Development of Piezoelectrically Driven Quasi-Static 2D MEMS Mirrors with Extremely Large FoV for Scanning LiDARs ^†

Abstract

In this paper, a piezoelectrically driven quasi-static MEMS mirror is developed for a scanning LiDAR system. Finite element method (FEM) simulations are used to optimize the design of the MEMS scanner. With special emphasis on the shape and thickness of the actuators, they are optimized to reach a maximum static total optical scan angle (TOSA) of 30°. Their influence on the resonance frequency at dynamic modes and the material stress are investigated. In this study, two designs are compared with respect to their FEM simulation results. Currently, the devices are manufactured in the cleanroom. The manufactured samples will be characterized and the measurement results will be published in future works.

1. Working Principle of a MEMS Scanner

The special feature of the quasi-static MEMS scanner [1] is its large actuators. The quasi-static tilting shown in Figure 1a is achieved by applying a DC voltage of ±100 V, as demonstrated in Figure 1b(i). A maximum field of view (FoV) as high as 30° × 30° is achieved when +100 V are applied to the actuators Q1 and Q2, leading to an inward motion, and when −100 V are applied to the actuators Q3 and Q4, leading to an outward motion. This leads to a tilt of the mirror plate, as shown in Figure 1a.

2. Design Optimization

The FoV is maximized by closely investigating the spring system and the actuators of the MEMS scanner to increase the scanning area of the LiDAR system.

2.1. Thickness

The thin film stack is deposited on a 725 µm thick Si substrate that is covered with an epitaxially grown poly Si (Epi-poly Si) layer (see Figure 1a). The manufacturing process is described in [2]. The influence of the Epi-poly Si thickness is investigated in order to optimize the device performance. Accordingly, a stiffer spring design with a poly Si thickness of 50 µm is compared to a softer design with a poly Si thickness of 25 µm. At an increased thickness, the resonance frequency of the dynamic modes is doubled from 770 Hz to 1564 Hz. In return, TOSA under DC drive decreases from 29.90° to 10.36°. Furthermore, the maximum material stress in the spring system increases by approximately 55% and reaches 2.91 GPa. In summary, a stiffer design shows larger resonance frequencies as well as material stresses at lower TOSA.

2.2. Actuators and Springs

In two of our own designs, the influence of the spring system and attachment of the actuators to the die frame is investigated. One stiffer design has 9 meanders in each spring system and actuators that are attached completely to the frame, as presented in [1], whereas the second softer design has 13 meanders in each spring system and partially released actuators, as shown in [3] and Figure 1b.

The results are comparable to the findings in Section 2.1. The softer spring system shows a TOSA of 29.90°, which is ten times larger than the TOSA of the stiffer system. In conclusion, the optimization of the spring system of the device is most important for the reachable scan angle.

2.3. Outlook

The next steps of the research include the manufacturing and characterization of the fully functional MEMS scanners. As a final task, the best-performing scanners will be integrated into a LiDAR system as part of a laser scanning unit.