Abstract:
A semiconductor device comprises a semiconductor substrate having a high voltage region and a low voltage region, at least a pair of adjacent high voltage MOS transistors disposed on the high voltage region of the semiconductor substrate, and low voltage MOS transistors disposed on the low voltage region of the semiconductor substrate. A first element isolator comprises a first shallow trench disposed on a surface of the low voltage, region of the semiconductor substrate, and a first dielectric embedded in the first shallow trench. A pair of second element isolators comprises two second shallow trenches spaced apart at an interval between a source region or a drain region of the pair of the adjacent high voltage MOS transistors and a source or a drain region of the other of the pair of the adjacent high voltage MOS transistors, and a second dielectric embedded in each of the second shallow trenches. The second shallow trenches are disposed on a surface of the high voltage region of the semiconductor substrate. A channel cut region having a high impurity concentration is disposed on the surface of the substrate between the second shallow trenches.

Description:
BACKGROUND OF THE INVENTION 
   1. Field of the Invention 
   The present invention relates to a semiconductor device in which high voltage MOS (hereinafter referred to as HVMOS) transistors and low voltage MOS (hereinafter referred to as LVMOS) transistors are mixedly mounted and formed on the same semiconductor substrate. HVMOS transistors are transistors normally used for driving liquid crystal display devices or thermal heads, and their operation voltage and output voltage are from 8 V to 50 V. Further, LVMOS transistors normally refer to transistors having a gate length in a sub-micron order, which are manufactured with a process rule of 0.25 μm or less, their operation voltage is on the order of 0.9 to 3.3 V, and they are applied in memory and logic circuits. 
   2. Description of the Related Art 
   In conventional techniques, element isolators are formed in order to isolate mutually adjacent transistors when a plurality of high voltage (HV) MOS transistors are formed on the same substrate. Parasitic transistors are generally formed by source regions or drain regions formed while being sandwiched between element isolators, and wirings formed on the element isolators. In particular, parasitic transistors are easily formed, and easily operated, in wirings used for HVMOS transistor regions because a high voltage is normally applied. 
   In particular, it is necessary to increase the threshold voltage of the parasitic MOS transistors formed in the HVMOS transistor formation regions for cases in which the high voltage (HV) MOS transistors and microscopic low voltage (LV) MOS transistors are mixedly mounted. Formation of an inverted region that becomes a channel between a source and a drain in the parasitic MOS transistors, at a voltage equal to or less than the operation voltage of the HVMOS transistors, can thus be prevented. Further, punch-through can be prevented by increasing the distance between the source and the drain of the parasitic MOS transistors, thus ensuring isolation between the elements. Specifically, Locos, recently an STI (shallow trench isolation) structure, or the like is used for the isolation in order to increase the threshold voltage of the parasitic MOS transistors, and doping (field doping) of an impurity used under an additionally thick (field) oxide film is performed, which functions as a channel stopper. 
   A conventional technique for a case of performing element isolation using a Locos field oxide film  103  having a film thickness from 4000 Å to 6000 Å and formed on an Si substrate  1  is shown in  FIG. 8A  (for example, refer to JP 9-312399 A (p. 20,  FIG. 3 )). For a parasitic transistor, low impurity concentration source and drain regions  31  existing in two HVMOS transistors that are mutually isolated by the Locos field oxide film  103 , a wiring  16 , and an interlayer insulating film  37  corresponding to a gate insulating film of the parasitic MOS transistor, are formed. Afield-dope region  104 , which is a high impurity concentration impurity region having the same conductivity type as the substrate, is then formed by ion injection directly under a channel region of the parasitic MOS transistor, that is, directly under the field oxide film  103 , in order to prevent inversion (channel formation). 
   A conventional technique is shown in  FIG. 8B  for a case of performing element isolation by an STI (shallow trench isolation)  17  having an oxide film with a film thickness on the order of 3000 Å and formed on the Si substrate  1  as a field oxide film (for example, refer to JP  10-27840  A (p. 5,  FIG. 1B )). Similar to  FIG. 8A , the elements are isolated by the STI  17  instead of the Locos formed between two mutually separated HVMOS transistors. The field-doped region  104  having a very high impurity concentration of an impurity element is formed by ion injection into a channel region formed directly under the STI  17 . That is, the STI  17  is formed by forming a shallow trench  17   a  on a surface of the Si substrate  1 , and embedding a dielectric film  17   b , which is an oxide film, in the trench. In addition, the field dope region  104 ′, in which a large amount of an impurity having the same conductivity type as the impurity of the Si substrate is introduced, is normally formed directly under the oxide film. 
