Chemistry: analytical and immunological testing – Optical result – With reagent in absorbent or bibulous substrate
Reexamination Certificate
1998-06-05
2001-11-20
Everhart, Caridad (Department: 2825)
Chemistry: analytical and immunological testing
Optical result
With reagent in absorbent or bibulous substrate
C438S680000, C438S681000
Reexamination Certificate
active
06319728
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention is directed toward the field of manufacturing integrated circuits. The invention is more particularly directed toward treating deposited thin films that form integrated circuits to improve their electrical characteristics.
2. Description of the Related Art
Presently, aluminum is widely employed in integrated circuits as an interconnect, such as plugs and wires. However, higher device densities, faster operating frequencies, and larger die sizes have created a need for a metal with lower resistivity than aluminum to be used in interconnect structures. The lower resistivity of copper makes it an attractive candidate for replacing aluminum. One challenge in employing copper instead of aluminum is the fact that copper does not adhere well to materials, such as titanium nitride, that are presently being used as diffusion barriers beneath the copper. Diffusion barriers are deposited between layers of the devices to prevent cross-contamination. For example, material from a conducting layer can migrate into neighboring insulating layers thereby compromising the intended insulative characteristic. Such a condition can lead to shorting or performance degradation of the device. Poor adhesion of the copper to a diffusion barrier results in portions of the copper being undesirably peeled away during polishing. This condition can also render an integrated circuit defective.
One solution to improve the adhesion of copper to an underlying diffusion barrier is to divide the process for depositing copper into two steps. During a first step, physical vapor deposition (PVD) is performed to deposit a seed layer of copper. PVD is a well established technique for depositing material to create thin films. Once the seed layer of copper is deposited using PVD, a bulk layer of copper is deposited. The bulk layer is deposited by either standard chemical vapor deposition (CVD) or electrical plating. The bulk layer of copper adheres relatively well to the copper seed layer.
Unfortunately, the use of the PVD process results in poor step coverage, which is unacceptable for devices that have a high aspect ratio (the ratio of the depth of a feature to its cross sectional width on the substrate surface). Further, the PVD process cannot be accomplished in the same chamber as either chemical vapor deposition or electrical plating. The need to have both a PVD chamber and either a CVD or electrical plating chamber increases integrated circuit manufacturing costs and reduces throughput (the number of substrates processed per unit of time). Alternately, adhesion can be improved by treatment of the seed layer by bombardment with ions prior to bulk deposition of CVD copper. However such thin films have an undesirably high electrical resistivity. Therefore, it is therefore desirable to employ chemical vapor deposition, for depositing both the seed and bulk layers, because CVD provides for a more conformal layer of copper.
The chemical vapor deposition of copper presents a further challenge. The challenge arises from a byproduct that is produced during the deposition of the copper. In one instance, the chemical vapor deposition of copper is achieved by using a precursor known as Cupraselect, which has the formula Cu(hfac)L. The L represents a Lewis base compound, such as vinyltrimethylsilane (VTMS). The (hfac) represents hexafluoroacetylacetonato, and Cu represents copper. During the CVD of copper using the Cu(hfac)L precursor, the precursor is vaporized and flowed into a deposition chamber containing a substrate (i.e. a semiconductor wafer). In the chamber, the precursor is infused with thermal energy at the wafer's surface, and the following reaction results:
2Cu(hfac)L→Cu+Cu(hfac)
2
+2L (Eqn. 1)
The resulting copper (Cu) deposits on the upper surface of the wafer, along with the Cu(hfac)
2
byproduct. The gaseous Lewis base byproduct (2L) is purged from the chamber. The presence of the byproduct as well as other contaminants on the wafer's surface increases the resistivity of the copper to an underlying diffusion barrier, such as titanium or tantalum nitride. Post annealing after deposition of the final layer of copper reduces the resistivity further but also degrades the adhesion. Consequently, a post annealing step is not entirely advantageous to formation of copper films.
Adding moisture (H
2
O) during the deposition of the seed layer by CVD increases the deposition rate which is highly desirable. Unfortunately, the addition of H
2
O also produces contamination from excessive oxygen (O
2
) which reacts with the copper forming copper oxide (CuO). The CuO incorporated into the seed layer increases the resistivity of the resulting film to an undesirable level. Further, a bulk layer of copper will not adhere well to a CuO interface.
Accordingly, it is desirable to provide a method for the treatment of moisture laden copper CVD layers so that the adhesion between the copper and the underlying diffusion barrier is strong and the electrical resistivity is reduced. It is also desirable for such a deposition to be performed in a single chamber (in situ).
SUMMARY OF THE INVENTION
In accordance with the present invention, a layer of material, such as copper, is formed on the surface of a wafer with reduced resistivity. In forming the layer of copper, a copper seed layer containing moisture (H
2
O) is first deposited by CVD on a surface of the wafer. Once the seed layer is deposited, the copper (containing moisture) is annealed by treatment with energy to reduce the resistivity of the copper on the surface of the wafer. The energy for annealing can be provided, for example, in the form of heat or bombardment with ions. A bulk layer of copper is subsequently deposited by CVD. To lower the resistivity of the copper bulk film without any post treatment, by heat or ion bombardment, it is essential to deposit the bulk copper layer in a moisture free chamber.
If the energy is supplied in the form of heat, any conventional heat source may be used. For example, the heat may be supplied by a resistive heating element, induction coil, radiant heat lamp or the like. A reducing agent such as hydrogen gas is also with the heat to remove the contaminants from the surface of the wafer. The hydrogen reacts with the contaminants on the hot surface reducing them to gaseous compounds which are removed from the chamber.
If the energy is supplied by ion bombardment, the copper seed layer is exposed, for example, to a plasma. The plasma is generated in one embodiment of the present invention using an inert gas, such as argon along with reactive gases such as hydrogen and nitrogen. Alternatively, a hydrogen
itrogen plasma can be employed along for bombarding the copper as well as providing for contaminant removal. The addition of the hydrogen provides for the removal of copper deposition byproducts such as carbon, oxygen, fluorine and the like, thereby reducing the resistivity of the copper seed layer and improving the adhesion of same.
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J. Norman, D. Roberts, A. Hochberg, P. Smith, G. Petersen,J. Parmeter,C.Apblett, T. Omstead, “Chemical Additives for Improved Copper Chemical Vapour Deposition Processing”, Thin Solid Films 262 (1995) pp. 46-51.
C. Marcadal,E.Richard,J. Mermet, J. Torres,J. Palleau,B. Alaux, M. Bakli, “Cu-CVD Process Optimised in a Cluster Equipment for IC Manufacturing:”, Microelectronic Engineering 33 (1997) pp. 3-13.
Bhan Mohan K.
Chang Mei
Chen Ling
Ganguli Seshadri
Jones Justin
Applied Materials Inc.
Everhart Caridad
Moser, Patterson & Sheridan L.L.P.
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