DESCRIPTION

ELECTROTHERMAL MICROACTU ATOR FOR LARGE VERTICAL DISPLACEMENT

WITHOUT TILT OR LATERAL SHIFT

This invention was made with government support under a grant awarded from the NSF under grant number BES0423557. The government has certain rights in the invention.

CROSS-REFERENCE TO RELATED APPLICATION The present application claims the benefit of U.S. Provisional Application Serial No.

61/007,762, filed December 14, 2008, which is hereby incorporated by reference herein in its entirety, including any figures, tables, or drawings.

BACKGROUND OF THE INVENTION Fast scanning micromirrors and microlenses with large vertical displacement (LVD) ranges are desired in a variety of applications, such as wave- front shaping in adaptive optics, interferometry systems, and spatial light modulators. Recently, endoscopic optical coherence tomography (OCT), optical coherence microscopy (OCM), nonlinear optical imaging, and confocal microscopy are emerging as powerful biomedical imaging modalities. These imaging modalities typically require hundreds of microns of vertical displacement for platforms incorporated with the micromirrors and/or microlenses. In OCM and confocal microscopy imaging systems, the vertical scanning ranges of the microlens's focal plane should be 200 μm up to about 2mm, for example. Linear translating retroreflectors can be dispersion-free and polarization-insensitive, but are typically bulky and slow. The Fourier domain rapid scanning optical delay line (RSOD) can achieve several kHz scans, but it is complex and lossy and requires dispersion compensation. Most existing OCT systems perform image scanning of samples by moving an OCT probe or rotating the distal end of an optical fiber, which is slow and has non-uniform optical coupling.

Currently, two main actuation mechanisms are used to obtain large vertical displacement of the micromirrors and microlenses for OCT, OCM, and confocal microscopy. The first is electrostatic actuation, which can only provide tens of microns of scanning range. The driving voltages required are usually hundreds of volts; thus, such actuation is generally not suitable for biomedical applications. The second is electrothermal actuation, which is

capable of hundreds of microns vertical scanning at low driving voltages. These electrothermal actuators conventionally have two sets of complementarily-oriented thermal bimorph beams, which have as a goal to keep the mirror plate or lens holder parallel to the substrate and take advantage of the large vertical displacement at the tip of the long rotational beams.

However, the designs using electrothermal actuation to date have had a serious lateral shift problem. As an example, a particular LVD microlens used for confocal microscopy can generate a maximum 0.71mm vertical displacement, but has a lateral shift as large as 0.42mm. Such large lateral shifts tend to greatly distort the microscopic image. The lateral shift in LVD micromirrors also significantly reduces the effective optical aperture size. Another disadvantage of the previous LVD designs using electrothermal actuation is that a certain ratio of the driving voltages for the two complementary actuators needs to be maintained in order to obtain a purely vertical motion during the actuation.

Thus, a need remains for a microactuator that is capable of providing a large vertical displacement with reduced tilting and/or reduced lateral shift.

BRIEF SUMMARY Embodiments of the invention relate to a method and apparatus for vertically displacing a platform relative to a substrate. Displacement of a platform relative to a substrate is often referred to as vertical displacement even when the platform is being displaced sideways. The actuation is based on bimorph (or bi-layer) structures which generate motions when temperature changes. The bimorph structures can also incorporate one or more added layers for mechanical support and/or providing an additional thermal expansion coefficient. The temperature-induced motion is caused by the thermal expansion coefficient difference of the two materials forming the bimorphs. The large displacement is achieved by using the angular amplification of a rotating rigid frame. The large vertical displacement is achieved by using a pair of bimorph beams which compensate each other's rotation and produce a translational vertical motion. The translational vertical motion can also be referred to as translational motion of the platform or other structure, attached to or moved by the third bimorph toward or away from a substrate or other structure the first bimorph is attached to or positioned on. The lateral shift of the platform during displacement can be controlled and, in a specific embodiment, is preferably small. In a preferred embodiment, the lateral displacement is negligible during displacement. In a specific

embodiment, negligible lateral displacement can be achieved by using three bimorphs. Further embodiments can incorporate more than three bimorphs. The tilting of the platform during displacement can be controlled and, in a specific embodiment, the platform is parallel to the top surface of the substrate after displacement. In specific embodiments, one-dimensional (ID) or 2D angular scanning of the platform can also be achieved by selectively activating the bimorphs of the device.

Embodiments of the invention also pertain to a method and apparatus for endoscopic optical coherence tomography (OCT), optical coherence microscopy (OCM), nonlinear optical (NLO) imaging, and/or confocal microscopy. Such embodiments can incorporate a method and apparatus for displacing a platform relative to a substrate in accordance with embodiments of the invention.

