Transitional dielectric layer to improve reliability and performance of high dielectric constant transistors

A gate dielectric structure (201) fabrication process includes forming a transitional dielectric film (205) overlying a silicon oxide film (204). A high dielectric constant film (206) is then formed overlying an upper surface of the transitional dielectric film (205). The composition of the transitional dielectric film (205) at the silicon oxide film (204) interface primarily comprises silicon and oxygen. The high K dielectric (206) and the composition of the transitional dielectric film (205) near the upper surface primarily comprise a metal element and oxygen. Forming the transitional dielectric film (205) may include forming a plurality of transitional dielectric layers (207) where the composition of each successive transitional dielectric layer (207) has a higher concentration of the metal element and a lower concentration of silicon. Forming the transitional dielectric layer (205) may include performing multiple cycles of an atomic layer deposition process (500) where a precursor concentration for each cycle differs from the precursor concentration of the preceding cycle.

FIELD OF THE INVENTION

The invention is in the field of semiconductor fabrication processes and, more particularly, fabrication processes that use high dielectric constant gate dielectrics.

RELATED ART

The use of dielectric materials having high dielectric constants (also referred to as high K materials) is well known. High K dielectrics are used to address reliability problems resulting from the use of very thin gate dielectrics. Historically, thermally formed silicon dioxide was the preferred material for the gate dielectric of an MOS (metal oxide semiconductor) transistor.

While silicon oxide is well understood and has many desirable properties as a gate dielectric material, the dielectric constant of silicon dioxide is relatively low. Accordingly, achieving sufficient gate capacitance in transistor that have a small area (W×L) requires a thin silicon dioxide film. (Capacitance is proportional to area and inversely proportional to the dielectric thickness). As the area of transistors is scaled smaller and smaller, the thickness of a silicon dioxide that would be required to achieve the desired capacitance is undesirably thin. Extremely thin dielectric films present reliability problems when subjected to the large electric fields often associated with small channel transistors.

High K materials address this problem by enabling the fabrication of a transistor having a dielectic film with a specified equivalent oxide thickness (EOT) using a film that is sufficiently thick to withstand high field breakdown, leakage, and other phenomena associated with thin films. Unfortunately, high K films, when formed overlying a silicon substrate inevitably form a thin silicon-oxide layer between the high K material and the substrate. Due to differences in dielectric constant between the silicon-oxide film and the overlying high K material, the electric field applied across the gate dielectric exhibits a disproportionate drop across the two films. Specifically, the electric field within the silicon oxide film is greater than the field within the high K film.

This electric field discrepancy is theorized to result in the formation of excessive border states at the interface between the two dielectrics that presents a serious reliability issue. In addition, the presence of at least some high K films in close proximity to the transistor channel in the underlying substrate channel is theorized to degrade transistor performance by reducing carrier mobility. It would be desirable, therefore, to implement a fabrication process that accommodated the use of high K materials without introducing reliability and performance problems for the resulting transistor.

DETAILED DESCRIPTION OF THE DRAWINGS

Generally speaking, the present invention encompasses a semiconductor fabrication method and a transistor gate dielectric structure in which a transitional dielectric film is formed between a semiconductor substrate and a high K dielectric or between a residual oxide film that lies on the substrate and the high K dielectric. The transitional dielectric film preferably includes a plurality of individual layers in which the composition of a layer differs from the composition of its adjacent layers. In the preferred implementation of the transitional dielectric film, the composition of the layers nearer to the residual oxide film interface resembles the composition of the residual oxide film and the composition of the layers nearer to the high K dielectric resembles the composition of the high K dielectric. For example, in an implementation using a HfO2high K dielectric, the transitional layers at the residual oxide film, which includes silicon and oxygen, are primarily silicon and oxygen with a relatively small concentration of hafnium while the transitional layers at the high K dielectric interface are primarily hafnium and oxygen with a relatively small concentration of silicon. The composition of each subsequently formed transitional layer has an increased concentration of hafnium and a decreased concentration of silicon.

The transitional dielectric layer provides two important benefits. First, by “smoothing” the electric field gradient, the transitional dielectric layer beneficially improves reliability by reducing the occurrence of states at the dielectric interface. Second, the presence of the transitional dielectric film physically displaces the high-K dielectric film from the channel region of the semiconductor substrate resulting in reduce carrier mobility degradation (i.e., better carrier mobility).

