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A magnetically actuated MEMS scanner with a microfabricated ferromagnetic nickel platform and thermosetting polydimethylsiloxane (PDMS) microlens is demonstrated. The device is driven by an external AC magnetic field, eliminating chip circuitry and thermal deformation from joule heating. The resonant frequency of 215.2 Hz and scanning angle of 23° of the scanner have been demonstrated. Experimental studies and optical modeling have shown that this microlens scanner achieves a scanning range of 125 µm when actuated by an external magnetic field of 22.2×10-3 Tesla flux density. The device has potential applications in in vivo medical imaging for minimally invasive diagnoses.
©2007 Optical Society of America
1. Introduction
Optical diagnoses of human tissues via fluorescent excitation, molecular absorption, and elastic and inelastic (Raman) scattering have been studied intensively over the last two decades [1]. Some technologies are based on in vivo macro-point spectral measurements [2], while others are based on in vivo macroscopic and microscopic imaging such as tissue autofluorescence imaging [2], optical coherence tomography (OCT) [3], laser scanning confocal microscopy (LSCM) [4], and two-photon excitation fluorescence (TPEF) microscopy [5]. Recent attempt has combined Raman spectroscopy with confocal imaging to perform microscopic scale spectral analysis of skin tissues in vivo [6]. These technologies, providing noninvasive or minimally invasive examination directly on tissue or cells without removal of tissue, have a great potential for clinical applications. However, these devices are initially developed with conventional bulky optical and opto-mechanical components and are not convenient for routine clinical uses. Miniaturization of these devices, especially the optical scanners will greatly enhance their clinical utility, especially for applications involving endoscopic imaging and spectroscopy of internal organs.
Optical beam scanners are one of the key components in these in vivo imaging devices; for example, OCT requires 1-D scanning of the illumination light beam, while LSCM and TPEF microscopy require 2-D raster scanning of the illumination beam. In the last few years, a couple of groups have worked on developing microelectromechanical systems (MEMS) based optical scanners for confocal, OCT, TPEF microscopy and other imaging applications [7–14]. Most of these miniaturized scanners are based on well developed reflective micro-mirrors using electrostatic or magnetic actuation. These micro-mirrors with the advantage of light weight and zero chromatic aberration provide various scanning capabilities such as adjustable mirror deformation for dynamic focal scanning and movable mirror for raster scanning from 165 Hz to 8 kHz. The disadvantage of reflective based device is that the optical axis is folded after reflecting off each mirror surface. This makes alignment and packaging of separate focusing and scanning elements difficult. In addition, an extra lens is required to work together with the mirrors for imaging. Transmissive scanners using moving lenses have alignment advantages for systems implementation and result in smaller MEMS packages. Their resonance frequencies vary from 550 Hz to 4.5 kHz and the scanning range of 100 µm is achievable [15–17]. However, the existing transmissive scanning microlenses are based on electrostatic actuation that needs high voltage, electrical wiring and wafer bonding. The scanning range is often limited by electrostatic pull-in.
In this paper, we demonstrate a magnetically actuated MEMS microlens scanner that integrates microfabricated ferromagnetic nickel platform and thermosetting elastomer microlens providing transmissive optical scanning capabilities. This scanner, employing external magnetic field for actuation in a non-contact fashion avoiding electrical wiring, high voltage supply, and joule heating, is particularly attractive for endoscopic catheter development. The device with demonstrated 1-D scanning capability should be applicable for OCT imaging. It can also be further developed to have 2-D scanning capabilities for applications in confocal imaging, TPEF microscopy, and other in vivo optical imaging and spectroscopy measurements.
2. Design and working principle
Fig. 1. A magnetically actuated microlens assembled with an optical fiber provides focal scanning capability.
Figure 1 illustrates a magnetically actuated microlens scanner assembled with an optical fiber having a collimation lens attached at the end for light delivery and collection. Incident light from the optical fiber is focused by the elastomer based scanning microlens onto the target specimen. Excited optical emissions from the specimen can be captured via the reverse optical path for spectroscopic analysis.
