- Email: [email protected]
Optical scanner using a MEMS actuator
Sensors and Actuators A 102 (2002) 176±184
Optical scanner using a MEMS actuator J.M. Zara*, S.W. Smith BME Department, Duke University, Box 90281, Durham, NC 27708, USA Received 21 November 2001; accepted 12 August 2002
Abstract Optical scanners have numerous applications from bar code readers and laser printers in industry to corneal resurfacing and optical coherence tomography in medicine. We have developed an optical scanner fabricated using photolithography on a polyimide substrate. This scanner uses an electrostatic microelectromechanical system (MEMS) actuator to tilt a gold-coated mirror resting on 3 mm thick polyimide torsion hinges to steer an optical beam. The linear actuator used is the integrated force array (IFA), a network of hundreds of thousands of deformable capacitors, which electrostatically contract with an applied differential voltage. IFAs are 2.2 mm thick patterned, metallized polyimide ®lms 1 cm long and either 1 or 3 mm wide depending on the application. The mirror support structures were modeled using onedimensional beam theory and ANSYS ®nite element analysis and then fabricated using a three-layer process with polyimide and gold on silicon wafers. Side scanning structures have been fabricated with tables 1.125 or 2.25 mm wide. The completed devices were coated with a Ê thick conformal coating of parylene for protection from the environment. These devices have demonstrated optical scan angles up to 500 A 1468 for applied voltages up to 50 V. These devices were also used to steer a laser beam in a prototype bar code reader to demonstrate functionality. # 2002 Published by Elsevier Science B.V. Keywords: Electrostatic microelectromechanical system; Integrated force array; Laser
1. Introduction Optical beam scanners are utilized in numerous applications. Industrial uses range from bar code scanners to laser printers and compact disk players. Medical applications include corneal resurfacing, optical imaging, and hair and tattoo removal [1], as well as optical coherence tomography (OCT), a medical imaging technique analogous to ultrasound and radar [2]. The majority of these applications use galvanometers and other large resonant scanners to steer the optical beam. While galvanometers offer a range of scan speeds and scan angles along with good precision, they require large magnetic bases and mirrors with relatively large masses to achieve the desirable performance characteristics [3]. There are many applications that would bene®t from scanners that are smaller, lighter, and consume less power. There have been several different approaches taken by other groups towards the development of miniature optical scanners. The devices described have used various methods to steer the mirror including electrostatic, magnetic, thermal, * Corresponding author. E-mail address: [email protected] (J.M. Zara).
0924-4247/02/$ ± see front matter # 2002 Published by Elsevier Science B.V. PII: S 0 9 2 4 - 4 2 4 7 ( 0 2 ) 0 0 3 0 2 - 3
and piezoelectric techniques. They were fabricated primarily out of silicon and polysilicon materials that have been etched to form the mirror structures on the wafer substrate. Electrostatic forces only act over short distances; so comblike structures and other designs that utilize electrodes separated by short distances have been developed [4±7]. These devices have been shown to produce optical displacements of 288 at a resonant frequency of 3 kHz [4], 24.98 at 34 kHz [5], 5±78 at 80 Hz [6] and 608 at 250 Hz [7]. Magnetic scanners using the galvanometric principle and an external magnetic ®eld have also demonstrated 228 optical scans at 1.5 kHz [8]. Devices based on the thermal bending of bimorph structures have demonstrated scans up to 908 at frequencies from 100 to 600 Hz [9]. Finally, devices utilizing asymmetrically balanced mirrors that vibrate due to piezoelectric actuators have produced optical scans of 448 at 40±120 Hz and 1208 at 10±22 kHz [10]. The scanners we are developing are fabricated out of polyimide ®lms and use linear electrostatic MEMS actuators to provide the scanning motion. These devices consist of gold plated silicon mirrors mounted on polyimide tables that pivot on very thin polyimide torsion hinges. The linear actuator contracts along its length and pulls on an attachment ¯ap on the top edge of the table to tilt the mirror and
J.M. Zara, S.W. Smith / Sensors and Actuators A 102 (2002) 176±184
177
Fig. 1. Schematic illustration of torsion hinged laser-scanning device.
scan the re¯ected beam. Fig. 1 shows a schematic illustration showing a front view of this optical scanning device with the linear actuator shown behind the mirror support structure. We have previously described similar devices that were used to steer an acoustic beam for ultrasonic imaging [11,12]. This work included both forward viewing and side scanning devices. Since the side looking scanners pivoted on torsion
hinges and kept a constant center of rotation, they were most suitable for laser-scanning applications. The linear microelectromechanical system (MEMS) actuator used to tilt the mirror, the integrated force array (IFA), is a network of hundreds of thousands of micron scale deformable capacitors. These capacitive cells contract due to electrostatic forces produced by a differential
Fig. 2. Schematic illustration of IFA actuator: (a) a group of cells in the array; (b) a close-up of a single capacitive cell; (c) contraction of the cells in the array; (d) contraction of a single capacitive cell.
