Micromachined Devices
4.4 Machine Design
4.4.1 Compliance Elements: Columns, Beams, and Flexures
4.4.1.9 Hinges and Hubs
A different type of rigid-body-mode machine is a vertical axis hinge. These hinges are used in surface micromachining to create three-dimensional structures out of a two-dimensional surface micromachin-ing process. In the surface micromachinmicromachin-ing process, hmicromachin-inges are constructed by staplmicromachin-ing one layer of poly-silicon over another layer of polypoly-silicon with a sacrificial layer between them. A cross section of a simple hinge is illustrated in Figure 4.16. A more complex hinge is shown in Figure 4.17. Hinges have some
π2EI 4L2
Sacrificial oxide
Polysilicon
Substrate
FIGURE 4.16 Cross section of a polysilicon mirror hinge. The top figure shows the device before release, the mid-dle drawing is after release, and the bottom depicts the actuated device.
advantages over flexible joints. One advantage is that no stress is transmitted to the hinged part so greater angles of rotation are possible. Also, the performance of hinged structures is not influenced by the thick-ness of the material and deformation of the machine is not required. The limitations of hinged structures are that the sacrificial layers must be thick enough for the hinge pin to rotate and at least two released lay-ers of material are required. Hinges are also susceptible to problems with friction, wear, and the associ-ated reliability problems. The same limitations and advantages are true for structures that rotate parallel to the plane of the substrate such as gears and wheels. A cross section of a simple hub and gear is shown in Figure 4.18 and a complex hub structure is shown in Figure 4.4.
4.4.1.10 Actuators
There are several different actuation techniques for surface micromachine mechanisms. Actuators for all types of MEMS devices are covered in Chapter 5, but the most common actuator types for surface micro-machines — electrostatic and thermal — are also covered here. Electrostatic actuators harness the attrac-tive Coulomb force between charged bodies. For a constant voltage between two parallel plates, the energy stored between the plates is given by:
W (4.37)
where ε
0is the dielectric constant of free space (8.8541012F/m),ε
ris the relative dielectric constant, for air it is 1.0,Ais the area,Vis the voltage, and yis the distance. The force between the plates is attractive and given by:
F ε (4.38)
0ε
rAV2 y2 dW
dy ε0ε
rAV2 2y
FIGURE 4.17 Hinged polysilicon micromirror fabricated in the Sandia National Laboratories SUMMiT™ process.
(Photograph courtesy of Sandia National Laboratories.)
Substrate
FIGURE 4.18 Cross section of a simple hub and gear fabricated in a two level surface micromachining process.
Note that this force is dependent on 1/y2. Therefore, the Coulomb force is very strong at small gaps, but drops off rapidly as the gap increases. If the plates are not fully engaged, as in Figure 4.19, there will be tangential as well as normal forces. The equation for the tangential motion is obtained by modifying Equation 4.37 by substituting the area term,A, with the product of the lateral dimensions zandx:
W (4.39)
The derivative of energy with respect to position is force.
F (4.40)
Note that this force is not dependent on the lateral position x. Comb-drive actuators utilize these tan-gential forces with banks of comb fingers. The surface micromachined comb-drive in Figure 4.20 typically operates at voltages of 90 V and has output forces of around 10µN. Comb-drives can operate at speeds up to 10 s of kHz and consume only the power necessary to charge and discharge their capacitive plates.
ε0ε
rzV2 2y dW
dx
ε0ε
rxzV2 2y ε0ε
rAV2 2y
y x z
Tangential forces Normal forces Area
FIGURE 4.19 Illustration of normal and tangential forces in an electrostatic actuator.
FIGURE 4.20 Electrostatic comb-drive actuator fabricated in the SUMMiT VTM process at Sandia National Laboratories.
Because the output force of an electrostatic comb-drive is proportional to the square of the applied voltage, these devices are often operated at higher voltages than most analog and digital integrated circuits. This has complicated their implementation into systems. It should be noted that Equations 4.37 through 4.40 are for electrostatic actuators connected to a power supply and in constant voltage mode.
