# Patent

## Title
Split gate semiconductor device with curved gate oxide profile

## Abstract

A split gate semiconductor device includes a trench gate having a first electrode region and a second electrode region that are separated from each other by a gate oxide layer and an adjacent dielectric layer. The boundary of the gate oxide layer and the dielectric layer is curved to avoid a sharp corner where the gate oxide layer meets the sidewalls of the trench.

## Background
REFERENCE TO RELATED U.S. APPLICATION
This application is a continuation (divisional) application of U.S. patent application Ser. No. 12/603,028, filed Oct. 21, 2009, by Y. Gao et al., now U.S. Pat. No. 9,419,129, which is hereby incorporated by reference in its entirety.


FIELD OF THE INVENTION
Embodiments in accordance with the present invention generally pertain to semiconductor devices.
BACKGROUND
To conserve power, it is important to reduce power losses in transistors. In a metal oxide semiconductor field effect transistor (MOSFET) device, and in particular in the class of MOSFETs known as power MOSFETs, power losses can be reduced by reducing the device's drain-to-source on-resistance (Rdson).
Split gate power MOSFETs, also known as shielded gate trench MOSFETs, utilize a greater doping concentration in the epitaxial layer to reduce Rdson. Split gate power MOSFETs incorporate trench gates that include a first electrode (e.g., polysilicon, or poly-1) that is separated from a second electrode (e.g., polysilicon, or poly-2) by an isolation layer (e.g., a differential oxide layer). From a manufacturing point of view, proper formation of the isolation layer can be challenging.
In one conventional manufacturing process, the isolation layer is grown on an exposed first polysilicon (poly-1) region at the same time that the gate oxide is formed. However, the isolation layer grows much faster on top of the poly-1 than along the sidewalls of the poly-1 region. As a result, a sharp corner is formed when the polysilicon (poly-2) for the second electrode is deposited onto the isolation layer. The sharp corner can affect the reliability of the isolation layer because of the point discharge effect, and can also increase the overlap between the gate-to-source and gate-to-drain, thus increasing Ciss (the sum of the gate-source capacitance and the gate-drain capacitance). Furthermore, because the isolation layer conforms to the profile of the underlying poly-1 region, any void or defect on the poly-1 surface will be translated into a distorted oxide profile, which may significantly reduce the poly-1/poly-2 breakdown. In addition, because the isolation layer and the gate oxide are formed simultaneously, their thicknesses are highly correlated. As a result, the isolation layer cannot be made thicker to compensate for defects or other manufacturing issues without also making the gate oxide thicker.
In another conventional manufacturing process, a sidewall oxide and the polysilicon for the first electrode are deposited in a trench. Then, the first polysilicon region is recess etched and the trench is refilled with a dielectric material that has a similar etch rate as that of the sidewall oxide. After planarization, the dielectric material and the sidewall oxide are etched back to form the isolation layer. However, it can be difficult to achieve a uniform etch rate for both the dielectric and the oxide. A difference in the etch rate could affect the uniformity of the isolation layer, which can affect the device's on-resistance as well as the input capacitance. Also, after the etch is performed, a sharp corner is formed where the material remaining in the trench meets the trench sidewalls. The sharp corner can significantly retard the gate oxide thickness, which in turn can significantly reduce gate oxide breakdown.
SUMMARY
Accordingly, a semiconductor device that avoids the shortcomings described above, and a viable method of producing such a device, would be advantageous.
In one embodiment according to the invention, a split gate semiconductor device (e.g., a power MOSFET) includes a trench gate having a first electrode (e.g., poly-1) region and a second electrode (e.g., poly-2) region that are separated from each other by a gate oxide layer and an adjacent dielectric layer. The boundary of the gate oxide layer and the dielectric layer is curved to avoid a sharp corner where the gate oxide layer meets the sidewalls of the trench.
During fabrication, in one embodiment, the poly-1 region is recess etched and the sidewall oxide is etched away. The recess is refilled with a dielectric material and planarized; thus, the region above the poly-1 region contains the same type of material (the dielectric material). The dielectric material is then etched back; because the same material is present throughout the region, the problem of trying to achieve a uniform etch rate for different materials is avoided. The dielectric region is etched to form a curved (e.g., concave) profile. When the overlying oxide layer is formed, it will conform to the shape of the dielectric region and thus will also have a curved profile. A sharp corner where the dielectric region and oxide layer meet the sidewalls of the trench is thereby avoided.
Relative to a conventional approach, there is a less overlap between gate-to-source and gate-to-drain, which reduces Ciss. Any defects or voids in the poly-1 surface will be filled when the recess is refilled with dielectric, thus achieving a proper profile instead of a distorted one. The absence of a sharp corner solves the gate oxide retardation problem associated with a conventional approach.
These and other objects and advantages of the present invention will be recognized by one skilled in the art after having read the following detailed description, which are illustrated in the various drawing figures.



