domingo, 25 de julio de 2010

Design and Development of a Package Using LCP for RF/Microwave MEMS Switches


Morgan Jikang Chen, Member, IEEE, Anh-Vu H. Pham, Senior Member, IEEE, Nicole Andrea Evers, Chris Kapusta, Joseph Iannotti, William Kornrumpf, John J. Maciel, Member, IEEE, and Nafiz Karabudak.

Abstract—We present the development of an ultrahigh moisture-resistant enclosure for RF microelectromechanical system (MEMS) switches using liquid-crystal polymer (LCP). A cavity formed in LCP has been laminated, at low temperature, onto a silicon MEMS switch to create a package. The LCP-cap package has an insertion loss of less than 0.2 dB at X-band. E595 outgas tests demonstrate that the LCP material is suitable for constructing reliable packages without interfering with the operation of the MEMS switch. The package also passes Method 1014, MIL-STD-883 gross leak, and fine leak hermeticity tests. Index Terms—Cavities, chip-on-flex, liquid-crystal polymer (LCP), microelectromechanical system (MEMS), microwave, packaging.

I. INTRODUCTION

PACKAGING is a critical part in bringing the RF microelectromechanical system (MEMS) into application at an affordable cost. MEMS switches are very sensitive to contamination and must be packaged with hermetic or near-hermetic seals in inert noble gas environments. These switches require hermetic packaging to prevent against contaminating particles and moisture. Invasion of particles into the MEMS device can cause the switch to be wedged open, stuck closed where the particle aggravates stiction, or simply degrade performance by acting as a resistive material.

A number of solutions are available for packaging MEMS switches. Several techniques used by industry to package MEMS devices include epoxy seals, glass frit, glass-to-glass anodic bonding, and gold-to-gold bonding. These techniques face two main problems. First, organic materials outgas inside the MEMS cavity during the bonding process due to wetting compounds in the glass, gold, or epoxy layers.

This contamination detrimentally affects the MEMS switch reliability. Second, to achieve a good seal, most bonding processes utilize high temperatures (300 C–400 C) that can degrade MEMS structures. Furthermore, available hermetic packages and ceramic/glass feed-throughs have significant parasitic losses at microwave frequencies, can be expensive, and add significant weight to a system. Packaging MEMS switches into an organic module, in which compact multilayer substrates house active and passive components present even more challenges. Although multilayer chip-on-flex modules using Kapton films are a proven technology for high-density packaging of microwave modules,2 3 Kapton is found to be incompatible with RF MEMS switch packaging due to its high moisture absorption, high out-gassing characteristics, and the need to use high outgassing epoxies for lamination.

In this paper, we present the development of an ultrahigh moisture-resistant package for RF MEMS switches in chip-on-flex modules using liquid-crystal polymer (LCP). We have developed a lamination process to adhere LCP onto silicon to form an enclosure for MEMS. Using multilayer flex and laser-drilled vias, the first level interconnect parasitic losses are negligible at X-band. The microwave measurements demonstrate that the LCP-package has less than 0.2-dB insertion loss and maintains the return loss of a switch to greater than 20 dB. The LCP MEMS package passes the E595 out-gassing test and Method 1014, MIL-STD-883 gross leak, and fine leak hermeticity tests.

Section II provides a brief review of multilayer organic modules, an introduction to LCP, and processes to create the LCP MEMS package. Section III demonstrates the experimental results of peeling strength tests, out-gassing tests, hermeticity tests, and lamination process evaluation. Section IV provides detailed analysis of the electrical performance of a package. Section V demonstrates the electrical performance of a packaged
RF MEMS switch in an LCP enclosure.

II. PACKAGE TOPOLOGY

The multilayer organic multichip module (MCM) is a potential candidate for integrating a system-in-package (SiP) at microwave and millimeter-wave frequencies. This technology has been utilized to package high-peed memory integrated circuits (ICs) and transceiver modules for communications. In 1998, Butler et al. had attempted to use thisMCMtechnology to package MEMS devices. However, the multilayer Kapton is not suitable for hermetic packaging of MEMS. In order to provide hermetic packaging of an RF MEMS switch, we investigate the feasibility of LCP as a multilayer interconnect layer in place of Kapton.

