Appl Phys A (2008) 93: 69–73DOI 10.1007/s00339-008-4636-6

Submicrometer grating fabrication in fused silicaby interferometric laser-induced backside wet etching technique

Csaba Vass · Károly Osvay · Tamás Véso · Béla Hopp ·Zsolt Bor

Received: 12 October 2007 / Accepted: 4 March 2008 / Published online: 31 May 2008© Springer-Verlag 2008

Abstract Submicrometer period fused silica gratings wereproduced by two-beam interferometric laser-induced back-side wet etching technique (TWIN LIBWE). The fourth har-monic of a Nd:YAG laser beam was spatially filtered in twosteps, and the smoothened beam was split into two parts andinterfered at incident angles of 60°, 30°, 14°, and 7.7°, re-spectively, on the backsides of fused silica plates that were incontact with a liquid absorber. The periods of the producedfused silica gratings were, respectively, 154 nm, 266 nm,550 nm, and 990 nm. In the next step, TWIN-LIBWE setupwas completed by using a coupling rectangular prism in or-der to reach immersion setup, which made possible to fabri-cate 104 nm period fused silica grating. This is the smallestlaser-generated grating constant in fused silica at present.

The morphology of the etched gratings was characterizedby atomic force microscope. Important parameters (modula-tion depth, low-pass filtered waviness, quality factor) of theproduced gratings were determined. Evolution of the gratingparameters was also studied in the 990 nm case: the depen-dence of modulation depth, waviness, and quality factor onthe number of laser pulses was investigated.

PACS 42.62 · 42.79 · 81.65

C. Vass (�) · K. Osvay · T. Véso · Z. BorDepartment of Optics and Quantum Electronics, Universityof Szeged, Dóm tér 9, 6720 Szeged, Hungarye-mail: VassCsaba@physx.u-szeged.hu

B. HoppResearch Group on Laser Physics of the Hungarian Academyof Sciences, Dóm tér 9, 6720 Szeged, Hungary

1 Introduction

The microstructured UV-transparent materials (fused silica,crystal quartz, sapphire, CaF2, BaF2, MgF2, etc.) can bewidely used in many fields of micro-optics (refractive anddiffractive optical elements: e.g., beam homogenizers, trans-mission gratings), optoelectronics, and microfluidics.

The microstructuring of UV-transparent materials can becarried out usually by lithography-based procedures, namelyRIE-reactive ion etching, powder blasting, and HF etching.These techniques are slightly complicated, expensive, andmulti-steps methods having several disadvantages such asrough etched surface and low lateral resolution. Laser-basedtechniques are alternative methods for micro- and nanostruc-turing of transparent dielectrics. The application of the con-ventional industrial UV-lasers makes possible a high lateralresolution and one step fabrication, but the direct machiningby these lasers is difficult, due to the high transparency ofthese materials, since the targets do not absorb the laser lightsignificantly and therefore the etching threshold fluence isvery high (e.g., ∼10 J/cm2 for fused silica). As alternativesolution is using femtosecond- and VUV, XUV, or soft X-raylasers that are suitable for micromachining of transparentmaterials, but these devices are too complicated, expen-sive, and not really established in industrial environments.A promising technique for the micro- and nanoprocessingof transparent dielectrics is an indirect laser etching, thelaser induced backside wet etching (LIBWE) [1–5]. It canbe applied for the micro-fabrication of a variety of UV-transparent materials using the irradiation of nanosecond-pulsed UV lasers (practically excimer lasers). The advan-tages of LIBWE procedure (one step method, fine controlla-bility, high lateral resolution, low roughness etched surface,low etching threshold fluence) help to find practical appli-

70 C. Vass et al.

cations of this technique in several fields of research andindustry.

We introduced a novel complex procedure to fabricatesubmicrometer period UV transparent gratings: the LIBWEprocedure was combined with the two-beam interferomet-ric method (TWIN-LIBWE) [6, 7]. The grating periodcan be further decreased with using immersion TWIN-LIBWE setup [8]. Submicrometer transmission gratings canbe applied in, e.g., spectroscopy and bio-sensing (e.g., inplasmon–resonant spectroscopy).

