A high-speed infrared microbolometer and a method of manufacturing the same

Abstract

The application belongs to the technical field of optoelectronic sensors, and particularly relates to a high-speed infrared microbolometer and a preparation method thereof. The application adopts a deep subwavelength super-structured film optical structure, integrates subwavelength-sized super-absorption structures and a thermal sensitive material on a wafer, enhances infrared band absorption, and effectively reduces the thermal capacity of the device; in the design of the pixel, a small-size structure is realized, the low thermal capacity characteristics of the response element are utilized, the overall thermal capacity of the device is maximally reduced, and the response speed is significantly improved; the device can realize a microsecond-level response time in a medium-long infrared wave band, and has the advantages of large-angle detection capability, response waveband tunability, room temperature operation and the like.

Description

Technical Field

[0001] This invention relates to the field of optoelectronic sensor technology, specifically to a high-speed infrared microbolometer and its fabrication method. Background Technology

[0002] An infrared microbolometer is an uncooled thermistor-sensitive infrared detector. Its working principle involves converting absorbed infrared radiation into temperature changes, which in turn cause a change in the detector's resistance, resulting in an electrical signal output. Due to its miniaturization and low cost, microbolometers are widely used in infrared sensing, industrial inspection, and medical testing. A microbolometer pixel unit typically consists of a multilayer thin-film structure, including an absorption layer for absorbing infrared radiation, a thermistor layer that converts temperature changes into resistance changes, and a supporting bridge structure layer. Traditional infrared microbolometers typically use black absorbers or quarter-cavity optical arrays to achieve efficient infrared radiation absorption. This optical design often results in high heat capacity, leading to a large time constant for the detector, typically on the order of 10 ms, thus limiting the detector's response speed. In recent years, metasurfaces, due to their excellent resonant absorption characteristics, have provided a new technological path in this field. Summary of the Invention

[0003] To address the problems existing in the prior art, the present invention aims to provide a high-speed infrared microbolometer based on a deep subwavelength (thickness less than λ / 10) metamaterial and its fabrication method. This invention integrates a subwavelength superabsorption structure with a thermistor material on-chip, significantly improving the detector's response speed while maintaining a high responsivity. This device achieves microsecond-level response times in the mid-to-long infrared band and possesses advantages such as wide-angle detection capability, tunable response band, and room-temperature operation.

[0004] The technical solution of the present invention is described in detail below.

[0005] This invention provides a method for preparing a high-speed infrared microbolometer, comprising the following steps:

[0006] (1) A silicon oxide film and a silicon nitride film are grown sequentially on the substrate to obtain a low-stress silicon oxide / silicon nitride composite film as a support layer;

[0007] (2) A thermistor layer is grown on a silicon oxide / silicon nitride composite film and patterned by a peeling method;

[0008] (3) Deposit metal on silicon oxide / silicon nitride composite film and pattern the metal to form two electrodes corresponding to the thermistor to connect to external photoelectric test circuit;

[0009] (4) A silicon oxide passivation layer is deposited on the thermistor layer at a temperature below 100°C to achieve electrical isolation between the optical absorption layer and the thermistor layer and to prevent secondary annealing of the thermistor material.

[0010] (5) A metal reflective layer and a bismuth thin film are sequentially deposited on the silicon oxide passivation layer to form a resonant absorption film, and the pattern is obtained by peeling to obtain an optical absorption layer with the same size as the thermistor layer.

[0011] (6) The silicon oxide / silicon nitride composite film is etched and patterned to obtain a bridge body composed of a thermistor layer and an optical absorption layer and a bridge arm connected to the substrate. The bridge body is a response unit.

[0012] (7) The substrate under the silicon oxide / silicon nitride composite film is etched, and the response unit is connected to the substrate by a bridge arm to form a cantilever or suspended film thermal isolation structure.

[0013] In step (1) above, the substrate is a single-crystal silicon substrate; silicon oxide film and silicon nitride film are grown sequentially on the substrate by plasma vapor deposition (PECVD) at a deposition temperature of 320-400℃, the thickness of silicon oxide film is between 50-150nm, and the thickness of silicon nitride film is between 100nm and 400nm; deposition at this deposition temperature is beneficial to reduce material stress.

[0014] In step (2) above, the thermistor layer is grown on the composite film using a magnetron sputtering process; the material of the thermistor layer is selected from vanadium oxide, amorphous silicon, or manganese cobalt nickel oxide, and its thickness is between 50-200 nm. The two electrodes corresponding to the thermistor in the thermistor layer are connected to an external photoelectric testing circuit. The resistance is controlled by controlling the aspect ratio of the thermistor film.

[0015] In step (3) above, metallic gold is deposited using electron beam evaporation and patterned to form an electrode with a thickness of 80-150 nm.

