Design, Fabrication, And Analysis Of MEMS Three-Direction .

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Design, Fabrication, and Analysis of MEMS Three-Direction CapacitiveAccelerometerKevin Petscha and Dr. Tolga KayaaaCentral Michigan University, Mount Pleasant, MI 48859Email: {petsc1k, kaya2t}@cmich.eduAbstractIn this project we present the design and fabrication of a MEMS three-direction capacitiveaccelerometer. The design of the motion sensor includes sensing acceleration in the x-, y-, and zdirections, tilting, and free-fall. This device achieves this function implementing four serpentinespring systems suspending a proof-mass. The structure of the sensor can then be modeled as amechanical mass-spring system where the mass is one side of a parallel plate capacitor and theframe of the accelerometer is the other. The device then has a nominal capacitance and willchange as the proof-mass responds to acceleration events. These changes can be monitored alongeach axis and then analyzed to provide information on the motion occurring.The design features a proof-mass with interdigitated fingers along each side that correspond toopposing interdigitated fingers extending from the frame. This structure then creates many smallparallel plate capacitors that will sense motions in-plane with the x- and y- axes. Beneath theproof-mass, separated by an air gap, is the bottom electrode acting as one parallel plate of thedifferential capacitor sensing along the z-axis. This structure is realized implementing the Siliconon Glass process developed at the University of Michigan’s Lurie Nanofabrication Facility.1. IntroductionHealth monitoring is becoming increasingly important as new advances in technology are made.These advances allow for smaller, more reliable devices that can aid in the analysis of a person’stemperature, blood pressure, brain activity, and physical [1]. Particular products such as drugdelivery systems are made possible using Microelectro Mechanical Systems (MEMS) devicesand include hypodermic needles and even pills coated with specialized materials that allow thedrug to be introduced into the system in controlled amounts over a controlled period of time.Advances in health monitoring create the unique opportunity for remote patient monitoring [2, 3,4].Proceedings of the 2012 ASEE North Central Section ConferenceCopyright 2012, American Society for Engineering Education1

Tremendous amounts of research have been conducted in health monitoring and MEMS devicescontinue to become more prevalent in the products and techniques developed from these efforts.Collaborations between researchers within the Department of Electronic and ComputerEngineering at the University of Limerick and the Rehabilitation Centre at St. Camillus’Hospital in Limerick to provide gait analysis for rehabilitation of older. The implementation ofsmall, cheap MEMS accelerometers has made the analysis of gait parameters such as such asstride time, stride symmetry, and speed a real possibility [1]. MEMS accelerometers have alsobeen used in discovering the loss of balance in older patients that lead to frequent falls andinjuries. Other than patients older in age, motion detectors can aid in analyzing other medicalrelated issues such as Parkinson’s disease and many others [5]. This has, in turn, created anemerging need of motion detection and signal processing so that the information from theseMEMS devices can be properly interpreted and analyzed.Motion detection is mainly achieved with accelerometers. The MEMS structures within apackage respond to motion events through bending, flexing, or somehow altering their structuresjust as a cantilever beam or spring system behaves in the real world scale. This, in turn, changesthe value within some electrical interface component of the design allowing for measurement totake place. MEMS design and fabrication techniques are used to realize these devices [4, 5].There are several different types of accelerometers that differ in their method of sensingmovement. These include capacitive, piezoelectric, piezoresistive and thermal [6, 7]. The thermaltype accelerometers use a gas producing material within an enclosed package. There are then atleast four temperature sensors arranged in the four corners of the device and one directly over thegas producing material itself. These types function by comparing the values from the temperaturesensors to determine the orientation of the device. The more common types of accelerometersuse capacitive techniques. The function is that a suspended proof mass acts as one plate of aparallel plate capacitor while the plate is stationary. The suspended proof mass deflects withmotion, much like a spring, and changes the distance between the parallel plates. This thenchanges the value of the capacitor and allows interface circuitry to detect the difference and thencalculate the force of the acceleration event. Many of these capacitive types are fabricated usingSilicon on Insulator (SOI) techniques that provide a suitable platform for the silicon device,comprising the accelerometer, to be fabricated upon. It is important that the substrate the deviceis on is an insulator so that the capacitance values are not interfered with on the package itself [8].The accelerometer presented in our design, fabrication, and analysis is of the capacitive sensingtype. There are many techniques to realizing three directions of motion which may includeProceedings of the 2012 ASEE North Central Section ConferenceCopyright 2012, American Society for Engineering Education2