In this study, we show our best quality gratings and thelimits of our procedure. We investigate the grating structuresand present a grating production procedure through the ap-plication of 990 nm period gratings as an example.

2 Experimental

The two-beam interferometric laser-induced backside wetetching (TWIN-LIBWE) is a combined method for fabri-cation of grating structures in transparent dielectric. Twocoherence laser beams interfered at a given incident angleon the backside of the transparent target, which is in con-tact with a liquid absorber to etch the interference fringesinto the transparent target (Fig. 1). The laser beam of a Q-switched, frequency-quadrupled Nd:YAG laser (s-polarized;λ = 266 nm; τFWHM = 8 ns; repetition rate: 10 Hz; coher-ence length: 1 cm) was split into two equal intensity partsand was steered to the backside surface of the transparenttarget (fused silica plate; thickness: 1 mm; refractive indexat 266 nm: nfs = 1.50; Suprasil II, Heraeus) contacting withsaturated solution of naphthalene and methyl-methacrylate(c = 1.85 mol/dm3;nliq = 1.28). The intensity profile of thelaser beam was smoothened in two steps to produce a uni-form and homogeneous beam: the second harmonic greenbeam (λ = 532 nm) was spatially filtered, and its diameterwas diminished before frequency doubling by CLBO crys-tal. The UV beam (λ = 266 nm) was also spatially filteredand diminished before the beam splitter. The homogeneousUV beams were steered to the target surface by dielectricmirrors. Two movable mirrors made possible to set negli-gible optical path difference (OPD) between the two inter-fering pulses. The OPD was not zero at the edges of thelaser spots but significantly less than the coherence lengthof the laser, therefore the visibility of the interference pat-terns ((Imax − Imin)/(Imax + Imin)) was constant across theetched area.

The period of the interference fringes and consequentlythe period of the grating structures (p) depend on the inci-dent angle at the backside of the fused silica (θfs) and thelaser wavelength in the target (λfs) as follows:

p = λfs

2 sin θfs= λair

2nfs sin θfs= λair

2 sin θair(1)

Fig. 1 Sketch of TWIN-LIBWE setup

where λair and θair is the wavelength and incident angleof the laser beam in the air, respectively. As can be seenfrom (1), the “theoretically” reachable minimal period isp = λair/2nfs = 88.7 nm (using θfs = 90◦).

In the followings, we summarize the limitation factorswhich constrain the reachable period in TWIN-LIBWEsetup: (1) The refractive index of the liquid is lower than thatof fused silica, therefore the effect of the total reflection lim-its the incident angle (θfs), and consequently the reachableminimal grating period, too. In our case, the critical angleof the total reflection was θcrit = 58.6◦, which correspondsto p = 103.9 nm period. According to our experiments, theintensity of the evanescence beam (entering a small inten-sity beam into the more rare optical material with incidentangle higher than the critical angle of the total reflection)is not high enough for the etching. (2) The incident angle(θfs) cannot be high enough (θfs < θcrit) in this setup (Fig. 1),since using the highest θair = 90◦ the incident angle is onlyθfs = 41.8◦, which would result in only p = 133 nm gratingperiod. (3) The reflection losses increase dramatically whenapproaching the θair = 90◦ incident angle (according to thewell-known Fresnel equations), therefore the laser energywould not be sufficient for the etching.

The first limitation could be eliminated by using absorb-ing liquid with high refractive index (at least equal to the re-fractive index of fused silica). To find or produce a suitableliquid absorber is not an easy project. The increase of the θfs

and the reduction of reflection losses (solutions of the sec-ond and third problems) can be carried out by the introduc-tion of immersion TWIN-LIBWE (Fig. 2): a fused silica rec-tangular prism was attached to the fused silica target by dis-tilled water (used as immersion liquid; nw = 1.37). Consid-ering that nw > nliq, the refractive index of the water cannotcause total reflection of the laser beam from the prism-waterboundary in our setup. After a simple calculation (writ-ing the condition of the total reflection (nfs sin θfs = nliq)

into (1)), the limit of the minimal grating period (pmin) ofthe I-TWIN-LIBWE arrangement can be obtained as

pmin = λair

2nliq(2)

Submicrometer grating fabrication in fused silica by interferometric laser-induced backside wet etching 71

Fig. 2 Sketch of I-TWIN-LIBWE setup

The minimal reachable grating period is 103.9 nm in ourexperiments, using the above mentioned materials and setup(Fig. 2).

