Abstract:
Provided is a semiconductor electrostatic discharge protection device with a high tolerance for an electrostatic discharge without increase in manufacturing cost and degradation in performance of a semiconductor device. Conductors are arranged over a semiconductor circuit formed on a semiconductor substrate, a part of the conductors are connected to a power supply line, and the rest of the conductors are connected to a ground line, in order to enhance the tolerance for the electrostatic discharge in an air discharge model. Further, an areal ratio of the conductors with respect to the semiconductor circuit is set at 40% or more, thereby forming a semiconductor electrostatic discharge protection device with excellent efficiency.

Description:
BACKGROUND OF THE INVENTION  
       [0001]     1. Field of the Invention  
         [0002]     The present invention relates to a semiconductor device having electrostatic discharge (ESD) protecting means, and more particularly, to a semiconductor device having electrostatic discharge protecting means applied to a CMOS circuit.  
         [0003]     2. Description of the Related Art  
         [0004]     A semiconductor element such as a transistor included in a semiconductor circuit constituting a semiconductor device is sometimes seriously damaged in an electrostatic discharge event. A main source of an electrostatic discharge exposed to the semiconductor circuit is a human body (“human body model”, abbreviated as HBM), and the discharge from the human body generates current with a peak of several amperes to the semiconductor circuit for about 100 nanoseconds.  
         [0005]     The second source of the discharge is a discharge from a metal object (“machine model”, abbreviated as MM), and this source quite possibly induces a transient in which a rise time is significantly larger than that of the electrostatic discharge in the HBM.  
         [0006]     The third source comes from “charge device model” (CDM). In this model, a semiconductor element and the like themselves, which are included in the semiconductor circuit, are charged. In order to protect the semiconductor device from the electrostatic discharge, the charged electricity, which causes the discharge, should be discharged to the ground.  
         [0007]     As all the devices are obliged to reduce dimensions thereof owing to demands for a faster operation speed, a lower operation voltage, a larger packing density, and less cost, protection from electrostatic discharge phenomena in the semiconductor circuit has become more important.  
         [0008]     A description will be made on a semiconductor device including conventional electrostatic discharge protecting means based on  FIG. 6 .  FIG. 6  shows the semiconductor device including the conventional electrostatic discharge protecting means (for example, refer to JP 5-180899 A).  
         [0009]     In  FIG. 6 , the most common protecting means employed in a semiconductor circuit of a metal-oxide semiconductor (MOS) is a parasitic bipolar transistor associated with an NMOS transistor  117  formed on a semiconductor substrate  110 . A drain  112  of the NMOS transistor  117  is connected to an input/output terminal  120  connected to a semiconductor circuit  121  to be protected, which is included in the semiconductor device, and a source  113  and gate  111  thereof are grounded. A protection level or a breakdown threshold value can be set by changing the gate length of the NMOS transistor  117  from the drain  112  to the source  113  below the gate  111  of the NMOS transistor  117 . Under a stress condition, the parasitic bipolar transistor of the NMOS transistor  117  provides a dominant current conduction path between the to-be-protected input/output terminal  120  and a ground line  118  (the ground). The parasitic bipolar transistor operates in a snapback region when the polarity of a ground stress event is positive, and operates in forward-bias condition of a diode when the polarity is negative. Specifically, as long as the snapback region and forward-bias characteristics of the parasitic bipolar transistor operate normally, such an electrostatic discharge stress applied to the input/output terminal  120  is discharged, and accordingly, the semiconductor circuit  121  does not break. Since normal operation of the parasitic bipolar transistor in the snapback region or forward-bias condition assures the discharge of an electrostatic discharge stress applied to the input/output terminal  120 , no breakdown occurs in the semiconductor circuit  121 .  
         [0010]     A main breakdown mechanism observed in an NMOS protection device operating as the parasitic bipolar transistor under such a snapback condition is a start of a second breakdown. The second breakdown is a phenomenon which always induces thermal runaway in the device when decrease of an impact ionization current is cancelled by thermal generation of carriers. The second breakdown starts in the device under the stress as a result of self-heating. It is known that the peak temperature of the NMOS device at which the second breakdown takes place rises together with the level of stress current.  
         [0011]     The brief description has been made above of technology about electrostatic discharge in the conventional model, and the electrostatic discharge phenomenon and the protection method for the semiconductor circuit are described, for example, in JP 5-180899 A and “IEC standard, IEC-61000-4-2”.  
