The invention concerns the usage of hybrid wafers consisting of a thin Si layer attached via wafer bonding to an underlying highly thermally conductive substrate, thereby constituting an advanced silicon-on-insulator wafer. Specifically, the invention concerns the fabrication of electronic devices in the Si layer, with the underlying substrate serving as a heat spreader not only for such Si devices, but also for electronics fabricated as discrete devices and then attached to the heat spreader via soldering or to the Si devices via bump bonding.
Heat buildup is an important concern for many modern electronics. High temperatures can lead to leakage currents and parasitic capacitances, which can in turn lead to device failures such as CMOS latch-up. Silicon-on-insulator (SOI) wafers avoid many of these problems by permitting individual circuit elements to be electronically isolated, which allows for higher operational temperatures (and not coincidentally, radiation hardness). However, the introduction of an electrically isolating material (typically SiO2) also decreases the overall thermal conductivity of the SOI Wafer compared to conventional Si.
Embodiments of the present invention concern an advanced form of SOI in which the underlying handle material has sufficiently high thermal conductivity to overcome any thermal resistance introduced by the intermediate isolation layer, and therefore the handle acts as a heat spreader. Furthermore, both SiC and diamond (two materials proposed for the heat spreader in accordance with at least some embodiments of the present invention) have lower dielectric constants than Si (9.7 ∈r and 5.5 ∈r respectively, versus 11.7 ∈r) and therefore using these materials as the handling substrate of an advanced SOI wafer means that thinner layers of intermediate oxide are necessary to obtain the same protection from parasitic capacitances as provided by conventional SOI with Si as the handle material.
Certain embodiments of the present invention concern the use of the advanced SOI material as more than just a means to dissipate heat from electronics fabricated in the Si layer. At least some embodiments of the present invention concern the advanced SOI material as a technology platform for combining diverse electronic elements into a compact package and dissipating heat from all such elements. Electronics may be fabricated in the underlying heat spreader material itself, either prior to or after Si layer bonding. Additionally, high-power compound semiconductor electronic devices fabricated separately may be directly integrated with Si control electronics fabricated in the advanced SOI wafer by means of contact pads and bump bonding. Finally, following fabrication of the advanced SOI wafer, sections of the Si layer may be removed to allow discrete high power electronic devices to be directly soldered to the underlying heat spreader.
The term “heat spreader” is used to differentiate embodiments of the present invention from conventional heat sinks. The larger size of the heat-spreading material compared to the attached electronics allows for heat to dissipate laterally before being transferred into a conventional heat sink. Accordingly, a heat spreader may be attached to a heat sink. The high thermal conductivity of the heat spreader material, such as SiC (k˜3.5 W/cmK) also means that no wafer thinning is necessary to enhance thermal dissipation, as the heat sink itself (typically Cu or Al) will not exhibit higher thermal conductivity than the heat spreader. Therefore the heat spreader itself can be used as a handling material during device fabrication.
A wafer bonding technique has been developed for creating hybrid wafers consisting of a thin (˜1 μm) film of Si on bulk SiC. Successfully fabricated hybrid Si-on-SiC wafers may employ an intermediate layer of SiO2 between the Si and SiC. The thickness of the SiO2 layer is controllable by the wafer fabrication process, and can range from 60 nm to 400 nm, but the process should provide for infinitely variable thickness. Several different polytypes and grades of SiC have been used.
Electronic elements can be fabricated in the Si layers of the hybrid Si-on-SiC wafers. Experiments with these elements have shown the Si layers to be device grade in terms of mobility. Furthermore, high-resolution cross-sectional transmission electron microscopy has shown the Si layer to be monocrystalline in nature, and that the SiC/SiO2 and SiO2/Si interfaces are uniform and substantially without voids.
Testing of the electronic elements has shown the device-to-device isolation properties of the hybrid Si-on-SiC wafers to be equal to that of conventional SOI wafers. Self-heating electronic elements fabricated in the Si layer exhibited up to 50% lower device temperatures at given power levels than identical elements fabricated in conventional SOI and bulk Si, thus proving the merit of the underlying SiC as a heat spreader.
A high power SiC BJT device can be bump-bonded to electrical contacts fabricated on the Si layer of a hybrid Si-on-SiC wafer. In such an embodiment, the BJT exhibits 125% higher gain levels in this configuration than when tested as a bare die, further indicating the heat-spreading properties of the underlying SiC. In testing, this same BJT was then removed and soldered directly to a bare piece of SiC, where it exhibited 150% higher gain levels than as a bare die.
Certain embodiments of the present invention concern the utilization of the hybrid Si-on-SiC wafer not only as an advanced form of SOI, but as a platform technology which allows Si electronics fabricated in the wafer itself to be closely incorporated with discrete electronic devices attached either by bump bonding or soldering. At least some embodiments of the present invention also concern future developments of this platform technology, such as integrating electronics fabricated in the underlying SiC itself (either before or after the wafer bonding process), utilizing other highly thermally conducive materials such as diamond (having a thermal conductivity of k˜18 W/cmK to k˜20 W/cmK, depending upon the purity of the diamond) for the hybrid wafer handle, and replacing the intermediate SiO2 layer with a more thermally conductive insulating material such as SiNx.
