The intrinsic properties of both wide bandgap (WBG) and ultrawide bandgap (UWBG) semiconductor materials make them suitable for a wide range of other applications in High-power and radio frequency (RF) electronics, deep ultraviolet (DUV) optoelectronics, quantum electronics, and high-temperature harsh environment applications. However, the full performance of these materials cannot be achieved due to the thick substrates they are typically grown so as to ensure single crystal quality. These substrates include Si, SiC, sapphire, AlN, and Ga2O3 among others, and the thickness is typically ˜400-500 μm. Although these thick substrates are convenient for growing and handling these semiconductors, they limit the figures-of-merits (FOMs) for device performance in the following application areas:
Thus, the removal of these thick substrates allows eliminating the above-mentioned drawbacks. However, handling thin active layers without damage is a challenge. There is currently no product that addresses this problem in the field of wide bandgap semiconductors. We propose the following approaches that have not been applied to UWBG power and RF devices. In some cases, these technologies have not been applied to DUV optics.
Presently disclosed subject matter relates to a novel laser lift-off (LLO) technique for AlN lift-off from sapphire substrates, which is highly desired for UWBG AlGaN HEMTs, which are always grown with AlN buffer layers.
Aspects and advantages of the presently disclosed subject matter will be set forth in part in the following description, or may be apparent from the description, or may be learned through practice of the presently disclosed subject matter.
Broadly speaking, the presently disclosed subject matter relates to laser-based lift-off (LLO) of high-electron mobility transistors (HEMTs). More particularly, the present disclosure is related to excimer laser lift-off of AlGaN/GaN HEMTs on thick AlN heat spreaders.
Still other aspects of the presently disclosed subject matter relate to innovations in advanced laser lift-off in GaN. For example, in some instances, presently disclosed subject matter relates to the transfer of wide and ultra-wide bandgap layers to engineered substrates.
Further, for example, we developed a novel laser lift-off (LLO) technique for AlN lift-off from sapphire substrates, which is highly desired for UWBG AlGaN HEMTs, which are always grown with AlN buffer layers.
This disclosure allows us to improve the thermal management and efficacy of power electronics devices in demanding applications that require high-current density operation such as hybrid electric vehicle (HEV) and aerospace applications by enabling us to manufacture smaller, lighter, more efficient high-power UWBG semiconductor devices in a cost-effective manner.
This technique also allows the full realization of III-nitride's potential in flexible electronics, such as power amplifiers for antenna/transmitters embedded in smartphone touchscreens, as well as in DUV optics for application areas such as COVID-19 disinfection, air treatment, and phototherapy, etc. A III-nitride compound semiconductor is formed due to the bonding of one of the group III elements, such as boron, aluminum, gallium or indium, with the group V element, nitrogen. The III-nitrides and their alloys are direct band gap semiconductors.
Further and generally speaking, this disclosure addresses the main performance/reliability problem in WBG and UWBG power devices. AlGaN/GaN HEMTs penetrated consumer electronics with first-order applications. However, the performance of the devices is currently limited by severe self-heating effects, which significantly reduce their efficacy in demanding applications that require high-current density operation. This requires an efficient heat dissipation strategy in high power handling devices employed in HEV, aircraft electronic systems, etc. Moreover, power devices are typically rated in $/Ampere from a commodities standpoint. By doubling the power handling with our LLO approach, we have effectively demonstrated how to cut the cost by 50%. In addition, improvements may be achieved through further optimization of die-attach.
Moreover, flexible devices are desired for several applications, such as biosensors, smartwatches, virtual reality (VR) and augmented reality (AR) devices, smart jackets, power amplifiers for antenna/transmitters embedded in smartphone touchscreens, etc. Removing thick substrate and transferring onto a flexible substrate with this approach can be applied to realize flexible electronics where III-nitride has not played a big role.
Laser Lift-Off (LLO) refers generally to a process where thin layers are separated from a substrate, such as polymers from glass, by use of a homogenized UV wavelength laser beam. One broad aspect of successful LLO is that the substrate be transparent to the working wavelength, and the target layer be opaque.
