Sequential chemical vapor deposition

Details Sequential chemical vapor deposition Sequential chemical vapor deposition

2001-05-24

2003-11-25

Meeks, Timothy H. (Department: 1762)

Coating processes

Direct application of electrical, magnetic, wave, or...

Plasma

C427S579000, C427S252000, C427S255310, C427S255394, C427S255700, C117S092000, C117S103000, C117S104000

Reexamination Certificate

active

06652924

ABSTRACT:

BACKGROUND OF THE INVENTION
CVD Reactor Technology
Chemical vapor deposition (CVD) reactors have been used for decades to deposit solid thin films and typical applications are coating tools, manufacture of integrated circuits, and coating jewelry. A. Sherman,
Chemical Vapor Deposition for Microelectronics
, Noyes Publications, New Jersey, 1987. Up to the 1960's many CVD reactors operated by exposing a heated object or substrate to the steady flow of a chemically reactive gas or gases at either atmospheric or reduced pressures. Since, in general, it has been desired to deposit films at as high a rate as possible as well as at as low a temperature as practical, the gases used to produce the film are extremely reactive (e.g., silane plus oxygen to deposit silicon dioxide). Then if the gases are allowed to mix for too long a time period before impinging the substrate, gas phase reactions can occur, and in extreme cases there can be gas phase nucleation and particles formed rather than deposition of continuous films. At the same time, the high rate of deposition and the reactive gases used makes it very difficult to coat large area substrates uniformly. This results in very complex and expensive commercial CVD reactors. A further complication with this method is that in some cases the films deposited do not conformally coat non-uniform surfaces. This can be particularly deleterious in the manufacture of integrated circuits.
In the 1960's it was realized that we could lower the temperature required for thin film deposition at acceptable rates by creating a low pressure glow discharge in the reactive gas mixture. The glow discharge produces many high energy electrons that partially decompose the reactive gases, and these gas fragments (radicals) are very reactive when they impinge on a surface even at moderate temperatures. Although using a glow discharge allows lower temperature operation, commercial reactors are very complex and expensive, since uniform deposition over large area substrates is even more difficult due to the inherent nonuniformity of glow discharges and due to the added expense of complex high frequency power supplies. Also, this technique can often lead to degradation of the film conformality, due to the highly reactive nature of the radicals.
In the 1970's atomic layer epitaxy (ALE) was developed in Finland by T. Suntola and J. Anston. U.S. Pat. No. 4,058,430 describes how they grew solid thin films on heated objects. This process involves exposing the heated surface to a first evaporated gaseous element, allowing a monolayer of the element to form on the surface, and then removing the excess by evacuating the chamber with a vacuum pump. When a layer of atoms or molecules one atom or molecule thick cover all or part of a surface; it is referred to as a monolayer. Next, a second evaporated gaseous element is introduced into the reactor chamber. The first and second elements combine to produce a solid thin compound monolayer film. Once the compound film has been formed, the excess of the second element is removed by again evacuating the chamber with the vacuum pump. The desired film thickness is built up by repeating the process cycle many (e.g., thousands) times.
An improvement to this technique was described in a later patent issuing in 1983 to T. Suntola, A. Paakala and S. Lindfors, U.S. Pat. No. 4,389,973. Their films were grown from gaseous compounds rather than evaporated elements so the process more closely resembles CVD. This was recognized to be especially advantageous when one component of the desired film is a metal with low vapor pressure, since evaporation of metals is a difficult process to control. With this approach, films were deposited by flow reactors similar to a conventional CVD reactor, where the excess of each gas is removed by flowing a purge gas through the reactor between each exposure cycle. This approach was limited to only a few films, depending on the available gaseous precursors, and all of these films were not as contamination free as desired. We will refer to this process as sequential chemical vapor deposition.
An alternative approach to operating a sequential chemical vapor deposition reactor would be to operate a non-flow vacuum system where the excess gaseous compound of each sequence is removed by vacuum pumps in a manner similar to the original Suntola 1977 process. H. Kumagai, K Toyoda, M. Matsumoto and M. Obara,
Comparative Study of Al
2
O
3
Optical Crystalline Thin Films Grown by Vapor Combinations of Al
(
CH
3
)
3
/N
2
O and Al
(
CH
3
)
3
/H
2
O
2
, Jpn. Appl. Phys. Vol. 32, 6137 (1993).
An early application of sequential chemical vapor deposition was for deposition of polycrystalline ZnS thin films for use in electrochromic flat panel displays. M. Leskela,
Atomic Layer Epitaxy in the Growth of Polycrystalline and Amorphous Films
, Acta Polytechnica Scandinvica, Chapter 195, 1990. Additional studies have shown that other commercially important solid films of different compounds, amorphous and polycrystalline, can be deposited by this technique on large area glass substrates. Among these other films are sulfides (strontium sulfide, calcium sulfide), transition metal nitrides (titanium nitride) and oxides (indium tin oxide, titanium dioxide). Elsewhere, this technique has been developed as a means of depositing epitaxial layers of group III-V (gallium indium phosphide) and group II-VI (zinc selenide) semiconductors, as an alternative to the much more expensive molecular beam epitaxy process.
To applicant's knowledge the only literature discussing sequential chemical vapor deposition of elemental films are those that deposit elemental semiconductors in group IVB such as silicon and germanium. One such study, S. M. Bedair,
Atomic Layer Epitaxy Deposition Process
, J. Vac. Sci. Technol. B 12(1), 179 (1994) describes the deposition of silicon from dichlorosilane and atomic hydrogen produced by a hot tungsten filament. By operating the process at 650° C. deposition of epitaxial films are described. Deposition of diamond, tin and lead films, in addition to silicon and germanium by an extraction/exchange method in conjunction with a sequential processing scheme similar to sequential chemical vapor deposition has also been reported M. Yoder, U.S. Pat. No. 5,225,366. Also although some of the studies reported have explored processes that may be useful at moderate temperatures, most require undesirably high substrate temperatures (300-600° C.) to achieve the desired sequential chemical vapor deposition growth of high quality films.
Conformal Films Deposited at Low Temperatures for Integrated Circuit Manufacture
A continuing problem in the commercial manufacture of integrated circuits is the achievement of conformal deposition of dielectric (e.g., silicon dioxide, silicon nitride) or conducting (e.g., aluminum, titanium nitride) thin solid films over large area wafers (e.g., 12 inches in diameter). A film is conformal when it exactly replicates the shape of the surface it is being deposited on.
In one paper by D. J. Ehrlich and J. Melngailis,
Fast Room
-
Temperature Growth of SiO
2
Films by Molecular
-
layer Dosing
, Appl. Phys.Lett. 58, 2675(1991) an attempt was reported of layer by layer deposition of silicon dioxide from silicon tetrachloride and water. Although the films appear to be very conformal, there is no discussion of film quality or density, and it is likely that these films are porous making them unsuitable for thin film applications. In support of this conclusion, we can refer to a study by J. F. Fan, K. Sugioka and K. Toyoda,
Low
-
Temperature Growth of Thin Films of Al
2
O
3
with Trimethylaluminum and Hydrogen Peroxide
, Mat. Res. Soc. Symp. Proc. 222, 327 (1991). Here, aluminum oxide deposited at 150° C. was compared to deposition at room temperature. In this case, the room temperature films thickness reduced from 2270 Å to 1200 Å upon annealing at 150° C. for 15 minutes confirming the high porosity of the film deposited at room temperature. Another attempt to deposit silicon dio

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