Ellipsometry and absorption: Queen's Physics Lab

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Introduction[edit | edit source]

A spectroscopic ellipsometer (SE) uses polarized light to determine optoelectronic properties of a material. Requiring some knowledge of the test specimen, such as the number and thickness of films deposited on a substrate transparent in the operating range of the SE, the results of analysis are very powerful.

Equipment Information[edit | edit source]

The ellipsometer used is a J.A. Woollam M-2000V variable-angle spectroscopic ellipsometer (VASE). The VASE is connected to a computer where WVase32 software is used for data collection. Data is saved by the program as a .txt file that can be imported into other software, such as Microsoft Excel, for analysis.

Protocol[edit | edit source]

  • Open the WVase32 software by clicking the icon on the desktop. Activate the Hardware window by clicking it.
  • Initialize the hardware by turning on the VASE power sources (two green buttons on the lower left side of the vibration isolating table) as well as the vacuum pump, then click Initialize and enter a user name.
  • Set the incident and reflected angles by clicking Setup, then Angle of Incidence. Enter the measured angles on each stage to the computer, and use the jog feature to zero the stages on those positions. Note that at this point, the detector should be at 180-deg and the stage at 90-deg; if this is not the case, move the stage and detector using Setup and Move.
  • To ensure proper alignment between the source and detector, ensure the stage is out of the beam line and perform an alignment. To ensure clearance for the stage, set the angle of incidence to 90-deg, and adjust the stage position using the micrometer dial or the beam alignment with the dials located on the source. To align the detector, click Acquire Data then Align. Move the sample stage using the small allen wrench until the absolute values of X and Y are within unity. It is important to not move the light source (i.e. by adjustment) once the alignment has been established, and only the detector should be modified if necessary.
  • Verify the signal strength, using Setup and Display Signal, then click Ok. The average AC should be approximately 35-40, and a gain of approximately 1 or 2.

Once the machine is set up, data can be collected.

  • Click Acquire Data then R&T Data.
  • Click Yes to replace the existing data from the last scan. Be sure to save any important data before clearing it to start a new test.
  • A baseline measurement can be subtracted from all other samples during testing. This allows the effects of, for example, the substrate or a film layer common to all samples be removed from the transmission and reflection data measured. This requires the first sample to be tested have all (and only) the layers to be omitted from the analysis and saved with a file name that can easily be recalled for future reference. When baseline data is used, select Baseline as the mode of data collection.

To determine the absorption properties of the material in the sample, note that

1 = T + R + A

To obtain absorption data, this can also be rearranged to

A = 1 - T - R

Where

         T = percentage of light transmitted through sample
         R = percentage of light reflected by sample
         A = percentage of light absorbed by sample
         1 = total incident light striking the sample surface


Tips[edit | edit source]

Quick Calibration[edit | edit source]

If uncertain about the date of the last machine calibration, perform a quick calibration. If the machine has recently been moved, or if it has been a significant while since the last complete calibration, preform a complete calibration on the system. The procedure for a quick calibration is as follows:

  • Set the angle of incidence to 65-deg.
  • Mount the provided silicon calibration wafer to the sample stage by carefully place it over at least one of the two ports connected to the vacuum pump. Take care in mounting the sample, and use the groove on the stage to work the tweezers in and around the sample.
  • Move the sample stage using micrometer dial to position and align the beam with the detector, in addition to moving the X and Y dials on the stage. Once the alignment is complete, press ESC or cancel.
  • Click Acquire data then Calibrate.
  • Pick calibration mode = normal.
  • If no message appears on the screen and data is displayed, the calibration has succeeded.

Transmission Measurements[edit | edit source]

In order to obtain measurements of the amount of light transmitted through the sample, the source and detector must be moved to be inline with one another and the sample stage must be perpendicular to the beam line between the source and detector. To do this, move the detector to 90-deg, then the stage to 0-deg.

  • Take care in placing the samples on the stage. For heavier samples, make sure it covers both ports connected to the vacuum pump.
  • When setting reference positions for both stages, click the Jog buttons slowly to align the stage and detector. It is crucial that these positions are set with care to ensure an accurate analysis.
  • To confirm the repeatability of measurements, or the consistency of your samples, be sure to test multiple spots on the same sample. Use caution in handling the sample, and do not slide the sample on the stage to avoid scratching of either the sample or the stage itself.

Other Equipment in the Materials Physics Cleanroom[edit | edit source]

For all booking information, rates and schedules, please see the Materials Physics cleanroom page.