A GaN-based HEMT and piezoelectric MEMS resonator monolithic integrated structure and a preparation method thereof

Abstract

This invention discloses a monolithic integrated structure and fabrication method of a GaN-based HEMT and piezoelectric MEMS resonator, comprising: an HEMT and a piezoelectric MEMS resonator; including: a substrate layer; an aluminum nitride layer disposed on the substrate layer; a gallium nitride layer disposed on the aluminum nitride layer; a gallium aluminum nitride layer disposed on the gallium nitride layer; a first metal electrode and a second metal electrode disposed on the gallium aluminum nitride layer; and a cavity structure formed in the substrate below the piezoelectric MEMS resonator. Beneficial effects: This invention integrates the HEMT and the piezoelectric MEMS resonator on the same substrate, simultaneously realizing the monolithic fabrication of active devices and RF MEMS devices using the GaN material system. This avoids the additional parasitic capacitance, inductance, and interconnection losses caused by traditional discrete packaging or heterogeneous integration, improves the transmission characteristics of high-frequency signals, and reduces impedance matching difficulty.

Description

Technical Field

[0001] This invention relates to the field of semiconductors, and more particularly to a monolithic integrated structure of GaN-based HEMT and piezoelectric MEMS resonator and its fabrication method. Background Technology

[0002] In recent years, with the rapid development of wireless communication technology, especially in cognitive radio, software-defined radio, and reconfigurable communication systems, the demand for high-frequency, highly integrated, low-power, and tunable radio frequency front-end devices has become increasingly urgent. Micromechanical resonators, due to their high quality factor, small size, and compatibility with integrated circuit processes, have become an important candidate technology for realizing high-performance filters and frequency reference sources. Meanwhile, gallium nitride-based high electron mobility transistors (GaN-based transistors) exhibit significant advantages in high-frequency and high-power applications due to their wide bandgap, high breakdown electric field, and high electron saturation drift velocity.

[0003] Currently, research has attempted to integrate GaN HEMTs with piezoelectric MEMS resonators to achieve monolithic integration of RF front-end modules. However, most existing integration schemes employ discrete device packaging or heterogeneous integration, resulting in significant parasitic effects, impedance matching difficulties, and large system size. More critically, current technologies lack the ability to flexibly control the resonator's operating state (e.g., on / off). Specifically, once a traditional piezoelectric MEMS resonator is fabricated, its electromechanical transduction characteristics are fixed, making it difficult to achieve on-demand switching or frequency reconfiguration within the system, thus limiting its application in reconfigurable RF front-ends. Summary of the Invention

[0004] Purpose of the invention: The purpose of this invention is to solve the technical problems in the prior art and provide a GaN... Monolithic integrated structure and fabrication method of HEMT and piezoelectric MEMS resonators.

[0005] Technical solution: In a first aspect, this application proposes a monolithic integrated structure of a GaN-based HEMT and a piezoelectric MEMS resonator, comprising: an HEMT and a piezoelectric MEMS resonator; including: Substrate layer; An aluminum nitride layer disposed on the substrate layer; A gallium nitride layer disposed on the aluminum nitride layer; A gallium aluminum nitride layer disposed on the gallium nitride layer; The first metal electrode and the second metal electrode are disposed on the gallium nitride aluminum layer; A cavity structure is formed in the substrate beneath the piezoelectric MEMS resonator.

[0006] Preferably, the substrate layer is made of silicon, doped silicon, germanium, silicon-germanium, or a semiconductor composite material, or any combination thereof.

[0007] Preferably, the piezoelectric MEMS resonator is a surface acoustic wave resonator or a Lamb wave resonator.

[0008] Preferably, the piezoelectric MEMS resonator excites the vibration of the gallium nitride aluminum / gallium nitride stack structure by applying an alternating electric field between the first metal electrode and the second metal electrode connected by a two-dimensional electron gas channel.

[0009] Preferably, by applying a negative bias voltage to the second metal electrode, the two-dimensional electron gas channel is depleted, the driving transducer capacitance is reduced, the electromechanical conversion is suppressed, and the piezoelectric MEMS resonator enters the off state.

[0010] Preferably, the cavity structure is formed by etching the substrate to release the piezoelectric micromechanical resonator, reduce energy loss, and improve the quality factor.

