EP1166084A1

EP1166084A1 - Method and apparatus for measuring internal transmittance

Info

Publication number: EP1166084A1
Application number: EP00918282A
Authority: EP
Inventors: Gregory L. Bucher; Stephan L. Logunov
Original assignee: Corning Inc
Current assignee: Corning Inc
Priority date: 1999-03-22
Filing date: 2000-03-22
Publication date: 2002-01-02
Also published as: WO2000057158A1; AU3911800A; CA2366739A1; JP2002540395A; KR20020011373A

Abstract

A method and apparatus is provided for measuring the below 300nm internal transmittance of an optical material (12) comprising measuring the transmittance of a below 300nm light source (14) through at least two different pathlengths (16, 16') within a single sample of the material. The below 300nm light is transmitted through two opposing surfaces of the sample, the two opposing surfaces being spaced apart in a non-parallel relationship. The invention provides improved measurements for below 300 nm optical microlithography.

Description

METHOD AND APPARATUS FOR

MEASURING INTERNAL TRANSMITTANCE

FIELD OF THE INVENTION

The present invention relates to property measurements of low loss optical materials. More particularly, the present invention relates to measuring the internal transmittance of low loss optical materials. Particularly the invention relates to measuring the below 300 nm wavelength transmission properties of low loss, microlithography optical materials such as silica glasses and fluoride crystals, and preferably the below 200 nm wavelength transmission properties such as at 193 nm and 157 nm. The invention includes a below 300nm transmission standard optical material device for calibration of transmission at below 300nm wavelengths.

BACKGROUND OF THE INVENTION

Low loss optical materials require precise and accurate evaluation of optical losses. These low loss optical materials include, but are not limited to, high purity fused silica, which is used for applications such as microlithography imaging lenses. One important measurement is the transmittance (optical power in/ optical power out) of an optical material such as microlithography silica glasses and microlithography fluoride crystals such as calcium fluoride. It is desirable for an optical lithography material (glass or crystal) used in the illuminating system or projection lens of a photolithographic stepper system to have an internal transmittance of 0.99 cm^"1 or higher at the microlithography wavelength. As light sources used with the stepper shifts towards KrF and ArF excimer lasers, higher transmittance materials such as silica and fluoride are required. At shorter wavelengths, below 300 nm, and below 200 nm it is very difficult to accurately measure the internal transmittance of optical microlithography materials because of instrumental limitations and greater contribution of surface losses. There are two conventional methods for determining the internal transmittance of a material. One known method measures transmittance through opposed, parallel surfaces of a single sample. A second known method measures transmittance of at least two such samples, each sample differing in pathlength. An unconventional third method uses a prism-shaped sample to provide a means of measuring transmittance as function of pathlength. In the second and third methods, the exponential slope of transmittance vs. pathlength provides the attenuation coefficient of the bulk sample, from which internal transmittance is calculated. The first method requires that surface losses (reflectance, scatter, absorption) and all optical system uncertainties be quantified in order to determine the bulk (internal) transmittance. Alternatively, comparison can be made to a known reference sample of the same material and geometry, or in certain cases, surface reflection and scatter can be eliminated by measurement of the sample immersed in an index-matching fluid. The second method with at least two samples requires that surface losses and optical system uncertainties remain constant for each sample. The third method requires only that the two surfaces remain constant throughout the test period.

In practice, using a below 300 nm, 248 nm excimer laser, the first two methods provide a measurement accuracy of no better than about +/- 0.1 % Transmittance/cm (T/cm), or 0.004 dB/cm. Using a 193 nm excimer laser, the accuracy provided by the first and second methods is no better than about 0.5% T/cm, or 0.02 dB/cm. Surface loss at the level of 0.1 % T to 0.2% T is variable over time, and from sample to sample. Other sources of optical system uncertainties (power response) are dependent on sample geometry and pathlength. The third method has not been demonstrated by prior art in the below 300 nm microlithography uv ranges. At long wavelength (633 nm) with a CW (continuous wave) (HeNe) laser -+/-0.004 dB/cm precision has been shown, with estimation of ~10^"5 dB/cm precision with equipment improvements. Accuracy was not determined.

In view of the above stated limits to measuring the internal transmittance of low loss optical microlithography materials, it would be desirable to provide a method and apparatus for providing an improved internal transmittance measurement accuracy at below 300 nm microlithography wavelengths. There is a need to provide a method and apparatus to accurately measure internal transmittance of low loss optical microlithography materials having an internal absorption of 0.001 cm^"1 or less, particularly in the short microlithography wavelength regions below 300 nm. Microlithography optical materials such as silica glasses and fluoride crystals with accurately measured internal transmittance at below 300 nm wavelengths will provide for beneficial microlithography optical elements such as lenses and allow for the adoption of below 300 nm and below 200 nm wavelength optical microlithography by the microlithography integrated circuit industry.

