A method for preparing a micro-bridge of a non-cooled infrared focal plane detector

Abstract

The present application relates to a kind of non-refrigeration infrared focal plane detector micro-bridge preparation method, comprising the following steps: coating polyimide, nano-imprinting, making micro-bridge pier hole, making support layer, preparing heat-sensitive layer, preparing first insulating layer, preparing electric connection layer, preparing second insulating layer, removing polyimide layer.The present application provides a kind of process for preparing micro-bridge pier hole using nano-imprinting and lithography etching two-step method, which reduces the process difficulty of micro-bridge structure pier hole, improves the uneven film thickness of micro-bridge pier deep hole sidewall and the resulting structural strength and electrical connection problem, optimizes the inaccuracy problem of pier hole etching.

Description

[0001] This invention relates to a method for fabricating an uncooled infrared focal plane detector, and more particularly to a method for fabricating a microbridge for an uncooled infrared focal plane detector. Background Technology

[0002] MEMS uncooled infrared detectors based on CMOS technology have been widely used in military and commercial fields, such as night vision, mining, autonomous driving, and fire prevention. The core technology of this detector is a thermistor chip with a high responsivity. Its working principle is that external infrared radiation shines on the thermistor layer of the chip. Due to the temperature coefficient of resistance of the thermistor material, its resistance changes with the energy of the external radiation and its own bias. By applying a certain electrical bias across the thermistor material, the change in resistance caused by the external infrared radiation can be read. Then, through a certain calibration method, the resistance change is mapped to the external temperature change, thus completing the infrared temperature measurement and imaging functions. The sensitivity of the thermistor chip depends on the properties of the thermistor material, the performance of the readout circuit, and the thermal isolation effect of the microbridge structure. To ensure good thermal isolation of the microbridge structure, a sacrificial layer technique is typically used on the readout circuit chip to fabricate a thermal isolation structure consisting of a dielectric support layer, a circuit-connecting metal layer, and a passivation layer. This structure only contacts the readout circuit chip at the bottom of the microbridge structure's piers; the other parts float above the readout circuit, forming a vacuum insulation between them. The height of the bridge piers in a microbridge structure depends on the thickness of the sacrificial layer, which controls the degree of thermal isolation and affects the chip's ability to absorb infrared radiation, ultimately impacting the chip's responsivity. Simultaneously, as the carrier of the entire chip, the bridge piers of the microbridge structure need to possess sufficient structural strength and stability on a relatively thin basis to prevent chip collapse or unstable output during detector use.

[0003] Due to the miniaturization requirements of chips, the pixel pitch (microbridge structure) of conventional chips has reached 17µm or even 12µm, while more advanced processes have successfully achieved pixel pitches of around 8µm. To ensure sufficient space for the thermal layer and microbridge legs, the size of the bridge hole is very limited, typically only a few micrometers. Therefore, the bridge hole needs to be machined almost perpendicular to the readout circuit chip. Such stringent dimensional requirements necessitate precise process control during mass production after process debugging to ensure product yield, resulting in process inconvenience and wasted time. Furthermore, the size of the bridge hole is difficult to control precisely, often causing mismatches with the electrical connection points of the readout circuit or interference with the second and subsequent multilayer film structures. Summary of the Invention

[0004] The technical problem solved by this invention is to construct a method for fabricating uncooled infrared focal plane detector microbridges, overcoming the technical problems of complex processes and low yield in the existing fabrication of uncooled infrared focal plane detector microbridges.

