Patent Abstract:
A semiconductor device includes: a semiconductor element; a protection diode for protecting the semiconductor element from a first reverse voltage applied to the semiconductor element; and a capacitor. The protection diode is coupled in parallel to the semiconductor element. The protection diode has a forward direction, which is equal to a first applying direction of the first reverse voltage. The capacitor is coupled in parallel to the protection diode. The capacitor is capable of absorbing a second reverse voltage applied to the protection diode. The second reverse voltage is larger than a breakdown voltage of the protection diode, and has a second applying direction opposite to the first applying direction.

Full Description:
CROSS REFERENCE TO RELATED APPLICATION 
   This application is based on Japanese Patent Application No. 2006-162769 filed on Jun. 12, 2006, the disclosure of which is incorporated herein by reference. 
   FIELD OF THE INVENTION 
   The present invention relates to a semiconductor device having a protection diode. 
   BACKGROUND OF THE INVENTION 
   A semiconductor circuit having a protection function against a reverse voltage is disclosed in JP-A-2003-124324. The semiconductor circuit includes an inner circuit and a protection circuit. The inner circuit is energized by a power source such as an exchangeable battery. The protection circuit includes a Schottky-barrier diode, which is connected in parallel to the inner circuit so that a current toward the protection circuit flows in a direction opposite to a current supplied to the inner circuit from the power source. 
   Accordingly, when the power source is correctly connected to the inner circuit, the current does not flow through the Schottky-barrier diode. However, when the power source is connected to the inner circuit in such a manner that the current flows reversely, the forward current flows through the Schottky-barrier diode so that the inner circuit is protected. Thus, the Schottky-barrier diode functions as a protection circuit, and electric power consumption of the protection circuit is reduced. 
   The Schottky-barrier diode as a protection diode is connected in parallel to a protection object circuit such as the inner circuit in such a manner that the forward direction of the protection diode is opposite to the forward direction of the protection object circuit. Thus, the protection object circuit is protected from being applied with an reverse voltage. 
   However, for example, when static electricity is discharged, a voltage more than a breakdown voltage of the protection diode may be instantaneously and reversely applied to the protection diode. In this case, characteristics of the protection diode may be changed, or the protection diode may malfunction. Thus, the protection diode does not function to protect the protection object circuit. Thus, it is required for the protection diode to secure the protection function. 
   SUMMARY OF THE INVENTION 
   In view of the above-described problem, it is an object of the present disclosure to provide a semiconductor device having a protection diode. 
   According to an aspect of the present disclosure, a semiconductor device includes: a semiconductor element; a protection diode for protecting the semiconductor element from a first reverse voltage applied to the semiconductor element; and a capacitor. The protection diode is coupled in parallel to the semiconductor element. The protection diode has a forward direction, which is equal to a first applying direction of the first reverse voltage. The capacitor is coupled in parallel to the protection diode. The capacitor is capable of absorbing a second reverse voltage applied to the protection diode. The second reverse voltage is larger than a breakdown voltage of the protection diode, and has a second applying direction opposite to the first applying direction. 
   In the above device, since the capacitor is capable of absorbing the second reverse voltage applied to the protection diode, the protection diode is protected from the second reverse voltage larger than the breakdown voltage of the protection diode. Thus, protection function of the protection diode is secured. 

   
     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 circuit diagraph showing a semiconductor device; 
       FIG. 2A  is a cross sectional view showing a temperature sensitive diode, and  FIG. 2B  is a plan view showing the temperature sensitive diode; 
       FIG. 3A  is a cross sectional view showing a protection diode, and  FIG. 3B  is a plan view showing the protection diode; 
       FIG. 4  is a cross sectional view showing a capacitor; and 
       FIG. 5  is a cross sectional view showing another temperature sensitive diode. 
   

   DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     FIG. 1  shows a semiconductor device according to an example embodiment of the present disclosure. The device includes a temperature sensitive diode  10  for functioning as a temperature detecting element. The diode  10  has temperature characteristic such that a relation ship between forward current and voltage is changed in accordance with temperature. The diode  10  is connected to a constant current supply (not shown) so that a forward constant current flows through the diode  10  from the constant current supply. Accordingly, when the constant current flows through the diode  10 , change of voltage drop of the diode  10  is detected by a comparator or the like (not shown) so that the temperature around the diode  10  is detected. 
   A protection diode  20  is connected in parallel to the temperature sensitive diode  10 . The forward direction of the temperature sensitive diode  10  is opposite to the forward direction of the protection diode  20 . The breakdown voltage Vz of the protection diode  20  is higher than the rising voltage Vf of the temperature sensitive diode  10  in the forward direction. Thus, when the voltage is generated in the temperature sensitive diode  10  in the forward direction, the temperature sensitive diode  10  firstly turns on. Thus, the current flows only through the temperature sensitive diode  10 , and no current flows through the protection diode  20 . 
   When static electricity is discharged, a voltage more than a breakdown voltage Vz of the temperature sensitive diode  10  is reversely applied to the diode  10 , the reverse voltage provides to turn on the protection diode  20  so that the forward current flows through the protection diode  20 . Thus, since an excess reverse current is not applied to the temperature sensitive diode  10 , the temperature sensitive diode  10  is protected from malfunctioning, and current-voltage characteristics of the temperature sensitive diode  10  are prevented from being changed. 
