Method of making suspended thin-film semiconductor piezoelectric devices

A process for forming a very thin suspended layer of piezoelectric material of thickness less than 10 microns. The device is made from a combination of GaAs and AlGaAs layers to form either a sensor or an electronic filter. Onto a GaAs substrate is epitaxially deposited a thin (1-5 micron) sacrificial AlGaAs layer, followed by a thin GaAs top layer. In one embodiment the substrate is selectively etched away from below until the AlGaAs layer is reached. Then a second selective etch removes the sacrificial AlGaAs layer, that has acted here as an etch stop, leaving the thin suspended layer of piezoelectric GaAs. In another embodiment, a pattern of small openings is etched through the thin layer of GaAs on top of the device to expose the sacrificial AlGaAs layer. A second selective etch is done through these openings to remove the sacrificial AlGaAs layer, leaving the top GaAs layer suspended over the GaAs substrate. A novel etchant solution containing a surface tension reducing agent is utilized to remove the AlGaAs while preventing buildup of gas bubbles that would otherwise break the thin GaAs layer.

CROSS-REFERENCE TO RELATED APPLICATIONS
 Not applicable.
 BACKGROUND OF THE INVENTION
 This invention relates to methods of making very thin, suspended layers of
 compound semiconductor materials, typically utilizing the GaAs/AlGaAs
 system. More particularly, this invention relates to methods of etching
 compound semiconductors to achieve thicknesses in the range of 10 microns
 or less for relatively large surface area layers. Still more particularly,
 this invention relates to methods of forming acoustic wave chemical
 microsensors and high frequency electronic filters made by these
 micromachining techniques.
 GaAs and quartz have been used as the piezoelectric acoustic wave elements
 for chemical sensors for many years. If a coating that is selective for a
 chemical analyte of interest is placed on the surface of the piezoelectric
 element and the coating is then presented to a fluid mixture that may
 contain the analyte, the resonant frequency of the coated sensor will
 change as the analyte builds up on the element. One class of these sensors
 is of the type known as surface acoustic wave (SAW) sensors in which a
 relatively thick substrate layer of quartz or GaAs is utilized. In this
 class of sensors, the acoustic wavelength is small compared to the
 substrate thickness. The chemical sensitivity of the SAW device scales
 inversely with acoustic wavelength and is therefor greatest for the
 smallest possible wavelength. The SAW wavelength is determined by the
 width of and the spacing between the interdigitated electrodes used to
 drive the acoustic wave in the crystal. Therefor, the chemical sensitivity
 of the SAW device is limited by the resolution of the microlithography
 process that sets a lower limit on the acoustic wavelength. In commonly
 used configurations, the area occupied by this sensor scales as the square
 of the acoustic wavelength, decreasing as the wavelength decreases and the
 sensitivity increases. The acoustic frequency of this sensor is determined
 by the acoustic wavelength of the device and the acoustic velocity of the
 substrate material such that the frequency increases as the wavelength
 decreases and the sensitivity increases.
 A second class of these sensors is of the type known as flexural plate wave
 (FPW) sensors. This class of sensors differs from SAW sensors in that the
 acoustic wavelength is comparable to or greater than the thickness of the
 substrate. For this class of sensors, the chemical sensitivity of the
 sensor increases as the thickness of the substrate decreases with constant
 acoustic wavelength. Therefor, the chemical sensitivity is limited by the
 ability to make thin substrates and is independent of the microlithography
 process used to form the interdigitated electrodes. The frequency of this
 device decreases for decreasing substrate thickness and increasing
 sensitivity. As with the SAW sensor, the area occupied by this sensor
 scales as the square of the wavelength.
 A third class of these sensors is of the type known as thickness shear mode
 (TSM) sensors in which again a relatively thick substrate layer of quartz
 or GaAs is utilized in typical devices. In this class of sensors, the
 chemical sensitivity scales inversely with the thickness of the substrate,
 increasing for thinner substrates. As with the FPW sensor, the chemical
 sensitivity is limited by the ability to make thin substrates and is
 independent of the microlithography process used to form the electrodes.
