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Abstract
Circular microdisk mechanical resonators vibrating in their various resonance modes have emerged as important platforms for a wide spectrum of technologies including photonics, cavity optomechanics, optical metrology, and quantum optics. Optically transduced microdisk resonators made of advanced materials such as silicon carbide (SiC), diamond, and other wide- or ultrawide-bandgap materials are especially attractive. They are also of strong interest in the exploration of transducers or detectors for harsh environments and mission-oriented applications. Here we report on the first experimental investigation and analysis of energetic proton radiation effects on microdisk resonators made of 3C-SiC thin film grown on silicon substrate. We fabricate and study microdisks with diameters of ∼48 µm and ∼36 µm, and with multimode resonances in the ∼1 to 20 MHz range. We observe consistent downshifts of multimode resonance frequencies, and measure fractional frequency downshifts from the first three flexural resonance modes, up to ∼-3420 and -1660 ppm for two devices, respectively, in response to 1.8 MeV proton radiation at a dosage of 1014/cm2. Such frequency changes are attributed to the radiation-induced Young’s modulus change of ∼0.38% and ∼0.09%, respectively. These devices also exhibit proton detection responsivity of ℜ ≈ -5 to -6 × 10−6 Hz/proton. The results provide new knowledge of proton radiation effects in SiC materials, and may lead to better understanding and exploitation of micro/nanoscale devices for harsh-environment sensing, optomechanics, and integrated photonics applications.
© 2023 Optica Publishing Group under the terms of the Optica Open Access Publishing Agreement
1. Introduction
Microdisk resonators have emerged as important platforms for controlling interactions between light, vibrations, and electrons, which can enable a wide spectrum of technologies including photonics, cavity optomechanics, optical metrology, and quantum optics [1,2,3,4]. Meanwhile, the suspended disk structure supports diverse resonance modes that can efficiently modulate the optical transduction, hence making it possible to optically read out the minuscule mechanical motions that are hardly probed by other means (e.g., electrical). For example, radial breathing (or contour) mode and other bulk modes of the microdisks have been widely exploited to couple to the optical whispering gallery modes, hence mechanical resonances up to GHz can be read out with ultrahigh displacement sensitivity (e.g., ∼10−17 m/√Hz) [5,6,7]. Meantime, flexural and torsional resonance modes of microdisk resonators have been studied as physical sensors, such as photodetection and gravimetric sensing [8,9,10]. To date, semiconductors and various advanced materials have been employed to construct microdisks, such as silicon [11,12], silicon carbide [3,13,14], diamond [15,16], silicon oxide [17], lithium niobate [18], gallium phosphide [19], etc. Among these, silicon carbide (SiC) has attracted great interest because of its fascinating material properties and its compatibility with micromachining using monolithic techniques [20,21]. For example, SiC has outstanding mechanical properties (e.g., high elastic modulus, EY ∼350-450 GPa), which are desirable for operating devices in the radio frequency (RF) and microwave ranges. SiC also exhibits excellent optical properties (wide bandgap of Eg ∼2.3-3.3 eV) and thermal properties (thermal conductivity k ∼360-490 W/[m·K]), which make it compatible with optical transduction and metrology, particularly with higher laser power ranges. Further, SiC is a chemically inert and biocompatible material. All these make SiC microdevices more appealing for operation in various environments, such as vacuum, gaseous, liquid, and especially harsh or even extreme environments, such as high-temperature and radiation conditions [20,22,23,24,25].
Fig. 1. Proton radiation effects on SiC microdisk resonators. (a) Schematic illustration of high-fluence proton radiation (1.8 MeV, 1014/cm2) on a SiC microdisk resonator, while its multimode resonance frequencies are measured by ultrasensitive laser interferometry, before and after the radiation exposure. (b) Frequency downshifts in response to the proton radiation. (c)-(d) The frequency shifts are associated with the ionizing effect and, more importantly, the displacement damage, which alters the elastic modulus in the constituent crystal.
As mentioned above, radiation situations are an important class of harsh or extreme environments, as many semiconducting devices are exposed to radiation in space or on earth. To date, noticeable endeavors have been made to explore radiation effects in SiC microelectronics (e.g., diodes, transistors) [26] and SiC micromechanical devices (e.g., diaphragms) [25]. In a few available studies (including SiC, Si, and GaN), it has been reported that both ionizing (trapped charge-induced strain or stress change) and non-ionizing effects (e.g., displacement damage) (Fig. 1) could degrade the MEMS resonator performance (causing reversible or irreversible frequency shifts) [25,27,28,29]. Although SiC microdisk resonators are very important platforms in optomechanics and optical MEMS research and technologies, the investigation of radiation effects on SiC microdisk resonators has been lacking.
