tutorial 16. modeling surface chemistrybarbertj/cfd training/fluent 12/tut16.pdf · tutorial 16....

Tutorial 16. Modeling Surface Chemistry

Introduction

In chemically reacting laminar flows, such as those encountered in chemical vapor deposi-tion (CVD) applications, accurate modeling of time-dependent hydrodynamics, heat andmass transfer, and chemical reactions (including wall surface reactions) is important.

In this tutorial, surface reactions are considered. Modeling the reactions taking place atgas-solid interfaces is complex and involves several elementary physico-chemical processeslike adsorption of gas-phase species on the surface, chemical reactions occurring on thesurface, and desorption of gases from the surface back to the gas phase.

This tutorial demonstrates how to do the following:

• Create new materials and set the mixture properties.

• Model surface reactions involving site species.

• Enable physical models and define boundary conditions for a chemically reactinglaminar flow involving wall surface reactions.

• Calculate the deposition solution using the pressure-based solver.

• Examine the flow results using graphics.

Prerequisites

This tutorial is written with the assumption that you have completed Tutorial 1, andthat you are familiar with the ANSYS FLUENT navigation pane and menu structure.Some steps in the setup and solution procedure will not be shown explicitly.

Before beginning with this tutorial, see Chapter 15 in the separate User’s Guide formore information about species transport, chemically reacting flows, wall surface reactionmodeling, and chemical vapor deposition. In particular, you should be familiar with theArrhenius rate equation, as this equation is used for the surface reactions modeled in thistutorial.

Release 12.0 c© ANSYS, Inc. March 12, 2009 16-1

Modeling Surface Chemistry

Problem Description

A rotating disk CVD reactor for the growth of Gallium Arsenide (GaAs) shown in Fig-ure 16.1 will be modeled.

Figure 16.1: Schematic of the Reactor Configuration

The process gases, Trimethyl Gallium (Ga(CH3)3) and Arsine (AsH3) enter the reactorat 293 K through the inlet at the top. These gases flow over the hot, spinning diskdepositing thin layers of gallium and arsenide on it in a uniform, repeatable manner.The disk rotation generates a radially pumping effect, which forces the gases to flow in alaminar manner down to the growth surface, outward across the disk, and finally to bedischarged from the reactor.

The semiconductor materials Ga(s) and As(s) are deposited on the heated surface gov-erned by the following surface reactions.

AsH3 + Ga s → Ga + As s + 1.5H2 (16.1)

Ga(CH3)3 + As s → As + Ga s + 3CH3 (16.2)

The inlet gas is a mixture of Trimethyl Gallium and Arsine and the mass fraction ofGa(CH3)3 is 0.15 and AsH3 is 0.4, respectively. The mixture velocity at the inlet is0.02189 m/s. The disk rotates at 80 rad/sec. The top wall (wall-1) is heated to 473 Kand the sidewalls (wall-2) of the reactor are maintained at 343 K. The susceptor (wall-4) is heated to a uniform temperature of 1023 K and the bottom wall (wall-6) is at303 K. These CVD reactors are typically known as cold-wall reactors, where only thewafer surface is heated to higher temperatures, while the remaining reactor walls aremaintained at low temperatures.

16-2 Release 12.0 c© ANSYS, Inc. March 12, 2009


In this tutorial, simultaneous deposition of Ga and As is simulated and examined.The mixture properties and the mass diffusivity are determined based on kinetic the-ory. Detailed surface reactions with multiple sites and site species, and full multi-component/thermal diffusion effects are also included in the simulation.

The purpose of this tutorial is to demonstrate surface reaction capabilities in ANSYSFLUENT. Convective heat transfer is considered to be the dominant mechanism comparedto radiative heat transfer, thus radiation effects are ignored.

Setup and Solution

Preparation

1. Download surface_chem.zip from the User Services Center to your working folder(as described in Tutorial 1).

2. Unzip surface_chem.zip.

The file surface.msh can be found in the surface chem folder created after unzip-ping the file.

3. Use FLUENT Launcher to start the 3D version of ANSYS FLUENT.

For more information about FLUENT Launcher, see Section 1.1.2 in the separateUser’s Guide.

