nmos using devedit

SILVACO International 152

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Chapter 6: Creating NMOS Structure Using DEVEDIT

6.1 Overview of DEVEDIT DEVEDIT is a device structure editor. It can be used to generate a new mesh on an existing

structure, modify a device or create a device from scratch. These devices can then be used by

SILVACO 2-D and 3-D simulators. DEVEDIT can be used through a Graphical User

Interface (GUI) or as a simulator under DECKBUILD.

6.1.1 The Problem A limitation of device simulators prior to DEVEDIT was inadequate or poor structure meshes. A mesh containing too many obtuse triangles or an insufficient number of triangles

(too coarse a grid) may provide an inaccurate result or no result at all. A mesh containing too

many triangles (too fine a grid) can result in excessive simulator processing time. Since the

time most simulators use grows geometrically with the number of triangles (or grid points), it

is very critical to keep the number of triangles down to a reasonable number. Using

simulators like SSUPREM4 to create non-uniform meshes tend to be very time consuming

and require a great deal of effort.

6.1.2 The Solution DEVEDIT resolves these problems by allowing structures to be created or read into

DEVEDIT in the form of Silvaco Standard Structure Files. The mesh contained in the file can

then be replaced using the MeshBuild algorithm. Refinement of the mesh is accomplished by

setting parameters that describe critical areas or by simply pointing to the areas which require

refinement. In the process of creating a structure, definition of a device can be accomplished

by simply drawing it on the screen. DEVEDIT can also perform analytic implants using built-

in equations or cut lines from other simulators. Constraints are then placed on the mesh, to

describe the critical areas of the device.

6.1.3 When to Use DevEdit

Use DEVEDIT when you want to perform the following operations:

• Define a device interactively on the screen for subsequent device or process

simulations.

• Remesh a device structure between process simulation and device test simulations,

when the process simulator does not create a good grid for the device simulator.



• Remesh a device structure during a process or device simulation, when the mesh is no

longer adequate for the next simulation step.

6.1.4: When Not to Use DevEdit

You should not use DEVEDIT to perform the following operations:

• Replacing numerical process simulations where accuracy is required.

• Meshing 1D device structures.



6.2 Getting Started With DEVEDIT DEVEDIT can be run from the UNIX prompt or from DECKBUILD. There are two file types

which DEVEDIT can read: SILVACO standard structure file format (common to all Silvaco

simulators) and command format (a list of DEVEDIT commands which create a structure).

The structure file format contains such information as triangles, impurity values, borders, etc.

The command format in normally used when starting a device mesh from scratch. It contains

the list of instructions that describe the current state of mesh development.

To start DEVEDIT in GUI mode use the following commands for a UNIX prompt:

devedit &

and the DEVEDIT base window display will appear as shown in Figure 6-1.

Figure 6-1 Base Window Display.



6.3 Creating a MOS Device Using DEVEDIT

Creating a device begins with the setting of the work area. In doing this, the overall size of

the device is established. Since zooming is limited to about a fiftieth (1/50) of this size, it

should be made as small as possible. You may wish to press the “Grid”, which adds some

reference marks in the work area.

6.3.1 The Work Area

To set the work area:

1. Pull down the Regions menu by holding down the Menu (Right) mouse button over

“Regions” and select “Resize Work Area...” by releasing the mouse button over that

selection as shown below. A new panel will appear on the right side of the screen.

Figure 6-2 Resize Work Area...

2. Set minimum y to -1.0 (the semiconductor will start at y=0). Remember to hit the

[RETURN] button on the board after entering the value.

3. Set minimum x to 0.0.



4. Set maximum x and y to 2.0 as shown in Figure 6-2.

Figure 6-3 Setting Work Area.

5. Press Apply and the work area scale should now reflect the change as shown in Figure

6-4.

Figure 6-4 Resized Area



6.3.2 Adding a Silicon Base Region

To add a silicon base region:

1. Pull down the Regions menu and select Add Region... The “Add Region” panel will

appear as shown in Figure 6-5. By default, Silicon should already be selected as the

Material.

