MEMS: Comparison With Micro Electronics

MEMS Comparison With Micro ElectronicsMicro Electro Mechanical Systems or MEMS is a destination coined around 1989 by Prof. R. Howe and others to suck up an emerging research, where mechanical elements, kindred coffin nailtilevers or tissue layers, had been make at a scale more identical to microelectronics circuit than to lathe machining. But MEMS is non the unaccompanied term practice sessiond to describe this and from its multicultural origin it is also known as Micromachines, a term often apply in Japan, or more broadly as Microsystem Technology (MST), in Europe.However, if the etymology of the word is more or less s nearly known, the dictionaries atomic crook 18 still mum about an exact definition. Actually, what could link an inkjet correspondent head, a video projector DLP system, a disposable bio-analysis bridle and an business linebag crash sensing element yes, they ar all MEMS, plainly what is MEMS?It appears that these winds sh be the presence of fea tures below ampere- jiffy micro metre that are non machined using standard machining but using other techniques globally called micro- parable technology. Of course, this simple definition would also ent locomote microelectronics, but there is a characteristic that electronic circuits do not share with MEMS. While electronic circuits are inherently solid and dense structures, MEMS spend a penny holes, perdition, channels, shadowertilevers, membranes, etc, and, in some way, imitate mechanical parts. This has a submit impact on their manufacturing bidding. Actually, even when MEMS are base on silicon, microelectronics process needs to be adapted to cater for thicker layer de put, deeper engraveing and to cite special steps to bare the mechanical structures. Then, many more MEMS are not based on silicon and can be manufactured in polymer, in glass, in quartz or even in metals 5, 6.Thus, if likeities in the midst of MEMS and microelectronics exist, they now clearly are tw o distinct. Actually, MEMS needs a completely different set of mind, where next to electronics, mechanical and material fellowship plays a fundamental role.1.2 MEMS technologyThe breeding of a MEMS comp wiznt has a cost that should not be misevaluated but the technology has the possibility to playact unique benefits. The reasons that prompt the use of MEMS technology can be en motivelessenify broadly in terce classesa) Miniaturization of existing devices, like for sample the production of silicon based gyroscope which reduced existing devices burden several kg and with a volume of 1000 cm3 to a chip of a few grams contained in a 0.5 cm3 package.b) Development of new devices based on principles that do not work at larger scale. A typical example is given by the biochips where galvanical are use to pump the reactant around the chip. This so called electro-osmotic outlet based on the creative activity of a drag suck in the fluid kit and caboodle only in channels with dimen sion of a fraction of one mm, that is, at micro-scale.c) Development of new tools to interact with the micro-world. In 1986 H. Rohrer and G. Binnig at IBM were awarded the Nobel price in physics for their work on scanning tunneling microscope. This work heralded the discipline of a new class of microscopes (atomic force microscope, scanning near optic microscope) that shares the presence of micro machined sharp micro-tips with radius below 50 nm. This micro-tool was use to position atoms in complex arrangement, writing Chinese character or helping verify some prediction of quantum mechanics. Another example of this class of MEMS devices at a slightly larger scale would be the development of micro-grippers to handle cells for analysis.2.Micro reflects2.1 History of Micro reflect In recent years, de realizeable mirror devices (DMDs) have emerged as a new micro-electromechanical (MEM) technology with tremendous potential for future applications. As shown in Fig. 1-1, the concept of d eformable mirrors was developed and utilized as early as 211 BC by Greek soldiers to destroy enemy ships 1.1However, it was not until 1973 that serious development of micromirror devices began to emerge. Currently, several foundings of deformable mirrors have been fabricated, some before a practical use had been identified. It is these devices that are now receiving serious attention as optical communication and link up fields are expanding.2.2 BACKGROUND reverberate devices are a specific type of spatial light modulator (SLM).Spatial light modulators are devices that can alter the phase, amplitude, and/or the direction of prolongation of an happening radiate of light. Deformable mirror devices do this by moving a pondering erupt to achieve the desired effect. Currently, two distinct types of micro-mirrors are apply 1. Continuous cake devices use one large ruminative membrane that is locally controlled by individual actuators to form a continuous reflective bug out. Circu s fun house mirrors are an example of such a device. Segmented devices, on the other hand, use a mirror prove that is divided into numerous individually controllable elfiner mirrors. Greek soldiers use segmented mirrors to