Microassembly of 3-D MEMS Structures Utilizing a MEMS Microgripper with a Robotic Manipulator Nikolai Dechev, William L. Cleghorn, James K. Mills Department of Mechanical and Industrial Engineering, University of Toronto, 5 King’s College Road, Toronto, Ontario, Canada, M5S 3G8
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Abstract This paper describes the process of bonding a MEMS (Micro-ElectroMechanical System) microgripper to the distal end of a robotic manipulator arm using a molten solder bonding technique. This task is part of ongoing work which involves the development of a general microassembly workstation. The goal of this workstation is to construct 3-D microstructures from MEMS subcomponents. The microgrippers bonded using the method described here are 1.5 mm by 0.6 mm in size. The methodology behind the solder bonding approach is presented, along with the design of a custom soldering device referred to as the contact head. The contact head is used as the interface between the robotic manipulator and the microgripper. Experimental results are given in a qualitative discussion. An explanation of the bonding procedure using automated calibration is described, along with pictures from the associated microscopy system, and some scanning electron microscope images.
mass-produce useful new products, similar to the way in which microelectronics research lead to the massproduction of IC (Integrated Circuit) chips in the past. However there is a fundamental difference between MEMS devices and IC devices. IC devices require no mechanically moving parts to function, while MEMS devices rely on micro-mechanical moving parts to provide their unique functionality.
1. Introduction This paper describes the process of bonding a MEMS (Micro-ElectroMechanical System) microgripper to the distal end of a robotic manipulator arm. The microgripper is mechanically and electrically bonded to the manipulator arm by solder bonding using tin-lead solder. The task of integrating the MEMS microgripper with the robotic manipulator is part of ongoing work described in [1], which involves the development of a general microassembly workstation. The goal of the workstation, shown in Figure 1, is to remove a micro-component from a MEMS chip, reorient the micro-component in space, move it to a secondary location, and join it to another micro-component. Micro-components are always fabricated parallel to a MEMS chip. However, after the workstation removes them from the MEMS chip, the workstation is able to rotate these micro-components in the α and β axes. In this way, complex 3-D microstructures can be assembled from a set of initially planar MEMS sub-components. MEMS design has received considerable attention in recent years. This is due to the hopes of the industrial and research communities that MEMS designs can be used to
Figure 1. MJMP Microassembly Workstation. Many of the MEMS devices being developed are fabricated using surface micromachining, which is adapted from IC fabrication technology. Surface micromachining, like IC fabrication, is based on the successive application and etching of thin films of material such as silicon nitride, silicon oxide, polysilicon
and gold. Due to the use of thin film fabrication techniques, the MEMS parts that are created are relatively thin, in comparison with their length and width. For example, a typical MEMS part made of one layer of material could be up to 3000 microns long or wide, but only a maximum of 2 microns thick. Surface micromachining allows only 3 to 5 layers of material to be used as structural layers for the construction of micromechanical mechanisms. Also, surface micromaching fabricates micro-components in their working locations and orientations. Although some of these microcomponents are able to translate slightly or to rotate, they remain in approximately the same region in which they were fabricated. These three limitations of surface micromaching place design restrictions on the type of devices that can be created. Another result of these limitations is that many MEMS devices operate along or near the plane of fabrication, which limits the diversity of device designs. A general process which can create fully 3-D MEMS would allow for more complex and useful devices. Through post-fabrication techniques, it is possible to create 3-D MEMS devices. Some of these techniques involve pop-up structures [2] or flip chip batch transfer microassembly [3]. The microassembly workstation described here uses sequential robotic operations to assemble 3-D MEMS microstructures.
grasping, over that of the macro-sized endeffectors. The micro-grasp tip of the microgripper is electrically driven to open and close, thereby allowing the microgripper to grasp other micro-components. Thermal resistive actuators [8] are used to convert electrical energy into mechanical energy that opens and closes the micro-grasp tip. The actuators are powered by applying +5 VDC to the central attachment pad and grounding the two outer attachment pads. Since these microgrippers are too small to be mounted by hand to the distal end of the manipulator (contact head), a solder bonding system has been developed, to bond the contact head to the three attachment pads.
