# Patent

## Title
Edge termination for super-junction MOSFETs

## Abstract

Edge termination for super-junction MOSFETs. In accordance with an embodiment of the present invention, a super-junction metal oxide semiconductor field effect transistor (MOSFET) includes a core super-junction region including a plurality of parallel core plates coupled to a source terminal of the super-junction MOSFET. The super-junction MOSFET also includes a termination region surrounding the core super-junction region comprising a plurality of separated floating termination segments configured to force breakdown into the core super-junction region and not in the termination region. Each termination segment has a length dimension less than a length dimension of the core plates.

## Background
RELATED APPLICATION
This Application claims priority to U.S. Provisional Patent Application No. 62/039,346, filed Aug. 19, 2014, entitled, “New Edge Termination for SJMOSFETs with Plates” to Pattanayak, which is hereby incorporated herein by reference in its entirety.


FIELD OF INVENTION
Embodiments of the present invention relate to the field of integrated circuit design and manufacture. More specifically, embodiments of the present invention relate to systems and methods for a termination of super-junction MOSFETs.
BACKGROUND
Super-junction metal oxide semiconductor field effect transistors (MOSFETs) comprise a drift region made up of both N-type and P-type regions. Super-junction MOSFETs critically depend on maintaining a definite charge relation between the N and P regions in the drift regions. In general, plate type structures in a drift region show less manufacturing variation in volume, and hence less variation in total charge, in comparison to pillar type structures, e.g., with a round or oval cross section. Accordingly, devices comprising plate type structures in a drift region generally present improved control of a desired charge relation, in comparison to pillar type structures. For this reason, plate type structures may be preferred over pillar type structures for a drift region of a super-junction MOSFET.
However, plate type regions have a directional asymmetry in the sense that in one direction the plates are floating, and in the other, perpendicular, dimension they assume source potential at very low currents. This characteristic requires development of a termination scheme within the constraints of charge balance requirements.
SUMMARY OF THE INVENTION
Therefore, what is needed are systems and methods for edge termination for super-junction MOSFETs. An additional need exists for edge termination for super-junction MOSFETs with increased breakdown voltage in a decreased distance. What is further needed are systems and methods for edge termination for super-junction MOSFETs comprising plate structures in the drift region. A still further need exists for systems and methods for edge termination for super-junction MOSFETs that are compatible and complementary with existing systems and methods of integrated circuit design, manufacturing and test. Embodiments of the present invention provide these advantages.
In accordance with an embodiment of the present invention, a super-junction metal oxide semiconductor field effect transistor (MOSFET) includes a core super-junction region including a plurality of parallel core plates coupled to a source terminal of the super-junction MOSFET. The super-junction MOSFET also includes a termination region surrounding the core super-junction region comprising a plurality of separated floating termination segments configured to force breakdown into the core super-junction region and not in the termination region. Each termination segment has a length dimension less than a length dimension of the core plates.
In accordance with another embodiment of the present invention, a metal oxide semiconductor field effect transistor (MOSFET) includes a core region formed in a substrate of first conductivity, below an active region of the MOSFET. The core region includes a plurality of parallel core plates of a second conductivity coupled to a source terminal of the MOSFET, each of the plates having a plate width. The core plates alternate with regions of the first conductivity having a width of about the plate width. The MOSFET also includes a termination region surrounding the core region. The termination region includes a plurality of separated floating termination segments of the second conductivity formed in the substrate, separated from one another by regions of the first conductivity. Each of the termination segments has a length dimension less than a length dimension of the core plates. The termination region is configured to have a higher breakdown voltage than the core region.
In accordance with yet another embodiment of the present invention, a vertical trench MOSFET includes a substrate of first conductivity, and a plurality of gate trenches descending beneath a surface of the substrate. Each gate trench includes one or more gates of the MOSFET. For example, a trench may comprise an active gate and optionally a shield gate, which may be coupled to the source. The vertical trench MOSFET also includes source and body regions of the MOSFET in a mesa between the gate trenches and a drift region below the gate trenches and below the source and body regions. The drift region includes a plurality of core plates of a second conductivity alternating with regions of the first conductivity, wherein the core plates are coupled to the source regions of the MOSFET. The vertical trench MOSFET further includes a termination region surrounding the drift region at about the same depth as the drift region. The termination region includes a plurality of separated floating termination segments of the second conductivity formed in the substrate, separated from one another by regions of the first conductivity. There are no gates of the MOSFET above the termination region, and each of the termination segments has a length dimension not greater than a length dimension of the core plates.