Referring now to the drawings,FIG. 1depicts a semiconductor wafer100suitable for use in a semiconductor fabrication process. Wafer100is shown at a stage in the fabrication process following the formation of a gate dielectric film according to the prior art. At the depicted stage in the fabrication process, wafer100includes isolation structures103formed in a semiconductor substrate102. Isolation structures103are typically silicon oxide shallow trench or local oxidation isolation structures well known in the field.

Wafer100includes a high K dielectric film (high K film)106overlying a semiconductor substrate102. A residual dielectric film104(also referred to herein as interfacial dielectric film104) is located between substrate102and high K film106. In a typical implementation, high K film is a metal-oxide dielectric and residual oxide film104is a silicon oxide containing film. (Note that the term silicon oxide as used herein includes stoichiometric SiO2as well as non-stoichiometric silicon oxide compositions).

As indicated above, formation of high K film106invariably results in the formation of the residual silicon oxide film104. The presence of high K film106directly on top of residual oxide film104is believed to result in a proliferation of border/interface states at the interface between the two films when a potential is applied across the composite structure. The presence of these states degrades the reliability of the dielectric structure. In addition, under normal operating voltages, the electric field across residual silicon oxide film104begins to exceed 10 MV/cm for structures having an equivalent oxide thickness of approximately 20 angstroms (2 nm) or less. It is generally undesirable from a reliability perspective to stress a silicon oxide film at greater than approximately 10 MV/cm.

The present invention address the interface/border state problem and the excessive electric field problem by introducing a transitional film, sometimes referred to as an interface film, between the residual oxide and the high K dielectric. In the preferred embodiment, the transitional film has a graded composition in which the composition of the transitional film proximal to the residual oxide film has a relatively low concentration of the metal (or other element) used in the high K dielectric while the composition proximal to the high K dielectric has a relatively high concentration of the metal in the high K dielectric. In other words, the composition of the transitional film is close to silicon oxide at the silicon oxide interface and close to MO (where M is the metal element in the high K) at the high K interface.

FIG. 2andFIG. 3are two representations of the a gate dielectric structure201according to an embodiment of the present invention. As depicted inFIG. 2, gate dielectric structure201includes a residual oxide film204overlying a semiconductor substrate202, a transitional dielectric film205, and a high K dielectric206. Isolation dielectric structures203have been formed in substrate202. Substrate202is preferably a single-crystal silicon substrate that may be doped with n-type or p-type impurities or both. Substrate202may be part of a silicon-on-insulator (SOI) wafer in which substrate202overlies a buried oxide (BOX) layer and a semiconductor bulk layer (not shown). In addition, substrate202may be any semiconductor material or combination of semiconductor materials, such as gallum arsenide or germanium. The depicted portions of substrate202may represent an NWELL region of substrate202over which PMOS transistors are formed or a PWELL region of substrate202over which NMOS transistors are formed.

In one embodiment, residual oxide film204has a thickness of less than approximately 10 Angstroms (1 nm), transitional dielectric film205has a thickness in the range of 10 to 20 Angstroms (1 to 2 nm), and high K dielectric206has a thickness in the range of approximately 20 to 100 Angstroms (2 to 10 nm). In the preferred embodiment, the overall EOT of gate dielectric structure201is less than approximately 25 Angstroms (2.5 nm). As indicated previously, residual oxide film204is most likely a silicon oxide compound. High K dielectric206is preferably a metal oxide compound or a metal oxynitride compound. The metal element in this embodiment of high K dielectric may be any suitable metal element including hafnium, aluminum, tantalum, zirconium, and yttrium. In a hafnium embodiment, high K dielectric is preferably HfO2. A dielectric constant of high K dielectric206is preferably in excess of approximately 20 (i.e., at least 5 times the dielectric constant of SiO2).

FIG. 3depicts additional detail of an implementation of transitional dielectric film205. In the depicted implementation, transitional dielectric film205includes multiple dielectric layers207-1through207-N (generically or collectively referred to as transitional dielectric layer(s)207). In this embodiment, the composition of each transitional dielectric layer207differs, albeit possibly only slightly, from the composition of its adjacent layers. The composition of transitional dielectric layers207in proximity to residual oxide film204(layer207-1for example) preferably includes primarily a semiconductor element such as silicon and oxygen or a semiconductor element such as silicon, oxygen, and nitrogen and a relatively small or zero concentration of a metal element. In contrast, the composition of dielectric layers207in proximity to high K dielectric206(layer207-N for example) preferably includes primarily the metal element and oxygen or the metal element, oxygen, and nitrogen, and a relatively small or zero concentration of the semiconductor element. If high K dielectric206is HfO2, for example, the composition of dielectric layer207-N is primarily hafnium and oxygen.

