In this paper we investigate the influence of tilted electrostatic actuators on the imaging in electron microscopes. Previous studies have shown image deformation by electrostatic actuation in electron microscopes, and suggested a strong influence of the tilt angle on the deflection of the electron beam. Our model suggests an influence of 1 \mu m/$^\circ$ of the tilt angle on the amplitude of deflection. Preliminary experimental results indicate an influence of the angle on the amplitude of deflection, but the observed effect is two orders of magnitude smaller than simulations predict. However, there are still some variables we have not explored that might influence the strength of the influence of tilt

Abelmann, L.

Been, A.J.

Vermeer, R.

Woldering, L.A.

University of Twente Research Information

EFFECTS ON IMAGING BY TILTING ELECTROSTATIC ACTUATORS IN ELECTRONMICROSCOPY.A.J. Been, R. Vermeer, L.A. Woldering and L. AbelmannMESA+ Institute for Nanotechnology, University of Twente, P.O. Box 217, 7500 AE Enschede, The Netherlands.Abstract — In this paper we investigate the in-fluence of tilted electrostatic actuators on the imag-ing in electron microscopes. Previous studies haveshown image deformation by electrostatic actuationin electron microscopes, and suggested a strong in-fluence of the tilt angle on the deflection of the elec-tron beam. Our model suggests an influence of 1 µm/◦of the tilt angle on the amplitude of deflection. Pre-liminairy experimental results indicate an influenceof the angle on the amplitude of deflection, but theobserved effect is two orders of magnitude smallerthan simulations predict. However, there are still somevariables we have not explored that might influencethe strength of the influence of tilt.Keywords: MEMS, Electron microscopy, Electro-static actuationI – IntroductionMicro Electro Mechanical System (MEMS) devicesare used in numerous applications, yet are still veryscarcely employed in electron microscopy. We investi-gate the potential of using MEMS devices inside elec-tron microscopes and the influence of their actuation onthe imaging in these microscopes. One of the forseenapplications of a MEMS device in an electron micro-scope is moving a small apperture with nanometer pre-cision.The disadvantage of imaging a MEMS device in anelectron microscope is that electrostatic actuation canstrongly affect the path of the electron beam, and thusdeform the image that is obtained by the microscope.An extensive study on this subject was reported by us atMME 2011 [1].We have continued our research to investigate theinfluence of several configuration parameters in a SEMon the amplitude of deflection of the electron beam,which is observed as a deformation in the images. Theamplitude of deformation in the SEM images was mea-sured and compared with simulation results. These re-sults show that the measured values are about four tofive times larger than the actual simulated values [2].Our suggested explanation for this observation is thata tilt of the device with respect to the horizontal plane,see Figure 1 for a situation overview, increases the mea-sured deformation significantly. Indeed simulations ofthis effect show a dependence of about 1 µm per degreeof tilt on the amplitude of deformation. The results ofthe simulations of the device under different tilt anglescan be found in Figure 2 (taken from [2]).Figure 1: Illustration of the situation inside the SEM. The elec-tron source emits electrons at the sample, which is actuated.Electric fields generated by the actuation are indicated bythe red arrow. The detector on the right counts the numberof secondary electrons emitted from the sample. The samplecan be tilted as shown, with the tilt axis perpendicular to theimage.To confirm that tilt has a great influence on the am-plitude of deflection, we are performing tests in a SEM.Preliminairy results of these tests are presented in thisabstract. In order to take into consideration any effectthe detector position may have, we also vary the experi-mental situations such that the observed edge is orientedtowards and away from the detector.II – Experimental DetailsTo measure the amplitude of deformation, we usedMEMS devices as described in [3]. These devices havea symmetrical nature, with a suspended table that isactuated by two identical combdrives on either side ofthe table. This device is shown on the left in Figure 3.The device is mounted on a custom home-built deviceholder and installed on the sample table of the SEM.Experiments were performed in a FEI Quanta 450 SEM,with an Everhart-Thornley Detector at 250 V bias to de-tect the secondary electrons [4]. The SEM is fitted withwiring that enables us to control the device from outsidethe vacuum chamber. The voltage applied to the comb-drive is generated by an Agilent 33120A waveform gen-erator, amplified by an ESyLAB LM3325 8 channel HVAmplifier with an ESyLAB LM3322 HV Power Supply.Connecting coax cables are of the RG58c/u MIL-C-17fand RG174/u MIL-C-17f lf types. Follow up measure-ments using biased actuation were performed using aP-265 HV amplifier (100 ×) and an Agilent 54622D os-cilloscope. We measured the amplitude of deformationin the SEM images under a constant working distanceFigure 2: The simulated amplitude of the deformation Adef asa function of the angle of the SEM sample table with respectto the