 In commonly used configurations, the area occupied by this device scales
 inversely with the sensitivity, decreasing with increasing sensitivity and
 decreasing substrate thickness.
 There exists a need in the art for a process to create thinner
 piezoelectric layers to increase chemical sensor sensitivity and to
 decrease area occupied by the sensor. In some applications, this need is
 coupled with an additional need to decrease the sensor frequency while in
 other applications this need is coupled with an additional need to
 increase the sensor frequency. Further, there exists a need in the art for
 a process to create acoustic wave chemical microsensors with increased
 performance in a manner that is compatible with the monolithic integration
 of microelectronic circuits that can control the sensors and extract data
 from them.
 These same piezoelectric materials can also be used as signal processing
 and signal conditioning components in high frequency electronic circuit
 applications, particularly filters. The same structures used in
 microsensor devices, namely SAW, FPW, and TSM structures, provide the
 signal processing and conditioning function. In this case, the devices do
 not require the application of chemically selective layers. As with the
 sensors, the operating frequency for some of these devices will increase
 as the substrate thickness decreases and the area occupied by these
 devices will decrease as the substrate thickness decreases. As with the
 sensors, there exists a need in the art for a process to create thinner
 piezoelectric layers to increase the operating frequency and reduce the
 size of electronic filters. In addition, there exists a need for the
 monolithic integration of the improved filters with microelectronic
 circuits.
 BRIEF SUMMARY OF THE INVENTION
 This invention is a process for constructing chemical microsensors and
 electronic circuit filters, among other things, on thin regions of
 piezoelectric compound semiconductor substrates in a manner that is
 compatible with the monolithic integration of microelectronic circuits.
 The thinning process produces piezoelectric material that is sufficiently
 thinner than other methods, resulting in devices with characteristics that
 are improved many-fold when compared to existing devices.
 In brief, first and second epitaxial layers are grown on a substrate. In
 one embodiment, the backside of the substrate is selectively patterned and
 etched away to expose the base of the first epitaxial layer. The first
 epitaxial layer is then selectively etched away from below to leave only
 the second epitaxial layer, which is then contacted by electrodes either
 on its top side only or on both its top and bottom sides. In another
 embodiment, the top of the second epitaxial layer is selectively patterned
 and etched down to the level of the first epitaxial layer. A second
 etching solution is then introduced through the openings in the second
 epitaxial layer to etch away the first epitaxial layer in the regions
 proximate to the etched openings in the second epitaxial layer. As a
 result, the second layer, in the area between the etched openings therein,
 is suspended above the substrate. The thickness of the suspended layer is
 typically less than about 10 microns and the ratio of the length of the
 suspended layer to its thickness is typically greater than 100:1.
 Electrodes are then emplaced either on the top side or on the top and
 bottom sides thereof. In either embodiment, this remaining second
 epitaxial layer can be made wide, long and uniformly thin to optimize its
 acoustic properties.

DETAILED DESCRIPTION OF THE INVENTION
 Piezoelectric chemical microsensors and piezoelectric electronic filters
 can be represented as a layer of piezoelectric material whose thickness is
 small compared to its lateral extent, coated on either one or both faces
 by an electrically conducting material, typically a metal, that makes
 electrical connection to the piezoelectric material. This is shown
 schematically in FIG. 1. As mentioned above, device performance improves
 for some classes of sensors and filters as the thickness of the
 piezoelectric layer is reduced. For compound semiconductor materials that,
 in general, are piezoelectric, it is possible to produce thin layers of
 piezoelectric material on a suitable substrate through processes
 collectively referred to as epitaxial growth. Epitaxial growth alone
 cannot produce the required thin piezoelectric material because the
 effective thickness of the piezoelectric material is the sum of the
 epitaxial layer thickness, which is quite thin, and the substrate
 thickness, which is relatively thick, as illustrated in FIG. 2 for a
 conventional SAW device. However, the proper and unique combination of
 substrate selection, epitaxial growth, photolithographic patterning,
 pattern etching, and metal deposition, all established processes in
 compound semiconductor microelectronics fabrication technology set forth
 herein, can produce isolated regions of the thin epitaxial layer that are
 similar in cross section to the ideal case shown in FIG. 1. One embodiment
 of the process results in the structure shown in FIG. 3 (but with a
 sacrificial epitaxial layer omitted for simplicity).
