Patent Publication Number: US-8114700-B2

Title: Method of fabricating an ultra-small condenser microphone

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     The present application is a Divisional of U.S. patent application Ser. No. 12/265,431, filed on Nov. 5, 2008, now U.S. Pat. No. 7,855,095 and claims the benefit of patent application number 2007-297687, filed in Japan on Nov. 16, 2007, the entire contents of each of which is incorporated herein by reference. 
    
    
     BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to a method of fabricating an ultra-small condenser microphone using a semiconductor process technology. 
     2. Description of the Related Art 
     An electret condenser microphone (ECM) is an acousto-electric transducer wherein an electret film having a semi-permanent electric polarity is formed by electretizing, and a DC (direct current) bias voltage is not needed to be applied to both electrodes of a condenser. An electret film is formed by electrically charging a dielectric film and fixing charges in the dielectric film so that a potential difference is generated between both electrodes by an electric field occurred by the fixed charges. Hereinafter, to fix charges in the dielectric film is referred to as ‘electretization’ and an amount of fixed charges is referred to as ‘an amount of deposited charges’. 
       FIG. 12  is a schematic cross-sectional view showing a principal part of a conventional electretization apparatus used for electretizing a dielectric film by injecting charges to form an electret film. In the apparatus shown in  FIG. 12 , a dielectric film is electretized by causing a corona discharge using a needle electrode. As shown in  FIG. 12 , a dielectric film  4  to be electretized is disposed on a ground electrode (a metal tray)  5 . In this state, a high voltage power source  7  applies a DC voltage to a needle electrode  6  arranged opposite to the ground electrode  5 , thereby causing the DC corona discharge between the needle electrodes  6  and the ground electrode  5 . Electretization is performed by charging and fixing ions resulting from the DC corona discharge in the dielectric film  4  (for example, see Japanese Patent Laid-Open Publication No. 2007-294858). 
     In resent years, ultra-small condenser microphones have been fabricated by processing silicon substrates utilizing a micro-processing technology for semiconductor integrated circuits. Such ultra-small condenser microphones have received attention as micro-electro-mechanical system (MEMS) microphones (hereinafter referred to as MEMS microphones). MEMS microphones are incorporatedly formed on a silicon substrate using a semiconductor process technology, so that it is impossible that a dielectric film alone is taken out from the microphones and separately electretized. Therefore, the electretization apparatus shown in  FIG. 12  can not be adopted. 
     The above mentioned Japanese Patent Laid-Open Publication No. 2007-294858 discloses that a dielectric film is electretized in a state that a MEMS microphone chip formed by micro-processing a silicon wafer is mounted on a substrate for packaging or in a state of an individual MEMS microphone chip which is separated by cutting a semiconductor substrate. In this technology, a dielectric film provided in a MEMS microphone chip is electretized by applying a corona discharge at least a time to a single or several MEMS microphone chips simultaneously by a needle electrode or a wire electrode. 
       FIG. 1  is a cross-sectional view showing a structure of a MEMS microphone chip fabricated by processing a silicon wafer using a semiconductor process technology. As shown in  FIG. 1 , a MEMS microphone chip  43  has a base  34  made of a silicon wafer (silicon diaphragm) having an opening in the center part. The opening in the base  34  is blocked by a vibration film  33 . An inorganic dielectric film  32  which is an object to be electretized is formed on a surface of the vibration film  33  opposite the surface in contact with the base  34  located. A fixed electrode  31  supported by a spacer  37  is arranged to be opposed to the inorganic dielectric film  32 . The fixed electrode  31  has a plurality of acoustic holes  35  (openings for transmitting an acoustic wave to the vibration film  33 ). An air gap  36  is provided between the inorganic dielectric film  32  and the fixed electrode  31 . The air gap  36  is formed by etching and removing a sacrificial layer embedded that area in a fabricating process of the silicon wafer. In this structure, the vibration film  33  functions as one of electrodes of a condenser and the fixed electrode  31  functions as the other one of electrodes of the condenser. The silicon wafer supports only the outskirts of the vibration film  33  and the surface of the vibration film  33  is exposed from the opening of the silicon wafer. 
       FIG. 2  is a plan view showing the MEMS microphone chip  43  in  FIG. 1 .  FIG. 1  is a cross-sectional view taken along A-A line in  FIG. 2 . As shown in  FIG. 2 , pads  40 ,  41  and  42  are formed on the surface having the inorganic dielectric film  32  in the base  34 . The pad  40  is electrically connected to the fixed electrode  31  (not shown). The pad  41  is connected to the vibration film  33  by an interconnection passed through the spacer  37 , and the pad  42  is electrically connected to the silicon wafer  34 . The pads  40 ,  41  and  42  are utilized to make contact with probe pins in inspections and are utilized for wire bonding in assembling. 
     SUMMARY OF THE INVENTION 
     In the above-discussed MEMS microphone, a dielectric film is electretized in a state that a MEMS microphone chip is mounted on the substrate for packaging or an individual MEMS microphone chip is diced. Thus, in the conventional electretization process, a MEMS microphone chip which is mounted on the substrate for packaging or diced (hereinafter referred to as a partially fabricated MEMS microphone) is transferred one by one from a supply tray and placed onto a position for processing and electretized with being held at that position. The electretized partially fabricated MEMS microphone is then transferred and held at a position to inspect an amount of deposited charges. After the inspection, the MEMS microphones are classified by a defective or a non-defective based on the inspection result and loaded onto trays according to the classifications. 
     In the mounted MEMS microphone chip, wires electrically connecting the MEMS microphone chip  43  with the substrate for packaging and other elements are exposed. While, the individualized MEMS microphone chip is a compound body containing the frame-shaped base  34 , the air gap  36 , the fixed electrode  31  comprised of a thin film, and the vibration film  33  comprised of a thin film. Therefore, the partially fabricated MEMS microphone chips are extremely apt to be damaged by outer stress. For this reason, a collet and the like can absorb the limited portions of the partially fabricated MEMS microphone for transferring and can grip the limited portions thereof for holding as described above. It is necessary to enhance accuracy of positioning by image recognition and the like and to adjust a stress to be added within an extremely narrow range in order to absorb or grip the limited portions securely. 
     Under the circumstances, production equipments which are used in the electretization process or in the assembling process thereafter should have been provided with a delicate and complicated mechanism in order to transfer and hold partially fabricated MEMS microphones having an extremely vulnerable structure in the past. In the production equipments, a difficulty in processing in high speed results in low throughput and gives limitations to enhance productivity. Therefore, it is difficult to utilize handlers and the like generally used for an assembly process of semiconductor integrated circuits, thereby increasing the equipment cost. 
     The present invention is suggested in consideration of the above discussed problems and has an object to provide a fabricating method of an ultra-small condenser microphone wherein a electretization process of a dielectric film of a MEMS microphone chip can be realized with low-cost and a simple equipment and productivity can be enhanced. 
