Ellipsometric Data Acquisiiton Options

Contents

1 Optical Components

1.1 Track Polarizer, tracking tolerance

1.2 Fixed Polarizer Setting

1.3 Zone Average Polarizer

1.4 Auto Retarder

1.5 Revs/Measurement

2 Dynamic Averaging

3 Sample Type

4 Monochromator Slit Width

5 Principal Angle Scan Configuration

6 Transmission Measurement

7 Slit grating changes across angles

1. Optical Components

This controls how the polarizer is positioned for data acquisition and how the variable retarder is used.

1.1 Track Polarizer, tracking tolerance

• This selection causes the polarizer azimuth to be tracked to psi value being measured, which minimizes errors in the measured data. Unless you have a good reason not to, always leave the polarizer tracking enabled for ellipsometric data acquisition.

1.2 Fixed Polarizer Setting

• This selection causes the polarizer angle to be held fixed at the value specified by the user in the adjacent box during the acquisition of data.

1.3 Zone Average Polarizer

• This selection causes the measured data to be averaged over data acquired with the input polarizer positioned in each of four quadrants. This can help to eliminate systematic errors due to errors in the knowledge of the polarizer calibration offset.

1.4 Auto Retarder

• The options are "Off" and "High Accuracy".
  • Off: the auto-retarder is not used for the measurement.
  • High Accuracy: the data is acquired at five different positions of the retarder.
    • Note: the Rev/Measurement value is divided between the different positions. For example, if "High Accuracy" is selected and Rev/Measurement is set to 100, then each data point will consist of five 20 rev measurements.

1.5 Revs/Measurement

• This is the number of analyzer cycles which will be averaged over each measurement.

2. Dynamic Averaging

• This box contains options which allow the user to use dynamic averaging of the data rather than averaging over the same number of analyzer cycles at each measurement.
• Dynamic averaging lets WVASE determine the number of analyzer cycles to average by averaging the data until the standard deviation of the noise on the averaged data falls below some cutoff level.
• This should yield experimental data sets with roughly constant standard deviations, although it may not be possible to obtain the specified minimum standard deviation at each data point.
  

• Dynamic averaging enable check box: When this check box is enabled, the data acquisition will be performed with dynamic averaging of the measured data.
• Maximum # of revs: This is the maximum number of analyzer cycles which will be averaged for any data point.
  • If the maximum number of analyzer cycles is reached, the measured data are calculated from the averaged data, and the appropriate standard deviation (larger than the cutoff value for convergence of the dynamic averaging) will be stored with the data values in the Experimental Data window.
• Threshold intensity: This is the minimum detector signal intensity for which dynamic averaging will be performed.
  • If the detector signal intensity is less than this value, the maximum # of revs will be averaged for the given data point.
• Min. Usable Intensity: This is the minimum intensity that must exist in order for a data point to be considered valid.
  • If the intensity for a data point is below the minimum, the data point is skipped.
  • The idea is to skip data points that would produce unsable data even if the full averaging was performed.
  • This is primarily intended for DUV work and to skip over the IR absorption band near 0.87 eV when using a UV enhanced fiber.

3. Sample Type

• The box is used to specify the type of measurement.
• The most common measurement type is "Isotropic Sample".
• This measurement assumes that an isotropic sample is being measured.
• There are several other types which are primarily for anisotropic samples.
• If an Auto-Retarder is installed, depolarization measurement types will be included in this list.
  