form a parabolic reflective surface which was employ to focus sunlight onto enemy ships.2Segmented devices are utilize today in the formation of large parabolic mirrors. As shown in class 1-2, the essential mirror of many modern optical scope systems is comprised of segmented deformable mirrors. In the past, the size-limiting grammatical constituent in such systems has been the size of the primary mirror which had to be mechanically stable yet light ample to propel to various positions throughout a full field of view. big mirrors were frequently damaged or caused damage to other components of the telescope when motion was attempted. With the application of segmented deformable mirror technology, the practical limit in telescopic primary mirror size can be extended since much(prenominal) lighter and small mirrors can be individually anchored, controlled, and position adjacent to severally other to form the necessary parabolic mirror.The segmented mirrors are not only placed at a slight angle to each other, but are shaped by the segmented actuators and are free to bend to form smaller parabolically curved surfaces. The segmented actuators are manipulated by the control electronics which receive information from the laser figure sensing element and the edge considerr which is then translated into a necessary change in the position or shape of the mirrors. These monitoring devices continually check the military position of the segmented mirrors to maintain the parabolic form of the entire device and to stop that no gaps or severe discontinuities are present in the surface of the primary mirror which would result in a distorted take in or a loss in image resolution.The basic principles of this macroscopical technology can also be used in microscopical applications which involve fabricating deformable mirrors on integrated circuits. Several forms of micromirrors have emerged that disencumberine on-chip addressing electronics with the micro-mechanical mirrors 1. The nonrepresentationalal and material variations of these devices demonstrate that deformable mirrors can be designed and implemented for a variety of specific uses. The micromirror devices legitimately used are segmented surface devices in which the propulsion of a small reflective mirror is controlled by a single address electrode. The metallized mirror and the address electrode of the device form a parallel menage capacitance. The voltage in the midst of the mirror and the electrode creates an stable force acting on the mirror in the downward direction. The flexures dimension the mirror are designed to deform, allowing the mirror to move vertically with utilize voltage. The resulting leaping force of the flexures acts on the mir ror in the upward direction, countering the motionless force of the capacitor.3.MICROMIRROR ACTUATION METHODS FOR SENSING3.1 Electromagnetic propulsionA micromirror can be deflected in two ways by electromagnetic actuation. first of all, by using Lorentz force to move a patterned coil by exerting external magnetic field. Second, by repulsive/ commitive forces to repel/attract the magnetic material attached to the mirror from/to the actuator. Advances in material fabrication to volunteer thick film deposition of magnetic material on the surface of micro actuators should reduce voltage and current requirements. Magnetic MEMS can offer non- contact operation, and can induce mechanical rapport by magnetic element excitation. However, caloric budget imposed by the current CMOS technology limits the fabrication of the magnetic film on the substrate from stretching the desired characteristics 3.3.2 piezo galvanising propulsionThe piezoelectric actuation takes profit of the agree physical deformation to applied electrical voltage property . It has relatively lower operation voltage (3-20 Volt DC) with low power consumption, remedy additiveity, and ready switching time 0.1 to 1.0 milli assists 3.3.3 Thermal ActuationThe main advantage of thermal actuation is the simplicity of the fabrication method. However, in general, thermal actuation tends to have higher power consumption and slow repartee time.The out-of-plane thermal micro actuator uses thermal expansion due to ohmic heating. A slew arm and wide arm configuration with one end meliorate to the substrate has nonlinear property due to temperature dependency .3.4 Electrostatic ActuationDespite suffering from the pull-in effect, nonlinear port, and higher operating voltage, the electrostatic actuations libertine response time (less than 0.1 ms), low power consumption, and the easiness of integrating and exam with electrical control system make the electrostatic actuation one of the preferred choi ces for micromirror actuation .The operation voltage of the micromirror can be lowered plot of land achieving more angular difference if the cruelty of deformation metre is reduced. However, when the stiffness is lowered, the natural frequency of the micromirror also decreases, thereby reducing operational band breadth.Say w, v, d scales as L1.Maximum Electrostatic emf Energy Stored is given by3Permitivity of vacuum and relative permitivity stiff unchanged with scaling.Assume Vb