Table 8.

Various frequency tuning methods for the comb-drive microresonators [19,21,89,298,305,310,311,313,314,315].

Reference	Tuning Mechanism/Method	Geometry Configuration	Active/Passive Method	Resonant Frequency	Tuning Range
Lee and Cho [21]	The DC-biased electrostatic tuning comb structures arranged in the triangular shape to adjust the resonant frequency using the linear electrostatic force.	Triangular comb arrays	Active method	2.42 kHz	−3.3% (measurement); −5.3% (estimation)
Syms [19]	The use of constrained thermal expansion to tune the resonant frequency. The tensile strains may be set up in the suspension using a folded geometry.	Folded geometry	Active method	1.56 kHz	−25% (at atmospheric pressure); 50% (at 10 mTorr)
Lee et al. [313]	A closed-form design approach for comb finger profiles to achieve constant electrostatic stiffness or linear electrostatic force.	Curved comb finger	Active method	19 kHz	−55% (measurement); −45.4% (theoretical result); −42.7% (simulation)
Jensen et al. [305]	Shaped fingers allow the design of resonators operating at a wide range of spring stiffness and tuning resonant frequency over a large range.	weakening fingers	Active method	4.3 kHz	165 Hz (downward)
		stiffening fingers			5.3 kHz (upward)
Xu and Tsai [310]	The basic idea is to synthesize the design of the supported springs and the releasing holes in the proof mass. Then the process-induced effective spring constant variation can be balanced by effective mass variation.	DRIE-induced variation	Active method	3.28 kHz	2.1%
Zine-El-Abidine and Yang [311]	The suspension configuration can be mechanically altered to change its spring constant.	Curved electrode	Active method	10.8 kHz	17.6 kHz (two actuators); 21.4 kHz (four actuators)
Scheibner et al. [298]	The principle is based on electrostatically generated, amplitude-dependent forces acting on the seismic mass, and the structures implement electrostatic softening effect.	Capacitive surfaces	Active method	2.89 kHz	901 Hz
Morgan and Ghodssi [314]	The vertically-shaped comb-fingers were designed as electrostatic springs without increasing the device area.	Shaped fingers	Active method	1.6 kHz	17% (bidirectional)
Joachim and Lin [315]	The selective deposition of polysilicon by silane decomposition on electrically heated, released microstructures.	Selectively deposited polysilicon	Passive method	86.6 kHz	1.96%
Chiao and Lin [89]	Post-packaging tuning process for microresonators by pulsed laser deposition (PLD). The advantages include precise process control, versatility and easy implementation.	donor film structure	Passive method	12.37 kHz	−1.2%