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.
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.
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.
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.
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.
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