Abstract:
An LDMOS transistor includes a gate insulation film formed on a semiconductor substrate, a gate electrode formed on the gate insulation film, a drain well of a first conductivity type formed in the substrate so as to include a gate region covered with the gate electrode, a channel well of a second conductivity type formed in the drain well in a partially overlapped relationship with the gate region, a source region of the first conductivity type formed in the channel well in an overlapping manner or adjacent with a side surface of the gate electrode, a medium-concentration drain region of the first conductivity type having an intermediate concentration level and formed in the drain well at a side opposing to the source region in a manner partially overlapping with the gate region, the medium-concentration drain region being formed with a separation from the channel well, a drain region of the first conductivity type formed in the medium-concentration drain region with a separation from the gate region, a low concentration well of the second conductivity type formed inside the drain well so as to include at least a part between said channel well and the medium-concentration drain region.

Description:
BACKGROUND OF THE INVENTION  
       [0001]     The present invention relates to semiconductor devices, the fabrication process thereof, and application thereof. Particularly, the present invention relates to a semiconductor device having a transistor of LDMOS structure, the fabrication process thereof, and application thereof.  
         [0002]     In this specification, an LDMOS transistor (lateral double diffuse insulation gate field transistor) is defined as a field effect transistor having a low impurity concentration layer formed so as to surround the source region with a conductivity type opposite to the conductivity type of the source or drain region. The LDMOS transistor thereby uses a surface part of the low impurity concentration layer right underneath the gate electrode as a channel region. Further, “conventional MOS transistor” used in this specification is defined as a MOS transistor having the structure in which a drain region is formed with an impurity concentration level higher than in a channel region.  
         [0003]     In the field of the semiconductor devices that include a circuit such as regulator or DC/DC converter, there is a demand for a semiconductor device of high output current in these days in view of the need of using the semiconductor device in wide variety of applications. An LDMOS transistor is characterized by low ON-resistance and thus draws attention because of its capability of increasing the output current. An LDMOS transistor has a unique feature, originating from its structure, in that miniaturization is possible while maintaining high breakdown voltage and it becomes possible to reduce the chip area with the use of such an LDMOS transistor.  
         [0004]     An LDMOS transistor is a field effect transistor in which a low impurity concentration layer (channel well) is formed so as to surround a source region with a conductivity type opposite to the conductivity type of source and drain regions. A channel is formed at a surface of the low impurity concentration layer right underneath the gate electrode.  
         [0005]      FIG. 1A  shows an example of an n-channel LDMOS transistor.  
         [0006]     Referring to  FIG. 1A , a polysilicon gate electrode  106  is formed over a silicon substrate  102  of n-type having a high resistivity via a gate oxide film (gate insulation film)  104 , wherein there is formed a channel well  108  in the silicon substrate  102  by introducing and activating a p-type impurity element while using an edge of the gate electrode  106  at the side of the source region as a mask. Thereby, a part of the channel well  108  thus formed serves for the channel region. Further, a source region  110  of n-type and a drain region  112  of n-type are formed in the silicon substrate  102  with low resistance by an ion implantation process of an n-type impurity element conducted while using the gate electrode  106  as a mask and subsequent thermal activation. Further, there is provided an interlayer insulation film  114  and electrodes  116  and  118  are formed respectively in connection with the n-type source region  110  and the n-type drain region  112 . Reference should be made to Patent Reference 1.  
         [0007]     In the case of using an LDMOS transistor for a high voltage transistor, it is generally practiced, in order to relax the electric field caused between the drain electrode and the gate electrode, to increase the thickness of a gate oxide film  104   a  as shown in  FIG. 1B  or by interposing a thick field oxide film  104   b  as shown in  FIG. 1C . Reference should be made to Patent Reference 2, for example. Here, it should be noted that the reference numeral  120  represents a drain region of medium concentration level of n-type formed underneath the thick oxide film  104   a  or  104   b  with an impurity concentration level higher than the n-type silicon substrate  102  but lower than the n-type drain region  112 .  
         [0008]     With the structure in which the thickness of the gate oxide film is not increased at the drain edge part as in the case of  FIG. 1A , the n-type drain region  112  is located right underneath the gate electrode and the breakdown characteristic is tend to be deteriorated because of the concentration of electric field in such a part.  
         [0009]     Thus, in order to improve the breakdown voltage of an LDMOS transistor, there is proposed a method of disposing the high concentration drain region with a separation from the gate electrode.  
         [0010]      FIG. 2  is a cross-sectional diagram showing a conventional n-channel LDMOS transistor.  
         [0011]     Referring to  FIG. 2 , there is formed a drain well  21  of n-type in a p-type semiconductor substrate (p-type substrate)  1 , wherein there is formed a p-type channel well  23  in the n-type drain well  21 , and there is further formed an n-type source region  11   s  in the p-type channel well  23 . Inside the n-type drain well  21 , there is formed a drain region lid of n-type with a separation from the p-type channel well  23  with a concentration level higher than the n-type drain well  21 . Further, there is formed an n-type gate electrode  11   g  of polysilicon over and across the n-type drain well  21  for the part between the n-type source region  11   s  and n-type drain region  11   d  and further the p-type channel well  23  but with a separation from the n-type drain region  11   d.  The gate electrode  11   g  is provided via a gate oxide film  11   ox.  With this structure, the surface of the p-type channel well  23  right underneath the n-type gate electrode  11   g  becomes the channel region.  
         [0012]     With the n-channel LDMOS of such a structure, the On-resistance is determined as a sum of the channel resistance, the drain resistance and the source resistance. Thus, with the present construction, the source resistance is eliminated by forming the n-type source region  11   s  in self-alignment with the n-type gate electrode  11   g.    
         [0013]     However, with the structure of  FIG. 2 , there appears a high drain resistance associated with the resistance component of the n-type drain well  21 , while there is caused a decrease of channel resistance when a high electric field is induced in the channel region with application of the gate voltage. Under such a situation, the drain resistance occupies the majority part of the transistor resistance, and there is caused a problem in that it is not possible to increase the drain current.  
         [0014]     Further, there has been a problem with the construction of  FIG. 2  in that, because the drain region is easily destroyed when a parasitic bipolar transistor has caused conduction as a result of the high electric field caused in the MOS transistor by the drain voltage. When this occurs, it is not possible to obtain a normal operational waveform.  
         [0015]     In order to improve the foregoing problem, there is proposed a method of forming the drain region and the source region in the form of double diffusion structure in the LDMOS transistor as proposed in Patent Reference 1.  
         [0016]     Further, there is proposed a method of conducting an impurity injection to the surface part of the well located between the gate electrode and the high-concentration drain region in a self-alignment manner for lowering the resistance thereof and for improving the drain breakdown voltage of LDMOS transistor. Reference should be made to Patent Reference 3. However, the LDMOS transistor disclosed in Patent Reference 3 lacks the feature of self-alignment at the source side, and thus, there arises a problem in that the source resistance is not eliminated completely. Thus, the construction of this prior art reference is not suitable for achieving low ON-resistance.  
         [0017]     In view of the foregoing problems of the prior art, the inventor of the present invention has attempted formation of both the source side and the drain side by a self-alignment process.  FIG. 3  shows the LDMOS transistor used in this related art.  
         [0018]      FIG. 3  is a cross-sectional diagram showing the n-channel MOS transistor of this related art, wherein those parts corresponding to the parts described previously are designated by the same reference numerals and the description thereof will be omitted.  
         [0019]     Referring to  FIG. 3 , the LDMOS transistor of the related art is constructed on a p-type substrate and includes therein an n-type source region  11   s,  an n-type drain region  11   d,  an n-type gate oxide  11   ox,  an n-type gate electrode  11   g,  and an n-type drain well  21 , wherein a p-type channel well  23  is formed inside the n-type drain well  21 . Thereby, it should be noted that the n-type source region  11   s  is formed in a self-aligned process with regard to the n-type gate electrode  11   g.  Further, an n-type drain region  4  of intermediate concentration level is formed on the surface part of the n-type drain well  21  at the side of the n-type drain region lid with regard to the n 0 type gate electrode  11   g.  It should be noted that the drain region  24 N of the intermediate concentration level is formed in a self-alignment process with regard to the n-type gate electrode  11   g.  The drain region  24  of the intermediate concentration level is formed shallower than the n-type drain region  11   d.    
         [0020]     Further, there is disclosed a construction in which the drain region  24  of the intermediate concentration and the gate electrode  11   g  are partially overlapped in the structure of  FIG. 16  Reference should be made to Patent Reference 4, for example.  
       REFERENCES  
       [0000]     Patent Reference 1  
         [0021]     Japanese Laid-Open Patent Application 7-302903 official gazette  
         [0000]     Patent Reference 2  
         [0022]     Japanese Patent 3,275,569  
         [0000]     Patent Reference 3  
         [0023]     Japanese Laid-Open Patent Application 10-335663 official gazette  
         [0000]     Patent Reference 4  
         [0024]     Japanese Laid-Open Patent Application 2003-86790 official gazette  
       SUMMARY OF THE INVENTION  
       [0025]     In a high voltage device in which an n-channel LDMOS transistor of low ON-resistance having p-channel LDMOS transistor are integrated on the same p-type substrate, it is possible to secure a breakdown voltage up to about 50V, for example, with regard to the p-channel LDMOS transistor by reducing the concentration level of the deep p-type well corresponding to the drain well.  
         [0026]     Similarly, it is possible to improve the breakdown voltage of an n-channel LDMOS transistor by reducing the concentration level of the deep n-type well forming the drain well or reducing the concentration level of the n-type medium concentration drain region and increasing the breakdown voltage of the p-n junction formed at the interface to the p-type channel well.  
         [0027]     However, in the case the concentration level of the n-type medium concentration drain region is decreased, there arises a problem that the drain region tends to cause thermal destruction with conduction of the parasitic bipolar transistor. Because of this, attempt has been made to improve the breakdown voltage by reducing the impurity concentration level of the n-type drain well.  
         [0028]     Thereby, attempt was made to share the fabrication process steps as much as possible by forming the n-type drain region of the n-channel LDMOS transistor and the n-type isolation well of the p-channel LDMOS transistor simultaneously in the prospect of reducing the number of impurity ion implantation process in the fabrication process of the semiconductor device. However, with such an approach, it was discovered that punch-through is tend to be caused between the p-type drain region and the p-type substrate when the impurity concentration level of the n-type isolation well is reduced in the p-channel LDMOS, while such punch-through invites decrease of the breakdown voltage of the p-channel LDMOS transistor. Thus, it was shown that it is not possible to reduce the concentration level of these n-type regions to the level of 8×10 15  cm −3  or less.  
         [0029]      FIG. 4  is a diagram showing the relationship between the n-type impurity concentration level of the n-type isolation well provided for device isolation and the drain breakdown voltage. In the drawing, the vertical axis represents the drain breakdown voltage in terms of volt, while the horizontal axis represents the n-type impurity concentration level (cm −3 ). In  FIG. 4 , the p-type drain well formed in the n-type isolation well has the concentration level of 1×10 −16  cm −3 .  