SUMMARY OF INVENTION

Accordingly, the present invention generally provides an improved apparatus and method for accurate measurement of below 300 nm internal transmittance of non- parallel sided prism-shaped optical lithography materials. The inventive method includes generating accurate transmittance standards for improving measurement accuracy of microlithography optical materials and microlithography optical elements thereof. Aspects of the invention include improved measurement apparatuses and measurements therewith of optical microlithography materials. In a preferred embodiment a non-parallel sided prism shaped wedge is translated relative to a below 300 nm light source to provide at least two different transmission path lengths. Preferably the method and apparatus does not utilize translation of mirror elements and inhibits the movement of reflective mirror optical elements and active moving of the beam.

In preferred embodiments the below 300 nm measurement light travels only in one-way through the material sample and two-way travel of the below 300 nm light through the material is avoided. The measurement beam transmits through the sample only once in a one-way transmit direction as opposed to being transmitted through the sample in a two-way direction with the light reflected back through by a mirror element in the second opposing direction. Preferably a below 300 nm pulsed laser with pulse by pulse reference compensation is utilized as the light source. The apparatus and method provide optimized measurements in the below 300 nm optical microlithography deep-uv wavelength ranges and provide accurately measured and selected microlithography optical materials and elements thereof. Aspects of the invention include generation of below 300 nm transmittance standards for improved accuracy of measuring below 300 nm optical lithography materials. The preferred below 300 nm optical lithography material standard sample is an optically polished and cleaned non-parallel sided prism-shaped wedge with an apex angle equal to the angle of minimum deviation, with triangular non-parallel side dimensions sufficiently long to give the desired pathlength difference, and with a length between parallel ends (end faces that are in a plane normal to the triangular non-parallel sides) equal to approximately the length of the typical average microlithography optical material element or blank to be measured and utilized in the intended below 300 nm optical microlithography system. For example, a transmittance below 300 nm lithography standard wedge made of Corning Incorporated HPFS™ type high purity fused silica glass for 193 nm optical microlithography would have an apex angle of about 65.24 degrees, triangular sides in the range of 1-6 cm, and a length between parallel ends in the range of 1-20 cm. Surface roughness is polished to be 5 angstroms rms, or better, for best accuracy.

A polarized below 300 nm light pulsed beam incident on the side of the prism- shaped wedge at the brewster angle is transmitted through the prism shape parallel to the base at various locations (a, b, c) by prism or source translation in a plane defined by the prism apex and base to yield transmittance vs. pathlength (pathlength a, b, c). This set of measurements is repeated along the length of the prism (A, B, C, D, E, F) (to account for optical material inhomogeneity).

The average attenuation coefficient (the slope of transmittance vs. pathlength) of all the coefficients at each measurement plane (A, B, C, D, E, F) along the length of the prism-shaped wedge is equal to the attenuation coefficient of a beam transmitted the full length of the prism-shaped wedge.

The transmittance of the prism-shaped wedge, with known average attenuation coefficient (and internal transmittance), is measured through the parallel faces such as with a spectrophotometer, to ascertain measurement error. Care is exercised in providing similarly polished and cleaned surfaces for both the prism-shaped standard and the typical average microlithography optical material element blank. Use of two, or more, different length prism-shaped wedge standards in this fashion preferably provides better compensation of surface effects, and a more accurate calibration of the spectrophotometer. The invention includes a set of wedge transmission standards with a variety of length useful for calibration of a spectophotometer.

Several important advantages will be appreciated from the foregoing summary. According to the present invention, a method and apparatus are provided in which it is possible to measure the below 300 nm internal transmittance of optical materials at a wavelength of 248 nm to an accuracy of at least 0.01% T/cm, and at 193 nm to an accuracy of at least 0.1% T/cm. Additional features and advantages of the invention will be set forth in the description which follows, and also may be learned by practice of the invention. It is to be understood that both the foregoing general description and the following detailed description are exemplary and explanatory and are intended to provide further explanation of the invention as claimed. The accompanying drawings are included to provide a further understanding of the invention and illustrate exemplary embodiments of the invention.

BRIEF DESCRIPTION OF THE DRAWINGS

Fig. 1 depicts a schematic of an apparatus for measuring the internal transmittance of a sample according to an embodiment of the present invention. Fig. 2 is a graph of data of -LnT (T= transmission) versus sample thickness obtained on high purity fused silica glass at 193 and 248 nm according to the present invention.

Fig. 3a is a schematic of how a prism-shaped standard wedge would be configured and measured. Fig. 3b is a plot of the internal transmittance along the prism length, with a line showing the average internal transmittance.

Fig. 3c illustrates a method of the invention. Fig. 3d illustrates a method of the invention.

DETAILED DESCRIPTION Reference will now be made in detail to the present preferred embodiment of the invention, an example of which is illustrated in the accompanying drawings.

