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On-chip self-sensing enhancing actuation for MEMS/NEMS with large efficiency
Meiyong Liao,1*Liwen Sang,2 Tokuyuki Teraji,1 Satoshi Koizumi1 and Yasuo Koide3
To intelligently fulfill the functionalities of MEMS/NEMS, all-electrical on-chip self- sensing and actuation are essential to provide the electrical interface with integrated circuits. However, the implementation of such on-chip MEMS/NEMS transducers for arbitrary resonators has been a challenge due to a number of difficulties such as material choice 1, large dissipation2,3, restriction in high frequency3, low sensitivity4,5, poor reliability2,3, and poor integrability6-7. Here we show a universal on-chip transduction scheme called self-sensing enhancing actuation (SEA), which can be adapted to any MEMS/NEMS resonator. Using the SEA transduction scheme, we achieve all electrical, on-chip MEMS/NEMS with unprecedented merits of high self- sensitivity, little dissipation, high-frequency operation. The SEA transduction scheme enables the high operation temperature of up to 873 K. The current work favors the development of ultrahigh-performance and robust MEMS/NEMS in the post-Silicon era based on emerging materials of diamond, SiC, III-nitrides etc superior to Si.
Microelectromechanical or nanoelectromechanical systems (MEMS/NEMS) combining with integrated circuits (ICs) are described as an important “More-than-Moore”
technology according to the International Technology Roadmap for Semiconductors8. By interacting with ICs, the physical, chemical or optical quantities of MEMS/NEMS can be controlled or processed intelligently. As a consequence, on-chip MEMS/NEMS transducers with all electrical sensing and actuation are necessary to provide electrical interfaces with ICs. Although off-chip actuation or sensing such as the inertia-based
1Research Center for Functional Materials, National Institute for Materials Science, Namiki 1-1, Tsukuba,305-0044, Ibaraki, Japan.2International Center for Materials Nanoarchitectonics (MANA), National Institute for Materials Science (NIMS), Namiki 1-1, Tsukuba, Ibaraki, 305-0044, Japan.3Research Network and Facility Services Division, National Institute for Materials Science (NIMS), 1-2-1 Sengen, Tsukuba, Ibaraki 305- 0047, Japan.* e-mail: [email protected]
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piezoelectric9, photothermal actuation6 and optical sensing7 methods do not suffer from energy dissipation, they are not compatible with ICs. On-chip actuation based on piezoelectric2,3, electrostatic4,5, magnetomotive10, and electrothermal11 techniques imposes restrictions on material choice or electrical conductivity due to doping impurities and thus induces dissipation. Similarly, the present on-chip sensing such as piezoelectric12, piezoresistive1, and capacitive3,4 methods have limitations in terms of material choice, geometric design, dissipation, and sensitivity. Therefore, universal on-chip, high- sensitivity self-sensing and actuation has not been realized, especially for high-frequency and high-temperature applications as well as integrability with ICs.
Here, we describe a universal transduction scheme with large efficiency called self- sensing enhancing actuation (SEA) for on-chip MEMS/NEMS, in which the self-sensing concomitantly enhances the electrical actuation. In our approach, we place ultrathin nanothick source-drain (S-D) electrodes on the resonator for self-sensing and gate electrode on the substrate surface vertically beside the resonators for actuation. In the on- chip MEMS/NEMS using SEA, the S-D sensing electrodes enable the resonator to function as a “high-k” dielectric material, greatly increasing the electrostatic force compared to those of conventional actuation methods. A heterodyne frequency down- conversion method is utilized to characterize the resonance frequency of the MEMS and NEMS cantilevers (Fig. 1a). Intrinsic single crystal diamond (SCD) cantilevers on a type- Ib SCD substrate fabricated via the IAL method 13,14 (Supplementary Fig.S1) are adopted to demonstrate the SEA transduction.
The cantilevers are actuated by the gate electrode and the motion is detected electrically by the S-D electrodes using the “1ω” method15. An rf signal with an amplitude of 𝑉𝑔𝑎𝑐 and a frequency of ω is applied along the bottom gate, while a second rf signal with an amplitude of 𝑉𝑑𝑎𝑐 and a frequency of ω+△ω is applied to the drain electrode.
