—In this paper , we discuss a novel, mixed mode 3D XYZ scanner built within a sin gle foundry process. The device has a large range of motion in X, Y and Z (14.0 μm in X,Y and 97.9 μm in Z) and can also rotate about two axes (7.4°) , making it a 5 degree of freedom scanner . Vertical actuation can be accomplished with both thermal actuators, which have a larger range of motion, and capacitive actuators, which are faster , responding full y up to 3.2 kHz. Although it is useful for many applications, including scanning probe microscopy, micrometer scale optical microscopy, and manipulation of biological objects, the device was designed to be a 3D scanner for spray -painting atoms upon a surf ace with nanoscale precision and resolution for nanofabrication . Demonstrating the ability to combine the device with other complicated MEMS systems, it is integrated with an XY scanner designed to serve as a shutter to control the flow of atoms. The full system has 7 degrees of freedom and 12 actuation motors, and because it is built in a low cost commercial foundry with a robust, stable process, it is easy and inexpensive to fabricate multiple copies or integrate into other complicated systems , making a system of systems . Index Terms — Comb Drive, Electrostatic Actuators, Nanopositioning, MEMS, Position Control, Thermal Actuators . I. INTRODUCTION HE ability to position and manipulate objects on the nanoscale is extremely valuable in modern technology. Applications of nanopositioning systems include micrometer scale optical microscopy [1]–[5], scanning probe microscopy (SPM) [6]–[8], optical beam steering [9], [10] , nanomanufacturing [11], [12] , and the manipulation of cells and other biological materials [13], [14] . This list is certainly not exhaustive and it is likely that many more applications will be created as nanotechnology continues to develop. MEMS have several potential advantages in nanopositioning. Due to their scale, they can have high resolutions, high resonant frequencies, high quality factors, and small footprints. In addition, their fabrication processes are low cost per device and allow for high device to device uniformity This work was funded by DARPA Atoms to Product (A2P) Program/Air Force Research Laboratory (AFRL) contract no. FA8650 -15-C-7545 , the National Science Foundation (NSF Grant No.1361948), and the Engineering Research Centers Program of the National Science Fo undation through NSF Cooperative Agreement under Grant EEC -0812056, Grant EEC -1647837, and Grant ECCS -1708283 . L. K. Barrett, T. Stark, R. Lally, and A. Stange are with the Division of Material Science, Boston University, Boston, MA 02215 USA (email: blawrenc@bu.edu ; Thomas.joseph.stark@gmail.com ; rlally@bu.edu ; stange@bu.edu). Fig. 1. Colorized SEM images of the full scanner system. The upper image shows the entire device while the two lower images show expanded views of its various parts. The device consists of two movable plates. The upper plate is the 5 DoF scanner and the lower plate is a 2 DoF or XY scanner . The device comprises 8 electrostatic comb drive motors and 4 thermal bimorph motors. Colors represent different device layers in the PolyMUMPs process by MEMSCAP. [15], [16] . Consequently, a variety of MEMS nanopositioners J. Reeves is with the Electrical and Computer Enginee ring Department, Boston University, Boston, MA 02215 USA (email: jbr2@bu.edu ). C. Pollock is with the Department of Mechanical Engineering, Boston University, Boston, MA 02215 USA (email: cpollock@bu.edu ) M. Imboden is with École Polytechnique Fédérale de Lausanne , Institute of Microengineering, Neuchâtel, Switzerland (email: matthias.imboden@epfl.ch ) D. J. Bishop is with the Electrical and Computer Engineering Department, Physics Dep artment, Division of Material Science, Department of Mechanical Engineering, Department of Biomedical Engineering, Boston University, Boston, MA 02215 USA (e -mail: djb1@bu.edu). A Large Range of Motion 3D MEMS Scanner with Five Degrees of Freedom Lawrence K. Barrett, Thomas Stark, Jeremy Reeves, Richard Lally, Alex ander Stange, Corey Pollock, Matthias Imboden, and David J. Bishop , Member, IEEE T > < 2 with th ree translational degrees of freedom (DoF) have been developed [1], [7], [17] –[21]. Most of these devices rely on electrostatic