Method for fabricating self-formed self-aligned microlens array on surface of silicon photomultiplier

Abstract

The application discloses a method for manufacturing a self-forming self-aligned microlens array on the surface of a silicon photomultiplier, which comprises the following steps: depositing a metal thin film layer on the surface of a silicon photomultiplier body; forming a photoresist pattern for windowing the metal thin film layer through a photoetching alignment process, wherein the window corresponds to and is aligned with the G-APD unit photosensitive region of the silicon photomultiplier body; forming a mesh structure by etching the metal thin film layer through an etching process; applying a coating layer which is not infiltrated by a transparent insulating packaging medium on the mesh structure; dropping liquid transparent insulating packaging medium on the upper surface of the mesh structure, so that the lower surface of the liquid transparent insulating packaging medium in each mesh structure forms a convex surface downward; and solidifying to obtain a microlens array on the surface of the silicon photomultiplier. The application solves the problems of high cost and complicated coupling alignment steps in the existing method for converging photons into the G-APD unit photosensitive region in a SiPM chip.

Description

Technical Field

[0001] This invention belongs to the field of semiconductor chip technology, specifically relating to a method for fabricating a self-forming and self-aligning microlens array on the surface of a silicon photomultiplier tube. Background Technology

[0002] Silicon photomultiplier tubes (SiPMs), as high-sensitivity, high-gain, and low-power photodetectors, are commonly used in applications such as photon counting, time measurement, and energy measurement. They consist of a series of tiny Geiger-mode avalanche photodiode (G-APD) units. When a photon is incident on a G-APD unit, the resulting photogenerated electron-hole pairs are amplified through an internal avalanche effect, resulting in an electrical pulse output. The electrical pulses output from multiple G-APD units can be directly added together, thus achieving photon count resolution. However, due to the necessary gaps between adjacent G-APD units, photons incident on the gap regions cannot trigger the Geiger avalanche process; therefore, the gap regions are insensitive to photons. These gap regions are often referred to as dead regions, and their existence is one of the important factors limiting the improvement of SiPM photon detection efficiency (PDE). Therefore, one important way to improve the PDE of SiPMs is to focus photons that would otherwise illuminate the dead region onto the photosensitive region. Currently, methods for focusing photons irradiating the dead zone onto the photosensitive region of the G-APD unit can be mainly divided into the following three categories: The first category is the fiber coupling method (Papa, Angela, et al. Scintillating fibres coupled to silicon photomultiplier prototypes for fastbeam monitoring and thin timing detectors. 824(2016):128-130.). This method aligns and couples a fiber bundle containing many optical fibers with the photosensitive region of the G-APD within the SiPM, thereby preventing photons from irradiating the dead zone. However, the fiber bundle and the G-APD unit within the SiPM require precise alignment, which is cumbersome and unsuitable for large-scale production and application. The second category is the independently fabricated optical lens array focusing method (Gramuglia, Francesco, et al. Towards 10ps SPTR and ultra-low DCR in SiPMs through the combination of microlenses and photonic crystals. IEEE, 2017.). This method uses a microlens array made of metamaterials or photonic crystals to focus incident photons onto the photosensitive region of the G-APD. However, the fabrication of metalenses requires complex processes and equipment, resulting in high manufacturing costs. It also requires precise position and angle control. Furthermore, the alignment process for microlens arrays is cumbersome and difficult to adapt to mass production.The third method is the light-trap method (Ward, John E., Juan Cortina, and Daniel Guberman. "Light-Trap: a SiPM upgrade for VHE astronomy and beyond." 11.11(2016):C11007.). This method uses a micro-mirror array to reflect photons incident on the dead zone of the SiPM to the photosensitive area of ​​the G-APD. This method also requires precise alignment operations, and the mirrors are relatively expensive. Therefore, existing methods for focusing photons onto the photosensitive area of ​​the G-APD unit within a SiPM chip generally suffer from high manufacturing costs, high fabrication difficulty, cumbersome coupling and alignment steps, and are difficult to adapt to mass production. Therefore, developing a simple and low-cost microlens array on the SiPM surface is of great significance for significantly improving the photon detection efficiency and overall performance of SiPM chips. Summary of the Invention