[0011] Preferably, the first metal electrode and the second metal electrode are one or more multilayer metal combinations selected from aluminum, nickel, molybdenum, gold, and titanium.

[0012] This application also proposes a method for fabricating a monolithic integrated structure of GaN-based HEMT and piezoelectric MEMS resonator, including the following steps: Step 1: Provide a substrate and deposit an aluminum nitride layer on the substrate surface; Step 2: Sequentially deposit a gallium nitride layer and a gallium aluminum nitride layer on the aluminum nitride layer, and perform patterned etching on the gallium aluminum nitride layer; Step 3: Etch gallium nitride and aluminum nitride layers to the substrate to form a groove and define the resonator region; Step 4: Etch gallium nitride aluminum layer and gallium nitride layer to the two-dimensional electron gas, deposit and pattern the first metal electrode to realize the two-dimensional electron gas electrical lead-out of HEMT and piezoelectric MEMS resonator; Step 5: Deposit and pattern the second metal electrode to achieve electrical lead-out; Step 6: Isotropically etch the substrate using a wet etching process to release the resonator structure.

[0013] Preferably, the gallium nitride layer and / or the gallium nitride aluminum layer are prepared using molecular beam epitaxy or metal-organic chemical vapor deposition.

[0014] Preferably, the aluminum nitride layer is prepared using plasma-enhanced deposition or magnetron sputtering. Beneficial effects

[0015] This invention integrates HEMT and piezoelectric MEMS resonators on the same substrate, and utilizes the GaN material system to simultaneously realize the monolithic fabrication of active devices and RF MEMS devices. This avoids the additional parasitic capacitance, inductance and interconnection losses caused by traditional discrete packaging or heterogeneous integration, improves the transmission characteristics of high-frequency signals, and reduces the difficulty of impedance matching.

[0016] This invention directly utilizes the inherent two-dimensional electron gas channel in the AlGaN / GaN heterojunction as the bottom electrode of the piezoelectric MEMS resonator, eliminating the need for additional bottom electrode material deposition, thus simplifying the process while ensuring excellent electroacoustic transduction efficiency. The two-dimensional electron gas exhibits high mobility and low resistance, which helps reduce the dynamic resistance of the resonator and improve the electromechanical coupling coefficient.

[0017] By applying a negative bias voltage to the top electrode (second metal electrode) of the resonator, the two-dimensional electron gas channel can be partially depleted, thereby cutting off the electromechanical transduction path and putting the resonator into a shut-off state. After the negative bias voltage is removed, the two-dimensional electron gas recovers, and the resonator operates normally. This mechanism enables flexible control of the resonator's operating state by DC voltage, providing a key device foundation for constructing reconfigurable filters, switching frequency sources, and tunable RF front-ends in cognitive radio systems.

[0018] A cavity structure is etched in the substrate below the resonator to effectively release the mechanical vibration region of the resonator, reduce the leakage of acoustic energy through the substrate, significantly reduce energy loss, thereby obtaining a higher quality factor and improving the frequency selection characteristics of the filter and the frequency stability of the oscillator.

[0019] The preparation method of this invention is based on mature GaN HEMT processes (such as MBE, MOCVD, metal stripping, wet etching, etc.). It only adds a few etching steps to achieve the release of the resonator and the formation of the cavity. It has strong process compatibility, is easy to promote on existing GaN production lines, and has good industrialization prospects.

[0020] The substrate can be made of various materials such as silicon, germanium, and semiconductor composites. The top electrode can be made of multilayer metal combination. The resonator can be designed as surface acoustic wave or Lamb wave mode, which can be flexibly adjusted according to specific application requirements to adapt to RF systems of different frequencies and power levels. Attached Figure Description

[0021] Figure 1 A schematic diagram of the method framework for this invention is provided; Figure 2 This is a schematic diagram of the structure after aluminum nitride layer deposition during the processing of the integrated structure proposed in this invention; Figure 3 This is a schematic diagram of the structure after gallium nitride layer growth in an embodiment of the present invention; Figure 4This is a schematic diagram of the structure after gallium nitride aluminum layer growth, gallium nitride etching, and gallium nitride aluminum layer etching in an embodiment of the present invention; Figure 5 This is a schematic diagram of the structure after the first deposition and patterning of the metal in an embodiment of the present invention; Figure 6 This is a schematic diagram of the structure after the second deposition and patterning of the metal in an embodiment of this disclosure; Figure 7 This is a schematic diagram of the structure after etching the substrate in an embodiment of this disclosure.

