Abstract:
Disclosed is a method of manufacturing a semiconductor device which includes a source region, a channel region, a drain region, a gate electrode formed on the channel region through a gate insulating film  6  and a drift region (N −  layer  22 ) formed between the channel region and the drain region, wherein the process of forming the drift region (N −  layer) comprises the steps of: ion-implanting and diffusing at least two kinds of second conduction type impurities (e.g. phosphorus and arsenic ions) having different diffusion coefficients in a P-type well region  21 ; ion-implanting at least one kind first conduction type impurities (e.g. boron ions) having a diffusion coefficient substantially equal to or larger than that of at least one of said second conduction type impurities (e.g. phosphorus); and diffusing the first conduction type impurities after the gate insulating film  6  has been formed.

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to a semiconductor device and a method of manufacturing it, and more particularly to LD (Lateral Double Diffused) MOS transistor technology which is used for a high voltage element for e.g. liquid crystal driving IC. 
     2. Description of the Related Art 
     Now, an LDMOS transistor structure refers to a structure in which impurities with a different conduction type are diffused in a diffused region formed on surface of a semiconductor substrate to form another diffused region and a difference in the horizontal diffusion between these diffused regions is employed as an effective channel length. This structure, in which a short channel is formed, can constitute an element with low “on” resistance. 
     FIG. 10 is a sectional view for explaining a conventional LDMOS transistor which are being developed by the inventors of the invention. A N-channel LDMOS transistor structure is illustrated. Although the structure of a P-channel LDMOS transistor structure is not explained here, it is well known that the same structure can be adopted except for its conduction type. 
     In FIG. 10A, reference numeral  1  denotes a semiconductor substrate with a first conduction type, e.g. P-type, and reference numeral  21  denotes a P-type well region. A P-type body region  3  is formed within the P-type well region  21 . An N-type diffused region  4  is formed within the P-type body region  3 . Another N-type diffused region  5  is formed apart from the N-type diffused region  4 . A gate electrode  7  is formed on the surface of the substrate  1  through a gate insulating film  6 . A channel region  8  is formed in the surface region of the P-type body region  3  immediately below the gate electrode. 
     The N-type diffused region  4  is used as a source region whereas the N-type diffused region is used as a drain region. A drift region (N −  layer  22 ) is formed which is composed of a shallow region (first N −  layer  22 A) below the gate electrode  7  and deep region (second N −  layer  22 B) in the vicinity of the drain region. Further, a source electrode and drain electrode (not shown) are formed on the surface so as to contact with source and drain region, respectively. A P-type diffused region  12  for assuming the potential of the P-type body region  3  is formed through an interlayer insulating film. 
     In the above LDMOS transistor, which has a high impurity concentration on the surface of the N-type drift region, a current is apt to flow in the surface of the N-type drift region, thereby realizing a high withstand voltage. The LDMOS transistor having such a configuration is referred to as a surface relax type (RESURF)LDMOS. 
     However, since the impurity concentration is high in the surface of the N-type drift region  22 , the P-type body region  3  cannot diffuse sufficiently. Therefore, as shown in FIG. 10B, the edge of the P-type body region  3  approaches the source region (N-type diffused region  4 ) so that the channel region B may be not be formed to have a suitable size and impurity concentration(see indicated arrow). 
     The inventors of the invention tried to solve the above problem by rearranging the manufacturing steps and eventually completed the present invention. Specifically, in the conventional process, the drift region (N −  layer  22 ) was formed in such a manner that at least two N-type impurities (phosphorus ions or arsenic ions) with different diffusion coefficients are ion-implanted and diffused in the surface layer of the P-type well region  21 , and thereafter P-type impurities (boron ions) are ion-implanted and diffused into the surface layer of the P-type well region  21  where the source region is to be formed so that the boron ions thus diffused cancel the phosphorous ions of the second N −  layer  22 B (diffused layer originating from phosphorus ions) formed at a relatively deep position of the P-type well region of the source region. 
     However, the following fact was confirmed. The drift region (N −  layer  22 ), after it has been formed, is subjected to thermal oxidation to form a gate insulating film  6 . Therefore, arsenic ions contained in the first N −  layer (with arsenicions diffused) are segregated on the substrate surface. Because of the segregated arsenic ions, the P-type body region does not diffuse sufficiently so that the channel region  8  cannot be formed to have a suitable size (see a hatched region showing segregated ions in the graph of FIG.  9 ). 