horizontal plane. A negative amplitude means a phaseshift of 180◦ with respect to the positive amplitude at an angleof the table of 0◦.of 12 mm and magnification of 20000×.In order to see a large effect of the electrostatic ac-tuation on the SEM imaging, we chose a low accelera-tion voltage of 2 kV. Accross all measurements, we keptthe beam current of 57 pA constant. SEM images weretaken for four actuation and measurement situations, seeFigure 3. These situations were chosen to investigatethe influence of the detector on the amplitude of de-formation for tilted devices. In addition we investigatedwhether the amplitude of deformation depends on thelocation of actuation. For each situation we varied thetilt of the sample between −4◦ and 4◦ with respect tothe horizontal plane.To determine the amplitude of deflection from theimage, we measured the distance between the peaks byhand. Since we align the device perpendicular to thedetector at the beginning of the experiment, we can takethe horizontal position of the two extremes and dividethe difference by the scale of the image to obtain theamplitude of deformation in micrometers.III – Results and DiscussionAs we performed the measurements of the four dif-ferent situations, we came across two situations wherethe edge of the structure could not clearly be measured.These are indicated as situation two (for positive an-gles) and situation four in Figure 3. When observingthe actuated side, the contrast difference is too largeto be able to see the edge throught a full cycle, seeFigure 4. Observations of the unactuated side are muchmore clear, see Figure 5. Our solution was to apply abias to the actuation, such that the applied voltage neverchanged sign. Using the oscilloscope we measured thatthe applied voltage was from −18.0 V to −55.3 V.We were unable to obtain clear images for experi-mental situation four, despite applying a bias to the ac-tuator. The images for experimental situation two wereclear enough for some measurements for both the biasedFigure 3: Top: Scanning electron micrograph of the device thatwas used for our experiments. In the middle a movable tableis shown that can be actuated by the electrodes on the left andthe right side. Contrast changes in the image are caused bythe electrode on the right that is connected to a 1 Hz, 40 Vppsine wave.Bottom: This schematic drawing shows the four differentmeasurement situations. For each situation the eye symbolshows the side which is observed and the source symbol whichside is actuated.Figure 4: Scanning electron micrograph of the actuated sideof the combdrive on a 1 Hz 40 Vpp sine. On the left side thedevice layer is shown, while on the right side the supportingsubstrate is observed. The device is tilted by 1.5◦ towardsthe detector and the observed edge is oriented towards thedetector. No clear edge can be defined to determine theamplitude of deformation, see arrow.Figure 5: Scanning electron micrograph of the unactuatedside of the combdrive. On the left side the device layer isshown, while on the right side the supporting substrate isobserved. The device is tilted by 1.5◦ towards the detectorand the observed edge is oriented towards the detector. Aclearly defined edge can be observed, see arrow. This edgehas a sinusoidal appearance due to the varying electric fieldgenerated by actuation of a combdrive approximately 1 mm tothe left of this structure.and unbiased cases. The results are shown in Figure 6.We observe that there is no significant difference be-tween the unbiased and the biased cases. The measuredresults of the experimental situations one and three areshown in Figure 7 and Figure 8 respectively. Our ob-servations suggest a dependence of the amplitude ofdeformation on the angle. However this dependence ismuch smaller than our models predict. The effect of thetilt is about 0.02 µm/◦ and only 0.5 % to 1 % of the totalamplitude of deformation. The results also indicate thatthe influence of tilt is about the same when both theactuated and the unactuated side are compared to eachother.Note that in the results the dependence on the an-gle is positive in Figure 7, while it is negative in Fig-ure 6 and Figure 8. The reason for this is that for situa-tion one the side of the device-layer is tilted away fromthe electron beam for positive angles. For situations twoand three the side of the device-layer is tilted towardsthe electron beam for positive angles. Apparently whenthe side is tilted away from the electron beam, the am-plitude of deflection is higher. A larger amplitude ofdeflection occurs when the actuated combdrive is closerto the electron beam than when the device is tilted theother way.Another phenomenon we encountered during our mea-surements is a difference in the amount of deflection ofthe electron beam for tilt angles where both edges ofthe device layer is shown. That is, when the device istilted in such a way that both the edge (A) between thetop of the device-layer and the side of the device layer,as well as the edge (B) between the side of the device-layer and the surface of the substrate can be observed. 