We design and fabricate pedestal-type, 15 μm diameter silicon nitride microdisk resonators on Si chip with horizontal air-slot using selective wet etching between Si, SiO2, and SiNx. As the slot structure is determined by deposition process, air slots that are as thin as 40 nm and as deep as 5 μm with ultra-smooth slot surfaces can easily be fabricated with photolithography only. Fundamental TM-like slot mode in which the E-field is greatly enhanced within slot was observed with an intrinsic Q factor of ~34,000 (λres = 1523.7nm) and energy overlap in slot region of 21.6%.
© 2010 OSA
Light confinement in nm-size region is essential for highly sensitive and compact devices for optoelectronics. Owing to this reason, slot-structures which consist of low refractive index region sandwiched with high refractive index material have attracted great attention . At the slot boundary, the magnitude of electric field which is polarized normal to the slot is enhanced with a ratio of dielectric constant to satisfy the continuity of electric displacement. If the width of slot is thin enough, and the index contrast large enough, we can obtain large electric field intensity within sub-wavelength thin region. By now, several applications such as sensing , optical modulation using enhanced nonlinear effect [3,4], and optical manipulation were reported using waveguides and resonators based on slot-structures [5,6].
These works utilized vertical-slot structures whose slot consisted of a narrow, vertical trench in a high-index waveguide. Such a structure has the advantage of an open slot into which functional materials can easily be introduced for enhanced photon-matter interaction confined in a sub-wavelength region. On the other hand, due to the difficulty of fabricating nm-wide, high aspect-ratio trench, the slot width tends to be in the range of ~100nm, with nm-sized sidewall roughness at the slot boundary that can lead to very high scattering losses .
Thus, horizontal-slot structures whose slot consists of a low-index layer in a multilayer thin film have been proposed as an alternative [8–12]. In this case, multiple ultra-thin slots can be formed just as easily as a single slot through deposition rather than etching [8,10]. Furthermore, as the slot boundaries are defined by deposition, precise control over the width of the slots with minimal interfacial scattering can be achieved easily as well. On the other hand, such horizontal slot-structures sacrifice one key advantage of the slot structure – the ability to introduce arbitrary materials of interest into the slot where E-field is greatly enhanced – due to the need to vertically support the multiple layers. In this paper, we design and fabricate pedestal-type, 15 μm diameter SiNx microdisk resonators on Si chip with horizontal air-slot formed by selective wet etching between Si, SiO2, and SiNx. As the slot structure is determined by deposition process, air slots that are as thin as 40 nm and as deep as 5 μm with ultra-smooth slot surfaces can easily be fabricated into the resonator sidewall without electron beam lithography. As the depth of the air slot region is larger than the width of the whispering gallery mode, the resonator effectively acts as two SiNx layers suspended in air and separated by a 40 nm thin layer of air. Fundamental TM-like slot mode in which the E-field is greatly enhanced within slot was observed with an intrinsic Q factor of ~34,000 at the resonance wavelength 1523.7nm and energy overlap in slot region of 21.6%.
2. Device fabrication
A 255nm-thin SiNx / 40nm-thin SiO2/ 255nm-thin SiNx multilayer thin film was deposited on a silicon substrate using reactive ion beam sputtering method with Ar, N2 and O2 gas flow. After deposition, the sample was annealed at 800°C during 30min in a flowing Ar environment to remove defects and increase film quality. Silicon nitride was chosen for the high-index layer due to its chemical and mechanical robustness that allow for compact micro resonators with intrinsic Q-factor of 25,000 near the 1.5 μm region . For PECVD-deposited, stoichiometric SiN, propagation loss of 2.2 ± 0.4 dB/cm had been reported . The Si content in the SiNx layers were increased beyond the stoichiometric value in order to increase the refractive index of the nitride layers, which was obtained to be as high as 2.308 at 1550nm using ellipsometry (data not shown). Finally, a ~600 nm thick amorphous Si layer was deposited on top to serve as the etch mask.
