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Abstract
We report the local crystallization of Y3Fe5O12 (YIG) thin films grown on Si substrates, and SOI waveguides by CO2 laser annealing (LA). The effect of laser power and oxygen pressure on the crystal structure of YIG films was systematically studied. Laser power dominated the YIG film crystallinity, while oxygen partial pressure during LA strongly influenced the crystal grain size and magnetic anisotropy. Fully crystallized YIG thin films with pure garnet phases were fabricated by LA. The refractive index n and extinction coefficient k were comparable to thin films fabricated by rapid thermal annealing (RTA). Propagation loss measured at 1550 nm wavelength on YIG/SOI waveguides and YIG/SiN ring resonators were comparable to RTA annealed films, promising device development for silicon photonics.
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1. Introduction
Yttrium iron garnet (YIG) and rare earth iron garnet (RIG) thin films are actively studied magneto-optical (MO) materials for nonreciprocal photonic devices in silicon photonic integrated circuits (PICs) [1,2]. Despite of progresses in growth of polycrystalline garnet thin films on silicon [3–6], a significant challenge remains in process integration due to the high thermal budget required for garnet crystallization [1,7–10]. Typically, crystallization of YIG films on silicon require annealing temperature over 700 °C, which well exceeds the highest temperature of 500 °C allowable in the BEOL process, making such materials inaccessible to complex silicon PICs. Although some studies have integrated magneto-optical materials into silicon photonic devices using wafer bonding methods, the relatively large die size may show disadvantage in large scale integration [11–14].
Thanks to highly localized energy profile, laser annealing (LA) allows targeted heat treatment of specific areas without affecting adjacent structures [15–19]. Laser annealing has been widely used in semiconductor industry such as crystallization of amorphous Si, Ge and ITO thin films, dopant-activation in Si and SiC materials, ohmic contact formation between metal and semiconductors and so on [20–24]. Recently, researchers started to use LA for garnet thin film crystallization. Gage et al. successfully crystalized polycrystalline YIG on SiO2 substrate deposited by RF reactive sputtering using a 343 nm wavelength laser. The films crystallized to the YIG phase, as observed in-situ under transmission electron microscopy (TEM) [7]. Sgibnev et al. crystallized Bi:YIG thin films deposited by metal–organic decomposition using 532 nm femtosecond laser pulses in air. The films showed Faraday rotation angles comparable to that of thermally annealed samples [25]. Hibiki et al. successfully crystallized Ce:YIG thin films on SiO2 substrate in vacuum using a 532 nm wavelength laser. The sample showed -0.04 deg/µm Faraday rotation angle at 1550 nm, which were however ∼62% lower compared to the thermally annealed samples [26].Although previous reports demonstrated the feasibility of garnet crystallization by LA, these studies predominantly employed short-wavelength lasers in the ultraviolet or visible. The optical field and temperature distributions may be influenced by the photonic device microstructures. Meanwhile, the laser absorption was inside the garnet films. The optical absorption and temperature profile strongly depends on the thickness of the garnet films. So far, there has been no report on device performance based on laser annealed garnet films.
In this study, we report YIG thin film crystallization using a CO2 laser source with the wavelength of 10.6 µm. The YIG thin films were indirectly annealed by CO2 laser absorption in the underlying SiO2 layer. SiO2, which is widely used as the cladding layers in silicon photonics, shows strong absorption at around 10 µm due to optical-phonon resonances, perfectly aligning with CO2 laser wavelengths. This approach mitigates issues related to uneven energy absorption caused by the thickness of YIG films and perturbation photonic device microstructures. We characterized the magnetic and optical properties of YIG films subjected to different laser powers and oxygen partial pressures. We further measured the propagation loss of the laser annealed YIG (LA-YIG) films on both straight YIG/SOI waveguides and SiN micro-ring resonators. Comparable crystallinity, magnetic, optical properties and propagation loss of LA-YIG to rapid thermal annealed YIG (RTA-YIG) films validated CO2 LA as a promising method for local crystallization of YIG on silicon photonic devices.
