Effects of glass forming conditions on the KrF-excimer-laser-induced optical damage in synthetic fused silica

J O U R N A l , OF

ELSEVIER

Journal of Non-CrystallineSolids 203 (1996) 69-77

Effects of glass forming conditions on the KrF-excimer-laser-induced optical damage in synthetic fused silica D.R. Sempolinski *, T.P. Seward, C. Smith, N. Borrelli, C. Rosplock Corning Inc., Corning, NY 14831, USA

Abstract

The optical damage induced by exposure to KrF (248 nm) excimer laser light was examined in synthetic fused silicas made under a variety of processing conditions. The SiC14-based glass exhibited increased UV absorption, increased red fluorescence and compaction after prolonged 248 nm exposure. The induced damage was most severe in glasses made with oxidizing deposition flames or in glasses with reduced levels of molecular hydrogen.The damage did not correlate with the OH level. Fused silica produced using a hydrogen/oxygen flame showed the same damage resistance as glass made with a natural gas/oxygen flame. Fused silica made with the chlorine-free precursor, octamethylcyclotetrasiloxane (OMCTS), exhibited the same general damage behavior as that seen in SiCl4-based glass except that the absorption damage shifted to a saturated condition without passing through the absorption transition prevalent in the SiCl4-based glass.

I. Introduction

The optical stability of fused silica under ultraviolet radiation has received increasing attention in recent years due to its growing use as optical elements in a variety of deep-UV applications. For example, the microlithography systems used to produce integrated circuits are moving to shorter wavelengths to achieve smaller feature sizes. The next generation systems will use KrF (248 nm) excimer lasers as their light source and require 2 0 - 3 0 fused silica lens elements in their imaging optics. Previous studies on synthetic (type III) fused silicas show that, under prolonged 248 nm exposure, these glasses exhibit increased UV absorption, in-

Corresponding author. Tel.: + 1-607 974 3210; fax: + 1-607 974 2103.

creased red (650 nm) fluorescence and compaction (densification) [1-5]. The absorption damage can include the formation of absorption bands centered approximately at 215 nm and at 260 nm. The 215 nm band is observed in all fused silicas and is caused by the E center [1,5,6]. The 260 nm band is usually prevalent in fused silicas which show poor damage resistance. It has been attributed to oxygen-related defects, such as N B O H C ' s (non-bridging oxygen hole centers) [1,7] and, in fused silicas with excess oxygen, to interstitial ozone [8]. Both defects have also been linked to the red (650 nm) fluorescence prevalent in fused silica. At low exposures, the 215 nm absorption and compaction increase linearly with the number of pulses and quadratically with laser intensity, indicating a two-photon mechanism [2,5]. The 215 nm absorption band relaxes partially at room temperature within minutes after laser expo-

0022-3093/96/$15.00 Copyright © 1996 Elsevier Science B.V. All rights reserved. PII S0022-3093(96)00480-2

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D.R. Sempolinski et al. ,/Journal qf Non-Co'stalline Solids 203 (1996) 69-77

sure is interrupted [3,9-11]. The relaxation time appears to be on the order of minutes. With higher exposure levels, the 215 nm absorption increases more rapidly and becomes permanent (i.e., shows no relaxation after laser exposure is stopped). Krajnovich et. al. have termed this change a 'sudden absorption transition' [3,4]. This transition is accompanied by a marked increase in the 260 nm absorption and in the red fluorescence. At still higher exposure levels, the absorption damage reaches a steady-state condition in which the levels of the laser-induced absorption and fluorescence stabilize

