BOLOMETER AND MANUFACTURING METHOD THEREOF TECHNICAL FIELD

[0001] The present invention is related to an infrared (IR) bolometer or a terahertz (THz) bolometer, particularly to an uncooled IR bolometer which can be operated under uncooled conditions and manufactured with a low cost, and to a manufacturing method thereof.

Priority is claimed on Japanese Patent Application No. 2012-038750, filed

February 24, 2012, the content of which is incorporated herein by reference.

BACKGROUND ART

[0002] A bolometer is a device which detects the temperature change of an object as a change of electrical resistance and is generally used as a key device for thermal detectors and cameras. Bolometers are mainly used in IR detectors which are applied in the areas of military technology and consumer product technology as key devices such as for missile-seeking, night vision, surveillance, security view, driver vision enhancement in automobiles, and predictive maintenance. In general, IR detectors operate with cooling systems, e.g., cryogenic cooling by liquid nitrogen, which increase the size of detectors and manufacturing costs. Uncooled IR bolometers are attractive because no cooling system is necessary for operations, and therefore it is possible to manufacture less expensive IR detectors and portable IR detectors.

[0003] FIG. 1 A is a drawing that illustrates a top view of a conventional bolometer and FIG. IB shows a three dimensional view of the bolometer. A silicon substrate 100 is used as a substrate material. The bolometer includes a thin film resistor 108 and a diaphragm layer 107, which are formed over the silicon substrate 100 to be elevated or suspended from the surface of the silicon substrate 100 using the diaphragm layer 107. For the conventional bolometer, the suspension structure of the thin film resistor 108 is indispensable to achieve high temperature sensitivity.

[0004] The suspension structure of the thin film resistor 108 is formed by use of a sacrificial layer (not shown in the figure). A reflector metal 110 is deposited on the silicon substrate 100 as shown in FIG. IB. The sacrificial layer is formed between the diaphragm layer 107 and the reflection metal 110, and subsequently, the sacrificial layer is selectively removed from the reflector metal 110 and the diaphragm layer 107 by an etching process. As a result, an optical cavity distance d 103 in FIG. IB is formed.

[0005] The thickness of the sacrificial layer is chosen according to a target wavelength of IR light. For example, when a target to be detected by the bolometer is a human, the thickness d is chosen to be approximately 2.5 μηι, which corresponds to 1/4 of the peak wavelength (λ = ΙΟμιτι) of the black body radiation from a human, assuming that the refractive index of air is approximately 1.

[0006] Electrodes 101 and 102 are formed on the sides of the thin film resistor 108 to measure the changes in the resistance of the thin film resistor 108 during operations of the bolometer. The electrodes 101 and 102 are connected to read-out circuits (not shown in the figure) provided on the silicon substrate 100.

[0007] The conventional bolometer also includes mechanical suspension arms 104 as shown in FIGS. 1 A and IB. Generally, silicon oxide or silicon nitride is used to form the mechanical suspension arms 104 since those materials are mechanically stable and compatible to IC manufacturing processes. The structure of the mechanical suspension arms 104 is effective for improving the thermal isolation of the bolometer, and thus reduces the thermal conductivity of the bolometer compared to a case in which the same bolometer is directly attached to the substrate 100. The suspension structure improves the sensitivity of temperature detection of the bolometer because heat generated in the bolometer while absorbing IR radiation in the thin film resistor 108 (or the bolometer material) of the bolometer is effectively isolated from the substrate 100, and therefore the heat is effectively converted to change the resistance of the thin film resistor 108.

However, the thermal conductivities of silicon oxide are not small enough compared to organic materials. For example, the thermal conductivity of silicon nitride is 1.85 (WV(m-K)), while the thermal conductivity of parylene-C is 0.082 (W/(m-K)). This shows a difference of magnitude approaching two orders of magnitude.

[0008] Since the change in the resistance of the thin film resistor 108 is proportional to the amount of the heat generated in the thin film resistor 108, the current flowing through the thin film resistor 108 can be detected as changes in temperature. Thus, a large increase in the temperature in the thin film resistor 108 makes it possible for the bolometer to detect the change in temperature of an object radiating the IR light using read-out circuits formed on the silicon substrate 100.

[0009] Electrodes 105 and 106 are formed over the mechanical suspension arms 104 and the diaphragm layer 107 by vacuum deposition. The electrodes 105 and 106 are connected to the read-out circuits through contacts 109 to measure the changes in the resistance of the thin film resistor 108.

[0010] As a material of the thin film resistor 108, for example, vanadium oxide is preferred, since vanadium oxide shows high TCR (temperature coefficient of resistor), being approximately -2 (%/K) and low noise characteristics. In this case, however, a vacuum deposition process and a post-annealing process are required to achieve stable operations of a vanadium oxide film, and therefore the manufacturing time increases, resulting high manufacturing costs.

[0011] In the manufacturing processes described above, MEMS (Micro

Electro-Mechanical Systems) process technology, which includes laborious and delicate processes, needs to be applied to form the diaphragm layer 107, the thin film resistor 108, and the contacts 109. The contacts 109 are formed through multiple lithographic processes. The number of lithographic processes often increases the risk of failure in forming such contacts.

[0012] When the bolometers are used for THz applications (wavelengths around 100 μιη), an optical cavity having the distance d, approximately equal to 2.5 μιη in the conventional bolometers, needs to be increased. Such a cavity formation process can be a big challenge in MEMS process technology. In addition, the contact formation process also increases the risk of failure in manufacturing bolometers, and thus results in a high cost of the bolometers.

