Research Academics  
 
   
 
An Electromagnetically Actuated Bi-stable Optical Microswitch
 
Contact Person : Dr.-Ing. Ha-Duong Ngo

Introduction

A new, large-angle, high-speed optical microswitch with electromagnetic actuation was developed and tested. The new fabrication concept combines LPCVD (Low Pressure Chemical Vapor Deposition)-polysilicon for the mirror structure and high aspect ratio 3D electroforming for the magnetic actuator system fabricated on different wafers. The completion of the device is facilitated by polymer bonding (BCB) using a CuBe spacer to adjust the distance between the mirror surface and the actuator. The mirror and actuator layouts were optimized using Finite Element Analysis for a mirror rotation up to ±5 degrees. The total size of the mirror device is approximately 1.1 mm x 1.5 mm x 0.9 mm. The mirror diameter is 500µm, whereas the torsional beam has a cross section of only 4 µm x 2 µm

The optical switch’s concept is illustrated in Fig. 1. The top wafer contains the mirror structure and the electroplated Ni-ring. The bottom wafer’s actuator consists of two spiral coil systems to be energized separately resulting in a rotation of the mirror structure: Exiting one of the systems generates a magnetic force towards the Ni-layer (Anchor)underneath the silicon mirror tilting the mirror towards the magnetic poles of the actuator [1]. The mirror’s movement ends at the bumps on top of the pole surface (Fig. 2). The actuator was fabricated using depth lithography and electroplating in several layers. As insulation and planarizing material, a photosensitive epoxy (SU-8) was used. To achieve constant magnetic and SU-8 layer thicknesses as well as flat surfaces, CMP (Chemical-Mechanical Polishing) processes were applied. The height of the actuator with double layer coils and magnetic flux guides was 61 µm. 

To fabricate the mirror, structures with minimal stresses are required. In this work we used LPCVD-polysilicon to produce the mirror structure. The polysilicon is nearly stress free and has been proven to be suitable for fabricating flat mirror structures, which were patterned by RIE (Reactive Ion Etching). One challenge of the release process is the capillary force, which makes the release of the small mirror structure very difficult. The mirror structure is round and has two torsion beams. To optimize the mirror layout, a FEM-Analysis was performed. Figure 3 shows the calculated rotation as a function of the generated force. A challenge was to preserve a flat mirror surface. One issue was stress in the Ni-layer, which led to a mirror deformation of about 4 µm. The rotation of the mirror is determined by the thickness of the BeCu-spacer. The assembly of the mirror system is performed by a BCB (Benzyocyclobutene) glue process: after spray coating the BeCu-spacer with BCB the actuator die and mirror die were bonded together. Figure 4 shows the assembled device.

To improve the optical properties of the mirror’s reflective surface, several combinations of metalization layers were sputtered and tested. The reflection coefficients of the metalized mirrors are shown in Tab. 1.    The measurement was carried out in MAT using a laser source He-Ne 633nm from SIOS GmbH. Figure 5 shows the measurement results with the fabricated device.

 

     

Figure. 1: Exploded view of the optical micro-switch with mirror die, CuBe-spacer, and electromagnetic actuator die.

Figure 2 : Schematic representation of a cross sectional view of the switch: distance d = 29 µm, angle a=5°

 

       

Figure 3 : FEM-simulation: actuator’s magnetic force and restoring force of the mirror’s torsion beams vs. the angle of rotation.

Type NiCr
[nm]
Ti
[nm]
Au
[nm]
%,7,5°
angle
%,23°
angle
1 25 - 22 58,5  
2 25 - 44 64  
3 50 - 44 65  
4 50 - 88 67,4  
5 - 15 60 67,4  
6 - - - - 50
Table 1. Reflection coefficients of different metalization layers. 

 

Figure 4. Picture of the assembled device

Figure 5. Measurement with the assembled device

The optical microswitch was developed in cooperation with the Institute for Microtechnology (IMT), Hannover.