Reduction of native oxides by annealing in reducing gas or plasma

Native oxide growth on germanium, silicon germanium, and InGaAs undesirably affects CET (capacitive equivalent thickness) and EOT (effective oxide thickness) of high-k and low-k metal-oxide layers formed on these semiconductors. Even if pre-existing native oxide is initially removed from the bare semiconductor surface, some metal oxide layers are oxygen-permeable in thicknesses below about 25 Å thick. Oxygen-containing species used in the metal-oxide deposition process may diffuse through these permeable layers, react with the underlying semiconductor, and re-grow the native oxide. To eliminate or mitigate this re-growth, the substrate is exposed to a gas or plasma reductant (e.g., containing hydrogen). The reductant diffuses through the permeable layers to react with the re-grown native oxide, detaching the oxygen and leaving the un-oxidized semiconductor. The reduction product(s) resulting from the reaction may then be removed from the substrate (e.g., driven off by heat).

BACKGROUND

Related fields include atomic layer deposition, high-performance logic, and mitigation of native-oxide effects on semiconductor device performance.

Traditional device scaling of logic devices based on silicon (Si) has encountered obstacles. Inherent material properties have placed limitations on further miniaturization, increases in processing speed, and other performance enhancements. For example, as gate conductor width decreases, gate dielectric thickness also needs to decrease to provide sufficient capacitance to control the transistor. Suppression of leakage current is critical to performance of a capacitor dielectric. Silicon oxide layers <2 nm thick are subject to unacceptably high leakage current due to tunneling effects.

Tunneling leakage decreases as a function of physical thickness. Therefore, there has been interest in gate dielectric materials that would exhibit the same capacitance as 1-2 nm thick silicon dioxide (SiO2) at a physical thickness too large for significant tunneling leakage (e.g., >=5 nm). Metal oxides with high dielectric constants (“high-k materials”) such as hafnium oxide (HfOx), aluminum oxide (Al2O3), and zirconium oxide (ZrOx) are, among others, being investigated as gate-dielectric candidates to replace silicon oxide.

Another avenue of exploration is replacement of Si channels with higher-mobility, lower-effective-mass materials such as germanium (Ge). Ge and Si—Ge are being explored for surface channels and strained buried channels. Indium gallium arsenide (InGaAs) is another Si substitute under consideration. The new materials, however, face various integration challenges. Several of these challenges are rooted in the susceptibility of these materials to growth of unstable native oxides that increase operational power consumption and decrease reliability.

Uncontrolled native oxide growth under a capacitor dielectric can unpredictably affect the effective oxide thickness (EOT=(kSiO2/k)t) and the capacitive effective thickness (CET˜EOT+(kSiO2/k)zavgfor an ultra-thin gate dielectric) of a logic stack. In the equations, k=dielectric constant of the actual material, t=physical thickness of the actual material, Zavg=average distance of inversion carriers from the gate-dielectric interface, and kSiO2=dielectric constant of SiO2˜3.9.

Removing the native oxide from Ge immediately before atomic layer deposition (ALD) of a high-k metal oxide layer has proven to be an incomplete solution. Although the ambient air that often triggers native GeOxgrowth is excluded from the ALD process chamber, the oxygen precursors (e.g., H2O) used for the high-k layer deposition can encourage the native GeOxto regrow. Some of the high-k materials, such as Al2O3and HfOx, are permeable in thicknesses less than about 25 angstroms (Å). Unbonded oxygen or oxidant molecules can diffuse through the high-k layers and grow native GeOxunderneath them even after they are partially (or, in some cases, fully) deposited.

Therefore, advanced logic technology would benefit if the unwanted dielectric effects of unstable native-oxide growth in materials such as Ge and InGaAs could be mitigated. In particular, a treatment that could work through permeable overlying layers would be desirable.

SUMMARY

The following summary presents some concepts in a simplified form as an introduction to the detailed description that follows. It does not necessarily identify key or critical elements and is not intended to reflect a scope of invention.

The oxidation reaction that creates unstable native oxides on Ge (and other semiconductors such as InGaAs) is countered by a reduction reaction of the native oxide with a reductant such as hydrogen. Like the oxygen-containing precursors that pass through permeable overlayers to react with the underlying semiconductor, reductants such as hydrogen-containing precursors or active species pass through the permeable overlayers to react with the underlying native oxide. The reductant detaches an oxygen component from the native oxide to form a reduction product. When the reduction products are removed, a non-oxidized semiconductor surface is left behind.

“Permeable,” as used herein, means at least permeable to oxygen and to the reductant used in the process. For some embodiments, it may also mean permeable to oxygen-containing compounds, gases, or species exposed to the substrate during various fabrication processes, and to any reduction products produced by the reaction of the reductant with the native oxide.