         [0030]     Thus, the present invention has its object of improving the drain breakdown voltage of the LDMOS transistor without decreasing the impurity concentration level of the first conductivity impurity element in the drain well of the first conductivity type and in the intermediate concentration well of the first conductivity type.  
         [0031]     In a first aspect, the present invention provides a semiconductor device comprising an LDMOS transistor, said LDMOS transistor comprising:  
         [0032]     a gate insulation film formed over a semiconductor substrate;  
         [0033]     a gate electrode formed over said gate insulation film;  
         [0034]     a drain well of a first conductivity type formed in said substrate so as to include a gate region covered by said gate electrode;  
         [0035]     a channel well of a second conductivity type opposite to said first conductivity type formed in said drain well in partially overlapped relationship with said gate region;  
         [0036]     a source region of said first conductivity type formed in said channel well in an overlapping manner or adjacent with a side surface of said gate electrode;  
         [0037]     a medium-concentration drain region of said first conductivity type having an intermediate concentration level and formed in said drain well at a side opposing to said source region in a manner partially overlapping with said gate region, said medium-concentration drain region being formed with separation from said channel well;  
         [0038]     a drain region of said first conductivity type formed in said medium-concentration drain region with a separation from said gate region;  
         [0039]     a low concentration well of said second conductivity type formed inside said drain well so as to include at least a part between said channel well and said medium-concentration drain region.  
         [0040]     According to the present invention, in which the low concentration well is formed inside the drain well in the LDMOS transistor so as to include at least the region between the channel well right underneath the gate electrode and the medium-concentration drain region, it becomes possible to improve the drain breakdown characteristics of the LDMOS transistor without reducing the concentration level of the first conductivity type impurity element in the drain well of the first conductivity type and the medium-concentration drain region.  
         [0041]     In a preferred embodiment, the medium-concentration drain region may be formed to have an impurity concentration level lower than said channel well. By forming the medium-concentration drain region to have a lower impurity concentration level as compared with the channel well, it becomes possible to avoid the loss of effective channel length even after applying thermal activation processing for forming the medium-concentration drain region and the channel well.  
         [0042]     In another preferred embodiment, the semiconductor device may be the one in which the semiconductor substrate comprises a p-type semiconductor substrate and the LDMOS transistor is a p-channel LDMOS transistor, the semiconductor device further includes an n-type isolation well and an n-channel LDMOS transistor on the n-type isolation well, the p-channel LDMOS transistor comprising: a drain well of p-type, a channel well of n-type, a medium-concentration drain region of p-type, a source region of p-type, and a drain region of p-type, wherein the first low concentration well of p-type constituting the n-channel LDMOS transistor is formed simultaneously with the second drain well of p-type constituting said p-channel LDMOS transistor.  
         [0043]     Thus, in the case the LDMOS transistor of the present invention is an n-channel transistor and there is further formed a p-channel LDMOS transistor on the same semiconductor substrate separately to the n-channel LDMOS transistor such that the p-channel LDMOS transistor includes the n-type isolation well and such that the n-type isolation well includes the drain well of p-type, the channel well of n-type, the medium-concentration drain region of p-type, the source region of p-type and the drain region of p-type, it becomes possible to form the low-concentration well without increasing the number of fabrication steps, by forming the p-type low-concentration well of the n-channel LDMOS transistor and the p-type drain well of the p-channel LDMOS transistor.  
         [0044]     In another embodiment, the semiconductor device may be the one in which the semiconductor substrate comprises a p-type semiconductor substrate and the LDMOS transistor is an n-channel LDMOS transistor, the semiconductor device further includes an n-type isolation well and an n-channel LDMOS transistor formed on the n-type isolation well, the p-channel LDMOS transistor comprising: a drain well of p-type; a channel well of n-type; a medium-concentration drain region of p-type; a source region of p-type; and a drain region of p-type, wherein the first drain well of n-type constituting the n-channel LDMOS transistor is formed simultaneously with the isolation well of n-type constituting the p-channel LDMOS transistor.  
         [0045]     Thus, in the case the LDMOS transistor of the present invention is an n-channel transistor and there is further formed a p-channel LDMOS transistor on the same semiconductor substrate separately to the n-channel LDMOS transistor such that the p-channel LDMOS transistor includes the n-type isolation well and such that the drain well of p-type, the channel well of n-type, the medium-concentration drain region of p-type, the source region of p-type, and the drain region of p-type are formed in the n-type isolation well, it becomes possible to reduce the number of fabrication steps, by forming the n-type drain well of the n-channel LDMOS transistor and the n-isolation well of the p-channel LDMOS transistor simultaneously.  
         [0046]     Further, the semiconductor device may be the one in which the semiconductor substrate comprises a p-type semiconductor substrate and the LDMOS transistor is an n-channel LDMOS transistor, the semiconductor device further comprising a conventional n-channel MOS transistor comprising a p-type well, wherein the low concentration well of p-type constituting the n-channel LDMOS transistor is formed simultaneously to the p-type well of the conventional n-channel MOS transistor.  
         [0047]     By forming, in the case in which the LDMOS transistor of the semiconductor device is an n-channel device and a conventional n-channel MOS transistor is formed separately therefrom on the same semiconductor substrate, the p-type low-concentration well constituting the n-channel LDMOS transistor concurrently with the p-type well constituting the conventional n-channel MOS transistor, it becomes possible to form a p-type low concentration well without increasing the number of the fabrication process steps.  
         [0048]     Further, the semiconductor device may be the one in which the semiconductor substrate comprises a p-type semiconductor substrate and the LDMOS transistor is an n-channel LDMOS transistor, the semiconductor device further comprising a conventional p-channel MOS transistor comprising an n-type well, wherein the n-type drain well constituting the n-channel LDMOS transistor is formed simultaneously to the n-type well of the conventional n-channel MOS transistor.  
         [0049]     By forming, in the case in which the LDMOS transistor of the semiconductor device is an n-channel device and a conventional p-channel MOS transistor is formed separately therefrom on the same semiconductor substrate, the n-type drain well constituting the n-channel LDMOS transistor concurrently with the n-type well constituting the conventional p-channel MOS transistor, it becomes possible to reduce the number of fabrication process steps of the semiconductor device.  
         [0050]     Further, in the present invention, it is possible to reverse the conductivity type with regard to the construction noted above.  
         [0051]     Further, in the semiconductor device of the present invention in which an LDMOS transistor and a conventional MOS transistor are integrated, the gate insulation film of the LDMOS transistor may have a thickness smaller than the conventional n-channel MOS transistor.  
         [0052]     By reducing the thickness of the gate insulation film of the LDMOS transistor as compared with the conventional MOS transistor, it becomes possible to reduce the ON-resistance of the LD MOS transistor and decrease the chip area.  
         [0053]     Further, in the LDMOS transistor of the present invention, the gate insulation film may be formed to have a uniform thickness.  
         [0054]     By forming the gate insulation film to have a uniform thickness, it becomes possible to form the source region, the channel well and the medium-concentration drain region with a self-alignment process.  
         [0055]     Further, the present invention is applicable to the device in which an edge of the gate electrode close to the drain region is formed on an insulation film having a thickness larger than a thickness of the gate insulation film.  
         [0056]     The semiconductor device of the present invention may be applied to an LDMOS transistor used in a power supply unit that includes a feedback circuit maintaining an output voltage constant by comparing the output voltage with a reference voltage.  
         [0057]     In another example, the semiconductor device of the present invention may be applied to a DC/DC converter having a charge pump circuit that charges and discharges a capacitor with a switching operation of internal switches, wherein the LDMOS transistor of the present invention is used for at least one of such internal switches.  
         [0058]     In another aspect, the present invention provides a fabrication process of a semiconductor device including an LDMOS transistor comprising the steps of:  
         [0059]     (A) forming a drain well of a first conductivity type in a semiconductor substrate;  
         [0060]     (B) forming a low-concentration well of a second conductivity type by introducing an impurity element of a second conductivity type into the drain well;  
         [0061]     (C) forming a gate insulation film on a surface of said low-concentration well and forming a gate electrode on the gate insulation film;  
         [0062]     (D) forming a channel well of the second conductivity by introducing and activating an impurity element of the second conductivity type into the low-concentration well at a side of the gate electrode in self-alignment to the gate electrode;  
         [0063]     (E) forming a medium-concentration drain region of said first conductivity type by introducing an impurity element of the first conductivity type to a part of said low-concentration well at a side opposite to said channel well with respect to the gate electrode; and  
         [0064]     (F) forming a drain region of the first conductivity type in said medium-concentration drain region and a source region of the first conductivity type in the channel well by introducing an impurity element of the first conductivity type to the medium-concentration drain region and the channel well.  
         [0065]     In a preferred embodiment, a thermal activation processing is conducted in the step (E) after introducing the impurity element for forming the medium-concentration drain region.  
         [0066]     Further, the step of activating of step (D) conducted after introducing the impurity element for forming the channel well and the thermal activation processing of the step (E) conducted after introducing the impurity element for forming the medium-concentration drain region are conducted simultaneously.  
         [0067]     With this, it is possible to reduce the number of the fabrication steps.  
         [0068]     Further, the step (D) of introducing the impurity element for forming said-channel well may be conducted with an amount exceeding an amount of the impurity element introduced in the step (E) for forming the medium-concentration drain region.  
         [0069]     Further, the step (A) may include a step of forming a well of the first conductivity, simultaneously to the step of forming said drain well, at a part of the semiconductor substrate different from a region where the LDMOS transistor is formed, for formation of another MOS transistor.  
         [0070]     By forming the well of the first conductivity type for formation of another MOS transistor simultaneously to the formation of the drain well, it becomes possible to reduce the number of the fabrication process steps.  
         [0071]     Here, “another MOS transistor” includes both a conventional MOS transistor and an LDMOS transistor.  
         [0072]     In another preferred embodiment, the well of the another MOS transistor formed in the step (A) may be formed for a second LDMOS transistor, and wherein the step (B) is conducted so as to form a drain well of the second conductivity type in the well of the second LDMOS transistor.  
         [0073]     By forming the drain well of the second conductivity type in the well of the second LDMOS transistor simultaneously to the formation of the low-concentration well, it becomes possible to form the low-concentration well without increasing the number of the fabrication process steps.  
         [0074]     Further, in a further preferred embodiment, the step (B) may be conducted so as to form a well of the second conductivity type simultaneously with formation of the low-concentration well, in a region of the semiconductor substrate different from a region where the LDMOS transistor is formed for another MOS transistor.  
         [0075]     By forming the formation of the low-concentration well concurrently with the step of forming another MOS transistor, it is possible to form the low-concentration well without increasing the number of the fabrication steps.  
         [0076]     According to the semiconductor device of the present invention, in which the low concentration well is formed inside the drain well of the LDMOS transistor, it becomes possible to improve the breakdown characteristics without reducing the concentration level of the medium-concentration drain region and drain well.  
         [0077]     By forming the medium-concentration drain region to have an impurity concentration level lower than said channel well, it becomes possible to avoid loss of channel effective length even after thermal activation processing is applied for forming the medium-concentration drain region and the channel wafer region.  
