Method of fabricating monolithic multifunction integrated circuit devices

Abstract: A method of selective molecular beam epitaxy for fabricating monolithically integrated circuit devices on a common substrate including combinations of PIN diode devices, HBT devices, HEMT devices and MESFET devices. The method includes depositing a profile layer of one of the devices on an appropriate substrate and then depositing a first dielectric layer over the profile layer. The profile layer and the dielectric layer are then etched to define a first device profile. A second profile layer for defining a second device is then deposited over the exposed substrate. The second profile is then selectively etched to define a second device profile. This process can be extended to more than two different device types monolithically integrated on a common substrate as long as the first developed devices are robust enough to handle the temperature cycling involved with developing the subsequent devices.

Description: BACKGROUND OF THE INVENTION 1. Field of the Invention This invention relates generally to a method of fabricating monolithic multifunction integrated circuits and the resulting integrated circuit and, more particularly, to a method of selective molecular beam epitaxy for fabricating different semiconductor device types in an integrated circuit format on a common substrate and the resulting integrated circuit. 2. Discussion of the Related Art It is known that the integration of multifunction circuit devices, i.e., circuits that include more than one device type, on a common substrate, in certain applications, such as microwave, millimeter wave, and optoelectronic applications, increases the performance of the integrated circuits incorporating the integrated devices over circuits that separately integrate the different device types. For example, by monolithically integrating a high electron mobility transistor (HEMT) and a heterojunction bipolar transistor (HBT), both well known devices in the art, the combination of the low-noise advantages of the HEMT and the high power, high linearity advantages of the HBT can provide microwave circuits with lower noise and higher power than could be realized by separately fabricating the HEMT and HBT in known baseline fabrication techniques and combining the devices in hybrid circuits. Many other circuits could benefit from the integration of multifunction circuit devices. PIN diode limiters are currently used to shield low-noise HEMTs in high-energy environments because HEMTs have a tendency to burn out from the high energy. However, because PIN diode limiters and HEMTs are currently separately integrated in hybrid circuits, there are additional transition losses that if the PIN diode limiter and the HEMT were monolithically integrated on the same substrate, such losses would be essentially eliminated. Additionally, digital function advantages of HBTs can be combined with microwave function advantages of HEMTs, so that for many applications, such as transistor-transistor logic (TTL) controlled phase shifters, HBTs and HEMTs can be generated with increased performance. In another example, HEMT amplifiers can be stabilized using HBT active regulation for improved reliability and stability. Essentially, any circuit application requiring multiple high performance semiconductor devices can be more effectively realized using monolithic integration rather than hybrid integration. Current semiconductor fabrication techniques are limited in the ability to fabricate more than one device type on a common substrate. Different techniques for integrating semiconductor devices having different functions are known in the art. For example, the integration of PIN diodes and metal semiconductor field effect transistors (MESFETs) or HEMTs has been accomplished using a single molecular beam epitaxial growth layer with certain interconnect schemes. See for example S. Miura et al., "A Monolithically Integrated AlGaAs/GaAs p-i-n/FET photoreceiver by MOCVD," IEEE Electron Dev. Lett., Vol. 4, pp. 375-376, 1983. Additionally, PIN diodes and MESFETs or HEMTs have been integrated using a molecular beam epitaxy (MBE) regrowth process. See Y. Zebda et al., "Monolithically Integrated InP-Based Front-End Photoreceivers," IEEE Trans. Elect. Dev., Vol. 38, pp. 1324-1333, 1991. Further, the integration of lasers and HBTs has been accomplished using buried epitaxial layers and MBE regrowth. See J. Shivata et al., "Monolithic Integration of InGaAsP/InGaAsP/InP Laser Diode With Heterojunction Bipolar Transistors," Appl. Phys. Lett., Vol. 45, pp. 191-193, 1984 and P.R. Berger et al., "GaAs Quantum Well Laser and Heterojunction Bipolar Transistor Integration Using Molecular Beam Epitaxial Regrowth," Appl. Phys. Lett., Vol. 59, pp. 2826-2828, 1991. GaAs-based Bi-FET technology has been reported using a single epitaxial growth with the FET merged into the HBT collector or emitter, or using AlGaAs overgrowth combined with beryllium implantation. See K. Itakura et al., "A GaAs Bi-FET Technology For Large Scale Integration," IEDM Technical Digest, pp. 389-392, 1989; D. Cheskis et al., "Cointegration of GaAlAs/GaAs HBTs and GaAs FETs With A Simple Manufacturable Processes," IEDM Technical Digest, pp. 91-94, 1992 and J.Y. Yang et al., "GaAs BIJFET Technology For Linear Circuits," Proceeding IEEE GaAs IC Symposium, pp. 341-344, 1989. In each of these examples, the FET performance is compromised due to the limitations of the profile and the process. InP based HEMT-HBT integration has also been attempted using a single epitaxial growth, but successful operation of both device types has not yet been reported. See W.E. Stanchina et al., "InP-Based Technology for Monolithic Multiple-Device, Multiple-Function ICs," GOMAC Digest of Papers, pp. 385-388, 1991. US Patent 5,159,413 discloses a monolithic integrated circuit having compound semiconductor layer epitaxially grown on ceramic substrate. Active components comprising HEMTs, MESFETs and diode devices and passive components are fabricated on the same substrate, wherein active components are formed on a first buffer formed on the substrate, while passive components are formed on a second buffer formed on the first buffer. EP 0 392 480 A1 discloses a method of manufacturing a semiconductor integrated circuit device by which a PIN photodiode crystal, a HBT crystal, and a HEMT crystal are manufactured on a compound semiconductor substrate. All layers of a crystal are formed before the layers of the next crystal to be formed subsequently are formed. Layers