2. Background This paper will focus on the development of the tools used to grasp micro-components on a MEMS chip. In order to grasp micro-components, the manipulator arm must be equipped with an endeffector suitable for the task. Since MEMS parts can be very small and fragile, the design of the endeffector is very important. The standard tool for micromanipulation is the tungsten probe, which is 50 mm long or larger. Some research groups use these probes as endeffectors to move micro-components from one place to another, or to re-orient the microcomponents [4]. Other groups have used two probes operating together, to manipulate small micro-blocks [5] with a kind of dexterous grasp. More advanced approaches include the use of HexSil or single crystal silicon fabricated milligrippers [6] to grasp the microcomponents. The smallest of the HexSil grippers are 5 mm in length. All of these systems use endeffectors that can be categorized as macro-scale objects. That is, the human hand can be used to mount the endeffector to the manipulator system. The work presented here uses surface micromachined endeffectors (microgrippers), which are 1.5 mm by 0.6 mm in size, as shown in Figure 2. The microgrippers are fabricated by the MUMPs (Multi-User MEMS Processes) surface micro-machining process [7]. Microgrippers of this size allow for some unique advantages [1] for
Figure 2. Composite Image of Actual Microgripper
3. The Manipulator System A five axis robotic manipulator, named the MJMP (Manipulator and Joiner of Micro Parts) is the basis of the microassembly workstation, shown in Figure 1. The axes of the MJMP are split into two groups with the x, y and α axes as one group on the granite base, and the z and β axes as the second group mounted on the granite post. The x, y and z axes are comprised of Danaher Precision Systems Ltd. crossed-roller bearing stages, driven by ball-bearing lead screws with a 2 mm lead. Vexta five-phase stepper motors are used for driving the three translation stages and are set to 20,000 steps per revolution, which provides a linear step distance of 0.1
µm. Linear encoders with a 0.1 µm resolution provide feedback to the operator. The translation stages have an open loop repeatability of +/- 0.2 µm. The α and β rotational axes are custom designed to have radial and axial runouts of less than 2 µm. This is achieved through the use of NSK-RHP P2 precision ball bearings. The motors driving the rotational axes have a resolution of 0.36°. In addition, there is an independent 3-axis manual translation stage on which the microscope system is mounted. This allows the microscope to be moved independently of the MJMP. The three translation and two rotation axes are commanded by a 5 axis Galil card, interfaced to a personal computer. The control strategy for positioning the axes of the MJMP varies, depending on the task. Work involving experimental or unproven microassembly procedures is carried out by manual operator control using a joystick. The operator relies on the microscopy system for visual feedback, and a readout display showing the linear encoder positions. For proven microassembly procedures, such as the solder bonding technique to be discussed later, the MJMP uses automatic control and relies on a program, and data capture through the Galil card. The goal of the microassembly workstation is to automate all microassembly operations, once the procedures have been established. This split configuration for the MJMP was chosen since it eliminates interference problems [1] between the macro-sized elements of the MJMP and the micro-sized components of the MEMS chip. This concept is illustrated in Figure 3. Note the contact head, which is used as the interface between the MJMP and the microgrippers. The contact head in Figure 3(a), is oriented 45 degrees below the horizontal. In this orientation the metal tips of the contact head are the lowest point of the manipulator attached to the granite post. This orientation allows the metal tips to probe the surface of the MEMS chip, and at the same time allows the metal tips to be in direct view of the microscope system. The contact head is a custom designed soldering iron that solders itself to the MEMS microgrippers, thereby electrically and mechanically joining them the distal end of the manipulator arm. Figure 3(a) illustrates how the contact head is attached to the MJMP. The distal arm is attached to the β axis rotary stage. The head clamp is bolted to the distal arm, and is used to hold the contact head, which is securely clamped within it using two bolts. The metal tips protruding from the bottom of the contact head are made of copper and are used for the solder bond to the microgripper. In order to bond a microgripper to the MJMP, the following procedure is used. With the distal arm in the orientation of Figure 3(a), the metal tips of the contact head are aligned in the x and y directions with the
attachment pads of the microgripper. The alignment is done with the aid of the microscopy system. Next, a localization procedure, described later, is carried out to locate the height of the metal tips, above the MEMS chip. Next, the soldering procedure, described later, is carried out to solder the MEMS microgripper to the bottom side of the metal tips. After soldering, the z axis is commanded up, thereby removing the microgripper from the MEMS chip. The microgripper is fabricated such that it is only attached to the MEMS chip by tethers, as shown in Figure 2. The tethers are attached to anchor pads which are permanently attached to the MEMS chip. The tethers are strong enough to hold the microgripper onto the MEMS chip during shipping but are designed to breakaway during the solder bonding procedure, thereby freeing the microgripper from the MEMS chip.