The structural design of the magnetically actuated scanning microlens is shown in Fig. 2. The device has a dimension of 2000µm×3000µm, consists of a plano convex polydimethylsiloxane (PDMS) microlens and a suspended ferromagnetic nickel platform with a pair of V-shaped springs hinged the platform to the substrate from both sides. A 1000 µm diameter aperture is formed at the center of the platform for optical transmission. The platform, the suspension springs, and the torsional hinges are electroplated nickel. An external magnetic field is used to actuate the scanning microlens. A magnetic moment is induced to rotate the nickel platform about the hinges creating a periodic focal scanning trajectory.
3. Device fabrication
The ferromagnetic platform and the PDMS microlens are fabricated separately from two steps. Microfabricated ferromagnetic platform are made from a commercial Metal MUMPS process [18] with a 20 µm-thick nickel free-standing structure. After the nickel structure is released, the PDMS microlens is formed using a thermal setting process and is transferred onto the nickel structure. The microlens and the platform are joined together by an adhesive PDMS layer. The final device is shown in Fig. 3.
3.1 Magnetic platform microfabrication
Figure 4 illustrates the device fabrication process. The ferromagnetic platform is fabricated on a 675µm±15µm thick, n-type (100) silicon wafer. A 2 µm-thick silicon dioxide (SiO2) is grown on a silicon wafer followed by a 200 nm phosphosilicate glass (PSG) sacrificial layer [Fig. 4(a)]. Afterwards, a 0.35 µm-thick low-stress silicon nitride layer and a 0.7 µm-thick polysilicon layer are deposited. Polysilicon is then patterned and etched by reactive ion etching, forming the pads for metal anchoring [Fig. 4(b)]. A second layer of 0.35 µm low-stress silicon nitride layer is deposited and dry etched together with the first silicon nitride layer, defining the optical pupil [Fig. 4(c)]. Another 1.1 µm-thick PSG layer is then deposited and annealed at 1050°C for 1 hour. It is then patterned for the sacrificial metal release. Anchoring metals consist of 10 nm-thick Cr and 25 nm-thick Pt layers are deposited on the polysilicon pads by a lift-off process [Fig. 4(d)]. The wafer surface is then coated and patterned with a thick layer of photoresist as the electroplating stencil. Nickel electroplating takes place inside the 20 µm-thick stencils and is followed by a 0.5 µm-thick gold layer deposition [Fig. 4(e)]. The photoresist stencil, the two PSG sacrificial layers and the silicon dioxide underneath the silicon nitride pupil are released by solvents and 49% hydrofluoric acid respectively. Finally and optionally, the pupil exposed silicon surface can be etched through by DRIE or wet etch process. The Metal MUMPS process only perform a 25µm trench etch [Fig 4(f)], but silicon is optical transparent in the infrared spectrum [19]. In the following experiments, we are using a 1064 nm Near Infrared (NIR) laser. Finally, the nickel platform is lifted up to 260 µm away from the substrate surface for 48 hours using a probe station. Permanent deformation in the vertical direction is created in the nickel suspension spring after the lifting process. The clearance distance between the lens and the substrate affects the maximum achievable scanning angle.
3.2 PDMS microlens
PDMS has greater than 90% optical transmittance from 380 nm to 1100 nm which is suitable for optical applications [20]. Formation of PDMS microlens is achieved by thermosetting of premixed 10:1 elastomer base and curing agent (Dow Corning. Sylgard Elastomer 184). Premixed PDMS elastomer is first degassed using a vacuum chamber to remove trapped air bubbles from stirring. A micropipette is pre-assembled onto a probe station for fine movement manipulation. On the probe station, a wafer handling chuck is connected to a digital controlled heater. A piece of double-side polished silicon wafer is placed on top of the heating surface as the microlens landing platform. A K-type thermocouples is attached on the silicon surface for temperature monitoring. In order to fabricate a spherical lens, the micropipette is carefully adjusted to make sure the pipette tip is perpendicular to the silicon surface. A 32.8 µl droplet of PDMS is disposed on the thermal controlled silicon surface. The polymerization takes 5 seconds at 140°C and forms a plano-convex microlens. The profile of the PDMS microlens has been measured by a profilometer (Tencor Alpha Step 200) as shown in Fig. 5.
