Electrostatic Tuned Filter Based on Circular Arc Cantilever Microbridge Technology and Its Fabrication Method

Abstract

An electrostatic tuning filter based on an arc-shaped cantilever microbridge process and its preparation method. The electrically tunable filter is composed of a silicon substrate layer, a bottom "hui"-shaped gold electrode layer, an air suspension layer, a top "S"-shaped silicon bridge surface layer, and a top gold "interdigital" electrode layer from bottom to top; the bridge surface cantilever corner and the bridge leg fixed connection of the silicon top bridge surface layer are in an "arc" structure, and each of the four inner edges of the top gold "interdigital" electrode layer has an "interdigital" electrode, and the arc radius of the fixed end of the "interdigital" electrode is the same as the bridge surface arc of the corner of the silicon top bridge surface layer. The arc-shaped microbridge structure of the present invention effectively solves the problems of excessive and concentrated stress at the 90° corner and the cantilever fixed point in the traditional "straight arm" structure, resulting in structural warping, tilting, collapse, etc., ensuring the overall consistency of the bridge surface and the overall stress of the bridge surface is uniform. The invention can be used for integrating a filter on the surface of an infrared detector to obtain a compact on-chip integrated dynamic filtering and tuning detection function.

Description

Technical Field

[0001] This invention relates to core dynamic spectral filters required in the field of infrared spectral detection, and in particular to an electrostatic tuning filter based on a circular arc cantilever microbridge process and its fabrication method. Background Technology

[0002] The development of infrared focal plane array (FLAS) detector chips is currently in its third generation, characterized by large-scale, high-density operation, and one of the next-generation development directions is gradually becoming clear: a novel "image-spectrum integration" detection technology based on intensity detection and incorporating spectral information. Compared to traditional intensity detection methods, which are ineffective in harsh environments such as cloud cover, fog, and sandstorms, this technology combines infrared spectral "fingerprint" identification information. It allows for proactive selection of spectral ranges to suppress background noise, improve the signal-to-noise ratio, and achieve effective target identification in complex environments. Furthermore, high-quality spectral selection increases the number of channels for acquiring spectral information.

[0003] The key to realizing a system-on-a-chip (SoC) is on-chip active-tuned spectral filtering technology. In recent years, developed countries, led by the United States, have conducted extensive research on this crucial technology. Currently, the main technical approach focuses on suspended microbridge filtering schemes based on the Fabry-Perot (FP) principle. Current technology uses an upper movable bridge surface, a middle suspended layer, and a lower fixed bridge surface as the main filtering structure. Under the bias of the upper and lower electrode voltages, the electrostatic attraction (or repulsion) changes the suspension height, thus adjusting the filter cavity length and achieving active selection of the filter. Traditional structures primarily use right-angle corner bridge surfaces. However, due to the special nature of the suspended structure, the upper bridge surface is prone to tilting and bending under the influence of external environmental factors such as temperature and pressure. The right-angle cantilever is a point of surface stress concentration, making it highly susceptible to collapse. These structural instabilities weaken the filtering effect and reduce the spectral quality factor.

[0004] For example, the conference paper "Study on the structure of bridge surface of the microFabry-Perot cavity tunable filter" (Journal of Physics: Conference Series. IOP Publishing, 2011, 276(1): 012079.) systematically studied the stress analysis and bridge deck displacement of right-angle cantilever. The stress analysis of other cantilever shapes was not conducted. Summary of the Invention

[0005] In order to improve the stability and balance of the suspended flat plate, this invention conducts research on the structural mechanics of this structure by simulating the coupling of multiple physical fields including force, electricity, and light. First, the corners and fixed points at the stress concentration of equivalent stress are structurally improved, and an arc structure is introduced. Second, the position of the upper electrode is reasonably designed to obtain the optimal position point for applying the bias voltage. Finally, while reducing the structural stress, the goal of ensuring uniform stress on the entire bridge deck and high consistency of the bridge deck displacement is achieved, and a highly stable spectral dynamic tunable filter is developed.

