RELATED APPLICATIONS 
     This application claims priority under 35 U.S.C. §119 to Japanese Patent Application No. JP2008-027250 filed on Feb. 7, 2008, the entire content of which is hereby incorporated by reference. 
     TECHNICAL FIELD 
     The present invention relates to a method of evaluating an insulating film of a semiconductor device and a measurement circuit used in the evaluation. 
     BACKGROUND ART 
     A method of measuring chronological dielectric breakdown characteristics of an insulating film, as in constant voltage time-dependent dielectric breakdown (TDDB) measurement, constant current TDDB measurement and the like, is commonly employed for evaluating an insulating film used in a semiconductor device. 
     The constant voltage TDDB measurement is an evaluation method that utilizes a time-dependent breakdown phenomenon in which a constant voltage not larger than the breakdown voltage is applied to the insulating film long enough to cause the dielectric breakdown of the insulating film dependent to a voltage application time. Constant current TDDB measurement is an evaluation method that utilizes a time-dependent dielectric breakdown phenomenon in which a constant amount of current is applied to the insulating film long enough to cause the dielectric breakdown of the insulating film dependent to a current application time. 
       FIG. 5  shows a diagram illustrating a measurement circuit for an insulating film evaluation through a constant current application. A semiconductor substrate  1  of one conductivity type includes a first diffusion layer  2  of the opposite conductivity type. An insulating film  3  and an electrode  4  are laminated on the first diffusion layer  2  to form a MOS diode. The electrode  4  of the MOS diode is connected to a terminal  5   a  and the diffusion layer  2  is connected to a terminal  5   b . The terminal  5   a  and the terminal  5   b  are wired to a current source  8  via a switch  6   a  and a switch  6   b , respectively, to apply current between the terminals  5   a  and  5   b . A voltmeter  7  is connected in parallel to the current source  8 . In this measurement circuit, a current is applied to the insulating film in one direction, and the direction of the current is chosen to be forward or backward to apply for the measurement of dielectric breakdown characteristics (see JP 06-201761 A, for example). 
     However, both the forward direction current and the backward direction current should be applied to the MOS diode in order to guarantee the endurance cycling of a semiconductor memory device. To apply both the currents in a conventional measurement circuit, the forward direction current and the backward direction current have to be applied alternately, taking a long period of time. Another problem is that, because a current is applied in both directions to the same insulating film, the evaluation result varies depending on the length of the current application time. 
     SUMMARY OF THE INVENTION 
     An object of the present invention is to provide a method of measuring the dielectric breakdown characteristics with respect to the forward and the backward direction current in one test, and predicting the lifetime for each direction. 
     In order to achieve the above-mentioned object, an insulating film evaluation method according to the present invention employs a measurement circuit which includes: 
     a first MOS diode including: a first diffusion layer formed on a semiconductor substrate of one conductivity type and having an opposite conductivity type, a first insulating film formed on the first diffusion layer, and a first electrode formed on the first insulating film; 
     a second MOS diode including: a second diffusion layer formed close to the first diffusion layer, a second insulating film formed on the second diffusion layer, and a second electrode formed on the second insulating film; 
     a first terminal connected to the first electrode and the second diffusion layer; 
     a second terminal connected to the second electrode and the first diffusion layer; 
     a current source interposed between the first terminal and the second terminal; and 
     a voltmeter placed in parallel to the current source, and the insulating film evaluation method includes applying an electric current in one direction between the first terminal and the second terminal to measure a change in voltage between the terminals. 
     Further, an insulating film evaluation method employs a measurement circuit which includes: 
     a first MOS diode including: a first diffusion layer formed on a semiconductor substrate of one conductivity type and having an opposite conductivity type, a first insulating film formed on the first diffusion layer, and a first electrode formed on the first insulating film; 
     a second MOS diode including: a second diffusion layer formed close to the first diffusion layer, a second insulating film formed on the second diffusion layer, and a second electrode formed on the second insulating film; 
     a first terminal connected to the first electrode; 
     a second terminal connected to the second diffusion layer; 
     a third terminal connected to the second electrode; 
     a fourth terminal connected to the first diffusion layer; 
     a first contact point to which the first terminal and the second terminal are connected via a first switch and a second switch, respectively; 
     a second contact point to which the third terminal and the fourth terminal are connected via a third switch and a fourth switch, respectively; 
     a current source interposed between the first contact point and the second contact point; and 
     a voltmeter placed in parallel to the current source, and the insulating film evaluation method includes applying an electric current in one direction between the first contact point and the second contact point to measure a change in voltage between the terminals. 
     Further, the insulating film evaluation method includes identifying a defective MOS diode by opening and closing the switches after the change in voltage between the terminals is detected. 
     Further, in the insulating film evaluation method, the first MOS diode includes a plurality of MOS diodes, and the first MOS diode includes a plurality of MOS diodes. 
