MEMS mirror for low cost laser scanners

U. Hofmann, J. Janes, Fraunhofer Institute for Silicon Technology ISIT

Abstract

Concept and design of a low cost two-axes MEMS scanning mirror with an aperture size of 7 millimetres

for a compact automotive LIDAR sensor is presented. Hermetic vacuum encapsulation and stacked

vertical comb drives are the key features to enable a large tilt angle of 15 degrees. A tripod MEMS mirror

design provides an advantageous ratio of mirror aperture and chip size and allows circular laser scanning.

1 Introduction

LIDAR sensors are becoming increasingly interesting for the realization and improvement of driver assistance

systems like pre-crash safety systems, intersection assistant, lane change assistant, blind spot assistant, parking

assistant or traffic jam assistant. A wide angular range and high angular resolution are key-features that scanning

LIDAR systems offer. Existing scanning LIDAR systems use bulky servo motors for rotation of a large aperture

scanning mirror making it difficult to demonstrate the required sensor dimensions and sensor costs for a series

automotive product. But cost reduction and a higher level of miniaturization seem to be possible by introduction of

MEMS technology. This paper describes the concept and the design of a low cost two-axis MEMS scanning mirror

that aims at replacing the bulky and expensive conventional laser scanner in an automotive LIDAR sensor

application.

Fig. 1. Hermetically vacuum packaged two-axis MEMS scanning mirrors fabricated on 8 inch silicon wafers

2 Basic optical concept

The key feature of the low-cost LIDAR sensor is an omnidirectional lens that integrates several reflective and

refractive functions within one single component (fig. 2). Omnidirectional scanning is achieved by first collimating

the divergent laser beam by passing the refractive centre area of the omnidirectional lens. The collimated beam then

impinges on a 2-axis MEMS scanning mirror.

Fig. 2. The optical concept of the LIDAR sensor is based on a circular scanning MEMS mirror and an omnidirectional lens

The tilted mirror reflects the beam back to propagate trough the lens again. After passing two internal reflections at

two reflective lens facets the beam exits the omnidirectional lens almost perpendicular to the optical axis of the

incoming divergent laser beam. According to the cylindrical symmetry of the overall configuration the laser beam

can be scanned within the whole range of 360 degrees. The optical concept requires a two-axis MEMS scanning

mirror which performs a circular scan at a constant tilt angle of 15 degrees resulting in a cylinder symmetric optical

deflection of 30 degrees. In order to enable a long measurement range of up to 80 metres the optical configuration

requires a mirror diameter of 7mm.

3 MEMS mirror design

MEMS scanning mirrors have been used in many different applications as for instance barcode scanners, laser

printers, endoscopes, laser scanning microscopes or laser projection displays [1]. Typically MEMS mirrors have a

mirror aperture size within the range of 0.5 to 2 millimetres. There are two major reasons for the limitation of

MEMS mirrors to such small dimensions: Firstly, static and dynamic mirror deformations rapidly increase with

increasing mirror diameter and secondly, the very low driving forces of MEMS actuators usually do not allow a

reasonable tilt angle of high inertia mirrors. Hence, to design and fabricate a 2D-MEMS scanning mirror with an

outstanding mirror size of 7 mm and a large mechanical tilt angle of +/-15 degrees is a challenge.

3.1 Static and dynamic mirror deformation

The optical conception of the LIDAR sensor requires that deformation of the MEMS mirror plate does not exceed

+/-500 nanometres. Deformations can be caused by stress gradients within the layers which the mirror is being made

of. Typically the uppermost reflective layer introduces mechanical stress that deforms the mirror to some extent. But

more often deformation is predominantly caused by the MEMS mirror dynamics. The dynamic mirror deformation

is known to scale proportional to the fifth power of mirror diameter [2]:

2

25

t

fDndeformatiomirror

(D= mirror diameter, f = tilting frequency, = tilt angle, t = mirror thickness)

This scaling law indicates that to keep the deformation of a mirror of 7 millimetres and tilt angle of 15 degrees

sufficiently low needs to correctly adjust the thickness of the mirror. For a more detailed investigation on how

different mirror geometries may effect the dynamic mirror deformation finite element analysis (FEA) was carried

out. Three different types of mirrors were simulated: 1) a mirror plate having a standard thickness of 80 microns

(typical MEMS device layer thickness), 2) a mirror plate identical to first type but additionally reinforced by a 500

micron thick and 200 microns wide stiffening ring underneath the mirror plate, 3) a solid mirror plate with a

thickness of 580 microns. For each type of mirror the simulation of mirror deformation was performed for four

different diameters (fig. 3). The FEA showed that a 7mm-mirror with a standard thickness of 80 microns would

experience unacceptably large deformations exceeding +/-6 microns. Considerable reduction of mirror deformation

to only +/-1.2 microns can be achieved by a narrow but 500 microns thick reinforcement ring underneath the mirror.

Finally a solid mirror plate with a thickness of 580 microns achieved the best result and showed a minimized mirror

deformation of only +/-0.2 microns. Thus, further design assessments and simulations only considered the two

reinforced mirror types.