LCP is an emerging low-cost dielectric material that is commercially available as single sheets or laminated substrates that have low moisture absorption (equivalent to glass). Table I compares the basic properties of LCP with Kapton. LCP can be manufactured to have different properties including a coefficient of thermal expansion (CTE) range from (8 -17).10^-6/ K and a glass transition temperature Tg from 280 C to well over 350 C. The use of low and high melting-point temperature LCP allows for layer-to-layer lamination processes without the use of adhesive materials. The main advantages of LCP compared to other organic substrate materials are low moisture absorption, low coefficient of hydroscopic expansion (CHE), excellent barrier properties, and adjustable CTE through thermal treatment processes. Moreover, LCP shows a very low dielectric constant and loss factor, over the frequency range of 1 GHz up to 110 GHz [11]. This unique combination of excellent electrical characteristics, excellent mechanical properties for harsh environment operation, and economical considerations make LCP a serious candidate for all MCM, SiP, and advanced packaging technology.

We have developed a process to laminate LCP onto silicon to form an enclosure for packaging an RF MEMS switch without the use of adhesives. One of the advantages of lamination is the low-temperature processing (below ~315 C), as compared to metallic or glass bonding ( ~400 C). Fig. 1 demonstrates our process flow for laminating LCP on silicon. The process starts with a bare 2-mil-thick LCP that has copper on one side. The copper serves as the roof of the cavity drilled in the LCP film.
 
The MEMS cavity is formed in the 2-mil-thick LCP using laser ablation to the copper lid. The ash is removed using isopropyl alcohol solvent. This cavity acts as a hermetic enclosure formed by the copper lid and LCP walls. The laser ablation is a convenient method to pattern the chemically stable LCP to provide very accurate vertical sidewalls. The single-sided copper-clad LCP film with the laser-drilled cavity is laminated onto an exposed and released silicon switch. The commercially available LCP films have a melting temperature from 240 C to 315 C, which, for robustness of process, is thermally well below any temperature that may impact the MEMS switch. Inert gas can be injected into the cavity to help improve the switch performance during the lamination process. Excellent lamination results have been obtained over a large range of pressures. Through our processing, we obtain 1 m of accuracy using conventional flipchip die bonding equipment.

Once the lamination is completed, square microvias 100- m long along each side and interconnects are formed on the LCP layer. The fabrication of vias and metal interconnects is similar to the process reported in Fig. 2 shows the three-dimensional (3-D) diagram of the LCP packaged RF MEMS switch, and Fig. 3 shows the actual packaged RF MEMS switch prototypes.


III. PROCESS AND PACKAGE EVALUATION


In order to demonstrate that LCP may be used as a package material, tests have been performed to address out-gassing, adhesion strengths, structural integrity, and hermeticity.

A. Out-Gassing Tests

Out-gassing is a major barrier in using polymer materials for packaging RF MEMS. During the processing of polymer in RF MEMS packaging, polymer materials tend to release gas particles that would degrade the reliability of the RF MEMS switch. The ASTM-E 595–93 (1999) tests were employed to evaluate the out-gassing characteristics of LCP materials.

These tests were conducted by measuring mass changes at 125 C under vacuum for 24 h. Results are given as total mass loss (TML), collected volatile condensable materials (CVCMs), and water vapor regain (WVR). TML is the percent difference of mass measured before and after the test. CVCM is the percentage of condensed mass measured on a collector plate over the initial specimen mass. WVR is calculated by placing the measured specimens through 50% relative humidity at 23 C for 24 h, and the value is given as the percentage of increase of specimen mass before and after humidity conditioning.

Historically, a TML of 1% and a CVCM of 0.1% are the maximum levels for materials used in spacecraft applications. As seen from Table II, the experimental results demonstrate that the LCP has passed the out-gassing tests and satisfies the requirements for spacecraft applications. More importantly, even though LCP is a polymer material, it has negligible out-gassing and is suitable for RF MEMS switch packaging.

B. Adhesion and Package Integrity

One of the advantages of LCP films is that they are able to adhere to other materials without the use of external adhesives in a lamination process. This feature not only simplifies the packaging process, but also reduces the electrical loss that is associated with lossy adhesive materials. Out of reliability concerns, adequate adhesion strengths are required because either a weak LCP-to-silicon or a weak LCP-to-metal bond could prevent vias from being formed and contacted correctly.

We have conducted a pulling test to evaluate the adhesion strength of LCP on silicon using a Chatillon pull tester. Fig. 4 shows a cross section of the test structure and how the experiment has been conducted. The experimental results demonstrate that the adhesion of LCP onto Si is more than 3 lbs/in. A comparison of sputter adhesion strengths is provided in Fig. 5, which indicates that the 3-lbs/in adhesion strength is adequate to provide a reliable enclosure. A photograph of a test sample after being subjected to a peel test is shown in Fig. 6. It is interesting to note that even though the Cu/LCP was being separated from Si, it was actually the Cu/LCP interface that came apart first, which attests to the high lamination strength between LCP and silicon.


