Infrared sensor having a bridge structure

Abstract: An infrared sensor has a bridge portion which bridges a cave portion formed in a sensor substrate. The bridge portion comprises an infrared receiving portion and supporting portions for supporting the infrared receiving portion. The plane shape of the infrared receiving portion is a square or a circle which has a greater side length or a greater diameter than the width of the supporting portions to increase the amount of infrared received by the infrared receiving portion.

Description: BACKGROUND OF THE INVENTION Field of The Invention The present invention relates generally to an infrared sensor for measuring the temperature of a subject, such as a tympanic membrane, without contact therewith. More specifically, the invention relates to an infrared sensor having a bridge structure on which a thermally sensitive portion is formed. Description of The Background Art In thermal type infrared sensors, the temperature of a substance is measured on the basis of the temperature increase of a thermally sensitive portion which is heated by infrared radiated from the substance. Various technologies for making the sensors compact and high-sensitive have been proposed. In particular, when the temperature of a substance is substantially equal to or less than a room temperature, the amount of infrared received by the sensor is very small. Therefore, it is necessary to decrease the heat capacity of the sensor and to prevent the energy of the received infrared from being scattered and lost as much as possible. In recent years, various infrared sensors reduced in size to decrease their heat capacity and having a bridge structure to prevent heat from being scattered and lost due to conduction of heat, have been proposed (JP-A 57-178149, 62-277528 and JP-A 3-136379). These sensors are designed to decrease heat capacity by compacting using semiconductor fine patterning or micro-machining, and they have a thermally sensitive portion on a bridge structure which does not make contact with a substrate in order to prevent heat from being scattered and lost due to conduction of heat. The infrared sensors disclosed in JP-A 57-178149 and JP-A 62-277528 are obtained by forming on a silicon substrate or a metallic substrate an insulating film of a laminated film consisting of a silicon oxide film (SiOx) and a silicon nitride film (SiNy), transcribing a pattern of a bridge portion on the insulating film by a photolithographic technique, forming and patterning a predetermined wiring and thermally sensitive portion on the insulating film which has not been etched, and selectively etching a part of the substrate to form a caved portion. These methods are referred to as "anisotropic etching" or "isotropic etching" in accordance with a manner for selectively etching a part of the substrate. The infrared sensor disclosed in JP-A 3-136379 is obtained by accurately forming a caved portion which is to be arranged below a bridge portion, on a substrate at a predetermined region prior to the formation of the bridge portion; filling in the caved portion with a metallic film which serves as a sacrificial layer for forming the bridge portion, to make the surface of the substrate even; forming on the whole even surface an insulating film of a laminated film of a silicon oxide film and a silicon nitride film, or an insulating film of an organic material; forming a pattern of a bridge on the insulating film by photolithography and etching; patterning metallic wirings and a thermally sensitive portion; and selectively etching only the sacrificial layer. However, since these conventional infrared sensors are provided to improve the yield of the formation of a bridge portion, the dimensional accuracy thereof, the uniformity of thermal characteristics of each sensor element and the accuracy of temperature measurement, they are not designed to increase the amount of received infrared to improve the sensitivity of the sensor. For that reason, these infrared sensors can not achieve fully high-sensitive measurement of temperature since the shape of the bridge portion is not designed to be optimized in view of the amount of received infrared energy and the amount of energy lost due to heat conduction of the bridge portion in order to increase the temperature increase of the thermally sensitive portion on the bridge portion. SUMMARY OF THE INVENTION It is therefore a principal object of the present invention to eliminate the aforementioned disadvantages and to provide an improved