Semiconductor device manufacturing: process – Coating with electrically or thermally conductive material – To form ohmic contact to semiconductive material
Reexamination Certificate
1997-02-04
2001-02-20
Thibodeau, Paul (Department: 1773)
Semiconductor device manufacturing: process
Coating with electrically or thermally conductive material
To form ohmic contact to semiconductive material
C438S655000, C438S656000, C438S682000, C438S685000, C438S688000
Reexamination Certificate
active
06191032
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to integrated circuit fabrication, and more particularly to the reduction of voiding in Al metal lines.
2. Description of the Related Art
Metallization lines in advanced integrated circuits are often fabricated from sputter-deposited Al/Ti bilayers. In particular, sputtered Ti is widely used in integrated circuits as an underlayer film for Al-based multilayer metallizations. The Al/Ti bilayer has improved electromigration resistance as compared to a single layer Al metallization. However, Ti and Al react at temperatures above 350° C. to form TiAl
3
, producing a 5.9% volumetric contraction. In a typical interconnect processing sequence, the Al/Ti alloy metallization stack is deposited at temperatures <300° C. followed by lithographic patterning and etching. Dielectric passivation deposition occurs at elevated temperatures >350° C. Upon cooling, the metal line cross-section, which is rigidly attached to the encapsulating material, is subject to a large tensile hydrostatic stress state since the Al has a larger coefficient of thermal expansion than the passivation. Subsequent thermal cycles often reach temperatures greater than 400° C., leading to further TiAl
3
formation and volume contraction. Strain induced by the volumetric contraction of TiAl
3
leads to increases in void formation in the Al metal line because voiding is a tensile hydrostatic stress relaxation mechanism. See Besser, P. R., Sanchez, J. e. Jr., and Alvis, R., .,
The Effect of Si on TiAl
3
Formation in Ti/Al Alloy Bilayers,
Proceedings of the 1994 MRS Fall Meeting, Vol. 355, p. 631, Materials Research Society, Pittsburg, Pa. (1995).
The reaction rate for the Ti+3Al=>TiAl
3
reaction has been shown to be a function of the concentration of alloying elements such as Cu and Si in the Al. As a result, Al:Si alloys have been used to reduce TiAl
3
formation. See Besser et al. Si has also been used in NMOS integrated circuits as an alloying element in Al for reduction of junction spiking in simple Al-to-Si ohmic contact structures. However, in both cases deposited films containing both Al and Si (typically, ~1% wt Si), exhibit Si precipitation problems. During the cooling cycle of a thermal anneal, the solid solubility of Si in Al decreases with decreasing temperature. The Al thus becomes supersaturated with Si, which causes nucleation and growth of Si precipitates out of the Al:Si solution. Si precipitates formed within the Al interconnect lines can increase the susceptibility of the lines to electromigration failure. In narrow Al lines, the precipitates can be large enough to obstruct a large fraction of the cross-sectional area of the metal line. In particular, the size of the Si precipitates in Al: 1% Si alloys can range from about 0.4-1.5 &mgr;m, depending on how slowly the films are cooled. At locations where such precipitates are formed, a large flux-divergence in the current is produced. This flux divergence, in turn, can lead to early failure of the conductor by an electromigration-induced open circuit. See generally, Wolf,
Silicon Processing for the VLSI Era: Volume
2-Process Integration, § 3.4.4, Lattice Press, 1990.
SUMMARY OF THE INVENTION
It has been observed that Si introduced into an Al metal line of an Al, Ti, and Si-containing layer stack of an integrated circuit, at concentrations uniformly less than the solid solubility of Si in Al, results in a reduction in Al metal line voiding. Some voiding is a stress induced phenomenon and the introduction of Si appears to reduce stresses in the Al metal lines. By controlling Ti deposition conditions to achieve a desired thickness and grain-size characteristics of the Ti underlayer, a self-regulating filter for introduction of Si into the Al metal layer is provided. Si is introduced into the Al metal layer by migration through a suitably deposited Ti layer. In this way Si is introduced into the Al and Al metal line voiding is reduced, while avoiding excess concentration of Si which can result in formation of Si precipitates in the Al metal lines, thereby avoiding related reductions in metal line cross-sections and reducing electromigration-induced open circuit failures.
