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Abstract
We reported on the study of the correlation between 193nm absorption under 1.5~5.0mJ/cm2 fluence irradiation and photoluminescence (PL) related defects for deep-ultraviolet (DUV) fused silica samples with different H2 and OH contents (0~1200ppm). Experimental results showed strong correlations between apparent nonlinear absorption at 193nm to 650nm PL band originated from a non-bridging oxygen hole center (NBOHC), and between apparent linear absorption at 193nm to 550nm PL band. In addition, only 650nm PL defects showed reversible concentration change under 193nm laser irradiation, indicating a possible link to the rapid damage process (RDP) under DUV irradiation. Experimental observation and theoretical calculations on the dependence of 650nm PL intensity on the laser fluence further demonstrated that the generation and annealing processes of NBOHC in these DUV fused silica samples are mainly due to two-photon excitation induced breakage of SiOH bond and combination of NBOHC with H2. These results give new insight into the influence of NBOHC and SiOH on fused silica’s DUV absorption and transmission properties, and therefore are helpful to the development of high-performance DUV fused silica materials.
© 2018 Optical Society of America under the terms of the OSA Open Access Publishing Agreement
1. Introduction
Defect induced absorption has a significant impact on the spectral performance of fused silica, which is today’s most important optical material in deep ultraviolet (DUV) spectral region. Generation and annealing of defects linking to strong DUV absorption lead to reversible absorption change during short-time DUV laser irradiation (described as rapid damage process RDP) [1] and irreversible transmittance deterioration after long-time DUV laser irradiation [2]. As a result, controlling concentrations of these defects, such as E’ center (6.0eV absorption band, ~0.4 oscillator strength) [3], oxygen deficiency centers (ODC, ~5.0eV and ~7.0eV absorption bands, >0.01 oscillator strength) [3] and NBOHC [4] (4.8~7.0eV wide absorption band, >10−18cm−2 absorption cross section), is essential to improve the long-term stability and lifetime of DUV fused silica optics used in micro-photolithography and DUV optical systems. Most DUV fused silica modification for performance improvement is connected to the adjustment of H2 and/or SiOH concentrations [5, 6]. The H2 group is mobile at room temperature and can rapidly combine with dangling bonds such as E’ and NBOHC or strained bonds [7] that as precursors [8] of these absorbing defects. The SiOH group can promote lattice relaxation in SiO2 network and reduce density of strained bonds [9]. Despite of these great efforts, effects of various defects on the spectral performance of DUV fused silica are still not fully clarified due to the complexity of these DUV absorbing defects. Optical performances of fused silica are totally different under different SiOH content [5] or even at different 193nm laser fluence [10].
The aim of our work is to investigate possible correlation between 193nm absorption and PL-related defects in silica network. Laser calorimetry (LCA) is employed to measure the linear and nonlinear absorption coefficients, as well as laser irradiation induced absorption change of fused silica samples [11]. PL spectroscopy is used to indicate the concentrations of PL-related defects. Overlapped PL bands are resolved by a lifetime spectrum method [12]. DUV spectrophotometry is employed to measure the transmittance spectra of fused silica samples. In addition, influences of SiOH and H2 groups on PL-related defects and absorption of fused silica are also experimentally investigated. Based on these results, a possible linkage between 193nm absorption and PL-related defects is established.
2. Experimental arrangement
2.1 Experimental setup
The experiment setup consists of an ArF excimer laser (Indystar 1000, Coherent, pulse width ~8ns), a laser calorimeter (Laser Zentrum Hannover, Germany) and a high-resolution spectrometer (iHR320, JOBIN YVON, grating grooves: 300 line/mm, spectral resolution: 0.79nm) equipped with an ICCD (intensified charge coupled device, response time: ~135ns). Fused silica samples to be tested are placed inside an adiabatic chamber purged with high-purity nitrogen (N2) gas. The measurements are performed at room temperature and N2 atmosphere. PL spectra are collected at a 45° detection angle from the incident laser beam and calibrated with a tungsten lamp. A 193nm filter is placed in front of the entrance of PL collection optics to avoid interference from 193nm laser excited fluorescence of optical fiber transmitting the photoluminescence signals to spectrometer. In addition, a set of long-wave pass filters are used to eliminate the influence of high-order diffractions of 193nm beam on the PL signals.
