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1 TCAD WORKSHOP USING SILVACO TCAD TOOLS Volume I This workshop will introduce you to process and device simulation using the Silvaco TCAD tools. It is assumed that you are familiar with basic concepts of Silicon device processing and the basic operation of Metal-Oxide-Semiconductor Field-Effect Transistor (MOSFET) and Bipolar Junction Transistor (BJT). SILVACO International 0

2 Chapter 1: Getting Started 1.1 Running ATHENA Under DeckBuild 1. To invoke ATHENA under DECKBUILD in interactive mode, enter the UNIX command: deckbuild an& 2. After a short delay, the main DECKBUILD window will appear: Figure 1-1 Main DECKBUILD Window SILVACO International 1

3 The lower text window of this window will contain the ATHENA logo and version number, a list of available modules, and a command prompt. ATHENA is now ready to run. To become familiar with the mechanics of running ATHENA under DECKBUILD you can load and run some of the ATHENA standard examples. 1.2 Loading and Running ATHENA Standard Examples DECKBUILD makes it possible to load and run a number of example simulation input files. To access the ATHENA examples: a. Click on the Main Control menu follows by the Examples menu item Figure 1-2 Loading DECKBUILD Examples. SILVACO International 2

4 b. The Deckbuild: Examples window will appear (Figure 1-3). Figure 1-3 DECKBUILD Examples Window Groups of DECKBUILD examples are listed in the Section menu and are grouped according to the simulation topic that the example demonstrates. Individual example input files are listed in the Sub-section menu. c. Select one of the simulation topics (e.g. MOS Application Examples) listed by double-clicking on this topic. A list of the MOS Application Examples will be listed as shown in Figure 1-4. SILVACO International 3

5 Figure 1-4 A List of MOS Application Examples d. Next, select the first input files (i.e. mos1ex01.in) by double-clicking on the input file name. A description of the selected input file will appear in the examples window as shown in Figure 1-5. Figure 1-5 Description of mos1ex1.in SILVACO International 4

6 e. Press the Load example button to load the selected input file into the DECKBUILD text window as shown in Figure 1-6. The input file, along with other files associated with the input file will be copied into your current directory. Figure 1-6 Selected input file being loaded into DECKBUILD Text Window f. Once the input file is loaded into the DECKBUILD text window, you can run the input file by pressing the run button on the DECKBUILD window. SILVACO International 5

7 Chapter 2: Creating an NMOS Device Structure Using ATHENA 2.1 Overview of the Procedure In this chapter, user will learn the basic operations required for creating a typical MOSFET input file. These operations include: Developing a good simulation grid Performing conformal deposition Performing geometric etches Performing oxidation, diffusion, annealing and ion implantation Structure manipulation Saving and loading structure information These operations are relevant to all individual ATHENA process simulators. 2.2 Creating An Initial Structure Defining Initial Rectangular Grid a. Clear the current text window of DECKBUILD by clicking the File menu follows by Empty Document as shown below. Figure 2-1 Clearing Text Window SILVACO International 6

8 b. To start ATHENA as the simulator, we need to start off with the go statement (refer to the ATHENA User s Manual) by typing the statement: go Athena in the text window as shown below. Figure 2-2 Starting with go athena Next, we shall specify the initial rectangular grid. The correct specification of a grid is critical in process simulation. The number of nodes in the grid has a direct influence on simulation accuracy and time. A finer grid should exist in those areas of the simulation structure where ion implantation will occur, where p-n junction will be formed, or where optical illumination will change photoactive component concentration. c. To define the rectangular grid, select the Mesh Define... menu item. The Mesh Define menu will appear as shown in Figure 2-3. SILVACO International 7

9 Figure 2-3 Invoking ATHENA Mesh Define Menu Creating Non Uniform Grid in a Rectangular 0.6 µm by 0.8 µm Simulation Area a. From the Mesh Define menu, the Direction Field is selected as X by default. b. Click on the Location field and enter a value of 0. c. Then, click on the Spacing field and enter a value of 0.1. d. For the Comment field, type Non-Uniform Grid (0.6um x 0.8um) as shown in Figure 2-4. Figure 2-4 Defining Grid Parameters. e. Click on the Insert button in the Mesh Define window and the line parameters will appear in the scrolling list as shown below. SILVACO International 8

10 Figure 2-5 Clicking on Insert Button f. In the same way, set the location of a second X line to 0.2 with a spacing of 0.01 and a third X line to 0.6 with a spacing of g. These X Line statements will define a grid that is very fine in the region on the right hand side i.e. from location x = 0.2µm to 0.6µm. This region will later be use for the active region of the NMOS transistor. h. Next, we shall proceed to create the grid in the Y direction. Select Y in the Direction field. i. Click on the Location field and enter a value of 0. Then, click on the Spacing field and enter a value of j. Click on the Insert button in the Mesh Define window. k. In the same way, set the location of a second Y line to 0.2 with a spacing of 0.01, a third Y line to 0.5 with a spacing of 0.05 and a fourth line to 0.8 with a spacing of 0.15 as shown in Figure 2-6. SILVACO International 9

