Silicon nitride layer deposited at low temperature to prevent gate dielectric regrowth high-K metal gate field effect transistors

[0001] The present invention relates generally to semiconductor device and more specifically High-k Metal Gate (HKMG) Replacement Gate Field Effect Transistors (RMG FETs) for use in CMOS technologies.

[0002] High-K dielectric with Metal Gate electrodes (HKMG) has been proven to be one of the most critical innovations that allows further gate thickness scaling, which in turn enables the active channel length scaling and the overall performance boost in advanced CMOS technology. Furthermore, the Replacement Gate (RMG) integration flow for HKMG provides additional advantage in terms of thermal budget control. However, in a typical HKMG RMG flow, gate dielectric, including the high-K gate can become exposed after the metal gate CMP process. This can provides an oxygen ingress path and lead to oxide regrowth at gate channel interface during the subsequent Middle of Line (MOL) dielectric deposition and Back End of the Line (BEOL) processing. The oxide regrowth increases the effective dielectric thickness and shifts the transistor threshold voltage, both of which can cause severe device degradation in terms of performance and variability. Thus, the oxygen ingress and regrowth at high-K gate dielectric to semiconductor channel interface should be avoided.

[0003] In the prior art, after the metal-gate CMP is completed, the next layer, usually silicon nitride, is typically deposited at temperatures high enough (>500° C.) that the exposed high-K gate dielectric material can absorb any oxygen present during the deposition process, and transport the oxygen towards the gate dielectric to channel interface region, where the oxygen causes an unwanted regrowth. The deposition temperature is important because higher temperatures increase the absorption and diffusion of oxygen into the high-K gate dielectric, towards the gate dielectric to channel interface.

[0004] FIGS. 1 to 8 illustrate prior art for High-K Metal Gate (HKMG) Field Effect Transistor (FET), using the Replacement Metal Gate (RMG) integration scheme, also known as Gate-Last integration.

[0005] FIG. 1 shows starting with a semiconductor substrate which can be formed of any semiconductor such as silicon, germanium, silicon-germanium or other compound semiconductors, the subsequent structures are built. Shown, is the formation of a sacrificial gate, which is formed by conventional deposition or growth of a sacrificial gate dielectric, such as SiO₂ or other suitable dielectric, and deposition of a poly-crystalline or amorphous semiconductor, such as silicon or germanium or any compound semiconductor, followed by standard gate lithography and etching to form gate structures. Also shown is a spacer formed by conventional deposition of silicon nitride or other suitable spacer dielectric and standard anisotropic Reactive Ion Etch (RIE) techniques.

[0006] FIG. 2 shows the formation of standard source/drain regions formed by ion implantation of dopants, or embedded source/drain formed by etching a trenched, then refilled by epitaxial deposition of doped semiconductor, such as silicon, silicon-germanium, silicon carbon, or other materials, or a combination of ion implantation and epitaxial source/drain formation. As mentioned previously, the source/drain region can be either n-type for NFET or p-type for PFET devices, which can be achieved by utilizing conventional lithography and patterning techniques.

[0007] FIG. 3 shows the deposition of an interlayer dielectric (ILD) such as silicon oxide or other suitable dielectric, which is then planarized by conventional Chemical Mechanical Polishing (CMP). The sacrificial gate is removed by a selective wet or plasma etch that removes the sacrificial gate material but not the ILD, nor the spacer material. The sacrificial gate dielectric can also be removed at the step if desired, or it can be retained and incorporated into the final structure. For the purposes of the description, the sacrificial gate dielectric is removed.

[0008] FIG. 4 shows the deposition of a gate dielectric stack which can have a single or multiple layers such as an interface oxide or oxynitride layer and high-K dielectric layers such as hafnium oxide, hafnium silicate or other high-K materials. Also shown is the deposition of a workfunction metal material such as TiN on top of the gate dielectric stack. The purpose of the workfunction metal layer is to set the appropriate workfunction to achieve the desired transistor threshold voltage (V_th). The workfunction layer can be a single layer or multiple layers. It should be understood that NFET and PFET transistors may require different workfunction metal layers, which can be achieved by conventional lithography and patterning techniques to place n-type workfunction metal layers in NFET transistor regions and p-type workfunction metal layers in PFET transistor regions.

