Abstract:
A semiconductor technique is disclosed. Particularly a low voltage high current power device for use in a lithium ion secondary battery protecting circuit, a DC-DC converter and a motor is disclosed. Further, a method for fabricating a high density trench gate type power device is disclosed. That is, in the present invention, a trench gate mask is used for forming the well and/or source, and for this purpose, a side wall spacer is introduced. In this manner, the well and/or source is defined by using the trench gate mask, and therefore, 1 or 2 masking processes are skipped unlike the conventional process in which the well mask and the source mask are separately used. The decrease in the use of the masking process decreases the mask align errors, and therefore, the realization of a high density is rendered possible. Consequently, the on-resistance which is an important factor for the power device can be lowered.

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to a semiconductor technique, and particularly to a low voltage high current power device for use in a lithium ion secondary battery protecting circuit, a DC-DC converter and a motor. Particularly, the present invention relates to a method for fabricating a high density trench gate type power device. 
     2. Description of the Prior Art 
     Generally, the power device based on the MOS (metal oxide semiconductor) technology is classified into: a VDMOS (vertical double-diffused metal oxide semiconductor) in which the source-gate-drain are disposed in the vertical direction; and an LDMOS (lateral double-diffused metal oxide semiconductor) in which the source-gate-drain are disposed in the horizontal direction. 
     The VDMOS can accommodate a larger electric current than the LDMOS, and therefore, it is used as a large current power device. Further, the VDMOS is classified in accordance with its type into a planar gate type and a trench gate type. 
     The trench gate type power device has the disadvantage that the fabricating process is complicated, because a trench has to be etched in a silicon substrate, and because a good quality gate oxide layer has to be grown. However, it can build a larger number of devices per unit area compared with the planar gate type power device, and therefore, the on-resistance which is an important factor of a power device can be lowered. Further, it can accommodate a large current with a low driving voltage. Therefore, it is the present trend that the use of the power device is being transferred from the planar gate type power device to the trench gate type power device. 
     FIG. 1 illustrates the layout of a trench gate type power device. Referring to this drawing, the trench gate type power device  100  is constituted such that a well  104  and a source  106  are defined across a trench gate  102 . All the drawings hereinafter will be sectional views taken along a line A-B. 
     FIGS. 2A to  2 C illustrate the fabricating process for the conventional N-channel trench gate type power device. This conventional fabricating process will be described in detail below. 
     First, as shown in FIG. 2A, an oxide layer  22  is grown upon an N − -epi-layer  21  /N +  silicon substrate  20 . Then a P-well mask is used to etch a portion of the oxide layer  22  where a P-well is to be formed. Then a screen oxide layer  23  is grown upon the exposed N − -epi-layer  21  in a thickness of 400 Å. Then an impurity ion implantation is carried out for forming a P-well, and then, a heat treatment is carried out, thereby forming a P-well  24  on the N − -epi-layer  21 . 
     Then as shown in FIG. 2B, a source mask is used to form an N +  source  25 , and then, an oxide layer  26  is deposited on the entire structure. Then a trench gate mask is used to etch a portion of the oxide layer  26  where a trench gate is to be formed. Then a hard mask is used on the patterned oxide layer  26  to form a trench which is deeper than the P-well. Then a gate oxide layer  27  is grown along the side wall of the trench, and then, a doped polysilicon film is deposited. Then a gate electrode mask is used on the polysilicon film to carry out an anisotropic etching, thereby forming a trench gate  28 . 
     Under this condition, although there is not illustrated in the drawings, a pad for realizing a gate contact is formed on the edge region. 
     Then as shown in FIG. 2C, a field oxide layer  29  is deposited on the entire structure, and then, a gate and source electrode contact mask is used to selectively etch the oxide layer  29 , thereby forming the gate and source electrode contact holes. Then a metal layer is deposited upon the entire structure, and then, a gate and source electrode mask is used to pattern the gate and source electrodes  30 . Then a drain electrode  31  is formed on the rear face of the substrate. 
     As described above, in the conventional trench gate type power device fabricating process, there are required six masks including the P-well mask, the source mask, the trench gate mask, the gate electrode mask, the gate and source electrode contact mask, and the gate and source electrode mask. Further, in the case where P +  ions are implanted into the source region, another sheet of mask is additionally required. 
