Method of Forming Thin Film

1. Field of the Invention

This invention relates to a method of forming a thin film, and particularly to a method of forming a tungsten thin film by chemical vapor deposition (hereinafter referred to simply as "CVD").

2. Description of Related Art

Recent semiconductor manufacturing technology features an advanced method for the densification and sophistication of integrated circuits. As a result, the load to individual integrated circuits has increased, and various problems have arisen. More precisely the fineness of the wiring pattern induces an increased current density, and electromigration, a type of wire breakage, is likely to occur. In particular, aluminum (Al) film which is presently used as a wiring material is formed by the sputtering method, and the method degrades step coverage in fine hole areas, which inhibits film formation at the bottom and the side wall of the holes and which results in a very thin film on these portions as compared with the film on a flat portion. As a result, wire breakage is further likely to occur on the sidewall surface and thus lowers the reliability of the integrated circuit.

For wiring in such fine hole areas, a tungsten (W) film formed by CVD is currently drawing attention. The W film formed by CVD provides a favorable conformal step coverage even on a fine hole having an aspect ratio of 2 or more, and is advantageous in that it has a high anti-electromigration resistance.

Generally, the CVD process forms a W film by introducing WF.sub.6 raw material gas and H.sub.2 reducing gas onto a substrate which has been heated to 400.degree. to 500.degree. C., and by reacting these gases. The underlying layer on which the W film is formed on often contains a TiN film. However, it is difficult to generate nuclei during the initial period of the reaction between WF.sub.6 and H.sub.2 on the TiN film, and results in a non-uniform distribution of the formed film. In that case, normally SiH.sub.4 gas is introduced during the initial period of the reaction to generate nuclei on the whole substrate surface based on the reaction between WF.sub.6 and SiH.sub.4, then high-speed film formation is carried out by the reaction between WF.sub.6 and H.sub.2.

The W film formed by the above-described conventional method has the problems described below. The W film formed by reacting WF.sub.6 and H.sub.2 provides the favorable characteristic of step coverage. However, since large crystal grains grow in the W film formation, the W film consisting of the large crystal grains has a rough surface and therefore a poor morphology. As a result, the W film has the disadvantage that the reflectance of the film's surface significantly decreases. This disadvantage causes several serious problems in lithography and etching.

Conventional W-film formation is carried out under a relatively high pressure ranging from several tens of Torr to around atmospheric pressure to improve the surface morphology. By this method, however, the morphology can not be fully improved. It is known that, in addition to applying such a high pressure, when N.sub.2 gas is added in the film-forming stage, the surface morphology is further improved. The addition of N.sub.2 gas, however, induces an increase in the specific resistivity of the film and increases the wiring resistivity so as to degrade the performance of integrated circuits.

Accordingly, the object of this invention is to provide a method for manufacturing a thin film, enabling the formation of a W film having a good surface morphology and high reflectance while avoiding the degradation of characteristics such as specific resistivity.

The object of this invention for manufacturing a thin film is to employ a method to deposit a W film on a heated substrate using CVD, wherein the method includes a first step, as the initial stage of film formation, of generating tungsten nuclei on the substrate surface by a reaction between WF.sub.6 and SiH.sub.4, and a second step, following the nuclei-generating first step, of forming the tungsten film by a reaction between WF.sub.6 and wherein H.sub.2, and the first step and the second step are alternately repeated.

In the above-described method, in the case that the final thickness of the W film is to be a specified thickness, the total process for forming the W film is divided into a number of unit processes, each unit process consisting of first and second steps, wherein the unit process is repeated for a number of times equal to the number of divided unit processes.

Generally, the growth of a W film by CVD is induced by the mass growth of W-crystal grains. Observing the size of the crystal grains in the W film formed by CVD, the film layer in a thickness range of from 1000 to 2000 .ANG., for example, consists of fine grains of 500 .ANG. or less. However, the film layer in a thickness range of 2000 .ANG. or more consists of the crystal grain size of larger than 2000 .ANG.. Furthermore, in the film layer having a thickness of 1 .mu.m, for example, the grain size becomes 4000 .ANG.. Accordingly, a feature of the W film is the grain size increases with the growth of the film.

It is presumed that the increase of crystal grain size during the film-growth stage in W-film formation by CVD is induced by the preferential growth of grains in a specific direction of the crystal plane. The formation of fine crystal grains during the initial period of the film-growth stage probably begins with the uniform generation of nuclei on the substrate by the reaction of WF.sub.6 and SiH.sub.4 occurring in the initial period of the film formation stage.