[0013] In general, it is preferable to use a wet etching process to remove the sacrificial layer, because the wet process can be simply performed and thus would have a low cost. In this case, however, while the wet etching is performed for the sacrificial layer, the etchant of the wet etching process generates the surface tension on the sacrificial layer, and which is applied to the suspension structure. This may often deform the suspension structure, resulting failure of the suspension structure formation. In order to avoid this problem, a dry etching process is usually used, in which plasma is employed to form the optical cavity-distance d 103. However, the dry etching process increases the cost of the manufacturing process.

DISCLOSURE OF INVENTION

[0014] The present invention was conceived in view of the above described circumstances and it is an object thereof in a bolometer and a manufacturing method thereof, an uncooled IR bolometer being capable of operating under uncooled condition.

[0015] In accordance with an embodiment of the present invention, a bolometer includes a thin film resistor configured to have a front side and a back side, the thin film resistor absorbing incident IR radiation from the front side, electrodes configured to connect with the thin film resistor, an optical cavity layer configured to contact to the thin film resistor from the back side, a reflector metal configured to contact to the optical cavity layer from the back side, a protection layer configured to contact to the reflector metal; and a photodefmable layer configured to partially support the protection layer.

[0016] In accordance with another embodiment of the present invention, a

manufacturing method of a bolometer includes forming a photodefmable substrate on a mother substrate, depositing an optical cavity layer on the photodefmable substrate, forming electrodes on the optical cavity layer, depositing a material of a thin film resistor on the optical cavity layer to cover the electrodes; and depositing a reflector metal below the optical cavity layer.

BRIEF DESCRIPTION OF THE DRAWINGS

[0017] The disclosure will provide details in the following description of preferred embodiments with reference to the following figures wherein:

FIG. 1 A a drawing that illustrates a top view of a conventional art bolometer;

[0018] FIG. IB is a schematic illustration that illustrates a three dimensional (3D) view of the conventional bolometer;

[0019] FIG. 2 A is a drawing that illustrates a cross-sectional view of a bolometer;

[0020] FIG. 2B is a drawing that illustrates a top view of the bolometer having a thin film resistor and electrodes connected to the thin film resistor;

[0021] FIG. 2C is a drawing that illustrates a bottom view of the bolometer, in which a protection layer, a hole region and a photodefinable material are indicated;

[0022] FIG. 3 A is a schematic illustration that illustrates a mother substrate used in a manufacturing process of the bolometer;

[0023] FIG. 3B is a schematic illustration that illustrates a sacrificial layer formed on the mother substrate;

[0024] FIG. 3C is a schematic illustration that illustrates a photodefinable substrate formed on the sacrificial layer;

[0025] FIG. 3D is a schematic illustration that illustrates a protection layer formed on the photodefinable substrate;

[0026] FIG. 3E is a schematic illustration that illustrates a photolithography process in the manufacturing process of the bolometer;

[0027] FIG. 3F is a schematic illustration that illustrates a reflector metal and a photoresist formed on the protection layer after the photolithography process and a metal etching process;

[0028] FIG. 3G is a schematic illustration that illustrates a self-aligned photolithography process of the photodefinable substrate using the reflector metal as a mask;

[0029] FIG. 3H is a schematic illustration that illustrates an exposed region and an unexposed region in the photodefinable substrate after the self-aligned photolithography process;

[0030] FIG. 31 is a schematic illustration that illustrates an optical cavity layer formed on the reflector metal and the protection layer;

[0031] FIG. 3 J is a schematic illustration that illustrates a row electrode is formed on the optical cavity layer and shows a top view of the row electrode; [0032] FIG. 3 K is a schematic illustration that illustrates insulation layers formed on the row electrodes at predetermined areas (right side);

[0033] FIG. 3 L is a schematic illustration that illustrates column electrodes formed on the insulation layers (right side);

[0034] FIG. 3M is a schematic illustration that illustrates a thin film resistor formed on the row electrode and the column electrode;

[0035] FIG. 3N is a schematic illustration that illustrates a cross sectional view of the bolometer;

[0036] FIG. 4A is a schematic illustration that illustrates a cross sectional view of a bolometer formed in accordance with a second embodiment of the present invention;

[0037] FIG. 4B is a schematic illustration that illustrate a top view of the bolometer including the row and column electrodes and the thin film resistor;

[0038] FIG. 4C is a schematic illustration that illustrates a shape of the backside metal, a hole and the reflector metal;

[0039] FIG. 5 A is a schematic illustration that illustrates a mother substrate in accordance with a second embodiment of the present invention;

[0040] FIG. 5B is a schematic illustration that illustrates a sacrificial layer formed on the mother substrate;

[0041] FIG. 5C is a schematic illustration that illustrates a photodefinable substrate formed on the sacrificial layer;

[0042] FIG. 5DA is a schematic illustration that illustrates a photolithography process to be performed to expose the photodefinable substrate through a mask;

[0043] FIG. 5DB is a schematic illustration that illustrates an alternative

photolithography process to be performed by an oblique UV exposure through a mask for forming a hole with inclined walls in the photodefinable substrate; [0044] FIG. 5E is a schematic illustration that illustrates an optical cavity layer formed on the photodefmable substrate;

[0045] FIG. 5F is a schematic illustration that illustrates row and column electrodes and a thin film resistor are formed on the photodefmable substrate;

[0046] FIG. 5GA is a schematic illustration that illustrates the photodefmable substrate after development, in which backside metals and a reflector metal is formed on a backside of the photodefmable substrate and the backside of the optical cavity layer, respectively; and

[0047] FIG. 5GB is a schematic illustration that illustrates an alternative cross sectional shape of the photodefmable substrate after a development process.

BEST MODE FOR CARRYING OUT THE INVENTION

[0048] Embodiments in accordance with the present invention will now be described below with reference to the drawings.

(FIRST EMBODIMENT)

[0049] FIGS. 2 A, 2B and 2C show different views of a bolometer 20 in accordance with a first embodiment of the present invention. FIGS. 2A, 2B and 2C are drawings that illustrate a cross sectional view, a top of view, and a bottom of view of the bolometer 20, respectively.