[0022] Figure 8 This is a schematic diagram of the complete structure in an embodiment of this disclosure.

[0023] 1. Substrate silicon; 2. Aluminum nitride; 3. Gallium nitride; 4. Aluminum gallium nitride; 5. First metal electrode; 6. Second metal electrode; 7. HEMT; 8. Piezoelectric MEMS resonator. Detailed Implementation

[0024] To make the technical solution of the present invention clearer, the present invention will be further described in detail below with reference to the accompanying drawings and specific embodiments. Example

[0025] To make the objectives, technical solutions, and advantages of this invention clearer, the technical solutions in the embodiments of this invention will be clearly and completely described below with reference to the accompanying drawings. Obviously, the described embodiments are only some, not all, of the embodiments of this invention. All other embodiments obtained by those skilled in the art based on the embodiments of this invention without inventive effort are within the scope of protection of this invention. Unless otherwise defined, the technical or scientific terms used herein should have the ordinary meaning understood by those skilled in the art. The terms "including" and similar expressions used herein mean that the element or object preceding the term covers the element or object listed after the term and its equivalents, but do not exclude other elements or objects.

[0026] In response to the problems existing in the current technology, such as Figure 1 As shown, a monolithic integrated structure of GaN-based HEMT7 and piezoelectric MEMS resonator 8 is proposed, comprising: HEMT7 (High Electron Mobility Transistor) and piezoelectric MEMS resonator 8 (RF piezoelectric micromechanical resonator); including: Substrate layer; An aluminum nitride 2 layer is disposed on the substrate layer; A gallium nitride 3 (GaN) layer disposed on the aluminum nitride 2 layer; A gallium nitride aluminum 4 layer is disposed on the gallium nitride 3 layer (forming a heterojunction with GaN, generating a high-concentration, high-mobility two-dimensional electron gas at the interface). A first metal electrode 5 and a second metal electrode 6 are disposed on the gallium nitride aluminum 4 layer (wherein the first metal electrode 5 forms an ohmic contact with the two-dimensional electron gas channel, serving as the source / drain electrode of the HEMT7 and the bottom electrode lead-out of the resonator; the second metal electrode 6 is disposed above the gallium nitride aluminum 3 layer in the resonator region, serving as the top electrode of the resonator. The electrode material is a titanium / gold (Ti / Au) bilayer metal, formed by electron beam evaporation and metal lift-off processes). A cavity structure is formed in the substrate below the piezoelectric MEMS resonator 8 (located in the substrate directly below the piezoelectric MEMS resonator 8, formed by isotropic wet etching, allowing the suspended film structure in the resonator region to vibrate freely). Although there is a cavity, conventional support beams are also provided for support (not shown in the figure) to ensure the stability of the structure.

[0027] During normal operation, no DC bias is applied to the second metal electrode 6, and the two-dimensional electron gas channel is in a conductive state with a high electron concentration. At this time, a small AC signal (such as 10mV) is applied between the first metal electrode 5 and the second metal electrode 6. The alternating electric field excites the resonator to vibrate through the piezoelectric effect, generating a significant admittance peak at the resonant frequency.

[0028] When the resonator needs to be turned off, a negative bias voltage (e.g., -5 V to -10 V, depending on the AlGaN layer thickness and doping) is applied to the second metal electrode 6. This negative bias voltage depletes the two-dimensional electron gas at the underlying AlGaN / GaN interface through the Schottky junction, causing a sharp increase in channel resistance and almost complete depletion of the two-dimensional electron gas. At this time, the capacitance between the first metal electrode 5 and the second metal electrode 6 decreases significantly, the AC electric field cannot effectively drive the piezoelectric stack vibration, the electromechanical conversion is suppressed, the resonator response disappears, and it enters the "off" state. After the negative bias voltage is removed, the two-dimensional electron gas automatically recovers, and the resonator is turned back on. This process can be repeated multiple times, exhibiting good reversibility and stability.