of the subsequently formed crystal are not formed in the area(s) of the crystal(s) already formed. EP 0 505 942 A1 discloses a method of integrating a HBT with a heterojunction FET and a PIN diode on a substrate. Some components of the devices are obtained by implantation in the substrate, other layers are grown by molecular beam epitaxy or by metal organic chemical vapour deposition (MOCVD). The method includes selectively implanting, into the substrate, first a sub-collector layer for an HBT and second a first layer for a PIN diode spaced from the sub-collector layer. Next, a i-layer is formed on the sub-collector layer, the first diode layer and the remaining substrate, the i-layer being a common layer for all the HBT, the FET and the diode. Subsequently, a collector layer for the HBT and a second diode layer a formed at the same time, followed by forming a base layer for the HBT in the regions of the HBT and the diode and etching the base layer in the diode region. The article "Gallium Arsenide - Materials, Devices, and Circuits" by M. J. Howes and D. V. Morgan, J. Wiley and Sons Ltd., 1985, pages 473-477, describes method for electrically isolating different devices integrated on a common substrate. United States Patent Number 5,262,335 issued to Streit et al., assigned to the assignee of the instant application discloses a method to produce complementary heterojunction bipolar transistors in which a first series of layer defining a NPN or PNP device ("NPN or PNP profile") is grown by selective molecular beam epitaxy on a substrate. A silicon nitride layer is then deposited over the profile, and the silicon nitride layer and the profile are selectively patterned and etched to define an NPN or PNP heterojunction bipolar transistor. The opposite NPN or PNP profile is then deposited on the substrate adjacent to the original profile such that the remaining portion of the silicon nitride layer protects the original profile from the growth of the second profile. The devices are patterned and etched such that the silicon nitride layers are removed and adjacent complimentary NPN/PNP profiles remain on the substrate. OBJECT OF THE INVENTION Fabrication processes in the prior art are limited in their ability to produce monolithic integrated devices. It is an object of the present invention to provide a molecular beam epitaxy fabrication method that enables a significant number of integrated devices to be monolithically integrated on a common substrate. SUMMARY OF THE INVENTION In accordance with the teaching of the present invention, a method is provided as defined in claim 1. Further, the object of the invention is achieved by an integrated circuit as defined in claim 4. Additional objects, advantages, and features of the present invention will become apparent from the following description and appended claims, taken in conjunction with the accompanying drawings. BRIEF DESCRIPTION OF THE DRAWINGS Figures 1(a)-1(d) show side views of a developing structure for fabricating an HEMT-HBT, HEMT-PIN diode, MESFET-HBT, or MESFET-PIN diode integrated circuit device provided for a better understanding of the invention ; Figures 2(a)-2(g) show side views of a developing structure for fabricating an HEMT-HBT-PIN diode or MESFET-HBT-PIN diode integrated circuit device provided for a better understanding of the invention ; Figure 3 shows a side view of a monolithically integrated HBT - Schottky diode - HEMT device that has been fabricated according to a method of the present invention; Figure 4 shows a side view of a monolithically integrated HBT-PIN diode-HEMT device that has been fabricated according to a method of the present invention; Figure 5 shows a side view of a monolithically integrated PIN diode-HEMT device provided for a better understanding of the invention ; Figure 6 shows a side view of a monolithically integrated HBT-Schottky diode-HEMT-PIN diode device that has been fabricated by a method of the present invention; Figure 7 shows a side view of a monolithically integrated HBT-Schottky diode-MESFET-PIN diode device that has been fabricated by a method of the present invention; Figure 8 shows another monolithically integrated HBT-Schottky diode-HEMT-PIN diode device that has been fabricated by a method of the present invention; Figure 9 shows a flow chart diagram of a method for fabricating a monolithically integrated multifunction device provided for a better understanding of the invention ; Figure 10 is a graph showing current (I) versus voltage (V) curves of a 2 x 10 µ m single-emitter HBT that has been fabricated by a method of forming a monolithically integrated HEMT-HBT device provided for a better understanding of the invention ; Figure 11 is a graph of cut-off frequency (f T ) versus collector current density for a 2 x 10 µ m quad-emitter HBT fabricated by a known baseline molecular beam epitaxy fabrication method and a method of forming a monolithically integrated HEMT-HBT device provided for a better understanding of the invention ; Figure 12 shows a graph of current versus voltage curves for a T-gate HEMT that has been monolithically fabricated with an HBT provided for a better understanding of the invention ; Figures 13(a)-13(b) show graphs of forward and reverse bias current and voltage characteristics for a PIN diode fabricated monolithically with an integrated HEMT-HBT device according to an embodiment of the present invention; Figure 14 shows a graph comparing gain and noise versus frequency between an HBT amplifier fabricated by a baseline technique and an HBT amplifier associated with a monolithically integrated HEMT-HBT provided for a better understanding of the invention ; Figure 15 shows a schematic diagram of a high performance amplifier that integrates a low-noise HEMT front end with HBT high intercept amplifiers; Figure 16 shows a schematic diagram of a PIN diode limiter integrated with an HEMT low-noise amplifier; Figure 17 shows a schematic diagram of an HBT regulator integrated with an HEMT low-noise amplifier; and Figure 18 is a schematic diagram of a transmit-receive module that uses an HEMT low noise amplifier for receive functions, a PIN diode switch using either an HBT base-collector PIN or a separate PIN, and an HBT power amplifier for the transmit function. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS The following discussion of the preferred embodiments concerning methods of generating monolithically integrated circuit devices, and the resulting integrated circuits, is merely exemplary in nature and is in no way intended to limit the invention or its applications or uses. First turning to Figures 1(a)-1(d), a series of side views of a monolithically integrated semiconductor structure 10 is shown as the structure 10 is being fabricated by steps in accordance with forming a monolithically integrated circuit device. The method of fabricating the structure 10 is applicable to form many monolithically integrated devices including, but not limited to, monolithically integrated HEMT-HBT, HEMT-PIN diode, MESFET-HBT or MESFET-PIN diode devices. Monolithically integrated devices fabricated by the process as will be discussed are formed on a common substrate 12. In the embodiment shown, the substrate 12 is either gallium arsenide (GaAs) or indium phosphide (InP), however, other substrates may be applicable. The process for fabricating the different semiconductor layers is performed by a selective molecular beam epitaxy (MBE) process, well understood by those skilled in the art. An HBT profile layer 14 is first grown on the substrate 12 by the MBE process as shown in Fig. 1(a). The HBT profile layer 14 is grown when fabricating an integrated HEMT-HBT or MESFET-HBT device. When fabricating an HEMT-diode or MESFET diode device, the profile layer 14 would be a diode profile layer. The profile layer 14 would include all of those semiconductor layers that make up an HBT or diode device. The diode profile could be any applicable diode profile, including, but not limited to, PIN diodes, schottky diodes, triodes, etc. After the layer 14 has been grown, a silicon nitride (Si 3 N 4 ) layer 16 is deposited on the structure 10 by, for example, a plasma enhanced chemical vapor deposition process, well understood to those skilled in the art. It is noted that silicon nitride is used by way of a non-limiting example as other dielectric layers, such as silicon dioxide, may be equally applicable. The silicon nitride layer 16 will be used as a blocking layer to shield the HBT device from the growth of HEMT or MESFET profile layers that are to be fabricated, as will become apparent from the discussion below. After the silicon nitride layer 16 has been deposited on the profile layer 14, a resist layer (not shown) is then deposited on the silicon nitride layer 16. A mask (not shown) is then used to pattern the resist layer, and thus the silicon nitride layer 16, into a pattern that will define either the diode device or the HBT device. The resist layer is exposed to radiation through the mask, and then is developed by an appropriate solvent such that the unmasked portions of the resist layer are dissolved and removed. An etchant solution is then applied to the exposed portions of the silicon nitride layer 16 to remove the silicon nitride in the areas which coincide with the removed regions of the resist layer in order to expose the profile layer 14 at these regions. The exposed regions of the profile layer 14 are then etched by an appropriate anisotropic etchant to expose the substrate 12. Figure 1(b) shows the structure 10 after the exposed regions of the layer 14 have been etched away. The remaining profile layer 14 defines an HBT device layer 18, as shown. A portion of the silicon nitride layer 16 remains over the device layer 18. The anisotropic etching step produces a retrograde profile that causes overhang areas of the remaining silicon nitride layer 16 to extend beyond a top surface of the device layer 18. This provides a clean separation between the device layer 18 and subsequent device profile layers, as will become apparent from the discussion below. The structure 10 is then removed from the MBE chamber (not shown), cleaned, and then reintroduced into the MBE system. The remaining silicon nitride layer 16 is densified during the normal de-gas in the MBE chamber as is well understood to those skilled in the art. A pseudomorphic InGaAs-GaAs HEMT profile is then grown over the structure 10 by a known HEMT MBE growth procedure. It is also possible to grow an MESFET profile at this place in the processing of the structure 10 instead of the HEMT profile. As shown in Figure 1(c), the HEMT material that is deposited on the remaining silicon nitride layer 16 forms a polycrystalline HEMT profile layer 20 because the HEMT material is not lattice matched with the silicon nitride layer 16. The HEMT material that is deposited on the cleaned substrate 12 forms a single crystalline HEMT profile layer 22 because the HEMT material is lattice matched to the cleaned surface of the substrate 12. The single crystalline profile layer 22 forms the HEMT device. The polycrystalline HEMT layer 20 is removed by a wet etch process and the remaining silicon nitride layer 16 is removed using a dry etch process, both known processes in the art. Portions of the single crystalline HEMT profile layer 22 can be removed by an appropriate masking and etching step so as to further define the device geometry of the HEMT device. The monolithically integrated structure 10, as shown in Figure 1(d), is then ready for device and circuit processing. By the above process, the HBT beryllium base dopant interstitial diffusion procedure associated with non-optimized MBE growth has been eliminated. This results in P-type profiles robust enough to withstand the additional temperature cycling associated with the HEMT growth, and is one key to the success of this selective MBE technique. In order for the HBT or diode profile to survive the HEMT temperature cycle during formation of the HEMT device, not only must the base layer of the HBT be stable, but also other layers in the HBT or diode profile must be stable. This includes an n=2x10 19 cm -3 contact layer and an n=5x10 17 cm -3 Al 0.3 Ga 0.7 As emitter layer. The techniques discussed above with reference to Figure 1(a) - 1(d) can be extended to a process for forming other multiple monolithic integrated devices. For example, Figures 2(a)-2(g) show sequential fabrication steps of a monolithically integrated semiconductor structure 28 that generates a monolithically integrated HEMT-HBT-diode device. The device is formed on a GaAs or InP substrate 30, as with the structure 10 above. As shown in Figure 2(a), a diode profile layer 32 is grown on the substrate 30. A first silicon nitride layer 34 is deposited, masked and etched, in the manner as discussed above, to form a diode device layer 36 as shown in Figure 2(b). A remaining portion of the silicon nitride layer 34 covers the device layer 36. Note that overhang areas of the silicon nitride layer 34 extend beyond the device layer 36 as a result of the retrograde profile produced