Chapter 8.52
Flexus Thin Film Stress Measurement
(flexus)
1.0
Title
Flexus Thin Film Stress Measurement
2.0
Purpose
Flexus (Tencor FLX-2320) is a thin-film stress measurement instrument. It accurately measures the changes in the radius of curvature of the substrate caused by the deposition of a stressed thin film on the substrate.
Flexus can also measure
the Elastic constant and thermal expansion coefficient of a thin film, if the
thickness of the film and the substrate are known.
3.0
Scope
This document describes the procedures of measuring thin-film stress and how to set up a measuring program. For film elastic constant and thermal expansion coefficient measurements, and advanced 3-D stress plot, please refer to Section 4.0.
4.0
Applicable Documents
Tencor FLX-2320 Thin Film Stress Measurement User Manual (one copy in office, one clean room copy in the drawer under the Flexus).
5.0
Definitions & Process
Terminology
5.1
Laser Interferometer
Flexus uses a laser interferometer to measure the curvature
of a wafer, which is used in the calculation of the stress in the film
deposited on the wafer.
5.2
Intrinsic Stress
The stress of a film at the deposition temperature. It is
mainly caused by the atomic structure mismatch between the film and substrate.
5.3
Thermal Stress
The film stress changes between the deposition and the
measurement temperatures. It is mainly caused by the difference of the thermal
expansion coefficients between the film and substrate.
6.0
Safety
6.1
Laser
The Flexus contains two solid-state Class III lasers. Do not
defeat the electrical or mechanical interlocks. When the interlocks are
defeated, laser beams of medium power could be present. It is hazardous. Do not
expose the laser beam directly which could damage to your eyes.
6.2
High Temperature
The hot plate and measurement platform are very hot (up to
500°C) when the heater is operating for film elastic constant and thermal
expansion coefficient measurements. Do not open the instrument door, touch the
inside components, or the wafer, if the temperature displayed on the upper left
corner of the instrument is above 40°C.
7.0
Statistical/Process Data
The Flexus is tested using two calibrated mirrors (flat and 20-meter curvature) monthly. Please contact the process staff for the data.
8.0
Available Processes, Gases,
Process Notes
8.1
Available Measurement Programs
8.1.1
Program 100: Standard program for 4” wafers.
50 measure points with 10 mm edge exclusion. Wafer thickness is set at 525 μm.
8.1.2
Program 150: Standard program for 6” wafers.
50 measure points with 15 mm edge exclusion. Wafer thickness is set at 690 μm.
8.2
Process Notes
8.2.1
Flexus stress measurement only works on blank wafers, not
patterned wafers.
8.2.2
Film stress measured is inversely proportional to square of
wafer thickness. Please verify that your wafer thickness matches the program
setting, especially if you are using test grade wafers.
8.2.3
The surface of the wafer must be reflective for the laser to
bounce back into the detecting device. Rough surfaces will scatter the laser
and affect the measurement.
8.2.4
The Flexus compares curvature of the wafer before and after
film deposition to calculate film stress. Both pre- and post-deposition
measurements are required. If wafer is coated on both side, e.g. deposited
in Tystar LPCVD furnaces, the backside film needs to be removed before
post-deposition measurements is done.
8.2.5
If you forgot to do pre-deposition measurement before film
deposition, you still can measure the film stress. First, strip the backside
film on the wafer, and measure it as pre-deposition. Then, strip the front side
film, and measure as post-deposition. The stress will reverse, e.g. tensile
measured is actually compressive, and vice versa. Since there will be no film
left on the wafer, this method only works on a test wafer.
8.2.6
Do not try to adjust any knobs on the Flexus. By doing so,
you may cause the laser to misaligned. The manufacture voids the warranty and
service agreement for un-authorized adjustment on the tool.
8.2.7
For high temperature measurements, the hot plate cover
(stored in the drawer under the Flexus) must be installed and secured with 4
thumbscrews. Make sure the chamber fan is ON. Failing to do so will overheat
the electronic parts and the laser.
9.0
Equipment Operation
9.1
System Description
The Flexus is controlled by a PC for measurements, data acquisition, and result calculation. The PC is running WINDOWS 3.1. The measurement results are stored in the user’s subdirectory, which should be set up when the user is qualified for the tool.
