Infrared sensor and preparation method thereof

Abstract

The invention relates to an infrared sensor and a preparation method thereof, in particular to an infrared sensor preparation method which comprises the following steps: S1, depositing a sacrificial layer on a semiconductor substrate, and photoetching and etching the sacrificial layer of a metal layer to form a first contact hole; s2, preparing a sensitive element of a piezoelectric microphone and a sensitive element of a micro-bolometer; s3, depositing a fourth insulating layer, and forming a second contact hole and a third contact hole through photoetching and etching; s4, depositing a metal layer, and performing metal patterning; depositing an insulating layer again, and exposing a metal electrode of the piezoelectric microphone through photoetching and etching; and S5, carrying out photoetching and etching again to form a release hole, and releasing the sacrificial layer of the metal layer to complete the preparation of the metal layer infrared sensor. The infrared sensor is prepared by combining the piezoelectric microphone and the resistance micro-bolometer, so that the power consumption of the system is reduced.

Description

Technical Field

[0001] The present invention belongs to the field of semiconductor technology, and in particular relates to an infrared sensor and a preparation method thereof. Background Art

[0002] Uncooled infrared focal plane detectors are primarily based on thermal sensors fabricated using microelectromechanical systems (MEMS) technology. These can be broadly categorized into thermocouples, pyroelectrics, and resistance microbolometers. Compared to pyroelectrics, resistance microbolometers do not require a modulator, reducing their structural complexity. Compared to thermocouple detectors, resistance microbolometers offer higher sensitivity and faster response speeds. However, their signal output relies on a bias voltage. This is achieved by applying a voltage across the resistance microbolometer pixel to generate a bias current. Consequently, resistance microbolometers consume a certain amount of power during off-peak hours.

[0003] A piezoelectric microphone is an acoustic sensor based on the piezoelectric effect, capable of converting sound signals into electrical signals. The core of a piezoelectric microphone is the microphone head, which consists of a piezoelectric crystal. When sound waves act on the piezoelectric crystal, they generate electric charges, which in turn form electrical signals. Signal processing circuits convert these charges into voltage signals, amplifying and filtering them to achieve better sound quality. Piezoelectric microphone circuits use differential inputs and require no bias voltage; they passively detect the output signal of the piezoelectric MEMS, resulting in low power consumption.

[0004] The present application provides an infrared sensor and a method for manufacturing the same, which reduces system power consumption by combining a piezoelectric microphone and a resistive microbolometer. Summary of the Invention

[0005] The present application provides an infrared sensor and a preparation method thereof, which combines a piezoelectric microphone and a resistive microbolometer. When there is no target intrusion, the piezoelectric microphone is in a real-time monitoring state, and the resistive microbolometer is in a dormant state, and no bias voltage needs to be applied. When a target intrudes, the piezoelectric microphone acts as a voice-controlled switch of the infrared detector, and quickly wakes up the resistive microbolometer after monitoring a voice signal, thereby achieving accurate observation of the intruding target. The piezoelectric microphone does not require a bias voltage, thereby reducing system power consumption.

[0006] Other purposes and advantages of the present invention can be further understood from the technical features disclosed in the present invention.

[0007] To achieve one, part, or all of the above purposes or other purposes, the present invention provides an infrared sensor and a method for preparing the same.

[0008] A method for preparing an infrared sensor comprises the following steps:

[0009] S1: providing a semiconductor substrate, depositing a sacrificial layer on the semiconductor substrate, photolithography and etching the sacrificial layer to form a first contact hole;

[0010] S2: sequentially depositing a first insulating layer, a metal electrode layer, and a sensitive material layer, and fabricating a sensitive element of a piezoelectric microphone by photolithography and etching; sequentially depositing a second insulating layer, an infrared sensitive layer, and a third insulating layer, and fabricating a sensitive element of a microbolometer by photolithography and etching;

[0011] S3: depositing a fourth insulating layer, and forming a second contact hole and a third contact hole by photolithography and etching;

[0012] S4: depositing a metal layer and performing metal patterning; depositing an insulating layer again, and exposing the metal electrodes of the piezoelectric microphone through photolithography and etching;

[0013] S5: performing photolithography and etching again to form release holes, release the sacrificial layer, and complete the preparation of the infrared sensor.