by the anisotropic etch that formed the device layer 36. An HBT profile is then grown over the structure 28 such that HBT material deposited on the substrate 30 forms a single crystalline HBT profile layer 38, and HBT material deposited on the remaining silicon nitride layer 34 forms a polycrystalline HBT profile layer 40, as shown in Figure 2(c). Figure 2(d) shows the developing structure 28 after the silicon nitride layer 34 and the polycrystalline HBT profile layer 40 have been dissolved by an appropriate wet etch and dry etch, respectively. A second silicon nitride layer 42 is then deposited over the structure 28. The silicon nitride layer 42 is then patterned and etched by an appropriate resist layer and mask layer (not shown), in the manner as discussed above, to expose a region on the substrate 30 that will accept an HEMT profile, as shown in Figure 2(e). HEMT material is then grown by the selective MBE process on the structure 28 such that a single crystalline HEMT layer 44 is deposited on the substrate 30, and a polycrystalline HEMT layer 46 is deposited on the remaining second silicon nitride layer 42, as shown in Figure 2(f). As with the process with reference to Figures 1(a)-1(d), it is possible to deposit an MESFET profile instead of the HEMT profile. An appropriate wet etch and dry etch selectively remove the polycrystalline layer 46 and the remaining silicon nitride layer 42, respectively, to expose and define an integrated HEMT-HBT-diode device, as shown in Figure 2(g). Figures 1(a)-1(d) show the processing steps necessary to fabricate dual monolithically integrated devices in which the first deposited device was robust enough to withstand the temperature cycling involved with fabricating the second deposited device, taking into consideration that the second deposited device is protected by the silicon nitride layer 16. Figures 2(a)-2(g) show the processing steps for fabricating three monolithically integrated devices in which the first deposited device is robust enough to withstand the temperature cycling involved with fabricating the second and third deposited devices, and the second deposited device is robust enough to withstand the temperature cycling involved with fabricating the third deposited device, taking into consideration that the second and third deposited devices are protected by the silicon nitride layer 42. This process can be extended to other semiconductor devices that satisfy this criteria, including extending the process to more than three monolithically integrated devices. For example, the process can be extended to fabricating a monolithically integrated HBT-diode device. The above-described process with reference to Figure 2 can also be extended to a monolithically integrated MESFET-HBT-diode device where an MESFET profile is grown instead of the HEMT profile. Further, after the fabrication steps involved in the selective molecular epitaxy process that forms the device profiles as shown in Figures 1(d) and 2(g), other fabrication steps, known in the art, can be incorporated to provide other device types. For example, an ion implantation isolation step can be performed to separate portions of the collector layers of the HBT device to form diodes, such as Schottky diodes and PIN diodes. Also, other devices can be deposited such as a superconducting-insulator-superconducting detector after the selective molecular beam epitaxy steps. Figures 3-8 show profile side views of six different monolithically integrated circuit devices after final device and circuit processing steps that could be fabricated by the selective MBE process according to the invention. It will be understood that the different device layers and profiles discussed below for specific devices is merely by way of a non-limiting example in that the device profiles for a particular device could be any applicable profile for that device that is known in the art. Because the different device layers and contacts are known in the art, the description given below for these layers is merely cursory. It is also noted that because the various devices have been formed by molecular beam epitaxy, the bottom layer of each device forms an epitaxial bond with the substrate. Figure 3 shows a monolithically integrated device 50 including an HBT 52, a THz Schottky diode 54, and an HEMT 56 that could have been fabricated by the process as discussed above with reference to Figure 1. Each of the different layers of the HBT 52, the Schottky diode 54 and the HEMT 56, are shown. As mentioned above, the HBT device layer 18 would include all of the layers of the HBT 52, and the HEMT profile layer 22 would include all of the layers of the HEMT 56. However, the HBT 52 has been further defined and processed more than the device layer 18, and the HEMT 56 has been further defined and processed more than the profile layer 22. The substrate 30 is shown as a GaAs substrate 58. The HBT 52 includes a heavily doped n-type GaAs subcollector layer 60 grown on the substrate 58 to a thickness of about 600 nm. Two ohmic collector contacts 62 are deposited and defined on the subcollector layer 60. A lightly doped n-type GaAs collector layer 64 is grown on the subcollector layer 60 to a thickness of about 700 nm. A heavily doped p-type GaAs base layer 66 is grown on the collector layer 64 to a thickness of about 140 nm. An ohmic base contact 68 is deposited and defined on the base layer 66. An n-type AlGaAs emitter layer 70 is grown on the base layer 66 to a thickness of about 180 nm. A heavily doped n-type InGaAs emitter contact layer 72 is grown on the emitter layer 70 to a thickness of about 85 nm. The combination of the emitter layer 70 and the emitter contact layer 72 form an emitter mesa 74. An ohmic emitter contact 76 is deposited and defined on the emitter contact layer 72. The different processing steps that generate each of the contacts and the collector, base and emitter mesas of the HBT 52 as shown in Figure 3 are well understood in the art. The layers that make up the Schottky diode 54 are deposited simultaneously with the collector layers 60 and 64 of the HBT 52 and the diode 54 is subsequently separated from the HBT 52. Particularly, a heavily doped n-type GaAs diode layer 80 of the diode 54 is grown at the same time the subcollector layer 60 of the HBT 52 is grown so that the layer 80 and the subcollector layer 60 have the same thickness and composition. The subcollector layer 60 and the diode layer 80 are separated by an implant isolation region 82 formed by an ion implantation step of a suitable ion, such