# Patent

## Title
High voltage lateral DMOS transistor with optimized source-side blocking capability

## Abstract

An integrated circuit and method having an extended drain MOS transistor with a buried drift region, a drain diffused link, a channel diffused link, and an isolation link which electrically isolated the source, where the isolation diffused link is formed by implanting through segmented areas to dilute the doping to less than two-thirds the doping in the drain diffused link.

## Background
CROSS-REFERENCE TO RELATED APPLICATIONS
The following co-pending patent application is related and hereby incorporated by reference: U.S. patent application No. 14/567,398 (Texas Instruments docket number TI-67676) filed concurrently with this application.


FIELD OF THE INVENTION
This invention relates to the field of integrated circuits. More particularly, this invention relates to MOS transistors in integrated circuits.
BACKGROUND OF THE INVENTION
An integrated circuit may contain a planar extended drain metal oxide semiconductor (MOS) transistor with a buried drift region, for example to provide an operating voltage above the dielectric strength of the gate dielectric layer in the MOS transistor. It may be desirable to form a low resistance drain portion connection between the buried drift region and the drain contact, a lightly doped channel portion link between the buried drift region and the channel of the MOS transistor, and a lightly doped isolation link between the buried drift region and the top surface of the substrate of the integrated circuit which electrically isolates the source and body of the MOS transistor from the substrate. It may further be desirable to minimize the number of photolithographic and ion implant operations in the fabrication sequence of forming the integrated circuit.
SUMMARY OF THE INVENTION
The following presents a simplified summary in order to provide a basic understanding of one or more aspects of the invention. This summary is not an extensive overview of the invention, and is neither intended to identify key or critical elements of the invention, nor to delineate the scope thereof. Rather, the primary purpose of the summary is to present some concepts of the invention in a simplified form as a prelude to a more detailed description that is presented later.
An integrated circuit may include a planar extended drain MOS transistor with a buried drift region between the drain contact and the channel of the MOS transistor. A drain portion link between the buried drift region and the drain contact is formed concurrently with a channel portion link between the buried drift region and the channel and an isolation link between the buried drift layer and a top surface of a substrate of the integrated circuit. The isolation link electrically isolates a source diffused region and a body region of the extended drain MOS transistor from the substrate. The drain portion link, the channel portion link and the isolation link are formed by ion implanting dopants followed by an anneal operation which diffuses the implanted dopants to make an electrical connection with the buried drift region. The average doping density in the isolation link is less than two-thirds the average doping density in the drain portion link. The isolation link is formed by segmenting the ion implanted area so that the diffused dopant distributions of implanted segments in the isolation link are more diluted than in the drain portion link after the anneal operation.



DESCRIPTION OF THE VIEWS OF THE DRAWING
FIG. 1A through FIG. 1D are perspective views of an integrated circuit formed according to an example, depicted in successive stages of fabrication.
FIG. 2A and FIG. 2B are top views of an integrated circuit containing a planar extended drain MOS transistor formed according to the first example, depicted in successive stages of fabrication.
FIG. 3A and FIG. 3B are top views of an integrated circuit containing a planar extended drain MOS transistor formed according to the second example, depicted in successive stages of fabrication.
FIG. 4 is a top view of an integrated circuit containing a planar extended drain MOS transistor formed according to an example, depicted after formation of a link ion implant mask.
FIG. 5 is a top view of an integrated circuit containing a planar extended drain MOS transistor formed according to an example, depicted after formation of a link ion implant mask.



