# Patent

## Title
Lateral DMOS device with dummy gate

## Abstract

An LDMOS transistor with a dummy gate comprises an extended drift region over a substrate, a drain region in the extended drift region, a channel region in the extended drift region, a source region in the channel region, a first dielectric layer with a first thickness formed over the extended drift region, a second dielectric layer with a second thickness formed over the extended drift region and the channel region, wherein the first thickness is greater than the second thickness, and wherein the first dielectric layer and the second dielectric layer form two steps, a first gate formed over the first dielectric layer and a second gate formed above the second dielectric layer.

## Background
This is a continuation application of U.S. application Ser. No. 13/351,295, entitled “Lateral DMOS Device with Dummy Gate” which was filed on Jan. 17, 2012 and is incorporated herein by reference.


BACKGROUND
The semiconductor industry has experienced rapid growth due to improvements in the integration density of a variety of electronic components (e.g., transistors, diodes, resistors, capacitors, etc.). For the most part, this improvement in integration density has come from shrinking the semiconductor process node (e.g., shrink the process node towards the sub-20 nm node). As semiconductor devices are scaled down, new techniques are needed to maintain the electronic components' performance from one generation to the next. For example, low gate-to-drain capacitance and high breakdown voltage of transistors are desirable for high power applications.
As semiconductor technologies evolve, metal oxide semiconductor field effect transistors (MOSFET) have been widely used in today's integrated circuits. MOSFETs are voltage controlled device. When a control voltage is applied to the gate a MOSFET and the control voltage is greater than the threshold of the MOSFET, a conductive channel is built between the drain and the source of the MOSFET. As a result, a current flows between the drain and the source of the MOSFET. On the other hand, when the control voltage is less than the threshold of the MOSFET, the MOSFET is turned off accordingly.
MOSFETs may include two major categories. One is n-channel MOSFETs; the other is p-channel MOSFETs. According to the structure difference, MOSFETs can be further divided into three sub-categories, planar MOSFETs, lateral double diffused MOS (LDMOS) FETs and vertical double diffused MOSFETs. In comparison with other MOSFETs, the LDMOS is capable of delivering more current per unit area because its asymmetric structure provides a short channel between the drain and the source of the LDMOS.
In order to increase the breakdown voltage of the LDMOS, the gate poly-silicon of the LDMOS may be extended to make an overlap with the drift region of the LDMOS. Such an overlap functions as a field plate to maintain the breakdown voltage of the LDMOS.



BRIEF DESCRIPTION OF THE DRAWINGS
For a more complete understanding of the present disclosure, and the advantages thereof, reference is now made to the following descriptions taken in conjunction with the accompanying drawings, in which:
FIG. 1 illustrates a simplified cross-sectional view of a lateral DMOS transistor with a dummy gate in accordance with an embodiment;
FIG. 2 illustrates a simplified top view of a lateral DMOS transistor with a dummy gate in accordance with an embodiment; and
FIG. 3 illustrates two curves illustrating the performance difference between a traditional LDMOS transistor without a dummy gate and a LDMOS transistor with a dummy gate in accordance with an embodiment.