2. Simulation and experimental details
We employed magnetron sputtering to grow YIG films on Si substrates with a 2 µm thick SiO2 layer at room temperature. Several YIG films were also grown on silicon-on-Insulator (SOI) waveguide devices and SiN micro-ring resonators. The thickness of the YIG film was 50 nm. The cross-section of the film was depicted in Fig. 1(b). Ceramic YIG targets were prepared using a solid-state reaction method with 99.999% pure Y2O3 and Fe2O3 powders. YIG was deposited using radiofrequency (RF) magnetron sputtering with 90 W sputtering power and 0.5 Pa argon ambient at room temperature. During sputtering, the substrate was rotated at a speed of 30 rpm. The deposition rate of was 0.55 nm/min.
Fig. 1. (a) Schematic diagram of the laser annealing setup. (b) Cross-sectional schematic of the sample. (c) Scanning trajectory of the laser on the chip.
Laser annealing of the deposited YIG thin film was performed using a custom-built laser annealing apparatus, as depicted in Fig. 1(a). During LA, different oxygen partial pressures (PO2) were introduced into the chamber. The sample was kept at 200 °C using a heating apparatus within the chamber to prevent excessive thermal shock. A quasi-continuous CO2 laser with maximum power of 30 W was used for YIG crystallization. The output power was adjusted by modulating the duty cycle of the laser pulses. The laser beam, initially Gaussian, passed through a beam expander and a beam homogenizer to transform into flat-top distribution. The homogenized laser spot then passed through a ZnSe focusing lens and projected onto the sample surface as a square spot of 2 × 2 mm area. The laser spot was then scanned back and forth along the x-axis using a stepper motor for 10 cycles. After completion, it was ascended by 1 mm along the y-axis to continue the scanning process across the whole 1 cm × 1 cm sample. The trajectory of the beam movement was depicted in Fig. 1(c). In this study, scanning speed of 5 mm/s was used to achieve short heat treatment time.
The laser annealing process was simulated using COMSOL Multiphysics. The thermophysical parameters and optical constants for SiO2 and Si were adopted from Refs [27–29]. The optical constants for YIG were obtained from our previous report [30]. Given the relatively thin SiO2 and YIG layers, a thin-layer approximation was applied for the heat transfer process. Thermal flux boundary conditions were utilized to simulate the laser absorption process. As the sample was located within a vacuum chamber, the impact of convective heat was neglected. Considering the small contact area at the edges of our thin film with the fixture, a thermal insulation condition around the edges was applied.
3. Simulation results of the CO2 laser annealing process
We first simulated the laser absorption process. Because Si and YIG layers exhibit much smaller absorption coefficients compared to SiO2 at 10.6 µm wavelength, for a given SiO2 layer thickness, the YIG layer and Si structure on the sample surface do not significantly affect the temperature distribution in YIG. As shown in Fig. 2(a) and (b), under the same incident conditions, there was only 4 °C difference of temperature in YIG at the steady state, for samples with and without SOI waveguide structures. The temperature distributed evenly across the YIG film thickness, thanks to the indirectly heating process.
Fig. 2. The temperature distribution at the sample cross-section when the incident laser wavelength is 10.6 µm for (a) a YIG/SiO2 sample and (b) a YIG/SOI sample. Temperature distribution of the YIG film surface when (c) a 2 mm × 2 mm laser spot or (d)a 50 µm × 50 µm laser spot is used. (e) Variation of the temperature at the film center with 9 W CO2 laser power and 2 mm × 2 mm laser spot size during scanning. (f) Variation of the temperature at the films center with different incident powers with 2 mm × 2 mm laser spot size.
