BACKGROUND OF THE INVENTION 
       [0001]    1. Field of Invention 
         [0002]    The present invention relates to a lateral double diffused metal oxide semiconductor (LDMOS) device and a manufacturing method thereof; particularly, it relates to such LDMOS device and a manufacturing method thereof wherein the breakdown voltage is increased. 
         [0003]    2. Description of Related Art 
         [0004]      FIGS. 1A-1C  show a cross-section view, a 3D (3-dimensional) view, and a top view of a prior art lateral double diffused metal oxide semiconductor (LDMOS) device  100 , respectively. As shown in  FIGS. 1A-1C , a P-type substrate  11  has multiple isolation regions  12  by which a device region of the LDMOS device  100  is defined. The isolation regions  12  and a field oxide region  12   a  for example are a local oxidation of silicon (LOCOS) structure or a shallow trench isolation (STI) structure, the former being shown in the figures. The LDMOS device  100  includes an N-type well  14 , a gate  13 , a drain  15 , a source  16 , a body region  17 , a body electrode  17   a,  and the field oxide region  12   a.  The well  14 , the drain  15  and the source  16  are formed by lithography process steps and ion implantation process steps, wherein the lithography process step defines the implantation region by a photoresist mask together with a self-alignment effect provided by all or part of the gate  13 , and the ion implantation process step implants N-type impurities to the defined region in the form of accelerated ions. The drain  15  and the source  16  are beneath the gate  13  and at different sides thereof respectively. The body region  17  and the body electrode  17   a  are formed by lithography process steps and ion implantation process steps, wherein the lithography process step defines the implantation region by a photoresist mask together with a self-alignment effect provided by all or part of the gate  13 , and the ion implantation process step implants P-type impurities to the defined region in the form of accelerated ions. Part of the gate  13  is above the field oxide region  12   a  in the LDMOS device  100 . The LDMOS device is a high voltage device designed for applications requiring higher operation voltages. However, for operating in the high voltage environment with a higher breakdown voltage, the conduction resistance is usually sacrificed (i.e., higher conduction resistance), and thus the application range of the LDMOS device is limited. It is a dilemma between the performance of the conduction resistance and the breakdown voltage; changing parameters of ion implantation process steps, or adding additional ion implantation process steps can reduce the conduction resistance, but the breakdown voltage will be sacrificed, or the manufacturing cost will be increased. 
         [0005]    In view of above, to overcome the drawbacks in the prior art, the present invention proposes an LDMOS device and a manufacturing method thereof, which increases the breakdown voltage without sacrificing the conduction resistance, so that the LDMOS device may have a broader application range, in which additional manufacturing process steps are not required. Besides, the parameters of the ion implantation process steps of the high voltage LDMOS device of the present invention can be adopted in a low voltage device, i.e., the ion implantation steps of the high voltage LDMOS device and the low voltage device may be integrated, such that the high voltage LDMOS device may be integrated with a low voltage device in one substrate. 
       TOTAL OF THE INVENTION 
       [0006]    A first objective of the present invention is to provide a lateral double diffused metal oxide semiconductor (LDMOS) device. 
         [0007]    A second objective of the present invention is to provide a manufacturing method of an LDMOS device. 
         [0008]    To achieve the objectives mentioned above, from one perspective, the present invention provides an LDMOS device, which is formed in a first conductive type substrate, wherein the substrate has an upper surface. The LDMOS device includes: a second conductive type high voltage well, which is formed in the substrate beneath the upper surface; a first field oxide region, which is formed on the upper surface, and is located in the high voltage well from top view; a gate, which is formed on the upper surface, wherein a first part of the gate is above the first field oxide region; a second conductive type source and a second conductive type drain, which are formed beneath the upper surface at two sides of the gate respectively; a first conductive type body region, which is formed in the substrate beneath the upper surface, at the same side as the source with respect to the gate, wherein the source is located in the body region; and at least one second field oxide region, which is formed on the upper surface, and is located between the first field oxide region and the drain from top view. 
