1Department of Chemical Engineering and Materials Science, University of Southern California, 2Department of Electrical Engineering-Electrophysics, University of Southern California
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Maker, A. J., Armani, A. M. Fabrication of Silica Ultra High Quality Factor Microresonators. J. Vis. Exp. (65), e4164, doi:10.3791/4164 (2012).
Whispering gallery resonant cavities confine light in circular orbits at their periphery.1-2 The photon storage lifetime in the cavity, quantified by the quality factor (Q) of the cavity, can be in excess of 500ns for cavities with Q factors above 100 million. As a result of their low material losses, silica microcavities have demonstrated some of the longest photon lifetimes to date1-2. Since a portion of the circulating light extends outside the resonator, these devices can also be used to probe the surroundings. This interaction has enabled numerous experiments in biology, such as single molecule biodetection and antibody-antigen kinetics, as well as discoveries in other fields, such as development of ultra-low-threshold microlasers, characterization of thin films, and cavity quantum electrodynamics studies.3-7
The two primary silica resonant cavity geometries are the microsphere and the microtoroid. Both devices rely on a carbon dioxide laser reflow step to achieve their ultra-high-Q factors (Q>100 million).1-2,8-9 However, there are several notable differences between the two structures. Silica microspheres are free-standing, supported by a single optical fiber, whereas silica microtoroids can be fabricated on a silicon wafer in large arrays using a combination of lithography and etching steps. These differences influence which device is optimal for a given experiment.
Here, we present detailed fabrication protocols for both types of resonant cavities. While the fabrication of microsphere resonant cavities is fairly straightforward, the fabrication of microtoroid resonant cavities requires additional specialized equipment and facilities (cleanroom). Therefore, this additional requirement may also influence which device is selected for a given experiment.
An optical resonator efficiently confines light at specific wavelengths, known as the resonant wavelengths of the device. 1-2 The common figure of merit for these optical resonators is the quality factor or Q. This term describes the photon lifetime (τo) within the resonator, which is directly related to the resonator's optical losses. Therefore, an optical resonator with a high Q factor has low optical losses, long photon lifetimes, and very low photon decay rates (1/τo). As a result of the long photon lifetimes, it is possible to build-up extremely large circulating optical field intensities in these devices. This very unique property has allowed these devices to be used as laser sources and integrated biosensors.10
A unique sub-class of resonators is the whispering gallery mode optical microcavity. In these devices, the light is confined in circular orbits at the periphery. Therefore, the field is not completely confined within the device, but evanesces into the environment. Whispering gallery mode optical cavities have demonstrated some of the highest quality factors of any optical resonant cavity to date.9,11 Therefore, these devices are used throughout science and engineering, including in fundamental physics studies and in telecommunications as well as in biodetection experiments. 3-7,12
Optical microcavities can be fabricated from a wide range of materials and in a wide variety of geometries. A few examples include silica and silicon microtoroids, silicon, silicon nitride, and silica microdisks, micropillars, and silica and polymer microrings.13-17 The range in quality factor (Q) varies as dramatically as the geometry. Although both geometry and high Q are important considerations in any field, in many applications, there is far greater leverage in boosting device performance through Q enhancement. Among the numerous options detailed previously, the silica microsphere and the silica microtoroid resonator have achieved some of the highest Q factors to date.1,9 Additionally, as a result of the extremely low optical loss of silica from the visible through the near-IR, both microspheres and microtoroids are able to maintain their Q factors over a wide range of testing wavelengths.18 Finally, because silica is inherently biocompatible, it is routinely used in biodetection experiments.
In addition to high material absorption, there are several other potential loss mechanisms, including surface roughness, radiation loss, and contamination loss.2 Through an optimization of the device size, it is possible to eliminate radiation losses, which arise from poor optical field confinement within the device. Similarly, by storing a device in an appropriately clean environment, contamination of the surface can be minimized. Therefore, in addition to material loss, surface scattering is the primary loss mechanism of concern.2,8
In silica devices, surface scattering is minimized by using a laser reflow technique, which melts the silica through surface tension induced reflow. While spherical optical resonators have been studied for many years, it is only with recent advances in fabrication technologies that researchers been able to fabricate high quality silica optical toroidal microresonators (Q>100 million) on a silicon substrate, thus paving the way for integration with microfluidics.1
The present series of protocols details how to fabricate both silica microsphere and microtoroid resonant cavities. While silica microsphere resonant cavities are well-established, microtoroid resonant cavities were only recently invented.1 As many of the fundamental methods used to fabricate the microsphere are also used in the more complex microtoroid fabrication procedure, by including both in a single protocol it will enable researchers to more easily trouble-shoot their experiments.