[0050] The bolometer 20 includes a photodefmable substrate 200, a protection layer 201, a reflector metal 202, an optical cavity layer 203 having a thickness d, electrodes 204 and 205, a thin film resistor 206, a hole region 207, and a diaphragm layer 208.

[0051] The photodefmable substrate 200 is formed on a mother substrate (not shown in the figure). The diaphragm layer 208 is formed on the photodefmable substrate 200. The diaphragm layer 208 includes the protection layer 201, the reflector metal 202, and the optical cavity layer 203. The electrodes 204 and 205, and the thin film resistor 206 are formed on the diaphragm layer 208.

[0052] In the first embodiment, it is required for the photodefinable substrate 200 to have lower thermal conductivity to effectively isolate the heat generated in the thin film resistor 206. It is preferable that the thermal conductivity of the photodefinable substrate 200 be lower than that of silicon oxide or silicon nitrides, for example, smaller than approximately 1 (W/(m K)). In addition, the photodefinable substrate 200 needs to be mechanically strong enough to support the diaphragm layer 208 including the electrodes 204 and 205 and the thin film resistor 206. For example, it is preferred that the Young's modulus of the photodefinable substrate 200 be greater than 0.5 GPa.

[0053] The photodefinable substrate 200 may be a negative tone UV photoresist, since the photoresist provides easy handling and is processed based on inexpensive processing methods such as an UV light exposure process or a developing process using etchants. As the negative tone UV photoresist, for example, SU-8 may be used. The thermal conductivity of SU-8 is around 0.2 (W/(m-K)) and the Young's modulus of SU-8 are between 2 GPa and 4 GPa depending on annealing conditions (baking conditions).

Those properties are suitable for the photodefinable substrate. Polyimide such as HD-7010 also matches to the criteria of the photoresist described above as well.

[0054] The protection layer 201 may be an oxide or nitride film composed of a semiconductor material, such as silicon oxide and silicon nitride, and organic materials such as polymers. It is preferable that the material of the protection layer 201 have sufficient mechanical strength and low thermal conductivity. Thus, a photoresist, which is composed of polymers, can be used as the protection layer 201. In addition, parylene-C (as a polymer) has a thermal conductivity smaller than that of silicon nitride, which is preferable as the protection layer 201. [0055] For example, the thickness of the photodefmable substrate 200 can be designed by adjusting the rotational speed of a spin-coating process while depositing the material of the photodefmable substrate 200 on the mother substrate (not shown in the figure). The diaphragm layer 208, on which the thin film resistor 206 is formed, is largely separated from the bottom of the photodefmable substrate 200 as shown in FIG. 2A. When a 100 μιη thick material of the photodefmable substrate 200 is formed, the separation between the bottom of the diaphragm layer 208 and the bottom of the photodefmable substrate 200 corresponds to 100 μηι. The obtained structure can achieve a significantly low thermal conductance through (the air in) the hole region 207 compared to its conventional counterparts. (Because of this fact, conventional bolometers need vacuum packaging). The thermal conductance of the remaining parts of the photodefmable substrate 200 is comparable to or lower than a conventional bolometer structure having a suspended diaphragm layer of 2.5 μιη formed over the surface of a silicon substrate on which the read-out circuits are formed.

[0056] The reflector metal 202 is arranged below the thin film resistor 206 and above the protection layer 201 to increase the absorption of incident IR radiation, as shown in FIG. 2A. The thickness d of the optical cavity layer 203 is designed to satisfy a condition of l/4n for the peak wavelength of the blackbody radiation from a human, where n denotes a refractive index of the material of the optical cavity layer 203 for the peak wavelength. By designing the thickness d of the optical cavity layer 203, the bolometer can be applied to THz applications (i.e., to longer wavelengths).

[0057] In the first embodiment of the present invention, it is preferable that the material of the optical cavity layer 203 allow high transmission of IR radiation and have low thermal conductivity.

[0058] An oxide or nitride of a semiconductor material such as silicon oxide or silicon nitride may be used as the material of the optical cavity layer 203. Further, organic material layers such as polymers may be used as the optical cavity layer 203. Polymers such as polyimide, parylene-C, or a photoresist are preferred as an optical cavity layer material because those materials can be simply coated on the underlying material and have lower thermal conductivities compared to those of the materials used for the mechanical suspension arms and the diaphragm layer of a conventional bolometer.

[0059] The thin film resistor 206 is formed on the optical cavity layer 203. The electrodes 204 and 205 are connected to the sides of the thin film resistor 206, as shown in FIGS 2 A and 2B. Note that the electrodes 204 and 205 are arranged to be separated so that an electric potential (an electric field) can be applied to the thin film resistor 206. The thin film resistor 206 absorbs IR light and detects changes in resistance of the thin film resistor 206 caused by the absorption of IR light. By measuring signals through the electrodes 204 and 205, changes in the temperature of the thin film resistor 206 are detected.

[0060] The electrodes 204 and 205 may be formed of metals, metal alloys, conductive semiconductors, conductive polymers or conductive composites. For forming the electrodes, vacuum deposition of the material of the electrodes 204 and 205 is preferable to achieve high electrical conductance and low noise characteristics.

[0061] The thin film resistor 206 may be formed from materials used for IR thin film resistors formed by use of a vacuum deposition process, such as vanadium oxide and amorphous silicon, conductive polymers, carbon nanotubes (CNTs), carbon nanohorns and composites. For forming the thin film resistor 206, it is preferable to use materials that can be coated with low-cost deposition processes. Such low-cost deposition processes may be printing, dispensing, spraying or spin-coating. If being applied to such materials of the IR thin film resistors, these process techniques can reduce the cost of the formation process of the thin film resistor 206 sufficiently compared to vacuum deposition process techniques that are used for manufacturing a conventional bolometer based on a MEMS process. In accordance with the first embodiment of the present invention, CNTs are preferred because CNTs are deposited by a low cost deposition process. In addition, CNTs have relatively high infrared absorption.

