Design of Microelectronics Systems - Laboratory - VIEEMA03 MEMS design task The task of the students is to create, design and simulate 1 pixel of a FIR sensor. As described in the presentation, there are several design possibilities, but the basic structure should be the same. The detectors must be located in/on a thermally isolated plane that is connected to the substrate via a suspension with high thermal resistance. Figure 1 Schematic of a thermal detector The suspension can be one or several beams with various shapes or the whole structure can be a membrane. As the incident light reaches the surface of the structure, it will be absorbed and it will heat the thermally isolated structure since the heat has to be transferred through the suspension towards the substrate with high thermal resistance. As the isolated structure heats up, the increased temperature has to be sensed by some kind of an electrical device. There is a couple of temperature- sensitive device like thermistor, thermocouple, diode, transistor, etc. In the design task, the students need to design a bolometer based FIR sensor structure where thermistors are responsible for temperature sensing. To create the thermally isolated structure, specific MEMS manufacturing technologies are to be utilized. It is advised to use AMS 0.35 CMOS compatible MEMS bulk micromachining technology, where the polysilicon bolometer and the contacting aluminum metallization is embedded inside a SiO2 sandwich structure. Besides the simulation, the project also includes the calculations of the electrical and thermal resistances based on the materials and the geometry. Figure 2 AMS 0.35 CMOS-MEMS manufacturing technology Specification In order to give more constraints to the design and to narrow the possibilities of the initial steps, please find a summary of the specifications that need to be fulfilled. • MEMS component: IR sensor • Micromachining technology: AMS 0.35 CMOS-MEMS micromachining technology • Sensing element: Bolometer o resistance: 1...5kΩ±20% tolerance o material: polysilicon o preferred shape: meander o minimum sensible electrical voltage difference on the contacts: 0.1mV o maximum applied current on the contacts: 0.1A • Suspended structure: plane, held by 2 or 4 L shaped arms • Incident radiation: 500-20000 W/m2 (step 500 W/m2) Figure 3 Layer structure Figure 4 Suspended structures Advised steps: 1. Identify the geometry and the manufacturing technology from the references 2. Create the 3D geometries a) Design the shape, dimensions of the bolometer structure based on the manufacturing technology and the material parameters. Use analytical calculations to estimate the value. b) Based on the shape of the arms holding the central plane give an estimation for the thermal resistances 3. Enter the material properties into the Engineering data after identifying them in the references. 4. Set up the coupled field simulations for a given input 5. Vary the geometry parameters and the loads and export the characteristics – parameter sweep 6. Documentation a) Generate report in ANSYS b) Calculations of the thermal and electrical resistances Example from last year The following example shows a MEMS bolometer designed by other students last year. It is visible that for the correct simulation, it is not necessary to design the substrate as well as it acts as the thermal ground. The inner structure needs to be connected to the thermal ground (e.g. substrate). In the inside, a polysilicon meander is representing the bolometer with the aluminum metallization connections to the substrate. The length, width of the meander was calculated analytically to fulfill the listed specifications. Based on the references, you need to identify the material properties. If you cannot find the values in the description of the manufacturing technology, try to find it elsewhere on the web. To design the MEMS as a home task, you need to use the ANSYS student version available for download at https://www.ansys.com/academic/free-student-products. Unfortunately, the problem size limitation of the student version for structural physics is 32K nodes/elements, so you have to be careful when creating the mesh. During the coupled field setup, the incident radiation can be modeled as a heat flux on the top surface, while the side of the structure, where it is connected to the substrate, should have a fixed temperature (isothermal boundary condition). To measure the change of the resistance value against the temperature of the bolometer, do not forget to set a temperature-dependent resistivity in the engineering data. In the setup, it is advised to set one end of the metallization contact for the resistor to 0 Volt while applying a current constraint on the other side. This way, a voltage distribution will be induced on the resistor as a consequence of the supplied current that will change as the temperature rises. Keep in mind that the applied current should not heat the structure by itself since it will give an offset to the measurements. Also, keep in mind the specification which says that the supplied current should not exceed 0.1mA and the minimal sensible voltage change is 1mV. Figure 5 Geometry of the design Figure 6 Temperature distribution for 1W/m2 heat flux Figure 7 Voltage distribution for 1W/m2 heat flux while the resistance is supplied by 0,033mA References The following references are just a basic collection to start with. It is expected to search the available web resources besides these articles as you would do it in an individual project ▪ S. Mir, L. Rufer, A. Dhayni, Built-in-self-test techniques for MEMS ▪ J. Thomas, J.S. Crompton, K.C. Koppenhoefer, Multiphysics Analysis of Infra Red Bolometer ▪ J. Teva, et. al., From VHF to UHF CMOS-MEMS monolithically integrated resonators ▪ https://mycmp.fr/IMG/pdf/cmp_processcat-18-2.pdf ▪ https://www.coventor.com/mems-solutions/micro-bolometers/ ▪ https://www.eet.bme.hu/~szabop/SSI/sim_cut_toolbar.swf ▪ https://www.eet.bme.hu/~szabop/SSI/parameters_toolbar.swf
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