as oxygen, well known to those skilled in the art. A set of ohmic contacts 84 are deposited and defined on the diode layer 80. A lightly doped n-type GaAs diode layer 86 is formed on the diode layer 80. The diode layer 86 is formed simultaneously with the collector layer 64 of the HBT 52, and as such, is the same thickness and composition as the layer 64. A top ohmic contact 88 is deposited and defined on the diode layer 86. The diode layer 86 is separated from the collector layer 64 and defined by any suitable patterning process well known to those skilled in the art. Separation of the Schottky diode 54 from the collector layers of the HBT 52 provide a convenient way of providing another monolithically integrated device after the molecular beam epitaxy steps that is useful for certain applications. Etching steps are used to remove the profile layers that would have been above the diode layers 80 and 86 after the MBE steps. The HEMT 56 has the different device layers as the HEMT profile layer 22 of Figure 1. Particularly, the HEMT 56 includes a superlattice buffer layer 92 grown on the substrate 52. An In 0.22 Ga 0.78 As channel layer 94 is grown on the superlattice buffer layer 92 to a thickness of about 15nm. A silicon planar doped layer 96 is deposited on the channel layer 94 to define the channel layer 94, as is well understood in the art. An Al 0.22 Ga 0.78 As donor layer 98 is grown on the planar doped layer 96 to a thickness of about 30nm. A heavily doped n-type GaAs contact layer 100 is grown on the donor layer 98 to a thickness of about 40nm. A source terminal 102 and a drain terminal 104 are deposited and defined by an electron beam lithography process on the contact layer 100 as shown. The contact layer 100 is etched to expose the donor layer 98, and a T-gate drain terminal 106 is deposited and etched on the donor layer 98 as shown. As discussed above, it is within the scope of the invention to replace the HEMT 56 with an MESFET. Figure 4 shows a side profile view of a monolithically integrated device 110 according to the invention. The integrated device 110 includes an HBT 112, a PIN diode 114 and an HEMT 116 formed on a common substrate 118. The HBT 112 is the same as the HBT 52 and the HEMT 116 is the same as the HEMT 56, discussed above, and thus, the different layers of the HBT 112 and the HEMT 116 will not be discussed. As above, the HEMT 116 could have been replaced with an MESFET. The PIN diode 114 is isolated from the HBT 112 by an implant isolation region 120 in the same manner that the Schottky diode 54 was isolated from the HBT 52 above. The PIN diode 114 includes a heavily doped n-type GaAs diode layer 122 and a lightly doped n-type GaAs diode layer 124 that were grown at the same time as the subcollector layer and the collector layer of the HBT 112, respectively, and therefore have the same thickness and composition as these layers. The layer 124 acts as the intrinsic layer in the PIN structure. A heavily doped p-type GaAs layer 126 is grown on the lightly doped n-type GaAs layer 124 to provide the PIN diode structure. The layer 126 was grown at the same time as the base layer of the HBT 112. However, the layer 126 has been etched to reduce its thickness to be less than the base layer of the HBT 112. Diode contacts are deposited and defined in connection with the layer 122 and the layer 126 as shown. As with the Schottky diode 54 above, the PIN diode 114 has been separated from the HBT 112 in a convenient manner so as to provide an additional integrated device than the two device circuit as discussed above with reference to Figure 1. Figure 5 shows a side profile view of a monolithically integrated device 130 not forming part of the claimed invention that could also have been fabricated by the process as discussed above with reference to Figure 1. The integrated device 130 includes a PIN diode 132 and an HEMT 134 formed on a common substrate 136. The HEMT 134 is the same as the HEMT 56, discussed above, and thus, the different layers of the HEMT 134 will not be discussed. The HEMT 134 could also be an MESFET. The PIN diode 132 shows the different device layers of a PIN diode profile. The PIN diode 132 and the HEMT 134 were grown on the substrate 136 as were the device layer 18 and the profile layer 22 on the substrate 12. The PIN diode 132 includes a heavily doped n-type GaAs contact layer 138 grown to a thickness of about 600nm on the substrate 136. First diode ohmic contacts 140 are deposited and defined on the contact layer 138. A GaAs intrinsic layer 142 is grown on the contact layer 138 to a thickness of about 2000nm. A p-type GaAs layer 144 is grown on the intrinsic layer 142 to a thickness of about 100nm. A heavily doped p-type GaAs contact layer 146 is grown on the layer 144 to a thickness of about 50 nm. A second ohmic contact 148 is deposited and defined on the contact layer 146. The PIN diode 132 is significantly different than the PIN diode 114 that was separated from the HBT 112. This is because the PIN diode 114 was conveniently separated as a result of the development of the HBT 112, but the PIN diode 132 was generated by the more detailed selective beam epitaxy process. Figure 6 shows a side view of a monolithically integrated device 156 according to the invention. The integrated device 156 includes an HBT 158, a THz Schottky diode 160, an HEMT 162, and a PIN diode 164 on a common substrate 166. Each of these different devices shows the different layers for each particular device, and as such, the HBT 158 could have been formed from the HBT profile layer 38, the HEMT 162 could have been formed from the HEMT profile layer 44 and the PIN diode 164 could have been formed from the PIN diode profile layer 36 of Figure 2. Because the HBT 158 and Schottky diode 160 combination and the HEMT 162 of Figure 6 are the same as the HBT 52 and the Schottky diode 54 combination and the HEMT 56 of Figure 3, the individual layers of the HBT 158, the Schottky diode 160, and the HEMT 162 will not be discussed. Likewise, because the PIN diode 164 of Figure 6 has the same layer configuration as the PIN diode 132 of Figure 5, the PIN diode 164 also will not be discussed. Figure 7 shows a side view of a monolithically integrated device 174 according to the invention. The integrated device 174 includes an HBT 176, a THz Schottky diode 178, an MESFET 180, and a PIN diode 182 formed on a common substrate 184. The integrated device 174 is the same as the integrated device 156 except that the HEMT 162 has been replaced by the MESFET 180. Therefore, only the