- Email: [email protected]
Ion beam etching of high resolution structures in Ta2O5 for grating-assisted directional coupler applications
Applied Surface Science 252 (2005) 1006–1012 www.elsevier.com/locate/apsusc
Ion beam etching of high resolution structures in Ta2O5 for grating-assisted directional coupler applications Andreas Perentos *, Arnan Mitchell, Anthony Holland Australian Photonics Cooperative Research Centre, School of Electrical and Computer Engineering, RMIT University, G.P.O. Box 2476V, Melbourne, Vic. 3001, Australia Received 23 October 2004; received in revised form 27 January 2005; accepted 27 January 2005 Available online 2 March 2005
Abstract An investigation on thin Ta2O5 films patterning using argon ion beam etching (IBE) is presented. The etch rates are characterised by varying the angle of incidence of the beam onto the substrate. Ta2O5 gratings with a period of 2.2 mm (1.1 mm linewidth) and 0.25 mm thickness are fabricated using an angle of incidence of 08. The resulting Ta2O5 grating cross sectional profiles are analysed using AFM and SEM imaging. A fabrication method is thus demonstrated which could be used to implement wavelength selective gratings in applications such as grating-assisted directional couplers (GADCs). # 2005 Elsevier B.V. All rights reserved. PACS: 77.55.+f; 79.20.R; 81.65.C; 85.40.H; 85.60 Keywords: Tantalum oxide Ta2O5; Ion beam etching IBE; Grating-assisted directional coupler; High resolution
1. Introduction Ta2O5 is a material of great interest for fabricating semiconductor [1,2] and photonic [3] devices. This is due to its unique properties such as high dielectric constant (k 32), low leakage current density (<10 8 A/cm2 at 1.5 V for 20 nm thick films), high index of refraction (2.1 at 1550 nm) and low optical propagation losses (<1 dB/cm at 632.8 nm) [1,4,5]. * Corresponding author. Tel.: +61 3 9925 3250; fax: +61 3 9662 1921. E-mail address: [email protected] (A. Perentos).
Its high dielectric constant and low leakage current density make it popular for use in next generation semiconductor electronics [1]. In gigabit DRAM cells, it can be used as a storage capacitor (high k) and a gate insulator (low leakage current) and is proposed to replace SiO2 as an alternative insulator [1,2] (20 nm thick Ta2O5 can replace 3 nm thick SiO2). Its popularity in photonic devices is due to its high refractive index and low optical losses. It is reported as an optical waveguide material in applications where high index contrast is required such as wavelength multiplexers based on arrayed waveguide gratings and multilayer thin films [3].
0169-4332/$ – see front matter # 2005 Elsevier B.V. All rights reserved. doi:10.1016/j.apsusc.2005.01.153
A. Perentos et al. / Applied Surface Science 252 (2005) 1006–1012
Wavelength selective gratings on the electro-optic material LiNbO3 would be of great interest. Titanium diffused LiNbO3 waveguide gratings have been implemented via UV irradiation (l = 248 nm) as surface-relief gratings [6]. Another similar technique that can be used to produce gratings on LiNbO3 is electron beam irradiation [7]. The robustness of LiNbO3 renders these gratings quite subtle. Photorefractive gratings on Fe:LiNbO3 waveguides have also been implemented holographically [8], however these gratings are inherently unstable. Due to the high index of refraction of Ta2O5 and the fact that it is very close to the refractive index of Ti diffused LiNbO3 waveguides (n = 2.15) at 1550 nm, a wavelength selective device using Ta2O5 on Ti diffused LiNbO3 could be conceived. Previously, we have proposed the use of a ‘‘hard’’ (fixed) Ta2O5 grating on Ti diffused LiNbO3 waveguides to realise highly asymmetric grating-assisted directional couplers (GADC) [9]. This paper investigates the fabrication of high resolution, Ta2O5 structures for such grating applications.
1007
Fig. 1. Cross section of proposed device.
and hence 1.2 mm resolution linewidth. At 850 nm [15], the same contrast coupler corresponds to a grating period of 1.3 mm and hence 0.65 mm resolution linewidth. These resolutions can be realized photolithographically without resorting to holography. To couple significant power using such a compact device, the gratings must be made sufficiently deep. For complete power coupling in a 2.5 cm long device, a grating depth of 0.3 mm will be required [9].