[0006] The purpose of this invention is to provide a "round-arc" cantilever microbridge spectral tunable filter with low stress and high stability and its preparation method, and to solve the following existing technical problems:

[0007] 1. Problems such as excessive stress at the corners and joints of traditional right-angle cantilever beams, uneven stress, resulting in bridge deck tilt, warping, etc.;

[0008] 2. The problem that the right-angle cantilever beam is prone to collapse after applying a static voltage bias.

[0009] The technical solution of this invention is as follows:

[0010] An electrostatic tunable filter based on a round-arc cantilever microbridge, characterized in that the electrostatic tunable filter is composed of a silicon substrate layer, a bottom "square frame" gold electrode layer, an air suspension layer, a top "S" - shaped silicon bridge layer, and a top gold "interdigitated" electrode layer from bottom to top in sequence; the top view of the electrostatic tunable filter is square, the bridge deck cantilever corners and the bridge leg fixed joints of the silicon top bridge layer are "round-arc" structures, each of the four inner sides of the top gold "interdigitated" electrode layer has an "interdigitated" electrode, the arc radius R of the fixed end of the "interdigitated" electrode is the same as the arc radius of the bridge deck at the corner of the silicon top bridge layer, that is, the arc radius at the bridge leg fixed point, the four "interdigitated" electrodes each extend counterclockwise towards the opposite side, the end of the "interdigitated" electrode is flush with the diameter of the corner of the bridge deck arc of the silicon top bridge layer of the adjacent inner side, and the bridge deck of the silicon top bridge layer under the "interdigitated" electrode directly faces the "square frame" area of the bottom gold electrode layer.

[0011] The preparation method of the above-mentioned electrostatic tunable filter based on the round-arc cantilever microbridge process is characterized by including the following steps:

[0012] 1) Using a lift-off process on the silicon substrate layer to fabricate the gold bottom electrode layer, the bottom electrode as a whole presents a "square frame", the middle area is the light-transmitting area of the filter; the surrounding areas are electrodes for applying an electrostatic bias voltage;

[0013] 2) Growing a sacrificial layer on the gold bottom electrode layer by CVD method;

[0014] 3) A silicon layer is grown on the sacrificial layer using CVD, and then a micro-nano fabrication method is used, with photoresist as the etching mask material, and reactive ion etching is used to etch and process the silicon top bridge layer pattern. Note that the cantilever corners and fixed connections of the bridge legs are "arc" shaped.

[0015] 4) The top gold electrode layer is fabricated using a photolithography lift-off process, including four top "intercalation" electrodes. The radius R of the arc at the fixed end of the top "intercalation" electrodes is the same as the radius of the arc of the top bridge layer, and the ends of the four "intercalation" electrodes are flush with the diameter of the arc corner.

[0016] 5) Use a gas dry etching method to etch and release the middle part of the sacrificial layer to obtain the air suspension layer, so that the silicon top bridge layer of the "intercalation" electrode and the "U"-shaped area of ​​the gold bottom electrode layer are directly facing each other.

[0017] The technical effects of this invention are as follows:

[0018] 1. The arc-shaped microbridge structure of this invention effectively solves the problems of excessive stress concentration at the 90° corner and cantilever fixing point in traditional "straight arm" structures, leading to structural warping, tilting, and collapse, thus making the cantilever microbridge structure more stable. The maximum stress is reduced by 53% compared to before optimization.

[0019] 2. This invention ensures the overall consistency of the bridge deck and uniform stress across the entire deck. The optimized bridge deck has a height difference of 52 nm / 50 μm, with a uniformity of ~1%.

[0020] This invention can be used to integrate filters on the surface of infrared detectors to achieve detection functionality with dynamic filtering and tuning integrated on a compact chip. Attached Figure Description

[0021] Figure 1 This is a schematic diagram of the circular arc cantilever microbridge structure of the present invention.

[0022] Figure 2 These are the processing steps of the circular arc cantilever microbridge of the present invention.

[0023] Figure 3 Comparison of simulation results before and after optimization using this invention.

[0024] (a) Stress and displacement analysis after optimization; (b) Stress and displacement analysis before optimization. Detailed Implementation

[0025] The present invention will be further described below with reference to embodiments and accompanying drawings, but this should not limit the scope of protection of the present invention.

