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Flexus 2320 Stress Gage, stresstest

The Flexus 2320 determines wafer curvature by measuring the angle of deflection of a laser beam off the surface of the substrate. Film stress is determined by comparing the change in radius of curvature of the substrate, with and without the film. This means that the substrate and the film must be optically reflective in the wavelength used. Because semi-transparent films may absorb light (depending on the incident wavelength/angle and the film thickness/RI) the Flexus 2320 has two wavelengths (670 nm and 750 nm).

Picture and Location

Flexus 2320

 The tool is located at BC3 on the Lab Map.

 

Substrates:  The stage can accommodate substrates up to 8” in diameter.  The substrate rests on three pins, which determined the minimum substrate size (3”).  Samples smaller than 3” could be placed on a wafer or other flat holder.  Substrates without film must be reflective in one or both of the available laser wavelengths.  The substrate thickness should be known to reasonable accuracy, since the stress measurement scale with the square of substrate thickness.

 

Films and film processing:  As film stress is determined by the change in curvature, with and without the film of interest, there are some important considerations:

 

  1. The default calculation assumes the first measurement is done on the bare wafer and the second measurement, on the deposited wafer.  You can do the reverse (first measurement with film; second without) but will need to reverse the sign of the stress calculated (tensile versus compressive.)
  2. Wafer curvature results only when a film with significant stress is present on one side of the wafer.  If, for example, using an LPCVD furnace, in which both sides generally receive film deposition, the film must be removed from the backside only before making wafer curvature measurements.
  3. The film should be of sufficient thickness and stress to yield a significant result for your experiment.
  4. The film should be optically reflective in the range of interest.  Certain semi-transparent films (such as silicon nitride) will absorb strongly at certain thickness given a specific incident wavelength and angle and may yield a “Low Intensity Alarm.”  The Flexus has two wavelengths to choose from, so it may be another wavelength is less absorbing.  But it also may be that a different film thickness should be used. 
  5. The film should be fairly uniform in thickness across the wafer.  Non-uniform films may generate anamalous curvature measurements and may be difficult for the standard fitting algorithm, which assumes a simple parabola.  In this case deposit film to the backside, or etch the film off the front side, measure the curvature of the frontside, and take the inverse of the stress value reported.

Process Capabilities

Cleanliness Standard

 The Flexus 2320 appears in all three equipment groups (clean, semiclean, and gold).

 

How to Become a User

 

  1. Read all material on the SNF website concerning the Flexus 2320.
  2. Contact SNF training contact on the Equipment Summary page.

 

 

Operating Procedures

Reference documents:

Flexus Incorporated Thin Films Stress Measurement System Operation Manual

About the desktop:  The PC that runs the Flexus 2320 is not networked and is shared by all users.  No measures are taken to protect or routinely backup data, so it is recommended that you store your own data on a USB stick or other portable memory.

  1. Power: 

    1. Turn on main red power switch on the unit.  It will light when powered.
    2. Turn the laser key switch clockwise.  It will light when powered.
    3. If not on already, switch on the computer and the monitor.  It will boot up automatically.
  2. Start the computer:

    1. If not already open, click on the “WIN FLX” icon.
    2. Login or create a login for yourself.  Your data files will be stored under the login folder you create.
    3. Retrieve a process program.
      From the Edit menu, choose Process Programs.  Login.
      The standard 100 mm program is “Std100.prc”. 
      Maximum scan points 50
      Low intensity alarm 0.2
      Elastic modulus (MPa)(for 100 Si)  
      Substrate thickness (microns) 525
      Wafer diameter (mm) 100
      Hole diameter(mm) 0
      Units
      MPa
      Wafer type Flat
      Laser selection Atomatic

      See Table 1 for definitions.  You may create your own program – make sure to use another file name!
      • Click on load.
      • Close.
  1. First measurement (“Bare wafer”):

    1. Open door and load your wafer on the stage.  Make sure to center your wafer and orient the same way each time. Close the door.
    2. From the Measure menu, choose First (No Film).  On the First Measurement dialog, you will be prompted for:
      File:  Enter a file name for your data up to 8 characters in length.  This will automatically have the DAT extension (DOS name).
      ID:  This can be a sample ID up to 16 characters in length.
      Orientation:  This is the angle of the scan relative to your flat, however you define it.
      Comment:  A comment up to 12 characters in length.
      Substrate Thickness:  Enter the thickness of your substrate in microns.  This needs to be accurate, as stress scales with the square of the substrate thickness.
    3. Click “Measure” and the machine will scan.
    4. After measurement, a graph menu bar will appear.  You can view and save these graphs.
    5. Select "First measurement" window and click on "cancel".
  2. Second measurement (“After deposition”):

    1. Load your wafer on the stage.  Make sure to center the wafer and orient the same way as before.
    2. From the Measure menu, choose Single.  The Single Stress dialog will look much like the First Measurement dialog.
      File:  Enter the same file name as entered in the first stress measurement.  If you don’t remember, click on the Files button to display available file names.
      ID:  Enter the same ID as entered in the first stress measurement. Available ID’s will be displayed once File is entered.
      Orientation:  This is the angle of the scan relative to your flat, however you define it.
      Comment:  A comment up to 12 characters in length.
      Film Thickness:  Enter the thickness of your deposited film in angstroms.  This should be accurate as stress scales inversely with the film thickness.
    3. Click on “Measure”. 
    4. After measurement is complete, graphs and data, including measured radius of curvature and stress calculations are displayed.  A negative stress value indicates compressive stress, whereas positive stress indicates tensile stress.  If your deposited wafer measurement was done (“First (No Film)”) before the bare wafer measurement, remember to reverse the sign.
  3. More features: 

    See the Tencor FLX-2320 manual for more information about how data can be tabulated and saved for further analysis.  Some additional capabilities of the system include:

    1. Stress measurements made over time (“Stress-Time Measurement.”) 
    2. Stress measurements with temperature (“Stress-Temperature Measurement.”)
    3. Data analysis for thermal stress, elastic & expansion coefficient calculations and determination of diffusion coefficients.

 Shutdown:

Turn off main red power switch on the unit.
Turn off the laser key switch (counter clockwise)
Close program WINFLX
Switch off monitor.

Table 1: Process Program Window

Maximum scan points:  50 points is standard, although you can specify up to 1250.  The software is set up so that if scan profiles are saved, only 50 data points are saved.

Low intensity alarm:  Measurement will be ignored for values below this level and the system will not measure if starting values are below this level.

Elastic modulus (MPa):  The biaxial elastic modulus is specific for a substrate material and its crystal orientation.  A list of pre-programmed materials appears on a pull-down menu.

Substrate thickness (microns):  The substrate thickness is entered here, although this is also entered when the “First” measurement is taken. 

Wafer diameter (mm):  The substrate size.

Hole diameter (mm):  This feature, for hard drive disks or CD’s, allows you to exclude measurement from the middle of the substrate.

Save scan:  To save data for analysis, this should be set to “On”.  Only up to 50 points will be saved, whatever the Maximum scan points setting above.

Auto Scan:  When “On”, this automatically sets the scan length to between 10% and 90% of the substrate diameter (10 mm to 90 mm across a 100 mm substrate, for example.)  When “Off”, you will be prompted to enter the start and stop scan positions.

Units:  MPa or dynes/cm2

Wafer type:  Flat

Laser selection:  You can choose the laser wavelength used for scan.

 

 

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