A novel prototype of an electrothermal chevron-beam actuator based microelectromechanical systems (MEMS) platform has been successfully developed for circumferential scan. Microassembly technology is utilized to construct this platform, which consists of a MEMS chevron-beam type microactuator and a micro-reflector. The proposed electrothermal microactuators with a two-stage electrothermal cascaded chevron-beam driving mechanism provide displacement amplification, thus enabling a highly reflective micro-pyramidal polygon reflector to rotate a large angle for light beam scanning. This MEMS platform is ultra-compact, supports circumferential imaging capability and is suitable for endoscopic optical coherence tomography (EOCT) applications, for example, for intravascular cancer detection.
© 2012 OSA
Optical coherence tomography (OCT) has demonstrated its potential as a powerful imaging technology due to its cellular or even subcellular resolution (1-10 μm) , which is considered suitable for early detection and diagnosis of cancer. For some clinic applications, for example, intravascular or gastrointestinal investigation, full circumferential scanning (FCS) is highly desired. Early research efforts on FCS such as spinning the entire catheter with build-in cables and fibers  were capable of scanning at a speed of around 4 revolutions per second, albeit with nonlinear motion due to the long cable rotation. Commercially-available micromotors have also been employed to spin mirrors or prisms to achieve FCS [3–5].
In recent times, MEMS technology [6–26] has demonstrated strong potential in biomedical imaging applications due to its outstanding advantages of, for instance, small size, fast scanning speed and convenience of batch fabrication. The recent development of a dual reflective MEMS micromirror based on bimorph actuators has further enriched and extended the MEMS technology for FCS in clinic applications [15,16].
In this paper, we propose a novel MEMS platform that is capable of circumferential scanning for optical coherence tomography (OCT) applications. The proposed configuration utilizes multiple parallel incident light beams to drastically reduce the required mechanical rotation angle to achieve 360-degree circumferential scanning with only minimal increase in package size. In our configuration, this compact MEMS platform is placed at the distal side of the endoscopic probe and orientated perpendicularly to the incident light beams (as shown in Fig. 1 ). A micro-pyramidal polygon reflector with four highly reflective facets is mounted on top of MEMS chevron-beam microactuators to redirect the focused light. A four-pieces-in-one fiber-pigtailed GRIN lens bundle is utilized to direct the focusing incident light beams to the slanted facets of the micro-reflector. Once the micro-reflector is driven to rotate, a circumferential light scan will be realized. A circumferential tissue image may be reconstructed by recording the data from the four fiber optic “channels” sequentially or simultaneously. The proposed configuration not only provides a way to achieve compact and miniature circumferential scanning probes, but also helps to reduce deformation that may be induced by residual stresses in traditional thin MEMS micromirrors as a solid reflector block is used here as the light reflector.
2. Device design
The bent-beam actuator [17–26] – usually called chevron-beam actuators – consists of two narrow beams each inclined at a small pre-bending angle θ in-plane as schematically shown in Fig. 2(a) . This pre-bending angle helps to encourage the beams to buckle in a preferred direction with motion amplification proportional to 1/θ for small angles. Normally, higher forces can be generated by placing multiple pairs of such actuators in parallel and larger displacements can be generated by cascading the actuators. Compared to other electrothermal devices that provide in-plane motion, bent-beam actuators are an attractive alternative in achieving large displacements and forces but, yet maintaining scalability and simplicity in design.
For cascaded actuators, if two primary base units are driven by the same voltage source, the current through the secondary unit will be nominally zero (Fig. 2(b)). Cascaded devices can provide significantly larger displacements than a simple single-stage chevron-beam pair. However, a drawback-associated with all large thin structures - of the proposed cascaded arrangements is that the critical force for vertical (out-of-plane) buckling is reduced. To compensate, the operating temperature for these devices must be reduced or the thickness of the devices increased. In order to make the device robust to external impact, the design thickness of the proposed device is selected to be 80 μm.