[0014] The specific process of step S1 includes: coating polyimide on the semiconductor substrate and curing it to complete the preparation of the polyimide sacrificial layer;

[0015] The thickness of the polyimide sacrificial layer is 2.0±0.2 μm.

[0016] The specific process of step S1 further includes: etching the polyimide sacrificial layer until the semiconductor substrate is exposed to form a first contact hole.

[0017] The specific process of step S2 includes:

[0018] Photolithography and etching the sensitive material layer until the metal electrode layer is exposed, thereby completing the preparation of the piezoelectric microphone sensitive element;

[0019] Photolithography and etching the metal electrode layer to complete the preparation of the lower electrode of the piezoelectric microphone sensitive element;

[0020] The sensitive material layer is a piezoelectric material layer.

[0021] The specific process of step S2 also includes:

[0022] Photolithography and etching the infrared sensitive layer and the third insulating layer until the second insulating layer is exposed, thereby completing the preparation of the sensitive element of the microbolometer;

[0023] The third insulating layer completely covers the infrared sensitive layer, and etching stops at the second insulating layer.

[0024] The specific process of step S3 includes:

[0025] A fourth insulating layer is deposited, and the insulating layer at the location of the first contact hole is photoetched and etched until the semiconductor substrate is exposed, thereby forming the second contact hole.

[0026] The step S3 further includes:

[0027] The insulating layer covering the piezoelectric microphone sensitive element and the microbolometer sensitive element is photolithographically and etched until the metal electrode layer, the sensitive material layer and the infrared sensitive layer are exposed, thereby forming the third contact hole.

[0028] In step S5 , a release hole is formed by photolithography and etching the insulating layer to expose the sacrificial layer and simultaneously form the support leg structure of the microbolometer.

[0029] The sacrificial layer is released through the release hole to form a microbridge structure.

[0030] An infrared sensor, comprising:

[0031] a substrate, and a microbridge structure formed on the substrate;

[0032] The microbridge structure includes a piezoelectric Mike bridge deck and a microbolometer bridge deck that shares a bridge pier with the piezoelectric Mike bridge deck, wherein the bridge deck is suspended above the substrate via two bridge piers;

[0033] The piezoelectric microphone bridge surface includes a sensitive element of the piezoelectric microphone and a contact hole exposing a metal electrode of the piezoelectric microphone;

[0034] The microbolometer deck includes sensitive elements of the microbolometer.

[0035] The base is a semiconductor substrate with a built-in readout circuit;

[0036] The sensitive element of the piezoelectric microphone includes a metal electrode layer, a sensitive material layer and a metal layer after metal patterning;

[0037] The sensitive element of the microbolometer includes an infrared sensitive layer and a metal layer after metal patterning, and the metal layer after metal patterning is arranged in contact with the infrared sensitive layer.

[0038] Compared with the prior art, the beneficial effects of the present invention mainly include:

[0039] This application combines a piezoelectric microphone and a resistive microbolometer. When there is no intruder, the piezoelectric microphone is in real-time monitoring mode, while the resistive microbolometer is in sleep mode, requiring no bias voltage. When a target intrudes, the piezoelectric microphone acts as a voice-activated switch for the infrared detector, quickly waking up the resistive microbolometer upon detecting a voice signal, enabling accurate observation of the intruder. Furthermore, the piezoelectric microphone does not require a bias voltage, thereby reducing system power consumption.

[0040] This application selects polyimide as the sacrificial layer material, and the thickness is set to 2.0±0.2μm. After removing the polyimide sacrificial layer, a cavity with a height of about 2μm can be formed, releasing the suspended structure, which can provide sufficient movement space and achieve a good balance between support and release, and help to obtain a flat and stable suspended structure; and the polyimide sacrificial layer of this application is deposited by spin coating. The thickness of about 2μm is relatively easy to achieve and has good uniformity in the spin coating process, and will not increase the process difficulty due to excessive thickness.

[0041] In order to make the above and other objects, features and advantages of the present invention more clearly understood, preferred embodiments are given below with reference to the accompanying drawings for detailed description. BRIEF DESCRIPTION OF THE DRAWINGS

[0042] In order to more clearly illustrate the technical solutions in the specific embodiments of the present invention, the following briefly introduces the drawings required for use in the description of the embodiments. Obviously, the drawings described below are only some embodiments of the present invention. For ordinary technicians in this field, other drawings can be obtained based on these drawings without paying any creative work.