The applied incident angles were 7.7◦, 14◦, 30◦, and 60◦in TWIN-LIBWE setup (Fig. 1) and 20◦ in I-TWIN-LIBWEarrangement (Fig. 2), resulting in 990 nm, 550 nm, 266 nm154 nm, and 104 nm grating periods. The number of laserpulses was varied between 5 and 150, while the applied laserfluences were altered between 285 and 700 mJ/cm2. Thelaser fluences were determined at the back side position ofthe target plate (in the immersion setup, the beam size wasmeasured before the rectangle prism, and from it the realbeam size was derived). Fluences mean average values, theintensity of interference patterns at the maximum, and mini-mum positions were not measured, since these intensities arenot measurable in a usual way at such high spatial frequen-cies. The morphology of the produced gratings was studiedby a PSIA XE-100 atomic force microscope in noncontactmode.

3 Results and discussion

On average, 40 gratings were produced with each setup (ateach incident angle) with different laser fluences and num-bers of laser pulses. During the complete analysis of the pro-duced structures, it was found that the best quality gratingscan be fabricated by 20–100 laser pulses, and the optimalfluence range was 300–500 mJ/cm2(depending on period)(Fig. 3). The characteristic parameters of the best qualitygratings were determined. The modulation depth, which isone of the most important parameters, is plotted in Fig. 4a asa function of period. It decreases with decreasing of the grat-ing period. This quality deterioration can be due to the de-creasing of the visibility of the interference pattern or to thelow thermal diffusion length. Below the curve fitted to themeasured data (Fig. 4a), grating can be produced with lowermodulation depth by less laser pulses than the line indicates.The production of higher modulation depth grating resultsin significant quality loss by using more laser pulses. Thethermal diffusion length (lT ≈ 2

√Dτ , where D is heat dif-

fusivity, and τ is laser beam dwell time [9]) was calculated

for the fused silica and liquid absorber, which was foundto be 167 nm and 50 nm, respectively. It can be seen thatlfused silicaT > pmin. The heat diffusion is the second limita-

tion factor, which reduces extremely the modulation depthat low period cases (104, 154 nm). It can be seen that thesmallest reachable grating period has two limitation factors:optical (2) and thermal limits.

The grating surfaces become wavy in the high fluenceand/or number of laser pulse ranges [6, 7]. The wavelengthof these waves is much lager than the grating period. Forcharacterization of this unevenness, the waviness (Wa) of thesurface was determined. The recorded AFM images werelow-pass Fourier-filtered at a cutoff frequency of 1/2p, andthen the Wa (numerically equal with the surface roughness(Ra)) was established (Fig. 4b). The Wa was determinedfrom different sizes of square images with 20, 10, 5, 2.5, and1 µm lateral dimensions in the cases of 990, 550, 266, 154,and 104 nm periods, respectively. Usually, the roughnessparameters increase with increasing the scan size [10], butwe could not find significant differences between the wavi-ness values calculated from different image sizes (20, 10,5, 2,5 µm lateral dimensions) at 550 nm period case. Thedetermined grating waviness increases with increasing theperiod. The Wa of the gratings was higher than the wavinessof the original fused silica surface (Waoriginal fs = 0.25 nm).

The waviness can be considered as a low frequency noise,and it is reasonable to introduce a general quality factor de-fined by the ratio of the modulation depth and the Wa para-meter (Fig. 4c). This factor confirms our qualitative impres-sion (according to the AFM images in Fig. 3) that the gratingquality deteriorate with the decreasing of the period.