specific device profile of the MESFET 180 will be described with the understanding that the other device profiles are the same as those discussed above. The MESFET 180 includes a GaAs buffer layer 186 deposited on the substrate 184. An n-type GaAs layer 188 is grown on the buffer layer 186 to a thickness of approximately 200nm. A heavily doped n-type GaAs layer 190 is grown on the layer 188 to a thickness of approximately 40nm. A source terminal 192 and a drain terminal 194 are deposited and defined by an electron beam lithography process on the layer 190. The layer 190 is etched to expose the layer 188, and a T-gate drain terminal 196 is deposited and etched on the layer 188 as shown. The availability of the process techniques to fabricate monolithic multifunction integrated circuits discussed above also allows the incorporation of superconducting films for microwave detection mixing and for digital applications. Superconducting films are typically deposited by sputtering, well known to those skilled in the art, but alternate deposition techniques also are available, such as by evaporation and laser ablation. Therefore, the substrate 166 of the integrated device 156 can also accept a superconducting-insulator-superconducting (SIS) detector 206, as shown in Figure 8. The SIS detector 206 includes a bottom superconducting film 210 deposited on the substrate 166. An insulator layer 212 is deposited on the superconducting film 210. Another superconducting film layer 214 is deposited on the insulator layer 212. The SIS detector 206 could also have been integrated with an HEMT only, a PIN diode-HEMT integrated device, or a PIN diode-HBT integrated device, depending on its specific application. The SIS detectors 206 can also be replaced by superconducting transmission line or super conducting digital circuits. Merged process technologies have been developed to achieve all of the integrated devices discussed above. Figure 9 shows a flow chart diagram 220 that provides a step-by-step discussion of the formation of monolithic integrated circuits with reference to the discussion above for Figures 1(a) - 1(d), as well as subsequent device processing steps. Box 222 represents the step of growing the HBT profile layer 14 or a PIN profile layer on the substrate 12. Box 224 represents the formation of the HBT device layer 18 by the deposition, the masking and the etching steps of the silicon nitride layer 16. Box 226 represents the growth of the single crystalline HEMT profile layer 22 and the polycrystalline HEMT profile layer 20 or the growth of a single crystalline MESFET profile layer and a polycrystalline MESFET profile layer. Box 228 represents the etching steps of the polycrystalline layer 20 and the remaining silicon nitride layer 16. The remaining steps of the flowchart diagram 220 are directed to device and circuit processing steps, such as forming contacts of the individual devices and connections between the devices. Box 230 represents a step of forming the ohmic metal contacts 102 and 104 of the HEMT 56 or the contacts of an MESFET. HEMT or MESFET ohmic metal is evaporated over the developing HEMT or MESFET structure and is rapidly thermally annealed. The evaporated metal is then patterned to form the contacts in a manner well understood to those skilled in the art. If an HEMT or MESFET is being fabricated integrally with a HBT having the structure shown in Figure 3 or Figure 4, then the flow diagram 220 moves from the Box 230 to Box 232. The box 232 represents a step where the emitter mesa 74 of the HBT structure is patterned and etched. Base metal is then evaporated on the developing HBT 52 and is patterned to form the base contact 68 as represented by Box 234. Next, the mesas that form the HBT base layer 66, the Schottky diode layers 80 and 86 and the PIN diode layers 122, 124 and 126 are patterned and etched as represented by Box 236. Ohmic metal is then evaporated on the developing HBT 52 structure and is patterned to form the collector contact 62 and the emitter contact 76 as represented by Box 238. Next, the emitter contact 76, the base contact 68 and the collector contact 62 are annealed as represented by Box 240. If an HEMT or MESFET is being monolithically integrated with a PIN diode having the structure as shown in Figure 5, then after the HEMT or MESFET metal deposition step of the Box 230, the mesas that form PIN diode layers 138, 142, 144 and 146 are etched as represented by Box 242. Next, the p-type ohmic contact 148 and the n-type ohmic contact 140 are evaporated, as represented by Box 244. The ohmic contacts 140 and 148 are then thermally annealed as represented by Box 246. After the mesas and ohmic contacts have been formed on either the HBT 52 or the PIN diode 112, as discussed above, the next step is device isolation by an oxygen ion implantation step as represented by 248. For the integrated device 50, the ion implantation region 82 separates the HBT 52 from the Schottky diode 54. For the integrated device 110, the ion implantation step separates the HBT 112 from the PIN diode 114. Next, the T-gate terminal 106 of the HEMT 56 or a T-gate terminal associated with an MESFET is written by electron beam lithography (EBL) to form its shape as represented by step 250. Next, a silicon nitride passivation layer (not shown) is deposited over the entire monolithically formed integrated circuit 50, 110 and 130, and vias are formed to the appropriate terminals as represented by box 252. Thin film resistors, capacitors, inductors, airbridges and interconnect metalization (not shown) are then defined within each HBT, HEMT, MESFET, PIN diode, and Schottky diode and between these devices as represented by Box 254. Pad vias and backside vias (not shown) are then formed as represented by Box 256. Finally, a backside metal planar layer (not shown) is formed over the entire integrated device 50 and 110 as represented by Box 258. For the integrated circuit 156 of Figures 6 and 8, and the integrated circuit 174 of Figure 7, after the step of the box 230, the HBT emitter mesa is etched as indicated by the box 232, and the HBT base contact and the p-type PIN diode contact of the boxes 234 and 244 are performed. The HBT base layer and the Schottky diode mesas are then formed as shown by the box 236, and the PIN diode mesa is formed as represented by the box 242. Next, the HBT emitter contact and collector contact are evaporated and annealed in accordance with the boxes 238 and 240. Next, the PIN diode contact is annealed in accordance with the box 246. The process then picks up at