EXPERIMENT 1 INTRODUCTION TO MEMS Pro v5.1: DESIGNING a PIEZO- RESISTIVE PRESSURE SENSOR

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1 EXPERIMENT 1 INTRODUCTION TO MEMS Pro v5.1: DESIGNING a PIEZO- RESISTIVE PRESSURE SENSOR 1. OBJECTIVE: 1.1 To learn and get familiar with the MEMS Pro environment and tools 1.2 To learn the basis of process setup and design. 2. INTRODUCTION 2.1 Introduction In these MEMS Pro tutorial you will go over the design flow of a MEMS device. This example tutorial is based on the design of a piezo-resistive pressure sensor. Figure 1: Piezo-resistive pressure sensor You will start by creating the schematic design using a piezo-resistor model, and analyze the system behavior with MEMS Pro s tools S-Edit and T-SPICE.

2 The goal is this design is to create a pressure sensor device with an output gain about 100mV/bar and gain linearity lower than 1%. In the second step, you will create the mask data and generate a 3D Model in L-Edit. This tutorial is delivered with files that will be used during each step of the design. We strongly recommend that you create a tutorial folder where you will save this material. 2.2 Starting with MEMS Pro Create the tutorial directory where you will store the tutorial libraries and save your work. Start by opening the MEMS Pro Project Manager by clicking on: Start -> Programs -> MEMS Pro v5.1 -> MEMS Pro The MEMS Pro Project Manager will help you in configuring and starting the different tools of the MEMS Pro suite. Figure 2: MEMS Pro Project Manager

3 3.0 SCHEMATIC DESIGN AND SIMULATION During this first part we will capture the schematic of the piezo-resistor Wheatstone bridge. Once designed, we will perform simulation to view the behavior of the device. From the MEMS Pro Project Manager, Select the Schematic Editor environment. Figure 3: Schematic Editor Environment As we will create a new design, leave the fields empty and click on Run button. This will automatically launch S-Edit Schematic Editor. Once opened, we will save the schematic as Pressure_Sensor.sdb in the working directory by clicking on: File -> Save As from the S-Edit Menu Bar. Then rename the module as Bridge by clicking on: Module -> Rename 3.1 Piezo-resistor Wheatstone Bridge Open the symbol Browser by clicking on the icon or from the menu bar: Module -> Symbol Browser Click on the Add Library button and select the PR_LIB.sdb library file delivered with the tutorial. Once loaded, select the rpiezo_behavioral symbol from the Modules part of the Symbol Browser.

4 Figure 4: Symbol Browser Click on Place button to instantiate the piezoresistor symbol in the schematic then Close to close the Symbol Browser window

5 Figure 5: Rpiezo symbol instanced in the schematic This instance will represent the piezoresistor placed on the top edge of the membrane. Use the Duplicate function from the menu bar: Edit -> Duplicate to copy the piezoresistor instance and place the right, the bottom and finally the left piezoresistors in the schematic as in Figure 6. Select the Top piezoresistor instance and from the menu bar, click on: Edit -> Flip -> Vertical to change the orientation of the instance. The Pressure pin must be place on the bottom of the device and Rpiezo on the top as on Figure 6.

6 Figure 6: Placement of symbols in the schematic Edit the properties of the piezoresistor by using the icon or the menu bar: Edit -> Edit Object For each piezoresistor instance set the property named position with PIEZO_TOP, PIEZO_RIGHT, PIEZO_BOTTOM, PIEZO_LEFT, according to its position in the layout. Figure 7: Instance properties

7 Figure 8: Piezoresistor instances with modified placement properties Once updated, connect the instances together using the Wire icon Wheatstone bridge as below: to create the full

8 Figure 9: Wheatstone bridge with connections and ports Once connected, we will add the Input and Output ports and add names on nodes. Add the Pressure, VBias+ and VBias- input ports by using the Input Port icon. Also add the VSense+ and VSense- output ports by using the Output Port icon. Finally, add node names R_Piezo1, R_Piezo2, R_Piezo3, R_Piezo4 on each unconnected pin of the rpiezo instances by using the Node Label icon. The R_Piezo pins correspond to the resistance of each piezoresistor element. 3.2 Loads and Electronics We will now add the Pressure load using the pulse width voltage source called Source_V_pwl from the PR_LIB library.