The evolution of grating structure was also studied. Con-stant fluence (330 mJ/cm2) was applied, and the number oflaser pulses was varied from 10 to 150 using 7.7◦ incidentangle, which resulted in 990 nm grating period. AFM im-ages of the gratings and their cross sections can be seen inFig. 5. First 10 pulses generate narrow periodic valleys witha wide of 300–400 nm. 60–70% of the surface is unmodi-fied, and the original surface can be seen in cross sections ofimage. The small peeks at the rim of valleys can be due tothe small error of the noncontact mode of the atomic forcemicroscope. The valleys became wider and deeper (the mod-ulation depth increases) with increasing the number of laserpulses (Fig. 6a). Between 50 and 75 pulses, the untreatedsurface disappeared, and the whole surface was etched,which can be attributed to the not unity visibility of the inter-ference patterns or to the ejected molten/softened transpar-ent material [5]. The waviness of the grating also increaseswith increasing the number of laser pulses (Fig. 6b). Thequality factor has a weak maximum around 50–100 pulses(Fig. 6c), but the determined data are relatively spread. Theobservation of this grating development is more difficult athigher incident angle (in low period cases).

72 C. Vass et al.

Fig. 3 The optimal quality gratings (a) p = 990 nm; F = 330 mJ/cm2; 100 pulses (b) p = 550 nm; F = 330 mJ/cm2; 50 pulses (c) p = 266 nm;F = 525 mJ/cm2; 50 pulses (d) p = 154 nm; F = 500 mJ/cm2; 50 pulses (e) p = 104 nm; F = 310 mJ/cm2; 20 pulses

Fig. 4 Characteristic parameters of best quality gratings vs. grating period (a) modulation depth; (b) low-pass filtered waviness (λcutoff = 2p);(c) quality factor

4 Summary

We introduced novel interferometric laser-induced back-side wet etching methods (TWIN-LIBWE and immersionTWIN-LIBWE) to produce fused silica gratings with dif-ferent periods. The reached minimum period was 104 nm,which is the smallest grating constant fabricated by lasertechniques in fused silica so far. We showed that there is

an optical limit of the reachable minimum grating period,

moreover it is also limited by the thermal diffusion. We per-

formed a comparing detailed analysis of several gratings

produced under different experimental conditions. During

this, we studied the dependence of the characteristic grating

parameters on the period, and it was demonstrated that the

modulation depth and the quality of the gratings decrease

Submicrometer grating fabrication in fused silica by interferometric laser-induced backside wet etching 73

Fig. 5 Pulse number dependence (p = 990 nm; θ = 7.7o; F = 330 mJ/cm2) The number of laser pulses is indicated in the right top corner ofeach image

Fig. 6 Grating evolution: characteristic grating parameters vs. pulse numbers (p = 990 nm, F = 330 mJ/cm2)

with decreasing the period. The grating evolution was alsostudied through the changing of the characteristic parame-ters of 990 nm period gratings. Our results demonstratedthat the highest quality can be produced by 100 laser pulsesat 330 mJ/cm2 laser fluence in this case.

Acknowledgements The authors gratefully acknowledge the finan-cial support of Hungarian Scientific Research Foundation (OTKA TS049872 K 67818).

References

1. J. Wang, H. Niino, A. Yabe, Appl. Phys. A 68, 111 (1999)

2. H. Niino, Y. Kawaguchi, T. Sato, A. Narazaki, T. Gumpenberger,R. Kurosaki, Appl. Surf. Sci. 252, 4387 (2006)

3. R. Böhme, A. Braun, K. Zimmer, Appl. Surf. Sci. 186, 276 (2002)4. G. Kopitkovas, T. Lippert, C. David, A. Wokaun, J. Gobrecht, Mi-

croelectron. Eng. 67–68, 438 (2003)5. C. Vass, T. Smausz, B. Hopp, J. Phys. D Appl. Phys. 37, 2449

(2004)6. C. Vass, K. Osvay, M. Csete, B. Hopp, Appl. Surf. Sci. 253, 8059

(2007)7. C. Vass, K. Osvay, B. Hopp, Opt. Express 14, 8354 (2006)8. C. Vass, K. Osvay, B. Hopp, Z. Bor, Appl. Phys. A 87, 611 (2007)9. D. Bäuerle, Laser Processing and Chemistry, 3rd edn. (Springer,

Berlin, 2000)10. D.L. Sedin, K.L. Rowlen, Appl. Surf. Sci. 182, 40 (2001)