B ACKGROUND AND LITERATURES SURVEY

1.2.1 Micromechanical resonator

For mechanically resonant signal processing, electrical signals (current or voltage) are converted into the mechanical signals (force or velocity), processed in the mechanical domain, and then converted back to the electrical forms [7]. Most of micromechanical resonators are constructed by the driving and sensing electrodes with a transducer gap less than 2μm and actuated by the electrostatic force and Si is chosen as the most common structural material for this application. Due to the high Q value (often in excess of 1000), it is very suitable for communication applications. In terms of its operational mode, the micromechanical resonators can be categorized into flexural, torsional, and bulk mode [8]. In Figure 1.3(a)-(c), comb [9], clamped-clamped beam [10], free-free beam [11] are the typical flexural mode design of the micromechanical resonators. The displacement of the flexural structure is orthogonal to the bending stress. Figure 1.3(d) is the typical torsional mode design of the micromechanical resonators [12]. The resonant structure is in the torsional motion driven by shear stresses. Bulk mode operation of the resonators is a representative of standing longitudinal wave which is driven by electrostatic force. Based on the high stiffness of the

bulk mode, the higher resonant frequency can be achieved. In Figure 1.3(e), disk resonator is the most common design in the bulk mode resonators [13]. The technology roadmap in the development of micromechanical resonator can be illustrated by Figure 1.4 [14]. The frequency-quality factor products have increased exponentially over past years.

The resonant frequency (f) of a micromechanical resonator can be expressed simply as following, Here, Keff and Meff are the effective stiffness and mass of resonator. Modulating the size of resonator, i.e. increasing the thickness or shrinking the length of beam type resonator, can directly lead to a high resonant frequency design. When the geometric size of a micromechanical resonator is limited by manufacture technique, using high order mode or the structural design with higher stiffness can be also helpful to increase its resonant frequency.

In equation (1), the K_eff/M_eff is proportional to the ration of Young’s modulus/density (E/ρ).

Therefore, by adopting the structural material with a higher E/ρ, the higher resonant frequency can be achieved.

Quality factor (Q) is defined as 2πW0/ΔW, where W0 is the total stored energy and ΔW is the energy lost per cycle. ΔW can be expressed as ΣΔWi, where i represents the energy dissipation mechanism. The inversed Q as follows can be written as the summation of the Q factor originated from each dissipation source:

∑

wave into substrate via anchor and this stress wave will carry energy away from the resonator [15]. If the resonator can be anchored to the substrate at its nodes, certain points with zero vibration, the energy loss can be diminished effectively. For example, free-free beam resonator anchored at its flexural nodal points with four torsional beams has been proposed and validated with a much higher Q factor performance than the clamped-clamped one [11].

In addition to the anchor loss, Thermoelastic dissipation (TED) [16, 17] and Akhieser effect (AKE) [8, 18], as shown in Figure 1.4, are brought up as the dominant energy loss mechanism when the frequency is above MHz and THz, respectively. TED is resulted by the heat flow generated by the compression-expansion in the elastic material. On the other hand, AKE is caused by the loss originated from the energy absorption while the phonons of an anharmonic solid shift to a new non-equilibrium distribution under stress. As long as the characteristic time (τ) is close to the period of resonator (1/ f), a maximum of internal friction of energy loss would take place.

1.2.2 Electroplated Ni for micromechanical resonator application

As a micromechanical structural material, electroplated Ni has drawn many research attentions in MEMS manufacture since it has the characteristics of high deposition rate, low process temperature, low manufacture cost, good electrical conductivity, and high mechanical strength very suitable for post-CMOS MEMS fabrication [19, 20]. A variety of high performance Ni-based MEMS micro-actuators have been demonstrated, such as the electro-thermal actuators with large output displacement for low power applications [21, 22]

and the micromechanical resonators with high quality factor for monolithic RF CMOS oscillator fabrication [19, 23]. Owing to the intrinsic ferromagnetic property of Ni, Ni-based MEMS devices can be also designed with a magnetic-force-driven function [24, 25], applicable for the use in highly conductive salty solutions, such as in-vivo biological systems [26]. Using

the multiple molding/electroplating technology [17], 3-D Ni-based MEMS structures can be constructed with a high aspect ratio of the structural thickness to width (>100)[28] which can effectively increase the sensitivity but also reduce the driving voltage in any capacitive type transducers [29].