     Thus a large number of masks is required in the conventional fabricating process, and therefore, the fabricating process becomes complicated, while the fabricating cost is increased. Further, due to the increase in the number of the masking processes, align errors are induced, with the result that the realization of a high density is hindered, and that the yield is lowered. Due to this difficulty of realizing the high density, the on-resistance which is an important factor of the power device is degraded. 
     SUMMARY OF THE INVENTION 
     The present invention is intended to overcome the above described disadvantages of the conventional technique. 
     Therefore it is an object of the present invention to provide a method for fabricating a trench gate type power device, in which the on-resistance is improved. 
     In achieving the above object, the method for fabricating a trench gate type power device according to the present invention includes the steps of: a) forming an insulating layer upon a semiconductor substrate; b) using a trench gate mask to pattern the insulating layer; c) carrying out an ion implantation by using the insulating layer (thus patterned) as an ion implantation mask, for forming a well; d) further using the insulating layer as an ion implantation mask to carry out an ion implantation for forming a source; e) forming a spacer insulating layer on side walls of the insulating layer; f) using the insulating layer and the spacer insulating layer as etch masks to form a trench on the semiconductor substrate and to define a source region; g) forming a gate insulating layer on inside walls of the trench; h) filling a gate electrode material into the trench, with the gate insulating layer having been formed thereon; and i) forming a source electrode electrically contacted to the source region, and forming a drain electrode electrically contacted to a rear face of the semiconductor substrate. 
     In another aspect of the present invention, the method for fabricating a trench gate type power device according to the present invention includes the steps of: a) forming a first insulating layer upon a semiconductor substrate; b) using a well mask to pattern the first insulating layer; c) carrying out an ion implantation by using the first insulating layer (thus patterned) as an ion implantation mask, for forming a well; e) forming a second insulating layer upon an entire structure after completing step c); f) using a trench gate mask to pattern the second insulating layer; g) using the patterned first and second insulating layers as ion implantation masks to carry out an ion implantation for forming a source; h) forming a spacer insulating layer on side walls of the first and second insulating layers; i) using the first and second insulating layers and the spacer insulating layer as etch masks to form a trench on the semiconductor substrate and to define a source region; j) forming a gate insulating layer on side walls of the trench; k) filling a gate electrode material into the trench, with the gate insulating layer having been formed thereon; l) selectively removing the second insulating layer; m) etching an exposed portion of the semiconductor substrate after the step l), to form a source contact region; and n) forming a source electrode electrically contacted to the source region, and forming a drain electrode electrically contacted to a rear face of the semiconductor substrate. 
     In still another aspect of the present invention, the method for fabricating a trench gate type power device according to the present invention includes the steps of: a) forming an insulating layer upon a semiconductor substrate; b) using a trench gate mask to pattern the insulating layer; c) carrying out an ion implantation by using the insulating layer (thus patterned) as an ion implantation mask, for forming a well; d) forming a spacer insulating layer on side walls of the insulating layer; e) using the insulating layer and the spacer insulating layer as etch masks to form a trench on the semiconductor substrate; f) forming a gate insulating layer on inside walls of the trench; g) filling a gate electrode material into the trench, with the gate insulating layer having been formed thereon; h) removing the spacer insulating layer; i) using the insulating layer as an ion implantation mask to carry out an ion implantation for forming a source region; and j) forming a source electrode electrically contacted to the source region, and k) forming a drain electrode electrically contacted to a rear face of the semiconductor substrate. 
