Dual Coil Patterned Ultra‐Thin Silicon Film Enable by Double‐Sided Process

Double‐sided microfabrication process on an ultra‐thin silicon film has rarely been attempted due to the challenges in terms of the preparation and handling of a thin film in spite of its promising fabrication potentials. Such a process allows for doubling the thin film device density or providing dual functionalities for a thin film depending on whether the front and back sides of a thin film are processed identically or distinctively. Here, a novel double‐sided thin film processing strategy is introduced by realizing a dual coil patterned ultra‐thin silicon film that is working as an actuating or energy harvesting system. Experimentally, a dual coil patterned thin film enabled using the introduced approach shows remarkably enhanced device performance when compared with a single coil patterned counterpart. Furthermore, a multiphysics simulation model is developed and the resultant modeling data validate the experimentally measured performance enhancement. Finally, the structural durability of the thin film upon cyclic loading is tested and its diverse vibration modes are investigated.


Dual Coil Patterned Ultra-Thin Silicon Film Enable by Double-Sided Process
vide a film with dual functionalities if two sides of the film are patterned differently. Moreover, it can be potentially used for providing more space on fabricating flexible or stretchable sensors or functional circuits that can be used as a wearable devices. [3][4][5][6][7] In accordance with this strategy, patterning devices on both sides of a UTS significantly increases device density that benefits integrated circuit research fields through giving a possibility to continue or pass beyond Moore's law. Especially, a recently introduced pattern transfer technique shows that a UTS can be readily doubleside processed upon controlling the adhesion between a temporary substrate and a UTS. [1] In the medium of air, the substrate-UTS adhesion is high enough to do conventional microfabrication processes on the UTS. Whereas if those adhered substrate and UTS are in the medium of a low surface energy liquid the adhesion in between decreases such that the UTS self-delaminates from the substrate. At this point, one can flip and transfer the UTS to another temporary substrate and proceed with further microfabrication on the back side.
To the best of our knowledge, the double-sided process besides the foregoing technique has been rarely studied. One research presents double-sided microelectrode arrays that were fabricated on 25-50 µm thick UTS films. The process includes bonding an UTS film to a carrier substrate, patterning the frontside, releasing, flipping, and patterning the backside. [8] Another study demonstrates a transistor array where its UTS film was thinned down to 6 µm by wet etching in tetramethylammonium hydroxide (TMAH). For wafer handling, a donutshaped silicon nitride was implemented to prevent the wafer edge from etching by TMAH and to form a supporting ring. [2] UTS preparation and handling is one of the key factors in fabricating dual-sided devices on UTS. A variety of UTS preparation methods have been developed yet to produce just singlesided ultra-thin chips. [9] Grinding the thick wafer into UTS is one of the popular methods. [10][11][12][13] Here the wafer is bonded to a chuck to be ground and polished. Depending on the tool and technique used, the resulting UTS can be thin as a few microns and the process can be done in a few minutes. [14,15] However, the warpage or deflection induced by high stress can be a problem while subsequential handling and processing. Therefore techniques such as Dicing-Before-Grinding (DBG) [16,17] and TAIKO [18,19] are used to avoid the downside of the grinding method. Moreover, dry etching such as reactive ion etching, [20,21] or XeF 2 etching [22] can also be used to get UTS. Dry etching is Double-sided microfabrication process on an ultra-thin silicon film has rarely been attempted due to the challenges in terms of the preparation and handling of a thin film in spite of its promising fabrication potentials. Such a process allows for doubling the thin film device density or providing dual functionalities for a thin film depending on whether the front and back sides of a thin film are processed identically or distinctively. Here, a novel doublesided thin film processing strategy is introduced by realizing a dual coil patterned ultra-thin silicon film that is working as an actuating or energy harvesting system. Experimentally, a dual coil patterned thin film enabled using the introduced approach shows remarkably enhanced device performance when compared with a single coil patterned counterpart. Furthermore, a multiphysics simulation model is developed and the resultant modeling data validate the experimentally measured performance enhancement. Finally, the structural durability of the thin film upon cyclic loading is tested and its diverse vibration modes are investigated.