9 Figure 10: Loads and Electrical supply Edit the properties of the Source_V_pwl instance and set the load pattern sequence as following: Pattern: 0 0 1u 0 2u 100k 3u 100k 10u 50k 11u 0 12u 0 Note: 100k Volt is equivalent to Pa, or 1000 hpa thus One atmosphere. Add a wire on the plus pin of the voltage source, and attach a label called Pressure. Naming nodes with the same name is equivalent to drawing a wire. Thu the node named Pressure will be connected to the input port called Pressure. Using the Global Symbol Instance icon, add the ground connected to the minus pin of the voltage source. Add two DC voltage source to supply the Wheatstone bridge using the Source_V_dc instance of the PR_LIB library. Edit the properties to make sure the supply voltage is 5 volts. Connect the middle node to the ground and add two wires called VBias+ on the top and VBias- on the bottom. VBias+ will generate +5V while VBias- will be 5V. We will add the output stage composed of an operational amplifier to combine the two output signals of the wheatstone bridge. Use the opamp symbol of the PR_LIB library.

10 Figure 11: Operational Amplifier Connect the positive input to VSense+, and negative input to VSense-. To output a positive signal, connect the negative output to the ground and add a wire called Vout on the positive output of the amplifier. 3.3 Simulation Directive In order to simulate the device, we will add some directive onto the schematic. These directives will be automatically exported to the spice netlist and used for the simulation.

11 Figure 12: Simulation Directives To set the simulation directives, we use the TSPICECOMMAND symbol of the PR_LIB library. Place the symbol on the schematic, and then edit the properties to set the SPICE OUTPUT parameter as follows:.model rpiezo external winfile= PATH/TO/RPIEZO.c Where PATH/TO/ corresponds to the path where you saved the tutorial material and the rpiezo.c file This first statement will determine which model to use to simulate the piezo resistor. Warning: The Paths delimiter is the / character (and not \ as in windows) A second instance will be set with:.include PATH/TO/process.sp This second statement indicates to the simulator to include the process.sp file that contains process dependant parameters. And a third instance with:.tran 0.1u 20u This last statement specifies the type of simulation to perform (transient), the maximum simulation step (0.1us) and the simulation end time (20us). We can save the design and run the simulation.

12 3.4 Simulation The T-Spice simulator can be ran directly from the S-Edit environment by clicking on T- Spice icon: Figure 13: T-Spice environment As the directives have already been set at the schematic level, we can directly run the simulation by clicking on the Run Simulation icon: The Run Simulation window should appear. Check the input and output file names, and click on Start Simulation button. During the simulation, the Simulation Status window allows you to monitor the simulation.

13 Figure 14: Simulation Status window The simulation is completed when the message Simulation completed with appears. At this step, you can run the Waveform viewer W-Edit from the T-Spice environment, or S-Edit by clicking on: 3.5 Probing Simulation Results: Measuring Gain Once the W-Edit environment is open, you can select from S-Edit the signals you want to plot. For this, use the Probe icon: Then, with the probe, select the nodes Pressure and Vout on the schematic. The W-Edit window automatically updates with the traces of the signals. Note: All the traces are gathered in the same graph. Use the Expand Chart icon display each trace in different charts. to

14 Figure 15: W-Edit Simulation Results Using the Markers button pressure., you can determine the output voltage as a function of the The output voltage value for a 1 bar pressure variation = mv The output gain, A1= mv/bar This value corresponds to the minimum gain of 100mV/bar we wanted to achieve. Close W-Edit and T-Spice. To measure the linearity of the gain, change in S-Edit the maximum Pressure load from 1 bar to 1 mbar and perform a new simulation. Again, using the Markers button pressure., determine the output voltage as a function of the The output voltage value for a 1 bar pressure variation = mv The output gain, A2= mv/bar

15 The gain linearity is given by the equation: Calculate the output linearity using the above equation. From the W-Edit window, click on: Chart -> New Chart To add a chart entry, then to add traces in this chart, click on: Chart -> Traces The Traces Windows appears: Figure 16: Adding traces From the Traces available trace list, select the R_Piezo voltages and click on Load button to copy them in the Traces in chart list. Click on OK to validate.