In this work, for the first time, we fabricate SiC microdisk resonators and detect their multimode frequency shifts caused by energetic proton radiation (Fig. 1). The wide bandgap of SiC enables perfect compatibility with ultrasensitive optical motion transduction schemes including both optothermal drive and laser interferometry readout (Fig. 1(a)). An amplitude-modulated shorter-wavelength laser (e.g., 405 nm diode laser) can be absorbed by SiC to photothermally excite the multimode resonances, while a longer-wavelength laser (e.g., 633 nm He-Ne laser) can transmit through the transparent SiC and be reflected by the SiC surface and interfaces, thus interferometrically transduce the flexural-mode vibrations. Such an all-optical excitation-detection scheme allows us to directly probe the proton radiation effects on SiC micromechanical resonators, without worrying about other parasitic effects from structures such as metallization or electrodes (otherwise required for electrical transduction of motion). Based on the multimode frequency shifts (Fig. 1(b)), we analyze the radiation effects on mechanical properties and the suggested Young’s modulus change, and correlate with the high-fluence proton radiation-induced displacement damage (Fig. 1(c)-(d)).
2. Device fabrication
The SiC microdisk resonators are fabricated on a 3C-SiC-on-Si platform. We employ ∼500 nm thick 3C-SiC film hetero-epitaxially grown on (100) single crystal Si substrate (Fig. 2(a)) by atmospheric pressure chemical vapor deposition (APCVD) [20,22,25]. The diameters of the SiC microdisks are defined by using focused ion beam (FIB) (FEI Helios NanoLab 650). The SiC layer is directly milled through, leaving the Si substrate exposed for the subsequent wet etching (Fig. 2(b)). We pattern two specific sizes of SiC microdisk resonators (∼48 µm in diameter and ∼36 µm in diameter). We then employ HNA (hydrofluoric, nitric, & acetic) solution to etch an appropriate amount of Si substrate, while the remaining Si underneath forms the pedestals for supporting the suspended microdisks (Fig. 2(c)). Figure 2(d) shows a top-view optical microscopy image of a typical SiC microdisk after fabrication. An aerial-view scanning electron microscopy (SEM) image of the SiC microdisk is shown in Fig. 2(e), in which the suspended microdisk structure can be observed. Two microdisk resonators are measured in this work, one having disk diameter of ∼48 µm and pedestal diameter of ∼17 µm (Device A), and the other having disk diameter of ∼36 µm and pedestal diameter of ∼4 µm (Device B).
Fig. 2. Fabrication process of the SiC microdisk resonators. (a) ∼500 nm thick SiC film grown on (100) single crystal Si through atmospheric pressure chemical vapor deposition (APCVD). (b) SiC microdisks with different diameters patterned by using FIB milling. (c) Suspended microdisk structures released by HNA etch of Si substrate. (d) Top-view optical microscopy image and (e) Aerial-view SEM image of a fabricated SiC microdisk resonator.
3. Experimental techniques
3.1 Optical interferometry for resonance characterization of SiC microdisks
The mechanical resonances of the SiC microdisk resonators are characterized by using a custom-built ultrasensitive optical interferometry system [3,8,9]. The microdisk resonators are tested in vacuum with a pressure level of ∼16mTorr, at room temperature. The interferometry system, as shown in Fig. 3(a), consists of a 405 nm amplitude-modulated diode laser (output laser power ≈800µW) to photothermally excite the flexural-mode vibrations of the microdisk resonators, and a 633 nm He-Ne laser focused on the device (with an on-device power of ∼1 mW or lower) to readout the multimode motions with a typical displacement sensitivity down to ∼10fm/√Hz [3]. The intensity of the interferometry signal, due to multiple reflections from device surface and interfaces, is modulated by the periodic gap variations between the microdisk and the substrate, hence the time-varying light intensity carries information of the resonance motions. A photodetector is exploited to convert the interferometry signal into an electrical signal, and a network analyzer to sweep the modulation frequency of the diode laser and convert the time-varying signal to the frequency domain. Thanks to the wide bandgap, outstanding thermal properties of SiC, and minimized laser powers, the parasitic heating effect (heating-induced frequency shift) is minimized and negligible.