178
J.M. Zara, S.W. Smith / Sensors and Actuators A 102 (2002) 176±184
voltage applied across the capacitor electrodes. The electrostatic force equation for a gap with polyimide and air as the dielectric is given in (1), where F is the force produced, A the surface area of the plate, V the applied voltage, e0 the dielectric constant of air and L the plate separation: e0 AV 2 F 1:2L2
(1)
The small scale of the cells allows these devices to utilize the short-range electrostatic forces effectively. The IFA has been discussed in detail in previous publications [13±15]. Fig. 2 shows a schematic illustration of a single cell and a portion of the cell network of the IFA in both contracted and relaxed positions. These devices were 3 mm wide and 1 cm long with active areas that were 3 mm 8 mm. These devices included more than 500,000 capacitive cells in three columns and were shown to produce strains of up to 20% and forces up to 13 dyn with applied voltages up to 65 V. Also, since the capacitive cells only draw current when contracting [14], power consumption is very low. We have measured currents of less than 200 mA when the devices are powered with 40 V triangle waves, leading to rms power consumption estimates of less than 5.33 mW. In this paper, we discuss the modeling, design, and fabrication of new scanning devices using integrated circuit fabrication technology. These devices are fabricated in millimeter scale dimensions out of polyimide and are intended for use in applications where small size and mass are desired, but silicon micromirrors are too small or have too high of a scan frequency. 2. Methods To begin device development, one-dimensional beam analysis and ®nite element analysis (ANSYS, Canonsburg, PA) were used as guiding tools to determine appropriate ranges of device dimensions. After preliminary dimensions were determined, devices were fabricated with several different hinge dimensions. Since operating scanning mirrors near resonance greatly increases the available scan angle, both one-dimensional and ANSYS models were used as a validation of the resonant frequencies of the structures. The fabricated devices were tested to determine their optical scan angles and resonance frequencies. To further determine that the optical scan was repeatable and predictable, the polyimide mirrors were inserted as the scanning elements in a prototype bar code reader. For the one-dimensional modeling both prior to and after fabrication, the torsion hinges were represented by rectangular bars of constant thickness ®xed at one end and subjected to a twisting torque on the other. We determined the angular displacement, Y, of the table when subjected to the moment, T, produced by the force acting on the edge of the
table using the equations below, which were discussed in [11] and originated in [16]: Tl tanh 4l Y l (2) 2 1 mDc 4l l
l p 1:5 1 m c
(3)
D
Eh3 12 1 m2
(4)
In Eq. (2), l is an aspect ratio parameter given by Eq. (3) in which l is the length of the hinges, 250 mm, c the hinge width, m the Poisson's ratio for polyimide (0.34), and D the local ¯exural stiffness that is described by Eq. (4), where E is the elastic modulus of polyimide (2600 MPa), and h the hinge thickness (3 mm). After determining the angular displacement and torsional stiffness of the hinges, the model was reduced to a mass suspended between two torsion springs to estimate mechanical resonant frequencies of the structures in air. For this model, the angular displacement from Eq. (2) was used to determine a torsion constant that related the applied moment to an angular displacement. This constant was used in a model of a mass of a known moment of inertia hanging from a torsional spring in order to determine the resonance frequency of the structure in air from Eq. (5): r 1 k f (5) 2p I In (5), k is the torsion constant and I the moment of inertia of the table. As another method to assist in device design and analysis, the devices were then modeled in three dimensions using ®nite element analysis with ANSYS. The ANSYS analysis was done under static loading conditions in order to approximate the displacement of the structures at frequencies away from resonance as a check of the one-dimensional models. The force applied by the IFA to the structure was assumed to be 2 dyn in order to make a conservative estimate of the displacements of the devices for modeling purposes. A ``modal'' analysis was then performed for each structure to determine the resonant frequencies of each of the structures in air for comparison with the previous model and the fabricated devices. Both the one-dimensional beam analysis and ®nite element modeling provided preliminary data to begin the fabrication of functioning devices. Since it was clear that the models used neglected major factors of the actual behavior of the devices, including hinge non-linearity at large displacements and curling in the devices due to internal strains in the polyimide that result from the curing process, it was necessary to fabricate and test devices of both large and small sizes with different hinge dimensions to evaluate their performance. The mirror assemblies were fabricated at North Carolina State University in the Biomedical Microsensors Laboratory
J.M. Zara, S.W. Smith / Sensors and Actuators A 102 (2002) 176±184
179
Fig. 3. Schematic diagram of fabricated layers on silicon wafer.
(BMMSL) using a two-layer process on 5 in. silicon wafers. Fig. 3 shows a cross-sectional schematic illustration of the layers of material on the silicon wafer. To form the thin hinge layer, a 3 mm layer of polyimide (PI-2723, HD Microsystems, Wilmington, DE) was spun onto the wafer and then patterned. The thicker supports and tables were made of a 30 mm thick patterned polyimide layer (Durimide, Arch Chemicals, Norwalk, CT). A sacri®cial silicon oxide layer was deposited on the wafer prior to processing to release the polyimide structures from the wafer. This sacri®cial layer was etched away using hydro¯uoric acid (HF). These devices were designed in two sizes, large devices with tables 2 mm 2:25 mm and small devices with tables 1 mm 1:125 mm. For all of the devices, the torsion hinges were made 3 mm thick and 250 mm long. The dimension that was varied for the various design iterations was the width of the hinge. Fig. 4 shows an enlarged schematic illustration of a torsion hinge demonstrating the hinge dimensions. Large devices were fabricated with three hinge widths, 90, 135, and 180 mm. The small devices had hinges that were 60, 90, and 120 mm wide. Devices were empirically selected that exhibited maximum ¯exibility to increase displacements, but were stiff enough to be mechanically stable in both quasi-static and resonance modes. The large scanner with 135 mm hinges and the small structure with 60 mm hinges met these considerations. After the devices were lifted off of the silicon wafers, gold plated silicon mirrors, 1 mm on a side were bonded to the small tables, and mirrors 1.5 mm on a side were bonded using a fast curing epoxy to the large tables. The gold plated silicon mirrors were 450 mm thick and had a surface ¯atness Ê across the 1.5 mm width of the large mirrors. within 500 A This ¯atness is less than 1/10 of a wavelength for all
applications in the red and infrared ranges. The devices were then mounted onto metal rods for handling. The ®nal step in the device assembly was to attach the IFA actuator to the attachment ¯ap on the table. Fig. 5 shows the photographs of front and side views of both the large and small laser-scanning devices. The top portion of the IFA was bonded to the attachment ¯aps using a water soluble glue for easy removal of the actuator if necessary, and the bottom end of the actuator was bonded to a polymer sheet that contained copper traces and solder pads to provide electrical connections to the IFA. For the smaller scanners, IFAs were designed and fabricated that are only 1 mm wide and 1 cm long. These devices were fabricated at MCNC in Research Triangle Park, North Carolina, in a multiplayer photolithography process similar to that described in [13]. These devices consist of only one column of cells as opposed to three columns in the 3 mm wide actuators and produce 1/3 the force, but produce equivalent strains. The 3 mm wide IFAs were attached to the large scanning mirrors. In order to prevent ambient water vapor from condensing on the hydrophilic polyimide of the IFA, a conformal coating of parylene, a hydrophobic polymer, was deposited on the actuator. A 0.05 mm thick parylene ®lm was deposited using a PDS 2010 vacuum parylene deposition system (Specialty Coating Systems, Indianapolis, IN). This coating step was conducted before the IFA was mounted to the mirror assembly. After the parylene coating, the devices were run for over half a million cycles with no decrease in performance. The optical displacements of the devices were determined using a calibrated bulls-eye target positioned at a known distance from the scanning mirror. The laser used for this experiment was a 2 mW HeNe laser (JDS Uniphase Model
Fig. 4. Close-up view of torsion hinge detailing relevant dimensions.