         [0012]     In recent years, a new electrostatic discharge model has attracted attention. This electrostatic discharge model is called “air discharge model”, and this breakdown model assumes an electrostatic breakdown caused by a direct discharge from a charged body to an IC package.  
         [0013]      FIG. 7  shows a simplified view of an electrostatic discharge measurement equivalent circuit for the air discharge model. In  FIG. 7 , static electricity is charged from a power supply  100  through a resistor  103  to a capacitance  105  when a switch  101  is on and a switch  102  is off. By putting off the switch  101  and putting on the switch  102 , the static electricity charged in the capacitance  105  is applied through a resistor  104  to a circuit board  106  implementing a semiconductor circuit therein. A measurement method of this model is described in “IEC standard, IEC-61000-4-2”.  
         [0014]      FIG. 8  shows a standard of an air discharge model in the IEC. Demand for Level 4 electrostatic discharge tolerance from the market increases the importance of an electrostatic discharge protection element.  
         [0015]     The above-described conventional semiconductor device has following problems. The electrostatic discharge protection in the HBM model, the MM model, and the CDM model can be made by the methods disclosed in JP 5-180899 A and “IEC standard, IEC-61000-4-2”. Insufficiency of the methods against the air discharge model, however, makes it difficult to respond to the market demand for Level 4 strength.  
         [0016]     Though increase in the area of an electrostatic discharge protection element can resolve the above-described problem, the larger electrostatic discharge protection element and the increase in the area of the chip turn out be a next problem.  
         [0017]     Connection of passive elements such as a resistor, a coil, and a capacitance to the electrostatic discharge protection element can yield higher electrostatic discharge voltage though, resulting not only possible increase in the chip area but also decrease in the CR time constant, which degrade the performance of the semiconductor device.  
         [0018]     Further, the electrostatic discharge stress in the air discharge model is applied not only to the input/output terminal but also directly to the semiconductor circuit  121  in the inside of the chip. At this time, a problem occurs that the conventional method can respond to the application of the electrostatic discharge stress to the input/output terminal, but not to the electrostatic discharge stress applied directly to the internal semiconductor circuit  121  of the chip.  
       SUMMARY OF THE INVENTION  
       [0019]     The present invention has been made in view of the points as described above, It is an object of the present invention to provide a semiconductor device including electrostatic discharge protecting means resistant to the air electrostatic discharge without increase in manufacturing cost or degradation in the performance of the semiconductor device.  
         [0020]     In order to solve the above-described problem, the present invention has adopted the following means.  
         [0021]     (1) A semiconductor device, comprising: a semiconductor circuit provided on a semiconductor substrate; and conductors for electrostatic discharge protection disposed over the semiconductor circuit, and having a ratio of a total area of the conductors disposed over the semiconductor circuit to an area of a chip occupied by the semiconductor circuit being 40% or more.  
         [0022]     (2) A semiconductor device according to (1) which includes electrostatic discharge protection means, in which the conductors are made of metal.  
         [0023]     (3) A semiconductor device according to (1) which includes electrostatic discharge protection means, some of the conductors are a conductor connected to a power supply line, and the rest of the conductors are connected to a ground line.  
         [0024]     (4) A method of designing a semiconductor device including electrostatic discharge protection means, including: providing a plurality of conductors for protecting a semiconductor element included in the semiconductor circuit from an electrostatic discharge breakdown, on an upper surface of a semiconductor circuit formed on a semiconductor substrate; connecting a part of the conductors to a power supply line to use the part of the conductors as VDD protection conductors, and connecting the protection conductors which are not connected to the power supply line to a ground line to use the protection conductors as GND protection conductors; and adjusting a ratio of a total area of the VDD protection conductors to a total area of the GND protection conductors to determine the strength for an ESD.  
         [0025]     In the semiconductor device including the electrostatic discharge protecting means according to the present invention, arranging conductors over the semiconductor circuit consisting of semiconductor devices on the semiconductor substrate causes the application of the electrostatic discharge stress in the air discharge model to the conductors, protecting the application of the electrostatic discharge stress to the semiconductor circuit and permitting the improvement in the electrostatic discharge strength without increase in the chip area or degradation in the performance of the semiconductor device. In the semiconductor device including the electrostatic discharge protecting means according to the present invention, use of the same metal for the wirings as the conductors can prevent the increase in manufacturing steps.  