1. Microsphere Fabrication
2. Microtoroid Fabrication
3. Representative Results
The microsphere and microtoroid devices can be imaged using both optical microscopy and scanning electron microscopy (Figure 1d, e and Figure 2h, i). In all images, the uniformity of the device surface is clearly evident.
To verify that the detailed approach creates ultra-high-Q devices, we also characterized the Q factor of several devices by performing a linewidth (Δλ) measurement and calculating the loaded Q from the simple expression: Q=λ/Δλ=ωτ, where λ=resonant wavelength, ω=frequency, and τ=photon lifetime. Representative spectra of each device fabricated using the previously detailed procedures1,9 and a comparison graph of several devices is shown in Figure 3. The quality factors of all devices are above 10 million, with the majority being above 100 million.
The spectrum of the microsphere was a single resonance, indicating that the light coupled into either the clockwise or counter-clockwise propagating optical mode. However, the spectrum of the toroid exhibited a split resonance, indicating that light coupled into both the clockwise and counter-clockwise modes simultaneously. This phenomenon occurs when there is a slight imperfection at the coupling site. By fitting the spectrum to a dual-Lorentzian, the Q factor of both modes can be determined. The split resonance phenomena can occur in both sphere and toroid resonators, but is more frequently observed in toroids as they are more susceptible to imperfections and have fewer optical modes compared to spheres.
Figure 1. Flow chart of the microsphere cavity fabrication process. a) Rendering and b) optical micrograph of a cleaned and cleaved optical fiber. c) Rendering, d) optical micrograph and e) scanning electron micrograph of a microspere resonator.
Figure 2. Flow chart of the microtoroid cavity fabrication process. a) Rendering, b) top-view optical micrograph and c) side-view scanning electron micrograph of the circular oxide pad, as defined by photolithography and BOE etching. Note the slight wedge-shape of the oxide that is formed by the BOE. d) Rendering, e) top-view optical micrograph and f) side-view scanning electron micrograph of the oxide pad after the XeF2 etching step. Note that the oxide disk maintains the wedge-shaped periphery. g) Rendering, h) top-view optical micrograph and i) side-view scanning electron micrograph of the microtoroid cavity.
Figure 3. Representative quality factor spectra of the a) microsphere and b) microtoroid resonant cavities as determined using the linewidth measurement method. In very high Q devices, one may observe mode-splitting or a double peak, in which light reflects off a small defect and circulates in both clockwise and counterclockwise directions. c) Comparison graph showing the Q factors of several microsphere and microtoroid resonant cavities. Click here for larger figure.
Figure 4. Schematic of the CO2 laser reflow set-up. The CO2 laser beam (solid blue line) is reflected and then focused on the sample. It passes through the 10.6 μm / 633 nm beam combiner, which transmits 10.6 μm and reflects 633 nm. The optical column images the reflection of the sample off of the beam combiner; therefore, the image is somewhat red. A list of the necessary parts for this setup is in Table 4.
Figure 5. Incorrectly reflowed a) microsphere and b) microtoroid resonant cavities. Due to incorrect placement within the beam, the device is mal-formed. c) As a result of a poor photomask or poor lithography, the toroid is moon-shaped.
As with any optical structure, maintaining cleanliness at every step of the fabrication process is of critical importance. As there are numerous textbooks written on the topic of lithography and fabrication, the suggestions below are not intended to be comprehensive, but highlight a few of the more common issues researchers have faced.19-20
Because the uniformity of the microtoroid's periphery is determined by the uniformity of the initial disk, it is very important to pattern very circular disks. Common problems specific to the microtoroid are: 1) pixilation of photo-masks, 2) poor photolithography (under or over exposure, under or over developing, and rough or uneven etching), and 3) poor adhesion of the photoresist to the silica; here we address each issue individually.
It is very important to acquire high resolution photo-masks. While low resolution photomasks or ink-jet photomasks are readily available, these will result in "pixilated" or jagged circles which will not reflow correctly, resulting in non-circular toroids. The present protocols give UV exposure times for very specific photoresist film thicknesses at specific UV intensities. If different film thicknesses are used or if the photoresist is expired, then a different exposure time will be necessary. It is also advisable to calibrate one's photoaligner to ensure the correct UV exposure is given. Similarly, the time required in developer may vary as it is specific to the photoresist's film thickness and assumes that the photoresist is fully exposed. Finally, if the silica is not exposed to HMDS immediately before the photoresist is applied, the photoresist will not adhere well to the wafer. As a result, when the sample is etched using BOE, it will experience a severe and non-uniform undercut.
There is one other issue which also frequently arises with the toroid fabrication process and is related to the XeF2 undercutting step. Because of the high degree of selectivity of XeF2 for silicon over silica, the XeF2 will not directly etch the native oxide which is inherently present on the silicon wafer. Therefore, it is important to make sure to minimize the potential growth of such an oxide and to further eliminate any further oxide growth by thoroughly purging the XeF2 etch chamber with Nitrogen. If this is not done, the XeF2 etch will be extremely rough or pocketed.