180
J.M. Zara, S.W. Smith / Sensors and Actuators A 102 (2002) 176±184
Fig. 6. Bar code scanner schematic illustration.
of opposite polarity that can be adjusted for both peak voltage and frequency of the waveform. Fig. 6 shows a schematic illustration of the prototype bar code reader. The 2 mW HeNe laser was used with a photodiode (Hamamatsu Model S1223) positioned to detect the re¯ected light from the bar codes. The received signals were digitized using an A/D board (Gage Compuscope 8012A) and a personal computer and compared with the bar code being evaluated. Fig. 5. Photographs of completed scanning devices: (a) front view of large scanner; (b) side view of large scanner; (c) front view of small scanner; (d) side view of small scanner.
1322, San Jose, CA). Displacements were measured as a function of frequency at a driving voltage of 40 V and as a function of driving voltage both at non-resonant frequencies and near the main resonant frequencies of the devices. These measurements were taken with the device a known distance away from a calibrated target. The optical displacements are twice the actual mechanical scan angle due to the fact that the re¯ection of the optical beam is equal to the angle of incidence. Optical displacements were determined by increasing the drive voltage at ®xed frequencies both near and away from resonance and by sweeping through drive frequencies while maintaining a ®xed voltage. For all of these tests, the driving voltage consists of two triangle waves
3. Results The one-dimensional and ANSYS analyses were used to analyze the selected structures to predict their resonant frequencies. Table 1 contains a summary of these results. Both models predicted resonances near the lower frequencies, 20.6 Hz for the large tables and 31 Hz for the small tables, leading us to believe that the ®rst measured resonances of each devices were the fundamentals. Both models only dealt with the initial resonance mode and ignored harmonics. The optical displacements of the devices were then measured as a function of drive frequency. The results for both the scanners are shown in Fig. 7. These measurements were taken at a peak differential voltage of 40 V. The large devices have a resonance at 20.6 Hz yielding an optical scan angle of 778 and a larger resonance at 41.2 Hz with a
Table 1 Summary of resonance frequencies of the structures
Large scanner Small scanner
One-dimensional model (Hz)
ANSYS (Hz)
First measured resonance (Hz)
Second measured resonance (Hz)
12.6 30.7
17.3 36.2
20.6 31
41.2 62
Fig. 7. Optical angle vs. frequency for the two types of scanners: (a) large scanner with 135 mm thick hinges; (b) small scanner with 60 mm thick hinges.
Fig. 8. Optical angle vs. driving voltage for the two types of scanners near and away from largest resonant frequencies: (a) large scanner; (b) small scanner.
182
J.M. Zara, S.W. Smith / Sensors and Actuators A 102 (2002) 176±184
displacement of 1428. Measurements of the small device show a resonance at 31 Hz of 648 and a larger resonance at 62 Hz yielding a displacement of 898. The second resonance of both the structures is believed to be a superposition of a harmonic of the ®rst resonance and other modes in the mirror and support structure, which increase the measured motion beyond the level of the fundamental. Fig. 8 shows the plots of optical displacements vs. an increasing drive voltage for both the large and small scanners. These displacement measurements were taken near the largest resonant frequencies of the structures (41.6 Hz for the large scanner and 62 Hz for the small scanner) and also at a 1 Hz frequency well away from the resonances. Near resonance, the optical scan angle of the large scanner ranges from 1128 at 25 V to 1468at 50 V and the small scanner displacements range from 238 at 25 V to 1188 at 50 V. Away from resonance, the large scanner has displacements range from 108 at 25 V to 458 at 50 V and the small scanner has displacements ranging from 58 at 25 V to 33.58 at 50 V. Displacements were also determined as the drive voltage was decreased. There was some hysteresis in
the displacement measurements at the lower drive voltages (50±60 V). This hysteresis was not noticed at the higher drive voltages, therefore, the scanner appears to be most repeatable at voltages between 70 and 100 V. For the results contained in Fig. 8, it is clear that the optical displacement of the scanner increases with an increasing drive voltage amplitude. This increase in scan angle is expected, but the expected form of the graphs is very dif®cult to evaluate. In a simple model, the electrostatic forces produced by the applied voltage (Eq. (1)) would increase as the square of the voltage. However, one must also take into account many other complicating factors, such as the fact that we are reporting scan angle rather than linear actuator displacement, the torsion hinges become non-linear as they twist beyond small displacements, and the forces from Eq. (1) increase as the capacitive cells contract. For these reasons, trying to accurately predict the optical scan angle of these devices is dif®cult. More detail on the actuator contraction with respect to voltage is contained in Ref. [13]. Fig. 9 shows the video captured images of the laser beam scanned across the calibrated bulls-eye target for both the
Fig. 9. Plots of optical beam on the calibrated target: (a) optical beam from large scanner at rest; (b) optical beam from large scanner displaced away from resonance; (c) optical beam from large scanner displaced near resonance; (d) optical beam from small scanner at rest; (e) optical beam from small scanner displaced away from resonance; (f) optical beam from small scanner displaced near resonance.
J.M. Zara, S.W. Smith / Sensors and Actuators A 102 (2002) 176±184
Fig. 10. Bar code read by scanning system and received signal: (a) portion of code 39 bar code; (b) received signal from optical detection system.
large and small scanner. These images were all taken with the differential drive voltage set at 40 V. The images show the beam at rest, steered at a frequency of 1 Hz (quasi-static operation), and at resonance. Fig. 9(a)±(c) shows the displacements for the large scanner and Fig. 9(d)±(f) are for the small scanner. The images in Fig. 9(c) and (f) were taken with the camera aperture open wider than the other images to clearly show the extent of the scanned beam. The scanning mirrors were then inserted as the moving element in a prototype bar code scanner to sector the beam and read a portion of a Code 39 (Code 3 of 9) bar code which is the typical non-food standard and the most popular symbology for ID, inventory and tracking of purchases [17]. The large bars in this code are 800 mm wide and the small bars are 400 mm wide, with all of the bars separated by 400 mm. Fig. 10 shows the schematic illustration of the bar code (Fig. 10(a)) and the received signal from the photodiode and ampli®er after being digitized by the A/ D board (Fig. 10(b)). The high voltages correspond to the dark bars and low voltages represent re¯ections from the white areas of the bar code. This bar code data was recorded at a drive frequency of 62 Hz and a differential drive voltage of 40 V.