         [0026]     Further, the conductors are connected to the optimum lines in response to an electrostatic discharge protection capability of the semiconductor device, thus making it possible to constitute a semiconductor device including electrostatic discharge protecting means having a good overload tolerance for the electrostatic discharge.  
         [0027]     Further, connecting the conductors to the optimum lines according to the electrostatic discharge protection capability of the semiconductor device enables the constitution of a semiconductor device including electrostatic discharge protecting means having sufficient electrostatic discharge strength.  
         [0028]     Then, 40% or more of the ratio of the area of the conductors arranged on the semiconductor circuit disposed on a chip (semiconductor substrate) to the area of the chip (semiconductor substrate) can not only provide a semiconductor device including electrostatic discharge protecting means with sufficient efficiency but also permit stable manufacturing of the conductors.  
         [0029]     As described above, according to the present invention a semiconductor device having high-performance electrostatic discharge protecting means can be realized without increase in manufacturing cost. 
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0030]     In the accompanying drawings:  
         [0031]      FIG. 1  is a schematic plan view showing a first embodiment of a semiconductor device including electrostatic discharge protecting means of the present invention;  
         [0032]      FIG. 2  is a cross-sectional view of the semiconductor device along a line A-A of  FIG. 1 ;  
         [0033]      FIG. 3  is a schematic view of the first embodiment of the semiconductor device at a time of an electrostatic discharge test;  
         [0034]      FIG. 4  is a graph showing a relationship between areal ratio of conductor and tolerance for an electrostatic discharge in an air discharge model in the semiconductor device;  
         [0035]      FIG. 5  is a graph showing polarity dependencies of tolerance for the electrostatic discharge to areal ratio in the air discharge model in the semiconductor device;  
         [0036]      FIG. 6  is a cross-sectional view showing a semiconductor device including conventional electrostatic discharge protecting means;  
         [0037]      FIG. 7  is a simplified view of an electrostatic discharge measurement equivalent circuit of the air discharge model; and  
         [0038]      FIG. 8  is a table for levels in the air discharge model in the IEC standard.  
     
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT  
       [0039]     A description will be made below of an embodiment of the present invention.  
         [0040]      FIG. 1  is a schematic plan view showing a first embodiment of a semiconductor device including electrostatic discharge protecting means of the present invention.  FIG. 2  is a schematic cross-sectional view of the semiconductor device when viewed from a plain indicated by a line A-A shown in  FIG. 1 . In this drawing, a contact and a wiring which are connected to a gate of a MOS transistor  7  are omitted.  
         [0041]     In  FIGS. 1 and 2 , a semiconductor device, for example, the MOS transistor  7  is formed on a semiconductor substrate  1 , for example, on the semiconductor substrate  1  with an impurity concentration in which a resistivity is 1 Ωcm to 20 Ωcm, and the MOS transistor  7  is composed of a diffusion layer  3   a  serving as a source, a diffusion layer  3   b  serving as a drain, and polycrystalline silicon  2  serving as a gate. Wirings  5   a ,  5   b , and  5   c  connected to the diffusion layers  3   a  and  3   b  and the polycrystalline silicon  2  are formed of aluminum, for example, to which silicon is added for preventing a spike, and are connected through contacts  4   a ,  4   b , and  4   c  to the diffusion layers  3   a  and  3   b  and the polycrystalline silicon  2 . On the MOS transistor  7 , a conductor  6   a  formed of aluminum, for example, to which silicon is added for preventing the spike, is disposed against the wiring  5   a  to have a space larger than the minimum dimension defined by the design rule. At this time, simultaneous formation of the wirings  5   a ,  5   b , and  5   c  and the conductor  6   a  can avoid additional manufacturing steps.  
         [0042]     A description will be made below of a phenomenon at the time when an electrostatic discharge test of the air discharge model shown in  FIG. 7  is performed by using the semiconductor device including the electrostatic discharge protecting means of the present invention, which has the above-described configuration, for example, while referring to a semiconductor circuit including two MOS transistors  7   a  and  7   b  shown in  FIG. 3 .  
