The Simulation Environment

Transcription

1 Institut für Integrierte Systeme Integrated Systems Laboratory Introduction The Simulation Environment WS 2011 The goal is to become familiar with the simulation package. We will use the tools SDE, DESSIS, TECPLOT and INSPECT. A simple pn-diode serves as example to demonstrate the course of simulation from mesh generation to visualization. Consider the numerics as a black box. Simulation results are finally to be used for the discussion of the physical processes in the pn-diode. Task Preparation. The physics of a pn-junction shall be understood. The following terms are important for the practical part on the computer: Doping Space Charge Region Majority and Minority Carriers Computer work starts by copying the necessary files into your working directory. Then SDE is started up, the diode will be designed from scratch and the mesh is generated from this newly obtained device. After this the device simulation can be started with S-DEVICE. The simulation results can be visualized with TECPLOT and INSPECT. The following commands will guide you through this process step by step. The prepared input files have to be copied from the master account into your own directory: >> mkdir tcad_intro >> mkdir tecplot_macro >> cp ~hlbe/diode/* tcad_intro/ >> cp ~hlbe/tecplot_macro/cut.mcr tecplot_macro/ >> cp ~hlbe/.alias. >> source.alias >> cd tcad_intro SDE is started with the command >> sde &. In the graphical user interface we first need to create the boundary geometry of the pn-diode. Therefore do the following: 1. Activate the use of exact coordinates with the button 2. Click Draw in the drop down menu and deactivate Auto Region Naming. This will force the program to prompt you for a name for every entity you create on your work area. 3. Click on the Create Rectangular Region Button 4. Now in the working area click and drag an arbitrary rectangle, you will be prompted for exact coordinates

2 5. In the appearing dialog box enter the following values First Vertex X: 0 Y: 0 Second Vertex X: 3.0 Y: 0.2 Confirm your entries by clicking OK. 6. Now you will be prompted for a name for this silicon body that you just created enter silicon bulk and confirm. You now see a block of silicon which will be transformed into a pn-diode in the following. Because the coordinate point P = (3.0, 0.2) lies outside the standard positioning of the viewport, you have to recenter the view. This can be done, either manually by using the Pan - tool ( ) or automatically by pressing the Zoom to Extents button ( ). For the grid generation a so-called refinement of the underlying structure has to be defined. These refinement definitions work as boundary conditions for the mesh generators, when generating the grid for the device. Click on Mesh Refinement Placement (in the drop down menu). A window pops up where you select Max. Element Size X Direction = 0.1, Min. Element Size X Direction = 0.01 and Max. Element Y Direction = 0.1, Min. Element Y Direction = Further choose Region in the Placement Type Section. Everything else can be left as it is. Finally click Add Placement and Close Now would be the right time to save the project. You should periodically save the project to secure against the possibility of program or system crashes. Click File Save Model and type diode as file name. Extensions will be handled by the program. After saving your progress a first mesh can be generated by clicking onto Mesh Build Mesh. In the upcoming dialog window the file name diode should show up in the path under the field Save Grid To File:, if not, type diode. Further choose MESH instead of SNMESH as meshing engine and confirm with Build Mesh. Your first mesh has been generated. The window that pops up now, can be ignored and closed. Clicking on the triangles of the mesh will bring up a window with information contained in this mesh cell. To proceed with the design, you first have to switch off the rendering of the mesh. Otherwise it is impossible to add any new geometry. You can do this by clicking the Mesh View button ( ) on the right side of your work area. If in any state of the work progress you changed something accidentally in your project, you can undo this change by clicking Edit Undo. If you changed / rotated the view in your workspace you can restore the default orientation with the button. It is even better to lock the view, when working on 2D devices by clicking View Gui Mode 2D Mode. To create the doping we need a construct called Reference Window. Click in the drop down menu Mesh Define Ref/Eval Window Rectangle or on the button. Draw a rectangle with the mouse over the left half of the diode and enter then the coordinates First Vertex X: 0 Y: 0 Second Vertex X: 1.5 Y: 0.2 When prompted for the name enter refwindow_p Repeat the last step, but this time with coordinates First Vertex X: 1.5 Y: 0 Second Vertex X: 3.0 Y: 0.2 and refwindow_n as name for the Reference Window. 2