 The lateral dimensions of the thin piezoelectric layer must be large enough
 to contain the device of interest. Given how thin the piezoelectric layer
 must be for improved device performance, this is a non-trivial constraint.
 The mechanical strength of the piezoelectric layer must be sufficient to
 survive the fabrication process and device use.
 One embodiment of the invention is illustrated in the structure shown in
 FIG. 4. In this example, a gallium arsenide (GaAs) substrate has had two
 epitaxial layers grown sequentially on one surface of the substrate. The
 first layer is an aluminum gallium arsenide (AlGaAs) alloy that is
 typically a few microns thick and contains at least 37 atomic percent
 aluminum (Al). A GaAs epitaxial layer, also a few microns thick, is grown
 on top of the AlGaAs layer. Both GaAs and AlGaAs are piezoelectric. This
 GaAs/AlGaAs/GaAs sandwich is the starting point for two different
 embodiments of this invention for producing thin suspended piezoelectric
 material.
 In the first embodiment, the exposed bottom surface of the GaAs substrate
 is photolithographically patterned to protect all but a selected region of
 this surface. This exposed region is then etched in a chemical mixture
 (for example, citric acid, hydrogen peroxide and water will work, as will
 other known etching solutions for GaAs) that dissolves GaAs much faster
 than AlGaAs. This has the effect of etching through the GaAs substrate and
 stopping on the AlGaAs layer. The etchant is then changed to one that
 dissolves the AlGaAs much faster than GaAs (for example, hydrofluoric
 acid). This removes the AlGaAs layer and leaves only the top most GaAs
 epitaxial layer suspended over the opening in the GaAs substrate. This is
 shown in FIG. 5. The resulting suspended GaAs layer has an extremely
 uniform thickness, owing to the thickness control of the epitaxial growth
 process and the highly selective hydrofluoric acid etch (not shown). Metal
 electrodes, typically gold, are then deposited on the top surface or on
 both the top and bottom surfaces of the GaAs epitaxial layer to form the
 microsensor or filter. Other compound semiconductors besides GaAs can be
 used for the suspended layer, e.g., indium gallium arsenide (InGaAs). The
 criterion in this embodiment is that the alternative compound
 semiconductor have a reaction to the various etchants that is similar to
 that of GaAs.
 A second approach to achieve this embodiment can be accomplished with the
 growth on the GaAs substrate of a single epitaxial layer of AlGaAs that is
 typically a few microns thick and contains at least 37 atomic percent Al.
 As before, the exposed bottom surface of the GaAs substrate is
 photolithographically patterned to protect all but a selected region of
 this surface. This exposed region is then etched in a chemical mixture
 (for example, citric acid, hydrogen peroxide and water will work, as will
 other known etching solutions for GaAs) that dissolves GaAs much faster
 than AlGaAs. This has the effect of etching through the GaAs substrate and
 stopping on the AlGaAs layer. This leaves only the AlGaAs layer suspended
 over the opening in the GaAs substrate. However, this approach suffers
 from the relatively poor selectivity of the currently available GaAs
 etches that results in an AlGaAs layer of non-uniform thickness, when
 compared to the uniformity of the GaAs layer in the first approach. In
 some cases, a non-uniform AlGaAs layer may not be suitable for microsensor
 and electronic filter applications. Nevertheless, AlGaAs is a better
 piezoelectric material that is GaAs. AlGaAs can be made with higher
 resistivity. Both these characteristics result in a higher performance
 piezoelectric layer when compared to GaAs. Where the higher performance of
 an AlGaAs suspended layer is needed, the non-uniformity of the AlGaAs
 suspended layer becomes less of a factor in the choice of this design.