Figure 6-5 Add Region Panel

2. Draw the silicon region by moving the mouse in the drawing window to 0.0, 0.0. The

location of the mouse is shown on the right side of the screen in the panel list.

3. Press and release the left button of the mouse. As the mouse moves, there will be a

connection line to the first point. Move the mouse to 2.0, 0.0, and again, press and

release the left button of the mouse. Then repeat at 2.0, 2.0 and at 0.0, 2.0.

4. There should now be three sides of a rectangle. The fourth side will automatically be

added later (see Figure 6-6).



Figure 6-6 Drawing the Region

The base impurities can now be set:

5. Change the current editing mode by pressing the right mouse button above the “New

Polygon.” Move the mouse to the “Set Base Impurity” item as shown in Figure 6-7.

Then, release the button to select the item.



Figure 6-7 Selecting “Set Base Impurity”

6. Using the right mouse button, change the “Silicon Donor” to “Silicon Acceptor”

7. Enter 1e15 on the Boron line as shown in Figure 6-8.

8. Press Apply.



Figure 6-8 Setting Base Impurity.

The region appears in orange as in Figure 6-9.

Figure 6-9 The Silicon Region



6.3.3 Additional Regions

To add an insulator region:

1. Pull down the Regions menu and select Add Region....

2. Pull down the Materials menu and select Silicon Oxide.

3. Draw points at (0,0), (0,-0.1), (2,-0.1), and (2,0).

4. Press on the Apply button.

The Silicon Oxide region is as shown in Figure 6-10.

Figure 6-10 Adding a Silicon Oxide Region



6.3.4 Adding Contacts

To add some contacts, proceed as follows:


2. Select the Electrode check box.

3. Click on the standard “Electrode names” list and select “source”.

4. Pull down the Materials menu and select Aluminum.

5. Draw points at (0,0), (0,-0.2), (0.3,-0.2), and (0.3,0). Overlapping regions are not a

concern since the new region will etch away the old region.

6. Select the Electrode check box. Figure 6-11 shows the setting for adding the source

electrode.

Figure 6-11 Adding Source Electrode

7. Click on the Apply button and the source electrode will appear in the DEVEDIT base window as shown in Figure 6-12.



Figure 6-11 Source Electrode Region.

To add a second electrode:



10. Click on the standard “Electrode names” list and select “drain”. The rest of the

fields should already be set.

11. Draw points at (1.7,0), (1.7,-0.2), (2,-0.2), and (2,0).

12. Click on the Apply button and this creates the drain electrode as well as shown in

Figure 6-12.



Figure 6-12 Drain and Source Electrode.



6.3.5 Adding Substrate Contact

To add a substrate contact:

1. Pull down the Regions menu and select Substrate Electrode....

2. Toggle on the “Substrate Electrode Exists” check box as shown in Figure 6-13.

3. Press the Apply button when done.

Figure 6-13 Substrate Electrode Panel



6.3.6 Adding A Gate

Finally, to add a gate:



3. Click on the standard “Electrode names” list and select “gate”.

4. Pull down the Materials menu and select PolySilicon.

5. Draw points at (0.7,-0.1), (0.7,-0.5), (1.3,-0.5), and (1.3,-0.1).

6. Click on the “New Polygon” and select “Set Base Impurities.”

7. Set the Arsenic field to 1e+19, if desired as shown in Figure 6-14.

8. Click on the Apply button and the add polysilicon gate region will appear in

DEVEDIT Base Window as shown in Figure 6-15.

Figure 6-14 Setting the Polysilicon Gate Parameters.



Figure 6-15 The Polysilicon Gate Region.

6.3.7 Adding Some Doping

Now that the regions are complete, some impurities can be added:

1. Pull down the Impurities menu and select Add Impurity.... The Add Impurity panel

will appear on the right hand window.