3 Now the doping can be defined. In the drop down menu click Device Constant Profile Placement (Button: ) in the pop up window enter the following data: Placement Name: pdopplacement Placement Type choose refwindow_p from the Ref Window list. Constant Profile Definition: Name: pdopdefinition Species: BoronActiveConentration Concentration: 4e+16 Click Add Placement. The window won t disappear giving you the chance to define more doping profiles. Repeat the last step. Change Ref Window to refwindow_n and replace pdop with ndop in the Placement Name and Definition Name and change species: ArsenicActiveConentration / Concentration: 1e+16. Click Add Placement Close. You can have an overview of the defined Placements and Regions by clicking View Entity Viewer. By clicking View Placement Viewer you can select any created doping and refinement placement and edit it by double clicking on it in the list. The doping concentrations are now shown with colors (blue = p-region, red = n-region). Don t forget to change the name of the grid file, if the program didn t remember the last choice. It should be diode. Extensions are added automatically. With Mesh Build Mesh the grid is renewed. The Grid Postprocessing window can be closed. At the pn-junction the mesh must be further refined. For this, again define a Ref Window as in the steps above by drawing a rectangle over the region of interest which is the space charge region of the diode (click to draw the rectangle). This time you can leave the coordinates as obtained from the drawing and name it junctionwindow. Now define a new Refinement Placement by clicking Mesh Refinement Placement or press. Enter Max. Element Size X Direction = 0.02, Min. Element Size X Direction = Further choose Ref/Eval Window in the Placement Type Section and select the junctionwindow that you created before from the list. The PlacementName and the DefinitionName need to be changed, otherwise you will overwrite the settings made in the beginning for the refinement of the silicon_bulk-region. Click Add Placement and Close In the last step we need to define the contacts of the diode. Therefore, click in the drop down menu Contacts Contact Sets In the pop up window enter Contact Name: Anode and click Set, now again type Cathode and change the Edge Color from to and again click Set. You should now see both contacts listed on the right side of the window. This step adds just the contact definitions. Click Close. 3

4 The final step is to assign the contact definitions to certain Edges of the geometry. As depicted above, choose Select Edge in the selection level list and Anode in the contact list. Now (with activated selection tool ) click on the left edge of your diode. It will be highlighted as bold reddish line, if you really hit the edge. Sometimes the selection doesn t work properly. A possible workaround for this problem is to drag a small rectangle (using the selection tool) overlapping with the inner of the corresponding edge to be selected. Now click on Contacts Set Edges. The edge will change its color to the one you selected when creating the contact definition. Now select Cathode from the list, click on the right edge of the diode and click again on Contacts Set Edges. Build the mesh once again with diode as file name. Save your model via the drop-down menu File->Save Model. The device simulator S-DEVICE will be started with an input script which is discussed below. The starting command is >> sdevice diode_des.cmd or >> dessis diode_des.cmd. When the simulation is finished, the IV-curve can be inspected. Type >> inspect diode_des.plt. In the middle left window select Anode. Click OuterVoltage below and assign to the x-axis with to X axis. In the same way assign TotalCurrent to the y-axis. The result is the IV-characteristics. As can be seen, the diode was ramped from 1V to +1V. The fields are visualized with TECPLOT (check the Visualization part on page 6 of this exercise). The S-DEVICE output file must be provided. During the simulation S-DEVICE wrote data files at 5 points of the IV-curve. We want to inspect the fields for the case of a reverse bias. Therefore, select the file which was written first (bias = -1V): >> tecplot &. Load the File: diode_000000_des.tdr To load the file click File Load. In the pop up window, either double-click the desired file and press OK or click the filename once and then press Add OK. Congratulation! You successfully finished your first simulation. Additional tasks: Inspect the band diagram by repeating the simulation with additional variables. Open the input script >> emacs diode_des.cmd& (see below) and add the keywords ConductionBandEnergy, ValenceBandEnergy, equasifermi, and hquasifermi to the plotsection. Note that the Fermi potential must be multiplied by -1 in order to obtain the Fermi energy. Check whether the discretization of the space-charge region has a sufficient resolution. All physical quantities should appear as smooth curves. Create a cut along the x-direction using the provided macro (follow the Visualization part on page 6 of this exercise). 4

5 SYNOPSYS-TCAD Overview Geometry / Mesh Generation SDE Physics / Numerics S-DEVICE Visualization TECPLOT INSPECT SDE SDE is the tool for the generation of the boundary and the grid. It provides a way to generically generate a virtual device usable for electrical device simulation. The other way to obtain such a virtual device is to execute a process simulation of the fabrication process. This in turn demands deep understanding of process techniques and is very time consuming on its own. For most of our purposes the generically designed devices shall proof to be sufficient. S-DEVICE Device Equations S-DEVICE numerically solves (in the simplest case) the following equations: Poisson equation (ɛ ϕ) = e(p n + N + D N A ) (1) Electron continuity equation J n = er + e n t (2) Hole continuity equation J p = er + e p t (3) Electron drift-diffusion current J n = eµ n n ϕ + ed n n (4) Hole drift-diffusion current J p = eµ p p ϕ ed p p (5) The symbols are defined as follows: ϕ electrostatic potential n, p electron and hole densities J n, J p electron and hole current densities N + D, N A ionized donor and acceptor concentrations µ n, µ p electron and hole mobilities e elementary charge D n, D p diffusion constants R recombination rate Nabla operator (1D: x ) Input script There is no graphical user interface for S-DEVICE. For the definition of the transport model, the physical models, the file names, the solve procedure, and so on, a script has to be written with a 5