Introduction
It is highly desired to enable the double-sided process on a thin film or patterning both the front and back sides of a thin film for these reasons. It may simply double the identical pattern on an ultra-thin silicon (UTS) film. [1,2] Additionally, it may pro-favorable to stress relief and it is more effective when combined with grinding. However, thinning down a thick silicon substrate into UTS solely by dry etching is not effective in terms of time, cost and environmental issues. A chemical reaction based wet etching can also be used to thin down a silicon substrate in an agent such as potassium hydroxide (KOH) and TMAH. The etch rate is controllable by changing concentration or temperature, and the etched area can be selected by covering the wafer with mask layers commonly made of SiN x or SiO 2 . [23][24][25][26] Wet etching can be promising owing to low cost and high scalability. Furthermore, wet etching can remove the surface stress or damage that has been generated from other thinning methods such as grinding. However, the removal of bubbles generated and precise temperature control while etching that is essential to get a very high thickness uniformity are yet challenging.
For UTS handling, bonding is the mainstream method. [8,[27][28][29][30] UTS can be bonded to a temporary handle layer and it can be debonded after all processes are complete. This can avoid UTS from breaking, cracking, bending, or warping. Otherwise, a custom designed carrier can be also used to handle UTS where electrostatic, [31,32] or Bernoulli [33] force is used to hold the wafer. While the aforementioned methods all require a sort of carrier to handle the UTS, TAIKO method is carrier-free since it leaves the peripheral ring unaffected when grinding such that the ring is utilized to handle the UTS. [18,19] In this paper, we explore the fabrication potential of the doublesided process that relies on the self-delamination based "pattern transfer" technique. [1] While the previous paper introduced this technique involving material deposition on the backside, it did not demonstrate subsequent material patterning. Here, we showcase a device with backside patterning achieved through photolithography and wet etching. As an example, we present a dual coil patterned ultra-thin silicon (DCUTS) that includes doubleside fabricated coils on a 15 µm thick UTS. The UTS film is pattern-transferred from an SOI (silicon on insulator) wafer by eliminating a sacrificial buried oxide layer. [1] Here, we adopt a unique transferring and flipping process along with traditional microfabrication processes on both the front and back sides of UTS to fabricate the DCUTS. The proposed method does not need a bonding step to handle UTS. Instead, the UTS is freely transferred, flipped, or fixed on a temporary mediator substrate by controlling the adhesion between the UTS and the mediator substrate. First, we summarize a fabrication procedure including UTS preparation, transfer, flipping, and coil patterning. Next, we show the fabricated DCUTS functioning as an actuator followed by functioning as a vibrational energy harvester. For each case, the displacement or current is collected based on the input current or movement, respectively. The overall mechanism is modeled employing MATLAB Simulink and compared with the experimental results. Lastly, the laser beam reflected from a vibrating DCUTS is captured and demonstrated, which shows a critical evidence for the multi-modal vibration of DCUTS.

Preparation of UTS Films
The 15 µm thick and 20 × 20 mm 2 wide and long UTS for DCUTS (Figure 1a) was prepared using a previously reported method. [1] Here, an SOI prime silicon wafer (Ultrasil Corp.) with a 15 µm thick device layer and a 1 µm thick buried oxide layer was used. The device layer was intrinsic, also known as undoped, such that the top-side and bottom-side patterns might be electrically isolated. The SOI wafer was defined with photoresist into desired UTS shape involving the array of etch holes and the device layer was etched with inductively coupled plasma deep reactive ion etcher (Plasmatherm, SLR 770). As shown in Figure 2a, the patterned SOI wafer was submerged in hydrofluoric acid to eliminate the buried oxide layer. The etch rate of the oxide layer was close to 1 µm min −1 , however, it could stay in the bath for a sufficient amount of time due to the highly selective etch rate between the oxide and the silicon. Once the oxide layer was completely removed, the wafer was cleaned with acetone, isopropanol (IPA), and deionized (DI) water, and transferred to an acetone bath. The device layer, which was now a UTS, self-delaminates and could be transferred to a silanized glass as a mediator substrate in the bath. The silanized glass here was prepared by coating a single layer of Perfluorodecyltrichlorosilane (FDTS) on a glass slide using a molecular vapor deposition (Applied MicroStructures, Model 100). The UTS on silanized glass taken out from the acetone bath was now ready for the traditional microfabrication processes.

DCUTS Fabrication by Double-Sided Process
In Figure 2b, 5/125 nm thick Cr/Au layers were deposited using a sputtering system(AJA, ATC Orion Series). Although there was sufficient adhesion between the UTS and the silanized glass for this process, one might wanted to eliminate any possibility of the UTS falling off since this deposition system holds the sample upside down. In this case, the UTS could be fixed onto silanized glass using water-soluble tape on two diagonal corners to prevent it from falling off. Next, the photoresist (Microchemicals, AZ 5214E) was spun on the UTS at 3000 rpm www.advmatinterfaces.de for 30 s and then soft baked for 2 min at 110 °C on a hotplate. This photoresist was selectively exposed to a laser of a maskless aligner (Heidelberg, MLA150) for 150 mJ cm −2 and developed in a bath of 1:4 AZ 400 K (Microchemicals) and DI water. Then the gold and chrome layers were selectively wet etched into a coil design. If the water-soluble tape was used, it could be removed in this step by immersing it under DI water and gently rubbing it off with a cotton swab. Then the front side patterned UTS was again submerged in the acetone bath and flipped over under the bath using a flat, wide, and sharp object such as a razor blade with the back side facing up after its selfdelamination. The patterning of the back side was identical to the foregoing patterning of the front side. Afterward, the UTS was placed to a Polytetrafluoroethylene (PTFE) tape covered glass using a tweezer under the acetone bath. A silver paste was used to establish an electrical connection and to minimize the contact resistance between the copper tape and the gold pad shown in Figure 1b. The silver paste was also used to build a through-silicon via (TSV) connection between the top and bottom Au coil center pads (Figure 1c). Finally, another PTFE tape covered glass was placed over the first glass forming the final clamped cantilever-shaped DCUTS.

