1-1 Motivation
Micro Electro-Mechanical Systems (MEMS) is a technique to fabricate mechanical structures at microscales. Mechanical structures, actuators, sensors, and optical elements in MEMS can be implemented by bulk micromachining [1], surface micromachining [2], and the LIGA processes [3]. Bulk micromachining fabricates micro-components by anisotropic or isotropic etching of the substrate which usually is silicon. Surface micromachining is the planar process by deposition and etching of substrate and sacrificial layers to fabricate mechanical structures. High planar resolution is the advantage of surface micromachining which is the most widely used technique to fabricate microstructures. The LIGA process is suitable for manufacturing structures with high aspect ratio (i.e., structures that are much higher than broad).
In the past years, MEMS techniques have been applied to optical engineering, and biomedical engineering. In particular, three-dimensional (3-D) out-of-plane structures on the silicon wafers are important components in applications such as free-space optical bench [4]. Figure 1-1 shows the scanning electron micrographs (SEM) of an optical pickup head and other 3-D applications [4-6]. Furthermore, many sensors and actuators also have the demand for 3-D structures. Batch fabrication and assembly processes are important issues for MEMS technique with new 3-D applications. Many techniques for fabricating complex 3-D structure have been proposed in the past 20 years. One of the most important issues for the 3-D structures is angular positioning of out-of-plane structures, especially for optical applications. Based on this demand, a micro assembly process with good positioning accuracy is needed for fabricating an
optical bench. In our previous study [7], 90° out-of-plane structures were fabricated and assembled by the proposed one-push method. However, the angular accuracy still needs to be improved and an assembly method for arbitrary angles is also desired.
Thus, solving the angular accuracy problem and developing the assembly method for arbitrary angles are the objectives of this thesis.
Figure 1-1 Examples of optical MEMS applications, (a) on-chip optical-disk pickup head system ,, (b) microlens scanner with integrated polymer microlens [5], (c) optical cross-connect switches [6].
(a) Micro-Fresnel lens
Beam-splitter 45o reflector
(b) (c)
1-2 Literature survey
Most 3-D components are fabricated by surface micromaching and then flipped up to form the 3-D microstructures. Micro hinges are a common solution to anchor the flip-up components. In the past years, many assembly methods for 3-D structures have been proposed. In addition to manual assembly, external forces powered by scratch drive actuators, magnetic force, electrostatic force, or ultrasonic agitation were also used to assemble 3-D structures. Pre-stressed bimorph beams and surface tension were also used for self-assembly. Recently, automatic assembly process was also demonstrated by standard or specially designed equipments. In this section, techniques for 3-D structure assembly are reviewed.
1-2-1 Microfabricated hinges
Surface micromachining is usually used to manufacture MEMS devices with the structure layer thickness less than 5µm. It was developed in the 80's for combining MEMS and planar integrated circuits (IC) technology on a common silicon wafer. In [8], the proposed micro hinge structures enabled the surface micromachined structures to rotate out of the plane. Two structural layers were used to fabricate the hinge. The poly-1 layer was used as structure layer and hinge pin, while the poly-2 layer was used to fabricate the hinge staple that could cover the hinge pin, as shown in Figure 1-2. This invention enabled micro-structures to rotate out of the plane and also opens an important door for future three-dimensional (3-D) applications.
Figure 1-2 (a) Cross section view of fabrication processes, (b) SEM figure of a hinged structure [8].
However, conventional microfabricated hinges have an important problem due to the play between the pin and the staple. Therefore the pin can not be precisely positioned. In order to solve this problem, an improved hinge was proposed [9], as shown in Figure 1-3. A cantilever beam was used to press and fix the hinge pin, which was made wider than the polysilicon layer thickness. The vertical hinge play was eliminated in this invention and the fabricated structure could not shift vertically anymore. However, the lateral play still existed and would cause shift in assembly process.
To eliminate the lateral play and improve the positioning accuracy, a solution was proposed in our previous study [7]. The V-shaped hinge locks the hinge pin in desired axis by the geometric design. The comparison between the V-shaped hinge and the conventional hinge will be presented in Chapter 4.