[0062] In the first embodiment of the present invention, the reflector metal 202 not only plays a role as a reflector of incident IR radiation but also a lithography mask for the UV exposure process of forming the photodefinable layer 200 in the manufacturing process of the bolometer. The reflector metal 202 is formed to effectively absorb the incident IR radiation by the thin film resistor 206. Although part of the incident IR radiation (passing-through IR radiation) passes through the thin film resistor 206 without being absorbed by the thin film resistor 206, the reflector metal 202 reflects the

passing-through IR radiation back to the thin film resistor 206 so that the passing IR radiation is absorbed by the thin film resistor 206. This is very important since the reflector metal 202 increases the absorption efficiency of the IR radiation with the thin film resistor 206 up to 90 %. In addition, when the UV exposure process is performed to form the photodefinable layer 200, the reflector metal 202 acts as a self-aligned mask, so that no mask alignment process is required to define the photodefinable substrate. This self-alignment process makes manufacturing very simple and reduces the risk of failure due to miss-alignment of a mask, and therefore the manufacturing cost of bolometers can be sufficiently reduced.

[0063] In accordance with the first embodiment, the stiction problem during removal of sacrificial layer by wet etchant can be avoided as the diaphragm layer 208 of the bolometer 20 is formed approximately 100 μπι above from the bottom of the

photodefinable substrate (from the top of the mother substrate). Therefore, a wet release process technique can be effectively applied to the manufacturing process of the bolometer 20.

[0064] FIG. 2C is a drawing that illustrates a bottom view of the bolometer 20. The photodefinable substrate 200 forms the hole region 207. The protection layer 201 is exposed at the bottom of the bolometer 20.

[0065] FIGS. 3A-3I are schematic illustrations to illustrate the manufacturing steps of the bolometer 20 in accordance with the first embodiment of the present invention.

[0066] FIG. 3A shows a mother substrate 300, on which the bolometer 20 is

manufactured. A substrate with or without active elements can be used as the mother substrate 300. The mother substrate 300 is preferred to be a hard material having a Young's modulus greater than 1 GPa, which allows easy handling of the bolometer manufacturing process.

[0067] Semiconductor materials such as silicon, germanium, GaAs, insulating materials such as glass, quartz, sapphire, plastics, and metals such as steel, may be used as a material of the mother substrate 300. In the first embodiment of the present invention, silicon or glass is used as the mother substrate 300 because those are typical materials used in semiconductor device manufacturing processes and allow low cost substrates.

[0068] A sacrificial layer 301 is formed on the mother substrate 300 as shown in FIG. 3B, and a photodefinable substrate 302 is formed on the sacrificial layer 301 as shown in FIG. 3C.

[0069] The sacrificial layer 301 may be typical materials such as silicon oxide, silicon nitrides, aluminum, silicon, germanium, photoresists, or polymers. In the first embodiment of the present invention, photodefinable materials such as UV photoresists, E-beam (electron beam) photoresists or the like may be used as a material of the sacrificial layer 301. Using a photodefinable material as a sacrificial layer 30 allows simple manufacturing processes and easy handling. In particular, a photodefinable material having the same properties as those of the photodefinable substrate 302 is more preferable for the sacrificial layer 301. UV photoresist is preferred as the sacrificial material of the sacrificial layer 301. In the first embodiment, for example, a negative tone resist, SU-8 photoresist, a AZ family photoresist, or a photodefinable polyimide (photo sensitive polyimide) such as HD-7010 may be used as a material of the

photodefinable substrate 301. Such material is formed by spin-coating on the mother substrate 300 with a few micrometers thickness.

[0070] As the photodefinable substrate 302, any materials may be used as long as the material is easily processed and easily handled in the bolometer manufacturing processes. UV photoresist matches the criteria of the photodefinable substrate 302. In the first embodiment in accordance with the present invention, a negative tone photoresist is preferable. SU-8 resist, some resists belonging to the AZ family, polyimides such as HD-7010 or the like are possible candidates for the material of the photodefinable substrate 302.

[0071] The photodefinable substrate 302 is required to have a low thermal conductivity and to be thermally and mechanically stable. The UV photoresist can be deposited by spin-coating, patterned by UV light radiation, and developed by wet developers. In accordance with the first embodiment of the present invention, SU-8 resist is preferable for the following reasons. It is (i) commercially available, (ii) one of the popular UV photoresists used in semiconductor manufacturing, (iii) the film thickness is simply chosen by spin-coating from a few hundred nanometers to a few hundred micrometers (the thickness of photodefinable material T is adjustable), (iv) UV radiation time (exposure time) is less than 60 seconds, (v) chemically resistant to other chemicals, (vi) thermally stable, (vii) reasonable mechanical strength being 4 GPa obtained after a baking process, which is comparable to commercial polyimide substrates, and (viii) organic or alkali based developers can be applied for a patterning process.

[0072] In the first embodiment, negative tone UV photoresist SU-8 may be used as the sacrificial layer 301 and the photodefinable substrate 302 as shown in FIGS. 3B and 3C, respectively. In the developing process, the soluble region (UV unexposed region) of the photodefinable substrate 302 and the sacrificial layer 301 can be removed by a developer in the same process. This can reduce the manufacturing time, and thus reduce the manufacturing cost.