[3]. Several researchers have correlated the damage behavior with various aspects of the glass chemistry [11-20]. Glass with excess oxygen is more susceptible to laser-induced damage due to the presence of peroxy linkages a n d / o r interstitial oxygen [12,13] which absorb near the wavelength of the KrF (248 rim) excimer laser. Low-OH ( < 10 ppm) fused silica also exhibits poor damage resistance [1,14]. Several studies have noted the beneficial effects of hydrogen additions [12,15-19]. The hydrogen can reduce the laser-induced damage caused by the formation of E' centers and NBOHC's by reacting with these defects to generate S i - H and S i - O H species. The purpose of this study is to examine how sensitive the optical damage induced in synthetic fused silica by exposure to KrF (248 nm) radiation is to changes in the glass processing conditions. Synthetic fused silica is normally produced by flame hydrolysis of SiC14. This manufacturing approach can, however, include forming variations, such as changes in the laydown flame and the chemical precursor, or the addition of post-processing treatments, such as high temperature thermal treatments and hydrogen anneals. This investigation seeks, as a practical goal, to identify what processing flexibility exists in the manufacture of synthetic fused silica for UV-applications but, in doing so, it has also generated fundamental information about the damage mechanism which is operative in fused silica during UV exposure. A key feature of this investigation is the use of an on-line technique to monitor the change which occurs to the primary UV absorption bands during 248 nm exposure. Since the absorption damage can relax after exposure is stopped, off-line measurements fail

to capture a significant fraction of the absorption signal [32].

2. Experimental set up

2.1. Sample description This study examined glass samples made by flame deposition and by a sol-gel/sintering approach. The flame-deposited samples were produced using a small (single burner) flame deposition furnace which produced boules approximately 15 cm in diameter by 5 cm thick. The chemical precursor was either SiC14 or OMCTS (octamethylcyclotetrasiloxane) [20]. The glass deposition experiments included runs made using either a natural gas/oxygen flame or a hydrogen/oxygen flame. The deposition runs made with natural g a s / o x y g e n flames were run with fuel/oxygen (molecular) ratios ranging from 0.45 to 0.75. Those made with hydrogen/oxygen flames were run only at a hydrogen/oxygen ratio of 2.06. The glass chemistry of these glasses was measured using a variety of techniques. The OH levels were measured by FTIR, the molecular hydrogen levels by Raman [22], the chlorine levels by pyrohydrolysis and the trace metal levels by atomic absorption. The reference point for the process study was the laydown conditions used to manufacture Corning's Code 7940 fused silica (i.e. SiCI 4 precursor and natural gas/oxygen flame with a slightly reducing fuel/oxygen mix). Glasses made under this condition had 900-950 ppm OH and 50-60 ppm C1. Preliminary tests show that this glass exhibits the same damage behavior as Code 7940 fused silica. Glasses made using a hydrogen/oxygen flame and the SiCI 4 precursor had 1130 ppm OH and 50-60 ppm CI. The higher OH level is typical of a hydrogen/oxygen flame deposition. Glasses made using a natural gas/oxygen flame and the OMCTS precursor had 850-900 ppm OH and < 10 ppm CI. All of the flame deposited glasses had an inherent molecular hydrogen level of 3-5 X 1017 molecules/cm 3. The total concentration of metal impurities in these glasses was < 150 ppb, with individual metal contamination levels at < 20 ppb.

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D.R. Sempolinski et al. / Journal of Non-Crystalline Solids 203 (1996) 69-77

As a contrast to the flame deposition process, samples were also prepared by sintering powder formed from the room temperature hydrolysis of TEOS (tetraethylorthosilicate) [21]. Since this process includes a chlorine drying step, the resulting fused silica is a low-OH ( < 1 ppm), high-C1 (--~ 400 ppm) glass. The molecular hydrogen level is estimated to be << l0 ~6 m o l e c u l e s / c m 3 based on fiber measurements. The concentration of metal impurities was equivalent to that seen in the flame-deposited glasses. The effects of post-forming thermal treatments in air and in hydrogen were also examined. The air treatment consisted of a 30 minute hold at 1630°C. It was set up to approximate the general conditions of a high temperature sagging operation. This treatment reduced the molecular hydrogen level to < 5 X 1016 m o l e c u l e s / c m 3 through degassing and the reaction of hydrogen with the glass matrix. The hydrogen treatment will be discussed in detail in a separate report [23]. Raman measurements show that this treatment produced glasses with an average molecular hydrogen level in excess of 102°/cm 3. Samples for laser damage testing were cut from the centers of the boules. The typical damage sample was a block 1.0 cm x 1.5 cm X 2.0 cm. All of the surfaces were given a commercial-grade polish. The surfaces which were to be exposed to the laser were polished to a 1 / 1 0 wave flatness.