   The threshold voltage of the parasitic transistor must be increased, depending on the voltage used, for cases of forming high voltage MOS transistors by the conventional element isolation method shown in  FIGS. 8A and 8B . If a high voltage is used, the field oxide film  103 , which corresponds to the gate oxide film of the parasitic transistor formed between the HVMOS transistors, must be made thick, or the (high) impurity (field) doped region  104 , which is used as a channel stopper, must be introduced in the channel region directly under the field oxide film of the parasitic transistor. 
   If the Locos field oxide film  103  of the HVMOS transistor portion is made thick, the element isolators of the mixedly mounted low voltage (LV) MOS transistor portion also come to use the thick field oxide film  103  from the view point of manufacturing steps. In addition, a large area is necessary for the element isolators used with the Locos because of the Locos bird&#39;s beak region, and it is difficult to mount them mixedly with the LVMOS transistors having gate lengths in a sub-micron order compactly. 
   Further, the distance between regions doped with the impurity at a low impurity concentration must be increased due to the reduction of the junction withstand voltage when the concentration of the doped impurity used for a channel stopper is high, and the area of the high voltage MOS transistor formation region is increased. 
   SUMMARY OF THE INVENTION 
   According to the present invention, two STIs are formed as element isolators between two adjacent HVMOS transistors and a channel (formation) region (active region) of a parasitic MOS transistor sandwiched between the two STIs is taken as an Si substrate surface, and an impurity (impurity having the same conductivity type as an impurity of the Si substrate) is doped in the channel formation region as a channel stopper. Through the doping, a high impurity concentration impurity can be doped at the same time as formation of sources and drains of the HVMOS transistors and LVMOS transistors. 
   Impurity formation is not only performed for channel stopper in the present invention (at the same time as source and drain formation), and therefore manufacturing processes can be simplified. Further, one STI element isolator can be used as an element isolator for microscopic transistors of an LVMOS transistor region, and therefore it is possible to compactly mount them mixedly with the LVMOS transistors having a gate length in the sub-micron order. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
     In the accompanying drawings: 
       FIG. 1  is a cross sectional diagram of a structure of an embodiment of the present invention; 
       FIG. 2  is a plan view of main parts of the embodiment of  FIG. 1  of the present invention; 
       FIGS. 3A and 3B  are plan views of main parts of another embodiment of the present invention; 
       FIGS. 4A to 4D  are cross sectional diagrams in a step order of the embodiment of  FIG. 1  of the present invention; 
       FIGS. 5A to 5D  are cross sectional diagrams in a step order of processes after those shown in  FIGS. 4A to 4D ; 
       FIGS. 6A to 6D  are cross sectional diagrams in a step order in another case; 
       FIG. 7  is a cross sectional diagram of another embodiment of the present invention; 
       FIGS. 8A and 8B  are cross sectional diagrams of a conventional technique; 
       FIG. 9  is a cross sectional diagram of an STI that is a main part of the present invention; and 
       FIG. 10  is a graph in which threshold voltages are compared. 
   

   DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     FIG. 1  is a cross sectional diagram of a structure of an embodiment of the present invention and the embodiment will be explained below. An HVNwell  2  and an HVPwell  3  formed within an HV region  18  for forming a high voltage (HV) MOS transistor, and an LVNwell  4  and an LVPwell  5  formed within an LV region  19  for forming an LVMOS transistor, are disposed on an Si substrate  1 . Elements are isolated within each of the HVPMOS and HVNMOS transistor element formation regions  18  by two STIs  17 . Elements are isolated with each of the LVPMOS and LVNMOS transistor element formation regions  19  by the one STI  17 . 
