Abstract:
A method for manufacturing a pressure sensor includes the steps of: preparing a semiconductor substrate; forming an insulation film on the substrate; forming a first metal film on the insulation film; forming a first protection film on the first metal film and the insulation film; forming a second protection film on the first metal film and the first protection film; performing reduction treatment of adhesive force on the second protection film, the force between the second protection film and a second metal film; forming the second metal film on the first metal film and the first protection film; and removing a part of the second metal film.

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
   This application is based on Japanese Patent Applications No. 2005-10340 filed on Jan. 18, 2005, and No. 2005-300299 filed on Oct. 14, 2005, the disclosures of which are incorporated herein by reference. 
   FIELD OF THE INVENTION 
   The present invention relates to a method for manufacturing a pressure sensor. 
   BACKGROUND OF THE INVENTION 
   There has been proposed a structure in which a sensor element comprising semiconductor and a bonding portion that is electrically connected to a pad provided on the sensor element. The sensor element and the bonding portion are covered with a metal diaphragm into which oil is filled. This is disclosed in, for example, JP-A-H7-243926, which corresponds to U.S. Pat. No. 5,595,939. 
   In a pressure sensor having such a structure, once a fluid as a pressure medium is introduced through a pressure introduction hole provided in the pressure sensor, pressure of the fluid is applied to the sensor element via a metal diaphragm and oil. Therefore, the diaphragm provided in the sensor element is distorted, and a gage resistance formed in the diaphragm is thus transformed by compression stress or tensile stress. Thereby, a detection signal in accordance with the pressure of the fluid is outputted from the sensor element. 
   In the case of the pressure sensor having such a configuration, there are problems that the number of components is increased because the metal diaphragm is necessary, and a structure of the pressure sensor is complicated because a structure for sealing the oil using the metal diaphragm is necessary. 
   On the contrary, a structure where the metal diaphragm is eliminated can be considered. However, when such a structure is used, when the pressure sensor is used in a situation where the sensor element comprising semiconductor is exposed to a corrosive liquid, for example, the pressure sensor is used for measurement of differential pressure of DPF that is an exhaust cleaning filer of diesel-powered automobiles or pressure measurement in an atmosphere within an engine room, a problem that corrosion occurs because material of a bonding pad is Al. 
   Therefore, the applicants previously proposed a structure in which a bonding portion is covered with a gel protection layer, and a surface of a bonding pad comprising Al is coated by an Au plating film. Thereby, contact probability between a corrosion medium and the Al as an object to be corroded is reduced. Consequently, durability against pad corrosion can be improved. This is disclosed in Japanese Patent Application No. 2004-562923. 
   However, even if the above structure is used, it has been newly found that since adhesion between the protection film comprising SiN formed in the periphery of the pad and the Au film is not good, the corrosion medium that has penetrated through the gel protection layer through an interface between the Au film and the protection film enters a pad side, and therefore, the corrosion medium causes erosion. 
   SUMMARY OF THE INVENTION 
   In view of the above-described problem, it is an object of the present invention to provide a manufacturing method of a pressure sensor with high sealing performance. 
   A method for manufacturing a pressure sensor includes the steps of: preparing a semiconductor substrate having a sensor element thereon, the sensor element generating a detection signal in accordance with pressure as a detection object; forming an insulation film having a contact hole on a surface of the semiconductor substrate, the contact hole connecting to the sensor element; forming a first metal film on a predetermined part of the insulation film, the first metal film electrically connecting to the sensor element through the contact hole; forming a first protection film on the first metal film and the insulation film to expose a pad region of the first metal film; forming a second protection film made of organic resin on the first metal film and the first protection film to expose the pad region of the first metal film and a part of the first protection film disposed around the pad region; performing reduction treatment of adhesive force on a surface of the second protection film after the step of forming the second protection film, the adhesive force being between the second protection film and a second metal film thereon; forming the second metal film on the pad region of the first metal film and the part of the first protection film disposed around the pad region after the step of performing the reduction treatment; and removing a part of the second metal film disposed on the surface of the second protection film. 
