High-Definition Vertical Scanning Interferometry Enables Greater Measurement Detail

Optical profilers are specialized interference microscopes that utilize the interference of two beams of light for characterizing surface topographies. For decades, Veeco has been the world's leading developer and manufacturer of interferometric optical profilers for a wide variety of research and industrial applications, from MEMS testing to tribology characterization of machined surfaces to the examination of biomaterials.

These highly precise, non-contact, full-field measurement instruments have been designed to deliver subnanometer measurement precision with accuracy, repeatability and reliability.

However, as the high-tech industry continues to pursue ever-shrinking dimensions, increasingly stringent quality demands, and faster throughput, it has been necessary to continue to expand the limits of interferometric technology. Metrology instrumentation manufacturers have had to respond to this challenge with continued advances in profiler technology. Veeco's new high-definition vertical scanning interferometry (HDVSI) mode utilizes an innovative algorithm to deliver subnanometer precision on a wide range of surfaces in a single measurement, significantly streamlining profiler operation for a range of applications (see figures 1 & 2).

Figure 1. Results from an HDVSI scan on a wavy surface. Note the fine details of the surface finish.

Figure 2. The low noise of HDVSI mode allows for the finely detailed measurement of this 3 nm tall grating.

Optical Profiler Operation Modes

Every measurement begins by evaluating the test sample and then determining the best way to measure it. Veeco's optical profilers have traditionally utilized two complementary modes of operation, phase shifting interferometry (PSI) and vertical scanning interferometry (VSI). PSI is very precise and is used to measure smooth, continuous surfaces, such as micromirrors, plastic films, and solar cell substrates. VSI can measure a wider range of surfaces, but with a somewhat lower level of precision than is possible with PSI.

Phase Shifting Interferometry, PSI

PSI is used to map optically smooth surface topographies and can achieve sub-nanometer vertical resolution better than any other optical method. Vertical resolution refers to the point where measurement data drops into the noise of the system. This mode can measure samples as tall as tens of microns, but samples with abrupt height discontinuities greater than about 150 nanometers on an otherwise smooth surface result in ambiguities that are difficult for this method to resolve. PSI mode uses a nearly monochromatic light source to generate interference fringes, and the surface topography is calculated by measuring the shape (position) of the fringes on the sample (see fi gure 3). Only a few frames are collected by the solid-state camera during the approximately 1 micron vertical scan, and the full-field measurement is completed in less than 200 milliseconds. The fringes generated represent a topography map of the sample's surface from which the shape is then derived. Veeco's PSI mode is fast, repeatable and highly accurate.

Figure 3. Fringes for a spherical surface in PSI mode (monochromatic illumination) are visible everywhere in the field of view.

Vertical Scanning Interferometry, VSI

Although less precise than PSI, VSI allows for the measurement of rough surfaces or those with larger height discontinuities. VSI mode works well for measuring samples that PSI cannot measure effectively, such as integrated circuit boards, paper, fabric or foam. Rough surfaces can be diffi cult to measure because only a little light is reflected back into the system. However, VSI mode is versatile enough to accept the high levels of illumination required to obtain measurements on rough surfaces while still providing good data for those areas on the sample where the fringe signal is somewhat saturated.

VSI typically uses a white light source and looks at the fringe contrast rather than the shape of the fringes as in PSI. During the VSI measurement the objective moves vertically down the full height range of the sample while collecting frames at the camera frame rate. although the scanner generally moves at speeds of about 5 microns per second, 100 microns-per-second scans are possible with reduced vertical resolution. During a VSI scan each pixel on the camera sees fringes only when the given point on the sample comes into focus (see figure 4). The position of maximum fringe contrast is then found for each pixel. Because the white light source has a short coherence length, fringes only appear around the best focus position. For this reason VSI can be considered an array of best-focus sensors. VSI is an extremely versatile mode, for it can measure the full range of most surfaces.

Figure 4. Fringes for spherical surface in VSI and HDVSI mode (white light illumination) are visible only very close to the best focus plane for three different scan positions: close to the bottom, middle and top of sample.

PSI and VSI are complementary methods and which mode to use depends on the sample surface. However, the advancement of new technologies and the presence of a wider range of applications for optical profilers have challenged the capabilities of both PSI and VSI. For example, for a MEMS device with a smooth surface and height discontinuities less than 150 nanometers, PSI mode could be used. A similar surface with step heights larger than 150 nanometers would require VSI mode; however, although VSI could measure the step height, noise inherent in VSI limits the vertical resolution to around 3nm, which is well below PSI mode's vertical resolution of 0.1nm. For these kinds of surfaces Veeco has developed a measurement mode that combines the accuracy of PSI with the versatility of VSI.

High Definition Vertical Scanning Interferometry, HDVSI

The new HDVSI mode combines the high vertical resolution of PSI with VSI's ability to measure discontinuous and rough surfaces. HDVSI further advances Veeco's surface mapping technologies in a number of significant ways. From a single set of data acquired during a VSI scan, both the position of maximum fringe contrast (VSI) and the position of the fringes on the sample (PSI) are calculated concurrently and independently of each other. The VSI data provides an approximate surface profile, while the PSI information imparts sub-nanometer precision to the measurement (see figure 5). System features such as Veeco's reference signal technology help overcome error sources such as scanner nonlinearity and mechanical vibration.

Figure 5. With HDVSI, the sharp features of these 90nm tall cross-hatch bars can be measured precisely and easily

In technical terms, the patent-pending HDVSI mode applies a unique PSI quadature-demodulation algorithm to the fringe data already contained in the VSI measurement. This procedure allows the position of the fringes (phase) to be calculated independently of the position of maximum fringe contrast. The VSI data is then combined with the PSI data to avoid the ambiguities inherent in PSI-only measurements on rough or discontinuous surfaces. The resulting topography map merges the sub-nanometer vertical resolution of PSI with the large vertical scanning range of VSI (see figure 6).

Figure 6. The wide measurable height range of HDVSI mode allows for the measurement of this charge vortex lens (about a 2 micron step size), and HDVSI's relatively low noise enables the markings of the ebeam process on the smooth surface of the vortex to be observed. (Vortex lens made by Daniel Wilson, JPL using ebeam lithography.

HDVSI's importance is its ability to deliver sub-nanometer precision on a wide range of surfaces in a single measurement. With HDVSI, randomly rough surfaces or surfaces that change over time can now be measured. For example, hip joint replacements require testing both right after production and after a period of wearing. While PSI would be used to measure the smooth post-production surface, the worn, corroded or roughened surface might necessitate VSI mode. The single mode, HDVSI, could be used to measure both surfaces. In other words, HDVSI can go from super-smooth to rough surfaces all in one measurement with near-PSI precision (see figure 7).

Figure 7. HDVSI mode was used to measure the roughness of this sample; the result was then compared to the calibrated value attained using a stylus profiler with a fine.

HDVSI performs particularly well on smooth surfaces that contain large discontinuities or slopes such as MEMS/ MOEMS devices, gratings and microoptics, which may be difficult to measure at the edges with other techniques. Measurements where changes in surface roughness over time can be tracked are another application where HDVSI delivers precise results. HDVSI provides a high level of measurement precision and flexibility to the material science, semiconductor and micro- and nano-technology industries-all in a single measurement mode.

This information has been sourced, reviewed and adapted from materials provided by Veeco.

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