[0016] In step (4) above, a silicon oxide passivation layer is deposited using a low-temperature plasma vapor deposition (PECVD) process at a temperature of 70-90°C, and the thickness of the silicon oxide passivation layer is 40-60 nm.

[0017] In step (5) above, a metal reflective layer and a bismuth thin film layer are deposited by electron beam evaporation. The metal reflective layer is made of any metal among Cr, Pt, Au, Al or Ag, and the thickness of the metal reflective layer is between 50-200 nm to ensure no transmission in the infrared band. The thickness of the bismuth thin film layer is between 50 nm and 500 nm. Electron beam evaporation is used for deposition and a water-cooled tray is used to ensure the uniformity of the deposition of low-melting-point bismuth material.

[0018] In step (6) above, the bridge area is 16 μm. 2 ~100μm 2 Between these elements, the detector's thermal capacity is reduced, and the device's response speed is improved; the bridge structure uses reactive ion etching (RIE) to etch silicon oxide / silicon nitride thin films.

[0019] In step (7) above, the detector surface is protected by photoresist during the etching process, and acetone is used to remove it after the sacrificial layer is released.

[0020] In step (7) above, the substrate at the bottom of the device is etched using a dry etching process of xenon difluoride (XeF2). During the etching process, the detector surface is protected by photoresist, and the sacrificial layer is removed with acetone after it is released.

[0021] In steps (2), (3), (5) and (6) above, methods such as laser direct writing and ultraviolet lithography are used for patterning.

[0022] The present invention also provides a high-speed infrared microbolometer prepared by the preparation method described above.

[0023] Compared with the prior art, the beneficial effects of the present invention are as follows:

[0024] This invention employs a deep subwavelength meta-thin-film optical structure to enhance infrared absorption while effectively reducing the device's heat capacity. In terms of pixel design, it achieves a small-size structure and utilizes the low heat capacity of the responder itself to minimize the overall heat capacity of the device, thereby significantly improving response speed. Furthermore, this invention fully considers compatibility with readout circuitry. Based on a silicon nitride / silicon oxide composite thin-film system, it utilizes XeF2 etching to release the suspended microbridge structure, ensuring structural integrity and process success rate of the small-size pixels during fabrication. This overcomes the limitations of traditional radiometric calorimeters that rely on organic sacrificial layers or back etching for thermal isolation. This invention is not only applicable to infrared detection but can also be extended to various cantilever or suspended film micro / nano devices, such as inertial sensors, pressure sensors, chemical / biological sensors, and other types of photodetectors. Attached Figure Description

[0025] Figure 1 This is a schematic diagram of the device structure and a flowchart of the fabrication process of the high-speed microbolometer of the present invention.

[0026] Figure 2 This is a scanning electron microscope image of the high-speed microbolometer in Example 1.

[0027] Figure 3 This is a graph showing the relationship between the response rate and modulation frequency of the high-speed microbolometer in Example 1.

[0028] Figure 4 This is a scanning electron microscope image of the high-speed microbolometer in Example 2.

[0029] Figure 5 This is a graph showing the relationship between the response rate and modulation frequency of the high-speed microbolometer in Example 2.

[0030] The numbers in the figure are: 1-substrate, 2-support layer, 3-thermometer layer, 4-electrode, 5-silicon oxide passivation layer, 6-optical absorption layer. Detailed Implementation

[0031] To make the objectives, technical solutions, and advantages of this invention clearer, specific embodiments are given below, and the invention is further illustrated in conjunction with the accompanying drawings.

[0032] Example 1

[0033] like Figure 1 As shown, this embodiment provides a method for fabricating a high-speed microbolometer, which is based on a metal reflective layer-bismuth deep subwavelength metafilm and XeF2 etching process, including the following steps:

[0034] (1) A 500 μm thick single-crystal silicon substrate 1 was used as the substrate. On the substrate 1, a 100 nm thick silicon oxide film and a 200 nm thick silicon nitride low-stress composite film were sequentially deposited at a temperature of 350 °C using plasma vapor deposition (PECVD) to form a support layer 2. The PECVD used SiH4 + NH3 reaction to deposit the silicon nitride film and SiH4 + N2O reaction to deposit the silicon oxide film.

[0035] (2) A 100 nm thick vanadium oxide thermistor layer 3 was sputtered onto the composite thin film using magnetron sputtering. A DC power supply and a vanadium metal target were used for sputtering, and oxygen was introduced during the sputtering process to react and generate the vanadium oxide thin film. Patterning was performed using a lift-off method. The thermistor dimensions were 4 μm × 4 μm.