infrared sensor. It is another object of the present invention to provide an infrared sensor which can increase the temperature increase of a thermally sensitive portion to achieve high-sensitive measurement of the temperature of a subject. It is another object of the present invention to provide an infrared sensor which can increase the temperature increase of a thermally sensitive portion by optimizing the shape of a bridge portion on which the thermally sensitive portion is formed. In order to accomplish the aforementioned and other objects, an infrared sensor has a bridge portion formed on a sensor substrate having a cave portion, the bridge portion comprising a supporting portion and an infrared receiving portion which is wider than the width of the supporting portion. The shape of the bridge portion may be selectively determined on the basis of a relationship between the temperature increase of a thermally sensitive portion formed on the bridge portion and the manufacturing yield of the sensor. An infrared sensor according to the invention is defined in claim 1. The infrared receiving portion may be integrally formed with the supporting portions. Although the plane shape of the infrared receiving portion may be optionally selected, it is preferably a square to facilitate the manufacturing of the sensor. The center of the infrared receiving portion is preferably arranged at the center of the bridge portion. The ratio of a side length of the infrared receiving portion to the whole length of the bridge portion is in the range of 0.1 to 0.9, preferably in the range of 0.2 to 0.6. The plane shape of the infrared receiving portion may also be a circle. In this case, the ratio of the diameter of the infrared receiving portion to the whole length of the bridge portion is in the range of 0.1 to 0.9, preferably in the range of 0.2 to 0.6. Although the number of the supporting portions may be optionally selected, it is preferably two or four since the quantity of heat lost due to heat conduction increases as the number increases. The sensor substrate may be a semiconductor substrate of silicon or germanium, it is preferably a silicon substrate which is easily and cheaply available. The thermally sensitive portion may be formed of a film selected from the group consisting of amorphous germanium (a-Ge), amorphous silicon (a-Si) and polycrystalline silicon films. The thermally sensitive portion may be formed by sputtering, ion-beam sputtering, CVD (chemical vapor deposition) method or the like. The bridge portion is formed of silicon oxynitride film (SiOxNy). It is formed of a silicon oxynitride film (SiOxNy) because it has both characteristics of a silicon oxide film and a silicon nitride film, and has a good stress balance to allow the formation of a stable bridge structure. The optimum condition of the formation of the silicon oxynitride film varies in accordance with the kind of sensor substrate to be used, since the thermal expansion coefficient varies in accordance with the plane orientation of the substrate material. BRIEF DESCRIPTION OF THE DRAWINGS The present invention will be understood more fully from the detailed description given herebelow and from the accompanying drawings of the preferred embodiments of the invention. However, the drawings are not intended to imply limitation of the invention to a specific embodiment, but are for explanation and understanding only. In the drawings: Fig. 1 is a perspective view of the preferred embodiment of an infrared sensor, according to the present invention; Fig. 2 is an enlarged plan view of a bridge portion of the infrared sensor of Fig. 1; Figs. 3(A) to 3(I) are sectional views illustrating a process for manufacturing the infrared sensor of Fig. 1; Fig. 4 is a graph showing results of computer simulation of a relationship between a/L and Δ T in the infrared sensor of Fig. 1; and Figs. 5(A) to (C) are plan views showing examples of a supporting structure when the shape of the infrared receiving portion is substantially square. DESCRIPTION Referring now to the drawings, particularly Figs. 1 and 2, the preferred embodiment of an infrared sensor of the present invention will be described below. As shown in Fig. 1, an infrared sensor, according to the present invention, comprises a silicon substrate 11 (300 to 400µm in thickness) serving as a sensor substrate and having a caved portion 12, and a bridge portion 13 which is formed on a surface of the silicon substrate 11 so as to bridge the caved portion 12. The bridge portion 13 