One basic concept behind laser lift-off generally is the difference in absorption of the laser light by one layer, typically a host substrate, and the filmstack being separated. For laser lift-off generally, a laser light source is projected through a transparent material and is absorbed in an adjacent material on the backside, such as GaN on sapphire. Confined plasma at the interface results in lift-off or separation of the materials. In the case of LEDs for example, short wavelength laser light passes through the sapphire and ablates the interface when it couples with the GaN, thereby releasing the two materials.
In the case of monolithic advanced lift-off in GaN, shorter wavelengths/higher energy photons are better absorbed, resulting in shallower optical penetration depth in certain materials such as GaN. UV pulses can be used for separation of thin film semiconductor materials when the heat penetration depth needs to be minimal.
Generally, an excimer laser is a form of ultraviolet laser sometimes used in the production of microelectronic devices, semiconductor-based integrated circuits, eye surgery, and micromachining. Aluminum nitride (AlN) is a technical ceramic material that features a combination of very high thermal conductivity and favorable electrical insulation properties. Such combination makes aluminum nitride suitable for use in power and microelectronics applications, for example, used as a circuit carrier (substrate) in semiconductors or as a heat-sink in LED lighting technology or high-power electronics. III nitride materials generally comprise AlN, GaN, InN, and their solid solutions.
The presently disclosed subject matter provides the following advantages:
The presently disclosed subject matter could be useful to companies that manufacture high-power semiconductor devices seeking an alternative material for SiC as substrate and that can reduce the cost significantly while providing similar performance metrics of devices. That list of companies could include developers of specialized devices and sensors for health care and military applications.
It is to be understood from the complete disclosure herewith that the presently disclosed subject matter equally relates to both apparatus and corresponding and related methodology and/or to devices made with such methods and methodology.
One presently disclosed exemplary methodology preferably relates to a method for transferring wide and ultrawide bandgap (WBG and UWBG) layers to an engineered substrate, comprising performing laser-based lift-off (LLO) on high-electron mobility transistors (HEMTs) with AlN heat spreading buffer layers grown over sapphire substrate material, to remove the sapphire substrate material; and applying a carrier substrate to the heat spreading buffer layers using a bonding agent, to collectively form an engineered substrate.
Another presently disclosed exemplary method relates to a double transfer method for fabricating WBG and UWBG semiconductor devices without requiring a final polishing step. Such method preferably may comprise forming AlGaN/GaN HEMTs on a layer of AlN heat spreaders having a thickness of at least 10 μm, grown over sapphire substrate materials; applying excimer laser lift-off to remove the sapphire substrate materials to expose the layer of AlN heat spreaders; and using a bonding agent to apply a heat sink layer to the exposed layer of AlN heat spreaders. Per such method, first transferring off the sapphire substrate materials and subsequently transferring on a heat sink layer results in engineered formation of WBG and UWBG power devices.
Yet another presently disclosed exemplary embodiment relates to methodology for forming a layered substrate, comprising performing laser-based lift-off (LLO) on AlGaN high-electron mobility transistors (HEMTs) with ceramic heat-spreading buffer layers having relatively high-thermal conductivity, and grown over sapphire substrate material, to remove the sapphire substrate material; and applying a copper heat sink to the ceramic heat spreading buffer layers using a bonding agent, to collectively form an engineered layered substrate.
Other example aspects of the present disclosure are directed to systems, apparatus, tangible, non-transitory computer-readable media, user interfaces, memory devices, and electronic smart devices or the like. To implement methodology and technology herewith, one or more processors may be provided, programmed to perform the steps and functions as called for by the presently disclosed subject matter, as will be understood by those of ordinary skill in the art.
