Optical: systems and elements – Single channel simultaneously to or from plural channels – By partial reflection at beam splitting or combining surface
Reexamination Certificate
2002-07-17
2004-10-26
Mack, Ricky (Department: 2873)
Optical: systems and elements
Single channel simultaneously to or from plural channels
By partial reflection at beam splitting or combining surface
C219S121600, C219S121850
Reexamination Certificate
active
06809871
ABSTRACT:
The following disclosure is based on German Patent Application No. DE 101 36 507.1 filed on Jul. 17, 2001, which is incorporated into this application by reference.
BACKGROUND OF THE INVENTION
1. Field of the Invention
The invention relates to a method for fabricating a geometric beamsplitter, a beamsplitter that may be fabricated employing that method, and an illumination system equipped with a beamsplitter of that type for use on an optical device, in particular, on a microlithographic projection exposure system.
2. Description of the Related Art
Numerous application areas of optics require splitting a light beam into two equal, or unequal, partial beams. Beamsplitters that are configured either as a geometric beamsplitter or a physical beamsplitter are employed for that purpose. While physical beamsplitters leave the cross-sectional areas of light beams unaltered, geometric beamsplitters split their cross-sectional areas into partial beams using, for example, a reflective surface that has openings.
In many applications, an effort is made to accurately determine, for example, in the case of so-called “dosimetry mirrors,” such as those employed on the illumination systems of optical devices, the relation between the reflected energy and the energy transmitted through those openings.
As an example, U.S. Pat. No. 6,028,660 discloses an illumination system equipped with a dosimetry device for use on a microlithographic projection exposure system for fabricating semiconductor devices and other types of microdevices. Systems of that type, which may be configured as wafer scanners or wafer steppers, are used for projecting patterns on photomasks (reticles) or ruled plates onto an object, for example, a semiconductor wafer, coated with a light-sensitive film, at high spatial resolutions. In order to obtain a well-defined pattern on the substrate, the substrate should be irradiated with an accurately determined quantity of energy, or dose. It is thus essential to determine the actual irradiation dose in order to allow accurately setting the necessary dose whenever necessary using a feedback device that controls the luminous output of the light source. Between its light source and its light exit, the illumination system has an output-coupling device for coupling out a fraction of the luminous energy from its light source toward an energy sensor whose energy measurements may be employed for controlling its light source, where it is essential to know the absolute transmittance of the beamsplitter in order to allow drawing conclusions regarding the absolute intensity of the light source based on the energy sensor's energy measurements, which is necessary in the case of, for example, wafer steppers, in order to be able to accurately set the open periods of their shutters or the durations of the illumination periods for exposures. In the case of wafer scanners, it is essential that illumination doses be uniform over any given illuminated field.
There is thus need for beamsplitters that have accurately defined transmittances. These should be simple and inexpensive to fabricate.
SUMMARY OF THE INVENTION
One object of the invention is to provide a method for fabricating a geometric beamsplitter that will allow inexpensively fabricating geometric beamsplitters having accurately defined transmittances. It is another object of the invention to provide a geometric beamsplitter having an accurately definable transmittance that may be inexpensively fabricated in large numbers.
As a solution to these and other objects, the invention, according to one formulation, provides a method for fabricating a geometric beamsplitter comprising: coating a surface of a substrate consisting of a transparent material with a reflective coating that contains at least one metallic layer; creating a pattern of holes comprising a large number of transparent holes in the reflective coating; wherein the holes of the pattern of holes are created by laser processing.
In the case of the method according to the invention, a surface of a substrate consisting of a transparent material is initially coated with a reflective coating that contains at least one metallic layer whose thickness is preferably chosen such that it is substantially opaque to the light to be employed. A pattern of holes having a large number of transparent holes whose number, size, and/or distribution largely determines the beamsplitter's transmittance is then created in that reflective coating. In accordance with the invention, those holes are created using laser processing, whereby the reflective coating is simultaneously or repeatedly irradiated at numerous locations with laser light having a suitable diameter and a suitable wavelength, energy, and duration such that transparent holes in the reflective coating arise, without destroying the substrate or areas of the reflective coating bordering on irradiated sections. The total transmittance is set mainly by varying the total number of holes and their average area.
The method is suitable for use on all types of reflective coatings that have absorptions at the laser wavelength employed for creating the pattern of holes that are sufficiently high to allow removing the reflective coating, largely via evaporation. As used here, the term “metallic layer” stands for, in general, layers of materials that absorb the light employed strongly enough to allow their evaporation. Such materials are not necessarily metallic. Compared to wet-chemical etching methods, under which patterns of holes may be created employing a photolithographic process involving coating, exposure, and subsequent etching, which may also be employed, dry laser creation of patterns of holes has the advantage that it causes no chemical changes and leaves no contaminants on the reflective surface. The method, which works both in a vacuum and in air, thus imposes no special requirements on the working environment and allows inexpensively, rapidly, fabricating geometric beamsplitters having well-defined transmittances.
A pulsed laser is preferably employed for creating the holes. It has proven beneficial to employ a first laser pulse, and at least a second laser pulse, for irradiating a given area, where the laser energy is set such that the first laser pulse largely creates a hole in the coating and the second cleans up the hole in order to smooth and even out its edges. It has been found that it will be sufficient if the laser pulses have substantially the same power density and duration, which means that no special requirements are imposed on controlling the laser during laser processing and that the method may be performed less expensively than, for example, known methods for creating blind and through holes in multilayer structures for multichip modules (WO 97/44155).
It is possible to create a regular hole pattern in the form of, for example, a two-dimensional grid. However, another preferred embodiment foresees that a pattern of holes having a random distribution of holes, i.e., a pattern other than a grid, where the spatial coordinates of individual holes are generated by a random number generator, is created. All holes will preferably be randomly distributed. However, both a group of randomly distributed holes and a group of regularly distributed holes may also be provided.
Random hole distributions yield significant benefits, both during fabrication and when beamsplitters are employed. When beamsplitters are employed, i.e., when large areas of their reflective surfaces are illuminated, diffractive effects and interference effects in the transmitted light, such as those that occur in the case of patterns of transmitting holes having regular grid layouts, are precluded. Moreover, random patterns of holes may be particularly simply reprocessed to yield other random patterns of holes having greater total transmittances by creating additional (randomly distributed) holes. In the following, we shall explain how random patterns of holes may be employed for fabricating beamsplitters having accurately prescribed transmittances with high
Heller Matthias
Kress Werner
Kuhn Matthias
Weissenrieder Stefan
Carl Zeiss Semiconductor Manufacturing Technologies AG
Mack Ricky
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