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Woollam M2000 Spectroscopic Ellipsometer, woollam

The Woollam M2000 is a spectrometer which will measure phase changes in polarized light to estimate the thickness and optical constants of films.

Picture and Location

 Woollam M2000

Background

 

The Woollam tool uses ellipsometry to characterize thin films.  Ellipsometry detects the phase change in polarized light as it reflects from or transmits through samples.  The Woollam does not directly measure thickness and optical constants, but rather uses collected data to solve models which estimate the thickness and optical parameters of interest.  The general sequence of operation is: 1) Measure, 2) Create Model, 3) Fit, 4) Results.

 

The Woollam measures Ψ and Δ which characterize the change in polarization as the light is reflected from the surface.  Ψ characterizes the amplitude and Δ characterizes the phase difference.  

Process Capabilities

Cleanliness Standard

 The Woollam M2000 Spectroscopic Ellipsometer appears in all three equipment groups (clean, semiclean, and gold).

Performance of the Tool

What the Tool CAN do

  • Measure reflected polarized light. 
  • Fit models of optical constants with the measured data to determine thickness and optical constants of films.
  • Complex modeling of optical data is possible with the powerful Woollam software.  The basics are covered here but the tool manual for advanced use is available to be checked out.  Contact Michelle Rincon (mmrincon@stanford.edu) if you would like to check out the manual.

 

What the Tool CANNOT do

  • Directly measure thickness or optical constants.

  

The Woollam is capable of using variable angles of incidence.  In order to optimize the reflection from the surface only (and not the substrate), measurements around the Brewster angle of the substrate is recommended to enhance sensitivity to film parameters.   Multiple angle measurements can be used to improve confidence in the final answer.

The Woollam measures reflected light across the 210-1600 nm spectrum.  The spot size varies with angle of incidence from 1.1-11.5mm (It is 2mm normal to the surface).

 

Process Monitoring

A calibration wafer is run after lamp changes and for error recovery.  Monthly calibration checks are to be scheduled.

The System

The SNF Woollam uses a Xe Arc Lamp as the light source.

The SNF Woollam has an 8-in chuck.  The standard set up for the guide pins is to accommodate a 4-in wafer.  Please contact staff if 8-in wafer set up is needed.  Wafer pieces can be used with both 4-in and 8-in set up.

 

Contact List and How to Become a User

Contact List

The following people make up the Tool Quality Circle:

  • Process Staff: Michelle Rincon (mmrincon@stanford.edu)
  • Maintenance: Gary Sosa (gsosa@stanford.edu)
  • Super-Users:

 

Training to Become a Tool User

  1.   Read all material on the SNF website concerning the Woollam M2000 Spectroscopic Ellipsometer.
  2.   Shadow lab users on Woollam.  After this step, the user should be familiar enough with the equipment to be able to run the equipment on their own.
  3.   Contact Michelle Rincon (mmrincon@stanford.edu) for qualification.  The final certification will consist of 2 tests: 1) written quiz, and 2) in-lab oral exam.

Operating Procedures

The software has uses 6 different windows to perform different functions.  First, the Hardware window is used to collect the data.  After data collection the Hardware window can be closed and the general work flow is in a counter clockwise direction starting in the upper right hand corner of the screen with the Data window.  As you click on each window, a different set of menu instructions is shown.  Right clicking in any window also shows the list of menu options for that window.

 

Typical work flow:

1.     Enable in Coral

2.     Turn on the lamp with “Lamp Ignition” button.

3.     Load wafer on chuck.  Turn on vacuum switch.

4.     Logon to computer screen.

5.     Open WVase32 software.

6.     In Hardware Window:

a.     Initialize (NEVER calibrate the system.  This function requires a calibration wafer which is not available to users. )

b.     Acquire data using Spectroscopic Scan.

                                      i.     Align Sample- this will find the center of a 4-in wafer.

                                     ii.     Translator move- if you wish to measure a different spot on the wafer, use the arrows to jog the sample table.  The default movement is 1mm per click, but there is a window available to change the distance/click.

                                   iii.     Select angle(s) for measurement.  In order to reduce the polarization shifts from the substrate it is best to chose angles near the Brewster Angle for your substrate.  (Typical selection for Si is 65-75 degrees).

                                   iv.     Select angle step size for measurement.  More angles will give you more data and improve confidence in the results, but will also take more time.  (Typical selection for a single thin film on silicon is 3 angles, so the angle step size would be 5 to measure at 65, 70, and 75 degrees).

                                     v.     Select revs/measurement.  The default setting is 5.  More revs/measurement will decrease the amount of noise in the data collection but will also take longer.  (Typical value is 40 revs/measurement)

                                   vi.     DO NOT CHANGE POLARIZER SETTING.  This is a hardware setting that is fixed.

c.      Close Hardware Window after data is collected.

7.     Verify Data is visible in Data window and curves displayed in Graph Window.  Default settings for graph are Ψ and Δ on the y-axes and nm on the x.  Right clicking on the graph will show options for changing the data that is graphed and the axes.

8.     Create model.

a.     Click in Model Window to select the window.

b.     Select Add layer.  For Si wafers, use Si.mat, 1mm default thickness.

c.      Add layers as required for your sample.  Select parameters for fitting in the pop up box.  Note some layers will not have data for the entire range of the Woollam spectrum.  Data outside of the range will be extrapolated.  If you do not wish to have the data extrapolated, go back to the Data window and use the menu options to select the range desired.

9.     Generate Data

a.     Click in Generate Data window to select the window.

b.     Select Generate Data.  Model data will be displayed in Graph Window with measured data.

10. Solve for fit parameters

a.     Click in Fit Window to select the window.

b.     Select Normal Fit.  Woollam will generate an answer for the fit parameters.  Verify fit curves against measured data and check MSE.

11. Modify model if necessary, then repeat steps 8 and 9 until you are satisfied with the answer.

12. Turn off vacuum and remove wafer.

13. Turn off lamp by powering down system, waiting for lights to go off, then turning power back on.  Lamp ignition light will remain off.

14. Close Vase software and lock computer screen.

 

Some options (more frequently used options- many more exist and please feel free to document any helpful procedures that might be useful to lab members!):

1.     Transmission Scan instead of Spectroscopic Scan.  Transmission scans require that the wafer be set perpendicular to the chuck.  SNF does not have a jig available for users but many users fashion their own out of lab-approved materials.

2.     Some films will not have the optical constants in any of the tool databases.  Modeling of the films as Cauchy layers, EMA layers or GENOSC layers can be done as well.  Please contact staff for training if you need help.

 

 

 

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