2. Highly asymmetric GADC concept 3. Grating realization A grating can be used to enhance coupling between otherwise isolated waveguides of differing refractive indexes. If the index contrast is high, the grating period required will be relatively short. The resulting gratingassisted directional coupler will only couple for a narrow range of optical wavelengths. We have proposed the use of a Ta2O5 grating to selectively couple wavelengths from a Ti diffused LiNbO3 waveguide (n = 2.15) to another waveguide of smaller refractive index (n = 1.5) [9], such as PMMA [10], SU8 [11] or even SiO2 (Fig. 1). With a high index contrast (Dn 0.65), wavelength selectivity can be as low as Dl = 0.8 nm (3 dB intensity) over a short coupling length (1 cm) [12]. This is desirable in wavelength division multiplexing (WDM) applications [13]. For coarse WDM, where Dl = 20 nm, shorter coupling lengths (100 mm) could be practical. Such wavelength selective couplers could be useful for densely integrated system-on-achip applications. According to theory [14], enhanced coupling of optical wavelengths in the vicinity of 1550 nm in a high contrast optical directional coupler (Dn 0.65) require a grating period of L = 2.4 mm
The gratings required for this investigation must have linewidths between 1.2 and 0.65 mm to obtain wavelength selection between 1.55 and 0.85 mm and must be at least 0.25 mm deep to attain significant coupling over lengths less than 1 cm. To realise these gratings, it will be necessary to deposit films of high refractive index Ta2O5 and then pattern it with relatively high resolution and high aspect ratio lithographic techniques. 3.1. Deposition of Ta2O5 thin film A high refractive index, low optical loss film with at least 0.25 mm thickness was desired for deposition on Si, glass and LiNbO3. This can be achieved either with RF sputtering deposition or e-beam evaporation. In this investigation, e-beam evaporation was selected over sputtering because it provides good uniformity over the wafer dimensions and can achieve a very smooth surface. To characterise the deposition, films of Ta2O5 with ˚ were desired on both (1 0 0) silicon, thickness 2500 A
1008
A. Perentos et al. / Applied Surface Science 252 (2005) 1006–1012
glass and LiNbO3. These films were formed using a BALZERS/BAK 600 e-beam evaporation batch coating system. The base pressure of the chamber was ˚ /s and a 1 10 7 Torr. The deposition rate was 2.5 A film of 0.25 mm was produced. The refractive index was then measured on a glass slide using reflectance spectroscopy and was found to be 2.0 at 1550 nm. This refractive index is deemed sufficient for the proposed application. The refractive index of Ta2O5 is dependent on the degree of oxidation and the film density achieved [4]. It may therefore be possible to produce a higher refractive index Ta2O5 film by introducing oxygen flow into the chamber during evaporation or by annealing the samples directly after evaporation. Methods to increase the refractive index of Ta2O5 will be investigated in the future to optimise the effectiveness of the gratings. 3.2. Patterning of Ta2O5 To form gratings, the films of Section 3.1 should be patterned with periodic structures with line-width resolutions in the range 0.65–1.2 mm. For practicality, this process should involve as few processing steps as possible, should provide relatively high aspect ratio (with minimal pattern distortion) while etching through depths in excess of 0.25 mm and should introduce minimal excess roughness. Several lithographic techniques can be considered. Reactive ion etching (RIE) of Ta2O5 has been reported in [16–18]. However, photoresist cannot be used as a masking material in this method. Wet chemical etching, although possible [19], is not as suitable because lateral etching causes significant undercutting of the pattern for the dimensions required for this investigation. Also, it is difficult to mask Ta2O5 with photoresist for wet etching as all reported wet etchants attack photoresist [20]. Therefore a metal mask would be required thus introducing more fabrication steps and complexity. Lift-off is another possible lithographic technique [21], however it is anticipated that the side-wall profile produced by this technique would be difficult to control. A thorough investigation of the realisation of grating patterns using lift-off lithography will be conducted in a separate investigation. Ion beam etching (IBE) offers some significant advantages such as accurate, high resolution etching
with no mask undercutting, ability to etch any material or multi-layer combination and controllable etched sidewall profile [22]. This section will thus investigate the use of IBE to realise the required grating structures in Ta2O5 films on (Si and LiNbO3) substrates. This will be done by first identifying the optimum IBE beam angle and etching characteristics and then demonstrating the etching of high resolution grating structures using this optimal configuration. 3.3. IBE characterisation of Ta2O5 Often IBE is conducted using a metallic mask such as Ti as it has a very low etch rate. This requires photolithographic patterning of the Ti mask on the surface of the film and then IBE to transfer this pattern onto the film [22]. Residual Ti must then be removed to minimise optical losses. It is likely that traces of Ti may remain and cause scattering or absorption. To simplify the fabrication process, it would be desirable to use a photoresist mask directly. Photoresist has been shown to etch relatively quickly during IBE. Very thin films of photoresist (0.7 mm) are required to obtain high resolution features and thus it is possible that the photoresist will be depleted before a sufficient etch depth (0.25 mm) is obtained in the Ta2O5. It is well known that the etch rates of various materials depend strongly on the beam angle used during IBE [22]. This investigation will thus characterise the dependence of the IBE etch rate of high resolution photoresist, Ta2O5 and Ti as a function of beam angle. From these characteristics, the optimal beam angle and maximum aspect ratio that can be achieved using a photoresist mask will be determined. The evaporated Ta2O5 films on LiNbO3 and Si (1 0 0) were coated with 0.7 mm thick AZ5206E photoresist. The photoresist was puddled on the wafer and then spun at 2500 rpm with a closed lid for 5 s at a ramping speed of 2500 rpm/s. These conditions provide a very uniform resist film. Pre-bake then followed at 90 8C for 25 min in a convention oven to remove most of the solvent. Several samples were prepared with simple step patterns by masking a portion of the sample with photoresist using an MJB 3 contact exposure system. This system provides both vacuum contact and UV radiation features of 320 nm which are essential to achieve 1 mm resolution. The exposed pattern was then developed in AZ400K
A. Perentos et al. / Applied Surface Science 252 (2005) 1006–1012
1009
Fig. 3. Measured etch rates of Ta2O5 and AZ5206E as a function of ion beam incidence angle. The angular dependence of Ti [22] is also presented for comparison. Fig. 2. IBE setup.
developer diluted in 1:4 ratio de-ionised water. Finally the samples were post-baked at 110 8C for 30 min in a convention oven. The IBE system setup can be seen in Fig. 2. Argon ions are generated and accelerated by an IOS-3000 ion source. The beam of the ion source is directed at the sample making an angle u with the sample normal. It is possible to adjust the angle u from normal incidence u = 0–808. Using a cryogenic pump, the base pressure in the IBE chamber was reduced to 1.5 10 6 Torr. The chamber pressure rose to 1.5 10 4 Torr with the introduction of the argon gas (4 sccm). Thermal paste was used below the samples as the photoresist masking layer could be damaged due to heating by ion bombardment. Overheating the resist layer leads to damaged patterns and makes the photoresist difficult to remove after etching. The emitter current was set at 165 mA and the emitter voltage at 470 V, the anode voltage at 500 V and the accelerator at 70 V. The etch rate was estimated by etching samples for 18 min.