The invention resides in a method and apparatus for microlithography capable of providing at least an order of magnitude better precision and accuracy than prior art methods of measuring internal transmittance in the below 300 nm microlithography wavelength regions, and use of such method and apparatus to generate accurate transmittance standards for optical spectrophotometry and microlithography use. This invention provides improved accuracy and precision and employs a prism-shaped sample wedge to make possible measurement of multiple pathlengths (lengths a, b, c) within a single sample. As used herein, the prism-shaped wedge comprises an optical material sample which provides a reasonable difference in the pathlength from top (apex) to bottom (base) of the sample, for example, at least about 5-10 degrees wedge is required for 6 cm sample. An apex angle equal to the minimum deviation angle is best, as it precludes the need to realign the sample at each measurement location to prevent displacement of the beam on the detector. The minimum deviation angle is calculated using A= 180-2 Θ _B> where Θ _B = arctan (n) and n is the refractive index of the prism optical material. According to the present invention, measuring transmittance involves only two surfaces of a single sample, which minimizes error due to surface inconsistency inherent in methods with multiple samples due to polishing and cleaning. Another advantage of the present invention is that transmittance as a function of pathlength can be obtained quickly by simple translation of the sample across the measurement light beam. This reduces errors due to time variance in surface losses, instrument system drift, and multiple sample alignments. Another advantage of the invention is that below 300 nm transmission is through the sample in only one direction. Such one-way beam transmission reduces error caused by necessary small displacement of a two-way measurement beam on reflection in a double-pass arrangement. Another advantage of the invention includes utilizing a below 300 nm pulsed laser light source and reference compensation using high-speed software and electronics and a beamsplitter which allows even within pulse compensation of a pulsed laser source. The present invention provides an economical cost effective below 300 nm transmission measurement requiring less material and less sample preparation than multiple sample, multiple path length techniques. Enhanced precision of the exponential fit of -LnT versus sample thickness, and information on short range very local inhomogenieties, is accomplished, simply, by translation in finer increments.

A preferred embodiment of the present invention uses a polarized, collimated below 300 nm measurement beam incident on the sample at Brewster's angle. Preferably the light source is a pulsed laser. The collimated beam minimizes pathlength dependent systematic error. The below 300 nm polarized beam with Brewster-angle incidence minimizes specular reflection at the surfaces. This measurement geometry can be realized in a single prism shaped sample by propagating the beam along parallel pathways (a, b, c), for example parallel to the prism base (Fig. 3). The present invention is superior to single sample techniques that utilize a sample having plane parallel surfaces, as disclosed in United States patent no. 5,776,219, the contents of which are incorporated herein by reference. Plane parallel surfaces, as conventionally used, create a host of systematic errors related to multiple surface reflections or Fabry-Perot interferences from the surface reflections. An exemplary embodiment of the measurement system present invention is shown in Fig. 1 and is designated generally by reference numeral 10. Referring to Fig. 1, measurement system 10 includes a single sample of optical microlithography material 12 and a below 300 nm light source 14 for generating a beam of light 16 and a detector 18 for detecting the beam of light transmitted though the sample 12. In an important aspect of the present invention, the prism-shaped wedge sample 12 has two surfaces through which the light beam 16 travels, the surfaces being spaced apart, non- parallel relationship. The apparatus shown in Fig. 1 also includes a beam splitter 20, and a second light detector 22 to measure the power of the light prior to transmission through the sample 12. As shown in Fig. 1, the sample 12 can be in the non -parallel sided shape of a prism, for example. Other sample shapes having non-parallel, opposed wedge surfaces are within the scope of the present invention, for example a prisim type shape with a truncated top. Transmittance is determined by comparing the optical power generated by the light source detected by detector 22 with the optical power of the light detected by detector 18. According to Fig. 1, the sample 12 is translated relative to the light beam 16 to provide measurement of transmittance through multiple paths lengths within a single sample, such as with a translating positioning stage 100. The following non-limiting example will further illustrate the principles of the present invention.

Example A 6.5 cm prism made of HPFS^® type high purity fused silica glass optical material available from Corning Incorporated, Corning, NY, was tested with the method shown in Fig. 1. The prism angle was selected such that below 300 nm polarized light is incident on both optically polished non-parallel surfaces of optical material sample 12 at the Brewster angle. Figure 2 plots the results of -LnT (Transmittance) versus sample thickness using a 193 nm and 248 nm laser light beams from a pulsed below 300 nm laser light source 14. A Lambda Physik multi-gas excimer laser was used. The 193nm source was operated at up to 400 Hz and an energy/pulse of up to 100 mJ. The 248 nm source was operated at 500 Hz and an energy/pulse up to about 10 to 50 mJ. The beam splitters 20 were calcium fluoride substrates having 50% reflectance. The pulse energy used for measurements preferably did not exceed 30 microjoules/cπT. Preferably with the invention the fluence levels of the measurement laser beam are low enough that transmission changes are not induced in the optical material.