[0009] FIG. 5 shows an additional metal gate deposition on top of the workfunction metal to completely fill the gate region. The metal gate material can be W, Al or other suitable metal which provides a low resistance gate contact or stacks of different metals. The metal gate material can be different between NFET and PFET transistors.

[0010] FIG. 6 shows the structure after a conventional metal gate CMP process has been performed to planarize the surface to the top of the gate. The CMP process removes the metal gate, workfunction metal and high-K gate dielectric stacks from the top surface of the wafer, leaving those materials only within the gate. Note also, that because of the CMP process, the top periphery of the high-K dielectric is exposed to the ambient environment during subsequent process steps. In particular, if that ambient environment in subsequent process steps contains any oxygen and the temperature is high enough (>500° C.), the exposed top periphery of the high-K dielectric stack will absorb that oxygen and transport it by diffusion to the gate dielectric to semiconductor channel interface, which causes several problems as will be described in a subsequent section.

[0011] FIG. 7 shows the deposition of a middle of line (MOL) dielectric such as silicon nitride or oxide or other suitable dielectric material, which is typically deposited by chemical vapor deposition (CVD) or other conventional techniques at a temperature exceeding 500° C. Because the ambient of the deposition process typically contains some oxygen, the oxygen will be absorbed by the exposed top periphery of the high-K dielectric and transported by diffusion to the gate dielectric to semiconductor channel interface, which causes several problems as will be described in a subsequent section.

[0012] FIG. 8 shows in additional detail the structure in FIG. 7, and the issues created by the standard replacement metal gate processing in prior art. As described previously, after the metal gate CMP process to planarize and remove the metal gate, workfunction metal and high-K gate dielectric materials from the top surface of the wafer, the top periphery of the high-K gate dielectric is exposed to ambient of the next process step. The next process step is a conventional MOL dielectric deposition of either silicon nitride or oxide or other suitable dielectric material which is performed at a temperature of 500° C. or more. Because the ambient of these deposition processes typically contains some residual oxygen, the oxygen will be absorbed by the exposed top periphery of the high-K dielectric and transported by diffusion to the gate dielectric to semiconductor channel interface. When oxygen diffuses to the gate dielectric to semiconductor channel interface, several problems can occur. First, the oxygen grows additional unwanted oxide at the critical gate dielectric to semiconductor channel interface, which thickens the effective oxide thickness (EOT) of the gate dielectric stack, which reduces transistor performance. Second, the oxygen shifts transistor V_th away from the desired V_th set point. Third, because the oxygen diffusion and ingress into the gate dielectric to semiconductor channel interface is not well controlled and varies significantly across wafer and from wafer to wafer during processing, both the EOT thickness increase and V_th shifts are highly variable, which severely degrades the overall semiconductor chip performance and manufacturability.

[0013] Accordingly, there is a need for a structure and method to minimize the oxygen ingress and diffusion into the gate dielectric to semiconductor channel interface in RMG FET structure and process.

[0026] In one aspect, in an embodiment of the invention, a structure and a method of fabrication thereof are provided that effectively suppress the oxide regrowth issue associated with a typical HKMG RMG flow.

[0027] In another aspect, in an embodiment, a standard replacement metal gate (RMG) flow, the top periphery of high-K gate dielectric is exposed after the metal gate CMP step. Since high-K gate dielectric materials such as HfO₂ and hafnium silicates transport oxygen effectively, the exposed high-K periphery region serves as a conduit for oxygen found in subsequent high temperature process steps to reach the gate dielectric/channel interface, causing an unwanted regrowth of the gate dielectric interface region.