         [0009]    From another perspective, the present invention provides a manufacturing method of an LDMOS device, including: providing a first conductive type substrate, wherein the substrate has an upper surface; forming a first field oxide region and at least one second field oxide region on the upper surface; forming a second conductive type high voltage well in the substrate beneath the upper surface, the first field oxide region and the at least one second field oxide region are within the range of the high voltage well from top view; forming a gate on the upper surface, wherein the gate includes a first part, which is above the first field oxide region; and forming a second conductive type source and a second conductive type drain beneath the upper surface at two sides of the gate respectively, and forming a first conductive type body region in the substrate beneath the upper surface, at the same side as the source with respect to the gate, wherein the source is located in the body region, and the drain is located outside the one second field oxide region or one of the second field oxide regions which is most away from the gate; wherein the high voltage well is formed after the first and second field oxide regions are formed, such that a distribution of the concentration of the second conductive type impurities in the high voltage well is related to the location of the at least one second field oxide region. 
         [0010]    In one preferable embodiment, at least one opening region is defined between the first field oxide region and the second field oxide region, wherein a second conductive type impurity concentration beneath the upper surface at the opening region is higher than that beneath the first field oxide region and that beneath the second oxide region. 
         [0011]    In the aforementioned embodiment, the gate may further include a second part, which is formed on the upper surface at the opening region, and has a gate dielectric layer connected to the upper surface. 
         [0012]    In the aforementioned embodiment, the gate may further include a third part, which is formed on the second field oxide region. 
         [0013]    In one preferable embodiment, the LDMOS device includes a plurality of second field oxide regions, wherein a plurality of opening regions are defined between the first field oxide region and its adjacent second field oxide region, and between the second field oxide regions, wherein second conductive type impurity concentrations beneath the upper surface at the opening regions are higher than those beneath the first field oxide region and the second oxide regions. 
         [0014]    In the aforementioned embodiment, the opening region which is relatively nearer to the drain has an area preferable larger than that of the opening region which is relatively nearer to the first field oxide region. 
         [0015]    In another embodiment, the body region and the substrate may be separated by the high voltage well, such that the body region and the substrate are not in direct contact with each other; or at least part of the body region may be directly connected to the substrate, or may be indirectly connected to the substrate by a first conductive type connecting well, such that the body region and the substrate are electrically connected. 
         [0016]    The objectives, technical details, features, and effects of the present invention will be better understood with regard to the detailed description of the embodiments below. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0017]      FIGS. 1A-1C  show a cross-section view, a 3D (3-dimensional) view, and a top view of a prior art lateral double diffused metal oxide semiconductor (LDMOS) device  100 , respectively. 
           [0018]      FIGS. 2A-2D  show a first embodiment of the present invention. 
           [0019]      FIG. 3  shows a second embodiment of the present invention. 
           [0020]      FIG. 4  shows a third embodiment of the present invention. 
           [0021]      FIG. 5  shows a fourth embodiment of the present invention. 
           [0022]      FIG. 6  shows a fifth embodiment of the present invention. 
           [0023]      FIG. 7  shows a sixth embodiment of the present invention. 
           [0024]      FIG. 8  shows a seventh embodiment of the present invention. 
           [0025]      FIG. 9  shows an eighth embodiment of the present invention. 
           [0026]      FIG. 10  shows a ninth embodiment of the present invention. 
           [0027]      FIG. 11  shows a tenth embodiment of the present invention. 
           [0028]      FIGS. 12A-12C  show characteristic curves of a prior art LDMOS device. 
           [0029]      FIGS. 13A-13C  show characteristic curves of an LDMOS device according to the present invention. 
       
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
       [0030]    The drawings as referred to throughout the description of the present invention are for illustration only, to show the interrelations between the regions and the process steps, but not drawn according to actual scale. 
