A preparation method of a monolithic integrated low-phase-noise wideband voltage-controlled oscillator and the monolithic integrated low-phase-noise wideband voltage-controlled oscillator

Abstract

This invention discloses a method for fabricating a monolithically integrated low-phase-noise broadband voltage-controlled oscillator (VCO) and the VCO itself. The method includes fabricating an integrated circuit on a main substrate; forming a varactor mesa structure on a varactor substrate; thinning the back side of the varactor substrate; permanently bonding the processed varactor wafer to the main substrate; depositing a metal interconnect layer on the surface of the bonded main substrate to form a complete oscillation circuit; and dicing the bonded wafer into individual chips using a wafer dicing process to obtain the monolithically integrated low-phase-noise broadband VCO. This invention integrates a high-performance superjunction variable capacitor diode, a compound semiconductor microwave negative resistance circuit, and an oscillator circuit onto a single chip using heterogeneous integration technology, achieving a monolithically integrated low-phase-noise broadband VCO with advantages such as lower phase noise, a wider tuning range, and a smaller chip area.

Description

Technical Field

[0001] This invention relates to the field of microwave and millimeter-wave integrated circuit technology, specifically to a method for fabricating a monolithically integrated low-phase-noise broadband voltage-controlled oscillator and the monolithically integrated low-phase-noise broadband voltage-controlled oscillator. Background Technology

[0002] As a core component of electronic systems such as communication and radar, the phase noise and tuning bandwidth of voltage-controlled oscillators (VCOs) directly affect system performance. Traditional monolithic microwave integrated circuit VCOs are typically implemented using single semiconductor processes such as Si CMOS or GaAs HBTs. However, due to material limitations, their phase noise performance and tuning bandwidth cannot simultaneously meet the demands of high-end applications. GaAs-based super-junction variable capacitor diodes (SJVDCs) possess extremely high capacitance tuning ratios and low-loss characteristics, making them ideal devices for achieving low-phase-noise broadband frequency tuning. However, traditional CMOS or compound semiconductor processes cannot directly integrate high-performance GaAs-based SJVDCs, limiting further improvements in oscillator performance. The current mainstream solution is to use hybrid integration technology, integrating independently packaged SJVDCs with the MMIC oscillator via bonding or flip-chip methods. This method suffers from problems such as large parasitic effects, large package size, and low reliability, failing to meet the high-performance, miniaturized application requirements of the millimeter-wave band. Summary of the Invention

[0003] Purpose of the invention: This invention proposes a method for fabricating a monolithically integrated low-phase-noise broadband voltage-controlled oscillator and the monolithically integrated low-phase-noise broadband voltage-controlled oscillator. By using heterogeneous integration technology, a resonant circuit composed of a super-abrupt junction variable capacitor diode and a compound semiconductor negative resistance circuit are integrated on the same chip to realize a low-phase-noise, broadband tuned voltage-controlled oscillator.

[0004] Technical Solution: This invention proposes a method for fabricating a monolithically integrated low-phase-noise broadband voltage-controlled oscillator, comprising the following steps:

[0005] Step 1: Form a mesa-type varactor on the varactor wafer substrate, and deposit an electrode on the upper surface of the mesa-type varactor;

[0006] Step 2: Coat the surface of the varactor wafer substrate with a temporary bonding agent. The temporary bonding agent wraps around the mesa varactor and the upper electrode. Set a temporary carrier and temporarily bond it to the temporary bonding agent.

[0007] Step 3: Thin the back side of the varactor wafer substrate and etch away the portion of the varactor wafer substrate that is not in contact with the mesa varactor.

[0008] Step 4: Deposit electrodes and bonding metal on the back side of the thinned and etched varactor wafer substrate to obtain a mesa-type varactor wafer;

[0009] Step 5: Permanently bond the mesa-shaped varactor wafer obtained in Step 4 to the main substrate, which has an integrated circuit, and the bonding metal is electrically connected to the integrated circuit.

[0010] Step 6: After separating the temporary carrier and temporary bonding agent from the mesa-type varactor and the upper electrode, the integrated structure is obtained;

[0011] Step 7: Deposit a metal interconnect structure on the surface of the integrated structure obtained in Step 6;

[0012] Step 8: Thin the back side of the main substrate and form a ground via and a back metal layer;

[0013] Step 9: Divide the integrated structure obtained in Step 8 into individual monolithic integrated low phase noise broadband voltage-controlled oscillators using a wafer dicing process.

