Optical: systems and elements – Light interference – Produced by coating or lamina
Reexamination Certificate
1997-01-28
2002-01-29
Spyrou, Cassandra (Department: 2872)
Optical: systems and elements
Light interference
Produced by coating or lamina
C359S586000, C359S589000, C359S900000, C349S080000, C349S106000, C348S273000, C348S280000
Reexamination Certificate
active
06342970
ABSTRACT:
BACKGROUND AND SUMMARY OF THE INVENTION
The present invention relates to a dielectric interference filter system having at least two filter elements built up on a common carrier and which spectrally operate differently, an Liquid Crystal Device (LSD), display as well as a charge coupled device (CCD) arrangement comprising a color filter system, a process for manufacturing the dielectric interference filter system, and use of the process for manufacturing interference color filter systems.
The term “dielectric interference filter system” refers herein to a system in which filter elements are provided side-by-side on a common carrier. When viewed above the wavelength, the filter elements have different transmission characteristics. They may be high-pass, low-pass, band-pass or band-stop filters.
“Transparent” layers refers to the fact that, in the spectral wavelength regions which are of interest with respect to the action of the filter elements, such layers have an essentially constant high transmission and low absorption values.
A “black matrix” is a layer or a layer system whose transmission vanishes for radiation in a spectral wavelength region which is specific with respect to the action of the filter elements.
Although the present invention specifically describes interference color filter systems, i.e., filter systems which operate with respect to colored wavelength ranges discernible by the human eye, it is to be clearly understood that the present invention may also be used in conjunction with interference filter systems which operate outside the wavelength range be discernable by the human eye.
A color filter system is an optical element which influences the spectral region of a light source visible to the human eye so that the resulting light radiation causes a specific color impression. The color may be expressed in the form of CIE-coordinates for whose computation the spectral characteristics (transmission or reflection) of the optical filter element, the spectral radiation distributions of the light source and the spectral sensitivity of the human eye are used, as defined in DIN-Standard 5033 (July 1970).
Basically, it is known, as described in K. Tsuda, “Color Filters for LCDs”,
Displays
, Vol. 14, No. 2, P. 115 (1993), to implement color filter elements by using spectrally selective absorbing layers of a defined thickness d which are applied to a broadband high-transmitting substrate.
Such selectively absorbing layers consist of organic materials with an approximately constant refractivity n and with a highly wavelength-dependent extinction coefficient k (&lgr;) In this case, the spectral transmission amounts to
T
(&lgr;)=exp [−4
·n·d·k
(&lgr;)·&lgr;
−1].
The above-mentioned article by K. Tsuda describes disadvantages of organic color filters or, within the scope of a filter system, organic color filter elements. These disadvantages are low color saturation, high absorption losses which may lead to an undesirable heating of the color filter or color filter element, insufficient chemical, mechanical and thermal stability, and insufficient geometric precision; i.e., fluctuations in the layer thickness and evenness of the surface.
A second approach, to which the present invention basically relates, is the implementation of optical filter systems, particularly color filter systems, with the use of dielectric thin-film systems which consist, for example, of alternately successive, relatively low-refracting layers, for example of SiO
2
, and high-refracting layers, for example, of TiO
2
, as described, for example, in H. A. Macleod, “Thin-Film Optical Filters”, Adam Hilger Ltd. (1986).
Such layer systems are normally produced by vacuum coating processes, for example, by (a) vaporization techniques, such as electron beam vaporization, (b) arc vaporization or by sputter coating in DC-, AC- or mixed AC- and DC-plasmas, (c) by ion plating, which are all PVD-processes which can be used reactively or non-reactively, or (d) by CVD-processes or PECVD-processes (Plasma Enhanced Chemical Vapor Deposition Processes).
A corresponding desired spectral characteristic, expressed, for example, by the spectral transmission T(&lgr;), occurs in this second approach by the interference of the light which is reflected on the different interfaces of the layer system and is transmitted. The absorption may, by approximation, be neglected. Typically, the resulting overall thickness of such layer Systems is a function of the spectral region, particularly of the color which is to be transmitted by the filter element Thus, for example, a dielectric interference filter element for blue is the thickest because here the long-wave region of the visible spectrum must be blocked. Correspondingly, a red filter element is the thinnest. Reference is made in this respect to the above-mentioned “Thin-Film Optical Filters” publication by H. A. Macleod, as well as to “An Active-Matrix Color LCD with High Transmittance Using an Optical Interference Filter”,
Japan Display '
89, Page 434 (1989), T. Unate, T. Nakagawa, Y. Matsushita, Y. Ugai and S. Aoki.
By way of optical monitoring methods, dielectric layer systems may be produced with a layer thickness precision of ±1%. In the case of typical overall thicknesses of dielectric interference color filter elements in the range of 1.5-3.5 &mgr;m, this tolerance an absolute precision deviation of, at most, 0.07 &mgr;m.
For structuring dielectric interference filter systems into individual filter elements, predominantly two techniques may be used. The first technique is etching where an applied layer system is etched in defined areas. For this purpose, an etching mask is applied to the initially unstructured layer system and, in areas not covered by the mask, the layer system situated underneath is removed by etching, as desired. In this technique, the etching may take place in a chemically wet manner but is preferably implemented by a vacuum process. For this purpose, reactive or non-reactive PVD-processes are suitable, such as DC-, AC- or AC- and DC-sputtering or reactive ion etching, the latter being of particular interest in the present context.
The second technique is the lift-off technique where a mask is applied to a carrier system situated underneath and the desired layer system is deposited over the mask. During the subsequent lift-off of the mask, the layer system, which is structured as desired, will then remain only on areas which previously had not been covered by the mask.
As previously above, the thickness of absorbing organic color filter elements fluctuates considerably because of the manufacturing tolerances of the organic material layers. In dielectric interference filter systems, the overall thickness of the filter element layer systems will fluctuate because of the numbers and thicknesses of the layers which are required for the endeavored spectral characteristics.
In many applications of filter systems in which filter elements, which spectrally have different effects, are constructed side-by-side on the same carrier, the reaching of the same levels for all provided filter elements which have different spectral effects would be highly desirable. A typical example and an application to which the present invention also relates specifically are LCD-displays.
FIG. 1
is a cross-sectional view of the basic construction of a known color-LCD-display. On a substrate
1
, color filter elements
3
are situated in the active range of the display, that is, in that range in which the image is created. For example, the color filter elements
3
are red “R”, green “G” and blue “B”. As shown in
FIG. 1
partly under and between the elements
3
, black matrix elements
5
, as above defined, are built in under, between or above the color filter elements
3
. The black matrix elements
5
are typically made of chromium and, depending on the desired optical density, have a thickness of from 0.1 to 0.2 &mgr;m. An electrically conductive transparent layer
7
, typically an indium-tin-oxide layer ITO, which, depending on the
Rudigier Helmut
Schoech Helmut
Sperger Reinhard
Wierer Peter
Crowell & Moring LLP
Jr. John Juba
Spyrou Cassandra
Unaxis Balzers Aktiengesellschaft
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