The front view of the FLX-2320 instrument is shown in Section 11.1. For normal stress measurement, the hot plate cover and
thumbscrews are not used. The user needs to select a proper Wafer Locator Ring,
which is shown in Section 11.2, for the size of the wafer to be measured. All the wafer
locator rings are stored in the drawer under the Flexus.
9.2
Making a Measurement
9.2.1
Enable Flexus on the WAND.
9.2.2
Check the temperature display on the upper left corner of
the instrument. Make sure it is at room temperature.
9.2.3
Double click the [WIN FLX] icon. The measurement
program will start and prompt you to enter your own subdirectory name.
9.2.4
If the computer has been rebooted, enter login Flexus and password FLX-2320 (case-sensitive). At the DOS prompt, enter win to start
Windows. Then double click on the [WIN
FLX] icon.
Pre-deposition Measurement
9.2.5
Open the instrument door. Place the wafer, face up, in the
wafer locator ring on the measurement platform. Change the locator ring if it
is not the right size for your wafer. Close the instrument door securely.
9.2.6
Choose [First (no Film)] from the Measure menu. The [First
Measurement] dialog box will pop out. Enter the file name, for data to be
stored, and all other fields. Change the substrate thickness to match your
wafer. There is a micrometer on the shelf on top of the Flexus if you do
not know the thickness of your wafer. Make sure the Process program is the
one you want to use. Refer to Section
9.4 to edit the current program or
load other program.
9.2.7
Click the [Measure] button in the [First
Measurement] dialog box. The first measurement starts and you can hear some
beeping sounds. When the measurement finishes, the PC displays two more windows
– substrate deflection and light intensity.
If you plan to do more graphic
operations, e.g. 3D contour graphs, you need to save the graphs for future use
by choosing [Save As] from the [File] menu. You do not have to do
so if you only want to measure the stress in the film.
9.2.8
Repeat the operation (Sections 9.2.4 to 9.2.6)
for other wafers. You can store the data of different wafers in the same
data file with different ID.
9.2.9
When you finished measuring all your wafers, close all the
window and exit the program. Disable Flexus on the WAND.
Post-deposition Measurement
9.2.10 Measure
the thickness of the film deposited on your wafer. Make sure the backside film
is stripped completely. Any residue left on the backside will affect your
stress measurement
9.2.11 Repeat Sections 9.2.1 to 9.2.4 to start the WINFLX program.
9.2.12 Choose [Single]
from the Measure menu. The [Single Stress] dialog box will pop out.
Click the [File] button to select the data file, which contains the
pre-deposition measurement. It is the one you entered in Section 9.2.5. Click the drop down button to select the wafer ID, which
must match the pre-deposition measurement ID for the wafer. Enter the thickness
of the film deposited.
If you have not measured the film
thickness, you can enter an approximate one. You can correct the entry after
you have measured it (Section 9.3).
9.2.13 Click
the [Measure] button in the [Single Measurement] dialog
box. Similar to the first measurement (Section 9.2.6), the
measurement starts and you can hear some beeping sounds. When the
measurement finishes, the PC displays two more windows – substrate deflection
and light intensity.
In addition, the measured wafer
curvature and stress values are displayed on the graphs. Negative values
indicate a convex surface and compressive stress. Positive values indicate a
concave surface and tensile stress.
If you plan to do more graphic operations,
e.g. 3D graphs, you need to save the graphs for future use by choosing [Save
As] from the [File] menu. You do not have to do so if you only want
to measure the stress in the film.
9.2.14 Repeat the
operation (Sections 9.2.9 to
9.2.12) for other wafers.
9.2.15 When you
finished measuring all your wafers, close all the windows and exit the program.
Disable Flexus on the WAND.
9.3
Editing, Printing, and Data Transfer of Measurement
Results
9.3.1
Perform Sections 9.2.1 to 9.2.4 to start the WINFLX program.
9.3.2
From the Edit menu, choose [Data Files].
Select the data file from the dialog box.
