# Patent

## Title
Semiconductor transistors having high-K gate dielectric layers and metal gate electrodes

## Abstract

A semiconductor structure and a method for forming the same. The semiconductor structure includes (i) a semiconductor substrate which includes a channel region, (ii) first and second source/drain regions on the semiconductor substrate, (iii) a final gate dielectric region, (iv) a final gate electrode region, and (v) a first gate dielectric corner region. The final gate dielectric region (i) includes a first dielectric material, and (ii) is disposed between and in direct physical contact with the channel region and the final gate electrode region. The first gate dielectric corner region (i) includes a second dielectric material that is different from the first dielectric material, (ii) is disposed between and in direct physical contact with the first source/drain region and the final gate dielectric region, (iii) is not in direct physical contact with the final gate electrode region, and (iv) overlaps the final gate electrode region in a reference direction.

## Background
FIELD OF THE INVENTION
The present invention relates generally to semiconductor transistors and more particularly to semiconductor transistors having high-K gate dielectric layers and metal gate electrodes.
BACKGROUND OF THE INVENTION
A typical semiconductor transistor having high-K gate dielectric layer and metal gate electrode usually has poor gate dielectric quality at bottom corners of the gate electrode. Therefore, there is a need for a structure (and a method for forming the same) in which the gate dielectric quality at bottom corners of the gate electrode has a higher quality than that of the prior art.
SUMMARY OF THE INVENTION
The present invention provides a semiconductor structure, comprising a semiconductor substrate which includes a channel region; a first source/drain region on the semiconductor substrate; a second source/drain region on the semiconductor substrate, wherein the channel region is disposed between the first and second source/drain regions; a final gate dielectric region, wherein the final gate dielectric region comprises a first dielectric material, wherein the final gate dielectric region is in direct physical contact with the channel region via an interfacing surface, and wherein the interfacing surface defines a reference direction perpendicular to the interfacing surface and pointing from the final gate dielectric region toward the channel region; a final gate electrode region, wherein the final gate dielectric region is disposed between and in direct physical contact with the channel region and the final gate electrode region, and wherein the final gate electrode region comprises an electrically conductive material; and a first gate dielectric corner region, wherein the first gate dielectric corner region comprises a second dielectric material that is different from the first dielectric material, wherein the first gate dielectric corner region is disposed between and in direct physical contact with the first source/drain region and the final gate dielectric region, wherein the first gate dielectric corner region is not in direct physical contact with the final gate electrode region, and wherein the first gate dielectric corner region overlaps the final gate electrode region in the reference direction.
The present invention provides a structure (and a method for forming the same) in which the gate dielectric quality at bottom corners of the gate electrode has a higher quality than that of the prior art.



BRIEF DESCRIPTION OF THE DRAWINGS
FIGS. 1A-1M show cross-section views used to illustrate a fabrication process of a semiconductor structure, in accordance with embodiments of the present invention.
FIGS. 2A-2L show cross-section views used to illustrate a fabrication process of another semiconductor structure, in accordance with embodiments of the present invention.