Based on the cascaded chevron-beam actuator working principle, we propose a novel two-level rotational MEMS micro-platform. The MEMS device is composed of a micro-pyramidal polygon reflector (Fig. 2(c)) and a cascaded two-stage chevron-beam actuator (Fig. 2(b)). For the cascaded two-stage chevron-beam actuator, two pairs of primary chevron beams are placed in direct opposition with their middle suspended shuttle apexes pointing inwards. The secondary chevron-beam pairs have their beam ends connected to the shutter apexes of the primary units. The beams of the secondary unit pairs are placed in opposite directions with their apexes facing outwards (see Fig. 2(b)). These two pairs of secondary chevron-beams have an offset in the x-direction in order to suspend a hollow ring-shaped platform holder in the center. When the same voltage is applied to the two primary chevron beams, an inward pushing motion along the x-direction will be produced due to thermal expansion of the chevron beams. This motion in turn results in the outward pulling motion of the secondary units. Subsequently, the circular ring-shaped holder suspended at the center will be driven to rotate by the pulling motion of members attached to the periphery of the holder.
In order to realize the capability of optical scanning, a highly reflective micro-pyramidal polygon reflector is fabricated. This reflector has four pieces of 45° slanted facets (see Fig. 2(c)) and is fully coated by Au on the surfaces. Suppose four parallel light beams normal to the device are incident on the four slanted factes of the pyramid, full circumferential scan (360°) can be achieved when the micro-pyramidal polygon reflector rotates an angle of 45°. A connection pillar is designed to facilitate the positioning and fixing of the pyramidal reflector onto the center circular ring-shaped holder. A specially-designed aligning jig is used to manually assemble these two parts together and they are subsequently bonded using epoxide resin.
An electro-thermo-mechanical finite element analysis (FEA) of the MEMS chevron-beam cascaded microactuator is conducted using CoventorWareTM 2010 to tailor the configuration and optimize geometric parameters. The structural material used is pure highly boron-doped single crystal silicon (110), and since the MEMS micro-platform is expected to be utilized in OCT applications, the overall die size is preliminary designed to be less than 5 mm (in this case, 4.5 mm). The width and length of the primary chevron beams used here are 15 μm and 1450 μm respectively, while those of the secondary chevron beams are 10 μm and 1050 μm.
Figure 3(a) shows the output displacement produced by a single-leg chevron beam in the primary unit for pre-bending angles ranging from 0° to 45° when a voltage of 8 V DC is applied. It is obvious that the output displacement decreases with increase in pre-bending angle, but the beams exhibit little or no deflection when this angle is smaller than 0.1°. For pre-bending angles less than 0.1°, the actuator refuses to move in-plane but instead attempts to buckle out-of-plane. Thus the pre-bending angle of 0.1° is predicted to provide the largest value of deflection, calculated at 35 μm. However, as a compromise between the resolution demands of the glass masks used and magnitude of the displacement generated, a pre-bending angle of 0.2° that can produce a displacement of 32 μm was selected for the primary chevron beams. Additionally, in order to provide a force large enough to push the secondary unit chevron-beam, twenty pairs of these chevron-beam actuators are placed in parallel.
Cascaded microactuators that are configured to provide displacement amplification have been reported many times in the literature [18–20,23,25,26]. Normally, the pre-bending angle of the secondary unit chevron beams has a significant effect on the amount of amplification of the input displacement from the primary unit. Assuming the lateral stiffness of the secondary chevron-beam mechanism has little constraint on its movement, the displacement in the y-direction (shown in Fig. 2(b)) has a geometric relation with that in the x-direction, given by:
The relationship between the displacement in the y-direction and the pre-bending angle of the secondary chevron beam is shown in Fig. 3(b) as a dashed line together with the FEA simulation results (solid line) for comparison. It is seen that these two curves match well beyond the 20° pre-bending angle point (right region of the central dashed line that is vertical to the x-axis), while exhibiting totally divergent behavior to the area left of this point. The reason for this deviation is probably due to the lateral stiffness of the beam, which governs the propensity of the chevron beam to deform. Though the pre-bending angle of less than 20° can theoretically provide at least tens of times of amplification, as seen from Eq. (1), the reaction force induced by the beam’s lateral stiffness in its x-axis counteract this effect. Furthermore, if the pre-bending angle in secondary beam is set lower than 5°, the amplification effect will no longer exist. An even more serious buckling issue may occur with the primary unit chevron beams when this angle is less than 2°.
Figure 4 shows the simulation results for the microactuator having a secondary pre-bending angle of 20 degrees at a temperature of 650K. It is clear that the rotation angle of the ring-shaped holder is around 45 degrees at this driving condition. If a micro-pyramidal polygon reflector with four 45-degree reflective slanted facets is mounted on top of the circular ring-shaped holder, a full 360°circumferential scan can be realized.