[0005] The technical solution of this invention is: to provide a method for fabricating an uncooled infrared focal plane detector microbridge, comprising the following steps: Polyimide coating: A first layer of polyimide is coated on the electrode layer of the readout circuit chip, and pre-baking is performed to convert the liquid polyimide into a solid state. Nanoimprinting: A nanoimprinting template with a convex mold is used to nanoimprint a microbridge pier hole on a polyimide layer, which has the same shape as the convex mold of the nanoimprinting template. Then the nanoimprinting template is demolded to obtain a polyimide layer with holes. Fabrication of microbridge pier holes: The imprinted polyimide layer is cured to form cured polyimide layer microbridge pier holes; Fabrication of the support layer: A thin film is prepared on the polyimide layer as a support layer; Preparation of thermal layer: Prepare thermal layer, etch away the thermal layer in the hole part of the micro bridge pier and the thermal layer in the leg part of the micro bridge, and retain the thermal layer of the micro bridge deck; Preparation of the first insulating layer: The first insulating layer is prepared on the thermistor layer, and a hole VA1 is opened at the bottom of the hole of the microbridge pier to expose the readout circuit electrode. A hole VA2 is opened at the electrical connection position of the thermistor layer to expose the thermistor layer. Fabrication of electrical connection layer: The electrical connection layer is set and patterned by photolithography etching to make the thermal layer and the readout circuit electrode electrically connected through the bridge hole; Preparation of the second insulating layer: A second insulating layer is formed on the electrical connection layer; Remove the polyimide layer: Remove the polyimide layer formed during the fabrication of the microbridges.

[0006] A further technical solution of the present invention is: in the step of making microbridge pier holes, if the height H1 of the punch and the depth D1 of the microbridge pier hole are equal, then the microbridge pier hole of the cured polyimide layer is a completed through hole.

[0007] A further technical solution of the present invention is: in the step of making microbridge pier holes, if the height H1 of the punch is less than the depth D1 of the microbridge pier hole, the part of the microbridge pier hole inside the nanoimprint hole with the depth D1 greater than the height H1 of the punch is photolithographically etched away to form a polyimide layer microbridge pier hole.

[0008] A further technical solution of the present invention is that the nanoimprint template punch is provided with an angle, the angle being between 0 degrees and 89 degrees.

[0009] A further technical solution of the present invention is that the nanoimprint template punch is a stepped shape with an angle.

[0010] A further technical solution of the present invention is: in the step of making the microbridge pier hole, a sidewall blocking layer is also provided on the polyimide layer.

[0011] A further technical solution of the present invention is: in the step of fabricating the microbridge pier hole, the sidewall blocking layer at the bottom of the blind hole is removed by etching patterning before the polyimide layer is etched.

[0012] A further technical solution of the present invention is: for a single-layer microbridge structure, the polyimide layer is removed directly through a polyimide release process.

[0013] A further technical solution of the present invention is as follows: For the multilayer microbridge structure, an upper polyimide layer is prepared again on the basis of the second insulating layer, and then other functional layers are prepared on the upper polyimide layer, and then the polyimide layer is removed by a polyimide release process.

[0014] A further technical solution of the present invention is that the first insulating layer and the second insulating layer are passivated insulating layers, and the second insulating layer serves as an infrared absorbing layer.

[0015] The technical effect of this invention is as follows: This invention provides a method for fabricating a microbridge for an uncooled infrared focal plane detector, comprising the following steps: coating a first layer of polyimide on the electrode layer of a readout circuit chip, and converting the liquid polyimide to a solid state after pre-baking; using a nanoimprint template with a convex mold to nanoimprint microbridge pier holes with the same shape as the convex mold on the polyimide layer; then demolding the nanoimprint template to obtain a polyimide layer with holes; curing the imprinted polyimide layer to form cured polyimide layer microbridge pier holes; and fabricating microbridge pier holes on the polyimide layer. A thin film is used as a support layer; a thermal layer is prepared, and the thermal layer of the microbridge pier hole portion and the microbridge leg portion is etched away, while the thermal layer of the microbridge surface is retained; a first insulating layer is prepared on the thermal layer, and a hole VA1 is opened at the bottom of the microbridge pier hole to expose the readout circuit electrode, and a hole VA2 is opened at the electrical connection position of the thermal layer to expose the thermal layer; an electrical connection layer is set, and the electrical connection layer is patterned by photolithography to make the thermal layer and the readout circuit electrode electrically connected and conductive through the pier hole; a second insulating layer is prepared on the electrical connection layer; the polyimide layer formed during the fabrication of the microbridge is removed. This invention provides a two-step process for preparing microbridge pier holes using nanoimprinting and photolithography etching. This process reduces the fabrication difficulty of the pier holes in the microbridge structure, improves the uneven film thickness on the sidewalls of the deep holes in the microbridge and the resulting structural strength and electrical connection problems, and optimizes the problem of inaccurate etching of the pier holes. Attached Figure Description