2.2. Damage testing The laser damage behavior of the above samples were studied using an accelerated damage test. Glass samples were exposed to 248 nm laser irradiation generated with a Lumonics Excimer 600 laser. All tests were carried out at room temperature but no attempt was made to control sample heating. The beam arrangement is pictured in Fig. 1. The laser was pulsed at 400 Hz and the beam intensity at the sample position was controlled with a focal lens. The test intensities ranged from 50 to 450 mJ/cm2/pulse. The samples were oriented so that the laser beam passed through the 1.5 cm dimension and the on-line transmission measurements were monitored across the 1 cm dimension. The volume exposed to the laser beam included the entire optical path of the transmission measurement.

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The quoted beam intensities are referenced to the apertures used to measure the transmission changes. Apertures were used for both the on-line and off-line transmission measurements which restricted the measurement field to an area 2 mm by 7 . 5 - 1 0 ram. The short (2 mm) dimension was oriented to center on the maximum of the (Gaussian) intensity profile. The reported intensity for any given test condition is the average intensity over the transmission measurement aperture. The optical damage was monitored by measuring the changes in the UV transmission, fluorescence and compaction. The bulk of the experimental work focused on the transmission changes induced by the laser exposure. The fluorescence and compaction measurements were performed on a selective basis. The change in the UV transmission was measured both on-line and off-line. The on-line measurements used a deuterium lamp, positioned perpendicular to the laser beam, and an arrangement of notch filters and photomultiplier tubes ( P M T s ) t o isolate the 215 and 260 nm absorption bands (see in Fig. 1). The 215 nm band could be detected clearly but the 260 nm required extra aperturing to cut down on laser beam scatter. Even with an optimized setup, the scatter typically caused a one percent increase in the light intensity detected by the 260 nm PMT, which did not vary during the exposure test. The off-line measurements were made using a UV-visible spectrophotometer (CARY model 3E). These measurements covered the wavelengths of 190 to 400 nm

D.R. Sempolinski et al. / Journal of Non-C~stalline Solids 203 (1996) 69- 77

72

and were made 5 min after the sample was removed from the beam. Changes in the absorption coefficient of 0.002 c m - ] could be detected with this arrangement. The fluorescence intensity was measured using a spectroradiometer (Optronic model #752) [2]. The measurement was made at 90 degrees to the laser beam path. A spectral scan included the wavelengths from 260 to 800 nm. Examples of typical fluorescence spectra are given in Ref. [2]. The compaction measurements were made using an interferometer (Zygo Mark IV). They are reported as the change in the optical path divided by the thickness of the undamaged sample. Estimates of the temperature changes during laser exposure were obtained for several samples using a thermocouple positioned on the top of the sample within 0.5 cm of the exposure zone.

3. Experimental results

3. I. SiCl4-based fused silica The laser damage resistance of the SiC14-based fused silica made under standard forming conditions is summarized in Fig. 2. The absorption damage includes a primary band centered at 215 nm and a secondary band at 260 nm (see Fig. 2 insert). The magnitudes of the two bands varied independently. 1o2