11 Figure 2-6 Mesh Define for Y Direction Notice that the finest grid is defined at the surface i.e. from location y = 0µm to y = 0.2µm. This region will later be use to form the surface active region of the NMOS transistor. Thus, it is essential to have finest grid in this region. l. To preview the rectangular grid, from the Mesh Define menu, select the View... button. The View Grid window will be displayed. (Notice that a total of 1786 points and 3404 triangles are generated.) Figure 2-7 View Grid Window. m. Finally, write mesh define information to the text window by pressing on the WRITE button on the Mesh Define menu. A set of lines will appear as shown below. SILVACO International 10

12 Figure 2-8 Line Statements for Generating Non-Uniform Grid 2.3 Defining the Initial Substrate The LINE statements specified by the Mesh Define menu set only the rectangular base for the ATHENA simulation structure. The next step is the initialization of the substrate region. To initialize the simulation structure: a. Select Mesh Initialize... from the ATHENA Commands menu. The ATHENA Mesh Initialize menu will popup as shown in Figure 2-9. SILVACO International 11

13 Figure 2-9 ATHENA Mesh Initialize Menu. b. By default, Silicon is selected as the Material with <100> Orientation. c. Click on the Boron impurity box so that Boron is selected as the background doping. d. For the Concentration field, select the desired concentration as 1.0 using the slider or by typing it, and select an exponent of 14 from the Exp: field. This will give a background concentration of 1.0 x atom/cm 3. (It is also possible to set background concentration using the By Resistivity specification in Ohm cm.) e. For the Dimensionality field, check the 2D box. This forces the simulation to be run in a two-dimensional calculation. f. For the Comment field, type Initial Silicon Structure with <100> Orientation as shown in Figure SILVACO International 12

14 Figure 2-10 Defining the Initial Substrate using the Mesh Initialize Menu. g. Press the WRITE button to write the mesh initialization information into DECKBUILD text window. The following Comment and Initialize statements will appear in the text window as shown in Figure Figure 2-11 INIT Statement Appears in Text Window. SILVACO International 13

15 2.4 Run ATHENA and Plotting Now, run ATHENA to obtain the initial structure. Press the run button DECKBUILD Control. The following output will appear in the simulator subwindow: on the Figure 2-12 Running DECKBUILD The line struct outfile=.history01.str is automatically produced by DECKBUILD via the History function. The History function allows moving backwards to any previous line in the input file and restarts execution. It provides an invaluable service when debugging new files, performing what if simulations, and in visualizing the device at different stages in the process flow. To visualize the initial structure: SILVACO International 14

16 1. Highlight the.history01.str file. Next, click on the Tools menu button, select Plot and then Plot Structure... as shown in Figure Figure 2-13 Plot History file structure. 2. After a short delay, TONYPLOT will appear as shown in Figure It will have only regional and material information. From TONYPLOT, click on the Plot menu button, followed by Display. SILVACO International 15

17 Figure 2-14 Selecting Display Menu from TONYPLOT. 3. The Display (2D Mesh) menu will appear as shown in Figure By default, the Edges and Regions icons will be selected. Click on the Mesh icon as well, follows by the Apply button. The initial triangular grid will appears as shown in Figure Figure 2-15 Tonyplot: Display (2D Mesh) Menu. SILVACO International 16

18 Figure 2-16 Initial Triangular Grid So, the previous INIT statement creates a <100> silicon region of size 0.6µm x 0.8µm, which is uniformly doped with boron concentration of 1 x atom/cm 3. This simulation structure is ready for any process step (e.g. implant, diffusion, Reactive Ion Etching, etc.). 2.5 Performing Gate Oxidation Next, we will grow an gate oxide layer on the Silicon surface by performing dry oxidation at 950 o C for 11 minutes in 3% HCL at 1 atmospheric pressure. To perform this gate oxidation step, from the ATHENA Commands menu, select the items Process Diffuse... The ATHENA Diffuse menu will appear as shown in Figure SILVACO International 17

19 Figure 2-17 ATHENA Diffuse Menu. a. From the Diffuse Menu, change the Time (minutes) from 30 to 11 and the Temperature (C) from 1000 to 950. Note: The Constant Temperature is selected by default (see Figure 2-18). b. From the Ambient field, click on the Dry O2 box. SILVACO International 18

20 c. Check on the Gas pressure and the HCL fields. Change the HCL to 3%. d. Add a comment Gate Oxidation in the Comment field and click on the WRITE button. Figure 2-18 Gate Oxidation Parameters defined using the Diffuse Menu. e. The gate oxidation information will be written into DECKBUILD text window as shown in Figure From this figure, it can be seen that the Diffuse statement is used to perform the gate oxidation. Figure 2-19 Gate Oxidation Step using Diffuse Statement. f. Continue the ATHENA simulation by clicking on the Cont button on the DECKBUILD control. Once the gate oxidation step is completed, another history file.history02.str will be saved as shown in Figure SILVACO International 19

21 Figure 2-20 Continue simulating the Gate Oxidation Step g. Plot this structure again by highlighting.history02.str file and then select the menu items Plot Plot Structure... from the Tools menu of DECKBUILD. Figure 2-21 Gate Oxide Structure. The resulting gate oxide structure will appears in TONYPLOT as shown in Figure From the plot, it can be seen that an oxide layer was deposited onto the silicon surface. SILVACO International 20