     That is, in the present invention, a trench gate mask is used for forming the well and/or source, and for this purpose, the side wall spacer is introduced. In this manner, the well and/or source is defined by using the trench gate mask, and therefore, 1 or 2 masking processes are skipped unlike the conventional process in which the well mask and the source mask are separately used. The decrease in the use of the masking process decreases the mask align errors, and therefore, the realization of a high density is rendered possible. Consequently, the on-resistance which is an important factor for the power device can be lowered. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The above object and other advantages of the present invention will become more apparent by describing in detail the preferred embodiment of the present invention with reference to the attached drawings in which: 
     FIG. 1 illustrates the layout of a high density trench gate type power device; 
     FIGS. 2A to  2 C illustrate the fabricating process for the conventional trench gate type power device; 
     FIGS. 3A to  3 G illustrate a first embodiment of the fabricating method for the trench gate type power device according to the present invention; 
     FIGS. 4A to  4 D illustrate a second embodiment of the fabricating method for the trench gate type power device according to the present invention; 
     FIGS. 5A to  5 F illustrate a third embodiment of the fabricating method for the trench gate type power device according to the present invention; 
     FIG. 6 is a sectional view showing an IGBT (insulated gate bipolar transistor) type power device as a fourth embodiment of the present invention; and 
     FIG. 7 is a sectional view showing a trench gate type power device as a fifth embodiment of the present invention. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     The preferred embodiments of the present invention will be described in such a manner that those ordinarily skilled in the art can easily carry out the present invention. 
     FIGS. 3A to  3 G illustrate a first embodiment of the N-channel trench gate type power device according to the present invention. The first embodiment will be described referring to these drawings. 
     First as shown in FIG. 3A, an N − -epi-layer  2  with a resistivity of 0.3-1 Ω-cm and with a thickness of 3-8 μm is grown on an N +  silicon substrate  1  which has a resistivity of 0.004 Ω-cm or less. Then an oxide layer  3  is grown in a thickness of 4000-5000 μm at a temperature of 900-1100° C. by using an electric furnace, and thereupon, a photoresist is spread. Then a portion where a trench gate is to be formed is defined by using a trench gate mask. Then a plasma ion etching process is applied to anisotropically etch the oxide layer so as to remove the photoresist. 
     Then as shown in FIG. 3B, a screen oxide layer  11  is grown in a thickness of 400 Å on the exposed N − -epi-layer  2 , and then, BF 2  is ion-implanted with an energy of 60-80 KeV and at a dose of 1-3E13/cm 2 . 
     Then as shown in FIG. 3C, a heat treatment is carried out at a temperature of 1000-1150° C., thereby forming a P-well  4  with a depth of 1.2-2 μm. Then in order to form a source region  6 , P or As is ion-implanted vertically or inclinedly with an energy of 60-80 KeV and at a dose of 3-5E15/cm 2 , and then, a heat treatment is carried out. 
     Then as shown in FIG. 3D, a TEOS (tetraethylotho silicate) oxide layer or LTO (low temperature oxide) layer is deposited in a thickness of 2000-5000 Å. Then the entire surface of the oxide layer is etched by applying a plasma ion etch process, thereby forming a spacer oxide layer  5 . 
     Then as shown in FIG. 3E, the oxide layer  3  and the spacer oxide layer  5  are used as etch masks to etch the exposed screen oxide layer  11  and the P-well  4  by applying a plasma ion etching process so as to form a trench structure. Under this condition, the P-well is etched at least as deep as the P-well. Then in order to remove the defects on the side walls of the trench, a sacrificial layer (not illustrated) is grown in a thickness of 500-1000 Å at a temperature of 850-1100° C., and is removed. 
     Then as shown in FIG. 3F, a gate oxide layer  7  is grown in a thickness of 300-500 Å on the inside wall of the trench, and then, a P (phosphorus)-doped polysilicon film is deposited. Then a gate electrode mask is used to anisotropically etch the polysilicon film, thereby forming a trench gate  8 . Then a gate protecting oxide layer  12  is grown in a thickness of 300-1000 Å on the surface of the exposed trench gate  8 . 
     Then as shown in FIG. 3G, a field oxide layer  9  is deposited in a thickness of 7000-8000 Å on the entire structure. Then a photo etch process is applied to form contact holes for the source electrode and the gate electrode. Then a metal layer is deposited on the entire structure, and then, a photo etching process is applied to form a source electrode  10 , and to form a drain electrode  13  on the rear face of the substrate. 
     In the case where the trench gate type power device is fabricated by the above described process, a well mask and a source mask are omitted, and therefore, the number of the masks can be reduced to four (including the trench gate mask, the gate electrode mask, the gate and source electrode contact mask, and the gate and source electrode mask). The decrease in the use of the masking process decreases the mask align errors, and therefore, the process margin can be increased. Thus if the area per unit device is decreased, the on-resistance is naturally lowered. 