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The 215 nm band was the dominant absorption peak at all exposure levels. At exposure levels which did not generate the absorption transition, the 215 nm absorption band relaxed rapidly at room temperature when laser exposure stopped. The absorption level typically dropped to < 30% of its on-line value within < 100 s after UV exposure was stopped. The more damage resistant glasses relaxed more rapidly. After the initial rapid recovery, the 215 nm absorption damage continued to relax at a slower rate, dropping to < 10% of its on-line value in < 20 h. These changes were not accompanied by temperature changes in the glass. The relaxation of the absorption damage is not permanent. The absorption band returns to its unrelaxed level immediately after the glass is re-exposed, indicating that the concentration of defects which are precursors to the E' center is permanently increased by the UV exposure. Prior to the absorption transition, the magnitude of the 215 nm band scaled roughly with the number of pulses and the square of the beam intensity. The 260 nm band forms only as the glass approaches the absorption transition. No significant relaxation of the 260 nm band was observed but small changes could have been masked by the light scattering effects described earlier. All of the SiC14-based samples exhibited an absorption transition at which the on-line and off-line readings of the 215 nm band converged. The transition was accompanied by an increase in the sample temperature. In samples with poor damage resistance, the temperature exceeded 60°C. Temperature increases of this magnitude complicated the damage state of the glass and could produce a more transmissive state during exposure. The possibility exists that the onset of the absorption transition is accompanied by an increase in the fluorescence near 260 nm. The on-line PMT which is filtered to detect the 260 nm transmission consistently shows a small (3-5%) increase in the total signal just before the drop associated with the formation of the 260 nm absorption band. This increase was not seen in the 215 nm signal. Attempts to verify the presence of a fluorescence peak with the on-line fluorescence measurement, however, were inconclusive because the wavelength of concern is at the UV limit of the fluorescence spectrophotometer and too close to the lasing wavelength. The absorption damage is accompanied by corn-

D,R. Sempolinski et a l . / Journal of Non-Crystalline Solids 203 (1996) 69-77

73

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paction and a change in the fluorescence intensity. The compaction of a given sample scales with the number of pulses and the square of the beam intensity [2]. The fluorescence spectra consists of a primary red (650 nm) peak and a secondary, green-yellow (495-560 nm) peak. At a given beam intensity, the red peak increases with exposure and the greenyellow peak decreases with exposure (see Fig. 3). The induced absorption damage varies with the fuel/oxygen ratio of the deposition flame and with the hydrogen content of the glass. Examples of these effects are given in Fig. 4. The absorption damage shows a sharp increase in the 215 nm and 260 nm

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bands when oxidizing flames are used to deposit the glass (see Fig. 5). The oxidizing flame is the only condition tested in this study in which the 260 nm absorption is larger than the 215 nm absorption. The effect of the hydrogen level was examined by varying its concentration using the post-forming treatments discussed earlier. Both treatments had a pronounced effect on the absorption damage. The air refiring at 1630°C, which reduced the molecular hydrogen level to < 5 × 1016 molecules/cm 3, produces a sharp increase in the induced absorption and in the red fluorescence. It also causes the glass to shift to the saturated condition at relatively low exposure levels. This increase in the laser damage occurred without a significant change in the OH level. The sagged glass retained an OH concentration of = 900 ppm OH. The deterioration in the laser damage resistance could be recovered with the hydrogen treatment. Raising the molecular hydrogen concentration to > 1020 molecules/cm 3 reduced the absorption damage to a level below that seen in the as-formed glass. Again, the hydrogen treatment did not change the OH level of the glass significantly. The laser damage resistance was not affected by the substitution of hydrogen for natural gas in the deposition flame, despite the increased OH content of the glass made with the hydrogen flame, as long as equivalent flame stoichiometries were maintained. The absorption damage of the glass made with the hydrogen/oxygen flame was equivalent to that seen in glasses deposited with a natural gas flame (see

74

D.R. Sempolinski et al. / Journal of Non-Crystalline Solids 203 (1996) 6 9 - 7 7

~" 101 x~ natural gas/O 2 flame ,¢O• H2/O2flame t~E 100

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Fig. 6) and the compaction was slightly higher (see Fig. 8).