183
example of their functionality. Advantages of the scanners we have developed include the ability to operate in both resonant and quasi-static (low frequency motion away from resonance) modes, a large optical scan angle at low frequencies, and being fabricated from polyimide rather than silicon. These devices are also small, lightweight, have low power consumption, and are intermediate in size between the very small silicon mirrors and the galvanometers. The nominal resonance frequencies of these scanners (20±60 Hz) are in the range of conventional bar code scanners 36 Hz [17], and 30 Hz video frame rates for optical imaging systems. Future work on these devices includes integrating the fabrication of the IFA actuator, the mirror, and the support structure on the same wafer. This would reduce both the fabrication cost and assembly time and result in devices that could be fabricated relatively easily. We are also developing new IFA devices that are twice as thick as the previous devices (4.4 mm) and therefore will produce twice the force for a given voltage. These stronger IFAs would allow for larger scan angles away from the resonance of the structures. New types of support structures are being developed that would utilize multiple IFAs to both balance out the scan of the devices and also increase the degrees of freedom of the scanner. The ®rst implementation would be to attach actuators to both sides of the scanning mirror to pull on both sides of the table in an alternating fashion. For this con®guration, it would most likely be necessary to bend the table and hinges at a right angle to produce a forward-looking scanner. Designs are also being investigated to scan the mirror in two dimensions for applications that could bene®t from an additional degree of freedom in order to produce
4. Discussion In this paper we described the design, fabrication, and testing of a new optical beam scanner. These devices were operated away from resonance as quasi-static scanners producing optical displacements up to 458, or as resonant scanners to produce optical scans up to 1458. The scanning mirrors were also used in a prototype bar code scanner as an
Fig. 11. Schematic illustration of the miniature Cardano suspension proposed to scan the mirror in two-dimensions.
184
J.M. Zara, S.W. Smith / Sensors and Actuators A 102 (2002) 176±184
two-dimensional scans. These designs would use miniature Cardano suspensions that consist of two tables with orthogonal sets of torsion hinges to tilt in both the directions as illustrated in Fig. 11. As a ®nal step towards reliable scanning devices for multiple applications we are investigating capacitive feedback from the actuator that would allow for a control loop to increase the precision of the devices motion. This is possible due to the fact that the actuator is a network of capacitors that will change capacitance as it contracts and the individual cells change their geometry. These capacitance measurements should be able to be used by a control system to accurately predict the compression of the actuator and therefore the location of the beam steered by the tilting mirror. This type of precise control of the optical beam location is particularly important in many therapeutic applications such as Lasik surgery and other thermal ablation procedures. Acknowledgements We would like to thank Ken Gentry for his assistance in the initial development of these optical scanners. We would also like to thank Professor Stephen Bobbio and Dr. Scott Goodwin-Johansson for development of and expertise regarding the IFA actuator. This work was supported in part by HHS grants HL-58754 and HL-64962. References [1] K. Leggett, Medical lasers are at the threshold of a new era, Biophotonics Int. 42±47 (1998). [2] D. Huang, E. Swanson, C. Lin, J. Schuman, W. Stinson, W. Chang, M. Hee, T. Flotte, K. Gregory, C. Puliafito, J. Fujimoto, Optical coherence tomography, Science 254 (1991) 1178±1181.
[3] M. Truex, Lasers in dermatology: galvanometer scanners allow more uniform treatment, Biophotonics Int. 54±56 (1998). [4] M. Kiang, O. Solgaard, R. Muller, K. Lau, Micromachined polysilicon microscanners for barcode readers, IEEE Photonics Technol. Lett. 8 (12) (1996) 1707±1709. [5] R. Conant, J. Nee, K. Lau, R. Muller, A fast flat scanning micromirror, in: Proceedings of the Solid-State Sensor and Actuator Workshop, Hilton Head, SC, 2000, pp. 6±9. [6] W. Lang, H. Pavlicek, Th. Mark, H. Scheithauer, B. Schmidt, Electrostatically actuated micromirror devices in silicon technology, Sens. Actuat. A 74 (1999) 216±218. [7] H. Schenk, P. Durr, T. Haase, D. Kunze, U. Sobe, H. Lakner, H. Kuck, Large deflection micromechanical scanning mirrors for linear scans and pattern generation, IEEE J. Select. Top. Quant. Electron. 6 (5) (2000) 715±722. [8] L. Ferreira, S. Moehlecke, A silicon micromechanical galvanometric scanner, Sens. Actuat. A 73 (1999) 252±260. [9] S. Schweizer, S. Calmes, M. Laudon, Ph. Renaud, Thermally actuated optical microscanner with large angle and low consumption, Sens. Actuat. A 76 (1999) 470±477. [10] K. Yamada, T. Kuriyama, A novel asymmetric silicon micro-mirror for optical beam scanning display, in: Proceedings of the IEEE Micro Electro Mechanical Systems, 1998, pp. 110±115. [11] J. Zara, S. Bobbio, S. Goodwin-Johansson, S. Smith, Intracardiac ultrasound scanner using a micromachine (MEMS) actuator, IEEE Trans. UFFC 47 (4) (2000) 984±993. [12] J. Zara, S. Smith, A micromachine high frequency ultrasound scanner using photolithographic fabrication, IEEE Trans. UFFC 49 (7) (2002) 947±958. [13] J.D. Jacobson, S.H. Goodwin-Johansson, S.M. Bobbio, C.A. Bartlett, L.N. Yadon, Integrated force arrays: theory and modeling of static operation, J. Microelectromech. Syst. 4 (3) (1995) 139±150. [14] S.M. Bobbio, M.D. Kellam, B.W. Dudley, S. Goodwin-Johansson, S.K. Jones, J.D. Jacobsen, F.M. Tranjan, T.D. Dubois, Integrated force arrays, in: Proceedings of the IEEE Conference on MEMS, 1994. [15] S.M. Bobbio, S.W. Smith, S. Goodwin-Johansson, R.B. Fair, T.D. DuBois, F.M. Tranjan, J. Hudak, R. Gupta, H. Makki, Integrated force array: interface to external systems, SPIE 3046 (1997) 248± 259. [16] E. Reisner, M. Stein, Torsion and transverse bending of cantilever plates, Technical Note 2369, National Advisory Committee for Aeronautics, June 1951. [17] A Bar Code Primer, Worth Data, 1997±2001.
