Pretreatment of Semiconductor Substrate

1. Field of the Invention

The present invention relates, in general, to a method for treating a semiconductor substrate prior to chemically depositing any material thereon and, more particularly, to use of a reaction gas comprising WF and subsequently a reducing gas along with a source gas for deposition material, thereby leading to uniform and fast nucleation.

2. Description of the Prior Art

A titanium nitride (TiN) thin film is usually used for a diffusion barrier layer in a metallization process when fabricating a semiconductor device. The titanium nitride film can be formed largely by two processes: physical vapor deposition (hereinafter referred to as "PVD") process and chemical vapor deposition (hereinafter referred to as "CVD").

In PVD process, a titanium nitride is formed in such a way that argon atoms accelerated in a reactor sputter into a titanium target, so as to react with nitrogen (N.sub.2). A representative example of PVD process includes reactive sputtering process.

As for CVD process, a titanium nitride is formed through reaction of titanium chloride (TiCl.sub.4) with ammonia (NH.sub.3). Typical examples of CVD process include low pressure CVD (LPCVD) and plasma enhanced CVD (PECVD). Besides, metal organic CVD (MOCVD), which uses metal organic precursor, has recently attracted attention.

Generally, the titanium nitride deposited by CVD or PVD slowly reacts in most chemical reactions owing to very strong bond between titanium (Ti) and nitrogen atom (N). After being deposited, the titanium nitride, if exposed to air, goes into natural oxidation, to form a thin oxynitride layer with a thickness of about 10 to 15 Angstrom, on the surface.

Thus, when a metal thin film is deposited on a substrate of titanium nitride by CVD, nucleation rate is very slow such that steady-state deposition cannot be accomplished at early reaction time. For example, when tungsten is deposited at 450.degree. C. by a chemistry of WF.sub.6 --H.sub.2, a time delay of about 7 to 10 min is observed. Upon deposition of copper by MOCVD, nucleation on the surface of the titanium nitride does not occur at a high rate and it is observed that islands of copper crystal granules are sparsely dotted.

FIGS. 1 and 2 show the time delays which are observed in appearance when a tungsten thin film and a copper thin film are deposited on a titanium nitride substrate at a predetermined temperature, respectively. In FIG. 1, there are plotted the thicknesses of the tungsten thin film with regard to deposition times. Likewise, the thicknesses of copper thin film are plotted in FIG. 2.

Not only does slow nucleation take much time to obtain normal deposition of the thin films as shown in the figures, deleteriously aggravating thickness uniformity, but also it shows a disadvantage of decreasing the throughput. In addition, since the thin film grown is low in density, resistivity increases when it is applied for interconnection, which results in a significant decrease of device reliability.

It is reported a more improved method which asserts that a tungsten thin film with a higher density can be formed on a titanium nitride substrate by the chemistry of WF.sub.6, without time delay. To the end, the depositing method suggests that, at initial stage, only WF.sub.6 is charged for a certain time to let WF.sub.x adhere to the surface of the titanium nitride substrate and then, WF.sub.6 and reducing gas such as H.sub.2 or SiH.sub.4, are added simultaneously. In more detail, a substrate covered with titanium nitride is placed in a reaction chamber which is, then, evacuated to 1 mTorr or less. Subsequently, WF.sub.6 is charged into the reaction chamber at a predetermined temperature, e.g. about 300.degree. to 450.degree. C., for a certain time, to form nucleus. Thereafter, mixture gas of WF.sub.6 and H.sub.2 or SiH.sub.2 is charged to form a tungsten thin film. This is, after nucleation is done by charging only WF.sub.6 or activation is done by changing only WF.sub.6, the thin film grows under a reducing gas such as H.sub.2 or SiH.sub.4.

Where a metal thin film is formed by the above-mentioned method, the continuity of thin film can be secured while reducing the time delay to some extent. However, there are several suggestions what happens when the surface of titanium nitride is exposed to SF.sub.6.

Based on the fact that titanium nitride is very stable and shows low reactivity with WF.sub.6, it was suggested that nucleation occurs via a mechanism that looks as if tungsten had been deposited from WF.sub.6 over the oxide film, SiO.sub.2. Another suggestion was that tungsten seeds and TiF.sub.3 of solid phase are formed upon WF.sub.6 --TiN reaction, which was based on thermodynamic calculation but not verified by experiment.

