Radar Love
By Bishnu Gogoi, BJ Lyman, Mike Purchine, Dave Rice; HVVi Semiconductors, Inc.

As I approached my colleague’s office the other morning, I noticed a caption that he had above his desk relating to our power transistors. I read it once and that was it; for the rest of the day I could not get the phrase out of my head! It simply said “We’ve got a thing that’s called Radar Love”. You are probably reciting the lyrics from that classic “Golden Earring” tune as we speak. It is a great song and an excellent segue for a Microwave Product Digest (MPD) submission, as the editorial focus for this September issue is “Radar.”

Much has changed in the field of Pulsed Power Transistors since HVVi’s initial article submitted to MPD in September, 2006 titled “New Silicon Microwave Power Transistor Simplifies RF Design.”1 Specifically, customers are seeking higher performance replacements for legacy silicon bipolar devices, technology advancements are providing for devices with higher operating voltages affording higher efficiency while yielding less complex implementation, and several power transistor suppliers are reaching beyond their primary wireless infrastructure markets and seek to enter alternative markets (such as Avionics) by attempting to supplant highly ruggedized power transistors with traditionally commercial devices.

Although the total high-power pulsed RF device revenue is predicted to grow year-over-year, with a Compound Annual Growth Rate (CAGR) of 7.6% from 2009 through 20132, it remains a crowded field. Factor in emergent technologies, such as Gallium Nitride (GaN) and Silicon Carbide (SiC), and semiconductor suppliers must continue to expand and differentiate their product offering to include higher performance devices that address the rigors of their target application, such as avionics and radar.

HVVi Semiconductors has done just that. Utilizing our highly-rugged enhanced-mode High Voltage Vertical Field Effect Transistor (HVVFET™) process, HVVi has added two new part numbers to our Avionics RF Power Transistor product offering; specifically HVV1011-600 and HVV1012-550. As supported in Table 1, the HVVFET process provides industry leading ruggedness, higher power density as compared with Laterally Diffused Metal Oxide Semiconductors (LDMOS) sharing a similar package, higher gain and efficiency as compared with silicon bipolar devices, and a full line-up of 50V devices which ease implementation.

HVV1011-600 is the highest output power device (600W) offered within our selection of power transistors optimized for the IFF and TCAS transponder/interrogator markets. Figure 1 identifies the gain and efficiency for a typical device at 1090MHz. Designed for L-band pulsed applications, HVV1011-600 operates at the frequencies between 1030 MHz and 1090 MHz and under the following pulse condition: Pulse Width = 50μsec, Pulse Duty Cycle = 2% at VDD = 50V, IDQ = 100mA. Assembled in a RoHS compliant “HV800” flanged package with liquid crystal polymer lid, this product is qualified for gross leak test – MIL-STD-883, Method 1014 (see Table 2).

Addressing Distance Measuring Equipment (DME) applications, HVVi offers the HVV1012-550. Figure 2 shows the gain and efficiency of a typical device used for DME at an operational frequency of 1150MHz. This 550W device operates at the frequencies between 1025 and 1150 MHz and under the following pulse condition; Pulse Width 10μsec, Duty Cycle = 1% at VDD = 50V, IDQ = 100 mA (see Table 3).

On the topic of diversification, HVVi has taken further steps to assure the integrity of our highly-ruggedized devices by completing an internal Pulse Reliability Study in support of our gold metallization assembly and manufacturing techniques. This independent study was undertaken for several important reasons. As noted previously, various power transistor semiconductor suppliers seek to introduce commercial-grade devices into the Avionics and Radar market. Although the performance of these cost-effective devices appears to be acceptable, device reliability remains a primary concern when addressing the critical nature of civilian and defense oriented airborne and radar applications. Based upon economy of scale, many LDMOS suppliers build their avionics and radar power transistors utilizing the same assembly techniques as their volume oriented wireless infrastructure devices; utilizing aluminum wire bonds for cost saving purposes. Addressing specifically the Pulsed Power Transistor market, where cycle fatigue in bond wires is pertinent, this critical topic was researched in detail within a technical paper titled “Prediction of High Cycle Fatigue in Aluminum Bond Wires: A Physics of Failure Approach Combining Experiments and Multi-Physics Simulations.”3 This paper highlights reliability concerns and discusses predictability models developed to identify mechanical fatigue with aluminum wire-bonds used within some LDMOS products that have been utilized in pulsed applications. The “fatigue” is linked to temperature and power cycling, causing the wires to expand and shrink in a cyclical way under certain pulsed conditions.

In support of highly reliable and ruggedized power transistors, HVVi is presenting a technical paper titled “All Gold Metallization System Enables High Power RF Pulsed Transistors with High Reliability”4 at the upcoming European Radar Conference in September (www.eumweek.com/2009). This paper is focused upon the reliability of our all-gold wire connection scheme, further supporting our position as an industry leader in the field of ruggedized pulsed power transistors for the Avionics and Radar markets.