[0016] Figure 1 This is a flowchart of the present invention.

[0017] Figure 2 This is a structural diagram of the present invention. Implementation

[0018] The technical solution of the present invention will be further described below with reference to specific embodiments.

[0019] like Figure 1 , Figure 2 As shown, a specific embodiment of the present invention provides a method for fabricating an uncooled infrared focal plane detector microbridge, comprising the following steps: Coating with polyimide: A first layer of polyimide is coated on the electrode layer of the readout circuit chip, and pre-baking converts the liquid polyimide into a solid state.

[0020] The specific implementation process is as follows: The first layer of polyimide is coated on the readout circuit chip using MEMS semiconductor technology. After baking, it changes from liquid to solid. However, at this time, due to the low baking temperature, the polyimide is not completely imidized and the hardness is not very high.

[0021] Nanoimprinting: A nanoimprinting template with a convex mold is used to nanoimprint a microbridge pier hole on a polyimide layer, which has the same shape as the convex mold of the nanoimprinting template. Then the nanoimprinting template is demolded to obtain a polyimide layer with holes.

[0022] The specific implementation process is as follows: A nanoimprint template is used to imprint onto a polyimide layer. A hole with the same shape as the nanoimprint template's punch is formed on the polyimide layer by an imprint template with a punch. Then, the nanoimprint template is demolded to obtain a polyimide layer with holes. Specifically, the nanoimprint template can be a single template with the same size as the readout circuit wafer, or it can be a partitioned template smaller than the readout circuit wafer. Repeatedly splicing and imprinting the partitioned templates onto the polyimide layer can also yield a whole wafer with holes. The punch shapes on the partitioned templates with punches can be the same or different. If the shapes are different, multiple corresponding polyimide holes of different shapes can be prepared on a single wafer. The hole generally has a circular cross-section, and the height H1 of the nanoimprint template punch is 1% to 100% of the overall polyimide thickness D1. In a preferred embodiment, the nanoimprint template punch has an angle between 0 and 89 degrees, and the nanoimprint template punch is a stepped shape with an angle. The nanoimprint stencil punch is designed with an angle, and the corresponding holes formed on the polyimide also have an angle, typically ranging from 0 to 89 degrees. Generally, the hole shape is wider at the top and narrower at the bottom, resulting in a funnel shape. Alternatively, the nanoimprint stencil punch can be designed with an angled, stepped shape, and the corresponding holes formed on the polyimide also have an angle, with a wider top and narrower bottom, resulting in a multi-level stepped funnel shape. The angle is typically 0 to 89 degrees, and the steps range from 1 to 50 levels.

[0023] This invention fabricates microbridge pier holes using a two-step method combining nanoimprinting and photolithography etching, avoiding the high process difficulty of the one-step photolithography etching method for fabricating deep holes in polyimide microbridges. In particular, since both polyimide and photoresist are organic materials, the etching process of the polyimide layer also results in photoresist loss. Therefore, a thicker layer of photoresist is required to support the deep holes in the polyimide layer during etching, leading to complex processes and increased costs. The method of this invention, by employing photolithography etching with adjustable etching depth for the polyimide layer holes, allows for the application of a thinner layer of photoresist compared to the one-step deep hole etching method.

[0024] Fabrication of microbridge pier holes: The imprinted polyimide layer is cured to form the cured polyimide layer microbridge pier holes.