     The present invention adopts following technical methods in order to solve the above-discussed problems and accomplish the objects. Firstly, the present invention is assumed that a method of fabricating an ultra-small condenser microphone including a vibration film of which an outer edge is supported by a base formed of a semiconductor substrate, a dielectric film formed on the vibration film and a fixed electrode arranged above the dielectric film via a space. Then, in a method of fabricating the ultra-small condenser microphone relating to the present invention, a sheet is stuck on an opposite surface of a surface whereon the vibration film is formed in the semiconductor substrate wherein a plurality of the ultra-small condenser microphones is formed. Next, the semiconductor substrate stuck on the sheet is disposed opposed to a discharge electrode. The dielectric film provided in the ultra-small condenser microphone on the disposed semiconductor substrate is electretized. The electretization for fixing charges in the dielectric film is realized by irradiating the dielectric film placed between the fixed electrode and the vibration film with ions resulting from a corona discharge of the discharge electrode in a state that a predetermined potential difference is given between the fixed electrode and the vibration film. The electretization for the dielectric film provided in the plurality of ultra-small condenser microphones on the semiconductor substrate is successively performed by relatively moving the semiconductor substrate and the discharge electrode. 
     In accordance with the method of an ultra-small condenser microphone, a plurality of ultra-small condenser microphones formed on a single substrate is electretized in a state of a substrate with a simple structure and without damaging the ultra-small condenser microphones. 
     Further, in other method of fabricating an ultra-small condenser microphone relating to the present invention, a sheet is stuck on an opposite surface of a surface whereon the vibration film is formed in the semiconductor substrate wherein a plurality of the ultra-small condenser microphones is formed. Next, the semiconductor substrate stuck on the sheet is disposed opposed to a discharge electrode. The dielectric film provided in the ultra-small condenser microphone on the disposed semiconductor substrate is electretized. The electretization for fixing charges in the dielectric film is realized by irradiating the dielectric film placed between the fixed electrode and the vibration film with ions resulting from a corona discharge of the discharge electrode in a state that a predetermined potential difference is given between the fixed electrode and the vibration film. Further, an inspection to inspect an amount of charges deposited in the dielectric film provided in an ultra-small condenser microphone completed the electretization on the semiconductor substrate is performed in parallel with the electretization. Then, the electretization for the dielectric film provided in the plurality of the ultra-small condenser microphones on the semiconductor substrate and the inspection of an amount of deposited charges in the dielectric film completed the electretization on the semiconductor substrate are successively performed by relatively moving the semiconductor substrate and the discharge electrode. 
     In accordance with the method of an ultra-small condenser microphone, a plurality of ultra-small condenser microphones formed on a single substrate is electretized in a state of a substrate and the amount of deposited charges is inspected in a state of a substrate with a simple structure and without damaging the ultra-small condenser microphones. Moreover, the inspection of the amount of deposited charges is performed in parallel with electretization, thereby productivity can be further enhanced. 
     Preferably, the semiconductor substrate and the discharge electrode move relatively in a time interval of that an amount of charges deposited in the dielectric film reaches a predetermined amount, and the above-described electretization is performed continuously. Also, preferably, the semiconductor substrate and the discharge electrode move relatively in a time interval required for the inspection of the amount of deposited charges, and the above-described electretization is performed continuously. 
     In the above inspection of the amount of deposited charges, a structure of measuring a capacitance of a condenser composed of the fixed electrode and the vibration film in every potential difference by changing the potential difference given between the fixed electrode and the vibration film in one direction within a predetermined range is adopted. In such a case, a potential difference of which a decreasing rate of the capacitance relative to the increase in the potential difference is maximum and a potential difference of which an increasing rate of the capacitance relative to the increase in the potential difference is maximum are obtained based on the measured capacitance, and the amount of deposited charges is inspected based on an intermediate value of the obtained respective potential differences. Preferably, the capacitance is measured by reducing a variation of the potential difference given between the fixed electrode and the vibration film around the potential difference of which the decreasing rate of the capacitance relative to the increase in the potential difference is maximum and the potential difference of which the increasing rate of the capacitance relative to the increase in the potential difference is maximum, thereby accurately inspecting the amount of deposited charges. Moreover, in finishing giving the potential difference to measure the capacitance, it is preferable that the potential difference to measure the capacitance is finished to give after a potential difference corresponding to the intermediate value is given to the fixed electrode and the vibration film. Accordingly, the inspection of the amount of deposited charges can be finished as a steady-state wherein load is not given to the vibration film of the ultra-small condenser microphone. Moreover, in giving the potential difference to measure the capacitance, it is preferable that the same potential given to the fixed electrode is applied to the base supporting the outer edge of the vibration film so as to stably measure the capacitance. 
     Further, it is preferable that at least one or more chips are interposed between an ultra-small condenser microphone to be electretized and an ultra-small condenser microphone to be inspected the amount of deposited charges in parallel with the electretization on the semiconductor substrate. 
     Furthermore, a needle electrode of which a tip is opposed to the semiconductor substrate can be adopted to a discharge electrode. 
     Well, in other method of fabricating an ultra-small condenser microphone relating to the present invention, a sheet is stuck on an opposite surface of a surface whereon the vibration film is formed in the semiconductor substrate wherein a plurality of the ultra-small condenser microphones is formed. Next, the semiconductor substrate stuck on the sheet is disposed opposed to a tip of a needle discharge electrode. The dielectric film provided in the ultra-small condenser microphone on the disposed semiconductor substrate is electretized. Here, electretization for fixing charges in the dielectric film is performed simultaneously to a plurality of the ultra-small condenser microphones which is all contained in an area where ions resulting from a corona discharge of the needle discharge electrode are to be reached on the semiconductor substrate. The electretization is performed by irradiating each dielectric film between each fixed electrode and each vibration film with the ions and fixing charges based on the ions in each dielectric film in a state that a predetermined potential difference is given between each fixed electrode and each vibration film. 
     Further, in other method of fabricating an ultra-small condenser microphone relating to the present invention, a sheet is stuck on an opposite surface of a surface whereon the vibration film is formed in the semiconductor substrate wherein a plurality of the ultra-small condenser microphones is formed. Next, the semiconductor substrate stuck on the sheet is disposed opposed to a linear discharge electrode. The dielectric film provided in the ultra-small condenser microphone on the disposed semiconductor substrate is electretized. Here, electretization for fixing charges in the dielectric film is performed simultaneously to a plurality of the ultra-small condenser microphones which is all contained in an area where ions resulting from the corona discharge of the linear discharge electrode are to be reached on the semiconductor substrate. The electretization is performed by irradiating each dielectric film between each fixed electrode and each vibration film with the ions and fixing charges based on the ions in each dielectric film in a state that a predetermined potential difference is given between each fixed electrode and each vibration film. 
     In the above-discussed method of fabricating an ultra-small condenser microphone, the ultra-small condenser microphone to be electretized is selected by giving the potential difference between the fixed electrode and the vibration film and in parallel a time to electretize is adjusted with a time to give the potential difference. Further, a ground potential may be given to the fixed electrode and the vibration film before giving the potential difference between the fixed electrode and the vibration film so as to remove static electricity. 