 Thin piezoelectric GaAs layers have been fabricated using the method of
 this first embodiment. FIG. 6 is a scanning electron microphotograph of
 the bottom side of the suspended GaAs epitaxial layer, viewed through the
 opening that was etched through the substrate and the AlGaAs epitaxial
 layers. The GaAs substrate is 100 microns thick, and the AlGaAs and GaAs
 epitaxial layers are each 1 micron thick. The AlGaAs layer is not visible
 in this view because it has been removed by etching. The opening in the
 substrate is about 1 mm by 1 mm.
 In a second embodiment, one etches away the AlGaAs layer by creating
 openings in the top GaAs epitaxial layer (instead of the GaAs substrate as
 is done in the first embodiment), leaving the substrate intact with the
 GaAs epitaxial layer suspended above it. The GaAs epitaxial layer is
 photolithographically patterned to protect all but small openings in the
 mask over the top surface of this layer. The appropriate GaAs etch
 solution is then applied to the mask-covered GaAs epitaxial layer with the
 result that corresponding small openings are created that extend through
 the GaAs epitaxial layer down to or through the top surface of the AlGaAs
 layer. A selective etchant is then used to dissolve the AlGaAs layer
 beneath the GaAs epitaxial layer in the regions proximate to the small
 openings with the effect of completely undercutting the GaAs epitaxial
 layer in the area when it is desired to create the suspended GaAs
 epitaxial layer. This technique is shown in top and side views in FIGS. 7A
 and 7B respectively. Metal electrodes are then deposited on the GaAs
 epitaxial layer. For some types of microsensors and filters, for example,
 microsensors based on flexural plate wave acoustic devices, it is
 sufficient to deposit metal only on the top surface of the GaAs epitaxial
 layer. This can be accomplished by any number of means widely known in the
 microelectronics industry. Metal deposition on both top and bottom
 surfaces of the GaAs epitaxial layer could be achieved with chemical vapor
 deposition techniques, with the edges of the GaAs epitaxial layer being
 treated to isolate the metal electrodes on the top and bottom surfaces.
 Other options might include electroless deposition on both surfaces or gas
 phase deposition by light activation or heating of the suspended layer,
 followed again by treatment to isolate the respective electrodes. An
 alternative approach would be to produce a thin region of conducting GaAs
 on the bottom of the epitaxial layer by either ion implantation techniques
 or by epitaxial growth of a highly doped GaAs layer during the initial
 epitaxial growth process.
 As with the first embodiment, one can also form an AlGaAs suspended layer
 instead of a GaAs suspended layer by suitable modification of the process
 steps. For this result, one would deposit a first layer of AlGaAs onto the
 GaAs to act as an etch stop. A sacrificial GaAs layer would then be
 deposited, followed by an AlGaAs layer to act as the suspended layer in a
 mask-defined area of the chip. A first mask would define vias about the
 perimeter of the suspended area of the top AlGaAs layer. An AlGaAs
 selective etch would create the vias down to the sacrificial GaAs layer to
 be followed by a selective GaAs etch that would remove the GaAs underneath
 the mask-defined suspended area of the top AlGaAs layer, leaving the
 suspended AlGaAs layer to be patterned with electrodes as above.
 One characteristic of epitaxial compound semiconductor growth that
 differentiates the epitaxial layers from bulk substrates of the same
 material is the incorporation of higher levels of impurities during growth
 into the epitaxial layers. These impurities have the effect of raising the
 electrical conductivity of the epitaxial layers, which degrades the
 performance of chemical microsensors and electronic filters. While
 epitaxial growth processes do produce materials of sufficiently low
 conductivity to fabricate useful microsensors and filters, improved
 performance would result if the conductivity could be reduced. This can be
 achieved through the process of ion implantation, a process that is well
 known in the field of microelectronics fabrication. The implantation of
 hydrogen or oxygen ions into the epitaxial layer that forms the sensor can
 bring about an improvement in performance. The implantation is performed
 after the epitaxial growth and in advance of other fabrication processes.