The finite element method (FEM) reveals that a strong electric field is confined around the edge of the SCD cantilever (Fig. 1b). We note that no additional dc bias is applied to the cantilever, as is normally done4,5,16 . An rf signal alone is sufficient to drive the MEMS cantilevers and can provide sufficient electrical readout for 𝑉𝑔𝑎𝑐=0.8 V and 𝑉𝑑𝑎𝑐 =10 V (Fig. 1c). At resonance, an abrupt change of the measured phase is observable. The electrical current is sensed on the modulation of the S-D electrodes resistance on the
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cantilever with a variation ∆𝑅0 due to the mechanical motion of the cantilever17. The signal generated at the source is a mixture of the gate and drain signals. We demodulate the output signal at a frequency of △ω=5 kHz, which can be expressed as 𝑉𝑜𝑢𝑡 =
∆𝑅0
𝑅+𝑅0𝑓(𝑉𝑑𝑎𝑐, 𝑉𝑔𝑎𝑐)cos (∆𝜔𝑡 − 𝜑) , where R is the circuit resistance, R0 is the resistance of the S-D electrodes on the cantilever, 𝑓(𝑉𝑑𝑎𝑐, 𝑉𝑔𝑎𝑐) ∝ 𝑉𝑑𝑎𝑐 in the case of non-coupling between 𝑉𝑑𝑎𝑐 and 𝑉𝑔𝑎𝑐, and 𝜑 is the phase. ∆𝑅0 depends linearly on 𝑉𝑔𝑎𝑐 for a fixed 𝑉𝑑𝑎𝑐 = 10 𝑉 (Supplementary Fig. S2). The resonance frequency shows no dependence on 𝑉𝑔𝑎𝑐. As shown later, the electrical readout increases squarely with the sensing source- drain voltage, revealing the self-sensing enhancing actuation mechanism. The SEA transduction is also successfully achieved for the NEMS (Fig. 1d), even for a gate amplitude of 50 mV without any dc bias applied to the gate. Due to the self-sensing enhancing actuation mechanism, the distance between the gate and the cantilever can be several micrometer, several tens larger than those of conventional electrostatic or dielectric actuation mechanism.5,16
The resonance frequency can be tuned by varying the rf signal amplitude applied to the drain, which shifts downward from 2.65069 MHz to 2.64959 MHz when 𝑉𝑑𝑎𝑐 is increased from 2 to 10 V with 𝑉𝑔𝑎𝑐 fixed at 1 V( Fig. 2 a and b). The “softening” of the cantilever is due to the local Joule heating from the S-D electrodes and was confirmed using the laser Doppler method of measuring the resonance frequency of a similar SCD cantilever when different dc biases were applied to the S-D electrodes (Supplementary Fig.S3). The frequency shift δf/f0 is found to follow a quadratic relationship with 𝑉𝑑𝑎𝑐 (Fig.S4a), consistent with 𝛿𝑓𝑓
0 ≈12𝛿𝐸𝐸 ≈12𝛼𝑑𝑇𝑑𝑃𝑃 , where E is the Young’s modulus of diamond, α is the thermal expansion coefficient of diamond, P is the electrical power to the electrode and dT is the temperature variation. Assuming the temperature distribution in the cantilever to be homogeneous given the high thermal conductivity of diamond, the temperature increment was evaluated to be around 60 K at 𝑉𝑑𝑎𝑐=10 V by calibrating the SCD cantilevers resonance frequencies as a function of measurement temperature. The slope of the derivative resonance frequency shift versus 𝑉𝑑𝑎𝑐 is calculated to be 50-200 Hz/V, depending on the amplitude of 𝑉𝑑𝑎𝑐 (Fig. 2c). The readout amplitude can also be tuned by 𝑉𝑑𝑎𝑐 , which increases quadratically with increasing 𝑉𝑑𝑎𝑐 due to the SEA
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transduction (Fig. 2d). The quality factor exhibits little dependence on 𝑉𝑑𝑎𝑐 and is slightly higher than those measured by using the optical method (Supplementary Fig.
S4b).