An embodiment of a microactuator for displacing a platform vertically with respect to a substrate includes a first rigid frame, a first flexible bimorph beam connecting the first frame to the substrate, a second rigid frame, a second flexible bimorph beam connecting the second frame to the first frame, and a third flexible bimorph beam connecting a platform to the second frame. Activation of the first, second, and third flexible bimorph beams causes vertical displacement of the platform with respect to the substrate. Preferably, the vertical displacement is achieved with negligible lateral shift.

A specific embodiment of a microactuator assembly includes a substrate, a plurality of first rigid frames, a plurality of first flexible bimorph beams, a plurality of second rigid frames, a plurality of second flexible bimorph beams, a platform, and a plurality of third flexible bimorph beams. Actuation of the first, second, and third pluralities of bimorph beams causes vertical displacement of the platform with respect to the substrate. In this way, the amount of lateral shift that occurs during the vertical displacement can be controlled. In a preferred embodiment, the vertical displacement is achieved with negligible lateral shift.

A specific embodiment incorporates four identical microactuator assemblies, for example, as described above, that are located respectively at the four sides of a central platform. Simultaneous activation of the four microactuators moves the central platform vertically without tilting and lateral shift. Selective activation of one or two adjacent microactuators generates tip-tilt motions, such that ID and 2D angular scanning of the platform can be achieved. Other embodiments incorporate one, two, three, or more microactuator assemblies, attached to one or more sides, or other locations, of the platform.

BRIEF DESCRIPTION OF THE DRAWINGS

Figure IA is an elevation view of a large-vertical-displacement (LVD) microactuator with lateral shift.

Figure IB is an elevation view of an exemplary embodiment of a lateral-shift-free (LSF) LVD microactuator of the present disclosure.

Figure 1C shows an embodiment incorporating SiO ₂ ZPt/ Al/SiO ₂ multilayer thin films with optimized thickness ratios.

Figure 2 illustrates the geometry design conditions for a flat and LSF platform. Figure 3A shows the layout of an exemplary optimized structure using four sets of identical actuators, one at each side of a square platform.

Figure 3B shows simulation results in Coventorware of the LSF-LVD design as shown in Figure 3A at room temperature.

Figure 3C shows simulation results in Coventorware of the vertical displacement motion with four actuators being actuated simultaneously by equal voltages. Figure 3D shows simulation result in Coventorware of the tip-tilt motion of the same design with only one actuator being actuated.

Figure 3E shows the simulation result in Coventorware of the tip-tilt motion of the same design with two adjacent actuators being actuated.

Figure 4A shows a schematic diagram of an exemplary configuration for optical coherence tomography.

Figure 4B shows a schematic diagram of an exemplary configuration for optical coherence microscopy.

Figure 4C shows a schematic diagram of an exemplary configuration for confocal microscopy. Figure 5 shows an exemplary fabrication process for an embodiment of a microactuator of the present disclosure.

Figure 6A is a scanning electron microscope (SEM) image of a fabricated embodiment of an LSF-LVD micromirror in accordance with the subject invention, with insets showing the Pt heater, Al/SiO2 bimorph beams, and frames. Figure 6B is a SEM image of a fabricated embodiment of an LSF-LVD microlens holder in accordance with the subject invention, with an inset showing the meshed SiO ₂ lens holder (polymer lens not shown).

Figure 6C is a SEM image of a wire bonded device in accordance with the invention, with a U.S. dime.

Figure 6D shows an SEM image of a fabricated LSF-LVD micromirror with the first and second frames incorporating SiO ₂ ZPt/ Al/SiO ₂ multilayer thin films, in accordance with an embodiment of the invention.

Figure 7A is a graph showing vertical displacement performance of an exemplary device in accordance with the subject invention.

Figure 7B is a graph showing lateral shift performance of an exemplary device in accordance with the subject invention. Figure 7C is a graph showing tilting angle performance of an exemplary device in accordance with the subject invention.

Figure 8 is a graph showing the measured frequency response of an exemplary LSF- LVD micromirror device in accordance with the subject invention.

Figure 9A and B show the Zygo interferometric measurement results of an exemplary device with one actuator and two adjacent actuators being actuated respectively, which verify the tip-tilt motion.

Figure 9C shows the static scanning performances by actuating each actuator of an exemplary device.

Figure 10 shows the dynamic 2D scanning patterns of an exemplary device, in which A and B show the scanning patterns with different phase shifts in both directions and C shows a 15° ^χ 30° raster scanning pattern.