22 2.6 Extracting Gate Oxide Thickness Next, we would like to extract the gate oxide thickness that was grown during the oxidation process step. This can be done using the Extract routines of DECKBUILD. Extract forms a function calculator that allows you to combine and manipulate values or entire curves quickly and easily. You can create your own, customized expressions, or choose from a number of standard routines provided for the process and device simulators. You can take one of the standard expressions and modify it as appropriate to suit your needs. Extract also has variable substitution capability so that you can use the results of previous Extract commands. To extract the thickness of gate oxide: a. From the Commands menu, just click on the Extract. as shown below and the ATHENA Extract menu will appears as shown in Figure Figure 2-22 ATHENA Extract Menu SILVACO International 21

23 b. By default, the Material thickness is selected in the Extract field. c. For the Name field, type in Gateoxide. d. For the Material field, click on the Material and select SiO~2. e. On the Extract location field, click on the X location and enter a value of 0.3 as shown in Figure Figure 2-23 Parameters used for Extracting Gate Oxide Thickness. f. Click on the WRITE button and the Extract statement will appear in the text window as shown in Figure Figure 2-24 Gate Oxide Thickness Extract Statement. In this Extract statement, all the parameters are self-explanatory except the mat.occno=1 which specifies the layer occurrence number. This parameter is optional in this case as there is only one silicon dioxide layer. However, in some cases SILVACO International 22

24 where there are many stacked silicon dioxide layers, we have to specify the specific silicon dioxide layer that we are interested in. g. Continue the ATHENA simulation by clicking on the Cont button on the DECKBUILD control. The runtime output of the Extract statement is as shown Figure Figure 2-25 Runtime Output of the Extract Statement. From the runtime output, we can see that the extracted gate oxide thickness was Angstroms. In the next section, we shall learn how to optimize the gate oxide thickness using the Optimizer function in DECKBUILD. 2.7 Optimizing Gate Oxide Thickness In this section, we will learn how to use the Optimizer function in DECKBUILD to optimize the gate oxidation process parameters. Assuming that the measured gate oxide thickness is 100 Angstroms, and both the diffusion temperature and partial pressure in the Gate Oxidation step needs to be tuned. To optimize this parameter, DECKBUILD Optimizer will be used as follows: a. Click on the Main Control menu follows by Optimizer as shown below. Figure 2-26 Invoking DECKBUILD Optimizer. SILVACO International 23

25 This invokes the DECKBUILD Optimizer as shown in Figure The first optimizer displays a table of the control parameters in the Setup Mode. The default parameters here are normally adequate, except that we can set the Maximum error (%) so as to fine tune the gate oxide thickness to 100Å. Change maximum error from 5 to 1. Figure 2-27 Setup Mode of DECKBUILD Optimizer. b. Therefore, select the value for Maximum Error on the Stop criteria column and change the value from 5 to 1. c. Next, we must define the parameter to optimize by using the Mode button to change from Setup mode to Parameters mode as shown below. SILVACO International 24

26 Figure 2-28 Changing to Parameters mode. c. For this tutorial, the optimization parameters are the temperature and partial pressure during the Gate Oxidation step. To define this to the Optimizer, go to DECKBUILD window and highlight the Gate Oxidation step as shown below. Figure 2-29 Highlighting Gate Oxidation Step. d. Then, within the Optimizer, click on the Edit menu follows by Add. The Deckbuild: Parameter define popup window will appear as shown in Figure 2-30, listing the items that may be used as parameters. SILVACO International 25

27 Figure 2-30 Defining the Parameter to be Optimized. e. Check on the temp=<variable> and press=<variable> items. Then, click Apply. The added optimization parameter will then be display as shown below. Figure 2-31 Added Optimization Parameter. f. Next, using the Mode button, change the mode from Parameters to Targets so as to define the target of the optimization. SILVACO International 26

28 Figure 2-31 Changing to Targets Mode. g. The Optimizer uses the value of Extract statement in DECKBUILD to define the Optimization target. Therefore, go to DECKBUILD text window and highlight the Extract gate oxide thickness statement as shown below. Figure 2-32 Highlighting Optimization Target. h. Then, within the Optimizer, click on the Edit menu follows by Add. This adds the target Gateoxide to the Optimizer target list as shown in Figure SILVACO International 27

29 Figure 2-33 Add target Gateoxide to the Optimizer Target List. i. You must define the target value for this entry in the Target List. Enter the Target value as 100Å. Figure 2-34 Enter Target value as 100Å. The Optimizer has now been configured to optimize the gate oxide thickness by varying the temperature and partial pressure during the Gate Oxidation process step. j. To monitor the optimization process, change from Targets mode to Graphics mode as shown below. SILVACO International 28

30 Figure 2-35 The Graphics mode in the Optimizer. k. Finally, to perform optimization, click on the Optimize button. The simulation will run again and will, after a little time, start to iterate the Gate Oxidation step. The Optimizer will then converged at a temperature of o C and a partial pressure of and the extracted oxide thickness is Å as shown in Figure SILVACO International 29

