Книги+1 / 2013 [Chandan_Kumar_Sarkar]_Technology_CAD
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FIGURE 4.12
Complete structure of the 2D MOSFET in Sentaurus Editor after proper meshing at different places.
(sdegeo:define-2d-contact (list (car (find-edge-id (position 0.075 0 0)))) “drain”)
Now the device requires proper doping at a different place to work as a MOSFET. For that purpose a different place has to be selected for doping. The techplots showing electrostatic potential and conduction band energy
FIGURE 4.13 (See color insert)
Tecplot_sv showing electrostatic potential across the device at VGS = 2.0 V, VDS = 2.0 V.
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FIGURE 4.14 (See color insert)
Tecplot_sv showing conduction band energy (eV) across the device at VGS = 2.0 V, VDS = 2.0 V.
FIGURE 4.15
Inspect showing VGS versus ID both in normal and logarithmic plots at VGS = 2.0 V, VDS = 2.0 V.
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tSub =50 tSi = 70
xsd = 50
Region 6
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Origin |
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TOX = 3 |
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Gate height = 20 |
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Region 8 |
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Region 9 |
Region 11 |
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Region 10 |
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Region 4 |
Region 3 |
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Region 5 |
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xsw = 100 |
Lg |
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xsw = 100 |
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Region 7 |
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Lg/2 |
Lg/2 |
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Region 2 |
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Region 1 |
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FIGURE 4.16
Construction of a proposed parameterized MOSFET.
FIGURE 4.17
Creation of Region 1 shown in Figure 4.16.
FIGURE 4.18
Creation of Region 2 shown in Figure 4.16.
FIGURE 4.19
Creation of Region 3 shown in Figure 4.16.
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FIGURE 4.20
Creation of Region 4 shown in Figure 4.16.
FIGURE 4.21
Creation of Region 5 shown in Figure 4.16.
FIGURE 4.22
Creation of Region 6 shown in Figure 4.16.
FIGURE 4.23
Creation of Region 7 shown in Figure 4.16.
FIGURE 4.24
Creation of Region 8 shown in Figure 4.16.
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FIGURE 4.25
Creation of Region 9 shown in Figure 4.16.
across the device are shown in Figures 4.13 and 4.14. The characteristic plots in normal and logarithmic scale are shown in Figure 4.15. The diagram of a proposed parameterized MOSFET is shown in Figure 4.16. Figures 4.17 to 4.27 show the construction steps of the 11 Regions of the parameterized MOSFET structure shown in Figure 4.16. The following command [1] will select a different place as a rectangular window, and doping in that window is possible according to the requirement and command:
(sdedr:define-constant-profile “ConstantProfileDefinition_1” “BoronActiveConcentration” 0.4e18) (sdedr:define-constant-profile-material “ConstantProfilePlacement_1”
“ConstantProfileDefinition_1” “Silicon”)
Here the profile name is “ConstantProfileDefinition_1” and the dopant is “BoronActiveConcentration” as boron is needed to be implanted in the bulk material whose doping concentration is 0.4e18 (0.4 × 1018) per cm3 and whose name is mentioned as “BoronActiveConcentration”. This
FIGURE 4.26
Creation of Region 10 shown in Figure 4.16.
FIGURE 4.27
Creation of Region 11 shown in Figure 4.16.
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“ConstantProfileDefinition_1” will be placed wherever it will find “Silicon” material [1]. In this way, the entire bulk material will be doped by boron with a doping concentration of 0.4 × 1018 per cm3.
(sdedr:define-refeval-window “RefEvalWin_1” “Rectangle” (position -0.1 0 0) (position -0.05 0.03 0)) (sdedr:define-constant-profile “ConstantProfileDefinition_2” “ArsenicActiveConcentration” 1e+20) position(sdedr:define-constant-profile-placement “ConstantProfilePlacement_2” “ConstantProfileDefinition_2” “RefEvalWin_1” 0 “Replace”)
Now source/drain doping is needed, and for this purpose two rectangular areas must be selected. For source doping, a rectangular window has been selected by the command line whose corner coordinates are (–0.1 0 0) and (–0.05 0.03 0). This is named “RefEvalWin_1” and a doping profile has been selected “ArsenicActiveConcentration” as Arsenic is required as a dopant whose doping concentration is 1020 per cm3, and finally this profile definition name is “ConstantProfileDefinition_2”. Similarly, using the command below, drain doping has been done. Here the “Replace” command [1] will replace previous boron doping by new arsenic doping with the mentioned doping profile.
