Patent Publication Number: US-2023155025-A1

Title: Semiconductor device

Description:
CROSS-REFERENCE TO RELATED APPLICATION 
     This application claims the priority benefits of Japanese application no. 2021-187723, filed on Nov. 18, 2021. The entirety of the above-mentioned patent application is hereby incorporated by reference herein and made a part of this specification. 
     BACKGROUND 
     Technical Field 
     The present invention relates to the structure of a semiconductor device, and more particularly relates to a technique effectively applied to a high breakdown voltage LDMOS transistor which requires a high breakdown voltage of 100V or higher. 
     Description of Related Art 
     There is an LDMOS (Lateral Double-diffused MOS) transistor with a breakdown voltage of about 30V or higher, which is formed on a semiconductor substrate, as the transistor used in drive circuits which use inductors, such as automotive solenoids and fan motors, and capacitive elements, such as piezoelectric elements, as the loads. 
     In order to realize low on-resistance while ensuring a high breakdown voltage in this LDMOS transistor, a transistor structure is known in which an impurity layer (buried layer) having an opposite conductivity type to the drift region is provided under the drift region while the impurity concentration of the drift region through which a current flows is increased (Patent Literature 1, Japanese Patent Laid-Open No. 2020-98883). 
     In the conventional N-type LDMOS transistor  400  illustrated in  FIG.  8   , a P-type buried layer  4  extends from a P-type body region to a position under an N-type drift region  7 , and a first field plate of a gate electrode  13  is provided on an insulating layer  8  above the drift region  7  and second field plates  16   a  and  19   a  composed of a plurality of wiring layers are provided to be formed on interlayer insulating films  14  and  17  above the first field plate. Then, the distance LB from the drain  10  to the P-type buried layer  4  is designed to be smaller than the distance LF 1  from the drain to the first field plate and greater than the distance LF 3  from the drain to the upper wiring layer  19   a  which constitutes the second field plate. 
     With such a configuration, even when the impurity concentration of the drift region  7  is set to a relatively high concentration of 1e16/cm 3  or more, the electric potential can be made uniform without causing concentration of the electric field in the drift region  7  in the off state of the transistor. As a result, it is possible to obtain transistor characteristics which achieve both a high breakdown voltage and low on-resistance. 
     However, in the N-type LDMOS transistor described in the above-mentioned Patent Literature 1, although a relatively high breakdown voltage can be obtained in the off state, there is a characteristic that the current increases as the source-drain voltage (Vds) increases without saturating in the saturation region of the drain current when the transistor is in the on state. 
       FIG.  9    illustrates the Vds dependence of the drain current (Ids) in the on/off state of the N-type LDMOS transistor illustrated in  FIG.  8   . In the on state in which a voltage is applied to the gate, the drain current is divided into a linear region (Region  1 ) where Ids changes linearly with respect to Vds, a saturation region (Region  2 ) where the change of Ids is small with respect to Vds, and an avalanche region (Region  3 ) where Ids changes greatly with respect to Vds, and in the saturation region (Region  2 ), there is a region (Region  2   b ) where Vds gradually increases as Vds increases. This region arises from a relatively low Vds with respect to the voltage of the off breakdown voltage (BVoff). When a transistor with such characteristics is used in a current mirror circuit, the mirror ratio changes depending on the amount of current. 
     In addition, in this transistor, as illustrated in  FIG.  9   , the Vds, which is the avalanche region (Region  3 ) where Ids changes greatly with respect to Vds, is also relatively small with respect to BVoff. 
     Therefore, for example, in a drive circuit which uses an inductor as the load, when a high Vds is applied during a transition period from the off state to the on state and the transistor enters the avalanche region (Region  3 ), there is a problem that a large drain current flows and destroys the element. 
     Furthermore, in the drive circuit using this transistor, when the operating condition is to transition through the saturation region of Region  2   b  during the switching operation, reliability issues arise as the characteristics of the transistor change over time. 
     SUMMARY 
     The present invention provides a high-performance high breakdown voltage LDMOS field effect transistor capable of achieving both a high breakdown voltage and low on-resistance in high breakdown voltage LDMOS field effect transistor. 
     Specifically, the present invention provides a transistor having the current characteristics that the source-drain voltage dependence of the drain current is small in the saturation region of the drain current of the high breakdown voltage LDMOS transistor having characteristics of off breakdown voltage and low on-resistance. 
