Patent Application
20180019169
Conventional techniques for manufacturing electronic devices, may involve the formation and manipulation of thin layers of materials. One example of such manipulation is the transfer of a thin layer of material from a first (donor) substrate to a second (target) substrate. This may be accomplished by placing a face of the donor substrate against a face of the target substrate, and then cleaving the thin layer of material along a sub-surface cleave plane formed in the donor substrate.
The donor substrate may comprise valuable, high quality crystalline material that is expensive to produce and may contain devices already processed onto a face. Thus, following such a layer transfer process, the donor substrate may be sought to be reclaimed for subsequent use in further layer transfer efforts. Accordingly, there is a need in the art for methods and apparatuses of processing a donor substrate to allow for its reclamation for subsequent layer transfer. There is also a need to mechanically and thermally stabilize a donor substrate to allow it to be subjected to high-power implant processes.
A donor substrate in a layer transfer process, is stabilized by attaching a backing substrate. When utilized within a high-power implant process, the assembly (donor and backing substrate) has enhanced mechanical stability and thermal heat spreading capabilities to allow for optimized backside heat extraction through convection or conduction mechanisms. Upon cleaving the donor substrate to release a thin layer of material to a target, the backing substrate prevents uncontrolled release of internal stress leading to buckling/fracture of the donor substrate. The internal stress may accumulate in the donor substrate due to processes such as cleave region formation, bonding to the target, and/or the cleaving process itself, with uncontrolled buckling/fracture potentially precluding reclamation/reuse of the donor substrate in subsequent layer transfer processes. In certain embodiments the backing substrate may exhibit a Coefficient of Thermal Expansion (CTE) substantially matching, or complementary to, that of the donor substrate. In some embodiments the backing structure may include a feature such as a lip, constraining lateral expansion of the donor substrate (e.g., in response to the application of thermal energy) and allowing mechanical fixturing of the assembly onto equipment such as a polishing or implant tool.
The process of transferring thin layers of material from a donor substrate to a target, may involve the formation of stress in the donor. For example, certain embodiments may employ bonding the donor substrate to a target, followed by controlled cleaving along a cleave region formed at a depth in the donor substrate.
Such a cleave region may result from the implantation of particles (e.g., hydrogen ions) into a face of the donor substrate. The resulting controlled cleaving may call for the application of energy to the donor substrate to initiate and/or propagate cleaving along the cleave region, leaving a thin film of transferred material remaining bonded to the target.
Stress in the donor may arise from a variety of sources. One possible source of stress may be formation of the cleave region. In particular, the energetic implantation of particles into the donor substrate creates a subsurface cleave region different from the surrounding material. This may give rise to stress at surface and subsurface locations.
In addition, it is noted that the cleave region itself may not be formed under uniform conditions, leading to internal stress. For example, edge/initiation portions of a cleave region may receive higher doses of implanted particles than other portions of the cleave region, leading to further stress within the donor substrate.
Another possible source of stress in the donor substrate may be the bonding process. In particular, the donor substrate may be exposed to conditions such as elevated temperatures, reduced pressures, and/or external energies (e.g., plasma) in order to accomplish bonding the implanted face of the donor substrate to the target. These conditions can give rise to internal stress being created within the donor substrate.
Still other sources of possible stress in the donor substrate may arise from the cleaving process itself. In particular, one or more forms of energy may be applied to the donor in order to release the thin layer along the cleave region. Examples of such energy can include but are not limited to thermal energy (e.g., an electron beam), optical energy (e.g., a laser), pneumatic energy (e.g., a pressurized gas jet), hydraulic energy (e.g., a pressurized water jet), and mechanical energy (e.g., application of a blade).
Moreover, certain controlled cleaving processes may involve an initiation phase creating a cleave front, followed by a propagation phase to cause the cleave front to migrate across the substrate, ultimately resulting in the complete detachment of a thin layer of material from the donor substrate. In such embodiments the same (or different) types of energy may be applied (at the same or different magnitudes) for cleave initiation, as are subsequently used to propagate a cleave front that has been formed.
Finally, it is noted that cleaving processes may operate differently in certain portions of the substrate than others. For example, the existence of a bevel in the target substrate may preclude contact with edge regions of the donor substrate. Upon cleaving, this can result in edge portions of the donor remaining bound to the donor rather than being transferred to the target. This and other phenomena associated with cleaving, can introduce stress internal to the donor substrate.