[0003] The purpose of this invention is to provide a method for fabricating a self-forming and self-aligning microlens array on the surface of a silicon photomultiplier tube, which solves the problem of cumbersome coupling and alignment steps in existing methods for focusing photons into the photosensitive area of ​​the G-APD unit within a SiPM chip.

[0004] The technical solution adopted in this invention is a method for fabricating a self-forming and self-aligning microlens array on the surface of a silicon photomultiplier tube, comprising the following steps:

[0005] Step 1: Deposit a metal thin film layer on the surface of the silicon photomultiplier tube;

[0006] Step 2: Form a photoresist pattern for opening windows in the metal thin film layer using a photolithography alignment process. The windows correspond one-to-one with the photosensitive areas of the G-APD units in the silicon photomultiplier tube body and are aligned in the center.

[0007] Step 3: The metal thin film layer is etched to form a mesh structure using an etching process;

[0008] Step 4: Apply a non-wetting coating to the mesh structure that does not wet the transparent insulating encapsulation medium;

[0009] Step 5: Drop liquid transparent insulating encapsulating medium onto the upper surface of the mesh structure, so that the lower surface of the liquid transparent insulating encapsulating medium in each mesh structure forms a convex surface facing downwards;

[0010] Step 6: Curing to obtain a microlens array located on the surface of the silicon photomultiplier tube.

[0011] The invention is further characterized in that,

[0012] The silicon photomultiplier tube body is composed of several G-APD unit arrays, and the photosensitive area of ​​the G-APD unit is circular.

[0013] The metal thin film layer forming the mesh structure is connected to the upper surface of the high-concentration ion-doped layer of the silicon photomultiplier tube body. The metal thin film layer has an upward convex step structure around all G-APD units.

[0014] The coating is composed of a suspension of silica powder and anhydrous ethanol.

[0015] The transparent insulating encapsulation medium is silicone resin or epoxy resin.

[0016] The curing method is either curing under ultraviolet light or solidifying under natural conditions.

[0017] The beneficial effects of this invention are as follows: The method for fabricating a self-forming and self-aligned microlens array on the surface of a silicon photomultiplier tube (SiPM) allows the formed microlens array to self-align with the G-APD unit inside the SiPM, eliminating the need for alignment adjustments. This solves the problem of cumbersome coupling and alignment processes between external microlens arrays and SiPMs, significantly improving the effective fill factor of the SiPM and thus increasing its photon detection efficiency. Furthermore, it substantially reduces the manufacturing cost of the microlens array, facilitating mass production. This invention can also be extended to other arrayed detectors, fabricating microlens arrays on their surfaces, demonstrating a degree of universality. Attached Figure Description

[0018] Figure 1 This is a schematic flowchart of the method for fabricating a self-forming and self-aligning microlens array on the surface of a silicon photomultiplier tube according to the present invention.

[0019] Figure 2 This is a schematic diagram of the structure obtained in step 1 of the method for fabricating a self-forming and self-aligning microlens array on the surface of a silicon photomultiplier tube according to the present invention;

[0020] Figure 3(a) is a schematic diagram of the longitudinal cross section of the structure obtained in step 2 of the method for fabricating a self-forming and self-aligning microlens array on the surface of a silicon photomultiplier tube according to the present invention.

[0021] Figure 3(b) is a top view of the structure obtained in step 2 of the method for fabricating a self-forming and self-aligning microlens array on the surface of a silicon photomultiplier tube according to the present invention.

[0022] Figure 4(a) is a schematic diagram of the longitudinal cross section of the structure obtained in step 3 of the method for fabricating a self-forming and self-aligning microlens array on the surface of a silicon photomultiplier tube according to the present invention.