4. Results 4.1. Characterisation of IBE etch rates In order to verify the suitability of photoresist as a masking material, the etch rates of Ta2O5, AZ5206E and Ti [22] for u = 0–808 are shown in Fig. 3. These were determined by profiling the step patterned
samples on an XP-2 AMBIOS profilometer before and after stripping the AZ5206E mask. For Ta2O5 the maximum and minimum etch rates are achieved at 308 and 808, respectively. However, between 0 and 708, the variation in etch rate is less than 30%. The etch rate of AZ5206E photoresist increases by 100% going from u = 08 to 608. From Fig. 3 the ratio of etch rates of AZ5206E and Ta2O5 is 3.0 at u = 608 and 1.7 at u = 08. The etch rate ˚ /min due to measurements have a tolerance of 5 A the profilometer’s accuracy. Ideally the etch mask should have a suitably low etch rate and was made thin to give good resolution. In this study, where the photoresist is 0.7 mm and the Ta2O5 is 0.25 mm, an etch rate difference of 2.8 or less between the photoresist and Ta2O5 is required. Therefore, an ideal u would be at 08. Hence, using the conditions outlined above and an IBE angle of 08, 0.7 mm films of AZ5206E photoresist is a suitable masking material for realising high resolution gratings in Ta2O5 with depths of up to 0.4 mm. This depth is more than sufficient for the proposed application. If features with depth exceeding 0.4 mm were required, it should be possible to use Ti as a mask. The angular dependence of the etch rate of Ti reported in [22] is also presented in Fig. 3 for comparison. It is evident Ti etches at approximately half the rate of Ta2O5 at all angles. Thus higher aspect ratio patterns could be achieved using a Ti mask however this would come at the expense of an extra lift-off processing step, adding cost complexity and reduction in precision.
1010
A. Perentos et al. / Applied Surface Science 252 (2005) 1006–1012
4.2. IBE of fine resolution Ta2O5 grating patterns
Fig. 4. Typical cross sectional profile of photoresist grating.
After characterizing the etch rates, LiNbO3 and Si ˚ Ta2O5 films as samples were coated with 2500 A described in Section 3.1. These were then masked with photoresist rectangular grating patterns of period 2.2 mm as described in Section 3.3. Fig. 4 shows a typical photoresist grating cross sectional profile on Si (but of lower linewidth, 0.8 mm) before it was etched. IBE was then performed at u = 08. The Ta2O5 structures realized on LiNbO3 substrate were analysed using an atomic force microscope (AFM). The AFM images are presented in Fig. 5a–d. The Ta2O5 structures realized on Si substrate were taken to a Philips xL-30 SEM for further examination. Before loading the samples into the SEM, they were coated with a thin layer of gold to prevent charging up
Fig. 5. Profiles of 0.25 mm deep and 1.1 mm wide Ta2O5 grating patterns on LiNbO3.
A. Perentos et al. / Applied Surface Science 252 (2005) 1006–1012
1011
Fig. 6. Cross sectional profiles of 0.25 mm deep and 1.1 mm wide Ta2O5 grating patterns on Si.
(since Ta2O5 is a dielectric). Cross sectional profiles of these gratings can be seen clearly in Fig. 6.
5. Discussion The AFM images (Fig. 5) of the ion beam etched Ta2O5 gratings on LiNbO3 substrate exhibit horns on the edges. These appear to be of significant height (50–100 nm) and it is believed that this is debris caused by redeposition of etched Ta2O5. This could be a problem as it can introduce higher grating scattering losses. However, the SEM images (Fig. 6) suggest that, although these debris spikes do exist, they are not of such significant scale (<50 nm) and hence the losses involved would be less significant. In Figs. 5a, c and 6c, d (less obvious), a development of trenches (facets) is evident around the base corners of previous mask edges. This is due to the fact that incident ions on the undercutted photoresist mask sidewalls are reflected down at the
base corners of the pattern [22]. As a result, the Ar ion flux density at the base corners is higher and therefore they etch faster than other points in between. Hence consistent trench features develop. These facet angles should be the same as the maximum etch rate angle for Ta2O5, that is 308. Had the maximum etch rate angle been 08, no consistent trenching would occur and nearly vertical side walls could be achieved [22]. The surface roughness along the grating is approximately 15 nm in the worst case. This roughness is unlikely to cause significant scattering in a grating-assisted directional coupler application.