Following a cleaving process, it may be desired to reclaim the remainder of the donor substrate for subsequent reuse in the transfer of additional thin films. However, internal stress that has developed within a donor substrate due to one or more of the above processes (e.g., cleave region formation, bonding, cleave initiation/propagation), can interfere with efficient reclamation.
In particular, internal stress accumulated within the donor substrate can find relief by uncontrolled roughening, buckling, or even fracture of the donor material. This in turn can render the donor substrate unsuited for future use.
Accordingly, in order to provide stress relief and stabilization, embodiments propose the attachment of a backing substrate to the donor substrate.
In alternative embodiments, attachment of the backing substrate to the donor may be accomplished under irreversible conditions. There, it is not foreseen that the backing substrate will be released from the donor. Examples of such generally irreversible processes can include but are not limited to permanent adhesives, thermo-compression bonding, Transient Liquid-Phase (TLP) bonding and fit-based ceramic bonding.
Within an implant process performed under vacuum, the donor substrate/backing substrate assembly can allow for higher power density implants with less temperature excursion. These benefits occur by having a stiffer assembly that allows for more gas cooling backpressure and/or mechanical pressure to be applied on the backside for thermal heat dissipation. For example, at a 3×1017 H+/cm2 dose with about 100 pieces of 2-inch GaN substrates over a 4,000 cm2 area scanned by a 150 keV, 60 mA proton beam, the areal power density will be about 2.25 W/cm2. If a temperature rise of no more than 40° C. temperature is desired from the assembly to implant cooling plate, a thermal conductance of 0.056 W/cm2-K is required. Assuming no more than 25 um mechanical bending in the center, a backside pressure of 10 Torr is required (see
Center gap=0.696pr4/Et3 (1)
Where p=pressure in Pa, r=wafer radius, E=Young's Modulus of Elasticity, t=assembly thickness.
For this configuration, the required thickness is less than the GaN substrate thickness which is typically 400-500 μm. The conclusion is that for this application, reclamation and general handling of fragile GaN substrates in a production environment would dictate the required backing plate thickness. A 1 mm Mo plate would thus be sufficient to satisfy the implant conditions above. A slightly larger diameter of the backing plate would allow edge clamping of the assembly without contacting the 2″ (50.8 mm) GaN substrate.
For a deep implant using a high-power implant beam impinging on 300 mm silicon substrates, the minimum backing plate thickness can be substantial to avoid excessive plate bending during backside gas application. As an example, a deep (750 keV) proton implant at 60 mA over 4 300 mm silicon wafers would apply a thermal load of 45 kW over 4,000 cm2 area or about 11.2 W/cm2. Assuming no more than 100° C. temperature rise, a thermal conductance of 0.112 W/cm2-K is required. According to
The donor substrate supported by the backing substrate is now available for reclamation processes. Examples of such reclamation can include but are not limited to grinding, polishing, plasma exposure, wet chemical exposure, and/or thermal exposure.
The presence of the backing substrate supporting the donor substrate, may further serve to stabilize the latter during such reclamation processing. That is, stress arising from the application of energy to prepare the donor substrate surface for subsequent implant, may be addressed by the backing substrate to prevent uncontrolled stress release giving rise to buckling, fracture, etc.
It is further noted that the backing substrate should be compatible with exposure to the conditions under which reclamation is to take place. For example, where reclamation involves exposure to a plasma, the use of certain kinds of metal for the backing substrate may be discouraged in order to avoid arcing. In another example, where the reclamation involves etching conditions, the backing substrate should not comprise a material susceptible to degradation by repeated exposure to the etching conditions, to the point that it is unable to perform its stabilizing function.
In most applications, the backing substrate may comprise a material matching in thermal expansion coefficient over the temperature range of interest. This would limit deformations and temperature induced stresses that can lower yield and achievable specifications such as surface flatness. Materials such as molybdenum, tungsten, aluminum nitride, Mullite, sapphire, and CTE-matched glasses could satisfy criteria to be used as a backing plate material. Apart from mechanical flatness, CTE-matching and stiffness, making the backing plate slightly larger in diameter can also have practical benefits. In some applications this may also be advantageous to choose a material which is electrically conductive. To secure the backing plate assembly for implant or polishing operations without touching the GaN surface, a lip of backing material extending from the GaN edge would allow mechanical clamping. For most applications, a lip of millimeter scale would be sufficient.