Fig. 3. Experimental systems. (a) Schematic illustration of the laser interferometry measurement system. The 405 nm laser is utilized to photothermally excite the multimode resonances of a microdisk. The 633 nm laser is focused on the disk surface to read out the tiny displacements as the reflected light intensity is modulated by the vibrating device and varying gap between the disk and substrate. (b) Optical image and illustration of the proton irradiation apparatus. The SiC microdisk chip is mounted and irradiated by protons with an energy of 1.8 MeV. The chip is irradiated for 8 hours (h), corresponding to a dosage of 1014/cm2.
The network analyzer first sweeps from 1 MHz to 30 MHz, in various 10 MHz spans, collecting broadband frequency spectra. This is done to ensure multiple modes are identified. We then zoom into the first three modes by optimizing the 633 nm laser spot focused onto the microdisks, maximizing the interferometry signal amplitude. All the laser interferometry measurement conditions are kept the same before and after radiation exposure of the devices.
3.2 Radiation exposure of the SiC microdisks
The devices have been irradiated in an S-Series Pelletron radiation system (Fig. 3(b)). Protons are used to irradiate the devices. The devices are placed into the radiation chamber and exposed for 8 h. This corresponds to a proton dosage of 1014/cm2 with a proton energy of 1.80 MeV.
An important consideration is to determine how many protons interacted with the SiC microdisks. The probability of a proton traveling a distance x without interacting is e-µx, where µ is the attenuation coefficient. It follows that the probability of a proton interaction is
where µ/ρ is the mass attenuation coefficient and ρ is the density. In the case of SiC, Where wi is the weight fraction of each of the constituent elements, Si and C.As summarized in Table 1, it is calculated, through Eq. (1), that the probability, P, of a proton interacting with a ∼500nm-thick SiC film, is between 7.16 × 10−6 and 8.32 × 10−6. Based on the probability, 1.87 × 109 protons are expected to impinge on the SiC microdisk with a diameter of 48.8µm (Device A), among which 1.34 × 104 to 1.55 × 104 protons will have radiative interaction with the device. Likewise, ∼1.03 × 109 protons are expected to impinge on the SiC microdisk with a diameter of 36.3µm (Device B), among which 0.74 × 104 to 0.86 × 104 protons will interact with the device and contribute to radiation effects.
Table 1. Probability, number of protons impinging on the SiC microdisks based on an implant dosage of 1014/cm2, and the approximate number of protons that interact with Device A and Device B
4. Results and discussions
4.1 Frequency stability of SiC microdisk resonators
As the interferometric measurements have been performed before and after the radiation, and the radiation exposure takes 8 hours, the frequency drift of the devices needs to be considered before precisely evaluating radiation effects on the SiC microdisk resonators. Therefore, we perform a frequency stability measurement over 24 hours, as shown in Fig. 4. We monitor the resonance frequency of a 48µm-diameter microdisk, showing typical f ∼ 4.5338 MHz and quality factor Q > 110 (Fig. 4(b)). Resonance curves are taken every 30 minutes, with the 633 nm laser spot re-optimized every time to counteract the mechanical drift of the system. The collected data can be separated into two periods. The first period covers the first 7 hrs within 24 hrs and the second period covers the last 7 hrs within the same 24 hrs. Figure 4(c) shows the resonance frequency change collected. In the first segment of 0 − 7 hr, f shows a mean of 4.5223 MHz and a standard deviation of 0.0025 MHz (∼553 ppm). For a time from 17 − 24 h, f shows a standard deviation of 0.0031 MHz with a mean of 4.5287 MHz (∼685 ppm). Therefore, for a time over 24 hrs, we eventually estimate a standard deviation of ∼0.0043 MHz, corresponding to a frequency drift of ∼966 ppm. Although such frequency fluctuations seem quite undesirable, these values could be reasonable for SiC microdisks with Q∼100 − 200, and the radiation-induced frequency shifts are still discernable since they are larger than the measured frequency instability level.
Fig. 4. Frequency stability measurement of a typical microdisk resonator over 24 hrs before proton radiation exposure. (a) Optical image of the microdisk with a diameter of 48 µm. (b) Measured f = 4.53379 MHz and Q = 113 for the device. (c) The resonance frequency is measured over 24 hrs, with a ∼10 hr idle time overnight.