C. Structural Integrity

From the peel testing, we discovered that optimal lamination strength actually occurred over a temperature range around the melting temperature ( ), as opposed to simply being above a certain threshold value. If the lamination temperature was too low, the lamination strength would be poor. Conversely, if the lamination temperature was too high, then widely varying nonuniform lamination strengths occurred along the interface of LCP and silicon. At the extremes, nonuniform lamination at the interface gave the appearance of good bonds speckled in regions of generally poor lamination. Under optimal conditions, our peel tests show LCP-to-silicon lamination to be in excess of 10 lbs/in.

Fig. 7(a) shows an open rectangular hole in LCP laminated on a silicon substrate that has interdigitated fingers. This rectangular hole is the same size as the cavity used in the MEMS switch enclosure. Fig. 7(b) shows the cross section of the laminated LCP onto Si. As can be seen from Fig. 7(a), after the lamination, the LCP has reflowed and altered the original shape of the sharp rectangular hole. The width of the rectangular hole is 200 m. The reflow is measured to be less than 5 m at the midpoint of the cavity sidewall and 25 m at the corners (noncritical features).


D. Hermeticity Tests

It is well known that polymer materials are usually unsuitable for hermetic packaging because of their high permeabilities, which cause failure during fine leak testing. In order to establish that LCP would be viable for hermetic enclosures, hand calculations are performed based on referred data. LCP has been reported to have a permeability of 2.19 10 cm s for helium in LCP. This value may be compared to the hermetic shielding material Corning 7740 glass in helium, which has a leak rate of 8.5 10 cm s. Package hermeticity is quantitatively analyzed by using the diffusion leak rate closed-form approximation equation Leak rate (1) where is the permeability, is the exposed package area, is the pressure difference, and is the package wall thickness.

Using a permeability of 2.19 10 cm s for helium in LCP, an exposed area of 0.22 mm , an effective wall thickness of 300 m, and pressure as specified for testing the package with 7.5 10 mm cavity volume, the leak rate is estimated to be 6.424 10 atm cm s. This value is significantly below the cutoff condition required by Method 1014, MIL-STD-883.

Gross and fine leak hermetic testing has been performed on five LCP-packaged MEMS switches at Six Sigma.4 These parts are fully functional with both dc and RF via connections. The gross and fine leak tests evaluate the hermetic properties of the LCP packages in accordance with Method 1014, MIL-STD-883. Gross leak is generally indicative of structural failure, while fine leak more generally detects contamination pathways by bulk diffusion mechanisms through materials. Gross-leak testing is performed under 60 pounds per square inch guage relative to atmosphere (PSIG) of perfluorocarbon fluid for 125 min and immediately vacuumed under 5 torr for 30 s. The parts are then submerged in a bubble tester and visually inspected for leaks, as indicated by the appearance of any bubbles from the parts. Fine leak testing is performed under 125 min, 60 PSIG helium soak, followed by a 5-torr vacuum for 1 min. The experimental results demonstrate that our packages have passed the gross and fine leak tests in accordance with Method 1014, MIL-STD-883. Due to the small volume size of our package ( 0.06 mm ), standard detection methods may not be capable of measuring the species inside the cavity. Hence, it is questionable if Method 1014, MIL-STD-883, which is the current standard test, can provide conclusive results on hermeticity for small-volume packages.

IV. ELECTRICAL PACKAGE DESIGN AND SIMULATIONS

In order to evaluate the effects of the package on RF MEMS switches, full-wave electromagnetic simulations have been conducted using Ansoft High Frequency Structure Simulator (HFSS) software that employs a finite-element method. The basic structure for studying insertion loss and return loss includes a bare microstrip transmission line on silicon with a bulk conductivity S m. This structure is considered as an unpackaged device, shown in Fig. 8(a). The bare microstrip line is then packaged in LCP ( , ) with a 2-mil height cavity capped by a copper lid, as shown in Fig. 8(b). Copper vias 100 m by 100 m with 5- m-thick walls form the first-level interconnect. Each metal layer is also 5- m thick.


The chip is 3000 m by 3000 m. Agilent's Advanced Design System (ADS) LineCalc, which uses close-form equations for calculating impedance and transmission-line geometry, is employed to determine the width of microstrip lines on 254- m-thick Si. The widths of 50- and 80-microstrip lines (unpackaged) are found to be 197 and 50.7 m, respectively. In the packaged simulation, the microstrip section feeding to the coplanar waveguide is deembedded at the port.