[0043] Figure 1 Schematic diagram of a method for preparing an infrared sensor provided in Example 1 of this application Figure 1 .

[0044] Figure 2 Schematic diagram of a method for preparing an infrared sensor provided in Example 1 of this application Figure 2 .

[0045] Figure 3 Schematic diagram of a method for preparing an infrared sensor provided in Example 1 of this application Figure 3 .

[0046] Figure 4 Schematic diagram of a method for preparing an infrared sensor provided in Example 1 of this application Figure 4 .

[0047] Figure 5 Schematic diagram of a method for preparing an infrared sensor provided in Example 1 of this application Figure 5 .

[0048] Figure 6 This is a schematic diagram of the infrared sensor structure provided in Example 2 of the present application. DETAILED DESCRIPTION

[0049] The foregoing and other technical aspects, features, and functions of the present invention are clearly presented in the following detailed description of a preferred embodiment with reference to the accompanying drawings. Directional terms such as up, down, left, right, front, and back, used in the following embodiments, are intended solely to refer to the directions in the accompanying drawings. Therefore, the directional terms used are for illustrative purposes only and are not intended to limit the present invention.

[0050] The following detailed description of the various embodiments of the present application is provided in conjunction with the accompanying drawings. However, those skilled in the art will appreciate that many technical details are provided in the various embodiments of the present application to facilitate a better understanding of the present application. However, even without these technical details and the various variations and modifications based on the following embodiments, the technical solutions claimed in the present application can still be implemented.

[0051] Example 1:

[0052] A method for preparing an infrared sensor comprises the following steps:

[0053] S1: providing a semiconductor substrate 1, depositing a sacrificial layer 2 on the semiconductor substrate, photolithography and etching the sacrificial layer 2 to form a first contact hole 21;

[0054] Specifically, if Figure 1 As shown, polyimide is coated on the semiconductor substrate 1 and cured at high temperature to complete the preparation of the polyimide sacrificial layer 2. In the present application, polyimide is selected as the sacrificial layer material, and the thickness is set to 2.0±0.2μm. After removing the polyimide sacrificial layer, a cavity with a height of about 2μm can be formed, releasing the suspended structure, which can provide sufficient movement space and achieve a good balance between support and release, thereby helping to obtain a flat and stable suspended structure. In addition, the polyimide sacrificial layer of the present application is deposited by spin coating. The thickness of about 2μm is within the range that is relatively easy to achieve and has good uniformity in the spin coating process, and will not increase the process difficulty due to excessive thickness.

[0055] Photolithography and etching the polyimide sacrificial layer 2 until the semiconductor substrate 1 is exposed to form a first contact hole 21;

[0056] S2: sequentially depositing a first insulating layer, a metal electrode layer, and a sensitive material layer, and fabricating a sensitive element of a piezoelectric microphone by photolithography and etching; sequentially depositing a second insulating layer, an infrared sensitive layer, and a third insulating layer, and fabricating a sensitive element of a microbolometer by photolithography and etching;

[0057] The insulating layer of the present application is one of silicon nitride, silicon dioxide, and silicon oxynitride. As a preferred embodiment of the present application, silicon nitride is selected as the insulating layer material. Silicon nitride has a high dielectric constant and breakdown field strength, and is suitable for use as an insulating layer in high-performance electronic devices. Silicon nitride also has high hardness and strong wear resistance, and can provide good structural support and protection. Silicon nitride remains stable in high-temperature environments and is better suited for high-temperature processes or high-temperature working environments. Silicon nitride can be prepared by a variety of methods (such as plasma-enhanced chemical vapor deposition (PECVD) and low-pressure chemical vapor deposition (LPCVD)) to meet different process requirements.