DETAILED DESCRIPTION OF EXAMPLE EMBODIMENTS
The following co-pending patent application is related and hereby incorporated by reference: U.S. patent application Ser. No. 14/567,398 filed concurrently with this application.
The present invention is described with reference to the attached figures, wherein like reference numerals are used throughout the figures to designate similar or equivalent elements. The figures are not drawn to scale and they are provided merely to illustrate the invention. Several aspects of the invention are described below with reference to example applications for illustration. It should be understood that numerous specific details, relationships, and methods are set forth to provide an understanding of the invention. One skilled in the relevant art, however, will readily recognize that the invention can be practiced without one or more of the specific details or with other methods. In other instances, well-known structures or operations are not shown in detail to avoid obscuring the invention. The present invention is not limited by the illustrated ordering of acts or events, as some acts may occur in different orders and/or concurrently with other acts or events. Furthermore, not all illustrated acts or events are required to implement a methodology in accordance with the present invention.
An integrated circuit may include a planar extended drain MOS transistor with a buried drift region in a substrate of the integrated circuit between a drain contact and a channel of the MOS transistor. A drain portion link between the buried drift region and the drain contact is formed concurrently with a channel portion link between the buried drift region and the channel, and an isolation link between the buried drift region and a top surface of the substrate, by ion implanting and annealing, which diffuses implanted dopants in each link to make electrical connections with the buried drift region. The isolation link is configured to electrically isolate a source diffused region and body region of the MOS transistor from the substrate. Ion implant areas in the isolation link are segmented so that dopants are diluted laterally during the anneal process, to reduce an average doping density compared to a non-segmented implanted area. Distributions of dopants of adjacent implanted segments in the isolation link overlap after the anneal operation. An average doping density in the isolation link is less than two-thirds the average doping density in the drain portion link. The segmentation of the ion implanted area for the isolation link may be adjusted to provide a desired breakdown voltage and series resistance of the MOS transistor.
In a first example, the isolation link extends laterally around the source diffused region and body region and connects to the channel portion link so as to electrically isolate the source diffused region and body region. In a second example, the channel portion link is configured in a closed loop which surrounds the drain portion link, and the isolation link is configured in a closed loop which surrounds the channel portion link, so that the source diffused region and body region are located between the channel portion link and the isolation link.
For the purposes of this description, the term “substantially equal” as applied to features of an integrated circuit is understood to mean equal within fabrication tolerances used to form the integrated circuit.
FIG. 1A through FIG. 1D are perspective views of an integrated circuit formed according to an example, depicted in successive stages of fabrication. Referring to FIG. 1A, the integrated circuit 1000 is formed in and on a p-type substrate 1002, which may be a single crystal silicon wafer, a silicon-on-insulator (SOI) wafer, a hybrid orientation technology (HOT) wafer with regions of different crystal orientations, or other material appropriate for fabrication of the integrated circuit 1000. An n-type buried drift region 1004 of a planar extended drain n-channel MOS transistor is formed in the substrate 1002. The buried drift region 1004 may be formed by ion implanting n-type dopants, for example phosphorus, into an existing top surface of the substrate 1002 followed by growing p-type epitaxial semiconductor material over the implanted n-type dopants. In one version of the instant example, a top surface of the buried drift region 1004 may be between 2 and 4 microns deep in the substrate 1002. A region of the substrate 1002 above the buried drift region 1004 may provide a RESURF region during operation of the integrated circuit 1000.