3.2. OMCTS-based fused silica The magnitudes of the absorption and the compaction damage in the fused silicas made with the OMCTS precursor are, in most respects, comparable to those seen in glass made with SiC14. The main differences were that, over the tested exposure range, the OMCTS-based glass did not show a transition at which the 215 nm absorption damage became permanent (see Fig. 7) and that the red fluorescence did

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[21. not increase with laser exposure (see Fig. 3). The absorption damage consisted only of the 215 nm band whose magnitude increases with laser exposure until it reaches saturation. There was no evidence of an absorption band at 260 nm, even in the saturated condition (see Fig. 7 insert). The magnitude of the compaction damage follows a quadratic dependence with exposure intensity (see Fig. 8). The fluorescence spectra include a red peak centered at 650 nm and a less intense, yellow peak at 560 nm. The intensities of both peaks decreased gradually with further UV exposure (see Fig. 3).

3.3. Ix~w-OHfused silica The glasses made by Cl-drying and sintering TEOS-derived powder exhibited poor damage resis-

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D.R. Sempolinski et al. / Journal of Non-Ct3,stalline Solids 203 (1996) 69 77

tance (see Fig. 9). Excessive absorption damage is evident after less than 100,000 pulses at 375 m J / c m 2. The absorption damage resistance can be improved, however, by using the hydrogen treatment referred to earlier. This treatment produced a glass whose damage resistance is comparable to that of the flame-deposited (high-OH) glasses. The hydrogen treatment did not increase the OH level. Both the as-formed and hydrogen-treated glasses had OH concentrations less than 1 ppm. 4. Discussion

The above findings set some restrictions on what options are available for manufacture of a fused silica for 248 nm applications. The keys to minimizing 248 nm absorption are to suppress the formation of the 260 nm band and to restrict the 215 nm (E' center) absorption so that its tail does not represent a significant absorption at 248 nm. These absorption bands can be controlled by forming the glass under reducing conditions and by maintaining the molecular hydrogen content of the glass. These processing changes, however, have their trade-offs, especially when the other material requirements of the 248 nm applications, such as index homogeneity and inclusions, are considered. Reducing deposition flames, for example, tend to produce seeds (bubbles) in the glass and the need to control the hydrogen level places limits on the thermal treatments which can be employed to improve index homogeneity. The deleterious effect of oxidizing forming conditions is consistent with previous studies which have looked at the redox state of the fused silica [29-31]. These results reinforce the observation that the presence of oxygen-excess defects, such as the peroxyl radical, accelerate the absorption damage induced by a KrF excimer laser because they generate strong absorptions near the lasing wavelength. The experimental results also have implications about the nature of the process by which the laser damage occurs. First, the absence of an absorption transition in OMCTS-based glass is a unique feature of the tested glasses. This behavior is not due to any differences in the OH or molecular hydrogen levels because they are comparable in the SiC14-based and OMCTS-based glasses. The main chemical difference between these glass types is the chlorine level.