The figures may not be drawn to scale. Moreover, where directional terms (such as above, over, left, right, under, below, etc.) are used with respect to the illustrations or in the discussion, they are used for ease of comprehension only and not as limitations. The elements of the devices may be oriented otherwise, as readily appreciated by those skilled in the art.

DETAILED DISCLOSURE

Embodiments of the invention relate to a method and apparatus for vertically displacing a platform relative to a substrate. Displacement of a platform relative to a substrate is often referred to as vertical displacement even when the platform is being displaced sideways. The lateral shift of the platform during displacement can be controlled and, in a specific embodiment, is preferably small. In a preferred embodiment, the lateral

displacement is negligible during displacement. The tilting of the platform during displacement can be controlled and, in a specific embodiment, the platform is parallel to the top surface of the substrate after displacement.

Embodiments of the invention also pertain to a method and apparatus for endoscopic optical coherence tomography (OCT), optical coherence microscopy (OCM), nonlinear optical (NLO) imaging, and/or confocal microscopy. Such embodiments can incorporate a method and apparatus for displacing a platform relative to a substrate in accordance with embodiments of the invention.

Embodiments of a method and apparatus incorporate an electrothermal large vertical displacement (LVD) actuator design that can generate up to, or more than, a half millimeter vertical displacement. Preferably, the vertical displacement is achieved with negligible lateral shift and tilting angle. Further embodiments can achieve up to 1 mm, up to 1.5mm, or up to 5mm vertical displacement. In a specific embodiment, a lateral-shift-free (LSF) LVD microactuator design greatly simplifies the driving of the device by using only one driving voltage for vertical motion actuation. In an exemplary embodiment, the design of the LSF- LVD actuator include four actuators where each of the four actuators includes the following: 1) three bimorph beams used to compensate for the angular motion; 2) two frames connected between the three beams, such that the frames rotate oppositely to compensate for the lateral shift while still taking advantage of large vertical displacement at the tips; and 3) heaters (e.g., platinum) embedded all along the beams and the frames for uniform heating of the bimorph beams. Each of the four identical actuators incorporating the three beams and two frames are placed at the edges of a square-shaped platform. By embedding the heaters along the beams and frames, the temperature on the three bimorph beams of each microactuator can be maintained the same by a single driving voltage, thus a vertical displacement at the end of each microactuator can be obtained by a single driving voltage. Other materials can be used as heaters other than platinum such as chrome and polysilicon. In this embodiment, a symmetric structure is formed and, thus, by simultaneously actuating the four identical microactuators, the residual tilting caused by the temperature difference and fluctuation on the bimorph beams can be compensated and a purely vertical motion can be obtained on the platform. Angular scanning motion of the platform can also be achieved from the same embodiment. Actuating one actuator or two adjacent actuators of the four microactuators at the four sides of the platform, results in vertical displacement at the corresponding sides of the platform. Thus, ID or 2D angular scanning of the platform can be obtained.

In another embodiment, the platform has actuators only on two sides. The actuator on each side may be a single microactuator or a plurality of the microactuators. In case of plural microactuators, the numbers of microactuators on both sides may be the same or different.

In alternative embodiments, the platform can be other shapes than square, the individual actuators can be driven in subgroups and/or individually, more or less than four individual actuators can be attached to the platform, the bimorph beams and frames of the different individual actuators can be of different lengths, widths, or other properties, and/or the bimorph beams can have different properties.

Figure IA is an elevation view of a conventional LVD microactuator, which suffers lateral shift upon vertical displacement. In comparison, Figure IB is an elevation view of an exemplary embodiment of a LSF-LVD microactuator in accordance with the present disclosure. In the embodiment shown in Figure IB, the platform, which in exemplary cases is a mirror or lens holder, is connected to the last set of bimorph beams. The frames and the platform have single-crystal silicon (SCS) underneath to provide structural rigidity and avoid expansion upon heating. In another embodiment, as shown in Figure 1C, the first and second frames incorporate SiO ₂ ZPt/ Al/SiO ₂ multilayer thin films with optimized thickness ratios to maintain the flatness upon heating, which still takes advantage of the amplified out-of-plane displacement at tip of the flat frame but avoids using thick Si layer for the purpose of a faster thermal response. A platinum (Pt) heater is embedded in all the bimorph beams for a uniform and efficient heating. SiO ₂ meshes are used to provide good thermal isolation, such as isolating a mirror plate on the platform from the heaters. The tilt of the platform can be controlled by the rotation angles of the three bimorph beams, and the lateral shift can be minimized by properly choosing the lengths of the frames and bimorph beams. The conditions for a flat and LSF platform can be derived from the geometry design shown in Figure 2.