     SUMMARY OF THE INVENTION 
     An object of the invention is to provide a method of manufacturing a semiconductor device which can satisfy the requirements of a high withstand voltage and reduced “on” resistance. 
     Namely the present invention intends to reduce a defect of channel caused by the segregation of the impurity ion for forming a drift region in a semiconductor device having the drift region near the channel region, thereby a reliable LDMOS type transistor can be obtained. 
     In order to achieve the above object, in accordance with the invention, there is provided a method of manufacturing a semiconductor device which includes a source region  4 , a channel region  8  and a drain region  5 , a gate electrode  7  formed on the channel region through a gate insulating film  6  and a drift region (N −  layer  22 ) formed between the channel region and the drain region, wherein the process of forming the drift region (N −  layer) comprises the steps of: ion-implanting and diffusing at least two kinds of second conduction type impurities (e.g. phosphorus and arsenic ions) having different diffusion coefficients in a P-type well region  21 ; ion-implanting at least one kind first conduction type impurities (e.g. boron ions) having a diffusion coefficient substantially equal to or larger than that of at least one of said second conduction type impurities (e.g. phosphorus); and diffusing the first conduction type impurities after the gate insulating film  6  has been formed. 
     Preferably, the step of diffusing the first conduction type impurities is executed after a film for forming said gate electrode (polysilicon film  17 ) has been formed. 
     In accordance with the invention, since the first conduction type impurities for forming a drift region with the first conduction type are diffused after the thermal oxidation for forming the gate insulating film, it is possible to solve the problem that the body region with the first conduction type does not diffuse sufficiently owing to a drift region with the second conduction type, thereby permitting a suitable channel region to be formed and providing a semiconductor device which can satisfy the requirement of high withstand voltage and reduced “on” resistance. 
     In addition, where the step of diffusing the first conduction type impurities for forming the drift region with the first conduction type is executed before the film for forming a gate electrode is made conductive, it is possible to solve the problem that the temperature of the viscous flow of the gate insulting film is made lower than that of the normal viscous flow by the impurities used to make conductive the film for forming the gate electrode, and hence while the first conduction type impurities are diffused, the gate insulating film of SiO 2  film is exposed to an atmosphere of the higher temperature than the temperature when the viscous flow occurs, thereby avoiding the occurrence of the phenomenon of reducing the withstand voltage of the oxide film. 
     The above and other objects and features of the invention will be more apparent from the following description taken in conjunction with the accompanying drawings. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIGS. 1 to  8  are sectional views for explaining a method of manufacturing a semiconductor device according to an embodiment of the invention; and 
     FIG. 9 is a graph showing concentration of various ions for explaining the theory of forming a drift region according to the invention; and 
     FIG. 10 is a sectional view of a conventional semiconductor device. 
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     Now referring to the drawings, an explanation will be given of an embodiment of a method of manufacturing a semiconductor device according to the invention. FIGS. 1 to  8  are sectional views for explaining a manufacturing process of a semiconductor device according to an embodiment of the invention. 
     The device structure according to the invention is substantially the same as the conventional device (FIG.  10 ). In order to avoid repetition of explanation, the explanation will be simplified without explaining the illustrated structure and referring like reference numerals to like parts in the conventional structure. 
     FIG. 10 shows an N-channel type LDMOS transistor as an example of LDMOS transistor. Although the structure of a P-type LDMOS transistor structure is not explained here, it is well known that the same structure can be adopted except for its conduction type. 
     First, in FIG. 1, after a pad oxide film  30  has been formed on a P-type semiconductor substrate  1 , using a photoresist film  31  as a mask, two kinds of N-type impurities (e.g. arsenic ions and phosphorus ions) are ion-implanted into a P type well region  21  to form a first ion-implanted layer  32  (one-dot chain line) and a second ion-implanted layer  33  (dotted line). These N-type impurities serve to form an N −  layer  22  constituting a drift region in later steps. In this step, the arsenic ions are implanted under the condition of an accelerating voltage of about 160 KeV and a dose of 3×10 12 /cm 2 , whereas the phosphorus ions are implanted under the conduction of the accelerating voltage of about 50 KeV and dose of 4×10 12 /cm 2 . 
     In FIG. 2, using a silicon nitride film (not shown) formed on the substrate as a mask, a certain region of the substrate surface is selectively oxidized to form LOCOS oxide films  9  each having a thickness of about 7300 A. Simultaneously, because of a difference in the diffusion coefficient between the diffused arsenic ions and phosphorus ions, the arsenic ions are diffused into the substrate  1  so that the first N −  layer  22  (illustrated one-dot chain line) is formed in a relatively substrate-surface layer. The phosphorus ions are diffused into the substrate  1  so that the second N −  layer  22 B (illustrated by solid line) is formed at a relatively deep position in the P type well region  21 . 
