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RF MEMS PACKAGING FOR SPACE APPLICATIONS

William P. Kornrumpf, GE Global Research, Schenectady, NY kornrumpf@crd.ge.com Nafiz N. Karabudak, Lockheed Martin Commercial Space Systems, Newtown, PA nafiz.n.karabudak@lmco.com

Abstract The Lockheed Martin/GE High Density Interconnect (HDI) packaging technology has been applied to packaging RF MEMS components for space applications. In previous work, HDI has been used to fabricate multi-chip modules (MCMs) for a wide range of applications from RF (70 MHz) to mm wave (94GHz) using GaAs MMICs and Si control circuits by embedding the active components in a polymeric structure. This packaging technology is an alternative to the use of LTCC and HTCC ceramic based MCMs. The GaAs MMICs used in these modules are fabricated with fragile air-bridges that require protection from physical damage and intrusion of dielectric materials. For this reason, an air-capping process has been developed that places an air pocket over the critical elements of each MMIC within the HDI MCM thereby protecting the fragile airbridge structures from physical damage. This same technique has now been used to provide an air pocket over the critical moving elements of RF MEMS devices. This packaging technology can be used to produce MCMs that contain MEMS and MMIC devices as well as Si control ICs.

Introduction Among the several packaging techniques studied for RF MEMS packaging with applications on space platforms, GE-LM High Density Interconnect (HDI) process offers to provide an excellent method of integrating RF MEMS and other circuitry elements into a compact, reliable, and low cost package. High Density Interconnect (HDI) process is a multi-chip module fabrication process that provides low-cost RF, DC, and control circuitry interconnections to active components in preformed cavities within rigid substrates. HDI enables integration of multiple functional components and provides the encapsulation of the devices from particulate and environmental contamination.

Advanced Chip Scale Packaging (CSP) for MEMS components

CSP has been used to package microwave devices such as GaAs MMICs that have sensitive mechanical structures similar to MEMS cantilevers. These sensitive structures are the air bridges used to provide low capacitance crossovers for the RF and DC signal. A typical GaAs MMIC is shown in figure 1 with both air bridges and gate regions that are sensitive to dielectric infiltration. Figure 1 Photograph of typical X-band GaAs MMIC with air bridges and sensitive gate regions. Conventional molded plastic packages would infiltrate polymer under these air bridges thereby increasing the cross talk and reducing the microwave performance of the packaged MMIC. A MMIC polymer based CSP packaging approach has been demonstrated by GE and Lockheed Martin that provides cavities at the location of each MMIC air bridge thereby preserving the RF performance of the MMIC. This packaging process is called the plastic encapsulated Chip-on-Flex (COF) 1 and is an extension to the LM/GE High Density Interconnect (HDI)2 process.

Air bridges

Sensitive gate regions

Air bridges

Sensitive gate regions

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The COF HDI process flow is shown in figure 2. A polyimide sheet is coated with an adhesive that is subsequently patterned where cavities are required. The MMICs are then placed on the adhesive and the adhesive is cured providing a sealed cavity at each air bridge and gate region. A conventional semiconductor plastic encapsulant is then formed over the backside of the MMIC providing physical support for the bare die. Connections to the MMIC bond pads are made by first drilling micro via holes (25 micron diameter) in the polyimide sheet 3. Figure 2 Chip Scale Packaging of GaAs MMIC circuits providing protection for sensitive air bridges and gate regions. The interconnect metal is then deposited on the polyimide sheet extending through the via holes to the device bond pads. This interconnect metal is then patterned to provide individual connections to each of the MMIC’s bond pads. A solder mask is then applied and solder balls formed on the surface. The packaged parts are then singulated into individual CSP devices that can be used in surface mount assemblies Figure 3 SEM photograph of MMIC CSP showing cavity in polyimide cover.

Figure 3 is a close up photograph of one of the cavities formed over a series of air bridges that are used in the gate and drain regions of the GaAs transistors. The process flow for packaging MEMS devices is similar to the MMIC packaging process with the exception that the cavit ies in the cover sheet are larger and deeper. The process flow is shown in figure 4 5. Figure 4 MEMS packaging process flow with deep cavities formed in the cover sheet. The only modification to the process flow is to increase the depth and size of the cavities in the polyimide cover sheet. These cavities are formed using a laser so complex shapes can be formed when required. Figure 5 is a photograph of a series of MEMS devices placed on a polyimide sheet that has had cavities formed on the surface. Figure 5 Photograph of MEMS CSP showing cavities in polyimide cover sheet at each MEMS location.

Adhesive on polyimide

Adhesive Laser Patterned

MIMC Devices Attached withRobotic pick and place machine

Back side encapsulated in rigid dielectric material

Turn over, laser drill holes to MMIC bond pads, metalize and pattern

HDI MMIC Chip Scale Packaging Concept with Airbridge Protection

Form solder balls and laser singulate

AirbridgeBond Pads

Solder balls

Rigid Encapsulation

Hole in Adhesive

Adhesive

Laser cut line

Laser drilled vias

Polyimide

Cavity in Adhesive

Adhesive on polyimide

Adhesive Laser Patterned

MIMC Devices Attached withRobotic pick and place machine

Back side encapsulated in rigid dielectric material

Turn over, laser drill holes to MMIC bond pads, metalize and pattern

HDI MMIC Chip Scale Packaging Concept with Airbridge Protection

Form solder balls and laser singulate

AirbridgeBond Pads

Solder balls

Rigid Encapsulation

Hole in Adhesive

Adhesive

Laser cut line

Laser drilled vias

Polyimide

Cavity in Adhesive

MMIC Airbridge

Cavity in Cover

Polyimide Cover MaterialEdge of Cavity in Cover

Close-up of Cavity in Cover showing depth and edge definition

MMIC Airbridge

Cavity in Cover

Polyimide Cover MaterialEdge of Cavity in Cover

Close-up of Cavity in Cover showing depth and edge definition

Adhesive on Thick (2 mil) polyimide

Adhesive Laser Patterned

MEMS Devices Attached withRobotic pick and place machine

Back side encapsulated in low loss dielectric material

Fabricate HDI/MEMS Filter modules (can add layers if needed)