   In the above method, the part of the second metal film on the second protection film is easily removed; and therefore, the other part of the second metal film adheres tightly on the first metal film and the first protection film so that sealing performance between the second metal film and the first metal film or the first protection film becomes higher. Thus, the sealing performance therebetween is improved; and therefore, corrosion medium is prevented from penetrating through an interface between the second metal film and the first protection film. Accordingly, no erosion occurs at a pad side. 
   Preferably, the step of performing the reduction treatment provides to increase adhesive force between the second metal film and the pad region of the first metal film or the part of the first protection film disposed around the pad region. 
   Preferably, the step of performing the reduction treatment of adhesive force is a plasma processing in a gas atmosphere including CF 4  gas and argon gas or nitrogen gas. 
   Preferably, the first metal film has a surface protrusion, a height of which is equal to or smaller than 0.5 μm. More preferably, the first metal film includes a titanium film and an aluminum based film. The step of forming the first metal film includes steps of: forming the titanium film; and forming the aluminum based film on the titanium film. The height of the surface protrusion is reduced by orientation of the titanium film. More preferably, the first metal film includes an aluminum-silicon-copper alloy film, and the height of the surface protrusion is reduced by thermal stability of the aluminum-silicon-copper alloy film. 
   Further, a method for manufacturing a pressure sensor includes the steps of: preparing a semiconductor substrate having a sensor element thereon, the sensor element generating a detection signal in accordance with pressure as a detection object; forming an insulation film having a contact hole on a surface of the semiconductor substrate, the contact hole connecting to the sensor element; forming a first metal film on a predetermined part of the insulation film, the first metal film electrically connecting to the sensor element through the contact hole; forming a first protection film on the first metal film and the insulation film to expose a pad region of the first metal film; forming a second protection film made of organic resin on the first metal film and the first protection film to expose the pad region of the first metal film and a part of the first protection film disposed around the pad region; performing increase treatment of adhesive force on a surface of the pad region of the first metal film and on the part of the first protection film disposed around the pad region after the step of forming the second protection film, the adhesive force being between the first metal film or the first protection film and a second metal film thereon; forming the second metal film on the pad region of the first metal film and the part of the first protection film disposed around the pad region after the step of performing the increase treatment; and removing a part of the second metal film disposed on the surface of the second protection film. 
   In the above method, the part of the second metal film adheres tightly on the first metal film and the first protection film so that sealing performance between the second metal film and the first metal film or the first protection film becomes higher. Thus, the sealing performance therebetween is improved; and therefore, corrosion medium is prevented from penetrating through an interface between the second metal film and the first protection film. Accordingly, no erosion occurs at a pad side. 

   
     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. 1  is a cross sectional view showing a pressure sensor according to a first embodiment of the present invention; 
       FIG. 2  is a partially enlarged cross sectional view showing a connection portion between a sensor element and a bonding wire in the pressure sensor according to the first embodiment; 
       FIGS. 3A to 3G  are partially enlarged cross sectional views explaining a method for manufacturing the sensor according to the first embodiment; 
       FIGS. 4A and 4B  are partially enlarged cross sectional views showing a connection portion between a sensor element and a bonding wire in a pressure sensor according to a second embodiment of the present invention; 
       FIG. 5  is a partially enlarged cross sectional view showing the connection portion between the sensor element and the bonding wire in the pressure sensor according to the second embodiment; 
       FIG. 6  is a partially enlarged cross sectional view showing a connection portion between a sensor element and a bonding wire in a pressure sensor according to a third embodiment of the present invention; and 
       FIG. 7  is a partially enlarged cross sectional view showing a connection portion between a sensor element and a bonding wire in a pressure sensor according to a modification of the present invention. 
   

   DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
   First Embodiment 
   Hereinafter, a pressure sensor to which an embodiment of the invention is applied will be described.  FIG. 1  is a cross section drawing of a pressure sensor S 1  in the embodiment, and description is made according to the drawing. The pressure sensor S 1  is, for example, used for differential pressure measurement of DPF that is an exhaust cleaning filter of diesel-powered automobiles. 
   As shown in  FIG. 1 , a connector case  10  as a first case is produced by die forming of resin such as PPS (poly phenylene sulfide) or PBT (poly butylene terephthalate) and formed in an approximately cylindrical shape in the embodiment. A recess  11  is formed at one end portion (end portion at a lower side in  FIG. 1 ) of the connector case  10  as the resin case. 