     In FIG. 3, after a photoresist film  34  has been formed on the substrate  1  of a region where a drain is to be formed (referred to as “drain-forming region”), using the photoresist film  34  as a mask, P-type impurities (e.g. boron ions) are ion-implanted in the substrate-surface layer of a region where a source is to be formed (referred to as “source-forming region). Thus, a third ion-implanted layer  23  (illustrated by xxx mark) is formed. 
     In FIG. 4, the substrate surface is pyro-oxidized at about 850° C. to form a gate oxide film  6  having a thickness of about 800 A on the substrate  1 . Thereafter, a polysilicon film  17  (or amorphous silicon film) having a thickness of 2000-3000 A is formed from the gate oxide insulating film  6  on the entire surface of the substrate inclusive of the LOCOS oxide film  9 . 
     Subsequently, diffusion of the boron ions which have been implanted in the third ion-implant layer  23  is executed. 
     Through this diffusion processing, the phosphorus ions constituting the second N −  layer  22 B of the source-forming region are canceled by the diffused boron ions, the second N −  layer  22 B disappears. In this step, for example, after the boron ions are implanted under the condition of an accelerating voltage of 80 KeV and dose of 8×10 12 /cm 2 , they are diffused for two hours in an atmosphere of N 2  in a range between 1000° C. and 1150° C., preferably at 1100° C. Now, FIG. 9 is a graph showing the distribution of the impurity concentration when arsenic ions (illustrated by solid line), phosphorus ions (illustrated by dotted line) and boron ions (illustrated by one-dot chain line) are diffused, respectively. As seen from the graph, the concentration distribution originating from the phosphorus ions are canceled by its merging with that from the boron ions. 
     In this way, in accordance with the present invention, using the difference in a diffusion coefficient between the arsenic ions and phosphorus ions when the drift region is formed, the second N −  layer  22 B of the source-forming region formed at a deep position of the substrate is caused to disappear by diffusing the boron ions implanted in the subsequent step. Thus, only the first N −  layer  22 A formed in the substrate surface layer is left in the source-forming region. Accordingly, the semiconductor device with reduced “on” resistance reduced can be manufactured through a relatively simple manufacturing process. 
     In addition, the step of diffusing the boron ions previously implanted into the substrate-surface layer is executed after the thermal oxidation step for forming the gate insulating film  6  has been executed. For this reason, it is possible to prevent occurrence of the conventional problem that the P-type body region  3  cannot diffuse sufficiently owing to the concentration of the N −  layer  22  of the drift region so that the channel region  8  cannot be to have a suitable size. 
     Specifically, in the conventional process in which the steps of implanting the boron ions in the substrate-surface layer and of diffusing them are successively executed and thereafter the step of thermal oxidation for forming the gate insulating film  6  is executed, during the thermal oxidation, the arsenic ions in the first N −  layer  22 A (arsenic ions diffused) are segregated on the substrate surface so that the P-type body region  3  cannot diffuse sufficiently owing to the segregated arsenic ions so that the channel region cannot be to have a suitable size. However, the present invention can avoid occurrence of such a problem, thereby providing a method of manufacturing a semiconductor device which can satisfy the requirements of the high withstand voltage and reduced on-resistance. 
     Incidentally, in the above embodiment, the step of diffusing the boron ions is executed after the polysilicon film  17  for forming the gate electrode has been formed on the gate insulating film  6 . However, this step may be executed after immediately after the gate insulating film  6  has been formed, immediately after the polysilicon film  17  has been made conductive (described later), or in a further later step. In short, the step of diffusing the boron ions may be executed after the thermal oxidation for forming the gate insulating film has been executed. In the case that the step of diffusing is executed immediately after forming the gate insulating film  6 , B diffusion can be executed during applying an inert gas such as N 2  gas immediately after oxidation. According to the process, pollution is not happened and operation efficiency can be improved, because of operation within the furnace without exposing from the furnace. 