[0014] Preferably, in step one, the thickness of the varactor wafer substrate is 300μm-750μm, the mesa-type varactor has a super-abrupt junction structure, the material of the mesa-type varactor is GaAs, the size of the upper mesa of the mesa-type varactor is 5μm-500μm, the height of the mesa is 2μm-5μm, and the upper electrode is one of TiPtAu and TiAu.

[0015] Preferably, in step two, the temporary bonding agent is one of photoresist and paraffin, and the temporary carrier is one of glass, sapphire, silicon or silicon carbide; the thickness of the temporary carrier is 500μm~1000μm; the temporary bonding temperature is 150~300℃, the temporary bonding pressure is 200MPa~2000MPa, and the time is 5-30 minutes.

[0016] Preferably, the method for thinning the varactor wafer substrate in step three includes any one or more combinations of grinding, polishing, and etching, and the thickness after thinning is 0μm~10μm.

[0017] Preferably, the lower electrode in step four is one of AuGeNiAu, TiPtAu, or WtiAuTi, and the bonding metal material is one of gold, copper, nickel, gold-tin, or gold-indium, with a thickness of 500nm-5μm.

[0018] Preferably, the permanent bonding method in step five is one of eutectic bonding, thermo-press bonding, and hydrophilic bonding; the bonding temperature is from room temperature to 500°C; the bonding time is from 10 minutes to 3 hours; and the bonding pressure is from 1000N to 60000N. The main substrate material in step five is a compound semiconductor, one of GaAs and InP. The main substrate size is 3 to 6 inches. The integrated circuit in step five includes a microwave negative resistance circuit and an oscillator circuit. The microwave negative resistance circuit and the oscillator circuit include active devices, passive components, and microstrip line interconnects and matching structures. The active devices include one of HEMT and HBT, and the passive devices include one or more of resistors, capacitors, and inductors.

[0019] Preferably, the separation method in step six includes one of heating separation, laser separation, and solution immersion separation.

[0020] Preferably, the metal interconnect structure forms a closed loop with each device in the integrated circuit, the bonding metal, the lower electrode, and the upper electrode. In step seven, the metal interconnect structure includes an air bridge, a dielectric bridge, and a microstrip line. The metal interconnect structure material is either gold or copper.

[0021] Preferably, in step eight, the thickness of the main substrate after thinning is 50μm-120μm, the diameter of the grounding via is 20μm-100μm, and the back metal layer material is either gold or copper.

[0022] A monolithic integrated low-phase-noise broadband voltage-controlled oscillator obtained according to the preparation method described above.

[0023] Beneficial Effects: This invention proposes a method for fabricating a monolithically integrated low-phase-noise broadband voltage-controlled oscillator and the monolithically integrated low-phase-noise broadband voltage-controlled oscillator itself. Through heterogeneous integration technology, a high-performance super-abrupt junction variable capacitor diode, a compound semiconductor microwave negative resistance circuit, and an oscillator circuit are integrated onto the same chip, offering the following significant advantages: 1) Low Phase Noise: The super-abrupt junction variable capacitor diode exhibits extremely low phase noise characteristics, reducing oscillator phase noise by 3-5 dBc / Hz compared to traditional varactor diodes. 2) Broadband Tuning: The capacitance tuning ratio of the super-abrupt junction variable capacitor diode can be extended to 20%-30%, significantly superior to traditional monolithic oscillators. 3) Miniaturization: Employing wafer-level heterogeneous integration technology, devices that previously required separate packaging are integrated onto the same chip, reducing the chip area by more than 50%, meeting system miniaturization requirements. 4) High Performance: Through optimized metal interconnect structure, parasitic effects are reduced, the circuit Q value is improved, and the phase noise and output power performance of the oscillator are further enhanced. Attached Figure Description

[0024] Figure 1 This is a schematic diagram of the structure of the integrated circuit after it has been fabricated on the main substrate according to the present invention;

[0025] Figure 2 This is a schematic diagram of the structure of the present invention after forming a mesa-shaped varactor and an upper electrode on a varactor wafer;

[0026] Figure 3 This is a schematic diagram of the structure of the varactor wafer after temporary bonding according to the present invention;

[0027] Figure 4 This is a schematic diagram of the structure of the varactor diode wafer substrate after thinning.

