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Cost is a significant driver in the development of phased array systems, where modest savings at the module level can result in significant savings at the array level. In a multichip module, monolithic microwave integrated circuit (MMIC) related costs can be significantly more than just the cost of the MMIC die, when factors such as off-chip components and assembly costs are taken into account. Multifunction MMICs can directly reduce cost through reduced MMIC die count and associated off-chip components and bond wires, while at the same time providing indirect savings through improved module yield and reliability. The purpose of this article is to describe the latest efforts by M/A-COM to provide highly integrated multifunction control MMICs, or MCMs, for phased array applications. The vehicle presented is a seven-gain stage design with six bits each of phase and attenuation control, transmit/receive (Tx/Rx) switching and an onboard serial-to-parallel controller (SPC). To our knowledge, this represents the highest level of integration available in the market for a standard gallium arsenide (GaAs)-based Tx/Rx MMIC for phased array applications.

This MMIC die, in conjunction with an integrated 10 W limiter/LNA MMIC and 20 W high-power amplifier (HPA) MMIC die, provides a low-cost, high-performance approach to implementing an X-band phased array radar front-end. Figure 1 presents a block diagram for such a front-end.

In addition, to maintain a competitive advantage into the future, efforts to provide improved microwave performance are ongoing and results will be presented. Performance enhancements of the latest generation of the MSAG amplifier and switch devices have resulted in MCM P_1dB and third-order intercept (TOI) improvements of 1 dB and 4 dB, respectively, and reduced passive losses of ~25%, for example.

Process description

The MSAG device process flow is represented in Figure 2. Advantages of the MSAG device process include:

1. Multiple device implant capability to allow optimization of FET characteristics for power (5A), gain and linearity (5G), low-noise (5N), switch (5B and 5C (low-loss)) or digital (5E/D) functions.

2. Robust low capacitance dielectric crossovers rather than air-bridges.

3. A thick (7 µm) topside polyamide buffer layer added at the completion of front-side processing for mechanical strength and scratch protection.

In addition, the refractory metal (TiWN) gate provides reduced hydrogen sensitivity as compared to other gate metallizations; and the post-gate anneal provides for a de facto device burn-in. Both are critical for long-term performance stability. Processing is performed on four-inch wafers at the company's Roanoke, VA fab facility. Typical wafer thicknesses are 3 mils or 5 mils, with the X-band MCM presented here being 3 mils. The block diagram and layout of the MCM are shown in Figures 3 and 4, respectively. The MCM is 4.6 × 6.1 mm in size.

Component description

Amplifiers: The seven gain stages are simple resistive feedback designs for wideband performance (gain, gain flatness and match) and reduced process sensitivity. They use 5 A (power) FETs biased at 5 V, ~40% Idss. Device peripheries are 300 µm. However, the final stage in the transmit path uses a 625 µm 5 A FET for improved output power performance.

Phase shifter: The 6-bit phase shifter provides full 360° coverage across the design band. A mix of topologies is used, chosen for performance based on the required bandwidth, phase shift, insertion loss, differential insertion loss, match and size. The low-order bits (5.6°, 11.25°, 22.5° and 45° bits) use switched-tee topologies, whereas the higher-order bits (90° and 180° bits) use switched HP/LP topologies. In these, the switch is integrated into the filter networks for size reduction. Optimization is a function of bandwidth, phase shift, match, insertion loss, differential loss between states and match. Optimizing for best match reduces VSWR interaction between adjacent components. Minimizing differential loss between states offers the potential for reduced system complexity by allowing for reduced look-up table and calibration complexity.

Attenuator: The attenuator provides 32 dB full range coverage across the design band. As with the phase shifter, the attenuator bits are implemented in a mix of topologies for best performance as a function of required bandwidth, attenuation, match and size. The MSB bit (0.5 dB) is implemented as a simple switched shunt resistance. The remaining bits are either single- or two-section switched tee topologies. Optimization is a function of bandwidth, attenuation, ‘off’ state insertion loss and match. As with the phase shifter, optimizing for best match reduces VSWR interaction between adjacent components.

Tx/Rx switch: The Tx/Rx switch topology was chosen for meeting required low insertion loss for ‘on’ paths and high isolation for ‘off’ paths in minimum area. The DPDT function is built up from a set of SPDT switch elements. A feature of the Tx/Rx switch is the non-reflective nature of the Tx and Rx ports of the switch. Matched termination of the unused port, and thus any I/O RF line to/from adjacent components, when not in use provides for much better behavior at the next higher level of operation (in the module).

Serial-to-parallel converter (SPC): The SPC provides overall MCM control. With its inclusion on the same MMIC as the microwave functions, significant benefit is derived. Module complexity, yield and ultimately cost are improved due to the reduction in GaAs/Si chip count, associated off-chip components (i.e., bypass capacitors) and bond wires. The SPC block diagram for this X-band MCM is shown in Figure 3b. SPC development uses a family of existing digital cells, providing flexible control development options. Operation is from a single -5 V supply. Control signals are TTL compatible. In this SPC instance, the user has the ability to load both the Tx and Rx word as a single word, allowingf fast switching between the two modes. In addition, the input word can be read back out, as a serial TTL word, either for daisy-chaining or verification purposes.