     A dielectric breakdown test for both the positive polarity and the negative polarity can thus be performed concurrently on an insulating film under test, easily shortening the measurement time. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a diagram of a measurement circuit according to a first embodiment of the present invention. 
         FIG. 2  is a diagram of a measurement circuit according to a second embodiment of the present invention. 
         FIG. 3  is a diagram of a measurement circuit according to a third embodiment of the present invention. 
         FIG. 4  is a diagram of a measurement circuit according to a fourth embodiment of the present invention. 
         FIG. 5  is a diagram of a conventional measurement circuit for evaluating an insulating film. 
     
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     Embodiments of the present invention are described below with reference to  FIGS. 1 to 4 . 
     First Embodiment 
     As illustrated in  FIG. 1 , a semiconductor substrate  1  of one conductivity type includes a first diffusion layer  21  and a second diffusion layer  22  each having an opposite conductivity type. An insulating film  31  and an electrode  41  are laminated on the first diffusion layer  21  to form a first MOS diode. An insulating film  32  and an electrode  42  are laminated on the second diffusion layer  22  to form a second MOS diode. The first MOS diode and the second MOS diode are placed close to each other. The electrode  41  of the first MOS diode and the diffusion layer  22  are wired together and connected to a terminal  5   a . The electrode  42  of the second MOS diode and the diffusion layer  21  are wired together and connected to a terminal  5   b . The terminals  5   a  and  5   b  are wired to a current source  8  via a switch  6   a  and a switch  6   b , respectively, and hence a current is applied between the terminals  5   a  and  5   b . A voltmeter  7  is connected in parallel to the current source  8 . 
     In the above-mentioned measurement system, after applying an electric current from the current source  8  in one direction by closing the switches  6   a  and  6   b , for example, current flows from the electrode side to the diffusion layer side in the oxide film  31  and current flows from the diffusion layer side to the electrode side in the oxide film  32 . The voltmeter  7  measures a change in voltage between the terminals  5   a  and  5   b , to thereby detect the dielectric breakdown of the oxide film  31  or the oxide film  32 . 
     While the switches  6   a  and  6   b  are put in the circuit in this embodiment, if the current source  8  has a switching function built therein, the switches  6   a  and  6   b  here can be read as this function, and putting the switches  6   a  and  6   b  in the circuit is not necessary. 
     Second Embodiment 
     As illustrated in  FIG. 2 , a semiconductor substrate  1  of one conductivity type includes a first diffusion layer  21  and a second diffusion layer  22 , which have an opposite conductivity type. An insulating film  31  and an electrode  41  are laminated on the first diffusion layer  21  to form a first MOS diode. An insulating film  32  and an electrode  42  are laminated on the second diffusion layer  22  to form a second MOS diode. The first MOS diode and the second MOS diode are placed close to each other. A terminal  5   a  connected to the electrode  41  and a terminal  5   d  connected to the diffusion layer  22  are connected to a contact point  9   a  via a switch  6   a  and a switch  6   d , respectively. A terminal  5   c  connected to the electrode  42  and a terminal  5   b  connected to the diffusion layer  21  are connected to a contact point  9   b  via a switch  6   c  and a switch  6   b , respectively. A current source  8  and a voltmeter  7  are connected to the contact points  9   a  and  9   b.    
     After closing the switches  6   a ,  6   b ,  6   c , and  6   d  to apply an electric current from the current source  8  in one direction causes, for example, current flows from the electrode side to the diffusion layer side in the oxide film  31 , and current flows from the diffusion layer side to the electrode side in the oxide film  32 . The voltmeter  7  measures a change in voltage between the contact points  9   a  and  9 , to thereby detect the dielectric breakdown of the oxide film  31  or the oxide film  32 . 
     In this embodiment, when the dielectric breakdown of the oxide film  31  or the oxide film  32  is detected, a broken oxide can be identified by applying a current and measuring the voltage with closing the switches  6   a  and  6   b  and opening the switches  6   c  and  6   d  to examine a dielectric breakdown in the oxide film  31 . The reverse switching operation is performed to examine a dielectric breakdown in the oxide film  32 . According to the examination data dependency of the insulating film strength on the direction of the applied current can be determined between current application from the electrode side to the diffusion layer and current application from the diffusion layer to the electrode side. 
     Third Embodiment 
     The measurement circuit in the first embodiment described above has two MOS diodes paired with each other. The measurement circuit may have more MOS diodes. 
