Optical: systems and elements – Lens – Microscope objective
Reexamination Certificate
1999-11-30
2004-01-27
Sugarman, Scott J. (Department: 2873)
Optical: systems and elements
Lens
Microscope objective
C359S649000, C359S754000
Reexamination Certificate
active
06683729
ABSTRACT:
CROSS REFERENCES TO RELATED APPLICATIONS
Not applicable.
STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH OR DEVELOPMENT
Not applicable.
BACKGROUND OF THE INVENTION
1. Field of the Invention
The invention relates to an objective with crystal lenses. Such objectives have been known for over a hundred years as Carl Zeiss apochromatic microscope objectives with fluorspar (CaF
2
) lenses.
In recent times, refractive projection objectives for microlithography in the DUV at wavelengths of 248 or 193 nm have been constructed, and contain lenses of quartz glass and CaF
2
.
2. Discussion of Relevant Art
An optical system is known from German Democratic Republic Patent DD 222 426 B5, with optical glasses and BaF
2
single crystal as optical media, and can be used for wavelengths of 150 up to 10
4
nm. The embodiment example is a planapochromat for 480-800 nm with several different glasses and BaF
2
.
The choice of materials for UV microlithographic objectives (centered on 248 nm wavelength) is described in G. Roblin, J. Optics (Paris), 15 (1984), pp. 281-285.
The outcome was that only combinations of quartz glass with CaF
2
or LiF are graded as usable.
In U. Behringer, F+M (Munich) 107 (1999), 57-60, fluorides such as CaF
2
, MgF
2
and LiF are described as suitable for 157 nm microlithography, with reservations regarding the double refraction of MgF
2
and regarding the handling of LiF.
In K. F. Walsh et al., SPIE Vol. 774 (1987), 155-159, excimer lasers for wavelengths of 248, 193 and 157 nm are presented, among other things. For 248 nm quartz glass, CaF
2
, BaF
2
, and MgF
2
are named as the only applicable lens materials. For wavelengths below 248 nm, quartz glass is expected to be the only one material which is usable.
A catadioptric 1:1 projection objective for microlithography at 248 nm is described in U.S. Pat. No. 5,031,977, and contains a concave mirror, a quartz glass lens, an LiF lens, and two CaF
2
deflecting prisms. Neither arguments for choice of materials are given nor information on modifications of the special construction.
However, the absorption edge of quartz glass is situated close to 157 nm. CaF
2
still usefully transmits at 157 nm, but has too high a dispersion for a purely CaF
2
objective for microlithography, even for a spectrally narrowed F
2
excimer laser. Heretofore objectives for wavelengths below 193 nm are therefore known only as catadioptric (see German Patent Document DE 196 39 586 A of the same Inventor, and U.S. Prov. Apln. Ser. No. 60/094579 of Jul. 29, 1998) or catoptric (see U.S. Pat. No. 5,686,728) systems. Here U.S. Pat. No. 5,686,728 gives a purely mirror objective for VUV microlithography with a 126 nm, 146 nm, or 157 nm excimer laser, for example.
SUMMARY OF THE INVENTION
The invention has as its object an alternative concept of an objective with a new composition of material which opens up new possibilities of application, particularly in microlithography at low wavelengths.
This object is attained by an objective having a plurality of lenses made of at least two different crystals. This object is also attained by a microlithographic projection objective corrected for illumination with a F
2
excimer laser at 157 nm, wherein the projection objective is purely refractive and comprises a plurality of lenses of a material selected from the group consisting of BaF
2
, SrF
2
, NaF, LiF, and KF. This object is also attained by a projection exposure equipment having a light source including an excimer laser with 100-160 nm wavelength, an illumination system including refractive optical elements comprising one or more fluorides, a reticle positioning and movement system, a projection objective with a plurality of lenses comprising at least two of crystalline materials selected from the group consisting of CaF
2
, BaF
2
, LiF, NaF, SrF
2
, KF and amorphous BeF
2
, and an object positioning and movement system. A process according to the invention for the production of microstructured components provides that a substrate provided with a photosensitive layer is exposed to ultraviolet light by means of a mask and a projection exposure equipment and if necessary after development, the photosensitive layer is structured corresponding to a pattern contained on the mask.