In order to keep the platform parallel to the substrate surface during the actuation, the rotation angles of the three bimorph beams should satisfy the following relation:

where θ], θ ₂ and θ ₃ are the arc angles of the three bimorphs and are proportional to the bimorph lengths and temperatures. If a uniform temperature is obtained on all the bimorph beams, the arc angles will be directly proportional to the bimorph lengths. Thus, Eq.(l) becomes / ₂ = h + h- For simplicity, in an embodiment, l _\ and / ₃ can be chosen as half of / ₂ to maintain a flat platform, i.e.,

_{/ / / /} ^

⁴ I ^{~~ l} 3 ^{~ l} 2 ' ^δ (2)

This also yields

θ _J = θ ₃ =θ ₂ /2. (3)

For a leveled platform, in the embodiment shown in Figure 2, there are five components contributing to the lateral shift: Actuator 1, Frame 1, Actuator 2, Frame 2 and Actuator 3. Assuming each bimorph beam has a uniform curvature, the lateral shift of each component can be expressed as

LS _actuatoλ = k -k{smθ _λ ) lθ^ ₍₄₎

LS _actuator2 = l ₂ - 2l ₂ (sinθ, )/θ _{2 λ6)}

LS frame! ^~ ^ ₂ ^~ L ₂ ^C0S ®i (η\

LS _actuator3 = I ₃ - I ₃ (^θ ₃ ) /θ _{3 ^ (8)} where Li and L ₂ are the lengths of Frame 1 and Frame 2, respectively.

By plugging Eqs.(2)-(3) into Eqs.(4)-(8), the total lateral shift of the platform is given by:

^ ~*-^actwtaλ ^ ^" -^"Jhmel ~ ^' ^" * actuator! ^" ^ frame! ^^aAtatorl

KZ ₁ -Z ₂ )(I-COS^ ₁ ) ₍₉₎

Thus, the lateral shift and the tilting of the platform can be controlled by selection of L] and Z ₂ for the case of / ₂ /2 — h = h and In a specific embodiment for the case of / ₂ /2 = / _/ = / ₃ and Gi= 0 ₃ =0 ₂ /2, the lateral shift can be substantially eliminated by simply choosing the same length for the two frames and the lengths of the bimorph beams to satisfy Eq.(2).

Figure 3 A shows the layout of an exemplary optimized symmetric structure with four sets of identical LVD actuators, one at each side of a square platform. With each actuator having a uniform heater, the four actuators can be driven simultaneously with a single driving voltage to maintain a purely vertical motion. Figure 3B and C show simulation results in Coventorware modeling software to verify the LSF-LVD structure design of the device of Figure 3A, in which Figure 3B is the simulation result of the structure at room temperature with no actuators being actuated, and Figure 3 C is the simulation result with all four actuators being actuated simultaneously by equal voltages. A purely vertical motion mode of the platform is demonstrated.

Tip-tilt motion can be also obtained from the same microactuator assembly as in

Figure 3A. Figure 3D and E show the Coventorware simulation results of the same design with one and two adjacent actuators being actuated, in which the corresponding platform edges displace downward thus the platform exhibits a tip-tilt motion with controlled tilting angles.

In alternative embodiments, such as when lateral shift and/or platform tilting is desired upon displacement, the actuators can be driven separately, in pairs, or otherwise use multiple voltage sources. In a further embodiment, the four actuators can be driven by multiple voltage sources and the multiple voltage sources can be controlled to control the tilt angle, vertical displacement, and lateral shift of the platform. Further, the relationship between θj, θ ₂ , and θ ₃ , and Z ₁ , / ₂ , and / ₃ can be adjusted as well to control the tilt angle, vertical displacement, and lateral shift of the platform.

With specific embodiments of an LSF-LVD actuator design in accordance with the subject invention, axial scanning can be precisely controlled for OCM and confocal microscopy. By controlling the axial scanning, three dimensional imaging via OCT systems can also be enabled, such as with endoscopic catheters.

Applications for embodiments of the subject microactuators include their use as key elements in miniature endoscopic imaging probes for in vivo diagnosis of early cancers including lung, bladder, colon, and gastrointestinal cancers and in miniature cardiovascular imaging catheters for plaque detection and intervention. Other applications include the use of embodiments of the subject microactuators as axial scanning micromirrors for optical delay lines in miniature OCT and OCM imaging probes and axial scanning microlens for OCM and confocal microscopy imaging probes.