The composition of transitional dielectric film205may be described quasi-formulaically. In one embodiment, each transition dielectric layer207has a composition of MXSiYOZwhere X is approximately 0 in transitional dielectric layer207-1and where Y is approximately 0 in transitional dielectric layer207-N. The relative concentrations of M and Si are increased and decreased respectively as each successive transitional layer207is formed. For an embodiment in which high K dielectric206is HfO2(i.e., M is hafnium), transitional dielectric layer207-1would be primarily silicon-oxide with a small amount of hafnium while transitional dielectric layer207-N would be primarily hafnium oxide with a small amount of silicon. In this manner, the composition of transitional dielectric film205transitions from a silicon-oxide-like composition to a hafnium-oxide-like composition.

In some embodiments that include nitrogen in transitional layers207-1through207-N, the nitrogen concentration is relatively constant in each of the transistional layers207-1through207-N. In other embodiments, however, the nitrogen concentration in transitional layers207-1through207-N may vary from layer to layer. In this embodiment, the nitrogen concentration may increase with each successively formed transitional layers or decrease with each successively formed transitional layer. The nitrogen concentration may also be related to the metal element concentration. For example, the nitrogen concentration may either increase or decrease as the metal element concentration increases. In still another embodiment, the nitrogen concentration may be relatively high at the both extremes of the transitional dielectric film205(i.e., high nitrogen concentration near transitional dielectric layer207-1and207-N) with a relatively low nitrogen concentration in the intermediate transitional dielectric layers.

Referring toFIG. 4, a graphical depiction of transitional dielectric film205is presented. InFIG. 4, the dielectric constant of the gate dielectric structure is plotted as a function of the distance from (above) the silicon substrate interface (i.e., the interface between silicon substrate202and residual oxide film204). As shown inFIG. 4, the dielectric constant of the gate dielectric structure is approximately 3.9 (the dielectric constant of SiO2) within the residual oxide film204. The dielectric constant is approximately 20 within the high K dielectric film206. In between,FIG. 4shows the dielectric constant of the gate dielectric structure increasing from 3.9 to 20 in discreet increments. Each “step” in the transitional portion of the dielectric structure represents a corresponding layer207-1to207-N in transitional dielectric film205.

The implementation of transitional dielectric film205represented inFIG. 4contemplates a series of discrete transitional dielectric layers207-1to207-N formed overlying residual oxide film204. In one embodiment of the invention, transitional dielectric layers207-1to207-N are monolayer films (a film having a depth of one atomic layer). Monolayer film embodiments of transitional dielectric layers207-1to207-N achieve the transition from silicon oxide to metal oxide with a desirably thin transitional dielectric film205. It is desirable to constrain the thickness of transitional dielectric film205to maintain a thin effective oxide thickness. Because monolayer films represent the thinnest films that may be formed feasibly, the use of monolayer transitional films constrains the thickness of transitional dielectric film205while simultaneously enabling the use of multiple films and multiple film compositions. The use of multiple film compositions beneficially spread the potential drop across the dielectric structure thereby reducing the electric field (E-field) experienced by residual oxide film204.

The embodiment described above encompasses dielectrics that may contain a metal element, a semiconductor element such as silicon, and oxygen. In another embodiment, transitional dielectric may also include nitrogen. In one implementation of a nitrogen embodiment, transitional dielectric205transitions from MX1SiY1OZ1NW1in transitional dielectric layer207-1to MX2SiY2OZ2NW2in transitional dielectric layer207-N wherein the ratio X2:Y2 is greater than the ratio X1:Y1 (i.e., the concentration of the metal element M is greater in dielectric layer207-N than in the dielectric layer207-1). In a variation of this implementation, transitional dielectric layer207-1is an SiO2layer (i.e., X1=W1=0). In this variation, the concentration of nitrogen and the metal element would be increased for transitional dielectric layer207-2so that transitional dielectric film205would transition from SiO2 to MSiON (e.g., HfSiON) with a low metal concentration to MSiON with a high metal concentration. In a second implementation of the nitrogen embodiment, transitional dielectric205transitions from MON (e.g., HfON) with a low concentration of M to MON with a high concentration of M. In a variation of this embodiment, transitional dielectric layer207-1is either SiO2(no metal, no nitrogen) or SiON (no metal). The silicon concentration is decreased to 0, while the metal concentration is increased.