Meanwhile, a experiment result was submitted that, since volatile TIF.sub.4 rather than TiF.sub.3 of solid phase is formed as a by-product by the chemistry of WF.sub.6 --TiN and easily evaporated from the surface of titanium nitride, no by-products are detected at the interface between tungsten and titanium nitride after deposition of tungsten.

In this experiment result, seed formation reaction was proposed as follows:

This reaction formula, however, cannot illustrate the time delay and tungsten islands locally formed on the surface of titanium nitride, neither. In addition, what influence the natural oxide film formed on the titanium nitride has on nucleation is not considered in the above-mentioned techniques.

In order to know the influence of the oxide layer of the titanium nitride on nucleation thermodynamic simulation was executed under WF.sub.6 --TiO.sub.2 system on the assumption that the oxide layer is TiO.sub.2 (s). FIG. 3 is the result of this thermodynamic simulation. As shown in this figure, when the temperature is 450.degree. C. or less, the oxygen of TiO.sub.2 (s) is transformed into WO.sub.3 (s) through the reaction formula ›II! as suggested below:

and Ti is eliminated in the form of TiF.sub.4 (g).

On the other hand, when the temperature is over 450.degree. C., tungsten-oxyfluoride (WOF.sub.4) is increasingly generated together with TiF.sub.4 as the consumption amount of the charged WF.sub.6 increases whereas WO.sub.3 (s) remains at the interface between tungsten and titanium nitride, deleteriously affecting the interface property. Thus, it is not desirable that only WF.sub.6 is charged for nucleation. What is worse, if WF.sub.6 is charged for long time, WF.sub.x penetrates along the grain boundaries of titanium nitride, having a serious influence on the lower layer of titanium nitride.

Therefore, it is an object of the present invention to overcome the above problems encountered in prior arts and to provide a method for pretreating the surface of a titanium nitride substrate, by which the nucleation rate of deposition material and the number of nucleation sites on the substrate can be increased.

It is another object of the present invention to provide a method for pretreating the surface of a titanium nitride substrate, by which a thin film with a uniform thickness and high density is formed on the substrate.

Based on the intensive and thorough research by the present inventor, the above objects could be accomplished by a provision of a method for pretreating a semiconductor substrate, comprising the steps of: placing a titanium nitride substrate in a reaction chamber and subjecting the reaction chamber to vacuum; purging the reaction chamber with an inert gas selected from the group consisting of N.sub.2, Ar and He and evacuating the reaction chamber into 1 mTorr or lower; treating the surface of the titanium nitride substrate with a reaction gas comprising WF; charging a reducing gas and a source gas for deposition material to form a thin film on the titanium nitride substrate.

The above objects and other advantages of the present invention will become more apparent by describing in detail the preferred embodiments of the present invention with reference to the attached drawings in which:

FIG. 1 is a plot of tungsten thin film thicknesses with regard to deposition times upon depositing a tungsten thin film on a titanium nitride at predetermined temperatures, showing apparent time delay to normal deposition;

FIG. 2 is a plot of copper thin film thicknesses with regard to deposition times upon depositing a copper thin film on a titanium nitride at predetermined temperatures, showing apparent time delay to normal deposition;

FIG. 3 shows thermodynamic simulation results in conventional WF.sub.6 --TiO.sub.2 system on the assumption that an oxide layer on a titanium nitride substrate is formed of TiO.sub.2 ;

FIG. 4 shows thermodynamic simulation results in the present WF.sub.6 --SiH.sub.4 --TiO.sub.2 system on the assumption that an oxide layer on a titanium nitride substrate is formed of TiO.sub.2 ;

FIG. 5 is Ti.sub.2p and N.sub.1s XPS spectra of titanium nitride surface in which a is Ti.sub.2p and N.sub.1s spectrum obtained by analyzing the titanium nitride thin film grown by reactive sputtering, b is Ti.sub.2p and N.sub.1s spectrum obtained by XPS analysis for the titanium nitride substrate which is surface-pretreated with WF.sub.6 --SiH.sub.4 /H.sub.2 gas mixture at 400.degree. C. for 20 seconds under 1 torr or lower; and

FIG. 6 is a plot showing exponentially decreased time delays with regard to increase of deposition temperatures upon using WF.sub.6 --SiH.sub.4 /H.sub.2 gas mixture under 1 torr, which is obtained by extrapolation.