Fig. 3. Calculated mirror deformation versus mirror diameter for three different mirror geometries: mirror without

reinforcement structure 80 µm thick, mirror with 500 µm thick stiffening ring underneath and solid mirror with

thickness of 580µm

3.2 Driving concept and fabrication process

In principle electromagnetic actuation would enable to achieve the highest driving forces and hence would be the

first choice for actuation of such a high inertia MEMS mirror. But the attractiveness is lowered by the fact that it

requires mounting of large permanent magnets on chip level resulting in a too large and too expensive scanning

device. A compact and cost effective solution is an electrostatically driven MEMS mirror since the whole device can

be produced completely on wafer level including hermetic packaging [3]. Figure 4 shows a two-axes MEMS

scanning mirror electrostatically actuated by stacked vertical comb drives. To drive such a large MEMS mirror with

an aperture size of 7millimetres to the required large tilt angles of +/-15 degrees it is necessary to apply resonant

actuation because it allows to achieve higher oscillation amplitudes. However, if the MEMS mirror works in

standard atmosphere damping by gas molecules is so high that even resonant actuation is not sufficient to achieve

the required scan angles. To meet the requirements of large mirror size and large tilt angle it is necessary to

minimize damping. This can be achieved by packaging the 2D-MEMS scanning on wafer level in a miniature

vacuum environment (see fig. 1). This allows the MEMS mirror to accumulate driving energy over many thousand

oscillation cycles. Electrostatically driven MEMS mirrors with Q-factors as high as 145,000 have already been

demonstrated [4]. Hence, the low-cost LIDAR MEMS scanning mirror will be fabricated in a dual layer thick

polysilicon process. Wafer bonding techniques will be applied to permanently protect each MEMS mirror against

contamination by particles, fluids or gases. A titanium getter will be integrated into each MEMS scanner cavity in

order to achieve a permanent miniature vacuum environment.

Fig. 4. Typical gimbal-mounted two-axes MEMS scanning mirror electrostatically driven by stacked vertical comb drives

3.3 Suspension concept

As shown in Fig. 4 the standard design to allow a MEMS mirror to scan a laser beam in two dimensions is a gimbal

mounted device. But the optical concept of the targeted low-cost LIDAR sensor requires a circular scan trajectory

and the MEMS mirror has to provide two perpendicular scan axes that have identical scan frequency. Practically,

this is difficult to be achieved using a gimbal mounted mirror design. For that reason a completely different design

was chosen which eliminates the need for an outer gimbal frame. Instead of suspending the mirror by two torsional

beams the mirror plate is movably kept by three long and circular bending beams (see fig. 5). This allows achieving

an advantageous ratio of mirror diameter and chip size which is an important factor for a low cost scanner. Because

of a considerably lower total mass with respect to a gimbal mirror design such a tripod design shows higher

robustness. Finite element analysis has shown that mechanical stress in the bending beams can be kept sufficiently

low to enable the required tilt angle of 15 degrees. Regardless of the three beams which are spatially separated by

angles of 120 degree the mirror builds two perpendicular tilt axes (two eigenmodes) that have almost identical

resonant frequencies. In comparison with a gimbal mounted mirror design the tripod approach shows a considerably

lower number of parasitic eigenmodes. Different variants of such a tripod MEMS mirror design will be fabricated

covering a range of scan frequency of 600Hz to 1.6kHz. This scan frequency depends on the stiffness of the three

suspensions and by the moment of inertia which is different for a solid reinforced mirror and for the ring reinforced

mirror. The whole 360 degree scenery is thus scanned at a rate of 600Hz or higher.

Fig. 5. Tripod MEMS mirror design. Deformations are minimized by stiffening rings underneath the mirror plate.

References

[1] Hofmann U., Oldsen M., Quenzer J., Janes J., Heller M., Weiss M., Fakas G., Ratzmann L., Marchetti E., D’Ascoli F., Melani M., Bacciarelli L., Volpi E., Battini F., Mostardini L., Sechi F., De Marinis M., Wagner B., Wafer-level vacuum packaged micro-scanning mirrors for compact laser projection displays, Proc. SPIE Vol. 6887, 2008

[2] Brosens P., Dynamic mirror distortions in optical scanning, Applied Optics, vol. 11, p. 2988-2989, 1972

[3] Oldsen M., Hofmann U., Quenzer J., Wagner B., A Novel Fabrication Technology for Waferlevel Vacuum Packaged Microscanning Mirrors, Proc. 9

th Electronics Packaging

Technology Conference, Singapore, 2007 [4] Hofmann U., Eisermann C., Quenzer J., Janes J., Schroeder C., Schwarzelbach O.,

Jensen J., Ratzmann L., Giese T., Senger F., Hagge J., Wagner B., Benecke W., MEMS scanning laser projection based on high-Q vacuum packaged 2D-resonators, Proc. SPIE 2011

Ulrich Hofmann

Fraunhofer Institute for Silicon Technology

Fraunhofer Strasse 1

D-25524 Itzehoe

Germany

E-mail: ulrich.hofmann@isit.fraunhofer.de

Joachim Janes

Fraunhofer Institute for Silicon Technology

Fraunhofer Strasse 1

D-25524 Itzehoe

Germany

E-mail: joachim.janes@isit.fraunhofer.de

Keywords: MEMS, mirror, LIDAR, laser, circular scan, electrostatic, vertical comb drives, low-

cost, hermetic package, vacuum