# Patent

## Title
Method for making a semiconductor device having a high-K gate dielectric and a titanium carbide gate electrode

## Abstract

A method for making a titanium carbide layer is described. That method comprises alternately introducing a carbon containing precursor and a titanium containing precursor into a chemical vapor deposition reactor, while a substrate is maintained at a selected temperature. The reactor is operated for a sufficient time, and pulse times are selected for the carbon containing precursor and the titanium containing precursor, to form a titanium carbide layer of a desired thickness and workfunction on the substrate.

## Background
This is a Divisional Application of Ser. No. 11/006,074 filed Dec. 7, 2004, now U.S. Pat. No. 7,064,066.


FIELD OF THE INVENTION
The present invention relates to methods for making semiconductor devices, in particular, those with titanium carbide containing gate electrodes or barrier layers.
BACKGROUND OF THE INVENTION
An MOS field-effect transistor may include a high-k gate dielectric and a metal gate electrode. The metal gate electrode may comprise a titanium carbide layer, which may be formed on the high-k gate dielectric using an atomic layer chemical vapor deposition (“ALCVD”) process. Although an ALCVD process may be used to deposit such a layer on such a dielectric, it may be difficult to generate a titanium carbide layer with the desired thickness and workfunction using such a process.
Accordingly, there is a need for an improved process for making a semiconductor device that includes a titanium carbide containing gate electrode or barrier layer. There is a need for an ALCVD process that may be tailored to produce a titanium carbide layer with the desired thickness and workfunction. The method of the present invention provides such a process.



BRIEF DESCRIPTION OF THE DRAWINGS
FIGS. 1a-1b represent cross-sections of structures that may be formed when carrying out an embodiment of the method of the present invention.
FIGS. 2a-2i represent cross-sections of structures that may be formed when carrying out an embodiment of the method of the present invention to make a semiconductor device using a replacement metal gate process.



