Patent Application
20050031970
The invention is directed to a method for predicting laser-induced damage caused to single crystals of ionic materials, and in particular to metal fluorides of formula MF2; using such predictions to identify metal fluoride crystals suitable for use in below 200 nm optical lithography; and metal fluoride crystals suitable for use in below 200 nm optical lithography.
Calcium fluoride (CaF2) single crystals have been expected to be the material of choice for the optics of next-generation photolithography techniques in use by the semiconductor industry. These lithographic systems will involve the use of high power lasers operating at vacuum ultraviolet (“VUV”) wavelengths below 200 nm, typically at 193 nm and 157 nm. However, one of difficulties involved in operating at these low wavelengths is that the damage that the laser can cause to the CaF2 single crystals that will be used in the optics. This damage results in a decrease in the optical transmission under ultraviolet (UV) laser irradiation. Since the photolithography technique requires very high optical transmission, the reduction of laser damage effect is an important part of product quality in the manufacturing of CaF2 crystals. The laser damage effect is measured in a Fluence Dependent Transmission (“FDT”) test by monitoring the transmission change under a series of UV excimer laser pulses. The typical behavior of CaF2 in FDT test has been found to have two aspects. The first is the “transmission decay speed”; that is the time required to reach the saturation with a fixed laser pulse energy. The second is “FDT slope”; that is, the linear dependence of the optical transmission at saturation under laser pulse energy variation. With regard to the decay speed, it has been found that CaF2 crystals can be sorted into two broad groups. One group of CaF2 products, single crystals and optical elements made therefrom, was found to have a fast decay behavior; for example, <10 seconds up to transmission saturation under 1 -50 mJ/cm2 per a pulse and 100 pulses per a second. The second group has a slow decay; for example, >100 second up to saturation. It has also been found that the FDT slope does not correlate with the decay speed. Moreover, the FDT slope is not a constant for CaF2 crystals with very low impurities (for example, impurities determined by optical absorption spectroscopy). With respect to the laser damage inflicted on crystals, the main problems are to identify the origins of the FDT behaviors (decay speed and slope) and to develop a method for quantification of the origins.
The radiation damage effect of CaF2 crystals by X-ray or electron-beam irradiation has been investigated over several decades. High energy irradiation (for example, X-ray, electron-beam, and short wavelength UV light) has been reported to produce F-centers which are free electrons trapped into a fluorine anion vacancy. The F-centers in CaF2 have strong optical absorption at 385 nm with a very broad width. In addition, the aggregation centers of F-centers with themselves and/or other impurities have optical absorption in UV spectral range. The optical absorption of the radiation-induced defects is considered as the origin of the transmission decreases observed in CaF2 crystals. However, what has not been clearly identified are the factors (impurities, defects, etc) that cause the FDT behaviors.
The measurement techniques use to characterize defects in CaF2 crystals are based on the optical spectroscopy (absorption and emission measurements) and magnetic resonance (NMR, EPR and similar techniques). In addition to these well-known methods, a method called the “thermally stimulated current” (“TSC”) technique was developed in the 1960's to characterize the defects of electric charges or dipoles in ionic crystals. [See C. Bucci and R. Fieschi, Physical Review Letters 12: 16 (1964)]. TSC method was reported to characterize the point structural defects (vacancies and interstitials) in CaF2. [See I. Kunze et al., Physica Status Solidi (A) 13: 197 (1972) and K. Tanaka et al, J. Physics and Chemistry of Solids 57: 307 (1996).] However, no investigations have been done to determine if there is any correlation between the FDT behaviors and the TSC results. Consequently, there is a need to find a method of correlating FDT behaviors and TSC results in order to aid in the determination of what crystals are suitable for less than 200 nm lithographic methods.
The present invention is directed to a method of correlating TSC measurement results with FDT behaviors. In particular, the invention is directed to a method of correlating TSC measurement results with FDT behaviors in order to provide a means for which determining metal fluoride crystals, and optical elements made therefrom, are suitable for use in optical lithography systems; for example, lithography systems operating below 200 nm. The metal fluoride crystals are Group IIA metal fluorides of formula MF2 where M is Ca, Mg, Ba and Sr (including mixtures thereof). Crystals of formula MF2 are suitable for below 250 nm lithographic applications and particularly for below 200 m lithography; for example, at 193 and 157 nm. The invention can also be used to diagnose and select alkali metal single crystals of formula M° F., where M′ is Li and K, suitable for use in systems requiring such crystals. Use of the method of the invention avoids excessive costs and time in manufacturing of metal fluoride single crystals suitable for optical lithography.