The temperature distribution on the YIG film surface during laser scanning was simulated as shown in Fig. 2(c) and 2(d) for laser spot sizes of 2 mm × 2 mm and 50 µm × 50 µm respectively. As shown in the inset of Fig. 2(d), controlling the laser spot size to 50 µm × 50 µm can significantly reduce the >500 °C temperature region within 200 µm diameter range. For convenience of material characterization, we used 2 mm × 2 mm laser spot size in experiments. Figure 2(e) illustrates the temperature at the YIG film surface at the center of the sample (1 cm × 1 cm in size) during repetitive scans with 9 W laser power. The temperature at this point rises and falls around 800 °C during laser scan, resembling a rapid thermal annealing process around 800 °C [7]. Since the spot size used is relatively large in this study, the temperature of the entire 1 cm × 1 cm sample during annealing exceeds 500 °C. The annealing temperature of the sample can be controlled by setting different laser powers, resulting in different annealing temperatures, as shown in Fig. 2(f).
4. Experiment results and discussion
To ensure crystallization of the YIG film under laser irradiation, we first conducted laser annealing of the YIG in vacuum conditions using different laser powers, and then characterized its structure and magnetic properties. Figure 3(a) shows the XRD pattern of LA-YIG films under 0 Torr oxygen partial pressure and different laser powers. By increasing laser power, the YIG film gradually crystallized from amorphous to crystalline state. At 6 W, only the diffraction peak of the Si substrate appeared. However, at 7.5 W and higher, three diffraction peaks corresponding to the (400), (420), and (422) crystal planes of the YIG films emerged. Figure 3(b) shows that the lattice constant of the YIG thin films after LA at different powers did not show significant changes. The lattice constants were slightly larger than our RTA samples (12.297 Å). This could be attributed to the presence of more oxygen vacancies in YIG crystallized in an oxygen-deficient environment. The lattice constant was smaller than the reported value of 12.376 Å, which may be attributed to thermal mismatch between YIG film (10 × 10−6/K) and SiO2 substrate (0.47 × 10−6/K) [31,32], causing in-plane tensile stress and smaller out-of-plane lattice constants. AFM images in Fig. 3(c) and (d) depicted the morphology of LA-YIG films after annealing with different laser power. The film annealed at 6 W exhibited a non-crystalline state with a low RMS roughness (0.290 nm). At 7.5 W power shown in Fig. 3(c), YIG crystal nuclei with large crystal size of over 8 µm appeared, indicating a partially crystallized state. Large grains imply low nucleation density of YIG thin films. At 9 W power and above, according to our simulation results, the temperature of the YIG film exceeded 800°C. The AFM image revealed the appearance of fully crystallized YIG films with grain boundaries and large grain sizes as shown in Fig. 3(c). The central regions of these grains exhibited a radiating groove pattern. This grain surface morphology aligns closely with previous reports [33]. As the power further increased, the surface roughness of the films also increased. At 10.5 W and 12 W power, the RMS reached 1.14 nm and 1.37 nm, respectively. Therefore, to prevent excessive damage to the YIG film, the laser power should not be excessively high.
Fig. 3. (a) XRD patterns of annealed YIG film under different laser power. (b) Lattice constants of annealed YIG film under different laser power. (c) AFM image of YIG film after 25% power laser annealing, (d) AFM image of YIG film after 30% power laser annealing.
To verify whether the LA-YIG films under vacuum conditions were fully crystallized, room temperature magnetic hysteresis loops of YIG films annealed at different laser powers were measured by vibrating sample magnetometry (VSM). As shown in Fig. 4(a-e), the crystallized YIG films exhibited easy-plane magnetic anisotropy dominated by the shape anisotropy. As shown in Fig. 4(f), the saturation magnetization (Ms) of the YIG film increased with the laser power. For 9 W laser power, the YIG film reached the maximum Ms of 136.7 emu/cm3, close to the saturation magnetization of pure YIG material of 137.3 emu/cm3 [34]. This observation was consistent with the crystal morphology observed by AFM. The saturation magnetization slightly decreased to 134.5 emu/cm3 with increasing laser power up to 12 W. Meanwhile, the out-of-plane saturation magnetic field HS, as shown in Fig. 4(f), increased with increasing laser power as the film fully crystallized, while the in-plane saturation magnetic field remained almost unchanged.