2.2 Sample preparation
Six high-purity (Cu content < 3 ppb, indicating 2.25eV PL from Cu can be neglected [13], Al content < 10ppb in fused silica and ~10ppm in quartz) SiO2 samples with 25mm diameter and 2mm thickness from Heraeus (FS 1 - 4 and Q1) and Corning (FS 5) are analyzed. FS 1 - 5 are fused silica samples. One quartz sample (Q1) is also measured for comparison. Before 193nm irradiation, SiOH contents of all samples are measured with a FTIR spectrophotometer [14], and H2 contents are provided by venders (Table 1). Besides, samples are pre-irradiated by 193nm laser at 400Hz repetition rate and 10.0mJ/cm2 fluence until relative changes of PL intensities and measured 193nm absorption coefficients are both within 5%. After pre-irradiation treatment, steady-state 193nm absorption and corresponding time-resolved PL spectra from fused silica and quartz samples are measured. In addition, transient changes in PL intensities over changing laser fluence are also measured and analyzed to find possible defects related to RDP.
Table 1. H2 content, SiOH content, and apparent linear α193nm and nonlinear absorption coefficients β193nm at 193nm of all SiO2 samples.
3. Results
3.1 IR transmittance spectra
Figure 1 shows the measured infrared (IR) transmittance spectra of all SiO2 samples. In IR spectra, the absorption bands ranging from 3000 - 5000cm−1 are mainly due to molecule water (3200 - 3450cm−1) and SiOH groups (~3673cm−1 and ~4522cm−1) [15]. For fused silica samples, there are two significant absorption bands, peaked at 3673 cm−1 and 4522 cm−1 respectively, which correspond to the stretching of SiO-H bonds [14]. However, for the quartz sample no 3673 cm−1 and 4522 cm−1 bands are observed, indicating very low SiOH content in quartz sample (much less than 5.0ppm) and 3000 - 5000cm−1 absorption bands are mainly due to molecule water. In addition, the adsorbed molecule water defects in fused silica samples could be quite different due to inhomogeneity of fused silica, leading to a wider nonlinear broadening in absorption spectra than quartz sample. SiOH contents of all SiO2 samples calculated from the 3673 cm−1 and 4522 cm−1 absorption bands are presented in Table 1.
Fig. 1 IR transmittance spectra of fused silica samples (a) and quartz sample (b) from 3000cm−1 to 5000cm−1 wavenumber.
3.2 193nm absorption
Previous experimental observation [16] showed that for DUV fused silica samples, absorption presented a complicated nonlinear increase with the increasing laser fluence. At the low fluence end, the absorption increased nonlinearly with the laser fluence with a decreasing rate versus fluence. When the laser fluence was higher than around 1.5 mJ/cm2, an approximately linear dependence of the absorption on the fluence was evident and from which “apparent” single-photon (linear) absorption coefficient α193nm and two-photon (nonlinear) absorption coefficient β193nm could be defined and extracted via a linear fit to the measured fluence dependence of absorptance:
where a and b are the y-intercept and slope of the linear fit, respectively. d is the sample thickness. τ is the pulse width of the laser beam. A reversible nonlinear increase of both apparent linear and nonlinear absorption coefficients for UV fused silica with increasing repetition rate from 50 to 1000Hz was also observed [16], indicating that both 193nm linear and nonlinear absorption coefficients consisted of not only intrinsic and defect-induced absorptions, but also the contribution from accumulation of laser induced absorption defects in fused silica at 400Hz repetition. Figure 2 shows the dependence of measured absorptance on laser fluence in the range from 1.5 to 5.0mJ/cm2 for FS1 and Q1 samples, which presents an approximately linear dependence and from which apparent linear and nonlinear absorption coefficients are obtained. Similar measurement results are obtained for other samples. The extracted apparent linear and nonlinear absorption coefficients at 193nm for all samples are presented in Table 1. From Table 1, the linear and nonlinear absorption coefficients of quartz sample are higher than that of most fused silica samples (except FS 4).Fig. 2 1.5~5.0mJ/cm2 fluence dependence of 193nm absorptance for FS 1 and Q1 measured at 400Hz repetition rate. Dotted lines are linear fit to data points for each sample.