[0026] Please refer to Figure 1 , Figure 1It is a schematic diagram of the arc-shaped cantilever microbridge structure of the present invention. As can be seen from the figure, the present invention is an electrostatic tuning filter based on an arc-shaped cantilever microbridge. The electrostatic tuning filter is composed of a silicon substrate layer 1, a bottom "hui"-shaped gold electrode layer 2, an air suspension layer 3, a top "S"-shaped silicon bridge layer 4, and a top gold "interdigitated" electrode layer 5 from bottom to top. The top view of the electrostatic tuning filter is square. The bridge cantilever corner 41 and the bridge leg fixed connection 42 of the silicon top bridge layer 4 are "arc" structures. Each of the four inner sides of the top gold "interdigitated" electrode layer 5 has an "interdigitated" electrode 51. The arc radius R of the fixed end 512 of the "interdigitated" electrode 51 is the same as the arc radius of the bridge surface arc 41 of the corner of the silicon top bridge layer 4, that is, the arc radius at the bridge leg fixed part. The four "interdigitated" electrodes 51 each extend counterclockwise towards the opposite side. The end 511 of the "interdigitated" electrode 51 is flush with the diameter of the corner of the bridge surface arc 41 of the silicon top bridge layer 4 of the adjacent inner side. The bridge surface of the silicon top bridge layer 4 under the "interdigitated" electrode 51 directly faces the "hui"-shaped area 2 of the bottom gold electrode layer.

[0027] The preparation method of the above-mentioned electrostatic tuning filter based on the arc-shaped cantilever microbridge process includes the following steps:

[0028] 1) On the silicon substrate layer 1, use the lift-off lithography process to fabricate the gold bottom electrode layer 2. The bottom electrode as a whole presents a "hui" shape, and the middle area is the light-transmitting area of the filter; the surrounding areas are electrodes for applying an electrostatic bias voltage.

[0029] 2) Grow a sacrificial layer 3' on the gold bottom electrode layer 2 by CVD method.

[0030] 3) Grow a layer of silicon on the sacrificial layer 3' by CVD method, and then use the micro-nano processing method. Using photoresist as the etching mask material, use the reactive ion etching (RIE) method to etch and process to obtain the pattern of the silicon top bridge layer 4. Note that the bridge cantilever corner 41 and the bridge leg fixed connection 42 are "arc" shapes.

[0031] 4) Use the lift-off lithography process to fabricate the top gold electrode layer 5, including four top "interdigitated" electrodes 51. The arc radius R of the fixed end 512 of the top "interdigitated" electrode 51 is the same as the arc radius R of the top bridge layer 4. The ends 511 of the four "interdigitated" electrodes 51 are flush with the diameter of the arc corner 41.

[0032] 5) Use the gas dry etching method to etch and release the middle part of the sacrificial layer 3' to obtain the air suspension layer 3, so that the silicon top bridge layer 4 of the "interdigitated" electrode directly faces the "hui" area 2 of the gold bottom electrode layer.

[0033] Example 1:

[0034] The thickness of Si substrate 1 is 300 μm;

[0035] The thickness of the Au bottom electrode layer 2 is 150 nm;

[0036] After the sacrificial layer SiO2 is released, the thickness of the Air suspended layer 3 is 2μm.

[0037] The Si top bridge layer 4 has a thickness of 0.22μm, a bridge surface size of 50μm×50μm, an arc radius R1=32μm at the fixed point of the bridge leg, and a small arc radius R2=4μm at the corner of the bridge surface connection.

[0038] The Au top electrode layer has a thickness of 0.05 μm, the radius R of the fixed end of the top electrode is consistent with the bridge surface, and the ends of the four "intercalated" electrodes are flush with the diameter of the arc corner.

[0039] Example 2:

[0040] The thickness of the Si substrate 1 is 300 μm;

[0041] The thickness of the Au bottom electrode layer 2 is 200 nm;

[0042] After the sacrificial layer SiO2 is released, the thickness of the Air suspended layer 3 is 1.8 μm.

[0043] The Si top bridge layer 4 has a thickness of 0.22μm, a bridge surface size of 200μm×200μm, an arc radius R1 = 32μm at the fixed point of the bridge leg, and a small arc radius R2 = 4μm at the corner of the bridge surface connection.