         [0078]     In the case the LDMOS transistor formed on the p-type semiconductor substrate is an n-channel transistor and there is further formed a p-channel LDMOS transistor on the semiconductor substrate separately to the n-channel LDMOS transistor such that the p-channel LDMOS transistor includes the n-type isolation well and the drain well of p-type, the channel well of n-type, the medium-concentration drain region of p-type, the source region of p-type, and the drain region of p-type are formed in the n-type isolation well, it becomes possible to form the low-concentration well without increasing the number of fabrication steps, by forming the p-type low-concentration well of the n-channel LDMOS transistor and the p-type drain well of the p-channel LDMOS transistor. Because it is not necessary to reduce the impurity concentration level of the mid-concentration drain and the drain well by forming the low-concentration well, there is no need of decreasing the concentration of the n-type isolation well for the p-channel LDMOS transistor, and it becomes possible to suppress occurrence of punch-through between the p-type medium-concentration drain and the p-type semiconductor substrate in the p-channel LDMOS transistor.  
         [0079]     In the case the LDMOS transistor of the present invention formed on the semiconductor substrate is an n-channel transistor and there is further formed a p-channel LDMOS transistor on the semiconductor substrate separately to the n-channel LDMOS transistor such that the p-channel LDMOS transistor includes the n-type isolation well and that the drain well of p-type, the channel well of n-type, the medium-concentration drain region of p-type, the source region of p-type, and the drain region of p-type are formed in the n-type isolation well, it becomes possible to reduce the number of fabrication steps, by forming the n-type drain well of the n-channel LDMOS transistor and the n-isolation well of the p-channel LDMOS transistor simultaneously. Because it is not necessary to reduce the impurity concentration level of the mid-concentration drain region and the drain well by forming the low-concentration well, there is no need of decreasing the concentration of the n-type isolation well for the p-channel LDMOS transistor, and it becomes possible to suppress occurrence of punch-through between the p-type medium-concentration drain region and the p-type semiconductor substrate in the p-channel LDMOS transistor.  
         [0080]     In the case the LDMOS transistor of the semiconductor device is an n-channel device and a conventional n-channel MOS transistor is formed separately therefrom on the same semiconductor substrate, it becomes possible to form the p-type low-concentration well without increasing the number of fabrication process by forming the p-type low-concentration well constituting the n-channel LDMOS transistor concurrently with the p-type well constituting the conventional n-channel MOS transistor.  
         [0081]     In the case in which the LDMOS transistor of the semiconductor device is an n-channel device and a conventional p-channel MOS transistor is formed separately therefrom on the same semiconductor substrate, it becomes possible to reduce the number of fabrication process steps of the semiconductor device by forming the n-type drain well constituting the n-channel LDMOS transistor concurrently with the n-type well constituting the conventional p-channel MOS transistor and the fabrication process of the semiconductor device is simplified.  
         [0082]     By reducing, in the case the semiconductor device of the present invention comprises a semiconductor device in which an LDMOS transistor and a conventional MOS transistor are integrated, the thickness of the gate insulation film of the LDMOS transistor as compared with the conventional MOS transistor, it becomes possible to reduce the ON-resistance of the LD MOS transistor and decrease the area of the LDMOS transistor. Thereby, it becomes possible to reduce the chip area.  
         [0083]     The semiconductor device of the present invention can be applied to the device in which the LDMOS transistor has a gate insulation film of uniform thickness or to the device in which an edge of the gate electrode of the LDMOS transistor close to the drain region is formed on an insulation film having a thickness larger than a thickness of the gate insulation film.  
         [0084]     According to a power supply unit of the present invention, ion which the LDMOS transistor constituting the present invention is used therein, it becomes possible to reduce the size of the MOS transistor used for the output driver because of the small ON-resistance, and it becomes possible to reduce the chip area.  
         [0085]     In a DC/DC converter of the present invention that uses the LDMOS transistor of the present invention for at least one of internal switches used therein, it becomes possible to reduce the size of the MOS transistor used for the internal switches because of the low ON-resistance, and it becomes possible to reduce the chip area.  
         [0086]     Further, according to the present invention, in which the low-concentration well of the second conductivity type is formed in the drain well by conducting the ion implantation of the impurity element of the second conductivity type, it becomes possible to improve the drain breakdown voltage of the LDMOS transistor without decreasing the impurity concentration level of the medium-concentration drain region and the drain well.  
         [0087]     By conducting a thermal activation processing in the step (E) after introducing the impurity element for forming the medium-concentration drain region, the medium-concentration drain region is increased and it becomes possible to increase the amount of the impurity element introduced to the medium-concentration drain region as compared with the case not conducting such thermal annealing process, and it becomes possible to reduce the medium concentration drain region and further the drain resistance while maintaining high breakdown voltage.  
         [0088]     By conducting the step of activating of step (D) after introducing the impurity element for forming the channel well and the thermal activation processing of the step (E) after introducing the impurity element for forming the medium-concentration drain region simultaneously, it becomes possible to increase the medium-concentration drain region without adding a thermal annealing process.  
         [0089]     By increasing the amount of impurity injection for forming the channel well in the step (D)as compared with the an amount of the impurity element introduced in the step (E), it becomes possible to avoid the loss of channel effective length even when thermal activation process is conducted for forming the mid concentration drain region and the channel well.  
         [0090]     By forming the well of the first conductivity type for formation of another MOS transistor simultaneously to the formation of the drain well, it becomes possible to reduce the number of the fabrication process steps.  
         [0091]     By forming the drain well of the second conductivity type in the well of the second LDMOS transistor simultaneously to the formation of the low-concentration well, it becomes possible to form the low-concentration well without increasing the number of the fabrication process steps.  
         [0092]     By forming the formation of the low-concentration well concurrently with the step of forming another MOS transistor, it is possible to form the low-concentration well without increasing the number of the fabrication steps. 
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0093]      FIGS. 1A-1C  are cross-sectional diagrams showing conventional LDMOS transistors;  
         [0094]      FIG. 2  is a cross-sectional diagram showing another conventional LDMOS transistor;  
         [0095]      FIG. 3  is a cross-sectional diagram showing further conventional LDMOS transistor;  
         [0096]      FIG. 4  is a diagram showing the relationship between an impurity concentration level and a drain breakdown voltage of an n-type isolation well for explaining the problems of the conventional art;  
         [0097]      FIG. 5  is a cross-sectional diagram showing an embodiment of the semiconductor device of the present invention;  
         [0098]      FIG. 6  is a cross-sectional diagram showing another embodiment of the semiconductor device;  
         [0099]      FIGS. 7A-7L  are cross-sectional diagrams showing the process steps for fabricating the semiconductor device of  FIG. 6 ;  
         [0100]      FIG. 8  is a cross-sectional diagram showing a semiconductor device of another embodiment of the present invention;  
         [0101]      FIGS. 9A-9L  are cross-sectional diagrams showing the process steps for fabricating the semiconductor device of  FIG. 6 ;  
         [0102]      FIG. 10  is a circuit diagram showing an embodiment of a power supply unit  
         [0103]      FIG. 11  is a circuit diagram showing a DC/DC converter to which the semiconductor device of the present invention is applied;  
         [0104]      FIG. 12  is a diagram showing the relationship between an impurity concentration level of the p-type low-concentration well and impurity concentration level of the LDMOS transistor of the present invention;  
         [0105]      FIGS. 13A-13D  are cross-sectional diagrams showing a further embodiment of the LDMOS transistor of the present invention. 
     
    
     DETAILED DESCRIPTION OF THE INVENTION  
       [0106]     With the present specification, a “conventional MOS transistor” may any MOS transistor of conventional type including those having an LDD (lightly doped drain) region of double diffusion structure, those having a DDD (double doped drain) structure, those having an LOCOS structure having a thick oxide film at the gate edge, or the like.  
         [0107]      FIG. 5  is a cross-sectional view showing a semiconductor device according to an embodiment of the present invention, wherein the semiconductor device of  FIG. 5  is an embodiment of the LDMOS transistor of the present invention applied to an n-channel LDMOS transistor.  
         [0108]     Referring to  FIG. 5 , a drain well  21  of n-type is formed in a p-type substrate (semiconductor substrate)  1 , wherein it can be seen that a p-type low-concentration well  22  is formed in the n-type drain well  21 . Further, a p-type channel well  23  is formed in the p-type low-concentration well  22  such that a part of the p-type channel well  23  forms a channel region, and an n-type source region  11   s  is formed in the p-type channel well  23 . Further, there is formed an n-type medium-concentration drain region  24  is formed inside the p-type low-concentration well  22  with a separation from the p-type channel well  23  such that the n-type medium-concentration drain region  24  is introduced with an n-type impurity element such as phosphorus with a concentration level exceeding the concentration level of the p-type low-concentration well  22 . Further, an n-type drain region  11   d  doped with an n-type impurity element such as phosphorus is formed inside the n-type medium-concentration drain region  24  with a concentration level exceeding the concentration level of the n-type medium-concentration drain region  24 .  
         [0109]     Further, there is formed a gate electrode  11   g  of n-type polysilicon via a gate oxide film (gate insulation film)  11   ox  so as to extend over a part of the n-type drain well  21  located between the n-type source region  11   s  and the n-type drain region  11   d  with a separation from the n-type drain region  11   d,  such that the gate electrode  11   g  further covers a part of the p-type channel well  23  and a part of the n-type medium-concentration drain region  24 . Thereby, the separation between the n-type gate electrode  11   g  and the n-type drain region  11   d  is set for example to 2.0 μm, while the gate oxide film  11   ox  may have a thickness of about 30 nm, for example. The surface of the p-type channel well  23  located underneath the n-type gate electrode  11   g  serves for the channel region. It should be noted that the n-type gate electrode  11   g  is introduced with phosphorus by an ion implantation process or solid diffusion process with a concentration level of 1.0×10 21  cm −1 . The n-type source region  11   s,  the p-type channel well  23  and the n-type medium concentration drain region  24  are formed in self-alignment to the n-type gate electrode  11   g.    
         [0110]     The p-type channel well  23  is introduced with boron as the p-type impurity element with a concentration level of 1.0×10 17  cm −3 ; The n-type drain well  21  is introduced with phosphorus as the n-type impurity element with the concentration level of 8.0×10 15  cm −3 . Further, the p-type low-concentration well  22  is introduced with boron as the p-type impurity element with a concentration level of 1.2×10 16  cm −3  for the part between the p-type channel well  23  and the n-type medium concentration region  23 .  
         [0111]     While the present embodiment uses a p-type semiconductor substrate, it is also possible to use an n-type semiconductor substrate. In this case, the conductivity type of various parts noted before is simply reversed.  
         [0112]      FIG. 6  is a cross-sectional diagram showing another embodiment of the semiconductor device of the present invention, wherein the semiconductor device of the present embodiment integrates a conventional p-channel MOS transistor and a conventional n-channel MOS transistor forming a low-voltage CMOS region and a p-channel LDMOS transistor and an n-channel LDMOS transistor forming a high voltage CMOS region on a common semiconductor substrate. Thereby, the semiconductor device of the present embodiment uses the LDMOS transistor of the present invention.  