DETAILED DESCRIPTION OF THE INVENTION
FIGS. 1A-1M show cross-section views used to illustrate a fabrication process of a semiconductor structure 100, in accordance with embodiments of the present invention. More specifically, with reference to FIG. 1A, the fabrication process of the semiconductor structure 100 can start with a silicon substrate 110.
Next, in one embodiment, a temporary gate dielectric layer 112 is formed on top of the silicon substrate 110. The temporary gate dielectric layer 112 can comprise silicon dioxide. If silicon dioxide is used, the temporary gate dielectric layer 112 can be formed by thermally oxidizing the top surface 118 of the silicon substrate 110 resulting in the temporary gate dielectric layer 112.
Next, in one embodiment, a temporary gate electrode layer 120 is formed on top of the temporary gate dielectric layer 112. The temporary gate electrode layer 120 can comprise poly-silicon. The temporary gate electrode layer 120 can be formed by CVD (Chemical Vapor Deposition) of poly-silicon on top of the temporary gate dielectric layer 112 resulting in the temporary gate electrode layer 120.
Next, in one embodiment, a cap layer 125 is formed on top of the temporary gate electrode layer 120. The cap layer 125 can comprise silicon dioxide. The cap layer 125 can be formed by CVD of silicon dioxide on top of the temporary gate electrode layer 120 resulting in the cap layer 125.
Next, in one embodiment, the cap layer 125 and the temporary gate electrode layer 120 are patterned resulting in the cap region 125 and the temporary gate electrode region 120 of FIG. 1B. More specifically, the cap layer 125 and the temporary gate electrode layer 120 can be patterned by anisotropically and selectively etching in a direction defined by an arrow 115 (hereafter can be referred to as the direction 115) resulting the cap region 125 and the temporary gate electrode region 120 of FIG. 1B. The direction 115 is perpendicular to the top surface 118 of the silicon substrate 110 and points from the temporary gate dielectric layer 112 toward the silicon substrate 110.
Next, with reference to FIG. 1B, in one embodiment, a thermal oxidization of the exposed surfaces of the structure 100 of FIG. 1B is performed resulting in dielectric regions 130a and 130b of FIG. 1C on side walls of the temporary gate electrode region 120. Also as a result of this thermal oxidization step, most of the portions of the temporary gate dielectric layer 112 of FIG. 1B increase in thickness in the direction 115 resulting in the temporary gate dielectric layer 112′. More specifically, the closer to the surrounding ambient a portion of the temporary gate dielectric layer 112 of FIG. 1B is, the thicker in the direction 115 this portion is. For instance, with reference to FIG. 1C, for the portions of the temporary gate dielectric layer 112′ sandwiched between the temporary gate electrode region 120 and the silicon substrate 110, the closer to the center point C a portion is, the thinner in the direction 115 this portion is.
The temporary gate dielectric layer 112′ comprises bird's beaks 112a and 112b at bottom corners of the temporary gate electrode region 120. The dielectric regions 130a and 130b and the temporary gate dielectric layer 112′ can comprise silicon dioxide.
Next, with reference to FIG. 1D, in one embodiment, extension regions 114a and 114b are formed in the silicon substrate 110. The extension regions 114a and 114b can be formed using a conventional ion implantation process.
Next, with reference to FIG. 1E, in one embodiment, a spacer layer 140 is formed on top of the structure 100 of FIG. 1D. The spacer layer 140 can comprise silicon nitride. The spacer layer 140 can be formed by CVD of silicon nitride on top of the structure 100 of FIG. 1D resulting in the spacer layer 140.
Next, in one embodiment, the spacer layer 140 and the temporary gate dielectric layer 112′ are anisotropically etched in the direction 115 until the top surface 118 of the silicon substrate 110 is exposed to the surrounding ambient resulting in the structure 100 of FIG. 1F. After the etching of the spacer layer 140 and the temporary gate dielectric layer 112 is performed, with reference to FIG. 1F, what remain of the spacer layer 140 are spacer regions 140a and 140b, whereas what remains of the temporary gate dielectric layer 112 is the temporary gate dielectric region 112′′ which includes the bird's beaks 112a and 112b. 
Next, with reference to FIG. 1F, in one embodiment, source/drain regions 116a and 116b are formed in the silicon substrate 110. The source/drain regions 116a and 116b can be formed using a conventional ion implantation process.
Next, with reference to FIG. 1G, in one embodiment, silicide regions 150a and 150b are formed on the source/drain regions 116a and 116b, respectively. More specifically, the silicide regions 150a and 150b can be formed by (i) depositing a metal layer (not shown) on top of the structure 100 of FIG. 1F, then (ii) heating the structure 100 resulting in the metal chemically reacting with silicon of the source/drain regions 116a and 116b, and then (iii) removing unreacted metal resulting in the silicide regions 150a and 150b. If the metal used is nickel, then the silicide regions 150a and 150b comprise nickel silicide.