scales linearly with d (Out of Paschen effect range), then4Electrostatic Forces Found to Scale as Square of L.Since mass and whence inertial forces scale as cube of L, Electrostatic Actuators are good in Scaled Down Sizes 3.Paschen Effect Breakdown of continuum theory enroll 3 -Vb v/s P,dPaschen Effect Breakdown of continuum theorya) Vb scales non linearly in Paschen effect range.b) Vb increases in Paschen effect range.c) Higher Vb implies higher stock of energy and so larger force.4.Summary of Ad vantages and Disadvantages of Each Actuation MechanismActuationAdvantagesDisadvantagesMagnetic suffering actuation voltage Relatively large angular deflection with lower driving power Difficult to assemble eternal magnets and coils with current CMOS technology Challenge in minimizing the size of devicePiezoelectric Higher switching speed Low power consumption rook actuation rangeThermal Ease of fabrication (require only one composite gleam) for bulk production-High power consumption Slow response time Fatigue due to thermal cycleElectrostatic Low power consumption Fast switching Ease of integration and testing with electrical control circuitry Nonlinear characteristics Limited by the pull-in effect High actuation voltage Fabrication complexity5.Proposed Designs5.1 ANALYTICAL posture OF THE STACKED MICROMIRRORSIn this section, micromirrors of different configurations are presented and compared in footing of their deflection angle and actuation voltage. The conceptual schematics of the three configurations analyzed are shown below. Figure 1(a) shows a conventional micromirror configuration. Figure 1(b) shows a unique configuration of the sonsy micromirror also denoted as the first stacked mirror configuration, and Figure 1(c) shows a novel configuration of the stacked micromirror with an offset, which is also known as the second stacked micromirror configuration 8.Figure 1. Schematics of Three Different Micromirror Configurations.The moving electrode (middle plate) in the stacked configurations is designed to be identical to the micromirror in size and material. Solutions for the pursuit analytical model are independent of the shape and size of the plate (micromirror) as long as the dimensions of each layer are identical.First, an analytical model of the micromirror is derived to better understand the relationship between each line of reasoning of the micromirror. The torque created by the electrostatic force between the micromirror and its electrodes, as denoted by M for each configuration, is derived from the following dynamic Equation (1)I (d2O/dt2) + C (dO/dt) + kO = M -(1)where,I is the moment of the inertia.C is the damping coefficient representing the squeeze-film.k is the torsional stiffness of the turn curved spring.M is the torque created by the electrostatic force between the micromirror and its electrodes.The moment of the inertia of the micromirror along the y-axis is adjoin to (1/12)*ml2.Second, the value for damping coefficient, c, representing the squeeze-film damping of the micromirror is derived from the linearized Reynolds comparability 13 and presented in Equation (2).C= -(48w3)/(6(b2+4)D3) (2)where, is the dynamic viscosity of the air.l is equal to the half length of the micromirror, .w is the width of the micromirror.b is the ratio of the width to the length of the micromirror.D is the initial air gap between the micromirror and its electrodes.Third, the torsional stiffness, k, of the rotated snakelike s pringK= (G Jp)/(2NLp+3Lp) (3)where,G is the shear modulus of the material used in the rotated serpentine spring.Jp is the torsion factor of a carry with rectangular cross-section 14 and can be derived from the Equation (4) below.N is the number of the loops or turns in the rotated serpentine spring.Lp is the length of the rotated serpentine spring segment that is parallel to the rotation axis.Jp= (tw3/3)*(1-(192w/3t)*t=1,2,3.1/t3* tanh(tt/2w)) (4)Fourth, for the pursuit of simplicity, the micromirror is considered to be a rigid body and the deflection of the rotated serpentine spring in the Z axis is assumed to be negligible. In dictate to find the torque created by the electrostatic force between the micromirror and its electrodes, the parallel plate capacitor theory is used to derive the differential gear force that acts on a small segment of the micromirror and its electrodesdF = 1V2 (wdx)/(D-x2 -(5) where, denotes the permittivity of air and V represents the potential differ ence.The torque, M, for each configuration is simplified with the normalized angle as represented by the following Equation (6), (7) and (8)MO = 0.5 wV2 (L2/D2 o2)*(o/1-o + ln(1-o)) (6)M1 = 0.5 wV2 (L2/D2 4o2)*(2o/1-2o + ln(1-2o)) (7)M2 = 0.5 wV2 (L2/D2 2)*(1/(1-2o+o2)) (8)where,M0 represents the torque created in the single mirror configuration. M1 and M2 denote the torque generated in the first and second stacked mirror configurations, respectively. To simplify the analysis, the furbish up cornerstone electrodes are not used to actuate the micromirrors in both(prenominal) stacked configurations 8.Figure-2. crookedness versus burthen Comparison Plot for Three Micromirror Configurations.To visualize the