As aforementioned, Si is the most common structural material for micromechanical application and, so far, two-chip packaging solution using wire bonding is taken for the fabrication of the oscillator with a Si-based micromechanical resonator. If the micromechanical resonator can be fully integrated with CMOS transistors, more chip area can be saved and small form factor can be realized. However, for the realization of the MEMS-CMOS integration, MEMS-last scheme is the best economic strategy [9, 23] because its process can be fully compatible with CMOS foundry. Nevertheless, as the CMOS technology advances, the low-k dielectric materials of BEOL (Backend of the Line) may not be able to keep its property if the post-CMOS processing temperature is over 400°C [30].

Since conventional high quality Si [9] for micromechanical resonator fabrication would be close to or even higher than the ceiling temperature, it is required to develop new structural materials with low processing temperatures as well as maintain the performance of micromechanical resonators, such as frequency response and quality factor. In this regard, the key properties of materials commonly used in fabricating micromechanical resonator are summarized in Table 1.1 [31] and it reveals the feasibility of the materials pertinent to process temperatures for the CMOS integration. Ni shows the superior characteristics in terms of deposition temperature and electrical conductivity for MEMS-last application.

Electroplated Ni has been utilized as the structural material of micromechanical resonator [19, 23, 32, 33]. Owing to the coefficient mismatch of thermal expansion between

times reduction over the polysilicon one and 7 times over the Ni one with the folded-equal-beam design. With in-situ localized annealing, the quality factor of Ni comb resonator can be boosted from few thousands to tens of thousands as shown in Figure 1.7 [33].

The Q factor of a Ni micromechanical resonator is strongly dominated by its anchor. In the other words, the attachment of Ni resonators to the Si substrate at their anchors is not as sturdy as the Si one. The poor adhesion might result in a weak anchor that ultimately dissipates more energy during vibration. Via the no stem design, Ni disk resonator resonating at 60 MHz shows that its Q can be improved up to 54,507 [23]. It verifies that Ni’s intrinsic material Q is quite high at very high frequency (VHF). Thus, MEMS-last Ni disk resonator can be integrated onto CMOS transistor directly for oscillator application [19].

Previous study showed that the phase noise of the Ni resonator oscillator is -95dBc/Hz at 10 kHz offset from the 10.92 MHz carrier frequency. This performance was sufficient for low end clock applications. While the micromechanical resonators were placed over the CMOS transistor directly, the footprint of this oscillator could be only dominated by the area of the resonators. The area of 9-disk-array was only 302 μm × 60 μm as shown in Figure 1.8(b).

1.2.3 Ni based nanocomposite

Recently, it has been found that the physical properties of Ni can be further reinforced by incorporating a secondary material such as Al₂O₃, SiC, SiO₂, diamond and CNTs within itself [34-37]. These hard particles further improve the wear resistance of Ni and make it suitable for surface coating applications. It may be attractive for MEMS device fabrication. The cordierite particles had been added into the electroless Ni solution for Ni-cordierite composite deposition [36]. The incorporation of cordierite particles in Ni film improved the thermal expansion coefficient compatibility with Si. Therefore, it was possible to fabricate a Ni comb resonator on Si substrate with less thermal stress because of the incorporation of cordierite

particles. At the same time, it was found that the electroplated Ni with higher diamond concentration makes the films more compressively stressed.

Previously, our group reported a simple process by adding nano-diamond or CNT nanoparticles into an electroplating bath to fabricate Ni-based nanocomposite electro-thermal micro-actuators [38, 39]. With appropriate incorporation of the secondary phase, such as nano-diamond or CNTs, the nanocomposite actuator can have superior performance including lower power consumption and larger output displacement due to the increase of Young’s modulus, hardness and coefficient of thermal expansion (CTE) even without sacrificing its intrinsic mechanical reliability [38, 40]. Further studies have found that the incorporation of nano diamond particles could reduce fatigue limit of Ni [41]. When the average diameter of incorporated nano diamond particles lowing to 50 nm, the nanocomposite shows a high fatigue limit as good as pure Ni. The Ni nanocomposite plated in a plating bath with 2 g/L nano-diamond has about 1.29 times higher Young’s modulus/density (E/ρ) ratio than pure Ni [40]. On the other hand, with 0.028 g/L CNTs in the plating bath, the plated nanocomposite reveals 1.47 times E/ρ [38]. The property enhancements have led such electroplated Ni-based nanocomposite films for more MEMS applications, especially in RF MEMS like the fabrication of MEMS switch, resonator and filter components.