A long-distance silicon waveguide release processing method based on SOI

Abstract

The invention discloses a long-distance silicon waveguide release processing method based on SOI, which is applied to the field of MOEMS sensors. The invention aims to solve the problem in the existing release processing technology that HF ​​gas corrodes the SiO2 layer below through the grooves on both sides of the silicon waveguide, and the silicon waveguide collapses due to lack of support, thus leading to processing failure. The invention utilizes the property of magnesium fluoride that it is highly resistant to hydrogen fluoride gas corrosion, and adopts a series of micro-nano processing technologies to protect the integrated long-distance silicon waveguide area, thereby greatly improving the success rate of silicon waveguide processing and avoiding processing defects such as silicon waveguide collapse, deviation, bending, etc., which are very easy to occur in the long-distance silicon waveguide release processing.

Description

Technical Field

[0001] The invention belongs to the field of MOEMS (micro-optical electromechanical system) sensors, and in particular relates to a micro-nano processing technology for a MOEMS sensor chip. Background Art

[0002] MOEMS sensors have gained more and more attention in the field of sensor research because they combine the advantages of easy integration of traditional MEMS (micro-electromechanical systems) and high optical measurement accuracy. Unlike the current mature large-scale integrated circuit processing, the micro-nano processing technology of MOEMS sensors is more difficult, and there is usually a saying that there is a product and a set of processes. In particular, the cavity optical force sensor designed based on the principle of the optical spring effect of the optical resonant cavity needs to use a silicon waveguide to guide the laser into the optical resonant cavity to couple with the micromechanical oscillator to achieve the measurement of physical quantities such as weak force, displacement, mass, acceleration, angular velocity, etc.

[0003] In order to realize the integration of on-chip laser sources and cooperate with chip end polishing and other processes, the silicon waveguide is usually designed to be long, about 2000um or more. This long-distance silicon waveguide needs to be integrated with a micromechanical oscillator. To achieve the resonance of the micromechanical oscillator designed on the top silicon of SOI (silicon on insulator), HF (hydrogen fluoride) gas is usually used to etch away the SiO2 below. 2 The silicon dioxide layer is used to suspend the micromechanical oscillator (called release processing). This process allows HF gas to pass through the grooves on both sides of the silicon waveguide to release the SiO2 layer underneath. 2 The silicon waveguide also collapses due to lack of support, resulting in processing failure. Summary of the invention

[0004] In order to solve the above technical problems, the present invention proposes a method for releasing a long-distance silicon waveguide, which can release the SiO2 below the silicon waveguide. 2 The silicon waveguide is protected and remains intact, greatly improving the success rate of the release process.

[0005] The technical solution adopted by the present invention is: a long-distance silicon waveguide release processing method based on SOI, comprising:

[0006] A1. Etching several micro-OPM sensor devices with integrated long-distance silicon waveguides on the top silicon layer of SOI wafer;

[0007] A2, cutting the SOI wafer processed in step A1 to obtain chip units including several devices;

[0008] A3. Measure the chip unit structure size and process the photoresist according to the measurement data;

[0009] A4, clean the chip unit, and completely cover the top silicon of the cleaned chip unit with photoresist;

[0010] A5. Use a photomask to develop the silicon waveguide area on the chip unit to remove the photoresist covering the silicon waveguide area.

[0011] A6. Use an optical coating machine to grow MgF on the surface of the chip unit after being processed in step A5. 2 film;

[0012] A7, soak the chip unit processed in step A6 in an acetone solution to strip the photoresist in the undeveloped area in step A5 and the MgF thereon. 2 Thin film, exposing the micro-oem-mechanical system devices and the edge of the silicon waveguide;

[0013] A8. Use HF gas to remove the SiO2 underneath through the etching holes on the micro-oEM system structure. 2 layer, so that the micro-oem-mechanical system structure is suspended in the air to facilitate its resonance.

