Infrared detecting device and infrared detecting element for use in the device

Abstract: An infrared detecting arrangement includes an infrared detecting element (10) having a heat insulating film (13) and an infrared detector (14) which are disposed between an interior space of a hermetically sealed casing (18) for the element and a cavity (11) formed within a substrate (12) of the element and communicating through a ventilator (19) with the interior space (17). In an infrared detecting device of this arrangement, any gas pressure difference between the interior space and the cavity can be remarkably effectively reduced to attain an extremely excellent sensitivity.

Description: This invention relates to a method of making an infrared detecting device. In conventional infrared detecting arrangement employing a thermal type infrared detecting element having a diaphragm structure, there have been suggested to take measures for improving the sensitivity of the infrared detecting element by attaining a vacuum in the interior of hermetically sealed casing or charging a thermally low conducting gas in the casing. Further, the infrared detecting element of this kind is constituted by providing a infrared detecting element on a substrate having a cavity and disposing an infrared detector on this infrared detecting element, so as to more improve the sensitivity to the infrared. For the infrared detector employed in this element, there has been a thermistor which varies electric resistance in response to variation in temperature, and a practical example is an amorphous semiconductor (silicon) film. As a prior art reference disclosing the infrared detecting arrangement, Japanese Utility Model Laid-Open Publication No. 3-78222 may be mentioned, and practical detecting elements have been disclosed in, for example, Japanese Patent Laid-Open Publications Nos. 58-170001, 61-30730 and 4-215022. DE 38 39 482 A1 discloses an infrared detection arrangement with a protective gasfilling, in which a cavity is provided beneath the film carrying the infrared detector chip. Overflow channels (Überströmkanäle) connect the cavity with the space above the detector chip, the cavity also being filled with protective gas. In order to attain a more excellent detection sensitivity with these infrared detecting arrangements, it is desired to attain the vacuum or to charge the thermally low conducting gas also in the cavity or hollow space made in the substrate in similar manner to the interior space of the hermetically sealed casing. Further, in exhausting the gas in the interior of the hermetically sealed casing, it is also desired to prevent the infrared detecting element and eventually the infrared detector on the film from being damaged by an excessive pressure difference between the interior space of the casing and the cavity included in the infrared detecting element since the film and detector are disposed just between these space and cavity. It is also desirable to reduce internal stresses in the device. A primary object of the present invention is, therefore, to provide an infrared detecting device of a shape and materials tending to meet these desiderata. According to the present invention, a method of making an infrared detecting device is as claimed in claim 1. According to the above infrared detecting arrangement, the sensitivity to the infrared which has been apt to be deteriorated in the conventional arrangement of the similar type for detecting the infrared can be extremely excellently improved, as will be readily appreciated. Other objects and advantages of the present invention shall become clear as the description of the invention detailed with reference to accompanying drawings in the followings advances. BRIEF DESCRIPTION OF THE DRAWINGS FIGURE 1 is schematic explanatory view of an infrared detecting device useful for understanding the present invention. FIGURE 2 is a schematic side view of the infrared detecting element in the device of FIGURE 1. FIGURE 3 is a schematic plan view of the element of FIGURE 2. FIGURES 4 to 7 are graphic diagrams of varying relationship between gas exhaust time and pressure difference with the conductance of the communicating means varied in the device shown in FIGURE 1. FIGURES 8 to 10 show in schematic side views other embodiments of a device. FIGURES 11 and 12 are explanatory views for an aspect of a manner in which the infrared detecting element in the device of FIGURE 1 is prepared. FIGURES 13 and 14 are explanatory views for another aspect of the manner in which the infrared detecting element in the device of FIGURE 1 is prepared. FIGURES 15 and 16 are explanatory views for still another aspect of the manner in which the infrared detecting element in the device of FIGURE 1 is prepared. FIGURE 17 illustrates the present invention by showing in a sectioned view an embodiment of an infrared detecting element to be employed in the device shown in FIGURE 1 according to the invention. FIGURE 18 is a plan view of the infrared detecting element shown in FIGURE 17. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS Referring to FIGS. 1 to 3, there is shown an embodiment of the infrared detecting device, in which the device includes an infrared detecting element 10 which comprises a substrate 12 having a cavity 11 defined therein, an insulating film 13 provided on the substrate 12 to cover the cavity 11, and an infrared detector 14 disposed on the insulating film 13. The substrate 12 is disposed preferably on a base plate 15 formed to have a peripheral stepped part, and a cap 16 is fitted over the base plate 15, while FIGURE 1 shows them as separated. The cap 16 is joined to the peripheral stepped part of the base plate 15 by means of an electric welding or the like, to form a hermetically sealed casing. The infrared detecting device is constituted in the following manner. The infrared detecting element 10 is accommodated within a hermetically sealed chamber 18 defining therein an interior space 17 attained the vacuum or charged with other gas of a low thermal conductivity than air. Here, the infrared detector 14 on the insulating film 13 is positioned to be between the cavity 11 formed in the substrate 12 and the interior space 17 defined in the chamber 18. Further, the base plate 15 is formed to have a ventilator 19 in the form of a hole or groove made in the base plate 15, so that the cavity 11 in the substrate 12 can communicate with the interior space 17 of the chamber 18 through the ventilator 19. In this case, the ventilator 19 as the communicating means may be provided alternatively in a spacer disposed between the substrate 12 and the base plate 15. Then, the cap 16 is joined to the base plate 15 to form the casing within the hermetically sealed chamber 18, so that the infrared detecting element 10 will be sealed in the hermetical casing having the same atmosphere as the interior space of the chamber 18. Assuming here that the gas inside the chamber 18 is to be exhausted at a flow rate Q0 and at an exhaust speed S0 in vacuum series, that the cavity 11 of the substrate 12 is of a volume V1 and a pressure P1 while the interior space 17 of the sealed chamber 18 is of a volume V2 and a pressure P2, and that the ventilator 19 between the cavity 11 and the interior space 17 is of a conductance C, a gas flow rate Q1 and a exhaust speed S, then there is satisfied between them such relationship as follows: S.P1 = Q1, C(P2-P1) = Q1 Further, variation in the pressure P2 inside the chamber 18 will be as follows: V2(dP2/dt) = -S0.P2 + Q1 Provided that, normally, V2 >> V1 and Q1 is negligibly small with respect to S0 and P2, then V2(dP2/dt) = -S0.P2 whereby, when initial pressure of interior space 17 is P0 P2 = P0 exp (-S0.t/V2) On the other hand, variation in the pressure P1 in the cavity 11 will be as follows: V1(dp1/dt) = -S.P1 Because, as in the above, S.P1 = Q1 and C(P2-P1) = Q1 , V1(dP1/dt) = C(P2-P1) and hence, P1 = P2 + (P0-P2) exp (-Ct/V1) in which expression coefficient (P0-P2) is fixed as the initial pressure in the cavity 11 equal P0. Consequently, the conductance C of the ventilator 19 is so set that P1 and P2 will be equal to each other. At this time, a tolerance limit of the pressure difference is determined by the pressure resisting break strength of the insulating film 13. Referring more specifically to the above with reference to a practical example, it is assumed here that the infrared detecting element 10 and a vacuumizing exhaust system satisfying the conditions of S0 = 10 cm 3 /S, V1 = 10 -3 cm 3 and V2 = 10 cm 3 are furnished, and the insulating film 13 of the element 10 is formed by a silicon oxide film or a multi-layered film of a silicon oxide film and silicon nitride film and 1µm thick, and this film is to be broken at the pressure difference of about 300 Torr. In this system, there is established such relationship of the gas exhaust time to the pressure difference between P1 of the cavity 11 and P2 of the interior space 17 at the time when the ventilator conductance C is 10 -6 cm 3 /S, 10 -5 cm 3 /S, 10 4 cm 3 /S and 10 -3 cm 3 /S, respectively, as shown in FIGURES 4 to 7 in view of which the gas exhaust is made possible under the pressure difference of P1 and P2 below 300 Torr, when C is 10 -3 cm 3 /S. On the other hand, required size of the ventilator 19 for attaining the optimum conductance C will be determined as in the followings. That is, the viscous fluidity of a circular conduit in general is to be assumed as in the followings, D 4 = L.C/182p Here, D is a diameter of the circular conduit, L is the length of the conduit, p is an average pressure of pressures at inlet and outlet ports of the conduit, and the conduit will have a sectional area of about 100 µm 2 when C = 10 -3 cm 3 /S, L = 0.05 cm and p = 380 Torr. In designing the ventilator 19, therefore, it is desirable that the ventilator is provided with a sectional area larger than this sectional area of the conduit. The ventilator 19 as the communicating means between the cavity 11 and the interior space 17 may be formed also in either one of such manners as shown in FIGURES 8 and 9, in the former of which the ventilator 19A is a groove made in bottom face of the substrate 12A abutting on the base plate 15A, and