     Further, the predetermined potential difference can be given between the fixed electrode and the vibration film through probe pins, for instance. Then, it is preferable that a probe card supporting the probe pins is provided with a cover having conductivity connected to a ground potential through a high resistance. The cover has a function to prevent ion irradiation on the dielectric film provided in an ultra-small condenser microphone except for an ultra-small condenser microphone to be electretized. Preferably, frosted black coating is applied to a surface of the cover opposed to the semiconductor substrate. Additionally, it is preferable that the probe pins are set through portions except for above the fixed electrode provided in an ultra-small condenser microphone to be electretized. In the same manner, the inspection of the amount of deposited charges can be performed through probe pins. 
     In order to prevent oxidation of the probe pins, a structure that an ion-shielding shutter having conductivity connected to a ground potential opens/closes a path of ions resulting from the discharge electrode to the semiconductor substrate can be adopted. Besides, a structure as below as well prevents oxidation of the probe pins in a case that a plurality of the ultra-small condenser microphones is electretized simultaneously. Namely, using a relay and the like for switching a conducting state and a non-conducting state interposed in each electrical path to apply a voltage in order to give the predetermined potential difference to the fixed electrode and the vibration film provided in each of the ultra-small condenser microphones to be simultaneously electretized, a part of the electrical paths is to be the non-conducting state in order to stop giving the potential difference selectively. The relay and the like may be arranged on a probe card or on a board to which the probe card is connected. 
     Additionally, the method of fabricating the ultra-small condenser microphone as discussed above is accomplished by arranging the discharge electrode in a wafer prober and the semiconductor substrate wherein the plurality of the ultra-small condenser microphones is formed is disposed on a stage provided in the wafer prober. In this case, preferably, a distance between the disposed semiconductor substrate and the discharge electrode is adjustable. The distance can be adjusted based on a scale which specifies a position of the discharge electrode. 
     In the above-discussed method of fabricating the ultra-small condenser microphone, it is applicable in a state that the semiconductor substrate stuck on the sheet is separated into pieces on the sheet and the separated semiconductor substrates are held together by the sheet. 
     In accordance with the present invention, each ultra-small condenser microphone is successively electretized in a short time and productivity thereof can be significantly enhanced as MEMS microphone chips are electretized in a state of a substrate. On the other hand, although a handler equipment on the market and the like which are frequently used for inspections of semiconductor integrated circuit devices in a state that an ultra-small condenser microphone is diced cannot be utilized as the diced ultra-small condenser microphones are damaged in the prior art, a wafer prober on the market frequently used for inspections of a state of a substrate of semiconductor integrated circuit devices can be used in accordance with the present invention. Therefore, an equipment cost can be reduced. 
     The foregoing and other objects, features, aspects and advantages of the present invention will become more apparent from the following detailed description of the present invention when taken in conjunction with the accompanying drawings. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a cross-sectional view showing a MEMS microphone chip. 
         FIG. 2  is a plan view showing a MEMS microphone chip. 
         FIG. 3  is a cross-sectional view showing a principal part of an electretization apparatus that relates to the first embodiment of the present invention. 
         FIG. 4  is a cross-sectional view showing a principal part of a modification of an electretization apparatus that relates to the first embodiment of the present invention. 
         FIG. 5  is a cross-sectional view showing a principal part showing a state that a separated semiconductor substrate is measured in an electretization apparatus relating to the first embodiment of the present invention. 
         FIG. 6  is a cross-sectional view showing a principal part of an electretization apparatus that relates to the second embodiment of the present invention. 
         FIG. 7  is a graphical representation showing a capacitance obtained by an inspection of an amount of deposited charges that relates to the second embodiment of the present invention. 
         FIG. 8  is a plan view showing an entire semiconductor substrate wherein a plurality of MEMS microphone chips is formed. 
         FIG. 9  is an enlarged view showing a principal part of an electretization apparatus that relates to the third embodiment of the present invention. 
         FIGS. 10A and 10B  are a view showing a structure of a principal part of an electretization apparatus that relates to the fourth embodiment of the present invention. 
         FIG. 11  is a schematic view showing a positional relationship between probe pins and a MEMS microphone chip on a semiconductor substrate. 
         FIG. 12  is a cross-sectional view showing a principal part of a conventional electretization apparatus. 
     
    
    
     DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS 
     Embodiments of the present invention will be described in detail hereinafter with reference to the drawings. In the following embodiments, the present invention is embodied in an electretization process in a fabricating process of an ultra-small condenser microphone (a MEMS microphone) as shown in  FIGS. 1 and 2 . 
     In an electretization process in a fabricating process of an ultra-small condenser microphone in accordance with the present invention, an electrical potential is applied to each electrode of a MEMS microphone chip which is an object to be electretized and a corona discharge is generated above a fixed electrode in a state that a plurality of MEMS microphone chips is incorporatedly formed on a same substrate. Ions resulting from the corona discharge reach a dielectric film through acoustic holes on the fixed electrode, and the dielectric film is electretized. In the meantime, an inspection to measure an amount of deposited charges of other MEMS microphone chips may be performed in parallel with electretization of a MEMS microphone chip in an electretization process. Further, a plurality of MEMS microphone chips is irradiated by ions resulting from a corona discharge so that a plurality of MEMS microphone chips can be simultaneously electretized. In such a case, in parallel, the amount of deposited charges of other plurality of MEMS microphone chips, which are on the same substrate but are different from a plurality of MEMS microphone chips irradiated by ions, may be measured. Embodiments of a fabricating method of an ultra-small condenser microphone in the present invention as described above will be discussed in detail hereinafter with reference to the drawings. 
     First Embodiment 
       FIG. 3  is a schematic cross-sectional view showing a principal part of a structure of an electretization apparatus used for an electretization process of an ultra-small condenser microphone in accordance with a first embodiment of the present invention. In the electretization apparatus in  FIG. 3 , a plurality of ultra-small condenser microphones (MEMS microphones), which is formed on a same semiconductor substrate, in a state of a substrate not being diced is electretized. The electretization to a MEMS microphone chip is performed by irradiating a single MEMS microphone chip formed on the semiconductor substrate with ions resulting from a corona discharge of a single needle electrode in this apparatus. 
     As shown in  FIG. 3 , the electretization apparatus in this embodiment is provided with a stage  81  on which a semiconductor substrate having a plurality of MEMS microphone chips  43  (only MEMS microphone chips  43   a ,  43   b  and  43   c  are shown in  FIG. 3 ) is disposed. The stage  81  has a structure of being movable in horizontal and up-and-down directions. In an opposite position to the stage  81 , a needle electrode (a discharge electrode)  51  is arranged. A high voltage power source  53  is connected to the needle electrode  51  so as to cause a corona discharge. The needle electrode  51  has a structure of being movable in a perpendicular direction so as to adjust the height from the semiconductor substrate disposed on the stage  81 . A probe card  75  having probe pins  70  and  71  which are arranged according to an arrangement of pads  40  and  41  formed on the MEMS microphone chip  43  is fixed between the stage  81  and the needle electrode  51 . The probe card  75  has an opening as a path to enable ions resulting from the corona discharge to reach the semiconductor substrate disposed on the stage  81 . The probe pin  71  is connected to a ground potential. The probe pins  70  and  71  are connected to a variable voltage power source  55  which gives a potential difference between the probe pins  70  and  71 . 