Figures 4A-C show the schematics of classical setups of (Figure 4A) microelectromechanical- (MEMS)-based endoscopic optical coherence tomography; (Figure 4B) MEMS-based endoscopic optical coherence microscopy; and (Figure 4C) MEMS-based endoscopic confocal microscopy, representing the most applicable elements in those systems to embodiments of the LVD microactuator in accordance with the subject invention. The LSF-LVD micromirror can be used for axial scanning both in OCT and OCM systems. The LSF-LVD scanning microlens can be used in the OCM, nonlinear optical imaging, and confocal microscopy systems. The LSF-LVD micromirrors can be used for lateral scanning in OCT, OCM, nonlinear optical imaging, and confocal microscopy.

The device can be fabricated by a combined surface- and bulk-micromachining batch process. Figure 5 shows an exemplary fabrication process for a microactuator of the present disclosure, starting with Pt sputtering and lift-off to form the heater pattern on a 1 μm thick thermal SiO ₂ coated wafer (a). Steps (b) and (c) show a plasma enhanced chemical vapor deposition (PECVD)-formed SiO ₂ (~0.1μm) layer and e-beam Al layer (~lμm) being deposited and patterned. A SiO ₂ plasma etch is performed to expose the front- side Si (d). A backside SiO ₂ etch and a deep silicon etch are performed to form a ~20μm thick SCS platform (e). After that, a front-side deep silicon trench etch, followed by a silicon undercut, is used to release the micro structure (f). Other fabrication techniques can also be used and are known to those skilled in the art.

Figure 6A is a scanning electron microscope (SEM) image of a fabricated LSF-LVD micromirror with insets showing the Pt heater, Al/SiO2 bimorph beams, and frames. Figure 6B is a SEM image of a fabricated LSF-LVD microlens holder with an inset showing the meshed SiO ₂ lens holder (polymer lens not shown). Figure 6C is a SEM image of a wire bonded device with a U.S. dime. Figure 6D shows an SEM image of a fabricated LSF-LVD micromirror with the first and second frames incorporating SiO2/Pt/Al/SiO2 multilayer thin films.

The performance of a fabricated LSF-LVD micromirror, including the vertical displacement, lateral shift, and tilting angle, has been characterized using an Olympus BX51 microscope and a QC200 geometry measuring system, with d.c. voltages being applied to the four actuators simultaneously. The device has a 0.8mm mirror size and a 2.5mm device footprint. It exhibited an initial elevation of the mirror plate of about 646μm due to the residual stresses and the temperature coefficient of expansion (TCE) difference of the two materials. A large vertical displacement as high as 621μm has been obtained at only 5.3Vdc (184m W), as shown in Figure 7A. Figure 7B and Figure 7C show the measured lateral shifting and tilting angle, with the maximum values of only about lOμm and 0.7° respectively during the entire vertical scanning range. The frequency responses have been measured using a Polytec OFV-511 laser Doppler vibrometer, with an applied voltage of (l+0.5sinωt) V, as shown in Figure 8. The first resonance peaks are observed at 453 Hz, which correspond to the vertical motion resonance mode.

The tip-tilt motion of the same design has been verified experimentally. An experimental setup with a laser beam incident on the mirror surface and voltage applied to four actuators individually has been used to measure the static scanning angles versus applied

voltage on each actuator. As shown in Figure 9, about 34° optical angles can be obtained from each actuator at only 4.3 Vdc.

Dynamic 2D scanning was performed by using the same experimental setup for the static scan. By driving the two sets of opposite actuators with two pairs of ac voltages, which respectively are Vp(l+sin(ωt)) and Vp(l-sin(ωt)), the two opposite sides of the mirror plate in each direction can be excited alternatively with a fixed 180° phase shift. Thus, an optical scanning range of more than ±30° can be obtained independently in both axes with controlled piston motion and no lateral center shift. Figure 1 OA and B show the scanning patterns with different phase shifts of the two ac voltage pairs for both directions. Figure 1OC shows a 15°x30° raster scanning obtained by driving one axis at resonant frequency (fast scan) with Vp = 1.5V and the other at 47Hz (slow scan) with Vp = 3.5V.

AU patents, patent applications, provisional applications, and publications referred to or cited herein are incorporated by reference in their entirety, including all figures and tables, to the extent they are not inconsistent with the explicit teachings of this specification. It should be understood that the examples and embodiments described herein are for illustrative purposes only and that various modifications or changes in light thereof will be suggested to persons skilled in the art and are to be included within the spirit and purview of this application.