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Film Bulk Acoustic Resonator (FBAR)

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Chloe Allison
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In this tutorial we will learn how to simulate a 2D Film Bulk Acoustic Resonator (FBAR).  We will analyse the impedance and mode shapes of the device.

You will learn:

  • The basic simulation workflow in OnScale Designer
  • How to set up a 2D model
  • How to create a simple geometry
  • How to edit material properties
  • How to add circuits into a design
  • How to display and post-process results

What is an FBAR?: FBAR is an acoustically isolated device consisting of a piezoelectric material sandwich between two electrodes. FBAR devices using piezoelectric films with thicknesses ranging from several micrometres down to tenth of micrometers resonate in the frequency range of roughly 100 MHz to 10 GHz. Aluminium nitride and zinc oxide are two common piezoelectric materials used in FBARs. Common applications of FBARs is radio frequency (RF) filters used in phones and other wireless applications. Filters are made from a network of resonators and are designed to remove unwanted frequencies from being transmitted whilst allowing specific frequencies to be received. These networks are usually in half-ladder, full-ladder, lattice or stacked topologies. FBARs can also be used in sensor applications in this case when a device is subject to some form of mechanical pressure its resonance frequency will shift.

Model Definition

model.png

Characteristics of the model

Model:

FBAR

Mesh Size:

20 elements per wavelength

Analysis Time:

10 us

Output Results:

- Y Displacement Mode Shape at 1.967 MHz

Material Data

Name

Code Name

Density 

Bulk Velocity 

Shear Velocity

Poling

Aluminium Nitride

aln

3230 kg.m-3

-

-

Y+

Molybdenum

moly

10220 kg.m-3

6649.79 ms-1

3509.29 ms-1

-

Silicon, Generic

si

2330 kg.m-3

7526 ms-1

4346 ms-1

-
 

Note: Two of these materials were taken from the Project Materials Database. Molybdedum is created by the user as it is not in the Database.

Why This Simulation?

It is useful to use simulation when designing FBARs because the thickness of the electrodes and piezoelectric layer determine the frequency range in which the device can operate. It allows designers to optimise their design for frequency range, quality factor and many other key performance characteristics.

This 2D FBAR model provides a simple starting structure consisting of a piezoelectric active layer (Aluminum Nitride), Molybdenum electrodes and silicon substrate material at the sides of the air cavity. In this simulation, we apply a voltage across the AlN layer and analyse the resulting impedance and mode shapes.

The Simulation Process

Let's go through the step by step tutorial and see how to simulate a 2D FBAR in OnScale!

Step 1 - Create a New Project

Open up OnScale in Designer Mode. The first step is to create a new project.

  1. In the Home tab of the ribbon, click New Project. The New Project window shows.
  2. Type a name for the project.
  3. If desired, change the save location and/or project file name by clicking … beside Project File.
  4. For Analysis, select Mechanical Dynamic.
  5. For Model Type, select 2D Model.
  6. Select the Advanced checkbox.
  7. For Distance, select μm.
  8. Click OK.
fbar-new-project.png

Step 2 - Set the Frequency of Interest

First we will specify the frequency of interest.

  1. Select Project Settings in the Model Tree
  2. Expand the Frequency of Interest property
  3. Tick the box
  4. Enter a value of 1.9e9
  5. Expand the Frequency of Damping property
  6. Tick the box
  7. Enter a value of 1.9e9
prjctset.png

Step 3 - Add the Materials from the Material DB

The second step is to add the required materials for the FBAR transducer to the Project Materials database.

Molybdenum

Add electrode material

  1. Click + icon, next to Materials to open the Material Database
  2. Select Add New Material > Add Project Material
  3. Give the Material Description - Molybdenum
  4. Set Material Name to moly
  5. Select Material Category to be Metal
  6. Toggle the Damping property
  7. Set Density to 10220
  8. Set Bulk Velocity to 6649.79
  9. Set Shear Velocity to 3509.29
  10. Set Damping Units to dB/MHz/cm
  11. Set Bulk Attenuation to 0.1
  12. Set Shear Attenuation to 0.3
  13. Set Bulk Power Law to 1
  14. Set Shear Power Law to 1
  15. Select Save
moly.png

Aln

Add piezoelectric material.

  1. Expand the 'Piezoelectric' materials dropdown
  2. Double click on 'Aluminium Nitride - aln' to add it to Project Materials
aln.png

Silicon

Now add the substrate material.

  1. Expand 'Misc' materials dropdown 
  2. Double click on 'Silicon, generic- si' to add it to Project Materials
sili.png

Molybdenum 2

Copy electrode material

  1. Right click 'moly' in Project Materials and select Copy
  2. Rename material to moly2
  3. Select OK
  4. Select Done
copymoly.png

Create Basic Geometric Shapes

We will make use of the geometric primitive shapes available in OnScale to build the FBAR.

Substrate

We will start by creating the substrate of the device. 

  1. Select Rectangle button
  2. Select primitive_1
  3. Set the Material property of primitive_1 to si
  4. Set the X End property to 230
  5. Set the Y End property to 2
  6. Right click in the workspace and select Reset View to snap the view to the geometry
prim1.png

Cavity

Next we will add another rectangle to create the cavity.

  1. Right click primitive_1 and select Duplicate Selection
  2. Set the Material property of primitive_2 to void
  3. Set X Begin property of to 10
  4. Set X End property of to 220
prim2.png

Bottom Electrode

Next we will add another rectangle to represent the bottom electrode.

  1. Right click primitive_1 and select Duplicate Selection
  2. Set the Material property of primitive_3 to moly
  3. Set Y Begin property of primitive_3 to 2
  4. Set Y End property of primitive_3 to 2.4
prim3.png

Piezoelectric Layer

The piezoelectric layer is made from AlN in this example.