         [0113]     With the present embodiment, a p-type semiconductor substrate (p-substrate)  1  having the substrate resistivity of about 20 Ωcm is used and a low-voltage CMOS region  3  and a high-voltage CMOS region  5  are formed on the semiconductor substrate  1 .  
         [0114]     Further, a p-channel MOS transistor  7  and an n-channel MOS transistor  9  are formed on the low-voltage CMOS region  3 . The p-channel MOS transistor  7  and the n-channel MOS transistor  9  are operated with a supply voltage of 5V, for example.  
         [0115]     On the high-voltage CMOS region  5 , there are formed an n-channel LDMOS transistor  11  and a p-channel LDMOS transistor  13 , wherein the n-channel LDMOS transistor  11  and the p-channel LDMOS transistor  13  may be operated with a supply voltage of 40V, for example.  
         [0116]     The p-channel MOS transistor  7  and the n-channel MOS transistor  9 , and the n-channel LDMOS transistor  11  and the p-channel LDMOS transistor  13 , are separated with each other by a field oxide film  15  formed on the surface of the p-type substrate  1  with a thickness of about 800 nm.  
         [0117]     In the part of the low-voltage CMOS region  3  corresponding to the p-channel MOS transistor  7 , there is formed an n-type well  17 , wherein a p-type source region  7   s  and a p-type drain region  7   d  are formed in the n-type well  17  with a mutual separation. Further, there is formed a p-type gate electrode  7   g  on a part of the n-type well  17  between the p-type source region  7   s  and the p-type drain region  7   d,  via a gate oxide film  7   ox,  wherein a channel region is formed in the surface part of the n-type well  17   b  right underneath the p-type gate electrode  7   g.  The gate oxide film  7   ox  may have a thickness of about 65 nm. The p-type gate electrode  7   g  is doped with a p-type impurity element such as boron with a concentration level of 1.0×10 18  cm −3 -1.0×10 19  cm −3 , particularly with the dose of 5.0×10 13  cm −2 . The p-type source region  7   s  and the p-type drain region  7   d  are formed in self-alignment with regard to the p-type gate electrode  7   g.    
         [0118]     The n-type well  17  is doped with an n-type impurity element such as phosphorus, with a concentration of 1.5×10 16  cm −3  for the channel region. The p-type source region  7   s  and the p-type drain region  7   d  are introduced with boron as the p-type impurity element, with a boron concentration level of 5.0×10 19  cm −3 . In the p-channel MOS transistor  7 , the p-type source region  7   s  and the p-type drain region  7   d  are formed with higher impurity concentration level as compared with the channel region.  
         [0119]     In the device region corresponding to the n-channel MOS transistor  9 , there is formed a p-type well  19  in the p-type substrate  1 , wherein an n-type source region  9   s  and an n-type drain region  9   d  are formed in the p-type well  19  with a mutual separation. Further, there is formed an n-type gate electrode  9   g  on a part of the p-type well  19  between the n-type source region  9   s  and the n-type drain region  9   d,  via a gate oxide film  9   ox,  wherein a channel region is formed in the surface part of the p-type well  19  right underneath the p-type gate electrode  9   g.  The gate oxide film  9   ox  may have a thickness of about 65 nm. The n-type gate electrode  9   g  is doped with an n-type impurity element such as phosphorus with a concentration level of 1.0×10 21  cm −3 , for example by an ion implantation process or solid phase diffusion process. The n-type source region  9   s  and the n-type drain region  9   d  are formed in self-alignment with regard to the n-type gate electrode  9   g.    
         [0120]     The p-type well  19  is doped with a p-type impurity element such as boron, with a concentration of 1.5×10 16  cm −3  for the channel region. The n-type source region  9   s  and the n-type drain region  9   d  are introduced with phosphorus as the n-type impurity element, with a phosphorus concentration level of 1.0×10 20  cm −3  for example. In the n-channel MOS transistor  9 , the n-type source region  9   s  and the n-type drain region  9   d  are formed with higher impurity concentration level as compared with the channel region.  
         [0121]     In the region of the n-channel LDMOS transistor  11  of the high-voltage CMOS region  5 , there is formed an n-channel LDMOS of the structure identical to that of  FIG. 1 . Because the structure of the n-channel LDMOS transistor  11  is identical with the structure of  FIG. 1 , the description thereof will be omitted.  
         [0122]     In the region of the p-channel LDMOS transistor  13 , there is formed an n-type isolation well  25  for isolating the p-channel LDMOS transistor  13  from the p-type substrate  1 .  
         [0123]     In the n-type isolation well  25 , there is formed a p-type drain well  27 , wherein an n-type channel region  29  is formed in the p-type drain well  27 . A part of the n-type channel well  29  constitutes a channel region. Further, there is a p-type source region  13   g  in the n-type channel well  29 . In the p-type well, there are formed a medium concentration drain region  30  is formed by introducing boron, for example, with separation from the n-type channel well  29  and with a concentration level higher than that of the p-type drain well  27 . Further, a p-type drain region  13   d  doped with a p-type impurity element such as boron is formed in the p-type medium concentration drain region  20  with a concentration level higher than in the p-type medium concentration drain region  30 .  
         [0124]     Further, a p-type gate electrode  13   g  of polysilicon is formed over a part of the p-type drain well  27  between the p-type source region  13   s  and the p-type drain region  13   d  via a gate oxide film  13   ox  so as to cover a part of the n-type channel well  29  and a part of the p-type medium drain region  30  but with separation from the drain region  13   d.  The separation between the p-type gate electrode  13   g  and the p-type drain region  13   d  is set for example to 1.5 μm. The gate oxide film  13   ox  is formed with a thickness of about 30 nm. Thereby, the part of the n-type channel well  29  underneath the p-type gate electrode  13   g  functions as the channel region. The p-type gate electrode  13   g  is introduced with a p-type impurity element such as boron with the concentration of 1.0×10 18 -1.0×10 19  cm −3 , particularly with the dose of 5.0×10 13  cm −2  with the present example. It should be noted that the p-type source region  13   s,  the n-type channel well  29  and the p-type medium concentration drain region  30  are formed in self-alignment to the p-type gate electrode  13   g.    
         [0125]     The n-type channel well  29  is introduced with phosphorus as the n-type impurity element, and the channel region contains phosphorus with the concentration level of 1.0×10 17  cm −3 . Further, the p-type drain well  27  is introduced with boron, wherein the p-type drain well  27  contains boron with the concentration level of 1.2×10 16  cm −3 . In the p-channel LDMOS transistor  13 , it should be noted that the drain region has a lower concentration level than in the channel region.  
         [0126]     Further, there is formed an interlayer CVD film  28  over the entire surface of the p-type substrate  1  by a CVD (chemical vapor deposition) process. The interlayer insulation film  28  is formed with a metal interconnection  26  of aluminum, for example. Thereby, the metal interconnection  26  is connected electrically to the p-type source regions  7   s  and  13   s,  n-type source regions  9   s  and  11   s,  p-type drain regions  7   d  and  13   d  and n-type drain regions  9   d  and  11   d  via respective contact holes.  
         [0127]     With this embodiment, the gate oxide films  7   ox  and  9   ox  have a thickness of about 65 nm in the p-channel MOS transistor  7  and the n-channel MOS transistor  9  wherein this film thickness is larger than the film thickness of the gate oxide film  11   ox  or  13   ox  of the n-channel LDMOS transistor  11  or the p-channel LDMOS transistor  13  of about 30 nm. It should be noted that the n-type well  17  of the p-channel MOS transistor  7 , the n-type drain well  21  of the n-channel LDMOS transistor  11  and the n-type isolation well  25  of the p-channel LDMOS transistor  13  are formed simultaneously. Further, the p-type well  19  of the n-channel MOS transistor  9 , the p-type drain well  27  of the p-channel LDMOS transistor  13  and the p-type low-concentration region  22  of the n-channel LDMOS transistor  11  are formed simultaneously. Because the gate oxide films  7   ox  and  9   ox  are formed with a smaller thickness as compared with the gate oxide films  7   ox  and  9   ox  in the p-channel MOS transistor  7  and the n-channel MOS transistor  9 , it is possible to use the n-type well  17  formed simultaneously with the n-type drain well  21  and the n-type isolation well  25 . Further, it becomes possible to user the p-type well  19  formed simultaneously to the p-type drain well  27  and the p-type low-concentration region  22 .  
         [0128]     With regard to the n-channel LDMOS transistor  11  and the p-channel LDMOS transistor  13 , the medium concentration drain regions  24  and  30  are formed respectively adjacent to the n-type gate electrode  11   g  and the p-type gate electrode  13   g  in self-alignment, and thus, it is possible to reduce the drain resistance and stabilize the transistor characteristics. Further, because the n-type source region  11   s  and the p-type source region  13   s  are formed respectively in self-alignment with regard to the n-type gate electrode  11   g  and the p-type gate electrode  13   g,  it is possible to reduce the source resistance and stabilize the transistor characteristics.  
         [0129]     Further, because the p-type low-concentration well is formed by conducting the impurity ion implantation process for forming the p-type well and the p-type drain well also in the n-type drain region  21 , there is no increase in the number of fabrication steps, and it becomes possible to increase the breakdown voltage of the n-channel LDMOS transistor without increasing the number of the fabrication steps.  
         [0130]     While the present embodiment describes the semiconductor device that includes the p-channel MOS transistor  7 , the n-channel MOS transistor  9 , the n-channel LDMOS transistor  11  and the p-channel LDMOS transistor  13  on the same p-type substrate  1 , the present invention is by no means limited to such a specific construction but may be applied to any semiconductor device that has an LDMOS transistor.  
         [0131]     Next, the fabrication process of the semiconductor device of the present embodiment will be described with reference to  FIGS. 7A-7L .  
         [0132]     (1) Referring to  FIG. 7A , a resist pattern is formed on the p-type substrate  1  such that the resist pattern has resist openings in correspondence to the device region of the p-channel MOS transistor  7  in the low-voltage CMOS region  3  and the device regions of the n-channel LDMOS transistor  11  and the p-channel LDMOS transistor  13  in the high-voltage CMOS region  5 , and ion implantation of phosphorus is conducted into the p-type silicon substrate  1  under the acceleration energy of 150 keV with the dose of about 4.2×10 12  cm −2  while using the resist pattern as a mask. Further, after removing the resist pattern, thermal activation is conducted at 1180° C. for 24 hours. With this, the n-type well  17 , the n-type isolation well  25  and the n-type drain well  21  are formed simultaneously.  
         [0133]     (2) Next, in the step of  FIG. 7B , a resist pattern is formed on the p-type substrate  1  such that the resist pattern has resist openings in correspondence to the device region of the n-channel MOS transistor  9  in the low-voltage CMOS region  3  and the device regions of the p-channel LDMOS transistor  13  in the n-type isolation well  25  and the n-channel LDMOS transistor  11  in n-type drain well  21 , and ion implantation of boron is conducted into the p-type silicon substrate  1  under the acceleration energy of 50 keV with the dose of about 3.0×10 12  cm −2  while using the resist pattern as a mask. Further, after removing the resist pattern, thermal activation is conducted at 1150° C. for 8 hours. With this, the p-type well  19 , the p-type drain well  27  and the p-type low-concentration well  22  are formed simultaneously, respectively in the device region of the n-channel MOS transistor  9 , the n-type isolation well  25  and in the n-type drain well  21 . Thereby, the p-type well  19 , the p-type well  27  and p-type well  22  contains the same p-type impurity element with the same concentration level.  