9.3.3
The selected data file will be displayed and you
can change all its contents, e.g. film thickness, and wafer thickness. elastic
modulus, and etc. Do not change the Id field, since it may corrupt the
link between the first and single measurements.
9.3.4
The stress will be recalculated automatically after you make
the change. If not, choose [Recalculate] from the Edit menu.
9.3.5
To print the data file, choose [Print] from the File
menu. Make sure the printer, which sits on top of the Flexus, is on and it has
paper.
9.3.6
To transfer data files through the network, first click on
the [Program Manager] icon,
and then on the [File Manager]
icon. Click on the C: drive, locate your personal directory and select the files you
want to transfer. The data files with
extension .grp are text files with the raw data and calculated stress
measurement.
9.3.7
Place a floppy disk in the 3.5” drive. Go to the [File] menu and click [Copy]. Enter A:
in the field for the destination drive and hit [OK]. Note: The M: drive is
no longer available as this computer is no longer connected to the
network.
9.4
Loading, Editing, And Saving Measuring Program
9.4.1
Perform Sections
9.2.1 to 9.2.4 to start the WINFLX
program
9.4.2
From the Edit menu, choose [Process Programs].
The Process Program dialog box will pop-up, with the current program name
displayed in the title bar.
9.4.3
To load another program, choose the [Load]
button. A list of available programs will be displayed. Select the desired
program then click [OK].
9.4.4
After loading the program, click [Cancel]
in the Process Program dialog box to return to the stress measurement window.
9.4.5
You can customize a process program after you
load it. The Process Program dialog box has the following fields that a user
can modify. Use the default if you do not know what to enter.
Field Description
Maximum Scan Points You can specify a maximum of 1250
points. But only 50 points, which is the default, will be saved by the computer
for future reference.
Low Intensity Alarm Use default.
Elastic Modulus The elastic modulus of the substrate.
The drop-down list has several for common substrates used in the semiconductor
research.
Substrate Thickness The thickness of the substrate.
Wafer Diameter The diameter of the wafer in
millimeters.
Save Scan If set to NO, the scan data will not be
saved.
Auto Scan If set ON, the program scans from 10% to
90% of the substrate diameter. To define your own limits, set it to OFF. You
will be prompted to enter the starting and ending scan positions after the Wafer
Type field is edited.
Hole Diameter The diameter of the center
region to be skipped (for hard or compact disk applications)
Units Select MPa or dynes/cm2.
Wafer Type Select whether your wafer has Flat or
Notch.
Laser Selection There are two lasers in the
system, 670nm and 750nm. Select Automatic allows the system to decide
which laser to use.
9.4.6
After you finish modifying a process program,
click [Save]. Make sure you do not overwrite other programs.
10.0 Troubleshooting
Guidelines
10.1
Problem: The computer
screen is blank.
Cause: The Flexus is not enabled.
Solution: Enable Flexus on the WAND.
10.2
Problem: Cannot find
the WIN FLX icon after enabling Flexus on the WAND.
Cause: Previous user changed the WINDOWS displays.
Solution: Select the [Windows] from the WINDOWS
toolbar menu. Click the [Application]. The WIN FLX icon is in the
Application window.
10.3
The computer displays a Low Laser Intensity alarm.
Cause: The wafer to be measured is not reflective,
i.e. dark film.
Solution: No solution. The wafer cannot be measured.
Cause: There are
certain combinations of film thickness/refractive index that makes the
reflected laser beams to be destructively interfered.
Solution: Select laser of different wavelength in the
process program.
11.0 Figures
and Schematics
11.1
Tencor FLX-2320 (Front View)
11.2
Locater Ring used with Three Hot Plate Pins
12.0 Appendix
Theory of Operation
The FLX-2320 measures the changes in the radius of curvature
of a substrate caused by deposition of a stress thin film. The stress in the
thin film is calculated from the radius of curvature of the substrate using the
following equation:
σ = [E/(1- ν)]
[ h2/6Rt]
where
σ is the film stress (Pa)
E/(1- ν) is the biaxial elastic modulus of the
substrate (1.805E11Pa for 100 Si)
h is the
substrate thickness (m)
t is the
film thickness (m)
R is the
substrate radius of the curvature (m)