Abstract:
A sensor includes a detector for detecting physical quantity, a membrane, and a stress relaxation area. A stress is expected to concentrate in the stress relaxation area in a case of manufacturing process of the sensor or a case of operating the sensor. The detector is disposed on the membrane except for the stress relaxation area.

Description:
CROSS REFERENCE TO RELATED APPLICATION 
   This application is based on Japanese Patent Applications No. 2002-310978 filed on Oct. 25, 2002, and No. 2003-275312 filed on Jul. 16, 2003, the disclosures of which are incorporated herein by reference. 
   FIELD OF THE INVENTION 
   The present invention relates to a sensor having a membrane. 
   BACKGROUND OF THE INVENTION 
   A sensor  100  having a membrane structure according to a prior art is shown in  FIGS. 8A and 8B . The sensor  100  is a thermopile type infrared sensor, and disclosed in Japanese Unexamined Patent Application Publication No. H04-98883. The sensor  100  has a thin film membrane  112 . A hot contact point  115   a  of a thermocouple  115  (i.e., a hot junction  115   a  of a thermocouple  115 ) is disposed on the membrane  112  so that thermal separation between the hot contact point  115   a  and a cold contact point  115   b  of the thermocouple  115  (i.e., a cold junction  115   b  of the thermocouple  115 ) is improved. 
   In the above sensor  100 , an upper surface of the sensor  100  disposed on the thermopile  115  is uneven, i.e., the upper surface has a wavy structure, as shown in  FIG. 7 . On the other hand, the other surface of the sensor  100  (not shown), where the thermopile  115  is not formed, is even, i.e., flat. By means of the wavy structure, stress is concentrated in this uneven portion. 
   When comparatively large stress is applied to the sensor  100  by means of thermal stress or distortion of the sensor  100  in a case of manufacturing process or a case of operating the sensor, the membrane  112  easily cracks because the mechanical strength of the membrane  112  is comparatively weak. Further, the membrane  112  may be broken by the large stress. 
   It is considered that thickness of a film for providing the membrane  112  becomes thicker so as to protect the sensor from cracking or being broken. However, in accordance with becoming thicker, thermo-conductivity of the film becomes large, so that thermal separation between the hot contact point  115   a  and the cold contact point  115   b  is deteriorated. Therefore, a sensitivity of the sensor is decreased. Further, it is considered that material of the film is changed to new material, which has comparatively low thermo-conductivity, so as to compensate the deterioration of the thermal separation. However, this makes the manufacturing cost increase. 
   SUMMARY OF THE INVENTION 
   In view of the above problem, it is an object of the present invention to provide a sensor having a membrane, stress on which is reduced so that the sensor is limited to crack and to be broken. 
   A sensor includes a detector for detecting physical quantity, a membrane, and a stress relaxation area. A stress is expected to concentrate in the stress relaxation area in a case of manufacturing process of the sensor or a case of operating the sensor. The detector is disposed on the membrane except for the stress relaxation area. 
   Since the above sensor has the stress relaxation area, in which the stress is concentrated, the sensor is limited to crack and to be broken. Moreover, the stress relaxation area is easily formed without adding a new part or adding a new manufacturing process, since the stress relaxation area can be formed by only changing a pattern of the detector. Therefore, the manufacturing process of the sensor is not changed substantially so that the manufacturing cost of the sensor is almost the same as that of a sensor without the stress relaxation area. 
   Preferably, the membrane has a rectangular shape. More preferably, the rectangular shape of the membrane has a width and a length, and the stress relaxation area has a rectangular shape having a width and a length. The width of the stress relaxation area is one-two hundredth of the width of the membrane, and the length of the stress relaxation area is one-fifteenth of the length of the membrane. The stress relaxation area is disposed in a middle of an edge of the rectangular shape, and disposed inside from the edge of the rectangular shape. 
   Preferably, the thermopile includes a plurality of thermocouples with a pair of a hot contact portion and a cold contact portion, and the hot contact portion is disposed on the membrane, and the cold contact portion is disposed outside of the membrane. 
   Preferably, the stress relaxation area is disposed in a range between 0 μm and 5 μm measured from an edge of the membrane. 
   Preferably, the membrane is composed of a thin film disposed on a semiconductor substrate, and the thermopile is disposed on the thin film. More preferably, the semiconductor substrate has a rectangular opening disposed opposite to the thin film so that the membrane has a rectangular shape. The hot contact portion is disposed on the thin film of the membrane so that the hot contact portion is disposed on the rectangular opening of the semiconductor substrate, and the cold contact portion is disposed on the thin film on the semiconductor substrate. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The above and other objects, features and advantages of the present invention will become more apparent from the following detailed description made with reference to the accompanying drawings. In the drawings: 
       FIG. 1A  is a schematic plan view showing a sensor according to a preferred embodiment of the present invention, and  FIG. 1B  is a cross-sectional view taken along line IB—IB in  FIG. 1A ; 
       FIG. 2  is a partial circuit diagram showing an equivalent circuit of a thermopile of the sensor according to the preferred embodiment; 
       FIG. 3  is a plan view of a sensor chip of the sensor explaining a position of a stress relaxation area, according to the preferred embodiment; 
       FIG. 4A  is a histogram showing a relationship between the number of cracks and a distance Y, and  FIG. 4B  is a schematic plan view of the sensor chip explaining the distance Y and a simulation area S, according to the preferred embodiment; 
       FIG. 5  is a partial plan view of a membrane in the simulation area S showing a contour of stress, according to the preferred embodiment; 
       FIG. 6  is a schematic plan view showing a sensor according to a comparison of the preferred embodiment; 
       FIG. 7A  is a cross-sectional view taken along line VIIA—VIIA in  FIG. 6 , and  FIG. 7B  is a cross-sectional view taken along line VIIB—VIIB in  FIG. 6 ; 
       FIG. 8A  is a plan view showing a sensor according to a prior art, and  FIG. 8B  is a cross-sectional view taken along line VIIIB—VIIIB in  FIG. 8A ; and 
       FIG. 9  is a partially enlarged cross-sectional perspective view showing the sensor according to the prior art. 
   

   DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
   A sensor  10  according to a preferred embodiment of the present invention is shown in  FIGS. 1A and 1B . The sensor  10  is composed of a sensor chip  11  for sensing infrared light. The sensor chip  11  is formed with fabricating a silicon substrate  11   a . On an upper surface of the sensor chip  11 , a thin film  13  is formed for providing a membrane  12 . The membrane  12  is formed in such a manner that a lower surface of the silicon substrate  11   a  is etched so as to expose the thin film  13 . On the lower side of the silicon substrate  11   a , the thin film  13  is exposed to have a rectangular shape. Therefore, the membrane  12  has the rectangular shape in a plan view. 
   On the thin film  13 , a thermopile  14  is formed. The thermopile  14  is provided by a plurality of thermocouples  15  connected with series connection. Each thermocouple  15  has a hot contact point  15   a  and a cold contact point  15   b . The hot contact point  15   a  is disposed on the membrane  12 , and the cold contact point  15   b  is disposed on the silicon substrate  11   a . The predetermined number of thermocouples  15  is disposed in a middle portion of each edge of the rectangular shape of the membrane  12 . 
   Here, at the middle of the edge of the rectangular shape of the membrane  12 , a stress relaxation area  16  is disposed. In the stress relaxation area  16 , the thermopile  14  is not formed. Stress is mainly concentrated and applied to the membrane  12  in this stress relaxation area  16  because of a construction of the membrane  12 . In other words, if the thermopile  14  is formed on the membrane  12  in this stress relaxation area  16 , a step structure is formed by the thermopile  14 . The membrane  12  becomes weaker by this step structure, so that the membrane  12  with the thermopile  14  may crack easily. 
   To compare with the sensor shown in  FIG. 1 , a comparison sensor  1  is prepared, as shown in  FIG. 6 . The comparison sensor  1  has no stress relaxation area  16  in the middle of the edge of the membrane  12 . As shown in  FIG. 7A , the thermopile  14  is insulated by an insulation film  5 . On the insulation film  5 , a passivation film  6  is formed. Therefore, a step is formed on the thermopile  15 . On the other hand, in a portion, which has no thermopile  15 , there is no step, as shown in  FIG. 7B . The stress is concentrated at the step, and the step makes the membrane  12  weaker. 
   The inventors have confirmed by the experiment that the strength of the membrane  12  with the thermopile  14  is weaker than 70% of the strength of the membrane  12  without the thermopile  14 . Moreover, as described later, the stress is easily concentrated in the middle of the edge of the membrane  12 , i.e., the stress relaxation area  16 . Therefore, the thermopile  14  is not formed on the membrane  12  in the stress relaxation area  16 . 
   Next, an infrared absorption film  17  is formed on the thermopile  14  on the membrane  12 . The infrared absorption film  17  is limited to reflect and to transmit the received infrared light substantially, so that the thermal absorption of the infrared absorption film  17  is promoted. Two output terminals  14   a ,  14   b  are disposed at both ends of the thermopile  14 , respectively. The output terminals  14   a ,  14   b  as a bonding pad electrically connect to an outer circuit outside the sensor  10 . Each output terminal  14   a ,  14   b  has a predetermined area for connecting a bonding wire or a bump. 
   The thermopile  14  has an equivalent circuit shown in  FIG. 2 . The thermopile  14  includes a plurality of thermocouples  15 . Each thermocouple  15  is composed of a pair of electrodes. Each electrode is made of material, which is different from each other. In  FIG. 2 , a pair of narrow and broad lines shows a pair of electrodes. A plurality of thermocouples  15  connects with a series connection. All of the hot contact points  15   a  are disposed on the membrane  12 , and all of the cold contact points are disposed on the silicon substrate  11   a.    
   When the sensor  10  catches an infrared light, the infrared light is absorbed in the infrared absorption film  17 . Then, the hot contact point  15   a  is heated, so that temperature of the hot contact point  15   a  becomes high. That is, because the hot contact point  15   a  has no thermal diffusion portion substantially for conducting heat to the outside. On the other hand, the cold contact point  15   b  is limited to heat, since the silicon substrate  11   a  works as a heat sink for conducting heat to the outside. Therefore, temperature of the hot contact point  15   a  is different from that of the cold contact point  15   b . This temperature difference causes difference of electromotive force (i.e., the potential difference) between the hot contact point and the cold contact point according to the Seebeck effect. Thus, each thermocouple  15  has each potential difference, respectively. And, all of the potential differences are summed up so that an output voltage VOUT is provided, because the thermocouples are connected with series connection. The output voltage VOUT is outputted from a pair of output terminals  14   a ,  14   b.    
   The stress relaxation area  16  is defined as follows. As shown in  FIG. 3 , the stress relaxation area  16  has a rectangular shape, which has a length L and a width W. Assuming each length of edges of the membrane  12  is A and B, respectively, the length L and the width W are provided as follows.
 