The aspheric nature of the microlens surface can be described by the conic constant k [21]:
where r is the lens radius, z is the sag height and c is the lens curvature [22]:
θc is the lens contact angle between the plano and convex surface and V is the lens volume. The calculated conic constant, k=0.361 shows the microlens has an oblate ellipsoid surface. (for a perfect spherical surface, k=0). The parameters for the calculations are listed in Fig. 5.
The refractive index and the numerical aperture of the microlens are correlated factors that affect the focal length and the image brightness. Different studies on the fabrication of microlens such as planner glass diaphragm, UV curable polymer, soft lithography and the technique of photoresist reflow have been reported with numerical apertures vary from 0.05 to 0.55 [22–27]. Although PDMS has been investigated for different optical application since 1995 [28], its refractive index values as a function of wavelength are not readily available in the literature. Most investigators assumed the refractive index to be 1.4. A recent study on polymer materials by spectroscopic ellipsometry shown that the refractive index of 10:1 mixed PDMS decreases with increasing wavelength, but measurements were only performed in several wavelengths in the visible wavelength range [20]. In this work, we estimated the refractive index of the PDMS microlens at 1064 nm as 1.272 based on the Sellmeier dispersion formula [29, 30]. The numerical aperture, NA, of the microlens for beam focused from the plano surface is calculated as [22]:
The NA at 1064 nm is calculated as 0.219. The resolving power of the PDMS mcirolens is estimated from the Rayleigh criterion for resolution [21]:
where d is the resolution of the lens showing the minimum resolvable distance between two point images, and λ is the laser wavelength (1064 nm). The calculated resolution is 2.96 µm.
4. Device characterization
The actuation magnetic field is generated from a solenoid (S-17-85-39QH, Magnetic Sensor system) inclined 30° from the ferromagnetic nickel platform. When the magnetic field is imposed on the platform, the nickel domain boundaries tend to line up along the field direction, induces a net magnetic moment for microlens tilting. This titling effect is illustrated by a coin size magnet sliding underneath from left to right hand side in Fig. 6.
When an AC magnetic field is applied, the microlens oscillates periodically. Figure 7 shows the motion of the oscillating devices.
The characteristic of this oscillation is measured by a laser doppler vibrometer (Polytec OFC-5000, Displacement decoder DD-200, resolution range: 2 nm to 32 nm). Figure 8 shows the frequency response under 0.6×10-3 Tesla magnetic flux density measured by a Gauss meter (F.W. BELL Model 4048 Gauss/Tesla meter and calculated probe no. 1448). The microlens has a resonance frequency of 215.2 Hz and a quality factor (Q-factor) of 107. The oscillation angle with imposed magnetic flux density is shown in Fig. 9. The result shows increase of the tilting angle with the magnetic field from 0 to 22.2×10-3 Tesla with a maximum 23° oscillation angle.
Fig. 9. The change of oscillation angle of the magnetic actuated microlens at different magnetic flux density.
The optical scanning behavior of this device is investigated by both computer modeling (Zemax-EE Optical design program, Ver. 2005) and experiment at the resonance frequency. The optical setting is illustrated in Fig. 10.
The incident beam is refracted by the PDMS microlens, the silicon wafer, and the glass shielding of the charge coupled device (CCD) before being focused onto the CCD sensor. Refractive indices of these components are 1.272, 3.6 [31], and 1.5 [32] respectively. The focal distance of the optical system measured from the microlens’s plano surface to the CCD is 4.21 mm. The experimental study is conducted at room temperature in a dark environment. The light beam is generated from a 1064nm NIR infrared diode pumped solid-state laser (Forte 1064, Laser Quantum). The scanning trajectories are captured using a board level digital camera based on a Sony ICX204AL CCD (4.65×4.65 µm pixel size, 1024×768 pixels, Matrix Vision, MvBLueFox-M121G) and analyzed by an imaging processing software (Maxtor Inspector, Ver 3.1). All the images are analyzed at 8 bit grey scale intensity. Figure 11 shows images of the optical scanning patterns with the microlens oscillating at around 0°, 4°, 10° and 20° from the modeling and experiment. Figure 12 shows the scanning ranges at different oscillation angles. The background noise of the experimental images is eliminated by intensity threshold value set to 70. The images are then rescaled to have a maximum grey level of 255 for better displaying. When the segmentation threshold is set at 230 for the modeling images, the scanning ranges determined from modeling and from the experiment matches closely (Fig. 12.). A 125.6 µm scanning distance is achieved experimentally from 22.2×10-3 Tesla external magnetic flux density.