[0073] As is shown in FIG. 3C through FIG. 3D, after forming the photodefinable substrate 302 on the sacrificial layer 301, a protection layer 303 is coated on the photodefinable substrate 302. A thickness T of the photodefinable substrate 302 may range from approximately 50 μιη to approximately 150 μιη as indicated in FIG. 3C. In the step of FIG. 3E, reflector metal 304 is deposited on the protection layer 303, and a lithography process using a photoresist 305 and a UV expose system is performed for resist patterning. A developing process is carried out to remove the UV exposed region of the photoresist 305 using a developer. A metal etching process is performed using a metal etchant and the patterned photoresist 305 as a mask for forming the reflector metal 304 as shown in FIG. 3E through FIG. 3F. As a result, the reflector metal 304 is patterned (FIG. 3F).

[0074] In the step of FIG. 3G, a self-aligned lithography process is performed using UV light radiation and the reflector metal 305 (patterned reflector metal 305) as a mask to expose a predetermined region of the photodefinable substrate 302. Through the lithography process, the region in the photodefinable substrate 302 exposed by UV radiation becomes an insoluble region as indicated in FIG 3H, while another region of the photodefinable substrate 302, where UV light was not exposed because the patterned reflector metal mask 304 prevented the UV light from penetrating the photodefinable substrate 302 during the UV light radiation, becomes a soluble region.

[0075] The protection layer 303 may be a material used in MEMS process, such as silicon dioxide and silicon nitride, and organic materials such as polymers and organic materials composed of polymers. A polymer is preferable since polymer has low thermal conductivity, and allows simple processing and simple handling. Polyimide, chemitite, CYTOP, photoresist and parylene may be used as the polymer. As an example, parylene-C is used since parylene-C has a thermal conductivity lower than that of silicon nitride and is simply formed using a coating process at room temperature.

[0076] The reflector metal 304 may be any material as long as the material can prevent UV light from penetrating the photodefinable substrate 302. The reflector metal 304 may be referred to as a reflecting material, an absorbing material, a mask material or the like. In other words, any material which reflects electromagnetic waves can be used as the reflector metal 304. The reflector metal 304 may be a metal, an alloy of metals, semiconductors, doped semiconductors, conductive polymers and the like. For example, vacuum deposition is used to deposit metals. The reflector metal 304 may be aluminum, gold, silver, and titanium. Aluminum is one of suitable materials for the reflector metal, because an aluminum film formation is performed under relatively simple conditions, allows simple processing, and is a material which is compatible with IC manufacturing. Further, aluminum material is provided at a low cost and has high reflective

characteristics for IR radiation. In the first embodiment of the present invention, as an example, aluminum is used and deposited on the protection layer 303 by a vacuum deposition process.

[0077] In the reflector metal 304 formation, it is preferable to employ a positive photoresist 305. A lithography mask 307 is used to selectively expose the positive photoresist 305 according to the pattern of the lithography mask 307 by the radiation of UV light 306. The region of the positive photoresist 305 which is exposed by the UV light radiation becomes soluble by alkali based wet developers. After developing the positive photoresist 305, the reflector metal 304 is etched. In the first embodiment of the present invention, as an example of general use, a positive photoresist, Shipley, is preferably used. As a material of the reflector metal 304, aluminum may be used. In this case, aluminum can also be etched in the developer of the positive photoresist, which forms a structure shown in FIG. 3F. Subsequently, irradiation of UV light 308 is performed through the structure as shown in FIG. 3G, and thus insoluble regions and soluble regions of the photodefinable substrate 302 are formed. The insoluble regions and the soluble regions are indicated in FIG 3H.

[0078] As an another example, the reflector metal 304 of FIG. 3G may be directly formed by depositing the material of the reflector metal 304 on the protection layer 303 through a metal mask that has openings corresponding to the shape of the reflector metal 304 of FIG. 3G. It is possible to achieve a resolution of 50 μιη or less using the metal mask. After forming the reflector metal 304, UV light irradiation is performed through the structure of FIG. 3G using a UV exposure system.

[0079] An optical cavity layer 309 is formed on the structure in the subsequent process as shown in FIG. 31. A thickness d of the optical cavity layer 309 is chosen to be l/4n of a peak wavelength of the blackbody radiation, where n represents a refractive index of the optical cavity layer 309. For example, when the peak wavelength is written as λ, the thickness d of the optical cavity layer 309 is expressed by d= 4n. When assuming that the wavelength λ= 10 μιη and n=1.6, the thickness d of the optical cavity layer 309 becomes d = λ/6.4 = 1.6 μηι.

[0080] A row electrode 400 is formed on the optical cavity layer 309 as shown in FIG. 3 J. A top view of the row electrodes 400 is also shown on the right side in FIG. 3 J.

[0081 ] The optical cavity layer 309 may be made from a material used in MEMS process, that is, semiconductor materials such as silicon, germanium, and compound semiconductors, insulator materials such as silicon oxide and silicon nitride, and organic materials such as polymers, photoresist, or polyimide. It is preferable that the material of the optical cavity layer 309 have high transmission characteristics for IR radiation and a low thermal conductivity. In this respect, polymers such as polyimide and parylene-C may be used as the material of the optical cavity layer 309. In the first embodiment of the present invention, parylene-C is used, and the thickness of parylene-C is determined to be approximately 1.6 μιη. CNTs are used as the bolometer material and deposited on predetermined areas as the thin film resistors 403 so as to contact the row electrode 400 and the column electrode 402, as shown in FIG. 3M.

[0082] The bolometer in accordance with the present invention is used for THz applications by choosing an appropriate thickness d for the optical cavity layer 309. For a bolometer operable at 3 THz, the thickness of the optical cavity layer 309 is preferably 16 μηι.