actuation methods which are fast, low power, and are highly repeatable [15], [16] . Some of these devices require extra assembly or off chip components [1], [18], [19] . Others have limited (<5 µm) vertical range of motion [7], [17] . One of the most complete electrostatic actuators has been demonstrated by Correa et al. [22] and Koo et al. [20] in their partner articles. Their device is capable of moving in a 20 µm cube using a silicon -on-insulator (SOI) process and closed loop control. Another device, demonstrated by Chen et al., is capable of moving in the three translational degrees of freedom with ranges of 8.4 μm × 12.8 μm × 8.8 μm and the three angular degre es of freedom with ranges of 1.1° × 1.0° × 1.9°. The device relies on thermal actuation methods and was made using deep reactive ion etching (DRIE) [21]. As part of an ongoing effort to develop a nano manufacturing technique based on MEMS systems [11], [12], [23] , we developed a system of nanopositioners or scanners consisting of a novel 5 DoF scanner capable of moving in X,Y,Z and rotating about two axis integrated with an XY scanner, as seen in Fig. 1 . The technique is called atomic c alligraphy, and t he idea is to mill nanoscale apertures into the plate of the 5 DoF scanner and use the modified plate as a dynamic stencil to direct the flow of atoms as they are evaporated through a hole in the MEMS substrate on to a target substrate. The 5 DoFs allow the plate to be moved in the desired pattern in X and Y while concurrently the plate can be held paralle l to the target using the two axis of rotation and the gap distance between them can be controlled with the Z actuation. The additional X Y scanner is useful here as a shutter that can start and stop the flow of atoms rapidly. The atomic calligraphy configu ration is outlined in supplemental materials section S1. In this paper, we focus on the design and behavior of the novel 5 DoF scanner. The XY scanner pictured in Fig. 1 is functional, and its presence demonstrates the ability to easily integrate the 5 DoF scanner with other complicated MEMS systems. It is not discussed in more detail because its design and behavior has been discussed elsewhere [11], [12], [24] . Though designed to be a dynamic stencil, the 5 DoF scanner is generally useful in applicatio ns that require positioning on the nanoscale. It utilizes both thermal and electrostati c actuation to position the plate central plate . It is fabricated using the MEMSCAP P olyMUMPs process with no significant post processing [25]. Using a low cost commercial foundry with a robust, stable process simplifies the production of devices and streamlines integration wi th complicated MEMS systems. The scanner is capable of moving 14.0 μm in both transl ational directions, 97.9 μm vertically , and rotating 7.4° in around both axis. Vertical actuation is achieved using three independent method s. The method providing the largest range of motion relies on thermal actuation and is limited to operating below 10 Hz by the thermal time constant of the system. The other methods use electrostatics with measured responses up to 3.2 kHz. II. DESIGN The scanner , as shown in F ig. 1, is fabricat ed using t he PolyMUMPs process by MEMSCAP. The process includes three polysilicon layers (two of which are moveable) and a gold layer that sit on a silicon nitride coated silicon handle. The device consists of four comb drive actuators each connected to a pair of polysilicon/gold thermal bimorphs. The bimorphs in turn are connected to a central polysilicon plate by a pair of polysilicon tethers which have bends to relieve tension. In the particular devi ce studied here, a second 2 DoF scanner was fabricated under the 5 DoF scanner . This gives the full d evice 7 DoF that comes from 12 different actuation motors. Devices come from the foundry with the movable polysilicon layers buried in sacrificial oxid e which is remove d by etching in hydrofluoric acid (HF) f or 15 min. This release time is longer than recommend for a P olyMUMPs device because no holes were put into the central plate to allow for rapid releasing. Release holes could be added to the design, but that is unde sirable when using the central plate as a mask . PolyMUMPs devices are known to have a problem with galvanic corrosion