TCAD Workshop Volume II - Printing - Weeblyonlinemanagementv2.weebly.com/.../5/8/9358123/tcad_workshop_volum… · SILVACO International 77 TCAD WORKSHOP Figure 3-11 The DECKBUILD:

SILVACO International 69

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Chapter 3: NMOS Device Simulation Using ATLAS 3.1 Overview of ATLAS

ATLAS is a physically-based two dimensional device simulator. It predicts the electrical behavior of specified semiconductor structures, and provides insight into the internal physical

mechanisms associated with device operation.

ATLAS can be used standalone or as a core tool in SILVACO’s VIRTUAL WAFER FAB simulation environment. In the sequence of predicting the impact of process variables on

circuit performance, device simulation fits between process simulation and SPICE model

extraction.

3.1.1 ATLAS Inputs and Outputs

Figure 3-1 shows the types of information that flow in and out of ATLAS. Most ATLAS

simulations use two inputs: a text file that contains commands for ATLAS to execute, and a structure file that defines the structure that will be simulated.

ATLAS produces three types of output. The run-time output provides a guide to the progress of simulations running, and it is where error messages and warning messages appear. Log

files store all terminal voltages and currents from the device analysis, and solution files store

two- and three dimensional data relating to the values of solution variables within the device

for a single bias point.

Figure 3-1 ATLAS Inputs and Outputs



3.1.2 The Order of ATLAS Commands

In ATLAS, each input file must contain five groups of statements in the correct order.

The order of these groups is as shown in Figure 3-2. Failure to do this will usually cause an

error message and termination of the program and it could lead to incorrect operation of the

program. For example, material parameters or models set in the wrong order may not be used

in the calculations.

Figure 3-2 ATLAS Command Groups with the Primary Statements in each Group

3.1.3 Getting Started With ATLAS

To start ATLAS under DECKBUILD, type:

deckbuild –as&

at the UNIX system command prompt. The command line option -as instructs DECKBUILD

to start ATLAS as the default simulator.

After a short delay, DECKBUILD will appear as shown in Figure 3-3. As can be seen from

the DECKBUILD output window, the command prompt is now ATLAS instead of ATHENA.



Figure 3-3 DECKBUILD Window with ATLAS simulator.

3.1.4 Defining Structure in ATLAS

A device structure can be defined in three different ways in ATLAS.

1. An existing structure can be read in from a file. The structure can have been created

by another program such as ATHENA or DEVEDIT.

2. The input structure can be transferred from ATHENA or DEVEDIT through the

automatic interface feature of DECKBUILD.

3. A structure can be constructed using the ATLAS command language.



The first and second methods are more convenient than the third and are to be preferred

whenever possible. In this tutorial, we shall use the second method to transfer an NMOS

structure from ATHENA into ATLAS using the auto interface feature of DECKBUILD.

3.2 ATLAS Simulation with NMOS Structure

In this tutorial, we shall demonstrates the following:

1. Generation of simple Id versus Vgs curve with Vds = 0.1V

2. Extraction of device parameters such as Vt, Beta and Theta.

3. Generation of Id versus Vds curves with Vgs = 1.1V, 2.2V and 3.3V

The NMOS structure that was created by ATHENA in Chapter 1 of the TCAD Workshop

Volume I will be used here to simulate the electrical characteristics of the NMOS device.

3.3 Creating ATLAS Input Deck File

To start the simulator with ALTAS, type the statement:

go atlas

To load the “nmos.str” structure file created by ATHENA:

a. From the ATLAS Commands menu, select Structure follows by the Mesh… item.

The ATLAS Mesh menu will popup as shown in Figure 3-5.

Figure 3-4 Invoking ATLAS Mesh Menu.

b. From the Type field, click on the “Read from file” box.



c. Next, specify the structure file name “nmos.str” in the File name field.

Figure 3-5 ATLAS Mesh Menu.

d. Press the WRITE button to write the MESH statement into DECKBUILD text window as shown in Figure 3-6.

Figure 3-6 MESH Statement written into DECKBUILD Text Window.

3.4 Models Specification Command Group

Since the NMOS structure has already been created in ATHENA, we shall skip the Structure Specification command group and start with the Models Specification command group. In

this command group, we have to specify the models, contact characteristics and interface

properties using the Model statement, Contact statement and Interface statement

respectively.

3.4.1 Specifying Models

For simple MOS simulation, the parameters SRH and CVT define the recommend model.

SRH is the Shockley-Read-Hall recombination model while CVT is the inversion layer model

from Lombardi (refer to ATLAS User’s Manual vol.1 pp. 3-43) and it sets a general purpose mobility model including concentration, temperature, parallel field and transverse field

dependence. To define the two models for the NMOS structure:

a. From the ATLAS Commands menu, select Models follows by Models … The Deckbuild: ATLAS Model menu will appear as shown in Figure 3-7.



Figure 3-7 The DECKBUILD: ATLAS Model Menu



a. Under the Category field, choose the “Mobility” model.

b. A list of mobility model appears and select CVT.

c. To print the model status while running in the runtime output region, click “Yes” for

Print Model Status option.

To change the default value of the CVT model (if needed):

1. Click on Define Parameters followed by CVT… The ATLAS Model – CVT menu

appears as shown in Figure 3-8.

