This paper presents the thermal behavior of a non-resonant (quasi-static) biaxial piezoelectric MEMS scanner. Such MEMS mirrors, whose design originates from [
1], use either a Bragg coating (multiple (n) dielectric bilayers of amorphous Si and SiO
2) or a reflective gold coating. In our case, they are intended for LIDAR (LIght Detection And Ranging) applications, using a 1550 nm pulsed laser with an average power of 2 W. LIDAR is a crucial sensor for autonomous driving as it can provide high-density point clouds with accurate three-dimensional information [
2]. For long-range LIDAR (>100 m), the MEMS-LIDAR approach, based on MEMS mirrors, seems to currently be the best solid-state beam-steering option [
2].
In this way, 2 × 2 mm
2 square mirrors were fabricated with Bragg and Gold reflectors. The best result obtained, using a quasi-static mode, is optical angles close to 8°, on both axes, for a driving voltage of 20 V, as reported in
Figure 1. The optical angles obtained with 1 × 1 mm
2 and 0.5 × 0.5 mm
2 square mirrors are also reported using the FoM (Figure of Merit) defined in [
2]. Our results reflect the state of the art, with FoM values between 0.6 and 0.7, as shown in
Figure 2.
This paper presents a series of experimental results concerning the thermal behavior of the Bragg reflector compared to a reflective gold coating. Experimental absorption measurements, performed at 1550 nm with p- and s-type laser polarization, showed, as seen in
Figure 3, that the use of the Bragg reflector can lead to up to 24 times lower incident power absorption than the best metallic (Au) reflective coating (s-polarization, four Bragg bilayers (
n = 4)). These results pave the way for reducing mirror heating and increasing the tolerable incident laser power. A key parameter of MEMS mirrors is the impact of mirror overheating, in particular the effect of reflector absorption on mirror flatness. To study such an evolution, 400 °C/10 min annealing in air was performed on a full sheet Bragg reflector. It showed a 50% decrease in absorption (p-polarization) after the annealing step, which means better resistance to incident power.
Regarding the evolution of mirror flatness with temperature, stress-compensated technology has been developed, which allows a static radius of curvature (SCR) of 250 m at room temperature (25 °C), as shown in
Figure 4. Measurements taken at 200 °C showed that the SCR remains close to 300 m, meaning that the mirror remains flat even during overheating.
Finally, the evolution of the 2D scan was carried out at mirror temperatures of 30 °C and 150 °C, showing no difference in terms of optical angles and 2D scan figures.
These MEMS scanners have been packaged and integrated into a LIDAR system and three-dimensional (3D LIDAR) point cloud images were obtained.
Author Contributions
L.M.: project administration, writing—original draft. C.D.: investigation. A.H.: investigation. F.G.: investigation. G.L.R.: validation, investigation. J.H.: validation, investigation. L.F.: validation, investigation. G.C.: investigation. All authors have read and agreed to the published version of the manuscript.
Funding
This work was conducted under the framework of the ECSEL VIZTA project funded by the ECSEL Joint Undertaking (JU) under grant agreement no. 826600.
Institutional Review Board Statement
Not applicable.
Informed Consent Statement
Not applicable.
Data Availability Statement
Data are contained within the article.
Acknowledgments
Part of this work, performed on the Platform for NanoCharacterisation (PFNC) at CEA, was supported by the “Recherche Technologique de Base” Program funded by the French Ministry for Research and Higher Education. The authors would like to thank Catherine Brunet-Manquat (University Grenoble Alpes, CEA, Leti) for characterizing the micro-mirrors.
Conflicts of Interest
The authors declare no conflicts of interest.
References
- Kobayashi, T.; Maeda, R.; Tsaur, J. Fabrication of Optical Micro scanner driven by PZT Actuators. Jpn. J. Appl. Phys. 2005, 44, 7078. [Google Scholar] [CrossRef]
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