Optical scanner using a MEMS actuator J.M. Zara*, S.W. Smith BME Department, Duke University, Box 90281, Durham, NC 27708, USA Received 21 November 2001; accepted 12 August 2002
Abstract Optical scanners have numerous applications from bar code readers and laser printers in industry to corneal resurfacing and optical coherence tomography in medicine. We have developed an optical scanner fabricated using photolithography on a polyimide substrate. This scanner uses an electrostatic microelectromechanical system (MEMS) actuator to tilt a gold-coated mirror resting on 3 mm thick polyimide torsion hinges to steer an optical beam. The linear actuator used is the integrated force array (IFA), a network of hundreds of thousands of deformable capacitors, which electrostatically contract with an applied differential voltage. IFAs are 2.2 mm thick patterned, metallized polyimide ®lms 1 cm long and either 1 or 3 mm wide depending on the application. The mirror support structures were modeled using onedimensional beam theory and ANSYS ®nite element analysis and then fabricated using a three-layer process with polyimide and gold on silicon wafers. Side scanning structures have been fabricated with tables 1.125 or 2.25 mm wide. The completed devices were coated with a Ê thick conformal coating of parylene for protection from the environment. These devices have demonstrated optical scan angles up to 500 A 1468 for applied voltages up to 50 V. These devices were also used to steer a laser beam in a prototype bar code reader to demonstrate functionality. # 2002 Published by Elsevier Science B.V. Keywords: Electrostatic microelectromechanical system; Integrated force array; Laser
1. Introduction Optical beam scanners are utilized in numerous applications. Industrial uses range from bar code scanners to laser printers and compact disk players. Medical applications include corneal resurfacing, optical imaging, and hair and tattoo removal [1], as well as optical coherence tomography (OCT), a medical imaging technique analogous to ultrasound and radar [2]. The majority of these applications use galvanometers and other large resonant scanners to steer the optical beam. While galvanometers offer a range of scan speeds and scan angles along with good precision, they require large magnetic bases and mirrors with relatively large masses to achieve the desirable performance characteristics [3]. There are many applications that would bene®t from scanners that are smaller, lighter, and consume less power. There have been several different approaches taken by other groups towards the development of miniature optical scanners. The devices described have used various methods to steer the mirror including electrostatic, magnetic, thermal, * Corresponding author. E-mail address: [email protected] (J.M. Zara).
0924-4247/02/$ ± see front matter # 2002 Published by Elsevier Science B.V. PII: S 0 9 2 4 - 4 2 4 7 ( 0 2 ) 0 0 3 0 2 - 3
and piezoelectric techniques. They were fabricated primarily out of silicon and polysilicon materials that have been etched to form the mirror structures on the wafer substrate. Electrostatic forces only act over short distances; so comblike structures and other designs that utilize electrodes separated by short distances have been developed [4±7]. These devices have been shown to produce optical displacements of 288 at a resonant frequency of 3 kHz [4], 24.98 at 34 kHz [5], 5±78 at 80 Hz [6] and 608 at 250 Hz [7]. Magnetic scanners using the galvanometric principle and an external magnetic ®eld have also demonstrated 228 optical scans at 1.5 kHz [8]. Devices based on the thermal bending of bimorph structures have demonstrated scans up to 908 at frequencies from 100 to 600 Hz [9]. Finally, devices utilizing asymmetrically balanced mirrors that vibrate due to piezoelectric actuators have produced optical scans of 448 at 40±120 Hz and 1208 at 10±22 kHz [10]. The scanners we are developing are fabricated out of polyimide ®lms and use linear electrostatic MEMS actuators to provide the scanning motion. These devices consist of gold plated silicon mirrors mounted on polyimide tables that pivot on very thin polyimide torsion hinges. The linear actuator contracts along its length and pulls on an attachment ¯ap on the top edge of the table to tilt the mirror and
J.M. Zara, S.W. Smith / Sensors and Actuators A 102 (2002) 176±184
177
Fig. 1. Schematic illustration of torsion hinged laser-scanning device.
scan the re¯ected beam. Fig. 1 shows a schematic illustration showing a front view of this optical scanning device with the linear actuator shown behind the mirror support structure. We have previously described similar devices that were used to steer an acoustic beam for ultrasonic imaging [11,12]. This work included both forward viewing and side scanning devices. Since the side looking scanners pivoted on torsion
hinges and kept a constant center of rotation, they were most suitable for laser-scanning applications. The linear microelectromechanical system (MEMS) actuator used to tilt the mirror, the integrated force array (IFA), is a network of hundreds of thousands of micron scale deformable capacitors. These capacitive cells contract due to electrostatic forces produced by a differential
Fig. 2. Schematic illustration of IFA actuator: (a) a group of cells in the array; (b) a close-up of a single capacitive cell; (c) contraction of the cells in the array; (d) contraction of a single capacitive cell.