Some methods to take advantage of this reduction effect include hydrogen (H2) plasma treatments. Some methods include soaks in H2or ammonia (NH3). The reductant (e.g., the NH3or H2) diffuses through the permeable layers (e.g., metal oxides such as HfOxor Al2O3) to reduce the GeOxunderneath, reducing the CET and EOT of the stack. As used in the ALD art, a “soak” may include introducing a gas in the chamber, then closing off the inlets and exhausts for a predetermined time while the gas adsorbs or reacts with the substrate surface. It may also be done as a very long pulse (for instance, about 30 seconds to about 10 minutes). During this type of soak, the gas inflow and outflow may be adjusted to keep the pressure in the chamber approximately (e.g., ±10%) constant.

The reduction treatment may be repeated at intervals during the formation of the permeable layers. Alternatively, it may be done just before the formation of the first oxygen-impermeable layer (e.g., a titanium nitride (TiN) layer) above the native oxide. If the oxygen-impermeable layer is formed by ALD, the reduction treatment may be postponed until (or may be repeated after) the deposition of an initial few monolayers if the initial few monolayers are partially permeable.

In some embodiments of reduction treatments, the substrate is heated to about 300-400 C. Heating may promote the reaction or help to drive the reduction products off the surface through any overlying permeable layers. Chamber pressure may be between about 0.1 and about 5 Torr. Treatment duration may be between about 1 and about 60 minutes. In some embodiments, the reductant does not react with one or more of the permeable layers through which it diffuses before reaching the underlying semiconductor. The permeable layers may be between about 2 and about 40 Å thick. The permeable layers may have dielectric constants (k) above or below 9.

DETAILED DESCRIPTION OF EXAMPLE EMBODIMENTS

Semiconductor fabrication generally requires many other processes before and after those described. This description omits steps that are irrelevant to, or that may be performed independently of, the described processes. To avoid unnecessarily obscuring the description, some technical material known in the related fields is not described in detail.

By default, singular articles “a,” “an,” and “the” (or the absence of an article) may encompass plural variations. For example, “a layer” may mean “one or more layers,” except where the text or context clearly indicates “only one layer.” “About” or “approximately” contemplates up to 10% variation. “Substantially” contemplates up to 5% variation.

FIGS. 1A-1Econceptually illustrate the removal of native oxide and its regrowth as permeable layers are formed over the host semiconductor in the presence of oxygen. InFIG. 1A, substrate101has a top semiconductor layer102. Substrate101may have any number of other layers under top semiconductor layer102. Top semiconductor layer102has a native oxide103on its upper surface. Such native oxides103routinely grow on semiconductors such as silicon, germanium, or InGaAs when the semiconductors are exposed to the oxygen in ambient air, but they can also result from exposure to other sources of oxygen.

InFIG. 1B, a removal process110removes native oxide103from the top surface of semiconductor102. A number of processes for this removal are known in the art, such as wet etches, polishing, laser ablation, plasma treatments, and others. Some of the processes may involve reducing agents. The methods described herein may be practical in some cases; for example, when the native oxide is initially thin.

InFIG. 1C, an ALD precursor124is introduced into a process chamber containing substrate101. For example, precursor124may be a metal precursor. When precursor124encounters the upper surface of top semiconductor102, deposited material104(e.g., the metal) is adsorbed onto the surface. As used herein, “adsorb” may include chemisorption, physisorption, electrostatic or magnetic attraction, or any other interaction resulting in part or all of the precursor adhering to the substrate surface. In some processes, the entire precursor molecule adsorbs to the surface. In other processes, a ligand129is initially part of precursor124, but detaches from deposited material104when deposited material104adsorbs to the surface. Typically, detached ligands129and any unabsorbed precursor124are then purged from the chamber.

InFIG. 1D, an oxygen-containing precursor135is introduced in the chamber. The oxygen may be the deposited material105, as illustrated, or it may be part of ligand139. In some processes, the oxygen may not even be part of a precursor, but may be part of some other process gas such as a purge gas, buffer gas, or catalyst. Because the layer being deposited onto semiconductor102is permeable, some oxygen105may diffuse through pores or gaps in the permeable layer to reach and oxidize semiconductor102. This reaction results in regrowth of native oxide103. The chamber is purged again after this step, but a normal purge generally cannot remove the oxygen that has reacted with substrate102.