[0028] In yet another aspect, in an embodiment, an additional low temperature (<500° C.) silicon nitride layer is added immediately after the metal gate CMP step in order to seal the top surface of the wafer and the high-K periphery to block oxygen ingress and interface regrowth. By using a low temperature (<500° C.) silicon nitride deposition process, an undesired regrowth issue is avoided both during the deposition process itself and in subsequent processings since silicon nitride is an excellent barrier to oxygen diffusion. After the low temperature deposited silicon nitride is in place, the FET will be resistant to the regrowth issue even when high temperature processes greater than 500° C. are used.

[0029] In a further aspect, in order to suppress the oxygen ingress and resulting interface regrowth, the following structure can be provided: Immediately after gate metal polish in RMG, a first deposit a thin silicon nitride layer 3 to 5 nm thickness, deposited at a low temperature (<500° C.) providing a buffer layer to suppress oxygen ingress and interface regrowth. Subsequently, a second silicon nitride layer is deposited, which is deposited at higher temperatures (>500° C.) since the first nitride layer is already present and will block oxygen from entering the high-K gate dielectric. A second silicon nitride layer can be optimized for other properties, such as low dielectric constant to reduce the parasitic capacitance or higher density without any concern of oxygen ingress into the high-K dielectric. The first low temperature silicon nitride beneath already protects the gate dielectric stack from oxygen ingress, In order to suppress the oxygen ingress and a resulting interface regrowth, the structure is provided immediately after gate metal polish in RMG by first forming a high-K gate dielectric recess, and then depositing a thin silicon nitride layer preferably having a 3 to 5 nm thickness, deposited at low temperature (<500° C.) as a buffer layer to suppress an oxygen ingress and an interface regrowth. The silicon nitride fills the previously etched recess, thus forming a silicon nitride plug forming a significantly robust barrier to oxygen ingress. Subsequently a second silicon nitride layer is deposited, which can be deposited at higher temperatures (>500° C.) because the first nitride layer is already present and will block oxygen from entering the high-K gate dielectric. The second silicon nitride layer can be optimized for other properties such as a low dielectric constant to reduce parasitic capacitance or higher density, without concern of oxygen ingress into the high-K dielectric because the first low temperature silicon nitride beneath is already protecting the gate dielectric stack from oxygen ingress.

[0030] In an embodiment, an integrated semiconductor structure a field effect transistor (FET) having a metal gate adjacent to an interlayer dielectric (ILD) on top of a source and drain, the metal gate surrounded by a gate work function metal surrounding a gate dielectric and abutting to a sidewall spacer, and a silicon nitride oxygen blocking layer above a planarized top surface of the metal gate and spanning across the gate dielectric, and the ILD.

[0031] A method of forming an integrated semiconductor structure comprising providing a field effect transistor (FET) having a metal gate adjacent to an interlayer dielectric (ILD) on top of a source and a drain, the metal gate surrounded by a gate work function metal surrounding a gate dielectric and abutting at a sidewall spacer; and depositing at a low temperature a silicon nitride (Si₃N₄) oxygen blocking layer on top of a planarized surface of the metal gate, the deposition spanning across the gate dielectric and the ILD.

[0032] The accompanying drawings, which are incorporated in and which constitute part of the specification, illustrate the presently preferred embodiments of the invention which, together with the general description given above and the detailed description of the preferred embodiments given below serve to explain the principles of the disclosure

[0033] FIG. 9 describes embodiments showing aspects of the inventive processes and structures. A silicon nitride buffer layer is deposited at low temperature (less than 500° C.) after the CMP process, but before the standard MOL dielectric deposition. Hence, as shown, the low temperature nitride layer is located between the gate which is below the nitride layer, and the MOL dielectric which is above the nitride layer. Because the additional silicon nitride buffer layer is deposited at a low temperature of 500° C. or less, oxygen ingress and diffusion into the high-K gate dielectric layer is effectively suppressed. Furthermore, once the top periphery of the high-K dielectric can be sealed by an additional low temperature silicon nitride layer, wherein a subsequent processing above 500° C. will not induce oxygen ingress into the high-K gate dielectric to semiconductor channel interface, particularly since silicon nitride is a highly effective barrier to oxygen diffusion. The silicon nitride layer can be deposited by molecular layer deposition (MLD), atomic layer deposition (ALD), CVD, or plasma-enhanced deposition, using precursors such as, but not limited to ammonia (NH₃), silane (SiH₄), tetrachlorosilane (SiCl₄), or dichlorosiline (SiH₂Cl₂) at a temperature of 500° C. or less. Thus, oxide regrowth can be effectively suppressed during the SiNx buffer layer formation.