BRIEF DESCRIPTION OF THE DRAWINGS
The accompanying drawings, which are incorporated in and form a part of this specification, illustrate embodiments of the invention and, together with the description, serve to explain the principles of the invention. Unless otherwise noted, the drawings are not drawn to scale.
FIG. 1 illustrates a plan view of an exemplary edge termination for a super-junction MOSFET, in accordance with embodiments of the present invention.
FIG. 2 illustrates an exemplary design of termination segments, in accordance with embodiments of the present invention.
FIG. 3 illustrates a plan view of an exemplary edge termination for a super-junction MOSFET, in accordance with embodiments of the present invention.
FIG. 4 is an exemplary cross-sectional view showing elements of a semiconductor device, according to an embodiment of the present invention.



DETAILED DESCRIPTION
Reference will now be made in detail to various embodiments of the invention, examples of which are illustrated in the accompanying drawings. While the invention will be described in conjunction with these embodiments, it is understood that they are not intended to limit the invention to these embodiments. On the contrary, the invention is intended to cover alternatives, modifications and equivalents, which may be included within the spirit and scope of the invention as defined by the appended claims. Furthermore, in the following detailed description of the invention, numerous specific details are set forth in order to provide a thorough understanding of the invention. However, it will be recognized by one of ordinary skill in the art that the invention may be practiced without these specific details. In other instances, well known methods, procedures, components, and circuits have not been described in detail as not to unnecessarily obscure aspects of the invention.
Notation And Nomenclature
The figures are not drawn to scale, and only portions of the structures, as well as the various layers that form those structures, may be shown in the figures. Furthermore, fabrication processes and operations may be performed along with the processes and operations discussed herein; that is, there may be a number of process operations before, in between and/or after the operations shown and described herein. Importantly, embodiments in accordance with the present invention can be implemented in conjunction with these other (perhaps conventional) processes and operations without significantly perturbing them. Generally speaking, embodiments in accordance with the present invention may replace and/or supplement portions of a conventional process without significantly affecting peripheral processes and operations.
As used herein, the letter “n” refers to an n-type dopant and the letter “p” refers to a p-type dopant. A plus sign “+” or a minus sign “−” is used to represent, respectively, a relatively high or relatively low concentration of such dopant(s).
The term “channel” is used herein in the accepted manner. That is, current moves within a FET in a channel, from the source connection to the drain connection. A channel can be made of either n-type or p-type semiconductor material; accordingly, a FET is specified as either an re-channel or p-channel device. Some of the figures are discussed in the context of an n-channel device, more specifically an n-channel vertical MOSFET; however, embodiments according to the present invention are not so limited. That is, the features described herein may be utilized in a p-channel device. The discussion of an n-channel device can be readily mapped to a p-channel device by substituting p-type dopant and materials for corresponding n-type dopant and materials, and vice versa. In addition, embodiments in accordance with the present invention are well suited to planar gate super-junction MOSFETs.
The term “trench” has acquired two different, but related meanings within the semiconductor arts. Generally, when referring to a process, e.g., etching, the term trench is used to mean or refer to a void of material, e.g., a hole or ditch. Generally, the length of such a hole is much greater than its width or depth. However, when referring to a semiconductor structure or device, the term trench is used to mean or refer to a solid vertically-aligned structure, disposed beneath a surface of a substrate, having a complex composition, different from that of the substrate, and usually adjacent to a channel of a field effect transistor (FET). The structure comprises, for example, a gate of the FET. Accordingly, a trench semiconductor device generally comprises a mesa structure, which is not a trench, and portions, e.g., one half, of two adjacent structural “trenches.”
It is to be appreciated that although the semiconductor structure commonly referred to as a “trench” may be formed by etching a trench and then filling the trench, the use of the structural term herein in regards to embodiments of the present invention does not imply, and is not limited to such processes.
Edge Termination for Super-Junction Mosfets
One function of an edge termination in a super-junction MOSFET is to drop the potential gradually from that of the source potential to the drain potential in a manner that does not stress the source metal above the ionizing potential of air. The source to drain potential may be on the order of 600 volts or more.
FIG. 1 illustrates a plan view of an exemplary edge termination 110 for a super-junction MOSFET 100, in accordance with embodiments of the present invention. The closed shapes of FIG. 1 indicate p-type material, for example, p-type material in an n-type epitaxial layer. It is appreciated that the area around and between p-type regions is typically n-type material. The termination region 110 comprises a plurality of termination segments 114 and termination plates 112 surrounding a core region 120. The p-type termination plates 112 and termination segments 114 are floating. There be more or fewer than the exemplary three rows of termination plates 112 and termination segments 114, in some embodiments.
The core region 120 is typically beneath active devices, e.g., MOSFETs, and is generally much larger in area than termination region 110. The p-type core plates 122 in the core region 120 are typically coupled to the source electrode, e.g., they are at source potential. For example, there will typically be many more core plates 122 than illustrated. The core plates 122, termination plates 112 and termination segments 114 have about the same vertical depth and vertical extent, e.g., in the plane perpendicular to the plane of FIG. 1.
The termination segments 114 are preferably square, e.g., their width is equal to their length, although that is not required. The termination plates 112 should be rectangular, e.g., their length is greater than their width. In general, the termination plates 112 should be the same length as, and aligned in parallel with, the p-plates 122 within the core region 120. The width of the segments 114 and the plates 112 are not necessarily the same.
The number of termination plates 112 and termination segments 114 and their spacing is determined by the desired source to drain potential of the super-junction MOSFET. In silicon, approximately 10 volts can be supported in a space of 1 μm. For a 600 volt device, the total space consumed by the gaps between the p-type structures (n-type material between termination plates 112 and/or termination segments 114) from one another should be of the order of 40 μm, e.g., supporting approximately 400 volts. It is appreciated that the spacing need not be regular. Assuming the width of the p-type structures to be 5 μm each, supporting about 50 volts, eight such segments are required to support 400 volts. Together therefore, for an 800 volt edge termination of the type described, the edge termination width can be as low as 80 μm. According to the conventional art, a commercially available product requires about 230 μm for about 600 volts of edge termination.
In accordance with embodiments of the present invention, the termination plates 112 and termination segments 114 and the inter-segment spacing should be designed in such a way that the charge balance condition is set so that all the segments are mostly depleted and the breakdown voltage of the edge termination is slightly higher than that of the core region. This will improve unclamped inductive-switching (UIS) capability of the device.
It is not necessary, however, that the edge segments be completely depleted. The depletion layers of the segments should merge with each other and support the voltage in both horizontal and vertical dimensions (in the view of FIG. 1). The segments surrounding the core area may be designed differently than the core plates so that the breakdown voltage of the edge is slightly higher than that of the core, for example, forcing breakdown into the core region and not in the termination region.
In accordance with embodiments of the present invention, segment spacing and width may be designed according to Relation 1, below:

δQn ̃2δQp  (Relation 1)

where δQn is the charge in the space between the termination segments, and δQp is the additional charge in the P plate segment, due do its greater width relative to the width of the core plate.