   A sensor element  20  for pressure detection is set on a bottom of the recess  11 . 
   The sensor element  20  is an element of a semiconductor diaphragm type, in which a diaphragm as a pressure receiving surface is provided. Pressure received by the diaphragm is converted into an electric signal by a gage resistance formed on a surface of the diaphragm. Then, the electric signal is outputted as a sensor signal. 
   The sensor element  20  is integrated to a base  20   a  comprising glass by anode bonding and the like, and the base  20   a  is adhered to the bottom of the recess  11  so that thereby the sensor element  20  is mounted on the connector case  10 . 
   A plurality of metal rod terminals  12  for electrically connecting between the sensor element  20  and an external circuit are pierced through the connector case  10 . 
   In the embodiment, the terminal  12  comprises a material of brass that has been subjected to plating (for example, Ni plating), and held in the connector case  10  by integrally molding with the connector case  10  by insert molding. 
   An end portion at one end side (lower end side in  FIG. 1 ) of each of the terminals  12  is disposed in a manner of protruding from the bottom of the recess  11  in the periphery of the region where the sensor element  20  was mounted. On the other hand, an end portion at the other end side (upper end side in  FIG. 1 ) of each terminal  12  is exposed in an opening  16  at the other end side of the connector case  10 . 
   The one end portion of each terminal  12  protruding in the recess  11  and the sensor element  20  are wired and electrically connected by a bonding wire  13  of gold or aluminum. The electrical connection structure between the sensor element  20  and the bonding wire  13  described herein is a portion described in detail later. 
   A sealing agent  14  comprising silicone base resin and the like is provided in the recess  11 , which seals a gap between a root of the terminal  12  protruding into the recess  11  and the connector case  10 . 
   At a side of one surface  10   a  of the connector case  10  as a housing, a gel protection layer  15  is provided such that it covers roots of the sensor element  20 , bonding wires  13  and terminals  12 . 
   On the other hand, in  FIG. 1 , the other end portion (end portion at an upper side in  FIG. 1 ) side of the connector case  10  includes an opening  16 , and the opening  16  is formed as a connector portion for electrically connecting the other end side of the terminals  12  to the external circuit (ECU of the vehicle and the like) via an external wiring member (not shown) such as a wiring harness. 
   That is, the other end side of each terminal  12  exposed in the opening  16  can be electrically connected to the outside by the connector portion. Thus, a signal is transmitted between the sensor element  20  and the outside via the bonding wires  13  and the terminals  12 . 
   As shown in  FIG. 1 , a housing  30  as a second case is assembled to the one end portion of the connector case  10 . Specifically, it is configured that a reception recess  30   a  as an accommodation concavity is formed in the housing  30 , and the one end side of the connector case  10  is inserted into the reception recess  30   a , thereby the housing  30  is assembled to the connector case  10 . 
   A casing  100  that is formed by integrally assembling the connector case  10  as the first case to the housing  30  as the second case is thus configured, and the sensor element  20  is provided in the casing  100 . 
   The housing  30  comprises a metal material containing, for example, aluminum (Al) as a main component, and has a pressure introduction hole  31  for introducing measurement pressure from a measurement object, and a screw portion  32  for fixing the pressure sensor S 1  to the measurement object. As described above, the measurement object includes the DPF that is the exhaust cleaning filter of the diesel-powered automobiles, and the measurement pressure includes differential pressure of the DPF. 
   Furthermore, the reception recess  30   a  in the housing  30  has one surface  30   b  facing an end face  10   a  of the connector case  10 . The connector case  10  is contacted to the one surface  30   b , thereby positioning of the connector case  10  is performed. 
   An annular groove (O-ring groove)  17  is formed on the end face  10   a  of the connector case  10  such that it encloses an outer edge of the pressure introduction hole  31 , and an O-ring  18  is set in the groove  17 , so that a fluid as a measurement object introduced from an interface between the end face  10   a  of the connector case  10  and the one surface  30   b  of the housing  30  does not leak. 
   As shown in  FIG. 1 , an end portion at a side of the reception recess  30   a  in the housing  30  is caulked to the one end portion of the connector case  10 , thereby a caulking portion  36  is formed, and as a result the housing  30  is fixed to and integrated with the connector case  10 . 