2. Press Apply after modifying the parameters.

Figure 3-8 ALTAS Model – CVT Parameters Menu.

To add recombination model:

a. Choose the “Recombination” option in the Category field. Three different

recombination models will then appear as shown in Figure 3-9. These models are the

Auger, SRH (Fixed Lifetimes) and SRH (Conc. Dep. Lifetimes)



Figure 3-9 The Recombination Model

b. Select the SRH (Fixed Lifetimes) model for the NMOS structure.

c. Next, click on the Write button and the Model statements will appear in the

DECKBUILD text window as shown in Figure 3-10.

Figure 3-10 The Model Statements

3.4.2 Specifying Contact Characteristics

An electrode in contact with semiconductor material is assumed by default to be ohmic. If a

work function is defined, the electrode is treated as a Schottky contact. The Contact statement is used to specify the metal workfunction of one or more electrodes. To specify the

workfunction of the n-type polysilicon gate contact using the Contact statement:

a. Select Models ⇒⇒⇒⇒ Contacts … from the ATLAS Commands menu. The

DECKBUILD: ATLAS Contact menu appears.

b. Enter ‘gate’ for the Electrode name field.

c. Then, select n-poly for n-type polysilicon as shown in Figure 3-11.



Figure 3-11 The DECKBUILD: ATLAS Contact Menu

d. Click the Write button and the following statement will appear in the input file:

contact name=gate n.poly

3.4.3 Specifying Interface Properties

To specify the interface properties of the NMOS structure, the Interface statement is used.

This statement is used to define the interface charge density and surface recombination

velocity at interfaces between semiconductors and insulators. To defines a fixed charge of 3 x

1010 cm

2 at the interface between silicon and oxide:

1. Select Models ⇒⇒⇒⇒ Interface… from the ATLAS Commands menu. The

DECKBUILD: ATLAS Interface menu appears.

2. Enter ‘3e10’ for the Fixed Charge Density field as shown in Figure 3-13.

3. Press the WRITE button to write the Interface statement into DECKBUILD text window.



The Interface statement should appears as:

interface s.n=0.0 s.p=0.0 qf=3e10

Figure 3-12 The DECKBUILD: ATLAS Interface Menu

3.5 Numerical Methods Selection Command Group Next, we shall select the type of numerical methods to be used for the simulation. Several

different methods can be used for calculating the solutions to semiconductor device

problems. For the MOS structure, the de-coupled (GUMMEL), and fully coupled

(NEWTON) methods are used. In simple terms, the de-coupled technique like the Gummel

method will solve for each unknown in turn keeping the other variables constant, repeating

the process until a stable solution is achieved. Fully coupled techniques such as the Newton

method solve the total system of unknowns together. The Method statement can be included

by:

a. Select Solutions ⇒⇒⇒⇒ Method … from the ATLAS Commands menu. The Deckbuild: ATLAS Method menu appears.

b. Select Newton and Gummel option in the Method field as shown in Figure 3-13.

c. The default setting of the maximum number of iterations is 25. Change this value if

necessary.

d. Press the WRITE button to write the Method statement into DECKBUILD text window.



Figure 3-13 The DECKBUILD: ATLAS Method Menu

e. The Method statement appears is as shown in Figure 3-14. This statement will cause

the solver to start with Gummel iterations and then switch to Newton, if convergence

is not achieved.

Figure 3-14 The Method Statement



3.6 Solution Specification Command Group In the Solution Specification command group, we need to include the Log statement to save

the log files which contains all the terminal characteristics calculated by ATLAS, the Solve statement for solving different bias conditions and the Load statement for loading the

solution files. All these can be done using the Deckbuild: ATLAS Test menu.

3.6.1 Obtaining Id versus Vgs curve with Vds = 0.1 V

In this tutorial, we wish to obtain a simple Id versus Vgs curve with Vds = 0.1 V for the NMOS

structure. To accomplish this:

a. From the ATLAS Commands menu, select the chain Solutions ⇒⇒⇒⇒ Solve…. The Deckbuild: ATLAS Test menu as shown in Figure 3-15.

Figure 3-15 Deckbuild: ATLAS Test menu.

b. Click on the Prop… button to invoke the ATLAS Solve properties menu.

c. Change the file name in the Log file field to “nmos1_” as shown in Figure 3-16.

Click OK when done.



d. Next, move the mouse within the Worksheet region, right click on the mouse and

select “Add new row” as shown in Figure 3-17.

Figure 3-16 ATLAS Solve properties menu.

Figure 3-17 Adding New Row.

e. A new row will be added in the worksheet as shown.



Figure 3-18 A New Row Added.

f. Move the mouse over the “gate” parameter, follow by right clicking on the mouse. A

list of electrode names will appear. Select the “drain” as shown below.

Figure 3-18 Changing “Gate” to “Drain.

g. Click on the value under the Initial Bias column and change it to 0.1 follows by the

WRITE button.

h. Next, move the mouse within the worksheet region again, right click on the mouse

and select “Add new row” again.

i. This adds another new row under the drain row as shown below.



Figure 3-19 Adding Another Row.

j. For the “gate” row, move the mouse over the CONST type, right click the mouse and

select “VAR1”. Change the Final Bias value to 3.3 and Delta value to 0.1 as shown in

Figure 3-20.

