SEMICONDUCTOR DEVICE

Abstract

An LDMOS transistor includes a P-type body region formed on a main surface of a semiconductor substrate, an N-type source region, an N-type drift region, an N-type drain region, a gate electrode formed via a gate insulating film, a first field plate formed on the drift region via a first insulating film, a plurality of second field plates being in contact with the source region or the gate electrode and formed on the first field plate via a second insulating film, a P-type first buried region, and a P-type second buried region having an impurity concentration lower than an impurity concentration of the first buried region. Distances of the first and second field plates from the drain region in the semiconductor substrate plane direction decrease toward the upper layers, and have a predetermined relationship with the distances between the first and second buried regions and the drain region.

Claims

1. A semiconductor device, comprising: 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, wherein 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.

2. The semiconductor device according to claim 1, wherein an impurity concentration of the drift region is greater than 1e10.sup.16/cm.sup.3, and an impurity concentration of the first buried region is greater than 1e10.sup.16/cm.sup.3, and the impurity concentration of the second buried region is set to a value of ⅓ to ⅔ of the impurity concentration of the first buried region.

3. The semiconductor device according to claim 1, wherein in the plurality of wiring layers constituting the second field plate, the distance between the lowermost wiring layer and the drain region is smaller than the distance between the first buried region and the drain region, and greater than the distance between the second buried region and the drain region.

4. The semiconductor device according to claim 2, wherein in the plurality of wiring layers constituting the second field plate, the distance between the lowermost wiring layer and the drain region is smaller than the distance between the first buried region and the drain region, and greater than the distance between the second buried region and the drain region.

5. The semiconductor device according to claim 3, comprising: a third buried region of the first conductivity type being adjacent to the second buried region and having an impurity concentration smaller than the impurity concentration of the second buried region formed under the drift region and extending in the direction of the drain region, wherein a distance between the third buried region and the drain region is greater than the distance between the uppermost wiring layer and the drain region in the wiring layers constituting the second field plate.

6. The semiconductor device according to claim 5, wherein the impurity concentration of the third buried region is set to a value of ⅓ to ⅔ of the impurity concentration of the second buried region.

7. The semiconductor device according to claim 1, wherein the semiconductor substrate comprises an SOI substrate having a buried insulating layer in a semiconductor layer.

8. The semiconductor device according to claim 2, wherein the semiconductor substrate comprises an SOI substrate having a buried insulating layer in a semiconductor layer.

9. The semiconductor device according to claim 3, wherein the semiconductor substrate comprises an SOI substrate having a buried insulating layer in a semiconductor layer.

10. The semiconductor device according to claim 5, wherein the semiconductor substrate comprises an SOI substrate having a buried insulating layer in a semiconductor layer.

11. The semiconductor device according to claim 6, wherein the semiconductor substrate comprises an SOI substrate having a buried insulating layer in a semiconductor layer.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

[0022] FIG. 1 is a diagram illustrating the cross-sectional structure of the semiconductor device according to the first embodiment of the present invention.

[0023] 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.

[0024] FIG. 3 is a diagram illustrating the drain current characteristics of the conventional semiconductor device.

[0025] 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.

[0026] FIG. 5 is a diagram illustrating the cross-sectional structure of the semiconductor device according to the second embodiment of the present invention.

[0027] FIG. 6 is a diagram illustrating the cross-sectional structure of the semiconductor device according to the third embodiment of the present invention.

[0028] 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.

[0029] FIG. 8 is a diagram illustrating the cross-sectional structure of the conventional semiconductor device.

[0030] FIG. 9 is a diagram illustrating the drain current characteristics of the conventional semiconductor device.

[0031] FIG. 10 is a diagram illustrating the equipotential distribution in the on state of the conventional semiconductor device.

DESCRIPTION OF THE EMBODIMENTS

[0032] 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

[0033] The semiconductor device according to the first embodiment of the present invention will be described with reference to FIG. 1 to FIG. 4.

[0034] 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.

[0035] In the SOI semiconductor substrate, a P-type semiconductor layer having an impurity concentration of 4e16/cm.sup.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.sup.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.

[0036] Further, a drift region 7 composed of an N-type semiconductor layer having an impurity concentration of 5e16/cm.sup.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.

[0037] 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.

[0038] Subsequently, an interlayer insulating film 14 is deposited on the SOI semiconductor substrate, and a portion thereof is perforated so that a contact 15a 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 15b is formed on the drain region 10.