         [0031]    Please refer to  FIGS. 2A-2D  for a first embodiment according to the present invention, wherein  FIGS. 2A-2B  are 3D schematic diagrams showing a manufacturing method of an LDMOS device  200  according to the present invention, and  FIGS. 2C and 2D  are a cross-section view and a top view of the LDMOS device  200  respectively. As shown in  FIGS. 2A and 2B , first, a substrate  21  with an upper surface  21   a  is provided, wherein the substrate  21  is for example but not limited to a P-type substrate (or an N-type substrate in another embodiment). The substrate  21  for example is a non-epitaxial silicon substrate, or an epitaxial substrate. Next, as shown in  FIG. 2A , an isolation region  22  and field oxide regions  22   a  and  22   b  are formed on the upper surface  21   a.  The field oxide regions  22   a  and  22   b  are located in a high voltage well  24  from top view (referring to  FIG. 2D ), wherein the high voltage well  24  is formed in a later process step. The isolation region  22  and field oxide regions  22   a  and  22   b  are, for example, a LOCOS or an STI structure (the former being shown in  FIGS. 2A-2C ). The isolation region  22  and field oxide regions  22   a  and  22   b  may be formed by for example but not limited to the same process steps. Next, an N-type high voltage well  24  is formed in the substrate  21  beneath the upper surface  21   a  by an ion implantation process step, wherein the ion implantation process step implants N-type impurities to a defined region in the form of accelerated ions. Note that the field oxide regions  22   a  and  22   b  have masking effect to the aforementioned accelerated ions, and therefore the distribution of the N-type impurity concentration in the high voltage well  24  is related to the location of the field oxide region  22   b.  According to this embodiment, an opening region  221  (referring to  FIGS. 2C and 2D ) is defined beneath the upper surface  21   a  between the field oxide regions  22   a  and  22   b,  wherein the N-type impurity concentration below the opening region  221  is higher than that below the field oxide regions  22   a  and  22   b.    
         [0032]    Next, referring to  FIGS. 2B ,  2 C, and  2 D, a gate  23 , a drain  25 , a source  26 , a body region  27 , and a body electrode  27   a  are formed. As shown in  FIG. 2B , the gate  23  is formed on the upper surface  21   a,  wherein a part of the gate  23  is above the field oxide region  22   a.  The drain  25  and the source  26  for example are N-type but not limited to N-type, and they are beneath the upper surface  21   a  and at different sides of the gate  23  in the high voltage well  24 . The drain  25  and the source  26  are separated by the gate  23  and the field oxide regions  22   a  and  22   b,  as shown by the top view of  FIG. 2D . The body region  27  is formed in the high voltage well  24  beneath the upper surface  21   a  at the same side as the source  26  with respect to the gate  23 , and the source  26  is in the body region  27 . The drain  25  is formed at the other side of the gate  23  in the high voltage well  24 . The N-type source  26  and drain  25  are formed beneath the upper surface  21   a  by lithography process steps and ion implantation process steps, wherein the lithography process step defines the implantation regions by a photoresist mask together with a self-alignment effect provided by all or part of the gate  23  and the field oxide regions  22   a  and  22   b,  and the ion implantation process step implants N-type impurities to the defined regions in the form of accelerated ions. The P-type body region  27  and body electrode  27   a  are formed beneath the upper surface  21   a  by lithography process steps and ion implantation process steps, wherein the lithography process step defines the implantation regions by a photoresist mask together with a self-alignment effect provided by all or part of the gate  23  and the isolation region  22 , and the ion implantation process step implants P-type impurities to the defined regions in the form of accelerated ions. The source  26  and drain  25  may be formed by the same or different lithography and ion implantation process steps, and besides, the sequence for forming the source  26 , the drain  25 , the body region  27 , and the body electrode  27   a  can be any order. 
         [0033]    In the prior art LDMOS device  100 , the drift region between the body region  17  and the drain  15  is entirely covered by the gate  13  and the field oxide region  12   a  from top view. This embodiment is different from the prior art LDMOS device  100  in that, the drift region of the LDMOS device  200  in this embodiment is not entirely covered by the gate  23  and the field oxide regions  22   a  and  22   b.  Part of the upper surface  21   a  between the field oxide regions  22   a  and  22   b  above the drift region is exposed, such that the ion implantation process step which forms the high voltage well  24  implants more impurities in the opening region  221 , and therefore, the N-type impurity concentration below the opening region  221  is higher than that below the field oxide regions  22   a  and  22   b.  This arrangement is advantageous over the prior art in that: First, the LDMOS device of the present invention has a relatively higher breakdown voltage, in particular a relatively higher ON breakdown voltage because the Kirk effect is mitigated according to the present invention. Second, in manufacturing process, no additional process step or mask is required, that is, the field oxide region  22   b  may be formed by the same process steps with the field oxide region  22   a  and the isolation region  22  without any additional process step. As such, the LDMOS device in the present invention has a higher breakdown voltage while it can be manufactured by a low cost. 