   The HVNwell  2  is an HVMOS formation region, and an n-type conductivity impurity region. The HVPwell  3  is an HVMOS formation region, and a p-type conductivity impurity region. The LVNwell  4  is an LVMOS formation region, and an n-type conductivity impurity region. The LVPwell  5  is an LVMOS formation region, and a p-type conductivity impurity region. 
   A plurality of p-channel HVMOS (HVPMOS) transistors  7  are formed in the HVNwell  2 . The HVPMOS transistors  7  each include an HV gate oxide film  26 , a gate electrode  28  formed on the HV gate oxide film, and a p-type low impurity concentration source and drain regions  32  formed on both sides of the gate-electrode  28 , and p-type high impurity concentration source and drain regions  23  formed on the outside of the p-type low impurity concentration source and drain regions  32 . The HVPMOS transistor  7 , which is a high voltage MOS transistor, is formed by forming the p-type low impurity concentration source and drain regions  32 . The structure of the high voltage HVPMOS transistors  7  and HVNMOS transistors  6  are not limited to the structure of this embodiment, and other conventional structures may also be used. In addition, an interlayer insulating film  37  is formed on the transistors, and a contact  38  is formed in the interlayer insulating film  37  for connecting a wiring  16  formed thereon to high impurity concentration source and drain regions  23 , and the like. This is a general PMOS transistor structure. A tap, which is not shown in the figures, is formed in the HVNwell  2  for fixing the electric potential thereof. 
   The two STIs  17  are formed with a gap, between two adjacent HVPMOS transistors  7  formed on the surface of the HVNwell  2 . Each of the STIs  17  extends longitudinally in a direction orthogonal to the direction in which the two HVPMOS transistors  7  are disposed.  FIG. 9  is an enlarged cross-sectional view thereof. The two STIs  17  have the following structure. Two shallow trenches  17   a  are formed between the two adjacent HVPMOS transistors  7 , and dielectrics  17   b  are embedded in each of the trenches. The dielectrics  17   b  are normally made of oxide films. High impurity concentration source and drain regions  23  having a p-type conductivity of each of the HVPMOS transistors  7  are formed at both outer sides of the two STIs  17 . The shallow trenches  17   a  each normally contact one of the high impurity concentration source and drain regions  23 . A surface of the HVNwell  2  appears between the two shallow trenches  17   a . An HVN+ channel cut layer (field dope)  39 , which has a relatively low impurity concentration impurity of an n-type conductivity (not shown in the figures), and is formed at the same time as formation of low impurity concentration source and drain regions  31  of the HVNMOS transistor  6  explained later, is formed on the surface of the HVNwell  2  between the two shallow trenches  17   a.    
   In addition, an HVN++ channel cut layer  14 , which has a high impurity concentration impurity of an n-type conductivity and is formed at the same time as formation of high impurity concentration source and drain regions  22  of the HVNMOS transistors  6 , is formed on the HVN+ channel cut layer  39 . Note that the effects of the present invention are obtained even if the HVN+ channel cut layer  39  having a relatively low impurity concentration impurity of an n-type conductivity is not formed. The interlayer insulating film  37  is formed on the STIs  17 , and wirings  16  are formed thereon. A high voltage (8 to 50 V) is applied to the wirings  16  because they are used for the HVPMOS transistors  7  and the HVNMOS transistors  6 . The HVN++ channel cut layer  14  is formed on the surface of the HVNwell  2  region sandwiched between the two shallow trenches  17   a  below the wirings  16 , to which the high voltage is applied, and therefore the polarity does not invert. That is, the parasitic transistor does not operate. 
   Referring back to  FIG. 1 , the plural n-channel HVMOS (HVNMOS) transistors  6  are formed on the surface of the HVPwell  3  region. The HVNMOS transistors  6  have a structure similar to that of the HVPMOS transistors  7 , but are structured by elements having different conductivity types. The n-type low impurity concentration source and drain regions  31  of the HVNMOS transistor are formed as a substitute for the p-type low impurity concentration source and drain regions  32  of the HVPMOS transistors  7 . N-type high impurity concentration source and drain regions  22  are formed on the n-type low impurity concentration source and drain regions  31  as a substitute for the p-type high impurity concentration source and drain regions  23  of the HVPMOS transistors  7 . 