Next, with reference to FIG. 1H, in one embodiment, a silicon nitride layer 160 and a BPSG (boro-phospho-silicate glass) layer 170 are formed in turn on top of the structure 100 of FIG. 1G. More specifically, the silicon nitride layer 160 and the BPSG layer 170 can be formed by (i) depositing silicon nitride on top of the structure 100 of FIG. 1G resulting in the silicon nitride layer 160 and then (ii) depositing BPSG on top of the silicon nitride layer 160 resulting in the BPSG layer 170.
Next, in one embodiment, a CMP (Chemical Mechanical Polishing) process is performed on top of the structure 100 of FIG. 1H until the top surface 122 of the temporary gate electrode region 120 is exposed to the surrounding ambient resulting in the structure 100 of FIG. 1I. After the CMP process is performed, what remain of the BPSG layer 170 are BPSG regions 170a and 170b, and what remain of the silicon nitride layer 160 are silicon nitride regions 160a and 160b. 
Next, with reference to FIG. 1I, in one embodiment, the temporary gate electrode region 120 is removed resulting in a trench 124 of FIG. 1J. The temporary gate electrode region 120 can be removed using a wet etching process.
Next, with reference to FIG. 1J, silicon dioxide on side walls and bottom walls of the trench 124 is removed (by using a wet etching process, for example) resulting in the top surface 118 of the silicon substrate 110 being exposed to the surrounding ambient, as shown in FIG. 1K. After the removal, what remain of the temporary gate dielectric region 112′′ are the bird's beaks 112a and 112b. 
Next, with reference to FIG. 1L, in one embodiment, a final gate dielectric layer 180 and a final gate electrode layer 190 are formed in turn on top of the structure 100 of FIG. 1K. The final gate dielectric layer 180 can comprise a high-K dielectric material, wherein K is dielectric constant and K is greater than 4. For example, the final gate dielectric layer 180 comprises hafnium silicon oxynitride (HfSiON). The final gate electrode layer 190 can comprise a metal such as tantalum nitride (TaN). The final gate dielectric layer 180 and the final gate electrode layer 190 can be formed by (i) CVD or ALD (Atomic Layer Deposition) of the hafnium silicon oxynitride on top of the structure 100 of FIG. 1K resulting in the final gate dielectric layer 180 and then (ii) CVD or ALD of tantalum nitride on top of the final gate dielectric layer 180 such that the trench 124 is completely filed with tantalum nitride resulting in the final gate electrode layer 190.
Next, in one embodiment, a CMP process is performed on top of the structure 100 of FIG. 1L until the top surface 170′ of the BPSG regions 170a and 170b is exposed to the surrounding ambient resulting in the structure 100 of FIG. 1M. After the CMP process is performed, what remain of the final gate dielectric layer 180 and the final gate electrode layer 190 are the final gate dielectric region 180 and the final gate electrode region 190, respectively. In one embodiment, each of the bird's beaks 112a and 112b overlaps the final gate electrode region 190 in the direction 115. A first region is said to overlap a second region in a reference direction if and only if there exits at least one point inside the first region such that a straight line going through that point and being parallel to the reference direction would intersect the second region.
Next, in one embodiment, interconnect layers (not shown) are formed on top of the structure 100 to provide electrical access to the source/drain regions 116a and 116b and the final gate electrode region 190.
With reference to FIG. 1M, the structure 100 shows a transistor having the final gate electrode region 190, the final gate dielectric region 180, the source/drain regions 116a and 116b and the channel 119. The presence of the bird's beaks 112a and 112b at corners of the final gate electrode region 190 increases the distances between the final gate electrode region 190 and the source/drain regions 116a and 116b and thereby helps reduce leakage currents between the final gate electrode region 190 and the source/drain regions 116a and 116b during the operation of the transistor.
FIGS. 2A-2L show cross-section views used to illustrate a fabrication process of a semiconductor structure 200, in accordance with embodiments of the present invention. More specifically, with reference to FIG. 2A, the fabrication process of the semiconductor structure 200 can start with the structure 200 of FIG. 2A. The structure 200 is similar to the structure 100 of FIG. 1B. The formation of the structure 200 is similar to the formation of FIG. 1B.
Next, in one embodiment, the structure 200 is annealed in ammonia (NH3) or ammonia plasma resulting in silicon dioxide of the cap region 225 and exposed portions of the temporary gate dielectric layer 112 being converted to SiON, as shown in FIG. 2B. More specifically, with reference to FIG. 2B, dielectric regions 230a and 230b of the temporary gate dielectric layer 112 now comprise SiON, whereas the temporary gate dielectric region 212 still comprises silicon dioxide. The cap region 225 now comprises SiON. In one embodiment, the annealing of the structure 200 is performed such that the dielectric regions 230a and 230b undercut the temporary gate electrode region 120. As a result, both the dielectric regions 230a and 230b overlap the temporary gate electrode region 120 in the direction 115.
Next, with reference to FIG. 2C, in one embodiment, extension regions 114a and 114b are formed in the silicon substrate 110. The extension regions 114a and 114b can be formed using a conventional ion implantation process.