[0039] Then, first wiring layers 16a and 16b composed of a metal layer such as aluminum (Al) are formed on the interlayer insulating film 14 and connected to the contacts 15a and 15b, respectively. The first wiring layer 16a connected to the contact 15a constitutes a source electrode and extends in the direction of the drain region to constitute a second field plate.

[0040] Furthermore, an interlayer insulating film 17 is deposited on the first wiring layers 16a and 16b, and a portion thereof is perforated so that wiring connection holes 18a and 18b composed of a metal layer such as aluminum (Al) are formed on the first wiring layers 16a and 16b, respectively. Then, second wiring layers 19a and 19b composed of a metal layer such as aluminum (Al) are formed on the interlayer insulating film 17 and connected to the wiring connection holes 18a and 18b, respectively. The second wiring layer 19a connected to the wiring connection hole 18a constitutes the source electrode and extends in the direction of the drain region to constitute the second field plate.

[0041] 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 LF1, the distance in the semiconductor substrate plane direction between the second field plate 16a composed of the first wiring layer and the drain region 10 is LF2, and the distance in the semiconductor substrate plane direction between the second field plate 19a composed of the second wiring layer and the drain region 10 is LF3, LF1, LF2, and LF3 have the relationship of Formula (1).

[Formula 1]

LF1>LF2>LF3  (1)

[0042] That is, the distances of the first and second field plates (13, 16a, 19a) from the drain region 10 in the semiconductor substrate plane direction decrease toward the upper layers.

[0043] Further, when the distance in the semiconductor substrate plane direction between the first buried region 4 and the drain region 10 is LB1, and the distance in the semiconductor substrate plane direction between the second buried region 5 and the drain region 10 is LB2, LB1, LB2, LF1, and LF3 have the relationships of Formulas (2) and (3).

[Formula 2]

LF1>LB1  (2)

[Formula 3]

LB2>LF3  (3)

[0044] That is, the distances LB1 and LB2 between the first and second buried regions (4, 5) and the drain region 10 are smaller than the distance LF1 between the first field plate 13 and the drain region 10, and greater than the distance LF3 between the second field plate 19a of the wiring in the uppermost layer and the drain region 10.

[0045] 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).

[0046] 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.sup.3, due to the resurf effect of the first field plate 13, the second field plate (16a, 19a), 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.

[0047] 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>LF2>LB2  (4)

[0048] Although the second field plate 16a 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 16a is electrically connected to the gate electrode and the first field plate 13.

[0049] 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.

[0050] 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

[0051] 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.

[0052] 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 (19a, 19b), wiring connection holes (21a, 21b) perforated in the interlayer insulating film 20 are connected to the second wiring layers (19a, 19b), and third wiring layers (22a, 22b) are formed on the interlayer insulating film 20. The third wiring layer 22a which becomes the source electrode extends in the direction of the drain region 10 to constitute the second field plate.

[0053] Here, as illustrated in FIG. 5, when the distance in the semiconductor substrate plane direction between the second field plate 22a composed of the third wiring and the drain region 10 is LF4, LF1, LF2, LF3, and LF4 have the relationship of Formula (5).

[Formula 5]

LF1>LF2>LF3>LF4  (5)

[0054] That is, the distances of the first field plate 13 and the second field plates (16a, 19a, 22a) from the drain region 10 in the semiconductor substrate plane direction decrease toward the upper layers.

[0055] Further, LF1, LB1, LB2, and LF4 have the relationships of Formulas (2) and (6).

[Formula 2]

LF1>LB1  (2)

[Formula 6]

LB2>LF4  (6)

[0056] That is, the distances LB1 and LB2 between the first and second buried regions (4, 5) and the drain region 10 are smaller than the distance LF1 between the first field plate 13 and the drain 10, and greater than the distance LF4 between the second field plate 22a of the wiring in the uppermost layer and the drain region 10.

[0057] 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

[0058] 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.

[0059] 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.

[0060] 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 LB3, LB1, LB3, LF1, and LF4 have the relationships of Formulas (2) and (7).

[Formula 2]

LF1>LB1  (2)

[Formula 7]

LB3>LF4  (7)

[0061] 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.

[0062] 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>LF2>LB2>LF3>LB3>LF4  (8)

[0063] 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.