3. Fabrication and assembly
The micro-pyramidal polygon reflector is fabricated using silicon-on-insulator (SOI) micromachining technology with a total of four photomasks used (as shown in Fig. 5 ). This SOI wafer has a 150 μm-thick silicon device layer, a 1 μm-thick buried oxide (BOX) layer, and a 680 μm-thick silicon substrate. A 2 μm plasma-enhanced chemical vapor deposition (PECVD) silicon oxide layer is first deposited on the backside of the SOI wafer. Then the backside is patterned by photolithography, followed by 2 μm oxide dry etching and a 280 μm silicon deep reactive-ion etching (DRIE) process to define the height of the pyramid. Subsequently, a photoresist (PR) strip and Piranha clean are carried out and the backside oxide is etched off by a buffered oxide etchant (BOE) solution (with six parts of 40% NH4F and one part of 49% HF). Next, 300 Å thermal oxide and 1500 Å low-pressure chemical vapor deposition (LPCVD) silicon nitride layers are deposited on both sides of the SOI wafer, followed by 2000 Å low-pressure tetraethylorthosilicate (LPTEOS) silicon oxide deposition. The 2300 Å silicon oxide and 1500 Å silicon nitride layers, which serve as hard masks for the subsequent wet-etching process, are patterned using reactive-ion etching (RIE) process on the backside. Subsequently, a 400 μm-thick micro-pyramidal polygon reflector with 54.74° slanted facets is formed by wet anisotropic Si etching in a KOH solution (35 wt%, 75°C). Considering the undercuts occurring at the convex corners of the micro-pyramidal platform, corner compensation structures were added. Figure 6(b) shows a typical compensation structure at a convex corner. This structure consists of three square patterns attached to the corner apex, whose dimensions are indicated in the figure. The length in the figure can be estimated using , where is the desired etching depth. In this paper, we set, hence. To enhance the surface reflectivity, the surface of the pyramid is covered by a 300 Å/5000 Å thick Cr/Au layer deposited by E-beam evaporation. After the backside processes, the connection pillar is patterned and formed on the front-side by silicon DRIE. Finally, the whole micro-pyramidal polygon reflector is released by etching through the BOX layer with RIE.
The electrothermal MEMS cascaded chevron-beam microactuator is also fabricated using SOI micromachining technology (as shown in Fig. 7 ). Five photomasks were used in this process. The SOI wafer used has an 80 μm-thick heavily doped silicon device layer, a 1 μm-thick BOX layer, and a 650 μm-thick silicon substrate. The overall die size is 4.5 mm × 4.5 mm. A 1 μm PECVD oxide layer is first deposited on the front-side of the SOI wafer. After that, the silicon dioxide in the pad area is patterned using photolithography and an RIE process. The front-side is then again patterned using photolithography followed by a 70 μm deep silicon etch to define the device structure. During this step, a 10 μm thick silicon layer is left in the open etch area to avoid deformation or cracking of the structure beams in the device layer that may occur due to residual stresses in the BOX layer underneath. After PR strip and wet Piranha cleaning, the pads area is patterned by 20 μm negative dry film followed by 500 Å/1 μm Cr/Au layer E-beam evaporation and a dry film liftoff process to metalize the pads area for wire bonding. Next, 2 μm PECVD oxide is deposited on the backside to serve as a hard mask for backside DRIE. 2000 Å PECVD oxide is deposited on the wafer’s top surface, followed by patterning to cover the metallic pad areas. Subsequently, the backside of the SOI wafer is first patterned using 2 μm RIE oxide etching process and followed by 620 μm Si DRIE process. After the wafer level process, the whole wafer is diced into chips for chip-level release to obtain a higher yield. Selected chips are then boned onto the top surface of an oxidized carrier wafer using thermal tape to ensure the 30 μm silicon etching process stops at the BOX layer. These chips are then transferred onto the top surface of another silicon carrier wafer for a 1 μm oxide etching by RIE. After that, the 10 μm remaining silicon layer left on the front-side of the chip was etched away by DRIE to release the structure. Finally, front-side 2000 Å oxide is etched away by RIE process to expose the metal pads.