Figure 3. Illustration of β Axis and Contact Head
After a microgripper is soldered onto the metal tips of the contact head, as shown in Figure 3(a), the microgripper becomes the lowest point of the distal arm and is able to probe the MEMS chip, while in direct view of the microscope. Note that when the distal arm is rotated 90 degrees counter clockwise about the β axis, as shown in Figure 3(b), the contact head is again 45 degrees below the horizontal, although the microgripper is now perpendicular to the MEMS chip, however, it is again the lowest point on the distal arm. In this orientation, it becomes the endeffector of the MJMP, and is able to grasp other micro-components on the MEMS chip. Since the microscope is mounted on its own three axis stage, it is manually moved to ensure that the microgripper is always in direct view of the microscopy system.
ceramic cylinder with three embedded copper strips protruding from each side, all electrically isolated from each other, as shown in Figure 4(a). There are many different formulations for castable ceramics. The ceramic used for the contact head is based on Al2O3, can be heated up to 1600 °C, is an electrical insulator and is also a good thermal conductor, compared to ordinary ceramics.
4. Solder Bonding System 4.1 Design of the Contact Head The goal of the bonding procedure is to mechanically and electrically fasten the microgripper to the contact head of the MJMP. Three separate electrical connections must be made with the microgripper, one for +5 VDC and the other two for ground. These three connections provide power to the thermal resistive actuators on the microgripper, allowing it to open and close the micrograsp tip. Although glue could be used for bonding to the contact head, it does not conduct electricity well, and is not easily removed when a microgripper must be changed. There are six design requirements for the contact head, which are: 1) rigidly hold three metal connectors that are electrically isolated from one another, 2) provide a uniform heat source to the metal connectors that is sufficient to melt tin-lead solder, 3) include a temperature sensor to control the heating of the contact head 4) ensure the material used for the contact head body does not melt, does not conduct current, and is a good conductor of heat 5) ensure the contact head body is mechanically strong enough to be clamped into the head clamp and lastly 6) ensure the entire contact head occupies a space/geometry small enough such that it does not interfere with the optics of the microscope system. The resulting contact head design is illustrated in Figure 4. The first step in creating the contact head is the construction of the three metal strip cluster. This consists of three strips of gold plated copper, which are 200 microns thick. The three strips are placed in a plastic mold and separated from each other by 400 micron gaps. Castable ceramic material is used to form the structure around the three metal tips and the body of the contact head. A ceramic powder is mixed with water to form a liquid-ceramic fluid that is poured into a mold and flows to fill all the gaps. The ceramic is then allowed to set for 24 hours, is removed from the mold, and is cured in an oven at 250 °C. The resulting metal strip cluster is a solid
Figure 4. Illustration of Contact Head After the metal strip cluster is complete, nickelchromium electrical-resistance heating wire is wrapped around the cylinder to form a coil. There are a minimum number of loops required for the coil, in order to achieve sufficient heat. Square pin connectors are crimped onto the ends to the resistance wire. Finally, the metal strip cluster with the encircling resistance heating wire coil, and a J-Type thermocouple are set into a second plastic mold. Castable ceramic liquid is poured into the second mold, is allowed to set for 24 hours, removed from the mold and is cured in an oven, at 450 °C. The resulting structure is illustrated in Figure 4(b). The contact head is then tested to ensure there are no electrical shorts between the thermocouple, the nickel-chromium wire, and the three copper strips. Finally the contact head is connected a Partlow MIC 2000 temperature controller, and tested for heating. The contact head used in these experiments uses a 57 watt heating coil and can reach steady state temperatures of up to 500 °C. It is controlled by time proportional control, and keeps the temperature steady to within 2 °C. A picture of the actual contact head, mounted to the distal arm, is shown in Figure 5.