         [0043]     In  FIG. 3 , input/output terminals of the semiconductor circuit, which do not directly relate to the present invention, are omitted. When the electrostatic discharge test has been performed, an electrostatic discharge stress is applied to the semiconductor circuit. The electrostatic discharge stress owing to an air discharge, which is applied to the semiconductor circuit, is not only applied to the input/output terminal but may also be applied to the wirings of the semiconductor circuit, which are not connected to the input/output terminal. The electrostatic discharge stress applied to the input/output terminal is released through an electrostatic discharge protection element of the conventional art, which is connected to the input/output terminal as in the HBM model or the MM model. Meanwhile, the electrostatic discharge stress applied to the wirings of the semiconductor circuit without passing through the input/output terminal is absorbed by the conductors  6   a ,  6   b ,  6   c , and  6   d , and is released to a ground line  8  and a power supply line  9 . From the above, the release of the electrostatic discharge stress applied to the portions other than the input/output terminal, through the conductors  6   a  to  6   d  to the ground line  8  or the power line  9 , prevents the application of the electrostatic discharge stress to the MOS transistors  7   a  and  7   b , permitting a sufficient tolerance against electrostatic discharge stress in the air discharge model. At this time, all the electrostatic discharge stress applied to the portions other than the input/output terminal is not applied to the conductors  6   a  to  6   d , and an amount of the stress applied thereto is determined by an areal ratio of the conductors with respect to the semiconductor circuit.  FIG. 4  shows a relationship between the areal ratio of the conductors  6  with respect to a chip area and the tolerance for the electrostatic discharge in the air discharge model. In  FIG. 4 , it can be observed that setting the areal ratio of the conductors at 40% or more makes it possible to obtain stable and high tolerance for the electrostatic discharge. By setting the areal ratio of the conductors at 40% or more, most of the electrostatic discharge stress applied to the chip is applied to the conductors  6 , preventing the electrostatic discharge stress from applying to the MOS transistor  7  formed on the semiconductor substrate  1  and making it possible to obtain a loading effect in a step for forming the conductors  6 , specifically, at the time of etching for patterning the conductors  6  as well.  
         [0044]     Further, since the semiconductor device including the electrostatic discharge protecting means of the present invention includes the conductors  6  connected to the ground line  8 , and the conductors  6  connected to the power supply line  9 , the semiconductor device can control the tolerance for the electrostatic discharge in the air discharge model.  FIG. 5  is a graph showing polarity dependencies of the tolerance for the electrostatic discharge in the air discharge model in the semiconductor device including the electrostatic discharge protecting means of the present invention. Referring to  FIG. 5A , description will be made below of the reason why the semiconductor device including the electrostatic discharge protecting means of the present invention can control the tolerance for the electrostatic discharge.  
         [0045]     Connection of only the power supply line  9  to the conductors  6  makes the potential difference between the electrostatic discharge stress of the positive polarity and the power supply line  9  smaller than that between the electrostatic discharge stress of the negative polarity and the power supply line  9 , thus making the current induced by the negative polarity electrostatic discharge stress easy to flow than that induced by the positive polarity electrostatic discharge stress. Accordingly, the tolerance for the electrostatic discharge of the negative polarity becomes larger than that for the electrostatic discharge of the positive polarity. In a case where the conductors  6  are connected to either the ground line  8  or the power supply line  9 , the tolerance for the electrostatic discharge is changed depending on potentials of the lines connected to the conductors  6 . Accordingly, at the time of designing the semiconductor circuit, it has been necessary to select whether the conductors  6  are to be connected to the ground line  8  or the power supply line  9  in considering the tolerance for the electrostatic discharge of the semiconductor device. However, in the present invention, it is possible to control the polarity dependencies of the overload tolerance for the electrostatic discharge in the air discharge model by a ratio of the power supply line and the ground line, which are connected to the conductors  6 , in considering the tolerance for the electrostatic discharge of the semiconductor device.  
         [0046]     In the first embodiment of the present invention, the description has been made of the case where the MOS transistor  7  is formed as the semiconductor device formed on the semiconductor substrate. However, it is possible to obtain a similar effect to that in the case of forming the MOS transistor  7  also in the case of forming a bipolar transistor as the semiconductor device. Further, the type and presence of the semiconductor device formed on the semiconductor substrate do not affect the essence of the present invention at all.  
         [0047]     Further, in the first embodiment of the present invention, the description has been made of the case where the metal which forms the conductors is aluminum to which silicon is added. However, it is possible to obtain a similar effect to that in the case of forming the conductors of aluminum to which silicon is added also in the case of forming the conductors of other metals. Further, the types of metals which form the conductors do not affect the essence of the present invention at all.