4. Enable Double Precision.

Note: The Display Options are enabled by default. Therefore, after you read in the mesh,it will be displayed in the embedded graphics window.

Step 1: Mesh

1. Read in the mesh file surface.msh.

File −→ Read −→Mesh...

Step 2: General Settings

General

1. Check the mesh.

General −→ Check

ANSYS FLUENT will perform various checks on the mesh and will report the progressin the console. Ensure that the reported minimum volume is a positive number.


http://www.fluentusers.com


2. Scale the mesh.

General −→ Scale...

Scale the mesh to meters as it was created in centimeters.

(a) Select cm (centimeters) from the Mesh Was Created In drop-down list in theScaling group box.

(b) Click Scale and verify that the domain extents are as shown in the Scale Meshdialog box.

The default SI units will be used in this tutorial, hence there is no need tochange any units.

(c) Close the Scale Mesh dialog box.

3. Check the mesh.

General −→ Check

Note: It is a good practice to check the mesh after manipulating it (i.e., scale,convert to polyhedra, merge, separate, fuse, add zones, or smooth and swap).This will ensure that the quality of the mesh has not been compromised.

4. Examine the mesh (Figure 16.2).

Extra: You can use the left mouse button to rotate the image and view it fromdifferent angles. Use the right mouse button to check which zone numbercorresponds to each boundary. If you click the right mouse button on oneof the boundaries in the graphics window, its name and type will be printedin the ANSYS FLUENT console. This feature is especially useful when youhave several zones of the same type and you want to distinguish between themquickly. Use the middle mouse button to zoom the image.



Figure 16.2: Mesh Display

5. Retain the default solver settings.

General



Step 3: Models

Models

In this problem, the energy equation and the species conservation equations will be solved,along with the momentum and continuity equations.

1. Enable heat transfer by enabling the energy equation.

Models −→ Energy −→ Edit...

(a) Enable Energy Equation.

(b) Click OK to close the Energy dialog box.

2. Enable chemical species transport and reaction.

Models −→ Species −→ Edit...

Although you enable reactions, you still run a non-reacting flow to produce an initialsolution. You will run reacting flow in step 8.



(a) Select Species Transport in the Model list.

The Species Model dialog box will expand to show relevant input options.

(b) Enable Volumetric and Wall Surface in the Reactions group box.

(c) Enable Mass Deposition Source in the Wall Surface Reaction Options group box.

Mass Deposition Source is enabled because there is a certain loss of mass dueto the surface deposition reaction, i.e., As(s) and Ga(s) are being depositedout. If you were to do an overall mass balance without taking this fact intoaccount, you would end up with a slight imbalance.

(d) Enable Inlet Diffusion in the Options group box.

(e) Retain the default setting for Diffusion Energy Source.

This includes the effect of enthalpy transport due to species diffusion in theenergy equation, which contributes to the energy balance, especially for the caseof Lewis numbers far from unity.

(f) Enable Full Multicomponent Diffusion and Thermal Diffusion.

The Full Multicomponent Diffusion activates Stefan-Maxwell’s equations andcomputes the diffusive fluxes of all species in the mixture to all concentrationgradients. The Thermal Diffusion effects cause heavy molecules to diffuse lessrapidly, and light molecules to diffuse more rapidly, toward heated surfaces.

(g) Click OK to close the Species Model dialog box.

ANSYS FLUENT will list the properties in the console, that are required for themodels that you have enabled.

An Information dialog box will open reminding you to confirm the propertyvalues that have been extracted from the database.

(h) Click OK in the Information dialog box.



Step 4: Materials

Materials

In this step, you will create the gas-phase species (AsH3, Ga(CH3)3, CH3, H2), the sitespecies (Ga s and As s), and solid species (Ga and As).

1. Create species arsine.

Materials −→ air −→ Create/Edit...

(a) Enter arsine in the Name text entry field.

(b) Enter ash3 in the Chemical Formula text entry field.

(c) Specify the properties as shown in Table 16.1.



