Multiwavelength irradiation effect in fused quartz ablation using vacuum-ultraviolet Raman laser

m

surface science ELSEVIER

Applied Surface Science 96-98 (1996) 347-351

Multiwavelength irradiation effect in fused quartz ablation using vacuum-ultraviolet Raman laser K. Sugioka a8*, S. Wada ‘, Y. Ohnuma b, A. Nakamura a The b Department

Institute

ofPhysical

ofAppliedPhysics,

b, H. Tashiro a, K. Toyoda a

and Chemical Research (RIKEN), Wake, Saitamcr 351-01, Japan

Science Unioersiry

of

Tokyo,

I-3

Kagurazaka, Shinjuku-ku, Tokyo 162, Japan

Received 22 May 1995

Abstract Simultaneous multiwavelength irradiation of a vacuum-ultraviolet (VUV) Raman laser presents high-quality microfabrication of fused quartz by ablation. In order to make the mechanism of high-quality ablation clear, both of stationary and transitional absorption changes of a fundamental beam (266 nm) in fused quartz induced by short wavelength components were examined. The stationary change is caused by photodissociation of Si-0 bonds and formation of metastable absorption sites, while the transitional change is attributed to excited-state absorption (ESA) due to coupling of the VUV components and the fundamental beams. Comparison of magnitude of the absorption changes between both effects indicates that the transitional effect is dominant in the process.

1. Introduction

Fused quartz is one of the most important materials in various industrial fields due to high hardness, high thermal stability, high transparency in the visible wavelength region, and so on. Phase shift masks, optical waveguide devices, and UV optics often require development of a sophisticated microfabrication technique for fused quartz [l]; however, the wide energy gap of - 9 eV makes it difficult. Recently, we have demonstrated microfabrication of fused quartz by the novel ablation technique using a vacuum-ultraviolet (VUV) Raman laser based on high-order anti-Stokes Raman scattering of the fourth harmonic of a Q-switched Nd:YAG laser in an H, gas Raman cell [2,3]. Simultaneous irradiation of all

* Corresponding author. Fax: + 81 46 [email protected]. O169-4332/96/$15.00 6 1996 SSDZ 0169-4332(95)00442-4

462 4703;

e-mail: ksug-

wavelength components of the VUV Ran-ran laser realized high-quality ablation with smooth etched surfaces and without cracks and thermal distortion. In the present paper, the mechanism of high-quality ablation of fused quartz by the VUV Raman laser is investigated. In this process, the short wavelength components in the VUV region may play important roles, although their energy is too small to ablate fused quartz by themselves. In order to discuss the short wavelength irradiation effect, both of transitional and stationary absorption changes of the fundamental beam in fused quartz induced by the VUV components are examined.

2. Experiment The VUV Raman laser, using high-order antiStokes Raman scattering of the fourth harmonic of a

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K. Sugioka et al./Applied

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Surjb

AS6

Science 96-98

11996) 347-351

VUV Raman Laser beams (AS, S, 266nm) 97%

266nm

Dichroic Attenuator ormation

Aperture

of metastable

Quartz Substrate

?>, kY ‘,.,q 266nm ‘y< i Y

“\ Biplanar

Biplanar Phototube

Phototube k

Signal

(a) Stationary absorption effect Fig.

Aperture

Signal

(b) Transitional absorption effect

I. Schematic illustrations of the absorption measurement for (a) the stationa

Q-switched Nd:YAG laser (a 266 nm wavelength and an 8 ns pulse width) in an H, gas Raman cell, was used for quartz ablation. It has a unique characteristic of simultaneous emitting of widely spread discrete lines from the 133 nm 9th order anti-Stokes beam to the 594 nm 5th order Stokes beam. About 45% of the total energy of the output beam are attributed to fundamental radiation of a 266 nm wavelength. The energy of the anti-Stokes beam almost exponentially decreases with increasing order. In the experiment, all wavelength components of the VUV Raman laser were simultaneously directed to the samples placed in a vacuum chamber. The intensity of each wavelength of the output radiation and a schematic diagram of the experimental setup have been published elsewhere [2]. In order to make the ablation mechanism clear, both of transitional and stationary effects induced by the VW components were investigated by measuring absorption changes of the fundamental beam in fused quartz. Fig. 1 shows schematic illustrations of the absorption measurement for (a) the stationary and (b) transitional effects. For the stationary measurement, 100 pulses of 6th order anti-Stokes (AS6) beams (160 nm) at 570 mJ/cm’ were, first, preirradiated onto the fused quartz through a 100 pm aperture in diameter. Then, the fundamental beam of at which the ablation did not takes 610 mJ/cm’,

and (b) transitional effects.