Additional objects and advantages of the presently disclosed subject matter are set forth in, or will be apparent to, those of ordinary skill in the art from the detailed description herein. Also, it should be further appreciated that modifications and variations to the specifically illustrated, referred and discussed features, elements, and steps hereof may be practiced in various embodiments, uses, and practices of the presently disclosed subject matter without departing from the spirit and scope of the subject matter. Variations may include, but are not limited to, substitution of equivalent means, features, or steps for those illustrated, referenced, or discussed, and the functional, operational, or positional reversal of various parts, features, steps, or the like.
Still further, it is to be understood that different embodiments, as well as different presently preferred embodiments, of the presently disclosed subject matter may include various combinations or configurations of presently disclosed features, steps, or elements, or their equivalents (including combinations of features, parts, or steps or configurations thereof not expressly shown in the figures or stated in the detailed description of such figures). Additional embodiments of the presently disclosed subject matter, not necessarily expressed in the summarized section, may include and incorporate various combinations of aspects of features, components, or steps referenced in the summarized objects above, and/or other features, components, or steps as otherwise discussed in this application. Those of ordinary skill in the art will better appreciate the features and aspects of such embodiments, and others, upon review of the remainder of the specification, and will appreciate that the presently disclosed subject matter applies equally to corresponding methodologies as associated with practice of any of the present exemplary devices, and vice versa.
A full and enabling disclosure of the presently disclosed subject matter, including the best mode thereof, directed to one of ordinary skill in the art, is set forth in the specification, which makes reference to the appended Figures, in which:
Repeat use of reference characters in the present specification and figures is intended to represent the same or analogous features or elements or steps of the presently disclosed subject matter.
It is to be understood by one of ordinary skill in the art that the present disclosure is a description of exemplary embodiments only and is not intended as limiting the broader aspects of the disclosed subject matter. Each example is provided by way of explanation of the presently disclosed subject matter, not limitation of the presently disclosed subject matter. In fact, it will be apparent to those skilled in the art that various modifications and variations can be made in the presently disclosed subject matter without departing from the scope or spirit of the presently disclosed subject matter. For instance, features illustrated or described as part of one embodiment can be used with another embodiment to yield a still further embodiment. Thus, it is intended that the presently disclosed subject matter covers such modifications and variations as come within the scope of the appended claims and their equivalents.
The present disclosure is generally directed to laser-based lift-off (LLO) of High-electron mobility transistors (HEMTs). More particularly, the present disclosure is related to excimer laser lift-off of AlGaN/GaN HEMTs on thick AlN heat spreaders.
Further, the present disclosure in some instances relates to the use of 193-nm excimer laser-based lift-off (LLO) of Al0.26Ga0.74N/GaN high-electron mobility transistors (HEMTs) with thick (t>10 μm) AlN heat spreading buffer layers grown over sapphire substrates. The use of the thick AlN heat spreading layer resulted in thermal resistance (Rth) of 16 Kmm/W for as-fabricated devices on sapphire, which is lower than the value of ≈25-50 Kmm/W for standard HEMT structures on sapphire without the heat-spreaders. Soldering the LLO devices onto a copper heat sink led to a further reduction of Rth to 8 Kmm/W, a value comparable to published measurements on bulk SiC substrates. The reduction in Rth by LLO and bonding to copper led to significantly reduced self-heating and drain current droop. A drain current density as high as 0.9 A/mm was observed despite a marginal reduction of the carrier mobility (≈1800 to ≈1500 cm2/Vs). This is the highest drain current density and mobility reported to-date for LLO AlGaN/GaN HEMTs.
AlGaN/GaN High-electron mobility transistors (HEMTs) have come a long way since their initial demonstration in 1993 and are desired for a multitude of applications in high-frequency and high-temperature power electronics.[1-8] Recently, AlGaN/GaN HEMTs penetrated the consumer electronics with first-order applications.[9,10] However, the performance of the devices is currently limited by severe self-heating effects that significantly reduce their efficacy in demanding applications that require high current density operation. One strategy to reduce the self-heating effects of GaN-based HEMTs is to use high thermal conductivity SiC or bulk AlN substrates. However, the cost of these substrates is ≈3-10 times that of sapphire substrates.[11] Hence, strategies to improve the thermal management of the devices are highly desired for the full realization of III-nitride-based device's potential in power electronics.