The invention starts from the discovery that novel kinds of objective properties can be provided by the use of two different crystals in an objective. In particular, included in this is the possibility of achromatizing at low wavelengths, at which each known glass, including quartz glass, absorbs strongly. The existing reservations in microlithography against BaF
2
relate to 248 nm and quartz glass as the partner.
Alkali and alkaline earth halides, especially the fluorides, and also other fluorides, are known as optical materials. Their partially difficult properties as materials have however led to their outstanding transmission properties in the deep UV being made use of only in an adjunct manner. It has been shown according to the invention that optical microlithography can be extended down to about 100 nm wavelength with these and similar materials.
A pair of materials for the achromatization of 157 nm optics can be provided for the first time by the pairing of two fluoride crystals, particularly of CaF
2
, BaF
2
, SrF
2
, LiF, NaF, or KF, but also of mixed crystal fluorides. The materials are already known in optical manufacture, as evidenced by the cited state of the art. Barium fluoride, strontium fluoride, or sodium fluoride are preferably used for negative lenses, and indeed only for individual ones, as this can be sufficient. Calcium fluoride then finds application not only for the positive lenses, but also for the remaining negative lenses.
It is particularly advantageous that numerical apertures greater than 0.5 are attained, even at 157 nm. The following example with a numerical aperture of 0.8 reveals this clearly. The advance in resolution of EUV microlithography caused by about 1/10 of the wavelength is thereby partially compensated, since three times the NA is attained. As against 193 mm, the resolution can be nearly halved with 109 nm, since the level of the NA is retained. For the accuracy of processing, the invented technology has dramatic advantages over EUVL caused by tenfold the wavelength.
The stitching process (exposure of the chips by columns) according to an advantageous feature of the invention has recently been talked about for microlithography at very low wavelengths, and permits image fields of reduced size as rectangles with a moderate aspect ratio and thus makes possible a dramatic reduction in size of the objectives. This in turn dramatically relieves the production problems for the lens crystals.
In a quite different kind of embodiment of the invention, it was surprisingly found that in DUV microlithography with 248 nm or 193 nm, an aging process termed “compaction” occurs in quartz glass during long-term operation; in it, the material becomes densified and consequently the refractive index and the shape of the lens become changed. This of course worsens the imaging power of the objective. Besides compensation by positionable elements, it was known that the most highly loaded image-side lenses concerned can be made of crystal, preferably of CaF
2
, SrF
2
or BaF
2
, which are substantially more stable against UV radiation.
Several embodiment examples of such a use of calcium fluoride lenses are contained in German Patent Application DE 19855157.6 of the Applicant, of the same priority date, which Application is incorporated herein by reference as part of the disclosure of the present Application.
BaF
2
as well as SrF
2
in this reference have the advantage of differing substantially less than CaF
2
from quartz glass in their optical properties (see Roblin, as cited hereinabove)—although this is a disadvantage in the context of achromatization. The design changes of a projection objective on replacing quartz lenses by BaF
2
or SrF
2
lenses near the image are therefore minimal. The projection objective is thus optimized by the use of two crys
Sugarman Scott J.
Zeiss Carl
LandOfFree
Objective with crystal lenses and projection exposure... does not yet have a rating. At this time, there are no reviews or comments for this patent.
If you have personal experience with Objective with crystal lenses and projection exposure..., we encourage you to share that experience with our LandOfFree.com community. Your opinion is very important and Objective with crystal lenses and projection exposure... will most certainly appreciate the feedback.
Profile ID: LFUS-PAI-O-3198329