Figure 3-20 Set Gate Bias Parameters.



k. Click the WRITE button and the following statements will then appears in

DECKBUILD text windows as shown in Figure 3-21:

solve init solve vdrain=0.1 log outf=nmos1_0.log solve name=gate vgate=0 vfinal=3.3 vstep=0.1

The statements as shown above start with the “solve init” statement. This statement provides

an initial guess for the potential and carrier concentrations at the zero bias (or thermal

equilibrium) case.

After solving for the zero bias condition, the second Solve statement i.e. “solve vdrain=0.1”

will perform a dc bias of 0.1V at the drain electrode. When the voltage on a particular

electrode is never defined on any Solve statement, that voltage is zero. Therefore, it is not

necessary to explicitly state the voltage on all electrodes on all Solve statements.

The third statement is a Log statement i.e. log outf=nmos1_0.log. This statement is use to

save all the simulation results calculated by ATLAS in the nmos1_0.log log file. These results include the current and voltages for each electrode in the DC simulations. To stop the

terminal characteristics being saved to this file use another Log statement with the “off”

option i.e. log off or with a different log filename.

Finally, the last Solve statement i.e. solve name=gate vgate=0 vfinal=3.3 vstep=0.1 will

ramp the gate voltage from 0V to 3V with a bias step size of 0.1V. Note that the name

parameter in this statement is required and the electrode name is case-sensitive.

Figure 3-21 Statements used for simulating Id versus Vgs curve with Vds = 0.1 V



3.6.2 Extracting Device Parameters

In this workshop, the second objective is to obtain some of the device parameters such as Vt,

Beta and Theta. This can be done using the ATLAS Extract menu as follows:

a. From the ATLAS Commands menu, select the chain Extract ⇒⇒⇒⇒ Device…. The Deckbuild: ATLAS Extraction menu appear as shown in Figure 3-22.

Figure 3-22 The Deckbuild: ATLAS Extraction menu.

b. By default, Vt is being select in the Test name field. Also, user are allows to modify

the default extract expression.

c. Click on the WRITE button and the Vt Extract statement will then appear in

DECKBUILD text windows as shown:

extract name="vt"(xintercept(maxslope(curve(abs(v."gate") , abs(i."drain")))) - abs(ave(v."drain"))/2.0)



d. Next, invoke the Deckbuild: ATLAS Extraction menu again. Then, click on the Test

name field and change it to Beta as shown in Figure 3-23

Figure 3-23 Setting Extract Statement for Beta.

e. Click on the WRITE button and the Beta Extract statement will then appears in


extract name="beta" slope(maxslope(curve(abs(v."gate") ,abs(i."drain")))) * (1.0/abs(ave(v."drain")))

f. Finally, we will invoke the Deckbuild: ATLAS Extraction menu again to set the

Extract statement for extracting the theta parameter. Then, click on the Test name

field and change it to Theta as shown in Figure 3-24.

Figure 3-24 Setting Extract Statement for Theta.

g. Click on the WRITE button and the Theta Extract statement will then appears in


extract name="theta" ((max(abs(v."drain")) * $"beta")/max (abs(i."drain")))-(1.0 / (max(abs(v."gate")) - ($"vt")))

Before running the simulation, we need to use the Tonyplot statement to plot the simulation

results. To automatically plot the Id versus Vgs, simply type in the following Tonyplot

statement after the last Extract statement as follows:

tonyplot nmos1.log



Now, we can start running the simulation. Press the run button on the

Deckbuild control to start the device simulator.

Once the simulation is completed, TONYPLOT will automatically be invoked with the Id versus Vgs characteristics as shown in Figure 3-25.

Figure 3-25 A Plot of Id versus Vgs for the NMOS device.

Also, the extracted device parameters i.e. Vt, Beta and Theta can be seen from the runtime

output window of DECKBUILD as shown in Figure 3-26.



Figure 3-26 DECKBUILD Runtime Output showing the Extracted Device Parameters.



3.6.3 Generating Families of Curves Using Log, Solve and Load Statements

The next objective of this tutorial is to generate families of Id versus Vds curves with Vgs = 1.1

V, 2.2 V and 3.3 V and Vds is ramp from 0 V to 3.3 V. In order to stop the previous

“nmos1.log” from saving the later terminal characteristics, we need to add another Log

statement as follows:

log off

To generate the family of curves, first, we need to obtain the solution for each Vgs using the

Deckbuild: ATLAS Test menu as follow:

a. From the ATLAS Commands menu, select the chain Solutions ⇒⇒⇒⇒ Solve… to invoke the Deckbuild: ATLAS Test menu.

b. Click on the Prop… button to invoke the ATLAS Solve properties menu.

c. Change the Write mode field to “Line” as shown below and click OK when done.

Figure 3-27 Change Write mode field to “Line” mode.

d. Set the following parameters for biasing the gate as shown in Figure 3-28.

