Chemistry: electrical and wave energy – Processes and products – Coating – forming or etching by sputtering
Reexamination Certificate
2002-03-12
2004-05-18
Versteeg, Steven (Department: 1753)
Chemistry: electrical and wave energy
Processes and products
Coating, forming or etching by sputtering
C204S298020, C204S298030, C204S298230, C204S298280
Reexamination Certificate
active
06736943
ABSTRACT:
BACKGROUND
Vacuum coating techniques are known and used in the production of coated substrates for numerous applications including, for example, thin film optical devices, such as optical substrates having reflective, anti-reflective, wavelength selective or other thin film coatings to perform as filters, mirrors, interleavers, wide band pass filters, narrow band pass filters, edge filters, short wave pass filters, long wave pass filters and other devices useful in optical communication systems, sensors and other applications.
Physical vapor deposition (PVD) of thin films covers a broad class of vacuum coating deposition processes in which material is removed from a target or source and deposited onto the surface of a substrate, the process being carried out in a vacuum or partial vacuum (both being referred to herein simply as a vacuum). Exemplary physical vapor deposition techniques include sputter deposition in which material is physically removed from a source or target by sputtering, transfers or travels across a vacuum zone within the vacuum chamber, and condenses or deposits as a coating or film on the surface of the substrate. In reactive sputtering the coating material generated at the target typically reacts with an introduced gas, such as oxygen as or after it deposits on the substrate surface as a coating of the reaction product. Variations of these and other vapor deposition techniques are known, including, ion assisted, e.g., diode or triode sputtering, ion beam sputtering, ion assisted evaporation, planar or cylindrical magnetron sputtering, direct current or radio frequency sputtering, electron beam evaporation, activated reactive evaporation and arc evaporation.
For many applications, the thin film coating on a substrate must be deposited with extreme precision. Optical elements for sensors and optical communication devices, such as wavelength selective thin film filters, mirrors, interleavers and the like useful in optical communication systems, are commercially produced by coating wafers using magnetron sputtering, ion beam sputtering or other vacuum coating deposition techniques. The coated wafer is cut into numerous small optical elements. Typically such optical elements comprise multiple Fabry-Perot film stacks, i.e., multiple resonant cavities each formed of numerous alternating thin films of high and low refractive index materials deposited in sequence onto the surface of an optically transparent wafer. Frequently such optical elements employ multi-cavity coating designs, as in a Fabry-Perot bandpass filter selectively transparent to the narrow wavelength band at the assigned center wavelength of a particular channel in a multi-channel system employing wavelength division multiplexing. To form such optical elements, each cavity (itself a filter) forming the multi-cavity Fabry-Perot coating must be resonant at precisely the same wavelength. That is, the cavities must be optically matched to each other. It is a well-known problem in this industry, however, that coating quality is difficult to adequately control. Typically, monitoring systems used during the deposition monitor the growing film thickness at the monitored point on the wafer. The uniformity and stability of the deposition substantially impacts the yield obtained from the rest of the wafer. The precise thickness and uniformity of each of the multiple layers deposited on a substrate wafer controls the optical properties of the resulting optical elements. Unfortunately, however, considerable uncontrolled variation occurs from one location to the other on the wafer surface during deposition. Variation occurs in film deposition rate from point to point on the surface of the substrate during the deposition, which can result in film thickness differences at different locations on the wafer. Also, considerable uncontrolled variation occurs in film deposition rate over time during a deposition run, potentially resulting in the different stacked cavities forming the filter coating being mismatched. Consequently, much of the surface area of a coated wafer produced in a typical deposition run either has the wrong passband or other optical property or is effectively opaque due to having optically mismatched Fabry-Perot cavities. Thus, yields of useable optical elements from a wafer coated in a typical deposition run are lower than desirable.
As a result of these difficulties, it is a significant problem that the cost of producing such optical elements is high. The problem is exacerbated by the growing need for increased quantities of such optical elements. Their use in sensors, medical devices and other applications is advancing and, most notably, the quantities used in optical communication systems are rapidly increasing in the face of shortages. So-called bandwidth-hungry applications such as the Internet, e-commerce, video-on-demand, public and private web sites and the like are driving rapid installation and upgrading of optical communication system capacity. Moreover, the problem is yet further exacerbated by the demand for ever narrower bandwidth optical elements to accommodate narrower channel spacing in optical communication systems to increase effective bandwidth by dividing the usable spectrum into an ever larger number of multiplexed channels. Equipment and processes currently used in the production of bandpass filters and other Fabry-Perot optical elements have exceedingly low yields of such higher quality optical elements, resulting in costs which are detrimental to implementation of higher density wavelength division multiplexing (WDM) in fiber optic telecommunication systems. As the wavelength spacing between adjacent WDM channels decreases, the precision and quality of the optical elements for thin film filters, mirrors, interleavers and the like must increase. This can be expected to reduce yields, as discussed above, yet yields of optical elements from each deposition run must be increased rather than decreased in order to reduce the cost of the optical elements.
Various suggestions have been made for improving and controlling film deposition.
A modified reactive magnetron sputtering apparatus is suggested in U.S. Pat. No. 4,931,158 to Bunshah et al, wherein a wire grid is positioned over the target and an auxiliary plasma is said to be produced adjacent to the substrate using a positively biased d.c. probe. Deposited film properties are said to be controlled by varying the d.c. probe voltage and the open area of the wire grid.
It is an object of the present invention to provide improved methods and apparatus for PVD vacuum coating deposition, especially sputter deposition. It is a particular object of certain preferred embodiments of the invention to provide methods and apparatus for vacuum coating deposition, which are suitable for producing high precision optical elements for fiber optic communication systems. Other objects and aspects of the invention, generally, or of certain preferred embodiments, will be apparent from the following summary disclosure and/or from the detailed description thereafter of certain preferred embodiments.
SUMMARY
In accordance with various aspects of the inventive technology disclosed here, film deposition is controlled to produce coated substrates having large areas of operative optical coating, for example wafers having large areas suitable to be cut into optical elements operative as high quality Fabry-Perot filters. Without being bound by theory, it may be useful to an understanding of this disclosure to consider that the coating material transferring to the substrate from the target in a sputtering or other PVD process, by application of the various aspects of this inventive technology, is better controlled to reduce variation in the individual films making up the finished Fabry-Perot coating on the substrate. As will be better understood from the following disclosure and the detailed discussion of certain preferred embodiments, the deposition apparatus and systems and the deposition methods disclosed here in some aspects minimize or reduce variatio
Greenberg Thomas L.
Kastanis William P.
Scobey Michael A.
Soberanis David L.
Wescott Elizabeth M.
Banner & Witcoff , Ltd.
Cierra Photonics Inc.
Versteeg Steven
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