[0083] Although it is simple in the present invention, fabricating this cavity in conventional bolometers is very challenging. In the conventional bolometers (FIG. IB), the diaphragm layer includes mechanical suspension arms with electrical contacts and the optical cavity layer 103. The diaphragm layer must be formed 25 μηι above the substrate 100. The formation of such a diaphragm structure by MEMS processes is a challenging task. In the first embodiment of the present invention, the optical cavity layer 309 is composed of polymer (n > 1), so that a thick optical cavity layer can be simply formed by spin-coating. When parylene-C (n ~ 1.6) is used as the optical cavity layer 309, a 16 μιη thick parylene-C film will be sufficient for 3 THz applications. The calculation of the thickness d of parylene-C layer is based on the equation of d= /4n by assuming that the refractive index n of parylene-C is approximately 1.6 and the wavelength λ = 100 μιη for 3 THz applications, so that d=100/6.4 ~ 16 μιη.

[0084] FIGS. 3J-3N illustrate the manufacturing process of the bolometer 20 in accordance with the first embodiment of the present invention. Top views of arrayed bolometers 20 are also shown on the right sides of the figures. In this case, the bolometers 20 are arrayed in 2 by 2 rows and columns. In FIGS. 3J-3N, Z-Z' correspond to cross sectional views of one of the arrayed bolometers 20 depicted along a broken line Z-Z' in the top views of the arrayed bolometers 20. The cross sectional views are shown on the left sides of the figures.

[0085] Row electrodes 400 are formed on the optical cavity layer 309 as shown in FIG 3 J. In FIG. 3K, insulation layers 401 are formed to cover the row electrodes 400 at predetermined intersection regions on which column electrodes 402 are arranged to be formed. Column electrodes 402 are formed on the insulation layers 401 at the predetermined intersections as shown in FIG 3L. In a process of FIG. 3M, each thin film resistor 403 is formed between the row electrode 400 and the column electrode 402. After etching the soluble regions of the photodefinable substrate 302, the bolometers 20 are finally released from the mother substrate 300, as shown in FIG 3N. A thickness dl of the optical cavity layer 309 may be approximately 1.6μπι, a thickness d2 of the protection layer 303 may be approximately 1 μιη, and a thickness of the photodefinable substrate 302 may range from approximately 50μηι to approximately 150μηι.

[0086] The row electrodes 400 may be formed by UV lithography or a metal mask. UV lithography is preferably used for achieving high resolution patterns. The material of the row electrodes 400 may be metals, alloys of metals, semiconductors, doped semiconductors, conductive polymers, conductive composites, and conductive nanoparticles such as gold, carbon nanotubes and carbon nanohorns. Metals are preferable to obtain high electrical conductivity and low noise characteristics. A vacuum deposition process may be used to deposit the metal. In the first embodiment of the present invention, vacuum deposition is used for metal deposition.

[0087] The column electrodes 402 may be formed in a similar way to the row electrodes 400. The material used for the formation of the row electrodes 400 may be used in the formation of the column electrodes 402. The row electrodes 400 and the column electrodes 402 are arranged to measure the resistance of the thin film resistors 403.

[0088] The row electrodes 400 and the column electrodes 402 are isolated from each other by the insulation layers 401 formed at the predetermined intersection regions, as shown in FIG. 3L. The insulation material of the insulation layers 401 may be materials generally used for MEMS process, such as silicon oxide, silicon nitride, metal oxide and some organic materials such as photoresist chemitite, CYTOP, and polymers. In the first embodiment of the present invention, a negative tone UV photoresist, SU-8, is used for simple processing.

[0089] The thin film resistors 403 may be semiconductor materials such as silicon, germanium, compound semiconductors, or oxides such as vanadium oxide, or organic materials such as conductive polymers, proteins (e.g. cytochrome-C), carbon nanotubes (CNTs), carbon nanohorns, or composites. A vacuum deposition technique may be used for forming the thin film resistors 403. Further, other deposition techniques, which are performed with lower cost than that of the vacuum deposition technique, may be used, such as spin-coating, dispensing, printing, or spraying. In the first embodiment of the present invention, CNTs may be used, because CNTs have high IR absorption capability and can be coated with such low cost deposition techniques. [0090] Material of the electrodes 400 and 402 may be chosen to form either ohmic contacts or a non-ohmic contact with the thin film resistors 403. By designing the combination of the work functions of the electrode material and the thin film resistor material, it is possible to form electrical potential barriers (e.g. Schottky barrier or tunnel barrier) at the interface of the thin film resistors 403 and the electrodes 400 and 402. The electrical potential barriers provide diode-like characteristics for the bolometer 20.

[0091] After forming the thin film resistors 403, an anti-reflection coating film (not shown in the figure) may be formed so as to cover the thin film resistor 403. The material used for an IR anti-reflection coating film may be zinc sulfide (ZnS), yttrium fluoride (YF3), and multiple layers of the materials. For THz applications, parylene-C and parylene related materials or those multiple layers are preferably used as a THz anti-reflection coating film.

[0092] The structure of the bolometer 20 having the diaphragm layer 208 is finally removed from the mother substrate 300 by removing the soluble region of the photodefinable substrate 302, which results in a suspension structure, as shown in FIG. 3N. In the first embodiment of the present invention, SU-8 organic developer is used for the removing process. The suspension structure having approximately 100 μηι height (gap) can be obtained through the manufacturing process described above. (SECOND EMBODIMENT)

[0093] FIGS. 4A, 4B, and 4C show different views of a bolometer 50 in accordance with a second embodiment of the present invention. FIGS. 4A, 4B, and 4C are drawings that illustrate a cross sectional view, a top of view, and a bottom of view of the bolometer 50, respectively. FIG. 4A shows a cross sectional view along the line X-X' of FIG. AC. [0094] Materials used for forming the bolometer 50 may be the same as those used in the bolometer 20 of the first embodiment and may be used for the same purpose. For processes which are identical to those used in the manufacturing process of the bolometer 20 in accordance with the first embodiment, the explanations will be omitted in the following descriptions.

