Integrated circuit switches used in 3D and other integrated circuits can be formed from solid state structures (e.g., transistors) or passive wires (MEMS). MEMS switches are typically employed because of their almost ideal isolation, which is a critical requirement for wireless radio applications where they are used for mode switching of power amplifiers (PAs) and due to their low insertion loss (i.e. resistance) at frequencies of 10 GHz and higher. MEMS switches can be used in a variety of applications, primarily analog and mixed signal applications. One such example is cellular telephone chips containing a power amplifier (PA) and circuitry tuned for each broadcast mode. Integrated switches on the chip would connect the PA to the appropriate circuitry so that one PA per mode was not required.
MEMS can be manufactured in a number of ways using a number of different tools. In general, though, the methodologies and tools are used to form small structures with dimensions in the micrometer scale with switch dimensions of approximately 5 microns thick, 100 microns wide, and 200 microns long. Also, many of the methodologies, i.e., technologies, employed to manufacture MEMS have been adopted from integrated circuit (IC) technology. For example, almost all MEMS are built on wafers and are realized in thin films of materials patterned by photolithographic processes on the top of the wafer. More specifically, the fabrication of MEMS use three basic building blocks: (i) deposition of thin films of material on a substrate, (ii) applying a patterned mask on top of the films by photolithographic imaging, and (iii) etching the films selectively to the mask. In any of these methodologies, the switches are fabricated in a horizontal orientation above the wafer/chip.
Depending on the particular application and engineering criteria, MEMS structures can come in many different forms. For example, MEMS can be realized in the form of a single cantilever structure such as, for example, shown in U.S. Pat. No. 5,578,976. In this cantilever application, a single cantilever arm (suspended electrode) is pulled toward a fixed electrode by application of a voltage. To manufacture such a cantilever structure, though, several extra and expensive processing steps are required, in addition to the building of the CMOS structure itself. For example, once all of the CMOS wiring is completed, additional process steps are required to form the MEMS switch, which adds considerable processing costs to the structure.
Also, as clearly shown in such application, the MEMS are horizontal cantilever type switches fabricated above the wafer/chip. These horizontal cantilever type switches are known to add costs to the fabrication of the device, as well as adding to package interaction issues. In addition, horizontal cantilever type switches, in many current applications, are known to stick, e.g., exhibit an inability to open the switch due to freezing closed during processing and the relatively small contact or actuation gap used in the switch, which on the order of 1 micron. This is known as sticktion.
Additionally, in known applications, the voltage required to pull the suspended electrode down to the fixed electrode by electrostatic force may be high. This has been seen to cause unwanted charging on the insulator after prolonged use and eventual failure of the switch. In certain applications, the high voltage, e.g., 100 volts, is also difficult to obtain since this has to be stepped up from about 1.5-5 volts to 30 to 100 volts using charge pumping or similar methods . The minimum voltage required for switching is called pull-in voltage, which is dependent on several parameters including the length of the suspended electrode, spacing or gap between the suspended and fixed electrodes, and spring constant of the suspended electrode, which is a function of the materials and their thickness.
Reducing the pull-in voltage without decreasing the gap and without softening the spring is desirable, as the spring provides the restoring force and determines the switching speed. In U.S. Pat. No. 7,265,429, a pair of side parallel-plate electrostatic actuators is implemented for lowering or eliminating of the bias voltages. These additional electrostatic actuators are used to reduce or eliminate the bias voltage to be applied on the fixed signal electrode. In implementation, the fixed electrode of the side parallel-plate electrostatic actuators can be elevated above a fixed signal electrode. Thus due to a smaller gap, the pull-in voltage required to pull the suspended electrode down to the fixed electrode can be lowered. However, the MEMS shown in U.S. Pat. No. 7,265,429 are not hermetically sealed, and the additional electrostatic actuators can increase fabrication costs. Also, the MEMS are horizontal cantilever type switches fabricated above the wafer/chip.
Accordingly, there exists a need in the art to overcome the deficiencies and limitations described hereinabove.