Fig. 4. Room temperature magnetic hysteresis loops of YIG film after annealing at different powers, (a) 6 W, (b) 7.5 W, (c) 9 W, (d) 10.5 W, (e) 12 W, (f) variation of Ms and out-of-plane saturation field HS of LA-YIG films with power.
Although YIG films can crystallize under vacuum conditions, they typically require annealing in an oxygen environment to achieve low propagation loss on waveguide devices. Therefore, we conducted laser annealing on YIG films under different oxygen pressure conditions. To ensure complete crystallization of the YIG film and small surface roughness, a laser power of 9W was chosen. Figure 5(a) shows the XRD pattern of LA-YIG films under different oxygen partial pressures. All YIG films fully crystallized to the garnet phase, showing no significant differences in peak intensities. Figure 5(b) shows the lattice constants of the YIG thin films after laser annealing at different oxygen partial pressures. The YIG films exhibited slightly smaller lattice constants in an oxygen-rich environment, which may be attributed to the reduction of oxygen vacancies in the YIG thin films. Figure 5(c) shows the surface morphology of LA-YIG films at PO2 = 1 Torr, showing clear polycrystalline grains and grain boundaries. The RMS roughness of the sample was 0.518 nm. However, unlike LA-YIG films under 0 Torr oxygen partial pressure, the YIG film's grain size noticeably reduced under oxygen atmosphere, with overall size ranging from 1 to 3 µm. The crystal morphology resembled our RTA-YIG samples and previous reports [7], indicating oxygen rich ambient induced higher nucleation density of the YIG crystals. We did not observe any cracks by large area scans of AFM, SEM and optical microscope on the entire sample.
Fig. 5. (a) XRD patterns of annealed YIG film under different oxygen partial pressures. (b) Lattice constants of annealed YIG film under different oxygen partial pressures. (c) AFM surface morphology of a LA-YIG film annealed at PO2 of 1 Torr. (d) AFM surface morphology of an RTA-YIG film annealed under PO2 of 2 Torr.
The oxygen pressure during annealing may influence the magnetic properties and crystallinity of the YIG films, so room temperature magnetic hysteresis loops of LA-YIG films annealed under different oxygen partial pressures were measured, as shown in Fig. 6(a-e). The Ms was not significantly influenced by the oxygen partial pressure, as shown in Fig. 6(f). However, the magnetic anisotropy changed drastically, showing clear perpendicular anisotropy (PMA). It has been reported that perpendicular anisotropy in YIG may be induced by strain [35,36]. Thanks to the negative magnetic striction coefficient of polycrystalline YIG (λs = -2 ppm), the manifestation of PMA in LA-YIG films may be due to in-plane tensile strain caused by thermal expansion coefficient mismatch between YIG and SiO2. Comparably, the magnetic anisotropy of an RTA-YIG film with 60 seconds annealing time also showed perpendicular anisotropy as shown in Fig. 6(e). The out-of-plane saturation magnetic field HS of the YIG film decreased with increasing oxygen partial pressure, as shown in Fig. 6(f). These results indicate that the CO2 LA-YIG films underwent a similar crystallization process with comparable magnetic properties as RTA-YIG films.
Fig. 6. Room temperature magnetic hysteresis loops of LA-YIG film after annealing under different oxygen partial pressures. (a) 0.5 Torr, (b) 1 Torr, (c) 1.5 Torr, (d) 2 Torr. (e) Magnetic hysteresis loops of RTA-YIG film annealed under PO2 = 2 Torr for 60 seconds. (f) Variation of Ms and out-of-plane saturation field Hs of LA-YIG films with different oxygen partial pressures.