InFIG. 1E, more ALD cycles have occurred. A new layer (e.g., a metal oxide) is being formed on semiconductor102. Because the new layer is permeable, every cycle involving oxygen-containing precursor135presents an opportunity for oxygen to diffuse through pores or gaps106to the underlying semiconductor102, where it reacts to form more native oxide103. Both high-k and low-k metal oxides can be permeable after deposition, especially when the layers are thin (<50 Å). While the growth of the ALD layer is tightly controlled, the incidental regrowth of native oxide103is uncontrolled and unpredictable. If the layer being grown is a dielectric for a capacitor, the regrown native oxide103decreases the dielectric constant k of the dielectric layer (GeO2is very low-k), and increases the CET and EOT of the finished device, as an uncontrolled variable.

FIGS. 2A and 2Bconceptually illustrate the effect of introducing a reductant through a permeable overlayer. InFIG. 2A, a layer including material components104and105has been formed on semiconductor layer102on substrate101. The layer is permeable. Regrown native oxide103has resulted from oxygen percolating through pores or gaps106. A reductant253is introduced into the chamber and allowed to diffuse through gaps106to reach and react with regrown native oxide103.

InFIG. 2B, reductant253has reduced regrown native oxide103, producing a reduction product259that includes oxygen205removed from regrown native oxide103. With the oxygen205removed, semiconductor102reverts to its un-oxidized state. For example, if the reductant is hydrogen, the reaction may be expressed by GeO2+2 H2→Ge+2H2O. If the reductant is ammonia, the reaction may be expressed by 3GeO2+4NH3→3Ge+6H2O+2N2

Reduction product259(e.g., H2O, N2) is expelled through gaps106, for example by applying heat253to substrate101. This process produces a permeable ALD layer or stack with no native oxide underneath. In some cases, if the reduction product is sufficiently stable to retain the oxygen and prevent it from re-oxidizing the semiconductor, and if the properties of the reduction product do not adversely affect device performance, the reduction product may not need to be removed from the surface.

Reductant259may be, for example, gaseous ammonia (NH3) or hydrogen (H2). Alternatively, reductant259may include plasma-activated species (e.g., plasma-activated hydrogen), and the substrate temperature may be about 25 C or higher. Reductant259may diffuse through more than one permeable layer; for example, a high-k layer and a low-k layer. The thickness of the permeable layers may be between 2 and 40 Å. In some embodiments, reductant259only reacts with native oxide103or regrown native oxide203on semiconductor102, not with the permeable layer(s) or any other material in the stack.

The ambient pressure in the chamber may range from about 0.1 to about 5 Torr. The treatment duration may be between about 1 minute and about 60 minutes. In some embodiments, the substrate may be heated to a temperature between about 300 and about 400 C.

FIG. 3is a schematic diagram of an example ALD chamber. Inside ALD chamber300, substrate301is held by a substrate holder310. Substrate holder310may be configured with vacuum312(for example, a vacuum chuck to grip the substrate); motion313in any direction, which may include tilt and rotation; a magnetic field source314; heater or temperature control315; or sources of AC316or DC317bias voltage, or static electrical charge for an electrostatic chuck to hold the substrate (not shown). Chamber300also has gas inlets321,322,323,324for precursors, buffer gases, and purge gases. Some of the inlets may feed through diffusers325,326. A remote plasma chamber330may generate reactive species that enter chamber300through input adapter331. Measurement system340may monitor substrate301through measurement ports342. The measurements from measurement system340may be collected by a monitoring system350and sent for analysis or storage to a data collection device such as computer370. Substrate holder310, gas inlets321-324, diffusers325-26, remote plasma chamber330, plasma input adapter331, exhausts327-28, measurement system340, and monitoring system350may jointly or individually be controlled by controllers such as computer370.

To form ALD layers (such as high-k or low-k metal oxides or metal nitrides), the substrate301is prepared and positioned on substrate holder310. Preparing substrate301may include removing pre-existing native oxides from a top semiconductor surface by any suitable method. Substrate301may be held on substrate holder310electrostatically, by vacuum, or by any other suitable means. Precursors for making the layers, as well as other process gases or species such as buffers or catalysts, may enter through plasma input adapter331, undiffused gas inlets321and322, or gas inlets323and324with diffusers325and326. Precursors may be introduced into chamber300in “pulses,” short periods of inflow followed by a delay to allow a portion of the precursor to adsorb on the surface of substrate301, or the inflow may be continuous. To promote or regulate the adsorption of the deposited material from the precursors, substrate301may be heated or cooled315, AC- or DC-biased316or317, or subjected to a magnetic field314by substrate holder310.

Exhausts327and328may equalize the pressure for continuously flowing precursors. Measurement equipment340may dynamically measure characteristics of the surface of substrate301so that monitoring equipment350may track the progress of precursor deposition. After each pulse or period of precursor inflow, chamber300may be purged by drawing any gaseous contents out through exhausts327and328. In some embodiments, a purge gas may be routed through chamber300. Purge gases are often inert gases such as nitrogen and argon, but other types of purge gases are sometimes used. The temperature, electric field, or magnetic field of substrate301may also be adjusted during the purge.