Referring to Fig. 2, a linear fit to the 50 data points (50 different pathlengths) provides a slope, or attenuation coefficient, with a standard deviation around 3 x 10^"5 /cm (0.003% T/cm). The absolute internal transmittance obtained was in Fig. 2, (0.9985 +/- 2 xlO^"5 at 248 nm (line 52), and 0.9905 +/- 3 x 10^"5 at 193 nm (line 54)) are in very good agreement with conventional spectrophotometric measurements of other Corning HPFS^® type glass, but compared to the best known accuracy of 0.1% T/cm at 248 nm for prior art methods, have more than one order of magnitude less uncertainty. In the Fig. 2 graph -lnT=a + (α)L, wherein a is the loss due to surface reflection and scatter, is the absorption coefficient, and L is sample thickness, and internal transmittance (Ti) is expressed as Ti = e ⁽-°^lxIcm).

The obtainable measurement uncertainty corresponds to a loss detection limit on the order of a few ppm, or approximately 1 dB/km.

The method and apparatus of the present invention is applicable to optical microlithography material samples and to below 300 nm optical microlithography wavelengths. Preferably a pulsed below 300 nm source is used, such as an excimer laser. For a preferred measurement precision and accuracy, a beam divergence of less than 10^"2 radians is preferred. In addition to transmittance of a homogeneous optical material, localized short-range inhomogeneities and variations in index are preferably detected in optical lithography materials. Scanning the sample in an X-Y mode with a small diameter beam is preferably used to generate a two dimensional transmittance map of a sample. Maximum pathlength is limited only by sample fixturing and degree of beam collimation, and amount of optical material available. In principle, an entire large optical material sample such as a cylindrical large boule (such as 1 to 2 meters diameter) could be fashioned into a single prism-shaped sample with unparalleled sides, and measured in layers to build a two dimensional transmittance profile of radius vs. depth.

It will be appreciated that the inventive measurement system can provide realtime compensation of below 300 nm pulse-to-pulse noise, and automatic rejection of pulse energies outside selected limits by use of a reference detector.

According to the present invention, the resultant plot of below 300 nm transmittance vs. thickness can contain as few, or as many, data points as needed to obtain the desired precision in the exponential fit (or linear fit of-lnT vs. L). It will be appreciated that in the conventional multiple sample methods of the prior art, the number of data points is limited, in a practical sense, by the amount of optical material available and the time and cost of sample preparation. In a preferred practice of the invention, the use of polarized light and choosing the prism angle and beam position such that light is incident on both surfaces at the Brewster angle reduces reflectance at each surface to approximately 10^"3 to 10^"4 as opposed to approximately 4xl0^"2 per surface for normal incidence. This minimizing of reflectance reduces the importance of total surface loss. Further, surface residues or films, left by cleaning or exposure to ambient atmospheres, and surface polish (micro-roughness), are more likely to be uniform over a single surface than between multiple surfaces of multiple samples. Therefore the present invention reduces the uncertainty of the exponential fit to the data. In other words, the present invention improves the accuracy of the determination of the bulk attenuation coefficient at below 300 nm wavelengths. Additionally, according to the present invention, the speed of measurement of multiple paths within a single sample is inherently faster than measuring multiple samples. The present invention requires only a single sample mount and alignment. Such a fast measurement reduces the requirements for instrument and sample-surface stability.

It will be understood by those skilled in the art of transmittance measurements, that the systems described above are exemplary only, and not limiting to the scope of the present invention. Accordingly, the present invention includes an apparatus that includes a below 300 nm light source for generating a below 300 nm light beam, a detector for detecting the light beam, and an optical microlithography material sample having two opposed, non-parallel surfaces which provide at least two pathlengths for the single sample. Further details on internal transmittance measurement systems may be found in: United States patent no. 5,776,219; J.P. Dakin, W.A. Gambling, "Transmission measurements in optical glass with an improved twin-beam spectrometer" Opto-Electronics, 1973, V.5, 4„ p.335-344; W. Heitmann, "Attenuation measurements in low-loss optical glasses by polarized radiation" Applied optics, V.14, 12, 1975, 3047-3052; J.D. Spear, R.E. Russo, "Transverse photothermal beam deflection within solids." J. Appl. Phys. 70, 1991, 580-586; and K.L. Eckerle, J.J. Hsia, K.D. Mielenz, V.R. Weidner, "NBS Measurement Services: Regular Spectral Transmittance," National Bureau of Standards (U.S.) Special Publication 250-6, July 1987.