In an embodiment in accordance with the present invention, a method for reducing voiding of Al metal lines in a Ti/Al metal stack includes depositing a Ti-comprising layer on a Si-comprising dielectric layer, depositing, essentially Si free, the Al-comprising overlayer on the Ti-comprising layer, and migrating an amount of Si from the Si-comprising dielectric layer through the Ti-comprising layer and into the Al-comprising overlayer. The amount of Si so migrated is greater than a trace amount but no greater than that soluble in the Al-comprising overlayer. Depositing of the Ti-comprising layer on the Si-comprising dielectric layer is under conditions to provide a filter for Si migration into the Al-comprising overlayer.
In a further embodiment, the Ti-comprising layer depositing step includes depositing the Ti-comprising layer to a predetermined thickness and with predetermined Ti grain sizes. The predetermined thickness and Ti grain sizes are selected to provide the filter for Si migration from the Si-comprising dielectric layer into the Al-comprising overlayer. In yet a further embodiment, the migrating step includes introducing a diffusion profile of Si into the Al-comprising overlayer. The diffusion profile exhibits a generally decreasing concentration of Si in the Al-comprising overlayer at increasing distances from an interface between the Al-comprising overlayer and the Ti-comprising layer. In still yet a further embodiment, the migrating step further includes introducing Si into the Al-comprising overlayer by channelling. The channelling of Si into the Al-comprising overlayer generally obscuring the diffusion profile of Si introduced into the Al-comprising overlayer by diffusion.
In another embodiment in accordance with the present invention, a method for introducing Si into an Al-comprising metal layer includes forming an only partially effective Si migration barrier layer between an essentially Si-free Al-comprising metal layer and a Si-comprising dielectric layer and migrating through the only partially effective Si migration barrier layer an amount of Si from the Si-comprising dielectric layer into the Al-comprising metal layer. The amount of Si so migrated is greater than a trace amount but no greater than that soluble in the Al-comprising metal layer.
In yet another embodiment in accordance with the present invention, a method for introducing a concentration of Si into Al metal lines of an Al/Ti/SiO
2
metal stack, wherein the concentration of Si so introduced being uniformly less than the solid solubility of Si in Al, includes depositing a Ti layer on SiO
2
, depositing an essentially Si-free Al layer on the Ti layer, and migrating an amount of Si from the SiO
2
through the Ti layer and into the Al layer. The amount of Si so migrated is greater than a trace amount but no greater than that soluble in the Al layer. In a further embodiment, the Al layer remains essentially free from localized Si precipitates after completion of subsequent steps to fabricate an integrated circuit. In various further embodiments, migrating Si into the Al layer begins during Al layer deposition, begins after Al layer deposition, occurs primarily during Al layer deposition, or occurs primarily after Al layer deposition.
In still yet another embodiment in accordance with the present invention, a metal stack includes Si-comprising dielectric, a Si-comprising Al metal line, and Ti-comprising material disposed between the Si-comprising dielectric and the Si-comprising Al metal line. Concentrations of Si in the Si-comprising Al metal line are greater than trace concentrations but uniformly less than the solid solubility of Si in Al and essentially all of the Si in the Al- and Si-comprising layer is introduced through the Ti-comprising material.
REFERENCES:
patent: 4998157 (1991-03-01), Yokoyama
Brennan William S.
Hossain Tim Z.
Smith Patrick L.
Soza David
Tiffin Don A.
Advanced Micro Devices , Inc.
Rickman Holly C
Skjerven Morrill & MacPherson LLP
Thibodeau Paul
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