Figure 3-28 Parameters for Biasing the Gate.

e. Click on the WRITE button and the Solve statement will then appears in DECKBUILD text windows as shown:



solve vgate=1.1

In order to save the output from this solution in an ATLAS solution file, add-in the

“outf=solve1” option in the Solve statement as shown:

solve vgate=1.1 outf=solve1

f. Repeat the above for gate bias = 2.2V and 3.3V and save the solution file as

“solve2 and “solve3”. The following Solve statements shown be obtained:

solve vgate=2.2 outf=solve2 solve vgate=3.3 outf=solve3

Next, we will use the ATLAS Test menu again to set the Solve statement for ramping the

drain voltage from 0 V to 3.3 V. To do so,

a. From the ATLAS Commands menu, select the chain Solutions ⇒⇒⇒⇒ Solve… to invoke the Deckbuild: ATLAS Test menu.

b. From the ATLAS Test menu, click on the Prop… button to invoke the ATLAS Solve

properties menu.

c. Change the Write mode field to “Test” as shown.

d. Change the file name in the Log file field to “nmos2_” as shown in Figure 3-29.

Figure 3-29 Setting Solve Properties.

e. Click OK when done.

f. In the worksheet area, change the “Name” column from “gate” to “drain”, the Type

column from “CONST” to “VAR1”, the “Initial Bias” column to 0, “Final Bias”

column to 3.3 and “Delta” column to 0.3 as shown in Figure 3-30.



Figure 3-30 Setting Drain Bias Condition Using ATLAS Test Menu.

g. Click on the WRITE button and the following statement will then appears in


solve init log outfile=nmos2_0.log solve name=drain vdrain=0 vfinal=3.3 vstep=0.3

Next, we will load in the solution file i.e. “solve1” of the gate bias at 1.1V using the Load

menu and replace it with the “solve init” statement as follows:

a. Highlight the “solve init” statement as shown in 3-31.

b. From the ATLAS Commands menu, select the chain Solutions ⇒⇒⇒⇒ Load… to invoke the Deckbuild: ATLAS Load menu.



Figure 3-31 Highlight the “solve init” statement.

c. From the Deckbuild: ATLAS Load menu, enter “solve1” for the File name field.

d. Next, from the Format field, select the SPISCES format.

Figure 3-31 Setting Load Statement Parameters.

e. Click on the WRITE button and a prompt will appears as shown in Figure 3-32. Click

on “Yes, replace selection” button.



Figure 3-32 Replace Current Selection Prompt.

f. The Load statement will then replace the “solve init” statement in the DECKBUILD text windows as shown below.

Figure 3-33 Load Statement Replacing Solve Init Statement.

Therefore, the statements starting from the Load statement i.e. “load infile=solve1” to the

Solve statement will generate the data of Id versus Vds for Vgs = 1.1V. To generate the data of

Id versus Vds for Vgs = 2.2V and 3.3V, simply copy these three statements and:

a. Change the input file name in the Load statement from “solve1” to “solve2” and

“solve3”.

b. Change the log file name in the Solve statement from “nmos2_0.log” to

“nmos3_0.log” and “nmos4_0.log”



c. The resulting statements are as shown below.

Figure 3-34 Statements for generating the data of Id versus Vds for Vgs = 2.2V and 3.3V.

To plot the family of curves i.e. plot all 3 log files results in a single plot, type the following

Tonyplot statement:

tonyplot –overlay nmos2_1.log nmos2_2.log nmos2_3.log -set nmos.set

In this statement, the –overlay option is to overlay all the 3 log files in a single plot and the –

set option is used to load the set file and restore the display to the condition that TONYPLOT was in when that set file was created.

3.6.4 Quitting from the Simulation Finally, type the statement:

quit

to quit from the simulation.



You can now continue running the device simulation by pressing the Cont button

on the Deckbuild control. Once the simulation is completed, TONYPLOT will automatically be invoked with the families of Id versus Vds characteristics as shown in Figure

3-35.

Figure 3-35 Families of Id versus Vds curves for NMOS.



Chapter 4: Creating a Polysilicon Emitter NPN BJT Device Structure using ATHENA

4.1 Overview of the Procedure

In this session, we shall create the input deck file to simulate a Polysilicon Emitter NPN

bipolar device. The process flow of the NPN bipolar transistor is summarized as follows:

a. First, a boron implant is carried out to forms the intrinsic base region;

b. A second boron implant is self-aligned to the polysilicon emitter region to form a

connection between the intrinsic base and p+ base contact regions;

c. Spacer-like structures are used on the side of the poly emitter to space the p+ base

contact and to provide self-alignment;

d. Only half of the full device is simulated. Mirroring of this half device into a full

structure is done with structure mirror left;

e. The final stage of the ATHENA syntax defines the electrode positions.

4.2 Creating An Initial Structure

First, we will create a non-uniform grid in a rectangular 0.8 µm by 1.0 µm simulation area.

With the help of the ATHENA Mesh Define menu, define the non-uniform rectangular as

follows as shown in Figure 4-1.

Figure 4-1 Defining Grids in the X Direction.

Next, click on the Y Direction and define the grids in the y direction as shown in Figure 4-2.



Figure 4-2 Defining Grids in the Y Direction.

Once done, click on the View... button on the Mesh Define menu to preview the rectangular

grid. The View Grid window will be displayed as shown in Figure 4-3. (Notice that a total of

782 points and 1452 triangles are generated.)

Figure 4-3 Previewing the Grids.

