Method of modifying source chemicals in an ald process

Details Method of modifying source chemicals in an ald process Method of modifying source chemicals in an ald process

2002-04-11

2004-07-27

Planalto, Bernard (Department: 1762)

Coating processes

Coating by vapor, gas, or smoke

Metal coating

C427S250000, C427S252000, C427S255390, C427S255391, C427S255393, C427S255394, C427S255395, C427S383100, C427S383300

Reexamination Certificate

active

06767582

ABSTRACT:

BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to metal nitride thin films. In particular, the present invention concerns a method of in situ reduction of source chemicals as well as a method of growing of metal nitride thin films. The present invention also relates to an apparatus for growing thin films on a substrate by an ALD type process.
2. Description of Related Art
The integration level of components in integrated circuits is increasing, which rapidly brings about a need for a decrease of the size of components and interconnects. Design rules are setting the feature sizes to ≦0.2 &mgr;m. Deposition of uniform thin films on wafer surfaces by Physical Vapor Deposition (referred to as PVD hereinafter) and Chemical Vapor Deposition (referred to as CVD hereinafter) methods has become difficult due to small feature sizes. As a result, complete film coverage on deep bottoms of vias and trenches cannot be obtained. PVD methods require more or less direct line-of-sight on the surfaces to be coated Traditional CVD methods require rather precise concentration control of the source chemicals and good temperature uniformity over the substrate. Deep bottoms may have a local “microclimate” where the variable concentration of source chemical vapors is causing non-uniform growth of thin film.
Integrated circuits contain interconnects which are usually made of aluminium or copper. Especially copper is prone to diffusion to the surrounding materials. Diffusion affects the electrical properties of the circuits and active components may malfunction. The diffusion of metals from interconnects into the active parts of the devices is prevented with an electrically conductive diffusion barrier layer. Favoured diffusion barriers are, e.g., amorphous transition metal nitrides, such as TiN, TaN and WN. The nitrides can be non-stoichiometric because nitrogen is located at interstitial position of the lattice.
Atomic Layer Deposition (ALD), originally, Atomic Layer Epitaxy (ALE) is an advanced variation of CVD. The method name was changed from ALE into ALD to avoid possible confusion when discussing about polycrystalline and amorphous thin films. The ALD method is based on sequential self-saturated surface reactions. The method is described in detail in U.S. Pat. Nos. 4,058,430 and 5,711,811. The reactor benefits from the usage of inert carrier and purging gases which makes the system fast.
The separation of source chemicals from each other by inert gases prevents gas-phase reactions between gaseous reactants and enables self-saturated surface reactions leading to film growth which requires neither strict temperature control of the substrates nor precise dosage control of source chemicals. Surplus chemicals and reaction byproducts are always removed from the reaction chamber before the next reactive chemical pulse is introduced into the chamber. Undesired gaseous molecules are effectively expelled from the reaction chamber by keeping the gas flow speeds high with the help of an inert purging gas. The purging gas pushes the extra molecules towards the vacuum pump used for maintaining a suitable pressure in the reaction chamber. ALD provides an excellent and automatic self-control for the film growth.
In case of transition metal nitrides, reduction of the metal source material is needed in order to increase the amount of metal in the nitride and, thus, to lower the resistivity of the nitride. “Reduction” can be defined as any reaction, wherein the metal of the source chemical receives electrons and its oxidation state decreases.
In the art, it is known to reduce the metal source material by pulsing a reducing agent after the metal source material pulse. A number of different chemicals have been used for the reduction. For example, tungsten compounds have been reduced by using hydrogen (H
2
) (U.S. Pat. No. 5,342,652 and EP-A2-899 779), silanes such as SiH
4
(U.S. Pat. No. 5,691,235) and chlorosilanes such as SiHCl
3
(U.S. Pat. No. 5,723,384). Low oxidation-state metal chlorides can also be synthesized by passing a gaseous mixture of hydrogen chloride (HCl) and hydrogen (H
2
) over a heated metal (U.S. Pat. No. 4,803,127).
Reduction of WF
6
into W metal on substrate surfaces by using a silane, Si
2
H
6
, is disclosed by J. W. Klaus (Atomic Layer Deposition of Tungsten and Tungsten Nitride Using Sequential Surface Reactions,
AVS
46
th
International Symposium
, abstract TF-TuM6, http://www.vacuum.org/symposium/seattle/technical.html, to be presented on the 26
th
of October, 1999 in Seattle, USA).
There are, however, drawbacks related to these prior art methods. Silanes may also react with WF
6
, thus forming tungsten silicides, such as WSi
2
. Hydrogen can reduce a tungsten compound into tungsten metal which has too low vapor pressure for being transported in gas phase onto substrates.
Various metal species adsorbed on substrate surfaces have been reduced with zinc in ALE processes (cf., e.g., L. Hiltunen, M. Leskelä, M. Mäkelä, L. Niinistö, E. Nykänen , P. Soininen, “Nitrides of Titanium, Niobium, Tantalum and Molybdenum Grown as Thin Films by the Atomic Layer Epitaxy Method”,
Thin Solid Films
, 166 (1988) 149-154). In the known processes, the additional zinc vapor used during the deposition decreased the resistivity of the nitride film either by increasing the metal-to-nitrogen ratio or by removing oxygen from the films. The known process comprised the following pulsing order: a metal source chemical vapor pulse/an inert gas purge/a zinc vapor pulse/an inert gas purge/a nitrogen source chemical vapor pulse/an inert gas purge. A basic problem related to reduction carried out with the zinc vapor method is that thin films contaminated with zinc metal and its compounds should be avoided in processes used for the manufacture of integrated circuits (referred to as IC hereinafter). Diffusing zinc can destroy the active components of the IC's. Additionally, the low end of the substrate temperature range is probably limited by the volatility of zinc metal and the sticking coefficient of zinc compounds on the surface.
In addition to zinc, hydrogen and magnesium have also been tested as reducing agents in ALE processes. The results have not been promising, Hydrogen is not capable enough of reducing the metal compounds at low substrate temperatures. Magnesium forms on the substrate surface a halide which has a very low vapor pressure and stops film growth. It seems that the applicability of elemental reduction on the substrate surface is rather limited. Few elements have high enough vapor pressure to be used as ALD source chemicals. Even fewer gaseous elements form a volatile byproduct during the reduction step.
A method for influencing the properties of CVD source chemicals is disclosed by C.-Y. Lee [The Preparation of Titanium-Based Thin Film by CVD Using Titanium Chlorides as Precursors,
Chem. Vap. Deposition
, 5 (1999) 69-73)]. According to the publication, in a CVD process TiCl
4
vapor was flowing over titanium metal which was heated to 900° C. The reaction produced TiCl
x
(x<4) subchlorides. These subchlorides were downstream thermally decomposed into Ti metal on a substrate which was heated to 500-800° C.
The CVD reducing apparatus described by C.-Y. Lee can not be used in ALD because of the required performance and character of ALD source chemicals and the location of the reducing agent. If a titanium reducing agent is covered by titanium halide molecules and it is exposed to reactive nitrogen containing source chemical like ammonia a layer of very inert titanium nitride will grow on its surface. Thus the formed titanium nitride layer prevents the desired reduction reaction of TiCl
4
gas pulses.
SUMMARY OF THE INVENTION
It is an object of the present process to eliminate the problems of prior art and to provide a novel method of reducing metal source materials in an ALD type process. It is another object of the invention to provide a novel method of preparing metal nitride thin films by an ALCVD type method
It is a further object of the

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