[0023] Figure 4(b) is a top view of the structure obtained in step 3 of the method for fabricating a self-forming and self-aligning microlens array on the surface of a silicon photomultiplier tube according to the present invention.

[0024] Figure 5 This is a schematic diagram of the silicon photomultiplier tube structure obtained by the method of fabricating a self-forming and self-aligning microlens array on the surface of a silicon photomultiplier tube according to the present invention.

[0025] Figure 6 This is an optical path diagram of a microlens unit in a silicon photomultiplier tube obtained by the method of fabricating a self-forming and self-aligning microlens array on the surface of a silicon photomultiplier tube according to the present invention.

[0026] In the figure, 1. Microlens array, 2. Metal thin film layer, 3. SiO2 layer, 4. G-APD unit, 5. Si substrate, 6. Cathode, 7. Photoresist. Detailed Implementation

[0027] The present invention will now be described in detail with reference to the accompanying drawings and specific embodiments.

[0028] Example 1

[0029] The silicon photomultiplier tube body used in this invention is composed of an array of several circular G-APD units 4. Below the G-APD units 4 are a Si substrate 5 and a cathode 6. A SiO2 layer 3 is deposited on the top edge of the silicon photomultiplier tube body. A contact hole is opened in the SiO2 layer 3 opposite to the high concentration ion doping layer.

[0030] Example 2

[0031] This invention provides a method for fabricating a self-forming, self-aligning microlens array on the surface of a silicon photomultiplier tube, such as... Figure 1 As shown:

[0032] Step 1: Deposit a metal thin film layer 2 on the surface of the silicon photomultiplier tube chip to prepare signal extraction electrodes and meshed metal steps, such as... Figure 2 As shown, the metal thin film layer 2 deposited directly above the SiO2 layer 3 protrudes upward to form a stepped structure as the anode, and the metal thin film layer 2 is connected to the upper surface of the high-concentration ion-doped layer of the silicon photomultiplier tube body.

[0033] Step 2: Form the photoresist pattern 7 for opening windows in the metal thin film using a photolithography alignment process, as shown in Figures 3(a) and 3(b).

[0034] Step 3: Then, the metal thin film is etched to form a metal stepped mesh structure, which is used as the support structure for the microlens array 1 and plays a supporting role for the transparent insulating protective medium, as shown in Figure 4(a) and Figure 4(b).

[0035] Step 4: Apply a coating (such as a suspension of silica powder and anhydrous ethanol) to the formed metal stepped mesh structure to produce a non-wetting effect on common surface transparent insulating encapsulation media (such as silicone or epoxy resin).

[0036] Step 5: Drop liquid transparent insulating encapsulating medium onto the upper surface of the metal stepped mesh structure. Due to the combined effect of liquid surface tension and gravity, the lower surface of the liquid transparent insulating encapsulating medium in each mesh will spontaneously form a convex surface. Therefore, after the liquid insulating transparent protective medium solidifies, it can form a lens (convex lens) structure that has a light-convex focusing effect. The metal stepped mesh structure has many meshes, and each mesh can form a convex lens, thus forming a lens array. Since the metal mesh structure has been aligned with the center of each G-APD unit, the center of the formed lens will naturally align with the center of the G-APD unit, such as... Figure 5 As shown, the formed lens array can focus light onto the photosensitive area of ​​the G-APD unit array within the SiPM chip, thereby improving the effective fill factor and thus enhancing the overall photon detection efficiency (PDE) of the SiPM chip.

[0037] Step 6: Place the formed microlens array under a UV lamp for rapid curing or place it in a natural environment for slow solidification to form a microlens array 1 with a certain mechanical strength on the surface of the SiPM chip.