A link ion implant mask 1006 is formed over an existing top surface of the substrate 1002. The link ion implant mask 1006 may include photoresist and/or a dielectric layer such as silicon dioxide. The link ion implant mask 1006 has a drain open area 1008 over a drain portion 1010 of the buried drift region 1004. The link ion implant mask 1006 has a plurality of segmented channel open areas 1012 over a channel portion 1014 of the buried drift region 1004. The link ion implant mask 1006 further has a plurality of segmented isolation open areas 1016 over an isolation portion 1018 of the buried drift region 1004. The isolation portion 1018 extends past a subsequently formed source diffused region of the MOS transistor. In one version of the instant example, each segmented isolation open area 1016 may have lateral dimensions between 1.5 and 3.0 microns and be spaced between 4 and 7 microns apart. In one version of the instant example, the segmented isolation open areas 1016 and the segmented channel open areas 1012 may have substantially equal lateral dimensions and spacings. In another version, the segmented isolation open areas 1016 and the segmented channel open areas 1012 may have different lateral dimensions and spacings, as depicted in FIG. 1A. In one version of the instant example, the segmented isolation open areas 1016 and the segmented channel open areas 1012 may have linear configurations as depicted in FIG. 1A. In another version, the segmented isolation open areas 1016 and the segmented channel open areas 1012 may have curved configurations.
Referring to FIG. 1B, a link ion implant operation is performed on the integrated circuit 1000 which ion implants n-type dopants such as phosphorus and possibly arsenic through the drain open area 1008, the segmented channel open areas 1012 and the segmented isolation open areas 1016 of the link ion implant mask 1006, concurrently, into the substrate 1002. In one version of the instant example, the link ion implant operation may have a dose between 8×1012 cm−2 and 1.5×1013 cm−2. The link ion implant operation concurrently forms a drain link implanted region 1020 under the drain open area 1008, channel link implanted regions 1022 under the segmented channel open areas 1012, and isolation link implanted regions 1024 under the segmented isolation open areas 1016. In one version of the instant example, the isolation link implanted regions 1024 do not touch or overlap each other. The link ion implant operation may be performed as two or more sub-implants at different energies, for example to provide a more uniform vertical distribution of the implanted dopants.
Referring to FIG. 1C, an anneal operation is performed on the integrated circuit 1000 which diffuses the dopants in the drain link implanted region 1020 of FIG. 1B to form a drain diffused link 1026 which extends to, and makes an electrical connection with, the buried drift region 1004 at the drain portion 1010. The anneal operation also diffuses the dopants in the channel link implanted regions 1022 of FIG. 1B to form a channel diffused link 1028 which extends to, and makes an electrical connection with, the buried drift region 1004 at the channel portion 1014. Further, the anneal operation diffuses the dopants in the isolation link implanted regions 1024 of FIG. 1B to form an isolation diffused link 1030 which extends to, and makes an electrical connection with, the buried drift region 1004 at the isolation portion 1018. In one version of the instant example, diffused regions from adjacent isolation link implanted regions 1024 overlap to form a contiguous isolation diffused link 1030 as depicted in FIG. 1C. In an alternate version, gaps may exist between the diffused regions from adjacent isolation link implanted regions 1024, such that during operation of the MOS transistor, depletion regions around the diffused regions of the isolation diffused link 1030 overlap to electrically isolate a source diffused region of the MOS transistor from the substrate 1002. An average doping density in the isolation diffused link 1030 is less than two-thirds an average doping density in the drain diffused link 1026. In one version of the instant example, the average doping density in the isolation diffused link 1030 may be between 20 percent and 50 percent of the average doping density in the drain diffused link 1026. In a further version, the average doping density in the isolation diffused link 1030 may be between 25 percent and 33 percent of the average doping density in the drain diffused link 1026. In one version of the instant example, the average doping density in the drain diffused link 1026 may be between 2.5×1016 cm−3 and 3.5×1016 cm−3, and the average doping density in the isolation diffused link 1030 may be between 5×1015 cm−3 and 1×1016 cm−3.
