METHOD OF FORMING HIGH VOLTAGE N-LDMOS TRANSISTORS HAVING SHALLOW TRENCH ISOLATION REGION WITH DRAIN EXTENSIONS
A method and structure is disclosed for a transistor having a gate, a channel region below the gate, a source region on one side of the channel region, a drain region on an opposite side of the channel region from the source region, a shallow trench isolation (STI) region in the substrate between the drain region and the channel region, and a drain extension below the STI region. The drain extension is positioned along a bottom of the STI region and along a portion of sides of the STI. Portions of the drain extension along the bottom of the STI may comprise different dopant implants than the portions of the drain extensions along the sides of the STI. Portions of the drain extensions along sides of the STI extend from the bottom of the STI to a position partially up the sides of the STI. The STI region is below a portion of the gate. The drain extension provides a conductive path between the drain region and the channel region around a lower perimeter of the STI. The drain region is positioned further from the gate than the source region.
This application is a division of U.S. patent application Ser. No. 10/991,936 filed on Nov. 18, 2004 which is a divisional of U.S. patent application Ser. No. 10/249,766, filed on May 6, 2003, which is now U.S. Pat. No. 6,876,035, which are both fully incorporated herein.
BACKGROUND OF THE INVENTION1. Field of the Invention
The present invention generally relates to transistors and more particularly to an improved N-type lateral double diffusion metal oxide semiconductor that has a shallow trench isolation region between the gate and the drain.
2. Description of the Related Art
Power semiconductor devices are currently being used in many applications. Such power devices include high-voltage integrated circuits, which typically include one or more high-voltage transistors, often on the same chip as low-voltage circuitry. A commonly used high-voltage component for these circuits is the lateral double diffused MOS transistor (LDMOS). LDMOS structures used in high-voltage integrated circuits may generally be fabricated using some of the same techniques used to fabricate the low voltage circuitry or logic circuitry. In general, these existing LDMOS structures are fabricated in a thick epitaxial layer of opposite conductivity type to the substrate or they use a thin epitaxial layer and apply the RESURF (reduced surface field) principle (e.g., see U.S. Pat. No. 6,242,787, that is incorporated herein by reference, for a complete description of RESURF) to equally distribute the applied drain voltage laterally across the silicon surface in the drift region of the device.
High-power applications call for the use of such lateral double diffused MOS transistors primarily because they possess lower “on” resistance (“Rdson”), faster switching speed, and lower gate drive power dissipation than their bi-polar counterparts. These devices have heretofore also been strongly associated with bi-polar based process flows when integrated into a Bi-CMOS (bipolar complementary metal oxide semiconductor) environment.
SUMMARY OF THE INVENTIONThe invention provides a transistor having a gate, a channel region below the gate, a source region on one side of the channel region, a drain region on an opposite side of the channel region from the source region, a shallow trench isolation (STI) region in the substrate between the drain region and the channel region, and a drain extension below the STI region. The drain extension is positioned along the bottom of the STI region and along the portion of sides of the STI. Portions of the drain extension along the bottom of the STI may comprise different dopant implants than the portions of the drain extensions along the sides of the STI. Portions of the drain extensions along sides of the STI extend from the bottom of the STI to a position partially up the sides of the STI. The STI region is below a portion of the gate. The drain extension provides a conductive path between the drain region and the channel region around a lower perimeter of the STI. The drain region is positioned further from the gate than the source region.
The invention also discloses a method of manufacturing a transistor. First, the method forms a trench in a substrate. Next, the method partially fills the trench with a sacrificial material, and then forms spacers in the trench above the sacrificial material.
The method removes the sacrificial material and implants a drain extension through the trench into exposed portions of the substrate. The drain extension is implanted to regions of the substrate along sides and the bottom of the trench. The method fills the trench with a shallow trench isolation (STI) material. The method defines a channel region in the substrate on one side the STI material. The invention forms a source region in the substrate on an opposite side of the channel region from the STI material. The method then forms a drain region in the substrate on an opposite side of the STI material from the channel region. Lastly, the method forms a gate above the channel region.
The implanting process includes a vertical implant which forms a drain extension in the portion of the substrate along the bottom of the substrate and an angled implant to form the drain extension in the portion of the substrate along the sides of the substrate.