75

Its possible role in triggering the absorption transition, however, is not clear. Chlorine itself does not absorb at or near 260 nm [24-27]. The absorption transition probably begins when the absorption at 248 nm is large enough to permit the glass to absorb a significant portion of the laser beam directly (single photon process). This condition can occur either with the formation of the 260 nm band or by the build up of the 215 nm band to a level such that its tail represents a significant 248 nm absorption. When this occurs, the temperature of the glass can increase and distort the damage process by changing the balance between the damage build up and recovery mechanisms. If this description is valid, chlorine may contribute to this process by tying up the molecular hydrogen so that the hydrogen is not available in the required quantity to passivate the laser-induced defects or by providing a precursor site for a defect which absorbs at or near 260 nm. The 260 nm band is fairly broad and probably has multiple sources. Second, the OH level, by itself, does not have a significant effect on the absorption damage resistance of fused silica. The chlorine-dried glass, which had been given the hydrogen treatment, exhibited relatively low absorption damage despite the fact that the OH level was less than 1 ppm and the sagged samples of SiC14-based fused silicas exhibited excessive absorption damage even though their OH levels were in excess of 900 ppm. Finally, the presence of red fluorescence in all of the 'as-formed' fused silicas, even at exposure levels at which the 260 nm absorption is not observed, indicates that the simple defect model, which argues that laser damage is due primarily to the breaking of strained S i - O bonds, does not give a complete picture of the damage process. Since the two defects normally associated with red fluorescence in fused silica, the NBOHC and ozone generated from interstitial oxygen, absorb at 257 nm [28], they cannot be the source of the red fluorescence seen in the reduced fused silicas at the lower exposure levels (i.e. prior to the onset of the absorption transition). The strained bond model requires that E' centers and NBOHC's form in equal concentrations. Since the optical absorption signal of the NBOHC is not observed, the NBOHC either passivates quickly under laser exposure or is not the companion species to the E' center.

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D.R. Sempolinski et al. / Journal of Non-Crystalline Solids 203 (1996) 69-77

5. Summary SiCl4-based fused silicas made under oxidizing flame conditions exhibited strong absorption bands at 260 nm and 215 nm for all tested exposure conditions. The induced absorption bands were permanent and their magnitudes, for a given exposure level, were typically more than two orders of magnitude larger than those seen in glasses formed under reduced flame conditions. The oxidized glasses were the only glasses studied in which the 260 nm band was stronger than the 215 nm band. The absorption damage induced in fused silicas made under reducing conditions exhibited different behaviors depending on which chemical precursor was used to make the glass. At low exposure levels, the SiC14-based and the octamethylcyclotetrasiloxane (OMCTS)-based fused silicas exhibited only a 215 nm absorption band which relaxed partially whenever laser exposure was stopped. With increasing laser exposure, the 215 nm absorption induced in the SiC14-based glasses underwent an absorption transition during which the damage became permanent. The transition was accompanied by an increase in sample temperature and by the formation of an absorption band at 260 nm. The 215 nm absorption induced in OMCTS-based glasses conversely showed a shift to a saturated condition without passing through an absorption transition. The 215 nm absorption damage continued to show relaxation at saturation. The shift to saturation occurred without an increase in glass temperature and without the formation of the 260 nm absorption band. The SiC14- and OMCTS-based glasses had comparable OH and molecular hydrogen levels. The OMCTS-based glasses had at least five times less C1 than the SiCl4-based fused silicas. The resistance of fused silica to absorption damage could be manipulated by varying its molecular hydrogen content. Molecular hydrogen reduces the level of absorption damage by passivating the laserinduced defects. The benefits of molecular hydrogen were observed in both high- (900 ppm) and low( < 1 ppm) OH fused silicas. The trends in the absorption damage of the tested glasses tracked with the molecular hydrogen content and were independent of the OH level. The laser damage resistance was also not affected by changing from a

hydrogen/oxygen to a natural gas/oxygen flame, as long as an equivalent level of molecular hydrogen was retained in the glass.

Acknowledgements The authors wish to thank Carl Ponader, Paul Schermerhorn and David Fladd for their help with some of the measurements made as part of this study.

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[29] H. Hosono and R.A. Weeks, J. Non-Cryst. Solids 116 (1990) 289. [30] Y. Morimoto, T. Igarashi, H. Sugahara and S. Nasu, J. Non-Cryst. Solids 139 (1992) 35. [31] H. Imai, K. Arai, J. Isoya, H. Honoso, Y. Abe and H. Imagawa, Phys. Rev. B48 (1993) 3116. [32] C. Smith, N. Borelli, T.P. Seward and D.R. Sempolinski, 'On-line monitoring of laser-induced damage centers in fused silica', presented at Annual Meeting of Opt. Soc. Am., Portland, Sept. 10-15, 1995.