Like the precursors, the reductant for the reducing treatment may be generated in plasma chamber330and introduced into chamber300through plasma input adapter331. Alternatively, a gaseous reductant may be introduced into chamber300through undiffused gas inlets321and322, or gas inlets323and324with diffusers325and326. The reductant may be introduced to the chamber as a pulse or as a soak. As used in the ALD art, a “soak” may refer to introducing a gas in the chamber, then closing off the inlets and exhausts for a predetermined time while the gas adsorbs or reacts with the substrate surface. It may also refer to a very long pulse (for instance, about 30 seconds to about 10 minutes). Exhausts327and328may regulate the chamber pressure. Substrate holder310may heat substrate301or apply an electric or magnetic field to promote or regulate the reduction reaction. The reduction products, any buffer gases or catalysts, and any unabsorbed reductant may then be purged through exhausts327and328, optionally using additional purge gases introduced through gas inlets321,322,323or324.

FIG. 4is an example flowchart of a reducing treatment integrated with the formation of overlayers. The substrate with the top semiconductor layer is prepared401and positioned in the chamber. The top semiconductor layer may include germanium, silicon germanium, or indium gallium arsenide. Any existing native-oxide is removed410. One or more permeable layers are formed440by a process that involves an oxidizing gas. The oxidizing gas may be an oxygen precursor, a different precursor with a ligand including oxygen, or an oxygen-containing process gas other than a precursor. The permeable layers may include high-k metal oxides, low-k metal oxides, or any other material that is permeable as deposited.

To eliminate or mitigate any native oxide regrowth that occurred during formation440of the permeable layers, the substrate is exposed450to a reductant. The reductant may be, for example, ammonia gas, hydrogen gas, or hydrogen plasma. The reductant reduces the regrown native oxide and the oxygen is removed. The reaction may be promoted or regulated by heating451the substrate. When a reduction cycle is complete (as estimated by its allowed duration or as concluded from the results of measurements), the chamber is again purged460. At least one oxygen-impermeable layer is formed470to seal the permeable layers and the semiconductor surface from any further exposure to oxygen. For example, the oxygen-impermeable layer may be an electrode or a capacitor plate or a capping layer, and may include a metal or a metal nitride. After the oxygen-impermeable layer is fully formed470, a next process499may begin.

Depending on the amount of expected native oxide regrowth, the reduction treatment450may be performed only once, just before the first oxygen-impermeable layer is formed470. Alternatively, as indicated by dotted line arrow459, the reduction treatment450may be repeated, and the repetitions may be interspersed between cycles or sets of cycles of permeable layer formation440.

Even some highly impermeable materials such as titanium nitride (TiN) are somewhat permeable in thicknesses of a few monolayers. Therefore, the reduction treatment450may in some embodiments be performed directly after depositing a few initial, permeable monolayers of the material that will become the oxygen-impermeable layer as its thickness increases. For example, the first1to10monolayers may be permeable, depending on the composition and deposition parameters of the oxygen-impermeable layer.

In some processes, if a reductant is used in the deposition of the oxygen-impermeable layer (as high-temperature NH3may be used in ALD of TiN), the reduction may be a by-product of the impermeable-layer deposition, or of a modified version of the first few cycles. Such a modification may include, for example, lengthening the reductant pulse or lengthening a subsequent soak or pause, so that the reductant has sufficiently high concentration and sufficient time to diffuse through the permeable layer and reduce the underlying native oxide.

FIG. 5is an example flowchart of a reducing treatment integrated with the formation of metal oxide layers and a capping layer. To eliminate or mitigate any native oxide regrowth that occurred while forming the permeable metal oxide layers, the substrate is exposed555to a reductant; for example, ammonia gas, hydrogen gas, or hydrogen plasma. The reductant reduces the regrown native oxide and the oxygen is removed. The reaction may be promoted or regulated by heating551the substrate; for example, to between about 300 and about 400 C. When a reduction cycle is complete, the chamber is purged.

Depending on the amount of expected native oxide regrowth, the reduction treatment555may be performed only once, just before the oxygen-impermeable capping layer begins to be formed571, or optionally just before enough capping-material monolayers are deposited to make the capping layer oxygen-impermeable572. Alternatively, reduction treatment555may be repeated, and the repetitions may be interspersed between cycles or sets of cycles of permeable metal oxide layer formation521-532.

Although the foregoing examples have been described in some detail to aid understanding, the invention is not limited to the details in the description and drawings. The examples are illustrative, not restrictive. There are many alternative ways of implementing the invention. Various aspects or components of the described embodiments may be used singly or in any combination. The scope is limited only by the claims, which encompass numerous alternatives, modifications, and equivalents.