         [0134]     (3) Next, in the step of  FIG. 7C , a field oxide film  15  is formed on the surface of the p-type substrate  1  by a LOCOS oxidation process with the thickness of 800 nm, such that the regions of the transistors  7 ,  9 ,  11  and  13  are defined. In this step, it is also possible to form an impurity region under the field oxide films  15  for the purpose of channel stopper.  
         [0135]     (4) Next, in the step of  FIG. 7D , a pre-gate oxide film  2  is formed over the surfaces of the n-type well  17 , the p-type well  19 , the n-type drain well  21  and the p-type drain well  17  with the thickness of 40 nm. Further, a resist pattern  4  is formed thereon such that the resist pattern  4  has a resist opening in correspondence to the device region of the p-channel MOS transistor  7 , and channel doping is conducted into the n-type well  17  for the purpose of threshold control while using the resist pattern  4  as a mask.  
         [0136]     (5) Next, in the step of  FIG. 7E , the resist pattern  4  is removed and a resist pattern  6  is formed so as to have an opening in correspondence to the device region of the n-channel MOS transistor  9 . Further, while using the resist pattern  6  as a mask, channel doping is conducted to the p-type well  19  for the purpose of threshold control.  
         [0137]     (6) Next, in the step of  FIG. 7F , the resist pattern  6  is removed, and a resist pattern  8  is formed such that the resist pattern  8  has openings in correspondence to the device regions of the n-channel LDMOS transistor  11  and the p-channel LDMOS transistor  13 . Further, the pre-gate oxide films  2  on the surface of the n-type drain well  21  and the p-type drain well  27  are removed by using a hydrofluoric acid.  
         [0138]     (7) Next, in the step of  FIG. 7G , the resist pattern  8  is removed and a thermal oxidation processing is conducted. With this, a gate oxide film  11   ox  and a gate oxide film  13   ox  are formed respectively on the surface of the n-type drain well  21  and the p-type drain well  27  with a thickness of about 30 nm. With this thermal processing, it should be noted that the surface of the n-type well  17  and the surface of the p-type well  9  are also oxidized and there are formed gate oxide films  7   ox  and  9   ox  respectively on the surfaces of the n-type well  17  and the p-type well  19  with a thickness of 65 nm. In the case it is desired to form the gate oxide films  7   ox,    9   ox,    11   ox  and  13   ox  all with the same film thickness, the pre-gate oxide film  2  is removed in the step (6) from all of the regions and newly form the respective gate oxide films with the thickness of 60-80 nm, for example, by conducting a thermal oxidation processing.  
         [0139]     (8) Next, in the step of  FIG. 7H , a polysilicon film  10  is formed on the entire surface of the p-type substrate  1  by a CVD process with a thickness of 500 nm. Further, a silicon oxide not illustrated is formed on the surface of the polysilicon film  10  with a thickness of 25 nm by a thermal oxidation processing conducted at a process temperature of 850° C.  
         [0140]     (9) Next, in the step of  FIG. 7I , a resist pattern  12  is formed on the polysilicon film  10  such that the resist pattern  12  has an opening in correspondence to the device region of the p-channel MOS transistor  7  and the device region of the p-channel LDMOS transistor  13 , and a p-type polysilicon film  10   p  is formed by introducing boron into the polysilicon film under the acceleration voltage of 15 keV with the dose of about 5.0×10 13  cm −2  while using the resist pattern  12  as a mask.  
         [0141]     (10) Next, in the step of  FIG. 7J , the resist pattern  12  is removed and a silicon oxide film  14  is formed on the entire surface of the semiconductor substrate  1  by a CVD process with the thickness of 200 nm. Further, photolithographic process is conducted and an opening is formed in the silicon oxide film  14  in correspondence to the device region of the n-channel MOS transistor  9  and the device region of the n-channel LDMOS transistor  11 . Further, phosphorus is deposited on the entire surface of the p-type substrate  1  and an n-type polysilicon film  10   n  is formed after conducting a thermal annealing process at 900° C. for 30 minutes. It should be noted that the formation of the n-type polysilicon film  10   n  may be conducted by an ion implantation process.  
         [0142]     (11) Next, in the step of  FIG. 7K , the silicon oxide film  14  is removed and the polysilicon film  10   n  of n-type and the polysilicon film  10   p  of p-type thus formed are subjected to a patterning process. With this, the p-type gate electrode  7   g  of p-type polysilicon is formed on the gate oxide film  7   ox  in the device region of the p-channel MOS transistor  7 , the n-type gate electrode  9   g  is formed on the gate oxide film  9   ox  in the device region of the n-channel LDMOS transistor  9 , the n-type gate electrode  11   g  of n-type polysilicon is formed on the gate oxide film  11   ox  in the device region of the n-channel LDMOS transistor  11 , and the p-type gate electrode  13   g  of p-type polysilicon is formed on the gate oxide film  13   ox  in the device region of the p-channel LDMOS transistor  13 , wherein the gate electrodes  7   g,    9   g,    11   g  and  13   g  are formed simultaneously.  
         [0143]     Further, a resist pattern is formed such that the resist pattern has an opening in correspondence to the region where the n-type channel well  29  of the p-channel LDMOS transistor  13  is to be formed including the gate electrode  13   g,  and ion implantation of phosphorus is conducted under the acceleration energy of 100 keV with the dose of about 3.3×10 13  cm −2  while using the resist pattern and the p-type gate electrode  13   g  as a mask, and with this, the n-type channel well  29  is formed adjacent to the p-type gate electrode  13   g.    
         [0144]     Further, a resist pattern having an opening in correspondence to the region where the p-type channel well  23  of the n-channel LDMOS transistor  11  is to be formed including the n-type gate electrode  11   g,  and ion implantation of boron is conducted under the acceleration voltage of 30 keV with the dose of about 3.3×10 13  cm −2  while using the resist pattern and the n-type gate electrode  11   g  as a mask. With this, the p-type channel well  23  is formed adjacent to the n-type gate electrode  11   g.    
         [0145]     Further, a resist pattern is formed such that the resist pattern has an opening in correspondence to the region where the p-type medium concentration drain region  30  of the p-channel LDMOS transistor  33  is to be formed including the p-type gate electrode  13   g,  and the p-type medium concentration drain region  30  is formed adjacent to the p-type gate electrode  13   g  by introducing boron into the p-type drain well  27  under the acceleration energy of 30 keV with the dose of about 4.0×10 12  cm −2  while using the resist pattern and the p-type gate electrode  13   g  as a mask.  
         [0146]     Further, a resist pattern is formed such that the resist pattern has an opening in correspondence to the region where the n-type medium concentration drain region  24  of the n-channel LDMOS transistor  11  is to be formed including the n-type gate electrode  11   g,  and the n-type medium concentration drain region  24  is formed adjacent to the n-type gate electrode  11   g  by introducing phosphorus into the n-type drain well  21  under the acceleration energy of 100 keV with the dose of about 3.0×10 12  cm −2  while using the resist pattern and the n-type gate electrode  11   g  as a mask.  
         [0147]     Thereafter, thermal activation processing is conducted at the process temperature of 1100° C. for the duration of 140 minutes, and with this, the p-type channel well  23 , the n-type medium concentration drain region  24 , the n-type channel well  29  and the p-type medium concentration region  30  are formed as a result of the diffusion of the respective impurity elements.  
         [0148]     (12) Next, in the step of  FIG. 7L , a resist pattern is formed over the p-type substrate  1  such that the resist pattern has openings respectively in correspondence to the n-type well  17  in the device region of the p-channel MOS transistor  7 , the region of the n-type channel well  29  of the device region of the p-channel LDMOS transistor  13  adjacent to the p-type gate electrode  13   g,  and further in the p-type medium concentration drain region  30 . Thereby, the resist pattern is formed such that there exists a resist pattern having a width of 1.5 μm on the p-type medium concentration drain region  30  adjacent to the p-type gate electrode  13   g.  Further, ion implantation of boron is conducted to the n-type well  17 , the n-type channel well  29  and further the p-type medium drain region  30  under the acceleration energy of 30 keV with the dose of about 1.5×10 15  cm −2  while using the resist pattern as a mask.  
         [0149]     After removing the resist pattern, a resist pattern is formed over the p-type substrate  1  such that the resist pattern includes openings in correspondence to the p-type well  19  in the device region of the n-channel MOS transistor  9 , the region of the p-type well  23  in the device region of the n-channel LDMOS transistor  11  adjacent to the n-type gate electrode  11   g,  and the n-type medium concentration drain region  24 . Thereby, the resist pattern is formed such that there exists a resist patter of the width of 2.0 μm over the n-type medium concentration drain region  24  adjacent to the n-type gate electrode  11   g.  Further, while using the resist pattern as a mask, ion implantation of phosphorous or arsenic is conducted simultaneously to the p-type well  19 , the p-type channel well  23  and the n-type medium concentration drain region  24  under the acceleration energy of 50 keV with the dose of 6.0×10 15  cm −2 .  
         [0150]     Further, after removal of the resist pattern, a thermal activation processing is conducted, and with this, the p-type source region  7   s  and the p-type drain region  7   d  are formed in the n-type well  17  in the device region of the p-channel MOS transistor  7 , the n-type source region  9   s  and the n-type drain region  9   d  are formed in the p-type well  19  in the device region of the p-channel MOS transistor  9 . Further, in the device region of the n-channel LDMOS transistor  11 , the n-type source region  11   s  is formed in the p-type channel well  23  and the n-type drain region  11   d  is formed in the n-type medium concentration drain region  24 . Further, in the device region of the p-channel LDMOS transistor  13 , the p-type source region  13   s  is formed in the n-type channel well  29  and the p-type drain region  13   d  is formed in the p-type medium concentration drain region  30 . Thereby, it should be noted that, in order to relax the gate electric field, the n-type drain region  11   d  of the n-channel LDMOS transistor  11  is formed with a separation of about 2.0 μm from the n-type gate electrode  11   g.  Similarly, the p-type drain region  13   d  of the p-channel LDMOS transistor  13  is formed with a separation of about 1.5 μm from the p-type gate electrode  13   g.    
         [0151]     Further, the interlayer CVD film  28  is formed over the entire surface of the p-type substrate  1 , and contact holes are formed in the interlayer CVD film  28  in correspondence to the p-type source regions  7   s  and  13   s,  n-type source regions  9   s  and  11   s,  p-type drain regions  7   d  and  13   d  and n-type drain regions  9   d  and  11   d,  and the metal interconnection  26  is formed in the contact holes and in the interlayer CVD film  28 . Reference should be made to  FIG. 2 .  