One example of the use of a backing substrate for a donor, is now given in connection with the fabrication of hetero-structure layers and 3D-IC semiconductor devices. In some high-performance digital applications, an InGaAs layer is transferred onto a silicon substrate to form a 3D monolithic integration assembly. The implant energy in this application could be on the order of 50-300 keV. Also, in some 3D-IC stacking processes, high-energy proton implantation of 300 keV to 1 MeV and even 2 MeV are used to position a cleave plane well below the device layers to allow cleaving and transfer of a device layer onto a target substrate which collects multiple layers to form a 3D-IC structure. For both applications, the use of a backing substrate would allow for high-power implantation and efficient manufacturing without overheating the donor substrate.
One example of the use of a backing substrate for a donor, is now given in connection with the fabrication of an opto-electronic device. Specifically, semiconducting materials find many uses, for example in the formation of logic devices, solar cells, and increasingly, illumination.
One type of semiconductor device that can be used for illumination is the high-brightness light emitting diode (HB-LED). In contrast with traditional incandescent or even fluorescent lighting technology, HB-LED's offer significant advantages in terms of reduced power consumption and reliability.
An optoelectronic device such as a HB-LED may rely upon materials exhibiting semiconductor properties, including but not limited to type III/V materials such as gallium nitride (GaN) and GaAs is available in various degrees of crystalline order. However, these materials are often difficult to manufacture.
Accordingly,
The backing substrate 204 comprises a material that is compatible with the high-quality GaN material of the donor substrate. In certain embodiments, the backing substrate may exhibit a Coefficient of Thermal Expansion (CTE) that substantially matches, or is complementary to, that of the donor substrate.
Specifically, the backing substrate may exhibit properties that serve to accommodate and/or relieve internal stress arising in the donor substrate as a result of being subjected to one or more reclamation processes conducted in various environments. Examples of such reclamation processes can include but are not limited to, grinding, polishing, plasma or ion beam assisted etching, wet chemistry, thermal, vacuum, implantation, and others.
According to certain embodiments, the backing structure may include a feature such as a lip 206. A lip feature can serve to hold the donor/backing substrate assembly onto a platen or holder without contacting or covering the front face of the donor substrate. The backing structure also has a thickness, selected to satisfy the larger of a minimum thickness requirement from implant or reclamation processes.
Examples of possible approaches for fabricating a template suitable for high quality GaN growth, are described in U.S. provisional patent application No. 62/181,947 filed Jun. 19, 2015 (“the '947 provisional application”), and in U.S. nonprovisional patent application Ser. No. 15/186,184 filed Jun. 17, 2016, both of which are incorporated by reference in their entireties herein for all purposes.
In this example, a donor substrate 302 comprises high-quality GaN material. A backing substrate 303 is attached to the donor substrate.
A cleave region 304 is located at a sub-surface region of the donor substrate. This cleave region may be formed, for example, by the energetic implantation 305 of particles such as hydrogen ions, into one face of the GaN donor substrate.
Here, it is noted that the crystalline structure of the GaN donor substrate, results in it having two distinct faces: a Ga face 302a, and an N face 302b.
In a next step of the process of
The release layer may comprise a variety of materials capable of later separation under controlled conditions. As described in the '947 provisional application, candidate releasable materials can include those undergoing conversion from the solid phase to the liquid phase upon exposure to thermal energy within a selected range. Examples can include soldering systems, and systems for Thermal Lift Off (TLO).
In certain embodiments the release system may comprise silicon oxide. In particular embodiments this bond-and-release system can be formed by exposing the workpieces to oxidizing conditions. In some embodiments this bond-and-release system may be formed by the addition of oxide, e.g., as spin-on-glass (SOG), or other spin on material (e.g., XR-1541 hydrogen silsesquioxane electron beam spin-on resist available from Dow Corning), and/or SiO2 formed by sputtering or Plasma Enhanced Chemical Vapor Deposition (PECVD) techniques.
In a next step of the process of
Following cleaving of the GaN,
The Ga face is exposed and available for growth of additional high quality GaN material under desired conditions. Additional GaN may be formed by Metallo-Organic Chemical Vapor Deposition (MO-CVD), for example. That additional thickness of GaN material (with or without the accompanying substrate and/or dielectric material) may ultimately be incorporated into a larger optoelectronic device structure (such as a HB-LED).
Returning to the third (cleaving) step shown in
However, before such re-use can properly take place, the GaN donor substrate may need to first be reclaimed so that it is suitable for the intended processing. In particular, the Ga face of the donor substrate may exhibit properties such as surface roughness, defects, and/or non-planarity resulting from the previous cleaving step, that render it unsuitable for immediate implantation and bonding.