Modeling using MATLAB Simulink
The established two different MATLAB Simulink models to analyze the mechanical and electromagnetic characteristics of the DCUTS when it functions as an actuator as well as an energy harvester. Figure S1b,c (Supporting Information) show simplified block diagrams for the DCUTS mechanism (Figure S1a, Supporting Information) for actuation and energy harvesting, respectively. The block diagram starts with the force equilibrium of the mechanical domain that is, Here, x, b, k, m and F M is displacement, damping coefficient, stiffness, mass of DCUTS and its feedback electromechanical force, respectively. my is an external vibrational force to harvest energy that does not exist in the actuator scenario. The acceleration x is integrated to get velocity x and again to get displacement x. The velocity and displacement not only feedback the acceleration due to damping and restoring force, respectively, but also create an induced electromotive force that is defined as, Figure 2. The fabrication process of the dual coil patterned ultra-thin silicon(DCUTS). a) Process flow from an SOI wafer to UTS on a silanized glass slide. b) The sequential front and back side coil patterning on UTS to form DCUTS that is clamped by PTFE tape-covered glass slides.

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where N is the number of windings of the coil, B is the magnetic flux density, and A is the area of the coil. [34] Since the DCUTS stays above a stationary permanent magnet, it is assumed dA dt 0 = . By applying chain rule, Equation (2) becomes, Since z = x + const, dz dt can be substituted by x . Also, is the transduction factor that is later used to determine the feedback electromechanical force in Equation (8). The axial(B z ) and radial(B ρ ) component of the magnetic flux density of a cylindrical magnet is each defined as, [ Here, R is the radius of the external magnet, (ρ, z) is the coordinate for the location of B ρ and B z as shown in Figure S1a (Supporting Information) and B r is the remanent flux density of the external magnet. The magnetic flux density along the center axis can be simplified as, [36,37] Where L coil and R coil is the inductance and resistance of the coil, and R load is the resistance of the load. Finally, the electromechanical force that feedbacks into the mechanical domain is represented as, Figure S1b (Supporting Information) represents the block diagram for the DCUTS being used as an actuator where the external current i(t) inserted as an input and the output displacement x is collected. Similarly, Figure S1c

Actuating System
The reported DCUTS can function as an actuator upon an external magnetic field when an alternating current (AC) is applied to it. The experimental setup is illustrated in Figure 3a where the DCUTS is placed over a 2 inch diameter and 2 inch thick grade N52 neodymium magnet (K&J Magnetics). The copper tape electrodes are connected to a function generator (GW Instek, AFG-2005) and the resulting displacement at the center of the DCUTS is measured using a laser Doppler vibrometer (Polytec, VibroGo). An example of raw data collected from the vibrometer is shown in Figure S2a (Supporting Information) when 53 Hz, 5 volt peak-to-peak sine wave is applied from the function generator. This is converted into a frequency domain through a fast Fourier transform as depicted in Figure S2b (Supporting Information). In Figure 3b,c, we collect and compile the maximum magnitude from the frequency domain for each different frequency applied from the function generator. Four lines from Figure 3b represent different combinations of a single or dual coil and 5 or 1.25 V inputs. The resulting resonance amplitude increased by 71.5% for 5 V input and 86.9% for 1.25 V input for the dual coil compared to the single coil. Doubling the number of turns of the coil increases the amplitude significantly but it does not exactly double the amplitude presumably due to the increase in coil resistance. The resonant frequency of the dual coil is higher than that of the single coil majorly because the difference in clamping length ( Figure S3, Supporting Information) that may vary due to the assembly alignment error may shift the resonant frequency. This clamping length-dependent frequency shift is supported by the finite element analysis that ≈6 Hz of resonant frequency increases as the clamping length increases by 1 mm (Figure S3, Supporting Information).
Furthermore, the resonant frequencies tend to shift down when a higher voltage source is given. This is due to the system nonlinearity that is occurred by the increase in magnetic force when the DCUTS more closely approaches the magnet. In Figure 3c we present a simulated result from a MATLAB Simulink model summarized in Figure S1b (Supporting Information) and the result also verifies the shifting down of the resonant frequency from 62 to 59 Hz as the applied voltage increases from 1.25 to 5 V.