[0014] Beneficial effects of the invention: The invention utilizes the strong resistance of magnesium fluoride to hydrogen fluoride gas corrosion, adopts a series of micro-nano processing technologies to achieve protection of the integrated long-distance silicon waveguide area, greatly improves the success rate of silicon waveguide processing, and avoids processing defects such as silicon waveguide collapse, offset, and bending that are very easy to occur in the release processing of long-distance silicon waveguides. It provides an effective method for the release processing of micro-opto-mechanical-mechanical system devices with integrated long-distance silicon waveguides. BRIEF DESCRIPTION OF THE DRAWINGS

[0015] Figure 1 It is a top view of the chip processing steps provided by the present invention;

[0016] Figure 2 It is a front view of the chip processing steps provided by the present invention;

[0017] Figure 3 It is a left view of the chip processing steps provided by the present invention;

[0018] Figure 4 This is a local SEM (scanning electron microscope) photo of a chip after successful release processing using the processing method provided by the present invention. DETAILED DESCRIPTION

[0019] To facilitate those skilled in the art to understand the technical content of the present invention, the present invention is further explained below with reference to the accompanying drawings.

[0020] like Figure 1 , Figure 2 , Figure 3They are respectively a top view, a front view, and a left view of the chip processing steps of the SOI-based integrated long-distance silicon waveguide release processing method provided by the present invention. Taking the chip containing three micro-opto-electromechanical system structures as an example (the ellipsis in the figure indicates that it can contain any number of micro-opto-electromechanical system structures), the specific processing steps are shown as follows.

[0021] S1: Select 8-inch SOI wafer, the top silicon (Si) thickness is 250nm, silicon dioxide (SiO 2 ) layer thickness is 3um, and the substrate silicon (Si) thickness is 700um. Etch the micro-opto-mechanical system structure and integrated silicon waveguide structure on the top silicon (Si) of SOI according to the design drawing, cut into several chip units containing multiple micro-opto-mechanical system structures, and polish the edge of the chip (corresponding to the silicon waveguide end face) to ensure that the silicon waveguide end face is complete and smooth. Take a SEM picture of the chip unit and determine the size of each part, and then make a photomask according to the photolithography process requirements.

[0022] In step S1, the micro-opto-electromechanical system structure and the integrated silicon waveguide structure are etched on the top silicon (Si) of the SOI according to the design drawing. In this embodiment, an accelerometer is taken as an example. The specific etching process includes the following steps:

[0023] ① Clean 4-inch SOI wafers using the standard RCA process;

[0024] ② Spin-coat a layer of electron beam photoresist on the surface of the cleaned SOI wafer, with a thickness of 400nm and model: PMMAA4;

[0025] ③ Perform optical edge ball removal - edge exposure and then baking;

[0026] ④ Use electron beam direct writing system for exposure, then develop, clean with ultrapure water, spin dry, and electron beam direct writing system

[0027] The system model is JEOL JBX-9500FS;

[0028] ⑤ Etch out the overall structure of the accelerometer using an ICP etching process; the overall structure of the accelerometer includes corrosion holes.

[0029] S2: The chip is soaked in acetone solution to remove the residual photoresist when the chip end surface is polished in step S1, and then the photoresist is spin-coated on the entire chip top silicon surface. In order to facilitate the photoresist stripping in the subsequent steps, a thick glue process is selected, and the photoresist thickness is 5-6um. In this step, the chip is soaked in acetone solution for tens of minutes to several hours. It is necessary to observe whether the residual photoresist is removed at any time. If the residual photoresist is denatured, it needs to be further soaked in NMP solution (N-methylpyrrolidone) to remove it.

[0030] S3: Use the photomask to develop the chip and remove the photoresist in the silicon waveguide area. Keep the photoresist in the micro-OPM system structure area so that it can be peeled off together with the magnesium fluoride on it later, so as to ensure that the micro-OPM system structure will not be affected by other substances covering it and affecting its resonance after release; keep the photoresist in the 10-15um width area of ​​the chip edge so that it can be peeled off together with the magnesium fluoride on it later, so as to ensure that the waveguide end face is exposed instead of being covered by magnesium fluoride to prevent affecting subsequent measurements.