One promising approach for better thermal management of HEMTs on sapphire substrates is the LLO and bonding to a substrate with higher thermal conductivity. This approach has been used for visible InGaN and ultraviolet (UV) AlGaN LEDs[12-19] and HEMTs (Table 1).[20-26] The laser lifted-off devices are typically mounted on an Si, AlN, or a metallic heat sink, such as copper, commonly used in power electronics.[27,28] This leads to further challenges in assuring bonding with low thermal impedance and preserving the structural integrity of the III-nitride epi-layers. If the thickness of the III-nitride layer is small compared to the solder thickness of 10-50 μm, it may wrinkle, crack, and be damaged during the solder reflow. However, too thick an epilayer can also introduce more thermal resistance. Ultraviolet LEDs with typical epilayer thicknesses of 2-3 μm when flip-chipped by LLO[15] are also susceptible to cracking. Due to this damage, LLO HEMTs typically are not soldered directly to highly thermally conductive metallic heat sinks.[21-25]
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355
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No field effect mobility was able to be extracted due to unavailability of device dimensions.
indicates data missing or illegible when filed
Further, for example, we developed a novel laser lift-off (LLO) technique for AlN lift-off from sapphire substrates, which is highly desired for UWBG AlGaN HEMTs which are always grown with AlN buffer layers. The general approach is shown in present
Appropriately sized trenches are required to remove the generated N2 gas during laser exposure. In our studies, ˜1 mm2 dimensions were suitable to be large enough for practical applications and for scaling (laser spot size ˜1 mm2) while being small enough to provide sufficient area for removal of N2 gas generated during the excimer laser decomposition of UWBG III-N. The HEMT epilayers were then grown on these SAG AlN template, which consists of a 3 μm undoped GaN channel layer and a 30 nm delta-doped Al0.26Ga0.74N layer with a 1 nm AlN spacer in between. Ohmic contact metal stack Ti/Al/Ti/Au (150/700/300/500 Å) was e-beam evaporated and annealed for 30 seconds at 950° C. under N2 followed by gate-stack Ni/Au (1000/2000 Å) metallization. The metal contact side of the sample was bonded to a UV tape, and the sapphire was removed by LLO. The lifted-off AlN surface was then cleaned with 1:1 dilute HCl and Cl2/Ar ICP. The lifted-off surface was bonded to a copper heat sink substrate using In—Pb solder by thermocompression bonding, and the UV tape was removed. The In—Pb solder temperature (˜175° C.) is low enough to be compatible with flexible electronics. The incorporation of thick AlN heat spreading buffer layer led to a thermal resistance (Rth) of ˜16 K-mm/W for as-fabricated devices on sapphire, which decreased further down to ˜8 K-mm/W, comparable to published measurements on SiC substrates, after transferring the devices onto a copper heat sink due to the high intrinsic thermal conductivity of AlN and removal of large series Rth of the sapphire substrate. Self-heating induced current droop in as-fabricated HEMTs on sapphire is significantly reduced after transfer onto copper heat sink. Moreover, the intermediary AlN layer provided physical integrity during the transfer preventing damage.
The mechanical transfer of WBG and UWBG active layers involves using 2D materials such as boron, nitride, graphene, MoS2 as a sacrificial release layer. This technique allows the realization of devices on large, flexible, and affordable foreign substrates on which direct growth of nitride semiconductors of sufficient quality is problematic. This technique has been used with limited success for LEDs [59,60,61,62] and could be useful for power electronics as well. Depending on the application, engineered substrates are required, e.g., Gorilla® glass for smartphones, polyethylene terephthalate (PET) for flexible electronics.