Corresponding numerals and symbols in the different figures generally refer to corresponding parts unless otherwise indicated. The figures are drawn to clearly illustrate the relevant aspects of the various embodiments and are not necessarily drawn to scale.
DETAILED DESCRIPTION OF ILLUSTRATIVE EMBODIMENTS
The making and using of the present embodiments are discussed in detail below. It should be appreciated, however, that the present disclosure provides many applicable inventive concepts that can be embodied in a wide variety of specific contexts. The specific embodiments discussed are merely illustrative of specific ways to make and use the embodiments of the disclosure, and do not limit the scope of the disclosure.
The present disclosure will be described with respect to embodiments in a specific context, a lateral double diffused metal oxide semiconductor (LDMOS) device with a dummy gate. The embodiments of the disclosure may also be applied, however, to a variety of metal oxide semiconductor field effect transistors (MOSFETs).
FIG. 1 illustrates a simplified cross-sectional view of a lateral DMOS transistor with a dummy gate in accordance with an embodiment. The lateral DMOS transistor 100 includes a substrate 102, preferably P-type, an N-type region 104 formed over the substrate 102, a P-type region 106 and a first N+ region 116 formed over the N region 104. A P+ region 112 and a second N+ region 114 are formed in the P-type region 106. The lateral DMOS transistor 100 further comprises a gate dielectric layer 122 with a first thickness and a dielectric insulating layer 124 with a second thickness. An active gate 134 and a dummy gate 136 are formed on the gate dielectric layer 122 and the dielectric insulating layer 124 respectively.
The substrate 102 may be formed of silicon, silicon germanium, silicon carbide or the like. The N-type region 104 is formed over the substrate 102. In accordance with an embodiment, the N-type region 104 may be an epitaxial layer. The N-type region 104 may be doped with an n-type dopant such as phosphorous to a doping density of about 1015/cm3 to 1018/cm3. It should be noted that other n-type dopants such as arsenic, nitrogen, antimony, combination thereof, or the like, could alternatively be used. It should further be noted that the N-type region 104 of the LDMOS 100 may be alternatively referred to as an extended drift region.
The P-type region 106 is formed by implanting p-type doping materials such as boron, gallium, aluminum, indium, combinations thereof, or the like. In accordance with an embodiment, a p-type material such as boron may be implanted to a doping density of about 1015/cm3 to 1018/cm3. Alternatively, the P-type region 106 can be formed by a diffusion process. The P-type region 106 of the LDMOS 100 may be alternatively referred to as a channel region.
A first N+ region 116 is formed in the N-type region 104. In accordance with an embodiment, the first N+ region 116 functions as a drain of the LDMOS 100. The drain region may be formed by implanting an n-type dopant such as phosphorous at a concentration of between about 1019/cm3 and about 1020/cm3. Furthermore, a drain contact 138 is formed over the first N+ region 116.
A second N+ region 114 is formed in the P-type region 106. In accordance with an embodiment, the second N+ region 114 may be a source of the LDMOS 100. The source region may be formed by implanting an n-type dopant such as phosphorous at a concentration of between about 1019/cm3 and about 1020/cm3. As shown in FIG. 1, the source region is formed adjacent to the active gate 134 on the opposite side from the drain (the first N+ region 116).
A P+ region 112 is formed adjacent to the second N+ region 114 in the P-type region 106. The P+ region may be formed by implanting a p-type dopant such as boron at a concentration of between about 1019/cm3 and about 1020/cm3. The P+ region 112 may contact the p-type body. In order to eliminate the body effect, the P+ region 112 may be coupled to the source 114 directly through the source contact 132.
A gate dielectric layer 122 is formed over the N-type region 104. The gate dielectric layer 122 may be formed of silicon oxide, silicon oxynitride, hafnium oxide, zirconium oxide or the like. As shown in FIG. 1, the gate dielectric layer 122 is partially on top of the P-type region 106. In accordance with an embodiment, the gate dielectric layer 122 is of a thickness of between about 100 Å and about 200 Å. An active gate 134 is formed on the gate dielectric layer 122. The active gate 134 may be formed of polysilicon, polysilicon germanium, nickel silicide or other metal, metal alloy materials.
The dielectric insulating layer 124 is on top of the N-type region 104. More particularly, the dielectric insulating layer 124 is located between the active gate 134 and the drain 116. In accordance with an embodiment, the dielectric insulating layer 124 is of a thickness of between about 500 Å and about 2000 Å. A dummy gate 136 is formed on top of the dielectric insulating layer 124. It should be noted that the dummy gate 136 may be formed in the same process step as the active gate 134. More particularly, the dummy gate 136 and the active gate 134 may be formed as a large single gate first. In order to reduce the gate to drain charge, a middle portion of such a large single gate may be removed by using suitable etching techniques such as dry etching. As a result, the remaining portions of the large single gate become the active gate 134 and the dummy gate 136 respectively.