L=A/15  (1)
 
W=B/200  (2)
 
   The stress relaxation area  16  is disposed at the middle of the edge of the membrane  12 . In other words, an upper residual length C of the edge is equal to a lower residual length D of the edge. 
   The stress is concentrated in this area, i.e., the stress relaxation area  16 , as described below. 
   A relation between the number of cracks and a position where the crack is provided is shown in  FIG. 4A . As shown in  FIG. 4B , the position of the crack is defined by Y, which is measured from an edge of the membrane  12 . In  FIG. 4A , the total number of cracks is  38 , average value of the position of the crack is 1.65 μm, deviation 3σ, i.e., three sigma of standard deviation, is 3.37 μm, the maximum value of the position of the crack is 4.0 μm, the minimum value of the position of the crack is 0.0 μm, and each length A, B of edges of the membrane  12  is 1000 μm. 
   As shown in  FIG. 4A , the crack is mainly arisen in a range between 0 μm, i.e., the edge of the membrane, and 3 μm. In a range between 0 μm and 4.5 μm, almost all of the cracks are arisen. According to this result, the width W of the stress relaxation area  16  is determined to 5 μm. This value, i.e., 5 μm, is one-two hundredth of the width B of the membrane  12 . Here, the width B of the membrane  12  is 1000 μm. Thus, the above formula (2) is derived. 
   On the other hand, the formula (1) is derived from a simulation performed by the inventors. The simulation is performed by a finite element method (i.e., FEM) so that an area where the stress is concentrated is confirmed. As shown in  FIG. 4B , the stress is analyzed in a simulation area S, which is disposed on an upper left side of the membrane  12 . The result of the simulation is shown in  FIG. 5 . In  FIG. 5 , a contour line of the stress is shown. Here, the stress along with the contour line is applied equivalently. In the middle of the edge of the rectangular shape of the membrane  12 , a large stress area M is formed. The large stress area M is shown as a slanting line portion in  FIG. 5 . A small stress area N is disposed on an area, which is separated from the edge of the rectangular shape of the membrane  12 . According to this result, the length L of the stress relaxation area  16  is determined to one-fifteenth of the width A of the membrane  12 . Here, the width A of the membrane  12  is 1000 μm. 
   Thus, the stress relaxation area  16  is defined. Since the sensor  10  has the stress relaxation area  16 , in which the stress is concentrated, the sensor  10  is limited to increase the stress so that the sensor  10  is limited to crack and to be broken. 
   In this embodiment, the stress relaxation area  16  is formed for protecting the sensor from cracking or being broken. The stress relaxation area  16  is easily formed without adding a new part or adding a new manufacturing process, since the stress relaxation area  16  can be formed with only changing a pattern of the thermopile  14 . Therefore, the manufacturing process of the sensor  10  is not changed substantially so that the manufacturing cost of the sensor  10  is almost the same as that of the sensor  1 . 
   Although the membrane  12  has a rectangular shape, other polygons such as pentagon and hexagon can be used as the shape of the membrane  12 . Especially, in a case where the membrane  12  has a certain shape in which the stress concentration is easily arisen, a portion where the stress is concentrated is defined as the stress relaxation area  16 , and the thermopile is not formed on the stress relaxation area  16 . Thus, the sensor  10  is limited to increase stress so as to protect from cracking and being broken. Moreover, the degree of freedom in the design of the sensor  10  is increased, because the sensor  10  is protected from cracking or being broken, comparing a case where the thermopile  14  is formed on the membrane  12  uniformly so that the sensor  1  easily cracks and is broken. 
   Although the sensor  10  is the infrared light sensor, other sensors such as collector type temperature sensor and pressure sensor, which has a membrane for detecting physical quantity, can be used as the sensor  10 . 
   Such changes and modifications are to be understood as being within the scope of the present invention as defined by the appended claims.