[0025] The specific implementation process is as follows: After the above-mentioned nanoimprinted polyimide holes are completed, the polyimide layer is baked again to completely cure the polyimide. If the height H1 of the punch is equal to the thickness D1 of the polyimide layer, the cured polyimide hole is the completed through-hole, and subsequent processes are carried out; if the height H1 of the punch is less than the thickness D1 of the polyimide layer, the thickness of the polyimide layer between the bottom surface of the nanoimprinted hole and the top surface of the readout circuit is the difference between D1 and H1. In this case, nanoimprinting and photolithography processes need to be combined. Adhesive is applied, photolithography is performed, and etching is carried out on the cured polyimide layer to etch away the thickness D1-H1 of the polyimide layer inside the nanoimprinted hole, forming a polyimide layer through-hole. Because the blind holes on the upper part of the polyimide layer are precisely sized, and the etching depth of the through-holes on the lower part is shallow, the overall through-hole size is relatively accurate, avoiding the mismatch problem between the through-hole and the electrical connection point of the readout circuit.

[0026] Fabrication of the support layer: A thin film is prepared on the polyimide layer as a support layer.

[0027] The specific implementation process is as follows: (1) If the microbridge pier hole is a through hole formed by one-step nanoimprinting on the polyimide layer, then a thin film is prepared on the polyimide as a support layer by plasma-enhanced chemical vapor deposition (PECVD). At this time, the support layer film will cover the polyimide layer, including the bottom of the through hole and the sidewall of the through hole formed above. The coating thickness and uniformity of the inner sidewall of the pier hole can be adjusted by the angle of the nanoimprinting punch, which enhances the electrical connection reliability between the MEMS chip and the readout circuit chip. (2) If the microbridge pier hole is a through hole prepared by a two-step method combining nanoimprinting and photolithography etching on the polyimide layer, then the specific implementation method of the support layer is as follows: (2.1) If the through hole is a funnel-shaped structure with a vertical structure at the bottom, the Si3N4 support layer also forms a combination structure of funnel and vertical. The thickness of the Si3N4 film formed on the funnel is greater than the thickness of the Si3N4 on the sidewall of the vertical through hole.

[0028] (2.2) If the through hole is funnel-shaped, the overall trend of the thickness of the through hole sidewall is that the upper part is thicker and the lower part is thinner, and the thickness of the horizontal surface of the step is greater than the thickness of the inclined surface.

[0029] (2.3) If the through hole is funnel-shaped at the top and vertical at the bottom, the thickness of the sidewall of the funnel-shaped step is greater than the thickness of the sidewall of the vertical bottom, and the internal thickness of the funnel-shaped step is consistent with that described in (2.2).

[0030] Preparation of the thermal layer: Prepare the thermal layer, etch away the thermal layer in the hole part of the microbridge pier and the thermal layer in the leg part of the microbridge, and retain the thermal layer on the bridge deck.

[0031] The specific implementation process is as follows: After the support layer process is completed, a heat-sensitive layer is prepared, and the piers, legs and other useless positions are removed graphically, leaving only the heat-sensitive layer of the micro-bridge deck.

[0032] Fabrication of the first insulating layer: A first insulating layer is fabricated on the thermistor layer, and a hole VA1 is opened at the bottom of the hole in the microbridge pier to expose the readout circuit electrode. A hole VA2 is opened at the electrical connection position of the thermistor layer to expose the thermistor layer. The first insulating layer is a passivated insulating layer.

[0033] Fabrication of the electrical connection layer: An electrical connection layer is formed by photolithographic etching to pattern the electrical connection layer so that the thermistor layer and the readout circuit electrodes are electrically connected through the pier holes. In a specific embodiment, this layer is formed of an alloy material, and the electrical connection layer is patterned to make the thermistor material and the readout circuit electrically connected through the pier holes.

