes formation of a seed on a non-native substrate, removal of the seed form the non-native substrate, polishing and cleaning of the seed, regrowth using HVPE on the seed to form a boule with a thickness greater than 5 mm, slicing of the boule into wafers, and polishing the wafers to make an epi-ready surface. Also because the bulk nitride wafers are sliced from a bowed thick growth, a variable miscut is created when a flat wafer is made. Since growth conditions are different for various miscut angles, the result is a reduction in useable surface area on the wafer. The thick nature of bulk nitride wafers and the processing required to make them generates very high stress gradients within the wafers themselves. In contrast the flexible nitride veneers are low stress and have a uniform crystal orientation across the surface of the veneer.
Lastly, any useful device will require thinning to reduce the thermal impedance of the device. Doubling the thickness doubles the temperature delta across the layer. The same can be said for series resistance in vertical devices and optical absorption in optical devices. In all these cases, the thicker the device the layer the performance. The need therefore exists for the disclosure of devices, methods, and equipment which is specifically design to take advantage of the benefits that freestanding nitride veneers offer.
This invention discloses the use of high temperature bonding and interconnect methods for devices based on freestanding nitride veneers. The use of inorganic glasses is a preferred embodiment of this invention. Even more preferred is the use of inorganic glasses which exhibit a CTE of between 20 and 100/C. Most preferred is the use of inorganic glasses, which exhibit a CTE, which substantially matches the CTE of the freestanding nitride devices being packaged. The use of heating means includes, but is not limited to, laser welding, brazing, ovens, kilns, torches, furnaces, and IR lamps to melt the inorganic glasses such that bonding occurs between the inorganic glasses and the freestanding nitride devices. This bonding step can adhere an electrical interconnect means to at least one surface of the freestanding nitride devices. The electrical interconnect means may consist of, but is not limited to, a wire, foil, rod, or ball. Glass sealing metals, such as Kovar, dumet, and platinum, can ensure compatibility with the inorganic glass. The ability to melt bond contacts onto the freestanding nitride devices using inorganic glasses is disclosed. In this manner contacts and/or full/or partial encapsulation of the freestanding nitride devices can be realized very rapidly. Unlike organic solutions, inorganic glasses can provide hermetic sealing of the freestanding nitride devices or at least the contact regions of the freestanding nitride devices. For LED and other optoelectronic devices the use inorganic glasses is critical to preventing solarization, yellowing, and other degradation effects that plague existing high intensity LED applications. In order for high temperature processing to be possible the LED die themselves must be capable of being processed at these high temperatures.
In the method disclosed by the authors in U.S. Pat. Nos. 7,727,790 and 8,163,582 (included by reference to this disclosure) flexible freestanding nitride veneers are harvested with an epi ready surface. By using the freestanding nitride veneer, subsequent growth can occur on an epi-ready surface, which does not require any additional polishing steps. In addition the substantially all nitride nature of this approach enables the high temperature thermal processing disclosed in this filing. By eliminating waferbonding and/or bimorphic nature of other nitride device fabrication techniques high temperature processes are made possible. The flexible nature of the freestanding nitride veneer allows for release and control of the stresses created in the nitride layer during initial growth and in subsequent high temperature processing steps. As also disclosed previously by the authors, this effect can be used to modify spectral output, current droop, as well as other device parameters. In general, the flexible freestanding nitride veneer allows the device designer some level of control over the spontaneous, piezoelectric, and induced polarization fields, which dominate nitride device performance. There are also indications that the lower surface stress in flexible freestanding nitride films enable epitaxial growth of materials with large lattice mismatches and enhanced indium incorporation compared to either template or bulk nitride wafers.
Previously disclosed by the authors are methods for rapid epitaxial growth of the nitride semiconductors based on novel reactor design and the use of freestanding nitride films. Based on this approach, typical epi growth cycle time can be reduced by up to a factor of 10. This approach uses a novel freestanding nitride veneer, which is substantially all nitride based. The intent of this invention is to disclose methods and approaches for very high speed packaging of the resulting freestanding nitride semiconductor devices. These techniques are enabled by the freestanding nature of the nitride veneer, which enables the use of high temperature glass encapsulation, rapid epi growth, novel device structures, and new interconnect means. This approach also allows for hermetically sealed devices, especially with regard to LEDs and laser diodes. The freestanding nature of the devices enables the use of these techniques in a wide range of applications ranging from illumination to 3D stacked semiconductors.