[0029] Without a cavity (i.e., the resonator is directly on a complete substrate), the mechanical vibration energy of the resonator dissipates through the substrate, resulting in a quality factor typically below 500. Through isotropic wet etching in step 6 (using a mixed solution of hydrofluoric acid, nitric acid, and acetic acid to laterally etch silicon), an undercut cavity is formed beneath the resonator, leaving only a few support points or complete suspension between the resonant diaphragm and the substrate. Experimental results show that the released Lamb wave resonator achieves a quality factor exceeding 1500 near 1 GHz, reduces energy loss by approximately 70%, and significantly improves the sharpness of the resonance peak, which is beneficial for realizing highly selective filters.

[0030] In some specific embodiments, the substrate layer is made of silicon, doped silicon, germanium, silicon-germanium, semiconductor composites, or any combination thereof. It possesses good mechanical strength and thermal conductivity.

[0031] In some specific embodiments, the piezoelectric MEMS resonator 8 is a surface acoustic wave resonator or a Lamb wave resonator, providing a good lattice-matching interface for subsequent GaN growth.

[0032] In this embodiment, the piezoelectric MEMS resonator 8 is a Lamb wave resonator. It utilizes the inverse piezoelectric effect of the AlGaN / GaN stacked structure. By applying an alternating electric field between the first metal electrode 5 (connected to 2DEG) and the second metal electrode 6, the entire stack is excited to generate stretching vibration in the thickness direction, thereby forming resonance at a specific frequency.

[0033] In some specific embodiments, the piezoelectric MEMS resonator 8 excites the vibration of the gallium nitride aluminum 4 / gallium nitride 3 stacked structure by applying an alternating electric field between the first metal electrode 5 and the second metal electrode 6 connected by a two-dimensional electron gas channel.

[0034] In some specific embodiments, by applying a negative bias voltage to the second metal electrode 6, the two-dimensional electron gas channel is depleted, the driving transducer capacitance is reduced, the electromechanical conversion is suppressed, and the piezoelectric MEMS resonator 8 is turned off.

[0035] In some specific embodiments, the cavity structure is formed by etching the substrate to release the piezoelectric micromechanical resonator, reduce energy loss, and improve the quality factor.

[0036] In some specific embodiments, the first metal electrode 5 and the second metal electrode 6 are one or more multilayer metal combinations selected from aluminum, nickel, molybdenum, gold, and titanium.

[0037] This application also proposes a method for fabricating a monolithic integrated structure of GaN-based HEMT7 and piezoelectric MEMS resonator 8, including the following steps: Step 1, Combining Figure 2 A substrate is provided, and two layers of aluminum nitride are deposited on its surface. These two aluminum nitride layers are deposited using magnetron sputtering. This seed layer promotes the subsequent preferential orientation growth of GaN and reduces dislocation density. Step 2, Combining Figure 2-4 Gallium nitride (GaN) layer 3 and GaN 3 aluminum layer are deposited sequentially on aluminum nitride layer 2, and the GaN 3 aluminum layer is patterned and etched. Then, the GaN 3 aluminum layer is patterned by photolithography and dry etching (such as Cl2 / BCl3 plasma etching) to define the mesa isolation between the HEMT7 device region and the resonator region. Step 3, Combining Figure 4The three layers of gallium nitride and the two layers of aluminum nitride are etched to the substrate to form a groove, defining the resonator region; a ring groove is formed, which partially separates the future resonator diaphragm from the surrounding area, facilitating subsequent release; Step 4, Combining Figure 5 The gallium nitride (GaN) aluminum layer and GaN 3 layer are etched down to the two-dimensional electron gas region. The first metal electrode 5 is then deposited and patterned to achieve the two-dimensional electron gas electrical lead-out for the HEMT7 and the piezoelectric MEMS resonator 8. A low-damage etching process (such as digital etching) is used to locally remove the GaN aluminum layer and part of the GaN 3 layer, exposing the two-dimensional electron gas channel region. Then, a Ti / Al / Ni / Au metal stack is deposited by electron beam evaporation and patterned using a lift-off process, followed by rapid thermal annealing to form ohmic contacts. This first metal electrode 5 serves simultaneously as the source / drain of the HEMT7 and the lead-out terminal of the resonator's bottom electrode.

[0038] Step 5, Combining Figure 6 The second metal electrode 6 is deposited and patterned to achieve electrical lead-out. A SiNx layer is deposited on the device surface as an insulating protective layer, then the contact hole is etched, and Ni / Au (20 / 150 nm) is deposited as the second metal electrode 6 (Schottky contact) for applying switching bias and excitation signal. This electrode also serves as the gate electrode of HEMT7 (if needed).