 This approach is well-suited to reducing the conductivity of these
 epitaxial layers because the ions can penetrate no more than a few microns
 into the surface of semiconductor materials, which is the typical
 thickness of the top epitaxial layer.
 The selective AlGaAs etch is a critical part of the fabrication process,
 particularly for the second embodiment. Typically, hydrohalic acids, for
 example, hydrofluoric acid (HF) or hydrochloric acid (HCl), are used to
 selectively etch the AlGaAs. These are preferred because of their
 extremely high selectivity: AlGaAs etches rapidly while GaAs shows no
 signs of etching. However, these etchants have proven to be unsuitable for
 this process. During the AlGaAs etching, HF and HCI generate gaseous
 hydrogen (H.sub.2) bubbles as a by-product. While undercutting the GaAs
 layer, these H.sub.2 bubbles become trapped under the GaAs layer, growing
 larger with time, until they exert enough force on the GaAs layer to break
 it. This is only a problem for GaAs layers with large lateral extents
 because they require long etch times. These large layers, however, are
 required for some microsensor and filter designs. To address this problem,
 we have developed a novel selective etch in which HF is mixed with
 isopropyl alcohol, typically in a ratio of 1:1, by volume. The isopropyl
 alcohol reduces the surface tension of the etch mixture, making it easier
 for the H.sub.2 bubbles to release themselves from the GaAs surfaces. With
 this new etchant, bubbles no longer grow large enough to break the GaAs
 layer. Much larger layers can be produced as a result. We find no change
 in the selectivity of this etch process, although the AlGaAs etch rate is
 reduced somewhat from that in the concentrated acid.
 Isopropyl alcohol is but one of a number of surface tension reducing agents
 that could be utilized to solve the previously unencountered problem of
 breakage of the very thin suspended layer by the trapped hydrogen bubbles.
 Other agents could include simple alcohols such as ethanol, methanol and
 n-propanol. Simple glycols such as ethylene glycol should also work. Other
 organic and inorganic compounds will be apparent to those skilled in the
 art and are included within the scope of this invention. Propanol and
 ethanol have been used as part of etchant solutions for their usual
 purposes of viscosity modification, control of pH, and dissolution of
 other organic constituents. The prior art does not contemplate their use
 as surface tension reducing agents as the solution to the unexpected gas
 bubble problem encountered for the first time in the context of the
 present invention.
 Thin piezoelectric layers have been fabricated using the approach of this
 second embodiment and are shown in the scanning electron microphotographs
 of FIGS. 8 and 9. In FIG. 8, the openings in the GaAs epitaxial layer are
 100 microns by 100 microns. The lateral dimensions of the suspended layer
 are 40 microns by 240 microns. The GaAs and AlGaAs epitaxial layers are
 both 1.0 micron thick. The GaAs substrate is 640 microns thick. The AlGaAs
 layer is 50 atomic percent Al. In FIG. 9, the overall dimensions of the
 suspended membrane are 3 mm by 0.5 mm. The GaAs epitaxial layer is 3
 microns thick. The AlGaAs layer is 2 microns thick and contains 70 atomic
 percent Al. The GaAs substrate is 640 microns thick. To produce suspended
 layers of this size requires the use of the novel HF/isopropyl alcohol
 etchant described above.
 The ability to produce piezoelectric layers that are simultaneously
 exceptionally thin, sufficiently large, crystalline, and composed of
 compound semiconductors is the critical aspect of this invention that
 differentiates it from other techniques for producing thin piezoelectric
 films. With other techniques (surface lapping and polishing, for example)
 it is possible to produce crystalline layers of compound semiconductor and
 other piezoelectric materials but these are typically 100 times thicker
 than the layers described herein. It is also possible to deposit
 comparably thin layers of some piezoelectric materials (zinc oxide, ZnO,
 and aluminum nitride, AlN, are examples) through sputtering techniques.