   In addition, the two STIs  17  are formed between the two HVNMOS transistors  6  in the HVPwell  3 . An HVP+ channel cut layer, which has a relatively low impurity concentration impurity of a p-type conductivity and is formed at the same time as formation of the low impurity concentration source and drain regions  31  of the HVPMOS transistors  7 , is formed between the two shallow trenches  17   a  (STIs  17 ). In addition, an HVP++ channel cut layer  15 , which has a low impurity concentration impurity of a p-type conductivity and is formed at the same time as formation of the high impurity concentration source and drain regions  23  of the HVPMOS transistors  7 , is formed on the HVP+ channel cut layer. As stated above, it is not necessary to form the HVP+ channel cut layer that has a relatively low impurity concentration impurity of a p-type conductivity. 
   The interlayer insulating film  37  is formed on the STIs  17 , and the wirings  16  are formed on the interlayer insulating film  37 . A high voltage (8 to 50 V) is applied to the wirings  16  because they are used for the HVPMOS transistors  7  and the HVNMOS transistors  6 . The HVP++ channel cut layer  15  is formed on the surface of the HVNwell  2  region sandwiched between the two shallow trenches  17   a , and therefore the polarity of the HVP++ channel cut layer  15  does not invert by applying the high voltage to the wirings  16 . That is, the parasitic transistor does not operate. In other words, the element isolators have nearly the same structure as the HVPMOS transistors  7 , except for their polarity in conductivity type. 
   Further, the low voltage MOS transistor formation region  19  is formed on the Si substrate  1 . The region  19  includes the LVNwell  4 , which is an n-type well region for forming a plurality of low voltage (LV) PMOS transistors  9 , and the LVPwell  5 , which is a p-type well region for forming a plurality of low voltage (LV) NMOS transistors  8 . Computational circuits and logic circuits are structured by this region, and therefore it is not required for the region to have high voltage output. 
   The plural LVPMOS transistors  9  having p-type high impurity concentration source and drain regions  25  of the LVPMOS transistor are formed in the LVNwell  4 . The plural LVNMOS transistors  8  having n-type high impurity concentration source and drain regions  24  are formed in the LVPwell  5 . Element isolation between the low voltage MOS transistors having the same conductivity type is achieved by one shallow trench  17   a , and the dielectric  17   b  embedded in the trench  17   a . Voltages applied to the wirings  16 , which become gate electrodes, are low in the low voltage LVMOS transistors, and therefore element isolation is possible by only the shallow trench  17   a  and the dielectric  17   b  embedded therein. Needless to say, element isolators of the LV region  19  may have the same structure as the STI  17 , which is the element isolator for the HV region  18 . However, in this case there are the two shallow trenches  17   a , and therefore the surface area becomes larger. 
   A comparison between the threshold voltage values of parasitic MOS transistor for the conventional technique shown in  FIG. 8B  and parasitic MOS transistor of the present invention shown in  FIG. 9  is shown in  FIG. 10 .  FIG. 10  shows the threshold voltage of this n-type parasitic MOS transistor, with a distance  33  between the source and drain regions of the parasitic MOS transistor taken as a variable. The HVP++ channel cut layer  15 , in which an impurity is doped a high impurity concentration, is disposed in the present invention, and therefore the parasitic MOS transistor has a high threshold voltage. A threshold value of 50 V is maintained even if the distance  33  between the p-type high impurity concentration source and drain regions  23  of adjacent HVPMOS transistors  6 , or the distance  33  between the n-type high impurity concentration source and drain regions  22  of adjacent HVNMOS transistors  7  (the distance between the source and the drain regions of the parasitic MOS transistor), is equal to or less than 1 μ. The threshold voltage value is on the order of 12 V with the conventional example shown in  FIG. 9B . 
     FIG. 2  is a plan view of main parts of the embodiment explained by using  FIG. 1 . Note that, the reduction scale differs from that of  FIG. 1 . The main parts include the HVNwell  2  and the HVPwell  3  formed within the HV region  18  for forming the HVMOS transistors, and the LVNwell  4  and the LVPwell  5  formed within the LV region  19  for forming the LVMOS transistors, which are disposed on the Si substrate  1 . 