Based on the above-described concept, this invention is derived by focusing on the formation of fine crystal grains during the initial period of the formation of the W-film growth by CVD, and this invention makes it possible to maintain the fineness of the crystal grains even when the W film increases in thickness by the introduction of a step to generate nuclei during the W-film growth stage.

According to the invention, during the initial period of the film formation stage, the reaction between WF.sub.6 and SiH.sub.4 is carried out to enable the uniform nuclei generation on the underlying layer TiN, then the growth of the W film is performed through the reaction of WF.sub.6 and H.sub.2. As the next step, before the W-film growth and before the crystal size increase, the nuclei generation step is again performed through the reaction of WF.sub.6 and SiH.sub.4, then the W-film growth is carried out by the reaction of WF.sub.6 and H.sub.2. If necessary, the nuclei generation step and the W-film formation step are repeated several times. By the formation of a thin film following the above-described procedure, the fineness of the crystal grains is maintained even when the thickness of the W film increases to some degree.

FIG. 1 is a photograph of a cross-section of a thin W film formed on a substrate according to the present invention.

FIG. 2 is a photograph of a cross-section of a thin W film formed on a substrate according to a conventional method.

This invention is described below referring to embodiments in the examples of film-forming conditions.

The total thickness of the W film was selected as 1 .mu.m, and the W-film growth of 1 .mu.m thickness was carried out by four operational cycles. Each of the four W-film growth cycles consist of a nuclei-generating step during the initial period of the film formation and a succeeding W-film growth step. The nuclei-generating step was performed by the introduction of WF.sub.6 gas at 10 sccm and SiH.sub.4 gas at 2 sccm for 10 sec., under an internal pressure of 1.5 Torr. The W-film growth step was carried out through the introduction of WF.sub.6 gas at 100 sccm and H.sub.2 gas at 1000 sccm for 20 sec., at a substrate temperature of 500.degree. C. and under 40 Torr of internal pressure. The period of the W-film growth was the time necessary for the W-film to reach a thickness of 2500 .ANG., which was equal to the film thickness large enough to maintain the fineness of the crystal grains. By the film formation following the above-described condition, the surface morphology and reflectance were improved.

The total thickness of W film was selected to be 4000 .ANG., and the growth of the W-film thickness was carried out in two operational cycles. Each of the two operational cycles contained a nuclei-generating step in the initial stage of the film formation and a succeeding W-film growth step, similar to Example 1. The conditions of the film formation as the nuclei generation and of the W-film growth as the W-film growth were the same as in Example 1.

According to the photograph shown in FIG. 1, it is clear that the W film which was prepared using the method of this invention for forming a thin film maintains the fineness of the crystal grains even after the growth of W film to a thick state. On the contrary, a W film prepared by the conventional thin-film-forming method, which is shown in FIG. 2, clearly gives large crystal grains after the growth of the W film to a thick state. Therefore, the method of this invention for forming a thin film improves the surface morphology. As an example of the characteristics, the W film formed by the method of this invention has a specific resistivity of 9.2 .mu..OMEGA.-cm, a reflectance of 85% (vs Si), and a film stress 4.1.times.10.sup.9 dyn/cm.sup.2, while the film formed by the conventional method has a specific resistivity of 8.7 .mu..OMEGA.-cm, reflectance of 45% (vs Si), and a film stress of 4.5.times.10.sup.9 dyn/cm.sup.2. These characteristics show that the method of this invention improves the reflectance of the film's surface.

With the application of the thin-film-forming method of this invention which has the advantages described above, the step coverage in fine hole areas is improved because the method of this invention decreases the overhanging of the film above the hole compared with the conventional method for forming a thin film. The phenomenon presumably comes from a change which occurs in the growth of crystal grain through the step of forming nuclei during the growth of the W film.

As clearly shown in the description given above, this invention applies the W-film-forming method by CVD, wherein the step for generating nuclei and the step of W-film growth to maintain the fineness of the crystal grains are put together as a single unit-process set and wherein the unit process is repeated as many times as necessary as determined by the target thickness of the W film to be formed. Consequently, the prepared W film has good surface morphology and has high reflectance while the degradation of other characteristics, such as specific resistivity, is avoided.