separate accelerometers oriented for each axis [6]. Our design features interdigitated sensingcapacitors. These capacitors, or fins, extend from a proof mass suspended by a serpentine springstructure. While interdigitated fins for sensing are a common technique used in MEMSaccelerometers, the actuation technique differs [7, 8, 9, 10]. As mentioned before, our proposeddesign encompasses all three directions of motion using a proof mass suspended by serpentinesprings. Other techniques for achieving actuation include folded flexures, looped springs, orcantilever beams [11, 12, 13].Aside from design and fabrication of MEMS devices, another daunting challenge is processingthe actual signals, or data, provided by the devices. The challenge is that these devices are sosmall that the signals being monitored are extremely small as well. This makes it challenging todetect even smaller changes and convert the analog signal into a digital signal that can beanalyzed. In our sensor, the femtofarad range capacitances need to be monitored while avoidingparasitic capacitances that will skew the data being provided from the MEMS sensor itself. Thework presented here does not tackle the conversion of the capacitance values into a digital signaland is part of a separate research topic.We designed, modeled, fabricated, and tested our MEMS capacitive accelerometers usingvarious tools including AutoCAD for mask design, COMSOL Multiphysics for modeling, andimplemented the Silicon on Glass (SOG) process at the Lurie Nanofabrication Facility (LNF)within the University of Michigan. The process for developing a MEMS device is expansive andbegins with design. Once a particular method is decided that will be the sensing element of theaccelerometer, the process steps for fabrication need to be established. In order to implement theSOG process steps, the proper masks for each layer of the device need to be carefully designedwith incredible precision. Verification of the design is necessary and done in COMSOLMultiphysics where different simulations are executed. This includes vibrational, electrostatic,and modal analysis. Finally, testing of the fabricated devices themselves becomes anotherchallenge in itself and must be painstakingly consistent. Specialized tools are needed to handlethe MEMS devices to ensure accuracy and safety of the devices. These include a clean roomenvironment, probe station, wedge wire bonder, and the instruments to measure various electricalcharacteristics of the device.2. Proposed DesignOur proposed MEMS capacitive accelerometer design features a proof mass suspended byserpentine spring structures, shown in Figure 1. The entire package is 2mm x 2mm x 100µmProceedings of the 2012 ASEE North Central Section ConferenceCopyright 2012, American Society for Engineering Education3

with a critical dimension of 10µm. It should be noted that each device is assumed to be 100µmthick because that is the given thickness of the silicon wafer used for processing. This springstructure was chosen to provide the greatest flexibility and allow for maximum displacement ofthe proof mass and sensing regions. Other designs were considered and included spiral springtypes as well as traditional spring types. The capacitor fins extending from the proof mass are200µm x 10µm. The proof mass itself is 395µm x 395µm featuring forty-nine 20µm x 20µmdamping holes. Each side of the proof mass features five fins interdigitated with an opposing sixanchored fins extending from the sides of the package.It is important to note that this structure is made of doped silicon and is conductive. This is animportant concept for the design because regions need to be isolated from each other to avoidshort circuits during electrical measurements and testing. The proof mass and spring structuresare isolated from the other components because the interdigitated fins and proof mass itself arepart of parallel plate capacitors for sensing. If they were not isolated and were touching othercomponents, the parallel plate capacitor would short circuit prohibiting proper function. Otherareas that require isolation include the bottom electrode and its associated contact pads. It iscrucial that the silicon structure be suspended over the bottom electrode for the same reason theproof mass and fins need to be separated from the anchor fins. For clarity, each anchored finsection must be individually isolated as well to prevent shorting and ensure functionality.Figure 1: Three dimensional model of MEMS capacitive accelerometer demonstrating serpentine springs, proofmass, and interdigitated fin structures.Proceedings of the 2012 ASEE North Central Section ConferenceCopyright 2012, American Society for Engineering Education4