magnitude of torques against the normalized angles, the normalized torques of M0, M1, and M2 are plot in the Figure 2. The red line shows an exponential increase in the normalized torque as the normalized angle grows. The black line (conventional single mirror configuration) sho ws relatively gradual increase. As expected, while the deflection angle is small there are negligible differences between the three configurations in terms of the torque created by the same actuation voltage. However, as the deflection angle increases, the torque acting on the first stacked mirror grows exponentially. On the other hand, the second stacked mirror configuration shows a 50% increase in torque when compared to the single mirror configuration.5.2 GEOMETRYThe size and geometry of the micromirror are pertinacious by the diameter of the optical beam as well as its application. For example, a micromirror used in an endoscope would require a smaller form factor. The micromirror discussed here is designed to be 1 mm in length, 1 mm in width and 10 m in thickness. withal, it is assumed to be make of polysilicon that has a Youngs modulus of 160 GPa, Poissons ratio of 0.22and closeness of 2330 kg/m3. Normally, the micromirror is designed to be hang over a cavity by two torsio n bars. Even though a serial torsion bar is simple to design and fabricate, it suffers from residual stress, which alters the stiffness of a torsion bar and the micromirrors frequency response. Furthermore, modification of the physical or geometric properties of the straight torsion bar is not straightforward since the geometry of the torsion bar such as the width and thickness are limited by the fabrication process. Hence, two rotated serpentine springs are chosen to possess the micromirror in place while the micromirror rotates. The serpentine springs stiffness can be easily customized regardless of the fabrication process. Thus, a rotated serpentine spring is employed in this analysis. The rotated serpentine spring used in this analysis is 4 m wide, 10 m thick, and 100 m in lengthfrom one end to some other end. The gap between each turn is 4 m. Figure.3 (a) shows the expanded view of the rotated serpentine spring, and Figure.3 (b) shows the relative size and location of the sp ring on the micromirror.Figure 3. (a) go around Serpentine Spring Torsion Bar and (b) the Micromirror.Two different configurations of the micromirror are presented in Figure 4. To simplify modeling and analysis, the geometry and material of the plates (micromirrors) are unbroken identical except the stacking configuration. As shown in Figure 4(a), a micromirror is placed 250 m presently higher up another square plate along the z-axis. In Figure 4(b), a micromirror is placed above another mirror with a 250 m gap in the z axis and a 500 m offset along the x- axis. The top plate is the micromirror, and the bottom plate is used as moving electrodes 8.The micromirror and its moving counterpart have two electrodes located on their bottom. The electrodes are assumed to be made of 1 m aluminium thin film. The rotated serpentine springs provide electrical connection between the electrodes and control circuitry.Figure-4. Stacked Micromirror Configurations.5.3 Flexure transmit Micro-Mirro rCUsersAjiteshDesktopUntitled1.jpgCUsersAjiteshDesktopUntitled.jpgFigure-5 Flexure Beam MicromirrorAPPROACHIn order to develop the characteristic model of the Flexure-Beam micromirror device, it moldiness first be characterized by equating the electrostatic actuation force of the parallel plate capacitor with the mechanical restoring force of the spring. Figure-6 shows a Flexure-Beam device in the resting ( V = 0 ) and alert ( V 0 ) modes where Zm represents the vertical height of the mirror above the address electrode. It is initially assumed that when no electrode potential is applied, the mirror rests firm in the resting position, Z0, where the deflection distance, d, at all points on the mirror is aught 1.Figure-6 Forces acting in flexure Beam MicromirrorThe Flexure-Beam device is a phase-only device since the direction of motion of the mirror is orthogonal to the reflective surface. Therefore, the optical trail length can be altered while the direction of propagation remai ns unchanged. This makes the piston device very appealing for phase modulate filters or for adaptive phase correcting optics.Figure-7 Cloverleaf MicromirrorOne design improvement is another cantilever device known as the Cloverleaf. As shown in Figure, the flexures holding the reflective surfaces are placed in the center of the geometry. This takes the basic design of the Inverted Cloverleaf and reduces some of the negative effects observed. Also, the electrodes are located directly beneath each mirror which allows the cantilever surfaces to be individually addressable. mournful the support for the mirrors to the center of the pixel cell allows for better use of general space. Now, the pixels can be placed so that adjacent cells nearly particle each other with only a small gap essential between the mirrors of one cell and the mirrors of another. Most of the jibe surface subject of the device is reserved for the active elements