Energy Harvesting System
The reported DCUTS can also function as an energy harvester when external vibration is applied. The experimental setup is illustrated in Figure 4a where a permanent magnet is fixed on a linear motor stage (Aerotech). The PTFE tape covered glass that holds DCUTS is adhered onto the flat surface of the magnet module and the copper electrode is connected to an oscilloscope (Keysight, DSOX2012A) to measure the output voltage. Additionally, a vibrometer is used to monitor and verify the input linear motion that is programmed from a linear stage machine controller (Aerotech, A3200) and the resonance of the DCUTS.

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The input linear motion with 9.8 m s −2 acceleration at various frequencies is applied and the measured output voltage resulting from this external vibration is shown in Figure 4b. A peak voltage at resonance for the single coil is 0.737 V that increases by 44% to 1.064 V through adopting the dual coil design. Moreover, related simulation results from the Simulink block diagram model in Figure S1c (Supporting Information) are plotted as dotted lines. Both single and dual coil cases have shown a similar result of a 38% increase in resonant voltage peak change.
To analyze the durability of the DCUTS its displacement change is tracked while the resonant vibration at 53 Hz upon 5 V input lasts for 23 million cycles as shown in Figure 5. The linear fit of the displacement has a decreasing trend. Nevertheless, it is only expected to decrease by 7 µm if the resonant vibration lasted for 1 billion cycles that is a little over 218 days of continuous run. Instead, the DCUTS has shown more sensitive displacement changes depending on temperature. The daytime temperature is high, thus the resistance rises that results in the decrease of the displacement. Undoubtedly, the nighttime displacement increases vice versa.

Vibrating Mirror System
Similar to an actuation system, the DCUTS may be exploited as a vibrating mirror system. To this end, a laser beam is shot on a vibrating mirror, i.e., the DCUTS and the reflecting laser beam is captured. The test setup is illustrated in Figure 6a where the DCUTS is attached to the fixed magnet and the copper electrodes are connected to a function generator. The laser source installed on a fixture shoots the beam with an angle to the DCUTS and the reflected beam reaches a black cardboard. The laser beam is aimed at two different spots A and B as illustrated in Figure 6b that represents the first three mode shapes with resonant frequencies at 59, 140, and 360 Hz, respectively. ( Figure S3, Supporting Information) These resonant frequencies are predicted employing FEA results assuming the clamping length (l) is 1200 µm. Figure 6c,d each show the reflected laser beam trajectories on spots A and B, respectively for different frequencies  www.advmatinterfaces.de (Movie S1 and S2, Supporting Information). It is noted that the afterimage of the laser trajectories for higher frequency reflection can be easily captured in one frame of the video. However, an image merging process using software is required for lower frequencies since one frame from the video cannot fully express the full cycle of the laser beam trajectory. Here, a few distinctive characteristics of the DCUTS have been observed. First, the case at 20 Hz shows a fairly vertical trajectory since the 1 st mode shape vibration is dominating. Once the resonant frequency of 53 Hz is reached the vertical trajectory peaks to its maximum and remarkable amounts of horizontal trajectory are added. It is noteworthy that reflections of spot B involve more frequent horizontal trajectories than spot A presumably since the 2 nd mode shape vibration is larger at spot B. Furthermore, as the frequency increases up to 130 Hz, the vertical trajectory diminishes and the horizontal trajectory takes a major role that can also be interpreted as the mitigation of 1 st mode shape vibration and the expansion of higher mode shape vibration at 130 Hz. As a control test, single-sided coil reflecting the laser beam is depicted in Figure S4 (Supporting Information). It can be noted that the size and resonant frequency of the reflected beam follows the actuation results shown in Section 3.1.

Conclusion
In conclusion, this work presents a method to enable the dual coil patterning on an ultra-thin silicon (UTS) where not only traditional microfabrication but also thin film transferring and flipping processes are combined. This method is one example of the previously reported double-sided process and its fabrication potential is introduced by realizing a dual coil patterned ultra-thin silicon (DCUTS). To promote the efficiency increase of the dual coil patterning, we demonstrate DCUTS used as the actuator and energy harvester where the resonance amplitude of actuation increased by 71.5 to 86.9% and the harvested voltage improved by 44%, respectively. A simulation model is also used to verify these experimental results. Moreover, the durability of DCUTS is tested by vibrating it at resonance for 23 billion cycles. Lastly, the use of DCUTS as a mirror is demonstrated by reflecting a laser beam on it. This reported method is expected to be further exploited to develop higher-density or multi-functional thin film devices for electronics, MEMS and biomedical engineering.

Supporting Information
Supporting Information is available from the Wiley Online Library or from the author.