Single-crystal – oriented-crystal – and epitaxy growth processes; – Forming from vapor or gaseous state
Reexamination Certificate
2002-03-15
2004-03-16
Chen, Bret (Department: 1762)
Single-crystal, oriented-crystal, and epitaxy growth processes;
Forming from vapor or gaseous state
C117S088000, C427S255394
Reexamination Certificate
active
06706115
ABSTRACT:
The present invention relates to the preparation of metal nitride thin films. More particularly, the present invention concerns a method according to the preamble of claim
1
for growing transition metal nitride thin films by Atomic Layer Deposition (ALD).
Atomic Layer Deposition, or ALD, is also generally known as Atomic Layer Epitaxy (ALE). The name was changed from ALE to ALD to avoid possible confusion when discussing polycrystalline and amorphous thin films. ALD also covers the more specific technology marketed under the trademark of ALCVD, as manufactured by ASM Microchemistry, Espoo, Finland. In this application, the designation “ALD” will be used.
ALD is a known method (U.S. Pat. No. 4,085,430) for growing high quality thin films. In the method, a thin film is grown with the aid of alternate, saturating surface reactions. These reactions are carried out by conducting the gaseous or vaporised source materials alternately into the reactor and by purging the reactor with an inert gas between the pulses of the source materials. Because the film grows through saturating surface reactions, the growth is self-controlling, which guarantees that controlling the thickness of the film by means of the number of reaction cycles is precise and simple. The films grown by the method are flat, even on large surface areas, and their conformality is excellent (M. Ritala et al., Chemical Vapor Deposition, 5 (1999) 7). Consequently, ALD is a very useful potential method for growing diffusion barrier layers, too.
The continual decrease in the size of microelectronics components means that greater requirements are placed upon the metal conductors used in them and processes for manufacturing them. Metal conductors must be made narrower and narrower in order to decrease their surface area. This leads to increased resistance thereof. So-called RC delays (R=resistance, C=capacitance) are becoming a limiting factor on the speed of integrated circuits. Another significant consequence arising from the decreasing of cross-sectional areas of conductors is the increase in the electric current densities, which in turn raises the probability of electron migration damage. In order to minimise the RC delays, the resistance of conductors and the capacitance between them should be minimised. Using a metal with a lower resistance can decrease resistance. Up to now, aluminium has been used as a conductor, though it is in the process of being replaced by copper, which is more conductive, and which is considerably more stabile against electro migration than aluminium. Similarly, capacitance can be decreased by replacing SiO
2
used as a dielectric material between the conductors with a dielectric material having a lower dielectric constant. Examples include SiO
2
alloyed with fluorine, porous SiO
2
, along with different polymers.
The problem with introducing new conductor and dielectric materials is their compatibility with each other and materials surrounding them. Copper has a strong tendency to diffuse as much to silicon, SiO
2
as to other materials. On the other hand, impurities migrating from dielectric materials to copper increase its resistivity and thus decrease the advantage achieved with copper. In order to avoid these problems, copper conductors must be surrounded all over with a suitable diffusion barrier layer, which prevents diffusion and reactions between materials. There is clear need to provide nitride thin layers serving as better diffusion barrier layers. The diffusion barrier layer must be as thin as possible in order to maintain the cross-sectional surface area of the copper conductor as high as possible, in order to minimise the resistance. Furthermore, the resistivity of the diffusion barrier layer must be as low as possible in order to avoid excessive resistance between the two copper layers. The most commonly used and studied diffusion barrier layer materials are transition metals and nitride compounds thereof (A. E. Kaleoyeros and E. Eisenbraun, Annual Review of Materials Science 30 (2000) 363; C. Ryu et al., Solid State Technology (1999) April, 53; S. -Q. Wang, MRS Bulletin (1994), August, 30).
In addition to the material properties, attention must also be paid to a method for preparing a diffusion barrier layer. Low processing temperature and excellent conformality, in other words step coverage, are crucial requirements placed on the manufacturing method.
Polymeric dielectric materials having low permittivity as well as the other stability factors of microcircuits require that a high quality diffusion barrier layer can be prepared at low temperature, i.e. at 400° C. or below. Because the dual damascene process is used for making patterns on copper conductors, wherein the diffusion barrier and copper are grown in deep trenches and holes trenched in the dielectric material (P. C. Andricacos et al., IBM Journal of Research and Development 43 (1998) 567; N. Misawa, MRS Bulletin (1994) August, 63), the diffusion barrier layer has to have a very good conformality. Due to this requirement, the generally used physical layer growing methods will not be useful in growing the diffusion barrier layers. However, the ALD process makes it possible to deposit a flat film also on surfaces comprising small featured patterns like trenches, pits and/or vias, the depths of which can be in the order of under 0.2 &mgr;m.
According to known technology, many transition metal nitrides can be grown by the ALD method: TiN, NbN, TaN, MoN
x
and W
2
N (M. Ritala and M. Leskelä, Journal de Physique IV France 9 (1999) Pr8-837; Thin Films Handbook, Academic Press, in press). The chief difficulty in the chemical processing of the transition metal nitrides is in reducing metal, since in the generally used source materials, such as metal halides, the oxidation state is higher than in the nitride to be produced. In some cases ammonia, which is generally used as the nitrogen source, serves as a reducing agent. However, tantalum cannot be reduced with ammonia, but the TaCl
5
-NH
3
process produces a dielectric Ta
3
N
5
phase instead of the desired conductive TaN (M. Ritala et al., Chemistry of Materials 11 (1999) 1712). The lowest resistivities of the transition metal nitride films are achieved by those ALD processes, in which zinc is used as a separate reducing agent. In that case, a TaN phase of tantalum nitride is achieved, as well. The use of zinc in microelectronic applications is not recommended since it diffuses into silicon and produces electrically active electronic states. Furthermore, the low vapour pressure of zinc requires the temperature of the ALD process to be quite high, i.e. over 380° C. Consequently, finding new, powerful reducing agents is crucial in development of low temperature transition metal nitride processes.
1,1-Dimethylhydrazide (DMHy) is known to be a more reactive and reductive nitrogen source material than ammonia in the processing of transition metal nitrides with ALD (M. Juppo et al., Journal of the Electrochemical Society 147 (2000) 3377). Films grown with DMHy at 400° C. are slightly better than those grown with the aid of ammonia. The problem is that a conductive TaN phase cannot be achieved and other transition metal nitride films grown from DMHy are poorer in quality than films grown from ammonia with help of zinc.
Trimethyl aluminium is also used as a separate reducing agent in ALD growing of both TiN and TaN films (Juppo et al., Chemical Vapour Deposition, manuscript has been sent, and Alen et al., Journal of the Electrochemical Society, manuscript has been sent). The problem also in this case is that although the tantalum nitride films are already quite well conductive at the growth temperature of 400° C., carbon and aluminium residues are detectable in the films.
It is an aim of the present invention to eliminate the problems of the prior art and to provide a new solution for growing conductive metal nitride thin films.
In particular, an aim of the present invention is to use in the ALD method such nitrogen compounds as the nitrogen source materials that have
Alén Petra
Juppo Marika
Leskelä Markku
Ritala Mikko
ASM International N.V.
Chen Bret
Knobbe Martens Olson & Bear LLP
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