31 Figure 2-36 Optimization completed. To complete this Optimization exercise, the optimized value for the temperature and partial pressure should be copied back to the input deck. l. To copy this value, go to the Parameters mode and click on the Edit menu follows by Copy to Deck to update the optimized parameter in the input deck as shown in Figure SILVACO International 30

32 Figure 2-37 Optimized parameters automatically update in the correct place. The input deck will be automatically updated in the correct place. 2.8 Performing Ion Implantation Ion implantation is the main method used to introduce doping impurities into semiconductor device structures. Adequate simulation of the ion implantation process is very important because modern technologies employ small CDs and shallow doping profiles, high doses, tilted implants and other advanced methods. In ATHENA, ion implementation is done using the IMPLANT statement which can be set using the ATHENA Implant menu. In this tutorial, we will perform a threshold voltage adjust implant using Boron with a dose of 9.5 x cm -2 at an energy of 10 kev, with the ion beam tilted at 7 o and rotated at 30 o. To do so, SILVACO International 31

33 a. From the Commands menu, select Process follows by Implant... The ATHENA Implant menu will appears as shown in Figure b. Select Boron from the Impurity field. Figure 2-38 ATHENA Implant Menu. c. Enter a value of 9.5 in the Dose field using the slider or by typing it and a value of 11 in the Exp: field for the exponent. d. Enter 10 for the Energy field, 7 for the Tilt field and 30 for the Rotation field. e. The Dual Pearson model is chosen by default. f. Select Material Type as Crystalline. g. Enter Threshold Voltage Adjust implant for the Comment field. h. Click on the WRITE button and the implant statements will appear in the text window as shown in Figure SILVACO International 32

34 Figure 2-39 Threshold Voltage Adjust Implant Statement. All of the parameters in the IMPLANT statement are self-explanatory except CRYSTAL. The CRYSTAL parameter indicates that for all analytical models, the range statistics extracted for a single silicon crystal will be applied. i. Press the Cont button on the DECKBUILD control and ATHENA will continue the simulation as shown in Figure Figure 2-40 Simulation of the Threshold Voltage Adjust Implant Step. In the next section, we shall proceed on to analyze the doping profile of the implanted Boron. 2.9 Analyzing Boron Doping Profile in TONYPLOT The doping profile of the implanted Boron may be viewed using the 2D Mesh menu or using the Cutline tool of TONYPLOT. In the 2D Mesh menu, the contour plot of the Boron doping profile may be visualize. On the other hand, performing a Cutline on the 2D structure create a 1D cross-section plot of the Boron doping profile. SILVACO International 33

35 First, we shall illustrate the use of the 2D Mesh menu to obtain the contour plot of the Boron doping profile. a. Plot the history file i.e..history05.str of the threshold voltage adjust implant step by highlighting it and then from the Tools menu of DECKBUILD, select the menu items Plot Plot Structure... b. From TONYPLOT, select the Plot menu follows by Display. The Display (2D Mesh) popup will appear. c. Select the Contours icon to plot the contour of the structure. d. Then, click on the Define menu and select Contours as shown in Figure Figure 2-41 Invoking TONYPLOT: Contour menu. SILVACO International 34

36 e. The TONYPLOT: Contours popup will appears. By default, Net doping is being selected in the Quantity option of the popup. Change the Quantity from Net doping to Boron. f. Click on the Apply button follows by the Dismiss button when finished. g. The contour plot of the Boron doping profile will appears as shown in Figure Figure 2-42 Contour Plot of the Boron doping profile after Ion Implantation. Next, we shall perform a cutline on the 2D structure to create a 1D cross section plot of the Boron doping profile. To do so, a. From TONYPLOT, select the Tools menu follows by Cutline. The Cutline popup appears as shown in Figure b. By default, the Vertical icon is selected. This will restrict the cutline to a vertical direction. SILVACO International 35

37 Figure 2-43 TONYPLOT Cutline Popup. c. In the structure plot, left click and drag the mouse starting from the oxide layer to the bottom of the structure. In this way, a 1-D cross section plot of the Boron doping profile appears in another window as shown in Figure Figure 2-44 Performing a Vertical Cutline across the structure. SILVACO International 36

38 2.10 Performing Conformal Deposition for Polysilicon Gate Conformal deposition can be used to generate multilayered structures. Conformal deposition is the simplest deposit model and can be used in all cases when the exact shape of the deposited layer is not critical. Knowing that the polysilicon layer thickness grown in the NMOS process is 2000 Angstroms, it is possible to substitute this with conformal polysilicon deposition. To set the conformal deposition step, from the ATHENA Commands menu, select the items Process Deposit Deposit... The ATHENA Deposit menu will appear (see Figure 2-45). a. From the Deposit Menu, the Conformal deposition is the selected by default. b. Select Polysilicon from the Material menu, and set its thickness to 0.2. c. In the Grid specification parameters, click on the Total number of grid layers checkbox and set its value to 10. (It is always useful to set several grid layers in a deposited layer. In this case, 10 grid layers are needed in order to simulate impurity transport through the polysilicon layer.) d. Add a comment Conformal Polysilicon Deposition in the Comment field and click on the WRITE button. Figure 2-45 ATHENA Deposit Menu. SILVACO International 37

