CkBooks Online – Free Online Books

The general topic of MEMS modeling is a vast and nontrivial one [57] due to the great variety of physical behavior encountered in MEMS devices. The field can be narrowed down, however, when we focus on MEMS devices for RF/microwave applications. In these cases, the great majority of devices of interest fall into one of [...]

Currently, the maximum reported resonance frequency of MEM resonators is about 200 MHz [2]. For applications requiring resonators at higher frequencies, particularly the 0.5 to 6 GHz range, attention is being drawn to FBARs, since they are compatible with IC fabrication processes and MEMS micromachining, and they exhibit high Qs and resonance frequencies. In addition, [...]

The fabrication of a clamped-clamped beam MEM resonator employs a polysilicon surface micromachining process[46]. The substrate is a silicon upon which oxide and silicon nitride have been deposited; the sacrificial layer is silicon dioxide; and the structural layer is polysilicon. Wafer preparation begins by depositing isolation oxide and silicon nitride layers on a silicon wafer. [...]

Circuit design based on MEM resonators is usually carried out with the help of electromechanical analogies. Table 3.2 shows a set of such analogies. This way, the well-established CAD tools of conventional RF/microwave circuit design may be exploited. Once the circuit in question meets the electrical response specifications, the analogies enable a path back to [...]

As indicated by (3.19), the spring-mass system embodied by an unbiased clamped-clamped beam resonator exhibits a different frequency when subjected to an applied bias VP. This implies that its equivalent mass-spring- damper is bias-dependent. The corresponding bias-dependent mechanical equivalent was derived by Bannon et al. [46] in two steps: first, the unbiased parameters were derived; [...]

Description and Operation: The clamped-clamped beam resonator (Figure 3.32) consists of a doubly supported cantilever beam disposed over a bottom electrode. The beam has length Lr , width Wr , and thickness h, and is made up of a material with Young?s modulus E and mass density r. The bottom electrode has width We and [...]

Micromechanical resonators offer the potential for very high Qs in the context of conventional IC processes [1]. A number of MEM resonator structures have been investigated?namely, the clamped-clamped resonator [46], the free-free resonator [47], and the contour-mode disk resonator [48]. The clamped-clamped resonator, which has been demonstrated with frequencies around 8 MHz, has the distinction [...]

Resonators are key elements in the realization of filters and oscillators [1], as their Q determines the insertion loss and phase noise, respectively. A number of approaches to resonators have been investigated in the context of MEMS technology: planar, volumetric or cavity resonators, micromechanical, and the film bulk surface acoustic wave (FBAR) type. Transmission Line [...]

While substrate removal and shielding and spiral elevation clearly improve inductor performance, the fact that there still remains a parasitic capacitance between the all-metal traces and the substrate poses the ultimate limitation on improvement. One structure that minimizes this parasitic capacitance is the solenoid. As pointed out by Yoon et al. [25], the solenoid inductor, indeed, [...]

Up to this point, all approaches to inductors that we have discussed attempt to improve performance by separating/decoupling the structure, as much as possible, from the underlying substrate. A radical approach to substrate decoupling was recently introduced by Dahlmann et al. [28], who used solder surface tension self-assembly to bring planar inductor structures perpendicular to [...]

Yoon et al. [26] fabricated solenoid inductors in a two-step process. First, the bottom conductors were formed via thick photoresist patterning and electroplating (Figure 3.8). Second, the air bridges were formed by a technique of multiple exposure and single development. The multiexposure consisted of performing a shallow exposure to ultraviolet (UV) light [to define the [...]

To fabricate the switches, Mihailovich et al. [41] employed a low temperature surface micromachining process, with all thin films deposited at temperatures below 250°C for compatibility with MMICs [1]. The substrate was GaAs, the sacrificial layer was polyimide, and the structural layer was silicon dioxide. The process began by defining the signal lines on the [...]

The fabrication process flow is shown in Figure 3.26 [40]. The beginning wafer was GaAs, upon which the signal line and the bottom electrodes were patterned by plating Au over a seed layer of Ti/Au [Figure 3.26(a)]. Then, upon removal of the seed metal, the sacrificial layer and anchors are defined. The spun photoresist (AZ5214) [...]

The fabrication of the structure is accomplished with a five-mask process [39]. Beginning with a high-resistivity 400-µm-thick silicon wafer, a 500Å/7,500Å Ti/Au layer is deposited and patterned via lift-off to define the CPW line. Next, 100Å of plasma-enhanced chemical vapor Si3N4 is deposited and patterned to define the areas underneath where insulation is required to [...]

Examination of the equation for the pull-in of a cantilever beam, VPull-in = 8 3 27 0 k d Cantilever / e , reveals that the pull-in voltage may be reduced, not only by lowering the spring constant, but also by increasing the dielectric constant. Approaches that deal with both mechanisms were addressed by Park [...]