   The p-type Si substrate  1  is explained here. The two HVPMOS transistors  7  are drawn in the HVNwell region  2 . Two STIs  17  including the two shallow trenches  17   a  between the two HVPMOS transistors  7 , and the dielectrics  17   b  embedded in the shallow trenches  17   a , are formed between the two HVPMOS transistors  7 . A surface of the HVNwell region  2  appears between the two STIs  17 , and the HVN++ channel cut layer  14  is formed in that surface. Further, the wrings  16  are formed so as to straddle the HVN++ channel cut layer  14 . 
   The HVNMOS transistors  6  and the like are formed in the HVPwell region  3 , similar to the above. The HVP++ channel cut layer  15  is formed on a surface between the two STIs  17  in this region. Only the polarity differs with the HVNwell region  2 . 
   The LVPMOS transistors  9  are formed in the LVNwell region  4 , and the LVNMOS transistors  8  are formed in the LVPwell region  5 . Only one STI  17  is used for the element isolation, in the same manner as the HV region  18  and the LV region  19 . 
     FIGS. 3A and 3B  are plan views of another embodiment of the present invention. Adjacent portions of two adjacent HVPMOS transistors  7  or two adjacent HVNMOS transistors  6  are shown. The two HVPMOS transistors  7  formed within the HVNwell 2  are drawn in  FIG. 3A . The gate electrode  28 , and the p-type low impurity concentration source and drain regions  32  of the HVPMOS transistor  7 , which are formed on both sides of the gate electrode  28 , are formed in each of the HVPMOS transistors  7 . The STI  17  surrounds the periphery of the p-type low impurity concentration source and drain regions  32 , except for the gate electrode  28  portion of each of the HVPMOS transistors  7 . Although not shown in the figures, there are also cases in which the p-type high impurity concentration source and drain regions  23  are formed between the p-type low impurity concentration source and drain regions  32  and the STI  17 . The HVN++ channel cut regions  14  are then formed in the periphery of each of the p-type low impurity concentration source and drain regions  32  (and regions including the p-type high impurity concentration source and drain regions which are not shown in the figures), except for the gate electrode  28  portions of the HVPMOS transistors  7 , through the STIs  17 . That is, the HVN++ channel cut regions  14  are formed so as to surround each of the HVPMOS transistors  7 . 
   The wirings  16  are then formed so as to straddle element isolation portions of the HVPMOS transistors  7  designed to be adjacent, as shown in  FIG. 3A . It is generally not necessary that the wirings  16  be electrically connected directly to the gate electrodes  28 . The wirings may be formed on the gate electrodes  28  through an intermediate insulating film, and may be formed on the interlayer insulating film  37  so as not to overlap with the gate electrodes  28 . That is, the HVPMOS transistors  7  are surrounded by using the HVN++ channel cut regions  14 , and channel formation of the parasitic transistor is blocked. 
   Further, the HVN++ channel cut region  14  may also be disposed only in a portion under the wiring  16 , which becomes a channel region of the parasitic MOS transistor, as shown in  FIG. 3B . The distance  22  between an end of the HVN++ channel cut region  14  and the wiring  16  at this point is determined by the voltage applied to the wiring  16  and the threshold voltage of the parasitic MOS transistor. 
     FIGS. 4A to 4D  are cross sectional diagrams in a step order of the present invention. A process for a case in which the LVMOS transistors  8  and  9 , which have a process rule of 0.25 μm and have an operation voltage of 2.5 V, and the HVMOS transistors  6  and  7 , which have an operation voltage of 20 V, are mixedly mounted. 
   The HVNwell  2  used for forming the HVPMOS transistors  7 , and the HVPwell  3  used for forming the HVNMOS transistors  6  are formed on the p-type Si substrate  1  as shown in  FIG. 4A . The depth of the HVNwell  2  depends on the operation voltage of the high voltage transistor used. The HVPwell  2  and the HVNwell  3  having a junction depth on the order of 4 μm with respect to a withstand voltage of 20 V are formed this time. The junction depth differs between the LVPwell  5  and the LVNwell  4  etc., and therefore they cannot also be formed at the same time here. 