3.3 193nm excited PL spectra
Figures. 3(a)-3(b) show the PL intensity spectra of all samples before and after pre-irradiation process. After pre-irradiation with ~3.6kJ/cm2 dose, significant intensity drops of PL bands from 250 to 420nm are observed in several samples, while less intensity changes are observed in the 500 - 700nm spectral range. Since the PL bands are highly overlapped, lifetime spectrum method is employed to resolve these overlapped PL bands. Lifetime spectra are obtained by nonlinear least square (NLS) single-exponential approximation fitting from time-resolved PL signals with delay time from 0.0 to 100.0μs at each wavelength. From the calculated lifetime spectrum the number of overlapped PL bands can be determined. Once the number of PL bands is determined, the initial intensity and lifetime of each PL band in the measured PL spectrum can be resolved via a multi-parameter fitting described in Ref [12], regardless of the line shape of each PL band. Resolved PL bands of a typical fused silica (FS4) and quartz sample are shown in Figs. 3(c)-3(d).
Fig. 3 PL spectra (delay 0.0μs, detector gate width 100.0μs) of fused silica samples (a) and quartz sample (b) excited by 193nm laser at 400Hz repetition rate and 5.0mJ/cm2 fluence. Dotted and full lines are PL spectra before and after pre-irradiation process (400Hz repetition rate, 10.0mJ/cm2 fluence, and ~3.6kJ/cm2 dose), respectively. Resolved PL band intensity spectra of FS4 and Q1 resolved via lifetime spectrum method are shown in (c) and (d), respectively.
For FS 4 sample, five PL bands are observed. Except the 500nm PL band, other PL bands are observed in all fused silica samples. Among these common PL bands, 650nm PL band is confirmed to be related to the well-known non-bridging oxygen center (NBOHC) [17]. 313nm and 363nm PL bands can be closely related to undefined variants of ODC groups [2, 18]. However, 550nm PL band still cannot be firmly confirmed to any known source. The origin of 500nm PL band (FWHM~0.45nm, lifetime 10.0 ± 2.8μs), which is observed only in FS 4, is also unknown. On the other hand, PL spectrum of quartz sample is different from that of fused silica samples, since arrangement of SiO4 tetrahedrons in quartz is much more periodical than that in fused silica [19]. PL bands in fused silica are mainly from ODC related groups and NBOHC, while PL bands in high-purity quartz crystal are reported to be mainly connected with emission of self-trapped exciton (STE) [20]. However, structure of 338nm and 428nm PL bands observed in quartz are still unknown, making it difficult for further comparison. Details of common PL bands observed in all fused silica and PL bands for quartz sample are presented in Table 2, respectively.
Table 2. 1. Peak position, FWHM (full width at half maximum) and lifetime of common resolved PL bands in fused silica and quartz samples excited at 193nm (400Hz repetition rate, 10.0mJ/cm2 fluence).
Figure 4 shows the excitation properties of common resolved PL bands for fused silica and quartz, respectively. A quadratic function model is used to fit the fluence dependence of PL band intensity. PL bands between 300 - 550nm in both fused silica and quartz all show a strong linear excitation behavior as the fitted nonlinear term is negligibly small (<10−8cm2•mJ−1), while 650nm PL band intensity shows a significant quadratic function of laser fluence for all fused silica samples, indicating a two-photon excitation behavior. Among these PL bands, only one-photon excited 311nm and 363nm PL bands show significant correlation, which is shown in Fig. 5, implies that both 311nm and 363nm PL bands may originate from the same emitter. No other significant correlations among PL bands of fused silica samples are observed.
Fig. 4 Fluence dependence of common PL band intensities for FS 4 (a) and Q1 (b) samples excited by 193nm laser at 400Hz repetition rate. The dotted lines represent quadratic fits to measured data.
Fig. 5 Peak intensities of 311nm PL band versus 363nm PL band at 193nm laser irradiation with 400Hz repetition rate and 5.0mJ/cm2 fluence.
4. Discussion
4.1 Steady-state 193nm absorption and PL bands
Figure 6 shows the transmittance spectra of all fused silica samples in 160 - 300nm spectral region measured by a DUV/VUV spectrophotometer (ML6500, Laser Zentrum Hannover, Germany) and the absorption coefficients at 160nm for samples with different SiOH contents. The absorption coefficients are calculated from transmittance spectra by Beer’s law [21] (taking into account Fresnel reflection loss ~15% at 160nm). In DUV region, dominant absorption bands ranging from 160nm to 250nm from E’ center at 5.8eV (~212nm) and ODC at 7.6eV (~162nm) were reported [22]. However, only an approximately linear relationship between 160nm absorption coefficient and SiOH content is observed in our experiment, as presented in Fig. 6(b), corresponding to VUV edge absorption (>7.5eV) of SiOH group [23]. On the other hand, characteristic absorption bands of NBOHC at 4.8eV (~257nm) [22] and 6.8 eV(~181nm) [24] are not observed, indicating that NBOHC’s PL band is mainly created by 193nm laser irradiation.