         [0034]      FIG. 3  shows a second embodiment of the present invention.  FIG. 3  is a schematic diagram showing a cross-section view of an LDMOS device  300  of the present invention. The LDMOS device  300  is formed in a substrate  31  and includes a device region defined by an isolation region  32 . The LDMOS device  300  includes field oxide regions  32   a  and  32   b,  a gate  33 , a high voltage well  34 , a drain  35 , a source  36 , a body region  37 , and a body electrode  37   a.  This embodiment is different from the first embodiment in that, as shown in  FIG. 3 , the gate  33  includes a first part  33   a  above the field oxide region  32   a,  a second part  33   b  above the opening region  321  of the upper surface  31   a,  and a third part  33   c  above the field oxide region  32   b.  Note that the second part  33   b  preferably includes a gate dielectric layer (i.e., the gate  33  includes a gate electrode and a gate dielectric layer), and the gate dielectric layer is connected to the upper surface  31   a  to prevent direct electrical connection between the gate electrode and the high voltage well  34 . In another embodiment of the present invention, the gate  33  does not have to include the third part  33   c.    
         [0035]      FIG. 4  shows a third embodiment of the present invention.  FIG. 4  is a schematic diagram showing a cross-section view of an LDMOS device  400  of the present invention. The LDMOS device  400  is formed in a substrate  41  and includes a device region defined by an isolation region  42 . The LDMOS device  400  includes field oxide regions  42   a,    42   b,  and  42   c,  a gate  43 , a high voltage well  44 , a drain  45 , a source  46 , a body region  47 , and a body electrode  47   a.  This embodiment is different from the first embodiment in that, as shown in  FIG. 4 , the LDMOS device  400  includes multiple field oxide regions  42   b  and  42   c  between the field oxide region  42   a  and the drain  45 . Multiple opening regions are defined between the field oxide region  42   a  and its adjacent field oxide region  42   b,  and between the field oxide regions  42   b  and  42   c,  as indicated by the opening regions  421  and  422  shown in the figure. The N-type impurity concentrations below the upper surface  41   a  within the opening regions  421  and  422  are higher than those below the field oxide regions  42   a,    42   b,  and  42   c.    
         [0036]      FIG. 5  shows a fourth embodiment of the present invention.  FIG. 5  is a schematic diagram showing a cross-section view of an LDMOS device  500  of the present invention. The LDMOS device  500  is formed in a substrate  51  and includes a device region defined by an isolation region  52 . The LDMOS device  500  includes field oxide regions  52   a,    52   b,  and  52   c,  a gate  53 , a high voltage well  54 , a drain  55 , a source  56 , a body region  57 , and a body electrode  57   a.  This embodiment is different from the third embodiment in that, similar to the second embodiment, as shown in  FIG. 5 , the gate  53  covers field oxide regions  52   a,    52   b,  and  52   c,  and multiple opening regions between these field oxide regions. Certainly, the part of the gate  53  above the opening regions preferably includes a gate dielectric layer (i.e., the gate  53  includes a gate electrode and a gate dielectric layer), and the gate dielectric layer is connected to the upper surface  51   a  to prevent direct electrical connection between the gate electrode and the high voltage well  54 . 
