Transistors having argon gate implants and methods of forming the same

Embodiments disclosed herein pertain to transistors, methods of forming the transistors and integrated circuits including transistors.

Conventional MOS technology employs transistor gates including a metal over a doped polysilicon that is, in turn, over a gate dielectric. With the scaling of transistors to smaller sizes, the gate dielectric has transitioned from conventional silicon dioxide to higher-dielectric constant materials, such as hafnium dioxide (HfO 2 ) and hafnium silicon oxide (HfSiO), to achieve higher oxide capacitance.

The conventional polysilicon gate that was used with silicon dioxide is difficult to use with Hf-based dielectrics due to the interaction at the interface of the gate and gate dielectric and to fermi-level pinning. To overcome these problems, a metal gate, such as a transition metal nitride, is often used. However, a major drawback of using metal gates is the inability to tailor the work function of the metal gates. Consequently, it is difficult to achieve a low threshold voltage with metal gates.

One prior solution was to dope the metal gate with arsenic. Arsenic is a donor atom used for doping silicon. Accordingly, in an nMOS device, for example, an arsenic implant can result in channel counterdoping, which results in the degradation of the transistor's performance.

It is therefore desirable to achieve a metal-gated nMOS transistor having a lower threshold voltage without such disadvantages.

Embodiments of the invention are described below with reference to the following accompanying drawings.

FIG. 1 depicts an integrated circuit including a transistor according to an embodiment described herein.

FIGS. 2A-2C depict the integrated circuit of FIG. 1 at various stages of processing.

In the following detailed description, reference is made to various embodiments that are described with sufficient detail to enable those skilled in the art to practice them. It is to be understood that other embodiments may be employed, and that various structural, logical and electrical changes may be made. Embodiments of the disclosure are now explained with reference to the figures. Throughout the figures, like reference numbers indicate like features.

In the context of the present document, “metal material” refers to a conductive material containing a metal compound or compounds (which compound may further include semimetals and/or nonmetals), an elemental metal, or a metal alloy. Elements considered to be metals in this document do not include semimetals. Accordingly, semimetals B, Si, As, Se, Te, and At are not considered to be metals. In the context of the present document, “work function” refers to the energy needed to pull an electron at its Fermi level into vacuum. “Fermi” level is the energy level at which a 50% probability exists of finding electrons at that energy level.

The term “substrate” used in the following description may include any supporting structure including, but not limited to, a semiconductor substrate that has an exposed substrate surface or a semiconductor substrate having one or more overlying material layers. A semiconductor substrate should be understood to include silicon, silicon-on-insulator (SOI), silicon-on-sapphire (SOS), doped and undoped semiconductors, epitaxial layers of silicon supported by a base semiconductor foundation, and other semiconductor structures, including those made of semiconductors other than silicon. When reference is made to a semiconductor substrate in the following description, previous process steps may have been utilized to form regions, junctions or material layers in or over the base semiconductor or foundation. The substrate also need not be formed of a semiconductor material, but may be any support structure suitable for supporting an integrated circuit, including, but not limited to, metals, alloys, glasses, polymers, ceramics, and any other supportive materials as is known in the art.

FIG. 1 depicts an integrated circuit 111 including a transistor 100 . The transistor 100 is an nMOS transistor on a substrate 101 having a p-type region 170 . Substrate 101 may be semiconductive. The transistor 100 includes n-type source/drain regions 160 , channel region 165 and a gate stack 190 . The gate stack 190 includes a gate dielectric 110 , a high dielectric (“high-k”) gate dielectric 120 , a metal material 130 , a cap material 140 and sidewall spacers 150 .

The gate dielectric 110 is provided between the substrate 101 and the gate dielectric 120 and can be any suitable dielectric material. In one embodiment, the gate dielectric 110 is an oxide, such as silicon oxide. In one embodiment, the gate dielectric 110 is an in-situ steam generated (ISSG) oxide. The thickness of the gate dielectric 110 can be, for example, less than about 5 nm. In one embodiment, the gate dielectric 110 is about 1 nm thick.

The high-k gate dielectric 120 is a material having a dielectric constant higher than that of silicon dioxide, such as, e.g., a high dielectric constant material, such as a Hf-based dielectric (e.g., hafnium dioxide (HfO 2 ) and hafnium silicon oxide (HfSiO)). In one embodiment the high-k gate dielectric 120 is HfSiO, with Hf comprising about 60% of the atomic concentration of the gate dielectric 120 . The thickness of the high-k gate dielectric 120 can be, for example, less than about 5 nm. In one embodiment the high-k gate dielectric 120 is between about 3 nm and about 4 nm thick. In one embodiment the high-k gate dielectric 120 is about 3.2 nm thick.

The metal material 130 is over the high-k gate dielectric 120 and can be, among other metal materials, any of the following: titanium, tantalum, tungsten, titanium nitride (TiN), tantalum nitride, tungsten nitride, aluminum nitride, titanium silicide, tantalum silicide, tungsten silicide, and tantalum silicon nitride. The metal material 130 may be also doped with an inert element, for example a noble gas, such as argon, neon, krypton, or xenon. In one embodiment the metal material 130 is TiN and is doped with Argon. In one embodiment, the thickness of the metal material 130 is between about 5 nm and about 15 nm. In one embodiment, the metal material 130 is about 10 nm.