Finally, write mesh define information to the DECKBUILD text window by pressing on the

WRITE button on the Mesh Define menu. A set of lines will appear as shown below:

go athena # Polysilicon Emitter Bipolar (NPN)



line x loc=0.00 spac=0.03 line x loc=0.2 spac=0.02 line x loc=0.24 spac=0.015 line x loc=0.3 spac=0.015 line x loc=0.8 spac=0.15 # line y loc=0.00 spac=0.01 line y loc=0.12 spac=0.01 line y loc=0.3 spac=0.02 line y loc=0.5 spac=0.06 line y loc=1.0 spac=0.35

4.3 Defining the Initial Substrate

Next, we shall initialize the substrate region using the ATHENA Mesh Initialize menu. Here,

we wish to initialize the simulation structure as the Silicon Material with <100> orientation

and a background doping of 2 x 1016 atom/cm

3 of Arsenic. To initialize the simulation

structure, set the initialization parameters in the Mesh Initialize menu as shown in Figure 4-4

Figure 4-4 Setting the Initialization Parameters of the BJT.



Note that for the Dimensionality field, the default “Auto” is being used. Once the

parameters have been set, 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:

# Initial Silicon Structure init silicon c.arsenic=2e16 orientation=100

In this tutorial, we will save all the simulated structures for individual process steps. To do

so, the ATHENA File I/O menu will be used. Select the Save button and specify a file name

“init_bjt.str” as shown in Figure 4-5.

Figure 4-5 Saving Structure File.

Press the WRITE button and the following line will appears in the input file:

struct outfile=init_bjt.str

Run the input deck file by pressing the run button on the DECKBUILD window. Then, plot the mesh of the initial structure by highlighting the “init_bjt.str” structure file, and

then click Tools ⇒ Plot ⇒ Plot Structure.... TONYPLOT will appear.

To plot the mesh, invoke the Display (Cross Section) menu by clicking Plot ⇒ Display …,

then click on the mesh icon as shown in Figure 4-6 follows by the Apply button.

Figure 4-6 Display (Cross Section) menu.



The initial triangular grid will appears as shown in Figure 4-7. As the “two.d” option was not included, all the process steps were done in a one-dimensional calculation as shown in

Figure 4-7.

Figure 4-7 BJT Initial Triangular Grids.

4.4 Boron Implantation

Next, we will perform an ion implantation using Boron with a dose of 2.5 x 1013 cm

-2 at an

implantation energy of 18 keV to form the intrinsic base region of the bipolar transistor.

To perform the implantation step, the ATHENA Implant menu as shown in Figure 4-8 will be

used. Define the implantation parameters in the Implant menu as show in this figure. Once

done, click on the WRITE button and the following Implant statement will then be

added into the input file:

# Implant Boron implant boron dose=2.5e13 energy=18 crystal



Figure 4-8 Implant Parameters for first Boron Implantation.

The implantation step is followed by a diffusion of 60 minutes at 920oC and 1 Atmospheric

pressure. To perform this diffusion step, invoke the ATHENA Diffuse menu and define the

parameters as shown in Figure 4-9.

Figure 4-9 Diffuse Parameters for first Boron Implantation.




# Diffuse Boron diffus time=60 temp=920 nitro

Next, invoke the ATHENA File I/O menu to save the structure of the first boron implantation

step. Specify the file name as “implant1_bjt.str” (see Figure 4-10).

Figure 4-10. Saving “implant1_bjt.str” file.


struct outfile=implant1_bjt.str

To continue simulation without re-running from the start,

a. Highlight or place the cursor on the INIT statement (i.e. init silicon

c.arsenic=2e16 orientation=100) as shown in Figure 4-11.

Figure 4-11 Highlighting the INIT statement.



b. Next, click on the Initialize button on DECKBUILD Command Bar. This automatically output an initialize statement which invokes the input file

“.history01.str” in the DECKBUILD output window as shown below.

Figure 4-12 Initialize Input History File.

c. Click on the Cont button on the DECKBUILD control. The simulation continues as shown:

Figure 4-13 Continue Simulation of DECKBUILD.

d. Highlight the “implant1_bjt.str” structure file and plot it using TONYPLOT as shown in Figure 4-14.



Figure 4-14 Forming the Intrinsic Base Region of the Bipolar Transistor.

4.5 Deposit Polysilicon

The next step is to perform conformal deposition of polysilicon for the polysilicon emitter

region. Here, a polysilicon region of 0.3 µm thickness is deposited using the ATHENA

Deposit menu.

Set the deposition parameters for polysilicon as shown in Figure 4-15 and press the WRITE

button once completed. The following line will appears in the input file:

# Deposit Polysilicon deposit polysilicon thick=0.3 divisions=6

Next, invoke the ATHENA File I/O menu to save the structure of the polysilicon deposition

step and specify the file name as “poly_bjt.str” (see Figure 4-16).



Figure 4-15 Deposition Parameters for BJT.

Figure 4-16 Saving “implant1_bjt.str” file.


struct outfile=poly_bjt.str

Continue the simulation by highlighting or placing the cursor on the Diffuse statement i.e.

diffus time=60 temp=920 nitro, follows by clicking on the init and

Cont button on DECKBUILD Command Bar and plot the structure as shown in Figure 4-17.



Figure 4-17 Depositing Polysilicon for the Emitter Region.

