Optical: systems and elements – Mirror – With support
Reexamination Certificate
2001-10-17
2003-01-21
Sikder, Mohammad (Department: 2872)
Optical: systems and elements
Mirror
With support
C359S838000, C359S599000
Reexamination Certificate
active
06508561
ABSTRACT:
FIELD OF THE INVENTION
The present invention relates generally to optical network, and more particularly to optical mirror coatings for high-temperature diffusion barriers and mirror shaping.
REFERENCES
The following references are used through this patent application, and are hereby incorporated herein by reference in their entireties:
[1] K. Nunan et. al., LPCVD and PECVD Operations Designed for iMEMS Sensor Devices, Vacuum Technology and Coating, January 2001, pp. 27-37;
[2] R. F. Bunshah et. al., Deposition Technologies for Films and Coatings;
Developments and Applications, Noyes Publications, 1982, pp. 376-378 and 526;
[3] J. M. Poate, Diffusion and reactions in gold films, Solid State Tech., April 1982, 227-234 and Gold Bull., 14(1), 1981, pp. 2-11;
[4] Z. Marinkovic and V. Simic, Journal of the Less Common Metals, 115, 1986, pp. 225-234;
[5] Z. Marinkovic and V. Simic, Thin Solid Films, 156, 1988, pp. 105-115;
[6] K. Masahiro and S. Noboru, Journal of Materials Science, 28, 1993, pp. 5088-5091;
[7] B. Doyle, et. al., Thin Solid Films, 104, 1983, pp. 69-79;
[8] Yong Tae Kim, et. al., Japan Journal of Applied Physics, 32, 1993, pp. 6126-6131 and H. Kattelus, et. al., Journal of Vacuum Science Technology, A, 3, 1985, 2246;
[9] P. J. Pokela, et. al., Journal of the Electrochemical Society, 138(7), 1991, pp. 2125-2129;
[10] H. Hieber and K. Pape, Gold Bull., 15(3), 1982, pp. 90-100;
[11] M. Kitada, et. al., Journal of Magnetism and Magnetic Materials, 123, 1993, pp. 193-198;
[12] M. Kitada, Thin Solid Films, 250, 1994, pp. 111-114;
[13] Stanley Wolf and Richard Tauber, Silicon Processing for the VLSI Era, Volume 1: Process Technology, 2nd Edition, Lattice Press, 2000, pp. 202-207;
[14] V. Malina, et. al., Semiconductor Science and Technology, 11, 1996, pp. 1121-1126.
[15] Donald L. Smiith, Thin-Film Deposition, McGraw-Hill, 1995, pp. 188-200.
[16] Stanley Wolf and Richard Tauber, Silicon Processing for the VLSI Era, Volume 1: Process Technology, 2nd Edition, Lattice Press, 2000, pp. 106-112; and
[17] R. F. Bunshah et. al., Deposition Technologies for Films and Coatings; Developments and Applications, Noyes Publications, 1982, pp. 63-72.
BACKGROUND OF THE INVENTION
Mirrors used in optical applications, such as optical networking applications, must be highly reflective in order to minimize dispersion and reduce optical signal loss. These optical mirrors often consist of a thin film of highly-reflective material, such as gold, platinum, or aluminum, layered directly or indirectly over a substrate, such as single crystal silicon or polysilicon. The mirror layer is typically very thin in order to reduce problems from film stress and thermal expansion. The mirror layer may be placed directly on the substrate, although the mirror layer is quite often separated from the substrate by one or more additional material layers. For convenience, the material onto which the mirror layer is placed (whether the substrate or an additional material layer) is referred to hereinafter as the “backing layer” for the mirror layer.
One reason for using a backing layer is to improve adhesion of the mirror layer onto a lower material layer (whether the substrate or an additional material layer). Depending on the types of materials used in the optical mirror, the mirror layer may not adhere well to the lower material layer if placed directly on top of the lower material layer. Therefore, a backing layer may be used to bond the mirror layer to the lower material layer. In order to effectively bond the mirror layer to the lower material layer, the backing layer material must adhere well to both the lower material layer and the mirror layer.
