Heterogeneous 3D Integrated RF Platform: III-V/III-N RF Blocks transfer onto CMOS via Die-to-Wafer Bonding Technology for High-Frequency and HighDensity Wireless Communication Systems

This paper presents the first demonstration of a heterogeneous 3D (H3D) integrated RF platform that combines high-performance III-V/III-N RF blocks (InGaAs and GaN HEMTs) with CMOS technology for high-frequency, high-density wireless communication systems. The integration is achieved using die-to-wafer bonding technology, and the process is performed at temperatures as low as 150°C to preserve the integrity of both the RF and CMOS components.

Key Findings:

1. H3D Integration Process:

• The H3D RF platform integrates InGaAs and GaN HEMTs as a single RF block and stacks it onto a CMOS wafer using die-to-wafer bonding. This approach minimizes interconnect parasitics, provides ultra-short vertical distances between the RF block and CMOS, and supports high-density integration.

• The process includes monolithic 3D integration of the InGaAs HEMTs onto GaN HEMTs, followed by wafer bonding at 150°C, which ensures compatibility with both III-V and CMOS materials.

2. Device Fabrication:

• The GaN HEMTs were fabricated on an 8-inch Si substrate with epitaxial layers of SiN, AlN, GaN, and AlGaN. After fabrication, the GaN HEMT surface was planarized using chemical mechanical polishing (CMP) to prepare for the InGaAs HEMT integration.

• The InGaAs HEMTs, which are grown on an InP substrate, are transferred to the GaN wafer using direct wafer bonding, with Al2O3 as the bonding dielectric and an etch stop layer removed through wet etching.

• Both HEMTs were fabricated with 130 nm gate lengths for InGaAs and 80 nm for GaN, demonstrating excellent performance metrics.

3. Performance Metrics:

• InGaAs HEMT: The InGaAs HEMT achieved a peak transconductance (gm,peak) of 1.6 S/mm at VDS = 0.5 V and fT of 320 GHz with fMAX of 293 GHz, indicating high current-driving capability and high-frequency performance.

• GaN HEMT: The GaN HEMT demonstrated a gm,peak of 420 mS/mm at VDS = 5 V, with a maximum drain current of over 500 mA/mm at VGS = 1 V. The RF gain performance showed fT of 108 GHz and fMAX of 115 GHz, confirming its suitability for high-power applications.

4. Integration Benefits:

• This integration offers compact, high-performance RF front-end modules that are capable of both high-power and low-noise operation, essential for 5G and beyond wireless systems.

• The combination of high-frequency GaN HEMTs (for power amplification) and InGaAs HEMTs (for low-noise amplification) on the same platform, integrated with CMOS for beamforming and control, provides a scalable solution for future wireless communication systems.

5. Benchmarking and Comparison:

• The H3D integration platform demonstrates superior performance in terms of high-frequency figures of merit (fT and fMAX) when compared to other state-of-the-art RF integration technologies, including InGaAs and GaN HEMTs on Si CMOS and GaN HEMTs on Si CMOS.

Conclusion:
The paper successfully demonstrates a new approach to integrating InGaAs and GaN HEMTs with CMOS technology in a 3D architecture, offering a scalable and compact solution for high-performance RF systems. This platform addresses the challenges of interconnect parasitics and device scaling, providing a robust foundation for next-generation wireless communication systems, such as 5G and beyond.