         [0152]     In the embodiment of  FIG. 6 , the low voltage CMOS region  3  and the high-voltage CMOS region  5  are formed on the p-type substrate  1 , while it is also possible to form these in a common p-type well.  
         [0153]     Further, it should be noted that the mode of the present invention applied to the embodiment of  FIG. 5  can be applied to the case of using an n-type semiconductor substrate or in the case of forming a low-voltage CMOS region and a high-voltage CMOS region in a common n-type well.  
         [0154]     As shown in the present embodiment, it is also possible to improve the drain breakdown voltage from the conventional value of about 33V to about 42V also with the n-channel LDMOS transistor  11  by forming the p-type low concentration well  22  in the n-type drain well  21  by conducting the ion implantation of p-type impurity element such as boron, which is conducted to the p-channel LDMOS transistor  13 .  
         [0155]     In the case the p-channel LDMOS transistor  13  and the n-channel MOS transistor  11  are formed on the same semiconductor substrate and further forming the p-type low concentration well  27  of the p-channel LDMOS transistor and the p-type low concentration well  22  of the n-channel LDMOS transistor  11  on the same semiconductor substrate in the same step simultaneously ad in the present embodiment, there occurs a decrease in the drain breakdown voltage of the p-channel LDMOS transistor and increase of the drain breakdown voltage in the n-channel LDMOS transistor  11  with increase of the amount of injection of the p-type impurity element as shown in  FIG. 12 . Thereby, the drain breakdown voltages of the LDMOS transistors  11  and  13  are balanced at about 42V when the p-type impurity concentration level has reached the value of approximately 1.2×10 16  cm −3 . Thus, with the present embodiment, it is concluded that the preferable impurity concentration level is 1.2×10 16  cm −3 .  
         [0156]     It should be noted that the drive capability of transistor is influenced primarily by the threshold voltage and the contribution of drain resistance per well resistance caused by change of well concentration level is small. Thus, it becomes possible to share the process of forming the n-type isolation well  25  of the p-channel LDMOS transistor  13  and the process of forming the n-type drain well  21  of the n-channel LDMOS transistor  11 , and it becomes possible to increase the breakdown voltage of both the n-channel LDMOS transistor and the p-channel LDMOS transistor to 40V or higher without increasing the fabrication process steps such as impurity doping process.  
         [0157]      FIG. 8  is a cross-sectional diagram showing a further embodiment of the semiconductor device of the present invention, wherein the semiconductor device of the present embodiment integrates a p-channel MOS transistor and a conventional n-channel MOS transistor forming a low-voltage CMOS region and a p-channel LDMOS transistor and an n-channel LDMOS transistor forming a high voltage CMOS region on a common semiconductor substrate. Thereby, the semiconductor device of the present embodiment uses the LDMOS transistor of the present invention.  
         [0158]     With the present embodiment, an n-type semiconductor substrate (n-substrate)  31  having the substrate resistivity of about 20 Ωcm is used and a low-voltage CMOS region  33  and a high-voltage CMOS region  35  are formed on the semiconductor substrate  31 .  
         [0159]     Further, a p-channel MOS transistor  37  and an n-channel MOS transistor  39  are formed on the low-voltage CMOS region  33 . The p-channel MOS transistor  37  and the n-channel MOS transistor  39  are operated with a supply voltage of 5V, for example.  
         [0160]     On the high-voltage CMOS region  35 , there are formed a p-channel LDMOS transistor  41  and an n-channel LDMOS transistor  43 , wherein the p-channel LDMOS transistor  41  and the n-channel LDMOS transistor  43  may be operated with a supply voltage of 40V, for example.  
         [0161]     The p-channel MOS transistor  37  and the n-channel MOS transistor  39 , and the p-channel LDMOS transistor  41  and the n-channel LDMOS transistor  43 , are separated with each other by a field oxide film  45  formed on the surface of the n-type substrate  31  with a thickness of about 800 nm.  
         [0162]     In the part of the low-voltage CMOS region  33  corresponding to the p-channel MOS transistor  37 , there is formed an n-type well  47 , wherein a p-type source region  37   s  and a p-type drain region  37   d  are formed in the n-type well  47  with a mutual separation. Further, there is formed a p-type gate electrode  37   g  on a part of the n-type well  47  between the p-type source region  37   s  and the p-type drain region  37   d,  via a gate oxide film  37   ox,  wherein a channel region is formed in the surface part of the n-type well  47   b  right underneath the p-type gate electrode  37   g.  The gate oxide film  37   ox  may have a thickness of about 65 nm. The p-type gate electrode  37   g  is doped with a p-type impurity element such as boron with a concentration level of 1.0×10 18  cm −3 −1.0×10 19  cm −3 , particularly with the dose of 5.0×10 13  cm −2 . The p-type source region  73   s  and the p-type drain region  37   d  are formed in self-alignment with regard to the p-type gate electrode  37   g.    
         [0163]     The n-type well  47  is doped with an n-type impurity element such as phosphorus, with a concentration of 1.5×10 16  cm −3  for the channel region. The p-type source region  37   s  and the p-type drain region  37   d  are introduced with boron as the p-type impurity element, with a boron concentration level of 5.0×10 19  cm −3 . In the p-channel MOS transistor  37 , the p-type source region  37   s  and the p-type drain region  37   d  are formed with higher impurity concentration level as compared with the channel region.  
         [0164]     In the device region corresponding to the n-channel MOS transistor  39 , there is formed a p-type well  49  in the n-type substrate  31 , wherein an n-type source region  39   s  and an n-type drain region  39   d  are formed in the p-type well  49  with a mutual separation. Further, there is formed an n-type gate electrode  39   g  on a part of the p-type well  49  between the n-type source region  39   s  and the n-type drain region  39   d,  via a gate oxide film  39   ox,  wherein a channel region is formed in the surface part of the p-type well  49  right underneath the n-type gate electrode  39   g.  The gate oxide film  39   ox  may have a thickness of about 65 nm. The n-type gate electrode  39   g  is doped with an n-type impurity element such as phosphorus with a concentration level of 1.0×10 21  cm −3 , for example by an ion implantation process or solid phase diffusion process. The n-type source region  39   s  and the n-type drain region  39   d  are formed in self-alignment with regard to the n-type gate electrode  39   g.    
         [0165]     The p-type well  49  is doped with a p-type impurity element such as boron, with a concentration of 1.5×10 16  cm −3  for the channel region. The n-type source region  39   s  and the n-type drain region  39   d  are introduced with phosphorus as the n-type impurity element, with a phosphorus concentration level of 1.0×10 20  cm −3 , for example. In the n-channel MOS transistor  39 , the n-type source region  39   s  and the n-type drain region  39   d  are formed with higher impurity concentration level as compared with the channel region.  
         [0166]     In the device region of the p-channel LDMOS transistor  41  in the high-voltage CMOS region  35 , there is formed a p-type drain well  51  in the n-type substrate  31 , and an n-type low-concentration well  52  is formed in the p-type drain well  51 . Further, there is formed an n-type channel well  53  in the n-type low-concentration well  52  wherein a part of the n-type channel well  53  functions as the channel region. A p-type source region  41   s  is formed in the n-type channel well  53 , wherein a p-type medium concentration drain region  54  doped with boron is formed in the p-type drain well  51  with separation from the n-type channel well  53  wherein the p-type medium concentration drain region  54  is formed with a concentration higher than the n-type low-concentration well  52 . Further, a p-type drain region  41   d  doped with a p-type impurity element such as boron is formed in the p-type medium concentration drain region  54  with a concentration higher than the p-type medium concentration drain region  54 .  
         [0167]     Further, a p-type gate electrode  41   g  of polysilicon is formed over a part of the p-type drain well  51  between the p-type source region  41   s  and the p-type drain region  41   d  via a gate oxide film  43   ox  so as to cover a part of the n-type channel well  53  and a part of the p-type medium drain region  54  but with separation from the drain region  41   d.  The separation between the p-type gate electrode  41   g  and the p-type drain region  41   d  is set for example to 1.5 μm. The gate oxide film  41   ox  is formed with a thickness of about 30 nm. Thereby, the part of the n-type channel well  53  underneath the p-type gate electrode  41   g  functions as the channel region. The p-type gate electrode  41   g  is introduced with a p-type impurity element such as boron with the concentration of 1.0×10 18 -1.0×10 19  cm −3 , particularly with the dose of 5.0×10 13  cm −2  with the present example. It should be noted that the p-type source region  41   s,  the n-type channel well  53  and the p-type medium concentration drain region  54  are formed in self-alignment to the p-type gate electrode  41   g.    
         [0168]     The n-type channel well  53  is introduced with phosphorus as the n-type impurity element, and the channel region contains phosphorus with the concentration level of 1.0×10 17  cm −3 . Further, the p-type drain well  51  is introduced with boron, wherein the p-type drain well  51  contains boron with the concentration level of 8.0×10 15  cm −3 . Further, the n-type low-concentration well  52  is introduced with an n-type impurity element such as phosphorus, wherein phosphorous concentration between the n-type channel well  53  and the p-type medium concentration drain region is 1.2×10 16  cm −3 .  
         [0169]     In the device region of the n-channel LDMOS transistor  43 , there is formed a p-type isolation well  55  in the n-type substrate  31  so as to isolate the LDMOS transistor  43  from the n-type substrate  31 . Further, an n-type drain well  57  is formed in the p-type isolation well  55 . In the n-type drain well  57 , there is formed a p-type channel well  59 , wherein a part of the p-type channel well  59  constitutes the channel region. An n-type source region  43   s  is formed in the p-type channel well  59 . In the n-type drain well  57 , there is formed an n-type medium concentration drain region  60  doped with an n-type impurity element such as phosphorus with a separation from the p-type channel well  59  wherein the n-type medium concentration drain region  60  contains phosphorus with a concentration level higher than the n-type drain well  57 . Further, an n-type drain region  43   d  is formed in the n-type medium concentration drain region  60  wherein the n-type drain region  43   d  is doped with an n-type impurity element such as phosphorus with a concentration level higher than the n-type medium concentration drain region  60 .  
         [0170]     Further, there is formed a gate electrode  43   g  of n-type polysilicon via a gate oxide film  43   ox  so as to extend over a part of the n-type drain well  57  located between the n-type source region  43   s  and the n-type drain region  43   d  with a separation from the n-type drain region  43   d,  such that the gate electrode  43   g  further covers a part of the p-type channel well  59  and a part of the n-type medium-concentration drain region  60 . Thereby, the separation between the n-type gate electrode  43   g  and the n-type drain region  43   d  is set for example to 2.0 μm, while the gate oxide film  43   ox  may have a thickness of about 30 nm, for example. The surface of the p-type channel well  59  located underneath the n-type gate electrode  43   g  serves for the channel region. It should be noted that the n-type gate electrode  43   g  is introduced with phosphorus by an ion implantation process or solid diffusion process with a concentration level of 1.0×10 21  cm −3 . The n-type source region  43   s,  the p-type channel well  59  and the n-type medium concentration drain region  60  are formed in self-alignment to the n-type gate electrode  43   g.    