A 193 nm transmission standard is preferably made in the shape of a wedge with an apex angle for use with the 193 nm wavelength of 65.3 degrees from HPFS^® type high purity fused silica glass optical material. Preferably all sides of the transmission standard is polish finished to a 193 nm wavelength polish quality to at least a surface roughness RMS of about 5 angstroms. Preferably the transmission standard length corresponds to the pathlength used with quality control transmission measurements of the optical lithography blank or element. Preferably the length of the prism wedge transmission standard ranges from 10 to 200 cm. The transmission standard prism shaped wedge is measured in accordance with Fig. 1 and 3 to provide the internal transmission of the standard which is used to calibrate later measurements of blanks and elements with pathlengths corresponding to the wedges length. The prism shaped wedges are measured in multiple locations to result in transmission vs. pathlength plots. With a fused silica glass optical material the transmission standards transmission may vary from top to bottom of the prism, so preferably the plot of transmission vs. location on the standard (T(L)) is generated as shown in Fig. 3b The average transmission for a transmission standard wedge T_ave(L) is calculated. This transmission represents an absolute value of transmission from top to bottom of the standard. This then is used as a reference in spectrophotometer readings, when light will pass through two parallel surfaces of the transmission standard (Fig. 3a).

The error for a transmission standard depends on the error obtained for the slope of transmission vs. pathlength at each individual location of the sample. The slope error of the transmission line is typically better than 10^"4/ cm base e. For the example shown in figure 2, the slope error is 3xlO^"5/cm base e). Another contribution to the error for the standard is the number of locations (N) measured which depends on the sample length (minimal step determined by the beam size, 2-3 mm diameter). The total error can be expressed as σ = N^"1/2 {V∑ (σj²)}, where σj is an error at the individual location, i is the number of the different pathlengths transmission was measured at, and N is total number of the transmission readings at different locations along the prism standard. From this equation the typical error bar on transmission value for transmission standard is about the same as error bar on a single point. Preferably the transmission standards prism apex angle is A, A=180-2Θβ.

Where OR is Brewester angle, where n is refractive index of the material under test at the desired wavelength.

For the necessary size, DD, of the prism base and target material transmission the following relation may be established: DD > 10 σ/ ε N (cm), where σ is precision of single point measurements, ε is transmission loss / cm, and N is number of the points in the transmission vs. pathlength plot. For the inventive apparatus σ ~ 10^"3, for example, for a transmission of 10^"3/ cm, the base's size is preferably bigger than 1.5 cm, if 50 points (pathlengths) are collected.

The invention includes a method of making a below 300nm optical lithography transmission standard. The method includes providing a below 300nm optical lithography material. The method includes forming the below 300nm optical lithography material into a wedge 102 having a longitudinal transmission standard length 101. Preferably the longitudinal standard length 101 correlates with an optical lithography optical element blank transmission path length. The wedge's longitudinal transmission standard length 101 terminates with a first (103) and a second (104) parallel faces 103 and 104. Wedge 102 is formed with a first (105) and a second (106) non -parallel sides 105 and 106 aligned along length 101 and normal to parallel faces 103 and 104. The non-parallel sides 105 and 106 provide a narrow wedge top 107 and a broad wedge bottom 108.

The method includes providing a positionor 100 and providing a below 300nm measurement beam 16. The method includes aligning wedge 102 on positionor 100 wherein measurement beam 16 is transmitted through non-parallel sides 105 and 106 with a first path length (such as a, b, or c) and measured at a first measurement plane (such as A, B, C, D, E, or F) normal to length 101. The method includes positioning the measurement beam 16 at a second path length location (such as a, b, or c) in the first measurement plane and transmitting the measurement beam 16 through the non-parallel sides 105 and 106 and measuring the second path length location transmitted beam. The method includes positioning wedge 102 relative to measurement beam 16 wherein the measurement beam is transmitted through non-parallel sides 105 and 106 at an at least second measurement plane (such as A, B, C, D, E, or F) normal to the standard length 101. The at least second measurement plane is spaced from the first measurement plane. Measurement beam 16 is transmitted through non-parallel sides 105 and 106 in the at least second measurement plane at a short path length location (such as a) and measured. The measurement beam 16 is transmitted through sides 105 and 106 in the at least second measurement plane at a long path length location (such as b or c) and measured. The method includes determining an average attenuation coefficient from the measured transmitted beams 16 and transmitting a below 300nm longitudinal length transmission beam 120 through parallel faces 103 and 104 and along longitudinal transmission standard length 101 to provide an internal transmission standard value for the below 300 nm optical material for a length equal to the longitudinal transmission standard length. The method preferably includes calibrating an internal transmission measuring device such as a spectrophotometer using wedge 102 and its internal transmission standard value. Preferably positionor 100 comprises a positional stage, preferably with the positional stage providing for alignment of wedge 102 longitudinal length 101 in a vertical direction. Preferably positioning wedge 102 relative to measurement beam 16 includes vertically translating the vertically aligned wedge 102 with the positional stage supporting an end face.

In a preferred embodiment providing a below 300nm measurement beam includes providing a 248nm producing excimer laser and producing the 248nm measurement beam. In a further embodiment providing the measurement beam includes providing a 193nm producing excimer laser and producing the 193nm measurement beam. In a further embodiment providing the measurement beam includes providing a 157nm producing excimer laser and producing the 157nm measurement beam.

