Selective Etching Process for Boron Nitride Films

1. Field of the Invention

The present invention relates to a process for etching a film of boron nitride with high selectivity to a layer of silicon dioxide or silicon nitride.

2. Description of the Prior Art

In recent years, silicon nitride films have been increasingly used as interlevel dielectrics in the fabrication of ULSI devices. Silicon nitride films have found favor because they provide various advantages, such as good insulation characteristics, low deposition temperature and high passivation effect against moisture and alkali metal ions, as well as satisfactory conformal step coverage, crack resistance, etc. However, the dielectric constant of silicon nitride is relatively high, and this causes relatively large parasitic capacitance and relatively long propagation delay times between devices. In order to reduce the wiring capacitance, a dielectric material having a low dielectric constant is required; this material should also provide the other advantages provided by silicon nitride films.

One approach to solving the above problem is to use silicon dioxide as an interlevel dielectric. While silicon nitride has a dielectric constant of 7, silicon dioxide has a dielectric constant of 4. Recently, however, it has also been demonstrated that boron nitride films, that are boron-rich and prepared by thermal CVD (1000.degree. C.), have a low dielectric constant of less than 4, with satisfactory insulation and thermal stability characteristics. For a review of various techniques for preparation of boron nitride films, see Arya, et al., "Preparation, Properties And Applications Of Boron Nitride Thin Films," Thin Solid Films, 157 (1988), pp. 267-282.

Boron nitride films having a low dielectric constant have also been prepared by introduction of a small amount of carbon or silicon. For example, in Yamada, et al., "Improvements Of Stress Controllability And Radiation Resistance By Adding Carbon To Boron-Nitride", J. Electrochem. Soc., Vol. 137, No. 7 (Jul. 1990), pp. 2242-2246, a conventional plasma-enhanced CVD system, using a gas mixture of diborane, ammonia and methane, was employed to deposit carbon-containing boron nitride films (400.degree.-500.degree. C.). In Maeda, et al., "Low Dielectric Constant Amorphous SiBN Ternary Films Prepared By Plasma-Enhanced Deposition", Jap. J. of Applied Physics, Vol. 26, No. 5 (May 1987), pp. 660-665, silicon-containing boron nitride films were deposited by conventional plasma-enhanced CVD using a diborane, ammonia and silane gas mixture.

In the fabrication of ULSI devices, interlevel boron nitride films are etched to form contact vias. A number of techniques are disclosed in the art for etching boron nitride films. For example, U.S. Pat. No. 4,226,665, issued Oct. 7, 1980 to Mogab, describes the use of a plasma formed from CF.sub.4, with a small amount of oxygen (8%), to etch boron nitride and silicon nitride films. Also Maeda, et al., "A Low-Permittivity Interconnection

Using An SiBN Interlayer", IEEE Transactions On Electron Devices, Vol. 36, No. 9 (Sep. 1989), pp. 1610-1614, show the use of oxygen or a CF.sub.4 /H.sub.2 mixture to etch a SiBN layer, overlying a metallization pattern (Al-Si).

During etching of the films, it is also desirable that the overlying mask be replicated in the boron nitride without dimensional loss, i.e. that the etch be anisotropic. Further, it is important that there be minimal etching of the mask, typically silicon nitride or silicon dioxide, i.e. that the etch be highly selective to the overlying mask. Selectivity to silicon nitride and silicon dioxide is also required in that the boron nitride film frequently overlies a layer of one of these materials.

Processes which selectively etch to overlying or underlying layers of silicon dioxide have been developed, for use where silicon nitride is the interlevel dielectric material. See, for example, U.S. Pat. No. 4,793,897, issued Dec. 27, 1988 to Dunfield, et al., which discloses a process for etching silicon nitride selectively to underlying silicon dioxide, by employing a gaseous mixture of a fluorine-containing gas, e.g. SiF.sub.4, and oxygen; the mixture contains a predominant amount of the fluorine-containing component. Such a process is also disclosed in U.S. Pat. No. 4,303,467, issued Dec. 1, 1981 to Scornavacca, et al., which discloses a plasma treatment with SiF.sub.4.

Other processes for etching silicon nitride or silicon dioxide are disclosed in U.S. Pat. No. 4,374,698, issued Feb. 22, 1983 to Sanders, et al. (etching of silicon nitride or silicon dioxide, by employing a mixture of CF.sub.4 and small amounts of oxygen and a halogen-containing compound, e.g. CF.sub.2 Cl.sub.2); and U.S. Pat. No. 4,814,041, issued Mar. 21, 1989 to Auda (etching of silicon dioxide with a fluorine-containing gas, e.g. CF.sub.4, and a small amount of oxygen).

Thus, there is a need in the art for a process for etching a film of boron nitride with high selectivity to a layer of silicon dioxide or silicon nitride, and which achieves anisotropic etch profiles, with reasonably high etch rates.

