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MetroPro Films Analysis Application

Introduction

Figure 1.

The NewView™ family of scanning white light interferometric (SWLI) microscopes creates topographic height data from interference signals produced when white light is reflected off of a surface and interfered with the light reflected off of a high quality reference surface. The interference objective scans perpendicular to the test surface while a camera records the interference fringes at discrete points in z within the scan.

A computer analyzes the fringes to precisely determine the height of the sample surface at every pixel. Historically, it has been important to ensure that there is only one modulation signal present at each point. This has made it difficult to measure certain samples with transparent films often found on samples in nearly every industry, but particularly semiconductor, MEMS, and flat panel displays.

These films create multiple modulation signals at the sample point. Using traditional analysis methods on these signals can potentially lead to incorrect or nonexistent data.

Figure 2.

The new Films Analysis option for the NewView using MetroPro™ 8.1.1 (or later) eliminates this restriction and allows the user to perform the following tasks:

  • Isolate and measure the film’s top surface
  • Isolate and measure the substrate surface
  • Measure film thickness, film surface, and substrate surface simultaneously

Films Analysis for MetroPro

In order to separate the multiple modulation signals, The Films Analysis software uses ZYGO's patented TopSlice and FilmSlice techniques, which compare the complex white light interferometry signal created by a thin film sample with the simpler system characterization signal created by a smooth sample such as an optical flat without a film. This method allows for excellent interpretation of film structures as thin as 1.5 µm optical thickness.* As the majority of transparent films have indices of refraction between 1.4 and 1.6, a 1.5 µm optical thickness converts to approximately 1 µm physical thickness lower limit.

Instrument Guidelines

All NewView 5000 and 6000 systems are capable of measuring the top surface of a film >1.5 µm optical thickness, regardless of the objective being used. Secondary surface and film thickness measurement capability are dependent upon the thickness of the film, the objective, and the type of NewView being used to make the measurement.

Illumination Considerations
High quality measurements through a transparent film require that particular attention be paid to the numerical aperture (NA) of the illumination used for the test. This is best accomplished through the use of an illumination aperture stop as found in the NewView 6300. In the absence of a true aperture stop, the microscope objective itself can be used to restrict the illumination NA, however this method, will restrict available objectives to those with lower magnifications.

Index of Refraction
In order to correctly measure the secondary surface or the film thickness, it is important to have a good understanding of the effective index of refraction of the film. If this is not known ahead of time, it may be found by measuring a step from the top of the film to the substrate and computing the ratio of the film thickness to this step height.

NewView 6300
The 6300 uses an enhanced illuminator consisting of an LED source and a variable aperture stop to restrict the numerical aperture of the illuminator. The stop allows for all magnifications to be used for secondary surface and film thickness measurements. Objectives with magnifications of 5x or less can characterize films between 1.5 µm and 75 µm of optical thickness for top surface, secondary surface, and thickness. Objectives greater than 5X magnification can measure these parameters as well; however, the recommended maximum film thickness will decrease as magnification increases. The following chart depicts this relationship. Based on material characteristics, the upper limit for a given objective may be higher or lower.

NewView 6200 and NewView 5000
All NewView systems are capable of measuring the top surface at all magnifications, as well as secondary surface and film thickness when an objective magnification of 5x or less. The NewView 6200 and NewView 5000 are not recommended for film thickness or secondary surface analysis at magnifications greater than 5X. For these applications, we recommend the 6300, which incorporates a variable aperture stop to enhance thin film performance.


Objective/System/Application Matrix.

Typical Performance

Repeatability over 20 measurements.

As film thickness measurements are a comparison of two surfaces, repeatability and accuracy performance can be compared to that of a step height measurement. Several transparent films ranging from 1 µm to 50 µm of physical thickness were measured using various objectives. This table shows the excellent repeatability that can be expected.

Typical Data

MetroPro Films Analysis simultaneously provides data for the top surface, second surface (usually the substrate surface), and film thickness in one unified application. The following image shows a typical interface. Top surface is in the upper half with film thickness in the lower left and second surface in the lower right.


Conclusion

The Films Analysis Application for MetroPro combined with ZYGO’s flagship profiler, the NewView 6300, is the fastest and most versatile tool available for quantification and visualization of film surface and thickness characteristics. ZYGO’s patented TopSlice and FilmSlice algorithms allow for the widest range of film thicknesses and unparalleled repeatability for total confidence in your film metrology.

* Optical thickness is the thickness seen by the microscope assuming a film index of refraction of 1.0. Physical thickness is optical thickness / index of refraction.

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