in the latter of which the ventilator 19B comprises holes made through the insulating film 13B. Further, it is another effective measure to provide, between the substrate 12C and the base plate 15C, a spacer 20C having a groove or hole as the ventilator as shown in FIGURE 10 in which event the spacer 20C should advantageously be provided on the bottom face of the substrate 12C. In these embodiments shown in FIGURES 8 to 10, all other constituents which are the same as those in the embodiment of FIGURES 1 to 3 are denoted by the same reference numerals as those given in FIGURES 1 to 3 but with a suffix "A", "B" or "C" added, and they are arranged substantially in the same manner except for the above respects, to achieve the same function. In FIGURES 11 and 12, there is shown one of working aspects for preparing the infrared detecting element, which should preferably be carried out in such steps as described in the followings. That is, an adhesive is applied to a surface of the base plate 15D, the infrared detecting element 10D is mounted onto the surface of the base plate 15D, and they are heat-treated. The infrared detecting element 10D comprises the substrate 12Da having the cavity 11D, and the heat insulating film 12Db provided to cover the cavity 11D. The insulating film 12Db is formed in similar manner to the foregoing embodiments. Then, such infrared detector 14D as the thermistor is provided to be above the insulating film 12Db, for detecting the infrared by utilizing a phenomenon that the resistance value is caused to vary by ambient temperature variation. Preferably, as shown in FIGURE 11, the infrared detector 14D is divided into four sections respectively disposed on the thinner parts 12Db of the silicon substrate, which are subjected to mutual bridge connection for the infrared detection. The grooves 19D are formed as the ventilating means in part of the substrate 12Da having the cavity 11D. The grooves 19D can be readily provided by means of an anisotropic etching with an aqueous solution of potassium hydroxide, a cutting with a dicing saw or the like, an electro arc machining or the like. It will be advantageous to form the grooves by such means carried out preliminarily with respect to the material substrate or wafer for forming the chips. The communicating means can be formed, as shown in FIGURES 13 and 14, in the form of a ventilating hole 19E penetrating through the substrate 12Ea having the cavity 11E and the heat insulating film 12Eb, with bottom edge portion of this hole 19E expanded to be contiguous to the cavities 11E disposed below the respective infrared detector sections 14e in the illustrated case. The ventilating hole 19E can be attained by means of the anisotropic etching, cutting, arc machining or the like carried out simultaneously with the formation of the cavity 11E. In another embodiment shown in FIGURES 15 and 16, it is possible to provide a ventilating gap 19F between the base plate 15F and the substrate 12F having the cavity 11F by means of an interposition member 19F disposed between the substrate 12F and the base plate 15F. In the respective embodiments of FIGURES 11 and 12, FIGURES 13 and 14 and FIGURES 15 and 16, the same constituents as those in the foregoing embodiment of FIGURES 1 to 3 are denoted by the same reference numerals as those used in FIGURES 1 to 3 but with a suffix "D", "E" of "F" added thereto, and are arranged substantially in the same manner as in FIGURES 1 to 3 except for the above respects, to attain the same function. Referring to FIGURES 17 and 18, there is shown an embodiment according to the present invention, in which in particular any distortion or damage of the insulating film can be reliably prevented from occurring. In the present instance, the cavity 11G substantially of a square shape is formed in the central part of the substrate 12G, and the insulating film 13G covering the cavity 11G is formed with a silicon oxide film or a multi-layered film of silicon oxide film and silicon nitride film, which so functions as not to cause any large internal stress. The infrared detector 14G is formed by a thermistor made of an amorphous silicon. In this case, electrodes 14Ga of chromium Cr are provided to both upper and lower surfaces of the infrared detector 14G, which electrodes are diagonally outward extended and are provided at their extended ends respectively with connecting pads 14Gb. Further, the top of the infrared detector 14G, that is, the upper side electrode 14Ga on the detector 14G is covered with an infrared absorbing film 14Gc. The infrared absorbing film 14Gc is formed by silicon oxynitride, which so functions as not to cause any large internal stress and is so high in the infrared absorptivity that the infrared irradiated on the detector 14G can be absorbed at a high efficiency. With the above arrangement of holding the amorphous silicon thermistor forming the infrared detector 14G between the electrodes 14Ga, the infrared detector 14G is rendered to be easily controllable in the resistance value, any fluctuation between the thermistors can be restrained, and any noise can be reduced.