     This electretization apparatus performs electretization in the state that the semiconductor substrate on which the MEMS microphone chips which are objects to be electretized are formed is disposed on the stage  81  by repeating horizontal movements of the stage  81  and up-and-down movements thereof so that the probe pins  70  and  71  can contact pads  40  and  41  of the MEMS microphone chip  43 . That electretization apparatus, for instance, may be composed of a wafer prober utilized for such as an electric characteristic inspection in fabricating process of general semiconductor integrated circuits. In such a case, the needle electrode  51  is set above the probe card  75  being movable in perpendicular direction in the wafer prober. 
     The semiconductor substrate having a plurality of MEMS microphone chips  43  is disposed on the stage  81  in a state that an adhesive sheet  80  is stuck on the back surface (the surface opposite to the surface whereon vibration films  33  is formed) of the semiconductor substrate. Then, a vacuum adhesion system (not shown) provided in the stage  81  fixes the adhesive sheet  80 . Only one surface of the adhesive sheet  80  has adhesion. The adhesive sheet  80  is fixed with tension to a ring frame having a larger inside diameter than the external form of the semiconductor substrate, and the semiconductor substrate is stuck on the adhesive surface thereof. The adhesion of the adhesive sheet  80  can be reduced by irradiating with ultraviolet light thereto. The adhesive sheet  80  prevents the vibration film  33  and the like from being damaged by absorption when the semiconductor substrate is fixed to the stage  81 . The adhesive sheet  80  further prevents the vibration film  33  and the like from being damaged by absorption when a transferring system to transfer the semiconductor substrate through a substrate inlet and outlet of the electretization apparatus (the wafer prober) transfers the semiconductor substrate. 
     Further, the above-described electretization apparatus is provided with a cover  57  made of a metal having conductivity to irradiate only a single MEMS microphone chip (the MEMS microphone chip  43   a  in  FIG. 3 ) with ions resulting from a corona discharge of the needle electrode  51 . In this embodiment, the cover  57  is arranged along the opening of the probe card  75  and is connected to the ground potential through a resistor  58  having relatively high resistance of approximately tens of M ohm. The cover  57 , which is a frame shape with a square opening corresponding to the plane shape of the MEMS microphone chip  43   a , shields a path of ions moving to MEMS microphone chips (MEMS microphone chips  43   b  and  43   c  in  FIG. 3 ) adjacent to the MEMS microphone chip  43   a  to be electretized (hereinafter referred to as the chip  43   a  to be electretized) on the semiconductor substrate. In this embodiment, frosted black coating is applied to the back surface of the cover  57  in order not to block a needle inspection in an alignment of the wafer prober. 
     When the MEMS microphone chip  43  is electretized using the above-described electretization apparatus, the semiconductor substrate having a plurality of MEMS microphone chips and being stuck to the adhesive sheet  80  is disposed and held on the stage  81 . Then, the stage  81  moves horizontally so that a first chip  43   a  to be electretized locates directly below the opening of the cover  57 . At this time, the needle electrode  51  opposes to the chip  43   a  to be electretized through the opening of the cover  57 . Thereafter, the stage  81  rises so that the probe pins  70  and  71  contact the pads  40  and  41  of the chip  43   a  to be electretized. 
     When the probe pins  70  and  71  contact the pads  40  and  41 , the variable voltage power source  55  applies a voltage to give a potential difference between probe pins  70  and  71 . As described above, the pad  40  in contact with the probe pin  70  is electrically connected to a fixed electrode  31  (see  FIG. 1 ) and the pad  41  in contact with the probe pin  71  is electrically connected to a vibration film  33  (see  FIG. 1 ), thereby the variable voltage power source  55  gives the potential difference between the fixed electrode  31  and the vibration film  33 . 
     In such a state, the high voltage power source  53  applies a voltage to the needle electrode  51 , thereby causing a corona discharge thereto. In the electretization apparatus as shown in  FIG. 3 , a negative potential is applied to the needle electrodes  51  and the fixed electrode  31  and the ground potential is applied to the vibration film  33 . Thus, an inorganic dielectric film  32  is irradiated with negative ions resulting from the corona discharge through acoustic holes  35  (see  FIG. 1 ) provided in the fixed electrode  31  in the chip  43   a  to be electretized. As the electretization of the inorganic dielectric film  32  develops, the potential difference between the fixed electrode  31  and the inorganic dielectric film  32  decreases. When the potential difference between the fixed electrode  31  and the inorganic dielectric film  32  comes to zero in the end, negative ions resulting from the corona discharge do not reach the inorganic dielectric film  32 . In this manner, the inorganic dielectric film  32  in the chip  43   a  to be electretized is electretized by negative ions by utilizing the electretization apparatus in this embodiment. Further, in a structure wherein a negative potential is applied to the fixed electrode  31 , it may be suppressed that negative ions instantly reach the inorganic dielectric film  32  in the chip  43   a  to be electretized in the corona discharge and the inorganic dielectric film  32  receives charges to be deposited that greatly exceeds a desired amount of charges to be deposited. Preferably, each probe pin  70  and  71  is arranged to the probe card  75  so as not to cut across above the fixed electrode  31  in the chip  43   a  to be electretized in order not to prevent ions resulting from the corona discharge by the needle electrode  51  from reaching the inorganic dielectric film  32 . 
     The semiconductor substrate (the MEMS microphone chip  43 ) may be charged with static electricity before being electretized (for example, in a process that the adhesive sheet  80  is stuck on the back surface of the semiconductor substrate). In case of being charged with the static electricity, an amount of charges to be deposited may not reach a desired amount of charges. In order to remove the static electricity, when the probe pins  70  and  71  contact pads  40  and  41 , the variable voltage source  55  preferably first applies the ground potential (zero volt) to the fixed electrode  31  and the vibration film  33  before applying the potential difference as discussed above. 
     When the electretization of the inorganic dielectric film  32  is completed after enough time is passed to deposit the desired amount of charges, the stage  81  goes down and moves horizontally so that a MEMS microphone chip adjacent to the MEMS microphone chip completed the electretization on the semiconductor substrate locates directly below the opening of the cover  57 . Then, the stage  81  goes up and the MEMS microphone chip is electretized using the above-discussed manner. 
     As discussed above, a use of the electretization apparatus according to this embodiment allows successive process of electretization to a plurality of MEMS microphone chips incorporated on the semiconductor substrate. 