       FIG. 3  illustrates a measurement circuit according to a third embodiment of the present invention where a semiconductor substrate  1  of one conductivity type includes a first diffusion layer  21 , a second diffusion layer  22 , a third diffusion layer  23 , and a fourth diffusion layer  24 . The diffusion layers  21  to  24  have an opposite conductivity type. An insulating film  31  and an electrode  41  are laminated on the first diffusion layer  21  to form a first MOS diode. An insulating film  32  and an electrode  42  are laminated on the second diffusion layer  22  to form a second MOS diode. An insulating film  33  and an electrode  43  are laminated on the third diffusion layer  23  to form a third MOS diode. An insulating film  34  and an electrode  44  are laminated on the fourth diffusion layer  24  to form a fourth MOS diode. The second diffusion layer  22 , the fourth diffusion layer  24 , the first electrode  41 , and the third electrode  43  are electrically connected to one another by a second conductive film to constitute a first terminal  5   a . The first diffusion layer  21 , the third diffusion layer  23 , the second electrode  42 , and the fourth electrode  44  are electrically connected to one another to constitute a second terminal  5   b . A current source  8 , a voltmeter  7 , a switch  6   a , and a switch  6   b  are connected such that a current is applied between the first terminal  5   a  and the second terminal  5   b.    
     When an electric current is applied from the current source  8  in one direction, for example, current flows from the electrode side to the diffusion layer side in the oxide film  31  and the oxide film  33 , and current flows from the diffusion layer side to the electrode side in the oxide film  32  and the oxide film  34 . The voltmeter  7  measures a change in voltage between the terminal  1  and the terminal  2 , to thereby detect the dielectric breakdown of the oxide film  31  or the oxide film  32 . This embodiment describes an example in which four MOS diodes are grouped together as a set, but more MOS diodes may be combined in the circuit. 
     While the switches  6   a  and  6   b  are put in the circuit in this embodiment, if the current source  8  has a switching function built therein, the switches  6   a  and  6   b  here can be read as this function, and putting the switches  6   a  and  6   b  in the circuit is not necessary. 
     Fourth Embodiment 
     The measurement circuit in the second embodiment described above has two MOS diodes paired with each other. The measurement circuit may have more MOS diodes. 
       FIG. 4  illustrates a measurement circuit according to a third embodiment of the present invention where a semiconductor substrate  1  of one conductivity type includes a first diffusion layer  21 , a second diffusion layer  22 , a third diffusion layer  23 , and a fourth diffusion layer  24 . The diffusion layers  21  to  24  have an opposite conductivity type. An insulating film  31  and an electrode  41  are laminated on the first diffusion layer  21  to form a first MOS diode. An insulating film  32  and an electrode  42  are laminated on the second diffusion layer  22  to form a second MOS diode. An insulating film  33  and an electrode  43  are laminated on the third diffusion layer  23  to form a third MOS diode. An insulating film  34  and an electrode  44  are laminated on the fourth diffusion layer  24  to form a fourth MOS diode. A terminal  5   a  is connected to the first electrode  41  and the third electrode  43 . A terminal  5   b  is connected to the first diffusion layer  21  and the third diffusion layer  23 . A terminal  5   c  is connected to the second electrode  21  and the fourth electrode  4 . A terminal  5   d  is connected to the second diffusion layer  22  and the fourth diffusion layer  24 . A switch  6   a , which is connected to the terminal  5   a , and a switch  6   d , which is connected to the terminal  5   d , are connected via a contact point  9   a . A switch  6   b , which is connected to the terminal  5   b , and a switch  6   c , which is connected to the terminal  5   c , are connected via a contact point  9   b . A voltmeter  7  and a current source  8  are connected to the contact points  9   a  and  9   b.    
     When the switches  6   a ,  6   b ,  6   c , and  6   d  are closed to apply an electric current from the current source  8  in one direction, for example, current flows from the electrode side to the diffusion layer side in the oxide film  31  and the oxide film  33 , while current flows from the electrode side to the diffusion layer side in the oxide film  32  and the oxide film  34 . The voltmeter  7  measures a change in voltage between the contact points  9   a  and  9   b , to thereby detect the dielectric breakdown of the oxide film  31 , the oxide film  32 , the oxide film  33 , or the oxide film  34 . 
     In this embodiment, when the dielectric breakdown of the oxide film  31 , the oxide film  32 , the oxide film  33 , or the oxide film  34  is detected, a broken oxide can be identified by applying a current and measuring the voltage with closing the switches  6   a  and  6   b  and opening the switches  6   c  and  6   d  to examine a dielectric breakdown in the oxide film  31  or the oxide film  33 . Accordingly dependency of the insulating film strength on the direction of the applied current can also be determined between current application from the electrode side to the diffusion layer and current application from the diffusion layer to the electrode side. 
     In addition, the test may be continued to subsequently examine the MOS diode group of the oxide films that are not suffering dielectric breakdown. 
     This embodiment describes an example in which four MOS diodes are grouped together as a set, but more MOS diodes may be combined in the circuit.
Provided is a method of evaluating dielectric breakdown by applying a current to an insulating film, in which measurement for a forward direction current and measurement for a backward direction current are performed in a short period of time. For this purpose two MOS diodes in which an electrode of one MOS diode and a base of another MOS diode are short-circuited respectively are prepared to form a circuit to which the current is applied, providing current flow in one insulating film reverse to current flow in another insulating film, which enables the application of both the forward direction current and the backward direction current.