Claims: Method of making an infrared detecting device comprising the steps of: providing an infrared detecting element (10) comprising a substrate (12Ga) having a cavity (11G) defined therein; the cavity (11G) being substantially of a square shape and formed in the central part of the substrate (12Ga); providing an insulating film (13G) on the substrate (12Ga) to cover the cavity (11G), the insulating film (13G) being formed of a silicon oxide film or of a multi layered film of silicon oxide film and silicon nitride film; disposing an infrared detector (14G) made of amorphous silicon on the insulating film (13G); disposing the substrate (12Ga) on a base plate (15), formed to have a peripheral stepped part; forming a ventilator (19) in the form of a hole or groove made in the substrate, the insulating film, the base plate (15) or a spacer between the substrate (12Ga) and the base plate (15) or by an interposition member there between so that the cavity (11G) in the substrate (12Ga) can communicate with the surrounding space (17) through the ventilator (19); providing electrodes of chromium (Cr) to both upper and lower surfaces of the infrared detector (14G), which electrodes (14Ga) are extended diagonally with respect to said square shape outward; providing connecting pads (14Gb) at the extended ends of the electrodes (14Ga); covering the upper side electrode (14Ga) on the detector (14G) with an infrared absorbing film (14Gc) of silicon oxynitride; accomodating the assembly created in the foregoing steps, together with a separate cap (16), within a hermetically sealed chamber (18) defining therein an interior space (17), the infrared detector (14G) on the insulating film (13G) being positioned to be between the cavity (11G) formed in the substrate (12Ga) and the interior space (17) defined in the chamber (18); exhausting the gas inside the chamber (18) at a flow rate (Q0) and an exhaust speed (S0) such that a tolerance limit of the pressure difference between the pressure (P1) within the cavity (11) and the pressure (P2) in the chamber (18) is determined by the pressure resisting break strength of the insulating film (13G), if the infrared detecting element is to be charged with a gas of a lower thermal conductivity than air, charging this gas into the chamber; and joining the cap (16) to the base plate (15) to form a casing within the hermetically sealed chamber (18) so that the infrared detecting element is hermetically sealed in the casing having the same atmosphere as the interior space of the chamber (18). Method according to claim 1, characterized in that the ventilator (19A) is formed as a groove made in the bottom face of the substrate (12A) abutting on the base plate (15A). Method according to claim 1, characterized in that the ventilator (19B) comprises holes made through the insulating film Method according to claim 1, characterized in that a spacer (20C) is provided between the substrate (12C) and the base plate (15C), having a groove or hole as the ventilator, the spacer (20C) being provided on the bottom face of the substrate (12C). Method according to claim 1, characterized in that the method comprises the further steps of: providing a ventilator gap (19F) between the base plate (15F) and the substrate (12F) having the cavity (11F) by means of an interposition member (19Fa) disposed between the substrate (12F) and the base plate (15F).

Inventors: AIZAWA KOUICHI, AWAI TAKAYOSHI, ICHIHARA TSUTOMU, ISHIDA TAKURO, KAKITE KEIZI, NAKAMURA TAKURO, SAKAI JUN, SUMI SADAYUKI, TAKAMI SHIGENARI, TOMONARI SHIGEAKI, YOSHIDA HITOSHI, AIZAWA, KOUICHI, AWAI, TAKAYOSHI, ICHIHARA, TSUTOMU, ISHIDA, TAKURO, KAKITE, KEIZI, NAKAMURA, TAKURO, SAKAI, JUN, SUMI, SADAYUKI, TAKAMI, SHIGENARI, TOMONARI, SHIGEAKI, YOSHIDA, HITOSHI, AIZAWA, KOUICHI, C/O MATSUSHITA ELEC. WORK, LTD, AWAI, TAKAYOSHI, C/O MATSUSHITA ELEC. WORK, LTD, ICHIHARA, TSUTOMU, C/O MATSUSHITA ELEC. WORK, LTD, ISHIDA, TAKURO, C/O MATSUSHITA ELEC. WORK, LTD, KAKITE, KEIZI, C/O MATSUSHITA ELEC. WORK, LTD, NAKAMURA, TAKURO, C/O MATSUSHITA ELEC. WORK, LTD, SAKAI, JUN, C/O MATSUSHITA ELEC. WORK, LTD, SUMI, SADAYUKI, C/O MATSUSHITA ELEC. WORK, LTD, TAKAMI, SHIGENARI, C/O MATSUSHITA ELEC. WORK, LTD, TOMONARI, SHIGEAKI, C/O MATSUSHITA ELEC. WORK, LTD, YOSHIDA, HITOSHI, C/O MATSUSHITA ELEC. WORK, LTD
Applicants: MATSUSHITA ELECTRIC WORKS LTD, MATSUSHITA ELECTRIC WORKS, LTD.
Application year: 1993
Main classification: G01J5
IPC codes: G01J5, H01C7