Fig. 11. The optical scanning trajectory from the magnetic actuated microlens at 215.2 Hz on a 4.65µm pixel size CCD: (a) Computer modeling, (b) Experiment.
Fig. 12. Experimental and computer modeling of the optical scanning distances generated from 1064 nm near infrared laser light. The microlens scanner is operated at resonant mode.
According to the U.S. Environment Protection Agency, magnetic noise generated from electronic devices in occupational environment ranges from 0.1×10-3 to 1.3×10-3 Tesla [33]. To eliminate this environmental susceptibility on the microlens’s practicability, the microlens should be operated close to the resonant frequency with a high Q-factor that can effectively filter out noises. Furthermore, magnetic interference may be blocked by magnetic shielding materials surrounding the target specimen [34].
The aberration effect of the mcrolens device has been analyzed by Zemax modeling with 0.3 mm entrance pupil as shown in Fig. 13. The focal spots are evaluated at the location of the CCD sensor as the mcirolens tiltes from 0° to the maximum 11.5° from the normal towards one end. The peak to valley optical path different (Pv) increases from 0.06447λ to 0.17397λ, where λ is the wavelength (1064 nm) of the near infrared light. The values are acceptable by the 0.25λ Rayleigh criteria for an optical focal spot. Root mean square wavefront error (RMS) is between 12.4% to 18.5% of the corresponding Pv value within the 33% requirement [21]. The Strehl ratio presents the ratio between the aberrated central intensity to the zeroaberration diffraction-limited case, changes 4.1% from 0.9975 to 0.9560 above the 0.9 level. The Hygens point spread functions present the logarithmic intensity of the focal spots on the left hand side of Fig. 13. A clear wavefront departs from non-tilting sphericity to aberration at 11.5° tilting are shown with corresponding wavefront surfaces on the right hand side.
Fig. 13. The Hygens point spread function and the wavefront surface of the 1064 nm focal spot from mcriolens tilting : (a) 0° and (b) 11.5° from the normal of the microlens device.
5. Summary
We have demonstrated a magnetic actuated MEMS microlens scanner for the first time. The device has a dimension of 2000µm×3000µm with a microfabricated ferromagnetic nickel platform and a thermosetting plano-convex PDMS microlens. It is actuated by an external AC magnetic field. This device does not require electrical wiring, high voltage supply, or joule heating and could have advantages for endoscopic catheter applications. It will result in safer, compacter, and more reliable catheters that are better suitable for in vivo applications. The PDMS microlens is 1.5 mm in diameter with an oblate ellipsoid surface and a 0.219 NA at 1064 nm. The microlens scanner has a resonance frequency of 215.2 Hz and Q-factor 107. Experiment and computer modeling have shown that the oscillation microlens provides focal scanning trajectory of 125.6µm scanning range from a 22.2×10-3 Tesla external magnetic flux density. The scanning range can be further increased by designing a magnetic platform with a larger tilting angle and adjusting the PDMS microlens curvature to minimize the loss of resolution and aberration from tilting. The demonstrated resonant frequency is low compared to electrostatic type scanning microlens. This can be increased in the future by further shrinking the microlens size and implement stiffer springs. With the demonstrated 1-D scanning capability, this device could be applicable for OCT imaging. Further development of the device for 2-D scanning capabilities will enable applications in confocal imaging, TPEF microscopy, and other in vivo optical imaging and spectroscopy measurements.
Acknowledgments
The Metal MUMPS microfabrication process is provided by the CMC Microsystems, a Canadian federally incorporated non-profit corporation. The authors would like to thank Prof. U. Hafeli in the Faculty of Pharmaceutical Sciences, UBC for the use of a Gauss meter. This work is supported in part by the Faculty of Applied Science, UBC and NSERC Discovery Grant to MC. MC is supported by Canada Research Chairs Tier 2. †Part of this paper was presented in the IEEE MEMS 2007 Conference, Kobe, Japan, 2007.
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