This invention also discloses the use of the freestanding nature of the nitride veneer. Freestanding nitride veneers provide access to both sides of the veneer, do not require additional thinning processes, can be laser cut, can be attached to non-flat surface, can be flexed during or after device growth, can be cleaved along polar, non-polar, and semi-polar crystal planes and can be processed at very high temperature. The use of these advantages in device structures, subsequent processing and equipment design are embodiments of this invention.
This invention covers methods and devices based on using freestanding nitride layer 16 as an epitaxial growth substrate. By using freestanding nitride layer 16 as the growth substrate for subsequent epitaxial growths, improved device performance is possible. In the cases where the sapphire is still attached to nitride layer, significant stresses are always present. This is due to the lattice and thermal mismatches that always exist between the growth substrate and the epitaxially grown layer. This is based on effects of the strain on the quantum wells and various other layers. Active region 17 in this case is grown on freestanding nitride layer 16. Because freestanding nitride layer 16 does not required additional polishing and is flexible in nature the growth quality of the active region 17 can be improved. In addition the flexible nature of the freestanding nitride layer 16 allows for modification of the spontaneous, piezoelectric and induced polarization fields in the device being grown. Active region 17 typically consists of, but is not limited to, a PN junction, MOSFET, MESFET, HEMT, single or double heterojunction, and/or quantum wells or dots layers. Active region 17 may function as a LED, laser diode, solar cell, diode, HEMT, FET, as well as other electronic and optoelectronic devices. InGaN, InAlGaN, AlGaN, or other dilute nitride alloys are used to create the active region in the case of LEDs. By epitaxially growing on a freestanding nitride layer 16, the stresses within the active region 17 can be reduced. Not only does the freestanding nitride layer 16 provide a better lattice and thermal match for the active region 17 but freestanding nitride layer 16 also typically exhibits less dislocation defects than thin template based approaches.
Contact layer 15 may consist of but not limited to transparent conductive oxides, nitrides, and other high temperature coatings. More preferably contact layer 15 is an epitaxially grown transparent conductive oxide. Most preferably contact layer 15 is doped zinc oxide. The contact layer 15 protects the backside of freestanding nitride layer 16 during subsequent growth processes. The contact layer 15 simultaneously serves as current spreading layer for the device and protects the freestanding nitride layer 16 during subsequent growth processes. Contact layer 15 consists of and/or contains a luminescent element.
The contact layer 15 may be patterned to be used as a etch mask for the freestanding nitride layer 16. The use of sequential depositions for contact layer 15 allows that contact layer 15 to consist of substantially different materials spatially distributed across freestanding nitride layer 16. The formation of color pixels based on sequential depositions of contact layer 15 on freestanding nitride layer 16 is also disclosed. It is an embodiment of this invention the use of high temperature processing in excess of 1000 degrees C. for freestanding nitride layer 16 and contact layer 15. This enables the formation of high quality luminescent materials and/or contact layers, which can not be done due to temperature limitation of active layer 17. Active region 17 cannot be processed at temperatures much above 1000 degrees C. due to diffusional effects and stability of the nitride alloys typically used. It is therefore an embodiment of this invention that contact layer 15 and freestanding nitride layer 16 can be processed at temperature greater than 1000 degrees C.
Luminescent properties in particular can be enhanced/activated only through the use of high temperature annealing in controlled atmospheres. As such the use of annealing steps to enhance luminescent properties of Contact layer 15 on freestanding nitride layer 16 within a controlled atmosphere prior to subsequent epitaxial growths is an embodiment of this invention. The annealed contact layer 15 and freestanding nitride layer 16 which is luminescent is an embodiment of this invention. The luminescent contact layer 15 and freestanding nitride layer 16 can be used as a growth substrate for making a light emitting device. The formation of the active region 17 after the formation of the luminescent contact layer 15 is an embodiment of this invention. In this manner, high temperature processing of the luminescent material can be done without degrading the LED or other optoelectronic device.