during the HF etch, and the issue is exacerbated by the long etching time. We mitigated the problem, using a solution previously demonstrated by Syed et al. Breakable tethers wer e added to electrically connect the device layers to the silicon handle which then serves as a sacrificial anode during the etch [26]. After release, the devic es were dried using a critical point dryer (CPD). The CPD prevents device layers from sticking to each other or the substrate. If a device layer does stick, it usually can be separated using a probe station. Once the oxide has been removed, the gold/polysi licon thermal bimorphs bend out of plane due to residual stresses. The bending can be increased by annealing . The devices in this paper were annealed for 5 min. at 210 °C on a hotplate. The device is designed to position the central plate. It does so usin g four actuator pairs, each in a separate quadrant, that consis t of a comb drive paired with two gold/polysilicon thermal bimorph s, as seen in Fig 1. Generally, the comb drives are used for lateral actuation, and the thermal bimorphs provi de vertical and a ngular motion. The thermal bimorphs consist of a U -shaped piece of the second polysilicon layer with gold on the entire length of one of the straight sides . The other side has a small piece of gold where it attaches to the comb drive. This small gold square is there to prevent that side of the U -shape from sticking or catching by providing it with a small amount of lift which also increases the total height of the thermal bimorph to a small degree. Most o f the height comes from the all -gold side. The tops of the bimorphs are attached to the plate via a 2.5 μm wide tether of the second polysilicon layer. Previous iterations of this design had straight tethers with no bends in them (Supplemental Materials, S2 ). At higher actuation voltages on the com b drive, this design would begin to catch on the substrate because the force of the tether on the top of the bimorph and the spring on the bottom generated enough of a torque to cause the edge of the base to sink and make contact with the substrate. Bends were added to the tethers to relieve > < 3 (a) (b) Fig. 2. Diagrams of the mechanics of the device. (a) Diagram of a spring model of the device, (b) Diagram of the device with different components labeled with their name and the name of their corresponding spring in the spring model. some of the strain and decr ease the torque on the bimorphs. The bends also cause the pl ate to lower a small amount (~5 μm). Th is occurs because the bimorphs twist the tethers as they lift and the bend stretches vertically to relieve that twist. Consequently, the bimorph side of the tether is higher than the plate side. It is possible to design the tethers so that the plate side is higher, as can be seen in s upplemental materials section S3. This would be desirable if the plate needs to be the highest part of the device. However, thes e bends are less effective at reducing torque on the bimoprhs. III. PERFORMANCE The 5 DoF scanner is actuated using nine independent electrical inputs: the four immovable comb voltages, the four movable comb voltages, and the substrate ground. Lateral positioni ng is controlled via the potential differences between the movable and immovable combs. The potential generates a force across the capacitive comb which generates a displacement as it is balanced by the mechanical response of the system. The force generate d by the comb drive is given below [15]. 𝐹𝑐𝑜𝑚𝑏=|𝜕𝑈 𝜕𝑥|=|𝜕 𝜕𝑥(1 2𝐶𝑉2)| (1) The mechanical response o f the system can be modeled using simple linear spring s, as seen in Fig. 2 . The spring constant includes contributions from the tethers that r un perpendicular to the direction of motion, the folded beam suspension attached to the actuation comb drive, the opposite folded beam suspension, and the bends in the tethers. The total lateral spring constant of the system is given below. 