In accordance with embodiments of the present invention, a super-junction MOSFET may advantageously have a charge imbalance, e.g., the charge of p-type material is not equal to the charge of n-type material, in the termination region. For example, Qp≠Qn. In accordance with embodiments of the present invention, for an n-channel MOSFET formed in n-type material, e.g., an n-type epitaxial layer, the charge of n-type material may be slightly larger than the charge of p-type material, e.g., Qp<Qn. The termination breakdown voltage should be higher than the core breakdown voltage, with different charge imbalance between these two regions.
FIG. 2 illustrates an exemplary design 200 of termination segments, e.g., corresponding to termination segments 114 of FIG. 1, in accordance with embodiments of the present invention. FIG. 2 is not drawn to scale. FIG. 2 illustrates a portion 222 of a core plate, e.g., corresponding to core plate 122 of FIG. 1. Core plate portion 222 has a width dimension 224. It is appreciated that the full horizontal length of core plate portion 222 may not be illustrated.
FIG. 2 also illustrates a plurality of termination segments 214, e.g., corresponding to termination segments 114 of FIG. 1. The termination segments 214 are wider than core plate 222. The increased width provides an increased charge of 2δQp (220) in comparison to a termination segment of the same width (224) as the core plate. The termination segments 214 are spaced at a distance from each other 210 that provides a charge of δQn (210). Thus, the conditions of Relation 1 are met.
It is appreciated that dimensions 210 and 220 are illustrated in terms of the charge available in such regions. The actual physical dimensions are a function of the dopant densities of both n-type and p-type materials and the width of the core plate 222. As previously presented, the number of termination segments 214 and their spacing 210 are functions of the desired breakdown voltage in the termination region.
In this novel manner, the charge balance condition is set so that all the termination segments 214 are mostly depleted and the breakdown voltage of the edge termination is slightly higher than that of the core region, thus improving the unclamped inductive-switching (UIS) capability of the device.
FIG. 3 illustrates a plan view of an exemplary edge termination 310 for a super-junction MOSFET 300, in accordance with embodiments of the present invention. The closed shapes of FIG. 3 indicate p-type material, for example, p-type material in an n-type epitaxial layer. It is appreciated that the area around and between p-type regions is typically n-type material. In contrast to FIG. 1, the terminal region 310 comprises all segment regions, for example, there are no plate regions in the termination region. The termination region 310 comprises a plurality of termination segments 314 surrounding a core region 320. The p-type termination segments 314 are floating. There may be more or fewer than the exemplary three rows of termination segments 314, in some embodiments.
The core region 320 is typically beneath active devices, e.g., MOSFETs, and is generally much larger in area than termination region 310. The p-type core plates 322 in the core region 120 are typically coupled to the source electrode, e.g., they are at source potential. For example, there will typically be many more core plates 322 than illustrated.
The termination segments 314 are preferably square, e.g., their width is equal to their length, although that is not required. The width of the termination segments 314 and the core plates 322 are not necessarily the same. The corners of termination segments 314 and/or the core plates 322 may be rounded, as illustrated, to increase breakdown voltage. It is to be appreciated that such rounding is not so extreme as to reduce the segment shapes to that of pillars. For example, the radius of the rounded corners should be much less than one-half of the diagonal distance across the segment. In addition, the corner termination segments, e.g., corner termination segments 350, may be designed differently than the edge cells as demanded by electrostatic considerations relating charge to field distribution, in some embodiments. For example, corner termination segments 350 may be scaled such that the array of termination segments 314 comprises rounded “corners,” as indicated by the dotted lines in the corners of the termination region 310.
The number of termination plates 312, their width and their spacing are determined by the desired source to drain potential of the super-junction MOSFET, as previously presented with respect to FIGS. 1 and 2.
