Optics: measuring and testing – By dispersed light spectroscopy – With sample excitation
Reexamination Certificate
2001-10-11
2003-08-05
Stafira, Michael P. (Department: 2877)
Optics: measuring and testing
By dispersed light spectroscopy
With sample excitation
C356S319000, C356S030000
Reexamination Certificate
active
06603547
ABSTRACT:
BACKGROUND OF THE INVENTION
The present invention relates to methods of determination of radiation stability of crystals, especially of crystals for optical elements, such as lenses, and to the use of crystals of a given radiation stability for manufacture of optical components and electronic units.
It has been shown that color centers arise in crystals at crystal defects cause by the presence of foreign atoms and at crystal lattice imperfections due to irradiation of the crystals. This means that the more light or electromagnetic radiation that is propagated in a crystal, the greater the number of color centers formed in the crystal. Because of this the light absorption of the crystal increases or the light transmittance decreases. The formation of color centers and the decrease in radiation transmission caused by that increase has proven troublesome or problematic, especially in optical components, through which energetic light, such as laser light, is conducted, as in steppers, with which structures of integrated circuits are optically projected on a photo-lacquer-coated wafer.
This formation of color centers plays an important role in optical structuring by lasers. It has already been attempted to make crystals of sufficient perfection and sufficient purity so that they are nearly free of foreign atoms and filter defects. However since impurities of less than 1 ppm in optical components, especially those used for DUV (Deep Ultra Violet, &lgr;<250 nm), already cause notable difficulties, each individual crystal must be tested for radiation stability prior to use in an optical component. The procedure up to now has been to cut a sample with a length of about 1 to 10 cm and a cross-section of about 2.5 cm×2.5 cm of the crystal to be tested. Subsequently the front face of the sample was finely polished and irradiated by means of a laser, usually with a working wavelength of 157 nm generated by an F
2
excimer laser and/or with a current working wavelength for a stepper of 193 nm generated by an ArF excimer laser. The usual energy density amounts to from 1 to 100 mJ/cm
2
with a pulse frequency of 50 to 500 Hz and a pulse rate of 10
4
to 10
7
. The absorption of the sample before and after laser irradiation at the respective working wavelengths was measured with a spectrophotometer. The laser-induced transmission decrease was calculated at both values. The conversion to the absorption coefficient was then performed by calculation according to the method described by K. R. Mann and E. Eva in “Characterizing the absorption and aging behavior of DUV Optical Material by High-resolution Excimer Laser Calorimetry”, SPIE, Vol. 3334, p. 1055.
The position can then be determined, which the optical elements made with the crystalline material can take, because of the established radiation resistance or stability. The energy density of the light irradiated into the optics is different at the respective application wavelengths at the different positions. Only those crystals may be used, which have high radiation stability, for the optical elements that are furthest to the outside, i.e. toward the radiation source. Also those optical elements in which the laser radiation is focused must have high radiation stability. The formation of many color centers leads furthermore to a higher absorption, i.e. that is more radiation energy is absorbed in the crystal. This has the consequence that the crystalline material and thus the optical lens are heated, whereby the refractive index and thus the imaging properties change. The higher the radiation stability, the less energy is converted into heat in the lens system.
To perform this testing process a high capacity of expensive excimer lasers must be provided, which requires great maintenance work.
The time interval between removing the crude crystals from the crystal growing vessel and establishing their suitability for a particular application, which presupposes high radiation stability, is thus extremely large because of the work-intensive preparation of the samples. That also means that not only is there a high material consumption, but also an additional expensive storage is required for the large crude crystals prior to the material allocation.
The build-up of color centers in optical materials by means of a cobalt source at a 1 megarad dosage is described by I. Toepke and D. Cope in “Improvements in Crystal Optics For Excimer Laser”, SPIE Vol. 1835 Excimer Lasers (1992), pp. 89 to 97. The radiation damage resulting therefrom was found to be well correlated with that produced by an excimer laser.
This method has the disadvantage that the stringent regulations for use of highly radioactive material must be considered, so that it is not suitable as a practical method.
Also this measurement method consumes a large amount of expensively grown crystalline material, whereby the total yield of the grown crystals is further reduced. Furthermore the crystalline samples must be prepared in a costly and time-consuming process by means of sawing and polishing the measured portion of them.
SUMMARY OF THE INVENTION
It is an object of the present invention to provide a rapid and economical method for reliable determination of the radiation stability of crystals, which is performed by simple inexpensive means without great loss of material.
According to the invention the resistance or stability of crystals to intense radiation at an arbitrary wavelength can be determined with any undefined and untreated crystal or a cleaved piece from it, when the surface integral of the difference spectrum formed from respective spectra taken prior to and after irradiation over a fixed wavelength range from a first wavelength &lgr;
1
to a second wavelength &lgr;
2
is calculated.
This procedure usual comprises measuring a first transmission spectrum (spectrum A) over an arbitrarily predetermined wavelength range from a first wavelength &lgr;
1
to a second wavelength &lgr;
2
and then stimulating or exciting the crystal, preferably with short-wave energetic radiation and preferably so that all or nearly all the theoretically possible color centers have been formed. After that a second transmission spectrum (spectrum B) of the crystal is measured in the same wavelength range from the first wavelength &lgr;
1
to the second wavelength &lgr;
2
. It has now been shown that the difference of the surface integrals from the first wavelength &lgr;
1
to the second wavelength &lgr;
2
of the transmission curve before and after irradiation is a measure of the radiation resistance or stability and is linearly related to the maximum change in the absorption coefficient &Dgr;k to be expected. The spectra are preferably taken with a spectrophotometer. Furthermore it has proven satisfactory to scale the spectrum in relation to the thickness of the crystal (length of the light path through the crystal).
This result is thus surprising, because the absorption or transmission spectrum for each individual crystal should be different, since different impurities and crystal defects produce it.
It is possible to prepare a calibration curve, when, as described above, the surface integral of the difference spectrum is determined and compared with or plotted versus the radiation damage measured with conventional methods for the same crystal. For example, the method as described by K. R. Mann and E. Eva, in “Characterizing the absorption and aging behavior of DUV Optical Material by High-resolution Excimer Laser Calorimetry”, SPIE, Vol. 3334, p. 1055, may be used as the conventional method. A linear calibration curve is obtained from the measured values. It is possible to eliminate the color centers formed in the conventional method by gentle heating, so that the measurement can be performed with the same crystal. It is of course possible to measure the calibration curve with the aid of a few pairs of values, however because of laser instabilities it is particularly preferred to determine the calibration curves of crystals of differing purity and/or radiation resistances. A measured calibrati
Kandler Joerg
Moersen Ewald
Speit Burkhard
Strenge Lorenz
Davis Willie
Schott Glas
Stafira Michael P.
Striker Michael J.
LandOfFree
Method for determination of the radiation stability of crystals does not yet have a rating. At this time, there are no reviews or comments for this patent.
If you have personal experience with Method for determination of the radiation stability of crystals, we encourage you to share that experience with our LandOfFree.com community. Your opinion is very important and Method for determination of the radiation stability of crystals will most certainly appreciate the feedback.
Profile ID: LFUS-PAI-O-3130923