     Further, the present invention provides a transistor having the characteristics that the source-drain voltage at which the drain current starts to increase rapidly is large in the avalanche region of the drain current of the high breakdown voltage LDMOS transistor having characteristics of off breakdown voltage and low on-resistance. 
     Further, the present invention provides a transistor having high reliability characteristics that the change over time of the characteristics such as on-resistance is small. 
     An embodiment of the present invention includes a body region of a first conductivity type formed on a main surface of a semiconductor substrate; a source region of a second conductivity type formed on a surface of the body region; a drift region of the second conductivity type formed to be in contact with the body region; a drain region of the second conductivity type formed on the drift region; a gate electrode formed on the body region between the source region and the drift region and the drift region on the side of the source region via a gate insulating film; a first field plate extending from the gate electrode in a direction of the drain region and formed on the drift region via a first insulating film; a second field plate composed of a plurality of wiring layers, and being in contact with the source region or the gate electrode and formed on the first field plate via a second insulating film; a first buried region of the first conductivity type being in contact with the body region and formed under the drift region; and a second buried region of the first conductivity type being adjacent to the first buried region and having an impurity concentration smaller than an impurity concentration of the first buried region formed under the drift region and extending in the direction of the drain region. In the plurality of wiring layers constituting the second field plate, a distance between an upper wiring layer and the drain region is shorter than a distance between a lower wiring layer and the drain region, and a distance between a lowermost wiring layer and the drain region is shorter than a distance between the first field plate and the drain region, a distance between an uppermost wiring layer and the drain region is shorter than a distance between the second buried region and the drain region, and the distance between the first field plate and the drain region is longer than a distance between the first buried region and the drain region. 
     According to the present invention, it is possible to realize a high-performance high breakdown voltage LDMOS field effect transistor capable of achieving both a high breakdown voltage and low on-resistance in high breakdown voltage LDMOS field effect transistor. 
     According to the present invention, it is possible to realize a transistor having the current characteristics that the source-drain voltage dependence of the drain current is small in the saturation region of the drain current of the LDMOS transistor having characteristics of off breakdown voltage and low on-resistance. 
     Further, according to the present invention, it is possible to realize a transistor having the characteristics of a large source-drain voltage of the avalanche region (Region  3 ) at which the drain current of the LDMOS transistor having characteristics of off breakdown voltage and low on-resistance starts to increase rapidly. 
     Further, according to the present invention, it is possible to realize a transistor having high reliability characteristics that the change over time of the transistor characteristics such as on-resistance is small. 
     Problems, configurations, and effects other than those described above will be clarified by the following description of the embodiments. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG.  1    is a diagram illustrating the cross-sectional structure of the semiconductor device according to the first embodiment of the present invention. 
         FIG.  2    is a diagram illustrating the impurity concentration distribution in the direction parallel to the substrate main plane along the broken line A-A′ of the semiconductor device of  FIG.  1   . 
         FIG.  3    is a diagram illustrating the drain current characteristics of the conventional semiconductor device. 
         FIG.  4    is a diagram illustrating the equipotential distribution in the on state of the semiconductor device according to the first embodiment of the present invention. 
         FIG.  5    is a diagram illustrating the cross-sectional structure of the semiconductor device according to the second embodiment of the present invention. 
         FIG.  6    is a diagram illustrating the cross-sectional structure of the semiconductor device according to the third embodiment of the present invention. 
         FIG.  7    is a diagram illustrating the impurity concentration distribution in the direction parallel to the substrate main plane along the broken line B-B′ of the semiconductor device of  FIG.  6   . 
         FIG.  8    is a diagram illustrating the cross-sectional structure of the conventional semiconductor device. 
         FIG.  9    is a diagram illustrating the drain current characteristics of the conventional semiconductor device. 
         FIG.  10    is a diagram illustrating the equipotential distribution in the on state of the conventional semiconductor device. 
     