Another reason for using a backing layer is as a diffusion barrier to prevent interdiffusion and/or the formation of intermetallics between the mirror layer and a lower material layer (whether the substrate or another material layer).
An intermetallic is a type of alloy containing two or more metal atoms. In studies of gold films, it has been found that gold and silicon react at temperatures considerably below the eutectic (363C) [3], that gold thin films react with metals at room temperature if both the metal and the resulting compound(s) have melting points below approximately 700C [4], and that the reaction rate of intermetallic formation is controlled by diffusion and the interdiffusion coefficient is linearly dependent on melting point [5]. This linkage between intermetallic formation and diffusion was also noted in a study of gold-titanium, where diffusion was measurable at temperatures above 175C, allowing formation of intermetallics at temperatures above 250C [6].
Interdiffusion is essentially the mixing of two materials due to random thermal motion. Interdiffusion can occur, for example, when atoms from the lower material layer diffuse into the mirror layer or when atoms from the mirror layer diffuse into the lower material layer. In order for interdiffusion to occur, an atom must acquire a sufficient amount of energy to leave its present material so as to become available for diffusion into the other material, and there must be a “free volume” in the other material into which the atom can diffuse. Heat can provide the energy required by the atom to leave its present material and/or the energy required (if any) to create “free volume”. As-deposited metal films have more lattice defects (vacancies, grain boundaries, and dislocations) than do annealed films that are in thermal equilibrium. It has been found that grain boundary diffusion in gold is at least an order of magnitude higher than bulk diffusion [3]. Thus, small grain size films may be more susceptible to diffusion than coarser films. One way to avoid grain boundary diffusion is to use an amorphous thin film as a diffusion barrier, since amorphous films have no grain boundaries. For example, it has been found that amorphous nickel-niobium [7], amorphous tungsten-nitride [8], and amorphous tantalum-silicon-nitride [9] do not readily interdiffuse with silicon or gold.
Depending on the types of materials used in the optical mirror, interdiffusion and/or the formation of intermetallics may occur between the mirror layer and the lower material layer if the mirror layer is placed directly in contact with the lower material layer. Interdiffusion and/or the formation of intermetallics may contaminate the mirror and reduce the mirror's reflectivity. Therefore, a backing layer may be used to prevent interdiffusion and/or the formation of intermetallics between the mirror layer and the lower material layer. In order to effectively prevent interdiffusion and/or the formation of intermetallics between the mirror layer and the lower material layer, the backing layer material must not interdiffuse or form intermetallics with the mirror layer and must prevent the lower material layer from interdiffusing and forming intermetallics with the mirror layer.
Another reason for using a backing layer is to physically strengthen the mirror layer to prevent undue stresses in the mirror layer. Stresses in the mirror layer can cause gradual changes in the microstructure of the mirror layer material [10]. In a study focusing on a nickel-iron alloy for magnetoresistive elements [11], an approximate relationship was found between the melting temperature of a metal and the onset of intermetallic formation, and a link was also found between the onset temperature and diffusion. In a study focusing on diffusion and intermetallic formation in gold-niobium electrical contacts [12], interdiffusion, intermetallics, and gold hillocks (which were objectionable because they interfered with electrical contacts) were observed at temperatures above 350C.
A common optical mirror configuration uses a thin gold film as the mirror layer and uses silicon as the substrate.
Alie Susan A.
Hartzell Allyson
Karpman Maurice
Martin John R.
Nunan Kieran
Analog Devices Inc.
Bromberg & Sunstein LLP
Sikder Mohammad
LandOfFree
Optical mirror coatings for high-temperature diffusion... does not yet have a rating. At this time, there are no reviews or comments for this patent.
If you have personal experience with Optical mirror coatings for high-temperature diffusion..., we encourage you to share that experience with our LandOfFree.com community. Your opinion is very important and Optical mirror coatings for high-temperature diffusion... will most certainly appreciate the feedback.
Profile ID: LFUS-PAI-O-3054572