     When the MEMS microphone chip  43  is of a different type, the amount of charges to be deposited to the inorganic dielectric film  32  varies. Therefore, it may be considered that time to deposit the desired amount of charges is excessively shortened or made longer. Controlling the amount of charges to be deposited would be difficult when the time to deposit the desired amount of charges is excessively shortened, and throughput of electretization would be decreased when the time to deposit the desired amount of charges is excessively made longer. However, the time to deposit the desired amount of charges is made longer when the distance between the surface of the semiconductor substrate and the needle electrode  51  is made longer, and the time is shorten when the distance is shortened. Therefore, changing the distance between the surface of the semiconductor substrate and the needle electrode  51  depending on types of the MEMS microphone chips  43  realizes the electretization performed in a proper time. In the above-discussed electretization apparatus, for instance, the distance between the semiconductor substrate and the needle electrode  51  can be easily adjusted by setting a scale (marks) to specify the height from the surface of the semiconductor substrate disposed on the stage  81 . 
     On the other hand, in the period until the next MEMS microphone chip starts to be electretized after a completion of the electretization of a single MEMS microphone chip, the probe pins  70  and  71  are irradiated with ions resulting from the corona discharge and current flows to the probe pins  70  and  71 , and that may cause oxidation of tips of the probe pins  70  and  71 . In case that such oxidation is developed, it is not preferable since a contact resistance between the probe pins  70  and  71  and the pads  40  and  41  formed on the MEMS microphone chip would be increased. In order not to oxide the probe pins  70  and  71 , it is preferable that the corona discharge of the needle electrode  51  is stopped in the period until the next MEMS microphone chip starts to be electretized after the completion of the electretization of the single MEMS microphone chip. In the meantime, in case that a period to stop the corona discharge is short or it is difficult to stop the corona discharge by controlling the high voltage power source  53 , an ion shielding shutter  54  made of a metal having conductivity may be set to be movable between the opening of the cover  57  and the needle electrode  51  as shown in  FIG. 3 . The ion shielding shutter  54  is set to be movable on and out of paths of ions resulting from the corona discharge to reach the semiconductor substrate ( FIG. 3  shows the ion shielding shutter  54  interposed on the path of ions). In this structure, interposing the ion shielding shutter  54  between the opening of the cover  57  and the needle electrode  51  prevents the probe pins  70  and  71  from being irradiated with ions resulting from the corona discharge only in the period until the next MEMS microphone chip starts to be electretized after the completion of the electretization of the single MEMS microphone chip. When such ion shielding shutter  54  is set, it is possible to irradiate with ions after the potential difference is given between the fixed electrode  31  and the vibration film  33  under the conditions that ions irradiation is not performed. As shown in  FIG. 3 , the ground potential is supplied to the ion shielding shutter  54 . 
     In this embodiment, the number of chips to be electretized by the corona discharge at a time is limited to a single chip due to an application of a voltage to the probe pins  70  and  71  and the cover  57 . Thus, each of a plurality of the MEMS microphone chips formed on the semiconductor substrate can be electretized with the predetermined amount of deposited charges. In this embodiment, ions are entered to the inorganic dielectric film  32  between the fixed electrode  31  and the vibration film  33  using the potential difference given between the fixed electrode  31  and the vibration film  33 . Therefore, even if the cover  57  is not set, the inorganic dielectric film  32  between the fixed electrode  31  and the vibration film  33  given the potential difference is selectively electretized. The time to electretizing the inorganic dielectric film  32  can be adjusted by the time to give the potential difference. 
     Further, in the electretization process in a fabricating method of ultra-small condenser microphones in this embodiment, the adhesive sheet to strengthen is stuck on the back surface of the semiconductor substrate wherein a plurality of MEMS microphone chips is incorporatedly formed, and electretization is performed in the state of the substrate. Therefore, it is not necessary to transfer diced partially fabricated MEMS microphones one by one as practiced conventionally, and a plurality of ultra-small condenser microphones is successively electretized in a short time. Also, a wafer prober utilized generally in a fabricating process of semiconductor integrated circuits can be used for the electretization, thereby reducing an equipment cost. 
     Incidentally, the inorganic dielectric film  32  provided in the chip  43   a  to be electretized is irradiated with negative ions in the above-described electretization apparatus. However, even when the inorganic dielectric film  32  is electretized by irradiating with positive ions, a same property as the MEMS microphone is obtained. 
       FIG. 4  is a schematic cross-sectional view illustrating a principal part of an electretization apparatus utilized in an electretization process in case an inorganic dielectric film  32  provided in a chip  43   a  to be electretized is electretized by irradiating with positive ions. In  FIG. 4 , components having the same function as those in  FIG. 3  are referred to by the same reference numbers. 
     The electretization apparatus shown in  FIG. 4  differs from the electretization apparatus shown in  FIG. 3  in that a high voltage power source  54  applies a positive potential to a needle electrode  51 . Additionally, a polarity of a potential difference that a variable voltage power source  55  connected between probe pins  70  and  71  gives between the probe pins  70  and  71  is reversed as compared with the electretization apparatus shown in  FIG. 3 . That is, the positive potential is applied to the fixed electrode  31  and the ground potential is applied to the vibration film  33 . The inorganic dielectric film  32  provided in the chip  43   a  to be electretized can be electretized with positive ions by utilizing the same method as discussed above using this electretization apparatus. 
     Each of the MEMS microphone chips electretized in the electretization apparatuses shown in  FIG. 3  or  4  is diced into individual MEMS microphone chips after completing the electretization of all of the MEMS microphone chips on the semiconductor substrate. In other words, the semiconductor substrate wherein a plurality of MEMS microphone chips  43  is formed is separated. Such separation of the semiconductor substrate, however, may be performed in the period until the electretization described above is performed after the semiconductor substrate wherein a plurality of MEMS microphone chips  43  is formed is stuck to an adhesive sheet  80 . Even in such a case, the electretization apparatus discussed above can successively electretizes each MEMS microphone chip in a short time as the same manner as the state of the substrate. 
     The  FIG. 5  shows a process of an electretization of each MEMS microphone chip after a semiconductor substrate is separated. An electretization apparatus shown in  FIG. 5  is substantially the same as those shown in  FIG. 3  or  4 . In this case, the semiconductor substrate wherein a plurality of MEMS microphone chips  43  is formed is stuck to an adhesive sheet  80 , and the substrate is separated to a respective MEMS microphone chip  43 . At this time, the separated respective MEMS microphone chip  43  is stuck to the adhesive sheet  80 . Thus, although the respective MEMS microphone chip is separated to one by one, these are held together by the adhesive sheet  80  so that the external form of the semiconductor substrate is maintained. Therefore, MEMS microphone chips are successively electretized as discussed above by disposing and holding the adhesive sheet  80  sticking each of the separated MEMS microphone chip  43  on the stage  81 . 