The spacers and the sacrificial material control the size and location of the drain extension. The implanting process forms portions of the drain extensions along sides of the STI from the bottom of the STI to a position partially up the sides of the STI. The forming of the gate extends a portion of the gate over the STI material. The drain extension provides a conductive path between the drain region and the channel region around a lower perimeter of the STI. The process of forming the drain region positions the drain region further from the gate than the source region. The process of implanting the drain extension includes protecting regions other than the trench to limit the implanting process to the trench.
The manufacturing process shown above is advantageous when compared to conventional manufacturing processes because the invention forms the drain extension directly through the trench opening. Thus, a lower energy implant can be used than is used conventionally. Further, the penetration depth and unwanted diffusion is easily controlled because the implant is being made through the trench opening. In addition, the trench opening aligns the impurity implant more precisely than conventional methods that must pass the higher energy implant through the recessed oxide. Therefore, as shown above, the invention provides improved channel length control with maskless trench aligned implant and reduced straggle (unwanted diffusion) of the deep implant. This reduces on resistance (Rdson), and overlap capacitance (Cov), and increases the on current (Ion).
BRIEF DESCRIPTION OF THE DRAWINGSThe invention will be better understood from the following detailed description of a preferred embodiment(s) of the invention with reference to the drawings, in which:
Integration of microelectronics into consumer appliances, automotive, space technologies, etc., drives a market utilizing high performance CMOS (complementary metal oxide semiconductor), BJT's (bipolar junction transistors) and power MOS (metal oxide semiconductor) drivers. The lateral DMOS (LDMOS) transistor is typically chosen as the driving transistor, capable of switching high voltages.
The performance parameters for an optimized design are low and controlled Rdson/Overlap Capacitance at the drain terminal, and high Vdrain breakdown voltage. This implies that a “high” drive current is the result of a minimized and controlled Rdson for the LDMOS device. In
The limitations described above also limit the minimum channel length design for such a device, further constraining the chip function due to I/O requirements. More specifically, the channel region 230 runs approximately between the n− layer 204 and the n+ doped region 222. The size and shape of the bird's beak of the field oxide 210 (e.g., the left and right sides of the field oxide 210 in
The invention provides control over the source to drain spacing by integrating shallow trench isolation (STI) and low energy pre-implants prior to isolating adjacent transistors. The pre-implant is performed to dope the drain extension. One additional control feature provided by the invention is a STI sidewall spacer formation that is used to control the out-diffusion of the self-aligned junction floor pre-implant.
The structure shown in
Turning to
The mask 60 is then removed, as shown in
In
The manufacturing process shown above is advantageous when compared to conventional manufacturing processes because the invention forms the drain extension 40, 42 directly through the trench opening 61. Thus, a lower energy implant (e.g., 10-40 Kev) can be used. This is a substantial improvement over the implants required through the Fox (210 in
While the invention has been described in terms of preferred embodiments, those skilled in the art will recognize that the invention can be practiced with modification within the spirit and scope of the appended claims.
Claims
1. A transistor comprising:
- a gate on a substrate;
- a channel region in said substrate below said gate;
- a source region in said substrate on one side of said channel region, a drain region in said substrate on an opposite side of said channel region from said source region;
- a shallow trench isolation (STI) region in said substrate between said drain region and said channel region, wherein said STI region comprises a trench in said substrate, sidewall spacers along walls of said trench, and an isolation material between said spacers filling said trench; and
- a drain extension below said STI region.
2. The transistor in claim 1, wherein said drain extension is positioned along a bottom of said STI region and along a portion of sides of said STI.
3. The transistor in claim 2, wherein portions of said drain extension along said bottom of said STI comprise different dopant implants that said portions of said drain extensions along said sides of said STI.
4. The transistor in claim 2, wherein portions of said drain extensions along sides of said STI extend from said bottom of said STI to a position partially up said sides of said STI.
5. The transistor in claim 1, wherein said STI region is below a portion of said gate.
6. The transistor in claim 1, wherein said drain extension provides a conductive path between said drain region and said channel region around a lower perimeter of said STI.
7. The transistor in claim 1, wherein said drain region is positioned further from said gate than said source region.
8. The transistor in claim 1, further comprising a gate oxide below said gate, wherein said STI region forms a portion of said gate oxide.
Type: Application
Filed: Aug 24, 2007
Publication Date: Dec 13, 2007
Inventors: Wagdi Abadeer (Jericho, VT), Jeffrey Brown (Middlesex, VT), Robert Gauthier (Hinesburg, VT), Jed Rankin (South Burlington, VT), William Tonti (Essex Junction, VT)
Application Number: 11/844,573
International Classification: H01L 29/78 (20060101);