[0028] Figure 5 This is a schematic diagram of the structure after the lower electrode and bonding metal are fabricated on the back side of the thinned varactor wafer substrate according to the present invention;

[0029] Figure 6 This is a schematic diagram of the structure of the varactor wafer after bonding with the main substrate according to the present invention;

[0030] Figure 7 This is a schematic diagram of the integrated structure of the varactor diode wafer and the main substrate of the present invention after the temporary support carrier is removed.

[0031] Figure 8 This is a schematic diagram of the structure after the deposition of the dielectric layer and the metal interconnect structure in the integrated structure of the varactor wafer and the main substrate according to the present invention.

[0032] Figure 9 This is a schematic diagram of the structure after thinning the main substrate wafer and forming a grounding via and a back metal layer according to the present invention. Detailed Implementation

[0033] Reference Figures 1-9 This invention proposes a method for fabricating a monolithically integrated low-phase-noise broadband voltage-controlled oscillator, comprising the following steps:

[0034] Step 1: Form a mesa-type varactor 3 on the varactor wafer substrate 5, and deposit an upper electrode 4 on the upper surface of the mesa-type varactor 3; the thickness of the varactor wafer substrate 5 is 300μm-750μm, the mesa-type varactor 3 is a super-abrupt junction structure, the material of the mesa-type varactor 3 is GaAs, the size of the mesa on the mesa-type varactor 3 is 5μm-500μm, the height of the mesa is 2μm-5μm, and the upper electrode 4 is one of TiPtAu or TiAu.

[0035] Step 2: A temporary bonding agent 7 is coated on the surface of the varactor wafer substrate 5. The temporary bonding agent 7 wraps around the mesa-type varactor 3 and the upper electrode 4. A temporary carrier 6 is set up and temporarily bonded to the temporary bonding agent 7. The temporary bonding agent 7 is one of photoresist and paraffin wax. The temporary carrier 6 is one of glass, sapphire, silicon or silicon carbide. The thickness of the temporary carrier 6 is 500μm~1000μm. The temporary bonding temperature is 150~300℃, the temporary bonding pressure is 200MPa~2000MPa, and the time is 5-30 minutes.

[0036] Step 3: Thin the back side of the varactor wafer substrate 5; and etch away the part of the varactor wafer substrate 5 that is not in contact with the mesa varactor 3. The thinning method of the varactor wafer substrate 5 includes any one or more combinations of grinding, polishing and etching, and the thickness after thinning is 0μm~10μm.

[0037] Step 4: Deposit a lower electrode 8 and a bonding metal 9 on the back side of the thinned and etched varactor wafer substrate 5 to obtain a mesa-type varactor wafer; the lower electrode 8 is one of AuGeNiAu, TiPtAu, WtiAuTi, and the bonding metal 9 is one of gold, copper, nickel, gold-tin, or gold-indium, and the thickness of the bonding metal 9 is 500nm~5μm.

[0038] Step 5: Permanently bond the mesa-type varactor wafer obtained in Step 4 to the main substrate 1 containing the integrated circuit 2; fabricate the integrated circuit 2 on the main substrate 1. The material of the main substrate 1 is a compound semiconductor, namely GaAs or InP. The size of the main substrate 1 is 3 to 6 inches. The integrated circuit 2 includes a microwave negative resistance circuit and an oscillator circuit. The microwave negative resistance circuit and the oscillator circuit include active devices, passive components, and microstrip line interconnects and matching structures. The active devices include one of HEMT and HBT. The passive devices include one or more of resistors, capacitors, and inductors. The permanent bonding method is one of eutectic bonding, thermo-press bonding, and hydrophilic bonding. The bonding temperature is from room temperature to 500°C, the bonding time is from 10 minutes to 3 hours, and the bonding pressure is from 1000N to 60000N.

[0039] Step 6: After separating the temporary carrier 6 and temporary bonding agent 7 from the mesa-type varistor 3 and the upper electrode 4, an integrated structure is obtained; the separation method includes one of heating separation, laser separation, and solution immersion separation.

[0040] Step 7: Deposit a metal interconnect structure 10 on the surface of the integrated structure obtained in Step 6; the metal interconnect structure 10 forms a closed loop with each device in the integrated circuit 2, the bonding metal 9, the lower electrode 8 and the upper electrode 4; the metal interconnect structure 10 includes an air bridge, a dielectric bridge and a microstrip line, and the material of the metal interconnect layer 10 is one of gold and copper.