[0058] The metal electrode layer of the present application is made of one of the materials of molybdenum, tantalum, titanium, titanium nitride, aluminum, aluminum copper, etc. As a preferred embodiment of the present application, the present application selects metal molybdenum as the metal electrode layer of the piezoelectric microphone sensor. Metal molybdenum has the advantages of low resistivity and high sound velocity, and can provide good adhesion between the aluminum nitride film and the low-stress silicon nitride film;

[0059] The sensitive material layer of the present application is one of aluminum nitride, aluminum scandium nitride, PZT film, barium titanate BT, modified lead zirconate titanate, lead metaniobate, lead barium lithium niobate PBLN, modified lead titanate PT, lithium gallate, lithium germanate, titanium germanate, and ferroelectric transistor lithium niobate, lithium tantalate, etc. As a preferred embodiment of the present application, the present application selects aluminum nitride as the sensitive material layer of the piezoelectric microphone sensor, and adopts a sputtering process below 400°C to obtain aluminum nitride film as the piezoelectric material. Aluminum nitride retains most of the properties and functions of the bulk material in the form of a thin film, and has advantages such as high elastic modulus, high dielectric strength, high resistivity, high thermal conductivity, and high sound velocity, and is compatible with CMOS technology;

[0060] The infrared sensitive layer of the present application is made of one of the materials selected from vanadium oxide, amorphous silicon, silicon germanium, yttrium barium copper oxide, and the like. As a preferred embodiment of the present application, vanadium oxide is selected as the infrared sensitive layer of the microbolometer sensor. Using vanadium oxide as a heat-sensitive material, compared to the amorphous structure of polycrystalline silicon, vanadium oxide has advantages such as high temperature coefficient of resistance (TCR), low 1 / f noise, and easier integration with readout circuits, further improving the sensitivity of the microbolometer.

[0061] Specifically, step S2 sequentially deposits a first silicon nitride layer 3, a metal molybdenum layer 4, and an aluminum nitride layer 5, and fabricates a piezoelectric microphone sensor by photolithography and etching. A second silicon nitride layer 6, a vanadium oxide layer 7, and a third silicon nitride layer 8 are sequentially deposited, and fabricates a microbolometer sensor by photolithography and etching.

[0062] like Figure 2As shown, a first silicon nitride layer 3, a metal molybdenum layer 4 and an aluminum nitride layer 5 are deposited in sequence, the aluminum nitride layer 5 is photolithographically and etched, and the etching stops at the metal molybdenum layer 4, thereby completing the preparation of the piezoelectric microphone sensitive element; the metal molybdenum layer 4 is photolithographically and etched to complete the preparation of the lower electrode of the piezoelectric microphone sensitive element; wherein, the aluminum nitride layer 5 is a piezoelectric material layer, and the aluminum nitride layer 5 does not completely cover the metal molybdenum layer 4.

[0063] Next, a second silicon nitride layer 6, a vanadium oxide layer 7, and a third silicon nitride layer 8 are sequentially deposited. The vanadium oxide layer 7 and the third silicon nitride layer 8 are then photolithographed and etched. The etching stops at the second silicon nitride layer 6, completing the preparation of the vanadium oxide microbolometer sensitive element. The third silicon nitride layer completely covers the vanadium oxide layer.

[0064] S3: depositing a fourth insulating layer, and forming a second contact hole and a third contact hole by photolithography and etching;

[0065] Specifically, step S3: depositing a fourth silicon nitride layer 9, and forming a second contact hole 91 and a third contact hole 92 by photolithography and etching;

[0066] like Figure 3 As shown, a fourth silicon nitride layer 9 is deposited, and the fourth silicon nitride layer 9, the second silicon nitride layer 6 and the first silicon nitride layer 3 at the location of the first contact hole 21 are photolithographically and etched, and the etching stops at the semiconductor substrate 1 to form a second contact hole 91;

[0067] Then, the silicon nitride layer covering the piezoelectric microphone sensor and the microbolometer sensor is photolithographically and etched until the metal molybdenum layer 4 , the aluminum nitride layer 5 and the vanadium oxide layer 7 are exposed, thereby forming a third contact hole 92 .

[0068] S4: depositing a metal layer and performing metal patterning; depositing an insulating layer again, and exposing the metal electrodes of the piezoelectric microphone through photolithography and etching;

[0069] The metal layer of the present application is one of materials such as titanium and titanium nitride. As a preferred embodiment of the present application, the present application selects titanium metal as the metal layer. Titanium metal has the advantages of high electrical conductivity and low thermal conductivity. As an electrode of the infrared detector, it can reduce the heat loss of the infrared detector and further improve the performance of the infrared detector.