In this novel manner, a plurality of separated floating termination segments are configured to force breakdown into the core super-junction region and away from the termination region. Accordingly, the core super-junction region will break down before the termination region, thereby providing an effective termination for the desired drain-source voltage.
FIG. 4 is an exemplary cross-sectional view showing elements of a semiconductor device 400 (e.g., an n-channel Super Junction trench power MOSFET device) according to an embodiment of the present invention. FIG. 4 is an exemplary cross sectional view of the semiconductor device illustrated in FIG. 1 or FIG. 3. The semiconductor device 400 comprises a drain electrode 402 on the bottom surface of an n+ drain layer or substrate 404. FIG. 4 illustrates a drift region 401, below the gates 411 and bodies 414 of device 400. Drift region 401 may also be known as or referred to as a core region. Alternating p− drift regions or p− type columns or plates 422 and n− drift regions or n-type columns are located above the substrate 404, e.g., in epitaxial n− material. The alternating p-type (p−) columns 422 and n-type (n−) columns form what is known as or referred to as a super junction.
In the example embodiment of FIG. 4, each p-type column 422 is located under a respective polysilicon (poly) trench gate 411 (gate poly 411). Generally speaking, each trench gate 411 is aligned above a corresponding p-type column 422. More specifically, each trench gate 411 may be aligned along the longitudinal axis of a corresponding p-type column 422 (given the orientation of FIG. 4, the longitudinal axis is a vertical line within a p-type column)—in one embodiment, the longitudinal axis of a trench gate 411 coincides with the longitudinal axis of a p-type column 422 such that the trench gate 411 is centered over the p-type column 422. In the FIG. 4 embodiment, the p-type columns 422 are separated from the trench gates 411 by a respective isolation layer, e.g., oxide 409.
A p− region (p-type body region 414) is situated in a mesa between the trench gates 411. Also, n+ regions (n-type source regions 415) are situated on opposite sides of each trench gate 411, above the p-type body regions 414. According to an embodiment of the invention, the p-type columns 422 are picked up and electrically shorted to the source metal layer in a well known manner, e.g., outside the plane of FIG. 4 (not shown).
FIG. 4 also illustrates termination segments 412. Termination segments 412 are located in a termination region 403, distinct from, e.g., “outside of,” the drift region 401 of semiconductor device 400. For example, there are no gates, e.g., FET gates 411, or FET bodies 414, above the termination region 403, in contrast to the presence of FET gates 411 and FET bodies 414 above the drift region 401. Termination segments are “floating,” e.g., not connected to any other terminals, in some embodiments. Termination region 403 is at about the same depth into the semiconductor device 400 as drift region 401, e.g., at a depth deeper than the gate 411 and body 414.
FIG. 4 further illustrates an active region 440 of semiconductor device 400, in accordance with embodiments of the present invention. Active region 440 generally comprises a source, e.g., source 415, a body or channel region, e.g., body 414, and/or a gate, e.g., gate 414.
Embodiments in accordance with the present invention provide systems and methods for trench metal-oxide-semiconductor field-effect transistors (MOSFETs) with self-aligned body contacts. In addition, embodiments in accordance with the present invention provide systems and methods for trench MOSFETs with self-aligned body contacts having increased separation between a body contact implant and a gate trench. Further, embodiments in accordance with the present invention provide systems and methods for trench MOSFETs with self-aligned body contacts having improved performance at finer, e.g., smaller, inter-gate pitch dimensions. Still further, embodiments in accordance with the present invention provide systems and methods for trench MOSFETs with self-aligned body contacts that are compatible and complementary with existing systems and methods of integrated circuit design, manufacturing and test.
Various embodiments of the invention are thus described. While the present invention has been described in particular embodiments, it should be appreciated that the invention should not be construed as limited by such embodiments, but rather construed according to the below claims.