         [0037]      FIG. 6  shows a fifth embodiment of the present invention.  FIG. 6  is a schematic diagram showing a cross-section view of an LDMOS device  600  of the present invention. As shown in  FIG. 6 , the LDMOS device  600  is formed in a substrate  61  and includes a device region defined by an isolation region  62 . The LDMOS device  600  includes field oxide regions  62   a,    62   b,    62   c,  and  62   d,  a gate  63 , a high voltage well  64 , a drain  65 , a source  66 , a body region  67 , and a body electrode  67   a.  This embodiment intends to show that in the LDMOS device  600  according to the present invention, the distribution of the N-type impurity concentration below the opening regions maybe adjusted by providing different-sized field oxide regions  62   a,    62   b,    62   c,  and  62   d,  such that the characteristics of the LDMOS device according to the present invention may be optimized. For example, the opening region relatively nearer to the drain  65  may have a larger area than that of the opening region relatively nearer to the field oxide region  62   a,  to obtain an optimized ON breakdown voltage. For another example, the field oxide region relatively nearer to the drain  65  may have a smaller size than that of the field oxide region relatively nearer to the source region  66 , as shown by this cross section view which crosses the opening regions. 
         [0038]      FIG. 7  shows a sixth embodiment of the present invention.  FIG. 7  is a schematic diagram showing a top view of an LDMOS device  700  of the present invention. As shown in  FIG. 7 , the LDMOS device  700  includes a device region defined by an isolation region  72 . The LDMOS device  700  includes field oxide regions  72   a  and  72   b,  a gate  73 , a high voltage well  74 , a drain  75 , a source  76 , a body region  77 , and a body electrode  77   a.  This embodiment intends to show that in the LDMOS device  700  according to the present invention, the field oxide region  72   b  may include multiple opening regions, and these opening regions may be formed at different locations with different densities, such that the breakdown voltage of the LDMOS device is increased. Different layout arrangement of the opening regions in the field oxide region  72   b  is also within the scope of the present invention. 
         [0039]      FIG. 8  shows a seventh embodiment of the present invention.  FIG. 8  is a schematic diagram showing a top view of an LDMOS device  800  of the present invention. As shown in  FIG. 8 , the LDMOS device  800  includes a device region defined by an isolation region  82 . The LDMOS device  800  includes field oxide regions  82   a,    82   b,    82   c,  and  82   d,  a gate  83 , a high voltage well  84 , a drain  85 , a source  86 , a body region  87 , and a body electrode  87   a.  This embodiment intends to show that in the LDMOS device  800  according to the present invention, widths of the opening regions may be determined according to different requirements by arranging the locations and sizes of the field oxide regions  82   a,    82   b,    82   c,  and  82   d.    
         [0040]      FIG. 9  shows an eighth embodiment of the present invention.  FIG. 9  is a schematic diagram showing a cross-section view of an LDMOS device  900  of the present invention. As shown in  FIG. 9 , the LDMOS device  900  is formed in a substrate  91  and includes a device region defined by an isolation region  92 . The LDMOS device  900  includes field oxide regions  92   a  and  92   b,  a gate  93 , a high voltage well  94 , a drain  95 , a source  96 , a body region  97 , and a body electrode  97   a.  This embodiment is different from the first embodiment. In the first embodiment, the body region  27  and the substrate  21  are separated by the high voltage well  24  such that the body region  27  is not in direct contact to the substrate  21 , and therefore the LDMOS device  200  may be used, for example, as a high side device in a power supply circuit. On the other hand, as shown in the figure, part of the body region  97  of this embodiment is directly connected to the substrate  91 , such that the body region  97  is electrically connected to the substrate  91 , and therefore the LDMOS device  900  may be used, for example, as a low side device in a power supply circuit. 
         [0041]      FIG. 10  shows a ninth embodiment of the present invention.  FIG. 10  is a schematic diagram showing a cross-section view of an LDMOS device  1000  of the present invention. As shown in  FIG. 10 , the LDMOS device  1000  is formed in a substrate  101  and includes a device region defined by an isolation region  102 . The LDMOS device  1000  includes field oxide regions  102   a  and  102   b,  a gate  103 , a high voltage well  104 , a drain  105 , a source  106 , a body region  107 , and a body electrode  107   a.  Different from the eighth embodiment, part of the body region  107  of this embodiment is connected to a P-type connecting well  109  and the P-type connecting well  109  is further connected to the substrate  101 , such that the body region  107  is electrically but indirectly connected to the substrate  101 , and therefore the LDMOS device  1000  may be used as a low side device in a power supply circuit. 
