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Manuel pour Optical profiler Veeco Wyko NT1100

Veeco Wyko NT1100

Table des matières: CMI

  1. Introduction
  2. Principle
  3. Equipment description
  4. User guide
  5. References and docs

I. Introduction CMI

The Veeco Wyko NT1100 is an optical profiler providing three-dimensional surface profile measurements without contact. Two working modes are available: VSI (Vertical Shift Interference) and PSI (Phase Shift Interference). The VSI mode, based on white light vertical scanning interferometry, is dedicated to device measurement. The maximum measurable topography is 1mm and there are some limitation to measure slopes depending on the optics numerical aperture and surface roughness (see table 1). The PSI mode, based on optical phase-shifting, is dedicated to roughness measurements with maximum 150nm pick to valley topography. The system is able to run automatic stitching to grab larger fields.

II. Principle CMI

How the Wyko Surface Profilers work ?

Wyko surface profiler systems are non-contact optical profilers that use two technologies to measure a wide range of surface heights.

PSI mode

A white-light beam is filtered and passed through an interferometer objective to the test surface. The interferometer beam splitter reflects half of the incident beam to the reference surface within the interferometer. The beams reflected from the test surface and the reference surface recombine to form interferences fringes. These fringes are the alternating light and dark bands you see when the surface is in focus.

Veeco Wyko NT1100

During the measurement, a piezoelectric transducer (PZT) linearly moves the reference surface a small, known amount to cause a phase shift between the test and reference beams. The system records the intensity of the resulting interference pattern at many different relative phase shifts, and then converts the intensity to wave front (phase) data by integrating the intensity data.

This technique for resolving surface heights is reliable when the fringe pattern is sufficiently sampled. When the surface-height difference between adjacent measurement points is greater than λ/4, height errors in multiples of λ/2 may be introduced and the wave front cannot be reliably reconstructed. Thus, conventional phase-shifting interferometry is limited to fairly smooth, continuous surfaces. To resolve rougher surfaces, Wyko surface profilers use vertical-scanning interferometry techniques.

VSI mode

A newer technique than PSI, vertical scanning interferometry was developed at Wyko. The basic interferometric principles are similar in both techniques : light reflected from a reference mirror combines with light reflected from a sample to produce interference fringes, where the best-contrast fringe occurs at best focus. However, in VSI mode, the white-light source is filtered with a neutral density filter, which preserves the short coherence length of the white light, and the system measures the degree of fringe modulation, or coherence, instead of the phase of the interference fringes.

The interferometric objective moves vertically to scan the surface at varying heights. A motor precisely controls the motion. Because white light has a short coherence length, interference fringes are present only over a very shallow depth for each focus position. Fringe contrast at a single sample point reaches a peak as the sample is translated through focus.

The system scans through focus (starting above focus) as the camera captures frames of interference data at evenly-spaced intervals. As the system scans downward, an interference signal for each point on the surface is recorded. Finally the vertical position corresponding to the peak of the interference signal is extracted for each point on the surface.

Operational differences between PSI and VSI

VSIPSI
Neutral density filter for white lightNarrow bandwidth filtered
Vertically scans the objective actually moves through focusPhase-shift at a single focus point the objective doesn't move
Processes fringe modulation data from the intensity signal to calculate surface heightsProcesses phase data from the intensity signal to calculate surface heights

Why do VSI and PSI use different types of light?

The light for both techniques originates from a white-light source; however, it is filtered during PSI measurements to produce red light at a nominal wavelength of 632nm. VSI measurements use a neutral density filter, preserving the short coherence length of the white light.

Because white light has a short coherence length, the fringe contrast is highest at best focus but falls off rapidly as you move away from focus. A white-light source works best for vertical-scanning interferometry because the technique requires high modulation at a precise focus point. On the other hand, white-light focusing does not work well for phase-shifting interferometry. A filtered light source works best for phase-shifting interferometry because it has a longer coherence length than white light, so high-contrast fringes are present through a larger depth of focus. This increases the accuracy of your measurements, especially when the objective has a short depth of focus or the sample has tilt that cannot be removed easily.

System Performance

Range

Range refers to the greatest vertical distance the profiler can accurately measure. The limits of dynamic range for each mode are listed in the table below.

In PSI, six intensity frames of data are recorded as the PZT is moved a distance of p/2 or λ0/4, between each frame ; where λ0 is the center wavelength of the bandpass filter, usually approximatively 632nm. The system determines phase data from the intensity data, then calculates surface heights. The surface height data is then integrated to remove 2p effects. If the surface is smooth and continuous, such that integration errors are not encountered, the resulting data is used directly to generate the surface map.

PSI is reliable for smooth surfaces in which the height change between two adjacent points is not more than approximately 160nm (λ0/4 for a nominal measurement wavelength of 640nm). If you try to use PSI mode for higher steps, you will see integration errors (lines of discontinuity) in your data.

VSI is reliable for rougher surfaces, because the range is limited only by the scan length allowed by the linear translator.
ModeRange
PSI160 nm
VSI1 mm

Resolution

Resolution refers to the smallest distance the Wyko surface profilers can accurately measure. It can be in terms of lateral or vertical resolution.