The present invention is also directed to metal fluoride single crystals suitable for use in below 200 nm optical lithographic processes, said crystals having a FDT slope that is linearly dependent on the TSC peak maximum. Such crystals are of formula MF2 and the metal is selected from the group consisting of calcium, barium, magnesium and strontium, or mixtures thereof. Such crystals have a TSC peak at 305° K. of 10−12 ampere or greater.
A thermally stimulated current (TSC) method had been developed for analyzing ionic crystals. Referring to
The laser damage effect to CaF2 crystals under UV irradiation (for example at 193 or 157 μm) is initiated by an incident photon ionizing a host ion to produce a pair of free electron and hole. When a free electron meets with an anion vacancy, the electron is trapped in the anion vacancy and an F-center is formed. Such F-centers have an optical absorption at 385 nm with a very wide bandwidth. F-centers are subsequently aggregated with themselves and with defects/impurities, and the aggregated centers have optical absorption in UV and visible spectral region. While not intending to limited by theory it is suspected that formation of such color centers is related to the Fluence Dependent Transmission (FDT) behaviors at 193 or 157 nm. Fluence Dependent Transmission behavior of a crystal can be determined by subjecting a crystal to a selected wavelength (for example, 193 or 157 nm, or other wavelength depending on the intended application) and varying the fluence levels between selected values. For example, using a laser producing a 193 nm beam, the fluence may be varied between 0.8 and 50 mJ/cm2/pulse, or other selected range. Transmission of the part of the laser exposure wavelength is monitored during exposure. Over the course of exposure the fluence is changes. This change in fluence yields a change in transmission. The transmission reaches a “steady state” value that is plotted in absorption (base 10) units vs. exposure fluence. The slope of the absorption vs. fluence plot is the metric of interest.
The process of laser-induced color center formation which produces FDT behaviors is not fully understood at this time. However, it is believed that at the beginning of this process host anion vacancies play a very important role in producing F-centers with laser-induced free electrons. Consequently, it is necessary to know or to be able to determine the concentration and thermal properties of host anion vacancies in order to predict or estimate FDT results.
In making a TSC/FDT correlation, only the TSC peak maximum at 305K (=32° C.) is used to make a correlation to FDT results.
The present invention is directed to a method of correlating TSC measurement results with FDT behaviors. In particular, the invention is directed to a method of correlating TSC measurement results with FDT behaviors in order to provide a means for which determining metal fluoride crystals, and optical elements made therefrom, are suitable for use in optical lithography systems; for example, lithography systems operating below 200 nm. The metal fluoride crystals are of general formulas MF2 and M° F., where M is Ca, Mg, Ba and Sr, and where M′ is Li and K.
The invention thus enables one to select a metal fluoride single crystals suitable for use in below 200 nm optical lithographic processes by measuring only the TSC peak strength of the selected single crystal sample and using the standard linear relationship between the TSC peak strengths and the FDT slopes. Such crystals are of formula MF2 and the metal is selected from the group consisting of calcium, barium, magnesium and strontium, or mixtures thereof. Such crystals have a TSC peak at 305° K. of 10−12 ampere or greater.
The present invention has been described in general and in detail by way of examples. Persons skilled in the art understand that the invention is not limited necessarily to the specific embodiments disclosed. Modifications and variations may be made without departing from the scope of the invention as defined by the following claims or their equivalents, including equivalent components presently known, or to be developed, which may be used within the scope of the present invention. Hence, unless changes otherwise depart from the scope of the invention, the changes should be construed as being included herein.
This application claims the benefit priority of U.S. Provisional Patent Application No. 60/492,539 filed Aug. 4, 2003.
| Number | Date | Country | |
|---|---|---|---|
| 60492539 | Aug 2003 | US |
