DOMAIN OF THE INVENTION

The invention relates to the field of methods for forming a low-resistivity tantalum (Ta) film, and more particularly the field of methods for forming a tantalum film having a resistivity of less than 20 μΩ·cm (microhm centimeter). One particularly advantageous application is in the field of semiconductor devices, in particular as a barrier layer for the metallisation of conductive tracks and other elements made of copper (Cu).

TECHNOLOGICAL BACKGROUND

For several years, the electrical and structural properties of tantalum thin films have aroused interest for these applications in semiconductor devices, in particular as barrier layers for the metallisation of conductive tracks and other elements made of copper as a result of tantalum's immiscibility with copper, and also since tantalum-based thin films have satisfactory chemical and thermal stabilities. Moreover, tantalum-based barrier layers increase the deposited copper fraction having a crystalline structure (111), and have good adhesion with the copper, and thus the copper-based conductive tracks.

Tantalum, in the metallic form thereof, has two crystalline phases:a. an alpha (α) phase, taking the form of a body-centred cubic lattice, having a low resistivity, typically in the range 12 to 60 μΩ·cm, and more particularly in the range 13 to 30 μΩ·cm, andb. a beta (β) phase, taking the form of a tetragonal lattice, having a higher resistivity, typically in the range 130 to 220 μΩ·cm.

As a result of the lower resistivity of the alpha phase, the latter is preferred over the beta phase for electronic applications, in particular for thin film interconnects, since it reduces the interconnection resistance.

In a manner known in the prior art, tantalum films are generally deposited by conventional methods involving magnetron cathode sputtering or physical vapour deposition (PVD), however the tantalum thin films thus deposited, and having a typical thickness, in particular a thickness of less than 300 nanometres (nm), are generally present in a metastable tetragonal beta phase.

However, depending on the deposition method and/or technique used, the crystalline structure of the deposited film can correspond to a beta phase, a mixture of beta phase and alpha phase, or solely to a pure alpha phase.

Numerous approaches and methods for forming the alpha phase of tantalum have been proposed.

For example, scientific literature (M. Stavrera et al., J. Vac. Sci. Technol., A 17, 993, (1999)—D. W. Face and D. E. Prober, J. Vac. Sci. Technol., A 5, 3408, (1987)—S. Morohashi, Jpn. J. Appl. Phys., Part 2, 34, L1352, (1995)) and the U.S. Pat. Nos. 5,281,485, 5,834,827, 6,429,524, 2014/0061918 and 6,911,124 disclose that an alpha-phase tantalum thin film can be obtained by magnetron cathode sputtering or PVD using sacrificial sub-layers such as tantalum nitride (TaN), niobium (Nb), titanium nitride (TiN), tungsten (W) or tungsten nitride (WN) sub-layers.

Moreover, L. Maissel and R. Glang, in “Handbook of Thin Film Technology”, McGraw-Hill, 18 (1970) disclosed that, when the temperature of the substrate during the deposition exceeds 600° C. (degrees Celsius), the tantalum thin film deposited is present in the alpha phase form thereof.

Other techniques and methods for forming alpha-phase tantalum thin films consist of heat treating the deposited films (M. H. Read and C. Altman, Appl. Phys. Letts. 7 (3), 51, (1965)—S. L. Lee et al., Surf. Coat. Technol. 120/121, 44, (1999)—D. W. Matson et al., Surf. Coat. Technol. 133/134, 411, (2000)) by exploiting the fact that metastable beta-phase tantalum is known to transform into alpha phase when it is heated to a temperature in the range 700 to 1000° C.

Furthermore, the use of noble (inert) sputtering gases that are heavy (atomic weight of greater than 40 grams/mole) such as krypton (Kr) or xenon (Xe) during the deposition of a tantalum layer has been reported to allow the deposited tantalum layer to take the alpha phase thereof (D. W. Matson et al., Surf. Coat. Technol. 146/147, 344, (2001)—D. W. Matson et al., J. Vac. Sci. Technol. A 10 (4), 1791, (1992)).

The methods for forming a tantalum thin film briefly described hereinabove suffer from at least one of the following drawbacks:a. the use of a sacrificial sub-layer becomes prohibitive when the target thickness of the barrier layer for the interconnection lines continues to reduce as a result of the shrinking of the technological nodes,b. the high substrate temperatures which are not compatible with the thermal budgets for the manufacture of certain devices (which often require maintaining the temperature at less than or equal to 400° C.),c. the impossibility of complying with the thermal budgets, in particular in methods using high-temperature heat treatments after the deposition of the tantalum layer, andd. the use of heavy noble sputtering gases, which is not economically viable for the microelectronics industry due to the high cost thereof.

One purpose of the present invention is thus to propose a method for forming a low-resistivity tantalum thin film which does not require the use of a sacrificial sub-layer, and/or of heavy noble sputtering gases, and/or high deposition temperatures on a substrate, and/or high-temperature post-deposition treatments that do not comply with the imposed thermal budgets.

Another purpose of the present invention is to propose a method for forming a low-resistivity tantalum thin film that allows tantalum thin films in a pure alpha phase to be formed, in a reproducible manner, having a resistivity of less than or substantially equal to 20 μΩ·cm.

Other purposes, characteristics and advantages of this invention will appear upon reading the following description and its accompanying drawings. It is understood that other advantages can be incorporated thereto.