[0034] Preparation of a second insulating layer: A second insulating layer is fabricated on the electrical connection layer.

[0035] The specific implementation process is as follows: A second insulating layer is prepared on the electrical connection layer to form a protective layer for the electrical connection layer, and also serves as an infrared absorption layer. The second insulating layer is a passivation insulating layer.

[0036] Remove the polyimide layer: Remove the polyimide layer formed during the fabrication of the microbridges.

[0037] A preferred embodiment of the present invention is as follows: In the step of fabricating the microbridge pier hole, if the height H1 of the punch and the depth D1 of the microbridge pier hole are equal, the cured polyimide layer microbridge pier hole is a completed through hole. If the height H1 of the punch is less than the depth D1 of the microbridge pier hole, the portion of the microbridge pier hole inside the nanoimprint hole where the depth D1 is greater than the height H1 of the punch is etched away to form the polyimide layer microbridge pier hole.

[0038] A preferred embodiment of the present invention is as follows: in the step of fabricating the microbridge pier hole, a sidewall blocking layer is further provided on the polyimide layer, and the sidewall blocking layer at the bottom of the blind hole is removed by etching patterning before the polyimide layer is etched.

[0039] The specific implementation process is as follows: During the formation of through-holes using nanoimprinting and photolithography, to prevent over-etching of the blind holes initially formed by nanoimprinting during polyimide etching, a sidewall barrier layer can be prepared on the polyimide layer. This sidewall barrier layer is then removed from the bottom of the blind holes through etching patterning before the polyimide layer is etched. This sidewall barrier layer limits over-etching of the upper blind holes during the polyimide layer through-hole etching process and also serves as a seed layer for the subsequent preparation of the support layer, resulting in better film quality. It also increases the thickness of the bridge pier sidewalls, enhancing the strength of the support structure.

[0040] A preferred embodiment of the present invention is as follows: For a single-layer microbridge structure, the polyimide layer is directly removed using a polyimide release process. For a multilayer microbridge structure, an upper polyimide layer is prepared again on top of the second insulating layer, and then other functional layers are prepared on the upper polyimide layer. Finally, the polyimide layer is removed using a polyimide release process. Specifically: If the MEMS chip is designed as a single-layer microbridge structure, the polyimide is removed through a polyimide release process to form a thermally insulating cavity between the microbridge and the readout circuit, thus completing the chip fabrication process. If the MEMS chip is designed as a double-layer or multi-layer microbridge structure, an upper polyimide layer is fabricated again based on step eight, and then other functional layers are fabricated on the upper polyimide layer. Similar to the aforementioned polyimide and support layer process, a multi-layer structure is formed with the first layer microbridge surface as a support platform. Finally, the multi-layer microbridge structure process is completed through a one-step polyimide release process, thus completing the chip fabrication.

[0041] The technical effect of this invention is as follows: This invention provides a method for fabricating a microbridge for an uncooled infrared focal plane detector, comprising the following steps: coating a first layer of polyimide on the electrode layer of a readout circuit chip, and pre-baking to convert the liquid polyimide to a solid state; using a nanoimprint template with a convex mold to nanoimprint microbridge pier holes with the same shape as the convex mold on the polyimide layer, and then demolding the nanoimprint template to obtain a polyimide layer with holes; curing the imprinted polyimide layer to form cured polyimide layer microbridge pier holes; and fabricating a microbridge pier hole on the polyimide layer. A thin film is used as a support layer; a thermal layer is prepared, and the thermal layer of the microbridge pier hole portion and the microbridge leg portion is etched away, while the thermal layer of the microbridge surface is retained; a first insulating layer is prepared on the thermal layer, and a hole VA1 is opened at the bottom of the microbridge pier hole to expose the readout circuit electrode, and a hole VA2 is opened at the electrical connection position of the thermal layer to expose the thermal layer; an electrical connection layer is set, and the electrical connection layer is patterned by photolithography to make the thermal layer and the readout circuit electrode electrically connected and conductive through the pier hole; a second insulating layer is prepared on the electrical connection layer; and the polyimide layer formed during the fabrication of the microbridge is removed. This invention provides a two-step process for fabricating microbridge pier holes using nanoimprinting and photolithography etching. This process reduces the fabrication difficulty of the pier holes in the microbridge structure, improves the uneven film thickness on the sidewalls of the deep holes in the microbridge and the resulting structural strength and electrical connection problems, and optimizes the problem of inaccurate etching of the pier holes.

