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We report on the fabrication and characterization of ridge waveguides in lithium niobate thin films by diamond blade dicing. The lithium niobate thin films with a thickness of 1 µm were fabricated by bonding a He-implanted lithium niobate wafer to a SiO2-coated lithium niobate wafer and crystal ion slicing. Propagation losses of 1.2 dB/cm for TE and 2.8 dB/cm for TM polarization were measured at 1550 nm for a 9.28 mm long and 2.1 µm wide waveguide using the Fabry-Perot method.
© 2016 Optical Society of America
1. Introduction
Since its first demonstration [1] single crystal lithium niobate (LiNbO3) thin films have found several applications and are a promising technology for miniaturization and integration of components for optical signal processing [2]. Lithium niobate films of micrometer or sub-micrometer thickness bonded to a lower refractive index material, typically SiO2 [3] or the polymer BCB [4], are frequently referred to as lithium niobate on insulator (LNOI), following the term of silicon on insulator (SOI). As in SOI, a high contrast in refractive index between the LiNbO3 and the layer below allows structuring of optical waveguides with very small cross sections (<1 µm2) and bending radii (<10 µm). Therefore, LNOI technology enables the development of highly efficient components such as nonlinear frequency converters (where efficiency scales inversely with the cross section) or micro-disc resonators [5].
In order to spatially structure LNOI and shape waveguides or components into planar LNOI thin films, several methods were introduced. Here, milling waveguides into LNOI, in comparison to proton exchange [6] or strip loaded waveguides [7], provides a higher index contrast between core and cladding and thus allows for superior mode confinement. Most waveguides in LNOI so far were fabricated by dry etching using a patterned metal or SiO2 film as a mask [8]. Also implantation-enhanced wet etching [9], focused ion beam (FIB) milling [10] or laser micromachining [11] were used to fabricate ridge-type waveguides. Dry-etched waveguides in lithium niobate can show losses as low as 0.5 dB/cm [12]. However, up to now dry-etched LNOI ridge waveguides with cross sections of ~1 µm2 show rather high propagation losses of 6–13 dB/cm [2]. This behavior may be qualitatively explained by larger sidewall roughness (when compared to non-implanted LiNbO3 substrates) that is due to increased etching susceptibility after implantation as well as by the fact that modes of LNOI ridges are more sensitive to surface roughness.
On the other hand, the method of precision diamond blade dicing has already proven its feasibility for fabrication of ridge waveguides in LiNbO3 with smooth sidewalls and low optical attenuation of about 1 dB/cm [13]. However, as this method implies significant mechanical stress on the bonded layer, application of dicing to LNOI requires overcoming issues related to adhesion of the bonded layer. Here we present our results on successful fabrication of narrow ridge waveguides in LNOI samples with improved adhesion using precision diamond blade dicing. This technique results in smooth side walls with very low surface roughness allowing for attenuation coefficients as low as 1.2 dB/cm.
2. Experiments
Figure 1 depicts the fabrication process of LNOI films using the smart cut method and precision optical grade dicing of ridge waveguides. The LNOI samples were fabricated by bonding a first He-implanted z-cut LiNbO3 wafer to another z-cut wafer covered with a SiO2 layer, and subsequent crystal ion slicing. First, a dose of 4 × 1016 ions/cm2 of He+ ions was implanted into the -z face of the first LiNbO3 wafer [14]. Energy of 350 keV was chosen which resulted in a damaged layer at a depth of about 1 µm. To prevent bubble formation during bonding, which occurs due to remaining gas in the evaporated SiO2 layer on the second LiNbO3 wafer, grooves were diced into the implanted wafer [15]. These grooves, cut with a wafer saw (Disco DAD322), were typically separated by 50 µm.
Fig. 1 Schematic of sample fabrication: a) He+ implantation (E = 350 kV, 4 × 1016 ions/cm2); b) preparation of grooves for gas out-diffusion during wafer bonding; c) electron beam evaporation of SiO2; d) wafer bonding; e) thermal treatment for split-off (5 K/min, 6 h at 270 °C); f) ridge fabrication.
