Woollam Ellipsometer

The J. A. Woollam Co. Inc. Variable Angle Spectroscopic Ellipsometer (VASE®) is used to determine the film thickness and optical properties of a sample. These data are obtained through measuring the change in polarization state of light that occurs when light is reflected off the surface of (or transmitted through) a sample. The VASE system accomplishes this by supplying a light beam of known wavelength and polarization state and determining the polarization state of the light after being reflected off of the sample.

Operating Instructions

Setup

1. Confirm that the Monochromator Power button is ON. If not, turn it on.
2. Press first the 'Lamp Power' and then 'Lamp Ignition' buttons on the Monochromator System.
3. Switch the vacuum on top of the ellipsometer control module to 'ON'.
4. Enter your user password at the ‘Log In’ Screen; see MiRC staff if you need an account on this machine.
5. Your username should appear at the top left under ‘Current User’.

Configuring the Software

1. In order for the ellipsometer to correctly process your sample some information must be entered:
• Project: Select ‘General Use’ (or select your group if it is present)
• Recipe: Select the recipe that best fits the sample you wish to analyze; keep in mind the type of substrate, type of film, and approximate film thickness. If there is no recipe matching the type of sample you wish to analyze, contact MiRC staff.
• NOTE: the choice of a recipe does not affect how the sample is scanned. After scanning, the data is archived where it can later be accessed and re-analyzed using a different recipe (see 'Reviewing Results' section below).
• Scan Pattern: The Woollam Ellipsometer is capable of automatically performing scans at several points along a wafer. Select the diameter and number of points of the scan you wish. Select ‘(none)’ for a single point scan.
• Sample Description: Enter a description of your sample which will allow you to find it later when you wish to view the results of the scan.
2. When the above information has been supplied, click MEASURE to continue.

Preparing the Ellipsometer for a Scan

1. The Ellipsometer will now provide you with a summary of the scan that is about to take place. If the information is correct, place your sample on the sample stage. It should be roughly centered on the sample holder stage. Flip the vacuum toggle switch to ‘Vacuum’ to insure your sample will be held in place. Select CONTINUE; otherwise, select CANCEL to abort the scan and re-enter your scan parameters.
• Alternatively, you may see a screen informing you that the system needs to be calibrated. See the section on calibrating the ellipsometer below.
2. When prompted, insert the alignment detector into slots on the center of the polarizer. Keep the guide pin (the longest pin on the detector) facing you and keep the detector centered with respect to the polarizer. Gently push the detector up into the slots on the polarizer.
• This can be tricky, especially at first. Keep in mind though, that you should NEVER force the detector. Doing so can break the pins and damage the detector. If the pins are properly aligned, the detector will EASILY slide into the polarizer and lock into place.
• With the detector in place, click CONTINUE.

3. When the system is ready, align the red crosshair with the center of the axes by turning the alignment knobs located on the sample stage. The crosshair must be within the centered box and the intensity meter on the left must be above the minimum line for the software to continue. Once inside the box, the coordinates of the crosshair will appear at the top. You should attempt to get the crosshair as close to perfectly centered as possible. Click CONTINUE when complete.
4. Using the shutter lever, change the size of the shutter until the iris (the open space in the center) is the same size as the beam of light passing through it. Hold a piece of paper up to the analyzer to view the beam and determine how wide to make the iris. Click CONTINUE when complete.
5. Adjust the height of the sample stage by rotating the height adjustment knob. Watch the blue bar and attempt to maximize the distance between the horizontal blue bar and the black one by turning the knob. The large, vertical blue bar should touch the horizontal one. The intensity must be greater than 5 to allow you to continue. If it is less than 5, try adjusting the height of the stage and check to make sure the shutter is open wide enough. If the intensity is still too low, contact MiRC staff. Click CONTINUE when complete.
6. The system will now scan your sample. Each point will be scanned in order and information about the scan will be presented on the screen. A medium to large point scan (13 or more points) can take a fair amount of time (1 hr. and up), depending on the material. The scan is fully automated, so you do not have to be present during the scan.

Removing Your Sample / Viewing Results

1. First, remove your sample from the sample stage by switching the vacuum toggle to ‘Vent’ and taking the sample off of the stage.
2. If this was your last sample to be scanned, turn off the ellipsometer lamp by pressing the lamp power button on the monochromator system. The lamp ignition button should turn off automatically as well.
3. Switch the vacuum pump on top of the ellipsometer control module to 'OFF'.
4. After the scan, the software will provide a summary of the results. The data provided includes the film thickness, refractive index (n), and mean squared error (MSE). The lower the MSE, the better your sample matches the model of the recipe selected prior to scanning. An MSE value > 100 typically suggests that the analyzation method did not accurately model your sample and that the information provided may not be accurate. In this case, you may wish to re-analyze your sample. Fortunately, this does not require scanning your sample again.
5. From the results pop-up, select OK if the results provided you with the information you required and you are done analyzing the current sample. You can then select 'Log Off' in the lower left of the dialog box. Alternatively, click VIEW RESULTS if you wish to view more detailed information about your sample or want to reanalyze your sample.
6. In the ‘Samples’ list box, select the sample you wish to view. The results are organized by date and the description entered prior to scanning is shown to the right.
7. After a sample is selected, the results are shown in a text box on the right side. Data for all analyzed points as well as statistical data of the points are shown.
8. The ‘Graph’ button towards the bottom will provide 3D graphs (or 2D slices) of different sample parameters and allow you to see how they change over the area of your sample. This graph is most useful for viewing the uniformity of your sample.
9. If your process had a large MSE value (>100), you can re-analyze your scan data on the results screen. Click RE-ANALYZE DATA, select a new analysis strategy, and click OK. The software will now re-analyze the scanned data with respect to the new strategy. If the MSE is lower, you know that the new analysis strategy better models your sample.

Calibrating the Ellipsometer

First locate the calibration wafer; it should be in close proximity to the ellipsometer. At the prompt, mount the calibration wafer onto the sample stage. Follow the onscreen instructions. Setting up the calibration wafer for a scan consists of the same steps as doing a normal scan. See the above instructions for information about the steps involved in preparing the machine to scan. After adjusting the height of the sample stage, the ellipsometer will begin the scan. This usually takes about 10 mins. Afterwards a summary of the calibration data will be shown. Make sure that the MSE of the calibration standard is reasonably low (< 10). Click OK and you will be prompted to mount your sample and prepare the system for scanning. Follow the above instructions for information about these steps.