PSG
Figure 1-3 (a) Cross section view of the improved hinge, (b) SEM figure [9].
1-2-2 Three dimensional MEMS by powered assembly
Assembly powered external forces such as scratch drive actuators, magnetic force, electrostatic force, and ultrasonic agitation are reviewed in this section.
Scratch drive actuator assembly
The scratch drive actuator (SDA), as shown in Figure 1-4, was proposed in 1993 [10]. A scratch drive actuator is usually composed of a plate, a bushing, an insulator and a substrate electrode, as shown in Figure 1-4 (a). With an applied voltage, the plate is pulled down to the substrate by the electrostatic force. The bushing is tilted at this stage and moves a small distance. Once the voltage is removed, the plate and bushing return to their initial position. With this motion in cycles, the SDA can go forward to the desired position. Figure 1-4 (b) shows the working principle step by step. Figure 1-5 illustrates a micromirror assembled by using an array of SDA [11].
cantilever anchor
slider
pin (rotated 90o) slider
cantilever
(a) (b) hinge
Figure 1-4 (a) Cross section view of a SDA, (b) working principle of SDA [10].
Figure 1-5 (a) SEM of a free-rotating hinged micromirror lifted by an array of SDA, (b) structure is lifted by SDA [11].
Electrostatic force assembly
Electrostatic force generated by parallel plates [12] or ultrasonic waves [13] can be used to assemble 3-D structures. The operation principle of generating electrostatic force by parallel plates is shown in Figure 1-6. When the voltage is applied, the electrostatic attraction force between the ground plane and the hinged plate enables the hinged plate to move out of the plane, as shown in Figure 1-6 (b). The advantage of electrostatic force assembly is easy control of voltage.
Push bar
SDA Force
Mirror
Rotated hinge Plate
Bushing Insulator Substrate
(a) (b)
(a) (b)
In addition to parallel plates, ultrasonic wave is another way to generate electrostatic force. Ultrasonic vibrations are generated with an attached piezoelectric actuator to vibrate polysilicon plates on silicon nitride or polysilicon surfaces [13].
The friction brings contact electrification charge on the substrate and the flap. The electrostatic repulsion force is generated to lift up the flap, as Figure 1-7 shows. The parts were first actuated in ambient pressure (Figure 1-7(a)). However, at atmospheric pressure the structures would not be lifted up due to the air drag on the flaps. As the pressure was low enough with the ultrasound still on, the hinged flaps were lifted up (Figure 1-7(b)).
(a) (b)
Figure 1-6 Schematics of electrostatic force assembly by parallel plates, (a) the voltage is off, (b) the voltage is on.
In air with ultrasound In vacuum with ultrasound (a) (b)
Figure 1-7 Sequence of assembly, (a) ultrasonic vibrations heat and charge the polysilicon parts, (b) electrostatic repulsion forces the plate up [13].
Surface charge raises the flap
Hinged plate Electrostatic
force
Magnetic force assembly
Magnetic forces can be applied to actuate assemble 3-D structures by passing current through them (Lorentz force) or by depositing magnetic material on them. As shown in Figure 1-8 (a), electroplated magnetic material such as Permalloy is integrated with the hinged flap [14]. When an external magnetic field is applied, the hinged flap will be lifted up and rotate around the rotational axis. The rotation angle is determined from the volume of the magnetic material and the applied external field.
Figure 1-8 (b) shows an assembled structure by the magnetic force. By choosing different volume of Permollay electroplated on the flap, a precise sequence assembly can be achieved by changing the magnetic field strength [14, 15]. An example is shown in Figure 1-9. As Hext is increased gradually, the primary flap will be lifted up to 90° first due to the greater volume of Permolloy. With the increase of Hext, the secondary flap will be lifted up and lock the primary flap by the friction force.
(a) (b)
Figure 1-8 (a) Magnetic force assembly, (b) SEM of an assembled structure [14].