In the illustrated embodiment, the metal material 130 is doped by an ion implantation process. In one embodiment, the implant dose is between about 1×10 14 atoms/cm 2 and about 1×10 16 atoms/cm 2 . In another embodiment, the implant dose is between about 4×10 15 atoms/cm 2 and about 6×10 15 atoms/cm 2 . In one embodiment the implant energy used is between about 1 keV and about 10 keV. In one embodiment the implant energy used is about 4 keV. In another embodiment the implant energy used is between about 3 keV and about 5 keV. In a further embodiment the implant energy used is about 4 keV.

The cap material 140 is over the metal material 130 and can be any suitable material, such as silicon. The thickness of the cap material 140 can be, for example, less than about 10 nm. In one embodiment the cap material 140 is about 5 nm.

The sidewall spacers 150 are on the lateral sidewalls of the stack of the gate dielectric 110 , high-k gate dielectric 110 , metal material 130 and cap material 140 . The sidewall spacers 150 can be any suitable dielectric material, such as an oxide, e.g., silicon dioxide, silicon nitride, an oxynitride, ON, NO, ONO, or TEOS, among others.

By doping the metal material 130 with an inert element, the threshold voltage of the transistor 100 can be reduced. In addition, since an inert element is used, if the inert element reaches the channel region 165 , it will not affect the active dopant concentration of the channel region 165 . Further, the inert dopant can improve carrier mobility (as compared to no implant), which can improve transistor performance.

NMOS transistors having gate stacks consisting of the following materials, in order from the surface of a silicon substrate were formed: 1 nm ISSG silicon oxide, 3.2 nm HfSiO, 10 nm TiN formed by a physical vapor deposition process, 5 nm silicon. For each transistor, the channel doping was conducted with an implant dose of 2×10 12 atoms/cm 2 . For the following transistors, the following additional dopant implants were conducted to dope the TiN gates:

A: none

B: Arsenic, 4×10 15 atoms/cm 2 dose with an implant energy of 6 keV

C: Arsenic, 6×10 15 atoms/cm 2 dose with an implant energy of 6 keV

D: Argon, 4×10 15 atoms/cm 2 dose with an implant energy of 3 keV

E: Argon, 4×10 15 atoms/cm 2 dose with an implant energy of 4 keV

F: Argon, 4×10 15 atoms/cm 2 dose with an implant energy of 5 keV

G: Argon, 6×10 15 atoms/cm 2 dose with an implant energy of 4 keV

The gate bias to current characteristics were compared and the threshold voltages were extracted using to know methods (1) the constant current method and (2) the maximum slope method. When the threshold voltage was extracted using the constant current method, the threshold voltage for transistors B and C having an arsenic implant had a lower threshold voltage than transistors E and F that have an argon implant. However, when the threshold voltage was extracted using the maximum slope method, the threshold voltages for the transistors B and C having an arsenic implant are similar to the threshold voltages for the transistors E and F having an argon implant.

A threshold voltage reduction of about 250 mV was observed for transistors D, E and F as compared to transistor A. Transistor G, which received a higher argon implant dose, showed less of a threshold voltage reduction.

Carrier mobility for transistors D, E and F was about double that for transistor A. Carrier mobility for transistor G was greater than that of transistor A, but less than that of transistors D, E and F. Data showed that carrier mobility was independent of the implant energy used.

FIGS. 2A-2C show process stages leading to formation of the integrated circuit 111 , having transistor 100 shown in FIG. 1 . While the process stages are shown in a particular order, it should be appreciated that the order can be changed.

In FIG. 2A , a gate dielectric material 110 is formed on substrate 101 that includes a p-type region 170 . A second dielectric material 120 is formed on the gate dielectric material 110 , a metal material 130 is formed over the high-k gate dielectric material 120 and a cap material 140 is formed over the metal material 130 .

Each of the gate dielectrics 110 , 120 , the metal material 130 and the cap material 140 can be formed by any suitable technique. In one embodiment, the gate dielectric 110 is formed by an ISSG process and is silicon dioxide. In one embodiment, the metal material 130 is formed by physical vapor deposition and is TiN.

In FIG. 2B , the gate dielectrics 110 , 120 , the metal material 130 and the cap material 140 are patterned and etched by any suitable technique to form a stack 190 .

As depicted in FIG. 2C , the metal material 130 is doped with an inert gas, as represented by arrows 166 . Alternatively, the metal material 130 can be doped prior to the patterning and etching process shown in FIG. 2B . As another alternative the metal material 130 can be doped immediately after formation of the metal material 130 and prior to the formation of the cap material 140 .

In the illustrated embodiment, the metal material 130 is doped by an ion implantation process. In one embodiment, the implant dose is between about 1×10 14 atoms/cm 2 and about 1×10 16 atoms/cm 2 . In another embodiment, the implant dose is between about 4×10 15 atoms/cm 2 and about 6×10 15 atoms/cm 2 . In one embodiment the implant energy used is between about 1 keV and about 10 keV. In one embodiment the implant energy used is about 4 keV. In another embodiment the implant energy used is between about 3 keV and about 5 keV. In a further embodiment the implant energy used is about 4 keV.

The source/drain regions 160 and sidewall spacers 150 can be formed by suitable techniques to achieve the structure 100 depicted in FIG. 1 . As is known, source/drain regions 160 can be formed by ion implantation. Sidewall spacers 150 can be formed by forming a dielectric material over the stack 190 and substrate 101 and etching the dielectric material leaving portions forming the sidewall spacers 150 . In addition the channel region 165 can be doped, for example by an ion implantation process.

The above description and drawings should only be considered illustrative of example embodiments that achieve the features and advantages described herein. Modification and substitutions to specific process conditions and structures can be made. Accordingly, the claimed invention is not to be considered as being limited by the foregoing description and drawings, but is only limited by the scope of the appended claims.