         [0171]     The p-type channel well  59  is introduced with boron as the p-type impurity element with a concentration level of 1.0×10 17  cm −3 . The n-type drain well  60  is introduced with phosphorus as the n-type impurity element with the concentration level of 1.2×10 16  cm −3  for the part between the n-type drain region  43   d  and the p-type channel well  59 . In the n-channel LDMOS transistor  43 , the drain region has an impurity concentration level lower than the channel region.  
         [0172]     Further, there is formed an interlayer CVD film  58  over the entire surface of the n-type substrate  31  by a CVD (chemical vapor deposition) process. The interlayer insulation film  58  is formed with a metal interconnection  56  of aluminum, for example. Thereby, the metal interconnection  56  is connected electrically to the p-type source regions  37   s  and  41   s,  n-type source regions  39   s  and  43   s,  p-type drain regions  37   d  and  41   d  and n-type drain regions  39   d  and  43   d  via respective contact holes.  
         [0173]     With this embodiment, the gate oxide films  37   ox  and  39   ox  have a thickness of about 65 nm in the p-channel MOS transistor  37  and the n-channel MOS transistor  39  wherein this film thickness is larger than the film thickness of the gate oxide film  41   ox  or  43   ox  of the p-channel LDMOS transistor  41  or the n-channel LDMOS transistor  43  of about 30 nm. It should be noted that the p-type well  49  of the n-channel MOS transistor  39 , the p-type drain well  51  of the p-channel LDMOS transistor  41  and the p-type isolation well  55  of the n-channel LDMOS transistor  43  are formed simultaneously. Further, the n-type well  47  of the p-channel MOS transistor  37 , the n-type drain well  57  of the n-channel LDMOS transistor  43  and the n-type low-concentration region  52  of the p-channel LDMOS transistor  41  are formed simultaneously.  
         [0174]     With regard to the p-channel LDMOS transistor  41  and the p-channel LDMOS transistor  43 , the medium concentration drain regions  54  and  60  are formed respectively adjacent to the p-type gate electrode  41   g  and the n-type gate electrode  43   g  in self-alignment, and thus, it is possible to reduce the drain resistance and stabilize the transistor characteristics. Further, because the p-type source region  41   s  and the n-type source region  43   s  are formed respectively in self-alignment with regard to the p-type gate electrode  41   g  and the n-type gate electrode  43   g,  it is possible to reduce the source resistance and stabilize the transistor characteristics.  
         [0175]     While the present embodiment describes the semiconductor device that includes the p-channel MOS transistor  37 , the n-channel MOS transistor  39 , the n-channel LDMOS transistor  41  and the p-channel LDMOS transistor  43  on the same n-type substrate  31 , the present invention is by no means limited to such a specific construction but may be applied to any semiconductor device that has an LDMOS transistor.  
         [0176]     The semiconductor device of the embodiment of  FIG. 8  can be fabricated similarly by reversing the conductivity type in the fabrication process explained from  FIG. 6  and  FIG. 7A-7L .  
         [0177]      FIGS. 9A-9L  are diagrams showing the fabrication process of the semiconductor device of  FIG. 8 . Hereinafter, the fabrication process of the present embodiment will be described with reference to  FIGS. 9A-9L  and further with reference to  FIG. 8 .  
         [0178]     (1) Referring to  FIG. 9A , a resist pattern is formed on the n-type substrate  31  such that the resist pattern has resist openings in correspondence to the device region of the n-channel MOS transistor  39  in the low-voltage CMOS region  33  and the device regions of the p-channel LDMOS transistor  41  and the n-channel LDMOS transistor  43  in the high-voltage CMOS region  35 , and ion implantation of boron is conducted into the n-type silicon substrate  31  under the acceleration energy of 50 keV with the dose of about 4.0×10 12  cm −2  while using the resist pattern as a mask. Further, after removing the resist pattern, thermal activation is conducted at 1180° C. for 24 hours. With this, the p-type well  49 , the p-type isolation well  55  and the p-type drain well  51  are formed simultaneously.  
         [0179]     (2) Next, in the step of  FIG. 9B , a resist pattern is formed on the n-type substrate  31  such that the resist pattern has resist openings in correspondence to the device region of the p-channel MOS transistor  37  and the device regions of the n-channel LDMOS transistor  43  in the p-type isolation well  55  and the p-channel LDMOS transistor  41  in p-type drain well  51 , and ion implantation of phosphorus is conducted into the n-type silicon substrate  31  under the acceleration energy of 150 keV with the dose of about 3.0×10 12  cm −2  while using the resist pattern as a mask. Further, after removing the resist pattern, thermal activation is conducted at 1150° C. for 8 hours. With this, the n-type well  47 , the n-type drain well  57  and the n-type low-concentration well  52  are formed simultaneously, respectively in the device region of the p-channel MOS transistor  37 , the p-type isolation well  55  and in the p-type drain well  51 . Thereby, the well  47 , the well  57  and the well  52  contains the same n-type impurity element with the same concentration level.  
         [0180]     (3) Next, in the step of  FIG. 9C , a field oxide film  45  is formed on the surface of the n-type substrate  41  by a LOCOS oxidation process with the thickness of 800 nm, such that the regions of the transistors  37 ,  39 ,  41  and  43  are defined. In this step, it is also possible to form an impurity region under the field oxide films  45  for the purpose of channel stopper.  
         [0181]     (4) Next, in the step of  FIG. 9D , a pre-gate oxide film  32  is formed over the surfaces of the n-type well  47 , the p-type well  49 , the p-type drain well  51  and the n-type drain well  57  with the thickness of 40 nm. Further, a resist pattern  34  is formed thereon such that the resist pattern  34  has a resist opening in correspondence to the device region of the p-channel MOS transistor  37 , and channel doping is conducted into the n-type well  47  for the purpose of threshold control while using the resist pattern  34  as a mask.  
         [0182]     (5) Next, in the step of  FIG. 9E , the resist pattern  34  is removed and a resist pattern  36  is formed so as to have an opening in correspondence to the device region of the n-channel MOS transistor  39 . Further, while using the resist pattern  36  as a mask, channel doping is conducted to the p-type well  39  for the purpose of threshold control.  
         [0183]     (6) Next, in the step of  FIG. 9F , the resist pattern  36  is removed, and a resist pattern  38  is formed such that the resist pattern  38  as openings in correspondence to the device regions of the p-channel LDMOS transistor  41  and the n-channel LDMOS transistor  43 . Further, the pre-gate oxide films  32  on the surface of the p-type drain well  51  and the n-type drain well  57  are removed by using a hydrofluoric acid.  
         [0184]     (7) Next, in the step of  FIG. 9G , the resist pattern  38  is removed and a thermal oxidation processing is conducted. With this, a gate oxide film  41   ox  and a gate oxide film  43   ox  are formed respectively on the surface of the n-type drain well  51  and the p-type drain well  57  with a thickness of about 30 nm. With this thermal processing, it should be noted that the surface of the n-type well  47  and the surface of the p-type well  49  are also oxidized and there are formed gate oxide films  37   ox  and  39   ox  respectively on the surfaces of the n-type well  47  and the p-type well  49  with a thickness of 65 nm.  
         [0185]     (8) Next, in the step of  FIG. 9H , a polysilicon film  40  is formed on the entire surface of the n-type substrate  31  by a CVD process with a thickness of 500 nm. Further, a silicon oxide not illustrated is formed on the surface of the polysilicon film  40  with a thickness of 25 nm by a thermal oxidation processing conducted at a process temperature of 850° C.  
         [0186]     (9) Next, in the step of  FIG. 9I , a resist pattern  42  is formed on the polysilicon film  40  such that the resist pattern  42  has an opening in correspondence to the device region of the p-channel MOS transistor  37  and the device region of the p-channel LDMOS transistor  41 , and a p-type polysilicon film  40   p  is formed by introducing boron into the polysilicon film under the acceleration voltage of 15 keV with the dose of about 5.0×10 13  cm −2  while using the resist pattern  42  as a mask.  
         [0187]     (10) Next, in the step of  FIG. 9J , the resist pattern  42  is removed and a silicon oxide film  44  is formed on the entire surface of the semiconductor substrate  1  by a CVD process with the thickness of 200 nm. Further, photolithographic process is conducted and an opening is formed in the silicon oxide film  44  in correspondence to the device region of the n-channel MOS transistor  39  and the device region of the n-channel LDMOS transistor  43 . Further, phosphorus is deposited on the entire surface of the n-type substrate  31  and an n-type polysilicon film  40   n  is formed after conducting a thermal annealing process at 900° C. for 30 minutes. It should be noted that the formation of the n-type polysilicon film  40   n  may be conducted by an ion implantation process.  
         [0188]     (11) Next, in the step of  FIG. 9K , the silicon oxide film  44  is removed and the polysilicon film  40   n  of n-type and the polysilicon film  40   p  of p-type thus formed are subjected to a patterning process. With this, the p-type gate electrode  37   g  of p-type polysilicon is formed on the gate oxide film  37   ox  in the device region of the p-channel MOS transistor  37 , the n-type gate electrode  39   g  is formed on the gate oxide film  39   ox  in the device region of the n-channel LDMOS transistor  39 , the p-type gate electrode  41   g  of p-type polysilicon is formed on the gate oxide film  41   ox  in the device region of the p-channel LDMOS transistor  41 , and the n-type gate electrode  43   g  of n-type polysilicon is formed on the gate oxide film  43   ox  in the device region of the n-channel LDMOS transistor  43 , wherein the gate electrodes  37   g,    39   g,    41   g  and  43   g  are formed simultaneously.  
         [0189]     Further, a resist pattern is formed such that the resist pattern has an opening in correspondence to the region where the p-type channel well  59  of the n-channel LDMOS transistor  43  is to be formed including the gate electrode  43   g,  and ion implantation of boron is conducted under the acceleration energy of 30 keV with the dose of about 3.3×10 13  cm −2  while using the resist pattern and the n-type gate electrode  43   g  as a mask, and with this, the p-type channel well  59  is formed adjacent to the n-type gate electrode  43   g.    
         [0190]     Further, a resist pattern having an opening in correspondence to the region where the n-type channel well  53  of the p-channel LDMOS transistor  41  is to be formed including the p-type gate electrode  41   g,  and ion implantation of phosphorus is conducted under the acceleration voltage of 100 keV with the dose of about 3.3×10 13  cm −2  while using the resist pattern and the p-type gate electrode  41   g  as a mask. With this, the n-type channel well  53  is formed adjacent to the p-type gate electrode  41   g.    
         [0191]     Further, a resist pattern is formed such that the resist pattern has an opening in correspondence to the region where the n-type medium concentration drain region  60  of the n-channel LDMOS transistor  43  is to be formed including the n-type gate electrode  43   g,  and the n-type medium concentration drain region  60  is formed adjacent to the n-type gate electrode  43   g  by introducing phosphorus into the n-type drain well  57  under the acceleration energy of 100 keV with the dose of about 3.0×10 12  cm −2  while using the resist pattern and the n-type gate electrode  43   g  as a mask.  
