S-PARAMETER MEASUREMENTS OF MEMS SWITCHES

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1 Radant MEMS employs adaptations of the JMicroTechnology test fixture depicted in Figure 1 to measure MEMS switch s-parameters. RF probeable JMicroTechnology microstrip-to-coplanar waveguide adapter substrates are wire bonded to each microstrip port of the DUT, and GSG (ground-signal-ground) RF probes with 150 um pitch make contact with the coplanar waveguide ports of the adapters. Radant MEMS uses the JMicroTechnology PP TM 1003 adaptor, which is fabricated on 10 mil thick alumina. This thickness is nearly the same as the 8 mil height of the Radant MEMS switch base substrate, so there is only a 2 mil height difference between the wire ball bonds on the switch and the wire wedge bonds on the alumina. The s-parameters are directly applicable to situations where the MEMS switch RF ports are wire bonded to 50 ohm microstrip lines on 10 mil alumina substrate(s) and where the MEMS and microstrip backside metallic grounds are attached to the same metal base. (a) (b) Figure 1. (a) Sketch of RF probeable test fixture with FET DUT wire bonded to two Jmicro- Technology microstrip to coplanar waveguide adapter substrates. (b) RF Test Fixture for SPST MEMS Switches An Anritsu 37397C 40 MHz to 65 GHz vector network analyzer (VNA) is used to make the s- parameter measurements. Prior to measurement, VNA calibration based on calibration constants supplied by JMicroTechnology is performed on a JMicroTechnology PP TM CM10LX 10 mil calibration substrate that is similar in appearance to the CM05LX cal substrate as shown in Figure 2. On the lower left corner of the substrate is an open circuit; directly above the open circuit are a short circuit and a 50 ohm load. In the lower right corner of the substrate is a 0 mm length thru-

2 line. These four structures enable the collection of the data necessary for an SOLT calibration. The remaining lines on the substrate have varying lengths for LRL calibrations. Radant MEMS employs SOLT calibrations for many measurements up to 20 GHz. For higher frequencies, LRL calibrations are used. In fact, for measurements taken at above 10 GHz, Radant MEMS regards LRL calibrations as superior to SOLT calibrations, providing better accuracy. For very wideband measurements, such as from 50 MHz to 40 GHz (see Figure 3), Radant MEMS uses SOLT calibration up to about 6 or 7 GHz and LRL calibration for measurements at or above that range. Figure 2. JMicroTechnology calibration substrate

3 Isolation Isolation & Return Loss (db) Return Loss Insertion Loss (db) Isolation (db) Return Loss (db) Insertion Loss (db) Insertion Loss GHz Figure 3. S-Parameter Data for the RMSW200HP SPSPT, DC-40 GHz MEMS Switch Radant MEMS uses two wire bonds to connect each MEMS RF bonding pad to a JMicroTechnology adapter. The effects of these two bond wires are included in the measured MEMS s- parameters and are not de-embedded. The pair of bond wires has been modeled at Radant MEMS as a series inductance of 0.16 nh. A 0.01 pf shunt capacitance is also included to model the fringing capacitance at the end of the microstrip line on the alumina. If one desires s- parameters that do not include the bond wire effects, they can be obtained by inserting the measured s-parameters between shunt capacitors and series inductors with negative values as shown in Figure 4 and obtaining the new s-parameters with a circuit analysis program. For applications where the MEMS switch is attached to the top metal of a PCB board, such as Radant MEMS s evaluation boards, the bond wires are longer, and bond wire inductance is greater than 0.16 nh. For these situations, Radant MEMS often uses three bond wires at each RF port of the switch instead of two in order to reduce bond wire inductance. This is particularly true in the case of the larger SP4T and SP6T switch dice.

4 Measured MEMS Switch S-Parameters -0.16nH 0.16nH nH -0.16nH -0.01pF 0.01pF 0.01pF -0.01pF Figure 4. Using negative valued lumped elements to obtain switch s-parameters without bond wire effects Because the Radant MEMS s VNA can only be calibrated for 1-port and 2-port measurements, there are some limitations on the s-parameters obtained for Radant MEMS s SP2T, SP4T and SP6T switches, which have 3, 5, and 7 ports respectively. Radant MEMS s s-parameters for these switches are compilations of measured 2-port s-parameters.

5 When testing SP2T switches, a GSG RF probe is connected to a JMicroTechnology adapter at each of the three RF ports. If we designate the two switches in the SP2T as Switch #1 and Switch #2, and if the s-parameter measurements are to be made between the common port and Switch #1, Switch # 2 is terminated with a coaxial 50 ohm load at the other end of the RF probe that contacts its output port, as shown in Figure 5. When Switch #1 is on and the insertion loss is measured between the common port and the output of Switch #1 along with the return loss at both ports, there is no problem, and the return loss at both ports will indicate low reflections over the operational bandwidth since the RF probe reflections at both ports have been calibrated and de-embedded. Figure 5. An SPDT under Test When Switch #1 is opened and isolation between the common port and the Switch #1 output is measured along with the return loss at the common port and the output of Switch #1, the return

6 loss at the output of Switch #1 will indicate a very high reflection, as would be expected for an open switch. However, the return loss at the common port will show a limited match to 50 ohms. This is because the RF signal passes from the common port to the output of Switch #2 when Switch #2 is closed and Switch #1 is open. However, Switch #2 is connected to its 50 ohm termination through an RF probe that does not have a good match to 50 ohms because its discontinuities have not been calibrated out. For this measurement, only the discontinuities of the other two probes have been calibrated out. If the measurements are between the common port and the output of Switch #2, the same considerations apply. For a normal application, this is not a problem if all ports are terminated with good matches to 50 ohms, and the return loss at the common port will be good over the operational bandwidth if Switch #1 is on and Switch #2 is off and vice versa. For SP4T and SP6T switch parameters, it is not possible space-wise to position more than 3 adjustable GSG RF probes around the dice along with the required number of bias probes. Three of the RF ports are therefore connected to RF probes, and the remaining RF ports (whose switches were biased open) are wired to 50 ohm chip terminations in order avoid creation of resonances on those arms. Measurements of all possible combinations of s-parameters have not yet been made for these switches only the most important ones, such as insertion loss and isolation between the common port and each RF output port, along with associated return losses at those ports.