Fig. 6 (a) Transmittance spectra in the spectral range of 160nm to 300nm for the 5 fused silica samples; (b) 160nm absorption coefficient versus SiOH content. The dotted line is a linear fit.
On the other hand, the 550nm PL band intensity shows a strong correlation to the apparent linear absorption coefficient, and 650nm PL band intensity is strongly correlated to the apparent nonlinear absorption coefficient. Figure 7 shows the relationship between these PL band intensities and the apparent linear and nonlinear absorption coefficients at 193nm of fused silica at steady state. The 650nm PL band is related to NBOHC defects, with PL intensity showing nearly linear relationship to the apparent linear absorption coefficient. The origin of the 550nm PL band with excitation energy near 6.4eV is still unclear, which was assigned to peroxy radical POR (≡SiOO·) [25], E’ (≡Si·) [26], = SiO2 and Si = O groups [27]. In our previous work [12], this 550nm PL band showed correlation to defects in both laser and mechanically damaged sites, indicating such defects may be associated with physical disorders in SiO2 network during damage processes. The result presented in Fig. 7(b) also indicates that NBOHC defect is a major source, though not the only source, of two-photon absorption in fused silica material. From a linear fit, the β193nm value corresponding to zero NBOHC’s PL intensity is ~2.5 × 10−9cm/W, which is close to the 193nm two-photon absorption coefficient reported in [28]. This observation further indicates that the fluctuation of the apparent nonlinear absorption coefficient for different fused silica samples is mainly due to NBOHC, while their intrinsic nonlinear absorption coefficients are close.
Fig. 7 (a) 550nm PL peak intensity versus 193nm apparent linear absorption coefficient α193nm. (b) 650nm PL peak intensity versus 193nm apparent nonlinear absorption coefficient β193nm. Dotted lines represent corresponding linear fits. The measurements are performed at 400Hz repetition rate and 5.0mJ/cm2 fluence.
4.2 Transient changes in PL-related defects
Changes of 550nm and 650nm PL-related defect concentrations with the laser fluence are analyzed via the temporal evolution of PL band intensity when the laser fluence abruptly changes. The results are shown in Fig. 8. For the 550nm PL band, the intensity follows immediately the fluence change without observable time delay. However, for 650nm PL band, A reversible time delay in tens of seconds is observed in the PL intensity change when the laser fluence changes abruptly, which is similar to the delay rate in 193nm absorption reported in RDP [1]. In addition, at different laser fluences, no significant difference between the lifetime of 650nm PL band is experimentally observed, indicating that laser irradiation induces no change in NBOHC’s energy structure. As the 650nm PL band intensity is related to NBOHC concentration, the temporal evolution of 650nm PL intensity represents that of the NBOHC concentration.
Fig. 8 Peak intensity changes of 550nm (green) and 650nm (red) PL bands for FS 3 (a) and FS 4 (b) samples following fluence change (c). Black dotted lines represent fits of intensity change of 650nm PL band. The measurements are performed at 400Hz repetition rate.