    
    
     DESCRIPTION OF THE EMBODIMENTS 
     Embodiments of the present invention will be described below with reference to the drawings. In addition, in each drawing, the same configurations are denoted by the same reference numerals, and detailed descriptions of the repeated parts will be omitted. 
     Embodiment 1 
     The semiconductor device according to the first embodiment of the present invention will be described with reference to  FIG.  1    to  FIG.  4   . 
     As illustrated in  FIG.  1   , the semiconductor device of this embodiment is an N-type LDMOS transistor  100 , and is formed on an SOI semiconductor substrate in which an insulating layer  2  is formed on a P-type semiconductor substrate  1  and a P-type semiconductor layer  3  is formed on the insulating layer  2 . 
     In the SOI semiconductor substrate, a P-type semiconductor layer having an impurity concentration of 4e16/cm 3 , for example, which becomes a first buried region  4 , is formed to be connected to a P body region  6  of a P-type semiconductor layer formed on the SOI semiconductor substrate, and a P-type semiconductor layer, which becomes a second buried region  5 , is adjacent to the first buried region  4  in a direction parallel to the substrate main plane.  FIG.  2    illustrates the impurity concentration distribution in the direction parallel to the substrate main plane along the broken line A-A′ in  FIG.  1   . The impurity concentration of the second buried region  5  is, for example, 2e16/cm 3  and is set to be in the range of ⅓ to ⅔ with respect to the impurity concentration of the first buried region  4 . 
     Further, a drift region  7  composed of an N-type semiconductor layer having an impurity concentration of 5e16/cm 3 , for example, is formed adjacent to the P body region  6  and above the first buried region  4  and the second buried region  5 , and a source region  9  of an N-type semiconductor layer and a P body connection region  11  are formed on a surface of the P body region  6 . Besides, an N-type drain region  10  is formed on the N-type drift region  7 . Then, a gate oxide film  12  composed of an insulating layer is formed on the SOI semiconductor substrate to be adjacent to the source region  9 , and a gate electrode  13  composed of N-type polysilicon is formed on a portion of the P body region  6  and a portion of the drift region  7  on the side of the source region  9  via the gate oxide film  12 . 
     Further, an STI (Shallow Trench Isolation) composed of an insulating layer  8  is formed on the drift region  7 , and the gate electrode  13  extends over a portion of the STI and constitutes a first field plate  13 . 
     Subsequently, an interlayer insulating film  14  is deposited on the SOI semiconductor substrate, and a portion thereof is perforated so that a contact  15   a  composed of a metal layer such as aluminum (Al) is formed on the source region  9  and the P body connection region  11 , and a contact  15   b  is formed on the drain region  10 . 
     Then, first wiring layers  16   a  and  16   b  composed of a metal layer such as aluminum (Al) are formed on the interlayer insulating film  14  and connected to the contacts  15   a  and  15   b , respectively. The first wiring layer  16   a  connected to the contact  15   a  constitutes a source electrode and extends in the direction of the drain region to constitute a second field plate. 
     Furthermore, an interlayer insulating film  17  is deposited on the first wiring layers  16   a  and  16   b , and a portion thereof is perforated so that wiring connection holes  18   a  and  18   b  composed of a metal layer such as aluminum (Al) are formed on the first wiring layers  16   a  and  16   b , respectively. Then, second wiring layers  19   a  and  19   b  composed of a metal layer such as aluminum (Al) are formed on the interlayer insulating film  17  and connected to the wiring connection holes  18   a  and  18   b , respectively. The second wiring layer  19   a  connected to the wiring connection hole  18   a  constitutes the source electrode and extends in the direction of the drain region to constitute the second field plate. 
     Here, as illustrated in  FIG.  1   , when the distance in the semiconductor substrate plane direction between the first field plate  13  and the drain region  10  is LF 1 , the distance in the semiconductor substrate plane direction between the second field plate  16   a  composed of the first wiring layer and the drain region  10  is LF 2 , and the distance in the semiconductor substrate plane direction between the second field plate  19   a  composed of the second wiring layer and the drain region  10  is LF 3 , LF 1 , LF 2 , and LF 3  have the relationship of Formula (1). 
       [Formula 1] 
       LF1&gt;LF2&gt;LF3  (1)
 