## Claims

1. A vertical trench MOSFET comprising:
a substrate of first conductivity;
a plurality of gate trenches descending beneath a surface of said substrate,
wherein each gate trench comprises one or more gates of said MOSFET;
source and body regions of said MOSFET in a mesa between said gate trenches;
a drift region below said gate trenches and below said source and body regions, said drift region comprising:
a plurality of core plates of a second conductivity alternating with regions of said first conductivity, wherein said core plates are coupled to said source regions of said MOSFET;
a termination region surrounding said drift region at about the same depth as said drift region, said termination region comprising:
a plurality of separated floating termination segments of said second conductivity formed in said substrate, separated from one another by regions of said first conductivity,
wherein there are no gates of said MOSFET above said termination region, and
wherein each of said termination segments has a length dimension not greater than a length dimension of said core plates.

2. The MOSFET of claim 1 wherein said termination segments are wider than said core plates.

3. The MOSFET of claim 1 wherein a charge of one said termination segments is about equal to the magnitude of a charge in a region separating said termination segment from an adjacent termination segment.

4. The MOSFET of claim 1 wherein a charge of said first conductivity is greater than a charge of said second conductivity in said termination region.

5. The MOSFET of claim 1 wherein said termination segments have rounded corners of radius much less than their diagonal dimension.

6. The MOSFET of claim 1 wherein termination segments at a corner of said termination region are reduced in extent to approximate a rounded corner of said termination region.