It is well known that there are several annealing processes of NBOHC at room temperature, includes recombinations with atomic oxygen O0 (>300°C) [29], atomic hydrogen H0 and H2 [30]. Since reaction with H2 is responsible for the decay of NBOHC in temperature >200K from experimental observations in both H2-free and H2-impregnated fused silica samples [31], and the spectral measurement in our experiment is too slow (2s) to analyze fast reaction (decreased rapidly within 1s [30]) associated with H0, only reaction of H2 is considered and discussed in annealing process of NBOHC at room temperature. As a result, laser irradiation induced generation of NBOHC can be described as:
where CNBO(t) represents the NBOHC concentration at time t. CH2 represents the initial concentration of H2. a represents the total consumption content of H2 from time 0 to t. k1(H) is the generation rate which is directly proportional to power function of laser fluence H determined by n-photon excitation process. kH2 is the annealing rate of H2, which is limited by diffusion coefficient of H2 [32, 33]. The annealing of NBOHC consists of that by radiation-generated and impregnated H2 shown in the second term in Eq. (2). The 650nm PL intensity can be directly obtained from CNBO(t) as it is linearly proportional to CNBO(t). In the case of fused silica samples with extremely high H2 content (FS 1-3), annealing rate of NBOHC can be simplified to kH2·CH2. Then 650nm PL intensity in high H2-content fused silica sample IH(H) can be obtained:SH(H) is the equilibrium PL intensity in high H2-content fused silica sample which is linearly proportional to k1(H)/(kH2·CH2). Parameter c is determined by the initial and equilibrium intensities. It is worth mentioning that even if there is no H2 presence in fused silica sample, the annealing process still exists due to the combination of radiation-generated H2 produced from SiO-H bond’s breakage. On the other hand, in the case of fused silica samples with extremely low H2-content (FS 4-5), annealing rate of NBOHC can be simplified to kH2·CNBO(t)2. Then 650nm PL intensity in low H2-content sample IL(H) can be obtained:SL(H) is the equilibrium PL intensity in low H2-content samples, which is linearly proportional to the square of k1(H)/kH2. From Eqs. (3) and 4, the generation rate k1(H) of NBOHC can be evaluated from the term SH(H)·kH2·CH2 in high H2-content fused silica samples and from the term SL(H)·kH2/2 in low H2-content fused silica samples. From the fits presented in Fig. 8, k1(H) is a quadric function of laser fluence, indicating the generation process of NBOHC could be related to a two-photon absorption process under 193nm irradiation.To investigate the physical mechanism of RDP in fused silica, the influence of SiOH in NBOHC-related RDP is analyzed. The results are shown in Fig. 9. Although no direct relationship between SiOH content and 650nm PL equilibrium intensity S0(H) is observed in Fig. 9(a), no 650nm PL band is observed for SiO2 samples without SiOH group. The generation rate of NBOHC taking into account both generation and annealing processes versus SiOH content is presented in Fig. 9(b). A strong linear relationship between S0(H) × k2 and SiOH content is observed, implying that the generation of 650nm PL-related defects in these fused silica samples is mainly due to SiOH groups. The dissociation energy related to SiOH bond’s breakage is >7.67eV, matching a two-photon excitation process under 193nm (6.4eV) irradiation observed in our experiment. In addition, diffusion coefficient of H2 derived from the annealing rate in high H2-content samples is estimated to be ~10−12cm2s−1 at room temperature, matching the reported value of 10−11~10−12 cm2s−1 under F2 laser irradiation [33], further demonstrating the correlation between NBOHC annealing and H2. As a result, laser irradiation induced NBOHC concentration change is due to the generation process induced by Si-O bond’s breakage and the annealing process related to H2, which may have a strong impact on apparent nonlinear absorption at 193nm especially in fused silica with a high SiOH content.
5. Conclusion
A detailed study on correlation between 193nm absorption and PL related defects in SiO2 network have been presented. Due to difference in structure of SiO2 network, large differences can be found in absorption and emission spectra of quartz and fused silica samples. For UV fused silica samples, correlations between apparent linear absorption (at 193nm) and 550nm PL related defects, and between apparent nonlinear (two-photon) absorption and 650nm PL-related defects (NBOHC) have been observed. Among these PL-related defects, only 650nm PL-related defects NBOHC showed reversible concentration change under 193nm laser irradiation with a rate similar to RDP. The experimental observation further supported the conclusion that the generation of NBOHC was due to a two-photon excitation induced SiO-H breakage, and the annealing of NBOHC was due to the combination of NBOHC with H2, leading to two-photon absorption at 193nm induced by Si-O bond’s breakage especially in high SiOH-content fused silica materials. These results provided new insight into the influence of PL-related defects on 193nm absorption, indicated that apart from E’ center, NBOHC and SiOH groups also had strong impacts on 193nm two-photon absorption especially in fused silica with a high SiOH content. However, origin of 550nm PL-related defect, which show a strong relationship with 193nm linear absorption, is still needed to be answered to further understand the absorption properties and long-term stability of DUV fused silica materials.
Acknowledgments
The authors thank Bodo Kuehn of Heraeus Quarzglas GmbH, Germany for providing fused silica and quartz samples.
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