0.65 0.7 0.75 0.8 0.85 0.9 0.95 1 1.05 1.1-4 -3 -2 -1  0  1  2  3  4Amplitude of deflection Adef (µm)Tilt angle  (o)Figure 6: Measured results for experimental situation two. Theblue results show the unbiased measurements, while the redresults show the biased measurements. The unbiased results(blue) were displaced in the graph by −0.1◦ tilt angle toprovide a clearer image. We observe that there is indeed aninfluence of the angle on the amplitude of deformation. Theline is a linear fit that serves as a guide to the eye. 1.4 1.5 1.6 1.7 1.8 1.9 2-4 -3 -2 -1  0  1  2  3  4Amplitude of deflection Adef (µm)Tilt angle  (o)Figure 7: Measured results for experimental situation one. Themeasured amplitude of deformation is shown against the tiltangle. The results suggest a small dependence on the angle.The line is a linear fit that serves as a guide to the eye. 0.8 0.85 0.9 0.95 1 1.05 1.1 1.15-4 -3 -2 -1  0  1  2  3  4Amplitude of deflection Adef (µm)Tilt angle  (o)Figure 8: Measured results for experimental situation three.The measured amplitude of deformation is shown against thetilt angle. The results suggest a small dependence on theangle. The line is a linear fit that serves as a guide to the eye.Figure 9: A scanning electron micrograph of a tilted device.The bottom side (B) shows a much higher deflection comparedto the top side (A) of the structure. The deflection for bottomside B is 0.9 µm, whereas for the top side the deflectionis 0.2 µm. This structure was actuated with a 1 Hz, 40 Vppsine wave, but with other settings of the SEM than the otherexperiments1.This phenomenon is shown in Figure 9 1. We expect thatthis effect is caused by strong electric fields between theside of the device and the substrate. The fields aroundthis area are illustrated in Figure 10 (taken from [2]).The observed effects are in agreement with the as-sumption made in our model that the incident electronbeam is deflected by the electric stray field. We do notunderstand at this point why the observed influence ofthe tilt angle is several orders of magnitude smaller thanestimated from our calculations.A. Future workOur next goal is to explore more parameters andwe are hopeful to find a clearer influence of the tilt.Also, we would like to improve the method that wasused to measure the amplitude of deformation by usingthe procedure of fitting a periodic function to the edgeof the device. Since the device is actuated with a sinewave, we can use this information to acquire deflec-tion results more accurately. Furthermore we are con-sidering whether our simulation model needs to be im-proved. Other subsequent steps of our investigation areto vary the working distance in the SEM, as well asthe acceleration voltage. We also would like to furtherinvestigate the influence of the postition of the detectoron the amplitude of deformation. Finally we suspectthat other structures on the device could be affectingour measurements and we would like to eliminate theirinfluence from our measurements.1This image was taken using different settings than described inExperimental Details: magnification 10000×, acceleration voltage2.00 kV, beam current 57 pA, working distance 14.9 mmFigure 10: Illustration of the electric field around the comb-drives. The circle shows strong fields between the side of thedevice and the substrate. Edges A and B are also indicated inFigure 9IV – ConclusionWe investigated the effect of the alternating electricstray field emerging from electrostatic actuators on theimage deflection in a SEM. Simulations show a strongdependence of 1 µm/◦ of the amplitude of deformationon the tilt of the device in an electron microscope. How-ever, preliminairy tests only show a 0.02 µm/◦ influence.At this point we do not understand why the experimen-tal results are two orders of magnitude smaller than oursimulations.Our measurements indicate that the results for ob-serving the actuated side of our device are comparableto the results for observing the non-actuated side, andno significant change was measured when applying abias to the actuation voltage.We observed that a larger amplitude of deformationoccurs when the actuated combdrive is tilted closer tothe electron beam than when the device is tilted theother way, which is as expected.Finally the bottom side of structures show a muchhigher deflection than the top side. This indicates thepresence of strong electric fields near the edge of thedevice layer, like our model predicts.References[1] R. Vermeer, L. A. Woldering, and L. Abelmann. Effect ofelectric fields generated by microactuators on the imagingin electron microscopy. In Proceedings of the 22ndMicromechanics and Microsystems Technology EuropeWorkshop, Tonsberg, Norway, pages 222–225, Norway,2011.[2] R. Vermeer. Electrostatic actuation of mems devicesin electron microscopy. Master’s thesis, University ofTwente, MESA+ Institute for Nanotechnology, Universityof Twente, PO Box 217, 7500AE Enschede, the Nether-lands, October 2011.[3] J.B.C. Engelen, H. de Boer, J.G. Beekman, L.C. Fort-gens, D.B. de Graaf, S. Vocke, and L. Abelmann. TheMicronium — A Musical MEMS Instrument. Journal ofMicroelectromechanical Systems, 21(2):262 –269, 2012.[4] H. Seiler. Secondary electron emission in the scan-ning electron microscope. Journal of Applied Physics,54(11):R1–R18, 1983.

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Effects on imaging by tilting electrostatic actuators in electron microscopy

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Effects on imaging by tilting electrostatic actuators in electron microscopy

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