4.6 Dope Polysilicon

After depositing polysilicon, the next step is to dope the polysilicon with Arsenic. Here, the

implantation is carried out with a high dose of 7.5 x 1015 cm

-2 Arsenic at an implantation

energy of 50 keV. Using the ATHENA Implant menu, set the implantation parameters to

implant Arsenic as shown in Figure 4-18.

Once done, press the WRITE button and the following line will appears in the input file:

# Implant to Dope Polysilicon implant arsenic dose=7.5e15 energy=50 crystal

Save the structure again by invoking the ATHENA File I/O menu and specify the file name as

“dopepoly_bjt.str” as shown in Figure 4-19.



Figure 4-18 Implantation Parameters for Doping Polysilicon.

Figure 4-19 Saving “dopepoly_bjt.str” file.


struct outfile=dopepoly_bjt.str

Continue the simulation by highlighting or placing the cursor on the Deposit statement i.e.

deposit polysilicon thick=0.3 divisions=6, follows by clicking on the init

and Cont button on DECKBUILD Command Bar and plot the structure as shown in Figure 4-20.



Figure 4-20 Doping Polysilicon with Arsenic.

4.7 Etching Polysilicon The next step in the process simulation is the polysilicon emitter definition. In this tutorial,

we will use a polysilicon emitter edge at x = 0.2 µm and set the center of the emitter at x =

0.0 µm for the initial grid. Therefore, polysilicon should be etched to the right from x = 0.2 as

shown in Figure 4-23.

To perform the geometrical etch, invoke the ATHENA Etch menu and set the etch parameters

as shown in Figure 4-21. When this is done, click on the WRITE button and the following line

will appears in the input file:

# Pattern Polysilicon etch polysilicon right p1.x=0.2



Figure 4-21 Defining Parameters for Etch Polysilicon.


“etchpoly_bjt.str” as shown in Figure 4-22.

Figure 4-22 Saving “etchpoly_bjt.str” file.


struct outfile=etchpoly_bjt.str

Continue the simulation by highlighting or placing the cursor on the Implant statement i.e.

implant arsenic dose=7.5e15 energy=50 crystal, follows by clicking on

the init and Cont button on DECKBUILD Command Bar and plot the structure as shown in Figure 4-23.



Figure 4-23 Etch Polysilicon.

4.8 Performing Polysilicon Oxidation

After etching the polysilicon, the next step is to perform polysilicon oxidation. The oxidation

recipe is dry oxidation for 25 minutes at 920oC and 1 Atmospheric pressure. To perform this

oxidation step, use the ATHENA Diffuse menu to define the oxidation parameters as shown

in Figure 4-24.

Click on the WRITE button when finished and the following line will appears in the input file:

# Polysilicon Oxidation method fermi compress diffus time=25 temp=920 dryo2 press=1.00

From the method statement, the fermi method is used for undamaged substrates with doping

concentrations < 1x1020 cm

-3 whereas the compress method is used to model oxidation on

non-planar structures and for 2-D oxidation.



Figure 4-24 Parameters defined for Polysilicon Oxidation.


“etchpoly_bjt.str” as shown in Figure 4-25.

Figure 4-25 Saving “polyox_bjt.str” file.


struct outfile=polyox_bjt.str



Continue the simulation by highlighting or placing the cursor on the Etch statement i.e.

etch polysilicon right p1.x=0.2, follows by clicking on the init and

Cont button on DECKBUILD Command Bar and plot the structure as shown in Figure 4-26.

Figure 4-26 Oxide Formed after the Oxidation Step.

4.9 Annealing Process

After the oxidation process, we will perform an annealing process. This annealing process is

carried out for 50 minutes at a temperature of 900oC in the nitrogen gas at 1 atmospheric

pressure. To perform this annealing step, use the ATHENA Diffuse menu again to define the

annealing parameters as shown in Figure 4-27.


# Annealing Process diffus time=50 temp=900 nitro press=1.00



Figure 4-27 Parameters defined for Annealing Process.

Save the structure and specify the file name as “anneal_bjt.str” as shown in Figure 4-28.

Figure 4-28 Saving “anneal_bjt.str” file.


struct outfile=anneal_bjt.str




diffus time=25 temp=920 dryo2 press=1.00, follows by clicking on the init

and Cont button on DECKBUILD Command Bar.

Next, we would like to see the change in the Net Doping before and after the annealing

process. To do this, first plot the annealed structure i.e. anneal_bjt.str and then plot the Net Doping of the current structure as shown in Figure 4-29.

Figure 4-29 After Annealing Process

Next, load the file “polyox_bjt.str” and overlay it onto the “anneal_bjt.str” using the File

menu in TONYPLOT. The overlay structure plots are as shown in Figure 4-30. After the two structure plots are overlay onto each other, perform a cutline using the Cutline menu in

TONYPLOT.

Then, click on the keyboard icon and enter the following values for X and Y as shown

in Figure 4-31.



Figure 4-30 Overlaying “polyox_bjt.str” onto “anneal_bjt.str”.

Figure 4-31 Specifying Cutline Location using the Keyboard Option.

Once done, hit the [RETURN] button on keyboard and TONYPLOT will prompt you for confirmation as shown in Figure 4-32. Click on the Confirm button.



Figure 4-32 Confirm Cutline

This result in the generation of a one-dimensional plot on the right hand side as shown in

Figure 4-33. From Figure 4-33, it can be seen that the doping profile has been broaden by

diffusion after the annealing process.