Table 16.1: Properties of arsineParameter ValueCp kinetic-theoryThermal Conductivity kinetic-theoryViscosity kinetic-theoryMolecular Weight 77.95

Standard State Enthalpy 0

Standard State Entropy 130579.1

Reference Temperature 298.15

Ignore the Density parameter as the density will be set to incompressible-ideal-gas for mixture.

Hint: Scroll down in the Properties group box to see all the parameters.

(d) Click Change/Create to create the new material.

A Question dialog box will open, asking if you want to overwrite air.

(e) Click No in the Question dialog box.

(f) Select arsine(ash3) from the FLUENT Fluid Materials drop-down list.

Properties group box will expand to show L-J Characteristic Length, L-J EnergyParameter, and Degrees of Freedom.



(g) Enter 4.145 for L-J Characteristic Length.

(h) Enter 259.8 for L-J Energy Parameter.

(i) Retain the default value of 0 for Degrees of Freedom.

(j) Click Change/Create and close the Create/Edit Materials dialog box.

2. Create other species following the same procedure as for AsH3.

Materials −→ air −→ Create/Edit...

(a) Enter the parameter values for each of the species as shown in Table 16.2.



Table 16.2: Properties of Species

Parameter Ga(CH 3) 3 CH 3 H 2 Ga s As s Ga AsName tmg ch3g hydrogen ga s as s ga as

Chemical For-mula

ga<ch3>3 ch3 h2 ga s as s ga as

Cp kinetic-theory kinetic-theory

kinetic-theory

520.64 520.64 1006.43 1006.43

Thermal Con-ductivity

kinetic-theory kinetic-theory

kinetic-theory

0.0158 0.0158 kinetic-theory

kinetic-theory

Viscosity kinetic-theory kinetic-theory

kinetic-theory

2.125

e-05

2.125

e-05

kinetic-theory

kinetic-theory

MolecularWeight

114.83 15 2.02 69.72 74.92 69.72 74.92

Standard StateEnthalpy

0 2.044

e+07

0 -3117.71 -3117.71 0 0

Standard StateEntropy

130579.1 257367.6 130579.1 154719.3 154719.3 0 0

ReferenceTemperature

298.15 298.15 298.15 298.15 298.15 298.15 298.15

L-J Character-istic Length

5.68 3.758 2.827 - - 0 0

L-J Energy Pa-rameter

398 148.6 59.7 - - 0 0

Degrees ofFreedom

0 0 5 - - - -

(b) Click Change/Create to create the new material.

(c) Click No in the Question dialog box when asked if you want to overwrite air.

To enter complex formulae such as Ga(CH3)3 in the text entry box, use ‘<’ and ‘>’instead of ‘(’ and ‘)’, respectively.

3. Set the mixture species.

Materials −→ mixture-template −→ Create/Edit...

(a) Enter gaas deposition for Name.

(b) Click Change/Create.

(c) Click Yes in the Question dialog box to overwrite the mixture-template.

(d) Set the Selected Species, Selected Site Species, and Selected Solid Species.



i. Click the Edit... button to the right of the Mixture Species drop-down listto open the Species dialog box.

ii. Set the Selected Species, Selected Site Species, and Selected Solid Speciesfrom the Available Materials selection list as shown in Table 16.3.

Table 16.3: Selected SpeciesSelected Species Selected Site Species Selected Solid Speciesash3 ga s gaga<ch3>3 as s asch3 - -h2 - -

! The species should appear in the same order as shown in Table 16.3. Ensurethat h2 is at the bottom in the Selected Species selection list.

To add/remove the species:

• Select the required species from the Available Materials selection list andclick Add in the corresponding species selection list (Selected Species,Selected Site Species, or Selected Solid Species) to add a particularspecies to the list.

• Select the species from the selection list (i.e., Selected Species, Se-lected Site Species, or Selected Solid Species) and click Remove in thecorresponding selection list to remove an unwanted species from theselection list.



iii. Click OK to close the Species dialog box after all the species are set underthe respective categories.

