Chemistry: electrical and wave energy – Apparatus – Coating – forming or etching by sputtering
Reexamination Certificate
2001-04-20
2002-12-03
VerSteeg, Steven H. (Department: 1753)
Chemistry: electrical and wave energy
Apparatus
Coating, forming or etching by sputtering
C204S298040, C204S298080
Reexamination Certificate
active
06488825
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates generally to vapor deposition methods and, in particular, a plasma sputtering apparatus by which thin or thick films of insulating, semiconductor, or conductive materials are formed. Potential applications exist in the fabrication of integrated circuits, optical elements, optoelectronic devices, and other such products requiring well-controlled physical properties in these same films. According to another aspect, the present invention relates generally to the field of ultraviolet light sources.
2. Description of the Related Art
In one aspect, the invention relates generally to the treatment of dispersed photoabsorbing media, such as gases, with ultraviolet radiation. An equipment geometry for this purpose, in fluid treatment, utilizes a flow-through geometry, wherein the media to be processed passes through a processing tube constructed of ultraviolet-transmitting material—such as fused silica—and wherein the tube is surrounded with one or several ultraviolet lamps, thereby creating a high radiative flux within the photoabsorbing media. The coupling efficiency of the ultraviolet radiation to the media may then be increased, by placing this coaxial arrangement within a reflective cavity. This latter reflective cavity becomes increasingly necessary as the extinction distance of the ultraviolet within the media becomes much greater than the relevant physical dimension of the apparatus, and the ultraviolet radiation must make many passes through the media before it is appreciably absorbed.
When the extinction distance is orders of magnitude longer than the relevant cavity dimension, the coupling efficiency of this ultraviolet source becomes inherently limited by parasitic losses within the reflective cavity, which “steal” the ultraviolet radiation away before it can be absorbed by the dispersed media. Similarly to a laser source, such losses can be largely attributed to a combination of the mirror quality, diffraction losses, and the propagation of the ultraviolet along optical paths which “walk off” the cavity mirrors in so-called “end-losses”. Unlike a laser, however, retention within the reflective cavity of these lamp sources is not greatly increased through the establishment of high-retention lasing modes that dominate the photoemission process. In addition, as clearance restrictions require the aforementioned processing tube to be increasingly short in length—or of increasingly smaller aspect ratio—the end-losses of the commensurately shorter reflective cavity become a serious limitation to the coupling efficiency of the processing apparatus. Conversely, ultraviolet radiation that is allowed to leak from the reflective cavity may adversely interact with other parts of the process. These sort of requirements have been partially dealt with in past fusion research, at least when using longer optical wavelengths, by implementing a laser cavity. In this approach, the photoabsorbing media is passed through the laser cavity itself. However, this approach becomes far more difficult for other, similarly configured, materials processes that require ultraviolet radiation; in part, because these latter processes would typically require much more economical solutions than those afforded fusion research, whereas, capital and maintenance costs for a high-power ultraviolet laser are likely to be prohibitively high.
The problems encountered with irradiating low absorption cross-section dispersed media become increasingly acute with lower pressure processes, wherein the dispersed media would typically be some gas or vapor which is rarified to a degree consistent with the level of vacuum. In these latter vacuum processes, one encounters situations wherein the absorbing constituent may have a vapor pressure of only 10
−6
atmospheres, with extinction distances in the range of 10
2
to 10
3
meters. At the same time, these vacuum processes will frequently involve one or several critical material surfaces that interact with the process quite differently when irradiated with the ultraviolet radiation. These same critical surfaces will typically be modified during the process, so that the result of irradiating these surfaces will change, as well. For instance, a thin film forming on one of these material surfaces can dramatically alter the absorption, scatter, or reflection of the ultraviolet radiation as its thickness increases. In ultraviolet-enhanced physical vapor deposition (PVD) processes, including reactive processes utilizing PVD sources, these issues have not been adequately addressed.
A prevalent PVD means in industry for the deposition of high quality thin films is through the utilization of sputtering techniques. The term “sputtering” refers to a group of mechanisms by which material is ejected from a solid, or sometimes a liquid, target surface into a vapor form; this latter effect being due, at least in part, in either physical or chemical sputtering, to the kinetic energy transferred to the target atoms or molecules by bombarding particles. These mechanisms are utilized in sputter deposition processes categorized generally as laser sputtering, ion beam sputtering, glow discharge (or diode) sputtering, and magnetron glow discharge sputtering. The present invention, in its preferred embodiment, concerns primarily plasma sputtering, and, in particular, magnetron plasma sputtering. The magnetic confinement of the sputtering plasma in the magnetron sputtering process allows for a far greater range of mean free paths than the earlier, capacitively coupled diode plasma sputtering process. Its high deposition rate, combined with its versatility in depositing a wide range of materials under a great range of conditions, has made magnetron plasma sputtering a preferred thin film deposition technique for many industrial applications.
Yet, there are several aspects of plasma sputtering which are seen as significant barriers in utilizing the technique for future industrial applications. Most commercially available plasma sputter sources provide a small proportion of ionized species to the depositing film (<5%). Most of the energy supplied for non-equilibrium growth is supplied by the thermal velocities of the depositing species. The thermal distribution of these velocities is necessarily broad, allowing little control over specific growth processes at the film growth interface. Because the energy supplied by the depositing species is kinetic, it is often difficult to provide high energies to the growth interface with out simultaneously causing subsurface damage, due to the recoil and implantation of the bombarding atoms.
Several modifications have been devised to render greater control over plasma sputtering processes wherein, as in the present invention, excited state and ion populations in the gas/vapor phase are increased and manipulated by means external to the sputtering plasma. This is most commonly accomplished by injecting electrons into the sputtering plasma to increase the plasma density and ion population, while simultaneously allowing a decrease of the target voltage. A resulting benefit is the ability to introduce a high proportion of relatively low energy ions to either etch or deposit on the substrate. This method has been made popular in the well-established triode and tetrode sputtering configurations, wherein electrons are usually supplied by a thermionic filament. This latter art has been found to work well for the deposition of metals, but is not compatible with reactive processes where electron emitting surfaces are prone to modification.
In recent years, plasma sputtering processes have also been developed that increase ionization through the utilization of secondary coils or antennas for RF or microwave excitation of the plasma. This latter prior art has also been found useful in the deposition of metals. However, difficulties arise, in that resonance conditions are effected by the inevitable modification of the process chamber surfaces during deposition, especially when depositing
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