HDI MEMS Packaging Concept

Rigid Encapsulation

Hole in Adhesive and Polyimide

Adhesive

Laser drilled vias

Thick Polyimide

Cavity in Adhesive and polyimide for MEMS protection

Adhesive on Thick (2 mil) polyimide

Adhesive Laser Patterned

MEMS Devices Attached withRobotic pick and place machine

Back side encapsulated in low loss dielectric material

Fabricate HDI/MEMS Filter modules (can add layers if needed)

HDI MEMS Packaging Concept

Rigid Encapsulation

Hole in Adhesive and Polyimide

Adhesive

Laser drilled vias

Thick Polyimide

Cavity in Adhesive and polyimide for MEMS protection

RSC Quad RF Switch With Initial HDI cover in place over switches

Cavity in Polyimide Lid

Polyimide cover material

RF GSG probe pads

DC control pad

RSC Quad RF Switch With Initial HDI cover in place over switches

Cavity in Polyimide Lid

Polyimide cover material

RF GSG probe pads

DC control pad

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The cavities must be large enough to clear all of the MEMS cantilever structure including the anchor points so that no forces are transmitted from the cover to the structures. Any microwave packaging technology must provide not only physical protection but also parasitic free interconnections to the MMIC bond pads. The HDI process has demonstrated operation to 74 GHz 4 due to the small size of the interconnection micro vias. The use of the HDI based CSP technology allows the packaging of RF MEMS devices such as low loss phase shifters without performance degradation and minimal additional packaging related loss. Typical return loss for HDI structures is less than –20 dB while the insertion loss is less than 0.05 dB/mm at 10 GHz depending on the structures used. The present work has focused on packaging multiple phase shifters fabricated by Rockwell Science Center (RSC). While the process flow shown in figure 4 indicates that individual MEMS devices are to be packaged, in reality an entire MEMS reticule or wafer can be packaged with the same process. Figure 6 is a photograph of an HDI processing carrier with 4 RSC phas e shifter reticules in process. Each reticule can be released and pretested before insertion in the COF package. After packaging, the parts will be singulated into individual packaged phase shifters. Figure 6 Photograph of 4 RSC phase shifter reticules on an HDI carrier. Figure 7 is a close up of a single reticule showing the 9 active phase shifter circuits and additional test structures to evaluate the performance of the COF packaging. While reticule packaging has been the focus of the present program, the lessons learned will be applied to full wafer scale packaging of MEMS circuits and devices. The successful wafer level packaging will enable wafer level release and test thereby significantly

reducing the manual labor in the packaging process as automated test procedures can be used. Figure 7 Close up photo of the RSC phase shifter reticule showing the individual 2 bit TTD phase shift elements and the cavities at each of the switch sites.

Conclusions The Lockheed Martin/GE High Density Interconnect (HDI) packaging process of RF MEMS offers excellent benefits for space-based applications. Some major advantages over conventional interconnect processes are the high density routing of mixed signal circuitry, low cost , and reduced mass. HDI packaged RF MEMS circuitry designed and fabricated to understand the challenges in developing reliable and affordable packaging.

Acknowledgment The authors would like to acknowledge the support of Dr. Larry Corey, Jacqueline Toussaint-Barker, Dr. John Smith, John Capone, John Logrando, and Bill Taft who supported the program. This work was supported by DARPA and AFRL.

References 1) G.A. Forman, H.S. Cole, B. Gorowitz, R.F. Kolc,

S.E. Weaver, T.A. Sitnik-Nieters, W.P. Kornru mpf, J.W. Rose, “Development of GE’s Multichip on Flex Circuit Technology”, Proceedings of Flexcon ‘94, International Conference on Flexible Circuits 1994, Sunnyvale, CA

Processing frame

Phase shifter reticules

Processing frame

Phase shifter reticules

2 bit TTD cell

Bond pads(GSG)

Test cells

MEMS cavity

2 bit TTD cell

Bond pads(GSG)

Test cells

MEMS cavity

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2) J.J. Sowers, W.P. Kornrumpf, “ Application of

Microwave High Density Interconnect Technology to EHF Terminal Components”, 1994 government Microcircuit Applications Conference (GOMAC), Nov. 7-10, 1994, San Diego, CA.

3) E. Balch, S. Weaver, L. Douglas, R. Fillion, H. Cole, "Scanning Laser Formed Microvias for Multichip Modules," IPC Works '97, Arlington VA, Oct. 5-9, 1997.

4) A. Pham, J. Laskar, V. Krishnamurthy, H.S. Cole,

and T. Sitnik-Nieters, “Ultra low loss millimeter wave MCM interconnects,” Electrical Performance of Electronic Packaging, 1997, IEEE 6th Topical Meeting on, pp. 213-216, 1997.

5) D. Albert, "Workshop on affordability and cost

reduction for radar systems", April 5 & 6, 2000, Redstone Arsenal, Alabama