Coating processes – Coating by vapor – gas – or smoke – Mixture of vapors or gases utilized
Reexamination Certificate
2001-10-24
2003-10-07
Meeks, Timothy (Department: 1762)
Coating processes
Coating by vapor, gas, or smoke
Mixture of vapors or gases utilized
C427S250000, C427S248100
Reexamination Certificate
active
06630201
ABSTRACT:
FIELD OF THE INVENTION
The present invention relates to advanced thin film deposition apparatus and methods used in semiconductor processing and related technologies.
BACKGROUND
As integrated circuit (IC) dimensions shrink, the ability to deposit conformal thin film layers with excellent step coverage at low deposition temperatures is becoming increasingly important. Thin film layers are used, for example, as MOSFET gate dielectrics, DRAM capacitor dielectrics, adhesion promoting layers, diffusion barrier layers, and seed layers for subsequent deposition steps. Low temperature processing is desired, for example, to prevent unwanted diffusion of shallow junctions, to better control certain reactions, and to prevent degradation of previously deposited materials and their interfaces.
The need for conformal thin film layers with excellent step coverage is especially important for high aspect ratio trenches and vias, such as those used in metallization layers of semiconductor chips. For example, copper interconnect technology requires a continuous thin film barrier layer and a continuous thin film copper seed layer to coat the surfaces of trenches and vias patterned in an insulating dielectric prior to filling the features with copper by electrochemical deposition (ECD or electroplating).
A highly conformal, continuous barrier layer is required to prevent copper diffusion into the adjacent semiconductor (i.e., silicon) material or dielectric. The barrier layer also often acts as an adhesion layer to promote adhesion between the dielectric and the copper seed layer. Low dielectric constant (i.e., low-k) dielectrics are typically used to reduce inter- and intra-line capacitance and cross-talk, but often suffer from poorer adhesion and lower thermal stability than traditional oxide dielectrics, making the choice of a suitable adhesion layer more critical. A non-conformal barrier layer, or one with poor step coverage or discontinuous step coverage, can lead to copper diffusion and current leakage between adjacent metal lines or to delamination at either the barrier-to-dielectric or barrier-to-seed layer interfaces, both of which adversely affect product lifetime and performance. The barrier layer should also be uniformly thin, to most accurately transfer the underlying trench and via sidewall profile to the subsequent seed layer, and have a low film resistivity (e.g., p<500 &mgr;&OHgr;-cm) to lessen its impact on the overall conductance of the copper interconnect structures.
A highly conformal, uniformly thin, continuous seed layer with low defect density is required to prevent void formation in the copper wires. The seed layer carries the plating current and acts as a nucleation layer. Voids can form from discontinuities or other defects in the seed layer, or they can form from pinch-off due to gross overhang of the seed layer at the top of features, both trenches and vias. Voids adversely impact the resistance, electromigration, and reliability of the copper lines, which ultimately affects the product lifetime and performance.
Traditional thin film deposition techniques, for example, physical vapor deposition (PVD) and chemical vapor deposition (CVD), are increasingly unable to meet the requirements of advanced thin films. PVD, such as sputtering, has been used for depositing conductive thin films at low cost and at relatively low substrate temperature. Unfortunately, PVD is inherently a line of sight process, resulting in poor step coverage in high aspect ratio trenches and vias. Advances in PVD technology to address this issue have resulted in high cost, complexity, and reliability issues. CVD processes can be tailored to provide conformal films with improved step coverage. Unfortunately, CVD processes often require high processing temperatures, result in the incorporation of high impurity concentrations, and have poor precursor (or reactant) utilization efficiency, leading to a high cost of ownership.
Atomic layer deposition (ALD), or atomic layer chemical vapor deposition (AL-CVD), is an alternative to traditional CVD methods to deposit very thin films. ALD has several advantages over PVD and traditional CVD. ALD can be performed at comparatively lower temperatures (which is compatible with the industry's trend toward lower temperatures), has high precursor utilization efficiency, can produce conformal thin film layers (i.e., 100% step coverage is theoretically possible), can control film thickness on an atomic scale, and can be used to “nano-engineer” complex thin films.
A typical ALD process differs significantly from traditional CVD processes. In a typical CVD process, two or more reactant gases are mixed together in the deposition chamber where either they react in the gas phase and deposit on the substrate surface, or they react on the substrate surface directly. Deposition by CVD occurs for a specified length of time, based on the desired thickness of the deposited film. Since this specified time is a function of the flux of reactants into the chamber, the required time may vary from chamber to chamber.
In a typical ALD process deposition cycle, each reactant gas is introduced sequentially into the chamber, so that no gas phase intermixing occurs. A monolayer of a first reactant is physi- or chemisorbed onto the substrate surface. Excess first reactant is pumped out, possibly with the aid of an inert purge gas. A second reactant is introduced to the deposition chamber and reacts with the first reactant to form a monolayer of the desired thin film via a self-limiting surface reaction. The self-limiting reaction halts once the initially adsorbed first reactant fully reacts with the second reactant. Excess second reactant is pumped out, again possibly with the aid of an inert purge gas. A desired film thickness is obtained by repeating the deposition cycle as necessary. The film thickness can be controlled to atomic layer (i.e., angstrom scale) accuracy by simply counting the number of deposition cycles.
Physisorbed precursors are only weakly attached to the substrate. Chemisorption results in a stronger, more desirable bond. Chemisorption occurs when adsorbed precursor molecules chemically react with active surface sites. Generally, chemisorption involves cleaving a weakly bonded ligand (a portion of the precursor) from the precursor, leaving an unsatisfied bond available for reaction with an active surface site.
The substrate material can influence chemisorption. In current dual damascene copper interconnect structures, a barrier layer such as tantalum (Ta) or tantalum nitride (TaN) must often simultaneously cover silicon dioxide (SiO
2
), low-k dielectrics, nitride etch stops, and any underlying metals such as copper. Materials often exhibit different chemical behavior, especially oxides versus metals. In addition, surface cleanliness is important for proper chemisorption, since impurities can occupy surface bonding sites. Incomplete chemisorption can lead to porous films, incomplete step coverage, poor adhesion between the deposited films and the underlying substrate, and low film density.
The ALD process temperature must be selected carefully so that the first reactant is sufficiently adsorbed (e.g., chemisorbed) on the substrate surface, and the deposition reaction occurs with adequate growth rate and film purity. A temperature that is too high can result in desorption or decomposition (causing impurity incorporation) of the first reactant. A temperature that is too low may result in incomplete chemisorption of the first precursor, a slow or incomplete deposition reaction, no deposition reaction, or poor film quality (e.g., high resistivity, low density, poor adhesion, and/or high impurity content).
Traditional ALD processes have several disadvantages. First, since the process is entirely thermal, selection of an appropriate process temperature is often confined to a narrow temperature window. Second, the small temperature window limits the selection of available precursors. Third, metal precursors that fit the temperature window are often halides (e.g., compoun
Babcoke Jason E.
Brown Jeffrey A.
Chiang Tony P.
Leeser Karl F.
Angstron Systems, Inc.
Fuller Eric B
Meeks Timothy
Ogonowsky Brian D.
Patent Law Group LLP
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