Figure 5. Contact Head within Distal Arm. 4.2 Integration of Contact Head with Distal Arm Figure 3(a) illustrates the contact head held within the head clamp located on the distal arm of the manipulator. Under ordinary soldering conditions, the contact head is heated to 300 °C, and must be thermally isolated from the distal arm. Several layers of woven fiberglass fabric are placed on either side of the contact head, within the head clamp. These layers significantly reduce the flow of heat from the contact head to the distal arm, such that the distal arm is warm to the touch while the contact head is at 300 °C. Fiberglass fabric was chosen since it can be heated up to 700 °C, is not a hazardous material, and remains somewhat compliant allowing the contact head to thermally expand without damage to itself. Before using a newly installed contact head, two additional steps are performed. Firstly, the contact head is subjected to a high temperature heating cycle to stabilize its position within the head clamp. Secondly, the entire distal arm assembly, with the clamped and thermally stabilized contact head, undergoes a precision grinding operation. The three metal tips are precision ground such that a line that intersects each of the three tips is parallel to the 5th axis. In this way, the three metal tips will touch down onto the surface of a MEMS chip at almost the same time. 4.3 Localization of MEMS Chip with the MJMP The distance in the z-direction, between the microgrippers on the surface of the MEMS chip and the metal tips of the contact head, must be calibrated. This distance is critical in forming a solder bond. A glass slide, onto which the MEMS chip is bonded, is mounted
onto the worktable of the α axis. The MEMS chips arrive from the fabrication facility bonded to glass slides, however, the gluing process results in an orientation of the MEMS chip such that the surface plane of the MEMS chip is not parallel to the plane of the glass slide. Therefore, the surface of the MEMS chip is not perpendicular to the α axis, when it is mounted on the worktable. Since the MEMS chip itself is single crystal silicon, it is flat. Therefore, by determining the z position of at least three different points on the chip, the z positions of all points of the chip are known. Four calibration pads, as shown in Figure 3(a), are designed into the perimeter of the MEMS chip. Each pad is comprised of a gold layer 300 by 300 microns, elevated 2.5 microns above the substrate, and all pads are electrically connected to each other. To perform localization of the MEMS chip with respect to the MJMP, the vertical (z-axis) distance, from a calibration pad to the metal tips of the contact head, is measured. The measurement is made by the translation of the z axis of the MJMP, such that contact is made by one of the three metal tips of the contact head with a calibration pad. Contact is detected with the closure of a circuit loop created by the calibration pad and the metal tips. Upon contact, the encoder position of the x, y and z axes is recorded. Three such measurements are carried out to locate the plane of the MEMS chip with respect to the contact head, and a fourth measurement is used to provide redundancy so as to check for error. 4.4 Preparation of Contact Head for Soldering The solder used for bonding is Sn63/Pb37, and has a melting temperature of 183 °C. This solder is used since it has the lowest melting temperature of any solder, and therefore minimizes the amount of heat necessary to perform the solder bonding operation. To apply the solder to the contact head, it is rotated to the position shown in Figure 3(b) and is heated to 300 °C. The solder is applied by hand directly to each of the three metal tips of the contact head. The correct amount of solder to apply is important. Too little solder will not create a solder bead large enough to allow a joint to be formed. Too much solder will cause a short circuit between two or three of the metal tips. Oxidation of the solder is also a problem. The solder bond must be performed quickly after the solder is applied, otherwise an oxide layer forms on the molten solder and can prevent a bond.