     Second Embodiment 
       FIG. 6  is a schematic cross-sectional view showing a principal part of a structure of an electretization apparatus used for an electretization process of an ultra-small condenser microphone in accordance with a second embodiment of the present invention. The electretization apparatus shown in  FIG. 6  has a structure that a chip  43  to be electretized is electretized and simultaneously an amount of deposited charges of a MEMS microphone chip  44  which has been completed the electretization (hereinafter referred to as the electretized chip  44 ) on the same semiconductor substrate is inspected. In  FIG. 6 , a MEMS microphone chip  43  locates several pieces left-hand from a MEMS microphone chip  43   a  as a reference chip on the semiconductor substrate, that is not particularly limited, is regarded as the electretized chip  44 . In  FIG. 6 , components having the same function as those in  FIG. 3  are referred to by the same reference numbers. 
     As shown in  FIG. 6 , in the electretization apparatus of this embodiment, the probe card  75  contains probe pins  72 ,  73  and  74  in addition to the structure of the electretization apparatus in  FIG. 3  as discussed above. The probe pins  72 ,  73  and  74  are arranged corresponding to the positions of pads  40 ,  41  and  42  provided on the electretized chip  44 . Each of the probe pins  72 ,  73  and  74  is connected to an inspection apparatus  56  that measures an amount of deposited charges. In this embodiment, a built-in camera in the wafer prober checks a state of contacts of the probe pins  72 ,  73  and  74  in the probe card  75  with the pads  40 ,  41  and  42  so that openings are not provided adjacent to the probe pins  72 ,  73  and  74  in the probe card  75 . 
     When the probe pins  70  and  71  contact the pads  40  and  41  on the chip  43   a  to be electretized in this electretization apparatus, each of the probe pins  72 ,  73  and  74  contacts each of the pads  40 ,  41  and  42  on the electretized chip  44  corresponding to the chip  43   a  to be electretized. Therefore, the probe pin  72  is connected to the fixed electrode  31  of the electretized chip  44 , the probe pin  73  is connected to the vibration film  33  of the electretized chip  44  and the probe pin  74  is connected to the base  34  of the electretized chip  44 , respectively. 
     The inspection apparatus  56  measures capacitance of the condenser constituting the MEMS microphone in a state that a potential difference is given between the probe pins  72  and  73 . In other words, the capacitance of the condenser constituted by the fixed electrode  31 , the vibration film  33  and the inorganic dielectric film  32  is measured in the electretized chip  44  in the state that the potential difference is given between the fixed electrode  31  and the inorganic dielectric film  32 . The capacitance of the condenser is measured in a state that a different potential difference is successively given between the fixed electrode  31  and the vibration film  33 . The inspection apparatus  56  that measures the amount of deposited charges applies the equivalent potential to the probe pins  72  and  74  when the capacitance is inspected. Thus, the fixed electrode  31  and the base  34  (a silicon substrate) have the equivalent potential so as to stably measure the capacitance. For instance, when the potential applied to the fixed electrode  31  and the base  34  is fixed to the ground potential, a potential ranging from negative to positive is applied to the vibration film  33 . 
       FIG. 7  is a graphical representation showing the capacitance obtained by the inspection of measuring the amount of deposited charges using the electretization apparatus shown in  FIG. 6 . In  FIG. 7 , an abscissa axis corresponds to the potential difference given between the fixed electrode  31  and the vibration film  33 , and an ordinate axis corresponds to the capacitance. A potential difference of 0V in the abscissa axis in  FIG. 7  indicates that the fixed electrode  31  and the vibration film  33  have an equivalent potential. 
     As shown in  FIG. 7 , the capacitance measured by the inspection apparatus  56  varies according to the potential difference given between the fixed electrode  31  and the vibration film  33 . That originates from a generation of electrostatic absorption power between the fixed electrode  31  and the vibration film  33  caused by the potential difference given between the fixed electrode  31  and the vibration film  33 . In other words, the vibration film  33  deflects toward the fixed electrode  31  due to the absorption power so that a distance between the vibration film  33  and the fixed electrode  31  (an interelectrode distance) varies. As a result, the capacitance changes to a U-shape according to changes of the potential differences. 
     In case of a MEMS microphone chip having the inorganic dielectric film  32  which is not electretized, charges are not accumulated to the inorganic dielectric film  32 . Thus, when the absolute value of the potential difference between the vibration film  33  and the fixed electrode  31  is equal even though the polarity of the potential difference is opposite, the electrostatic absorption power generated between the vibration film  33  and the fixed electrode  31  becomes almost equal. Therefore, with respect to the MEMS microphone chip having the inorganic dielectric film  32  which is not electretized, when the dependency of the capacitance to the potential difference is obtained, the capacitance is minimized when the potential difference is 0V. 
     On the other hand, in case of a MEMS microphone chip having the inorganic dielectric film  32  which is electretized, charges are accumulated to the inorganic dielectric film  32 . Thus, when the polarity of the potential difference between the vibration film  33  and the fixed electrode  31  is opposite even though the absolute value thereof is equal, the electrostatic absorption power generated between the vibration film  33  and the fixed electrode  31  do not become equal. Therefore, the potential difference when the capacitance is minimized (hereinafter referred to as the minimal potential difference) varies according to the amount of deposited charges in the inorganic dielectric film  32  due to the electretization. As a result, the amount of deposited charges can be inspected (a propriety determination of the amount of deposited charges) by obtaining the minimal potential difference  90  and comparing the minimal potential difference  90  with a predetermined reference value. 
     In this embodiment, the minimal potential difference  90  is obtained in the following. Here, the right side of the minimal potential difference  90  corresponding to a bottom part of the U-shape in  FIG. 7  is expedientially referred to as a positive side and the left side thereof is referred to as a negative side. First, changing the potential difference given between the vibration film  33  and the fixed electrode  31 , the inspection apparatus  56  measures the capacitance every time the potential difference is applied so as to obtain a relationship between the capacitance and the potential difference shown in  FIG. 7 . Here, the minimum potential difference  93  that is the lowest limit in the measuring range and the maximum potential difference  94  that is the upper limit, and these are set in advance in the inspection apparatus  56 . Preferably, noises have to be excluded to accurately obtain the relationship between the capacitance and the potential difference. Here, the inspection apparatus  56  excludes an influence of noises by smoothing (leveling) the obtained data. 
     Next, the inspection apparatus  56  calculates the potential difference  91  (the differential minimum potential difference  91 ) of which the decreasing rate of the capacitance relative to the increase in potential difference is maximum, and the potential difference  92  (the differential maximum potential difference  92 ) of which the increasing rate of the capacitance relative to the increase in potential difference is maximum. For instance, the differential minimum potential difference  91  can be found as a potential difference of which a differential value in a curve in  FIG. 7  is minimized and the differential maximum potential difference  92  can be found as a potential difference of which a differential value in the curve in  FIG. 7  is maximized. Preferably, the inspection apparatus  56  measures the capacitance by reducing a variation of the potential difference around the differential minimum potential difference  91  and the differential maximum potential difference  92  in order to more accurately obtain these potential differences. 