In an embodiment of the invention, providing a below 300nm optical material comprises providing a silica glass. In a further embodiment the silica glass is a fluorinated silica glass. In an embodiment providing a below 300nm optical material comprises providing a fluoride optical material, preferably a fluoride crystal, most preferably a calcium fluoride crystal. In an embodiment the fluoride optical material is a fluoride glass.

Preferably forming the optical material into a wedge includes forming a wedge wherein non-parallel sides 105 and 106 have an apex melting angle in the range from about 60° to 70°. Non-parallel sides 105 and 106 do not have to meet at a top point apex in terms of having an apex meeting angle, since the apex meeting angle can be the meeting intersection of the planes of sides 105 and 106. Preferably the below 300nm transmission standard is for determining the below 300nm transmission of an optical lithography element blank which has an optical lithography path length and the longitudinal transmission standard length of formed wedge 102 corresponds to the optical lithography element blank optical lithography pathlength. Preferably the wedge is formed with a longitudinal transmission length in the range form about 1 to 100cm, and preferably about 1 to 25 cm. Preferably the wedge is formed with non-parallel sides 105 and 106 having a side length from the narrow wedge top to the broad wedge bottom in the range of about 1 to 100cm, and more preferably about 1 to 10cm. Preferably the wedge is formed with the parallel faces 103 and 104 having a polished surface with a roughness no greater than about 5 angstroms rms.

Preferably providing the below 300nm measurement beam includes providing a below 200nm measurement beam. In a further aspect the invention includes a below 300nm optical lithography transmission standard, preferably for below 300 nm optical lithography wavelength transmission measurements. The below 300nm optical lithography transmission standard is comprised of a below 300nm optical material having a below 300nm transmission and formed into a wedge 102 having a longitudinal transmission standard length 101. The standard length 101 terminating with first (103) and second (104) parallel wedge end faces 103 and 104. Wedge 102 having first and second non-parallel wedge sides 105 and 106 aligned along the length 101 and normal to end faces 103 and 104. Wedge non-parallel sides 105 and 106 providing a narrow wedge top and a broad wedge bottom. The non-parallel sides having an apex meeting angle and the wedge faces 103 and 104 having a polished surface with a roughness no greater than 5 angstroms rms. The longitudinal transmission standard length 101 corresponding to a below 300nm optical transmission targeted path length with the standard wedge length having an internal transmission standard value.

Preferably the optical material below 300nm transmission is greater than 99%/cm.

Preferably the apex meeting angle is in the range from about 60° to 70°. In an embodiment the optical material comprises a silica glass, preferably fused silica glass. In a further embodiment the silica glass is a fluorinated silica glass. In an embodiment the optical material comprises a fluoride optical material, preferably a fluoride crystal, and most preferably calcium fluoride. In a further embodiment the fluoride optical material comprises a fluoride glass.

With the below 300nm optical material silica glass, preferably high purity fused silica glass, the apex meeting angle is in the range of about 64° to 68° and more preferably in the range of about 65° to 67.5°. With silica glass, preferably the standard has a 193nm internal transmission standard value and the apex meeting angle approximates 65.2°. With silica glass a 248nm transmission standard has a 248nm internal transmission standard value and the apex meeting angle approximates 67°. With the below 300nm optical material calcium fluoride crystal the apex meeting angle is in the range of about 64° to 69°, and more preferably about 65° to 69°. With a calcium fluoride crystal a 157nm transmission standard has a 157nm internal transmission standard value and the apex meeting angle approximates 65.3°. With a calcium fluoride crystal a 193nm transmission standard has a 193 nm internal transmission standard value and the apex angle approximates 67.3°. With a calcium fluoride crystal a 248nm transmission standard has a 248 internal transmission standard value and the apex angle approximates 68.5°.

With below 300nm optical material fluorinated silica glass a 157nm transmission standard has a 157nm internal transmission standard value and the apex meeting angle approximates 62.2°.

It will be apparent to those skilled in the art that various modifications and variations can be made in the method and apparatus of the present invention without departing from the spirit or scope of the invention. Thus, it is intended that the present invention cover the modifications and variations of this invention provided they come within the scope of the appended claims and their equivalents.

Claims

What is claimed is:

1. A method of measuring the below 300 nm internal transmittance of an optical material, the method including providing an optical material sample having two opposing surfaces being spaced apart in a non-parallel relationship, providing a below 300 nm light source for transmitting a below 300 nm light beam in a one-way direction through said two opposing spaced apart non-parallel surfaces, translating the provided optical material sample relative to said light beam and measuring the transmittance of below 300 nm light through at least two different pathlengths within the provided optical material sample.