[0038] Through the above method, this invention uses a metal stepped mesh structure as the support structure and liquid transparent insulating encapsulation medium and surface modifier as the main materials to prepare a self-forming and self-aligned microlens array on the surface of a silicon photomultiplier tube body composed of several G-APD unit arrays. The self-forming and self-aligned microlens array method, which achieves the self-forming and self-aligned microlens array through the combined effect of surface tension and gravity of the SiPM chip surface encapsulation protective layer, solves the problems of high cost of microlens manufacturing, cumbersome steps and difficulty in mass production when coupling with G-APD units in SiPM chips in the existing technology.

[0039] Example 3

[0040] The principle of the method for fabricating a self-forming, self-aligning microlens array on the surface of a silicon photomultiplier tube (SiPM) provided by this invention is as follows: A metal thin film of a certain thickness is deposited on the surface of the silicon photomultiplier tube (SiPM) chip. A window pattern is formed above the photosensitive area of ​​the G-APD unit within the SiPM using an overlay process in microelectronic photolithography. Then, the metal thin film directly above the G-APD unit is etched away, leaving only a certain width of metal thin film in the spacing region between the G-APD units to form a metal stepped mesh structure. A surface modifier is coated on the SiPM surface to change the wetting characteristics of the metal stepped mesh structure. Then, a liquid transparent protective medium commonly used in photodetector packaging is dropped onto the SiPM surface. Due to the surface tension of the liquid transparent protective medium, large, approximately spherical liquid droplets are formed on the surface-modified SiPM surface. Furthermore, due to the gravity of the large liquid droplets, the liquid surface between the metal steps bends downwards to form a certain arc shape, thereby forming a convex lens. Because multiple mesh-structured metal steps are pre-formed on the SiPM surface, convex lenses are formed within each of the metal mesh openings, thus creating a convex lens array. This is the principle behind the present invention's ability to form a self-forming, self-aligning micro-convex lens array. Figure 6 As shown, the convex lens array can focus photons that were originally illuminating the non-photosensitive area onto the photosensitive area of ​​the G-APD unit, thereby improving the photon detection efficiency.

Claims

1. A method for fabricating a self-forming, self-aligning microlens array on the surface of a silicon photomultiplier tube, characterized in that, Includes the following steps: Step 1: Deposit a metal thin film layer on the surface of the silicon photomultiplier tube body. The silicon photomultiplier tube body is composed of several G-APD units arrayed together, and the photosensitive area of ​​the G-APD unit is circular. Step 2: Form a photoresist pattern for opening windows in the metal thin film layer using a photolithography alignment process. The windows correspond one-to-one with the photosensitive areas of the G-APD units in the silicon photomultiplier tube body and are aligned in the center. Step 3: The metal thin film layer is etched to form a mesh structure through an etching process. The metal thin film layer with the mesh structure is connected to the upper surface of the high-concentration ion-doped layer of the silicon photomultiplier tube body. The metal thin film layer has an upward protrusion forming a stepped structure around all G-APD units. Step 4: Apply a non-wetting coating to the mesh structure that does not wet the transparent insulating encapsulation medium; Step 5: Drop liquid transparent insulating encapsulating medium onto the upper surface of the mesh structure, so that the lower surface of the liquid transparent insulating encapsulating medium in each mesh structure forms a convex surface facing downwards; Step 6: Curing to obtain a microlens array located on the surface of the silicon photomultiplier tube.

2. The method for fabricating a self-forming, self-aligning microlens array on the surface of a silicon photomultiplier tube as described in claim 1, characterized in that, The coating is composed of a suspension of silica powder and anhydrous ethanol.

3. The method for fabricating a self-forming, self-aligning microlens array on the surface of a silicon photomultiplier tube as described in claim 1, characterized in that, The transparent insulating encapsulation medium is silicone resin or epoxy resin.

4. The method for fabricating a self-forming, self-aligning microlens array on the surface of a silicon photomultiplier tube as described in claim 1, characterized in that, The curing method is curing under ultraviolet light or solidifying under natural environment.

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