39 e. The following lines will appear in the text window: # Conformal Polysilicon Deposition deposit polysilicon thick=0.2 divisions=10 f. Continue the ATHENA simulation using the Cont button on the DECKBUILD control. g. Plot this current structure again by selecting the menu items Plot Plot Structure... from the Tools menu of DECKBUILD. A three layers structure is created as shown in Figure Figure 2-46 Conformal Deposition of Polysilicon Layer Simple Geometrical Etches The next step in the process simulation is the polysilicon gate definition. In this tutorial, we will use a polysilicon gate edge at x = 0.35 µm and set the center of the gate at x = 0.6 µm for SILVACO International 38

40 the initial grid. Therefore, polysilicon should be etched to the left from x = 0.35 as shown in Figure a. Go to the Commands menu of DECKBUILD and select the chain Process Etch Etch... The ATHENA Etch menu (Figure 2-47) appears. Figure 2-47 ATHENA Etch Menu. b. From the Etch menu, click on Left for the Geometrical type field. c. From the Material field, select Polysilicon. d. Set the Etch location to e. Add the comment Poly Definition for the Comment field. f. Click on the WRITE button and this will give the following statement: # Poly Definition etch polysilicon left p1.x=0.35 g. Continue the ATHENA simulation using the Cont button on the DECKBUILD control and plot the etched structure as shown in Figure SILVACO International 39

41 Figure 2-48 Etching Polysilicon to form the Gate Performing Polysilicon Oxidation The next step is to perform polysilicon oxidation prior to doping of the polysilicon by ion implantation. The oxidation recipe is wet oxidation for 3 minutes at 900 o C at 1 Atmospheric pressure. Since the oxidation is on a patterned (non-planar) and undamaged polysilicon, the method used will be the fermi and compress methods. The fermi method is used for undamaged substrates with doping concentrations < 1x10 20 cm -3 whereas the compress method is used to model oxidation on non-planar structures and for 2-D oxidation. To perform this oxidation step, the Diffuse menu is used by selecting the items Process Diffuse... from the ATHENA Commands menu. a. From the Diffuse Menu, change the Time from 11 to 3 and the Temperature from 950 to 900. SILVACO International 40

42 b. From the Ambient field, click on the Wet O2 box. c. Check on the Gas pressure checkbox and uncheck the HCL checkbox. d. Click on the Models setting from the Display field. The models available will then be displayed. e. Check on both the Diffusion and Oxidation models and select the Fermi and Compressible boxes. f. Add a comment Polysilicon Oxidation in the Comment field and click on the WRITE button. Figure 2-49 Polysilicon Oxidation Parameters defined using the Diffuse Menu. g. The following Diffuse statement will then be added into the input file: # Polysilicon Oxidation method fermi compress diffus time=3 temp=900 weto2 press=1.00 h. Continue the ATHENA simulation using the Cont button on the DECKBUILD control and plot the structure as shown in Figure Thus, it can be SILVACO International 41

43 seen that the poly oxidation step has formed oxide layer on top of the polysilicon and also on the substrate. Figure 2-50 Oxide Formation After the Polysilicon Oxidation Step Polysilicon Doping After the polysilicon oxidation step, the next step is to dope the polysilicon with phosphorus so as to create a n+ polysilicon gate. Here, the dose of the phosphorus used was 3x10 13 cm -2 with an implant energy of 20 KeV. To perform the polysilicon doping step, the ATHENA Implant menu is used again. a. From the Commands menu, select Process follows by Implant... The ATHENA Implant menu (see Figure 2-51) will appears. b. From the Impurity field, change the implantation impurity from Boron to Phosphorus. SILVACO International 42

44 c. In the Dose field, enter a value of 3 using the slider or by typing it and a value of 13 in the Exp: field for the exponent. d. Enter 20 for the Energy field, 7 for the Tilt field and 30 for the Rotation field. e. The Dual Pearson model is chosen by default. f. Select Material Type as Crystalline. g. Enter Polysilicon Doping for the Comment field. Figure 2-51 Defined Polysilicon Doping Parameters using the Implant Menu. h. Click on the WRITE button and the implant statements will appear in the text window as follows: # Polysilicon Doping implant phosphor dose=3e13 energy=20 crystal i. Continue the ATHENA simulation using the Cont button on the DECKBUILD control and plot the Net Doping of the structure by selecting the Contours icon on the Display (2D Mesh) menu follows by the Apply button as shown below. SILVACO International 43

45 Figure 2-52 Net Doping Contour Plot After Polysilicon Implantation Step. j. To view the implanted phosphorus contour plot, from the Display (2D Mesh) menu, click on the Define menu and select Contours as shown in Figure Figure 2-53 Invoking TONYPLOT: Contour menu. SILVACO International 44

46 k. The TONYPLOT: Contours popup will appears. By default, Net Doping is being selected in the Quantity option of the popup. Change the Quantity from Net Doping to Phosphorus. Figure 2-54 Changing Quantity Field from Net Doping to Phosphorus. l. Click on the Apply button follows by the Dismiss button when finished. m. The contour plot of the implanted phosphorus doping profile will appear as shown in Figure SILVACO International 45