Additionally, in order to form a circular structure, it is very important to use an isotropic silicon etchant. While XeF2 is the most commonly used etchant in the microtoroid fabrication process, there are others, such as HNA which is a mixture of hydrofluoric acid, nitric acid and acetic acid.20 However, because it contains HF, it is not as selective for silicon as XeF2 is, and the etching of the silica must be taken into account.
The CO2 laser reflow process used must be done very precisely to successfully fabricate microspheres and microtoroids. One standard and simple reflow setup is shown in Figure 4 with a list of parts in Table 4. There are many possible ways to build such a setup, and the layout and parts used can vary. However, the design must satisfy two important criteria. First, the distance between the sample and CO2 laser's focusing lens must be equal to the lens's focal length, so that the sample is located in the focus of the laser beam. Second, the uniformity of the CO2 laser across the spot and the placement of the device in the center of the spot are extremely important. This requires that all of the free space optics are in alignment, and of course, free space optics can drift with temperature and humidity fluctuations. Example devices which were fabricated with incorrectly aligned optics are in Figure 5. To help avoid these alignment problems, cameras and stages can be used to allow easier, more accurate positioning of a sample under the beam. While using an optical table or vibration isolation is not required, having the reflow components integrated and secured on a breadboard can improve alignment.
If a CO2 laser is not available, alternative reflow methods can be used. For the microsphere, a hydrogen torch could be used as an alternative method. If this approach is used, it is very important to follow all requisite safety protocols when building the reflow set-up, such as incorporating a flashback arrestor on the hydrogen tank and using a hydrogen torch, to eliminate the potential risk of an explosion. Typically, when this approach is used, a similar imaging system to that described for the CO2 laser set-up is used for monitoring the reflow process. However, a hydrogen torch will not work for the microtoroid, as the melting temperature of silicon is less than that of silica. The CO2 laser overcomes this problem, because silica strongly absorbs the laser light while silicon does not. Therefore, we have found that reflow with a properly aligned CO2 laser beam allows us to obtain the most consistent reflow needed for high quality factor microsphere and microtoroid resonators.
The pair of methods presented here enable the fabrication of ultra-high-Q silica resonant cavities. As a result of their long photon lifetimes, these devices have numerous important applications, particularly within the biological sciences.
No conflicts of interest declared.
A. Maker was supported by an Annenberg Foundation Graduate Research Fellowship, and this work was supported by the National Science Foundation [085281 and 1028440].
|Optical fiber||Newport||F-SMF-28||Any type of optical fiber can be used.|
|Fiber coating stripper||Newport||F-STR-175||Wire strippers can also be used|
|Ethanol||Any vendor||Solvent-level purity||Methanol or Isopropanol are substitutes|
|Table 1. Microsphere Fabrication Materials.|
|Silicon wafers with 2μm thermally grown silica||WRS Materials||n/a||We use intrinsic8, <100>, 4" diameter|
|Buffered HF - Improved||Transene||n/a||The improved buffered HF gives a smoother, better quality etch than plain BOE or HF|
|Acetone, Methanol, Isopropanol||Any vendor||99.8% purity|
|Table 2. Microtoroid Fabrication Materials.|
|Spinner||Solitec||5110-ND||Any spinner can be used.|
|Aligner||Suss Microtec||MJB 3||Any aligner can be used.|
|XeF2 etcher||Advanced Communication Devices, Inc.||#ADCETCH2007|
|Table 3. Microtoroid Fabrication Equipment.|
|CO2 Laser||Synrad||Series 48|
|3-Axis stage||OptoSigma||120-0770||Available from other vendors as well.|
|Si Reflector 1" diameter)||II-VI||308325||Available from other vendors as well.|
|Kinematic gimbal mount (for Si reflector)||Thor Labs||KX1G||Available from other vendors as well.|
|Beam combiner (1" diameter)||Meller Optics||L19100008-B0||Available from other vendors as well.|
|4" Focal length Lens (1" diameter)||Meller Optics or II-VI||Available from other vendors as well|
|Assorted posts, lens mounts||Thor Labs, Newport, Edmund Optics or Optosigma|
|Zoom 6000 machine vision system||Navitar||n/a||Requires generic USB camera and computer for real-time imaging. This is purchased as a kit.|
|Focuser for Zoom 6000 system||Edmund Optics||54-792||Available from other vendors as well.|
|X-Z Axis Positioners for Zoom 6000||Parker Daedal||CR4457, CR4452, 4499||CR4457 is X-axis, CR4452 is Z-axis, 4499 is mounting bracket.|
|Table 4. CO2 Laser Reflow Set-up.|