The sensing mechanism produces a capacitance that will serve as the raw output data for thesystem. It is important to understand the nominal capacitance while the entire system is at rest.Since the device behaves as a simple parallel-plate capacitor, it can be realized with Equation 1.Eq. 1This shows that the capacitance of a region, C, is equal to the dielectric constant of air, ε,multiplied by the area of the overlapping sensing regions, A, divided by the distance separatingthe two sensing regions, d. For an interdigitated fins section, there are eleven small differentialcapacitors that dominate the sensing for the x- and y- axes. The actual area overlapping per set ofdifferential capacitors is 180µm x 100µm separated a distance of 10µm. For an individual pair ofinterdigitated fins, the nominal capacitance is 15.9 femtofarads. For a set of eleven, thecapacitance is 175.4 femtofarads. The entire sensing system composed of all four sets ofinterdigitated fins is 701.2 femtofarads.The remaining sensing region is in the z-direction and is composed of the parallel platecapacitance formed between the bottom electrode and the suspended proof mass with dampingholes. There are forty-nine 20µm x 20µm damping holes that must be removed from the areacapacitance of the proof mass. This nominal capacitance is calculated to be 303.7 femtofarads,separated by a distance of 3µm.Aforementioned was the explanation of the serpentine spring structure with emphasis onflexibility in all axes of motion. The air gap for the z-sensing region is set to 3µm for a particularreason. The SOG process developed at the LNF calls for an etching step with a hazardoussolution, Hydrofluoric acid. This solution etches the glass at a certain rate and creates a taperedslope from the bottom of the recess created to the original planar surface of the glass wafer. Ithas been found that this depth creates a repeatable, reliable etch that allows the deposited bottomelectrode to extend from the recess, up the tapered slope, and to the original surface withuniformity. If etched too long, uniformity is sacrificed and the bottom electrode deposition maynot be successful.The bottom electrode itself has been purposefully designed in such a way to focus the dominantsensing capacitances in their appropriate regions. Since parallel plate capacitors only create avalue for regions that are overlapping, it does not make sense to coat the entire glass recess withmetal. Also, there are very small capacitances formed between the bottom electrode and theProceedings of the 2012 ASEE North Central Section ConferenceCopyright 2012, American Society for Engineering Education5

overlapping serpentine spring structures. By minimizing the area of the region extending frombeneath the proof mass to its contact pad that is outside the recess, the capacitances are furtherminimized. The bottom electrode extends symmetrically to each of its contact pads to maintainthe devices symmetry and provide four distinct chances at measuring the z-capacitance in casesome regions of the bottom electrode not protected by silicon during a Deep Reactive IonEtching (DRIE) process step were damaged.The 20µm pin holes serve an important purpose as well. During a z-motion event, air moleculesare forced out and away from the proof mass that is oscillating back and forth over the bottomelectrode. In such small scales, such as MEMS systems, air molecules are rather large and aredifficult to move. These pin holes provide another route for air to escape during z-motion eventsso that the proof mass is able to be displaced without resistance due to displacing air molecules.2.1 ModelingAfter initial design, Finite Element Analysis (FEA) modeling was used. COMSOL Multiphysicswas implemented to do such analysis. The models were imported from AutoCAD and physicsmodels were applied in the FEA software. The model was then manipulated for simplicity whilekeeping all the same structural characteristics and is shown in Figure 2 [14]. In Table 1 thematerial properties and geometric parameters are shown that are used throughout the COMSOLsimulations.Proceedings of the 2012 ASEE North Central Section ConferenceCopyright 2012, American Society for Engineering Education6