47 Figure 2-55 Contour Plot of Implanted Phosphorus Doping Profile Spacer Oxide Deposition Prior to the source and drain implants, spacer oxide deposition has to be performed. Here a spacer oxide will be deposited with thickness of 0.12 µm. This can be accomplished using the ATHENA Deposit menu as follows: a. From the ATHENA Commands menu, select the items Process Deposit Deposit... The ATHENA Deposit menu (see Figure 2-56) will appear. b. Select Oxide from the Material menu, and set its thickness to c. For the Grid specification parameters, set the Total number of grid layers to 10. d. Add a comment Spacer Oxide Deposition in the Comment field and click on the WRITE button. SILVACO International 46

48 Figure 2-56 Defined Spacer Oxide Deposition Parameters. e. The deposit statements will appear in the DECKBUILD text window as follows: # Spacer Oxide Deposition deposit oxide thick=0.12 divisions=10 f. Continue the ATHENA simulation using the Cont button on the DECKBUILD control and plot the structure with meshes as shown in Figure SILVACO International 47

49 Figure 2-57 Mesh of the Structure After Spacer Oxide Deposition Sidewall Spacer Oxide Formation To form the sidewall oxide spacer, a dry etch step has to be performed. This is done using the ATHENA Etch menu (see Figure 2-58) as follows: a. From the Etch menu, click on Dry thickness for the Geometrical type field. b. From the Material field, select Oxide. c. Enter 0.12 for the Thickness field. d. Add the comment Spacer Oxide Etch for the Comment field. e. Click on the WRITE button and this will give the following statement: SILVACO International 48

50 # Spacer Oxide Etch etch oxide dry thick=0.12 f. Continue the ATHENA simulation and plot the etched structure as shown in Figure Figure 2-58 Sidewall Spacer Oxide Formation after Dry Etching Source/Drain Implant and Annealing To form the n+ source/drain of the NMOS transistor, an arsenic implantation will be performed. In this workshop, the dose of arsenic used is 5 x cm -3 at an implantation energy of 50 KeV. To perform this implantation step, the ATHENA Implant menu will be used again. After invoking the Implant menu, perform the following: a. Change the implantation impurity from Phosphorus to Arsenic for the Impurity field. SILVACO International 49

51 b. In the Dose field, enter a value of 5 and a value of 15 in the Exp: field. c. Enter 50 for the Energy field, 7 for the Tilt field and 30 for the Rotation field. d. Select Material Type as Crystalline. e. Enter Source/Drain Implant for the Comment field. Below shows the parameters used for the source/drain implant. Figure 2-59 Source/Drain Implant Parameters using the Implant Menu. f. Click on the WRITE button and the implant statements will appear in the text window as follows: # Source/Drain Implant implant arsenic dose=5e15 energy=50 crystal The source/drain implant is then followed by a short annealing process in nitrogen for 1 minutes at 900 o C and 1 atmospheric pressure. This annealing process can be carried using the Diffuse menu (see Figure 2-60) as follows: a. From the Diffuse Menu, set the Time to 1 and the Temperature to 900. b. From the Ambient field, click on the Nitrogen box. SILVACO International 50

52 c. Check on the Gas pressure checkbox and set the value to 1. d. Click on the Models setting from the Display field. The models available will then be displayed. e. Check on the Diffusion model and select the Fermi box. Uncheck the Oxidation model. f. Add a comment Source/Drain Annealing in the Comment field and click on the WRITE button. Figure 2-60 Annealing Process Parameters The following diffuse statement will appear in the text window: # Source/Drain Annealing method fermi diffus time=1 temp=900 nitro press=1.00 g. Continue the ATHENA simulation using the Cont button on the DECKBUILD control and plot the doping of the structure as shown in Figure SILVACO International 51

53 Figure 2-61 Source/Drain Implant and Annealing Process. Next, we would like to see the change in the Net Doping before and after the annealing process. To do this, a. From the TONYPLOT of the Source/Drain Annealed structure (Figure 2-61), click on the File menu, follows by the Load Structure b. To load the previous history file generated during the implant arsenic dose=5e15 energy=50 crystal step (i.e..history12.str), key in the.history12.str for the Filename field. c. Then, click on the Load menu follows by Overlay as shown in Figure d. The previous implanted structure i.e..history12.str will then overlay onto the annealed structure i.e..history13.str as shown in Figure Notice that the subtitle of the plot indicates Data from multiple files SILVACO International 52

54 Figure 2-62 Loading Previous Implant Step Structure File and overlaying it. Figure 2-63 Overlaying Structures. SILVACO International 53

55 e. After the two structure plots are overlay onto each other, perform a Cutline by selecting the Tools menu in TONYPLOT follows by Cutline f. The Cutline menu will appear. Click on the keyboard icon and enter the following values for X and Y as shown in Figure Figure 2-64 Using the Keyboard option of the Cutline menu. g. Once done, hit the Return button on keyboard and TONYPLOT will prompt you for confirmation as shown below. Click on the Confirm button Figure 2-65 Confirm Cutline. This result in the generation of a one-dimensional plot on the right hand side as shown in Figure It can be seen that the short annealing process has moves the dopants away from the surface of the MOS structure. SILVACO International 54