2. A method as claimed in claim 1 , wherein providing a below 300 nm light source includes providing a pulsed laser.

3. A method as claimed in claim 1 , wherein providing a below 300 nm light source includes providing an excimer laser.

4. A method as claimed in claim 1 , wherein providing a below 300 nm light source includes providing a 248 nm wavelength source.

5. A method as claimed in claim 4, said method including generating a 248 nm transmission measurement which has an accuracy of at least 0.01% T/cm.

6. A method as claimed in claim 1, wherein providing a below 300 nm light source includes providing a 193 wavelength source.

7. A method as claimed in claim 1, wherein providing a below 300 nm light source includes providing a 157 wavelength source.

8. A method as claimed in claim 1, wherein providing a optical material sample includes providing a fused silica glass.

9. A method as claimed in claim 1, wherein providing a optical material sample includes providing a fluoride crystal.

10. A method as claimed in claim 1, wherein providing a optical material sample includes providing a calcium fluoride crystal.

11. A method as claimed in claim 1 , said method including inhibiting the movement of the below 300 nm light beam.

12. A method as claimed in claim 2, wherein said provided pulsed laser below 300 nm light source produces a below 300 nm light beam comprised of successive pulses, and the method includes splitting off a pre-transmittance part of a pulse prior to transmitting through said optical material sample.

13. A method as claimed in claim 12 further including monitoring said pre- transmittance part of successive pulses.

14. A method as claimed in claim 13 further including compensating for pulsed laser flucation differences between the pre-transmittance part of successive pulses.

15. An apparatus for measuring the below 300 nm internal transmittance of an optical microlithography material comprising: a below 300 nm light source for emitting a below 300 nm beam of light; a below 300 nm light detector; a single sample of optical microlithography material positioned between the below 300 nm light source and the below 300 nm light detector, the sample having a pair of non-parallel, opposed surfaces and being positioned so that a light beam travels through the non-parallel, opposed surfaces, and the apparatus measures the transmittance of the light source through at least two different path lengths of the single sample.

16. The apparatus of claim 15, wherein the sample has a prism-shape.

17. The apparatus of claim 15, said apparatus including a sample translation stage for receiving and translating said sample.

18. The apparatus of claim 17, wherein the translation stage translates said sample relative to the light source to provide the at least two different path lengths.

19. A method of making a below 300 nm transmission standard, said method including: providing a below 300 nm optical material; forming said below 300 nm optical material into a wedge having a longitudinal transmission standard length said longitudinal transmission standard length terminating with a first and a second parallel faces, said wedge having a first and a second non- parallel sides aligned along said longitudinal transmission standard length and normal to said parallel faces with said non-parallel sides providing a narrow wedge top and a broad bottom; providing a positionor; providing a below 300 nm measurement beam; aligning said wedge on said positionor wherein said measurement beam is transmitted through said non-parallel sides with a first path length and measured at a first measurement plane normal to said longitudinal transmission standard length and positioning said measurement beam at a second path length location in said first measurement plane and transmitting said measurement beam through said non-parallel sides and measuring said second path length location transmitted beam, positioning said wedge relative to said measurement beam wherein said measurement beam is transmitted through said non-parallel sides at an at least second measurement plane normal to said longitudinal transmission standard length, said at least second measurement plane spaced from said first measurement plane, and transmitting said measurement beam through said non-parallel sides and in said at least second measurement plane at a short path length location and measuring said at least second plane short path length location transmitted beam, and transmitting said measurement beam through said non-parallel sides in said at least second measurement plane at a long path length location and measuring said at least second plane long path length location transmitted beam, determining an average attenuation coefficient from said measured transmitted beams, and transmitting a below 300 nm longitudinal length transmission beam through said first and second parallel faces and along said longitudinal transmission standard length to provide an internal transmission standard value for said below 300 nm optical material for a length equal to said longitudinal transmission standard length.

20. A method as claimed in claim 19, further including calibrating an internal transmission measuring device using said wedge and said internal transmission standard value.

21. A method as claimed in claim 19, wherein providing a positionor includes providing a positional stage.

22. A method as claimed in claim 21 wherein providing a positional stage includes providing a positional stage for aligning said wedge longitudinal length in a vertical direction.

23. A method as claimed in claim 22 wherein positioning said wedge relative to a measurement beam at an at least second measurement plane includes vertically translating said positional stage.

24. A method as claimed in claim 19, wherein providing a below 300 nm measurement beam comprises providing a 248 nm producing excimer laser.

25. A method as claimed in claim 19, wherein providing a below 300 nm measurement beam comprises providing a 193 nm producing excimer laser.

26. A method as claimed in claim 19, wherein providing a below 300 nm measurement beam comprises providing a 157 nm producing excimer laser.

27. A method as claimed in claim 19, wherein providing a below 300 nm optical material comprises providing a fused silica glass.

28. A method as claimed in claim 19, wherein providing a below 300 nm optical material comprises providing a fluoride crystal.

29. A method as claimed in claim 19, wherein providing a below 300 nm optical material comprises providing a calcium fluoride crystal.