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. Like numbers denote like elements throughout the drawings and specification.
FIGS. 1A, 1B, and 1C illustrate a flowchart of a process that is used in the fabrication of a semiconductor device according to embodiments of the present invention.
FIGS. 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, 13, 14, 15, 16, 17, and 18 are cross-sectional views showing selected stages in the fabrication of a semiconductor device according to embodiments of the present invention.
FIG. 19 is a cross-sectional view showing elements of a semiconductor device in an embodiment according to the present invention.



DETAILED DESCRIPTION
In the following detailed description of the present invention, numerous specific details are set forth in order to provide a thorough understanding of the present invention. However, it will be recognized by one skilled in the art that the present invention may be practiced without these specific details or with equivalents thereof. In other instances, well-known methods, procedures, components, and circuits have not been described in detail as not to unnecessarily obscure aspects of the present invention.
Some portions of the detailed descriptions that follow are presented in terms of procedures, logic blocks, processing, and other symbolic representations of operations for fabricating semiconductor devices. These descriptions and representations are the means used by those skilled in the art of semiconductor device fabrication to most effectively convey the substance of their work to others skilled in the art. In the present application, a procedure, logic block, process, or the like, is conceived to be a self-consistent sequence of steps or instructions leading to a desired result. The steps are those requiring physical manipulations of physical quantities. It should be borne in mind, however, that all of these and similar terms are to be associated with the appropriate physical quantities and are merely convenient labels applied to these quantities. Unless specifically stated otherwise as apparent from the following discussions, it is appreciated that throughout the present application, discussions utilizing terms such as “forming,” “performing,” “producing,” “depositing,” “growing,” “etching” or the like, refer to actions and processes (e.g., flowchart 100 of FIGS. 1A, 1B and 1C) of semiconductor device fabrication.
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 steps may be performed along with the processes and steps discussed herein; that is, there may be a number of process steps before, in between and/or after the steps shown and described herein. Importantly, embodiments in accordance with the present invention can be implemented in conjunction with these other (perhaps conventional) processes and steps without significantly perturbing them. Generally speaking, embodiments in accordance with the present invention can replace portions of a conventional process without significantly affecting peripheral processes and steps.
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 the dopant.
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 n-channel or p-channel device. Some of the figures are discussed in the context of an n-channel device, specifically an n-channel power MOSFET; however, embodiments according to the present invention are not so limited. That is, the features described herein can 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.
FIGS. 1A, 1B, and 1C illustrate a flowchart 100 of one embodiment of a process that is used in the fabrication of semiconductor devices such as the device partially illustrated in FIG. 19. Although specific steps are disclosed in the flowchart 100, such steps are exemplary. That is, embodiments according to the present invention are well suited to performing various other steps or variations of the steps recited in the flowchart 100. The flowchart 100 is discussed in conjunction with FIGS. 2-18, which are cross-sectional views showing selected stages in the fabrication of a semiconductor device according to an embodiment of the present invention.
In block 102 of FIG. 1A, with reference also to FIG. 2, an epitaxial (epi) layer 204 is grown over a substrate 202 (which may not be shown in subsequent figures). In an n-channel device, the epitaxial layer includes p-dopant and is grown over an n+ substrate. The structure includes a drain region 203 on the bottom surface of the substrate 202.