     Successively, the inspection apparatus  56  calculates an average value (an intermediate value) of the differential minimum potential difference  91  and the differential maximum potential difference  92  as the minimal potential difference  90 . The inspection apparatus  56  may calculate the differential minimum potential difference  91  and the differential maximum potential difference  92  in parallel with obtaining the capacitance. Preferably, the inspection apparatus  56  finishes giving the potential difference between the vibration film  33  and the fixed electrode  31  after the vibration film  33  becomes a steady-state (a state that the distance between the fixed electrode  31  and the vibration film  33  is maximum and the capacitance is minimum) through the application of the obtained minimal potential difference  90 . 
     As described above, the electretized chip  44  to be inspected the amount of deposited charges is apart more than one chip from the chip  43   a  to be electretized, because it should be avoided that ions with which the chip  43   a  to be electretized is irradiated reach the electretized chip  44  and are re-deposited to the electretized chip  44  during the inspection. Although the electretization apparatus in this embodiment has the cover  57  so that only the chip  43   a  to be electretized is irradiated with ions in this structure as discussed above, the electretized chip  44  to be inspected the amount of deposited charges is selected in this manner thereby more accurately inspecting the amount of deposited charges. 
       FIG. 8  is a plan view showing the entire semiconductor substrate whereon a plurality of MEMS microphone chips is formed. Only two chips are shown in  FIG. 8 ; one is the chip  43   a  to be electretized and the other is the electretized chip  44  which is inspected the amount of deposited charges in parallel with electretization of the chip  43   a  to be electretized. An arrow in  FIG. 8  shows a direction of movements of the stage  81  to place next chips directly below each of the probe pins  70  to  74 , after the semiconductor substrate is disposed on the stage  81  in the electretization apparatus shown in  FIG. 6  and the electretization of the chip  43   a  to be electretized and the inspection of the amount of deposited charges of the electretized chip  44  are completed. In  FIG. 8 , the stage  81  moves one-chip distance to the left. Then, a MEMS microphone chip adjacent to (the right side in  FIG. 8 ) the MEMS microphone chip which is completed the electretization on the semiconductor substrate is a chip to be electretized next. Further, a MEMS microphone chip adjacent to (the right side in  FIG. 8 ) the MEMS microphone chip which is completed the inspection of the amount of deposited charges on the semiconductor substrate is a chip to be inspected the amount of deposited charges next. 
     In such a case, the stage  81  can move smoothly particularly when the time to deposit the required amount of charges in the inorganic dielectric film  32  in the electretization process and the time to inspect the amount of deposited charges are equal. Under this condition, the probe pins  70  and  71  set in the same probe card  75  can successively electretizes MEMS microphone chips, and in parallel the amount of deposited charges in electretized MEMS microphone chips can be successively inspected. 
     As discussed above, in accordance with this embodiment, the amount of charges deposited to the electretized MEMS microphone chips are inspected in parallel with the electretization of MEMS microphone chips to be electretized. Additionally, a wafer prober utilized generally in a fabricating process of semiconductor integrated circuits can be used for the electretization, thereby reducing an equipment cost. 
     Third Embodiment 
     In the first and the second embodiments, the electretization is performed by a corona discharge using a needle electrode. The electretization, however, can be performed using a linear electrode in stead of the needle electrode. Thus, the electretization that is performed by a corona discharge using a linear discharge electrode will be discussed in this embodiment. 
       FIG. 9  is a schematic enlarged view showing a principal part of an electretization apparatus used in an electretization process of an ultra-small condenser microphone in accordance with a third embodiment. The electretization apparatus in  FIG. 9  is provided with a wire electrode  52  (discharge electrode) which is a linear-formed in stead of the needle electrode  51  in the electretization apparatus described in the first embodiment. The wire electrode  52  simultaneously irradiates a plurality of MEMS microphone chips  43   a  to be electretized arranged linearly in one direction with ions resulting from a corona discharge on a semiconductor substrate whereon a plurality of MEMS microphone chips  43  is formed. The wire electrode  52  is connected to a negative electrode of a high voltage power source  53  to apply a voltage to generate a corona discharge. 
     The electretization apparatus in this embodiment has a probe card, although not shown in  FIG. 9 , wherein a plurality of sets of probe pins to apply the potential as discussed in the first embodiment is fixed. Each set of probe pins contacts the pads  40  and  41  provided in each of chips to be electretized which are simultaneously electretized by the corona discharge of the wire electrode  52  during electretization. The probe card has an opening which is a path of ions resulting from the corona discharge of the wire electrode  52  to reach the semiconductor substrate disposed on the stage  81 . The probe card is further provided with a cover made of a metal to shield a path of ions to reach MEMS microphone chips adjacent to a plurality of chips  43   a  to be electretized on the semiconductor substrate. In this case, the cover is a frame-shaped component having a rectangular-shaped opening corresponding to a plane shape of outer edges of a plurality of MEMS microphone chips to be simultaneously electretized. Other structures of the electretization apparatus in this embodiment are the same as those described in the first embodiment. 
     When the MEMS microphone chips  43  are electretized using the electretization apparatus, the semiconductor substrate having a plurality of MEMS microphone chips and being stuck to an adhesive sheet is disposed and held on the stage  81 . Then, the stage  81  moves horizontally so that the wire electrode  52  is located above a plurality of chips  43   a  to be electretized which are arranged in a line on the semiconductor substrate. After that, the stage  81  rises so that the pads  40  and  41  provided in each of the plurality of MEMS microphone chips  43   a  to be electretized contact the probe pins in the probe card. 
     When the probe pins contact the pads  40  and  41  on each of the chips  43   a  to be electretized, the potential difference discussed in the first embodiment is given between a fixed electrode  31  and a vibration film  33  on each of the chips  43   a  through the probe pins. Under the state, the high voltage power source  53  applies a voltage to the wire electrode  52 . Then, a corona discharge is generated from the wire electrode  52 . As a result, the plurality of chips  43   a  to be electretized arranged in a line on the semiconductor substrate is simultaneously electretized. 
     The semiconductor substrate is moved one after another, in a direction shown by an arrow in  FIG. 9 , every time the amount of charges needed by the inorganic dielectric film  32  is deposited or, in other words, every time electretization is completed so that a plurality of chips  43   a  to be electretized can be continuously electretized. 
     In this embodiment, electretization can be simultaneously performed to a plurality of MEMS microphone chips, thereby enhancing a throughput of the electretization as compared with the first embodiment. Therefore, in addition to the effects obtained in the first embodiment, an effect of increasing in productivity can be obtained. The electretization apparatus in this embodiment can employ a structure wherein the electretization is performed in parallel with an inspection of an amount of deposited charges as discussed in the second embodiment. In such a case, it is preferable that the amount of deposited charges is simultaneously inspected in terms of a plurality of the MEMS microphone chips which is simultaneously electretized. 
     Fourth Embodiment 
     In the third embodiment, the structure wherein a plurality of chips to be electretized is simultaneously electretized using a wire electrode has been discussed. However, even using a needle discharge electrode, a plurality of chips to be electretized can be simultaneously electretized. 