It is thus one aspect of the present invention to provide for the attachment of discrete electronic dies to electronics in the Si layer of advanced SOI wafers by means of bump bonding.
It is another aspect of the present invention to provide for the attachment of discrete electronic dies to the handle material of advanced SOI wafers by means of backside soldering.
It is still another aspect of the present invention to provide for the fabrication of electronics in the handle material of advanced SOI wafers prior to wafer fabrication.
It is still another aspect of the present invention to provide for the fabrication of electronics in the handle material of advanced SOI wafers after wafer fabrication.
It is still another aspect of the present invention to provide for a combination of the above-described aspects.
It is still another aspect of the present invention to incorporate discrete electronic dies with Si-layer electronics in advanced SOI wafers by means of wire bonding.
It is still another aspect of the present invention to incorporate the handle material electronics with Si-layer electronics in advanced SOI wafers by means of wire bonding.
It is still another aspect of the present invention to utilize oversized (larger in area than the constituent electronics) advanced SOI chips in conjunction with certain embodiments of the present invention to permit lateral heat spreading before dissipation into a heat sink.
It is still another aspect of the present invention to utilize any highly-thermally conductive material as the handle material for the advanced SOI wafer.
It is still another aspect of the present invention to utilize any highly thermally conductive, electrically insulating material as the intermediate layer in the advanced SOI wafer.
The summary is not intended to provide an exhaustive description of all embodiments of the present invention. Namely, additional features and advantages of embodiments of the present invention will become more readily apparent from the following description, particularly when taken together with the accompanying drawings.
Methods for incorporating electronics fabricated in the Si layer of an advanced SOI wafer with high power electronic devices of a different material, such as SiC, GaAs or any other semiconductor element or compound are provided. At least some embodiments of the present invention allow for the electronics to be combined in close proximity, thereby reducing overall device footprint and potentially decreasing signal noise between the Si electronics and the high power devices. The underlying highly thermally conductive handle material of the advanced SOI wafer serves as a heat spreader for both the Si electronics and the incorporated high-power electronics. By providing a thermally conductive handle layer that effectively spreads the heat away from the Si electronics and incorporated high-power electronics, embodiments of the present invention allow Si-based electronics to operate at a greater efficiency, for a longer time, all while decreasing the risk of having the electronics fail.
The incorporation of the high-power electronics with the Si electronics can be accomplished in a number of different ways in accordance with at least some embodiments of the present invention. Three possible ways of incorporating the high-power electronics with the Si electronics will be discussed in further detail herein without intending to limit the scope of the invention.
Referring initially to
Electronics 7 fabricated in the discrete device are shown on the bottom side of a discrete semiconductor electronics die 6. These may be directly incorporated by bump bonding (flip-chip) or solder connections 8 with electronics 5 fabricated in the Si layer of the hybrid wafer.
Referring now to
The semiconductor electronics 7 can be separately fabricated and cut into a die, the electrical contacts of which are then flip-chip bump-bonded 8 to contacts on the Si electronics 5. This allows the two individual electronic circuits 5 and 7 to be directly incorporated with each other without wire bonds. However, if the high-power device 6, 7 possesses backside contacts, these may be connected to contacts in the Si electronics by wire bonding. As an example, with this method a high-power SiC transistor die could be connected to Si gate control electronics to create a power converter.
With reference now to
As can be seen in
It should be appreciated that the figures are not necessarily drawn to scale. For example, the handle material 4 of a hybrid wafer 1 may be about 300 μm in thickness, while the Si layer 2 may be about 1 μm.
Certain embodiments of the present invention also cover any combination of the above three methods, such as a device containing both backside-soldered and bump-bonded high-power dies. Furthermore, at least some embodiments of the present invention cover not only high-power applications, but any application where the above described methods improve device performance.
An X-band GaAs amplifier was backside-soldered to a SiC substrate to test the concept of heat spreading in relation to certain embodiments of the present invention. The M/A-COM MAAPGM0079 GaAs die used for this experiment is a three stage GaAs power amplifier utilizing field-effect transistors (FETs) fabricated with silicon-like manufacturing processes. It is rated at 20 watts of saturated output power over the 8-10 GHz band and has a power added efficiency rating of 30% (meaning that 70% of the combined RF and DC input power goes directly into waste heat which must be removed from the device to avoid performance degradation or failure).
Semi-insulating B-grade 6H on-axis SiC substrates measuring 360 μm thick were diced into squares measuring 12.76×12.76 mm for use in these experiments. A SiC square was coated with 1 μm of Au to prevent signal loss into the SiC. The GaAs amplifier was soldered directly upon the Au-coated SiC with an 80/20 Au/Sn solder perform.