Lateral resolution

Lateral resolution is a function of the magnification objective and the detector array size you choose. Each magnification objective has its own optical resolution based on the magnification and numerical aperture (NA) of the objective. Optical resolution refers to the smallest surface feature the objective can distinguish.

If you select an objective and array configuration in which the detector sampling interval is much smaller than the optical resolution, you will be oversampling the surface. In this case, the resolution is optically limited. The resulting surface map may show blurry images because the features cannot be resolved optically. Generally, you should select a configuration in which the detector sampling interval is larger than the optical resolution. In this case, the resolution is limited by the detector. However, if the detector sampling interval is considerably larger than the optical resolution, you will be undersampling the surface, which could result in undetected surface features.

Vertical resolution

Each mode has different resolution limits. Resolution values for PSI and VSI are listed in the table below. The values are in terms of Rq and are based on measurements of a smooth surface (Rq of 1.5Å or less).
ModeVertical Resolution
Single measurementMultiple measurements (averaged)
PSI3 Å1 Å
VSI3 nm< 1 nm

Accuracy

Accuracy refers to how closely a measured value matches the true value. It is determined relative to a known, traceable standard.

To correct for aberrations in the interferometer's optics, a measurement of the internal reference mirror is generated and subtracted from measurements. This removes aberrational effects, which are significant for very smooth surfaces.

Stitching

The system is able to run automatic stitching to grab larger fields.

III. Equipment description CMI

Figures 1 and 2 show the main parts of the profiler hardware: Objectives main characteristics are given in table 1 and field size for objective/FOV combination are given in table 2.

The NT1100 system
Figure1: the NT1100 system.

FOV checking
Figure 2: FOV checking.

Magnification5X20X50X
Interferometer typeMichelsonMirau
Numerical aperture0.120.40.55
Working distance (mm)6.74.73.4
Optical resolution (μm)2.50.750.55
Practical maximum slope* (deg)5.217.725

Table 1: Objectives characteristics
* As measured on an optically smooth surface. Practical slope limit for non specular surface may be higher

 5X20X50X
FOV 0.52.47 x 1.880.62 x 0.470.25 x 0.19
FOV 1.01.24 x 0.940.40.55
FOV 1.50.82 x 0.634.73.4

Table 2: field size (mm x mm) for the different objectives/FOV combination.

IV. User guide CMI

  1. Login on Veeco Wyko NT1100 using the zone 4 main PC.
  2. Open Vision32 software on the NT1100 PC, select LAB mode, password is "wyko",
  3. Select the "sun icon" in Vision 32, and increase the window by hitting the yellow rectangle in the down right corner,
  4. You may calibrate the tool before starting. Calibration procedure is available near the tool and in chapter V.
  5. Hardware and setup selection:
    • Chose your hardware (filter, objective and FOV),
    • Select the correct setup. One setup is a combination of one mode, one objective, one FOV, one filter and one stitching field (for example: V50bN_1x1). Table 3 gives a sum up of the main different combinations.
  6. Modes:
    Roughness or device
    Objectives:
    Slopes or device size
    FOV:
    Usually 1.0
    FiltersStitching fields
    PSI -> P
    VSI -> V
    5X -> 5
    20X -> 20
    50X -> 50
    0.5 -> a
    1.0 -> b
    1.5 -> c
    High mag (orange) -> H
    No filter (VSI) -> N
    Low mag (red) -> L
    1 x 1
    2 x 2
    3 x 3

    Table 3: combinations to determine a setup.

  7. Find fringes:
    • Put the substrate on the stage (mind the objectives, no touch is allowed), move the sample under the objective, adjust the light intensity to see some contrast on the monitor, and drop the optics with the focus lever at about 2mm from the substrate top surface,
    • Lift the objective with the focus lever until some fringes appears (you are focusing at the same time),
      • For the VSI mode adjust the stage tilt to get only 3 or 4 fringes on a flat surface,
      • For the PSI mode adjust the stage tilt to get only 1 fringe and adjust the focus to get the maximum brightness,
    • Start the measurement,
    • Check quickly the result with the 2D Analysis or the 3D Interactive Plot tools.
    • If necessary, save the data on the hard disk (C:) in the public folder.
  8. Before leaving the profiler:
    • Drop the light intensity to the minimum and switch off the TFT monitor (the small one),
    • Close the vision 32 and Galil,
    • Lift the optics, pull the stage out of the optics, and unload your sample (mind the objectives, no touch is allowed),
    • Logout on Veeco Wyko NT1100 using the zone 4 main PC.
  9. Outside of the clean room:
    • Data can be imported from your desktop using the network: server sti, "\\cmipc51\@public",
    • You need to install the vision32 software on your desktop for data post treatment. The software is available on the sti file server and located in the folder "\\sti1files\cmi-transfert\Veeco Wyko NT1100 - optical profiler". At the end of the installation, read the EPFL-CMI.doc to complete the installation.

V. Calibration procedure

VSI - calibration

 

PSI - calibration

 

VI. References and docs CMI