178
J.M. Zara, S.W. Smith / Sensors and Actuators A 102 (2002) 176±184
voltage applied across the capacitor electrodes. The electrostatic force equation for a gap with polyimide and air as the dielectric is given in (1), where F is the force produced, A the surface area of the plate, V the applied voltage, e0 the dielectric constant of air and L the plate separation: e0 AV 2 F 1:2L2
(1)
The small scale of the cells allows these devices to utilize the short-range electrostatic forces effectively. The IFA has been discussed in detail in previous publications [13±15]. Fig. 2 shows a schematic illustration of a single cell and a portion of the cell network of the IFA in both contracted and relaxed positions. These devices were 3 mm wide and 1 cm long with active areas that were 3 mm 8 mm. These devices included more than 500,000 capacitive cells in three columns and were shown to produce strains of up to 20% and forces up to 13 dyn with applied voltages up to 65 V. Also, since the capacitive cells only draw current when contracting [14], power consumption is very low. We have measured currents of less than 200 mA when the devices are powered with 40 V triangle waves, leading to rms power consumption estimates of less than 5.33 mW. In this paper, we discuss the modeling, design, and fabrication of new scanning devices using integrated circuit fabrication technology. These devices are fabricated in millimeter scale dimensions out of polyimide and are intended for use in applications where small size and mass are desired, but silicon micromirrors are too small or have too high of a scan frequency. 2. Methods To begin device development, one-dimensional beam analysis and ®nite element analysis (ANSYS, Canonsburg, PA) were used as guiding tools to determine appropriate ranges of device dimensions. After preliminary dimensions were determined, devices were fabricated with several different hinge dimensions. Since operating scanning mirrors near resonance greatly increases the available scan angle, both one-dimensional and ANSYS models were used as a validation of the resonant frequencies of the structures. The fabricated devices were tested to determine their optical scan angles and resonance frequencies. To further determine that the optical scan was repeatable and predictable, the polyimide mirrors were inserted as the scanning elements in a prototype bar code reader. For the one-dimensional modeling both prior to and after fabrication, the torsion hinges were represented by rectangular bars of constant thickness ®xed at one end and subjected to a twisting torque on the other. We determined the angular displacement, Y, of the table when subjected to the moment, T, produced by the force acting on the edge of the
table using the equations below, which were discussed in [11] and originated in [16]: Tl tanh 4l Y l (2) 2 1 mDc 4l l
l p 1:5 1 m c
(3)
D
Eh3 12 1 m2
(4)
In Eq. (2), l is an aspect ratio parameter given by Eq. (3) in which l is the length of the hinges, 250 mm, c the hinge width, m the Poisson's ratio for polyimide (0.34), and D the local ¯exural stiffness that is described by Eq. (4), where E is the elastic modulus of polyimide (2600 MPa), and h the hinge thickness (3 mm). After determining the angular displacement and torsional stiffness of the hinges, the model was reduced to a mass suspended between two torsion springs to estimate mechanical resonant frequencies of the structures in air. For this model, the angular displacement from Eq. (2) was used to determine a torsion constant that related the applied moment to an angular displacement. This constant was used in a model of a mass of a known moment of inertia hanging from a torsional spring in order to determine the resonance frequency of the structure in air from Eq. (5): r 1 k f (5) 2p I In (5), k is the torsion constant and I the moment of inertia of the table. As another method to assist in device design and analysis, the devices were then modeled in three dimensions using ®nite element analysis with ANSYS. The ANSYS analysis was done under static loading conditions in order to approximate the displacement of the structures at frequencies away from resonance as a check of the one-dimensional models. The force applied by the IFA to the structure was assumed to be 2 dyn in order to make a conservative estimate of the displacements of the devices for modeling purposes. A ``modal'' analysis was then performed for each structure to determine the resonant frequencies of each of the structures in air for comparison with the previous model and the fabricated devices. Both the one-dimensional beam analysis and ®nite element modeling provided preliminary data to begin the fabrication of functioning devices. Since it was clear that the models used neglected major factors of the actual behavior of the devices, including hinge non-linearity at large displacements and curling in the devices due to internal strains in the polyimide that result from the curing process, it was necessary to fabricate and test devices of both large and small sizes with different hinge dimensions to evaluate their performance. The mirror assemblies were fabricated at North Carolina State University in the Biomedical Microsensors Laboratory
J.M. Zara, S.W. Smith / Sensors and Actuators A 102 (2002) 176±184
179
Fig. 3. Schematic diagram of fabricated layers on silicon wafer.
(BMMSL) using a two-layer process on 5 in. silicon wafers. Fig. 3 shows a cross-sectional schematic illustration of the layers of material on the silicon wafer. To form the thin hinge layer, a 3 mm layer of polyimide (PI-2723, HD Microsystems, Wilmington, DE) was spun onto the wafer and then patterned. The thicker supports and tables were made of a 30 mm thick patterned polyimide layer (Durimide, Arch Chemicals, Norwalk, CT). A sacri®cial silicon oxide layer was deposited on the wafer prior to processing to release the polyimide structures from the wafer. This sacri®cial layer was etched away using hydro¯uoric acid (HF). These devices were designed in two sizes, large devices with tables 2 mm 2:25 mm and small devices with tables 1 mm 1:125 mm. For all of the devices, the torsion hinges were made 3 mm thick and 250 mm long. The dimension that was varied for the various design iterations was the width of the hinge. Fig. 4 shows an enlarged schematic illustration of a torsion hinge demonstrating the hinge dimensions. Large devices were fabricated with three hinge widths, 90, 135, and 180 mm. The small devices had hinges that were 60, 90, and 120 mm wide. Devices were empirically selected that exhibited maximum ¯exibility to increase displacements, but were stiff enough to be mechanically stable in both quasi-static and resonance modes. The large scanner with 135 mm hinges and the small structure with 60 mm hinges met these considerations. After the devices were lifted off of the silicon wafers, gold plated silicon mirrors, 1 mm on a side were bonded to the small tables, and mirrors 1.5 mm on a side were bonded using a fast curing epoxy to the large tables. The gold plated silicon mirrors were 450 mm thick and had a surface ¯atness Ê across the 1.5 mm width of the large mirrors. within 500 A This ¯atness is less than 1/10 of a wavelength for all
applications in the red and infrared ranges. The devices were then mounted onto metal rods for handling. The ®nal step in the device assembly was to attach the IFA actuator to the attachment ¯ap on the table. Fig. 5 shows the photographs of front and side views of both the large and small laser-scanning devices. The top portion of the IFA was bonded to the attachment ¯aps using a water soluble glue for easy removal of the actuator if necessary, and the bottom end of the actuator was bonded to a polymer sheet that contained copper traces and solder pads to provide electrical connections to the IFA. For the smaller scanners, IFAs were designed and fabricated that are only 1 mm wide and 1 cm long. These devices were fabricated at MCNC in Research Triangle Park, North Carolina, in a multiplayer photolithography process similar to that described in [13]. These devices consist of only one column of cells as opposed to three columns in the 3 mm wide actuators and produce 1/3 the force, but produce equivalent strains. The 3 mm wide IFAs were attached to the large scanning mirrors. In order to prevent ambient water vapor from condensing on the hydrophilic polyimide of the IFA, a conformal coating of parylene, a hydrophobic polymer, was deposited on the actuator. A 0.05 mm thick parylene ®lm was deposited using a PDS 2010 vacuum parylene deposition system (Specialty Coating Systems, Indianapolis, IN). This coating step was conducted before the IFA was mounted to the mirror assembly. After the parylene coating, the devices were run for over half a million cycles with no decrease in performance. The optical displacements of the devices were determined using a calibrated bulls-eye target positioned at a known distance from the scanning mirror. The laser used for this experiment was a 2 mW HeNe laser (JDS Uniphase Model
Fig. 4. Close-up view of torsion hinge detailing relevant dimensions.