[0044] See Figure 3 , Figure 3 This is a comparison chart of simulation results before and after optimization using the present invention. Figure 3 (a) shows the equivalent von Miss stress diagram of the bridge deck cantilever corner 41 and the fixed connection 42 of the bridge leg of the silicon top bridge deck layer 4, which adopts an "arc" structure. Figure 3 (b) shows the Von Meys stress diagram of a traditional right-angle corner bridge deck. Comparing the two diagrams reveals that for this S-shaped cantilever microbridge, stress concentration points are located at the corner 41 and the fixed connection point of the bridge leg 42. These two locations are prone to collapse and fracture in the actual sample. For the traditional right-angle corner structure (Figure (b)), the maximum stress is 3.8 × 10⁻⁶. 7 N / m 2 After optimization, this invention adopts an "arc" structure for the cantilever corner 41 of the bridge deck and the fixed connection 42 of the bridge leg, with a maximum stress of 1.8 × 10⁻⁶. 7 N / m 2 , a reduction of 50%.

[0045] Experiments show that the arc-shaped microbridge structure of this invention effectively solves the problems of excessive stress concentration at the 90° corners and cantilever fixing points in traditional "straight arm" structures, leading to structural warping, tilting, and collapse. This invention ensures overall bridge deck consistency and uniform stress across the entire bridge deck. This invention can be used to integrate filters on the surface of infrared detectors, achieving dynamically filtered and tuned detection capabilities integrated on a compact chip.

Claims

1. An electrostatic tuned filter based on a circular arc-shaped cantilever microbridge, characterized in that, The electrostatic tuning filter is composed of a silicon substrate layer (1), a bottom "return" shaped gold electrode layer (2), an air suspension layer (3), a top "S" shaped silicon bridge layer (4), and a top gold "finger" electrode layer (5) from bottom to top in sequence; the top view of the electrostatic tuning filter is square, the bridge cantilever corner (41) and the bridge leg fixed connection (42) of the silicon top bridge layer (4) are "arc" structures, each of the four inner sides of the top gold "finger" electrode layer (5) has a "finger" electrode (51), the arc radius R of the fixed end (512) of the "finger" electrode (51) is the same as the arc radius of the corner of the silicon top bridge layer (4), that is, the arc radius (R1) at the bridge leg fixed part, the four "finger" electrodes (51) extend towards the opposite sides in the counterclockwise direction respectively, the end (511) of the "finger" electrode (51) is flush with the diameter of the corner of the arc of the silicon top bridge layer (4) of the adjacent inner side, and the bridge surface of the silicon top bridge layer (4) under the "finger" electrode (51) directly faces the area of the bottom "return" shaped gold electrode layer (2).

2. The method for fabricating an electrostatic tuned filter based on a circular arc-shaped cantilever microbridge as described in claim 1, characterized in that... It includes the following steps: 1) Use the lift-off lithography process on the silicon substrate layer (1) to fabricate the bottom "return" shaped gold electrode layer (2). The bottom electrode as a whole presents a "return" shape, the middle area is the light-transmitting area of the filter; the surrounding areas are electrodes for applying electrostatic bias voltage; 2) Grow a sacrificial layer (3') on the bottom "return" shaped gold electrode layer (2) by CVD method; 3) Grow a layer of silicon on the sacrificial layer (3') by CVD method, and then use micro-nano processing method. Using photoresist as the etching mask material, use reactive ion etching (RIE) method to etch and process to obtain the pattern of the silicon top bridge layer (4). Note that the bridge cantilever corner (41) and the bridge leg fixed connection (42) are "arc" shaped; 4) Use the lift-off lithography process to fabricate the top gold "finger" electrode layer (5), including four "finger" electrodes (51). The arc radius R of the fixed end (512) of the "finger" electrode (51) is the same as the arc radius R of the top bridge layer (4), and the ends (511) of the four "finger" electrodes (51) are flush with the diameter of the bridge cantilever corner (41); 5) Use the dry gas etching method to etch and release the middle part of the sacrificial layer (3') to obtain the air suspension layer (3), so that the silicon top bridge layer (4) of the "finger" electrode directly faces the bottom "return" shaped gold electrode layer (2).

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