Features shown in these figures are not intended to be drawn to scale.
DETAILED DESCRIPTION OF THE PRESENT INVENTION
A method for making a titanium carbide layer is described. That method comprises alternately introducing a carbon containing precursor and a titanium containing precursor at selected pulse times into a chemical vapor deposition reactor, while maintaining a substrate at a selected temperature. The reactor is operated for a sufficient time, and pulse times are selected for the carbon containing precursor and the titanium containing precursor, to form a titanium carbide layer of a desired thickness and workfunction on the substrate.
In the following description, a number of details are set forth to provide a thorough understanding of the present invention. It will be apparent to those skilled in the art, however, that the invention may be practiced in many ways other than those expressly described here. The invention is thus not limited by the specific details disclosed below.
FIGS. 1a-1b represent cross-sections of structures that may be formed when carrying out an embodiment of the method of the present invention. FIG. 1a represents substrate 100 upon which is formed high-k gate dielectric layer 101 and titanium carbide layer 102. Substrate 100 may comprise any material that may serve as a foundation upon which a semiconductor device may be built. Substrate 100 may, for example, comprise silicon and/or germanium.
High-k gate dielectric layer 101 may comprise, for example, hafnium oxide, hafnium silicon oxide, lanthanum oxide, lanthanum aluminum oxide, zirconium oxide, zirconium silicon oxide, titanium oxide, tantalum oxide, barium strontium titanium oxide, barium titanium oxide, strontium titanium oxide, yttrium oxide, aluminum oxide, lead scandium tantalum oxide, and lead zinc niobate. Particularly preferred are hafnium oxide, lanthanum oxide, zirconium oxide, and aluminum oxide. Although a few examples of materials that may be used to form high-k gate dielectric layer 101 are described here, that layer may be made from other materials that serve to reduce gate leakage.
High-k gate dielectric layer 101 may be formed on substrate 100 using a conventional ALCVD process. In such a process, a metal oxide precursor (e.g., a metal chloride) and steam may be alternately fed at selected flow rates into a CVD reactor, which is operated at a selected pressure while substrate 100 is maintained at a selected temperature. The CVD reactor should be operated long enough to form a layer with the desired thickness. In most applications, dielectric layer 101 should be less than about 40 angstroms thick, and more preferably between about 5 angstroms and about 20 angstroms thick.
In the method of the present invention, titanium carbide layer 102 is formed on high-k gate dielectric layer 101 by alternately introducing a carbon containing precursor and a titanium containing precursor into a CVD reactor, while substrate 100 is maintained at a selected temperature. Pulse times should be selected for the carbon containing precursor and the titanium containing precursor, and the reactor should be operated for a sufficient time, to generate a titanium carbide layer of a desired thickness and workfunction.
The carbon containing precursor may comprise a metal alkyl or metalloid alkyl complex, such as trimethylaluminum (“TMA”) or triethylboron. The titanium containing precursor may comprise a titanium halide, such as titanium tetrachloride (“TiCl4”). The pulse time for the carbon containing precursor for a given growth cycle, when introduced into the CVD reactor, may be less than about 1 second, on the order of 10 to 20 seconds, or somewhere in between—depending upon whether the desired titanium carbide layer will be a p-type, n-type, or mid-gap film.
The pulse time for the titanium containing precursor should be sufficient to provide enough titanium to generate the desired titanium carbide layer. In many applications, a pulse time between about 2 and 3 seconds should be adequate. The substrate temperature preferably should be maintained at between about 100° C. and about 700° C. The optimum substrate temperature, like the optimum pulse time for the carbon containing precursor, may depend upon whether the desired titanium carbide layer will be a p-type, n-type, or mid-gap film.
The method of the present invention may be tailored to produce titanium carbide layers with different thicknesses and workfunctions. For example, to form a p-type titanium carbide layer, the temperature of substrate 100 preferably should be maintained between about 100° C. and about 250° C. To form a p-type titanium carbide layer using a TMA precursor, that precursor preferably should be introduced into the reactor for a relatively short pulse time, e.g., a pulse time that is between about 0.5 seconds and about 1 second. As mentioned above, the titanium containing precursor (e.g., titanium tetrachloride) may have a pulse time that is between about 2 and 3 seconds.
