Stress tunable tantalum and tantalum nitride films

Details Stress tunable tantalum and tantalum nitride films Stress tunable tantalum and tantalum nitride films

1999-11-04

2002-12-03

Kalafut, Stephen (Department: 1745)

Chemistry: electrical and wave energy

Processes and products

Coating, forming or etching by sputtering

C204S192220

Reexamination Certificate

active

06488823

ABSTRACT:

BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention pertains to tantalum and tantalum nitride films which can be stress-tuned to be in tension or in compression or to have a particularly low stress, and to a method of producing such films. These stress-tuned films are particularly useful in semiconductor interconnect structures, where they can be used to balance the stress within a stack of layers, which includes a combination of barrier layers, wetting layers, and conductive layers, for example. The low stress tantalum and tantalum nitride films are particularly suited for the lining of vias and trenches having high aspect ratios.
2. Brief Description of the Background Art
A typical process for producing a multilevel structure having feature sizes in the range of 0.5 micron (&mgr;m) or less would include: blanket deposition of a dielectric material; patterning of the dielectric material to form openings; deposition of a diffusion barrier layer and, optionally, a wetting layer to line the openings; deposition of a conductive material onto the substrate in sufficient thickness to fill the openings; and removal of excessive conductive material from the substrate surface using chemical, mechanical, or combined chemical-mechanical polishing techniques. Future technological requirements have placed a focus on the replacement of aluminum (and aluminum alloys) by copper as the conductive material. As a result, there is an increased interest in tantalum nitride barrier layers and in tantalum barrier/wetting layers, which are preferred for use in combination with copper.
Tantalum nitride barrier films, Ta
2
N and TaN, have been shown to function up to 700° C. and 750° C., respectively, without the diffusion of copper into an underlying silicon (Si) substrate. Tantalum barrier/wetting films have been shown to function at temperatures of approximately 500° C. It is advantageous in terms of processing simplicity to sputter the barrier and/or Wetting layers underlying the copper. Tantalum nitride barrier layers are most commonly prepared using reactive physical sputtering, typically with magnetron cathodes, where the sputtering target is tantalum and nitrogen is introduced into the reaction chamber.
S. M. Rossnagel et al. describe a technique which enables control of the degree of directionality in the deposition of diffusion barriers in their paper titled “Thin, high atomic weight refractory film deposition for diffusion barrier, adhesion layer, and seed layer applications”,
J. Vac. Sci. Technol
. B 14(3) (May/Jun. 1996). In particular, the paper describes a method of depositing tantalum (Ta) which permits the deposition of the tantalum atoms on steep sidewalls of interconnect vias and trenches. The method uses conventional, non-collimated magnetron sputtering at low pressures, with improved directionality of the depositing atoms. The improved directionality is achieved by increasing the distance between the cathode and the workpiece surface (the throw) and by reducing the argon pressure during sputtering. For a film deposited with commercial cathodes (Applied Materials Endura® class; circular planar cathode with a diameter of 30 cm) and rotating magnet defined erosion paths, a throw distance of 25 cm is said to be approximately equal to an interposed collimator of aspect ratio near 1.0. In the present disclosure, use of this “long throw” technique with traditional, non-collimated magnetron sputtering at low pressures is referred to as “gamma sputtering”.
Gamma sputtering enables the deposition of thin, conformal coatings on sidewalls of a trench having an aspect ratio of 2.8: 1 for 0.5, &mgr;m wide trench features. However, we have determined that gamma-sputtered TaN films exhibit a relatively high film residual compressive stress, in the range of about −1.0×10
+10
to about −5.0×10
+10
dynes/cm
2
. High film residual compressive stress, in the range described above, can cause a Ta film or a tantalum nitride (e.g., Ta
2
N or TaN) film to peel off from the underlying substrate (typically silicon oxide dielectric). In the alternative, the film stress can cause feature distortion on the substrate (typically a silicon wafer) surface or even deformation of a thin wafer.
A method of reducing the residual stress in a Ta barrier/wetting film or a Ta
2
N or TaN barrier film would be beneficial in enabling the execution of subsequent process steps without delamination of such films from trench and via sidewalls or other interconnect features. This reduces the number of particles generated, increasing device yield during production. In addition, a film having a near zero stress condition improves the reliability of the device itself.
SUMMARY OF THE INVENTION
We have discovered that the residual stress residing in a tantalum (Ta) film or a tantalum nitride (TaN
x
, where 0<x≦1.5) film can be controlled (tuned) by controlling particular process variables during deposition of the film. Process variables of particular interest for sputter-applied Ta and TaN
x
films include the following: An increase in the power to the sputtering target (typically DC) increases the compressive stress component in the film. An increase in the process chamber pressure (i.e., the concentration of various gases and ions present in the chamber) increases the tensile stress component in the film. An increase in the substrate DC offset bias voltage (typically an increase in the applied AC as substrate bias power) increases the compressive stress component in the film. When the sputtering is IMP sputtering, an increase in the power to the ionization coil increases the compressive stress component in the film. The substrate temperature during deposition of the film also affects the film residual stress. Of these variables, an increase in the process chamber pressure and an increase in the substrate offset bias most significantly affect the tensile and compressive increases the compressive stress components, respectively.
The most advantageous tuning of a sputtered film is achieved using Ion Metal Plasma (IMP) sputter deposition as the film deposition method. This sputtering method provides for particular control over the ion bombardment of the depositing film surface. When it is desired to produce a film having minimal residual stress, particular care must be taken to control the amount of ion bombardment of the depositing film surface, as an excess of such ion bombardment can result in an increase in the residual compressive stress component in the deposited film.
Tantalum (Ta) films deposited using the IMP sputter deposition method typically exhibit a residual stress ranging from about +1×1
+10
dynes/cm
2
(tensile stress) to about −2×10
−10
dynes/cm
2
(compressive stress), depending on the process variables described above. Tantalum nitride (TaN
x
) films deposited using the IMP method typically can be tuned to exhibit a residual stress within the same range as that specified above with respect to Ta films. We have been able to reduce the residual stress in either the Ta or TaN
x
films to low values ranging from about +1×10
+9
dynes/cm
2
to about −2×10
+9
dynes/cm
2
using tuning techniques described herein. These film residual stress values are significantly less than those observed for traditionally sputtered films and for gamma-sputtered films. This reduction in film residual compressive stress is particularly attributed to bombardment of the film surface by IMP-generated ions during. the film deposition process. Heavy bombardment of the film surface by IMP-generated ions can increase the film residual compressive stress, so when it is desired to minimize the film compressive stress, the ion bombardment should be optimized for this purpose.
Other process variables which may be used in tuning the film stress include the spacing between the sputter target and the substrate surface to be sputter deposited; ion bombardment subsequent to film deposition; and annealing of the film during

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