comprises a circular infrared receiving portion 14 and four supporting portions 15 for supporting the infrared receiving portion 14. Both of the infrared receiving portion 14 and the supporting portions 15 are integrally formed of silicon oxynitride film having a thickness of 3µm. A thermally sensitive portion 16 is formed on the infrared receiving portion 14 at the center thereof. The thermally sensitive portion 16 is formed of a material having electrical resistance which varies in accordance with the variation of temperature thereof (thermistor effect), for example, amorphous germanium (a-Ge). The thermally sensitive portion 16 is connected to a measuring system (not shown) via comb-shaped wiring layers 17a and 17b of a metal, such as aluminum. An infrared guiding aperture 18 is formed in the silicon substrate 11 on the reverse side thereof opposite to the infrared receiving portion 14. Infrared radiated from a subject passes through the infrared guiding aperture 18 to be received by the infrared receiving portion 14. The infrared received by the infrared receiving portion 14 is transmitted to the thermally sensitive portion 16 to cause the variation of the electrical resistance value thereof. The electrical resistance value of the thermally sensitive portion 16 is detected by measuring the electrical current or voltage value of the measuring system connected to the wirings 17a and 17b via electrode pads which are provided on the ends of the wirings 17a and 17b. On the basis of this current or voltage value, the amount of infrared radiated from the subject, i.e. the temperature thereof, can be determined. According to the present invention, the infrared sensor can receive a greater amount of infrared, since the infrared receiving portion 14 is wider than the supporting portions 15 so as to increase the area for receiving light. Therefore, the temperature of the thermally sensitive portion 16 can be effectively increased, so that the sensitivity of the sensor can be improved. Although the shape of the infrared receiving portion 14 is a circle in the illustrated embodiment, it should not be limited to a circle. According to the present invention, it may be selected from various shapes if the infrared receiving portion 14 is wider than that of the supporting portions 15. As the width of the supporting portions 15 decreases, the quantity of heat which gets scattered and lost from this portion decreases. At this point, it is preferable to make the width of the supporting portions 15 thin, as long as a desired mechanical strength thereof is maintained. In order to decrease heat loss due to conduction of heat, the thickness of the infrared receiving portion 14 and the supporting portions 15 is preferably thin (for example, not more than 18µm). According to the present invention, it may decrease to about 3µm since the infrared receiving portion 14 and the supporting portions 15 are formed of a silicon oxynitride film having a superior strength. A process for manufacturing an infrared sensor, according to the present invention, will be described below. First, a silicon substrate 11 having a crystalline plane orientation (100) as shown in Fig. 3(A) was prepared. Then, a silicon oxynitride film (SiOxNy) 20 of 1µm in thickness was formed on both surfaces of the silicon substrate 11 by plasma CVD method as shown in Fig. 3(B). That is, the silicon substrate 11 was heated to 500°C to cause the vapor phase growth of the silicon oxynitride on both surfaces of the silicon substrate under a film forming condition which maintains a pressure of 60 Pa (0.45Torr) and a high-frequency output of 400W, using as reaction gases 10SCCM of monosilane (SiH 4 ), 207SCCM of nitrogen (N 2 ) and 28SCCM of nitrous oxide gas (N 2 O). Then, the sputtering was carried out using germanium as a target to deposit an amorphous germanium (a-Ge) film 21 on the silicon oxynitride film 20 on the surface of the silicon substrate 11 as shown in Fig. 3(C). The sputtering was carried out for 10 minutes at a gas flow rate of argon (Ar)=2SCCM, under a film forming pressure of 0.4Pa (3x10 -3 Torr) at a high-frequency output of 200W. Then, annealing was carried out to promote the polycrystallization of amorphous germanium. Subsequently, the polycrystallized amorphous germanium film 21 was patterned by reactive ion etching (RIE) to form a thermally sensitive portion 16 as shown in Fig. 3(D). The reactive ion etching was carried out using as etching gases freon (CF 4 ) and oxygen (O 2 ), at gas flow rates of CF 4 =38SCCM and O 2 =2SCCM under an etching pressure of 27 Pa (0.2Torr) at a high-frequency output of 150W for an etching time of 10 minutes. Then, as shown in Fig. 3(E), a metallic film, e.g. an aluminum film 22, was formed on the surface of the silicon substrate 11 by a vapor deposition method. Subsequently, as shown in Fig. 3(F), the aluminum film 22 was patterned by the reactive ion etching to form wirings 17a and 17b on the thermally sensitive portion 16. Thereafter, under the same condition as that of Fig. 3(B), a silicon oxynitride film (SiOxNy) 23 of 2µm in thickness was formed on the surface of the silicon substrate 11 by plasma CVD method as shown in Fig. 3(G). Then, the silicon oxynitride films 20 and 23 were selectively etched by the reactive ion etching to form openings 24 as shown in Fig. 3(H). In this reactive ion etching, methane trifluoride (CHF 3 ), freon (CF 4 ) and oxygen (O 2 ) were used as etching gases, the flow rates thereof were CHF 3 =30SCCM, CF 4 =8SCCM and O 2 =2SCCM, the pressure for the etching was 10.0 Pa (0.075Torr), the highpressure for the the high-frequency output was 300W and the etching time was 50 minutes. Furthermore, in the etching of the silicon oxynitride films 20 and 23, buffered hydrofluoric acid (e.g. NH 4 F:HF=6:1) may also be used. Finally, the silicon substrate 11 was etched out from both sides thereof through the openings 24 to form a caved portion 12 and a bridge portion 13 which has an infrared receiving portion 14 and a supporting portions 15 as shown in Figs. 1 and 3(I). This etching was carried out by anisotropic etching using an aqueous of hydrazine. Fig. 4 shows a relationship between a/L and a relative value of temperature increase ΔT at the center of the infrared receiving portion 14, using as parameters the width and the number of the bridge portion 13 and the supporting portion 14, which relationship was obtained by computer simulation on the basis of finite element method. In Fig. 4, a is a diameter when the plane shape of the infrared receiving portion 14 is a circle, or a length of a side when it is a square, and L is the whole length of the bridge portion 13. Furthermore, in order to simplify the calculation, the shape coefficient of each element on the bridge portion 13 was assumed to be 1. In the initial condition in the simulation model, the temperature of the whole bridge portion 13 was assumed to be 20°C, and in the boundary condition, the temperature of the silicon substrate 11, i.e. the temperature of the base end of the supporting portion 15, was assumed to be constant at 20°C. The temperature of a heat source (a subject, the temperature of which is to be measured) was assumed to be 36.5°C, and the heat source was assumed to be separated from the center of the infrared receiving portion 14 by a vertical distance of 2mm. Furthermore, Δ T shows a saturated value of temperature increase of the thermally sensitive portion 16 about 10 seconds after radiation by infrared. In Fig. 4, A is the case in which the infrared receiving portion 14 is a square and the number of the supporting portions 15 (35µm in width) is two, B is the case in which the infrared receiving portion 14 is a circle and the number of the supporting portions 15 (40µ m in width) is two, C is the case in which the infrared receiving portion 14 is a circle and the number of the supporting portion 15 is four (the width of the opposite two supporting portions is 40µm and the width of the other two supporting portions is 20µm), and D is the case in which the infrared receiving portion 14 is a square and the number of the supporting portions 15 (35µ m in width) is two. In either case of A to D, the thickness of the bridge portion 13 was assumed to be 3µ m, and the whole length (L) of the bridge portion 13 was assumed to be 2mm in either case of A to C and 1mm in the case of D. As can be clearly seen from Fig. 4, regardless of the width and the number of the supporting portions 15, Δ T reaches a maximum when a/L is about 0.7. When the value of a/L is small, i.e. when the area of the infrared receiving portion 14 is small, the amount of the received infrared is small so that the temperature increase Δ T is also small. On the other hand, when the value of a/L exceeds 0.7, the length of the supporting portions 15 which have a small width, i.e. a small heat conduction, is short relative to the size of the infrared receiving portion 14, so that the amount of heat getting scattered and lost increases, thereby the temperature increase Δ T decreases. That is, from a relationship