     FIGS. 4A to  4 D illustrate a second embodiment of the trench gate type power device according to the present invention. 
     This second embodiment will be described. First as shown in FIG. 4A, an N − - epi-layer  52  with a resistivity of 0.3-1 Ω-cm and with a thickness of 3-8 μm is grown on an N +  silicon substrate  51  which has a resistivity of 0.004 Ω-cm or less. Then an oxide layer  53  is grown in a thickness of 4000-5000 Å at a temperature of 900-1100° C. by using an electric furnace, and thereupon, a photoresist is spread. Then a portion where a trench gate is to be formed is defined by using a trench gate mask. Then a plasma ion etching process is applied to anisotropically etch the oxide layer so as to remove the photoresist. Then a screen oxide layer  55  is formed in a thickness of 400 Å on the exposed N − -epi-layer  52 . Then BF 2  is ion-implanted with an energy of 60-80 KeV and at a dose of 1-3E13/cm 2 . Then a heat treatment is carried out at a temperature of 1000-1150° C., thereby forming a P-well  54  with a depth of 1.2-2 μm. 
     Then as shown in FIG. 4B, a silicon nitride layer  56  with a thickness of 3000-5000 Å is deposited on the entire structure. Then a trench gate mask is used to define a portion where the trench gate is to be formed. Then the silicon nitride layer  56  is selectively etched, and then, in order to form a source region, P or As is ion-implanted vertically or inclinedly with an energy of 60-80 KeV and at a dose of 3-5E15/cm 2 . Then a TEOS oxide layer or an LTO layer is deposited in a thickness of 2000-5000 Å, and then, a plasma etching process is carried out to etch the entire surface, thereby forming a spacer oxide layer  57 . 
     Then as shown in FIG. 4C, by using the silicon nitride layer  56  and the spacer oxide layer  57  as the etch masks, a plasma ion etching process is carried out to etch the exposed screen oxide layer  55  and the P-well  54 , so as to form a trench structure. Under this condition, the trench is etched as deep as the P-well  54 . In order to remove the defects on the inside wall of the trench, a sacrificial layer (not illustrated) is grown in a thickness of 500-1000 Å at a temperature of 850-1100 Å and is removed. Then a gate oxide layer  59  is grown in a thickness of 300-500 Å on the inside wall of the trench. Then a phosphorus-doped polysilicon film is deposited, and then, a gate electrode mask is used to anisotropically etch the polysilicon film so as to form a trench gate  60 . Then a gate protecting layer  61  is grown in a thickness of 3000-5000 □ on the surface of the exposed trench gate  60 . Reference code  58  indicates an N +  source. 
     Then as shown in FIG. 4D, the silicon nitride layer  54  is removed, and by using the spacer oxide layer  57  and the protecting oxide layer  61  as masks, a plasma etching is carried out on the N +  source  58  and the P-well  54 , thereby forming a source contact region. Then a P +  impurity is ion-implanted into the source contact region, and then, a heat treatment is carried out. Then a metal layer is deposited upon the entire structure, and the metal layer is patterned so as to form the gate and source electrode  62 , while a drain electrode  63  is formed on the rear face of the substrate. 
     If the above fabricating process is carried out, there are required only 5 masks (the well mask, the trench gate mask, the gate electrode mask, the gate and source contact mask, and the gate and source electrode mask) in total. Therefore, like in the first embodiment, the fabricating process can be simplified, and the on-resistance can be improved. 
     FIGS. 5A to  5 F illustrate a third embodiment of fabricating method for the trench gate type power device according to the present invention. 
     This third embodiment will be described. First as shown in FIG. 5 a,  an N − -epi-layer  72  with a resistivity of 0.3-1 Ω-cm and with a thickness of 3-8 μm is grown on an N +  silicon substrate  71  which has a resistivity of 0.004 Ω-cm or less. Then an oxide layer  73  is grown in a thickness of 4000-5000 Å at a temperature of 900-1100 Å by using an electric furnace, and thereupon, a photoresist is spread. Then a portion where a trench gate is to be formed is defined by using a trench gate mask. Then a plasma ion etching process is applied to anisotropically etch the oxide layer so as to remove the photoresist. Then a screen oxide layer  74  is formed in a thickness of 400 Å on the exposed N − -epi-layer  72 . Then BF 2  is ion-implanted with an energy of 60-80 KeV and at a dose of 1-3E13/cm 2 . Then a heat treatment is carried out at a temperature of 1000-1150 Å, thereby forming a P-well  75  with a depth of 1.2-2 μm. 