56 Figure 2-66 One-dimensional Net Doping Plot Metallization ATHENA can attribute an electrode to any metal, silicide or polysilicon region. A special case is the backside electrode which can be placed at the bottom of the structure without having a metal region there. In this tutorial, the metallization of the half NMOS structure is done by, first forming the contact window in the source/drain region and next, depositing and patterning the Aluminum. To form the contact window in the source/drain region, the oxide layer is etch to the left at x = 0.2 µm. using the ATHENA Etch menu as follows: c. From the Etch menu, click on Left for the Geometrical type field. d. From the Material field, select Oxide. c. Enter 0.2 for the Etch location field. SILVACO International 55

57 e. Add the comment Open Contact Window for the Comment field. e. Click on the WRITE button and this will give the following statement: # Open Contact Window etch oxide left p1.x=0.2 f. Continue the ATHENA simulation and plot the etched structure as shown in Figure Figure 2-67 Defining Parameters to Open Contact Window. Figure 2-67 Forming the Contact Window before Metallization. Next, an aluminum layer of thickness 0.03 µm will be deposited on the half NMOS structure using the ATHENA Deposit menu as follows: a. Select Aluminum from the Material menu, and set its thickness to d. For the Grid specification parameters, set the Total number of grid layers to 2. SILVACO International 56

58 d. Add a comment Aluminum Deposition in the Comment field and click on the WRITE button. e. The following deposit statements will appear in the DECKBUILD text window: # Aluminum Deposition deposit aluminum thick=0.03 divisions=2 f. Continue the ATHENA simulation using the Cont button on the DECKBUILD control and plot the structure as shown in Figure Figure 2-68 Aluminum Deposition on the Half NMOS structure. Finally, the aluminum layer is etch to the right starting from x = 0.18 µm using the Etch menu as follows: f. From the Etch menu, click on Right for the Geometrical type field. g. From the Material field, select Aluminum. SILVACO International 57

59 c. Enter 0.18 for the Etch location field. h. Add the comment Etch Aluminum for the Comment field. e. Click on the WRITE button and this will give the following statement: # Etch Aluminum etch aluminum right p1.x=0.18 f. Continue the ATHENA simulation and plot the etched structure as shown in Figure Figure 2-69 Etching Aluminum on the Half NMOS structure. SILVACO International 58

60 2.17 Extracting Device Parameters In this session, we will extract some of the device parameters of the half NMOS structure. These include the junction depth, n++ source/drain sheet resistance, LDD sheet resistance under oxide spacer and the long channel threshold voltage This can be done by using the Extract menu on DECKBUILD Extracting Junction Depth To extract the junction depth, a. From the Commands menu, click on the Extract. The ATHENA Extract menu will appears. b. For the Extract field, select the Junction depth. c. Type in nxj for the Name field d. For the Material field, click on the Material and select Silicon. e. On the Extract location field, click on the X location and enter a value of 0.2 as shown in Figure Figure 2-70 Parameters used for Extracting Gate Oxide Thickness. f. Click on the WRITE button and the Extract statement will appear in the text window as follows: extract name="nxj" xj material="silicon" mat.occno=1 x.val=0.2 junc.occno=1 SILVACO International 59

61 In this extract statement, name="nxj" is the n-type, source/drain junction depth; xj indicates that junction depth is to be extracted; material="silicon" is material containing the junction. In this case, the material is Silicon; mat.occno=1 indicates to extract the junction depth at the first occurrence of Silicon layer; x.val=0.2 is to extract the source/drain junction depth at x = 0.2 µm; junc.occno=1 is to extract the junction depth at first occurrence of the junction. In a more complex structure, there may have more than one junction within the same material layer. For example, an n+ source/drain region within a p well on an n substrate would, on a line through the source/drain region, have two junctions. source drain junction 1 p Well junction 2 n substrate For the MOSFET structure, there is only one junction. Therefore, the junction occurrence number in this case is optional Extracting N++ Source/Drain Sheet Resistance To extract the sheet resistance, invoke the ATHENA Extract menu again and perform the following: a. Change the Extract field from Junction depth to Sheet resistance. b. Type in n++ sheet res for the Name field. c. On the Extract location field, with the X location selected, enter a value of 0.05 as shown in Figure d. Click on the WRITE button and the Extract statement will appear in the text window as follows: extract name="n++ sheet res" sheet.res material="silicon" mat.occno=1 x.val=0.05 region.occno=1 In this statement, sheet.res indicates that sheet resistance is to be extracted; and mat.occno=1 and region.occno=1 specify the material occurrence number and the region occurrence number to be 1; and x.val=0.05 tell the extract routine where the n++ region is. This is done by giving the location of a point within the region at x = 0.05 µm. SILVACO International 60