16 Figure 17: W-Edit Simulation Results with Piezo Resistors From the new chart you can see that the variation of the piezo-resistors resistances vary in opposition according to their position on the design. R_Piezo1 and R_Piezo3, the top and bottom piezoresistors are varying negatively while R_Piezo2 and R_Piezo4 are varying positively. Moreover the amplitude is not the same. These variations behave as given by the following equation: Where In our case, the transverse and longitudinal coefficient values are for a p-type substrate along the [110] direction: Longitudinal: 71.75e-11 /Pa Transverse: -66.3e-11 /Pa Thus for R_Piezo1 and R_Piezo3, the transverse value is predominant while for R_Piezo2 and R_Piezo4, it is the longitudinal value. You can easily modify the parameters of the piezo-resistors in the Schematic, run a new simulation and quickly see how they will impact on the target you want to reach.

17 4. LAYOUT CAPTURE 4.1 Starting MEMS L-Edit Pro From the MEMS Pro Project Manager, go in the Settings section to setup the process file we will use to capture the layout. Figure 18: MEMS Pro Project Manager In the Technology part, set the Technology name to Sensor and using the browse button, select the PR_Sensor_Tech.tdb file delivered with the tutorial material. Finally, from the Layout Editor section click on Run button to start L-Edit. Figure 19: Layout Editor section

18 4.2 Create Layout The process Layers is described as below Sub: Substrate Bulk-Etch: Substrate Etching NEpi: N-Type Silicon P-Diff: P-Type Diffusion (Piezoresistor) Contact Metal Cross Section View of the Process: We will first define the membrane: Figure 20: Cross Section view of the process Draw a NEpi box of 1600 * 1600 um using the Draw Box icon. Centered on the NEpi, add a 1000*1000 um Back_Ecth Box that corresponds to the membrane area. We will add the 4 piezoresistors. Select the Pdiff Layer, and draw 4 boxes of 100 * 20 um. Place the piezoresistors on the top, right, bottom and left inner border of the membrane. Finally, add the Metal pads and the connections with the piezoresistor.

19 Once completed save your design. Figure 21: Final Layout 4.3 3D Model Generation Before generating the 3D model, you need to generate a derived layer that will properly define the opening area for the back etch step as a function of the wafer thickness. Use the menu bar: Tools -> Generate Layers... Then from the Generate Layers window click on OK to perform. To generate the 3D model, click on the View 3D Model icon Pro Toolbar, click on:, or from the MEMS 3D Tools -> View 3D Model

20 The generator goes down the process steps to finally create the 3D model according to your mask layout. Figure 22: 3D Model of the Pressure sensor Using the orbit view icon, you can rotate the 3D model to see the other side. Figure 23: Backside view of the membrane By clicking on the Cross-Section icon 3D Model., you can perform a cutaway view from your In the Generate 3D Model Cross Section window Click on Horizontal button to perform a horizontal cutaway. Finally click on OK.

21 Figure 24: Cross Section view of the membrane By zooming close to the edge of the membrane, you can see the piezo-resistor. Figure 25: Zoom on the Piezo-resistor From the Generate 3D Model Cross Section you can also perform 3D Cross Sections. This feature is a mix between a traditional cutaway view and the 3D Model as it only cuts the 3D model along the cross-section line. Figure 26: 3D Cross Section View By using the MEMS Pro Palette menu: 3D Tools -> Export 3D Model you can save your model in SAT format to perform a structural analysis in a FEM Solver such as ANSYS.

22 EMT 432/4 MEMS Design and Fabrication Laboratory Module Name : Date : Matrix No : 3.5. RESULT: - Using 1 bar pressure: The output voltage value for a 1 bar pressure variation = mv The output gain, A1= mv/bar - After changing the maximum Pressure load from 1 bar to 1 mbar: The output voltage value for a 1 bar pressure variation = mv The output gain, A2= mv/bar The gain linearity is given by the equation: Calculate the output linearity using the above equation. Gain = A1 A2 Gain = A2 2

23 EMT 432/4 MEMS Design and Fabrication Laboratory Module 5. DISCUSSION: List and discuss all the MEMS Pro Tools that were used during piezo-resistive pressure sensor designing processes. 6. CONCLUSION: The conclusion for this lab is 3