Fig. 9 shows the simulation results of the unpackaged and packaged microstrip lines. When the 80- microstrip line is packaged with a 2-mil-high metal lid, the characteristic impedance is tuned down closer to 50 . In this case, the return and insertion losses of the packaged 80- microstrip lines improves from 13 to 25 dB and from 0.76 to 0.42 dB, respectively, at 10 GHz. The insertion and return losses of the 50- microstrip line worsens from 0.581 dB unpackaged to 0.624 dB packaged and from 24.1 dB unpackaged to 20.6 dB packaged, respectively at 10 GHz.


Table III compares simulation results of unpackaged and packaged 50- and 80- microstrip lines in 1- and 2–mil-high metal lids. High characteristic impedance microstrip lines are tuned closer to 50- transmission lines when they become striplines with 1- and 2-mil high metal lids. The capacitance per unit length of the striplines increases, which, in turn, decreases the characteristic impedance. This phenomena is described by the well-known equation for characteristic impedance.

In our research, the MEMS switch has been designed to have a high characteristic impedance ( 80 ) without a package. Hence, we expect that the package will improve the matching of the device to a 50- system. For mechanical robustness, we have chosen a 2-mil-high cavity. An equivalent-circuit model for the microvia interconnect has been developed from simulations using the Sonnet Software that employs the method of moments. This model targets the -band to understand the switch performance. The interconnect model is shown in Fig. 10 to have fF, pH, and models the capacitance between the via to the surrounding ground, and models the inductance associated with the narrow via constructed through the LCP thin film from the outer package to the metal trace on chip-parameters are measured from a packaged thru line.


An analytical method (ADS) is used to deembed all elements in the path other than the interconnect using the technique shown in. Fig. 11 compares modeled and measured -parameters of the transition. This is an agreement to 0.02 dB between model and measurement insertion losses at 10 GHz, which is our frequency of interest. Model and measurement both show less than 0.07-dB insertion loss per package transition at 10 GHz. Return loss shows agreement to less than 4-dB difference between modeled and extracted measurement. This lumped circuit strictly models the via interconnect. When devices are packaged, the interconnects and the additional copper over the packaged device together can cause tuning effects.

V. MEASURED RESULTS

S-parameter measurements have been performed with a Cascade probe station, an Agilent PNA E8364B network analyzer, and Picoprobe coplanar-waveguide probes. A load-reflect-match (LRM) calibration was performed to establish the reference planes to be at the RF probe tips. A dc probe is used to electrostatically bias the switch on with 90 V. The measured results of the LCP packaged switch in the closed state for insertion loss are provided in Fig. 12 over the -band region and plotted up to 18 GHz. Our packaged switches show a total insertion loss of 0.45 dB at -band due to the low-loss LCP material, microvias, and excellent shielding. This includes the additional 0.07 dB loss per interconnection at the input and output with 0.3 dB being attributed to the MEMS switch at -band. In addition, the measured return loss is better than 25 dB. The metal cap of the package tunes the characteristic impedance of the switch closer to 50 . Hence, the return loss of the packaged switch is improved to less than 25-dB return loss.


The -parameters of the packaged MEMS switch had also been measured in the open or off states (0 V \#\bias). Fig. 13 shows the measured -parameters of the off-state switch. The measured isolation of the packaged switch is 15 dB, which remains relatively the same as the unpackaged switch to within 1 dB.

Since the particular switches we use had been optimized for an 80- characteristic impedance system, rather than a 50- system, the isolation is a better metric of the packaging.
VI. CONCLUSION

This paper has successfully demonstrated an ultrahigh moisture-resistant RF MEMS switch enclosure using LCP. Simulations show that the entire package introduces miniscule electrical degradation to the overall circuit performance. Insertion loss of the LCP packaged switch is roughly 0.5 dB at -band with return loss greater than 25 dB and isolation loss of 14 dB.

ACKNOWLEDGMENT

The authors wish to acknowledge the collaborative work between the Microwave Microsystems Laboratory, University of California at Davis, the General Electric Global Research Center, Niskayuna, NY, Radant MEMS Inc., Stow, MA, and Lockheed Martin Commercial Space Systems, Newtown, PA.

Fuente: http://www.ece.ucdavis.edu/mml/papers/J11.pdf
Ver blogger original: http://nubia-anc.blogspot.com/
Materia: CRF

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