[0034] In an embodiment, the low-temperature silicon nitride deposition is an MLD process preferably performed by heating the wafer at a temperature ranging between 400° C. and 500° C., alternately flowing an NH₃ remote plasma to provide nitrogen on the wafer surface. Next, a purge using N₂ flow removes the NH₃ from the gas phase, and a flow of SiH₂Cl₂ provides the silicon at the wafer surface. The two precursors react at the wafer surface and deposit a single molecular layer of silicon nitride. The cycle is repeated as many times as necessitated to reach the desired thickness of the silicon nitride, with an additional N₂ purge flow between each cycle to ensure that the NH₃ and SiH₂Cl₂ do not react in the gas phase to avoid a silicon nitride particle formation. In this manner, silicon nitride is only to be deposited at the wafer surface.

[0035] The thickness of the low-temperature silicon nitride layer can be any thickness required to be compatible with the semiconductor manufacturing flow, preferably 3 to 5 nm thick. On top of the low-temperature silicon nitride layer, a number of additional layers can be deposited including higher-temperature silicon nitride layers deposited at a temperature greater than 500° C., without concern of oxygen ingress and high-K gate dielectric interface regrowth at the semiconductor channel, because the underlying low-temperature silicon nitride layer is highly effective at blocking the oxygen. The ability to deposit higher-temperature silicon nitrides without inducing unwanted gate dielectric interface regrowth is desirable because higher-temperature silicon nitrides can have beneficial properties such as higher density, improved the mechanical properties or improved the barrier properties to avoid contamination.

[0036] FIG. 10 shows an additional inventive process and structure. In this case, before the low temperature silicon nitride deposition, the exposed high-K gate dielectric after CMP is selectively etched by either a wet or plasma-based chemistry to recess the high-K gate dielectric 2 to 10 nm lower than the other materials, although other recess depths can be contemplated as well. In an embodiment, a wet or plasma-based chemistry is used to etch the high-K gate dielectric layer significantly faster than other exposed materials such that a recess in the high-K is efficiently formed without a significant loss of other materials. Thereafter, the same low temperature silicon nitride deposition at a temperature 500° C. or less, as previously described, is used to seal that recess region, as well as the top surface of the wafer. Generally, the oxygen ingress is present at the aforementioned temperature. It is by way of the oxide regrowth that the problem can be effectively suppressed by a silicon nitride layer formation that further avoids the oxygen ingress through the gate High-K dielectric.

[0037] Still referring to FIG. 10, the recess allows the formation of a silicon nitride plug even more effective at blocking oxygen diffusion and ingress into the high-K gate dielectric to channel interface regions. On top of the low-temperature silicon nitride layer, additional layers can be deposited, including higher-temperature silicon nitride layers deposited at a temperature greater than 500° C., without concern of oxygen ingress and high-K gate dielectric interface regrowth at the semiconductor channel, because the underlying low-temperature silicon nitride layer is highly effective at blocking oxygen. The ability to deposit higher-temperature silicon nitrides without inducing unwanted gate dielectric interface regrowth is desirable because higher-temperature silicon nitrides can have beneficial properties such as higher density, improved mechanical properties, or improved barrier properties to avoid contamination.

[0038] While the present invention has been particularly shown and described with respect to preferred embodiments thereof, it will be understood by those skilled in the art that the foregoing and other changes in forms and details may be made without departing from the spirit and scope of the present invention. It is therefore intended that the present invention is not to be limited to the exact forms and details described and illustrated, but fall within the scope of the appended claims.