Design and Fabrication of Quartz MEMS Based Double-ended Tuning Fork Resonator

This paper presents the design and fabrication of double-ended tuning fork (DETF) resonator based on quartz. DETF resonator was designed with two tines structure, due to this configuration has the advantage of providing dynamic balance. Both surface and side electrode were designed on DETF tines in order to enhance the piezoelectric effect. DETF resonator was fabricated from a quartz substrate by chemical wet etching and photolithography processes, the key feature of fabrication is to create Cr/Au metal films, which should be strong enough to withstand the aggressive quartz etchant, meanwhile, the wet etching parameters was optimized in order to reduce the sidewall structure in +X orientation which can greatly deteriorate vibrating characteristics of DETF tines. Experimental results demonstrated that the Q value would be lowered and motional resistance be increased with the force sensitivity increase of DETF tines.


Introduction
Double-ended tuning fork (DETF) resonator is a kind of force transducer which can be used to measure force, acceleration, pressure and so on [1] [2] [3].Many materials can be used to manufacture DETF such as quartz, silicon, etc.Among those, the choice of quartz crystal was made because of its piezoelectric characteristics, superb structural stability and excellent elastic linearity [4] [5]. when designing a sensor composed of a quartz DETF resonator, measurement range, resolution and nonlinearity are important parameters to be considered, these sensor parameters are mainly determined by the force-frequency characteristics of DETF resonator.As the axial force is applied to the DETF, the frequency increases under tensile loading and decreases under comprehensive loading, the behavior of the change in frequency versus input load is dependent almost entirely on geometry of DETF resonator.This paper presents three different dimension parameters of DETF resonator which are suitable for different senor application environments.

Design of DETF resonator
Geometry of DETF resonator we use DETF resonator with two tines structure described in Fig. 1, the two tines vibrate in precise opposition to provide near perfect cancellation of dynamic forces.Therefore, no significant reaction forces are applied to the surrounding structure and energy loss from the resonator is negligible.As a result, the resonator has high Q factor, yielding high performance.No mechanical isolation is required for two tines structure because the dynamic and mechanical stresses cancel at the root of the tines, so the sensor can be designed much smaller and simpler, leading to lower production costs.The zero-load resonant frequency is dependent almost entirely on the dimensions of the tines and the mechanical and physical properties of the quartz as described by eq.( 1) [6].The frequency of the tuning fork as a function of the axial load in the tines is given by eq.( 2).l0, t0, w0 is corresponding to the length, width and thickness of DETF tines, E is the Young's modulus, ρ is density of α crystalline quartz, T is the axial load.
Three geometry of DETF resonator was showed in Tab.1.

Electrodes design of DETF resonator
The design of the electrode structure is crucial for driving the quartz resonator by fully utilizing the piezoelectric effect of the crystal.Through a well-designed electrode layout, an efficient electric field is formed to achieve the bending vibration of the resonator beam.DETF typically uses a Z-cut quartz crystal, as shown in Figure 2, with the length of the resonator along the Y-axis of the crystal and the width along the X-axis.The vibration mode of DETF is bending vibration along the width direction.There are two vibration nodes along the length of the resonator forks, and the polarity of the electrodes changes at these nodes.As shown in Figure 3, the electrode structure is designed with a four-sided layout, which maximizes the electromechanical conversion efficiency of the device.The electrode distribution on the two forks is symmetrical, resulting in a 180°phase difference vibration in the plane, reducing energy loss.Tab. 2 summarized the measured results of three kinds of DETF resonator.From Tab. 2 we can learn that the Q value would be lowered from 4185 to 2297 and motional resistance be increased from 263 kΩ to 637 kΩ with the force sensitivity increase from 2700 Hz/N to 11000 Hz/N of DETF tines, however, we can seal the DETF in vacuum to improve the Q value and motional resistance and maintain the corresponding force sensitivity for different senor application environments.

Summary
The design and fabrication process of DETF resonator based on quartz was discussed.Three types of DETF resonator have been developed, which can be expected to be applied in force transducer to measure force, acceleration, pressure and so on.

Fig. 6
Fig.6 Sidewall profile in crystal +X orientation