• Isotropic Sample: The standard ellipsometric parameters psi and delta are measured, based on the assumption that the sample Jones matrix is diagonal. This should be the case unless the sample is very rough or anisotropic.
• Isotropic + Depolarization: This is available if the ellipsometer has an auto-retarder installed. During acquisition, both ellipsometric and depolarization data types will be stored in the Experimental data window.
  • The depolarization can be used in the modeling of layer thickness non-uniformity, patterned substrates, finite spectral bandwidth, finite angular bandwidth, and backside effects for transparent substrates.
• Anisotropic [Slightly]: This mode works best if the sample Jones matrix is dominated by the diagonal values Rpp>>Rps and Rss>>Rsp. This mode assumes that the anisotropic effect is small enough that placing the Polarizer near psi value is still a useful technique. Therefore, tracking the polarizer is allowed for the anisotropic measurement mode.
• Anisotropic [Highly]: This mode is a regressed data like the slightly anisotropic mode, but a fixed pattern of polarizer positions is used (-60, -30, 0, +30, +60, and +90 degree). This acquisition type can use the Auto-Retarder.
• Anisotropic [no regression]:
  • This mode was implemental early in the development of generalized ellipsometry. Although this mode can be used if light reaching the detector from the anisotropic sample is an incoherent mixture of light beams, it is recommended that you use one of the Mueller-Matrix measurement modes instead.
  • This mode robust anisotrpic measurement mode, but it does not allow the Auto-Retarder to be used and the measured psi and delta values do nto have a direct Jones matrix representation. This anisotropic measurement mode acquires data with the polarizer set at +45, -45, 0, and 90 degree, and then stores data of types E+45, E-45, E0, and E90. These stored data are simply direct conversions of the measured alpha and beta values to psi and delta with the polarizer set to the 4 values listed above. (For an explanation of alpha and beta and their relationship to psi and delta, see Eqs. (2-27) and (2-28) in the Jones matrix analysis of the rotating analyzer ellipsometer" section of Chapter 2 of the manual). These E+45, E-45, E0, and E90 data types can be modeled, but they do not have a direct physical significance.
• Fixed-Pol Anisotropic: No description in the manual.
• Mueller Matrix [General]: For VASE with AutoRetarder instruments, the Mueller Matrix Mode measures the first 3 rows of the sample's Mueller Matrix. The Mueller matrix data types are real-valued equations that described how the sample transforms the Stokes vector of the incident light beam. From the Mueller Matrix elements, WVSAE also automatically calculates and records the generalized ellipsometric parameters AnE, Aps and Asp, as well as the Jones Quality factor qJ data type.
• Isotropic M-Matrix: No description in the manual.

4. Monochromator Slit Width

• This box contains options which are only available for users with CVI monochromators, and allow the user to control the monochromator slit width used for data acquisition.
  

• Max. slit width: This is the maximum slit width which will be allowed at any data point during the scan.
  • The maximum allowed value is 2000 mm.
• Auto slit adjustment:
  • If auto slit adjustment is not enabled, all data points will be acquired with the monochromator slit set at the value specified in the max. slit width.
  • If auto slit adjustment is enabled, the max slit width will be used at each data point unless the detector is overloaded.
  • If a detector overload occurs, the slit width is decreased to half of its current value, and the measurement is repeated. This process is repeated until the slit width is small enough to prevent overloading of the detector, after which the measurement is performed.
  • The slit width is then automatically increased at the next data point to provide the maximum possible signal without overloading the detector.

5. Principal Angle Scan Configuration

• VASE system can acquire (and graph) ellipsometric data in which the angle of incidence is automatically varied to maintain delta close to 90 degree (or within any specified range) during the spectroscopic scan.
• The option is enabled checking the "Enabled" box.
  • The "Min" and "Max" boxes specify the tracking range for the measured Delta value, i.e. the angle of incidence will be adjusted until the measured Delta is within this range.
    

• When the principal angle scan mode is enabled, the angle range values specified on the VASE Scan dialog box have different meanings:
  • These values are used to guide the angle search algorithm.
  • The starting and ending angles of incidence are used to bound the angle search range, the current angle of incidence is used to start the search, and the angle increment specifies the step size when searching for a delta=90 degree crossing (a disection algorithm is used to narrow in on the correct angle after a delta=90 degree crossing is located).

6. Transmission Measurement

• Selecting this option causes the ellipsometry measurement to be in transmission-mode.
• During measurement, the detector will remain fixed at 180 degree, and the resulting data will be marked as ellipsometryc transmission data.

7. Slit grating changes across angles

• This option is used to speed data acquisition when using a CVI monochromator by acquiring data for all of the angles before switching gratings (CVI monochromators take about three minutes to switch gratings).