Referring to FIG. 1D, a gate dielectric layer 1032 of the MOS transistor is formed on the substrate 1002 between the channel diffused link 1028 and the isolation diffused link 1030. A gate 1034 of the MOS transistor is formed on the gate dielectric layer 1032. A drain diffused region 1036 is formed in the substrate 1002 at a top of the drain diffused link 1026. A source diffused region 1038 is formed in the substrate 1002 adjacent to the gate 1034, opposite the channel diffused link 1028, and between the channel diffused link 1028 and the isolation diffused link 1030. A back gate contact diffused region 1040 is formed in the substrate 1002 between the channel diffused link 1028 and the isolation diffused link 1030. During operation of the integrated circuit 1000, the channel diffused link 1028 provides an electrical connection from the buried drift region 1004 to a channel under the gate dielectric layer 1032. The isolation diffused link 1030 in combination with the buried drift region 1004 electrically isolates the source diffused region 1038 from the substrate 1002 outside the MOS transistor. Lateral dimensions and spacing of the segmented isolation open areas 1016 of FIG. 1B may be adjusted to provide a desired breakdown voltage of the MOS transistor.
It will be recognized that a p-channel version of the MOS transistor described in reference to FIG. 1A through FIG. 1D may be formed with appropriate reversals of doping polarities. It will be recognized that a second planar extended drain MOS transistor with a buried drift region may be formed in the integrated circuit 1000, so that an isolation diffused link of the second MOS transistor is formed concurrently with the isolation diffused link 1030 of the first MOS transistor, with a different configuration of segmented channel end open areas, so as to provide a breakdown different from the first MOS transistor with no additional process operations.
FIG. 2A and FIG. 2B are top views of an integrated circuit containing a planar extended drain MOS transistor formed according to the first example, depicted in successive stages of fabrication. Referring to FIG. 2A, the integrated circuit 2000 has a link ion implant mask 2002 formed over a substrate 2004 on which the integrated circuit 2000 is formed. A buried drift region 2006 has been formed in the substrate 2004. The link ion implant mask 2002 has a linear drain end open area 2008 over a drain portion of the buried drift region 2006. The link ion implant mask 2002 also has a linear array of segmented channel open areas 2010 over a channel portion of the buried drift region 2006 and a plurality of segmented isolation open areas 2012 over an isolation portion of the buried drift region 2006. The segmented isolation open areas 2012 extend laterally to the segmented channel open areas 2010 so as to laterally enclose a source area 2014. A link ion implant operation is performed on the integrated circuit 2000 which ion implants dopants through the drain open area 2008, the segmented channel open areas 2010 and the segmented isolation open areas 2012, concurrently, into the substrate 2004, to form a drain link implanted region, not shown, under the drain open area 2008, channel link implanted regions, not shown, under the segmented channel open areas 2010, and isolation link implanted regions, not shown, under the segmented isolation open areas 2012, as described in reference to FIG. 1B.
Referring to FIG. 2B, an anneal operation is performed on the integrated circuit 2000 which diffuses the dopants in the drain link implanted region to form a drain diffused link 2016 which extends to, and makes an electrical connection with, the buried drift region 2006 as described in reference to FIG. 1C. The anneal operation also diffuses the dopants in the channel link implanted regions to form a channel diffused link 2018 which extends to, and makes an electrical connection with, the buried drift region 2006 as described in reference to FIG. 1C. Further, the anneal operation diffuses the dopants in the isolation link implanted regions to form an isolation diffused link 2020 which extends to, and makes an electrical connection with, the buried drift region 2006 as described in reference to FIG. 1C. Subsequently, a gate 2022, source diffused region 2024 and optional back gate diffused contact region 2026 are formed in the source area 2014.
In the instant example, the isolation diffused link 2020 extends laterally around the source area 2014 and contacts the channel diffused link 2018 so as to electrically isolate the source diffused region 2024 from the substrate 2004 outside the MOS transistor. It will be recognized that other isolating elements such as deep trench isolation elements filled with silicon dioxide may also be used in combination with the isolation diffused link 2020 to electrically isolate the source diffused region 2024.