2. In the Add Impurity panel, pull down the Impurity menu and select Arsenic.

3. Enter the points (0,0) and (0.4,0.0) as has been done for regions. This will be the peak

concentration area and there will be both a vertical and a lateral drop off formula.

4. Enter 1e+20 as the Peak Value and 1e+15 as the Reference Value. Since the base

impurity level in the silicon region is 1e+15, this reference values describes the

junction value for this impurity. See Figure 6-16 for the parameters set for the

impurities.



Figure 6-16 Add Impurity Panel

To set the roll off functions:

1. Pull down the Y Rolloff menu and select “Gaussian (Dist)” (not “Gaussian”). This

function will require a distance between the peak concentration and the reference

value.

2. Enter 0.5 in the Y “Distance” field.

3. Pull down the X Rolloff menu and choose “Error Function (Dist)”.

4. Enter 0.3 as the distance (see Figure 6-16).

5. Click on the Apply button and the added impurity will appear in appear in the “User

Added Impurities” list as shown in Figure 6-17.

To enter the same distribution under the drain electrode:

1. Pull down the Impurities menu and select Add Impurity….

2. Enter the points (1.6,0) and (2,0). The rest of the settings will remain the same.



3. Press the Apply button.

Figure 6-17 User Added Impurity.

To display these impurities:

1. Pull down the Show: Net Doping and choose “Fine” (or “Very Fine” if you have

time), the Net Doping will then appears as shown in Figure 6-18.

2. To add the legend of the contour plot, pull down the Contour Legend field and

choose a location for the legend. (Refer Figure 6-18)



Figure 6-18 Net Doping Example.

Note: Before proceeding set the Show Net Doping menu to Off to prevent slow repainting of

the screen after future functions. At a later point, Show Net Doping can be turned back on.

6.3.8 Creating the Mesh

Before creating the mesh, the meshing controls must first be set first. To do this,

1. Pull down the Mesh menu and choose Mesh Parameters... The Mesh Parameters

panel will appear.

2. Set the Base Mesh Height and Base Mesh Width to 0.2 Microns as shown in Figure

6-19.

3. Press on the Apply button. (The mesh is not created until later)



Figure 6-19 Mesh Parameters Panel.

Next, set the refinement parameters.

1. Pull down the Mesh menu and choose the Refinement On Quantities.... This panel

controls which areas will have a finer mesh, based on the value of the impurity.

2. The Min. Mesh Spacing field is set to 0.02 Microns by default.

3. For the Refinement Quantities field, pull down the Add menu and select Net

Doping as shown in Figure 6-20.

4. Click on the Done button.

Note: Refining on impurities only takes place in the Semiconductor region.

Finally, to create the mesh:

1. Pull down the Mesh menu and choose Meshbuild. DEVEDIT will now produce a mesh. A Cancel button is shown during meshing, in case an unreasonable meshing

parameter was supplied and it is readily apparent that too many triangles are being

produced.



Figure 6-20 Refinement on Quantities Panel.

Figure 6-21 Mesh Example.



6.3.9 Saving the Silvaco Standard Structure File

To save this file for use by simulators:

1. Pull down the “File” menu and choose “Save as...”.

2. Specify a name to save the file “mos.str” as shown in Figure 6-22.

Figure 6-22 Save DEVEDIT Structure File.

3. Click on the “Save Structure” button, which produces a file readable by all

SILVACO 2-D Simulators.

6.3.10 Saving the Devedit File

To save this file for later editing:

1. Pull down the File menu and choose Save as...

2. Specify the file as “mos.de”.

3. Click on the Save Commands button.

Note: Almost all files created by DEVEDIT should be saved as both a Command File and a

SILVACO Standard Structure File. If both exist, read the command file as it contains more

information about the structure than the SILVACO standard structure file.

To read in a command file, type:

devedit test.de &

Amit

Note

save this file as mos.str or nmos.str

nmos using devedit

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