Example
The plate in Figure 4.19 is 50µm long, 6µm thick, and 3µm wide. It has a neighboring electrode that is 1µm away. If 80 V are applied between the electrode and its identical neighbor and fringing is ignored, what are the forces in the normal and tangential directions if the combs are aligned in the width dimen-sion and overlap by 40µm and 10µm in the length dimension? How much does the force change in both directions if the distance is reduced to 0.1µm? If there are 30 electrodes at 80 V interlaced with 31 elec-trodes at 0 V as in Figure 4.20with a separation of 1µm between the electrodes what is the net tangential force? Assume thatε
r 1.0.
Parallel plate electrostatic forces for the 40µm of overlap case are:
Fnormal 14106N
and for the 10µm case:
Fnormal 4.4106N
For the tangential force, the lateral dimension,x, is not included in Equation 4.40 so both calculations yield the same result.
Ftangential 1.7107N
The reader should note that the parallel plate force is much higher than tangential force. If the separation is reduced to 0.1µm, then the normal force is increased by a factor of 100 to 440µN and the tangential force is increased by a factor of 10 to 1.7µN. For 30 energized electrodes and 31 non-energized electrodes, the tangential force is multiplied by the number of energized electrodes and then doubled to account for both faces of the electrode:
F30fingers170 nN30210µN
Note that the parallel plate force of one electrode engaged 40µm is still higher than the tangential force of 30 electrodes. However, the tangential force comb-drive has a force that is independent of position, while the parallel plate case falls off rapidly with distance. However, the high forces at small separations makes parallel plate actuators useful for electrical contact switches.
Thermal actuators have higher forces (hundreds ofµN up to a few mN) and operate at lower voltages (1 V to 15 V) than electrostatic actuators. They operate by passing current through a thermally isolated actuator. The actuator increases in temperature through resistive heating and expands, thus moving the load. One type of thermal actuator has two beams that expand different amounts relative to each other [Guckel et al, 1992; Comtois, 1998]. These pseudo bimorph or differential actuators use a wide beam and a narrow beam that are electrically resistors in series and mechanically flexures in parallel. Because the narrow beam has a higher resistance than the wide beam, it expands more and bends the actuator in an arc around the anchors.
Another type of thermal actuator uses two beams that are at a shallow angle. Bent beam thermal actu-ators generally have strokes of between 5µm and 50µm [Que et al., 2001; Cragun and Howell, 1999].
Unlike the pseudo bimorph devices, they move in a straight line instead of an arc. Like the pseudo bimorph actuators, they have an output force that falls off quickly with displacement. Both types of thermal actua-tors are shown in Figure 4.21.
8.8541012F/m140106m(80V)2
4.5 Packaging
This section covers only aspects of packaging that are especially relevant to surface micromachines. One of the challenges of packaging surface micromachines is that the packaging is application specific and varies greatly between different types of devices. This is one reason that packaging tends to be the most expensive part of surface micromachined devices. Therefore it is very important that designers of surface micromachines understand packaging and collaborate with engineers specializing in packaging while designing their device. Some of the main purposes of packages for surface micromachines include elec-trical and mechanical connections to the next assembly and protection from the environment.
The mechanical attachment between a die and a package can be achieved in several different ways. The main criteria for choosing a die-attach method include: the temperature used during the die-attach process; the amount of stress the die-attach process induces on the die; the electrical and thermal prop-erties of the die-attach; the preparation of the die for the die-attach process; and the amount of out-gassing that is emitted by the die-attach. Die-attach methods that do not outgas include silver-filled glasses and eutectics (gold–silicon). Both of these induce a large amount of stress onto the die as well as requiring temperatures of around 400°C. Epoxy-based die-attaches use much lower temperatures (up to 150°C) and induce lower stress on the die than eutectics or silver-filled glasses. The low stress means that they can be used for larger die. However, depending on the type of epoxy, they do outgas water and cor-rosive chemicals such as ammonia. As an alternative, flip-chip processes combine the process of die-attach and electrical interconnection by mounting the die upside-down on solder balls. A note of caution:
surface micromachines are strongly influenced by coatings and contamination or removal of these coat-ings, which can happen during temperature cycling, is detrimental to the micromachine.