Next, a 1.6 µm thick SiO2 layer, serving as the optical barrier, was deposited onto the + z face of a second wafer with an electron beam evaporator. It turned out that adhesion of the different layers of the LNOI structure is crucial for the subsequent dicing process. Therefore, different adhesion promoting layers between the lower substrate and the SiO2 layer were tested, using Cr alone or in combination with Ti, as well as different substrate temperatures during evaporation. Best results were achieved for a combination of 5 nm of Ti and 1.5 nm of Cr using a substrate temperature of 200 °C. After deposition the wafer was annealed at 450 °C for 6 h and then underwent a chemical-mechanical polishing (CMP) with Ultra-Sol 500S for 10 min to reduce surface roughness to typical values of 0.4 nm (rms). The final SiO2 thickness was about 1.4 μm.
The standard size of waveguide samples was 1 × 1 cm2. These wafer pieces were cleaned and activated in oxygen plasma for 300 s at a pressure of 9 × 10−2 Torr before they were pre-bonded. Ion slicing was performed then by heating the bonded sandwich in an oven to 270 °C with a ramp of 5 K/min and keeping at this temperature for 6 h. After cooling down slowly, the implanted wafer had split at the implantation depth, and 50 µm wide stripes of 1 µm thick single-crystalline LiNbO3 thin film were transferred onto the SiO2/LiNbO3 substrate wafer. By polishing with the above named agent the roughness could be typically reduced from initial values of ~2 nm down to ~0.6 nm (rms). Finally, we cut ridges with widths ranging from 2 µm to 10 µm and heights from 1 µm to 20 µm into these LNOI stripes to form waveguides. We used a 100 µm thick resin-bonded blade at 20,000 rpm and a cutting speed of 0.1 mm/s. These conditions had turned out to minimize the chipping, observed at the edges of the ridges. Similarly, the end faces were prepared for fiber-to-waveguide coupling. A SEM image of a completed and functional sample is displayed in Fig. 2(b). In contrast the sample in Fig. 2(a) exhibited delamination of the SiO2 layer from the substrate during dicing. This demonstrates the necessity of sufficient adhesion to enable dicing of LNOI.
Fig. 2 a) Microscope image of a LNOI film with a SiO2 film directly evaporated onto the lower LiNbO3 substrate, see Fig. 1(c). b) SEM image of successfully diced LNOI ridges where adhesion has been improved by an additional Ti/Cr intermediate layer.
It was found that deep cut ridges, although having better verticality of their sidewalls, were more prone to cracking and exhibited a more pronounced tilt of their end faces. We believe that the shallow cut waveguides receive better support from the substrate than deep cut ones and are therefore better protected against rounding or cracking during end face polishing. Using a white light interferometer a tilt of about 10° for the deep diced waveguides’ end faces was estimated, while the tilt for shallow cut waveguides is <2°. Such a tilt makes the waveguide loss measurement by the Fabry-Perot method inaccurate, as well as represents a significant disadvantage for most applications. Electron microscope (SEM) images of a shallow cut and a deep cut waveguide are shown in Fig. 3. The deeper cut one exhibits a quite rectangular profile with high verticality, while the other one has a rather trapezoidal shape.
Fig. 3 SEM images: a) shallow and b) deep diced waveguide with end face preparation by a shallow cut. The insets show measured (top) and calculated (bottom) fundamental TE modes.
3. Characterization
As a light source for loss measurements we used a fiber-coupled swept-wavelength tunable laser (New Focus Venturi TLB6600H-CL) with a tuning range of 1520–1630 nm. The laser light was coupled into the sample with a lensed fiber (Nanonics) with a spot diameter of 1.7 µm and a working distance of 4 µm while the polarization was controlled with a paddle polarization controller. To position the lensed fiber to optimal in-coupling, we used a 3-axis piezo auto-align positioning system (Thorlabs Nanomax MAX341). The output light of the waveguide was then collected by an anti-reflection-coated lens (NA = 0.68, working distance 1.7 mm) and imaged through a polarizer and a beam splitter onto an IR camera and a calibrated photodetector.
In Fig. 3 measured intensity distributions of the fundamental TE modes are shown (top inset) and compared to the calculated ones (lower inset). For taking these mode images a 100 × objective and immersion oil were used. It can be seen that the mode of the shallow cut waveguide does not extend much into the flanks.