(sdedr:define-refeval-window “RefEvalWin_2” “Rectangle” (position 0.05 0 0) (position 0.1 0.03 0)) position (sdedr:define-constant-profile “ConstantProfileDefinition_3” “ArsenicActiveConcentration” 1e+20) (sdedr:define-constant-profile-placement “ConstantProfilePlacement_3” “ConstantProfileDefinition_3” “RefEvalWin_2” 0 “Replace”)
4.3.2 Meshing
The input and output files of Sentaurus Structure Editor are:
• Scheme script file (.scm)
This is a user-defined script file that contains scheme script commands describing the steps to be executed by Sentaurus Structure Editor in creating a device structure. This file can be edited to change its contents.
• ACIS SAT file (.sat)
This file contains the model geometry in native ACIS format and cannot be edited directly.
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• DF-ISE boundary file (.bnd)
This is a boundary representation file written in the DF-ISE format. It can be directly loaded into Sentaurus Structure Editor and then to mesh engines.
• DF-ISE doping and refinement file (.cmd)
This is a DF-ISE format file containing doping and mesh refinement information that, in conjunction with the corresponding boundary file, uniquely defines the geometry of the model.
4.4 Mesh
Mesh Generation Tools is a suite of tools that produce finite-element meshes for use which are required in semiconductor device simulation or process simulation. Once the device structure is created, meshing is usually required before the device can be numerically solved for its electrical pro perties. The Mesh Generation Tools are composed of three mesh generation engines: Sentaurus Mesh, Noffset3D, and Mesh. The choice of which mesh generator to use in an application depends largely on the geometry of the device. These mesh generators generate high-quality spatial discretizations for 1D, 2D, and 3D devices using a variety of mesh generation algorithms and procedures. Meshing basically involves defining a meshing strategy where the maximum and minimum sizes of the meshes are defined. These definitions are then placed in a specific region that may be a material or a device region or a user-defined refinement/evaluation (Ref/Eval) window. Ref/Eval windows are areas in which a certain mesh refinement or doping profile is to be applied. In some cases, the mesher can be instructed to refine the mesh in areas of steep doping gradients or near interfaces. For example, in the channel of a MOS transistor, a dense meshing is suitable near the silicon-oxide interface. The tightness of the grid spacing may be relaxed toward the bulk. This keeps the problem at a minimum of central processing unit (CPU) time.
Files in the Mesh Generator tool:
Input files: *.bnd or *.tdr and *.cmd files from SSE
Output Files: *_msh.dat (contains the doping information), *_msh.grd (contains the mesh geometry information), and *_msh.log (used as log file)
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To summarize, after meshing, the device is divided into numbers of crisscross points (which depend on the size of the mesh). Every point contains the following information:
1.Location of the point
2.Material of the point
3.Doping concentration of the point
This information may also be stored in a single *_msh.tdr file instead of *_msh.grd and *_msh.dat to enable the user to input a single file instead of two files to the next stage (i.e., Simulation).
4.4.1 Design Continuation
The following commands are used to mesh the generated structure:
(sdedr:define-refeval-window “RefEvalWin_4” “Rectangle” (position -0.1 -0.08 0) (position 0.1 0.2 0)) ;global meshing
(sdedr:define-refinement-size “RefinementDefinition_1” 0.005 0.005 0 0.005 0.005 0)
(sdedr:define-refinement-placement “RefinementPlacement_1” “RefinementDefinition_1” “RefEvalWin_4”)
(sdedr:define-refeval-window “RefEvalWin_3” “Rectangle” (position -0.05 -0.002 0) (position 0.05 0.03 0))
(sdedr:define-refinement-size “RefinementDefinition_2” 0.001 0.001 0 0.001 0.001 0)
(sdedr:define-refinement-placement “RefinementPlacement_2” “RefinementDefinition_2” “RefEvalWin_3”)
(sde:build-mesh “mesh” “-P -discontinuousData -f -t -d -F tdr “ “crc”);
This last command will generate a file name crc_mesh.tdr which is required for further simulation of the device.