When titanium carbide layer 102 will set the workfunction for a PMOS gate electrode, titanium carbide layer 102 must be sufficiently thick to set the workfunction for the gate electrode. To form a sufficiently thick titanium carbide layer, a minimum number of growth cycles (each growth cycle comprising a TMA pulse followed by a purging gas pulse, and a TiCl4 pulse followed by a purging gas pulse) must be performed. After completing between about 20 and about 40 growth cycles using the operating conditions described above, a p-type titanium carbide layer that is between about 10 angstroms and about 20 angstroms thick, and that sets a workfunction that is between about 4.9 eV and about 5.2 eV, may result.
If, alternatively, titanium carbide layer 102 should be an n-type layer, the temperature of substrate 100 preferably should be maintained above about 400° C. (e.g., between about 400° C. and about 700° C.) when forming that layer. To form an n-type titanium carbide layer using a TMA precursor, that precursor preferably should be introduced into the reactor for a relatively long pulse time, e.g., a pulse time that is between about 10 seconds and about 20 seconds. In some embodiments, a single pulse of TMA per growth cycle may be preferred; whereas, in other embodiments, multiple pulses of TMA per growth cycle (e.g., four separate 5 second pulses per growth cycle instead of a single 20 second pulse per growth cycle) may be preferred.
When forming an n-type titanium carbide layer using the method of the present invention, as when forming a p-type titanium carbide layer, the titanium containing precursor (e.g., titanium tetrachloride) may have a pulse time that is between about 2 and 3 seconds. One may form an n-type titanium carbide layer that is between about 50 angstroms and about 250 angstroms thick by completing between about 70 and about 250 growth cycles using these operating conditions. An n-type titanium carbide layer with that thickness may set a workfunction for an NMOS gate electrode that is between about 3.9 eV and about 4.3 eV.
When forming a titanium carbide layer with a mid-gap workfunction using the method of the present invention, the temperature of substrate 100 preferably should be maintained between about 250° C. and about 400° C. To make such a mid-gap titanium carbide layer, the pulse time for the carbon containing precursor preferably lies between the pulse times that may be applied when making p-type or n-type titanium carbide layers. Similarly, the number of growth cycles that may be required to form a mid-gap titanium carbide layer of the desired thickness may lie between the number of growth cycles required to make p-type or n-type titanium carbide layers.
FIG. 1b represents a semiconductor device that includes NMOS gate electrode 115 and PMOS gate electrode 120. NMOS gate electrode 115 comprises n-type titanium carbide layer 105. PMOS gate electrode 120 comprises p-type titanium carbide layer 110. N-type titanium carbide layer 105 and p-type titanium carbide layer 110 are each formed on high-k gate dielectric layer 101. Metal layer 121 is formed on n-type titanium carbide layer 105 and metal layer 118 is formed on p-type titanium carbide layer 110. Metal layers 121 and 118 may comprise, for example, titanium nitride. A replacement metal gate process, which may be used to form a structure like the one that FIG. 1b illustrates, is described in detail below.
Although the method of the present invention may be used to form a metal gate electrode that includes a titanium carbide layer, the method may alternatively be used to form titanium carbide containing barrier layers or liners. Such barrier layers or liners may serve to insulate one film from another to prevent them from reacting. When forming a titanium carbide barrier layer using the method of the present invention, it may be desirable to use triethylboron instead of TMA to generate a titanium carbide film with less impurity. Such a titanium carbide barrier layer may be relatively thin, e.g., between about 10 angstroms and about 40 angstroms thick.
FIGS. 2a-2i illustrate how the method of the present invention may be applied to make a semiconductor device using a replacement metal gate process. FIG. 2a represents an intermediate structure that may be formed when making a CMOS device. That structure includes first part 201 and second part 202 of substrate 200. Isolation region 203 separates first part 201 from second part 202. Polysilicon layers 204 and 206 are formed on dielectric layers 205 and 207, respectively. Polysilicon layer 204 is bracketed by sidewall spacers 208 and 209, and polysilicon layer 206 is bracketed by sidewall spacers 210 and 211. Dielectric 212 separates layers 204 and 206.
Substrate 200 may comprise any material that may serve as a foundation upon which a semiconductor device may be built. Isolation region 203 may comprise silicon dioxide, or other materials that may separate the transistor's active regions. Dielectric layers 205 and 207 may each comprise silicon dioxide, or other materials that may insulate the substrate from other substances. Polysilicon layers 204 and 206 preferably are each between about 100 and about 2,000 angstroms thick, and more preferably between about 500 and about 1,600 angstroms thick. In this embodiment, polysilicon layer 204 is doped n-type (e.g., with arsenic, phosphorus or another n-type material), while polysilicon layer 206 is doped p-type (e.g., with boron or another p-type material). Spacers 208, 209, 210, and 211 preferably comprise silicon nitride, while dielectric 212 may comprise silicon dioxide or a low-k material.