Figure 2: Simplified CAD model of accelerometer structure. Table 1: Parameters and material properties used inCOMSOL Simulations.Simulations performed include modeling of the air dampening effect during a z-motion event,maximum deflection in x- or y- directions, von mises stress analysis of serpentine springs, modalanalysis, change of capacitance due to deflection, and electrostatic potential analysis of thedevice in sensing regions [14]. The values achieved during these simulations are shown in Figure3.It was found, as expected, that the presence of air molecules would damp the behavior of themotion during a z-motion event. The more air molecules to displace, higher pressures, relates toa greater damped motion. It was discovered that it would be beneficial to damp our motionbecause the initial event shows very similar maximum displacements. To avoid what seems to bea harmonic oscillation and continuously changing capacitance values, the presence of airmolecules and removing damping holes from the design would be advantageous. The von misesstress analysis was performed to identify the problem regions of the spring design during largeacceleration events. The spring design accommodates for stresses up to -5 g and limits areas ofelevated stress. The capacitance of the device itself as a function of displacement is crucial aswell and was simulated up to 2.5µm. Modal analysis was used to identify resonance within thestructure at certain frequencies. Resonance frequencies were found to be 2 kHz along the x-, y-,and z-axes. Finally, electrostatic potential was analyzed to verify that these forces would allowfor displacement and therefore change in capacitance that would serve as the data to beprocessed from the device.Proceedings of the 2012 ASEE North Central Section ConferenceCopyright 2012, American Society for Engineering Education7

ABCDEFFigure 3: (A) Results of modeling air dampening in z-motion event for different pressures. (B) Von mises stressanalysis of serpentine spring structure. (C) Model used preparing electrostatic testing and change in capacitance dueto deflection. (D) Electric potential of sensing fin region. (E) Change in capacitance as a function of displacement.(F) Values obtained during modal analysis along each axis in and out-of-plane.Proceedings of the 2012 ASEE North Central Section ConferenceCopyright 2012, American Society for Engineering Education8

2.2 FabricationThe next great challenge comes in the form of the actual fabrication of these devices. Achievingprecise, controlled structures on the order of just a few microns requires a great deal of resourcesand understanding. It is important to perform these processing steps in an environment that willeliminate particles in the air that may destroy the functionality of the devices being fabricated.The fabrication of these structures was done within the LNF at the University of Michigan. Thisis a National Science Foundation facility part of the National Nanotechnology InfrastructureNetwork featuring a class 100 clean room and the necessary equipment to perform the SOGprocess. This facility was crucial in the creation of these MEMS accelerometers because particlesin unfiltered air could become lodged within the fin structures or beneath the proof mass anddestroys its functionality.The SOG process developed by the LNF provides the steps necessary in achieving suspendedsilicon structures on the sub-micron scale. A cut-view of a finalized sample created using theSOG process is shown in Figure 4. It should be noted that there is a glass recess with anelectrode deposited that extends from the recess to the original surface of the glass wafer and thatthe silicon is suspended over the recess with an air gap of 3µm below.Figure 4: Cut-view of finished SOG structure.The SOG process can be summarized by examining each processing step required. A bare 4”glass wafer 500µm thick is used as the substrate for the process. A chrome layer is deposited toserve as a mask layer for a later processing step. Photolithography is then performed to patternphotoresist in the glass recess pattern. After developing the photoresist, the exposed areas ofchrome are etched away exposing the original glass surface in a pattern consistent with the glassrecess. A hydrofluoric acid solution is used to etch away the exposed areas of glass. Thephotoresist and chrome are then stripped away revealing the original glass wafer with recesses.Photolithography is then performed again to patter for the bottom electrode metal. Thephotoresist is then developed to realize the bottom electrode pattern. Using e-beam evaporationProceedings of the 2012 ASEE North Central Section ConferenceCopyright 2012, American Society for Engineering Education9

techniques again, a chrome and platinum layer is deposited across the entire wafer. This isimportant because a lift-off technique will be used to reveal the bottom electrode metal in itsintended pattern at the bottom of the recess extending outward. Since the metal is deposited overall surfaces, the exposed glass where photoresist has been developed away and on top of thephotoresist itself, the acetone bath to strip the photoresist also eliminates the metal on top aswell. The sample is now a bare glass wafer with the bottom electrode patterned in the recesses ofthe glass extending outward.Photoresist is spun onto the entire sample to protect it during the next processing step. Pre-dicingis necessary so that once fabrication is

Design, Fabrication, and Analysis of MEMS Three-Direction Capacitive Accelerometer Kevin Petscha and Dr. Tolga Kayaa aCentral Michigan University, Mount Pleasant, MI 48859 Email: {petsc1k, kaya2t}@cmich.edu Abstract In this project we present the design and fabrication of a MEMS three-direction capacitive accelerometer.

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