between the amount of the received infrared and the amount of heat lost due to heat conduction, Δ T reaches a maximum when a/L is a specific value (0.7). As mentioned above, when the value of a/L=0.7, the value of Δ T reaches a maximum. However, by experiments by the inventors, it was found that when a/L=0.7, the supporting portion 15 tends to be damaged in the step for etching the silicon substrate 11 to form the bridge portion 13, so that the yield decreases to be about 20%. As shown in Fig. 4, if a/L is not 0.7, e.g. if a/L=0.5, the decrease of the temperature increase Δ T is only about 8% compared with that when a/L=0.7. Therefore, it is clearly advantageous to increase the yield of the sensor by designing the sensor so as to facilitate the formation of the supporting portion 15 when a/L is less than 0.7. In the sensor described in Fig. 1, the value of a/L is 0.25. In experiments by the inventors, when 0.2 < a/L < 0.6, a yield greater than 95% was obtained at the final manufacturing step. That is, the shape of the bridge portion 13 should be determined in view of both of Δ T and the yield. Fig. 5(A) to (C) show examples of a plane shape of the bridge portion 13 when the infrared receiving portion 14 is a square and the number of the supporting portion 15 is four. If the area of the infrared receiving portion 14 is same in either case, the value of Δ T reaches a maximum in the case of Fig. 5(C) since the length of the supporting portion 15 is the maximum in this case. However, in this case, the value of a/L is limited to be less than 0.5. In order to obtain a sensor having a/L of greater than 0.5, the sensor must have the shape shown in Fig. 5(A) or 5(B). The shape of Fig. 5(A) is better than that of Fig. 5(B) since the former can be easily formed. Although the infrared sensor (thermobolometer) utilizing thermistor effect was disclosed in the aforementioned embodiment, the present invention can be applied to other thermocouple type, p-n junction type and so forth.

Claims: An infrared sensor comprising: a sensor substrate (11) having a cave portion (12); a bridge portion (13) formed on a surface of said sensor substrate so as to bridge said cave portion, said bridge portion comprising an infrared receiving portion (14) for receiving infrared and supporting portions (15) for supporting said infrared receiving portion, said infrared receiving portion having a greater width than said supporting portions; and a thermally sensitive portion (16) formed on said infrared receiving portion, said thermally sensitive portion having the characteristic that its electrical resistance varies in accordance with the infrared received by said infrared receiving portion, characterized in that said infrared receiving portion and said supporting portions are formed of silicon oxynitride and the ratio of the length (a) of the infrared receiving portion to the whole length (L) of said bridge portion is in the range of 0.1 to 0.9. An infrared sensor according to claim 1 wherein said infrared receiving portion (14) is integrally formed with said supporting portions (15). An infrared sensor according to claim 1, wherein said infrared receiving portion (14) is arranged at the center of said bridge portion (13). An infrared sensor according to anyone of claims 1 to 3, wherein said infrared receiving portion (14) has a plane shape of a square. An infrared sensor according to anyone of claims 1 to 3, wherein said infrared receiving portion (14) has a plane shape of a circle. An infrared sensor according to anyone of claims 1 to 5, wherein said ratio is in the range of 0.2 to 0.6. An infrared sensor according to anyone of claims 1 to 6, wherein said sensor substrate (11) is a silicon or germanium substrate. An infrared sensor according to anyone of claims 1 to 7, wherein said thermally sensitive portion (16) is formed of a film selected from the group consisting of amorphous germanium, amorphous silicon and polycrystalline silicon films. An infrared sensor according to anyone of claims 1 to 8, which further comprises an infrared guiding aperture (18) for guiding infrared to said infrared receiving portion via said cave portion (12).

Inventors: KOMATSU KIYOSHI, KUDO TAKESHI, MORI TAKEHISA, KOMATSU, KIYOSHI, KUDO, TAKESHI, MORI, TAKEHISA, KOMATSU, KIYOSHI, C/O TERUMO KABUSHIKI KAISHA, KUDO, TAKESHI, C/O TERUMO KABUSHIKI KAISHA, MORI, TAKEHISA, C/O TERUMO KABUSHIKI KAISHA
Applicants: TERUMO CORP, TERUMO KABUSHIKI KAISHA
Application year: 1992
Main classification: H01L31
IPC codes: H01L31, G01J5, G01J1