Conventional process steps, materials, and equipment may be used to generate the FIG. 2a structure, as will be apparent to those skilled in the art. As shown, dielectric 212 may be polished back, e.g., via a conventional CMP step, to expose polysilicon layers 204 and 206. Although not shown, the FIG. 2a structure may include many other features (e.g., a silicon nitride etch stop layer, source and drain regions, and one or more buffer layers) that may be formed using conventional processes.
After forming the FIG. 2a structure, n-type polysilicon layer 204 is removed. In a preferred embodiment, that layer is removed by applying a wet etch process. Such a wet etch process may comprise exposing layer 204 to an aqueous solution that comprises a source of hydroxide for a sufficient time at a sufficient temperature to remove substantially all of that layer without removing a significant amount of p-type polysilicon layer 206. That source of hydroxide may comprise between about 2 and about 30 percent ammonium hydroxide or a tetraalkyl ammonium hydroxide, e.g., tetramethyl ammonium hydroxide (“TMAH”), by volume in deionized water.
N-type polysilicon layer 204 may be removed by exposing it to a solution, which is maintained at a temperature between about 15° C. and about 90° C. (and preferably below about 40° C.), that comprises between about 2 and about 30 percent ammonium hydroxide by volume in deionized water. During that exposure step, which preferably lasts at least one minute, it may be desirable to apply sonic energy at a frequency of between about 10 KHz and about 2,000 KHz, while dissipating at between about 1 and about 10 watts/cm2. For example, if n-type polysilicon layer 204 is about 1,350 angstroms thick, it may be removed by exposing it at about 25° C. for about 30 minutes to a solution that comprises about 15 percent ammonium hydroxide by volume in deionized water, while applying sonic energy at about 1,000 KHz—dissipating at about 5 watts/cm2.
After removing n-type polysilicon layer 204, dielectric layer 205 is removed. If dielectric layer 205 comprises silicon dioxide, it may be removed using an etch process that is selective for silicon dioxide. Such an etch process may comprise exposing layer 205 to a solution that includes about 1 percent HF in deionized water. The time layer 205 is exposed should be limited, as the etch process for removing that layer may also remove part of dielectric layer 212. With that in mind, if a 1 percent HF based solution is used to remove layer 205, the device preferably should be exposed to that solution for less than about 60 seconds, and more preferably for about 30 seconds or less. As shown in FIG. 2b, removal of dielectric layer 205 forms trench 213 within dielectric layer 212 positioned between sidewall spacers 208 and 209.
After removing dielectric layer 205, high-k gate dielectric layer 215 is formed within trench 213 and on substrate 200. Any of the materials identified above may be used to make high-k gate dielectric layer 215. High-k gate dielectric layer 215 may be formed using a conventional ALCVD process, as described above. High-k gate dielectric layer 215 preferably should be less than about 40 angstroms thick, and more preferably between about 5 angstroms and about 20 angstroms thick. As shown in FIG. 2c, when an ALCVD process is used to form high-k gate dielectric layer 215, that layer will form on the sides of trench 213 in addition to forming on the bottom of that trench, and will form on dielectric layer 212.
In the illustrated embodiment, n-type titanium carbide layer 216 is formed directly on high-k gate dielectric layer 215 to generate the FIG. 2d structure. Like high-k gate dielectric layer 215, part of n-type titanium carbide layer 216 lines trench 213 while part of that layer spills over onto dielectric layer 212. When forming n-type titanium carbide layer 216 on high-k gate dielectric layer 215 using an ALCVD process, the temperature of substrate 200 preferably is maintained above about 400° C. While maintaining substrate 200 at the appropriate temperature, TMA and TiCl4 are alternately pulsed into the reactor. The pulse time for TMA is preferably between about 10 seconds and about 20 seconds. That pulse time may reflect a single pulse of TMA per growth cycle or, alternatively, multiple pulses of TMA per growth cycle.
When forming n-type titanium carbide layer 216, TiCl4 may have a pulse time that is between about 2 and 3 seconds. By continuing this process for between about 70 and about 250 growth cycles, n-type titanium carbide layer 216 may reach a thickness that is between about 50 angstroms and about 250 angstroms. The resulting n-type titanium carbide layer may have a workfunction that is between about 3.9 eV and about 4.3 eV.
In this embodiment, after forming n-type titanium carbide layer 216 on high-k gate dielectric layer 215, fill metal 221 is formed on n-type titanium carbide layer 216. Fill metal 221 fills the remainder of trench 213 and covers dielectric layer 212, as illustrated in FIG. 2e. Fill metal 221 preferably comprises a material that may be easily polished, and preferably is deposited over the entire device using a conventional metal deposition process. Such a fill metal may comprise, for example, titanium nitride, tungsten, titanium, aluminum, tantalum, tantalum nitride, cobalt, copper, or nickel.
