Figure 1 shows the schematic view of the capacitive shunt switch of single pole single throw (SPST) type. The coplanar waveguide (CPW) is along B-B' in Fig.1(a). The movable metal electrode is on the silicon plate which is actuated by electrostatic force between he silicon plate and the electrode above the plate.
Unlike most of capacitive switches, the actuator is separated from the signal path. By separating the actuator electrode from the signal electrode, the design of electrodes can be more flexible, because the actuation electrodes do not have to meet the requirements for keeping RF characteristics. With zero actuation voltage, the movable electrode is on the CPW through insulating stopper layer (Fig.1 (b),(d)). Elastic force by silicon spring presses the
The S-parameters were measured by network analyzer (8510XF of Agilent Technologies) for the frequency range of 50 to 100 GHz. The switch chip was placed on the evaluation board. No electromagnetic seal was installed around the chip, and no obvious resonance due to the substrate was observed. The probes were put on the edge of CPW, and the length of CPW is 2.8mm. The driving voltage of about 50V was also applied from the pad on the CPW substrate. The insertion loss is shown in Fig.7 together with calculated result by HFSS. The measured insertion losses are 1.06dB for the single electrode and 0.77dB for the tuned electrode at 76.5GHz. At least around this frequency, the measured values agree well with calculated ones (1.19dB for single electrode and 0.68dB for tuned electrode). The loss for tuned electrode is lower than that of single electrode as
The measured isolations were 15.6dB for the single electrode and 36.8dB for the tuned electrode. The latter value of isolation is available for many applications. The measured return losses were 15.1dB for the single electrode and 23.3dB for the tuned electrode at 76.5GHz. Return loss should be controlled in many applications, and that of 20dB or better is important for them.
The prototype MEMS switch shows a good performance for millimeter-wave range and it can be available for some applications if the conditions are ulfilled such as reliability, cost, size, etc. In designing the switch, the separate actuation mechanism can ease the restriction on the shape of electrode. If the CPW is used as actuation electrode, he size of movable electrode is determined so that the both conditions for mechanical force and RF performance can be fulfilled. So the enlargement of electrode for increasing force, for example, is not necessarily possible when RF condition is not fulfilled. For the present structure, pull-up force is partly determined by the areas of the movable silicon plate and the upper electrode on the lid, and they can be much larger than the movable electrode. The MEMS switches are superior to PIN diode or FET in insertion loss, isolation, and return loss. The equivalent circuits shows the parameters which determines the RF characteristics. When the resonance occurs, the loss is determined by the equivalent esistance Rs. The Rs is mostly determined by the esisitivity of the movable electrode, so the conductivity of the electrode metal must be high enough. By bulk micromachining, flexible condition can be employed for deposition and after-treatment, because restriction is less than surface micromachining process where movable electrode is made after the CPW is finished.
Performance improvement by the tuned electrode can be significant. As shown in Fig.4, tuned electrodes are on the same silicon plate, and they are actuated by one actuator, so only a slight increase in size is necessary for improvement by matching circuit. This s one of the advantages of the present switch. It is desirable to extend the application frequency to such lower range as 2 to 5GHz. In principle, if we can make much larger capacitance and much higher on/off atio, the structure of the present switch can be available. It is a challenge and some breakthrough will be necessary in design and fabrication process.
Monolithic integration with circuit
The above switch is fabricated by bulk-micromachining technology, which is mainly used for discrete MEMS devices. But many reports have already been made about RF MEMS devices based on surface-micromachining technology which is suitable for integration with circuit. The surface-micromachining technology utilize deposition of base materials such as polysilicon, and it is very similar to usual IC fabrication process. Obviously the monolithic integration of RF MEMS devices with circuit is very attractive. One of the most attractive advantages is to be able to use the well defined process technology. Some companies are already developing the IC for wireless communication which integrates MEMS devices. If the one-chip or System-on-Chip (SoC) solution is available, significant reduction of size and cost can be expected.
But there are many issues to overcome for achieving this kind of integration. As for process, when the MEMS devices are fabricated after finishing circuit, severe restriction is sometimes forced in order not to damage the circuit. Another issue is that the base material is limited to silicon with resistivity suitable for circuit. If high-resistivity silicon is necessary as in the above MEMS shunt switch, special processes such as epitaxial deposition is inevitable, which leads to higher cost and degrade the advantage of integration. The packaging is also an important issue for integrated MEMS devices with movable parts in it. The low-price package such as plastic mold cannot be applied and the more expensive package such as ceramic package is necessary. The area of the integrated device is larger than discrete one, so the relative cost of package can be higher if the circuit area is dominant. At present, monolithic integration is not so easy, and many approaches are made for discrete devices and researches about System-in-Package (SiP) solution is active . A variety of technology for SiP have been proposed to improve the flexibility of packaging procedure.
Development of RF MEMS devices Several issues remain for the application of the RF MEMS devices. As for the devices with static displacement like switches, the following issues should be considered.
1. High driving voltage: For electrostatic actuation, relatively high voltage of around 10 to 60V is necessary for stable actuation of moving part. Though the supply current is very small, the circuit for power supply is costly. 2. Low speed: Since the mechanical displacement is necessary, operation time of less than 1µs is difficult. Typical operation time is 1 to 100µs, which limits the application of the device.
3. Relatively low reliability: The mechanical movable and the fixed parts have the possibility of sticking to each other. The mechanical strength of the movable part is generally less than that of solid devices such as diodes or transistors. So relative reliability of MEMS device is less than that of the conventional devices.
4. Packaging complexity: The package must not degrade the mechanical movement of the movable part. So the ambient material must be gas or vacuum, and corresponding package is necessary. In most cases, hermetic sealing is necessary for avoiding stiction.
5. Relatively high cost: The fabrication process is not standardized, and a variety of processes which depend on each device are inevitable. So cost reduction is more difficult than usual integrated circuits. The seriousness of the above issues depend on the device or the application. The MEMS devices show the some superior RF characteristics to that of the solid devices, but many issues must be resolved to meet the requirements for certain applications. The reliability must be guaranteed for its application, and the cost reduction is one of the most important issues. The suitable selection or creation of the application is essential for the expansion of the application.