Figure 4 shows the normalized transmission through a waveguide over the swept wavelength. From the contrast K = (Imax – Imin) /(Imax + Imin) of this curve the attenuation of the waveguide can be calculated using the Fabry-Perot method [16]:
Here L denotes the sample’s length. The reflectivity R at the end faces of LNOI waveguides strongly deviates from the bulk Fresnel reflectivity of 14 % because of the small cross section. Therefore, to find this parameter, FDTD simulations [17] were conducted for different ridge widths and polarizations. Results are given in Fig. 5(a). Furthermore, we performed simulations on the influence of the end face tilt angles θ1,2 , see Fig. 5(b). As displayed in Fig. 5(c), especially for TE polarization, small tilt angles θ1 of a few degrees result in significant reflectivity changes. On the other hand, for the typically achieved precision in dicing rectangular end faces in the lateral direction with θ2 = 90° ± 0.1°, no noticeable influence on the reflectivity is found (see Fig. 5(d)).Fig. 5 a) Simulation results for the end face reflectivity of 1 µm high LNOI waveguides; b) definition of angles θ1,2; c) and d) influence of the end face tilt on the reflectivity for a 2 µm wide waveguide.
For a shallow cut waveguide with a height of 1 µm, an effective width of 2.1 µm (top width: 0.7 µm) and a precisely measured length of 9.28 mm we determined attenuation values of 1.2 dB/cm for the TE and 2.8 dB/cm for the TM mode. The effective width was measured at half height of the lithium niobate thin film. Optical attenuation coefficients of other selected waveguides are given in Table 1. When wider waveguides were checked, only quite low contrast could be found. This can be understood due to the superposition of several excited modes as rectangular ridges are single mode for widths between 0.6 and 1.3 µm [16,18]. Also for the deeper cut waveguides, which exhibited quite rectangular cross sections, we could only measure curves with low contrast. We believe that this is a consequence of the stronger tilt of the end faces which goes along with lower effective reflectivity. Therefore we could not quantify low attenuations for deep diced waveguides or for wide multimode ones. We have to note that also the shallow cut waveguides’ end faces may have a slight tilt or other imperfections which could reduce reflectivity. This would lower the contrast in the measurements and result in an overestimated attenuation [16,18]. Hence, the attenuation values in Table 1 only mark an upper limit. The insertion losses for a shallow cut waveguide of effective width 2.1 µm were determined to be 6.2 dB (8.7 dB) for TE (TM) polarization and for a deep cut one of width 2 µm we measured 7.9 dB (9.6 dB). These values are not corrected for the limited NA of the objective lens.
Table 1. Results of Fabry-Perot loss measurements for 9.28 mm long waveguides
To discuss the accuracy of loss measurements we have to take into account that the width of the waveguides was measured with an optical microscope with an accuracy Δx ≈0.5 µm which goes along with inexact determination of the reflection coefficient ΔR ≈1 %. For the shallow cut waveguides the end faces have a tilt <2° which leads to an error ΔR ≈0.3 %. Also the measured Fabry-Perot spectra showed slight beats that cause an inaccuracy of the measured contrast of ~0.01. We therefore estimate the error of the above mentioned measurements to be ± 0.5 dB/cm.
4. Conclusion
In conclusion, we have successfully fabricated ridge waveguides in lithium niobate on insulator (LNOI) using diamond blade dicing. To prevent delamination of the thin film layers from the substrate during dicing, appropriate fabrication parameters were found. The fabricated ridge waveguides with a length of ~1 cm show low optical attenuation of only ~1.2 dB/cm. This is clearly superior to former results on LNOI waveguides with similar cross sections of ~1 µm2 obtained by e.g. proton exchange and FIB milling (where lengths were limited to only ~0.8 mm [6,10]), dry etching (losses of 6-13 dB/cm [2]) or dicing of a rather thick (3 μm) LNOI film (showing losses >10 dB/cm [13]). No significant degradation due to the ion implantation process or roughness of sidewalls is observed.
The fabricated waveguides confine the optical mode to an area two orders of magnitude smaller than that of proton exchanged or Ti in-diffused channel waveguides. Beside their potential use for highly-efficient nonlinear frequency conversion they are also promising candidates for the development of ultra-low-voltage electro-optic modulators which may benefit from both small waveguides widths (electrodes’ separation) and increased opportunities for group velocity matching in high frequency applications.
Acknowledgments
We would like to thank H. Hu from Shandong University, Jinan, P.R. China for helpful discussions on the bonding process. We acknowledge T. Breckwoldt from Helmut Schmidt University, Hamburg for the SEM examination. This work was financially supported by the Deutsche Forschungsgemeinschaft (DFG, grant Ki482/15-1).
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