[0042] The above description, in conjunction with specific preferred embodiments, provides a further detailed explanation of the present invention. It should not be construed that the specific implementation of the present invention is limited to these descriptions. For those skilled in the art, various simple deductions or substitutions can be made without departing from the concept of the present invention, and all such modifications and substitutions should be considered within the scope of protection of the present invention.

Claims

1. A method for fabricating a microbridge for an uncooled infrared focal plane detector, characterized in that, Includes the following steps: Coating with polyimide: A first layer of polyimide is coated on the electrode layer of the readout circuit chip, converting the liquid polyimide into a solid state; Nanoimprinting: A nanoimprinting template with a convex mold is used to nanoimprint a microbridge pier hole on a polyimide layer, which has the same shape as the convex mold of the nanoimprinting template. Then the nanoimprinting template is demolded to obtain a polyimide layer with holes. Fabrication of microbridge pier holes: The imprinted polyimide layer is cured to form a cured polyimide layer microbridge pier hole. If the height H1 of the punch is less than the depth D1 of the microbridge pier hole, the part of the microbridge pier hole with a depth D1 greater than the height H1 of the punch inside the nanoimprinted hole is photolithographically etched away to form a polyimide layer microbridge pier hole. It also includes setting a sidewall barrier layer on the polyimide layer. The sidewall barrier layer at the bottom of the blind hole is removed by etching patterning before the polyimide layer is etched. Fabrication of the support layer: A thin film is prepared on the polyimide layer as a support layer; Preparation of thermal layer: Prepare thermal layer, etch away the thermal layer in the hole part of the micro bridge pier and the thermal layer in the leg part of the micro bridge, and retain the thermal layer of the micro bridge deck; Preparation of the first insulating layer: The first insulating layer is prepared on the thermistor layer, and a hole VA1 is opened at the bottom of the hole of the microbridge pier to expose the readout circuit electrode. A hole VA2 is opened at the electrical connection position of the thermistor layer to expose the thermistor layer. Fabrication of electrical connection layer: The electrical connection layer is set and patterned by photolithography etching to make the thermal layer and the readout circuit electrode electrically connected through the bridge hole; Preparation of a second insulating layer: A second insulating layer is formed on the electrical connection layer; Remove the polyimide layer: Remove the polyimide layer formed during the fabrication of the microbridges.

2. The method for fabricating the uncooled infrared focal plane detector microbridge according to claim 1, characterized in that, The nanoimprint template punch is provided with an angle, which is between 0 degrees and 89 degrees.

3. The method for fabricating the uncooled infrared focal plane detector microbridge according to claim 1, characterized in that, The nanoimprint template punch is stepped with an angle.

4. The method for fabricating the uncooled infrared focal plane detector microbridge according to claim 1, characterized in that, For single-layer microbridge structures, the polyimide layer is removed directly through a polyimide release process.

5. The method for fabricating the uncooled infrared focal plane detector microbridge according to claim 1, characterized in that, For the multilayer microbridge structure, an upper polyimide layer is prepared on the basis of the second insulating layer, and then other functional layers are prepared on the upper polyimide layer. Finally, the polyimide layer is removed by a polyimide release process.

6. The method for fabricating the uncooled infrared focal plane detector microbridge according to claim 1, characterized in that, The first insulating layer and the second insulating layer are passivated insulating layers, and the second insulating layer serves as an infrared absorbing layer.

Images