In block 104 of FIG. 1A, with reference also to FIG. 3, a photoresist (PR) mask 306 is formed over selected portions of the epi layer 204. Then, the exposed portions of the epi layer 204 are etched away to form trench-like cavities, referred to as trenches 308. The mask 306 is then removed, as shown in FIG. 4.
In block 106 of FIG. 1A, with reference also to FIG. 5, a thermal oxide layer 510 is produced (grown) over the epi layer 204 and along the sidewalls and bottom surfaces of the trenches 308. In one embodiment, the thermal oxide layer has a thickness of approximately 300 Angstroms (Å).
In block 108, a dielectric layer 512 (also referred to herein as the first dielectric layer) is formed over the thermal oxide layer 510, extending into the trenches 308 along their sidewalls and bottoms as shown in FIG. 5. In one embodiment, the dielectric layer 512 includes sub-atmospheric undoped silicon glass (SAUSG). In one such embodiment, the thickness of the dielectric layer is approximately 1300 A.
In block 110 of FIG. 1A, with reference also to FIG. 6, a first polysilicon layer 614 is formed over the dielectric layer 512 and into the trenches 308 (in FIG. 6 and subsequent figures, the dielectric layer 512 and the thermal oxide layer 510 may be represented as a single layer 511). The first polysilicon layer may also be referred to herein as poly-1.
In block 112 of FIG. 1A, with reference also to FIG. 7, the poly-1 is removed using, for example, a chemical-mechanical planarization or polishing (CMP) process. The remaining poly-1 regions 714 can also be etched back a relatively small amount (e.g., about 0.1 microns) so that the exposed (e.g., top) surface of the poly-1 is slightly recessed relative to the layer 511.
In block 114 of FIG. 1B, with reference also to FIG. 8, a PR mask 816 is formed over selected poly-1 regions—that is, the core region of the structure is left exposed, while the pickup or termination region of the structure is covered by the mask 816. After the mask is applied, the exposed poly-1 is etched back (recess etched) to form a poly-1 region 818 in the core region that fills the trench 308 to a lesser height than the poly-1 region 714 in the pickup or termination region.
In block 116 of FIG. 1B, with reference also to FIG. 9, the PR mask 816 is removed, and then a blanket buffered oxide etch (BOE) is performed to remove portions of the layer 511. More specifically, the layer 511 is removed along the mesas of the epi layer 204, and from the sidewalls of the trenches 308, partially exposing the sides of the poly-1 regions 714 and 818 as shown in FIG. 9. The layer 511 is removed to a height that is slightly less than the height of the regions 714 and 818.
In block 118 of FIG. 1B, a dielectric layer 1020 is deposited over the exposed surfaces of the epi layer 204 and into the trenches 308 as shown in FIG. 10 using, for example, a sub-atmospheric pressure chemical vapor deposition (SACVD) process. In one embodiment, the dielectric layer 1020 includes SAUSG (e.g., 6K SAUSG), which is densified. Any defects or voids in the surface of the poly-1 regions 714 and 818 will be filled when the recess is refilled with dielectric, thus achieving a proper profile instead of a distorted one.
In block 120 of FIG. 1B, some of the dielectric layer 1020 is removed using a CMP process, for example, and the remainder of the dielectric layer is removed using a dry etch, such that the exposed surfaces of the epi layer 204 and the dielectric layer 1020 are planarized as shown in FIG. 11. Thus, the same type of material (e.g., SAUSG) is in the region above the poly-1 region 818.
In block 122 of FIG. 1B, a PR mask 1222 is formed over the pickup or termination region of the structure, leaving the core region exposed as shown in FIG. 12. A proprietary oxide etch is then performed to remove a portion of the dielectric layer 1020 to form the dielectric layer 1226 (also referred to herein as the second dielectric layer). Because the same material is present throughout the dielectric layer 1020, the problem of trying to achieve a uniform etch rate for different materials is avoided.
Significantly, the surface 1224 of the dielectric layer 1226 is curved. In FIG. 12, a cross-section of the structure is illustrated; in three dimensions, the surface 1224 of the dielectric layer 1226 is concave. Because the surface 1224 is curved, a sharp corner where the dielectric layer 1226 meets the sidewalls 1228 of the trench is thereby avoided.
In block 124 of FIG. 1B, the PR mask 1222 is removed and a gate oxide layer 1330 is produced (grown) over the exposed surfaces of epi layer 204, dielectric layer 1226, and dielectric layer 1020, as shown in FIG. 13. Because the surface 1224 is curved, the gate oxide layer 1330 will also be curved (concave-shaped) above the dielectric layer 1226. Thus, the gate oxide retardation problem, associated with the presence of sharp corners in conventional approaches, is avoided. Also, relative to conventional approaches, there is a less overlap between gate-to-source and gate-to-drain, which reduces Ciss.