The on-chip SEA transduction scheme demonstrates much higher sensitivity than that of external actuation using a piezo-ceramic disk (without consideration about the adhesion here). The resonance frequency was also characterized by off-chip actuation as a function of the driving voltage 𝑉𝑝𝑎𝑐 applied to the piezo-ceramic with 𝑉𝑑𝑎𝑐 fixed at 10V (Fig. 3a). The shift of the resonance frequency with 𝑉𝑑𝑎𝑐 is similar to that in on-chip actuation. Even with a large 𝑉𝑑𝑎𝑐, i.e.=10 V, the resonance signal is buried in the background noise at 𝑉𝑝𝑎𝑐 = 1 𝑉 . In addition, the electrical readout exhibits a linear relationship with 𝑉𝑑𝑎𝑐 unlike in the on-chip SEA transduction. It is noticed that a strong resonance signal is evident at 𝑉𝑔𝑎𝑐=1V in the on-chip actuation case (Fig. 2 a). The electrical readouts obtained using the on-chip actuation and external piezo-actuation are compared (Fig.3b). We define the electrical readout sensitivity as S=Vout/(Vdriv𝑉𝑑𝑎𝑐), where Vdriv represents the amplitude of the rf signal applied to either the on-chip gate electrode or the piezoelectric disk. S is around 1.1×10-5/V for on-chip actuation, about 45 times larger than its value of 2.4×10-7 /V for external piezo-actuation at 𝑉𝑑𝑎𝑐=10 V. The efficient on-chip actuation is further supported by the optical readout of the SCD cantilever mechanical resonance (Fig. 3c). We note that the adhesion between diamond plate and the piezoceramic may affect the sensitivity of the external actuation method.
The advantage of the SEA is that we do not need consider about such kind of placement in external actuation method. We characterize the optical readout sensitivity as the slopes of the plots of the optical readout versus driving voltage. The slope in the on-chip actuation case is determined to be 0.32, much larger than the value of 0.06 in the external piezo-actuation case (Fig.3 d). Similar sensitivities are also achieved for on-chip NEMS with SEA transduction.
Due to the specific features of geometry and the nano-thickness of the sensing electrode, little energy dispersion occurs in the SEA transduction. A SCD 60 µm-long MEMS cantilever with and without S-D electrodes was investigated to demonstrate this feature. The resonance frequency f0 of the 60 µm-long MEMS cantilever without S-D electrodes was measured to be 2.67533 MHz by using laser Doppler method. The
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resonance frequency exhibited a downward shift to 2.65093 MHz after deposition of S- D electrodes (Supplementary Fig. S5a). The quality factor was about the same after the electrodes were fabricated as it was (~12 000). The Young’s modulus of the SCD cantilevers was evaluated to be around 1100 GPa based on the fit curve of the resonance frequency versus the cantilever length. For the NEMS, the f0 of the 60µm-long cantilever also experienced a downshift (Supplementary Fig. S5b), and the quality factor changed little, from 5 522 to 5 478 before and after the S-D electrodes deposition. It will be shown later that the on-chip SEA transduction does not degrade the quality factors of both MEMS and NEMS.
The on-chip SEA enables MEMS/NEMS operation at high temperatures. The resonance frequency of another similar SCD cantilever with L=60 µm, W=12 µm, and d=2.8 µm was measured at different measurement temperatures T ranging from room temperature (RT) to 873 K (Fig. 4). Here, T denotes the temperature of the sample stage under the SCD MEMS chip. The resonance frequency of the cantilever was optically measured to be 2.63074 MHz with a quality factor around 13 422 at RT (Supplementary Fig. S6). The resonance frequency shifts slightly downward to 2.63069 MHz at 𝑉𝑑𝑎𝑐=3 V with a higher quality factor around 17 000 (Fig. 4a) with SEA transduction. At 𝑉𝑑𝑎𝑐=3 V, the increment of the temperature of the SCD cantilever is less than 10 K. We investigate the on-chip MEMS based on the SEA transduction at elevated temperatures by fixing the amplitude of the rf signal applied to the drain. The frequency response at 323 K to 673 K was measured at 𝑉𝑑𝑎𝑐=3 V (Fig. 4b). Due to the increase in the resistance of the S-D electrodes with increasing temperature, 𝑉𝑑𝑎𝑐 is raised to be 10 V as T increases from 773 K to 873 K (Fig. 4c). It is obvious that the on-chip MEMS is capable of operating at least up to 873 K (600 oC). The resonance frequency decreases with increasing measurement temperature (Fig. 4d). The temperature coefficient of frequency is 5-11 ppm/K. The quality factor of the cantilever decreases from 17 000 to 1 300 as the measurement temperature increases from RT to 873 K. The temperature dependence of the quality factor is possibly due to thermal elastic damping18.