Figure 4-33 Net Doping Before and After Annealing Process.



4.10 Second Boron Implantation

Next, we will perform another Boron implantation with a dose of 2.5 x 1013 cm

-2 at an

implantation energy of 18 keV. This second Boron implant is self-aligned to the polysilicon

emitter region to form a connection between the intrinsic base and p+ base contact regions.

To perform this implantation step, use the ATHENA Implant menu to set the Implant

statement as shown in Figure 4-34.

Figure 4-34 Implantation Parameters for Second Boron Implant.


# Second Boron Implantation implant boron dose=2.5e13 energy=18 crystal


“implant2_bjt.str” as shown in Figure 4-35.






diffus time=50 temp=900 nitro press=1.00, follows by clicking on the init

and Cont button on DECKBUILD Command Bar and plot the structure as shown in Figure 4-36.


Figure 4-36 Net Doping After Second Boron Implantation.



4.11 Spacer Formation

We will now define a spacer-like structure on the side of the poly emitter to space the p+ base

contact and also to provide self-alignment. This done by first depositing a layer of 0.4 µm

thick oxide and a grid setting of 10 using the ATHENA Deposit menu as shown below.

Figure 4-37 Parameters Setting for Spacer Deposition.

Click on the WRITE button and the following line will appears in the input file:

# Deposit Spacer deposit oxide thick=0.4 divisions=10

Save the structure using the ATHENA File I/O menu and specify the file name as

“depositspacer_bjt.str” as shown in Figure 4-38.


struct outfile=depositspacer_bjt.str



Figure 4-38 Saving “depositspacer_bjt.str” file.

Next, the spacer is dry etch by 0.5 µm thick to form the poly emitter side wall using the

ATHENA Etch menu as shown below.

Figure 4-39 Parameters Setting for Etching Spacer.


# Etch the Spacer Back etch oxide dry thick=0.50


“etchspacer_bjt.str” as shown in Figure 4-40. Press the WRITE button and the following

line will appears in the input file:



struct outfile=etchspacer_bjt.str

Figure 4-40 Saving “etchspacer_bjt.str” file.


implant boron dose=2.5e13 energy=18 crystal, follows by clicking on the

init and Cont button on DECKBUILD Command Bar and plot the structure as shown in Figure 4-41.

Figure 4-41 Forming the Spacer.



4.12 Third Boron Implant with Annealing

A third boron implant is carried out to form the p+ base region. In this implantation process,

boron with a high dose of 1 x 1015 cm

-2 is implanted using an energy of 30 keV. The

command statement is set using the ATHENA Implant menu again as shown in Figure 4-42.

Figure 4-42 Setting Parameters for Third Boron Implant.


# Forming P+ Base Region implant boron dose=1.0e15 energy=30 crystal


“implant3_bjt.str” as shown in Figure 4-43. Press the WRITE button and the following line






Continue the simulation by highlighting or placing the cursor on the Etch statement i.e.

etch oxide dry thick=0.50, follows by clicking on the init and Cont

button on DECKBUILD Command Bar and plot the Net Doping and Junction of the structure as shown in Figure 4-44.

Figure 4-44 Forming the p+ Base Region.



After the Boron implantation, it is follow by another annealing process using the a

temperature of 900oC for a period of 60 minutes in Nitrogen gas. To perform this annealing

process, use the ATHENA Diffuse menu to set the Diffuse statement as shown below.

Figure 4-45 Setting Parameters for Second Annealing Process.


# Second Annealing Process diffus time=60 temp=900 nitro press=1.00

Save the structure again using the ATHENA File I/O menu and specify the file name as

“anneal2_bjt.str” as shown in Figure 4-46. Press the WRITE button and the following line


struct outfile=anneal2_bjt.str


implant boron dose=1.0e15 energy=30 crystal, follows by clicking on the

init and Cont button on DECKBUILD Command Bar and plot Net Doping and Junction of the structure as shown in Figure 4-46.



Figure 4-46 Saving “anneal2_bjt.str” file.

Figure 4-47 Net Doping of the Structure After Second Annealing Process.

Again, we would like to see the change in the Net Doping before and after the annealing

process. To do so, load the file “implant3_bjt.str” and overlay it onto the “anneal2_bjt.str”

using the File menu in TONYPLOT. Then, perform a vertical cutline at Start X=0.7, Y=0 and End X=0.7 and Y=1. The one-dimensional plot will appear on the right hand side as shown in

Figure 4-48.



Figure 4-48 Net Doping of the Base Region Before and After Second Annealing Process.

From the above plot, it can be seen that the P+ base region has been broaden by diffusion

after the second annealing process but with the junction still remaining at the same depth.

4.13 Reflecting the Half Structure in the “Y” plane

We have been building one half of a NPN Bipolar Transistor. After defining all the important

regions in the device structure, it is necessary to obtain the full transistor. To obtain the full

bipolar transistor structure, mirror it at its left boundary using the ATHENA Mirror menu as

shown below.

Figure 4-49Reflecting the half BJT Structure using Mirror Menu.




struct mirror left


“full_bjt.str” as shown in Figure 4-50.

Figure 4-50 Saving “full_bjt.str” file.


struct outfile=full_bjt.str


diffus time=60 temp=900 nitro press=1.00, follows by clicking on the init

and Cont button on DECKBUILD Command Bar and plot Net Doping and Junction of the full structure as shown in Figure 4-51.