In block 126 of FIG. 1C, a second polysilicon layer 1332 is then formed over the gate oxide layer 1330. The second polysilicon layer may also be referred to herein as poly-2.
In block 128 of FIG. 1C, with reference also to FIG. 14, the poly-2 is removed using, for example, a CMP process to form a poly-2 region 1434. The poly-2 region 1434 can also be etched back a relatively small amount so that the exposed (e.g., top) surface of the poly-2 is slightly recessed relative to the gate oxide layer 1330. The poly-1 region 818 and the poly-2 region 1434 correspond to the first and second electrodes of a split gate in a split gate power MOSFET, also known as a shielded gate trench MOSFET.
In block 130 of FIG. 1C, with reference also to FIG. 15, a body implant is performed to form the body regions 1536. In an n-channel device, for example, the body regions 1536 include p-type (p−) dopant. A mask (not shown) can then be formed to shield the pickup or termination region, and then a source implant is performed to form the source region 1538. In an n-channel device, for example, the source region 1538 includes n-type (n+) dopant. The mask is then removed.
In block 132 of FIG. 1C, a layer of low temperature oxide (LTO) followed by a layer of borophosphosilicate glass (BPSG) are deposited—these layers are collectively identified as layer 1640 in FIG. 16.
In block 134 of FIG. 1C, with reference also to FIG. 17, a mask (not shown) is used to selectively remove portions of the layer 1640, the source region 1538, and the body region 1536, forming an opening 1742. At the bottom of the opening, in an n-channel device, dopant is then implanted to form the p-type (p+) contact region 1744.
In block 136 of FIG. 1C, another mask (not shown) can be formed over selected areas and material can be removed beneath openings in the mask to form pickups (not shown) to the poly-1 regions 818 and 714 and the poly-2 region 1434. A metal layer 1846 can be deposited over the structure as shown in FIG. 18, and then another mask (not shown) can be formed to selectively remove portions of the metal layer to form electrical connections. A passivation layer (not shown) can be optionally deposited, and then another mask (not shown) can be applied to etch the passivation layer to define gate and source pads.
FIG. 19 illustrates an embodiment of a trench or split gate 1900. The gate 1900 includes a first electrode (e.g., polysilicon, poly-1) region 818 and a second electrode (e.g., polysilicon, poly-2) region 1434 that are separated from each other by a gate oxide layer 1330 that is adjacent to a second dielectric layer 1226. The second dielectric layer 1226 is formed after the first dielectric layer 512 and separates the first dielectric layer 512 from the gate oxide layer 1330.
Significantly, the boundary 1224 of the gate oxide layer 1330 and the second dielectric layer 1226 is curved. The boundary 1224 traverses the width of the gate trench 308. More specifically, the boundary is concave in shape relative to the underlying dielectric layer 1226 (and therefore convex in shape relative to the gate oxide layer 1330).
The absence of a sharp corner where the second dielectric layer 1226 and the gate oxide layer 1330 meet the sidewalls of the trench 308 is avoided, thus addressing the gate oxide retardation problem associated with a conventional approach. Also, relative to a conventional approach, there is a less overlap between gate-to-source and gate-to-drain, which reduces Ciss.
In summary, embodiments of power MOSFET devices, and embodiments of methods for fabricating such devices, are described. The foregoing descriptions of specific embodiments of the present invention have been presented for purposes of illustration and description. They are not intended to be exhaustive or to limit the invention to the precise forms disclosed, and many modifications and variations are possible in light of the above teaching. The embodiments were chosen and described in order to best explain the principles of the invention and its practical application, to thereby enable others skilled in the art to best utilize the invention and various embodiments with various modifications as are suited to the particular use contemplated. It is intended that the scope of the invention be defined by the claims appended hereto and their equivalents.