The dummy gate 136 functions as a field plate, which helps to maintain the breakdown voltage of the LDMOS 100. In addition, by separating the dummy gate 136 from the active gate 134, the overlap between the gate region and drain region is reduced accordingly. Such a small overlap between gate region and drain region helps to reduce the gate-to-drain charge of the LDMOS 100. Furthermore, the dummy gate 136 may be biased by a voltage source. For example, when the LDMOS 100 is a 20V transistor, the bias voltage coupled to the dummy gate 136 may be up to 20V. Such a bias voltage helps to reduce the on resistance of the LDMOS 100.
The first isolation region 142 and the second isolation region 144 are used to isolate active regions so as to prevent leakage current from flowing between adjacent active regions. The isolation region (e.g., 142) can be formed by various ways (e.g., thermally grown, deposited) and materials (e.g., silicon oxide, silicon nitride). In this embodiment, the first isolation region 142 and the second isolation region 144 may be fabricated by a shallow trench isolation (STI) technique.
FIG. 2 illustrates a simplified top view of a lateral DMOS transistor with a dummy gate in accordance with an embodiment. In FIG. 2, the N-type region 104, the first N+ region 116, the second N+ region 114, the P+ region 112, the active gate 134 and the dummy gate 136 are shown. In addition, dimensions of the LDMOS transistor 100 are shown. In particular, the distance between the active gate 134 and the dummy gate 136 is illustrated. In accordance with an embodiment, the distance D1 is a range from 0.1 um to 1 um.
One skilled in the art will recognize that FIG. 1 illustrates an ideal profile as doped. The distance D1 may vary after subsequent diffusion processes. The distance D1 shown in FIG. 2 is used to illustrate the inventive aspects of the various embodiments. The present invention is not limited to any particular distance between the active gate 134 and the dummy gate 136.
FIG. 3 illustrates two curves illustrating the performance difference between a traditional LDMOS transistor without a dummy gate and LDMOS transistor with a dummy gate. The horizontal axis of FIG. 3 represents the breakdown voltage across an LDMOS transistor. The vertical axis of FIG. 3 represents a performance index of the LDMOS transistor. More particularly, the performance index is the produce of On-resistance (RON) and gate-to-drain charge (QGD). In accordance with an embodiment, the conduction losses of an LDMOS transistor may be proportional to the on resistance of the LDMOS transistor. On the other hand, the switching losses of the LDMOS transistor may be proportional to the gate-to-drain charge of the LDMOS transistor. In order to achieve an efficient LDMOS transistor, a low RON×QGD is desirable for reducing conduction losses as well as switching losses.
A curve 302 and a curve 304 illustrate the RON×QGD difference between a traditional LDMOS transistor without a dummy gate and LDMOS transistor with a dummy gate. As shown in FIG. 3, at the same breakdown voltage level, an LDMOS transistor with a dummy gate is of a low RON×QGD. For example, when the dummy gate LDMOS transistor has a 31V breakdown voltage, the corresponding RON×QGD is about 10 mOhm-nC. By contrast, with the same breakdown voltage level, the LDMOS transistor without a dummy gate has a higher RON×QGD. As illustrated by curve 302, the RON×QGD of the traditional LDMOS transistor is about 20 mOhm-nC.
FIG. 3 shows an LDMOS transistor having a dummy gate may achieve a higher breakdown voltage without sacrificing its RON×QGD. As such, an advantageous feature of this embodiment is that a dummy gate helps to maintain the breakdown voltage of an LDMOS transistor. In addition, the dummy gate can reduce the gate charge of the LDMOS transistor so that the switching losses of the LDMOS transistor may be reduced.
Although embodiments of the present disclosure and its advantages have been described in detail, it should be understood that various changes, substitutions and alterations can be made herein without departing from the spirit and scope of the disclosure as defined by the appended claims.
Moreover, the scope of the present application is not intended to be limited to the particular embodiments of the process, machine, manufacture, composition of matter, means, methods and steps described in the specification. As one of ordinary skill in the art will readily appreciate from the present disclosure, processes, machines, manufacture, compositions of matter, means, methods, or steps, presently existing or later to be developed, that perform substantially the same function or achieve substantially the same result as the corresponding embodiments described herein may be utilized according to the present disclosure. Accordingly, the appended claims are intended to include within their scope such processes, machines, manufacture, compositions of matter, means, methods, or steps.