[0041] Step 8: Thin the back side of the main substrate 1 and form a grounding via 11 and a back metal layer 12; the thickness of the thinned main substrate 1 is 50μm-120μm, the diameter of the grounding via 11 is 20μm-100μm, and the material of the back metal layer 12 is either gold or copper.

[0042] Step 9: Divide the integrated structure obtained in Step 8 into individual monolithic integrated low phase noise broadband voltage-controlled oscillators using a wafer dicing process.

[0043] Example 1

[0044] Step 1: Fabricate GaAs HBT active devices, thin-film resistors, planar capacitors and inductors, as well as microstrip matching circuits on a 4-inch GaAs HBT main substrate 1 with a thickness of 625μm.

[0045] Step 2: Form a mesa-type varactor 3 on a 4-inch GaAs varactor wafer substrate 5 with a thickness of 500μm. The varactor adopts a super-abrupt junction structure. The mesa height of the mesa-type varactor 3 is 2.5μm. The upper electrode metal is TiPtAu, and SiN is used for passivation protection of the sidewalls.

[0046] Step 3: After completing Step 2, spin-coat appropriate amounts of paraffin wax onto both the upper surface and the 500μm thick surface of the 4-inch GaAs mesa varactor wafer. Sapphire serves as the temporary support substrate 6, and paraffin wax as the temporary bonding agent 7. The spin-coating speed of the paraffin wax is set to 2000 rpm, the acceleration to 1500 rpm, and the spin-coating time to 30 seconds. Place the paraffin-coated GaAs mesa varactor wafer face up on a hot plate for pre-baking. The hot plate temperature is set to 130℃ for 10 minutes. Then, place the upper surface of the GaAs superjunction varactor wafer face up against the sapphire temporary support substrate and place it in a bonding machine for temporary bonding. The temporary bonding temperature is 250℃, the temporary bonding pressure is 1500MPa, and the time is 5 minutes.

[0047] Step 4: Thin the temporary bonded varactor wafer substrate 5. First, thin it to 100μm using a polishing machine, then further thin it to 2μm using mechanical polishing and whole-wafer dry etching. Finally, use photolithography patterning and re-etching to separate the remaining GaAs varactor wafer substrate 5 between the diodes.

[0048] Step 5: AuGeNiAu is sequentially evaporated on the lower surface of the GaAs super-abrupt junction varactor wafer to prepare the lower electrode 8, and a 1μm thick layer of AuSn 20% is evaporated as the bonding metal 9.

[0049] Step 6: Place the lower surface of the GaAs super-abrupt junction varactor wafer opposite the upper surface of the GaAs HBT main substrate 1 from Step 1, and apply bonding agent to perform AuSn eutectic bonding. The bonding temperature is 320℃, the bonding time is 20 minutes, and the bonding pressure is 20000N.

[0050] Step 7: Place the bonded mesa-type varactor wafer and the integrated structure of the main substrate 1 into a debonding machine for separation, and clean them sequentially with acetone, alcohol and deionized water.

[0051] Step 8: Prepare a 230nm SiN dielectric on the integrated structure surface of the bonded mesa varactor wafer and the main substrate 1, and electroplate an air bridge with a height of about 5μm and a thickness of about 2.5μm as a metal interconnect structure to interconnect the mesa varactor 3 with the GaAs HBT active device and other passive devices and microstrip matching circuits.

[0052] Step 9: Thin the GaAs HBT main substrate 1 to 80μm and etch a grounding via 11, then electroplate an 8μm thick gold layer as the back metal layer 12.

[0053] Step 10: The integrated structure obtained in Step 9 is divided into individual monolithic integrated low-phase-noise broadband voltage-controlled oscillators using a wafer dicing process.