[0036] (3) Electron beam evaporation deposits 100 nm thick gold on a thermistor film and patterns the gold to form electrode 4;

[0037] (4) A 50 nm thick silicon oxide passivation layer 5 was deposited at a temperature of 75 °C using low-temperature plasma vapor deposition (PECVD).

[0038] (5) A 50 nm thick Al layer and a 250 nm thick bismuth thin film layer were sequentially deposited by electron beam evaporation to form a resonant absorption film, and the film was patterned by peeling to obtain an optical absorption layer 6; the absorption layer area was 4 μm × 4 μm; the absorption peak was around 10.6 μm.

[0039] (6) The support layer 2 is etched and patterned using reactive ion etching, and the support layer 2 forms a bridge leg and a bridge body; the bridge body is located in the center and is connected to the substrate 1 through the bridge leg; the bridge surface size is 4μm×4μm, the bridge leg width is 20μm, and the bridge leg length is 80μm;

[0040] (7) The silicon at the bottom of the silicon nitride / silicon oxide composite film was etched using the XeF2 process; the etching time was 11 cycles and the lateral etching width was 15 μm; during the etching process, AZ5214 photoresist was used to protect the optical absorption layer 6.

[0041] The final microbolometer morphology is as follows: Figure 2 As shown, the 4μm×4μm central region (bridge) is the detector's response unit, including an optical absorption layer 6 and a thermistor layer 3. This region is connected to the substrate 1 by two 20μm wide bridge arms. When the microbolometer receives infrared radiation, it causes a change in the resistance of the thermistor layer, generating an electrical signal. This electrical signal is transmitted to an external test circuit and amplified into a voltage signal by a current preamplifier. The electrical signal at the chopping frequency f is detected by a lock-in amplifier, and the responsivity of the microbolometer can be calculated by combining it with the power of the incident infrared light. The ratio of the electrical signal to the infrared radiation intensity is the responsivity.

[0042] By testing the responsivity at different chopping frequencies f from low to high frequencies, the infrared frequency response of the detector can be obtained. Fitting the data reveals a cutoff frequency of 2690 Hz, corresponding to the frequency at which the detector's responsivity drops to 0.707 times its maximum response. Figure 3 As shown; the time constant of the thermal detector is equal to 1 / 2πf = 59μs.

[0043] Example 2

[0044] like Figure 1 As shown, this embodiment provides a method for fabricating a high-speed microbolometer, which is based on a metal reflective layer-bismuth deep subwavelength metafilm and XeF2 etching process, including the following steps:

[0045] (1) A 500 μm thick single-crystal silicon substrate 1 was used as the substrate. On the substrate 1, a 100 nm thick silicon oxide film and a 200 nm thick silicon nitride low-stress composite film were sequentially deposited at a temperature of 350 °C using plasma vapor deposition (PECVD) to form a support layer 2. The PECVD used SiH4+NH3 reaction to deposit the silicon nitride film and SiH4+N2O reaction to deposit the silicon oxide film.

[0046] (2) A 100 nm thick vanadium oxide thermistor layer 3 was sputtered onto the composite thin film using magnetron sputtering. A DC power supply and a vanadium metal target were used for sputtering, and oxygen was introduced during the sputtering process to react and generate the vanadium oxide thin film. Patterning was performed using a lift-off method. The thermistor dimensions were 10 μm × 10 μm.

[0047] (3) Electron beam evaporation deposits 100 nm thick gold on a thermistor film and patterns the gold to form electrode 4;

[0048] (4) A 50 nm thick silicon oxide passivation layer 5 was deposited at a temperature of 75 °C using low-temperature plasma vapor deposition (PECVD).

[0049] (5) A 50 nm thick Al layer and a 250 nm thick bismuth thin film layer were sequentially deposited by electron beam evaporation to form a resonant absorption film, and the film was patterned by peeling to obtain an optical absorption layer 6; the absorption layer area was 10 μm × 10 μm; the absorption peak was around 10.6 μm.

[0050] (6) The support layer 2 is etched and patterned using reactive ion etching, and the support layer 2 forms a bridge leg and a bridge body; the bridge body is located in the center and is connected to the substrate 1 through the bridge leg; the bridge surface size is 10μm×10μm, the bridge leg width is 5μm, and the bridge leg length is 110μm;

[0051] (7) The silicon at the bottom of the silicon nitride / silicon oxide composite film is etched using the XeF2 process; the etching time is 10 cycles and the lateral etching width is 10 μm (the total width of bidirectional etching is 20 μm); during the etching process, AZ5214 photoresist is used to protect the optical absorption layer 6.

