Coating apparatus – Gas or vapor deposition – With treating means
Reexamination Certificate
1997-07-30
2001-08-21
Nguyen, Nam (Department: 1722)
Coating apparatus
Gas or vapor deposition
With treating means
C118S725000, C427S457000, C427S561000, C438S760000, C438S795000
Reexamination Certificate
active
06276295
ABSTRACT:
BACKGROUND OF THE INVENTION
The present invention relates to the processing of semiconductor wafers. More particularly, the present invention relates to a thermal reflow technique that reduces the time necessary for reflow of a gap-filling metal thin-film without substantially heating the substrate underlying the metal thin-film. The present invention is particularly useful for the reflow of metallization layers deposited using conventional semiconductor processing methods.
BACKGROUND OF THE INVENTION
Thin-films are widely used to fabricate various electronic, optical and magnetic structures, because the processing of the thin-films may be precisely controlled allowing the manufacture of complex components. These films are typically thermally grown or deposited from a vapor phase. Often the films are formed from metals, semiconductors or insulator and must satisfy rigorous chemical, structural and electrical requirements. Patterned conductors and dielectric layers typically produce stepped topography. It is difficult to deposit films over stepped topography while avoiding defects in the same, such as voids in the dielectrics and opens in metal interconnect layers. Such defects may result in a reduction in production yields due to, inter alia, shorts between adjacent interconnect lines. To avoid these problems planarization techniques are implemented to reduce topographic undulations produced by the patterned metallization and dielectric layers.
Thermal reflow is a well known planarization technique in which the temperature of a metal thin-film layer is raised sufficiently to achieve diffusion of the metal atoms that comprise the metal thin-film layer. While in the diffusion state, the metal atoms of the thin-film layer flow so as to achieve equilibrium, filling all gaps and voids of the underlying layer and providing a substantially smooth surface. Thermal reflow is achieved by employing a conventional furnace to raise the temperature of both the metal thin-film layer and the underlying substrate. This requires a substantially longer process time than would be the case if only the metal thin-film layer were heated, due to the heating of the additional mass of the substrate. The structural integrity of the substrate may be compromised, resulting from exposure to the high temperatures necessitated to reach the melting-point of the metal thin-film.
Partial thermal isolation of the substrate during a thermal reflow technique has been achieved by employing lasers. For example, excimer lasers, such as an XeCl laser producing 308 nm wavelength beam, are used in thermal reflow of alumina layers. The laser is pulsed and the process is carried out in a vacuum chamber evacuated to approximately 7.5×10
−7
torr, with the substrate being maintained in the range of 250 to 450° C. It is necessary to provide a sufficient amount of laser energy to prevent boundary separation and metal cracking. To that end, the laser impinges upon the substrate surface as a localized spot which is scanned across the entire surface to achieve thermal reflow. Such a scanning process increases the time necessary to reflow the metal thin-film.
Recently developed is another thermal reflow technique which partially isolates, thermally, a substrate from the energy employed to heat a metal thin-film disposed thereon. This technique involves the use of microwave energy to reflow a copper thin-film disposed on a silicon substrate which is approximately 3 mm in diameter. See Ruth A. Brain, “Capillary-Driven Reflow of Thin Cu Films with Submicron, High Aspect Ratio Features”, Doctoral Thesis Submitted to the California Institute of Technology, Pasadena, Calif., pp. 102-113 (1995). As described therein, recesses in a SiO
2
substrate, having aspect ratios of up to 2.5:1, were filled with a copper thin-film without substantial thermal radiation reaching the substrate. Id. at page 113. This necessitated pulsing a microwave field at an appropriate duty cycle to prevent overheating of the substrate, thereby increasing the time required to achieve planarization. See id. The process disclosed, however, is not practical for fabrication of standard-size substrates which are typically greater than one hundred millimeters in diameter.
What is needed, therefore, is a thermal reflow technique that allows rapid diffusion of a metal thin-film so as to fill the steps on the surface of a standard-size substrate, of the type typically measuring at least one hundred millimeters in diameter, without substantially heating the substrate.
SUMMARY OF THE INVENTION
The present invention provides a thermal reflow system and method to achieve thermal reflow of a metal thin-film layer disposed on a semiconductor substrate so as to fill the gaps present therein, without substantially heating the substrate. The present invention does so by providing a process chamber designed as a high-Q resonant cavity for microwave energy, defining a reflow chamber. The microwave energy present in the reflow chamber rapidly heats the metal thin-film layer sufficiently to cause diffusion of the metal atoms present in the thin-film layer. Rapid heating is achieved by maximizing the mount of microwave energy absorbed by the metal thin-film layer. To that end, the substrate is supported within the chamber on an electrically non-conductive susceptor which is adapted to move within the cavity, and the atmospheric pressure of the reflow chamber is controlled to prevent a plasma from being formed in the reflow chamber.
The thermal reflow system includes a reflow chamber having conductive walls, defining a resonant cavity; a source of microwave energy; and a waveguide. A susceptor is formed from a dielectric and disposed within the cavity to support a substrate. The waveguide is coupled between the cavity and the source of microwave energy so as to allow the microwave energy to enter the cavity in a region thereof, defining an input aperture, disposed opposite to the susceptor. The spatial relationship between the region and the susceptor is chosen so that the susceptor is centrally disposed with respect to electric fields of the microwave energy. To maximize the energy absorbed by the substrate per unit time, the susceptor is moveably disposed within the cavity to move transverse to the electric fields and vary the distance between the substrate and the input aperture. Additionally, the pressure within the reflow chamber is established to prevent unwanted dissipation of the microwave energy due to, e.g., plasma formation and/or arcing. After reflow of the metal thin-film layer is achieved, the pressure in the reflow chamber is increased to rapidly cool the metal thin-film layer and the substrate. In this fashion, the temperature of the substrate is maintained below a predetermined temperature to prevent the compromise of the substrate's structural integrity. To further reduce the time necessary to heat the metal thin-film on the substrate, the susceptor may be placed in thermal communication with a heater so that it may be heated, provided that the temperature to which it is heated does not degrade the structural integrity of the substrate.
The method in accordance with the present invention includes the steps of evacuating the reflow chamber; introducing, into the processing chamber, microwave energy having a predetermined frequency at a region disposed opposite to the substrate; isolating, electromagnetically, the susceptor from the chamber walls; adjusting a distance between the substrate and the region to maximize absorption of the microwave energy absorbed by the metal thin-film; and maintaining process conditions, proximate to the surface of the substrate, for a predetermined amount of time to allow the metal thin-film. In one embodiment, the step of evacuating the reflow chamber takes place subsequent to the step of placing the substrate in the reflow chamber to reflow over the surface. Finally, a heating step may be included in which the susceptor is heated to reduce the time necessary to reflow the metal thin-film.
REFERENCES:
patent: 4792772 (1
Chen Ling
Li Steven T.
Applied Materials Inc.
Nguyen Nam
Nguyen Thu Khanh T.
Townsend and Townsend / and Crew LLP
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