Hext Hinge
Flap Magnetic material
Pin
Figure 1-9 Sequence of magnetic force assembly [14].
1-2-3 Three dimensional MEMS by self assembly
Aside from external applied forces, 3-D microstructures can be assembled by the intrinsic force of the fabricated structures. In other words, the fabricated structure can be assembled to 3-D structure itself. Self assembly by pre-stressed bimorph beams, surface tension, and thermal shrinkage of polyimide in V-grooves are reviewed in this section.
Self assembly by thermal shrinkage of polyimide in V-grooves
Thermal shrinkage of polyimide in V-grooves can be used for self assembly as illustrated in Figure 1-10 [16]. V-grooves are etched through a silicon membrane and filled with polyimide. The polyimide in V-grooves shrinks when it is cured. With a lager lateral length of polyimide at the top of the V-groove than at the bottom, the structure bends as a result of to different amount of shrinkage of polyimide. With a series of V-grooves, large bending can be achieved, as shown in Figure 1-11 [16].
Primary flap
Secondary flap
Friction effects
(a) (b) Figure 1-10 Principle of the polyimide V-groove joint [16].
Figure 1-11 Large bending by connecting a series of V-grooves [16].
Surface tension powered self assembly
In surface tension powered self assembly [17], meltable material is patterned on the hinge joint. The plate is flipped up by surface tension when the material is melted.
The fabrication process and a fabricated structure are shown in Figure 1-12. The rotation angle of the assembled structures are determined by the geometric features of the meltable material [18].
(a) (b)
Figure 1-12 (a) Fabrication process for surface tension assembly, (b) self-assembled mirror by surface tension [17].
Stress-induced self assembly
Residual stress of thin film can be used as motive force to assemble 3-D microstructure, as Figure 1-13 shows [19]. The assembly mechanism is composed of stress-induced bimorph beams and locking components. The locking mechanism fabricated in different structure layers is engaged when the micromirror is raised.
Bimorph stress beams can be fabricated by using different materials. However, the stress relaxation is a critical problem. Reliability tests showed that the problem of stress relaxation could be reduced by replacing metal films by dielectric films [20].
photoresist Spin resist
Pattern and Premelt resist
Release
Flap
Figure 1-13 (a) Schematic of the structures, (b) assembled structure by combining bimorph beams with locking mechanism [19].
1-2-4 Manual or robotic assembly
Microprobes are the first tool for manual assembly of fabricated micro parts. It is time consuming and has low yields. In recent years, specially designed robotic equipments are also developed for automatic assembly. These assembly techniques are reviewed in this section.
Conventional manual assembly
The conventional manual assembly of a flip-up mirror is illustrated in Figure 1-14.
The structure is composed of a mirror plate, two mechanical locks, and a number of hinges. First a microprobe is used to lift up the micromirror to be perpendicular to the substrate. Then the mechanical side locks are picked up by another microprobe while the micromirror is held by the original microprobe. Finally the side locks are folded and locked onto the plate by the V-shaped opening. Precise manipulation of microprobes is needed during the assembly process in this conventional manual assembly.
Figure 1-14 Conventional assembly process.
Assembly by microgrippers
Compliant passive microgrippers were proposed to assemble out-of-plane 3-D microstructures [21]. The surface-micromachined microgripper is originally attached to the substrate by tethers, which are weak enough to be pulled apart from the substrate. Figure 1-15 shows the surface micromachined microgripper and the solder bonded microgripper. First, the solder is melt by the heated metal tip which is attached to a robotic arm. Then the metal tip with melt solder on it is aligned and pressed on the solder pad of the microgripper. After cooling, the pad is bonded to the metal tip.
The tethers can be easily broken when the arm moves away from the substrate (Figure 1-16 (a)). Next, the microgripper is aligned to the micromachined microparts. The microgripper is inserted into the microparts (Figure 1-16 (b)). After inserting the microparts into the slots on the substrate, the microgripper is released and the assembly is complete (Figure 1-16 (c)). Figure 1-17 shows the SEM figures of the assembled devices.
