Surface Micromachining on Three-Dimensional Bulk Si Structures
Minoru Sasaki, Associate Professor, School of Engineering, Tohoku University
1. Introduction
In order to develop devices for micro-optics applications, elements of thedevice is often required to have 3D structures so as to form a space for theoptical path that is decided by the laws of optics, such as the law ofreflection. Few optical systems can be integrated on the surface of a single
substrate. Strategies for arranging devices three-dimensionally often haveproblems in assembly that reduce productivity. Such problems can bealleviated by prealigning the microactuator with the micro-optical benchusing an alignment guide. We developed an optical device having a thin film
construction layer on a three-dimensional bulk Si structure.
2. Optical Scanner
We prepared a micro-mirror on a Si(111) surface obtained by anisotropic
etching and fabricated an optical scanner incorporated with a temperaturedriven actuator. We applied a 3D photolithography technique capable offorming a SiN mask and forming patterns in the wall surfaces. Figs. 1(a) and1(b) show the structure of the optical scanner and an actual photo of thefabricated scanner, respectively. The left side of the structure shows a laser
diode (LD) chip mounted on a terrace, which serves as a 3D micro-opticalbench. The right side of the structure is a V-shaped cantilever microactuator
constructed with SiN sandwiched between a doped poly-Si heater layer andAl. When heated by electricity, the structure arches upward as the Al layeron the bottom expands. As the structure bends, the mirror rotates to aposition reflecting light emitted from the LD away from the surface of thewafer. A photodiode (PD) provided on the Si substrate makes the structure
applicable for barcode readers. The LD, mirror, and PD can be alignedlinearly. The ribs are prepared around the periphery of the scanning mirror
to strengthen the thin film mirror and make it resistant to warping. The
operations of the LD chip were confirmed after mounting the LD chip on theterrace, wherein a laser scanning angle of 30° was obtained at 120 mW. The
cutoff frequency was 100 Hz, close to the required value for a barcode reader.
3. Variable Optical Attenuator
For optical communications, we fabricated a bridge-type device shown inFig. 2(a) that functions on an optical fiber. An optical fiber fixed in a V
groove is polished down near its core, and a surface micromachined structure
is prepared on the polished surface. When a SiN film having a high refractiveindex (~ 2) is brought near the optical fiber core, some coupling loss of the
propagating light inside the fiber occurs. The transmittance varies according
to the position of the bridge. Fig. 2(b) shows one end of the device, whereinthe bridge is 0.25 x 3 mm, and the gap between the bridge and the opticalfiber is 3µm. The bridge is aligned precisely in relation to the optical fibercore. Since the position of the optical fiber is set by the V groove, the mask
can be aligned to achieve an accurate bridge position. The resulting
attenuation was about 1 dB. This small variation is attributed not only to a
problem of particles, but also the difficulty of positioning the entire longbridge uniformly near the optical fiber.
4. Conclusion
Two types of devices were successfully constructed by combining thinfilm microactuator and been aligned on the bulk optical base having threedimensional structures.

(a)

(b)
Fig. 1 Optical scanner formed on a 3D micro-optical bench

(a)

(b)

Fig. 2 Movable bridge-type variable attenuator prepared on the polished surface of an
optical fiber

Basic Study for Microactuators Controled by Wetting and Driven by Capilary
Force between Liquid-Liquid-Gas Interface
Izumi Hirasawa, Professor, Department of Applied Chemistry, Waseda University
Masato Sakurai, Senior Researcher, National Aerospace Laboratory of Japan
1. Introduction
In fluid phenomena, the effects of wettability and surface tension
become dominant as the scale becomes smaller. Since these forces become
dominant in a microgravitational field, this study was focused on thesimilarities between microscale systems and a microgravitational field. We
attempted to control a fluid by actively controlling the wettability andsurface tension.
2. Fluid Handling by Wettability
Next, we will give an example of fluid handling through wettability
distribution. A metal plate in the shape of a letter was placed on top of a
Peltier element for cooling, and a culture dish coated with the temperatureresponsive polymer IPAAm was placed over the letter-shaped plate. Since
the bottom surface of the dish had been coated with the IPAAm, only thecooled parts showed good wettability. Hence, water droplets only adhered
to areas with good wettability, forming a path in the shape of the letter on
the coated dish, as shown in Fig. 1.
3. Liquid-Liquid-Gas Marangoni Convection
It was discovered that a spontaneous flow occurs when a drop of silicon
oil is placed on a layer of fluorinert, as shown in Fig. 2. This flow is
attributed to evaporation, since the flow stops when the system isaccommodated in a hermetically sealed vessel. The thickness of the silicon
oil drop is thought to be about 1 mm. As can be seen from the velocity
distribution in Fig. 3, the driving force of this flow appears to originate at thepoint of intersection between a gas and two liquids. An experimental device
that applies this driving force was manufactured in order to produce
rotational force. The impeller of this device distributes wettability. Whenthe impeller was floated on the fluorinert and when a silicon droplet dyed
red with a syringe was adhered only to one blade of the impeller, as shown in
Fig. 4, the impeller began to rotate spontaneously.
4. Conclusion
Developing a microscale system for handling fluids is easy, since fluids aretreated without mechanical structures in this study. Future studies will attempt
to elucidate the mechanisms of the spontaneous flow and produce linear
motion, while developing driving forces that work effectively on a microscale.

2Fig. 1 The path of flow is formed by wettability
distribution

Fig. 2 Liquid-liquid-gas Marangoni convection

Fig. 3 Velocity distribution in liquid-liquidgas Marangoni convection

Fig. 4 Impeller employing liquid-liquid-gas
Marangoni convection