FIG. 3A and FIG. 3B are top views of an integrated circuit containing a planar extended drain MOS transistor formed according to the second example, depicted in successive stages of fabrication. Referring to FIG. 3A, the integrated circuit 3000 has a link ion implant mask 3002 formed over a substrate 3004 on which the integrated circuit 3000 is formed. A buried drift region 3006 has been formed in the substrate 3004. The link ion implant mask 3002 has a drain open area 3008 over a drain portion of the buried drift region 3006. The link ion implant mask 3002 also has an array of segmented channel open areas 3010 over a channel portion of the buried drift region 3006 around the drain open area 3008, and a closed loop array of segmented isolation open areas 3012 over an isolation portion of the buried drift region 3006. The array of segmented channel open areas 3010 may have a closed loop configuration as depicted in FIG. 3A. The segmented isolation open areas 3012 laterally surround the segmented channel open areas 3010 so as to laterally enclose a source area 3014 between the segmented channel open areas 3010 and the segmented isolation open areas 3012. A link ion implant operation is performed on the integrated circuit 3000 which ion implants dopants through the drain open area 3008, the segmented channel open areas 3010 and the segmented isolation open areas 3012, concurrently, into the substrate 3004, to form a drain link implanted region, not shown, under the drain open area 3008, channel link implanted regions, not shown, under the segmented channel open areas 3010, and isolation link implanted regions, not shown, under the segmented isolation open areas 3012, as described in reference to FIG. 1B.
Referring to FIG. 3B, an anneal operation is performed on the integrated circuit 3000 which diffuses the dopants in the drain link implanted region to form a drain diffused link 3016 which extends to, and makes an electrical connection with, the buried drift region 3006 as described in reference to FIG. 1C. The anneal operation also diffuses the dopants in the channel link implanted regions to form a channel diffused link 3018 which extends to, and makes an electrical connection with, the buried drift region 3006 as described in reference to FIG. 1C. Further, the anneal operation diffuses the dopants in the isolation link implanted regions to form an isolation diffused link 3020 which extends to, and makes an electrical connection with, the buried drift region 3006 as described in reference to FIG. 1C. Subsequently, a gate 3022, source diffused region 3024 and optional back gate diffused contact region 3026 are formed in the source area 3014.
In the instant example, the channel diffused link 3018, the gate 3022, the source diffused region 3024, the optional back gate diffused contact region 3026 if present and the isolation diffused link 3020 are configured as concentric racetrack shaped closed loops, so that the isolation diffused link 3020 electrically isolates the source diffused region 3024 from the substrate 3004 outside the MOS transistor. It will be recognized that other closed loop shapes, such as circular, rectangular, or fingered shapes may be used.
FIG. 4 is a top view of an integrated circuit containing a planar extended drain MOS transistor formed according to an example, depicted after formation of a link ion implant mask. The integrated circuit 4000 has the link ion implant mask 4002 formed over a substrate 4004 on which the integrated circuit 4000 is formed. The link ion implant mask 4002 has segmented isolation open areas 4006. The segmented isolation open areas 4006 are disposed in a staggered configuration, for example to obtain a desired doping density and total width of an isolation diffused link subsequently formed below the isolation open areas 4006. It will be recognized that a staggered configuration of segmented isolation open areas may be formed in an MOS transistor with a non-linear isolation diffused link, such as a circular or racetrack shaped isolation diffused link.
FIG. 5 is a top view of an integrated circuit containing a planar extended drain MOS transistor formed according to an example, depicted after formation of a link ion implant mask. The integrated circuit 5000 has the link ion implant mask 5002 formed over a substrate 5004 on which the integrated circuit 5000 is formed. The link ion implant mask 5002 has a plurality of first segmented isolation open areas 5006 with a first length and first width, and a plurality of second segmented isolation open areas 5008 with a second length and second width. The first segmented isolation open areas 5006 in combination with the second segmented isolation open areas 5008 may provide, for example, an advantageous grading of a doping density in an isolation diffused link subsequently formed below the first and second segmented isolation open areas 5006 and 5008. It will be recognized that a staggered configuration of segmented isolation open areas may be formed in an MOS transistor with a non-linear isolation diffused link, such as a circular or racetrack shaped isolation diffused link.
While various examples of the present invention have been described above, it should be understood that they have been presented by way of example only and not limitation. Numerous changes to the disclosed examples can be made in accordance with the disclosure herein without departing from the spirit or scope of the invention. Thus, the breadth and scope of the present invention should not be limited by any of the above described examples. Rather, the scope of the invention should be defined in accordance with the following claims and their equivalents.