     Further, it should be noted that the step of diffusing the boron ions is preferably executed after the polysilicon film  17  for forming the gate electrode has been formed and before the polysilicon film  17  is made conductive (by phosphorus-doping from the thermal diffusion source of POCl 3  or by ion-implantation of N-type impurities). Specifically, where the polysilicon film  17  made conductive is present on the gate insulating film  6 , the impurities contained in the polysilicon film  17  exude into the gate insulating film  6  so that the gate insulating film appears a phosphorus-doped film. Meanwhile, the “viscous flow” of the phosphorus-doped insulating film which occurs at a lower temperature than the melting point) occurs at about 900° C. whereas that of the non-doped insulating film normally occurs at about 1400° C. Therefore, the phosphorus-doped insulating film is exposed to the atmosphere of diffusing the boron ions at a temperature of 1100° C. so that pointed deposits continue like a bridge, thereby deteriorating the withstand voltage of the oxide film. The state where pointed deposits continue like a bridge is called “Oxide Ridge” (coinage of Oxide film and Bridge with “B” removed). 
     Further, where the step of diffusing the boron ions is executed after the polysilicon film  17  has been made conductive, the concentration distribution of the polysilicon film  17  (gate electrode  7 ) will vary. 
     Therefore as a file for forming the gate electrode  7 , non-doped polysilicon film instead of doped polysilicon film is required to be used. 
     Thus, as described above, in accordance with the invention, the step of diffusing the boron ions is executed after the gate insulating film  6  and the polysilicon film  17  have been formed and before the polysilicon film is made conductive, thereby solving the conventional problem and avoiding occurrence of the above phenomenon. 
     In this way, in the invention, by a minimum process change of only rearranging the order of manufacturing steps (simulation of optimizing a process condition such as ion-implantation and diffusion may be included), the above problem can be solved. 
     The following techniques can be proposed in order to solve the above problem. First, a technique can be proposed which increases the dose of boron ions in the P-type body region  3 . However, this technique excessively increases the concentration of the P-type body region  3 , thereby also increasing the threshold voltage. This hinders realization of the low on-resistance which is a feature of the LDMOS transistor. Secondly, another technique can be proposed which changes the thermal processing during the diffusion of the P-type body region  3 . However, this technique varies the impurity concentration distribution of another transistor (the DMOS transistor is not necessarily required), and hence requires a considerable process change of setting the impurity concentration distribution again. 
     As described above, the above proposed techniques have disadvantages. On the other hand, the invention can solve the problem without doing the considerable process change. 
     Returning to the process of manufacturing the semiconductor device, after the polysilicon film  17  has been made conductive using POCl 3  as a thermal diffusion source, as shown in FIG. 5, a gate electrode patterned so as to extend from gate insulating film  6  to the LOCOS oxide film  9 . 
     Next, in FIG. 6, using, as a mask, a photoresist film  35  formed so as to cover the gate electrode  7  and drain region, P-type impurities (e.g. boron ions) are implanted and diffused, thereby forming the P-type body region  3  to be adjacent to the one edge of the gate electrode  7 . Incidentally, in this step, after the boron ions have been implanted under the condition of an accelerating voltage of about 40 KeV and dose of 5×10 13 /cm 2 , they are diffused for two hours at 900-1100° C. preferably 1050° C. 
     Further, in FIG. 7, using, as a mask, a photoresist mask  37  having openings on the source region to be formed in the P-type body region  3  and the drain region, N-type impurities are diffused to form N-type diffused regions  4  and  5  which constitute the source and drain regions, respectively. In this step, where the source/drain regions are formed having an LDD structure, first, with the photoresist film  35  removed in FIG. 6, for example, phosphorus ions are implanted under the condition of an accelerating voltage of 40 KeV and dose of 3.5×10 13 /cm 2 , as shown in FIG. 7, a side wall spacer film  36  is formed on the side wall of the gate electrode  7 . Using the photoresist film  37  as a mask, for example, arsenic ions are implanted under the condition of an accelerating voltage of 80 KeV and dose of 5×10 15 /cm 2 . Incidentally, in this embodiment, it is needless to say that the source/drain region should not be limited to the LDD structure. 
     In FIG. 8, in order to form a P-type diffused region  12  adjacent to the N-type diffused region for the purpose of assuming the potential of the P-type body region  3 , using a photoresist film as a mask, P-type impurities (e.g. boron difluoride ions) are implanted to form the P-type diffused region  12 . In this step, the boron difluoride ions are implanted under the conduction of an accelerating voltage of 60 KeV and dose of 4×10 15 /cm 2 . 
     Subsequently, like the conventional structure, after the source electrode  10  and drain electrode  11  have been formed, an interlayer insulating film  13  is formed to complete the semiconductor device. 
     In the above embodiment, impurity ion is implanted by ion-implantation, but impurity ion can be supplied from gas phase or solid phase.