The refractive index and extinction coefficient of YIG thin films are key parameters in the design of magneto-optical waveguide devices, so we measured the refractive index and extinction coefficient of the YIG films annealed under different oxygen pressures. Since the Faraday rotation angle of the YIG film at 1550 nm is relatively low, we used a laser with a wavelength of 633 nm to measure the polar magneto-optical Kerr effect of the LA-YIG films. The incident angle of the laser during the measuring process was 72 degrees. Figure 7(a) and 7(b) show the refractive indices and extinction coefficients of LA-YIG films under different PO2, compared with an RTA-YIG films annealed under 850 °C and PO2 = 2 Torr for 5 min. The optical constants of LA-YIG are similar compared to RTA films. The refractive indices were slightly higher than RTA-YIG films. The polar Kerr rotation of LA-YIG films increases with increasing oxygen pressure, as shown in Fig. 7(c). In an oxygen-rich environment, LA-YIG films exhibit larger polar Kerr rotation angle than that of RTA-YIG films.
Fig. 7. (a) Refractive index and (b) extinction coefficient of YIG films under different oxygen pressure annealed using LA and RTA. (c) The polar Kerr rotation of LA-YIG films under different oxygen pressures and the magneto-optical Kerr hysteresis loops of LA-YIG, RTA-YIG, and unannealed YIG films at 633 nm. (d) Optical microscope image of the SOI waveguides used for loss characterization. (e) Optical microscope image of the micro-ring resonator used for loss characterization. (f) The |Hx| field distribution of the fundamental TM0 mode of a YIG/SOI waveguide and transmission loss of YIG at different oxygen pressure measured on SOI waveguides. (g) Transmission spectra of micro-ring resonator with YIG films annealed by laser under oxygen pressure of 1 Torr.
To assess the propagation loss of LA-YIG films, we deposited 50 nm YIG on a SOI channel waveguide with width and height of 3.5 µm and 100 nm, respectively. The device was annealed with 9 W laser power for 145.2 seconds. The optical microscope image of the straight waveguide is shown in Fig. 7(d). An index matching fluid with n = 1.45 was coated on the waveguide as the top cladding. We employed the cut-back method to evaluate the waveguide propagation loss for the fundamental transverse magnetic (TM0) mode. The cross-section of the straight waveguide and the distribution of the Hx field are illustrated in Fig. 7(f). The confinement factors and ${\mathrm{\Gamma }_{YIG}}$ for the Si and YIG layers were simulated based on the measured LA-YIG optical constants as 4.33% and 5.35% respectively. The waveguide propagation loss can be expressed as:
5. Conclusions
In summary, we successfully crystallized YIG thin films deposited on oxidized silicon substrates using laser annealing with a 10.6 µm wavelength CO2 laser. The films crystallized to pure garnet phase when incident laser power exceeded 9 W. When annealed in vacuum, LA-YIG films showed large grains with average diameter of 8 µm and easy-plane magnetic anisotropy. When annealed in oxygen, the LA-YIG films showed much smaller grains with average diameter of 2 µm and perpendicular magnetic anisotropy, comparable to an RTA-YIG sample. The LA-YIG films showed comparable optical constants to RTA-YIG films. The propagation loss of LA-YIG films measured on SOI waveguides and SiN micro-ring resonators were 89.7 dB/cm and 80.99 dB/cm, respectively, also comparable to RTA-YIG films. These results demonstrate promising potential of CO2 laser annealing for the development of integrated magneto- photonic devices.
Funding
Ministry of Science and Technology of the People's Republic of China (2021YFB2801600); National Natural Science Foundation of China (51972044, 52021001, 52102357, U22A20148); Science and Technology Department of Sichuan Province (99203070).
Disclosures
The authors declare that there are no conflicts of interest related to this article.
Data availability
Data underlying the results presented in this paper are not publicly available at this time but may be obtained from the authors upon reasonable request.
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