4.2 Multimode frequency shifts by exposure to proton radiation
We then characterize the radiation-induced frequency shifts for Device A (disk diameter of 48.8 µm) and Device B (disk diameter of 36.3 µm).
Figure 5 presents the multimode frequency changes of Device A measured before and after the proton radiation. We measure the three modes of Device A in the 4 − 6 MHz range before radiation, which are f1 = 4.6996 MHz, f2 = 4.9026 MHz, and f3 = 4.9701 MHz. After proton irradiation, we observe the frequency downshifts to f1′ = 4.6835 MHz, f2′ = 4.8888 MHz, and f3′ = 4.9580 MHz, respectively. We then calculate the fractional frequency shifts, which are Δf1/f1= -3426 ppm, Δf2/f2= -2815 ppm, and Δf3/f3= -2435 ppm, respectively (also summarized in Table 2). These values are much larger than the measured frequency instability (from Fig. 4), confirming that the high-fluence proton radiation indeed causes the frequency to clearly downshift in SiC microdisk resonators.
Fig. 5. Multimode frequency shifts measured from a 48.8µm-diameter microdisk resonator (Device A) in response to proton radiation. (a) Frequency spectrum of the device showing the first three resonance modes in the 4 − 6 MHz range. (b)-(c) Frequency shifts directly illustrated based on the measured first three modes, before and after proton radiation.
Table 2. Multimode frequency shifts of Device A and B in response to the 1.8 MeV proton radiation at a dosage of 1014/cm2
Figure 6 shows the multimode frequency changes of Device B measured before and after the proton radiation. We also measure the three modes of Device B in the 12 − 14 MHz range before radiation, which are f1 = 13.1167 MHz, f2 = 13.3485 MHz, and f3 = 13.7054 MHz. Note that in the wide-range frequency spectrum, three modes may not be seen simultaneously because the laser interferometric measurement is position-dependent and very sensitive to the mode shape. Therefore, we have always made effort to optimize the laser position, and zoom in with smaller frequency spans, to resolve the resonance modes one by one. After proton irradiation, we observe the frequency downshifts to f1′ = 13.1000 MHz, f2′ = 13.3302 MHz, and f3′ = 13.6826 MHz, respectively. We then calculate the frequency shifts, which are Δf1/f1= -1273 ppm, Δf2/f2= -1371 ppm, and Δf3/f3= -1664 ppm, respectively (as summarized in Table 2). Once again, the high-fluence proton radiation effect on the SiC microdisk resonators is clearly discernable as the frequency shifts are larger than the measured frequency instability. It is also worth noting that, in comparison to single mode cases, the consistent frequency downshifts of multiple resonances of each of the devices provide even more convincing evidence affirming the proton radiation effects upon the mechanical resonators.
Fig. 6. Multimode frequency shifts measured from a 36.3µm-diameter microdisk resonator (Device B) in response to proton radiation. (a) Frequency spectrum of the device showing the first three resonance modes in the 12 − 14 MHz range (note that the signal strengths of the three modes depend on the laser spot position and sometimes zooming into narrower frequency spans is needed to clearly view all the modes). (b)-(c) Frequency shifts directly determined by the measured first three modes before and after proton radiation.
Meanwhile, as listed in Table 1, given the estimated number of protons that interact with the SiC device layer, we expect more frequency shifts induced by proton radiation in Device A than in Device B. This also agrees well with the experimental results. The quality (Q) factor, and the product of optomechanics resonance frequency and quality factor (f × Q) are also important figures of merit to characterize a microdisk resonator. As shown in Fig. 7, we summarize the fs and Qs for Device A and Device B before and after proton radiation. Although we observe slight changes in Q values, no substantial changes in f × Q values are observed before and after proton radiation.
Fig. 7. (a)-(b) Multimode resonance spectra of Device A and Device B measured after finishing post-radiation annealing. (c) Measured multimode resonance frequencies and their quality (Q) factors for Device A and B before and after proton radiation, and after the post-radiation annealing. Dashed lines indicate the f × Q = 108, 109, 1010 contours.