It is against this background that the present invention presents an improved process for etching a film of boron nitride. In accordance with the process, the film is exposed to a plasma formed from a mixture of an oxygen-containing gas and a small amount of a fluorine-containing gas, and it is etched with a high selectivity to a layer of silicon dioxide or silicon nitride. In practice, etch selectivities to silicon dioxide or silicon nitride, ranging generally from about 10/1 to about 30/1, can be achieved. In addition, the film of boron nitride is etched at highly satisfactory etch rates, which are not significantly depressed when the films are doped with carbon or silicon. And, the etch provides anisotropic profiles, and is suitable for use in a wide range of processes for etching boron nitride in a controlled manner.

Accordingly, in a preferred embodiment, the invention also provides an improved process for fabricating a semiconductor device, in which a film of boron nitride is formed on a processed semiconductor substrate, and a masking layer of silicon dioxide or silicon nitride is formed on the film. The substrate is exposed to a plasma formed from a mixture of an oxygen-containing gas and a small amount of a fluorine-containing gas, so as to etch the film at a higher rate than the masking layer, the exposing being terminated when a predetermined portion of the film has been removed .

In the description to follow, reference will be made to the accompanying drawings:

FIGS. 1-3 diagrammatically show successive stages of manufacture of a semiconductor device, using the etching process of the present invention;

FIG. 4 shows the rates at which silicon nitride, silicon dioxide, boron nitride and silicon doped boron nitride disposed on a silicon substrate are etched, using the etching process of the present invention;

FIG. 5 shows the rates at which silicon nitride, silicon dioxide, boron nitride and silicon doped boron nitride disposed on a silicon substrate are etched, using the etching process of the present invention, including a plasma formed from a mixture of 10% CF.sub.4 /O.sub.2 ; and

FIGS. 6A and 6B illustrate the anisotropic profiles which result from etching a film of boron nitride with an overlying masking layer of silicon nitride, using the etching process of the present invention.

Referring to FIGS. 1-3, the etching process of the invention is illustrated by showing various stages in the fabrication of a semiconductor device. As shown in FIG. 1, a layer 2 is formed on a processed semiconductor substrate 1. Generally, the substrate 1, while for ease of illustration is shown as featureless, may actually include transistors, capacitors and other integrated circuit elements, as is well known to those skilled in the art. The layer 2 is typically a dielectric, such as silicon dioxide, silicon nitride, a spun-on glass (e.g. borophosphosilicate glass or phosphosilicate glass), a polymer (e.g. polyimide), or aluminum oxide, but may also be a wide variety of other materials, such as metals, and so forth.

Next, a film of boron nitride 3 is applied to the layer 2. Preferably, the film 3 is deposited by plasma enhanced chemical vapor deposition by reacting diborane (B.sub.2 H.sub.6) with ammonia in a carrier gas, e.g. nitrogen, employing a relatively high ammonia/diborane ratio of greater than about 5/1, and more preferably, greater than about 7/1, and a temperature of about 400.degree. C. However, a wide variety of preparation techniques, such as disclosed in Arya, et al., "Preparation, Properties And Applications Of Boron Nitride Thin Films", Thin Solid Films, 157 (1988), pp. 267-282; and in Yuzuriha, et al., "Structural And Optical Properties Of Plasma-Deposited Boron Nitride Films", Thin Solid Films, 140 (1986), pp. 199-207, may be employed as should be apparent to those skilled in the art. Further, the film 3 may also contain a dopant, such as silicon or carbon; such films are also known to those skilled in the art as described above. Typically, the films 3, which contain silicon or carbon, are formed by plasma-enhanced chemical vapor deposition, by adding silane or methane to the ammonia/diborane reaction mixture.

A masking structure is then formed on the boron nitride film 3. Still referring to FIG. 1, the masking structure preferably includes a lower layer 4 of silicon dioxide or silicon nitride, and an upper layer 5 of conventional photoresist. After the photoresist is exposed and developed using conventional photolithographic techniques, the underlying layer 4 is etched to form a desired pattern. Typically, the layer 4 is etched by exposure to plasma formed from, e.g. CF.sub.4 or CHF.sub.3, in accordance with well known techniques. The photoresist layer 5 is then stripped to provide the structure shown in FIG. 2.

After patterning the layer 4, the exposed boron nitride film 3 is exposed to a plasma formed from a mixture of an oxygen-containing gas and a small amount of a fluorine-containing gas, in accordance with the process of the present invention. Preferably, the oxygen-containing gas is oxygen, CO or CO.sub.2, with oxygen being the most preferred; while the fluorine-containing gas is a fluorocarbon, e.g. CF.sub.4 or CHF.sub.3, SF.sub.6 or NF.sub.3, where CF.sub.4 is particularly preferred.