[0031] S4: Using an optical coating machine, a magnesium fluoride film is grown on the chip surface with a thickness of about 500nm. Magnesium fluoride will fill the grooves on both sides of the waveguide (250nm deep) and fully cover the waveguide area, thereby preventing the HF gas in step S6 from entering the grooves to corrode the silicon dioxide under the silicon waveguide, thereby protecting the silicon waveguide. Magnesium fluoride has a low refractive index and its effect on the optical transmission of the silicon waveguide is negligible.

[0032] S5: Soak the chip in acetone solution to strip off the photoresist and magnesium fluoride above the micro-OPM system structure area and the chip edge area, so that the micro-OPM system structure and the silicon waveguide end face are exposed. In this step, the chip is soaked in the acetone solution for several hours to one day.

[0033] S6: Use HF gas to remove the SiO2 underneath through the etched holes on the micro-oEM system structure. 2 layer, so that the micro-oem-mechanical system structure is suspended in the air to facilitate its resonance. The silicon waveguide area has MgF 2 The protection of SiO 2 It is not corroded and supports the silicon waveguide, thus preventing the silicon waveguide from collapsing or breaking.

[0034] like Figure 4 The local SEM photos of the chip processed according to the above processing method are given. It can be seen that the silicon waveguide end face at the edge of the chip is intact and exposed. After the micro-optical electromechanical system structure in the middle of the chip is completely released, there is no MgF 2 The protected silicon waveguide is hollowed out below, with MgF 2 The silicon dioxide under the silicon waveguide in the protected area is not corroded, thus avoiding processing defects such as collapse and bending of the silicon waveguide.

[0035] Those skilled in the art will appreciate that the embodiments described herein are intended to help readers understand the principles of the present invention, and should be understood that the scope of protection of the present invention is not limited to such specific statements and embodiments. For those skilled in the art, the present invention may have various changes and variations. Any modifications, equivalent substitutions, improvements, etc. made within the spirit and principles of the present invention should be included in the scope of the claims of the present invention.

Claims

1. A long-distance silicon waveguide release processing method based on SOI, characterized in that: include: A1. Etching several micro-OPM sensor devices with integrated long-distance silicon waveguides on the top silicon layer of SOI wafer; A2, cutting the SOI wafer processed in step A1 to obtain chip units including several devices; A3. Measure the chip unit structure size and process the photoresist according to the measurement data; A4, clean the chip unit, and completely cover the top silicon of the cleaned chip unit with photoresist; A5. Use a photomask to develop the silicon waveguide area on the chip unit, remove the photoresist covering the silicon waveguide area, and retain the photoresist in the micro-opto-electromechanical system structure area; A width of 10-15um needs to be reserved at the edge of the chip unit to maintain photoresist coverage; A6. Using an optical coating machine, grow a MgF2 film on the surface of the chip unit treated in step A5; MgF2 fills the grooves on both sides of the waveguide and fully covers the waveguide area; A7, soaking the chip unit processed in step A6 in an acetone solution to strip the photoresist in the undeveloped area in step A5 and the MgF2 film thereon, exposing the micro-opto-electromechanical system device and the edge of the silicon waveguide; A8. Use HF gas to remove the SiO2 layer underneath the micro-ophthalmic-electrical system structure through the etching holes, so that the micro-ophthalmic-electrical system structure is suspended in the air to facilitate its resonance.

2. The method for releasing a long-distance silicon waveguide based on SOI according to claim 1, characterized in that: Step A2 also includes polishing the edges of the cut chip units.

3. A long-distance silicon waveguide release processing method based on SOI according to claim 2, characterized in that: The thickness of the photoresist in step A4 is 5-6 um.

4. The method for releasing a long-distance silicon waveguide based on SOI according to claim 3, characterized in that: The thickness of the MgF2 film on the waveguide surface is 500nm.

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