   The shallow trenches  17   a  used for the STIs  17  of the element isolation region are formed next as shown in  FIG. 4B . The STIs  17  used for the HVMOS transistors  6  and  7 , and the STIs  17  used for the LVMOS transistors  8  and  9 , which have a microscopic structure, have the same depth, and are formed at the same time. The shallow trenches  17   a  are formed independently in the HV region  18  so as to surround each of the HVMOS transistors  6  and  7 . That is, the two shallow trenches  17   a  are formed between two adjacent HVMOS transistors  6  and  7 , sandwiching the surface of the HVNwell  2  or the surface of the HVPwell  3 . Further, the shallow trenches  17   a  are formed so as to surround the LVMOS transistors  8  and  9  in the LV region  19 . However, a point of difference with the HV region  18  is that the surface of the LVNwell  4  and the LVPwell  5  is not exposed between the shallow trenches  17   a  that surround two adjacent LVMOS transistors  8  and  9 . The depth of the shallow trenches  17   a  is from 2000 Å to 5000 Å. In addition, the dielectric  17   b  is embedded in each of the shallow trenches  17   a , forming the STIs  17 . Note that the dielectric films  17   b  are formed of a silicon oxide film, a silicon nitride film, a BPSG film, or a laminate film of these films. 
   Next, as shown in  FIG. 4C , an n-type impurity is ion-implanted through the dielectric films  17   b  of the STIs  17  using a mask to form the n-type low impurity concentration source and drain regions  31  of the HVNMOS transistor  6  and the HVN+ channel cut layer  39  of the HVNwell  2 . In addition, a p-type impurity is ion-implanted through the dielectric films  17   b  of the STIs  17  using a mask to form the p-type low impurity concentration source and drain regions  32  of the HVPMOS transistors  7  and a HVP+ channel cut region  40  of the HVPwell  3 . The n-type low impurity concentration source and drain regions  31 , and the p-type low impurity concentration source and drain regions  32  may be formed with a larger depth than the STIs  17  as in  FIG. 1 , and may also be formed with a smaller depth than the STIs  17 . Note that the effects of the present invention can be obtained even if the HVN+ channel cut layer  39  and HVP+ the channel cut region  40  are not formed. 
   In addition, impurities are doped through the dielectric films  17   b  of the STIs  17  to form the LVNwell  4  and the LVPwell  5  used for the LVMOS transistors  8  and  9 , as shown in  FIG. 4D . The junction depths of the LVNwell  4  and the LVPwell  5  are approximately 1 μm. 
   In addition, the thick HV gate oxide film  26  used for the HVMOS transistors  6  and  7  is formed with a thickness on the order of. 700 Å, and after removing only the thick HV gate oxide film  26  of the LV region  19 , the thin LV gate oxide film  27  used for the LVMOS transistors  8  and  9  is formed on the LV region  19  with a thickness on the order of 50 Å, as shown in  FIG. 5A . 
   The gate electrodes  28  of each of the MOS transistors  6 ,  7 ,  8 , and  9  are then patterned and formed as shown in  FIG. 5B . 
   The n-type high impurity concentration source and drain regions  22  of the HVNMOS transistor  6 , the HVN++ channel cut region  14  formed on the surface of the HVNwell  2  region and sandwiched between two STIs  17 , and source and drain regions  24  of the LVNMOS transistor  8  are then formed at the same time using a mask through ion injection, as shown in  FIG. 5C . In addition, the p-type high impurity concentration source and drain regions  23  of the HVPMOS transistor  7 , the HVP++ channel cut layer  15  formed on the surface of the HVPwell  3  region and sandwiched between two STIs  17 , and the source and drain regions  25  of the LVPMOS transistor  9  are then formed at the same time using a mask through ion injection. Low impurity concentration impurity doping is performed by ion injection of As (arsenic) on the order of up to 10 15 /cm 2  for the n-type impurity, and B (boron) or BF 2  (boron fluoride) on the order of up to 10 15 /cm 2  for the p-type impurity. 