Claims

1. A method for fabricating a monolithically integrated low-phase-noise broadband voltage-controlled oscillator, characterized in that, Includes the following steps: Step 1: Form a mesa-type varactor (3) on the varactor wafer substrate (5) and deposit an electrode (4) on the upper surface of the mesa-type varactor (3). Step 2: A temporary bonding agent (7) is coated on the surface of the varactor wafer substrate (5). The temporary bonding agent (7) wraps the mesa varactor (3) and the upper electrode (4). A temporary carrier (6) is set up to temporarily bond with the temporary bonding agent (7). Step 3: Thin the back side of the varactor wafer substrate (5) and etch away the part of the varactor wafer substrate (5) that is not in contact with the mesa varactor (3); Step 4: Deposit the lower electrode (8) and bonding metal (9) on the back side of the thinned and etched varactor wafer substrate (5) to obtain a mesa varactor wafer; Step 5: Permanently bond the mesa-type varactor wafer obtained in Step 4 to the main substrate (1), which has an integrated circuit (2) on it, and the bonding metal (9) is electrically connected to the integrated circuit (2); Step 6: After separating the temporary substrate (6) and temporary bonding agent (7) from the mesa-type varactor tube (3) and the upper electrode (4), an integrated structure is obtained; Step 7: Deposit a metal interconnect structure (10) on the surface of the integrated structure obtained in Step 6; Step 8: Thin the back side of the main substrate (1) and form a ground via (11) and a back metal layer (12). Step 9: Divide the integrated structure obtained in Step 8 into individual monolithic integrated low phase noise broadband voltage-controlled oscillators using a wafer dicing process.

2. The method for fabricating a monolithically integrated low-phase-noise broadband voltage-controlled oscillator according to claim 1, characterized in that, The thickness of the varactor wafer substrate (5) in step one is 300μm-750μm, the mesa varactor (3) is a super-abrupt junction structure, the material of the mesa varactor (3) is GaAs, the size of the upper mesa of the mesa varactor (3) is 5μm-500μm, the height of the mesa is 2μm-5μm, and the upper electrode (4) is one of TiPtAu and TiAu.

3. The method for fabricating a monolithically integrated low-phase-noise broadband voltage-controlled oscillator according to claim 1, characterized in that, In step two, the temporary bonding agent (7) is one of photoresist and paraffin, and the temporary carrier (6) is one of glass, sapphire, silicon or silicon carbide; the thickness of the temporary carrier (6) is 500μm~1000μm; the temporary bonding temperature is 150~300℃, the temporary bonding pressure is 200MPa~2000MPa, and the time is 5-30 minutes.

4. The method for fabricating a monolithically integrated low-phase-noise broadband voltage-controlled oscillator according to claim 1, characterized in that, The method for thinning the varactor wafer substrate (5) in step three includes any one or more combinations of grinding, polishing, and etching, and the thickness after thinning is 0μm~10μm.

5. The method for fabricating a monolithically integrated low-phase-noise broadband voltage-controlled oscillator according to claim 1, characterized in that, The lower electrode (8) in step four is one of AuGeNiAu, TiPtAu, WtiAuTi, and the bonding metal (9) material is one of gold, copper, nickel, gold-tin, or gold-indium, and the thickness of the bonding metal (9) is 500nm-5μm.

6. The method for fabricating a monolithically integrated low-phase-noise broadband voltage-controlled oscillator according to claim 1, characterized in that, In step five, the permanent bonding method is one of eutectic bonding, thermo-press bonding, and hydrophilic bonding. The bonding temperature is from room temperature to 500°C, the bonding time is from 10 minutes to 3 hours, and the bonding pressure is from 1000N to 60000N. The main substrate (1) in step five is a compound semiconductor, one of GaAs and InP. The size of the main substrate (1) is 3 to 6 inches. The integrated circuit (2) in step five includes a microwave negative resistance circuit and an oscillator circuit. The microwave negative resistance circuit and the oscillator circuit include active devices, passive components, and microstrip line interconnects and matching structures. The active devices include one of HEMT and HBT, and the passive devices include one or more of resistors, capacitors, and inductors.

7. The method for fabricating a monolithically integrated low-phase-noise broadband voltage-controlled oscillator according to claim 1, characterized in that, The separation method in step six includes one of heating separation, laser separation, and solution immersion separation.

8. The method for fabricating a monolithically integrated low-phase-noise broadband voltage-controlled oscillator according to claim 1, characterized in that, The metal interconnect structure (10) forms a closed loop with each device in the integrated circuit (2), the bonding metal (9), the lower electrode (8) and the upper electrode (4). In step seven, the metal interconnect structure (10) includes an air bridge, a dielectric bridge and a microstrip line. The material of the metal interconnect structure (10) is one of gold and copper.

9. The method for fabricating a monolithically integrated low-phase-noise broadband voltage-controlled oscillator according to claim 1, characterized in that, The thickness of the main substrate (1) after thinning in step eight is 50μm-120μm, the diameter of the grounding via (11) is 20μm-100μm, and the material of the back metal layer (12) is either gold or copper.

10. A monolithic integrated low-phase-noise broadband voltage-controlled oscillator obtained by the preparation method according to any one of claims 1 to 9.

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