This technique is used to transfer very fine layers of crystalline materials from a donor substrate onto a mechanical support using the Smart-Cut™ technology (Soitec® patented) and has been used for Johnson's FOM improvements in silicon [63,64,65]. This approach has not been applied to III-nitrides but could be applicable to AlN substrates leading to a transformative reduction in cost in UWBG AlGaN devices, as multiple engineered High-quality AlN templates can be produced from a single wafer. Our double transfer approach eliminates the need for a final polishing step which is essential for smart-cut technology [Alam et. al (accepted)].
For effective soldering to copper heat sink, Ill-nitride epilayers >10 μm thick are required. We recently demonstrated the growth of such thick ultra-wide bandgap (UWBG) AlN layers on sapphire substrates with a room temperature thermal conductivity 320 W/m-K.[29,30] This is much higher than the measured thermal conductivity values for GaN.[31-33] These thick AlN/sapphire templates, therefore, not only are a suitable high-thermal conductivity platform for AlGaN/GaN HEMTs but can also provide protection during the soldering of lifted-off devices to copper heat sink. However, it is more difficult to release AlN than GaN from the sapphire substrate because of its hardness and higher melting temperature.[15] It also requires a high-fluence short wavelength deep ultraviolet (DUV) λ=193 nm excimer laser. The hardness and the high-laser fluence lift-off invariably lead to excessive layer cracking. Developing LLO techniques for AlN lift-off from sapphire substrates is also highly desired for UWBG AlxGa1-xN (x>0.6) HEMTs, which are always grown with AlN buffer layers. Thus, many previously demonstrated LLO approaches (Table 1) for AlGaN/GaN HEMTs are not applicable to emerging UWBG IIIN devices.[34-36]
In this disclosure, we demonstrate the LLO of AlGaN/GaN HEMTs that were fabricated with >10 μm-thick high-quality AlN buffer layers on sapphire substrates. The lifted-off layers were then soldered to copper heat sink to improve their capability to operate at high-drain currents without a thermal droop attributed to self-heating.[37] We show that the thermal performance is improved substantially and is like that of devices on bulk SiC substrates, the current gold-standard in heat sinks.
The AlGaN/GaN heterostructures used in this study were grown on c-plane sapphire by metalorganic chemical vapor deposition (MOCVD). A 2-μm AlN seed layer was first grown followed by the selective area growth (SAG) of 14 μm-thick AlN in 1×1 mm2 window openings in a SiO2 masking layer. The SiO2 mask was then etched off using HF, and the first 2 μm-thick AlN seed layer was also etched down by inductively coupled plasma (ICP), leaving a template with fully disconnected 16 μm-thick 1×1 mm2 blocks of AlN on the sapphire substrate. HEMT epilayers were then grown on these SAG AlN templates by MOCVD, with a 3 μm undoped GaN channel layer and a 30 nm delta doped Al0.26Ga0.74N layer with a 1 nm AlN spacer in between. Delta doping was done by sandwiching a 10 nm Si-doped Al0.26Ga0.74N layer between two undoped 10 nm Al0.26Ga0.74N layers. Delta doping separates the dopants from the AlGaN/GaN 2 DEG interface enabling higher sheet carrier concentration (ns), while minimizing carrier-impurity scattering that provides enhanced carrier mobility at high ns.[38] These effects lead to an overall lowering of the sheet resistance. The device source/drain ohmic contact metal stack Ti/Al/Ti/Au (150/700/300/500 Å) was e-beam evaporated and annealed for 30 s at 950° C. under N2. This was followed by the gate-stack Ni/Au (1000/2000 Å) metallization. Source-to-drain spacing was 6 μm, with a gate length of ≈2 μm.
For the LLO process, the epitaxial side of the processed sample was bonded to UV tape, and a 193-nm excimer laser fluence of ≈1 J/cm2 was used. This yielded HEMT devices with 16 μm-thick AlN heat spreading layers. The lifted-off surface was etched with 1:1 dilute HCl and Cl2/Ar ICP to remove the damaged AlN layer. The sample was then transferred to copper using thermocompression bonding. The In—Pb solder temperature (≈175° C.) [39] is low enough to be compatible with flexible electronics. This procedure is schematically represented in
This strain relaxation is supported by HRXRD (
where q is the electron charge, VT is the threshold voltage, CG1 is the gate capacitance per unit area; and VGS is the gate-source voltage.