the box 248 as discussed above. DC and microwave performance results of discrete HEMTs and HBTs fabricated using the merged HEMT-HBT process described above are equivalent to those of discrete devices fabricated using known baseline single device technology processes. Figure 10 shows a graph of current (I) in the vertical axis and voltage (V) on the horizontal axis displaying the I-V characteristics of a 2 x 10 µ m single-emitter HBT fabricated by the HEMT-HBT monolithic process discussed above. The HBT breakdown voltage V ceo is greater than 10V. The early voltage is greater than 500 volts, and β is about 56 at I c = 4.5 mA. The knee voltage is normal indicating that no additional collector resistance is induced during the selective MBE process of the invention. The base-emitter voltage V be = 1.1539V at 1 mA, and is typical for this device geometry at this current density. Any significant base dopant diffusion into the graded AlGaAs emitter, or any increased emitter resistance would result in an increased V be . The emitter specific contact resistance is 1.1 x 10 -7 Ω-cm 2 , typical for a device of this type, and indicates that no degradation of the HBT InGaAs emitter contact during the subsequent HEMT growth. Large HBT devices with 75 x 75 µ m 2 emitters had an average β = 102 at I c =1 mA (19A/cm 2 ) and β = 175 at I c = 40 mA (711 A/cm 2 ), with an ideality factor for the base-emitter junction diode of n = 1.03. HBT devices having a quad-emitter of 2 x 10 µm 2 had a cut-off frequency f T = 21.4 GHz and f max = 50 GHz at I c = 16 mA (2x10 4 A/cm 2 ) typical for these types of devices. A plot of f T versus collector current density for baseline and selective MBE HBTs is shown in Figure 11. The recorded values are essentially identical across the measured current range. The equivalence of dc and rf results for HBTs fabricated by baseline and selective MBE processes indicates that no significant degradation in HBT material properties has occurred during the HEMT-HBT integration process. Low-noise pseudomorphic InGaAs-GaAs 0.2 µ m T-gate HEMTs fabricated monolithically with GaAs-AlGaAs HBTs have dc and rf characteristics equivalent to those of HEMTs fabricated by known baseline processes. The I-V characteristic curves for a 2-finger 80 µ m gate-width HEMT device is shown in Figure 12. Forward and reverse I-V characteristics for the THz Schottky diode 54 fabricated from the HBT collector-subcollector region of the integrated device 50 are shown in Figures 13(a) - 13(b). The diode ideality factor of n=1.04, series resistance of 12 Ω, and a breakdown voltage of approximately 13V are typical for a Schottky diode of this type when fabricated by a known baseline HBT process. The performance of a monolithic HEMT-HBT integrated circuit device that uses active HBT regulation of an HEMT low-noise amplifier is shown in Figure 14. The amplifier was designed for 5-10 GHz bandwidth, nominal gain greater than 10 dB, and noise figure of less than 3 dB. The HEMT low-noise amplifier is a single stage feedback design using a single 0.2 µ m T-gate HEMT which is 200 µ in length. The bias current of the HEMT device can be regulated to within 5% over a threshold variation of ±0.5V using an on-chip HBT current regulator, which consumes 5 mA through a 10V positive supply voltage. The same amplifier without the HBT regulator was fabricated in the HEMT-only technology for comparison. Gain and noise figure for amplifiers fabricated using a selective MBE process, and a baseline single-technology are nearly identical as shown in Figure 14. The HEMT amplifiers fabricated by the merged HEMT-HBT process actually have a slight gain advantage near 10 GHz. This is probably due to normal variations in the process. Monolithic integration of HEMTs and HBTs on the same integrated circuit provides microwave circuits with performance levels unattainable using separate device generation technology. The design opportunities when using both HEMT and HBT devices on the same chip are extensive, especially when combined with THz Schottky diodes and PIN diodes. For example, the monolithic integration of microwave and digital functions can significantly improve receiver performance. Novel circuit designs, such as TTL-controlled phase shifters, low-noise high-power transmitter-receive modules, FMCW single-chip radar circuits that incorporate HBT variable control oscillators with HEMT low-noise amplifiers are all possible using monolithic integration of high-performance HEMTs and HBTs. In addition, superconducting detectors, mixers, transmission lines and digital circuits can be incorporated with MESFET, HEMT, HBT or PIN-diode circuits to increase performance to cryogenic temperatures. Figures 15-18 show circuit examples of HEMT-HBT, PIN diode-HEMT and PIN diode-HBT-HEMT circuits that could gain advantages from being monolithically integrated. Each of the circuits of Figures 15-18 are known in the art in that hybrid or separate chip implementations have been extensively developed. With the invention as discussed above, the individual circuit components of these circuits can be monolithically integrated on a common substrate. In other words, one skilled in the art could interconnect the integrated circuits discussed above to arrive at the circuits of Figures 15-18. By monolithically integrating the different circuit components on a common substrate, many advantages including, but not limited to, lower transition losses between the circuit components, less area requirements, less cost, compactness and higher performance can be realized with the monolithically integrated circuits. Figure 15 shows a high performance amplifier 260, known in the art, that provides a low-noise front end, high impedance and low distortion of the third harmonic, optimum sensitivity and high power. The amplifier 260 includes an HEMT low noise amplifier 262 that receives an rf signal and applies an amplified output signal to first and second HBT high intercept amplifiers (HIA) 264 and 266. By monolithically integrating the amplifiers 262, 264 and 266 on a common substrate, the known advantages of this circuit can be increased. Because HEMT amplifiers burn out readily easily at high input power, it is known in the art to provide a PIN diode limiter connected to the base (B) and emitter (E) terminals of an HEMT amplifier in order to protect the HEMT amplifier against such high input signals. Figure 16 shows a circuit 270 of this type of HEMT amplifier. The circuit 270 includes a PIN diode limiter 272 where the anode of the diode limiter 272 is connected to