   In the pressure sensor S 1  configured by assembling the connection case  10  with the housing  30  in this way, once the fluid as the measurement object is introduced through the pressure introduction hole  31 , pressure of the fluid is applied to the sensor element  20 , bonding wires  13 , and terminals  12  via the gel protection layer  15 . 
   Next, an electrical connection structure between the sensor element  20  and the bonding wires  13  in the pressure sensor S 1  configured as above is described.  FIG. 2  is a diagram showing a section structure of an electrical connection portion between the sensor element  20  and the bonding wires  13 . 
   As shown in  FIG. 2 , an insulating film  22  comprising SiN is formed on a surface of the semiconductor chip  21  as a semiconductor substrate on which the sensor element  20  has been formed. An Al film  23  as a first metal film is formed on a surface of the insulating film  22 . The Al film  23 , which corresponds to a first metal film, is in a structure where it is electrically connected to a desired region of the sensor element  20  through a not-shown contact hole formed in the insulating film  22 . 
   An Au film  24  as a second metal film having corrosion resistance is formed on a surface of the Al film  23 . The Au film  24 , which corresponds to a second metal film in the invention, is formed on an exposed portion of the first protection film  25  formed on the surfaces of the insulating film  22  and the Al film  23 , that is, it is formed on a surface of a region as a pad in the Al film  23  and the periphery of the pad in the first protection film  25 . 
   Specifically, it is configured that while the second protection film  26  comprising polyimide is formed on the surface of the first protection film  25 , the second protection film  26  is not formed on the region as the pad in the Al film  23  and the periphery of the region and the Al film is exposed therein; and a structure where the Au film  24  is formed on the exposed region as the pad and the periphery thereof is made. Thus, in the configuration, an end portion of the Au film  24  approximately corresponds to an end portion of the second protection film  26 . 
   The Au film  24  has high adhesion force to the Al film  23  and the first protection film  25  comprising SiN by being subjected to treatment for improving the adhesion force as described later. 
   In the structure, the bonding wires  13  are bonded to the surface of the Au film  24 , and the bonding wires  13  are electrically bonded to the sensor element  20  formed on the semiconductor chip  21  via the Au film  24  and the Al film  23 . 
   Then, a manufacturing method of the pressure sensor S 1  in the embodiment is described. However, since a basic manufacturing method of the pressure sensor S 1  is same as in the conventional method, description herein is made on the electrical connection portion between the sensor element  20  and the bonding wires  13 , which is a feature of the invention. 
     FIG. 3  shows a manufacturing process of the electrical connection portion between the sensor element  20  and the bonding wire  13  as shown in  FIG. 2 . 
   First, the sensor element  20  such as a gage resistance is formed on the semiconductor chip  21  in a conventionally known method, and then a diaphragm is formed in a method such as electrochemical etching. Then, as shown in  FIG. 3A , the insulating film  22  is formed, and then as shown in  FIG. 3B , the Al film  23  that is the first metal film is formed by deposition and the like, and then patterned to be left in a desired position. 
   Subsequently, as shown in  FIG. 3C , the first protection film  25  comprising a silicon nitride film or films of a silicon oxide film and a silicon nitride film stacked thereon is deposited on the surfaces of the Al film  23  and the insulating film  22 , and then a portion of the first protection film  25  formed over the region as the pad in the Al film  23  is removed by photo-etching and the like. Thus, the region as the pad in the Al film  23  is in an exposed condition. 
   Next, as shown in  FIG. 3D , the second protection film  26  comprising polyimide is deposited, and then a portion of the second protection film  26  formed on the region as the pad in the Al film  23  and a portion formed on the first protection film  25  situated on the periphery of the region are removed by photo-etching and the like. Thus, the region as the pad in the Al film  23  and the first protection film  25  situated on the periphery of the region are in an exposed condition. 
   Then, in parallel-plate plasma processing apparatus, while it is not shown, plasma processing is performed in a condition of a gas atmosphere comprising argon (Ar) or nitrogen (N 2 ) and fluorocarbon (CF 4 ). Thus, as shown in  FIG. 3E , adhesion force of the Al film  23 , first protection film  25  and second protection film  26 , which are bases of the Au film  24  formed later, can be changed. 
