77GHz Phased-Array Transceiver in Silicon Natarajan, A. Babakhani, A. Komijani, X. Guan, and, A. Hajimiri California Institute of Technology

Outline Motivation On-chip Antennas Local LO-Path Phase-Shifting Architecture A 77GHz Phased-Array Transceiver in SiGe Measurement Results Conclusion

Wireless Communications Motivation 24GHz 60GHz 77GHz Wireless Communications Vehicular Radar Fully-integrated silicon-based multiple-antenna systems enable widespread commercial applications at high frequencies. Complex, novel architectures can be realized on silicon with greater reliability and lower cost.

Challenges of mm-Waves in Si Substrate high dielectric constant (absorbs the fields). Conductive substrate (substrate losses). Low breakdown voltages (power challenges). Poor metal conductivity. High-frequency interface.

Outline Motivation On-chip Antennas Local LO-Path Phase-Shifting Architecture A 77GHz Phased-Array Tranceiver in SiGe Measurement Results Conclusion

Antenna on Silicon Top-Side Air, ε=1 <5 % >95% Silicon, εr=11.7 Silicon’s high dielectric constant (er~11.7) and conductivity of silicon substrate are the major design challenges Most of the power gets absorbed into silicon It may appear that ground shields might solve this problem

On-Chip Ground Shield Air, ε=1 SiO2, ε=4 h Silicon, ε=11.7 Typical distance between the top and bottom metal layers is very small (less than 15μm) Even for 15μm ground distance, the radiation resistance is around 0.02Ω (efficiency of 1-2%)

Dielectric Lens Air, ε=1 Top-Side SiO2, ε=4 Silicon, ε=11.7 Air, ε=1 Silicon Lens Back-Side A dielectric lens on the backside combines most of the surface wave power and couples it into air Reflection from silicon-air boundary can be eliminated by a matching layer Is a good thermal heat sink (Si thermal conductivity = 149 W/(m.k) better than Brass (120W/(m.k)) ["Integrated-Circuit Antennas," by D.B. Rutledge, et al., Infrared and Millimeter Waves, 1983.]

Outline Motivation On-chip Antennas Local LO-Path Phase-Shifting Architecture A 77GHz Phased-Array Transceiver in SiGe Measurement Results Conclusion

Earlier Implementations 24GHz Phased-Array Transmitter 24GHz Phased-Array Receiver Multiple-phase VCO Distribution Network Multiple phases of VCO were generated and distributed to each element, where one phase was selected. Well-suited for low-resolution beam steering with few elements. H. Hashemi, X. Guan, and A. Hajimiri, “A Fully-Integrated 24GHz 8-Path Phased-Array Receiver in Silicon,” ISSCC 2004. A. Natarajan, A. Komijani, and A. Hajimiri, “A 24GHz Phased-Array Transmitter in 0.18mm CMOS,” ISSCC 2005.

Local LO Phase-Shifting Architecture The desired phases are generated locally by interpolating between I and Q phases using phase rotator. Phase shift resolution is limited by the interpolator rather than by number of phases generated by the VCO. Scales with larger number of elements as it reduces area and complexity of LO signal distribution network. .

Local LO Phase-Shifting Architecture The desired phases are generated locally by interpolating between I and Q phases using phase rotator. Phase shift resolution is limited by the interpolator rather than by number of phases generated by the VCO. Scales with larger number of elements as it reduces area and complexity of LO signal distribution network. .

Outline Motivation On-chip Antennas Local LO-Path Phase-Shifting Architecture A 77GHz Phased-Array Transceiver in SiGe Measurement Results Conclusion

Transceiver Architecture Fully-integrated 4-element 77GHz phased-array transceiver. Two-stage frequency translation. (LO1: 52GHz, IF=LO2:26GHz) Local phase shifting in each element enables beam steering. A. Babakhani et al., “A 77GHz 4-Element Phased Array Receiver with On-Chip Dipole Antennas,” ISSCC 2006.

52GHz Phase Rotator Quadrature signal generated locally using a delay. Phase-shifter resolution limited by DAC resolution.

Die Micrograph 0.12mm SiGe transistors in BiCMOS process, ft : 200GHz. 7 metal layers: Top two layers are 4mm and 1.25mm thick.

Outline Motivation On-chip Antennas Local LO-Path Phase-Shifting Architecture A 77GHz Phased-Array Transmitter in SiGe Measurement Results Conclusion

LNA Gain and NF Measured LNA peak gain @ 77GHz = 23dB BW = 6GHz, NF = 6dB

Receiver Conversion Gain and NF More than 35dB gain between 78.5GHz and 80GHz 3-dB bandwidth is more than 2GHz Minimum NF of 8.0dB measured at 79.2GHz

System Packaging and Setup Silicon chip is thinned down to 100μm Floorplan issues lead to edge antennas A 500μm silicon wafer for mechanical stability Low frequency signals using wire-bond and board traces

Antenna Gain Peak Gain of +2dBi has been achieved in the E-plane Lens improves the gain by more than 10dB

Transmitter Test Setup Combination of waveguide probe testing and internal self- test mechanisms. Stand-alone PA testing and mixer output tested through internal test pads.

PA Measurements 3dB bandwidth larger than 15GHz (20% fractional BW). Small-signal Gain Large-signal @ 77GHz 3dB bandwidth larger than 15GHz (20% fractional BW). Output-referred 1dB compression point: 14.5dBm. Simulated peak power and PAE: 16dBm, 14%.

Transmitter Performance Output-referred 1dB compression point is +10.6dBm. 40dB conversion gain from baseband to RF.

Loopback Testing In loopback mode, output of upconversion mixer connected to input of downconversion mixer. Pattern measurement possible with baseband input and baseband output. A. Babakhani et al., “A 77GHz 4-Element Phased Array Receiver with On-Chip Dipole Antennas,” ISSCC 2006.

2-element Loopback Array Pattern Array pattern measured with two elements active in the receiver and the transmitter.

His visionary prophecy is fulfilled in silicon 40 years later. What the Future Holds Silicon systems at yet higher frequencies (mm-wave and beyond). Systems that leverage the benefits of integration to realize complex architectures that include mm-wave, analog, and digital circuits, Elimination of all high frequency interfaces to the outside world The last paragraph of Gordon Moore’s seminal paper published in 1965: “Even in the microwave area, structures included in the definition of integrated electronics will become increasingly important. … The successful realization of such items as phased-array antennas, for example, using a multiplicity of integrated microwave power sources, could completely revolutionize radar.” His visionary prophecy is fulfilled in silicon 40 years later. G. E. Moore, “Cramming more components onto integrated circuits,” Electronics, vol. 38, no. 8, pp. 114–117, Apr. 1965.

Acknowledgements Lee Center for Advanced Networking, Caltech, Prof. Rutledge (Caltech), Dr. Analui (Caltech/Luxtera) and Theodore Yu (Caltech/UCSD), Dr. Weinreb of JPL, Prof. Hashemi of USC, The DARPA Trusted Foundry program and IBM T. J. Watson for chip fabrication Software Assistance: Cadence, Agilent Technologies, and Zeland Software, Inc.