Performance results

Figures 5 and 6 show a typical transmit and receive small-signal performance for the X-band MCM, respectively. Small-signal gain is 22 ±0.5 dB and 28 ±0.5 dB in receive (Rx COM) and transmit (COM Tx), respectively; the difference due to the extra gain stage following the Tx/Rx switch in transmit. Input and output match in transmit and receive exceed 13 dB, a function of the switch implementation.

Figures 7 and 8 present phase and attenuation most-significant bit (MSB) performance. Excellent bit performance is noted, due to the well-matched characteristics of individual elements as well as the isolation of the internal amplifiers. Overall rms errors (uncorrected) for the MCM were 2.5° and 0.25 dB.

Advanced MSAG device development

Looking to maintain a competitive advantage against competing technologies, significant work to improve MSAG device performance has recently been undertaken. This includes optimizing device geometry as well as processing modifications, with the focus on optimizing the power (5A) device for improved 5 V control MMIC performance and the switch (5B) device for reduced insertion loss.

A comparison of X-band MCM receive output TOI fabricated with the baseline power (5A) or enhanced gain/linearity (5G) FET, Figure 9, shows an average 4 dB increase with the optimized device, a significant linearity improvement. Breakout (BO) data for individual amplifiers TOI improvement is significantly better than 4 dB. Transmit P_1dB also showed improvement with the 5G device. On average, an increase of ~1 dB, from 21.6 dBm to 22.7 dBm, was noted for the 5G X-band MCM P_1dB as compared to the 5A version.

In addition, BO data for phase, attenuation and switch elements show improved performance when using the enhanced, low-loss (5C) switch FET. Figure 10 presents data for a standard evaluation circuit (SEC), in this case a broadband switch, comparing insertion loss and isolation when fabricated using the baseline (5B) and low-loss (5C) FET. Insertion loss improves ~25%, on average, across the band with the low-loss device.

Conclusion

An overview of the development of a complex multifunction MMIC for phased array applications in MSAG product technology has been presented. To our knowledge, this represents the highest level of integration available in the market for a standard Tx/Rx GaAs MMIC for phased array applications. In addition, data has been presented for a version of the MMIC incorporating advanced MSAG device technology, which has resulted in significant performance improvements, especially in the areas of reduced passive losses and improved output TOI. These device enhancements ensure MSAG's competitiveness in the development of complex control MMICs into the foreseeable future.

Acknowledgment

The authors wish to thank the Roanoke, VA layout, fab and test groups, without whom the development of this complex MCM would not have been possible. In addition, thanks go to Rex Frye for managing this development activity.

References

1. H. Singh et al, “Integrated Digitally Controlled 6-Bit Phase Shifter, 4-Bit Attenuator, and T/R Switch Using Multifunction Self Aligned Gate Process,” 1991 Microwave and Millimeter-Wave Monolithic Circuits Symposium Digest, pp. 39-42, June 1991.

2. E. L. Griffin, “X-band GaAs MMIC Size Reduction and Integration,” 2000 IEEE MTT-S International Microwave Symposium Digest, Volume 2, pp. 709-712, June 2000.

ABOUT THE AUTHORS

Hausila P. Singh received his B.S. degree with honors, M.S. degree in physics and Ph.D. degree in solid-state physics from the Banaras Hindu University, India, in 1963, 1965 and 1968, respectively. He was a Post-doctoral Research Fellow at the Massachusetts Institute of Technology, Cambridge, MA, and at the University of Southern California, Los Angeles, CA, from 1969 to 1972. In his current position as a distinguished fellow of technology, he is responsible for GaAs combined digital and microwave product design and development engineering. He has designed and developed more than a dozen complex, state-of-the-art L, S and X-band highly integrated control MMICs. He has served as chairman of the IEEE VMS Microwave Theory and Techniques, Electron Devices and Power Engineering Society Chapters. He has also served on technical program committees for the IEEE GaAs IC Symposium.

Michael Ashman received his BS in electrical engineering from the WV Institute of Technology in 1985. From 1996 until 1998, he was with Sonnet Software Inc. as a sales engineer. Prior to that, he was with Lockheed Martin's Electronics Laboratory and Northrop's Defense Systems Division as a MIC/MMIC design engineer. Currently a senior principal engineer with M/A-COM's Integrated Products Business Unit, he is involved primarily with the development of highly integrated control MMICs for radar and communications systems. Other areas of interest include integrated transceiver and broadband amplifier design.

Monte Drinkwine received his Ph.D. in Surface Physics from the University of Wisconsin-Milwaukee in 1977, followed by two years of post-doctoral studies in surface analysis of micron-size particles. From 1985 to 2000 he was with ITT, where he helped develop the MSAG process for making RF and microwave multifunction GaAs ICs; established production capability and process controls for MSAG IC fabrication and module assembly; directed IC design efforts for military and commercial product development; and developed and transferred to production new MSAG process and device families for higher-performance applications. Since 2000, he has been with M/A-COM where he directs manufacturing and development engineering efforts for GaAs IC device and process development and for improving manufacturing yield and efficiency at the Fab in Roanoke, VA.