The on-chip MEMS/NEMS using the SEA scheme has the following advantages over the systems using conventional transduction schemes. First, the self-sensing couples with the actuation, strongly improving the signal-to-noise ratio and simplifying the electronic
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circuit. The rapid increase of the on-chip readout with 𝑉𝑑𝑎𝑐 supports this merit. Dielectric actuation, an on-chip transduction for a dielectric resonator can prevents the doping problem from occurring in SCD16. However, the dielectric force in this transduction scheme is very weak. We simulate the electric field around a SCD cantilever using FEM for on-chip dielectric actuation (Supplementary Fig.S7). The electrostatic force on the cantilever in our SEA method is enhanced by more than 1 000 times larger than that in the dielectric actuation. Experimentally, we observe that the optical readout achieved using SEA actuation is greatly enhanced by more than 10 times compared with that resulting from using dielectric actuation (Supplementary Fig.S8). Therefore, the micrometer vertical distance from the gate electrode to the resonator in our case offers the facile fabrication of on-chip MEMS/NEMS. Electrostatic or capacitive on-chip actuation technique is also commonly adopted. However, this method requires the resonator itself to be electrically conductive. Due to the large thermal activation energies of the dopants in wide-bandgap (WBG) semiconductors, it is difficult to achieve sufficient electrical conductivity by doping without inducing dissipation19. The on-chip SEA transduction circumvents the notorious doping problem. Therefore, the SEA can be applied to any semiconductor. In the conventional electrostatic actuation, the electrical force is quite week resulting from the vacuum/air gap in the capacitor. The SEA enables the MEMS/NEMS resonator itself to act as the dielectric in the actuation, which increases the electrostatic force up to 5.7 times (i.e. for diamond) that in conventional electrostatic actuation. Therefore, the second advantage of the SEA transduction scheme is that it enables actuation with an extremely low rf driving voltage in the order of millivolts level due to the enhanced electrical force provided by the dielectric resonator. This feature will facilitate the MEMS/NEMS integration with ICs. The SEA transduction mechanism also bring the third merit of the high electrical sensitivity, much larger than that resulting from using the external piezoelectric method and on chip readout. A method able to perform self-sensing and actuation on-chip involves the integration of a piezoelectric thin film on MEMS/NEMS devices. However, the deposition of a piezoelectric thin film on foreign substrates itself is difficult due to the problem of material phase control20.Alternatively, placing sensing and actuation electrodes on the same piezoelectric layer generates large energy dissipation. In contrast, the SEA transduction induces little dissipation in the MEMS/NEMS (Supplementary Fig. S9). In addition, high-frequency sensing via
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piezoelectric method is not settled yet due to internal leakage21. Furthermore, the fabrication of piezoelectric actuators capable of operating at temperatures higher than 473 K (200 oC) has been difficult22. The above merits of the SEA transduction along with the wide bandgap of diamond offer the capability of operating SCD MEMS at high frequency and high temperatures at least up to 873 K (600oC).
In conclusion, the on-chip all-electrical SEA transduction scheme is a universal transduction method with self-improvement effect for any MEMS/NEMS resonator from silicon to the emerging WBG semiconductors of diamond, SiC, and III-nitrides etc. The SEA transduction exhibits the merits of low actuation voltage in the order of millivolts, high-sensitivity readout, little energy dissipation, high-temperature and high-frequency operation. The integration of MEMS/NEMS with ICs can thus be achieved based on the SEA scheme. The SEA transduction circumvents the challenges in WBG semiconductors MEMS/NEMS, opening the avenue to overcome the intrinsic limitations of Si MEMS, such as poor performances and reliability23-25. The current work paves a pathway to develop novel MEMS/NEMS devices based on emerging materials in the post-Silicon era.