     Then as shown in FIG. 5B, a silicon nitride layer  76  in a thickness of 1000-4000 Å and a silicon oxide layer  77  in a thickness of 1000-4000 Å are sequentially deposited on the entire surface of the structure. Under this condition, the silicon oxide layer  77  may be TEOS or LTO. 
     Then as shown in FIG. 5C, the entire areas of the silicon oxide layer  77  and the silicon nitride layer  76  are etched to form side wall spacers  76  and  77  on the oxide layer  73 . Then by using the oxide layer  73  and the side wall spacers  76  and  77  as etch masks, a plasma ion etching is carried out on the exposed oxide layer  74  and the P-well  75  so as to form a trench with depth at least same as that of the P-well  75 . Then in order to remove the defects of the inside walls of the trench, a sacrificial layer (not illustrated) is grown in a thickness of 500-1000 Å at a temperature of 850-1100° C., and is removed. 
     Then as shown in FIG. 5D, a gate oxide layer  78  is grown in a thickness of 300-500 Å on the inside walls of the trench. Then a phosphorus-doped polysilicon film is deposited, and then, a gate electrode mask is used to anisotropically etch the polysilicon film to form a trench gate  79 . Then a gate protecting layer  80  is grown in a thickness of 1000-4000 Å on the surface of the exposed trench gate  79 . 
     Then as shown in FIG. 5E, the side wall spacers  76  and  77  are removed. Then P or As is ion-implanted with an energy of 60-80 KeV and at a dose of 3-5E15/cm 2 , and then, a heat treatment is carried out, thereby forming a source  81 . 
     Then as shown in FIG. 5F, an oxide layer  82  for serving as a field oxide layer is deposited upon the entire structure in a thickness of 7000-8000 Å. Then a photo etching process is carried out to form contact holes for the source and the gate. Then a metal layer is deposited, and a photo etching process is carried out to form the gate and the source electrode  83 , and to form a drain electrode  84  on the rear face of the substrate. 
     In the case where the trench gate type power device is fabricated by the above described process, the number of the masks can be reduced to four (including the trench gate mask, the gate electrode mask, the gate and source electrode contact mask, and the gate and source electrode mask). 
     Meanwhile, FIG. 6 illustrates an IGBT (insulated gate bipolar transistor) manufactured in a 4th embodiment of the present invention. In the first and third embodiments, the N − -epi-/N +  substrate was used, but in this 4th embodiment, an N − -epi/N + -epi/P +  substrate structure is used. Except this fact, the fabricating process is same as those of the first and third embodiments. 
     FIG. 7 is a sectional view showing a trench gate type power device as a fifth embodiment of the present invention. The fabricating process for this device is same as that of the first and third embodiments of the present invention, except that a thin N − -epi-layer of 2-3 μm is used. The difference lies in the fact that when forming the trench structure, an etching is carried out deeper than the N − -epi-layer, down to the N +  substrate. If such a thin N − -epi-layer is used, although the breakdown voltage is lowered, the resistance of the drift region can be reduced, and therefore, the on-resistance which is an important factor for the power device can be lowered, this being an advantage. 
     In the above-described first to fifth embodiments, the material of the side wall spacer can be substituted. Further, when the oxide layer patterning is carried out (e.g., the oxide layer  73  of the third embodiment), if the side wall is made inclined, then a gradient of the impurity profile can be formed during the ion implantation, and therefore, the electrical characteristics of the power device can be improved. 
     According to the present invention as described above, the number of the masking processes is decreased, and therefore, it will be advantageous for achieving a high density, while the on-resistance which is an important factor of the power device can be lowered. Further, owing to the decrease of the number of the masking processes, the cost for the masks can be curtailed. 
     In the above, the present invention was described based on the specific embodiments and the drawings, but it should be apparent to those ordinarily skilled in the art that various changes and modifications can be added without departing from the spirit and scope of the present invention which will be limited only by the appended claims.