6. Conclusions We have proposed a method for fabricating Ta2O5 gratings with periods between 2.4 and 1.3 mm for application in Highly asymmetric GADCs. We have characterised the ion beam etch rates of Ta2O5 for different ion beam angles of incidence. Photoresist can
1012
A. Perentos et al. / Applied Surface Science 252 (2005) 1006–1012
be used as a good masking material for the desired aspect ratios and resolutions. The optimum ion beam angle of incidence was found to be 08. Finally we have successfully fabricated 0.25 mm deep Ta2O5 gratings at a linewidth of 1.1 mm using Argon IBE with an angle of incidence of 08. The resulting grating structures are characterised by some horn and facet angle effects, but have a maximum surface roughness of 15 nm which is acceptable for this application. Future work will involve realization of these gratings by lift-off techniques and comparison with IBE, demonstration of the grating mechanism in photonic devices and perhaps testing of the resolution limits of Ar IBE on Ta2O5.
[9]
[10]
[11]
[12]
[13] [14]
References [1] E. Atanassova, D. Spassov, Thermal Ta2O5—alternative to SiO2 for high density dynamic memories, Presented at Microelectronics, 2002, MIEL 2002, 23rd International Conference on, 2002. [2] E. Atanassova, G. Tyuliev, A. Paskaleva, D. Spassov, K. Kostov, XPS study of N2 annealing effect on thermal Ta2O5 layers on Si, Appl. Surf. Sci. 225 (2004) 86–99. [3] H. Takahashi, S. Suzuki, I. Nishi, Wavelength multiplexer based on SiO2-Ta2O5 arrayed-waveguide grating, J. Lightwave Technol. 12 (1994) 989–995. [4] C. Chaneliere, J.L. Autran, B. Balland, R.A.B. Devine, Tantalum pentoxide (Ta2O5) thin films for advanced dielectric applications, Mater. Sci. Eng. R: Rep. 22 (1998) 269–322. [5] J.D.C.D.H. Hensler, R.J. Martin, P.K. Tien, Optical propagation in sheet and pattern generated films of Ta2O5, Appl. Opt. 10 (1971) 1037–1042. [6] B. Wu, P.L. Chu, H. Hu, Z. Xiong, UV-induced surface-relief gratings on LiNbO3 channel waveguides, IEEE J. Quantum Electron. 35 (1999) 1369–1373. [7] C. Restoin, C. Darraud-Taupiac, J.L. Decossas, J.C. Vareille, J. Hauden, Ferroelectric-domain-inverted gratings by electron beam on LiNbO3, Mater. Sci. Semicond. Processing 3 (2000) 405–407. [8] S.M. Kostritskii, O.G. Sevostyanov, P. Moretti, Optimization of photorefractive LiNbO3 waveguides fabricated by com-
[15]
[16] [17]
[18]
[19]
[20] [21] [22]
bined techniques of ion exchange and implantation, Opt. Mater. 18 (2001) 77–80. A. Perentos, A. Mitchell, M.W. Austin, Polymer and LiNbO3 Waveguide Asymmetric Grating Assisted Directional Couplers, Presented at COIN/ACOFT, Melbourne, Australia, 2003. A. Holland, H. Mendis, A. Mitchell, S. Huntington, D. Simon, G. Amarasinghe, R. Shanks, M.W. Austin, Fabrication of Polymer Waveguide by Direct plasma Etching, Presented at COIN/ACOFT, Melbourne, Australia, 2003. V. Balkunje, A. Holland, A. Mitchell, Fabrication of Polymer rib Waveguides Using SU-8, Presented at ACOFT, Canberra, 2004. J.C. Dainty, Optical waves in crystals, in: A. Yariv, P. Yeh (Eds.), Optics & Laser Technology, Wiley, 1984, pp. xi + 589, UK pound 47.45, vol. 17, 217–218, 1985. D. Sadot, E. Boimovich, Tunable optical filters for dense WDM networks, IEEE Commun. Mag. 36 (1998) 50–55. W.P.O. Juptner, Integrated optics—theory and technology, in: R.G. Hunsperger (Ed.), fifth ed., Advanced Texts in Physics, University of Delaware, Newark, DE, USA, Springer, Berlin, 2002, p. 446, ISBN No.: 3-540-43341-4 (Hardback), UK pound 56.00, Optics and Lasers in Engineering, vol. 42, 375–376, 2004. A. Mitchell, M.W. Austin, P.J. Edwards, L.N. Binh, L.A. Bui, S. Mansoori, W.J. Lai, Ultra-broadband shortwave polarization modulators for high-speed free-space quantum cryptography, Presented at Quantum Communications and Quantum Imaging, San Diego, CA, USA, 2004. T.U.a.B.T. Shunji Seki, Reactive ion etching of tantalum pentoxide, J. Electrochem. Soc. 130 (1983) 2505–2506. Y. Kuo, Reactive ion etching of sputter deposited tantalum oxide and its etch sensitivity to tantalum, J. Electrochem. Soc. 139 (1992) 579–583. F.-X. Jian, S.K. Kurinec, Tantalum oxide thin films for microelectronic applications, Proceedings of the Eleventh Biennial 1995, Presented at University/Government/Industry Microelectronics Symposium, 1995. M.J. Chuang, K.Y. Hsieh, High-temperature wet chemical etching of Ta2O5 in NaOH-based solutions for fabricating antiresonant reflecting optical waveguides, J. Electrochem. Soc. 145 (1998) 1020–1022. W.H.T. Perrin Walker, CRC Handbook of Metal Etchants, CRC Press, 1991. B.J.C.M. Hatzakis, J.M. Shaw, Single-step optical lift-off process, IBM J. Res. Dev. 24 (1980) 452–460. R.E. Lee, Microfabrication by ion-beam etching, J. Vac. Sci. Technol. 16 (1979) 164–170.