8 References
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20. Liao M. Y. et al. Improved ferroelectric properties of Pb(Zr0.52,Ti0.48)O3 thin film on single crystal diamond using CaF2 layer, Appl. Phys. Lett. 96, 012910 (2010).
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Acknowledgements
This work was partially supported by by JSPS KAKENHI (Grant Number 15H03999, 15H03980) and Nanotechnology Platform projects sponsored by the Ministry of Education, Culture, Sports, and Technology (MEXT) in Japan.
Competing financial interests
The authors declare that they have no competing financial interests.
Author contributions
M. L conceived the idea and designed the project. M. L and L. S. performed the experiments, simulated the device, and analyzed the data. M. L and T. T grew the samples. S. K and M. L. performed the high- temperature measurements. M. L wrote the paper. All authors discussed the results and commented on the manuscript.
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Methods
The fabrication of the single crystal diamond (SCD) MEMS/NEMS were started with an ion-implanted high-pressure high-temperature (HPHT) type-Ib SCD substrate. The ion implantation into the HPHT diamond substrate was conducted by carbon ions at an energy of 180 keV with a dose of 1016cm-3. After a chemical cleaning (H2SO4+HNO3) of the ion implanted HTHT SCD substrate, a microwave plasma chemical vapor deposition (MPCVD) system was utilized to grown the diamond homoepitaxial layer. The growth conditions for the MPCVD were as follows: a methane concentration of 0.1%, a hydrogen gas flow of 500 sccm. The microwave power was 900W and the temperature was about 920oC. During the CVD growth, the damaged layer caused by ion implantation was transformed into graphite.
After the CVD homoepitaxial growth, the diamond sample was annealed in UHV chamber for 6 hrs at 1100oC. A laser photolithographic process was performed to pattern the SCD MEMS/NEMS devices. An aluminum layer with a thickness of 150 nm was deposited on the patterned SCD substrate as a mask for the reactive ion etching (RIE) of the diamond sample by an inductively couple plasma (ICP) apparatus. The RIE conditions were: an oxygen flow of 90 sccm, a rf power of 800 W and a bias power of 20W. The thickness d of the MEMS cantilevers is around 2.8 µm and of the NEMS cantilevers is around 530 nm. The width W is 12 µm and the length L is 60-140 µm for both of the MEMS and NEMS cantilevers. The etching depth is around 4.6 µm for the MEMS and 1.2 µm for the NEMS. The metallization was then chemically removed by a boiling acid of a mixture of H2SO4+HNO3. This released the formation of the SCD cantilevers in the implanted region, leading to the all-diamond MEMS/NEMS. Finally, a laser photolithographic method was used to pattern the SCD wafer with SCD cantilevers. A Ti layer with a thickness of 3 nm follower by a Au layer with a thickness of 15 or 30 nm (denoted by Au/Ti later) was then deposited on SCD wafer as source, drain and gate electrodes by an e-beam vaporator.
The SEA transduction of the SCD resonators at room temperature was measured by a probe station in a vacuum chamber with a pressure lower than 10-3Pa. For high- temperature sensing and actuation, a probe station with a ultra-high vacuum lower than 10-6 Pa was adopted. The vacuum was kept to be lower than 10-5Pa even at 873 K.
The optical readout of the resonance frequency of the SCD resonator was performed by a laser Doppler velocity (LV 1710) technique in a high vacuum chamber with a pressure of 10-3 Pa. A piezoelectric ceramic was placed below the SCD MEMS wafer to actuate the cantilever vibration. A rf signal was applied to the piezoelectric ceramic and the optical signal reflected from the vibrating cantilever was readout by a lock-in amplifier.
Alternatively, on-chip actuation was also performed for optical readout of the on-chip MEMS/NEMS.
The FEM simulation of the electric field distribution was performed by the software of Quickfield Professional. The calculation was conducted in static condition. For the SEA transduction, the top surface of the SCD resonator was set to be electrically ground as the boundary condition. The dielectric constant of 5.7 was utilized. For the dielectric actuation, the resonator was set to be suspended as the boundary condition.