the base terminal of an HEMT 274 and the cathode of the diode limiter 270 is connected to the emitter terminal of the HEMT amplifier 274. The diode limiter 272 shunts input overload signals applied to the base of the HEMT 274 so as to protect the HEMT 274 from signal overload and burnout. By monolithically integrating the diode limiter 272 and the HEMT 274 on a common substrate many advantages over prior art circuits of this type can be realized. Figure 17 shows an HBT-regulated HEMT low-noise amplifier 278. Because the characteristics of an HEMT amplifier significantly changes over time, it is known in the art to provide an HBT operational amplifier (OPAMP) in order to regulate the input signal being applied to the base terminal of the HEMT amplifier. For the amplifier 278, input signals are applied to the input terminals of an HBT operational amplifier 280. An output signal of the HBT operational amplifier 280 is applied to a base terminal of an HEMT low-noise amplifier 282 to achieve the regulation. By monolithically integrating the HBT operational amplifier 280 and the HEMT amplifier to 282 on a common substrate, certain advantages, such as lower transition losses between circuit components, can be realized over the prior art HBT-regulated HEMT low-noise amplifiers. Figure 18 shows a transmit-receive module 284, known in the art, in a block diagram form, that uses an HEMT low-noise amplifier 286 for the receive function, PIN diode switches 288 and 290, and an HBT power amplifier 292 for the transmit function. The operation of a transmit-receive module configured in this manner is well known in the art. By monolithically integrating the HEMT amplifier 286, the PIN-diode switches 288 and 290, and the HBT power amplifier 292 on a common substrate according to the invention, certain advantages over the prior art transmit-receive modules incorporating these components can be realized. The applications for monolithic PIN-HEMT-HBT integrated circuits and for superconducting film integration with each of the discussed technologies is not limited to the specific circuits or applications disclosed herein. The techniques disclosed are applicable to fabrication of a variety of novel microwave and optoelectronic circuits that incorporate HEMT, HBT, PIN-diode, MESFET, or superconducting films in a variety of combinations to achieve a variety of advantages not currently available with single fabrication technologies alone.

Claims: A method of fabricating monolithically integrated circuit devices on a common substrate by a process of molecular beam epitaxy (MBE), the method comprising the steps of: forming a heterojunction bipolar transistor (HBT) (52, 112), a diode (54, 114) and either a metal semiconductor field effect transistor (MESFET) or a high electron mobility transistor (HEMT) (56, 116) on a common substrate (58, 118), wherein said diode is formed at a position lateral between said HBT and said semiconductor field effect transistor, all semiconductor layers (80, 122; 86, 124; 126) of said diode and respective lower layers (60, 64, 66) of said HBT are formed simultaneously, and said forming of said HEMT or said MESFET is performed subsequently to the forming of all semiconductor layers of said diode and of said HBT, and said simultaneous forming of said HBT and said diode including the steps of: depositing the semiconductor layers for said HBT on the substrate by the MBE process; depositing a first dielectric layer on the semiconductor layers for said HBT, removing a portion of the first dielectric layer and said semiconductor layers for said HBT at selected areas so that a portion thereof remains on the substrate and a first portion of the substrate is exposed; separating the diode from the HBT by implanting an implant isolation region (82,120) into said semiconductor layers for said HBT, and etching the semiconductor layers above first and second diode layers; whereby the sub-collector layer (60) of said HBT and said first diode layer (80, 122), both being separated by the implant isolation region, have the same predetermined thickness and composition and an epitaxial bond with said substrate; whereby the collector layer (64) of the HBT and said second diode layer (86, 124), both being separated patterning, have the same predetermined thickness and composition, and forming semiconductor layers of said HEMT or MESFET by selective MBE on said exposed first portion of the substrate. The method of Claim 1, wherein said simultaneous forming of said HBT and said diode includes the step: etching a third diode layer (126) to reduce its thickness to be less than the base layer (66) of said HBT, whereby said base layer and (60) said third diode layer have the same composition. The method of Claim 1 or 2, wherein said forming of said diode includes: forming of a PIN diode or forming of a Schottky diode. A monolithically integrated circuit comprising: a heterojunction bipolar transistor (HBT) (52, 112), a diode (54, 114) and a metal semiconductor field effect transistor (MESFET) or a high electron mobility transistor (HEMT) (56, 116) on a common planar substrate (58, 118) and each having an epitaxial bond to said substrate, wherein the sub-collector layer (60) and a first diode layer (80, 122) are simultaneously formed on said planar substrate by molecular beam epitaxy, whereby said sub-collector layer and said first diode layer have the same thickness and composition, and the collector layer (64) and a second diode layer (86, 124) are simultaneously formed by molecular beam epitaxy on said sub-collector layer and said first diode layer, respectively, whereby both said collector layer and said second diode layer have the same thickness and composition, and said diode being arranged at a position lateral between said HBT and said MESFET or HEMT and is isolated from the HBT by an implanted isolation region (82, 120) between said subcollector layer and said first diode layer. The monolithically integrated circuit of Claim 4, wherein said HBT and said diode comprise: a base layer (66) being formed on said collector layer (64), said base layer having the same composition as a third layer formed on said second diode layer. The monolithically integrated circuit of Claim 4 or 5, wherein said diode is a PIN diode or a Schottky diode.

Inventors: KOBAYASHI KEVIN W, OKI AARON K, STREIT DWIGHT C, UMEMOTO DONALD K, KOBAYASHI, KEVIN W., OKI, AARON K., STREIT, DWIGHT C., UMEMOTO, DONALD K.
Applicants: TRW INC, TRW INC.
Application year: 1995
Main classification: H01L27
IPC codes: H01L21, H01L29, H01L27