SUMMARY

In order to reach this objective, according to one embodiment, the present invention provides a method for forming a low-resistivity tantalum thin film, preferably having a resistivity of less than 20 μΩ·cm, comprising the following steps of:a. Depositing a tantalum layer on a substrate, the tantalum of the layer having a β phase,b. Treating the deposited tantalum layer by exposure to a radio frequency hydrogen (H) plasma, such that the layer has tantalum in a mixed β-α phase, andc. At least partially desorbing the hydrogen by carrying out at least one of the following steps:i. Exposure to a radio frequency inert gas plasma, andii. Thermal annealing,
the treatment step being configured such that the tantalum layer is subjected to temperatures of less than or equal to 300° C., preferably less than or equal to 250° C., and more preferably less than or equal to 150° C.

More particularly, the method according to one embodiment of the invention provides for:a. depositing, by conventional, known techniques, a beta-phase tantalum layer on the substrate at an ambient temperature,b. transforming the beta phase of the tantalum layer, by exposure to a radio frequency hydrogen plasma, into a mixed beta-alpha (β-α) phase, andc. forming an alpha-phase tantalum layer, preferably in the pure alpha phase, by exposing the mixed beta-alpha phase (β-α) to a radio frequency argon plasma.

The method according to the various embodiments of the invention can be carried out in one or more vacuum “cluster” type systems formed by a plurality of chambers, without a vacuum break or with a vacuum break between the steps thereof.

It thus appears that the method according to the invention allows a low-resistivity tantalum thin film to be formed using neither a sacrificial sublayer nor heavy noble sputtering gases (Kr, Xe).

Moreover, in particular in the absence of thermal annealing, the method according to the invention allows a low-resistivity tantalum thin film to be formed at temperatures and/or without the need for high-temperature post-deposition treatments, and thus complies with the thermal budgets imposed.

The method according to the invention thus allows tantalum thin films to be formed, in a reproducible manner, in an alpha phase that is relatively or perfectly pure, having a resistivity of less than or equal to 20 μΩ·cm.

Optionally, the invention can further have at least one of the following features.

The tantalum layer can be deposited by magnetron cathode sputtering or by physical vapour deposition.

The method can be free of any chemical vapour deposition (CVD) step involving the β-phase tantalum layer. The method according to this feature avoids the use of this deposition technique, which would result in a tantalum layer comprising contaminants originating from the one or more chemical precursors used to form the tantalum layer, for example carbon or oxygen, and would result in a less dense tantalum layer, and thus in a much higher resistivity (greater than 1000 μΩ·cm for a TaN film according to Table 3 of the U.S. Pat. No. 6,268,288) than that obtained by magnetron cathode sputtering or by PVD.

The tantalum layer can be deposited directly on the substrate.

The substrate can be free of any sacrificial layer or any activating layer for the tantalum layer.

The substrate can be free of any tantalum nitride layer (or Nb, TiN, W, WN layer, etc.)

Preferably, exposure to a radio frequency inert gas plasma is carried out in a chamber maintained at a temperature that is substantially equal to ambient temperature.

The deposition of the tantalum layer and the two plasma treatments (hydrogen then argon) of the tantalum layer are preferably carried out in a subsequent manner, the tantalum layer being firstly deposited, then treated.

The deposition of the tantalum layer can be carried out in a chamber maintained at a pressure of less than 40 millitorr (mT), preferably less than 30 mT, more preferably substantially equal to 5 mT.

The deposition of the tantalum layer is preferably configured such that the tantalum layer has a thickness in the range 10 to 40 nm.

The deposition of the tantalum layer (in β phase) is preferably configured such that the tantalum layer has a resistivity substantially equal to 180 μΩ·cm.

The treatment by exposure to a radio frequency hydrogen plasma can be carried out with pure hydrogen in a vacuum chamber at a pressure in the range 2 to 10 Torr (T), and more preferably substantially equal to 6 T. The exposure of the radio frequency hydrogen plasma can thus be carried out at a pressure significantly greater than that at which the deposition of the beta-phase tantalum layer is carried out. This may or may not require a vacuum break as a result of changing chambers between the chamber wherein the deposition of the tantalum layer is carried out and the chamber wherein the tantalum layer is exposed to the radio frequency hydrogen plasma.

The treatment by exposure to a radio frequency hydrogen plasma can be carried out with an incident radio frequency power in the range 100 to 1500 Watts (W), and more preferably substantially equal to 1000 W.

The treatment by exposure to a radio frequency hydrogen plasma can be carried out for a duration in the range 30 to 300 seconds, and more preferably substantially equal to 60 seconds.

The treatment by exposure to a radio frequency hydrogen plasma is preferably configured such that the mixed β-α phase tantalum layer has a resistivity substantially equal to 55 μΩ·cm.

The exposure to a radio frequency inert gas plasma can be carried out with argon, preferably pure, as an inert gas.

The exposure to a radio frequency inert gas plasma can be carried out in a vacuum chamber at a pressure in the range 0.1 to 10 mT, and more preferably substantially equal to 0.5 mT.

The exposure to a radio frequency inert gas plasma is preferably carried out at a temperature that is substantially equal to ambient temperature.

The exposure to a radio frequency inert gas plasma can be carried out with an incident radio frequency power in the range 50 to 500 W, and more preferably substantially equal to 200 W.

The exposure to a radio frequency inert gas plasma can be carried out for a duration in the range 5 to 30 seconds, and more preferably substantially equal to 10 seconds.

The exposure to a radio frequency inert gas plasma is preferably configured such that the α-phase tantalum layer has a resistivity of less than 20 μΩ·cm, and preferably substantially equal to 15 μΩ·cm

