MEMS self-mixing interferometer for displacement sensing

Principal investigator: Franck Chollet,

Co-investigator: Anand Asundi (NTU), Liu AiQun (IMRE), Lin LiWei (U of Michigan)

Funding: U of Michigan, NTU, IMRE

Date started: 07/1998

Date completed:

Introduction

Acceleration or displacement sensing is an interesting application of microsystem technology. The existing product are mainly based on capacitive sensing of displacement, which require a state-of-the art electronics for the detection. In the other hand interferometric optical method may yield simple displacement detection by coding it in light intensity.

An external cavity laser configuration, where the external mirror is moving, has shown here an advantage over standard arrangement by improving the displacement sensitivity. It has been shown that the sensitivity of the device is better with stronger feedback, by increasing the slope of the transduction curve. In order to obtain a simple device a silicon suspended flat mirror should be used and external coupling optics should be avoided, thus the strong feedback needs a very short external cavity. A length below 10 m should be suitable with a high reflectivity mirror. It has been shown that the change of intensity induced by the displacement of the external mirror has a periodicity of lambda/2. Thus, to simplify the packaging and the assembly of the mirror with the diode laser, the mirror should allow accurate displacement in a similar range.

Standard mirror design can not fulfil easily this target and we have designed a new type of folding polysilicon mirror, integrating a new type of precision position-lock (top lock), new hinges for improved compactness and a bi-directional electrostatic actuator to bias the position of the mirror with respect to the laser output facet. The mirror has been fabricated with a standard process using three layer of polysilicon, two sacrificial layers and a gold layer for increased reflectivity of the mirror.

After probe assembly, the mirror has been actuated and presented a static displacement range of 16m with a +/-60V supply. The comb-drive actuator used only 60 pairs of teeth with a 2m gap, to decrease the sensitivity to polarisation noise. Thus, the range of lambda/2 (0.4 m) needed for the external cavity laser optimal operation was obtained with a bit less than 10V.
The spring constant of the mirror suspension along the sensing axis was designed at 0.5 N/m, and preliminary measurement revels a lower experimental value of 0.230 N/m. The mass of the suspended mirror is about 1.5 g. If the feedback of the mirror is strong enough, a displacement resolution better than 0.1 nm is expected, that would correspond to an acceleration of less than 1mg, with a full range larger than 3g. Linear range will be smaller, but depends on the amount of optical feedback. The next phase of the project currently under way is the integration of the mirror with the LD at 0.8m.

SEM View

Nice SEM Pic of the device!

Publication

Last updated: 20.10.99
Webmaster