Generally speaking, a predominant amount of the oxygen-containing gas is employed, and the fluorine-containing gas is used in an amount ranging from about 1 to about 30 percent by volume of the mixture. It has been found that by increasing the proportion of fluorine-containing gas in the mixture, higher etch rates of the boron nitride film 3 can be obtained. However, at the same time, it has also been found that the etch becomes less selective to the boron nitride film 3. That is, at higher proportions of the fluorine-containing gas, the etch rate of an overlying or underlying layer of silicon dioxide or silicon nitride will also increase, resulting in a loss of selectivity. Specifically then, to maximize the boron nitride etch rate while also providing high selectivity, it is more preferred to employ the fluorine-containing gas in an amount ranging from about 5 to about 15 percent by volume of the mixture.

The etch is continued until the desired amount of the boron nitride film 3 has been removed. Generally, the pattern is transferred to the boron nitride film 3, without appreciable etching of the underlying layer 2, to produce the structure as shown in FIG. 3. Also, the etch produces anisotropic profiles, without significant undercutting of the boron nitride film 3 beneath the overlying layer 4.

The following examples are provided to further illustrate the invention.

Blanket films of boron nitride, silicon doped boron nitride, silicon nitride and silicon dioxide were formed on silicon substrates. The boron nitride and silicon doped boron nitride films were formed by plasma enhanced chemical vapor deposition (PECVD), using an AME 5072 SWE (Applied Materials Etcher 5072, Single Wafer Etcher; available from Applied Materials, Inc. Santa Clara, Calif.), under the following conditions:

The silicon nitride films were formed by low pressure chemical vapor deposition (LPCVD), using a conventional LPCVD system, under the following conditions:

The silicon dioxide films were thermally grown at 1000.degree. C. and 1 atm pressure in HCl/O.sub.2 ambient (HCl, 19 sccm, and O.sub.2, 232 sccm).

The blanket films were then etched, using an AME 5072 SWE, under the following conditions:

The etch rates of the films were as follows:

The etch rates for the blanket films are also shown in FIG. 4. As shown, the etch rates for the boron nitride and silicon doped boron nitride films generally increase with increasing proportions of CF.sub.4. However, the rates of increase are greatest between about 10-30% CF.sub.4 (by volume). Greater selectivity to the boron nitride and silicon doped boron nitride films is also shown at lower proportions of CF.sub.4, i.e. at less than about 30% CF.sub.4. Selectivities of greater than 10/1 are shown at 10 and 15% CF.sub.4, as compared with the silicon dioxide films, i.e. 632/52 and 921/74, respectively.

Blanket films of boron nitride, silicon doped boron nitride, silicon nitride and silicon dioxide were formed on silicon substrates, as in Example 1. These films were then etched, as in Example 1, except that he etch time varied and the plasma was formed form a mixture of 10% CF.sub.4 /O.sub.2. The etch rates of the films were as follows:

The etch rates for the blanket films are also shown in FIG. 5. As shown, the etch was highly selective; selectivities generally greater than 10/1, or in some cases greater than 20/1, as compared with the silicon dioxide and silicon nitride films, were obtained.

The following procedure was carried out to etch silicon doped boron nitride films masked with a layer of silicon nitride:

A film of silicon doped boron nitride was formed on a P- silicon wafer, as in Example 1, to a thickness of 8000 .ANG.. A layer of silicon nitride was then formed on the film by LPCVD as in Example 1, to a thickness of 2000 .ANG.. Next, a layer of photoresist was spin applied to the silicon nitride layer to a thickness of 0.6.mu., and exposed and developed to form a desired pattern. After etching the silicon nitride using a plasma formed from CF.sub.4, the remaining photoresist was stripped. The pattern was then transferred to the silicon doped boron nitride film, using the etch conditions of Example 1 and a plasma formed from a mixture of 10% CF.sub.4 /O.sub.2.

The results showed an etch rate of silicon doped boron nitride of 3052 .ANG./min, with a high selectivity with respect to the LPCVD silicon nitride. Also, vertical side walls were obtained, with good etch uniformity. The etch profiles are shown in FIGS. 6A and 6B.

It should also be apparent to those skilled int he art that various modifications can be made without departing form the spirit and scope of the invention. For example, the actual etch conditions may vary over wide ranges, such as the following preferred conditions:

Also, the thicknesses of the boron nitride films and the overlying and/or underlying layers may depend on many factors, depending upon the particular application. For example, thicknesses of 50-2000 .ANG. and 10-5000 .ANG. for the boron nitride films and the layers, respectively, would be typical. In any event, a rapid etch of boron nitride, with high selectivity to silicon nitride and/or silicon dioxide, demonstrating anisotropic profiles and good uniformity, is achieved.