180
J.M. Zara, S.W. Smith / Sensors and Actuators A 102 (2002) 176±184
Fig. 6. Bar code scanner schematic illustration.
of opposite polarity that can be adjusted for both peak voltage and frequency of the waveform. Fig. 6 shows a schematic illustration of the prototype bar code reader. The 2 mW HeNe laser was used with a photodiode (Hamamatsu Model S1223) positioned to detect the re¯ected light from the bar codes. The received signals were digitized using an A/D board (Gage Compuscope 8012A) and a personal computer and compared with the bar code being evaluated. Fig. 5. Photographs of completed scanning devices: (a) front view of large scanner; (b) side view of large scanner; (c) front view of small scanner; (d) side view of small scanner.
1322, San Jose, CA). Displacements were measured as a function of frequency at a driving voltage of 40 V and as a function of driving voltage both at non-resonant frequencies and near the main resonant frequencies of the devices. These measurements were taken with the device a known distance away from a calibrated target. The optical displacements are twice the actual mechanical scan angle due to the fact that the re¯ection of the optical beam is equal to the angle of incidence. Optical displacements were determined by increasing the drive voltage at ®xed frequencies both near and away from resonance and by sweeping through drive frequencies while maintaining a ®xed voltage. For all of these tests, the driving voltage consists of two triangle waves
3. Results The one-dimensional and ANSYS analyses were used to analyze the selected structures to predict their resonant frequencies. Table 1 contains a summary of these results. Both models predicted resonances near the lower frequencies, 20.6 Hz for the large tables and 31 Hz for the small tables, leading us to believe that the ®rst measured resonances of each devices were the fundamentals. Both models only dealt with the initial resonance mode and ignored harmonics. The optical displacements of the devices were then measured as a function of drive frequency. The results for both the scanners are shown in Fig. 7. These measurements were taken at a peak differential voltage of 40 V. The large devices have a resonance at 20.6 Hz yielding an optical scan angle of 778 and a larger resonance at 41.2 Hz with a
Table 1 Summary of resonance frequencies of the structures
Large scanner Small scanner
One-dimensional model (Hz)
ANSYS (Hz)
First measured resonance (Hz)
Second measured resonance (Hz)
12.6 30.7
17.3 36.2
20.6 31
41.2 62
Fig. 7. Optical angle vs. frequency for the two types of scanners: (a) large scanner with 135 mm thick hinges; (b) small scanner with 60 mm thick hinges.
Fig. 8. Optical angle vs. driving voltage for the two types of scanners near and away from largest resonant frequencies: (a) large scanner; (b) small scanner.
182
J.M. Zara, S.W. Smith / Sensors and Actuators A 102 (2002) 176±184
displacement of 1428. Measurements of the small device show a resonance at 31 Hz of 648 and a larger resonance at 62 Hz yielding a displacement of 898. The second resonance of both the structures is believed to be a superposition of a harmonic of the ®rst resonance and other modes in the mirror and support structure, which increase the measured motion beyond the level of the fundamental. Fig. 8 shows the plots of optical displacements vs. an increasing drive voltage for both the large and small scanners. These displacement measurements were taken near the largest resonant frequencies of the structures (41.6 Hz for the large scanner and 62 Hz for the small scanner) and also at a 1 Hz frequency well away from the resonances. Near resonance, the optical scan angle of the large scanner ranges from 1128 at 25 V to 1468at 50 V and the small scanner displacements range from 238 at 25 V to 1188 at 50 V. Away from resonance, the large scanner has displacements range from 108 at 25 V to 458 at 50 V and the small scanner has displacements ranging from 58 at 25 V to 33.58 at 50 V. Displacements were also determined as the drive voltage was decreased. There was some hysteresis in
the displacement measurements at the lower drive voltages (50±60 V). This hysteresis was not noticed at the higher drive voltages, therefore, the scanner appears to be most repeatable at voltages between 70 and 100 V. For the results contained in Fig. 8, it is clear that the optical displacement of the scanner increases with an increasing drive voltage amplitude. This increase in scan angle is expected, but the expected form of the graphs is very dif®cult to evaluate. In a simple model, the electrostatic forces produced by the applied voltage (Eq. (1)) would increase as the square of the voltage. However, one must also take into account many other complicating factors, such as the fact that we are reporting scan angle rather than linear actuator displacement, the torsion hinges become non-linear as they twist beyond small displacements, and the forces from Eq. (1) increase as the capacitive cells contract. For these reasons, trying to accurately predict the optical scan angle of these devices is dif®cult. More detail on the actuator contraction with respect to voltage is contained in Ref. [13]. Fig. 9 shows the video captured images of the laser beam scanned across the calibrated bulls-eye target for both the
Fig. 9. Plots of optical beam on the calibrated target: (a) optical beam from large scanner at rest; (b) optical beam from large scanner displaced away from resonance; (c) optical beam from large scanner displaced near resonance; (d) optical beam from small scanner at rest; (e) optical beam from small scanner displaced away from resonance; (f) optical beam from small scanner displaced near resonance.
J.M. Zara, S.W. Smith / Sensors and Actuators A 102 (2002) 176±184
Fig. 10. Bar code read by scanning system and received signal: (a) portion of code 39 bar code; (b) received signal from optical detection system.
large and small scanner. These images were all taken with the differential drive voltage set at 40 V. The images show the beam at rest, steered at a frequency of 1 Hz (quasi-static operation), and at resonance. Fig. 9(a)±(c) shows the displacements for the large scanner and Fig. 9(d)±(f) are for the small scanner. The images in Fig. 9(c) and (f) were taken with the camera aperture open wider than the other images to clearly show the extent of the scanned beam. The scanning mirrors were then inserted as the moving element in a prototype bar code scanner to sector the beam and read a portion of a Code 39 (Code 3 of 9) bar code which is the typical non-food standard and the most popular symbology for ID, inventory and tracking of purchases [17]. The large bars in this code are 800 mm wide and the small bars are 400 mm wide, with all of the bars separated by 400 mm. Fig. 10 shows the schematic illustration of the bar code (Fig. 10(a)) and the received signal from the photodiode and ampli®er after being digitized by the A/ D board (Fig. 10(b)). The high voltages correspond to the dark bars and low voltages represent re¯ections from the white areas of the bar code. This bar code data was recorded at a drive frequency of 62 Hz and a differential drive voltage of 40 V.