After the thin film deposition, 15 μm diameter disks were patterned using photolithography and dry etching. Pedestal-type disk resonators were then fabricated by using a KOH solution heated to 60°C to selectively etch the Si substrate and undercut the multilayer disk, leaving a Si center post that is more than 3 μm high. This also removes the remaining a-Si hardmask at the same time. The air-slot region was then formed by etching silicon oxide spacer layer with buffered hydrofluoric acid (BHF) from the disk edge to a depth of ~5 μm. Finally, the fabricated disks were dried using CO2 critical point drying method to avoid capillary-force driven collapse of the air-slot during the drying process.
The optical properties of the resonators were measured using a U-bent tapered fiber  with 1.5 μm diameter mounted to a 3-axis picomotor translator. Light from a tunable laser (1475 nm~1545 nm) with linewidth <300 kHz and controlled polarization was then coupled into and out of the resonator via the tapered fiber, and the transmitted intensity was monitored using a powermeter. In all cases, the coupling fiber was in a light contact with the microresonator sidewall, resulting in overcoupling condition. The input polarization was controlled with a fiber polarization controller, and excitation of TE- and TM-like modes were determined by comparing the obtained transmission spectra with simulated results. More detailed description of the measurement setup can be found in Ref .
3. Results and discussion
Figure 1(a) shows a scanning electron microscope (SEM) image of the multilayer film prior to disk formation. The two 255-nm thick SiNx layers and the 40-nm thin SiO2 spacer layer are clearly visible. The SEM image of an actual slot resonator after CO2 critical point drying is shown in Fig. 2(b) . The inset shows the edge of the resonator in more detail. We can clearly see that the SiO2 spacer layer has been etched away along the edge of the disk, leaving a 40 nm air slot around the circumference of the disk. Note, however, that the sidewalls are not quite vertical due to the use of photolithography in defining the disk patterns. The air slot structure is shown in more detail in Fig. 1(c) that shows the cross-section transmission electron microscope (TEM) image of the fabricated air-slot microdisk. The outer edges of the disk stick together during the sample preparation process. Still, the air-gap is clearly seen to extend 5.3 μm into the disk. The slope of the sidewall of the disk is determined from the TEM image to be ~10°. Finally, Fig. 1(d) shows the overall optical microscope image of the fabricated disk. The dark gray and blue circles in the center indicate the Si post.
Figure 2 shows the effects of remaining spacer layer in the center and the non-vertical slope of the resonator sidewall on the performance of the air-slot resonators, as investigated using finite-difference time-domain (FDTD) simulations. As shown in Fig. 2(a)-(c), we find that a slot-depth of only 3.5 μm is sufficient to diminish the effect of the center spacer region on the fundamental whispering gallery mode of the resonator, with same peak positions and mode numbers. The effect of 10° slope of the sidewall is shown in Fig. 2(d)-(e). The mode shifts slightly inward and upward, the overall E-field distribution remains largely unchanged, with radiation-limited Q-factor of >107. The resonance positions shift with the changes in the sidewall, while the free-spectral range (FSR) remains nearly the same at 23.2 nm. We note that because Figs. 2(c) and (e) were obtained by Fourier-transform of |E(t)|2. Thus, the wide linewidth is due to the limited number of data points, not due to low Q-factor . The Q-factor of the resonant modes, as determined by the decay time of the excited resonance modes, were > 107 in all cases.
As such, the resonator provides an electric energy overlap defined as below:
For a vertical edge, the energy overlap in slot region was calculated as 23.4%. In case of 10° sloped side-wall, the overlap factor in slot region was decreased to 21.6% due to lifting of the mode profile. However, this is still a much higher value than can be achieved by a conventional, single-disk resonator within same area.
The transmission spectra of the resonators, both before and after the air-slot formation, are shown in Fig. 3 . We observe sharp dips in transmission, indicating the excitation of resonance modes. In case of the TE-like mode, the FSR remains largely unchanged upon air slot formation, increasing from 22.0 nm to 22.1 nm only. In the case of TM-like mode, the FSR increases strongly from 21.4nmto 23.1nm upon air-slot formation corresponding to a modal index of only 1.51. Furthermore, the resulting resonant positions agree very well with calculated positions of TM-like slot modes using the resonator structure as determined by TEM (indicated by red arrow), indicating that we have indeed excited the fundamental TM-like slot modes.