   Specifically, in the Al film  23  and the first protection film  25 , adhesion force to the Au film  24  is secured; and in the second protection film  26 , adhesion force to the Au film  24  is reduced. The mechanism of such change of adhesion force has not been clarified, however, it is considered that in the second protection film  26  comprising polyimide, since carbon in the second protection film  26  is bonded with fluorine in the atmosphere and into a stable state, adhesion force to the Au film is not obtained when the Au film  24  is formed thereon. In the Al film  23  and the first protection film  25  comprising SiN, it is considered that an oxide film that is naturally formed on a surface is removed by vapor treatment, and fluorine is remained in the portion from which the oxide has been removed, and the fluorine acts to improve adhesion force when the Au film  24  is formed. In particular, it is considered that when the Au film  24  is formed, Au is moved into Al by an effect of the remained fluorine, which results in metal-to-metal bonding, consequently adhesion force is improved. 
   If the plasma processing apparatus herein is not only used in this process, but also used continuously from a process before the process, and respective processes are continuously performed in the plasma processing apparatus, air exposure of the semiconductor chip  21  can be prevented, therefore particles and the like contained in the air can be prevented from being adhered to the surface of the semiconductor chip. 
   Subsequently, as shown in  FIG. 3F , the Au film  24  is deposited on the surfaces of the Al film  23 , first protection film  25  and second protection film  26  in a sputter process or a vacuum-deposition process. Then, a fluid having original pressure of 1 MPa or more such as water is contacted to the surface of the semiconductor chip  21 , thereby as shown in  FIG. 3G , a portion of the Au film  24  formed on the surface of the second protection film  26  having weak adhesion force is separated, consequently the Au film is remained only on the surfaces of the Al film  23  and the first protection film  25 . 
   Then, the bonding wire  13  is bonded to the surface of the Au film  24 ; as a result the electrical connection structure as shown in  FIG. 2  is completed. 
   According to the manufacturing method of the pressure sensor S 1  of the embodiment described hereinbefore, treatment of changing adhesion force of the Al film  23 , the first protection film  25  and the second protection film  26  as the bases of the Au film  24  is performed. Therefore, the Au film  24  on the second protection film  26  of which the adhesion force is reduced can be separated, consequently the Au film  24  can be formed only on the surfaces of the Al film  23  and the first protection film  25  of which the adhesion force is secured. 
   Moreover, the Al film  23  and the first protection film  25  are in a condition that they are bonded with the Au film  24  with high adhesion force by the treatment of changing adhesion force. Therefore, entering of corrosion medium into a pad side through an interface between the Au film  24  and the first protection film  25  can be prevented; consequently corrosion of the pad can be prevented. 
   Second Embodiment 
   A second embodiment of the invention is described. While the case that the Al film  23  is used as the first metal film was described in the first embodiment, when the Al film  23  is used, self diffusion of atoms may occur at grain boundaries due to heat treatment after forming the Al film  23 , which sometimes causes gathering of the atoms in the surface of the Al film  23  and formation of a protrusion called hillock (hereinafter, referred to as hillock)  23   a . Height of the hillock  23   a  as a surface protrusion of the aluminum film  23  sometimes reaches 2 μm to 3 μm. 
   When the height of the hillock  23   a  is increased, a problem may occur as shown in a cross section drawing of a connection structure shown in  FIG. 4A : the hillock  23   a  reduces covering performance of a film including the Au film that is the second metal film, therefore the Al film  23  can not be protected from a corrosive atmosphere in the corrosive atmosphere, consequently a portion that has not been sufficiently covered is corroded, for example, as shown in a cross section diagram of the connection structure shown in  FIG. 4B . The method according to the second embodiment is to prevent the problem. 
     FIG. 5  is a cross section diagram of an electrical connection structure in a bonding portion between a sensor element and a bonding wire in a pressure sensor of the embodiment. The embodiment is an embodiment in which the electrical connection structure in the bonding portion is changed from that in the first embodiment, but a basic structure of the pressure sensor is same as in the first embodiment, therefore only different portions from the first embodiment are described. 
   As shown in  FIG. 5 , a Ti film  23   b  is formed on the surface of the insulating film  22 , then the Al film  23  is formed on the Ti film  23   b , thereby the first metal film is configured. 