   The low impurity concentration source and drain regions  31  and  32  are formed in the HVMOS transistors  6  and  7 , respectively, in this embodiment, and the source and drain regions  24  and  25  of the LVMOS transistors, respectively, and the high impurity concentration source and drain regions  22  and  23  of the HVMOS transistors  6  and  7 , respectively, may be formed at the same time. The contact  38  is then formed in order to connect the interlayer insulating film  37  formed of the PSG film, the BPSG film, or the like, and the elements. 
   The metallic wirings  16  can be disposed freely as in  FIG. 5D . The figure shows formation of the wirings  16  so as to straddle the channel cut regions. 
     FIGS. 6A to 6D  are cross sectional diagrams of another embodiment. There is a case of disposing the wirings made of polysilicon (polycide or suicide), which constitute the same layer as the gate electrodes, on the element isolation region. In this case, the STIs  17  are structurally the same as those of  FIG. 1 , but the polysilicon wirings are formed on the element isolation region and the high impurity concentration HVN++ channel cut region  14  and the high impurity concentration HVP++ channel cut region  15  are formed by ion implantation through the polysilicon wirings. Therefore, the ions are not injected under the polysilicon layer and the channel cut regions cannot be formed by the processes shown in  FIGS. 4A to 4D  and  5 A to  5 D. 
   Processing up to the step of  FIG. 4B  is the same. Next, the LVN well  4  and the LVP well  5  are formed by ion injection through the dielectrics  17   b  of the STIs  17  using a mask, as in  FIG. 6A . 
   The HV gate oxide film  26  and the LV gate oxide film  27  are formed as in  FIG. 6B , after which the gate electrodes  28  are formed, and wirings are formed simultaneously when the gate electrodes  28  are formed, as in  FIGS. 6B and 6C . The wirings may be wirings that straddle the channel cut regions. The low impurity concentration source and drain regions  31  of the HVNMOS transistor, the low impurity concentration source and drain regions  32  of the HVPMOS transistor, the high impurity concentration source and drain regions  22  of the HVNMOS transistor, the high impurity concentration source and drain regions  23  of the HVPMOS transistor, the LVNMOS source and drain regions  24 , the LVPMOS source and drain regions  25 , the interlayer insulating film  37 , and the contact  38  are then formed, as in  FIG. 6D . Another metallic wiring  16  is then formed. 
   Further, a cross sectional diagram of another embodiment is presented, although not explained with reference to the figure. The cross sectional shapes of the shallow trenches  17   a  of the STIs  17  formed in the HV region  18  and the LV region  19  differ, and the incline of the shallow trench  17   a  of the HV region is relatively gentle (flat), while the slope of the shallow trench  17   a  of the LV region  19  has a steep angle (standing). That is, it is necessary to form the shallow trenches  17   a  separately. This is made in order to attain the satisfactory performance of the element isolation of the HV region  18 . The STIs  17  are used in order to make the element isolation region smaller. There are cases where a desired voltage cannot be obtained because the electric field of the ends of the active region becomes strong for cases of manufacturing high voltage (HV) transistors. The slope of the STIs  17  of the HV region  18  is made gentle in this embodiment, while the shape of the STIs  17  of the LV region  19  is changed. 
   Further,  FIG. 7  is a cross sectional diagram of the structure of yet another embodiment. Compared to the embodiment of  FIG. 1 , this is an embodiment in which an n-type punch-through blocking region  35  and a p-type punch-through blocking region  36  are formed: under the HVN++ channel cut region  14 , which is sandwiched by two STIs  17 , and the HVP++ channel cut layer  15 , which is sandwiched by two STIs  17 , respectively; and under the low impurity concentration HVN+ channel cut layer  39  having a relatively high impurity concentration and the low impurity concentration HVP+ channel cut region  40  having a relatively high impurity concentration, respectively. 
   In accordance with the present invention, element isolators of HVNMOS transistors and HVPMOS transistors become more compact than Locos element isolators because two STIs are formed, and a channel cut region is formed on a surface portion of a well sandwiched between the two STIs. Further, doping for the channel cut region is performed at the same time as the impurity doping process for the source and drain regions of the MOS transistors, and therefore the process becomes simple. The impurity introduction for the channel cut region becomes simple as compared to the case of forming one STI and a channel cut region thereunder, and costs are reduced.