The Q-factor, on the other hand, does not change significantly after the air-slot formation. The measured Q factor of the TM-like slot mode was 2230 (λres,oxide = 1524.6nm) and 2240 (λres = 1523.7nm) before and after the air-slot formation, respectively. These values are much lower than the values obtained in Fig. 2. The main source of optical loss, we believe, is the coupling fiber that is in contact with the resonator. In fact, direct simulation of a slot resonator in direct contact with a 1.5μm-diameter tapered fiber has shown that the Q-factor decreases to ~4,500, in agreement with the experimentally observed results (data not shown). The intrinsic cavity loss (1/τcavity) independent of the fiber coupling loss (1/τfiber) was calculated with the coupled mode theory . Assuming that we are in the over-coupled regime, the intrinsic cavity Q factors were obtained using the following equations to be 37,000 and 34,000 before and after the air-slot formation, respectively.
The intrinsic Q-factor of 34,000 agrees well with previously reported values for comparable SiNx microdisk resonators , and is comparable to the value reported from a vertical air-slot, SOI ring resonator coupled with gap-controlled bus waveguide fabricated using e-beam lithography . This indicates that the air-slot structure does not introduce any excess optical loss, and that higher Q-factors could be achieved with better fabrication and fiber coupling conditions [8,18]. More importantly, the fact that the Q-factor does not degrade significantly upon air-slot formation indicates that the scattering from the disk surfaces is negligible. In fact, as the image of atomic force microscopy (AFM) in Fig. 4(b) shows, the RMS roughness of the top surface of the bottom disk after removing the top disk through completely etching away the SiO2 spacer layer is 0.51nm only, which is about seven times lower than the reported line edge roughness of optimized silicon waveguide .
It should be pointed out that the E-field distribution shown in Fig. 2(d) is quite distinct from recent results involving double-disk structures that used coupled (bonding and anti-bonding) TE-like modes that are supported individually by top and bottom disks [17–19]. In fact, a single 255 nm thick SiNx disk by itself does not support a high-Q TM-like mode at all. As shown in Fig. 4(a), TE-like mode can still be excited in a single-disk fabricated by completely etching off the top SiNx disk, but not the TM-like mode. Indeed, FDTD simulation has shown that the Q-factor of a single disk coupled with a fiber is only ~150 (data not shown), which is too low to be detected. This, together with the increased FSR, low modal index, and good agreement between calculated and observed resonance wavelengths demonstrates that we have successfully excited the fundamental slot-mode where the top and bottom disks act together as a single resonator with an E-field that is greatly enhanced along the entire circumference of the disk.
In conclusion, we have designed and fabricated pedestal-type, 15 μm diameter SiNx microdisk resonators on Si chip with horizontal air-slot that ultra-smooth, are as thin as 40 nm, and reach as deep as 5 μm in from the circumference of the disk with selective wet etching process. As a consequence, the top and bottom disks act together as a single resonator with an air gap, with a corresponding fundamental, TM-like slot mode whose E-field is greatly enhanced along the entire circumference of the disk, with an energy overlap in slot region of 21.6%. The measured Q-factor for such slot-mode is ~2240 due to strong coupling loss. The calculated intrinsic Q-factor is 34,000, comparable with conventional, single-disk SiNx resonators and vertical-slot ring resonators fabricated using e-beam lithography. This combination of Q-factor with the large electric field concentration indicates a great potential of such air-slot disk structure for many applications such as sensing, non-linear optics, and nano-manipulation.
This work was supported in part by the Korea Science and Engineering Foundation (KOSEF), grant No. R01-2007-000-21036-0 of the Korea Science and Engineering Foundation (KOSEF), the Basic Science Research Program through NRF funded by MEST (2009-0087691), the Top Brand R&D program of MKE (09ZC1110: Basic Research for the Ubiquitous Lifecare Module Development), and KICOS (GRL, K20815000003). J. H. Shin acknowledges support by WCU (World Class University) program, grant No. (R31-2008-000-10071-0).
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