   When the Ti film  23   b  is used in this way, since Ti has high orientation, orientation of the Al film  23  formed thereon can be improved compared with the case that the Al film is directly formed on the insulating film  22 , consequently the Al film can be thermally stabilized. Therefore, the self diffusion of Al due to heat treatment is hard to occur; consequently height of the hillock  23   a  becomes less than 0.5 μm. 
   When the height of the hillock  23   a  is made to be less than 0.5 μm, even if the Au film  24  that is the second metal film is formed using typical sputter apparatus, covering can be sufficiently made even over the periphery of the hillock  23   a . Therefore, the Al film  23  can be prevented from being corroded even if it is exposed to the corrosive atmosphere. 
   Third Embodiment 
   A third embodiment of the invention is described. The invention is to reduce the height of the hillock  23   a  formed in the Al film  23 , as in the case of the second embodiment. 
     FIG. 6  is a cross section drawing of an electrical connection structure in a bonding portion between a sensor element and a bonding wire in a pressure sensor of the embodiment. The embodiment is an embodiment in which the electrical connection structure in the bonding portion is changed from that in the first embodiment, but a basic structure of the pressure sensor is same as in the first embodiment, therefore only different portions from the first embodiment are described. 
   In a connection structure shown in  FIG. 6 , the Al film  23  is changed from a pure Al film to an alloy film containing Al as a main component such as Al—Si—Cu. Specifically, the Al film  23  is configured by using an alloy of Al—Si—Cu containing Al as a main component, Si of 0.01% to 4% (for example, 0.4%), and Cu of 0.01% to 5% (for example, 1%). 
   Such an Al alloy is thermally stable compared with pure Al. Therefore, the Al film  23  is configured by using the Al alloy, thereby the height of the hillock  23   a  can be decreased, and consequently the height of the hillock  23   a  is less than 0.5 μm. 
   When the height of the hillock  23   a  is less than 0.5 μm, even if the Au film  24  that is the second metal film is formed using typical sputter apparatus, covering can be sufficiently made even over the periphery of the hillock  23   a . Therefore, the Al film  23  can be prevented from being corroded even if it is exposed to the corrosive atmosphere. 
   (Modifications) 
   In the embodiments, description was made on the case using the electrical connection structure in which the Au film  24  is formed on the surface of the Al film  23 . However, such a structure was merely shown as an example, and other structures can be also used. 
   For example, as shown in  FIG. 7 , a configuration where an Al film  27 , a Ti film  28  and a Ni film  29  are disposed on the surface of the Al film  23 , and then Au film  24  is disposed thereon, that is, a configuration where the second metal film is configured by a stacked film of many types of metals can be also used. Alternatively, a configuration where only the Ti film  28  is interposed between the Al film  23  and the Au film  24  can be also used. 
   While the invention was described by giving the metal containing Al as the main component for the first metal film as an example in each of the embodiments, the invention can be also applied to the case of using other metals, for example, a metal containing Cu as the main component. In the first and second embodiments, at least part of the first metal film is configured by the Al film  23 . The Al film  23  may be not only the film comprising pure Al, but also a film comprising an Al alloy containing Al as the main component. 
   Moreover, the described materials of the first protection film  25  and the second protection film  26  were merely shown as an example, and the first protection film  25  may not necessarily comprise SiN, and the second protection film  26  may not necessarily comprise polyimide. The point is that in the treatment for changing the adhesion force, the first protection film  25  may comprise a material of which the adhesion force is improved, and the second protection film  26  can comprise a material of which the adhesion force is reduced. In this case, in a point of view of the material of which the adhesion force is improved, the first protection film  25  may comprise any material, if the material is a material of which the adhesion force is not reduced even if fluorine is remained after the treatment, that is, a material containing no carbon. Moreover, the second protection film  26  may comprise any material of which the adhesion force is reduced, such as an organic resin material. 
   While the invention has been described with reference to preferred embodiments thereof, it is to be understood that the invention is not limited to the preferred embodiments and constructions. The invention is intended to cover various modification and equivalent arrangements. In addition, while the various combinations and configurations, which are preferred, other combinations and configurations, including more, less or only a single element, are also within the spirit and scope of the invention.