  1. Right click primitive_3 and select Duplicate Selection
  2. Set the Material property of primitive_4 to aln
  3. Set Y Begin property of primitive_3 to 2.4
  4. Set Y End property of primitive_3 to 3.2
prim4.png

Top Electrode

Lastly, we will add the top electrode.

  1. Right click primitive_4 and select Duplicate Selection
  2. Set the Material property of primitive_5 to moly2
  3. Set X Begin property to 15
  4. Set Y Begin property to to 3.2
  5. Set X End property to to 215
  6. Set Y End property to to 3.6
prim5.png
  1. Right click primitive_5 and select Duplicate Selection
  2. Set Y Begin property of primitive_6 to 3.6
  3. Set X End property to to 17
  4. Set Y End property to to 3.8
prim_6.png
  1. Right click primitive_6 and select Duplicate Selection
  2. Set the Material property of primitive_7 to moly
  3. Set X Begin property of primitive_7 to 213
  4. Set X End property to to 215
prim_7.png

Step 5 - Define a Time Function

The next step is to add a drive function. We will use a Ricker Wavelet time function with a frequency of 1.9 GHz.

  1. Click '+' to open the Drive Function dialogue
  2. Change function type from Sinusoidal to Ricker Wavelet
  3. Set Drive Frequency to 1.9e9
  4. Click Insert to close the window. A record called timefunc_1 will be added to the Model Tree
time.png

Step 6 - Define a Circuit

It is now time to define a circuit.

  1. Click '+' to open the Circuit dialogue
  2. Select the line between points 2 and 3
  3. Set Element to Resistor
  4. Set Resistance to 0.001
  5. Select Insert to close window. A record called circuit_1 will be added to the Model Tree
circ.png

Step 7 - Choose The Right Mesh Size

It is time to set up the meshing of the model. 

  1. Expand Mesh in the Model Tree
  2. Select Configuration
  3. Set Definitions to Advanced
  4. Set Elements per Wavelength to 20
  5. Set Mesh Velocity to Defined
  6. Set Value to 6000
mesh.png

Step 8 - Create Voltage Loads Between Electrodes and Piezo

We will now create loads on each side of the piezoelectric layer.

Create Load 1

  1. In the Model Tree, expand Boundary Conditions and then, beside Loads, click +.
  2. For Creation Mode, select Geometry Interface.
  3. For Geometry, select primitive_3 (moly) or click it in the model.
  4. For Interfacing Item, select primitive_4 (aln).
  5. For Load Type, select Voltage.
  6. For Area Scaling, type 0.0002.
  7. For Termination, select Ground.
  8. Click Create Load.
fbar-load-1.png

Create Load 2

The Load Definition window should still be open.

  1. For Geometry, select primitive_4 (aln) or click it in the model.
  2. For Interfacing Item, select primitive_5 (moly2).
  3. For Circuit, select circuit_1.
  4. For Termination, select timefunc_1.
  5. Click Create Load.
fbar-load-2png.png

Step 9 - Define the Boundary Conditions

We need to change the boundary conditions so that the tail of the transducer is fixed and the water load has absorbing boundaries.

  1. Click Domain Boundaries in the Model Tree
  2. Set X Minimum to Fixed
  3. Set X Maximum to Fixed
  4. Set Y Minimum to Fixed
  5. Set Y Maximum to Free
boun.png

Step 10 - Define the Type of Analysis 

We will now set the model simulation time to be 10 us seconds

  1. Click Analysis 
  2. Change Simulation Run Time to 1e-5
analysis.png

Step 11 - Define the Simulation Output Results

We will now define an output so we are able to see the displacement mode shape of the device at a 1.967 GHz.

  1. Click '+' to create a new output
  2. Set Output Type to Shape Data
  3. Set Array Type to Displacement
  4. Set Array Component to Y
  5. Set Frequency to 1.967e9
out.png

Step 12 - Run the Simulation on the Cloud

At this point the model is completely set up and it can now be run on the cloud.

  1. Click Run on Cloud 
  2. Select Estimate 
  3. Select Run
run.png

How to Get the Simulation Results?

Once the simulation has finished, the results are available in your storage to download.

  1. Click the Storage button to open the cloud storage
  2. Select your job from the dropdown menu
  3. Expand the simulation folder
  4. CTRL + select the -shape.flxdato & flxhst file, right click and select Download Selection
download.png

Choose an appropriate save location and close the cloud storage.

Step 13 - Check the Simulation Results

Switch to the Post Processor 

  1. Click this icon to access the Post Processor to analyse simulation results

Open Results

  1. Click File Explorer
  2. Expand the job simulation folder
  3. Double click the data out file to open the mode shape results
  4. Double click the history file to open the time history results
file_explorer.png

Plot Impedance

  1. From Results Manager, select 'pize load2: Voltage'
  2. Select Impedance button
  3. Double click on 'Impd:load2.amp'
  4. Toggle Log yAxis
  5. Set xAxis Minimum to 2.1e9
  6. Set xAxis Maximum to 2.25e9
impd.png

Plot Mode Shape at 1.967 GHz 

  1. Expand Mode Shapes
  2. Expand Mode-1
  3. Select ydsp
  4. Right click and select Plot Shape Movie
  5. Select Continue
  6. Set Scale Factor to 0.001
  7. Select Play button to play mode shape animation
data_out.png

Try for Yourself 

Now that we have introduced you to the tutorial, try have a play around with some of the settings, add some other outputs, or use this model as a starting point of your own.

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