## Claims

1. A semiconductor device comprising:
an extended drift region over a substrate;
a drain region in the extended drift region;
a channel region in the extended drift region;
a source region in the channel region;
a first dielectric layer with a first thickness formed over the extended drift region;
a second dielectric layer with a second thickness formed over the extended drift region and the channel region, wherein the first thickness is greater than the second thickness, and wherein the first dielectric layer and the second dielectric layer form two steps, and wherein a first step is over a top surface of the extended drift region and at least a portion of a second step is below the top surface of the extended drift region;
a first gate formed over the first dielectric layer; and
a second gate formed above the second dielectric layer, wherein the first step is between the first gate and the second gate and the first step is free of a gate material thereon.

2. The semiconductor device of claim 1, wherein:
the second dielectric layer extends from an edge of the source region to an edge of the first dielectric layer; and
the first dielectric layer extends from an edge of the second dielectric layer to an edge of the drain region.

3. The semiconductor device of claim 1, wherein:
at least a portion of the first dielectric layer is below the top surface of the extended drift region.

4. The semiconductor device of claim 1, wherein:
the channel region is p-type;
the drain region is n-type; and
the source region is n-type.

5. The semiconductor device of claim 4, further comprising:
a p-type region between and in contact with an isolation region and the source region.

6. The semiconductor device of claim 5, wherein:
the p-type region and the source region share a same contact.

7. The semiconductor device of claim 1, wherein:
the first gate is a dummy gate; and
the second gate is an active gate.

8. The semiconductor device of claim 7, wherein:
the first gate is configured to be biased by a voltage source.

9. The semiconductor device of claim 8, wherein:
a bias voltage supplied by the voltage source is substantially equal to a rated voltage of a transistor formed by the drain region, the source region, the first gate and the second gate.

10. A device comprising:
a first drain/source region and a second drain/source region over a substrate;
a first gate over a first dielectric layer; and
a second gate over a second dielectric layer, wherein:
a thickness of the second dielectric layer is greater than a thickness of the first dielectric layer;
the first dielectric layer and the second dielectric layer form a step;
the first dielectric layer extends from an edge of the first drain/source region to the step;
the second dielectric layer extends from the step to an edge of the second drain/source region; and
the step is free of a gate material thereon.

11. The device of claim 10, wherein:
the first drain/source region, the second drain/source region, the first gate and the second gate form a lateral double diffused metal oxide semiconductor (LDMOS) device, and wherein:
the first drain/source region is a source;
the second drain/source region is a drain;
the first gate is an active gate; and
the second gate is a dummy gate.

12. The device of claim 10, further comprising:
an epitaxial layer over the substrate; and
a channel region in the epitaxial layer, wherein:
the first drain/source region is in the channel region; and
the second drain/source region is in the epitaxial layer.

13. The device of claim 10, further comprising:
a first contact formed over the first drain/source region; and
a second contact formed over the second drain/source region.

14. The device of claim 13, further comprising:
a body contact region between and in contact with an isolation region and the first drain/source region, wherein the body contact region is coupled to the first drain/source region through the first contact.

15. The device of claim 10, wherein:
an edge of the first dielectric layer is vertically aligned with an edge of the first drain/source region; and
an edge of the second dielectric layer is vertically aligned with an edge of the second drain/source region.

16. A transistor comprising:
an extended drift region over a substrate;
a drain region in the extended drift region;
a channel region in the extended drift region;
a source region in the channel region;
a first dielectric layer with a first thickness formed over the extended drift region;
a second dielectric layer with a second thickness formed over the extended drift region and the channel region, wherein:
the first thickness is greater than the second thickness;
at least one portion of the first dielectric layer is below a top surface of the extended drift region; and
a boundary between the first dielectric layer and the second dielectric layer is free of a gate material thereon;

a first gate on the first dielectric layer; and
a second gate on the second dielectric layer.

17. The transistor of claim 16, wherein:
a width of the second dielectric layer is greater than a width of the second gate; and
the second gate occupies a middle portion of the second dielectric layer.

18. The transistor of claim 16, wherein:
a bottom surface of the first dielectric layer is lower than the top surface of the extended drift region with reference to a top surface of the substrate.

19. The transistor of claim 18, further comprising:
a body contact region between and in contact with an isolation region and the source region.

20. The transistor of claim 16, wherein:
the first gate is configured to be biased by a bias voltage.