30. A method as claimed in claim 19, wherein providing a below 300 nm optical material comprises providing a fluorinated silica glass.

31. A method as claimed in claim 19, wherein forming said below 300 nm optical material into a wedge comprises forming a wedge wherein said first and second non- parallel sides have an apex meeting angle in the range from about 60° to 70°.

32. A method as claimed in claim 19, said below 300 nm transmission standard for determining the below 300 nm transmission of an optical lithography element blank having an optical lithography path length, wherein said longitudinal transmission standard length of said formed wedge corresponds to said optical lithography element blank optical lithography pathlength.

33. A method as claimed in claim 19, wherein said wedge is formed with a longitudinal transmission length in the range of about 1 to 100 cm.

34. A method as claimed in claim 19, wherein said wedge is formed with a longitudinal transmission length in the range of about 1 to 25 cm.

35. A method as claimed in claim 19, wherein said wedge is formed with said non- parallel sides having a side length from said narrow wedge top to said broad wedge bottom in the range of about 1 to 100 cm.

36. A method as claimed in claim 19, wherein said wedge is formed with said non- parallel sides having a side length from said narrow wedge top to said broad wedge bottom in the range of about 1 to 10 cm.

37. A method as claimed in claim 19, wherein said wedge is formed with said faces having a polished surface with a roughness no greater than 5 angstroms rms.

38. A method as claimed in claim 19, wherein providing a below 300 nm measurement beam includes providing a below 200 nm measurement beam.

39. A method as claimed in claim 19, wherein providing a below 300 nm optical material comprises providing a fluoride optical material.

40. A method as claimed in claim 39, wherein providing a below 300 nm fluoride optical material comprises providing a fluoride glass.

41. A below 300 nm transmission standard, said below 300 nm transmission standard comprised of a below 300 nm optical material having a below 300 nm transmission and formed into a wedge having a longitudinal transmission standard length, said longitudinal transmission standard length terminating with a first and a second parallel wedge end faces, said wedge having a first and a second non-parallel wedge sides aligned along said longitudinal transmission standard length and normal to said parallel end faces, said wedge non-parallel sides providing a narrow wedge top and a broad wedge bottom, said first and second non-parallel wedge sides having an apex meeting angle, said wedge end faces having a polished surface with a roughness no greater than 5 angstroms rms, said longitudinal transmission standard length corresponding to a below 300 nm optical transmission targeted path length with said longitudinal transmission standard length having an internal transmission standard value.

42. A below 300 nm transmission standard as claimed in claim 41, wherein said below 300 nm transmission > 99%/cm.

43. A below 300 nm transmission standard as claimed in claim 41, wherein said apex meeting angle is in the range from about 60° to 70°.

44. A below 300 nm transmission standard as claimed in claim 41, wherein said below 300 nm optical material comprises a silica glass.

45. A below 300 nm transmission standard as claimed in claim 41, wherein said below 300 nm optical material comprises a fluoride optical material.

46. A below 300 nm transmission standard as claimed in claim 45, wherein said fluoride optical material is a fluoride crystal.

47. A below 300 nm transmission standard as claimed in claim 45, wherein said fluoride optical material is a fluoride glass.

48. A below 300 nm transmission standard as claimed in claim 41 , wherein said below 300 nm optical material comprises a calcium fluoride crystal.

49. A below 300 nm transmission standard as claimed in claim 41, wherein said below 300 nm optical material comprises a fluorinated silica glass.

50. A below 300 nm transmission standard as claimed in claim 44, wherein said apex meeting angle is in the range of about 64°to 68°.

51. A below 300 nm transmission standard as claimed in claim 50, wherein said angle is in the range of about 65° to 67.5°.

52. A below 300 nm transmission standard as claimed in claim 44, wherein said standard having a 193 nm internal transmission standard value and said apex meeting angle approximates 65.2°.

53. A below 300 nm transmission standard as claimed in claim 44, wherein said standard having a 248 internal transmission standard value and said apex meeting angle approximates 67°.

54. A below 300 nm transmission standard as claimed in claim 48, wherein said apex meeting angle is in the range of about 64° to 69°.

55. A below 300 nm transmission standard as claimed in claim 48, wherein said apex meeting angle is in the range of about 65° to 69°.

56. A below 300 nm transmission standard as claimed in claim 48, said standard having a 157 nm internal transmission standard value and said apex meeting angle approximates 65.3°.

57. A below 300 nm transmission standard as claimed in claim 48, said standard having a 193 nm internal transmission standard value and said apex meeting angle approximates 67.3°.

58. A below 300 nm transmission standard as claimed in claim 48, said standard having a 248 nm internal transmission standard value and said apex meeting angle approximates 68.5°.

59. A below 300 nm transmission standard as claimed in claim 49, said standard having a 157 nm internal transmission standard value and said apex meeting angle approximates 62.2°.