4.5 Sentaurus Device
Sentaurus Device is a comprehensive device simulator framework for simulating the electrical, thermal, and optical characteristics of silicon-based and compound semiconductor devices. The device simulation tool simulates the
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characteristics of the devices as a response to external electrical, thermal, or optical signals and boundary conditions.
4.5.1 Input-Output Files of the Tool
The input command file of Sentaurus Device consists of several command sections, each of which executes a relatively independent function. The default extension of this file is _des.cmd. The input command file typically consists of the following sections:
1.File
2.Electrode
3.Physics
4.Plot
5.Math
6.Solve
The File section describes the various input files and output files. The essential input file consists of the information regarding the device geo metry and field values (_msh.tdr), for example, the doping on the structure. In addition, an optional parameter file can be specified. The device geometry information consists of the regions and materials of the device, location of the contacts, and the mesh points including the location of nodes and vertices. The optional parameter file (.par) consists of the user-defined model parameters. The Sentaurus Device simulation tool produces several output files. The Current file contains the electrical output data, such as currents, voltages, and charges at each of the contacts. The default extension of this file is _des.plt. In addition, a log file is generated that contains all the informative texts that the tool has downloaded during a run, including the error messages. The default extension of this file is _des.log.
The Electrode section consists of the definitions of the various contacts of the device, together with their initial bias conditions. Any special boundary condition for a contact can also be defined here. It may be noted with care that each electrode defined here must match (case sensitive) an existing contact name in the structure file, and only those contacts that are named in the Electrode section are included in the simulation process.
The Physics section consists of a declaration of the physical models that are to be used in the simulation procedure. Typically it consists of the carrier mobility model, the band-gap narrowing model, the carrier generation and recombination model, etc. With the use of a qualifier in the Physics section, it can be specified in which material or regions the models are to be activated. For example, Material = “[material name]”, Region = “[region name]”.
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In this section, the models are declared on activation only. The model parameters, if different from the default, are defined and loaded using the optional Parameter file specified in the File section. The Physics section typical of a simple NMOSFET simulation is given here.
Physics {
Mobility (DopingDep HighFieldSat Enormal) EffectiveIntrinsicDensity(OldSlotboom)
}
The Plot section is used to specify the solution variables that are to be saved in the Plot file after the simulation process is completed. The solution of these variables can later be visualized using tools like Tecplot SV.
The Math section is used to control the numeric solver involved in the simulation process. A typical Math section is shown below which may be used as a guideline.
Math { Extrapolate
RelErrControl
NotDamped=50
Iterations=20
}
•Extrapolate: In quasi-stationary bias ramps, the initial guess for a given step is obtained by extrapolation from the solutions of the previous two steps (if they exist).
•RelErrControl: Switches error control during iterations from using internal error parameters to more physically meaningful parameters.
•NotDamped = 50: Specifies the number of Newton iterations over which the right-hand side (RHS) norm is allowed to increase. With the default of 1, the error is allowed to increase for one step only. It is recommended that NotDamped > Iterations be set to allow a simulation to continue despite the RHS-norm increasing.
•Iterations = 20: Specifies the maximum number of Newton iterations allowed per bias step (Default = 50). If convergence is not achieved within this number of steps, for a quasi-stationary or transient simulation, the step size is reduced by the factor decrement and simulation continues.
The Solve section defines a sequence of solutions to be obtained by the solver. To simulate the Id–Vg characteristic, it is necessary to ramp the gate bias and obtain solutions at a number of points. By default, the size of the step between solutions points is determined by Sentaurus Device. As the