After the micro-pyramidal polygon reflector and the electrothermal chevron-beam based actuation mechanism have been fabricated, they are assembled manually (refer Fig. 9 ). Initially, the microactuator chip is manually placed on top of the polygonal reflector and held with a vernier caliper with the actuator facing downwards. The position of the vernier caliper can be adjusted in both transverse and longitudinal in-plane directions as well as vertical out-of-plane direction by a three-axis precision positioning stage. Next, the circular ring-shaped holder of the actuation mechanism is then aligned under a microscope to confirm that the mounting hole and the connection pillar are concentric before the actuator chip is lowered down and the connection pillar inserted into the circular ring-shaped holder. Finally, the connection pillar and the circular ring-shaped holder are bonded together by Araldite 2012 epoxy adhesive (Fig. 9(c)).
Figure 10 shows a scanning electron microscope (SEM) image of the device after assembly. An optical microscope image showing the device top view is also given in the inset. Circumferential scan will be realized by rotating the highly reflective micro-pyramidal polygon reflector to change the direction of the reflected light beams.
The relationships between current-voltage and current-optical-scanning-angle relationship curves of the prototype electrothermal MEMS chevron-beam based micro-platform obtained through measurement in a probe station. The rotation angles are measured by a microscope that is integrated with a digital camera. The results are illustrated in Fig. 11 . When a voltage is applied between the pairs of pads A and B, and A’ and B’ (as shown in the inset of Fig. 11), the apexes of the primary chevron-beam pairs are pushed inwards due to thermal expansion caused by joule heating. Simultaneously, the apexes of the secondary unit chevron beams are pushed outwards, multiplying the displacement output of the primary chevron-beam pairs several times and achieving a maximum displacement of 120 μm. The pulling forces generated by the outward movements of the apexes of the secondary unit chevron beams in turn drive the circular ring-shaped holder together with the micro-pyramidal polygon reflector mounted on it to rotate in plane to scan the light beams. The maximum optical scanning angle is measured to be 328° under a current of 600 mA or DC voltage of 5.95 V.
In order to find the transient response time of the MEMS micro-platform, two DC power supplies (Agilent E3647A) and a digital oscilloscope (DL1520) are connected to the device. A position sensitive detector (PSD) is utilized to detect the light trajectory scanned by the MEMS micro-platform. Figure 11(b) shows the input step voltage signal (change from 1 V to 2 V) and the corresponding measured transient response signal for this MEMS micro-platform. The response time, generally defined as the time taken to attain 90% of the saturation value, is found to be around 100 ms. In addition, for OCT applications, since there are four simultaneous channels for data acquisition, this speed is increased four times.
To demonstrate the circumferential scanning capability, the prototype device is driven into an oscillatory motion using a function generator (FC300, YOKOGAWA) and a current amplifier (OPA548T). A laser beam illuminates the micro-pyramidal reflector from above. The reflected beams from the slanted facets of the reflector project four laser spots on a cylindrical screen when the device is stationary as shown in Fig. 12(a) . The device is then driven to oscillate through applying a sinusoidal input voltage with an amplitude of 6 Vpp and a frequency of 10 Hz. The captured scanning pattern is shown in Fig. 12(b). Due to the limitation of the camera shooting angle, only three scan lines are visible in the figure. It is observed that under such a driving condition a near-360° circumferential scanning range can be obtained.
A novel electrothermal microactuator based MEMS platform to achieve near-360° light beam scanning for OCT purposes has been successfully developed. It consists of a pair of two-stage cascaded chevron-beam electrothermal actuators and a micro-pyramidal polygon reflector with high reflectivity. This micro-reflector is assembled onto the top of a ring-shaped holder that connects the chevron-beam actuators through flexures. A displacement amplification mechanism is employed in the cascaded two-stage chevron-beam microactuator design, resulting in a large mechanical in-plane rotation angle of around 41° of the micro-reflector. The transient response of the device is measured to be less than 100 ms and a 328° circumferential scanning range has also been experimentally demonstrated by the current prototype. In summary, it is possible to achieve miniature compact FS-EOCT probes with the proposed opto-mechanical configuration utilizing the MEMS-driven micro-reflector as a light manipulator. At the same time, existing problems such as degradation of imaging performance due to mirror deformation induced by residual stress are alleviated.
The authors would like to thank all the technical staff from Institute of Microelectronics, Agency for Science, Technology and Research (A*STAR), Singapore for their help and assistance. Financial support by the Singapore Ministry of Education (MOE) Academic Research Fund under grant R-265-000-416-112 is gratefully acknowledged.
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