Rth was measured using thermochromic paint that changes its color for a certain temperature under steady state electrical power. From
ΔT=RthP, (2)
where ΔT is the channel temperature rise and P is the applied power.
The as-fabricated devices on sapphire show Rth of ≈16K mm/W, which is lower than the typical 25-50K mm/W [44-47] seen in GaN, HEMTs grown directly on sapphire. This lower Rth is attributed to the better heat spreading in the ˜16 μm-thick AlN due to its high intrinsic thermal conductivity.[29,30] AlN layers <6 μm-thick showed ≈½ the thermal conductivity compared to the thicker films, leading to less effective heat removal attributed to poorer AlN quality at the sapphire/AlN interface.[29] After LLO and soldering to the copper heat sink, Rth is ≈8K mm/W, which is comparable to or less than the 10K mm/W for SiC substrates[45,46,48] using steady state techniques. The remaining Rth after sapphire removal and transfer onto copper heat sink is likely dominated by the poor thermal conductivity of In—Pb die-attach solder [≈41 W/mK [49] compared to the excellent thermal conductivities of AlN (≈320 W/mK) [29-30] and copper (≈386 W/mK). [50]
The carrier mobility (μn) is extracted from the IDS-VGS transfer curves (
where gm is the transconductance, L is the gate-length, and W is the width.
LLO of Al0.26Ga0.74N/GaN HEMT with >10 μm-thick AlN templates from sapphire substrate was performed by a 193-nm ArF excimer laser and transferred onto a copper heat sink bonded by In—Pb solder. Incorporating a thick AlN heat spreading buffer layer instead of GaN led to a Rth of ≈16K mm/W for as-fabricated devices on sapphire, which decreased further down to ≈8 Kmm/W, comparable to published measurements on SiC substrates, after transferring the devices onto a copper heat sink. This is due to improved heat spreading in the thick AlN buffer with high intrinsic thermal conductivity and removal of large series Rth of the sapphire substrate. After LLO, the mobility decreased from ≈1800 to ≈1500 cm2/Vs due to the introduction of traps during transfer. Drain current droop attributed to self-heating in as-fabricated HEMTs on sapphire is significantly reduced after transfer onto copper heat sink.
The following discussion refers to material for the Raman mapping images of both GaN E2 (High) and AlN E2 (High) mode in the access regions of both as-fabricated and LLO HEMT structures; lattice constants a and c of different epitaxial films, as well as this sample before and after LLO determined by HRXRD measurements; room temperature Raman shifts vs corresponding residual stress change indicated by both E2 (High) and A1 (Lo) modes; and TLM measurement results and transfer characteristics before and after LLO.
This written description uses examples to disclose the presently disclosed subject matter, including the best mode, and also to enable any person skilled in the art to practice the presently disclosed subject matter, including making and using any devices or systems and performing any incorporated methods. The patentable scope of the presently disclosed subject matter is defined by the claims and may include other examples that occur to those skilled in the art. Such other examples are intended to be within the scope of the claims if they include structural and/or step elements that do not differ from the literal language of the claims, or if they include equivalent structural and/or elements with insubstantial differences from the literal languages of the claims.
The present application claims the benefit of priority of U.S. Provisional Patent Application No. 63/300,729, titled “Transfer of Wide and Ultrawide Bandgap Layers to Engineered Substrate,” filed Jan. 19, 2022, which is fully incorporated herein by reference for all purposes.
This disclosure was made with government support under W911NF-18-1-0029, awarded by ARMY/ARO; N00014-18-1-2429, awarded by ONR; and 1711322, 1810116, and 1831954, awarded by NSF ECCS. The government has certain rights in the disclosure.
Number | Date | Country | |
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63300729 | Jan 2022 | US |