[0052] The final microbolometer morphology is as follows: Figure 4 As shown, the 10μm × 10μm central region (bridge) is the detector's response unit, including an optical absorption layer 6 and a thermistor layer 3. This region is connected to the substrate 1 by two 5μm wide bridge arms. When the microbolometer receives infrared radiation, it causes a change in the resistance of the thermistor layer, generating an electrical signal. This electrical signal is transmitted to an external test circuit and amplified into a voltage signal by a current preamplifier. The electrical signal at the chopping frequency f is detected by a lock-in amplifier, and the responsivity of the microbolometer can be calculated by combining it with the power of the incident infrared light. The ratio of the electrical signal to the infrared radiation intensity is the responsivity.

[0053] By testing the responsivity at different chopping frequencies f from low to high frequencies, the infrared frequency response of the detector can be obtained. Fitting the data reveals a cutoff frequency of 548 Hz, corresponding to the frequency at which the detector's responsivity drops to 0.707 times its maximum response. Figure 5As shown; the time constant of the thermal detector is equal to 1 / 2πf = 291μs.

[0054] The specific embodiments described above further illustrate the purpose, technical solution, and beneficial effects of the present invention. It should be understood that the above descriptions are merely specific embodiments of the present invention and are not intended to limit the present invention. Any modifications, equivalent substitutions, improvements, etc., made within the spirit and principles of the present invention should be included within the protection scope of the present invention.

Claims

1. A method for preparing a high-speed infrared microbolometer, characterized in that, Includes the following steps: (1) A silicon oxide film and a silicon nitride film are grown sequentially on the substrate to obtain a low-stress silicon oxide / silicon nitride composite film as a support layer; (2) A thermistor layer is grown on a silicon oxide / silicon nitride composite film and patterned by a peeling method; (3) Deposit metal on silicon oxide / silicon nitride composite film and pattern the metal to form two electrodes corresponding to the thermistor to connect to external photoelectric test circuit; (4) A silicon oxide passivation layer is deposited on the thermistor layer at a temperature below 100°C to achieve electrical isolation between the optical absorption layer and the thermistor layer and to prevent secondary annealing of the thermistor material. (5) A metal reflective layer and a bismuth thin film are sequentially deposited on the silicon oxide passivation layer to form a resonant absorption film, and the pattern is obtained by peeling to obtain an optical absorption layer with the same size as the thermistor layer. (6) The silicon oxide / silicon nitride composite film is etched and patterned to obtain a bridge body composed of a thermistor layer and an optical absorption layer and a bridge arm connected to the substrate. The bridge body is a response unit. (7) The substrate under the silicon oxide / silicon nitride composite film is etched, and the response unit is connected to the substrate by a bridge arm to form a cantilever or suspended film thermal isolation structure.

2. The preparation method according to claim 1, characterized in that, In step (1), the substrate is a single-crystal silicon substrate; a silicon oxide film and a silicon nitride film are grown sequentially on the substrate using plasma vapor deposition (PECVD) at a deposition temperature of 320-400℃. The thickness of the silicon oxide film is between 50-150nm, and the thickness of the silicon nitride film is between 100nm and 400nm.

3. The preparation method according to claim 1, characterized in that, In step (2), the thermistor layer is grown on the composite film by magnetron sputtering; the material of the thermistor layer is selected from vanadium oxide, amorphous silicon or manganese cobalt nickel oxide, and its thickness is between 50-200 nm.

4. The preparation method according to claim 1, characterized in that, In step (3), metallic gold is deposited using electron beam evaporation and patterned to form an electrode with a thickness of 80-150 nm.

5. The preparation method according to claim 1, characterized in that, In step (4), a silicon oxide passivation layer is deposited using a low-temperature plasma vapor deposition (PECVD) process at a temperature of 70-90°C. The thickness of the silicon oxide passivation layer is 40-60 nm.

6. The preparation method according to claim 1, characterized in that, In step (5), a metal reflective layer and a bismuth thin film layer are deposited by electron beam evaporation. The metal reflective layer is made of any metal among Cr, Pt, Au, Al or Ag, and the thickness of the metal reflective layer is between 50-200 nm to ensure no transmission in the infrared band. The thickness of the bismuth thin film layer is between 50 nm and 500 nm.

7. The preparation method according to claim 1, characterized in that, In step (6), the bridge area is 16 μm. 2 ~100μm 2 Between; Reactive Ion Etching (RIE) is used to etch silicon oxide / silicon nitride composite films.

8. The preparation method according to claim 1, characterized in that, In step (7), the substrate at the bottom of the device is etched using a dry etching process with xenon difluoride (XeF2). During the etching process, the detector surface is protected by photoresist, and the sacrificial layer is removed with acetone after it is released.

9. The preparation method according to claim 1, characterized in that, In steps (2), (3), (5) and (6), patterning is performed using laser direct writing or ultraviolet lithography.

10. A high-speed infrared microbolometer prepared by the method described in any one of claims 1-9.

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