The present invention is concerned with a CVD process for forming on a titanium nitride substrate a metal thin film such as aluminum or tungsten, in which the substrate is surface-treated by use of a reaction gas including WF, to increase the nucleation rate and site number of the metal material.

In more detail, first, a titanium nitride-deposited substrate is placed in a reaction chamber, which is then evacuated and purged up to 3 times with an inert gas such as nitrogen (N.sub.2), argon (Ar) or helium (He) gas.

Subsequently, the pressure of the reaction chamber is reduced into 1 mTorr or less and WF.sub.6 --SiH.sub.4 is charged for a predetermined time, to completely remove the oxide surface layer of the titanium nitride.

After ceasing to charge WF.sub.6 --SiH.sub.4, a source gas for deposition and a reducing gas are charged to deposit a thin film on the titanium nitride substrate.

During elimination of the oxide surface layer, the substrate is maintained at a temperature of about 300.degree. to 600.degree. C. under 10.sup.-3 -several tens Torr in the reaction chamber with about 0.1-1.0 partial pressure ratio of SiH.sub.4 to WF.sub.6.

Herein, besides WF.sub.6 --SiH.sub.4, WF.sub.6 --SiH.sub.4 --H.sub.2 and WF.sub.6 --H.sub.2 can be used as the gas mixture for the pretreatment of the oxide surface layer. Important in the case is the partial pressure ratio of SiH.sub.4 to WF.sub.6. As mentioned previously, H.sub.2 is charged in such a way that the total pressure within the reaction chamber may be in a range of about 10.sup.-3 to several tens torr while the partial pressure ratio of SiH.sub.4 to WF.sub.6 is maintained at about 0.1 to 1.0.

Gas mixture based on WF.sub.4 Cl.sub.2, for example, WF.sub.4 Cl.sub.2 --SiH.sub.4, WF.sub.4 Cl.sub.2 --H.sub.2 or WF.sub.4 Cl.sub.2 --SiH.sub.4 --H.sub.2, can be used for pretreatment, in which case the SiH.sub.4 gas may be substituted by Si.sub.2 H.sub.6 or Si.sub.3 H.sub.8.

In order to increase the effect of pretreatment as highly as possible, the deposition of thin film must be done immediately after the pretreatment by an in-situ process in which the vacuum is not broken. Examples of the thin film to be deposited comprise a copper film or aluminum film, which can be deposited by MOCVD using metal organic precursor, a tungsten film from WF.sub.6 source, and various metal silicides, e.g. WSi.sub.x and TiSi.sub.x.

A more detailed description will be given of surface pretreatment technology which comprises removing the oxide layer of titanium nitride surface by use of WF.sub.6 --SiH.sub.4 to facilitate the nucleation of the material to be deposited, in conjunction with the accompanying drawings.

When WF.sub.6 --SiH.sub.4 gas mixture is charged into a reaction chamber for a predetermined time on the assumption that the oxide layer on the titanium nitride surface is of TiO.sub.2 as mentioned previously, the reaction in WF.sub.6 --SiH.sub.4 --TiO.sub.2 (s) system will be considered.

Referring to FIG. 4, there are plotted the concentrations of reaction products with regard to temperatures which are obtained by thermodynamically simulating the reaction in WF.sub.6 --SiH.sub.4 --TiO.sub.2 (s) system. In this reaction, the concentrations of WF.sub.6 and SiH.sub.4 are 1 and 0.5 mole, respectively and pressure is maintained at 1 torr.

As shown in FIG. 4, it is apparent that tungsten is not formed independently of temperature, which is true of the reaction in WF.sub.6 --TiO.sub.2 (s) system of FIG. 3.

Meanwhile, the oxygen originating from TiO.sub.2 (s) is partially transferred to a form of WO.sub.3 (g) at 300.degree. C. (573K) or lower but is removed in a form of WO.sub.2 (g) at the entire temperature range.

In contrast with the reaction of FIG. 3, the reaction of FIG. 4 leads to the formation of two types of titanium fluoride, which is very sensitive to temperature. Nonvolatile TiF.sub.3 (s) is formed at low temperature range. When temperature reaches about 300.degree. C. (573K), the amount of TiF.sub.3 (s) drops while volatile TiF.sub.4 (s) increases. At about 320.degree. C. (593K) only TiF.sub.4 (s) forms.