Similarly, it has been previously disclosed by the authors that the use of freestanding nitride layer 16 enables high temperature device formation followed by lower temperature device formation for solar cell and electronic applications. In general, the freestanding nitride layer 16 can be used as a both a high temperature nitride growth substrate and a subsequent low temperature growth substrate either on the same side as the high temperature growth substrate or the other side of the high temperature growth substrate. As an example, high quality nitride solar cells can be grown on freestanding nitride layer 16 followed by lower temperature silicon, GaAs, as well as other low bandgap materials. The resulting integrated multi junction solar cell does not suffer from the process constraints of nitride on silicon approaches where the nitride device growth adversely affects the underlying silicon devices.
Vias can be formed by etching, laser ablation, mechanical means, as well as cutting means, to enable interconnects between devices grown on different sides of freestanding nitride layer 16. Subwavelength structures cane be formed including, but not limited to quantum dots, gratings, diffusers, and polarization elements, on either and/or both sides of contact layer 15 and freestanding nitride layer 16 prior to subsequent growth processes. Addressing elements can be formed on or within contact layer 15.
Freestanding nitride layer 16 may consist of n type, p type, and/or semi-insulating material. Freestanding nitride layer 16 maybe uniformly doped, gradient doped, and stepwise doped. The annealing processes on freestanding nitride layer 16 reduce bowing, improve doping uniformity, and modify surface morphology. The formation of surface texture using but not limited to laser patterning, lithography, chemical etching, and/or mechanical means as known in the art to improve extraction efficiency, enhances epitaxial growth (e.g. lateral overgrowth etc.) and/or modifies the stresses in nitride layer 16. Using these techniques an enhanced growth substrate is disclosed. Subsequent growth steps including active region 17, barrier layer 18, and doped layer 19 are used to form the desired device.
Because the growth substrate is substantially an all nitride layer 16, flexible very rapid thermal processing can be used to improve the interfaces between the subsequent growth layers. This becomes critical especially in the cases where quantum wells are being formed. The various layers must exhibit significant changes in composition in layers, which are only a few nanometers thick. This requires rapid changes in the growth conditions at the epitaxial surface. In the case of nitrides, growth temperatures determine the composition of the layers. As an example, 20% indium content InGaN requires a much lower growth temperature than GaN. Since MQWs typically consist of alternating layers of various nitride alloys 100s of degrees C. temperature shifts must occur in seconds. The combination of low thermal mass, thinness, and high thermal conductivity enables freestanding nitride layer 16 with or without contact layer 15 enables the formation of improved device structures. The use of nitride layer 16 with or without contact layer 15 as an enhanced growth substrate to allow for more rapid changes in growth conditions is an embodiment of this invention. Most preferred is a freestanding nitride layer 16 with or without contact layer 15, which is less than 100 microns thick. Even more preferred is a freestanding nitride layer 16 with or without contact layer 15, which is less than 50 microns thick.
Contact layer 20 consists of but is not limited to, transparent conductive oxide, luminescent layer, and/or active addressing element. The use of degenerative doping levels in one or both contact layers 15 and 20 is also an embodiment of this invention. The epitaxial growth methods forms contact layer 15 and 20 for reduced alpha. The epitaxial growth of contact layer 15 and 20 via MOCVD either separately or simultaneously is also an embodiment of this invention. Mechanical, laser, etching and waterjet means can scribe, cut and/or break the freestanding nitride semiconductor layer into smaller devices. More preferably, the cleaving along cleave planes can form triangular and/or triangular based shapes.
While the invention has been described in conjunction with specific embodiments and examples, it is evident to those skilled in the art that many alternatives, modifications and variations will be apparent in light of the foregoing description. Accordingly, the invention is intended to embrace all such alternatives, modifications and variations as fall within the spirit and scope of the appended claims.
This application claims the benefit of U.S. Provisional Patent Application Ser. No. 61/572,768, which was filed on Jul. 21, 2011, which is herein incorporated by reference.
| Number | Date | Country | |
|---|---|---|---|
| 61572768 | Jul 2011 | US |