Micromirror
Side locks
Microprobe
Friction force
Figure 1-15 (a) Microgripper is attached to substrate by tethers, (b) metal tip bonded to solder pad [21].
Figure 1-16 (a) Solder bonded metal tip is attached to the pad, (b) the micro-part is grasped by the microgripper, (c) the micro-part is inserted in the slot [21].
(a) (b)
(c)
(a) (b)
Figure 1-17 SEM of (a) assembled out-of-plane structures, (b) microcoil [21].
Hingeless 90° out-of-plane microstructures
Another novel technique for microassembly is the assembly of hingeless 90°
out-of-plane microstructures with the use of automated probing system [22]. Staple hinges are replaced by compliant hinges created using springs or torsion beams, which can produce out-of-plane motion by redirecting lateral displacement into rotation. The structure can be pushed up by a single lateral push. While the conventional springs are complaint in all direction, the serpentine springs are designed to be more compliant in the out-of-plane direction than the in-plane direction, as shown in Figure 1-18. When a lateral force is applied on the actuation pad, the spring is twisted and a 90° out-of-plane motion can be produced. The assembly process is presented in Figure 1-19. The lateral actuation force (FA) and the restoring force of the springs (FS) provides an out-of-plane torque after the bottom of the device touches the substrate (Figure 1-19(b)). As the probe moves in lateral direction, the rotational torque (which is associated to FS) increases because of the deformation of the springs (Figure 1-19(c)(d)). When the plate reaches a particular angle, the spring will begin to
(a) (b)
pull the plate until the structure finally reaches the upright position (Figure 1-19(e)(f)).
Figure 1-20 (a) shows the manual assembly procedure. Figure 1-20 (b) is an assembled device.
Figure 1-18 Comparison between (a) conventional beam and (b) serpentine spring [22].
Figure 1-19 Assembly procedure (a) no actuation force, (b) the bottom of the device touches the substrate, (c)(d)(e) the conceptual force diagram when the probe moves in lateral direction, (f) finally the structure is at upright position [22].
Effective axis of rotation
Rotational compliance (out of plane) Point of actuation
Point of actuation Axis rotation
Compliant in all direction
(a) (b)
(c) (d) (e) (f)
Anchor Anchor
(a)
(b)
Figure 1-20 (a) Optical microscope view of assembly process, (b) SEM image [22].
1-2-5 Conclusion
Although there are a number of methods for 3-D microstructure assembly, the applications of these methods are limited. Assembly by SDA needs large chip area.
Assembly by magnetic or electrostatic force needs materials which are not compatible with IC processes or demands a large electrical field. Parameters of thermal shrinkage of polyimide or surface tension forces are difficult to control. Bimorph stress beams have the problem of stress relaxation. Manual assembly by micromanipulators is time consuming. However, once the steps of manual assembly are simplified enough and automatic equipments can be used, systematic assembly can be achieved.
(a) (b)
(i) (ii)
(iii) (iv)
1-3 Objective and organization of the thesis
In our previous [7], a novel assembly method with one-push operation was proposed to lift up 3-D microstructures. SU-8 was used to fabricate locking mechanism. A novel hinge design was also introduced to improve the positioning accuracy.
Batch assembly in wafer level will be demonstrated in this thesis. The accuracy problem that was found in [7] will be solved. A corner cube reflector is demonstrated based on this method. Furthermore, 45° positioning method needed for optical benches will be introduced. Therefore, the objectives of this thesis are to:
1. demonstrate batch assembly;
2. verify the hinge positioning accuracy;
3. solve the angular accuracy problem;
4. apply the method to a corner cube reflector;
5. demonstrate the assembly method for arbitrary angles.
The basic principles and simulation of the proposed microstructures and assembly method are presented in Chapter 2. The fabrication processes and process issues are discussed in Chapter 3. The experiment and measurement results of the fabricated and assembled devices are showed in Chapter 4. Conclusion and future work are presented in Chapter 5.