Thus, the thermal treatment that employs WF.sub.6 --SiH.sub.4 gas mixture at a substrate temperature of not less than 300.degree. C. (573K) allows all the fluoride impurities to be removed in a form of TiF.sub.4 and can also eliminate the oxide surface layer in a form of WO.sub.2, resulting in a surface with a clear surface.

FIG. 5 shows an example that the oxide surface layer of the titanium nitride substrate is removed at 400.degree. C. by surface treatment using WF.sub.6 --SiH.sub.4 gas mixture. In this figure, a is Ti.sub.2p and N.sub.1s spectrum, which is obtained by analyzing the titanium nitride thin film grown by reactive sputtering through X-ray photoelectron spectroscopy (XPS). Ti.sub.2p2/3 peak shown at 458.3 eV on Ti.sub.2p spectrum is responsible for TiO.sub.2 while the peak shown at 455.0 eV is for Ti of TiN.

On N.sub.1s spectrum, the peak at 395.7 eV is responsible for oxynitride while the peak at 397.3 eV is for N of stoichiometric TiN.

As apparent from the result, an oxide layer is formed on the surface of TiN.

In FIG. 5, b is Ti.sub.2p and N.sub.1s spectrum, which is obtained by XPS analysis for the titanium nitride substrate which is surface-treated with WF.sub.6 --SiH.sub.4 /H.sub.2 gas mixture at 400.degree. C. for 20 seconds under 1 torr or lower. After reaction with the gas mixture, the Ti peak corresponding to titanium oxide almost disappears, whereas the peak at 456.4 eV, responsible for Ti of TiN, increases. Also, N.sub.1s spectrum is differentially shown. That is, the peak strength at 395.7 eV, corresponding to oxynitride film, is significantly decreased.

Therefore, the surface treatment of titanium nitride substrate with WF.sub.6 --SiH.sub.4 /H.sub.2 allows the oxide layer of the titanium nitride to be almost completely removed.

It is very important to choose appropriate times for the amount of WF.sub.6 and SiH.sub.4. For example, when the surface of titanium nitride is treated with WF.sub.6 --SiH.sub.4, if the treatment time is too short, the oxide surface layer is not removed completely. On the other hand, if the surface of titanium nitride is treated for too long time, WF.sub.x penetrates along grain boundary of titanium nitride, affecting other layers below the titanium nitride. In addition, a tungsten layer, undesirable layer, may be formed.

Following is of the choice of appropriate treatment time, which will be illustrated through an example. When tungsten is deposited on a titanium nitride surface as shown in FIG. 1, extrapolation is first done at any temperatures by linear fitting from the plot of thin film thickness with regard to deposition time, to investigate apparent time delay, which is then plotted by deposition temperatures.

FIG. 6 is an example of such extrapolation in which time delays are plotted with regard to deposition temperatures upon WF.sub.6 --SiH.sub.4 /H.sub.2 gas mixture under 1 torr. As shown in this plot, the time delay decreases exponentially as the deposition temperature increases.

In accordance with the present invention, about 60-80% of the time delay investigated is considered as the time suitable for surface treatment.

As described hereinbefore, the use of a reaction gas based on WF.sub.6, such as WF.sub.6 --SiH.sub.4, WF.sub.6 --H.sub.2 or WF.sub.6 --SiH.sub.4 --H.sub.2 can prevent oxide film such as WO.sub.3 from being formed at the interface of semiconductor substrate. In addition, the oxide surface layer of titanium nitride can be controlled appropriately, which results in restriction of the penetration of WF.sub.x into grain boundary of titanium nitride. The oxide layer-free titanium nitride substrate which is accomplished by the control of surface treatment time in accordance with the present invention facilitates not only nucleation of subsequent thin film but also brings about uniform distribution of nucleation sites, thereby forming a thin film with uniform thickness and remarkably increasing the density of the thin film.

Other features, advantages and embodiments of the invention disclosed herein will be readily apparent to those exercising ordinary skill after reading the foregoing disclosures. In this regard, while specific embodiments of the invention have been described in considerable detail, variations and modifications of these embodiments can be effected without departing from the spirit and scope of the invention as described and claimed.