with the exception of the posts which hold the mir rors in place. This increases the active area of the device to as much as 86% which is similar to the remaining devices described in this chapter. This device, however, maintains the side effect of redirecting an incident beam of light in four distinct directions.CUsersAjiteshDesktopUntitled3.pngFigure-8 The Quad-Cantilever deformable micromirror deviceThe epoch-making advantage over the Cloverleaf devices is that the mirrors are aligned so that the redirection of the incident beam of light is in a common direction. This allows the device to be clear of switching or redirecting the incident light with little loss in amplitude. One characteristic similar to the Inverted Cloverleaf and Cloverleaf devices is the slanted behavior of the deflected mirror. This behavior is typical with cantilever devices and creates a non-uniform phase response across the surface of each mirror 1.ELECTROSTATIC FORCEIn order to compute the electrostatic force on the mirror, it must first be determined by which means this force will be calculated. More specifically, it must be decided whether the pick distribution, which is not uniform over the mirror surface, will be considered. The charge distribution will change with the position of the mirror surface and will also be altered by any mirror surface deformations or discontinuities such as etch holes. This leads to a complicated solution when integrating across the mirror.As an alternative, since both the charge distribution of the mirror and the applied electrode voltage are related to the electric field deep down the device, it is possible to express the potential energy, of the electric charge distribution solely in terms of this fieldCUsersAjiteshDesktopUntitled4.jpgWhere, a is the surface charge distribution on the mirror, V is the actuation voltage between the mirror and address electrode, A is the area of the mirror, e0 is the free space dielectric constant and E is the electric field speciality at any point in the volume v within the device .By assigning an electric energy density of V-2coloumbs to each point in space within the device, the physical effect of the charge distribution on the mirror surface is preserved. From this approach it is easy to identify that the non-uniform charge distribution on the mirror surface and the fringing effects of electric fields around the edges of the mirror are complementary descriptions of the same electrical phenomenon.5.4 Dual Axis Micro-MirrorFigure-9 Dual-Axis micromirrorMicromirror working principleThe micromirror is made up by a circular polysilicon micromirror plate that is connected to a gimbal frame by a pair of polysilicon torsion springs (Fig. 9). The gimbal frame is supported by a pairs of polysilicon springs too. The structure is a dual axis micromirror the slow axis works at the resonance frequency of 300 Hz while the fast axis works at the resonance frequency of 30 kHz. The fast axis allows the micromirror to be tilted around y direction while t he slow axis allows the micromirror to be tilted around x direction. some(prenominal) the two axis are actuated by electrostatic vertical comb drives. Vertical comb drives provide a motion in and out of the plane and present several advantages if compared to lateral comb drives. First of all, they generate a vertical force larger than lateral comb drives ,then they achieve larger scan angle at high resonance frequencies and finally they directly apply the torque to the micromirror without needing any hinges to couple their linear motion into torsional micromirror motion 4.Each vertical comb drive consists of a set of moving mechanical polysilicon electrodes and a set of rigid electrodes suspended over an etched pit. The rigid electrodes are bound to the substrate, while the chattel electrodes are linked to the axis. When a voltage is applied between the fixed fingers and the transferrable fingers, an electrostatic Torque arises between the two electrodes 4. Consequently the movab le fingers rotate around the torsional axis until the Electrostatic Torque (Te) and the Mechanical restoring Torque (Tm) of the springs are equal. These two torques can be expressed by (1) and (2).CUsersAjiteshDesktopUntitled5.jpgCUsersAjiteshDesktopUntitled 6.jpgFigure-10 Forces acting in a Dual-Axis Micromirror5.5 Micromirror with Hidden Vertical ransack DrivesThe actuators and the torsion springs are hidden underneath the mirror to achieve high-fill factor in micromirror arrays. In this case, the fringing capacitance is significant and cannot be ignored 2. The total capacitance as a function of angle can be calculated by integrating over the finger length. Fig. 11 shows the three-D design of thisCUsersAjiteshDesktopUntitled7.pngFigure-11 Hidden Vertical-Comb Drive Micromirror6.CONCLUSIONIn this report, the first three phase of the project have been completed. The different actuation principles , their advantages and disadvantages have been discussed. Also four designs have been proposed and analytical study of them has been done. We can now move on to the next phase which comprises of modeling as well as analysis of the designs chosen.