Ion beam etching of high resolution structures in Ta2O5 for grating-assisted directional coupler applications Andreas Perentos *, Arnan Mitchell, Anthony Holland Australian Photonics Cooperative Research Centre, School of Electrical and Computer Engineering, RMIT University, G.P.O. Box 2476V, Melbourne, Vic. 3001, Australia Received 23 October 2004; received in revised form 27 January 2005; accepted 27 January 2005 Available online 2 March 2005
Abstract An investigation on thin Ta2O5 films patterning using argon ion beam etching (IBE) is presented. The etch rates are characterised by varying the angle of incidence of the beam onto the substrate. Ta2O5 gratings with a period of 2.2 mm (1.1 mm linewidth) and 0.25 mm thickness are fabricated using an angle of incidence of 08. The resulting Ta2O5 grating cross sectional profiles are analysed using AFM and SEM imaging. A fabrication method is thus demonstrated which could be used to implement wavelength selective gratings in applications such as grating-assisted directional couplers (GADCs). # 2005 Elsevier B.V. All rights reserved. PACS: 77.55.+f; 79.20.R; 81.65.C; 85.40.H; 85.60 Keywords: Tantalum oxide Ta2O5; Ion beam etching IBE; Grating-assisted directional coupler; High resolution
1. Introduction Ta2O5 is a material of great interest for fabricating semiconductor [1,2] and photonic [3] devices. This is due to its unique properties such as high dielectric constant (k 32), low leakage current density (<10 8 A/cm2 at 1.5 V for 20 nm thick films), high index of refraction (2.1 at 1550 nm) and low optical propagation losses (<1 dB/cm at 632.8 nm) [1,4,5]. * Corresponding author. Tel.: +61 3 9925 3250; fax: +61 3 9662 1921. E-mail address: [email protected] (A. Perentos).
Its high dielectric constant and low leakage current density make it popular for use in next generation semiconductor electronics [1]. In gigabit DRAM cells, it can be used as a storage capacitor (high k) and a gate insulator (low leakage current) and is proposed to replace SiO2 as an alternative insulator [1,2] (20 nm thick Ta2O5 can replace 3 nm thick SiO2). Its popularity in photonic devices is due to its high refractive index and low optical losses. It is reported as an optical waveguide material in applications where high index contrast is required such as wavelength multiplexers based on arrayed waveguide gratings and multilayer thin films [3].
0169-4332/$ – see front matter # 2005 Elsevier B.V. All rights reserved. doi:10.1016/j.apsusc.2005.01.153
A. Perentos et al. / Applied Surface Science 252 (2005) 1006–1012
Wavelength selective gratings on the electro-optic material LiNbO3 would be of great interest. Titanium diffused LiNbO3 waveguide gratings have been implemented via UV irradiation (l = 248 nm) as surface-relief gratings [6]. Another similar technique that can be used to produce gratings on LiNbO3 is electron beam irradiation [7]. The robustness of LiNbO3 renders these gratings quite subtle. Photorefractive gratings on Fe:LiNbO3 waveguides have also been implemented holographically [8], however these gratings are inherently unstable. Due to the high index of refraction of Ta2O5 and the fact that it is very close to the refractive index of Ti diffused LiNbO3 waveguides (n = 2.15) at 1550 nm, a wavelength selective device using Ta2O5 on Ti diffused LiNbO3 could be conceived. Previously, we have proposed the use of a ‘‘hard’’ (fixed) Ta2O5 grating on Ti diffused LiNbO3 waveguides to realise highly asymmetric grating-assisted directional couplers (GADC) [9]. This paper investigates the fabrication of high resolution, Ta2O5 structures for such grating applications.
1007
Fig. 1. Cross section of proposed device.
and hence 1.2 mm resolution linewidth. At 850 nm [15], the same contrast coupler corresponds to a grating period of 1.3 mm and hence 0.65 mm resolution linewidth. These resolutions can be realized photolithographically without resorting to holography. To couple significant power using such a compact device, the gratings must be made sufficiently deep. For complete power coupling in a 2.5 cm long device, a grating depth of 0.3 mm will be required [9].