Next, with reference to FIG. 2D, in one embodiment, a spacer layer 240 is formed on top of the structure 200 of FIG. 2C. The spacer layer 240 can comprise silicon nitride. The spacer layer 240 can be formed by CVD of silicon nitride on top of the structure 200 of FIG. 2C resulting in the spacer layer 240.
Next, in one embodiment, the spacer layer 240 and the dielectric regions 230a and 230b are anisotropically etched in the direction 115 until the top surface 118 of the silicon substrate 110 is exposed to the surrounding ambient resulting in the structure 100 of FIG. 2E. After the etching of the spacer layer 240 and the dielectric regions 230a and 230b is performed, with reference to FIG. 2E, what remain of the spacer layer 240 are spacer regions 240a and 240b, whereas what remain of the dielectric regions 230a and 230b are gate dielectric corner regions 230a and 230b. 
Next, with reference to FIG. 2F, in one embodiment, source/drain regions 116a and 116b are formed in the silicon substrate 110. The source/drain regions 116a and 116b can be formed using a conventional ion implantation process.
Next, in one embodiment, silicide regions 250a and 250b are formed on the source/drain regions 116a and 116b, respectively. More specifically, the silicide regions 250a and 250b can be formed in a manner similar to the manner in which the silicide regions 150a and 150b are formed on the source/drain regions 116a and 116b of the structure 100 of FIG. 1G.
Next, with reference to FIG. 2G, in one embodiment, a silicon nitride layer 260 and a BPSG layer 270 are formed in turn on top of the structure 200 of FIG. 2F. More specifically, the silicon nitride layer 260 and the BPSG layer 270 can be formed by (i) depositing silicon nitride on top of the structure 200 of FIG. 2F resulting in the silicon nitride layer 260 and then (ii) depositing BPSG on top of the silicon nitride layer 260 resulting in the BPSG layer 270.
Next, in one embodiment, a CMP process is performed on top of the structure 200 of FIG. 2G until the top surface 122 of the temporary gate electrode region 120 is exposed to the surrounding ambient resulting in the structure 200 of FIG. 2H. After the CMP process is performed, what remain of the BPSG layer 270 are BPSG regions 270a and 270b, and what remain of the silicon nitride layer 260 are silicon nitride regions 260a and 260b. 
Next, with reference to FIG. 2H, in one embodiment, the temporary gate electrode region 120 is removed resulting in a trench 224 of FIG. 2I. The temporary gate electrode region 120 can be removed using a wet etching process.
Next, with reference to FIG. 2I, in one embodiment, the temporary gate dielectric region 212 is removed resulting in the top surface 118 of the silicon substrate 110 being exposed to the surrounding ambient, as shown in FIG. 2J. The temporary gate dielectric region 212 can be removed by a conventional wet etching process.
Next, with reference to FIG. 2K, in one embodiment, a final gate dielectric layer 280 and a final gate electrode layer 290 are formed in turn on top of the structure 200 of FIG. 2J. The final gate dielectric layer 280 can comprise a high-K dielectric material. For example, the final gate dielectric layer 280 comprises hafnium silicon oxynitride (HfSiON). The final gate electrode layer 290 can comprise a metal such as tantalum nitride (TaN). The final gate dielectric layer 280 and the final gate electrode layer 290 can be formed by (i) CVD or ALD (Atomic Layer Deposition) of the hafnium silicon oxynitride on top of the structure 200 of FIG. 2K resulting in the final gate dielectric layer 280 and then (ii) CVD or ALD of tantalum nitride on top of the final gate dielectric layer 280 such that the trench 224 is completely filed with tantalum nitride resulting in the final gate electrode layer 290.
Next, in one embodiment, a CMP process is performed on top of the structure 200 of FIG. 2K until the top surface 270′ of the BPSG regions 270a and 270b is exposed to the surrounding ambient resulting in the structure 200 of FIG. 2L. After the CMP process is performed, what remain of the final gate dielectric layer 280 and the final gate electrode layer 290 are the final gate dielectric region 280 and the final gate electrode region 290, respectively. In one embodiment, each of the gate dielectric corner regions 230a and 230b overlaps the final gate electrode region 290 in the direction 115.
Next, in one embodiment, interconnect layers (not shown) are formed on top of the structure 200 to provide electrical access to the source/drain regions 116a and 116b and the final gate electrode region 290.
With reference to FIG. 2L, the structure 200 shows a transistor having the final gate electrode region 290, the final gate dielectric region 280, the source/drain regions 116a and 116b and the channel 119. The presence of the gate dielectric corner regions 230a and 230b at corners of the final gate electrode region 290 increases the distances between the final gate electrode region 290 and the source/drain regions 116a and 116b and thereby helps reduce leakage currents between the final gate electrode region 290 and the source/drain regions 116a and 116b during the operation of the transistor.
While particular embodiments of the present invention have been described herein for purposes of illustration, many modifications and changes will become apparent to those skilled in the art. Accordingly, the appended claims are intended to encompass all such modifications and changes as fall within the true spirit and scope of this invention.