(e) Set the mixture reactions.

i. Click the Edit... button to the right of the Reaction drop-down list to openthe Reactions dialog box.

ii. Increase the Total Number of Reactions to 2, and define the followingreactions using the parameters in Table 16.4:

AsH3 + Ga s → Ga + As s + 1.5H2 (16.3)

Ga(CH3)3 + As s → As + Ga s + 3CH3 (16.4)

CH3 further reacts with H (3CH3 + 1.5H2 → 3CH4) on the substrateproducing CH4.



Table 16.4: Reaction ParametersParameter For Equation 16.3 For Equation 16.4Reaction Name gallium-dep arsenic-dep

Reaction ID 1 2

Reaction Type Wall Surface Wall SurfaceNumber of Reactants 2 2

Species ash3, ga s ga<ch3>3, as sStoich. Coefficient ash3=1, ga s=1 ga<ch3>3=1, as s=1

Rate Exponent ash3=1, ga s=1 ga<ch3>3=1, as s=1

Arrhenius Rate PEF=1e+06, AE=0,TE=0.5

PEF=1e+12, AE=0,TE=0.5

Number of Products 3 3

Species ga, as s, h2 as, ga s, ch3Stoich. Coefficient ga=1, as s=1, h2=1.5 as=1, ga s=1, ch3=3

Rate Exponent as s=0, h2=0 ga s=0, ch3=0

Here,PEF = Pre-Exponential Factor,AE = Activation Energy, andTE = Temperature Exponent.

Set the ID to 2 in order to set the parameters for the second reaction.

iii. Click OK to save the data and close the Reactions dialog box.

(f) Set the reaction mechanisms for the mixture.

i. Click the Edit... button to the right of the Mechanism drop-down list toopen the Reaction Mechanisms dialog box.



ii. Retain Number of Mechanisms as 1.

iii. Enter gaas-ald for Name.

iv. Select Wall Surface in the Reaction Type group box.

v. Select gallium-dep and arsenic-dep from the Reactions selection list.

vi. Set Number of Sites to 1.

vii. Enter 1e-08 kgmol/m2 for Site Density for site-1.

viii. Click the Define... button to the right of site-1 to open the Site Param-eters dialog box.

A. Set Total Number of Site Species to 2.

B. Select ga s as the first site species and enter 0.7 for Initial Site Cover-age.

C. Select as s as the second site species and enter 0.3 for Initial SiteCoverage.

D. Click Apply and close the Site Parameters dialog box.

ix. Click OK to close the Reaction Mechanisms dialog box.

(g) Retain the default selection of incompressible-ideal-gas from the Density drop-down list.

(h) Retain the default selection of mixing-law from the Cp drop-down list.

(i) Select mass-weighted-mixing-law from the Thermal Conductivity drop-down list.

(j) Select mass-weighted-mixing-law from the Viscosity drop-down list.

(k) Select kinetic-theory from the Mass Diffusivity drop-down list.

(l) Select kinetic-theory from the Thermal Diffusion Coefficient drop-down list.

(m) Click Change/Create and close the Create/Edit Materials dialog box.



Step 5: Boundary Conditions

Boundary Conditions

1. Retain the default settings for outlet.

Boundary Conditions −→ outlet −→ Edit...



2. Set the conditions for velocity-inlet.

Boundary Conditions −→ velocity-inlet −→ Edit...

(a) Retain the default selection of Magnitude, Normal to Boundary from the VelocitySpecification Method drop-down list.

(b) Retain the default selection of Absolute from the Reference Frame drop-downlist.

(c) Enter 0.02189 m/s for Velocity Magnitude.

(d) Click the Thermal tab and enter 293 K for Temperature.

(e) Click the Species tab.



i. Set the Species Mass Fractions for ash3 to 0.4, ga<ch3>3 to 0.15, andch3 to 0 respectively.

(f) Click OK to close the Velocity Inlet dialog box.

3. Set the boundary conditions for wall-1.

Boundary Conditions −→ wall-1 −→ Edit...

(a) Click the Thermal tab.

i. Select Temperature in the Thermal Conditions group box.

ii. Enter 473 K for Temperature.

(b) Click OK to close the Wall dialog box.











(a) Select Moving Wall in the Wall Motion group box.

(b) Select Absolute and Rotational in the Motion group box.

(c) Enter 80 rad/s for Speed.