         [0192]     Further, a resist pattern is formed such that the resist pattern has an opening in correspondence to the region where the p-type medium concentration drain region  54  of the p-channel LDMOS transistor  41  is to be formed including the p-type gate electrode  41   g,  and the p-type medium concentration drain region  54  is formed adjacent to the p-type gate electrode  41   g  by introducing boron into the p-type drain well  51  under the acceleration energy of 30 keV with the dose of about 4.0×10 12  cm −2  while using the resist pattern and the p-type gate electrode  41   g  as a mask.  
         [0193]     Thereafter, thermal activation processing is conducted at the process temperature of 1100° C. for the duration of 200 minutes, and with this, the n-type channel well  53 , the p-type medium concentration drain region  54 , the p-type channel well  59  and the n-type medium concentration region  60  are formed as a result of the diffusion of the respective impurity elements.  
         [0194]     (12) Next, in the step of  FIG. 9L , a resist pattern is formed over the n-type substrate  31  such that the resist pattern has openings respectively in correspondence to the p-type well  49  in the device region of the n-channel MOS transistor  39 , the region of the p-type channel well  59  of the device region of the n-channel LDMOS transistor  43  adjacent to the n-type gate electrode  43   g,  and further in the n-type medium concentration drain region  60 . Thereby, the resist pattern is formed such that there exists a resist pattern having a width of 2.0 μm on the n-type medium concentration drain region  60  adjacent to the n-type gate electrode  43   g.  Further, ion implantation of phosphorus or arsenic is conducted to the p-type well  49 , the p-type channel well  59  and further the n-type medium drain region  60  under the acceleration energy of 50 keV with the dose of about 6.0×10 15  cm −2  while using the resist pattern as a mask.  
         [0195]     After removing the resist pattern, a resist pattern is formed over the n-type substrate  31  such that the resist pattern includes openings in correspondence to the n-type well  47  in the device region of the p-channel MOS transistor  37 , the region of the n-type well  53  in the device region of the p-channel LDMOS transistor  41  adjacent to the p-type gate electrode  41   g,  and the p-type medium concentration drain region  54 . Thereby, the resist pattern is formed such that there exists a resist patter of the width of 1.5 μm over the p-type medium concentration drain region  54  adjacent to the p-type gate electrode  41   g.  Further, while using the resist pattern as a mask, ion implantation of boron is conducted simultaneously to the p-type well  49 , the n-type channel well  53  and the p-type medium concentration drain region  54  under the acceleration energy of 30 keV with the dose of 1.5×10 15  cm −2 .  
         [0196]     Further, after removal of the resist pattern, a thermal activation processing is conducted, and with this, the p-type source region  37   s  and the p-type drain region  37   d  are formed in the n-type well  47  in the device region of the p-channel MOS transistor  37 , the n-type source region  39   s  and the n-type drain region  39   d  are formed in the p-type well  49  in the device region of the n-channel MOS transistor  39 . Further, in the device region of the p-channel LDMOS transistor  41 , the p-type source region  41   s  is formed in the n-type channel well  53  while the p-type drain region  41   d  is formed in the p-type medium concentration drain region  54 . In the device region of the n-channel LDMOS transistor  43 , the n-type source region  43   s  is formed in the p-type channel well  59  while the n-type drain region  43   d  is formed in the n-type medium concentration drain region  60 . Thereby, it should be noted that, in order to relax the gate electric field, the p-type drain region  41   d  of the p-channel LDMOS transistor  41  is formed with a separation of about 1.5 μm from the p-type gate electrode  41   g.  Similarly, the n-type drain region  43   d  of the n-channel LDMOS transistor  43  is formed with a separation of about 2.0 μm from the n-type gate electrode  43   g.    
         [0197]     Further, the interlayer CVD film  58  is formed over the entire surface of the n-type substrate  31 , and contact holes are formed in the interlayer CVD film  58  in correspondence to the p-type source regions  37   s  and  41   s,  n-type source regions  39   s  and  43   s,  p-type drain regions  37   d  and  41   d  and n-type drain regions  39   d  and  43   d,  and the metal interconnection  56  is formed in the contact holes and in the interlayer CVD film  58 . Reference should be made to  FIG. 6 .  
         [0198]     In the embodiment of  FIG. 6 , the low voltage CMOS region  33  and the high-voltage CMOS region  35  are formed on the n-type substrate  31 , while it is also possible to form these in a common n-type well.  
         [0199]      FIG. 10  is a circuit diagram showing an embodiment of a constant voltage circuit according to the present invention in which the semiconductor device of the present invention is used.  
         [0200]     Referring to  FIG. 10 , there is provided, between an input terminal (Vin)  61  connected to a power supply and an output terminal (Vout)  63  connected to a load, a p-channel LDMOS transistor  65  that constitutes an output transistor.  
         [0201]     Further, there is provided a differential amplifier circuit  67 , and an output terminal of the differential amplifier circuit  67  is connected to the gate electrode of the p-channel LDMOS transistor  65 . Further, the differential amplifier circuit  67  has an inverting input terminal connected to a reference voltage generator circuit (Vref)  69 . Thereby, a reference voltage is supplied to the inverting input terminal from the reference voltage generator. Further, the in-inverting input terminal is supplied with a voltage formed by dividing the output voltage of the p-channel LDMOS  65  by the resistances R 1  and R 2  forming a voltage divider. The power of the differential amplifier  67  and the reference voltage generator  69  is supplied from the input terminal  61 . The differential amplifier  67 , the reference voltage generator  69  and the resistance R 2  are grounded.  
         [0202]     With this embodiment, the LDMOS transistor of the present invention is used for the p-channel LDMOS transistor  65 . Thus, it should be noted that the p-channel LDMOS transistor  65  has a gate oxide breakdown voltage of 40V.  
         [0203]     In the case of obtaining an output voltage lower than the input voltage to the input terminal, the output voltage is produced by dividing the input voltage by a voltage divider, wherein it is necessary to change the ON resistance of the p-channel LDMOS transistor  65  according to the output current supplied to an external load from the output terminal in order to maintain the output voltage constant with such a constant voltage circuit. Thus, with the present embodiment, the reference voltage from the reference voltage generator is compared with the output voltage divided out by the resistances R 1  and R 2  by using the differential amplifier  67 .  
         [0204]      FIG. 11  is a circuit diagram showing an example of an inversion type charge-pump DC/DC converter that uses the semiconductor device of the present invention.  
         [0205]     Referring to  FIG. 11 , the DC/DC converter includes an input terminal (Vin)  71 , an output terminal (Vout; inverted output)  73 , a ground terminal (GND)  75 , a plus-side terminal (CP+)  77  of pump capacitor, and a minus-side terminal (CP−)  79  of pump capacitor, wherein there is provided an external capacitor not illustrated between the terminals  77  and  79 .  
         [0206]     Inside the DC/DC converter, there are provided a p-channel LDMOS transistor  81  and an n-channel MOS transistor  83  consecutively between the input terminal  71  and the ground terminal  75 , and the positive-side terminal  77  of the pump capacitor is provided between the p-channel LDMOS transistor  81  and the n-channel MOS transistor  83 . Further, the node between the n-channel MOS transistor  83  and the ground terminal  75  is connected to a ground potential  85 .  
         [0207]     Between the ground potential and the output terminal, there are connected consecutively n-channel MOS transistors  87  and  89 . Further, a pumping capacitor  79  is connected between the n-channel MOS transistors  87  and  89 .  
         [0208]     Further, there is provided an oscillator circuit (OSC)  96  that produces alternately a voltage (Vin voltage) identical in magnitude to the voltage at the input terminal  71  and a voltage (GND voltage) identical to the voltage at the ground terminal  75 . Further, the output terminal of the oscillator circuit  93  is connected directly to the gate electrodes of the n-channel MOS transistors  83  and  89  and further to the gate electrode of the n-channel MOS transistor  87  via an inverter  95  and to the gate electrode of the p-channel LDMOS transistor  81  via inverters  95  and  97 .  
         [0209]     With this inversion type charge pump DC/DC converter, switching is caused in the four transistors  81 ,  83 ,  87  and  89  by providing a voltage to the respective gate electrodes thereof by way of the oscillator circuit  93 . Thereby, the pump capacitor connected between the positive-side terminal  77  and the negative-side terminal  79  undergoes charging and discharging and the current associated with this charge/discharge operation of the capacitor appears as an output at the output terminal  73 , wherein this output voltage becomes inversion of the input voltage supplied to he input terminal  71 .  
         [0210]     When the oscillator circuit  93  has produced the ground voltage GND, the p-channel LDMOS transistor  81  and the n-channel MOS transistor  87  are turned ON while the remaining n-channel MOS transistors  83  and  89  are turned OFF. In this state, the capacitor connected between the positive-side terminal  77  and negative-side terminal  79  undergoes charging.  
         [0211]     On the other hand, when the oscillator circuit  93  has produced the Vin voltage, the p-channel LDMOS transistor  81  and the n-channel MOS transistor  87  are turned off, while the remaining two n-channel MOS transistors  83  and  89  are turned ON. In this state, the capacitor thus charged in the previous phase causes discharging, and the electric charges accumulated in the capacitor are output from the output terminal  73  in the form of inverted voltage in view of the fact that the output terminal  73  is set to a voltage lower than the ground terminal  75 .  
         [0212]     By repeating the foregoing operations, the current continues to flow in the form of inversion of the input voltage.  
         [0213]     It should be noted that the present invention is applicable to other LDMOS transistors that have a construction different from those of the foregoing embodiments. For example, the present invention can be applicable to the device of the related art shown  FIG. 16  in which drain region  11   d  is formed deeper than the medium concentration drain region  24  as shown in  FIG. 13A . Alternatively, the present invention is applicable to the related art structure of  FIG. 14B  as shown in  FIG. 13B , in which the thickness of the gate oxide film  104   a  is increased at the drain edge. Further, as shown in  FIG. 13C , it is possible to apply the present invention to the related art structure of  FIG. 14C  in which the field oxide film  104   b  is formed at the drain edge with a thickness larger than the thickness of the gate oxide film. In any of these, it becomes possible to improve the drain breakdown characteristics without decreasing the impurity concentration level in the drain well  21  and the medium concentration drain region  24  or  120 , by forming the low-concentration drain region  22  inside the drain well  21 .  
         [0214]     Further, the LDMOS transistor of the present invention is not limited to those explained heretofore in which the p-type low-concentration well  22  includes all of the p-type channel well  23  and the n-type medium concentration drain region  24 , but also to those in which the low-concentration well includes only one of the channel well and the low-concentration drain region for the entirety thereof and the other of the channel well and the medium concentration drain region is included only partially. Alternatively, as shown in  FIG. 13D , it is possible to construct such that the low-concentration well  22  includes only a part of the channel well and the low-concentration drain region. Such modifications can be made easily by modifying the mask used for forming the low-concentration well.  
         [0215]     It is needless to say that the conductivity type can be reversed in the structures of  FIGS. 13A-13D .  
         [0216]     Further, the present invention is by no means limited to the embodiments described heretofore, but various variations and modifications may be made without departing from the scope of the invention.  
         [0217]     The present invention is based on Japanese priority application No. 2005-062427 filed on Mar. 7, 2005, which is incorporated herein as reference.