Bi-resonant scanning mirror with piezoresistive position sensor for WVGA laser projection systems

Abstract

Fraunhofer IPMS developed a new type of small-sized scanning mirror for Laser projection systems in mobile applications. The device consists of a single crystal mirror plate of 1 mm diameter in a gimbal mounting enabling a biresonant oscillation of both axes at a resonance frequency of about 100 Hz and 27 kHz respectively. The mechanical scan angle (MSA) achieved is ± 7° for the slow and ± 12° for the fast axis. The mirror angle position and phase can be read out via two piezo-resistive sensors located at the torsion axes. In order to allow for a minimum device size of the resonantly driven slow axis the sensor of the inner fast axis was connected by a new kind of thin silicon conductors. Those are created by means of an etch stop in TMAH etch and kept as thin as possible in order to reduce their contribution to the mechanical stiffness of the mirror-supporting structures. This new system enables to lead six (or even more) independent electrical potentials onto the moving parts of the device, whereas the mechanical properties are mainly determined by only 2 torsion axes. The devices were subsequently characterized and tested. Technology details, simulation results, pictures of the device and the new conductor structures as well as measurement results are presented.

Wafer level vacuum packaging of scanning micro-mirrors using glass-frit and anodic bonding methods

“…The mechanical scan angle (MSA) of the micro-mirrors is 12°, which is equivalent to an optical scan angle (OSA) of 48°. The fabrication technology of such micro-mirrors was described elsewhere [1,2] and is not the target of this paper.…”

An electrostatically driven 2D micro-scanning mirror with capacitive sensing for projection display

“…Contrary to the DLP technology in which majority of the tasks for image production are realized optically, electronic design plays a more important role for scanning-type projectors. As demonstrated in prior studies, micro-scanning mirrors may be driven by electrostatic [5][6][7][8][9][10][11][12][13][14], electromagnetic [15][16][17][18] or piezoelectric [19][20][21][22][23] mechanisms. Each approach has its own advantages and disadvantages.…”

“…Adding another electrically independent set of sensing electrodes would complicate the structural design and require more manufacturing steps [12]. The other plausible sensing schemes using piezoresistive [14] and piezoelectric [21] materials also face similar challenges in parasitic feedthrough and limited routing paths that would necessitate more manufacturing steps. The main objective of the present study is to demonstrate simultaneous capacitive driving and position sensing with the same comb-drive electrodes for a 2D Lissajous scanning mirror.…”

“…Few studies have demonstrated micro-scanning mirrors with on-chip capacitive [12][13], piezoresistive [14] and piezoelectric [21] sensing capabilities. Drabe et al [14] reported a bi-axial electrostatic scanning mirror with piezoresistive sensors.…”

An Electromagnetically-Driven Piezoresistively Sensed CMOS MEMS Scanning Mirror for Projection Display

“…However, the idea is often hindered by the microfabrication capability to provide simultaneous driving and sensing. Therefore only few micro-scanners have demonstrated on-chip sensing capabilities [1][2][3]. As a first attempt to integrate CMOS in this work, the implementation conveniently provides piezoresistive sensing to the electromagnetically-driven scanning mirror.…”