Electrical interconnect to surface micromachines is typically accomplished by wire bonding.
Wirebonders use a combination of heat, force, and ultrasonic energy to weld a wire (normally aluminum or gold) to a bondpad on the surface micromachined die. The other end of the wire is welded to the pack-age. Although, wirebonds in high volume production have been done on less than 50µm centers, for low volume applications it is easier if the bondpads are fairly large. Metal coated bond pads that are 125µm or larger on a side with 250µm center-to-center spacing allow reworking of the wire bonds and for non-automated wire bonding equipment to be used. These numbers are on the conservative side but could be followed in the absence of process specific information.Figure 4.22 shows an Analog Devices ADXL50 surface micromachined accelerometer with its wirebonds and epoxy die-attach.
Cold
Cold Electrical current
Hot
Hot Electrical current
Absolute actuator
Differential actuator
Motion Motion
Cold
FIGURE 4.21 Drawings of absolute and differential thermal actuators.
While electrical and mechanical connections to surface micromachines are similar to other types of micromachines and integrated circuits, the mechanical protection aspects of the packaging can be quite different. The package must protect the surface micromachine from handling by people or machines, from particles and dust that might mechanically interfere with the device, and from water vapor that can induce stiction. Packages for some resonant devices must maintain a vacuum and all packages must keep out dust and soot in the air. However, sensor packages must allow the MEMS device to interact with its environment. Because surface micromachines are very delicate and fragile after release, even a careful packaging process can damage a released die. Therefore, one of the trends in packaging is to encapsulate and mechanically protect the devices as early as possible in the manufacturing process. Henry Guckel at the University of Wisconsin was one of the first developers of an integrated encapsulation technique [Guckel, 1991]. In this design, the surface micromachine was covered by an additional layer of structural material during the fabrication process. This last structural layer completely encapsulates the surface micromachine with the exception of a hole used to permit removal of the sacrificial layer by the release etchant. After the release etch, the hole is sealed with materials ranging from LPCVD films, to sputtered films, to solders. There is a good summary of this and other sealing techniques in Hsu (2004).
Another method of encapsulating the device is to use wafer bonding. In this technique a cap wafer is bonded to the device wafer forming a protective cover. One common method involves anodic bonding of a glass cover wafer over the released surface micromachines. A second involves the use of intermediate layers such as glass frit, silicon gold eutectic, and aluminum. The wafer bonding techniques are more independent of the fabrication process than the wafer level deposition processes. Because of the lack of stiction forces between the cap and the substrate and because film stresses in the cap are not a problem, bonded caps can be used for larger devices than deposited caps. A package formed by Corning 7740 glass (pyrex) bonded to a surface micromachine is shown in Figure 4.23.
Conventional packaging such as ceramic or metal packages also protect surface micromachines from the environment. In this case, the package provides electrical and mechanical interconnections to the next assembly as well as mechanical protection. These types of packages tend to be more expensive than plas-tic packages, which can be used if encapsulation is done on the wafer level. These packages are typically sealed using either a welding operation that keeps the surface micromachine at room temperature or a FIGURE 4.22 Analog Devices accelerometer after the package lid has been removed. The epoxy die-attach and wire bonds are clearly visible. This device had its package lid removed at Sandia National Laboratories. (Photograph cour-tesy of Jon Custer of Sandia National Laboratories.)
belt sealing operation that elevates the temperature of the entire assembly to several hundred degrees Centigrade.
4.6 Applications
The applications section of this chapter will present some surface micromachined mechanisms and dis-cuss them with regard to some of the mechanical concepts disdis-cussed earlier. The chapter concludes with some design rules and lessons learned in the design of surface micromachined devices.