place, was directed to the same region. A signal from the fundamental beam transmitted by the preirradiated fused quartz was detected by a biplanar phototube. For the transitional measurement, the VUV Raman laser beams having fifteen different wavelengths were simultaneously incident to fused quartz through a dichroic attenuator and a 100 pm aperture in diameter. About 97% of the fundamental beam (266 nm) energy were reflected by the dichroic attenuator in order to avoid ablation. The other wavelengths had no absorption in this optical device, so that the most of their energy continued on with the attenuated fundamental beam. The laser beams passing through the quartz, including all wavelengths, were separated by a prism, and then the signal from the separated fundamental beam was detected by a biplanar phototube behind an aperture.

3. Observation of ablated regions Fig. 2(a) shows a photograph of a sample ablated by simultaneous irradiation of all wavelength components of the VUV Raman laser through a contact mask of a Ni mesh with 25 X 25 pm2 apertures observed using a scanning electron microscope (SEM). The laser fluence and the number of laser pulses were 3.0 J/cm2 and 60 pulses, respectively.

K. Sugioka et al. /Applied Surjh

Science 96-98 (1996) 347-351

(a) VUV Raman laser 3.0 J/cm*, 60 Pulses

(b) 4w of Nd:YAG laser 3.0 J/cm*, 60 Pulses

(c) VUV Raman laser 20 J/cm*, 10 Pulses

(d) 4w of Nd:YAG laser 20 J/cm*, 10 Pulses

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Fig. 2. SEM photographs of the samples ablated at various conditions. (a) VUV Raman laser, 3.0 J/cm’. 60 pulses; (b) fourth harmonic of Eid:YAG laser, 3.0 J/cm2. 60 pulses; (c) VUV Raman laser, 20 J/cm’, 10 pulses; and (d) fourth harmonic of Nd:YAG laser, 20 J/cm’, 10 pulses.

For comparison, also shown is the relative result for the sample ablated by single wavelength irradiation of the fourth harmonic of the Nd:YAG laser in Fig. 2(b). The sample ablated by the VUV Raman laser shows a square pattern with about 25 X 25 km2 size which is the almost same as the size of mask patterns used. Regular ripple patterns are formed on the ablated surface, which may be attributed to diffraction of the laser beam at the mask edge. Therefore, the ablated profile reflects the spatial energy distribution of the laser beam, suggesting little thermal influence during the process. Additionally, deposition of ablated particles scarcely occurred around the ablated regions, although a little deposition of debris is observed. On the other hand, the surface ablated using the fourth harmonic of the Nd:YAG laser has irregular roughness. In the case of ablation using a

KrF excimer laser of a 248 nm wavelength, irregular microroughness and damage have also been observed [4,5]. Furthermore, many particles were deposited on the sample. The difference of morphology of the ablated surface notably takes place in the case of larger laser fluence. Fig. 2(c) and (d) show SEM photographs of the surfaces ablated by the VUV Raman laser and the fourth harmonic of the Nd:YAG laser at the laser fluence of 20 J/cm* and 10 pulses, respectively. The surface corresponding to the VUV Raman laser seems in some measure smooth, while irregular microstructure like resolidificated swelling was created for the (d) sample. Thus, simultaneous irradiation of the VUV Raman laser beams are much attractive for high-quality microfabrication of fused quartz. In the process, the

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VUV components may have very important though their energy is too small.

Surjke

Science 96-98 (1996) 347-351

role -Transmittance of 266nm .