(d) Retain the other default settings.

(e) Click the Thermal tab.



(f) Click the Species tab.



i. Enable Reaction.

ii. Retain the selection of gaas-ald from the Reaction Mechanisms drop-downlist.

(g) Click OK to close the Wall dialog box.



(a) Select Moving Wall in the Wall Motion group box.

(b) Select Absolute and Rotational in the Motion group box.

(c) Enter 80 rad/s for Speed.

(d) Click the Thermal tab.



(e) Click OK to close the Wall dialog box.









8. Disable diffusion at the inlet.


(a) Disable Inlet Diffusion and close the Species Model dialog box.

You can also use the define/models/species/inlet-diffusion? text com-mand to disable inlet diffusion. Enter no when asked if you want to includediffusion at the inlet.

Step 6: Operating Conditions

Boundary Conditions

1. Specify the operating conditions.

Boundary Conditions −→ Operating Conditions...

(a) Enter 10000 Pa for Operating Pressure.



(b) Enable Gravity.

(c) Enter 9.81 m/s2 for Gravitational Acceleration in the Z direction.

(d) Enter 303 K for Operating Temperature.

(e) Click OK to close the Operating Conditions dialog box.

The Operating Conditions dialog box can be accessed from the Cell Zone Conditions taskpage as well as the Boundary Conditions task page.

Step 7: Non-Reacting Flow Solution

1. Disable Volumetric for solving non-reacting flow.


(a) Disable Volumetric in the Reactions group box.

(b) Click OK to close the Species Model dialog box.

You will be running a non-reacting solution to establish the flow.

2. Retain the default Under-Relaxation Factors.

Solution Controls



3. Enable residual plotting during the calculation.

Monitors −→ Residuals −→ Edit...

(a) Retain the default settings and close the Residual Monitors dialog box.

4. Initialize the flow field using the boundary conditions set at velocity-inlet.

Solution Initialization

(a) Select velocity-inlet from the Compute from drop-down list.

(b) Click Initialize

5. Save the case file (surface-non-react.cas.gz).

File −→ Write −→Case...



6. Start the calculation by requesting 200 iterations.

Run Calculation

(a) Enter 200 for Number of Iterations and click Calculate.

The solution will converge in approximately 120 iterations.

Step 8: Reacting Flow Solution

1. Enable Volumetric for the reacting flow solution.




(a) Enable Volumetric and Wall Surface in the Reactions group box.

(b) Enable Mass Deposition Source in the Wall Surface Reaction Options group box.

(c) Click OK to close the Species Model dialog box.

2. Retain the default convergence criteria for calculation.


3. Request 250 more iterations.

Run Calculation




4. Compute the mass fluxes.

Reports −→ Fluxes −→ Set Up...

(a) Retain the default selection of Mass Flow Rate in the Options group box.

(b) Select outlet and velocity-inlet from the Boundaries selection list.

(c) Click Compute and close the Flux Reports dialog box.

5. Display contours of surface deposition rate of ga (Figure 16.3).

Graphics and Animations −→ Contours −→ Set Up...



(a) Enable Filled in the Options group box.

(b) Select Species... and Surface Deposition Rate of ga from the Contours of drop-down lists.

(c) Select wall-4 from the Surfaces selection list.

(d) Click Display and close the Contours dialog box.

Rotate the display with the mouse to obtain the view as shown in (Figure 16.3).

Figure 16.3: Contours of Surface Deposition Rate of ga

6. Reduce the convergence criteria.




(a) Enter 5e-06 for Absolute Criteria for continuity.

(b) Click OK to close the Residual Monitors dialog box.

7. Request 300 more iterations.

Run Calculation


8. Check the mass fluxes.

Reports −→ Fluxes −→ Set Up...

(a) Retain the default selection of Mass Flow Rate in the Options group box.

(b) Retain the selection of outlet and velocity-inlet in the Boundaries selection list.

(c) Click Compute and close the Flux Reports dialog box.

9. Save the case and data files (surface-react1.cas.gz and surface-react1.dat.gz).

File −→ Write −→Case & Data...