The Device
To support these findings, an overview of our experiment and abstract of the complete paper is provided below. To verify the reliability of the gold wire connection scheme, HVV1011-300 was selected as it was the highest power device, of 300W P1dB, offered at the time of this stress test. This device is characterized at 1030MHz with 48V drain supply voltage operating at 100mA of drain quiescent current under DC conditions (see Table 4).

Stress Test Conditions
The RF Stress Test conditions identified for this evaluation involve input signal conditions of a 1030 MHz pulsed signal with a 50µs pulse width and 1 ms pulse period. Under these conditions, the devices were subjected to 1000 pulses per second, which equates to over 86 million pulses per day. As the device pulses to its rated output power, the device draws several amps of current, causing the wires to heat and expand. In the device’s off state, the wires are allowed to cool to their ambient temperature and contract to their original state until the next cycle. The continuous heating and cooling stresses the bond-wires and can lead to premature device failures from wire sag, wire fatigue and breakage. The repetitive heating and cooling cycles are the main cause of wire fatigue failures with aluminum wires5. The devices were tested for nearly four months continuously, with the exception of a one-week break to establish a read point halfway through the experiment. More than 10 billion pulses were applied to the device over the course of the experiment to determine the reliability of the gold wires under a high power pulsed environment.

Device Assembly
To ensure the reliability of our devices under harsh pulsed conditions, HVVi utilizes a flip-chip package strategy to optimize thermal performance, assuring the high reliability of the HVVFET. Gold bumps are fabricated on both source and gate contacts, and the gate bumps align with an integrated “interposer” (Figure 4) which provides a means to connect the device input. The source bumps are bonded directly to the copper flange using a gold-tin eutectic process, acting both as an electrical ground and heat sink. The drain contact is the entire top side of the die in this package configuration. To reduce device/circuit mismatch, MOSCAPs are utilized for internal matching while all-gold bond wires complete the assembly of the internal device components. Prior to the initiation of this 10 billion pulse RF stress test, DC test data was recorded on all units to capture threshold voltage (VGS(TH)), as it is a measure of the minimum gate voltage required to turn each device from off to the on state. Also captured was drain to source breakdown voltage (BVDss) of the subject devices, as this is a measure of the maximum voltage that can be applied to the drain of the device when the device is in the fully turned off state.

Visual Inspection
After the devices were subjected to the 10 billion pulse RF stress test, a visual inspection was performed. The control unit and one stress tested unit (randomly selected) were visually evaluated and measured. The comparison identified that no sign of wire fatigue or wire sag were noted in the test device as compared with the control unit. (Figure 3).

Also of interest is wire-pull data gathered on the two HVV1011-300 devices studied. As supported in Table 5, the wire-pull data identifies minimal variance between these two devices. The average measurement of the 12 wire-bonds, for each of the devices, measured 23.2 grams of force for the control device and 23.6 grams of force for the pulsed device. One would expect after 10 billion pulses that wire fatigue would be noted in the pulsed unit, as compared with the original control unit. The close correlation of this comparative information further supports the integrity of an all-gold wire connection scheme.

Additionally, the average threshold voltage (VTH) for the devices measured 1.20 volts. The average threshold variation, both before and after stress testing of the control and subject devices, varied between 2-3mV. This variation is less than 1% of the total threshold voltage and well within an acceptable range. The average shift in breakdown voltage of the control device and that of the RF stress test units measured less than 1% of their original measurement, which is also within an acceptable range.

As identified within Figure 5, further RF data was compiled and is introduced in a power drive-up curve. This graph supports that the change in RF input power levels is never more than 3W and 1.5% from the baseline data. Overall, the RF data captured suggests that little or no change is measured in RF performance before and after the RF pulse stress testing.

Conclusion
The HVVFET structure and packaging technology combine to produce a novel RF Power FET that significantly advances state-of-the-art performance of pulsed power transistors with superior gain, power density, and reliability. With the recent introduction of part numbers HVV1011-600 and HVV1012-550, HVVi continues in the tradition of developing highly ruggedized transistors that meet the strict rigors of the Avionics and Radar markets. As the song goes, we’ve got a thing that called Radar Love!

Acknowledgement: The authors wish to acknowledge the contributors of the “All Gold Metallization System Enables High Power RF Pulsed Transistors with High Reliability” paper. In addition, we thank Mike Purchine for reminding us all of that 1970s hit by Golden Earring.

References
[1] “New Silicon Microwave Power Transistor Simplifies RF Design,” Microwave Product Digest, September 2009.
[2] “High Power RF Semiconductors for Pulsed Applications,” ABI Research 1Q 2009.
[3] “Prediction of High Cycle Fatigue in Aluminum Bond Wires: A Physics of Failure Approach Combining Experiments and Multi-physics Simulations,” Jeroen Bielen, Jan-Joris Gommans, and Frank Theunis, Phillips Semiconductor.
[4] “All Gold Metallization System Enables High Power RF Pulsed Transistors with High Reliability”, HVVi Semiconductors, Brian Battaglia, Dave Rice, Bishnu Gogoi, 2009
[5] M. Greenelsh. (2007) California Fine Wire Company homepage. (Online). Available: http://www.calfinewire.com/mag_ics.asp

HVVi Semiconductors, Inc.
www.hvvi.com
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