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Figure 1
Figure 1 | SEA transduction scheme for on-chip MEMS/NEMS. a, Schematic diagram of the double source 1ω method. An actuation rf signal with an amplitude of 𝑉𝑔𝑎𝑐 and a frequency ofωis applied to the gate. A carrier rf signal with an amplitude of 𝑉𝑑𝑎𝑐 and a frequency of ω+△ω is applied to the drain. The signal through the drain of the SCD MEMS/NEMS is demodulated at △ω by a lock-in amplifier. The self-sensing S-D electrodes on the SCD cantilever contain 3 nm-thick Ti followed by 15 or 30 nm-thick Au. The width of the S-D electrodes is 3 µm and length is 50 µm. Scale bar: 30 µm. b, FEM simulation of the electric field distribution of the SCD MEMS with SEA transduction. The S-D electrodes are grounded and 𝑉𝑔𝑎𝑐=0.8 V. c, Resonance frequency spectrum for on-chip MEMS cantilever with dimensions of L=60 µm, W=12 µm, and d=2.8 µm. A concomitant abrupt change of the measured phase appears at the resonance.
d, SEA transduction for on-chip NEMS cantilever with dimensions of L=100 µm, W=12 µm, and d=530 nm.
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Figure 2
Figure 2 | Tuning the resonance frequency of the MEMS by using the SEA transduction and adjusting 𝑉𝑑𝑎𝑐. a, Electrical readout of the resonance frequency at various 𝑉𝑑𝑎𝑐 with 𝑉𝑔𝑎𝑐fixed at 1 V. The resonance frequency shifts downward as 𝑉𝑑𝑎𝑐 increases. b, Relationship between the resonance frequency and 𝑉𝑑𝑎𝑐. The resonance frequency is tailored from 2.65069 to 2.64959 MHz as 𝑉𝑑𝑎𝑐 increases from 2 to 10 V. c, Relationship between δf/δ𝑉𝑑𝑎𝑐 and 𝑉𝑑𝑎𝑐, showing a linear dependence. d, Nearly quadratic increase of the on-chip electrical readout with 𝑉𝑑𝑎𝑐 , disclosing the effect of self-sensing enhancing actuation.
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Figure 3
Figure 3 | Comparison of the readout sensitivities of the MEMS with on-chip SEA actuation and external piezoelectric actuation. a, Electrical readout of the MEMS with external piezoelectric actuation. 𝑉𝑑𝑎𝑐 is fixed at 10 V. When 𝑉𝑝𝑎𝑐 = 1 𝑉, the resonance signal is buried in the noise. The resonance frequency is consistent with that of on-chip actuation at the same 𝑉𝑑𝑎𝑐. b, Plots of the electrical readout sensitivity with on-chip SEA actuation and external piezoelectric actuation at 𝑉𝑑𝑎𝑐=10 V. The on-chip sensitivity S=Vout/(Vdriv𝑉𝑑𝑎𝑐) is more than 45 times larger in the SEA case than in the external piezoelectric actuation case. Vdriv denotes the rf amplitude applied to either the on-chip gate electrodes or the piezoelectric ceramic. c, Optical readout of the cantilever with S-D electrodes by on-chip actuation at different 𝑉𝑔𝑎𝑐 . Note that the S-D electrodes on the cantilever are suspended. d, Plots of the optical readout sensitivity in the on-chip actuation and external piezoelectric actuation. The on-chip sensitivity, defined as the slope of the plot, is more than 5 times larger than in the external actuation. The optical signal is measured with the same geometrical alignment.
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Figure 4
Figure 4 | High-temperature frequency response of another on-chip SCD MEMS with similar dimensions using SEA. Electrical readout a, at RT with 𝑉𝑑𝑎𝑐=3 V and 𝑉𝑔𝑎𝑐=2 V. The dark plot is the measured phase. b, from 323K to 673 K in 50 K step with 𝑉𝑑𝑎𝑐=3 V and 𝑉𝑔𝑎𝑐=2 V. c, from 773 K to 873 K in 50 K step with 𝑉𝑑𝑎𝑐=10 V and 𝑉𝑔𝑎𝑐=1 V. The SEA transduction is achieved up to 873 K. d, Resonance frequency shift with the measurement temperature. The temperature coefficient of frequency is 5-11 ppm/K.