[0095] A line W-W indicates a cross sectional line that cuts out a structure of the bolometer 50 along the line W-W.

[0096] The bolometer 50 in accordance with the second embodiment of the present invention includes another structure that effectively reduces a thermal conductance to a bolometer.

[0097] A back side metal 507 and a reflector metal 508 are deposited on a

photodefinable substrate 500 and a back side of an optical cavity layer 501 without a mask after a mother material (not shown in the figure) is removed from a structure of the bolometer 50.

[0098] A diaphragm structure of the bolometer 50 does not include a protection layer unlike that of the bolometer 20 in accordance with the first embodiment and is formed on the photodefinable substrate 500. Electrodes 502 and 503, and a thin film resistor 504 are formed on an optical cavity layer 501, as shown in FIG. 4A. A soluble region of the photodefinable substrate 500 is removed to form a hole 505 by use of wet developer. The photodefinable substrate 500 may be a UV photoresist, such as negative tone photoresist. In the second embodiment, SU-8 may be used. This process allows forming a suspension structure of the diaphragm layer 506. Subsequently, backside metallization is performed to form the back side metal 507 and the reflector metal 508 as shown in FIG. 4A. FIG. 4B is the top view of the bolometer 50. The optical cavity layer 501 is seen under the electrodes 502 and 503. The electrodes 502 and 503 are arranged at two sides of the thin film resistor 504. The thin film resistor 504 is formed so as to cover the electrodes 501 and 502. FIG. 4C is the bottom view of the bolometer 50. The back side metal 507 and the reflector metal 508 are shown. A square shape of the hole 505 is indicated in the figure.

[0099] Manufacturing processes of a bolometer 50 in accordance with the second embodiment of the present invention are shown in FIGS. 5A-5GB in detail. FIGS. 5DB and 5GB are manufacturing processes of an alternative structure of the bolometer 60. A mother substrate 600 is provided in FIG. 5 A and a sacrificial layer 601 is formed on the mother substrate 600 as shown in FIG. 5B. Further, a photodefinable substrate 602 is formed on the sacrificial layer 601. A thickness T of the photodefinable substrate 602 may range from approximately 50 μιη to approximately 150 μπι as indicated in FIG. 3C. In the second embodiment of the present invention, the series of manufacturing processes shown in FIGS. 5A-5C are the same as those shown in FIGS. 3A-3C. Structures formed in the manufacturing processes shown in FIGS. 5DA and 5DB do not include either a protection layer or a reflector metal unlike the structure of the bolometer 20 which includes the protection layer 303 and the reflector metal 304.

[0100] A lithography process is shown in FIG. 5DA where UV light 603 is irradiated on the photodefinable substrate 602 through a lithography mask 604. Exposed regions and an unexposed region are indicated in FIG. 5DA. As the alternative structure of the bolometer 50, another UV irradiation method is performed for the photodefinable substrate 602, in which UV light irradiation 605 is made from predetermined oblique angles as shown in FIG. 5DB. It is noted in FIG. 5DA the exposed regions are approximately square shapes, while the exposed regions shown in FIG. 5DB are tapered square shapes. In both structures, the unexposed regions correspond to soluble regions when the etching process is performed. An optical cavity layer 606 is formed on the photodefinable substrate 602 as shown in FIG. 5E. A function of the optical cavity layer 606 is as the same as that of the optical cavity layer 203 described in the first

embodiment of the present invention, so that the geometrical design and the material of the optical cavity layer 606 may be similar to those of the optical cavity layer 203. In the second embodiment of the present invention, parylene-C is preferable as a material of the optical cavity layer 606.

[0101] Subsequently, row electrodes 607 are formed on the optical cavity layer 606 and insulation layers (not shown in the figure) are formed to cover the row electrodes 607 at predetermined intersection regions on which column electrodes 608 are to be arranged. The column electrodes 608 are formed on the optical cavity layer 606 so as to cross on the predetermined intersection regions. These processes are similar to those of the bolometer 20 shown in FIGS. 3J-3L.

[0102] Each of thin film resistors 609 is formed on the optical cavity layer 606, the row electrodes 607, and column electrodes 608 so as to bridge the row electrode 607 and the column electrode 608 as shown in FIGS 5GA and 5GB. FIG. 5GB shows the alternative structure of the bolometer 60. In the second embodiment of the present invention, the thin film resistors 609 may be formed from CNTs.

[0103] The structure of the bolometer 50 is released from the mother substrate 600 by removing the sacrificial layer 601 and the soluble region of the photodefinable substrate 602 with the developer. The developer may be SU-8 organic developer in the second embodiment of the present invention. This removing process forms a hole 610 under the optical cavity layer 606. The suspension structure of the bolometer 50 is completed by a backside metallization process, in which back side metals 611 and a reflector metal 612 are deposited on the bottom of the photodefinable substrate 602 and the bottom of the optical cavity layer 606, as shown in FIG. 5GA and FIG. 5GB. FIG. 5GB shows another bolometer 60 having a hole 710 having alternative cross sectional structure in the photodefinable substrate 602. In FIG. 5GA and FIG. 5GB, it is preferable that the area of the reflector metal 612 be approximately identical to or larger than the area of the thin film resistor 609 so that the reflector metal 612 receives and reflects the whole IR light having passed through the thin film resistor 609 toward the thin film resistor 609. A width of the reflector metal 612 may be approximately the same as a separation of the electrodes 607 and 608 or greater than the separation of the electrodes 607 and 608. In this case, it is preferable that the reflector metal 612 be disposed to correspond to the thin film resistor 609. Such geometrical condition of the reflector metal 612 improves an absorption of incident IR light to be detected by the thin film resistor 609 because part of incident IR light penetrating the thin film resistor 609 is effectively reflected to the thin film resistor 609 by the reflector metal 612 and the reflected IR light is absorbed by the thin film resistor 609.