   The semiconductor device further includes a capacitor  30 , which is connected in parallel to the protection diode  20 . 
   The voltage generated by discharge of static electricity may be applied to the protection diode  20  reversely. In this case, the forward current flows through the temperature sensitive diode  10 . At this time, there is a slight time delay until the temperature sensitive diode  10  turns on by the voltage. Accordingly, a voltage more than the breakdown voltage Vz may be applied to the protection diode  20 . 
   In view of the above difficulty, the capacitor  30  is connected in parallel to the protection diode  20 . Accordingly, even when the voltage more than the breakdown voltage Vz of the protection diode  20  is instantaneously and reversely applied to the protection diode  20 , the voltage is absorbed by charging the capacitor  20 . Accordingly, the excess reverse voltage more than the breakdown voltage Vz of the protection diode  20  is prevented from applying to the diode  20  although the excess reverse voltage may be applied to the diode  20  for a short time. Thus, the protection diode  20  is protected from malfunctioning, and current-voltage characteristics of the protection diode  20  are prevented from being changed. Protection function of the protection diode  20  is maintained. 
   Here, this capacitor  30  also absorbs the reverse voltage, which is applied to the temperature sensitive diode  10 . Thus, the excess reverse current is prevented from applying to the diode  10 . 
   Next, a concrete structure of the temperature sensitive diode  10 , the protection diode  20  and the capacitor  30  formed in a semiconductor substrate is explained. 
     FIGS. 2A and 2B  show the temperature sensitive diode  10 . The diode  10  includes a support substrate  1 , an insulation layer  2  and an active layer  3 , which are stacked in this order so that a SOI substrate  100  is formed. The insulation layer  2  is made of, for example, silicon oxide, and the active layer  3  has a P −  conductive type. 
   The diode  10  further includes a P +  conductive type layer  11  and a N +  conductive type layer  12 , which are alternately disposed in a surface portion of the active layer  3 . Each of the P +  conductive type layer  11  and the N +  conductive type layer  12  has a rectangular shape. A connection electrode  14  is coupled between the P +  conductive type layer  11  and the N +  conductive type layer  12  so that conduction of current from the N +  conductive type layer  12  to the P +  conductive type layer  11  is secured. A direction from the N +  conductive type layer  12  to the P +  conductive type layer  11  is a forward direction of the temperature sensitive diode  10 . 
   Further, one electrode  13   a  is formed on the SOI substrate  100  so that the electrode  13   a  contacts the P +  conductive type layer  11  disposed on one end. The other electrode  13   b  is formed on the SOI substrate  100  so that the electrode  13   b  contacts the N +  conductive type layer  12  disposed on the other end. The electrodes  13   a ,  13   b  and the connection electrode  14  are made of, for example, aluminum. The electrodes  13   a ,  13   b  and the connection electrode  14  are electrically coupled with the P +  conductive type layer  11  and the N +  conductive type layer  12  through openings, respectively. The openings are formed in an insulation film  7  made of an oxide film. 
   The active layer  3  in the SOI substrate  100  includes a trench  4 , which reaches the insulation layer  2 . An insulation film  5  is formed on a sidewall of the trench  4 . The insulation film  5  is made of a silicon oxide film formed by, for example, thermal oxidation method, a CVD method or a sputtering method. Alternatively, the insulation film  5  may be made of a silicon nitride film. Further, the insulation film  5  may be made of a combination film of a silicon nitride film and a silicon oxide film. After the insulation film  5  is formed on the sidewall of the trench  4 , a concavity in the trench  4  is filled with a poly crystalline silicon film  6  so that a surface of the substrate  100  is flattened, i.e., flatness of the substrate  100  is improved. 
   Thus, a region in which the temperature sensitive diode  10  is formed is electrically separated from other regions around the region with both of the insulation layer  2  in the SOI substrate  100  and the insulation film  5  on the sidewall of the trench  4  reaching the insulation layer  2 . Thus, since the temperature sensitive diode  10  is formed in the region isolated from the other regions, the temperature sensitive diode  10  is protected from being affected by electric potential of the other region. Accordingly, the temperature sensitive diode  10  can detect temperature with high accuracy. 
   Next, the method for manufacturing the temperature sensitive diode  100  is explained. 
   First, the SOI substrate  100  having the support substrate  1 , the insulation layer  2  and the active layer  3  is prepared. Then, a photo resist is formed on the active layer  3 , so that the photo resist is patterned in order to have an opening corresponding to the P +  conductive type layer  11 . By using the photo resist as a mask, a boron ion is implanted through the mask so that the P +  conductive type layer  11  is selectively formed in a surface portion of the active layer  3 . Similarly, another photo resist as a mask having an opening corresponding to the N +  conductive type layer  12  is formed on the active layer. Then, a phosphorous ion is implanted in the active layer  3 . Thus, the N +  conductive type layer  12  is selectively formed in another surface portion of the active layer  3 . 