2. Highly asymmetric GADC concept 3. Grating realization A grating can be used to enhance coupling between otherwise isolated waveguides of differing refractive indexes. If the index contrast is high, the grating period required will be relatively short. The resulting gratingassisted directional coupler will only couple for a narrow range of optical wavelengths. We have proposed the use of a Ta2O5 grating to selectively couple wavelengths from a Ti diffused LiNbO3 waveguide (n = 2.15) to another waveguide of smaller refractive index (n = 1.5) [9], such as PMMA [10], SU8 [11] or even SiO2 (Fig. 1). With a high index contrast (Dn 0.65), wavelength selectivity can be as low as Dl = 0.8 nm (3 dB intensity) over a short coupling length (1 cm) [12]. This is desirable in wavelength division multiplexing (WDM) applications [13]. For coarse WDM, where Dl = 20 nm, shorter coupling lengths (100 mm) could be practical. Such wavelength selective couplers could be useful for densely integrated system-on-achip applications. According to theory [14], enhanced coupling of optical wavelengths in the vicinity of 1550 nm in a high contrast optical directional coupler (Dn 0.65) require a grating period of L = 2.4 mm
The gratings required for this investigation must have linewidths between 1.2 and 0.65 mm to obtain wavelength selection between 1.55 and 0.85 mm and must be at least 0.25 mm deep to attain significant coupling over lengths less than 1 cm. To realise these gratings, it will be necessary to deposit films of high refractive index Ta2O5 and then pattern it with relatively high resolution and high aspect ratio lithographic techniques. 3.1. Deposition of Ta2O5 thin film A high refractive index, low optical loss film with at least 0.25 mm thickness was desired for deposition on Si, glass and LiNbO3. This can be achieved either with RF sputtering deposition or e-beam evaporation. In this investigation, e-beam evaporation was selected over sputtering because it provides good uniformity over the wafer dimensions and can achieve a very smooth surface. To characterise the deposition, films of Ta2O5 with ˚ were desired on both (1 0 0) silicon, thickness 2500 A
1008
A. Perentos et al. / Applied Surface Science 252 (2005) 1006–1012
glass and LiNbO3. These films were formed using a BALZERS/BAK 600 e-beam evaporation batch coating system. The base pressure of the chamber was ˚ /s and a 1 10 7 Torr. The deposition rate was 2.5 A film of 0.25 mm was produced. The refractive index was then measured on a glass slide using reflectance spectroscopy and was found to be 2.0 at 1550 nm. This refractive index is deemed sufficient for the proposed application. The refractive index of Ta2O5 is dependent on the degree of oxidation and the film density achieved [4]. It may therefore be possible to produce a higher refractive index Ta2O5 film by introducing oxygen flow into the chamber during evaporation or by annealing the samples directly after evaporation. Methods to increase the refractive index of Ta2O5 will be investigated in the future to optimise the effectiveness of the gratings. 3.2. Patterning of Ta2O5 To form gratings, the films of Section 3.1 should be patterned with periodic structures with line-width resolutions in the range 0.65–1.2 mm. For practicality, this process should involve as few processing steps as possible, should provide relatively high aspect ratio (with minimal pattern distortion) while etching through depths in excess of 0.25 mm and should introduce minimal excess roughness. Several lithographic techniques can be considered. Reactive ion etching (RIE) of Ta2O5 has been reported in [16–18]. However, photoresist cannot be used as a masking material in this method. Wet chemical etching, although possible [19], is not as suitable because lateral etching causes significant undercutting of the pattern for the dimensions required for this investigation. Also, it is difficult to mask Ta2O5 with photoresist for wet etching as all reported wet etchants attack photoresist [20]. Therefore a metal mask would be required thus introducing more fabrication steps and complexity. Lift-off is another possible lithographic technique [21], however it is anticipated that the side-wall profile produced by this technique would be difficult to control. A thorough investigation of the realisation of grating patterns using lift-off lithography will be conducted in a separate investigation. Ion beam etching (IBE) offers some significant advantages such as accurate, high resolution etching
with no mask undercutting, ability to etch any material or multi-layer combination and controllable etched sidewall profile [22]. This section will thus investigate the use of IBE to realise the required grating structures in Ta2O5 films on (Si and LiNbO3) substrates. This will be done by first identifying the optimum IBE beam angle and etching characteristics and then demonstrating the etching of high resolution grating structures using this optimal configuration. 3.3. IBE characterisation of Ta2O5 Often IBE is conducted using a metallic mask such as Ti as it has a very low etch rate. This requires photolithographic patterning of the Ti mask on the surface of the film and then IBE to transfer this pattern onto the film [22]. Residual Ti must then be removed to minimise optical losses. It is likely that traces of Ti may remain and cause scattering or absorption. To simplify the fabrication process, it would be desirable to use a photoresist mask directly. Photoresist has been shown to etch relatively quickly during IBE. Very thin films of photoresist (0.7 mm) are required to obtain high resolution features and thus it is possible that the photoresist will be depleted before a sufficient etch depth (0.25 mm) is obtained in the Ta2O5. It is well known that the etch rates of various materials depend strongly on the beam angle used during IBE [22]. This investigation will thus characterise the dependence of the IBE etch rate of high resolution photoresist, Ta2O5 and Ti as a function of beam angle. From these characteristics, the optimal beam angle and maximum aspect ratio that can be achieved using a photoresist mask will be determined. The evaporated Ta2O5 films on LiNbO3 and Si (1 0 0) were coated with 0.7 mm thick AZ5206E photoresist. The photoresist was puddled on the wafer and then spun at 2500 rpm with a closed lid for 5 s at a ramping speed of 2500 rpm/s. These conditions provide a very uniform resist film. Pre-bake then followed at 90 8C for 25 min in a convention oven to remove most of the solvent. Several samples were prepared with simple step patterns by masking a portion of the sample with photoresist using an MJB 3 contact exposure system. This system provides both vacuum contact and UV radiation features of 320 nm which are essential to achieve 1 mm resolution. The exposed pattern was then developed in AZ400K
A. Perentos et al. / Applied Surface Science 252 (2005) 1006–1012
1009
Fig. 3. Measured etch rates of Ta2O5 and AZ5206E as a function of ion beam incidence angle. The angular dependence of Ti [22] is also presented for comparison. Fig. 2. IBE setup.
developer diluted in 1:4 ratio de-ionised water. Finally the samples were post-baked at 110 8C for 30 min in a convention oven. The IBE system setup can be seen in Fig. 2. Argon ions are generated and accelerated by an IOS-3000 ion source. The beam of the ion source is directed at the sample making an angle u with the sample normal. It is possible to adjust the angle u from normal incidence u = 0–808. Using a cryogenic pump, the base pressure in the IBE chamber was reduced to 1.5 10 6 Torr. The chamber pressure rose to 1.5 10 4 Torr with the introduction of the argon gas (4 sccm). Thermal paste was used below the samples as the photoresist masking layer could be damaged due to heating by ion bombardment. Overheating the resist layer leads to damaged patterns and makes the photoresist difficult to remove after etching. The emitter current was set at 165 mA and the emitter voltage at 470 V, the anode voltage at 500 V and the accelerator at 70 V. The etch rate was estimated by etching samples for 18 min.