The SiC substrate and the attached die were then epoxied directly upon a copper housing. Epoxy was employed rather than solder to avoid problems associated with the differences in coefficient of thermal expansion between SiC and Cu. The epoxy used was H20E-HC, which has a relatively high thermal conductivity (for such materials) of 9 W/mK. The thickness of the epoxy layer after adhesion of the SiC to the Cu was approximately 50 μm. The copper housing was attached directly to an aluminum heat sink, which was cooled via the small fan. No thermal grease or solder was employed between the Cu and Al.
For the testing, the gate voltage was set to the factory-recommended −2.2 V while the drain voltage was ramped to the recommended 10 V. The gate voltage was then adjusted to set the quiescent drain current (no RF input) to optimal levels. RF input was then applied at 9 GHz at various power levels to observe the resulting power output. No signal tuning was attempted.
It is apparent from
While saturation power in FETs is highly dependent upon device temperature, FET gain levels are far less temperature sensitive. This is apparent in both our test of the SiC-bonded amplifier and the factory specs by the relatively unchanging slope in the curves before saturation power is achieved; if temperature was impacting gain in a significant way, these slopes would curve downward gradually over their entire range, rather than the abrupt change that is observed when saturation power is reached. Therefore the lower gain levels seen in the SiC-mounted substrate are likely due to a testing parameter. Because the device is three-stage, this difference of approximately 5 dBm between the SiC-mounted and unmounted (factory) performance translates into a difference of approximately 1.71 dBm per stage (the lower gain at each stage is compounded by the next one, i.e.: 1.713=5). This amount is typical for the variance that a single-stage device might exhibit depending on the level of tuning that is performed. Given that no device tuning was performed in our tests, and that it is likely that M/A-COM tuned their devices as well as possible for the die's published performance levels, it is not unexpected that the gain levels revealed by our tests would not equal the factory claims.
A SiC Bipolar Junction Transistor (BJT) die was backside-soldered to a SiC substrate to test the concept of heat spreading in relation to certain embodiments of the present invention. The BJT die used for these tests was a non-commercially available experimental device supplied by Mirosemi, Inc. The die measured 1×3 mm and approximately 300 μm thick, and consisted of six individual BJT structures.
Semi-insulating D-grade 6H on-axis SiC substrates measuring 360 μm thick were cleaved into pieces measuring approximately 5×5 mm for use in these experiments. A SiC piece was coated with 100 nm of Au to allow for solder adhesion. The SiC die was soldered directly upon the Au-coated SiC with an 80/20 Au/Sn solder perform.
Electronic testing of the active die was performed with the bonded SiC piece sitting directly on the vacuum chuck of the same parameter analyzer previously used to test the die on-wafer and as flip-chip bump-bonded to a hybrid Si-on-SiC wafer (both tests had been performed for an earlier research project). The vacuum chuck consists of a large steel plate and serves as a heat sink for the device being tested. The original results of the on-wafer BJT and as a cut die bump-bonded to hybrid Si-on-SiC can be seen in
In general, as device temperature increases, device performance is impaired and transistor gain levels drop. Therefore, the gain levels exhibited by a transistor can be used to examine how well certain device configurations and mountings dissipate heat. The I-V curves seen in
In an effort to determine the overall effect that lateral heat spreading has on the performance of an on-wafer device, the backside-bonded die was removed from the bulk SiC and tested directly on the stage. The removal was achieved by heating the bonded die and SiC substrate to over 282 C to re-melt the solder; the fact that the die could not be laterally displaced on the substrate with tweezers until the melting point had been reached is further evidence of the robustness of the backside bonding technique. The I-V test results of the unbonded die can be seen in
The fact that even the bump-bonded results are superior to those of the unbonded die suggests that placing a very high thermal conductivity material below a power device can significantly improve performance, even with the presence of an intermediate heat ‘bottleneck’ such as the bump bonds themselves. The bump-bonding technique could still prove useful for situations where direct incorporation of a power device to Si electronics is desired without any wire-bonding. However, as
While the above-descriptions have been discussed in relation to a particular sequence of events, it should be appreciated that changes to this sequence can occur without materially effecting the operation of the invention. Additionally, the exact sequence of events need not occur as set forth in the exemplary embodiments. The exemplary techniques illustrated herein are not limited to the specifically illustrated embodiments but can also be utilized with the other exemplary embodiments and each described feature is individually and separately claimable.
In addition to or in place of the described power converter and radar T/R module systems, methods and protocols of this invention can be implemented on electronics such as are used in a special purpose computer, a programmed microprocessor or microcontroller and peripheral integrated circuit element(s), an ASIC or other integrated circuit, a digital signal processor, a hard-wired electronic or logic circuit such as discrete element circuit, a programmable logic device such as PLD, PLA, FPGA, PAL, a communications device, such as a phone, any comparable means, or the like.
It is therefore apparent that there has been provided, in accordance with the present invention, systems and methods for creating and utilizing advanced hybrid-wafer electronics. While this invention has been described in conjunction with a number of embodiments, it is evident that many alternatives, modifications and variations would be or are apparent to those of ordinary skill in the applicable arts. Accordingly, it is intended to embrace all such alternatives, modifications, equivalents and variations that are within the spirit and scope of this invention.
Number | Date | Country | |
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60940122 | May 2007 | US |