## Claims

1. A method of fabricating a split gate in a semiconductor device, said method comprising:
forming a trench gate within a trench-like cavity in said semiconductor device, wherein said forming said trench gate comprises:
forming a first layer comprising a thermal oxide layer and a first dielectric region along sidewalls of said trench-like cavity, wherein said thermal oxide layer is along the bottom surface and surfaces of said sidewalls of said trench-like cavity, and wherein said first dielectric region is over said thermal oxide layer;
forming a first gate electrode region within said cavity and adjacent said first dielectric region;
removing a portion of said first layer from said sidewalls so that the height of said first layer is less than the height of said first gate electrode region;
forming a second dielectric region within said cavity and adjacent said first dielectric layer and said first gate electrode region, said second dielectric region having a uniform composition throughout;
etching back said second dielectric region to form a concave surface that traverses the entire width of said trench-like cavity, said concave surface highest where it meets said sidewalls of said trench-like cavity;
forming a gate oxide layer adjacent said second dielectric layer on said concave surface, wherein the boundary of said gate oxide layer and said second dielectric region is thereby concave; and
forming a second gate electrode region within said cavity and adjacent said gate oxide layer.

2. The method of claim 1 wherein said a gate oxide layer is also formed along said sidewalls prior to forming said second gate electrode region.

3. The method of claim 1 further comprising forming source and drain regions.

4. The method of claim 1 wherein said semiconductor device comprises a power metal oxide semiconductor field effect transistor (MOSFET) device.

5. A method of fabricating a split gate in a semiconductor device, said method comprising:
producing a thermal oxide layer along the bottom surface and sidewall surfaces of a trench;
forming a first dielectric region over the thermal oxide layer to form a first layer comprises said thermal oxide layer and said first dielectric region;
forming a first gate electrode region over said first dielectric region;
removing portions of said first layer from said sidewall surfaces so that the height of said first layer is less than the height of said first gate electrode region;
forming a second dielectric region in said trench by depositing a dielectric layer over said first layer and said first gate electrode region;
etching back said second dielectric region to form a concave surface that extends across the entire width of said trench and that meets said sidewall surfaces of said trench, wherein said concave surface is highest relative to said bottom surface of said trench where it meets said sidewalls of said trench;
forming a gate oxide layer adjacent said second dielectric region on said concave surface, wherein the boundary of said gate oxide layer and said second dielectric region is thereby concave; and
forming a second gate electrode region within said trench.

6. The method of claim 5 wherein said gate oxide layer is also formed along said sidewall surfaces prior to said step of forming said second gate electrode region.

7. The method of claim 5 further comprising forming source and drain regions.

8. The method of claim 5 wherein said semiconductor device comprises a power metal oxide semiconductor field effect transistor (MOSFET) device.

9. The method of claim 5 wherein a same material is present throughout said second dielectric region.