5. Experimental Results Results showing a microgripper that has been successfully joined to the contact head of the MJMP and removed from the MEMS chip is shown in Figure 6. A majority of soldering attempts between the metal tips and
the microgrippers are successful, using a technique described here as ‘hot calibration’. 5.1 Manual Soldering of Microgrippers Initial soldering attempts were unsuccessful due to the thermal expansion of the distal arm, and the unexpectedly large thermal expansion of the worktable, the glass slide and the MEMS chip. During the manual solder bonding process, the temperature of the various components is dynamic throughout the procedure. In these experiments, the cold (unheated) contact head is lined up with the microgripper attachment pads. The contact head heater coil is turned on, until the temperature reaches 300 °C, at which point the solder is presumed to have melted and bonded to the microgripper, and the heater coil is turned off. Upon heating of the contact head, thermal expansion begins in the distal arm due to conduction, and in the worktable, glass slide and MEMS chip due to radiation and conduction. Because the temperature never achieves a steady state, the thermal expansion is difficult to predict. Due to the uncertainty of the thermal expansion, the exact tip position of the contact head is uncertain. Therefore, during a bonding experiment, if the contact head is closer to the MEMS chip than anticipated, it would crush the microgripper and break it. If the contact head is farther from the MEMS chip than anticipated, no bond would be made. 5.2 Automated Hot Calibration Soldering The ‘hot calibration’ solder bonding approach is successful at joining the microgripper to the contact head. In this approach, the contact head temperature is held constant at 300 °C, using the temperature controller, while the calibration procedure of Section 4.3 is carried out. The thermal expansion of the contact head and distal arm reaches steady state, and is accounted for during the calibration procedure. However, thermal expansion of the worktable, the glass slide and the MEMS chip will be dynamic during the calibration. In order to account for the dynamic thermal expansion, the speed and motion of the calibration procedure is such that it matches the speed and motion of the solder bonding procedure. In this way, as the contact head approaches the MEMS chip, and makes contact, the rate of heating of the chip due to radiation, and the subsequent thermal expansion will be equal in both cases. Therefore the thermal expansion of system is accounted for by the calibration procedure. The matching of speed and motion is not possible without automatic control, and therefore the ‘hot calibration procedure’ is automated. It is assumed that the radiation heating of MEMS chip is even regardless which calibration pad is contacted, since the contact head is much larger than the chip. Since the position of all microgrippers is accurately known with respect to the calibration pads in the x and y directions, and the z
position is calibrated, no manual alignment using the microscope is necessary. The coordinates of the microgripper to be joined are entered into the controller and the MJMP automatically aligns with the microgripper and moves down in the z-direction. The metal tips stop 5 microns above the calibrated height of the microgripper, since the height of the solder bead is not accounted for by the calibration procedure. The actual height of the solder bead is usually 30 to 50 microns in the z-direction, which ensures that the molten solder contacts the microgripper. An SEM (Scanning Electron Microscope) picture of a successfully bonded microgripper is shown in Figure 6.
Figure 6. SEM of Contact Head Bonded to Microgripper A voltage of +5VDC, at a frequency of 1 Hz was applied across the central metal tip and the outer two metal tips, of the contact head shown in Figure 6. The thermal actuators operate well, however, the desired gripper tip motion is not always achieved. The microgripper design of Figure 6 is sensitive to the relative thermal contraction between the three metal tips, which causes a slight bending in the microgripper. This bending is often sufficient to misalign the thermal actuators, leading to poor tip deflection. Figure 7 is a series of images taken by the optical microscope (shown in Figure 5), of the microgripper. Note that the microgripper is positioned several microns above a micro-component on the surface of the MEMS chip. The depth of focus of the microscope image is 1.5 microns, therefore all objects closer or beyond this range appear out of focus. Figure 7(b) is the same image, but with the micro-component in focus. Figure 7(c) shows the microgripper tips located above a grid, used to locate the tips with respect to the MEMS chip, after the solder bonding operation. Figure 7(d) shows the central attachment pad of the microgripper. The dark blur on the lower right is the central metal tip of the contact head that
is bonded to the pad. The field of view of all the images in Figure 7 is 320 by 240 microns. Figure 8 shows a series of SEM pictures of a successful microassembly operation. Figure 8(a) shows the initial fabrication position of a micro-part. The microgripper used for the operations of Figure 8(b) to (d) was solder bonded to the contact head using the same procedure described in this paper, however the operation of this microgripper is not described here. Figures 8(c) and (d) show completed assembly operations.
6. Conclusion The process of joining a MEMS microgripper to the distal end of a robotic manipulator arm using a hot calibration, molten solder bonding technique, has been described. The methodology and equipment used for the soldering approach were presented, along with the design of the contact head. The contact head acts as an interface between the microgripper and the MJMP, provides three electrical paths to the microgripper, and has a built in heater coil to solder the metal tips to the microgripper. The experimental results show an SEM image of a typical successful solder bond. Using this bonding technique various types of microgrippers can be joined to a robotic manipulator, to perform microassembly operations using MEMS micro-components.
Acknowledgements This material is based upon work supported under a Natural Sciences and Engineering Research Council of Canada (NSERC) equipment grant. In addition, the authors wish to thank the Canadian Microelectronics Corporation (CMC) for allocating chip area.
References Figure 7. Microgripper Soldered to Metal Tip Above Chip
Figure 8. SEM Images of Microassembly Operation
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