 However, these layers are generally polycrystalline and do not retain the
 physical properties found in the bulk single-crystal materials. Mechanical
 strength is a particularly important property in this application. Thin
 polycrystalline piezoelectric films lack the intrinsic mechanical strength
 to be self-supporting and must be deposited onto other non-piezoelectric
 films to produce the types of membranes described above. In the particular
 case of compound semiconductors, sputtering is ineffective as a deposition
 technique. In fact, epitaxial growth is the only technique capable of
 producing thin layers of compound semiconductors that retain the important
 electronic, optoelectronic, piezoelectric, and mechanical properties found
 in the bulk materials.
 The development of the selective etch that yields the GaAs layers depicted
 in FIGS. 7, 8 and 9 also differentiates this approach from other
 GaAs/AlGaAs microfabrication processes. While similar, smaller structures
 have been fabricated prior to this work, it has not been possible
 previously to produce GaAs layers suspended over the GaAs substrate with
 lateral dimensions as large as these. The HF/isopropyl alcohol etch
 described above is the advance that has enabled the fabrication of GaAs
 layers large enough for microsensor and high frequency electronic filter
 applications.
 There are two principal advantages of this approach for producing high
 sensitivity chemical microsensors and high frequency electronic filters
 over competing methods. The first is the simplified fabrication scheme
 afforded by the use of epitaxial layers of piezoelectric compound
 semiconductors. The second is that it allows for significantly improved
 system performance through the monolithic integration of improved
 microsensors and electronic filters with high speed compound semiconductor
 microelectronics and optoelectronics.
 As was discussed above, there are other methods for producing thin
 piezoelectric films that can be, and are being, used to produce these
 sensors and filters. However, these approaches require the formation of
 two separate thin films: one for mechanical strength and the other for
 piezoelectric properties. With this new approach described herein, the
 properties of mechanical strength and piezoelectric effect are combined in
 a single layer because the epitaxially deposited material retains its bulk
 crystalline properties. This simplified process reduces the number of
 process steps, thereby reducing complexity, increasing yield and lowering
 costs.
 Monolithic integration is potentially the more significant improvement.
 Compound semiconductors, particularly GaAs, are the substrates of choice
 for high frequency, low power microelectronics and optoelectronics. Using
 this approach to fabricate microsensors, these devices can be built on the
 same substrate as the components necessary to control them, analyze the
 data produced and communicate the results. Using this approach to
 fabricate filters enables these devices to be built on the same substrate
 as the other elements in the electronic circuit. The advantages of
 monolithic integration are widely recognized and include: smaller size,
 greater functionality, lower power requirements, improved reliability,
 tighter manufacturing tolerances, and simplified packaging. Competing
 methods also have the potential for monolithic integration. Other types of
 microsensors and filters can be fabricated on GaAs substrates. However,
 these do not have the sensitivity or high frequency performance of this
 new approach. Other high sensitivity microsensors and high frequency
 filters can be fabricated on silicon (Si) substrates. However, Si is not
 as capable as GaAs for the high speed microelectronics required for
 integration with both these devices. Finally, other high sensitivity
 microsensors and high frequency filters can be fabricated on GaAs
 substrates with high speed GaAs microelectronics, but the fabrication
 complexity of this integration would make the approach unattractive by
 comparison. In some instances, the deposition of other piezoelectric
 materials, by sputtering ZnO for example, would be incompatible with the
 fabrication of microelectronics in either GaAs or Si because the
 piezoelectric material introduces impurities or the deposition process
 produces temperatures that degrade the performance of the
 microelectronics.
 The invention has been described in the context of several embodiments.
 Variations upon the process will be apparent to those skilled in the art.
 The true scope of the invention is to be found in the appended claims.