In a particularly preferred embodiment, fill metal 221 comprises titanium nitride. Titanium nitride may be deposited using an appropriate CVD or PVD process that does not significantly affect underlying n-type titanium carbide layer 216 or high-k gate dielectric layer 215. In addition, when polysilicon layer 206 is subsequently removed (as described below), titanium nitride may be more resistant than other metals to the etch chemistry used to remove that layer.
After forming the FIG. 2e structure, fill metal 221, n-type titanium carbide layer 216, and high-k gate dielectric layer 215 are removed from above dielectric layer 212 to generate the FIG. 2f structure. An appropriate CMP or etch process may be used to remove those layers from dielectric layer 212. In a preferred embodiment, a combination of CMP and etch processes are used, e.g., a CMP step to remove fill metal 221 followed by an etch step (or steps) to remove n-type titanium carbide layer 216 and high-k gate dielectric layer 215.
After removing fill metal 221, n-type titanium carbide layer 216, and high-k gate dielectric layer 215 from above dielectric layer 212, p-type polysilicon layer 206 is removed. P-type polysilicon layer 206 may be removed selectively to fill metal 221 by exposing layer 206 to a solution that comprises between about 20 and about 30 percent TMAH by volume in deionized water for a sufficient time at a sufficient temperature (e.g., between about 60° C. and about 90° C.), while applying sonic energy.
After removing polysilicon layer 206, dielectric layer 207 is removed, e.g., by using the same process that was used to remove dielectric layer 205. Removing dielectric layer 207 generates trench 214, as FIG. 2g illustrates. Following the removal of that dielectric layer, high-k gate dielectric layer 217 is formed within trench 214 and onto dielectric layer 212. The same process steps and materials used to form high-k gate dielectric layer 215 may be used to form high-k gate dielectric layer 217.
In this embodiment, p-type titanium carbide layer 220 is then deposited on high-k gate dielectric layer 217. Like n-type titanium carbide layer 216, p-type titanium carbide layer 220 may be formed using an ALCVD process. Unlike the process for forming n-type titanium carbide layer 216, however, when forming p-type titanium carbide layer 220, substrate 200 preferably should be maintained at a substantially lower temperature, e.g., between about 100° C. and about 250° C.
In this embodiment, TMA and TiCl4 are alternately pulsed into the reactor while maintaining substrate 200 at the appropriate temperature. The pulse time for TMA is preferably between about 0.5 seconds and about 1 second and the pulse time for TiCl4 may be between about 2 and 3 seconds. By continuing this process for between about 20 and about 40 growth cycles, p-type titanium carbide layer 220 may reach a thickness that is between about 10 angstroms and about 20 angstroms. The resulting p-type titanium carbide layer may have a workfunction that is between about 4.9 eV and about 5.2 eV.
After forming p-type titanium carbide layer 220 on high-k gate dielectric layer 217, fill metal 218 may be formed on p-type titanium carbide layer 220 to generate the FIG. 2h structure. The same process steps and materials used to form fill metal 221 may be used to form fill metal 218. In a preferred embodiment, fill metal 218 comprises titanium nitride. Fill metal 218, p-type titanium carbide layer 220 and high-k gate dielectric layer 217 are then removed from dielectric layer 212 to generate the FIG. 2i structure. The same CMP and/or etch steps used to remove fill metal 221, n-type titanium carbide layer 216 and high-k gate dielectric layer 215 from above dielectric layer 212 may be used to remove fill metal 218, p-type titanium carbide layer 220 and high-k gate dielectric layer 217 from above dielectric layer 212.
After removing fill metal 218, p-type titanium carbide layer 220 and high-k gate dielectric layer 217 from above dielectric layer 212, a capping dielectric layer (not shown) may be deposited onto the resulting structure using a conventional deposition process. Process steps for completing the device that follow the deposition of such a capping dielectric layer, e.g., forming the device's contacts, metal interconnect, and passivation layer, are well known to those skilled in the art and will not be described here. Although a few examples of process steps for forming the FIG. 2i structure are presented here, many other process steps may be used to make that structure, as will be apparent to those skilled in the art.
The method of the present invention may enable one to make titanium carbide layers with different thicknesses and workfunctions, which may be used in various applications. Although the foregoing description has specified certain steps and materials that may be used in the method of the present invention, those skilled in the art will appreciate that many modifications and substitutions may be made. Accordingly, all such modifications, substitutions and additions fall within the spirit and scope of the invention as defined by the appended claims.