𝑘𝑡𝑜𝑡𝑎𝑙=𝑘𝑓𝑜𝑙𝑑𝑒𝑑+𝑘1 (2) Where k folded is the spring constant of the folded beam suspension, and k 1 is given by 𝑘1=(1 2𝑘𝑟𝑒𝑙𝑖𝑒𝑓+1 𝑘2+4𝑘𝑡𝑒𝑡ℎ𝑒𝑟)−1 (3) and represents the spring constant of all flexures in parallel with the folded beam suspension attached to the actuation comb drive, as seen in F ig. 2. The spring constant of the strain relief is represented by k relief, the spring constant of the off axis tether is represented by k tether (this includes th e bending of the off axis bimorphs), and the cumulative spring constant of the strain reliefs and the folded beam suspension on the opposite side of the plate from the actuation comb. It is given by 𝑘2=(1 2𝑘𝑟𝑒𝑙𝑖𝑒𝑓+1 𝑘𝑓𝑜𝑙𝑑𝑒𝑑)−1 (4) The strain relief on the tether attached to the actuation comb drive stretches during actuation and this causes the plate to move less than the comb drive. > < 4 Fig. 3. Lateral actuation of the device obtained using the comb drives. Data and V 2 fits are shown for both air and vacuum actuation, for both the central plate and the comb drive itself. Because of an asymmetry in the electrostatic potential on the fingers of the comb drive and a torque on the bimorphs, there is a small vertical motion of the plate when the drives are actuated. Error bars represent twice the standard deviation above and below the average of three measurements. The ratio of the comb drive displacement to the lateral plate displacement is given by 𝑥𝑝𝑙𝑎𝑡𝑒 𝑥𝑐𝑜𝑚𝑏=𝑘1 𝑘2+4𝑘𝑡𝑒𝑡ℎ𝑒𝑟(5) The predicted ratio of x plate to x comb using optical measurements of dimensions and C omsol Multiphysics to predict the spring constants is 47%. The measured ratio is 64%. We attribute the difference largely to errors in component dimension measurements. The displacement of the comb drive and the plate is seen in Fig. 3 , and plate actuation is shown in air and vacuum. Fig. 3 also shows the vertical motion of the plate as it is moved laterally. The vertical motio n is undesirable; however, as we will discuss bel ow, there are multiple verti cal actuation methods that can be used to correct it . Fits of the lateral data are also included. The motion follows a well known 𝑥=𝑐𝑉2(6) relationship where x is lateral displ acement, c is a constant and V is the potential difference across the comb drive. This relationship stems from (1) and the derivative of capacitance with respect to lateral displacement being constant. Comb drive actuation was measured via optical microsco py, plate actuation in vacuum was measured with a scanning electron microscope (SEM), lateral plate actuation in air was measured with optical microscopy, and vertical plate actuati on in air was measured with an optical interferometer. All error bars are t wo standard deviations in both directions. The error bars for the plate motions are from three measurements (per data point) of different sections of a single scan. The error bars for comb drive motions come from three measurements as well, but each is measured from a different optical microscope image. For many data points, the error bars are small er than the point. Supplemental Movie 1 (SM1) is a stop motion video of lateral actuation of a device. It is compo sed of a series of SEM images at different actu ation potentials. Vertical actuation is achieved by three different methods. 1) All comb drive actuation where all of the comb drives are actuated simultaneously causing a vertical drop in the plate. 2) Parallel plate actuation where a potential is placed on the top plate and the bottom plate and substrate are grounded causing the top plate to pull down toward s the substrate. 3) B imorph actuation where the bimorphs are heated causing them to lower towards the substrate. In method one (all comb drive actuati on), t here are two effects that cause vertical displacement , as demonstrated in Fig. 4c. First, the lateral force applied at the base of the bimorphs by the comb drive and the restoring force applied at the top by the tethers generate a torque on the bimor phs that causes them to tilt and lowers the central plate. Second, there is a well -known levitation effect at the comb drive that occurs because of the grounding plane shielding fringe fields [27], [28], [24] . This effect results in a vertical force on the comb fingers and also causes the bimorphs to tilt, loweri ng the plate. The total range due to both of these effects is 9.2 µm. The actuation voltage vs. height data is plotted in Fig. 4a. Also plotted is the height data for a single comb that is also plotted in Fig. 3 . Both datasets were measured using a white light interferometer, and the error bars are twice the standard deviation of three samples taken from different sections of the same scan. Supplemental Movie 2 (SM2) is a stop motion video of this actuation method, similar to SM1. Also plotted in Fig 4a are a series of fits showing that the shape of the displacement curve is well predicted by the theoretical shapes that would be expected by the explanations given. At low voltages, the torque is low and the comb drive levitation effect dominates the slope. This switches at 50 V. The levitatio n effect’s curve was obtained using (1) and setting it equal to the restoring force of the spring for vertical and lateral displacements as seen in (7) and (8) . 