       FIG. 10A  is a schematic cross-sectional view showing a principal part of an electretization apparatus used in an electretization process of an ultra-small condenser microphone in accordance with a fourth embodiment, and  FIG. 10B  is a plan view showing a principal part thereof. As shown in  FIGS. 10  ( a ) and  10  ( b ), a structure of a cover  57  in the electretization apparatus in this embodiment differs from that in the electretization apparatus in the first embodiment. The cover  57  provided in the electretization apparatus in this embodiment has an opening in order that ions resulting from a corona discharge reach only a plurality of MEMS microphone chips, and all of the plurality of the MEMS microphone chips are contained within an area on the semiconductor substrate whereon irradiation with ions resulting from the corona discharge of a needle electrode  51  is performed (within a circle on the semiconductor substrate, a center of which is an intersection point of a perpendicular line pulled down from a tip of the needle electrode  51 , corresponding to an angle  95  to discharge ions irradiated by the needle electrode  51 ). As shown in  FIG. 10B , regarding a chip presented at the intersection point of the perpendicular line pulled down from the tip of the needle electrode  51  as the center chip, the cover  57  has a rectangular opening corresponding to the outer edges of nine pieces of the MEMS microphone chips which are distributed point symmetry with respect to the center chip within the area. 
     The electretization apparatus in this embodiment also has a probe card wherein a plurality of sets of probe pins to apply the potential as discussed in the first embodiment is fixed. Each set of probe pins contacts the pads  40  and  41  provided in each of chips  43   a  to be electretized which are simultaneously electretized by the corona discharge of the needle electrode  51  during electretization. The probe card has an opening which is a path of ions resulting from the corona discharge of the needle electrode  51  to reach the semiconductor substrate disposed on the stage  81 . Other structures of the electretization apparatus in this embodiment are the same as those described in the first embodiment. 
     When the MEMS microphone chips  43  are electretized using the electretization apparatus, the semiconductor substrate having a plurality of MEMS microphone chips and being stuck to an adhesive sheet is disposed and held on the stage  81 . Then, the stage  81  move horizontally so that the needle electrode  51  is located above a plurality of chips  43   a  to be simultaneously electretized (here, above the center chip of the nine pieces of the MEMS microphone chips). After that, the stage  81  rises so that the pads  40  and  41  provided in each of the plurality of MEMS microphone chips  43   a  to be electretized contact the probe pins in the probe card. 
     When the probe pins contact the pads  40  and  41  on each of the chips  43   a  to be electretized, the potential difference discussed in the first embodiment is given between a fixed electrode  31  and a vibration film  33  on each of the chips  43   a  through the probe pins. Under the state, the high voltage power source  53  applies a voltage to the needle electrode  51 . Then, a corona discharge is generated from the needle electrode  51 . As a result, the plurality of chips  43   a  to be electretized contained within the area on the semiconductor substrate whereon irradiation with ions resulting from the corona discharge of the needle electrode  51  is performed is simultaneously electretized. 
     The semiconductor substrate is moved one after another every time the amount of charges needed by the inorganic dielectric film  32  is deposited or, in other words, every time the electretization is completed so that a plurality of chips  43   a  to be electretized can be continuously electretized. 
     In this embodiment as well as the third embodiment, the electretization is simultaneously performed to a plurality of MEMS microphone chips, thereby enhancing a throughput of the electretization as compared with the first embodiment. Therefore, in addition to the effects obtained in the first embodiment, an effect of increasing in productivity can be obtained. The electretization apparatus in this embodiment can employ a structure wherein the electretization is performed in parallel with an inspection of an amount of deposited charges as discussed in the second embodiment. In such a case, it is preferable that the amount of deposited charges is simultaneously inspected in terms of a plurality of the MEMS microphone chips which is simultaneously electretized. 
     In this embodiment, the plurality of sets of probe pins is provided in order to simultaneously electretize a plurality of the MEMS microphone chips. Thus, in electretizing a MEMS microphone chip formed on the outer edge part on the semiconductor substrate, a few probe pins could be exposed to ions without contacting pads of the MEMS microphone chips. When current flows to such probe pins by irradiating with ions, tips of the probe pins could be oxidized as discussed above. Therefore, in this embodiment, in electretizing MEMS microphone chips formed on the outer edge part on the semiconductor substrate, the potential difference as described above is applied only to the probe pins which contact the MEMS microphone chips to be electretized. 
       FIG. 11  schematically shows a positional relationship between each set of probe pins and the MEMS microphone chip on the semiconductor substrate when MEMS microphone chips in the outer edge of the semiconductor substrate is electretized. Only one set of probe pins among nine sets of probe pins contact a chip  43   a  to be electretized on a semiconductor substrate  46  in  FIG. 11 . In  FIG. 11 , each set of probe pins is shown by illustrating external forms of the MEMS microphone chips with a rectangular shape drawn by broken lines whereto each set of probe pins applies a potential difference. 
     As shown in  FIG. 11 , this embodiment employs a structure wherein the potential difference as discussed above is not applied to the probe pins (the probe pins belong to a region  45  filled with slanted lines in  FIG. 11 ) which do not contact any MEMS microphone chip to be electretized. Here, interposing a relay and the like between each set of probe pins and a variable voltage power source  55  (see  FIG. 3 ), the relay and the like cut an electrical connection between each set of probe pins and the variable voltage power source  55 . The relay which is an object to be cut will be easily specified based on position coordinate information of the MEMS microphone chip to be electretized on the semiconductor substrate  46  outputted from the wafer prober. Further, the relay and the like, for instance, can be arranged on the probe card or on a board connected to the probe card. In accordance with this structure, the probe pins which are not on the MEMS microphone chip to be electretized can be in an electrically-open state, thereby preventing the tips of the probe pins from oxidizing. This method of preventing oxidization is as a matter of course applicable to the electretization apparatus discussed in the third embodiment. 
     In accordance with the present invention as discussed above, electretization of each of ultra-small condenser microphones is successively performed in short time, thereby increasing productivity of ultra-small condenser microphones significantly. Further, although a handler equipment on the market and the like which are frequently used for inspections of semiconductor integrated circuit devices in a state that an ultra-small condenser microphone is diced cannot be utilized as the diced ultra-small condenser microphones are damaged in the prior art, a wafer prober on the market frequently used for inspections of a state of a substrate of semiconductor integrated circuit devices can be used in accordance with the present invention. Therefore, an equipment cost can be reduced. 
     The embodiments discussed above do not restrict the technical range of the present invention and various modifications and applications can be available within the effective range of the present invention besides the embodiments discussed above. For example, although a dielectric film to be electretized is an inorganic dielectric film in the above description, the present invention is applicable in case that a dielectric film to be electretized is an organic dielectric film. 
     The present invention contributes to an increase in productivity of electretization of MEMS microphone chips fabricated using a fine processing technology and reduction of equipment costs, and is useful as fabricating method of ultra-small condenser microphones used for fabricating micro-miniature sized MEMS microphones equipped in mobile communication devices.