     That is, the distances of the first and second field plates ( 13 ,  16   a ,  19   a ) from the drain region  10  in the semiconductor substrate plane direction decrease toward the upper layers. 
     Further, when the distance in the semiconductor substrate plane direction between the first buried region  4  and the drain region  10  is LB 1 , and the distance in the semiconductor substrate plane direction between the second buried region  5  and the drain region  10  is LB 2 , LB 1 , LB 2 , LF 1 , and LF 3  have the relationships of Formulas (2) and (3). 
       [Formula 2] 
       LF1&gt;LB1  (2)
 
       [Formula 3] 
       LB2&gt;LF3  (3)
 
     That is, the distances LB 1  and LB 2  between the first and second buried regions ( 4 ,  5 ) and the drain region  10  are smaller than the distance LF 1  between the first field plate  13  and the drain region  10 , and greater than the distance LF 3  between the second field plate  19   a  of the wiring in the uppermost layer and the drain region  10 . 
     With such a configuration, the saturation current characteristics when the transistor  100  is in the on state are as illustrated in  FIG.  3   , and the Vds dependence of the drain current (Ids) in the saturation region (Region  2 ) can be reduced. In addition, the Vds voltage, which is the avalanche region (Region  3 ) where Ids greatly increases with Vds, can be set to a value close to the off breakdown voltage (BVoff). 
     Next, the reason why such characteristics are obtained will be described.  FIG.  4    illustrates the electric potential distribution when 5 V is applied between the gate and the source of the transistor  100  and a relatively high voltage (Vds) of 300 V is applied between the source and the drain so that the drain current is in the saturation region (Region  2 ). Although the impurity concentration of the drift region  7  is relatively high at 5e16/cm 3 , due to the resurf effect of the first field plate  13 , the second field plate ( 16   a ,  19   a ), and the first and second buried layers ( 4 ,  5 ), the electric potential of the drift region  7  is distributed without the electric field being locally concentrated. Further, in the transistor  400  having the conventional structure of  FIG.  8   , as illustrated in  FIG.  10   , the electric potential of the buried layer  4  is not uniform, and the electric field concentrates in the direction close to the drain region  10 , whereas the electric potential is uniformly distributed in the first buried layer  4  and the second buried layer  5 , and electric field concentration is suppressed. Therefore, as a result of suppressing the avalanche current due to the impact ionization phenomenon, the Vds dependence of the drain current (Ids) in the saturation region (Region  2 ) can be reduced, and the Vds at which the avalanche region (Region  3 ) starts can be increased. 
     In order to make the electric potential in the drift region  7  more uniform and obtain a higher breakdown voltage, it is more desirable to have the relationship given by Formula (4), but the present invention is not limited thereto. 
       [Formula 4] 
       LB1&gt;LF2&gt;LB2  (4)
 
     Although the second field plate  16   a  is electrically connected to the N-type source region  9  (body region  6 ) in this embodiment, a similar effect can be obtained when the second field plate  16   a  is electrically connected to the gate electrode and the first field plate  13 . 
     In addition, although this embodiment illustrates an example of an N-type MOS transistor, a similar effect can be obtained with a P-type MOS transistor. 
     Furthermore, even if a PN junction structure is provided in the N-type drain region  10  of the N-type MOS transistor to form an IGBT structure, by suppressing the electric field concentration in the drift region, it is possible to increase the breakdown voltage while downsizing the element. In this case, in the structure illustrated in  FIG.  1   , the N-type source region  9  becomes the “emitter region” and the N-type drain region  10  becomes the “collector region.” 
     Embodiment 2 
     The semiconductor device according to the second embodiment of the present invention will be described with reference to  FIG.  5   , mainly focusing on the difference from the first embodiment. 
     The difference from the first embodiment is that the N-type LDMOS transistor  200  has an interlayer insulating film  20  deposited on the second wiring layers ( 19   a ,  19   b ), wiring connection holes ( 21   a ,  21   b ) perforated in the interlayer insulating film  20  are connected to the second wiring layers ( 19   a ,  19   b ), and third wiring layers ( 22   a ,  22   b ) are formed on the interlayer insulating film  20 . The third wiring layer  22   a  which becomes the source electrode extends in the direction of the drain region  10  to constitute the second field plate. 
     Here, as illustrated in  FIG.  5   , when the distance in the semiconductor substrate plane direction between the second field plate  22   a  composed of the third wiring and the drain region  10  is LF 4 , LF 1 , LF 2 , LF 3 , and LF 4  have the relationship of Formula (5). 
       [Formula 5] 
       LF1&gt;LF2&gt;LF3&gt;LF4  (5)
 