Accordingly, subjecting the donor substrate to reclamation may permit its re-use. Donor substrate reclamation procedures may comprise exposure to one or more of the following environments: grinding, polishing, plasma or ion beam assisted etching, wet chemistry, thermal, vacuum, and others.
In a second step 404, a backing substrate is attached to the donor substrate. In a third step 406, the donor substrate attached to the backing substrate, is exposed to conditions giving rise to internal stress. The presence of the backing substrate serves to stabilize the donor substrate under these conditions, thereby allowing reclamation of the donor substrate in connection with subsequent processing.
Such a reclamation is shown as step 408 in
When mechanical processes are used for reclamation such as grinding, polishing and CMP, the donor assembly (backing and donor substrates) may need to meet flatness and stiffness requirements. Depending on the stresses generated by the prior processes, a donor substrate could exhibit excessive bow and warp that can result in non-uniform reclamation of the donor surface. After attaching a backing substrate of minimum stiffness, the donor surface is stabilized in flatness and can be reclaimed in a manner that meets surface specifications. As an example, a 2″ diameter GaN substrate of 470 um thickness was modeled using finite element analysis. The GaN substrate was given an initial bow value of 74 um (center to edge bow across the principal face). This level of bow is representative of a stress level of approximately 700 MPa extending 5 um into the GaN substrate from the top surface. This represents a stress state of the GaN substrate that must be removed through reclaim. Attaching a backing substrate can allow uniform lapping, polishing and CMP processes to remove this stress layer by lowering the bow value to about the same order as the target layer removal value (in this case about 5 um). When bonded to a 3 mm Mo backing substrate, the bow is reduced from 74 um to 3.9 um. A 5 mm Mo backing substrate would reduce the bow to 1.6 um. Bow reduction of this magnitude would make the reclamation processes uniform and predictable.
Returning to
However, other embodiments are possible. For example some applications (e.g., power electronics) may call for growth of GaN material from the N face, rather than from the Ga face. Incorporated by reference herein for all purposes are the following articles: Xun Li et al., “Properties of GaN layers grown on N-face free-standing GaN substrates”, Journal of Crystal Growth 413, 81-85 (2015); A. R. A. Zauner et al., “Homo-epitaxial growth on the N-face of GaN single crystals: the influence of the misorientation on the surface morphology”, Journal of Crystal Growth 240, 14-21 (2002). Accordingly, template blank structures of some embodiments could feature a GaN layer having an N face that is exposed, rather than a Ga face. Alternatively, an N face donor assembly could be used to fabricate a Ga face final substrate when bonded to a final substrate instead of a releasable transfer substrate as in
Such embodiments could be particularly amenable to the use of a backing substrate to stabilize the donor substrate after cleaving. In particular, the N-face of a GaN crystal is more chemically reactive compared to the Ga-face. Accordingly, the presence of a backing substrate could serve to flatten the assembly and reduce undesired enhanced etching of surfaces due to bow and warp high areas acting upon the CMP processes.
While the above discussion has focused upon the use of a backing substrate for GaN transfer processes, embodiments are not limited to such approaches. Certain embodiments may employ a backing substrate for fabrication processes involving a different Group III/V material such as GaAs. In particular embodiments, sapphire may be particularly suited to serve as a backing substrate for the transfer of GaAs material from a donor.
While the above is a full description of the specific embodiments, various modifications, alternative constructions and equivalents may be used. Although the above has been described using a selected sequence of steps, any combination of any elements of steps described as well as others may be used. Additionally, certain steps may be combined and/or eliminated depending upon the embodiment. Furthermore, the particles of hydrogen can be replaced using co-implantation of helium and hydrogen ions or deuterium and hydrogen ions to allow for formation of the cleave region with a modified dose and/or cleaving properties according to alternative embodiments. Still further, the particles can be introduced by techniques such as a diffusion process rather than an implantation process. Of course there can be other variations, modifications, and alternatives. Therefore, the above description and illustrations should not be taken as limiting the scope of the present invention which is defined by the appended claims.
The instant nonprovisional patent application claims priority to U.S. Provisional Patent Appl. No. 62/361,468 filed Jul. 12, 2016 and incorporated by reference herein in its entirety for all purposes.
| Number | Date | Country | |
|---|---|---|---|
| 62361468 | Jul 2016 | US |