   Next, a silicon nitride film having an opening corresponding to the trench  4  is formed on the substrate  100 . By using the silicon nitride film as a mask, the active layer  3  is dry-etched so that the trench  4  reaching the insulation layer  2  is formed. Then, a silicon oxide film as the insulation film  5  is formed on the sidewall of the trench  4  by a CVD method or the like. Then, the poly crystalline silicon film  6  is deposited so that the poly crystalline silicon film  6  fills a concavity of the trench  4  by a CVD method or the like. 
   Then, a silicon nitride film for covering a region, on which the electrodes  13   a ,  13   b  and the connection electrode  14  are formed, is formed on the substrate  100 . Then, by using the silicon nitride film as a mask, the active layer  3  is thermally oxidized. Thus, the insulation film  7  is selectively formed on the surface of the active layer  3 . After the silicon nitride film is removed, the electrodes  13   a ,  13   b  and the connection electrode  14  are formed on the active layer  3 . 
   Thus, the temperature sensitive diode  10  is formed. 
   Next, the structure of the protection diode  20  is explained with reference to  FIGS. 3A and 3B . 
   The protection diode  20  is similar to the temperature sensitive diode  10 , and difference between the protection diode  20  and the temperature sensitive diode  10  is such that the protection diode  20  only includes one P +  conductive type layer  21  and one N +  conductive type layer  22 . Specifically, the protection diode  20  includes the P +  conductive type layer  21  and the N +  conductive type layer  22 , which are disposed in a surface portion of the active layer  3 , and a pair of electrodes  23   a ,  23   b , which is electrically coupled with the P +  conductive type layer  21  and the N +  conductive type layer  22 , respectively. 
   A region, in which the protection diode  20  is formed, is electrically separated from other regions with both of the insulation layer  2  in the SOI substrate  100  and the insulation film  5  on the sidewall of the trench  4  reaching the insulation layer  2 . Thus, since the protection diode  20  is formed in the region isolated from the other regions, the protection diode  20  is protected from being affected by electric potential of the other regions. Accordingly, the protection diode  20  can protect the temperature sensitive diode  10  sufficiently. 
   Next, the structure of the capacitor  30  is explained. As shown in  FIG. 4 , the capacitor  30  is formed on the insulation film  7  arranged on the active layer  3 , which is isolated with the trench  4 . 
   The capacitor  30  includes a pair of a lower electrode  31  and an upper electrode  34 , which is made of conductive material such as poly crystalline silicon. Between the pair of lower electrode  31  and upper electrode  34 , an interlayer insulation film  32  made of, for example, silicon oxide, is formed. The interlayer insulation film  32  functions as a dielectric film. The capacitor  30  further includes an upper insulation film  32 , which covers the upper electrode  34  and the like. Furthermore, the capacitor  30  includes wiring layers  33 ,  36 , which are electrically connected to the lower electrode  31  and the upper electrode  34  through openings  32   a ,  35   a  in the interlayer insulation film  32  and the upper insulation film  35 , respectively. Thus, the capacitor  30  is formed on the SOI substrate  100 . 
   (Modifications) 
   Although the temperature sensitive diode  10  and the protection diode  20  are formed in and on the active layer  3  of the SOI substrate  100 , which is made of a single crystal silicon layer, the temperature sensitive diode  10  and the protection diode  20  may be formed from poly crystalline silicon layer.  FIG. 5  shows the temperature sensitive diode  10  formed from a poly crystalline silicon layer. 
   When the temperature sensitive diode  10  is formed from a poly crystalline silicon layer, firstly, an insulation film  202  is formed on a principal surface of a semiconductor substrate  201 . A poly crystalline silicon film is formed on the insulation film  202  by a CVD method or the like. Then, the poly crystalline silicon film is etched, so that the poly crystalline silicon film has a rectangular shape. 
   Next, a thermal oxidation film is formed on the patterned poly crystalline silicon film. Then, a photo resist is coated on the oxidation film, an exposure step is performed, the resist is selectively removed, and an ion implantation is performed. Thus, a P +  conductive type layer  203  and a N +  conductive type layer  204  are formed in the poly crystalline silicon film. The P +  conductive type layer  203  and the N +  conductive type layer  204  are alternately formed to have a rectangular shape, respectively. Further, the P +  conductive type layer  203  and the N +  conductive type layer  204  are adjacent to each other. 
   Next, an interlayer insulation layer  205  is formed on the poly crystalline silicon film. Further, an opening is formed in the interlayer insulation layer  205 , and then, a pair of electrodes  206   a ,  206   b  and a connection electrode  207  are formed. 
   Thus, the temperature sensitive diode  10  is formed on the semiconductor substrate  201  by using the poly crystalline silicon film. Since the temperature sensitive diode  10  is also formed on the insulation film  202 , which covers principal surface of the substrate  201 , the temperature sensitive diode  10  can function without affecting other regions and other elements. 
   In the above embodiment, the protection diode  20  protects the temperature sensitive diode  10  as a protection object. Alternatively, the protection diode  20  may protect a transistor or the like. 
   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.

Technology Classification (CPC): 7