     That is, the distances of the first field plate  13  and the second field plates ( 16   a ,  19   a ,  22   a ) from the drain region  10  in the semiconductor substrate plane direction decrease toward the upper layers. 
     Further, LF 1 , LB 1 , LB 2 , and LF 4  have the relationships of Formulas (2) and (6). 
       [Formula 2] 
       LF1&gt;LB1  (2)
 
       [Formula 6] 
       LB2&gt;LF4  (6)
 
     That is, the distances LB 1  and LB 2  between the first and second buried regions ( 4 ,  5 ) and the drain region  10  are smaller than the distance LF 1  between the first field plate  13  and the drain  10 , and greater than the distance LF 4  between the second field plate  22   a  of the wiring in the uppermost layer and the drain region  10 . 
     With such a configuration, the electric potential distribution in the drift region  7  can be made more uniform than in the transistor  100  of the first embodiment, and in the transistor with a higher breakdown voltage, as illustrated in  FIG.  3   , it is possible to obtain saturation current characteristics that the Vds dependence of the drain current (Ids) is small. 
     Embodiment 3 
     The semiconductor device according to the third embodiment of the present invention will be described with reference to  FIG.  6   , mainly focusing on the difference from the second embodiment. 
     The difference from the second embodiment is that, in the N-type LDMOS transistor  300 , a third buried region  23  composed of a P-type semiconductor layer is provided in the semiconductor substrate to be adjacent to the second buried region  5  and under the drift region  7 .  FIG.  7    illustrates the impurity concentration distribution in the direction parallel to the substrate main plane along the broken line B-B′ in  FIG.  6   , but the impurity concentration of the third buried region  23  is, for example, 1e16/cm 3, which is about ⅓ to ⅔ with respect to the impurity concentration of the second buried region  5 . 
     Further, as illustrated in  FIG.  6   , when the distance in the semiconductor substrate plane direction between the third buried region  23  and the drain region  10  is LB 3 , LB 1 , LB 3 , LF 1 , and LF 4  have the relationships of Formulas (2) and (7). 
       [Formula 2] 
       LF1&gt;LB1  (2)
 
       [Formula 7] 
       LB3&gt;LF4  (7)
 
     With such a configuration, the electric potential distribution in the buried regions ( 4 ,  5 ,  23 ) can be made more uniform than in the transistor  200  of the second embodiment, and in the transistor with a high breakdown voltage, it is possible to obtain saturation current characteristics that the Vds dependence of the drain current (Ids) is smaller. 
     In order to make the electric potential in the drift region  7  more uniform and obtain a higher breakdown voltage, it is desirable to have the relationship given by Formula (8), but the present invention is not limited thereto. 
       [Formula 8] 
       LB1&gt;LF2&gt;LB2&gt;LF3&gt;LB3&gt;LF4  (8)
 
     In addition, the present invention is not limited to the above-described embodiments, and includes various modifications. For example, the above-described embodiments have been described in detail in order to explain the present invention to be easily understandable, and are not necessarily limited to those having all the described configurations. In addition, it is possible to replace part of the configuration of one embodiment with the configuration of another embodiment, and it is also possible to add the configuration of another embodiment to the configuration of one embodiment. Moreover, it is possible to add, delete or replace part of the configuration of each embodiment with another configuration.