4. Results 4.1. Characterisation of IBE etch rates In order to verify the suitability of photoresist as a masking material, the etch rates of Ta2O5, AZ5206E and Ti [22] for u = 0–808 are shown in Fig. 3. These were determined by profiling the step patterned
samples on an XP-2 AMBIOS profilometer before and after stripping the AZ5206E mask. For Ta2O5 the maximum and minimum etch rates are achieved at 308 and 808, respectively. However, between 0 and 708, the variation in etch rate is less than 30%. The etch rate of AZ5206E photoresist increases by 100% going from u = 08 to 608. From Fig. 3 the ratio of etch rates of AZ5206E and Ta2O5 is 3.0 at u = 608 and 1.7 at u = 08. The etch rate ˚ /min due to measurements have a tolerance of 5 A the profilometer’s accuracy. Ideally the etch mask should have a suitably low etch rate and was made thin to give good resolution. In this study, where the photoresist is 0.7 mm and the Ta2O5 is 0.25 mm, an etch rate difference of 2.8 or less between the photoresist and Ta2O5 is required. Therefore, an ideal u would be at 08. Hence, using the conditions outlined above and an IBE angle of 08, 0.7 mm films of AZ5206E photoresist is a suitable masking material for realising high resolution gratings in Ta2O5 with depths of up to 0.4 mm. This depth is more than sufficient for the proposed application. If features with depth exceeding 0.4 mm were required, it should be possible to use Ti as a mask. The angular dependence of the etch rate of Ti reported in [22] is also presented in Fig. 3 for comparison. It is evident Ti etches at approximately half the rate of Ta2O5 at all angles. Thus higher aspect ratio patterns could be achieved using a Ti mask however this would come at the expense of an extra lift-off processing step, adding cost complexity and reduction in precision.
1010
A. Perentos et al. / Applied Surface Science 252 (2005) 1006–1012
4.2. IBE of fine resolution Ta2O5 grating patterns
Fig. 4. Typical cross sectional profile of photoresist grating.
After characterizing the etch rates, LiNbO3 and Si ˚ Ta2O5 films as samples were coated with 2500 A described in Section 3.1. These were then masked with photoresist rectangular grating patterns of period 2.2 mm as described in Section 3.3. Fig. 4 shows a typical photoresist grating cross sectional profile on Si (but of lower linewidth, 0.8 mm) before it was etched. IBE was then performed at u = 08. The Ta2O5 structures realized on LiNbO3 substrate were analysed using an atomic force microscope (AFM). The AFM images are presented in Fig. 5a–d. The Ta2O5 structures realized on Si substrate were taken to a Philips xL-30 SEM for further examination. Before loading the samples into the SEM, they were coated with a thin layer of gold to prevent charging up
Fig. 5. Profiles of 0.25 mm deep and 1.1 mm wide Ta2O5 grating patterns on LiNbO3.
A. Perentos et al. / Applied Surface Science 252 (2005) 1006–1012
1011
Fig. 6. Cross sectional profiles of 0.25 mm deep and 1.1 mm wide Ta2O5 grating patterns on Si.
(since Ta2O5 is a dielectric). Cross sectional profiles of these gratings can be seen clearly in Fig. 6.
5. Discussion The AFM images (Fig. 5) of the ion beam etched Ta2O5 gratings on LiNbO3 substrate exhibit horns on the edges. These appear to be of significant height (50–100 nm) and it is believed that this is debris caused by redeposition of etched Ta2O5. This could be a problem as it can introduce higher grating scattering losses. However, the SEM images (Fig. 6) suggest that, although these debris spikes do exist, they are not of such significant scale (<50 nm) and hence the losses involved would be less significant. In Figs. 5a, c and 6c, d (less obvious), a development of trenches (facets) is evident around the base corners of previous mask edges. This is due to the fact that incident ions on the undercutted photoresist mask sidewalls are reflected down at the
base corners of the pattern [22]. As a result, the Ar ion flux density at the base corners is higher and therefore they etch faster than other points in between. Hence consistent trench features develop. These facet angles should be the same as the maximum etch rate angle for Ta2O5, that is 308. Had the maximum etch rate angle been 08, no consistent trenching would occur and nearly vertical side walls could be achieved [22]. The surface roughness along the grating is approximately 15 nm in the worst case. This roughness is unlikely to cause significant scattering in a grating-assisted directional coupler application.