62 Figure 2-71 Parameters used for Extracting N++ Source/Drain Sheet Resistance Extracting LDD Sheet Resistance To extract the LDD sheet resistance under the oxide spacer, we simply move the point of interest to under the spacer. Referring to the structure simulated as shown in Figure 2-69, the value of 0.3 is reasonable. We will name the extracted resistance as `ldd sheet res'. Simply invoke ATHENA Extract menu and perform the following: a. Change the Name field to ldd sheet res. b. Change the value of the Extract location field to 0.3, with the X location selected. c. Click on the WRITE button and the Extract statement will appear in the text window as follows: extract name="ldd sheet res sheet.res material="silicon" mat.occno=1 x.val=0.3 region.occno= Extracting Long Channel Threshold Voltage To extract the long channel threshold voltage of the NMOS at x = 0.5 µm: SILVACO International 61

63 a. From the ATHENA Extract menu, change the Extract field from Sheet resistance to QUICKMOS 1D Vt. b. For the Name field, type in 1dvt. c. For the Device type field, click on the NMOS. d. Check on the Qss field, and enter a value of 1e10. e. On the Extract location field, enter a value of 0.5 as shown in Figure Figure 2-72 Parameters used for Extracting Long Channel Threshold Voltage. f. Click on the WRITE button and the Extract statement will appear in the text window as follows: extract name="1dvt" 1dvt ntype qss=1e10 x.val=0.5 In this statement, 1dvt instructs the Extract routine to extracts the 1D threshold voltage; ntype is the device type. In this case, we have a n-type transistor; x.val=0.5 is point that lies within the channel of the device; qss=1e10 is the trapped charge, Q ss, which is given as 1 x cm -2. By default, the gate bias setting is 0-5 V for a 0.25V step with the substrate at 0V and a default device temperature of 300 Kelvin. Continue the ATHENA simulation and all the extracted values will appears in the DECKBUIKLD output window as shown in Figure These information are also written to the file `results.final' in your current working directory. SILVACO International 62

64 Figure 2-73 Extracted Values Appear in the DECKBUILD Output Window Reflecting the Half NMOS Structure This tutorial has been building one half of a MOSFET-like structure. At some point in the simulation, it will be necessary to obtain the full structure. This must be done prior to exporting the structure to a device simulator or setting electrode names. To mirror the half MOSFET at its right boundary: a. From the Commands menu, select Structure follows by Mirror. The ATHENA Mirror menu will appear. b. Select Right for the Mirror field as shown in Figure Figure 2-74 ATHENA Mirror Menu c. Press the Write button to write the following statement to the input file: struct mirror right SILVACO International 63

65 d. Continue the ATHENA simulation using the Cont button on the DECKBUILD control and plot the resulting full NMOS structure as shown in Figure Figure 2-75 The Full NMOS Structure. From this figure, it can be seen that the right half of the structure is a complete mirror copy of the right part, including node coordinates, doping values, etc Specification of Electrodes To enable biasing in the device simulator ATLAS, it is essential to label the electrodes for the NMOS transistor. The electrodes of the structure can be defined using the ATHENA Electrode menu. To invoke this menu, a. Go to the Commands menu, select Structure follows by Electrode... The ATHENA Electrode menu will appears. SILVACO International 64

66 b. From the Electrode Type field, select the Specified Position button. c. For the Name field, type in source. d. Click on the X Position and set the value to 0.1 as shown in Figure e. Press the WRITE button and the following statement will appear in the input file: electrode name=source x=0.1 Figure 2-76 Specifying Source Electrode. Similarly, specify the drain electrode at x=1.1µm using the ATHENA Electrode menu to obtain the following statement: electrode name=drain x=1.1 The polysilicon gate electrode specification has the same format. For this structure it can be done the same way as for source or drain: electrode name=gate x=0.6 In ATHENA, a backside electrode can be placed at the bottom of the structure without having a metal region there. To specify a backside electrode, select Backside from the Electrode Type of the ATHENA Electrode menu. Then, type in the name backside. The following backside electrode statement will appear in the input file: electrode name=backside backside The syntax backside specifies that a flat (zero height) electrode will be placed on the bottom of the simulation structure. Continue running the input file. From the output window of DECKBUILD as shown in Figure 2-77, the following notes can be seen. SILVACO International 65

67 Figure 2-77 Assign Electrode Name. With the specification of the electrodes, the NMOS structure is completed. The last session will describe on how to save this final structure file. SILVACO International 66

68 2.20 Saving Structure File for Plotting or Initializing an ATHENA Input file for Further Processing Although, the DECKBUILD history function saves structure files after each process step. However, in many cases it is necessary to save and initialize structures independently. There are several reasons for this: i. The stack for the history files is limited (25 by default) ii. It is not usually desirable to keep dozens of history files on disc (each of which occupy dozens or hundreds of Kbytes) after the DECKBUILD session ends iii. Users often want to save the structure information generated after key process steps (e.g. final structure). To save or load a structure, use the ATHENA File I/O menu which is invoked by: a. Selecting File I/O... from the Commands menu. b. Press the Save button and specify a file name nmos.str. Figure ATHENA File I/O Menu. c. Press the WRITE button and the following line will appears in the input file: struct outfile=nmos.str d. Continue running the input file and plot the nmos.str structure file. Select the Electrodes icon Figure 2-80). to view the drain, gate, source and backside electrodes (refer SILVACO International 67

69 Figure 2-80 Full NMOS Structure. SILVACO International 68