Design and fabrication of a resonant scanning micromirror suspended by V shaped beams with vertical electrostatic comb drives
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- Li, X., Jin, Q., Qiao, D. et al. Microsyst Technol (2012) 18: 295. doi:10.1007/s00542-011-1384-x
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A scanning micromirror suspended by a pair of V-shaped beams with vertical electrostatic comb drives was designed, modeled, fabricated and characterized. The dynamic analyses were carried out by both theory calculation and FEM simulation to obtain frequency response, stiffness characteristics, oscillation modes and their resonance frequencies. The device was fabricated using the silicon-on-insulator process by only two photolithography masks. Some problems during the process such as the micromirror distortion and the side sticking of the comb fingers were effectively solved by thermal annealing and alcohol-replacement methods, respectively. Based on the fabricated device, the typical scanning patterns for 1-D and 2-D operation were obtained. The experimental results reveal that the micromirror can work in resonant mode with the resonant frequency of 2.38 kHz. The maximum deflection angles can reach ±4.8°, corresponding to a total optical scanning range of 19.2° at a driving voltage of 21 V.
Scanning micromirrors have been widely used in diverse applications such as barcode scanning (Arslan et al. 2010), head-up and head-worn displays (Ilgar and Ipek 2010), telecommunication systems (Toshiyoshi and Fujita 1996), display systems (Davis et al. 2008), adaptive optics (Huang et al. 2004), and etc. Their prominent advantages of small size, low power consumption, high-resolution and high-scanning speed make them attractive to increase the performance of existing devices and to expand the application fields of scanners (Schenk et al. 2001).
Vertical electrostatic comb drives is the most desirable actuation for scanning micromirrors because of its design flexibility, low driving voltages, large displacement, high resonant frequency and absence of pull-in phenomenon (Tsou et al. 2005). Theoretically, an oscillation would start only with an initial deflection of the mirror plate. So the previous configuration with an additional starting electrode on top of the driving electrode separated by an oxide layer was adopted to generate asymmetric force (Schenk et al. 1999). The initial deflection could be achieved when the mirror plate was excited by a DC voltage. The other method to start an oscillation by dual comb drives was presented by Lin et al. (2005). It used the polysilicon layer separated from device layer by insulated layer as a starting electrode which could provide an effective initial deflection before oscillation. The scanning micromirrors with these asymmetric configurations could work in both static mode and resonant mode. However, the fabrication processes of both methods were relatively complex.
The challenge of designing a scanning micromirror is to realize the relatively simple and feasible structure which can meet with the excellent performances such as large scanning angle, low driving voltages, mechanical stability, and etc. Furthermore, the limitations of the fabrication process should be taken into consideration. In order to increase finished product rate, the fabrication process should be as simple as possible. In this paper, a scanning micromirror realized by simple process was proposed and its structural design, SOI fabrication and performance test were implemented.
2 Device design and operations principle
In practice, the fabrication factors such as residual stress, alignment deviation and process variance could generate asymmetries which were sufficient to start the oscillation. For this reason all the additional starting electrodes have been abandoned since this actuation method was much easier to implement. Only exciting the driving electrode the micromirror plate could oscillate in a resonant scanning mode.
3 Modeling and analysis
3.1 Geometric modeling
Geometric dimensions of the micromirror
3.2 Modal analysis
FEM simulated frequencies of the first five modes and theoretically calculated frequency of the first mode for the 1-D scanning micromirror
3.3 Capacitance analysis
In ANSYS environment, with the element type solid122, capacitance for different tilt angle of one pair of comb fingers was extracted using the ‘CMATRIX’ command. Then at different tilt angle, the total capacitance value was multiplied by 2Nf. In our design, Nf was set to be 63.
Coefficients of the 3rd order cubic fit to C′(θ)
3.4 Stress analysis
4.1 Fabrication technology
The process started with a SOI wafer has been mentioned in Sect. 2. In fact there was another oxide layer below the handle layer which must be removed in the first step. But during this step remarkable stress would be introduced, which would lead to the micromirror plate distortion after the released step. To release the stress, the thermal annealing at 1,050° for 2 h was followed. After that, a 200 nm Al film was deposited by sputter deposition and patterned (mask #2) on the handle layer to form the etching window of the backside. Then a DRIE etching was employed to etch the silicon substrate using ICP where etching stop would occur at the BOX layer. The removal of the Al film at the backside of the handle layer was followed to prevent the chemical attacking of Al in the HF etchant which would be used in the released step. After that, the device structures were defined through lithography with mask #1 and formed by the deep silicon etching. Finally, the structures were released with HF etching to remove the BOX layer below the micromirror plate and the comb fingers. To prevent the side sticking of the comb fingers produced in HF released step, an alcohol-replacement method was used.
4.2 Fabrication results
5 Experiments and results
The experimental setup for 1-D operation was also used to measure the characteristics of the scanning micromirror. The scanning micromirror only works in the resonant scanning mode at a resonant frequency of 2.38 kHz. The maximum deflection angles can reach ±4.8°, corresponding to an optical scan range of 19.2° when actuated at twice its natural frequency at a rectangular pulsed voltage of 21 V.
A resonant scanning micromirror driven by vertically electrostatic comb drives, suspended by a pair of V-shaped beams was designed, modeled, fabricated and characterized. The dynamic analyses including modal analysis, resonant frequency analysis, capacitance analysis and stress analysis were carried out to obtain the device characteristics. The device which has comb fingers placed around the perimeter of the scanning micromirror was fabricated on a SOI wafer with a CMOS compatible process. Difficulties during the process such as the micromirror distortion caused by the stress, the side sticking of the comb fingers caused by the HF released step were effectively eliminated by thermal annealing and alcohol-replacement method, respectively. The typical scanning patterns for 1-D operation and 2-D operation were obtained by experimental setups. The maximum optical scan range can reach 19.2° when actuated at twice its natural frequency at a rectangular pulsed of 21 V, which proves that the scanning micromirror has good modulation performances.
This work was sponsored by the Program for the New Century Excellent Talents in University, Ministry of Education of China (Grant No. NCET-10-0075) and the National Natural Science Foundation of China (Grant No. 50805123)