183
example of their functionality. Advantages of the scanners we have developed include the ability to operate in both resonant and quasi-static (low frequency motion away from resonance) modes, a large optical scan angle at low frequencies, and being fabricated from polyimide rather than silicon. These devices are also small, lightweight, have low power consumption, and are intermediate in size between the very small silicon mirrors and the galvanometers. The nominal resonance frequencies of these scanners (20±60 Hz) are in the range of conventional bar code scanners 36 Hz [17], and 30 Hz video frame rates for optical imaging systems. Future work on these devices includes integrating the fabrication of the IFA actuator, the mirror, and the support structure on the same wafer. This would reduce both the fabrication cost and assembly time and result in devices that could be fabricated relatively easily. We are also developing new IFA devices that are twice as thick as the previous devices (4.4 mm) and therefore will produce twice the force for a given voltage. These stronger IFAs would allow for larger scan angles away from the resonance of the structures. New types of support structures are being developed that would utilize multiple IFAs to both balance out the scan of the devices and also increase the degrees of freedom of the scanner. The ®rst implementation would be to attach actuators to both sides of the scanning mirror to pull on both sides of the table in an alternating fashion. For this con®guration, it would most likely be necessary to bend the table and hinges at a right angle to produce a forward-looking scanner. Designs are also being investigated to scan the mirror in two dimensions for applications that could bene®t from an additional degree of freedom in order to produce
4. Discussion In this paper we described the design, fabrication, and testing of a new optical beam scanner. These devices were operated away from resonance as quasi-static scanners producing optical displacements up to 458, or as resonant scanners to produce optical scans up to 1458. The scanning mirrors were also used in a prototype bar code scanner as an
Fig. 11. Schematic illustration of the miniature Cardano suspension proposed to scan the mirror in two-dimensions.
184
J.M. Zara, S.W. Smith / Sensors and Actuators A 102 (2002) 176±184
two-dimensional scans. These designs would use miniature Cardano suspensions that consist of two tables with orthogonal sets of torsion hinges to tilt in both the directions as illustrated in Fig. 11. As a ®nal step towards reliable scanning devices for multiple applications we are investigating capacitive feedback from the actuator that would allow for a control loop to increase the precision of the devices motion. This is possible due to the fact that the actuator is a network of capacitors that will change capacitance as it contracts and the individual cells change their geometry. These capacitance measurements should be able to be used by a control system to accurately predict the compression of the actuator and therefore the location of the beam steered by the tilting mirror. This type of precise control of the optical beam location is particularly important in many therapeutic applications such as Lasik surgery and other thermal ablation procedures. Acknowledgements We would like to thank Ken Gentry for his assistance in the initial development of these optical scanners. We would also like to thank Professor Stephen Bobbio and Dr. Scott Goodwin-Johansson for development of and expertise regarding the IFA actuator. This work was supported in part by HHS grants HL-58754 and HL-64962. References [1] K. Leggett, Medical lasers are at the threshold of a new era, Biophotonics Int. 42±47 (1998). [2] D. Huang, E. Swanson, C. Lin, J. Schuman, W. Stinson, W. Chang, M. Hee, T. Flotte, K. Gregory, C. Puliafito, J. Fujimoto, Optical coherence tomography, Science 254 (1991) 1178±1181.
[3] M. Truex, Lasers in dermatology: galvanometer scanners allow more uniform treatment, Biophotonics Int. 54±56 (1998). [4] M. Kiang, O. Solgaard, R. Muller, K. Lau, Micromachined polysilicon microscanners for barcode readers, IEEE Photonics Technol. Lett. 8 (12) (1996) 1707±1709. [5] R. Conant, J. Nee, K. Lau, R. Muller, A fast flat scanning micromirror, in: Proceedings of the Solid-State Sensor and Actuator Workshop, Hilton Head, SC, 2000, pp. 6±9. [6] W. Lang, H. Pavlicek, Th. Mark, H. Scheithauer, B. Schmidt, Electrostatically actuated micromirror devices in silicon technology, Sens. Actuat. A 74 (1999) 216±218. [7] H. Schenk, P. Durr, T. Haase, D. Kunze, U. Sobe, H. Lakner, H. Kuck, Large deflection micromechanical scanning mirrors for linear scans and pattern generation, IEEE J. Select. Top. Quant. Electron. 6 (5) (2000) 715±722. [8] L. Ferreira, S. Moehlecke, A silicon micromechanical galvanometric scanner, Sens. Actuat. A 73 (1999) 252±260. [9] S. Schweizer, S. Calmes, M. Laudon, Ph. Renaud, Thermally actuated optical microscanner with large angle and low consumption, Sens. Actuat. A 76 (1999) 470±477. [10] K. Yamada, T. Kuriyama, A novel asymmetric silicon micro-mirror for optical beam scanning display, in: Proceedings of the IEEE Micro Electro Mechanical Systems, 1998, pp. 110±115. [11] J. Zara, S. Bobbio, S. Goodwin-Johansson, S. Smith, Intracardiac ultrasound scanner using a micromachine (MEMS) actuator, IEEE Trans. UFFC 47 (4) (2000) 984±993. [12] J. Zara, S. Smith, A micromachine high frequency ultrasound scanner using photolithographic fabrication, IEEE Trans. UFFC 49 (7) (2002) 947±958. [13] J.D. Jacobson, S.H. Goodwin-Johansson, S.M. Bobbio, C.A. Bartlett, L.N. Yadon, Integrated force arrays: theory and modeling of static operation, J. Microelectromech. Syst. 4 (3) (1995) 139±150. [14] S.M. Bobbio, M.D. Kellam, B.W. Dudley, S. Goodwin-Johansson, S.K. Jones, J.D. Jacobsen, F.M. Tranjan, T.D. Dubois, Integrated force arrays, in: Proceedings of the IEEE Conference on MEMS, 1994. [15] S.M. Bobbio, S.W. Smith, S. Goodwin-Johansson, R.B. Fair, T.D. DuBois, F.M. Tranjan, J. Hudak, R. Gupta, H. Makki, Integrated force array: interface to external systems, SPIE 3046 (1997) 248± 259. [16] E. Reisner, M. Stein, Torsion and transverse bending of cantilever plates, Technical Note 2369, National Advisory Committee for Aeronautics, June 1951. [17] A Bar Code Primer, Worth Data, 1997±2001.