[0070] like Figure 4 As shown, step S4: depositing a titanium metal layer 10 and performing metal patterning to produce metal connecting wires;

[0071] Depositing a fifth silicon nitride layer 11 again, and exposing the metal electrode 101 of the piezoelectric microphone by photolithography and etching;

[0072] Specifically, photoresist is first applied and photolithography is used to define the position and shape of the metal connection lines. Then, the titanium metal portion not protected by the photoresist is removed through an etching process, leaving only the required metal connection line pattern. After the patterning of the titanium metal layer is completed, a fifth layer of silicon nitride (Si3N4) is deposited to provide additional protection or serve as an insulating layer to prevent electrical interference between different circuits.

[0073] A photolithography process is performed on the fifth silicon nitride layer 11 to define the locations where the piezoelectric microphone metal electrodes 101 need to be exposed. An etching process is then used to remove the fifth silicon nitride layer 11 at these locations until the underlying titanium metal electrodes 101 are exposed. This ensures that the electrodes of the piezoelectric microphone can be properly connected to other circuit components, thereby guaranteeing signal transmission.

[0074] like Figure 5 As shown, step S5: photolithography is performed again and the fifth silicon nitride layer 11, the titanium metal layer 10, the fourth silicon nitride layer 9, the second silicon nitride layer 6 and the first silicon nitride layer 3 are etched. The etching stops at the polyimide sacrificial layer 2 to form a release hole 111 to expose the sacrificial layer 2; and at the same time, a support leg structure of the microbolometer is formed to suspend the sensitive element of the vanadium oxide microbolometer and isolate it from the semiconductor substrate 1 to reduce heat conduction loss and improve detection efficiency.

[0075] The polyimide sacrificial layer 2 is released through the release hole 111, and the sacrificial layer is removed to leave the required suspended structure, which can effectively separate the sensitive components, reduce unnecessary heat conduction paths, enhance the performance of the device, and complete the preparation of the infrared sensor.

[0076] Example 2:

[0077] An infrared sensor includes: a substrate, and a microbridge structure formed on the substrate; the microbridge structure includes a piezoelectric microphone bridge surface and a microbolometer bridge surface that shares a bridge pier with the piezoelectric microphone bridge surface, and the bridge surface is suspended above the substrate via two bridge piers; the piezoelectric microphone bridge surface includes a sensitive element of the piezoelectric microphone and a contact hole that exposes a metal electrode 101 of the piezoelectric microphone; the microbolometer bridge surface includes a sensitive element of the microbolometer.

[0078] The infrared sensor base is a semiconductor substrate 1 with a built-in readout circuit. The sensitive element of the piezoelectric microphone includes a metal electrode layer, a sensitive material layer, and a metal layer after metal patterning. The sensitive element of the microbolometer includes an infrared sensitive layer and a metal layer after metal patterning, and the metal layer after metal patterning is arranged in contact with the infrared sensitive layer.

[0079] Specifically, if Figure 6As shown, an infrared sensor includes: a semiconductor substrate 1, a first silicon nitride layer 3, a metal molybdenum layer 4, an aluminum nitride layer 5, a second silicon nitride layer 6, a vanadium oxide layer 7, a third silicon nitride layer 8, a fourth silicon nitride layer 9, a titanium metal layer 10 and a fifth silicon nitride layer 11. When there is no target intrusion, the piezoelectric microphone is in a real-time monitoring state. At this time, the microbolometer is in a dormant state and does not need to be biased. Since the piezoelectric microphone does not require a bias voltage, the power consumption of the entire system is very low. When a target intrudes, the piezoelectric microphone acts as a voice-controlled switch of the infrared detector and quickly wakes up the resistive microbolometer after monitoring the voice signal, thereby achieving accurate observation of the intruding target.

[0080] The present application provides an infrared sensor and a preparation method thereof, which combines a piezoelectric microphone and a resistive microbolometer. When there is no target intrusion, the piezoelectric microphone is in a real-time monitoring state, and the resistive microbolometer is in a dormant state, and no bias voltage needs to be applied. When a target intrudes, the piezoelectric microphone acts as a voice-controlled switch of the infrared detector, and quickly wakes up the resistive microbolometer after monitoring a voice signal, thereby achieving accurate observation of the intruding target. The piezoelectric microphone does not require a bias voltage, thereby reducing system power consumption.

[0081] In order to facilitate the description of the present invention, some common English nouns or letters are used for illustrative reference only and are not intended to be restrictive or specific. The scope of protection of the present invention should not be limited by their possible Chinese translations or specific letters.