10. Display contours of surface deposition rate of ga (Figure 16.4).





Figure 16.5: Scaled Residuals



Step 9: Postprocessing

1. Create an iso-surface near wall-4.

Surface −→Iso-Surface...

(a) Select Mesh and Z-Coordinate from the Surface of Constant drop-down lists.

(b) Click Compute.

(c) Enter 0.075438 m for Iso-Values.

(d) Enter z=0.07 for New Surface Name.

(e) Click Create and close the Iso-Surface dialog box.



2. Display contours of temperature on the plane surface created. (Figure 16.6).


(a) Ensure that Filled is enabled in the Options group box.

(b) Select Temperature... and Static Temperature from the Contours of drop-downlists.

(c) Deselect wall-4 from the Surfaces selection list.

(d) Select z=0.07 from the Surfaces selection list.

(e) Click Display.



Figure 16.6: Temperature Contours Near wall-4

Figure 16.6 shows the temperature distribution across a plane just above the rotatingdisk. You can see that the disk has a temperature of 1023 K.

3. Display contours of surface deposition rates of ga (Figure 16.7).


(a) Select Species... and Surface Deposition Rate of ga from the Contours of drop-down lists.

(b) Select wall-4 from the Surfaces selection list.

(c) Deselect z=0.07 from the Surfaces selection list.

(d) Click Display.

You may need to use the left mouse button to rotate the image so that you can seethe contours on the top side of wall-4 where the deposition takes place.

Figure 16.7 shows the gradient of surface deposition rate of ga. The maximumdeposition is seen at the center of the disk.




4. Display contours of surface coverage of ga s (Figure 16.8).


(a) Select Species... and Surface Coverage of ga s from the Contours of drop-downlists.

(b) Retain the selection of wall-4 in the Surfaces selection list.

(c) Click Display and close the Contours dialog box.

Figure 16.8: Contours of Surface Coverage of ga s

Figure 16.8 shows the rate of surface coverage of the site species ga s.



5. Create a line surface from the center of wall-4 to the edge.

Surface −→Line/Rake...

(a) Enter the values for x0, x1, y0, y1, z0, and z1 as shown in the Line/Rake Surfacedialog box.

You can also select the points by clicking Select Points with Mouse. Then, inthe graphic display, click at the center of wall-4 and at the edge using the rightmouse button.

(b) Click Create.

(c) Close the Line/Rake Surface dialog box.



6. Plot the surface deposition rate of Ga v/s radial distance (Figure 16.9).

Plots −→ XY Plot −→ Set Up...

(a) Disable Node Values in the Options group box.

(b) Select Species... and Surface Deposition Rate of ga from the Y Axis Functiondrop-down lists.

The source/sink terms due to the surface reaction are deposited in the celladjacent to the wall cells, so it is necessary to plot the cell values and not thenode values.

(c) Select line-9 from the Surfaces selection list.

(d) Click Plot and close the Solution XY Plot dialog box.

The peak surface deposition rate occurs at the center of wall-4 (where the concen-tration of the mixture is highest).



Figure 16.9: Plot of Surface Deposition Rate of Ga

Extra: You can also perform all the postprocessing steps to analyze the depositionof As.

7. Save the case and data files (surface-react2.cas.gz and surface-react2.dat.gz).

File −→ Write −→Case & Data...

Summary

The main focus of this tutorial is the accurate modeling of macroscopic gas flow, heatand mass transfer, species diffusion, and chemical reactions (including surface reactions)in a rotating disk CVD reactor. In this tutorial, you learned how to use the two-stepsurface reactions involving site species, and computed simultaneous deposition of galliumand arsenide from a mixture of precursor gases on a rotating susceptor. Note that thesame approach is valid if you are simulating multi-step reactions with multiple sites/sitespecies.

Further Improvements

This tutorial guides you through the steps to reach an initial solution. You may be ableto obtain a more accurate solution by using an appropriate higher-order discretizationscheme and by adapting the mesh. Mesh adaption can also ensure that the solution isindependent of the mesh. These steps are demonstrated in Tutorial 1.


tutorial 16. modeling surface chemistrybarbertj/cfd training/fluent 12/tut16.pdf · tutorial 16....

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