Figure 4-51 Net Doping and Junction of the Full BJT Structure.

4.14 Forming Emitter and Base Contacts After obtaining the full BJT structure, the next step is to deposit and etch the aluminum on

the structure surface so as to form the Emitter and Base contacts. In this tutorial, first, a layer

of aluminum with a thickness of 0.05 µm will be deposit using ATHENA Deposit menu as


Click on the WRITE button when done and the following line will appears in the input file:

# Put down Aluminum and Etch to form the Emitter/Base Contacts deposit aluminum thick=0.05 divisions=2


“anneal2_bjt.str” as shown in Figure 4-53.



Figure 4-52 Defining Parameters for setting Aluminum Deposition.

Figure 4-53 Saving “deposit_Al_bjt.str” file.


struct outfile=deposit_Al_bjt.str

Continue the simulation by highlighting or placing the cursor on the Structure statement i.e.

struct mirror left, follows by clicking on the init and Cont

button on DECKBUILD Command Bar and plot the structure with mesh as shown in Figure 4-54.



Figure 4-54 Depositing Aluminum on the BJT Full Structure.

Next, we need to etch the aluminum to form the emitter and base contacts. The portion of

aluminum to be etch away is indicated by the shaded boxes as shown in Figure 4-55.

Figure 4-55 Aluminum in the Shaded Regions Are to be Etch Away.

Point 2

(x=-0.16,y=0.1) Point 3

(x=-0.6,y=0.1)

Point 1

(x=-0.16,y=-0.35)

Point 4

(x=-0.6,y=-0.35)

Base

Emitter



First, we will perform etching to the left hand side shaded box defined at (-0.16, -0.35), (-

0.16, 0.1), (-0.6, 0.1) and (-0.6, -0.35). Use the ATHENA Etch menu, define these location as


Figure 4-56 Setting Parameters to Etch to the Left.

Once finished, click on the WRITE button and the following line will appears in the input file:

# Etching the Left Hand Side Shaded Region etch aluminum start x=-0.16 y=-0.35 etch cont x=-0.16 y=0.1 etch cont x=-0.6 y=0.1 etch done x=-0.6 y=-0.35

The first Etch statement begins with the start option indicating the starting point (at x = -

0.16, y = -0.35) of the etch process and alumimum indicates that only this material will be

etch. Therefore, other materials such as the silicon, polysilicon and silicon dioxide will not be

etched.

The second and third Etch statements are to specify the second and third coordinate points of

the shaded box. The continue option is used in both statements. The last coordinate point is

defined together with the done option in the last Etch statement.


“etchleft_bjt.str” as shown in Figure 4-57.



Figure 4-57 Saving “etchleft_bjt.str” file.


struct outfile=etchleft_bjt.str

Continue the simulation by highlighting the Deposit statement i.e. deposit aluminum

thick=0.05 divisions=2. Then, click on the init and Cont

button on DECKBUILD Command Bar and plot the structure as shown in Figure 4-58.

Figure 4-58 Etching the Left Hand Side Shaded Box.



Next, we will etch the right hand side shaded box. In this case, we can simply etch the

aluminum starting at x = 0.16 all the way to the right using the Etch menu again as shown in

Figure 4-59.

Figure 4-59 Setting Parameters to Etch to the Right.


# Etching the Right Hand Side Shaded Region etch aluminum right p1.x=0.15


“etchright_bjt.str” as shown in Figure 4-60.

Figure 4-60 Saving “etchright_bjt.str” file.



Press the WRITE button to write the following into the input file:

struct outfile=etchright_bjt.str

Continue the simulation by highlighting the Etch statement i.e. etch done x=-0.6 y=-

0.35. Then, click on the init and Cont button on DECKBUILD Command Bar and plot the structure as shown in Figure 4-61.

Figure 4-61 Etched Structure.

4.15 Electrode Specification The purpose of specifying the electrodes for the BJT structure is to enable the structure to be

bias in the device simulator ATLAS. To place the emitter electrode at x = 0 µm, use the ATHENA Electrode menu and set the parameters as shown in Figure 4-62.


electrode name=emitter x=0.00



Figure 4-62 Setting Emitter Electrode Location.

Similarly, set the base electrode at x = -0.65 µm as shown in Figure 4-63.

Figure 4-63 Setting Base Electrode Location.

The following statement should be obtained after clicking on the WRITE button:

electrode name=base x=-0.65

Finally, the collector electrode at the bottom of the structure is set using the Backside option

in the Electrode Type field as shown below.

Figure 4-64 Setting Collector Electrode at Backside.

The following statement should be obtained after clicking on the WRITE button:

electrode name=collector backside



The final structure is then saved using the ATHENA File I/O menu as shown in Figure 4-65.

Figure 4-65 Saving Final BJT Structure file.

Press the WRITE button to write the following into the input file:

struct outfile=bjt.str

Continue the simulation by highlighting the Etch statement i.e. etch aluminum right

p1.x=0.15. Then, click on the init and Cont button on DECKBUILD Command Bar and plot the final BJT structure as shown in Figure 4-66.

Figure 4-66 Final Emitter Polysilicon NPN BJT Structure.

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