[0082] It should also be noted that, in this article, relational terms such as "first" and "second" are merely used to distinguish one entity or operation from another entity or operation, and do not necessarily require or imply any actual relationship or order between these entities or operations.

Claims

1. A method for preparing an infrared sensor, characterized in that: The steps include: S1: providing a semiconductor substrate, depositing a sacrificial layer on the semiconductor substrate, photolithography and etching the sacrificial layer to form a first contact hole; S2: depositing a first insulating layer, a metal electrode layer, and a sensitive material layer in sequence, and preparing a sensitive element of the piezoelectric microphone by photolithography and etching; Depositing a second insulating layer, an infrared sensitive layer, and a third insulating layer in sequence, and fabricating a sensitive element of a microbolometer by photolithography and etching; S3: depositing a fourth insulating layer, and forming a second contact hole and a third contact hole by photolithography and etching; S4: depositing a metal layer and performing metal patterning; depositing an insulating layer again, and exposing the metal electrodes of the piezoelectric microphone through photolithography and etching; S5: performing photolithography and etching again to form release holes, release the sacrificial layer, and complete the preparation of the infrared sensor.

2. The method for preparing an infrared sensor according to claim 1, wherein: The specific process of step S1 includes: coating polyimide on the semiconductor substrate and curing it to complete the preparation of the polyimide sacrificial layer; The thickness of the polyimide sacrificial layer is 2.0±0.2 μm.

3. The method for preparing an infrared sensor according to claim 2, wherein: The specific process of step S1 further includes: etching the polyimide sacrificial layer until the semiconductor substrate is exposed to form a first contact hole.

4. The method for preparing an infrared sensor according to claim 1, wherein: The specific process of step S2 includes: Photolithography and etching the sensitive material layer until the metal electrode layer is exposed, thereby completing the preparation of the piezoelectric microphone sensitive element; Photolithography and etching the metal electrode layer to complete the preparation of the lower electrode of the piezoelectric microphone sensitive element; The sensitive material layer is a piezoelectric material layer.

5. The method for preparing an infrared sensor according to claim 4, wherein: The specific process of step S2 also includes: Photolithography and etching the infrared sensitive layer and the third insulating layer until the second insulating layer is exposed, thereby completing the preparation of the sensitive element of the microbolometer; The third insulating layer completely covers the infrared sensitive layer, and etching stops at the second insulating layer.

6. The method for preparing an infrared sensor according to claim 1, wherein: The specific process of step S3 includes: A fourth insulating layer is deposited, and the insulating layer at the location of the first contact hole is photoetched and etched until the semiconductor substrate is exposed, thereby forming the second contact hole.

7. The method for preparing an infrared sensor according to claim 6, wherein: The step S3 further includes: The insulating layer covering the piezoelectric microphone sensitive element and the microbolometer sensitive element is photolithographically and etched until the metal electrode layer, the sensitive material layer and the infrared sensitive layer are exposed, thereby forming the third contact hole.

8. The method for preparing an infrared sensor according to claim 1, wherein: In step S5 , a release hole is formed by photolithography and etching the insulating layer to expose the sacrificial layer and simultaneously form the support leg structure of the microbolometer.

9. The method for preparing an infrared sensor according to claim 8, wherein: The sacrificial layer is released through the release hole to form a microbridge structure.

10. An infrared sensor, characterized in that: include: a substrate, and a microbridge structure formed on the substrate; The microbridge structure includes a piezoelectric Mike bridge deck and a microbolometer bridge deck that shares a bridge pier with the piezoelectric Mike bridge deck, wherein the bridge deck is suspended above the substrate via two bridge piers; The piezoelectric microphone bridge surface includes a sensitive element of the piezoelectric microphone and a contact hole exposing a metal electrode of the piezoelectric microphone; The microbolometer deck includes sensitive elements of the microbolometer.

11. The infrared sensor according to claim 10, characterized in that: The base is a semiconductor substrate with a built-in readout circuit; The sensitive element of the piezoelectric microphone includes a metal electrode layer, a sensitive material layer and a metal layer after metal patterning; The sensitive element of the microbolometer includes an infrared sensitive layer and a metal layer after metal patterning, and the metal layer after metal patterning is arranged in contact with the infrared sensitive layer.

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