[0039] Step 6, Combining Figure 7-8 The resonator structure is released by isotropic etching of the substrate using a wet etching process. An isotropic silicon wet etching solution (e.g., an HNA solution: hydrofluoric acid, nitric acid, and acetic acid in a volume ratio of 1:3:8) is used to laterally etch the silicon substrate 1 through the grooves formed in step 3, creating an undercut cavity. After etching, the resonator diaphragm is completely released. Finally, the substrate is rinsed with deionized water and dried to complete device fabrication.

[0040] In some specific embodiments, the gallium nitride 3 layer and / or the gallium nitride 3 aluminum layer are prepared by molecular beam epitaxy or metal-organic chemical vapor deposition.

[0041] In some specific embodiments, the aluminum nitride layer 2 is prepared using plasma-enhanced deposition or magnetron sputtering techniques.

[0042] The above description is merely a specific implementation of the embodiments of the present invention, but the protection scope of the embodiments of the present invention is not limited thereto. Any changes or substitutions within the technical scope disclosed in the embodiments of the present invention should be covered within the protection scope of the embodiments of the present invention. Therefore, the protection scope of the embodiments of the present invention should be determined by the protection scope of the claims.

Claims

1. A monolithic integrated structure of GaN-based HEMT and piezoelectric MEMS resonator, characterized in that, include: HEMT and piezoelectric MEMS resonators; These include: Substrate layer; An aluminum nitride layer disposed on the substrate layer; A gallium nitride layer disposed on the aluminum nitride layer; A gallium aluminum nitride layer disposed on the gallium nitride layer; The first metal electrode and the second metal electrode are disposed on the gallium nitride aluminum layer; A cavity structure is formed in the substrate beneath the piezoelectric MEMS resonator.

2. The integrated structure of claim 1, wherein, The substrate material is one or any combination of silicon, doped silicon, germanium, silicon-germanium, and semiconductor composites.

3. The integrated structure of claim 1, wherein, The piezoelectric MEMS resonator is a surface acoustic wave resonator or a Lamb wave resonator.

4. The integrated structure of claim 1, wherein, The piezoelectric MEMS resonator excites the vibration of the gallium nitride aluminum / gallium nitride stack structure by applying an alternating electric field between the first and second metal electrodes connected by a two-dimensional electron gas channel.

5. The integrated structure of claim 1, wherein, By applying a negative bias voltage to the second metal electrode, the two-dimensional electron gas channel is depleted, the driving transducer capacitance is reduced, the electromechanical conversion is suppressed, and the piezoelectric MEMS resonator enters the off state.

6. The integrated structure of claim 1, wherein, The cavity structure is formed by etching the substrate to release the piezoelectric micromechanical resonator, reduce energy loss, and improve the quality factor.

7. The integrated structure of claim 1, wherein, The first metal electrode and the second metal electrode are one or more multilayer metal combinations selected from aluminum, nickel, molybdenum, gold, and titanium.

8. A method for fabricating a monolithic structure of a GaN-based HEMT and a piezoelectric MEMS resonator, characterized by, Including the following steps: Step 1: Provide a substrate and deposit an aluminum nitride layer on the substrate surface; Step 2: Sequentially deposit a gallium nitride layer and a gallium aluminum nitride layer on the aluminum nitride layer, and perform patterned etching on the gallium aluminum nitride layer; Step 3: Etch gallium nitride and aluminum nitride layers to the substrate to form a groove and define the resonator region; Step 4: Etch gallium nitride aluminum layer and gallium nitride layer to the two-dimensional electron gas, deposit and pattern the first metal electrode to realize the two-dimensional electron gas electrical lead-out of HEMT and piezoelectric MEMS resonator; Step 5: Deposit and pattern the second metal electrode to achieve electrical lead-out; Step 6: Isotropically etch the substrate using a wet etching process to release the resonator structure.

9. The production method according to claim 8, characterized by, The gallium nitride layer and / or the gallium nitride aluminum layer are prepared using molecular beam epitaxy or metal-organic chemical vapor deposition techniques.

10. The preparation method according to claim 8, characterized in that, The aluminum nitride layer is prepared using plasma-enhanced deposition or magnetron sputtering.

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