𝑘𝑥𝑥=1 2𝜕𝐶 𝜕𝑥𝑉2(7) 𝑘𝑧𝑧=1 2𝜕𝐶 𝜕𝑧𝑉2(8) The derivatives of capacitance were obtained by modeling change in capacitance with displacement i n Comsol Multiphysics. A simple second order polynomial function was used to fit this data and then obtain the derivatives. This simulation data and polynomial fit is in s upplemental materials section S 4. By solving the system of equations , vertical displacement (z) in terms of V was found. There is an additional scaling factor necessary to convert between vertical motion of the comb drive and vertical motion of the plate. A Comsol FEA simulation predict ed the scaling factor to be -1.94. The scalin g factor that fit the data was -3.58. We attribute the difference largely to errors in measurement s of physical dimensions . > < 5 The torque effect was modeled simply by multiplying the comb drive fit in Fig. 3 by a linear scaling factor . A Comsol model predicted that the scaling factor should be between - 0.199 and -0.658 depending on if the force at the comb drive is applied to the side walls or to a point which allows more rotation . The value that best fits the data is -0.335. Method two uses parallel plate capacitive actuation by putting a potential on the plate and gro unding the bottom plate and substrate . The height vers us potential is measured wit h white light interferometry and plotted in figure 4 b. The measured actuation range is 39.5 µm after which pull in occurs. This is approximately one third of the initi al height, which is (a) (b) (c) (d) (e) Fig. 4. Three methods of vertical actuation. (a) Plot of actuating all of the comb drives simultaneously also included is a fit based on two different mechanisms which are plotted individually and combined. (b) Plot of actuating by putting a potential difference between the plate and th e substrate and electrostatically pulling the plate down. The plate pulled in at about one third the initial s eparation. (c) A diagram of one quadrant of the device showing the two mechanisms used to fit the data in (a). (d) Plot of actuating the thermal bimorphs by running a current through the tethers and folded beam suspensions in both air and vacuum. (e) SEM i mage of the device with the potentials and currents used in (d) labeled. The potential difference plotted in (d) is the total pote ntial difference. > < 6 (a) (b) (c) Fig. 5. Shown is the tip/tilt operation. (a) An SEM image of the device with the potentials and current paths used in tip/tilt operation labeled. (b) SEM image of the device at maximum tilt in vacuum. The bimorphs on the right are near the substrate. (c) Plot of the tilt angle versus voltage in vacuum and air. The actuation voltage is the total potential difference between V+ and V - in (a). the pull in distance predicted for a parallel p late actuator balanced by a spring [15]. This method is complicated in applications where there is a second substrate above the plate is present, as in atomic calligraphy and SPM. Method three utilizes the thermal bimorphs. As the bimorphs are heated, they bend towards the substrate due to the greater thermal expansion coefficient of the gold. In this design, the bimorphs are heated by running current through the tethers and the folded bea m sus pensions, as seen in Fig. 4e. By heating all of the bimorphs simultaneously the plate is moved down towards the substrate. Fig. 4d shows the height vs actuation voltage on the bimorphs in both air and vacuum. Unlike electrostatic methods that yield almost indistinguishable results in air and vacuum, the vacuum actuation of the thermal bimorphs has a larger range and occurs at much lower voltages than the air a ctuation. This is because at atmospheric pressure additional he at is lost by conduction . The actuation range achieved in air is 44.6 µm and in vacuum is 97.9 µm. Air heights were measured with optical interferometry, and vacuum heights were measured in a SEM. In both cases, the error bars represent plus or minus twice the standard deviation of th ree different scans. For the vacuum data, only three data points have err or bars plotted because the data points without error bars represent si ngle measurements. Supplemental Movie 3 (SM3) is a stop motion video of this actuation method in vacuum. Simila r to the third method of vertical actuation, angular actuation is achieved using the thermal bimorphs. Running current between the returns of a pair of adjacent co mb drives, as outlined in Fig. 5a, moves four of the eight thermal bimorphs towards the subst rate causing the plate to tilt, as seen in Fig. 5b. Additionally, Supplemental M ovie 4 (SM4) is a stop motion video of this actuation method. Because this method relies on the thermal bimorphs , different performance is found between air and vacuum environments. Both environments performances are plotted in Fig. 5c. Vacuum actuation was measured using an SEM and air actuation was measured by white light interferometry. The maximum actuation angle s measured were 3.6° and 1.5° for vacuum and air respectively. Because the plate can be tilted in the opposite direction as well using the opposite four bimorphs, the total actuation range is twice the measured, or 7.4° and 2.1° for vacuum and air respecti vely. The maximum operating frequency is different for the thermal and electrostatic actuation methods. Thermal methods are limited by the thermal time constant of the system. The bimorph can only move back up as quickly as it can cool. Electrostatic force s on the other hand are applied extremely rapidly, and the limit to the maximum operating frequency is the mechanica l response of the system. Fig. 6 shows the normalized frequency response for a plate being actuated with the thermal bimorphs and being actu ated with the all of the comb drives (vertical actuation methods 3 and 1 respectively) . The response was measured by a laser vibrometer in air and both actuation methods moved the plate a similar amplitude. It can be seen in the figure that the response of the thermal bimorph begins falling off al most immediately, after 10 Hz, while the response of the all comb drive method does not begin to fall off until after the second major resonance peak at 3.2 kHz. > < 7 Fig. 6. Frequency response of the 5 DoF scanner. Comb drive actuation responds fully up to 3.2 kHz while actuation using thermal bimorphs begins to fall off above 10 Hz because the response speed is limited by the cooling rate. Below the graph are the various modes associat ed with the peaks (generated by a FEA in Comsol Multiphysics). Mode 1 is an up and down oscillation of the bimorphs where the bimorphs move in phase with each other. Mode 2 is also an up and down oscillation of the bimorphs, but the bimorphs move out of ph ase with each other. Mode 2 also possesses some small tilting of the structure. Mode 3 is a lateral in and out oscillation of the comb drive. The peak associated with this mode only appears in the response to the comb drive actuation because the bimorph ac tuation does not excite it. This result implies that the device can be positioned in <1 ms, potentially as fast as ~100 µs using advanced drive techniques [29], [30] . Also, shown are the modes corresponding to each of the major peaks . These were obtained using Comsol Multiphysics. IV. CONCLUSION We have presented a MEMS scanner with five degrees of freedom. It is capable of moving 14.0 μm in both translational directions, 97.9 μm vertically, and 7.4° in both tip and tilt, in vacuum. In a ir, it can move 14.0 μm transitionally, 44.6 μm vertically, and 2.1° in tip and tilt. Vertical actuation can be achieved both with therma l methods that have long ranges but are slow and with electrostatic methods that have shorter ranges, but are capable o f responding fully up to 3.2 kHz. Because this device is built by a low cost commercial foundry, it can inexpensively be modified and integrated into a variety of applications, and manufactured in both large and small quantities. ACKNOWLEDGMENT We would like to thank the Boston University Photonics Center staff.