6 text editor. We start S-DEVICE with the (already existing) input script diode_des.cmd by the command >> dessis diode_des.cmd In the following we briefly discuss the input script. It consists of different Sections which start with a typical keyword and contain the arguments in braces. 1 Electrode { 2 {Name="Anode" Voltage=-1 AreaFactor=10} 3 {Name="Cathode" Voltage=0 AreaFactor=10} 4 } 5 6 File { 7 Grid="diode_msh.tdr" Doping="diode_msh.tdr" 8 Plot="diode" Output="diode" Current="diode" 9 } Plot { 12 Doping 13 Potential 14 ElectricField/Vector 15 ecurrent/vector hcurrent/vector 16 SpaceCharge edensity hdensity 17 } Math { 20 Iterations=20 Digits=5 21 } Physics { 24 Mobility(DopingDependence) 25 Recombination(SRH Auger) 26 } Solve { 29 Coupled { poisson electron hole } Quasistationary ( 32 InitialStep=0.02 MaxStep=0.02 Minstep= Plot { range=(0,1) intervals=4} 34 Goal {name="anode" voltage=1.8}) { 35 Coupled { Electron Hole Poisson } 36 } 37 } In the Electrode Section the electrical contacts and the voltages at the beginning of the simulation are defined. Here, the anode voltage is negative to obtain the entire IV-curve in a single sweep. In most cases one starts with zero voltage at all contacts (thermodynamic equilibrium) to avoid convergence problems. Then the contact voltages are ramped to the working point of interest. The File Section contains the file names of the input and output files. Name convention: base name followed by the name of the tool (_des for S-DEVICE, _msh for SNMESH or MESH which 6

7 was invoked by SDE when you built the mesh), followed by the type of file (.grd for grid files,.dat for data files,.tdr for a binary format integrating grid (grd), boundary (bnd) and doping information (dat) in one file,.plt for x-y plot files,.cmd for command files). The Plot Section defines which quantities shall be written into files. In the Math Section settings for the numerical solver can be given. The Physics Section contains the keywords of the physical models used in the simulation. In the Solve Section one may notice two Coupled statements which contain in braces the equations to be solved coupled. First, this command appears alone, which results in the correct initial state of the device. Then the Quasistationary keyword is used to ramp the voltage and to obtain the IV-curve. The coupled device equations are solved for each new voltage of the sweep. In order to control the ramp, some statements for the step size, the Plot command, and the final voltage (Goal) must be given as arguments. Visualization TECPLOT is the graphics tool for the visualization of multi-dimensional fields. To load the data, files containing grid and data information (usually.grd and.dat or.tdr) must be loaded via File > Load >... Creating cuts orthogonal to the Cartesian axis is easy, since TECPLOT provides 3 buttons in the left sidebar, labeled X, Y, and Z. Just click on the respective button and then at the required position in the frame. In order to create a cut along an arbitrary direction, one can use the predefined macro cut.mcr : Open your structure in TECPLOT Load the Macro: File > Macro > Play > ~/tecplot_macro/cut.mcr Enter the coordinates of the start point: If the structure is 2D, set z = 0 Enter the coordinates of the end point: If the structure is 2D, set z = 0 A frame should pop up and an additional menu window, titled Mapping Style. Click on X- Axis Variable and choose Distance. Then click on Y-Axis Variable and choose the wished quantity. To fit the axis scale, click on Fit Frame in the left sidebar. IV-curves are visualized with INSPECT. In the call one has to specify so-called plot files (extension.plt). Data sets can be assigned to the x- or y-axis. Tutorial/Manuals It is recommended to try the Sentaurus TCAD Tutorials which provide a deeper insight into the available tools. The latest tutorials are accessible via >>firefox /usr/pack/sentaurus wi/tcad/e sp1/sentaurus_training/index.html. The latest manuals are accessible via >>acroread /usr/pack/sentaurus wi/tcad/e sp1/manuals/pdfmanual/front.pdf. 7