[0104] The alternative cross sectional structure of the hole 710 in FIG. 5 GB shows an oblique (slope) wall. The oblique wall of the hole 710 in FIG. 5GB can be formed by oblique irradiation of UV light during the lithography process as shown in FIG. 5DB.

[0105] The shape of the hole 710 in FIG. 5GB is useful to secure the separation between the backside metals 611 and the reflector metal 612 during and after deposition of the material of the backside metals 611 and the reflector metal 612 because the material is deposited from the bottom of the bolometer structure. If parts of the backside metals 611 and the reflector metal 612 are connected each other, the thermal isolation of the thin film resistor 609 is degraded, because the material of the backside metals 611 and the reflector metal 612 has a high thermal conductivity. Poor thermal isolation degrades the characteristics of bolometers. Therefore, the oblique (inclined) wall of the hole 710 can improve a yield of manufacturing of the bolometers. [0106] In addition, a cover layer may be deposited on the thin film resistor 609 (not shown in figures). The cover layer may be an insulator layer such as silicon dioxide, silicon nitride, titanium nitride, aluminum oxide or some polymers such as polyimide or parylene. The cover layer functions to increase IR absorption and passivation. In order to further increase IR absorption, highly IR absorptive materials such as CNTs may be deposited on the cover layer.

[0107] The bolometers 50 may be formed as array formats, which allows to take an thermal picture image of a targeted object. Such structure can increase the varieties of application of the bolometers.

[0108] The present invention provides a photodefinable substrate for a bolometer device with a simple suspended diaphragm layer fabrication with a good thermal isolation. With a photodefinable substrate, the suspended diaphragm layer can be separated from the bottom of the photodefinable substrate by 100 μιη (more or less), which provides a significantly lower thermal conductance through air compared to a conventional bolometer in which the suspended diaphragm layer is positioned 2.5 μιη above the silicon substrate. The photodefinable substrate and the optical cavity layer made of a polymer film with very low thermal conductance compared to silicon oxide or silicon nitride provides good thermal isolation (e.g. SiN: 1.85 W/(m K), parylene-C: 0.082 W/(m K), SU-8: 0.2 W/(m K)). Reflector metal placed below the thin film resistor by adjusting the thickness d of the optical cavity layer to the peak wavelength of the blackbody radiation of human increases the absorption. Refractive index of the optical cavity layer is an important consideration to reach the maximum absorption to adjust the thickness. (d=l .6 μιη with parylene-C, -1/6 of the peak wavelength). By changing the thickness of the optical cavity layer made by polymer, the device can be used in THz applications as well, without increasing the risk of failure in the fabrication processes.

[0109] While preferred embodiments of the invention have been described and illustrated above, it should be understood that these are examples of the invention and are not to be considered as limiting. Additions, omissions, substitutions, and other modifications can be made without departing from the scope of the present invention. Accordingly, the invention is not to be considered as being limited by the foregoing description, and is only limited by the scope of the appended claims.

[0110] The whole or part of the exemplary embodiments disclosed above can be descried as, but not limited to, the following supplementary notes.

[0111] (Supplementary note 1 ) A bolometer comprising a thin film resistor configured to have a front side and a back side, the thin film resistor absorbing incident IR radiation from the front side, electrodes configured to connect with the thin film resistor, an optical cavity layer configured to contact to the thin film resistor from the back side, a reflector metal configured to contact to the optical cavity layer, a protection layer configured to contact to the reflector metal, and a photodefinable layer configured to partially support the protection layer.

[0112] (Supplementary note 2) A bolometer according to Supplementary note 1 , wherein the photodefinable layer is formed of a negative tone UV photoresist.

[0113] (Supplementary note 3) A bolometer according to Supplementary notes 1 through 2, wherein the electrodes are arranged to measure an electrical signal generated in the thin film resistor.

[0114] (Supplementary note 4) A bolometer according to Supplementary notes 1 through 3, wherein the electrodes are arranged to apply an electric potential barrier or tunnel barrier to the thin film resistor through the electrodes.

[0115] (Supplementary note 5) A bolometer according to Supplementary notes 1 through 4, wherein the photodefinable layer includes a hole that is formed so that a size of an opening of the hole increases from an entrance of the hole toward the protection layer.

[0116] (Supplementary note 6) A bolometer according to Supplementary notes 1 through 4, the bolometer further comprising a cover layer configured to protect the front side of the thin film resistor and to enhance the absorption.

[0117] (Supplementary note 7) A bolometer according to Supplementary note 6, wherein a thickness of the cover layer is adjusted to effectively absorb the IR radiation using the thin film resistor.

[0118] (Supplementary note 8) A bolometer according to Supplementary notes 1 through 6, wherein the reflector metal has an area that is approximately identical to or larger than an area of the thin film resistor.

[0119] (Supplementary note 9) A manufacturing method of a bolometer comprising forming a photodefinable substrate on a mother substrate, depositing an optical cavity layer on the photodefinable substrate, forming electrodes on the optical cavity layer, depositing a material of a thin film resistor on the optical cavity layer to cover the electrodes, and depositing a reflector metal below the optical cavity layer.

[0120] (Supplementary note 10) A manufacturing method of a bolometer according to Supplementary notes 1 through 8, wherein the reflector metal reflects UV light during UV exposure.

[0121] (Supplementary note 11) A manufacturing method of a bolometer according to Supplementary note 9, wherein said depositing of the reflector metal is performed from a bottom side of the optical cavity layer.

[0122] (Supplementary note 12) A manufacturing method of a bolometer according to Supplementary note 11 , UV exposure is performed by oblique UV light. INDUSTRIAL APPLICABILITY

[0123] This invention can be applied to a bolometer which is a device that detects the temperature change of an object.