         [0042]      FIG. 11  shows a tenth embodiment of the present invention.  FIG. 11  is a schematic diagram showing a top view of an LDMOS device  1100  of the present invention. As shown in  FIG. 11 , the LDMOS device  1100  includes a device region defined by an isolation region  112 . The LDMOS device  1100  includes field oxide regions  112   a  and  112   b,  a gate  113 , a high voltage well  114 , a drain  115 , a source  116 , a body region  117 , and a body electrode  117   a.  This embodiment intends to show that in the LDMOS device  1100  according to the present invention, the field oxide region  72   b  may include multiple opening regions, and these opening regions may have different shapes. The shape of the opening region from top view is not limited; any shape of the opening region from top view is within the scope of the present invention. 
         [0043]      FIGS. 12A-12C  show characteristic curves of a prior art LDMOS device.  FIG. 12A  shows a characteristic curve of a drain current Id versus a drain voltage Vd of the prior art LDMOS device in the OFF operation. The breakdown voltage of the prior art LDMOS device in the OFF operation is around 76V, as indicated by the dash line shown in the figure.  FIG. 12B  shows characteristic curves of the drain current Id (left vertical axis) and conductance (right vertical axis) versus a gate voltage Vg. The threshold voltage of the prior art LDMOS device is around 1V.  FIG. 12C  shows a characteristic curve of the drain current Id versus the drain voltage Vd of the prior art LDMOS device in the ON operation. The breakdown voltage of the prior art LDMOS device in the ON operation is around 54V, as indicated by the dash line shown in the figure. 
         [0044]      FIGS. 13A-13C  show characteristic curves of an LDMOS device according to the present invention, wherein the operation voltage is the same as the LDMOS device shown in  FIGS. 12A-12C .  FIG. 13A  shows a characteristic curve of a drain current Id versus a drain voltage Vd of the LDMOS device according to the present invention in the OFF operation. The breakdown voltage of the LDMOS device according to the present invention in the OFF operation is around 100V, as indicated by the dash line shown in the figure.  FIG. 13B  shows characteristic curves of the drain current Id (left vertical axis) and conductance (right vertical axis) versus a gate voltage Vg. The threshold voltage of the LDMOS device according to the present invention is around 1V, and the conductance is comparable to the prior art LDMOS device shown in  FIG. 12B .  FIG. 13C  shows a characteristic curve of the drain current Id versus the drain voltage Vd of the LDMOS device according to the present invention in the ON operation. The breakdown voltage of the LDMOS device according to the present invention in the ON operation is around 75V, as indicated by the dash line shown in the figure. 
         [0045]    According to the characteristic curves shown in  FIGS. 12A-12C  and  13 A- 13 C of the prior art and the present invention respectively, the LDMOS device according to the present invention enhances the breakdown voltage without sacrificing the conductance. 
         [0046]    The present invention has been described in considerable detail with reference to certain preferred embodiments thereof. It should be understood that the description is for illustrative purpose, not for limiting the scope of the present invention. Those skilled in this art can readily conceive variations and modifications within the spirit of the present invention. For example, other process steps or structures which do not affect the primary characteristic of the device, such as a threshold voltage adjustment region, etc., can be added; for another example, the lithography step described in the above can be replaced by electron beam lithography, X-ray lithography, etc.; for another example, the shape of the LDMOS device from top view according to the present invention is not limited to rectangular, it may be circular or serpent. In view of the foregoing, the spirit of the present invention should cover all such and other modifications and variations, which should be interpreted to fall within the scope of the following claims and their equivalents.
The present invention discloses a lateral double diffused metal oxide semiconductor (LDMOS) device and a manufacturing method thereof. The LDMOS device is formed in a first conductive type substrate, and includes a high voltage well, a first field oxide region, at least one second field oxide region, a source, a drain, a body region, and a gate. The second field oxide region is located between the first field oxide region and the drain from top view. The distribution of the concentration of the second conductive type impurities in the high voltage well is related to the location of the second field oxide region.