## Claims

1. A semiconductor device comprising:
a first gate electrode on a first gate dielectric layer, wherein said first gate electrode is comprised of a first titanium carbide layer, and wherein said first titanium carbide layer has a first workfunction; and
a second gate electrode on a second gate dielectric layer, wherein said second gate electrode is comprised of a second titanium carbide layer, wherein said second titanium carbide layer has a second workfunction, and wherein said first workfunction and said second workfunction are different.

2. The device of claim 1 wherein said first workfunction and said second workfunction are different by at least about 0.6 eV.

3. The device of claim 1 wherein said first workfunction is between about 4.9 eV and about 5.2 eV.

4. The device of claim 3 wherein said first titanium carbide layer is between about 10 angstroms and about 20 angstroms thick.

5. The device of claim 1 wherein said first workfunction is between about 4.3 eV and about 4.9 eV.

6. The device of claim 5 wherein said first titanium carbide layer is between about 20 angstroms and about 50 angstroms thick.

7. The device of claim 1 wherein said first workfunction is between about 3.9 eV and about 4.3 eV.

8. The device of claim 7 wherein said first titanium carbide layer is between about 50 angstroms and about 250 angstroms thick.

9. The device of claim 1 wherein said first titanium carbide layer is between about 10 angstroms and about 20 angstroms thick and has a workfunction between about 4.9 eV and about 5.2 eV, and wherein said second titanium carbide layer is between about 50 angstroms and about 250 angstroms thick and has a workfunction between about 3.9 eV and about 4.3 eV.

10. The device of claim 1 wherein said first titanium carbide layer is between about 10 angstroms and about 20 angstroms thick and has a workfunction between about 4.9 eV and about 5.2 eV, and wherein said second titanium carbide layer is between about 20 angstroms and about 50 angstroms thick and has a workfunction between about 4.3 eV and about 4.9 eV.

11. The device of claim 1 wherein said first titanium carbide layer is between about 20 angstroms and about 50 angstroms thick and has a workfunction between about 4.3 eV and about 4.9 eV, and wherein said second titanium carbide layer is between about 50 angstroms and about 250 angstroms thick and has a workfunction between about 3.9 eV and about 4.3 eV.

12. The device of claim 2 wherein said first gate electrode has a p-type workfunction and said second gate electrode has an n-type workfunction.

13. The device of claim 1 wherein said first and said second gate dielectric layers are high-k gate dielectric layers comprised of a material that is selected from the group consisting of hafnium oxide, hafnium silicon oxide, lanthanum oxide, lanthanum aluminum oxide, zirconium oxide, zirconium silicon oxide, tantalum oxide, titanium oxide, barium strontium titanium oxide, barium titanium oxide, strontium titanium oxide, yttrium oxide, aluminum oxide, lead scandium tantalum oxide, and lead zinc niobate.

14. A semiconductor device comprising:
a first gate electrode on a first high-k gate dielectric layer, wherein said first gate electrode is comprised of a first titanium carbide layer, and wherein said first titanium carbide layer has a workfunction between about 4.9 eV and about 5.2 eV; and
a second gate electrode on a second high-k gate dielectric layer, wherein said second gate electrode is comprised of a second titanium carbide layer, and wherein said second titanium carbide layer has a workfunction between about 3.9 eV and about 4.3 eV.

15. The device of claim 14 wherein said first titanium carbide layer is between about 10 angstroms and about 20 angstroms thick and said second titanium carbide layer is between about 50 angstroms and about 250 angstroms thick.

16. The device of claim 15 wherein said first and said second gate dielectric layers are high-k gate dielectric layers comprised of a material that is selected from the group consisting of hafnium oxide, hafnium silicon oxide, lanthanum oxide, lanthanum aluminum oxide, zirconium oxide, zirconium silicon oxide, tantalum oxide, titanium oxide, barium strontium titanium oxide, barium titanium oxide, strontium titanium oxide, yttrium oxide, aluminum oxide, lead scandium tantalum oxide, and lead zinc niobate.

17. A semiconductor device comprising:
a PMOS gate electrode on a first high-k gate dielectric layer, wherein said PMOS gate electrode is comprised of a first titanium carbide layer, and wherein said first titanium carbide layer has a p-type workfunction; and
an NMOS gate electrode on a second high-k gate dielectric layer, wherein said NMOS gate electrode is comprised of a second titanium carbide layer, and wherein said second titanium carbide layer has an n-type workfunction.

18. The device of claim 17 wherein said first titanium carbide layer has a workfunction between about 4.9 eV and about 5.2 eV and said second titanium carbide layer has a workfunction between about 3.9 eV and about 4.3 eV.

19. The device of claim 18 wherein said first and said second high-k gate dielectric layers are comprised of a material that is selected from the group consisting of hafnium oxide, hafnium silicon oxide, lanthanum oxide, lanthanum aluminum oxide, zirconium oxide, zirconium silicon oxide, tantalum oxide, titanium oxide, barium strontium titanium oxide, barium titanium oxide, strontium titanium oxide, yttrium oxide, aluminum oxide, lead scandium tantalum oxide, and lead zinc niobate.

20. The device of claim 18 wherein said first titanium carbide layer is between about 10 angstroms and about 20 angstroms thick and said second titanium carbide layer is between about 50 angstroms and about 250 angstroms thick.