It should be noted that the specific concentrations of Si, O, N, and the metal element in each layer207is an implementation detail. The concept is to provide a transitional dielectric film that includes multiple discreet and possibly monoatomic layers where the composition of each subsequent layer differs from the composition of the prior layer. The transition is generally from an SiO2or SiON dielectric to a metal oxide or metal oxynitride dielectric.

Referring toFIG. 5, a flow diagram is presented to illustrate a method500of forming transitional dielectric film205according to one embodiment of the invention. In the preferred embodiment, method500employs an atomic layer deposition (ALD) apparatus capable of depositing monolayer films. ALD systems deposit films using controlled amounts of reactant in a self limiting process.

Method500as depicted inFIG. 5includes initially purging an ALD reactor chamber and setting (block502) values for variables X, Y, Z, and W where X, Y, Z, and W represent the relative concentrations of M (a metal such as hafnium), a semiconductor element such as silicon, oxygen, and nitrogen respectively. For an embodiment in which the composition of transitional dielectric layers207in proximity to residual oxide film204is close to the composition of residual oxide film204, the value of X is low relative to Y (representing the concentration of silicon).

After X, Y, Z, and W are initialized, a controlled amount of a precursor is introduced (block504) into the reactor chamber. The precursor components are controlled (according to the values of X, Y, Z, and W) to provide a precursor that will result in the deposition of a film having the desired composition of metal, silicon, oxygen, and nitrogen. One embodiment of the invention contemplates the use of a metal precursor component, a silicon or another semiconductor precursor component, nitrogen, and oxygen. The precursor components, oxygen, and nitrogen are introduced into the chamber in precisely controlled quantities to achieve the desired composition and to ensure that the deposition process consumes the available reactants after forming a monolayer film.

After the precursor is introduced into the chamber and appropriate environmental conditions (temperature, pressure, etc.) are established, a monoatomic transitional dielectric layer207of transitional dielectric film205is deposited (block506). Other embodiments (not depicted) may elect to use two or more monolayers for a transitional dielectric layer207, but minimizing the number of monolayers (i.e., the thickness) of any transitional dielectric layer207is desirable to minimize the effective oxide thickness of gate dielectric structure201as a whole.

Following deposition of a monolayer of transitional dielectric film205, method500purges (block508) the chamber and determines (block510) whether additional monolayers are desirable. In one embodiment, transitional dielectric film205includes a specified number of transitional layers207. In this embodiment, block510simply compares the number of monolayers already deposited to the specified number of layers. If additional layers are required, the values of X and Y are adjusted so that the X:Y ratio is increased. In the preferred embodiment, the number of monolayers within transitional dielectric layer205determines the amount by which the X:Y ratio is adjusted each cycle. If a relatively larger number of monolayers are used, smaller increments in X:Y ratio may be accommodated resulting in a less severe E-field gradient.

Method500completes when the number of deposited monolayers equals the specified number of monolayers. Exemplary embodiments may use as many as approximately 40 monolayers where each subsequent monolayer has an greater concentration of metal and a lesser concentration of silicon. Method500of forming transitional dielectric film205thus includes the formation of discreet layers using ALD and the precise control of precursors to vary the monolayer composition from layer to layer.

Although the depicted embodiment of method500adjusts the silicon and metal concentrations only in block512, the concentrations of oxygen and/or nitrogen may also be adjusted in block512. Moreover, the concentration of one or more of the dielectric components (i.e., silicon, nitrogen, oxygen, and the metal) may be 0 during a particular step. For example, embodiments of transition dielectric205that include an SiO2 layer or an SiON layer will have zero concentration of the metal element during the formation of the initial layer or layers207. Similarly, while the silicon concentration may be non-zero during an initial step, it may be zero for the formation of subsequent layers.

In the foregoing specification, the invention has been described with reference to specific embodiments. However, one of ordinary skill in the art appreciates that various modifications and changes can be made without departing from the scope of the present invention as set forth in the claims below. For example, although the depicted embodiment describes a metal oxide high K dielectric, other high K dielectrics including metal silicates and metal nitrides may also be used. In addition, although the described embodiment implies that the metal element used in the transitional dielectric layers207is the same element as the metal element used in high K dielectric206, other embodiments may use a first metal in the dielectric layers207and a second metal in the high K dielectric206. Accordingly, the specification and figures are to be regarded in an illustrative rather than a restrictive sense, and all such modifications are intended to be included within the scope of present invention.