## Claims

1. A method of forming an integrated circuit, comprising:
forming a planar extended drain MOS transistor in a substrate of a first conductivity type, by:
forming a buried drift region in the substrate below a top surface of the substrate, so that the buried drift region has a second conductivity type opposite the first conductivity type, and so that the buried drift region has a drain portion, an isolation portion, and a channel portion;
forming a link ion implant mask over the substrate, so that the link ion implant mask has a drain open area over the drain portion of the buried drift region, has a plurality of segmented channel open areas over the channel portion of the buried drift region, and has a plurality of segmented isolation open areas over the isolation portion of the buried drift region;
ion implanting dopants through the drain open area, the segmented channel open areas and the segmented isolation open areas concurrently into the substrate to form a drain link implanted region under the drain open area, to form a plurality of channel link implanted regions under the plurality of segmented channel open areas, and to form a plurality of isolation link implanted regions under the plurality of segmented isolation open areas;
performing an anneal operation which:
diffuses the dopants in the drain link implanted region to form a drain diffused link which extends to, and makes an electrical connection with, the drain portion of the buried drift region;
diffuses the dopants in the channel link implanted regions to form a channel diffused link which extends to, and makes an electrical connection with, the channel portion of the buried drift region and is spaced apart from the drain diffused link; and
diffuses the dopants in the isolation link implanted regions to form an isolation diffused link which extends to, and makes an electrical connection with, the isolation portion of the buried drift region, and extends to the top surface of the substrate, so that an average doping density in the isolation diffused link is less than two-thirds an average doping density in the drain diffused link;


forming a gate dielectric layer on the substrate between the channel diffused link and the isolation diffused link;
forming a gate on the gate dielectric layer;
forming a drain diffused contact region in the substrate so as to make an electrical connection to the drain diffused link; and
forming a source diffused region in the substrate adjacent to the gate and between the channel diffused link and the isolation diffused link, so that the source diffused region is electrically isolated by the isolation diffused link from the substrate outside of the MOS transistor.

2. The method of claim 1, wherein:
the conductivity type of the substrate is p-type;
the conductivity type of the buried drift region is n-type;
the conductivity type of the drain diffused link is n-type;
the conductivity type of the channel diffused link is n-type; and
the conductivity type of the isolation diffused link is n-type.

3. The method of claim 1, wherein the isolation diffused link contacts the channel diffused link so as to electrically isolate the source diffused region from the substrate outside of the MOS transistor.

4. The method of claim 1, wherein the isolation diffused link is configured in a closed loop around the channel diffused link so as to electrically isolate the source diffused region from the substrate outside of the MOS transistor.

5. The method of claim 1, wherein the segmented isolation open areas in the link ion implant mask are disposed in a staggered configuration.

6. The method of claim 1, wherein the segmented isolation open areas in the link ion implant mask include a plurality of first segmented isolation open areas with a first length and first width, and a plurality of second segmented isolation open areas with a second length and second width.

7. The method of claim 1, wherein the average doping density in the isolation diffused link is between 20 percent and 50 percent of the average doping density in the drain diffused link.

8. The method of claim 1, wherein the average doping density in the isolation diffused link is between 25 percent and 33 percent of the average doping density in the drain diffused link.

9. The method of claim 1, wherein:
the average doping density in the drain diffused link is between 2.5×1016 cm−3 and 3.5×1016 cm−3; and
the average doping density in the isolation diffused link is between 5×1015 cm−3 and 1×1016 cm−3.

10. The method of claim 1, wherein the segmented isolation open areas have lateral dimensions between 1.5 and 3.0 microns and are spaced between 4 and 7 microns apart.