## Claims

1. A semiconductor structure fabrication method, comprising:
providing a semiconductor structure which includes (i) a semiconductor substrate which includes a channel region, (ii) a first source/drain region on the semiconductor substrate, (iii) a second source/drain region on the semiconductor substrate, wherein the channel region is disposed between the first and second source/drain regions, and (iv) a first gate dielectric corner region, wherein the first gate dielectric corner region is in direct physical contact with the channel region;
forming a final gate dielectric region on the semiconductor substrate, wherein the final gate dielectric region comprises a first dielectric material, wherein the final gate dielectric region is in direct physical contact with the channel region via an interfacing surface, and wherein the interfacing surface defines a reference direction perpendicular to the interfacing surface and pointing from the final gate dielectric region toward the channel region; and
forming a final gate electrode region on the final gate dielectric region, wherein the final gate electrode region comprises an electrically conductive material, wherein the final gate dielectric region is disposed between and in direct physical contact with the channel region and the final gate electrode region, wherein the first gate dielectric corner region comprises a second dielectric material that is different from the first dielectric material, wherein the first gate dielectric corner region is disposed between and in direct physical contact with the first source/drain region and the final gate dielectric region, wherein the first gate dielectric corner region is not in direct physical contact with the final gate electrode region, and wherein the first gate dielectric corner region overlaps the final gate electrode region in the reference direction,
wherein said providing the semiconductor structure comprises:
providing the semiconductor substrate,
forming a first temporary gate dielectric layer on top of the semiconductor substrate,
forming a temporary gate electrode region on top of the first temporary gate dielectric layer, and
after said forming the first temporary gate dielectric layer and said forming the temporary gate electrode region are performed, increasing thicknesses of portions of the first temporary gate dielectric layer resulting in a second temporary gate dielectric layer, wherein the second temporary gate dielectric layer comprises the first gate dielectric corner region.

2. The method of claim 1,
wherein the semiconductor structure further includes a second gate dielectric corner region,
wherein the second gate dielectric corner region comprises the second dielectric material,
wherein the second gate dielectric corner region is disposed between and in direct physical contact with the second source/drain region and the final gate dielectric region,
wherein the second gate dielectric corner region is not in direct physical contact with the final gate electrode region, and
wherein the second gate dielectric corner region overlaps the final gate electrode region in the reference direction.

3. The method of claim 2, wherein the second dielectric material comprises silicon dioxide.

4. The method of claim 1, wherein said providing the semiconductor structure further comprises, after said increasing the thicknesses of the portions of the first temporary gate dielectric layer is performed, removing the temporary gate electrode region.

5. The method of claim 4, wherein said providing the semiconductor structure further comprises, after said removing the temporary gate electrode region is performed, removing portions of the second temporary gate dielectric layer.

6. The method of claim 1, wherein the first and second temporary gate dielectric layers comprise silicon dioxide.