100 4. Absorption change induced by the VUV beams Fig. 3 shows pulse forms of the fundamental beam transmitted by fused quartz with (solid line) and without preirradiation of the AS6 beam (dashed line). The preirradiation caused intensity reduction of about 20% at the peak. The reduction; i.e., increase of absorption, is attributed to photodissociation of Si-0 bonds and creation of SiO, (x < 2) caused by the preirradiation, which have been confirmed by X-ray photoelectron spectroscopy (XPS) analysis [3]. This absorption sites are metastable with a life time as long as several days. Thus, the stationary effect of the VUV components in the process was clarified. But, assuming that the absorption increase is proportional to the pulse number of the preirradiation, the increase per pulse is estimated to be less than 1%. For investigation of the transitional effect, pulse forms of the fundamental beam transmitted by the quartz with and without the simultaneous irradiation by the multiwavelength beams. The ratio of the beam intensities of the both pulse forms at each time gives temporal variation of the transmittance of the fundamental beam as shown in Fig. 4. For reference, the pulse form of the simultaneously irradiated AS6

10

20 30 Time [ ns ]

40

Fig. 3. Pulse forms of the fundamental beam transmitted by fused quartz with (solid line) and without preirradiation of the AS6 beam (dashed line).

‘: “‘.. 16Onmput% !.,.-.. --c ‘.. 0 0

5

10

15

20

Time [ns] Fig. 4. Temporal variation of the transmittance of the fundamental beams during the simultaneous irradiation of the VW beams. For reference, the pulse form of the simultaneously irradiated AS6 beam is also shown by the dashed line.

beam is also shown by the dashed line. Although the transmittance is originally lOO%, it decreases as soon as the fused quartz is irradiated by the VUV beams, and reaches a minimum value less than 40%. Immediately after the end of the VUV pulses, it recovers to almost 100%. Therefore, the maximum absorption change is estimated to be more than 60%. This phenomenon may be explained as excited-state absorption (ESA) induced by coupling of the short wavelength components and the fundamental beams. The band structure of fused quartz gives good explanation of ESA in this process. The energy gap and the electron affinity are N 9 and N 0.9 eV, respectively. When light beams with a photon energy more than 9 eV, corresponding to a wavelength of - 138 nm, are incident, electrons are excited from the valence band to the conduction band. The excited electrons are easily raised beyond the vacuum level by photons more than 0.9 eV, which correspond to N 1.38 km. Accordingly, the excited state may strong absorb the fundamental beam of 266 nm. Supposing band-to-band excitation, formation of the excited-state is only possible by AS9 (133 nm) in the VUV Raman laser beams. However, the absorption edge of fused quartz is around 1,70 nm, which is ascribed to impurities and defects. The longer VUV beams may contribute excitation through such defect levels.

K. Sugioka et al./Applied Sur$ace Science 96-98 (1996) 347-351

5. Conclusions The mechanism of high-quality ablation of fused quartz by simultaneous multiwavelength irradiation of the VUV Raman laser has been investigated. In the process, the short wavelength components have two roles; that is, formation of the excited-state (ESA, transitional effect) and the photodissociation of Si-0 bonds (metastable absorption sites, stationary effect). As the basic mechanism, it is considered that the transitional effect is dominant in the process, since the transitional absorption change during the simultaneous irradiation is much larger than the stationary change. Thus, simultaneous irradiation of multiwavelength beams presented a novel scheme

for high-quality microfabrication having wide energy gaps.

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of the materials

References [l] K.O. Hill, B. Malo, F. Bilodeau, DC Jhonson and _I.Albert, Appl. Phys. Lett. 62 (1993) 1035. [2] K. Sugioka, S. Wada. A. Tsunemi, T. Sakai, H. Takai, H. Moriwaki, A. Nakamura, H. Tashiro and K. Toyoda, Jpn. J. Appl. Phys. 32 (1993) 6185. [31 K. Sugioka, S. Wada, H. Tashiro, K. Toyoda and A. Nakamum, Appl. Phys. Lett. 65 (1994) 1510. [4] B. Braren and R. Srinivasan, J. Vat. Sci. Technol. B 6 (1988) 417

[5] J. Ihlemann, B. Wolff and P. Simon, Appl. Phys. A 54 (1992) 363.