Organic compounds -- part of the class 532-570 series – Organic compounds – Silicon containing
Reexamination Certificate
2001-02-07
2002-06-25
Shaver, Paul F. (Department: 1621)
Organic compounds -- part of the class 532-570 series
Organic compounds
Silicon containing
C556S478000, C556S487000
Reexamination Certificate
active
06410770
ABSTRACT:
BACKGROUND OF THE INVENTION
Trimethylsilane and other methylhydridosilanes are utilized as precursors for chemical vapor deposition (CVD) of low dielectric constant silicon dioxide-rich films in a variety of process protocols. These films are sometimes referred to as “carbon doped silicon dioxide” and “hydrogenated oxidized silicon carbon.” Such low dielectric constant (“low-k”) films, which also exhibit thermal stability, may be useful in advanced semiconductor integrated circuit technology. Although the method of deposition is not a subject of this patent, a discussion of the technology is useful in understanding the critical nature of precursor purity for the technology.
In the CVD process, the trimethylsilane or methylhydridosilane precursor is introduced into a deposition chamber containing the substrate in the presence of an oxidizing ambient. At elevated substrate temperatures (typically greater than about 300° C.), N
2
O and O
2
are used in a plasma assisted oxidation of the trimethylsilane, and the process is known as plasma enhanced CVD (PECVD).
The first step in the oxidation of the silane precursors is thought to be the conversion of the hydride substitution to an oxygen substitution, either by direct oxidation of the silyl hydride or through a trimethylsilyl radical intermediate. A mechanistic study, which examines the oxidation of trimethylsilyl radicals by N
2
O, has been reported by Lein and Potzinger (
Organometallics
, 19, 4701, 2000). The second step is the further oxidation of the silane, resulting in removal of the methyl groups and the formation of films dominated by O—Si—O bonds, with some Si-C bonds maintained.
The resulting films thus contain a silicon-oxygen network that is interrupted by organic groups such as methyl. It has been shown (N. Hendricks, Proc. Of the 6
th
International Dielectrics for ULSI Multilevel Interconnection Conference (DUMIC), Santa Clara, Calif., February 2000, p. 17), that the incorporation of methyl or other organic groups into the silicon-oxygen network has a significant effect on the thermal, physical, and chemical properties of the films. For example, the films exhibit reduced densities relative to silicon dioxide, resulting in lower dielectric constants. Typically, low dielectric constant films contain about 3 to 20 atom % carbon (atom % carbon=C/(C+Si+O)).
An example of a nitrous oxide PECVD method is provided by Loboda, et al. (1998 Fall Mtg Electrochemical Society Preprint). Using such a process, films containing a random network of C—Si—C and O—Si—O bonds were prepared. These films were shown to exhibit k<3.0, as well as low stress, low leakage current density, and high thermal and oxidative stability.
U.S. Pat. No. 6,147,009 of Gill provides an example of a PECVD process using a gas that contains oxygen. In this process, utilizing an organosilicon compound having a ring structure, the precursor gas contains silicon, carbon, nitrogen, and optionally oxygen, and may also be mixed with germanium, nitrogen, and/or fluorine-containing gases. The result is a non-polymeric hydrogenated silicon carbon or non-polymeric hydrogenated oxidized silicon carbon film on a substrate. These films are disclosed to exhibit dielectric constants<4.0, to be thermally stable up to 400° C., and to display low crack propagation in water.
It is also possible to perform chemical vapor deposition at temperatures below 300° C. For example, European Patent No. 1,050,600 of Xia, et al. discloses a thermal CVD process in which a carbon-doped silicon oxide layer is deposited from a process gas of ozone and an organosilane precursor containing at least one Si—C bond. The substrate is heated to less than 250° C. during the deposition, and results in the formation of a material that may be useful as the dielectric layer in integrated circuits.
Another low temperature CVD process is described by Yaue, et al. in U.S. Pat. Nos. 6,054,379 and 6,072,227. The low-k dielectric (2.5≦k≦3.0) material produced by such a process, known as Black Diamond™, is referred to as oxidized organo-silane. The CVD process, which is carried out at temperatures less than 100° C., uses gaseous precursors of an organo-silane and an oxidizer, and is carried out in a low-power capacitively coupled plasma. It is taught that the resulting material has excellent barrier properties.
As noted above, methylhydridosilanes are promising precursors for the chemical vapor deposition of low dielectric constant films which exhibit the properties needed for use in high-volume manufacturing processes. These compounds, also known as methylsilanes, have the general formula (CH
3
)
n
SiH
(4−n)
, in which n=1-3, and are typically prepared by the reduction of analogous halosilanes such as (CH
3
)
n
SiCl
(4−n)
, in which n=1-3. Alternatively, trimethylsilane can be prepared via dimethylchlorosilane, a multi-step synthetic route. In both cases, the resulting products typically contain small amounts of chlorides, which can be detrimental to device performance. Specifically, chloride and other mobile ions reduce the electrical properties of the films. In the worse case, at high levels, such impurities can cause current leakage. Therefore, to ensure optimal device performance, it is desirable to remove all traces of chloride from the silane precursors. As a result, there is a need in the art for a process for preparing alkylsilanes such as methylsilane in which chloride content is substantially minimized or eliminated.
BRIEF SUMMARY OF THE INVENTION
The invention includes a process for producing an alkylsilane, comprising reducing an alkoxysilane in the presence of an alkali metal hydride in the presence of a solvent to form an alkylsilane, wherein the alkylsilane has a boiling point lower than a boiling point of the solvent.
The invention also includes a chlorine-free alkylsilane formed from the reduction of an alkoxysilane in the presence of an alkali metal hydride.
DETAILED DESCRIPTION OF THE INVENTION
This invention provides a new route for the synthesis of alkylsilanes that does not involve the use of chloride or other halogen-containing compounds. As a result, there is no detectable chlorine in the resulting products, and the silanes exhibit low levels of other impurities as well. Another advantage to the synthetic route according to the present invention is that in the absence of metal halides, particularly aluminum chloride, rearrangement reactions are less likely to occur. Such rearrangements may lead to the formation of undesirable and/or potentially dangerous silane byproducts. Alkylsilanes made in accordance with the present invention are thus useful for microelectronic applications and minimize or eliminate the negative effects of chloride.
The method of the present invention may also be applicable to the synthesis of arylsilanes. Such a chloride-free synthetic route would be particularly desirable because in the presence of metal halides such as aluminum chloride, rearrangements of arylchlorosilanes can occur, producing undesirable and potentially dangerous silane byproducts.
The process involves the reduction of alkoxysilanes, such as methoxysilane, ethoxysilane and similar compounds, in a high-boiling solvent, preferably diglyme (bis(methoxyethyl)ether) or a similar high-boiling solvent, using lithium aluminum hydride or a similar non chlorine-containing alkali metal hydride. Examples of other alkali metal halides within the scope of the invention are sodium hydride, potassium hydride, and sodium aluminum hydride, though the use of other reducing agents is possible as well. Preferably, the reducing agent is an alkali metal aluminum hydride, and most preferably, is lithium aluminum hydride. As the reduction proceeds, the alkylsilane product is collected, preferably by distillation. The process preferably occurs at temperatures lower than about 90° C., and more preferably at temperatures below about 75° C. However, temperatures above 90° C. may also be used.
As an example, the overall reaction to form methylsilane is shown in Formula
Arkles Barry C.
Larson Gerald
Pan Youlin
Akin Gump Strauss Hauer & Feld L.L.P.
Gelest Inc.
Shaver Paul F.
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