6. Conclusions We have proposed a method for fabricating Ta2O5 gratings with periods between 2.4 and 1.3 mm for application in Highly asymmetric GADCs. We have characterised the ion beam etch rates of Ta2O5 for different ion beam angles of incidence. Photoresist can
1012
A. Perentos et al. / Applied Surface Science 252 (2005) 1006–1012
be used as a good masking material for the desired aspect ratios and resolutions. The optimum ion beam angle of incidence was found to be 08. Finally we have successfully fabricated 0.25 mm deep Ta2O5 gratings at a linewidth of 1.1 mm using Argon IBE with an angle of incidence of 08. The resulting grating structures are characterised by some horn and facet angle effects, but have a maximum surface roughness of 15 nm which is acceptable for this application. Future work will involve realization of these gratings by lift-off techniques and comparison with IBE, demonstration of the grating mechanism in photonic devices and perhaps testing of the resolution limits of Ar IBE on Ta2O5.
[9]
[10]
[11]
[12]
[13] [14]
References [1] E. Atanassova, D. Spassov, Thermal Ta2O5—alternative to SiO2 for high density dynamic memories, Presented at Microelectronics, 2002, MIEL 2002, 23rd International Conference on, 2002. [2] E. Atanassova, G. Tyuliev, A. Paskaleva, D. Spassov, K. Kostov, XPS study of N2 annealing effect on thermal Ta2O5 layers on Si, Appl. Surf. Sci. 225 (2004) 86–99. [3] H. Takahashi, S. Suzuki, I. Nishi, Wavelength multiplexer based on SiO2-Ta2O5 arrayed-waveguide grating, J. Lightwave Technol. 12 (1994) 989–995. [4] C. Chaneliere, J.L. Autran, B. Balland, R.A.B. Devine, Tantalum pentoxide (Ta2O5) thin films for advanced dielectric applications, Mater. Sci. Eng. R: Rep. 22 (1998) 269–322. [5] J.D.C.D.H. Hensler, R.J. Martin, P.K. Tien, Optical propagation in sheet and pattern generated films of Ta2O5, Appl. Opt. 10 (1971) 1037–1042. [6] B. Wu, P.L. Chu, H. Hu, Z. Xiong, UV-induced surface-relief gratings on LiNbO3 channel waveguides, IEEE J. Quantum Electron. 35 (1999) 1369–1373. [7] C. Restoin, C. Darraud-Taupiac, J.L. Decossas, J.C. Vareille, J. Hauden, Ferroelectric-domain-inverted gratings by electron beam on LiNbO3, Mater. Sci. Semicond. Processing 3 (2000) 405–407. [8] S.M. Kostritskii, O.G. Sevostyanov, P. Moretti, Optimization of photorefractive LiNbO3 waveguides fabricated by com-
[15]
[16] [17]
[18]
[19]
[20] [21] [22]
bined techniques of ion exchange and implantation, Opt. Mater. 18 (2001) 77–80. A. Perentos, A. Mitchell, M.W. Austin, Polymer and LiNbO3 Waveguide Asymmetric Grating Assisted Directional Couplers, Presented at COIN/ACOFT, Melbourne, Australia, 2003. A. Holland, H. Mendis, A. Mitchell, S. Huntington, D. Simon, G. Amarasinghe, R. Shanks, M.W. Austin, Fabrication of Polymer Waveguide by Direct plasma Etching, Presented at COIN/ACOFT, Melbourne, Australia, 2003. V. Balkunje, A. Holland, A. Mitchell, Fabrication of Polymer rib Waveguides Using SU-8, Presented at ACOFT, Canberra, 2004. J.C. Dainty, Optical waves in crystals, in: A. Yariv, P. Yeh (Eds.), Optics & Laser Technology, Wiley, 1984, pp. xi + 589, UK pound 47.45, vol. 17, 217–218, 1985. D. Sadot, E. Boimovich, Tunable optical filters for dense WDM networks, IEEE Commun. Mag. 36 (1998) 50–55. W.P.O. Juptner, Integrated optics—theory and technology, in: R.G. Hunsperger (Ed.), fifth ed., Advanced Texts in Physics, University of Delaware, Newark, DE, USA, Springer, Berlin, 2002, p. 446, ISBN No.: 3-540-43341-4 (Hardback), UK pound 56.00, Optics and Lasers in Engineering, vol. 42, 375–376, 2004. A. Mitchell, M.W. Austin, P.J. Edwards, L.N. Binh, L.A. Bui, S. Mansoori, W.J. Lai, Ultra-broadband shortwave polarization modulators for high-speed free-space quantum cryptography, Presented at Quantum Communications and Quantum Imaging, San Diego, CA, USA, 2004. T.U.a.B.T. Shunji Seki, Reactive ion etching of tantalum pentoxide, J. Electrochem. Soc. 130 (1983) 2505–2506. Y. Kuo, Reactive ion etching of sputter deposited tantalum oxide and its etch sensitivity to tantalum, J. Electrochem. Soc. 139 (1992) 579–583. F.-X. Jian, S.K. Kurinec, Tantalum oxide thin films for microelectronic applications, Proceedings of the Eleventh Biennial 1995, Presented at University/Government/Industry Microelectronics Symposium, 1995. M.J. Chuang, K.Y. Hsieh, High-temperature wet chemical etching of Ta2O5 in NaOH-based solutions for fabricating antiresonant reflecting optical waveguides, J. Electrochem. Soc. 145 (1998) 1020–1022. W.H.T. Perrin Walker, CRC Handbook of Metal Etchants, CRC Press, 1991. B.J.C.M. Hatzakis, J.M. Shaw, Single-step optical lift-off process, IBM J. Res. Dev. 24 (1980) 452–460. R.E. Lee, Microfabrication by ion-beam etching, J. Vac. Sci. Technol. 16 (1979) 164–170.