The fabrication of the digitally controlled varactor combines the familiar silicon surface micromachining MUMPs process together with flip-chip bonding to an Alumina substrate via indium bums [34]. The first step is the deposition of indium bums on gold pads defined on a prepatterned receiving substrate (e.g., Alumina) that contains the fixed bottom plate . The [...]

The digitally controlled parallel-plate varactor represents a clever idea for extending the tunability range of parallel-plate tunable capacitors [34]. In essence, this approach varies both the interplate distance d and the capacitor area A. Accordingly, in this structure the top plate is segmented into multiple individual plates of equal area, but suspended by springs with [...]

There are three essential aspects of this fabrication [33]: the definition of the bottom plate, the definition of the dielectric plate and the gap between it and the plates, and the definition of the top plate. A simplified schematic of the low-temperature fabrication process (all process steps at less than 200°C for post-processing compatibility with [...]

Addressing the issue of tunability, Yao [32] demonstrated an interdigitated varactor concept in which, instead of varying the interplate distance, what varies is the effective capacitor area A. The device was fabricated in a bulk crystalline silicon technology, which is not compatible with commercial IC processes, but exhibited excellent results: at a capacitance value of [...]

With the proliferation of multimode, multistandard wireless appliances, the need for high-quality varactors capable of large tunability range, at low tuning voltage spans, is a rather pressing one. Traditionally, the monolithic implementation of functions requiring tunability, such as high-performance voltage controlled oscillators (VCOs), has been precluded by the unavailability of high-quality on-chip varactors [1]. Since [...]

The starting substrate was a silicon wafer upon which a 650-nm-thick silicon nitride film was grown as an isolation layer. This was followed by the opening of windows, where the cavities were to be formed, by deep reactive ion etching (DRIE). The 30-mm-deep cavities thus created were then filled up via thermal oxidation. This was [...]

While bulk-micromachined inductors exhibit a clearly improved performance over their conventional counterparts, a number of questions have been raised regarding (1) their mechanical ruggedness to withstand subsequent wafer processing, (2) their lack of a good RF ground, and (3) the susceptibility of their characteristics to electromagnetic coupling. The first issue is elicited upon observing the [...]

The starting point of the suspended inductor [23] fabrication was the opening of vias on the wafer?s SiO2 surface passivation down to the substrate; the remaining passivation acting as a mask for the subsequent etchant. The substrate under the inductors was then removed by selective wet etching with ethylenediamine-pyrocatechol (EDP) in a procedure that lasted [...]

The pioneering work of Chang, Abidi, and Gaitan [20] first demonstrated the bulk-micromachined inductor suspended on an oxide layer and attached at four corners to the rest of the silicon wafer (Figure 3.5). The inductor, intended to have a value of 100 nH, was designed using Greenhouse?s formulas [21] as a 20-turn square spiral of 4-mm-wide [...]

Inductors are playing an ever-increasing role in RFICs [11, 12]. In addition to being frequently employed in passive tuning circuits or as high impedance chokes, many novel techniques to achieve low voltage operation in advanced silicon IC processes rely on the negligible dc voltage drop across inductors [11, 12] when employed as loads or as emitter/source [...]

To fabricate their capacitors, Muller et al. [9] began by depositing a thin 1.5-mm dielectric membrane on a 400-mm-thick high-resistivity <100> silicon substrate (Figure 3.3). The membrane comprised a three-layer structure, including a first thermal oxide 7,000Å-thick followed by a 3000Å-thick CVD-grown Si3N4 layer deposited at 700°C, and finally, a 5,000Å-thick CVD layer of silicon [...]

Capacitors are frequently employed for dc blocking and in matching networks. Two types of capacitors are normally employed in microwave circuits: (1) the interdigital capacitor for realizing values of the order of 1 pF and less, and (2) the meta-insulator-metal (MIM) capacitor for values greater than 1 pF [1]. In what follows, we examine these [...]

Micromachining, the fabrication technique to elaborate small 3-D structures in the context of planar processes, has been exploited extensively to implement high-performance passive devices [5?7]. The enhancement in RF/microwave properties usually results from the suspension of the structures, either by removal of the substrate supporting them, or from their elevation above the substrate anchoring them. [...]

Invariably, planar RF/microwave devices and circuits are mechanically supported by a substrate. The nature of the substrate (i.e., whether it is conductive, semi-insulating, or insulating) plays a major role in the ultimate performance of the devices and circuits disposed on it. The quality of a substrate may, perhaps, be most easily exposed by an examination [...]

Introduction: The application of MEMS technology in the field of RF/microwave circuits brings to within the reach of the designer the potential to achieve unprecedented levels of performance [1?3]. Indeed, by exploiting the versatility afforded by MEMS fabrication techniques [1] (in particular, bulk and surface micromachining and LIGA) the possibility of surgically removing the perennial [...]