4.3 Proton radiation effects on SiC microdisk resonators
We attribute the multimode frequency shifts in the SiC microdisks to the lattice (displacement) damage induced by proton irradiation and implantation, which can alter Young’s modulus of the SiC film. The multimode resonances of a SiC microdisk can be modeled as [30]:
According to Eq. (3), the percentage change in EY (${{\Delta {E_Y}} / {{E_Y}}}(\%)$) corresponding to the frequency shift is given by:
For Device A (48.8µm-diameter in Fig. 5), we estimate Young’s modulus change of ${{\Delta {E_Y}} / {{E_Y}}}(\%)$ = -0.49%, -0.37%, -0.29% using the first three modes (considering frequency instability of 966 ppm), thus we obtain an averaged ${{\Delta {E_Y}} / {{E_Y}}}(\%)$ ≈ -0.38%. Similarly, for Device B (36.3µm-diameter in Fig. 6), we estimate Young’s modulus change of ${{\Delta {E_Y}} / {{E_Y}}}(\%)$ = -0.06%, -0.08%, -0.14% using the first three modes (considering frequency instability of 966 ppm), thus we obtain an averaged ${{\Delta {E_Y}} / {{E_Y}}}(\%)$ ≈ -0.09%.
We further estimate the proton responsivity of these two devices (assuming that the protons uniformly impinge on the SiC microdisk),
where Δf is the frequency shift before and after radiation, A is the area of the disk, and N is the dosage. Therefore, we calculate ℜ ≈ -6.2 × 10−6 Hz/proton, -4.8 × 10−6 Hz/proton, and -3.9 × 10−6 Hz/proton for the first three modes of Device A. Similarly, we estimate ℜ ≈ -3.9 × 10−6 Hz/proton, -5.2 × 10−6 Hz/proton, and -9.3 × 10−6 Hz/proton for the first three modes of Device B (after subtracting 966 ppm frequency instability). Overall, we can obtain ℜ ≈ -5 to -6 × 10−6 Hz/proton for the SiC microdisk resonators in this work.4.4 Effect of post-radiation annealing on SiC microdisk resonators
We further explore the effect of high-temperature annealing on the SiC microdisk resonators after radiation. As shown in Fig. 7, we observe that resonance frequencies shift up and quality factors increase significantly for both Device A and Device B, after post-radiation annealing in a moderate vacuum at 600°C for 1 hr. Hence, > 10 × and >2 × improvements in f × Q values are observed in Device A and Device B, respectively. These results suggest that the microdisk resonators have been reset and transformed into new ones after the post-radiation annealing. The proton radiation-induced damage effects could be alleviated or even eliminated, and the crystal quality may be fully restored and enhanced; all as results of the high-temperature annealing after the radiation exposure. Such annealing processes can be highly useful for restoring the SiC optomechanical devices and resetting radiated devices for future repeated radiation experiments.
5. Conclusion
In summary, we have investigated the proton radiation effects in multimode SiC microdisk resonators using optical interferometry techniques. In 3C-SiC microdisks with diameters of ∼48 µm and ∼36 µm, we have measured multimode frequency shifts in response to energetic (1.8 MeV) proton radiation at a dosage of 1014/cm2. We attribute such frequency changes to the radiation-induced Young’s modulus lowering by ∼0.38% and ∼0.09% for ∼48 µm and ∼36 µm devices, respectively. These devices also exhibit proton detection responsivity of ℜ ≈ -5 to -6 × 10−6 Hz/proton, considering uniform radiation upon the microdisks. As microdisk resonators have been extensively investigated as the key components in integrated photonics and optomechanics, and SiC has become a highly attractive and promising candidate for building next-generation photonic, optomechanical, and electromechanical systems, the results in this study may help lead to better understanding and exploitation of proton radiation effects in such SiC micro/nanodevices, especially for harsh-environment applications. The specifications and performance of future devices, especially the Q factors may be greatly improved by using high-resolution photolithography and dry etching, to attain more geometrically symmetric disk structures with cleaner and sharper edges that sustain high-Q resonances. By further exploiting closed-loop measurement and integrating high-sensitivity optomechanical signal transduction, the radiation on SiC optomechanics may be directly measured in situ, such that radiation effects could also be more precisely investigated, and monitored in real time.
Funding
National Science Foundation (DUE-2142552, EFMA-1641099); Defense Threat Reduction Agency (HDTRA1-15-1-0039, HDTRA1-19-1-0035).
Acknowledgments
The authors thank the helpful discussions with Dr. Jacob Calkins.
Disclosures
The authors declare no conflicts of interest.
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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