NON-CONTACT VAMFO FILM THICKNESS MEASUREMENTS
MODEL 2010/M OPTION #2010/M-VO

Option #2010/M-VO for Metricon's Model 2010/M prism coupler allows the 2010/M to function as a VAMFO analyzer providing rapid, accurate, and non-contact measurement of thickness only on films thicker than 2 microns. The VAMFO technique is general and can be used with virtually any transparent film on virtually any polished substrate material, including transparent materials. The only advance knowledge required is an approximate value for the film index.

Option #2010/M-VO permits rapid conversion between prism coupling measurements of thickness/index and VAMFO measurements of thickness only by installation (via a single knurled thumbscrew) of the VAMFO sample holder which supports samples by vacuum from the rear. Changeover time between the two techniques* is less than one minute, enabling both types of measurements to be performed on a single system.

In the VAMFO technique, monochromatic light is directed onto the sample to be measured so that interference minima occur in the light reflected from the film surface as the angle of incidence of the light is varied. Film thickness is then computed from the angular locations of two interference minima. In order to guarantee that a film possesses two or more interference minima, film thickness must exceed a minimum threshhold value, typically 2 microns.

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Since the VAMFO technique is monochromatic, it offers major advantages in accuracy and generality compared to conventional thickness-measuring instruments based on spectrophotometry. In spectrophotometric instruments, interference in light reflected from the sample is obtained by varying the wavelength of the incoming light at a fixed angle of incidence. However, since all films exhibit dispersion (variation of refractive index with wavelength), spectrophotometric instruments must make detailed assumptions about how film index varies with wavelength, and they usually offer a fixed menu of film/substrate types for which pre-programmed index vs. wavelength assumptions are provided. If new or unusual film types are encountered, or if processing or deposition conditions cause film dispersion characteristics to vary from the preprogrammed assumptions, significant measurement errors can result.

In contrast to techniques relying on spectrophotometry, the only requirement for making accurate VAMFO measurements is that film index be known at a single wavelength, the measuring wavelength. This is easy to ensure, since refractive index can always be measured and re-checked periodically by operating the Model 2010/M in the prism coupling index-measurement mode. Thus, the VAMFO technique offers excellent accuracy even when unusual film types are encountered, or when deposition conditions cause variations in film dispersion characteristics.

Films thicker than a few microns usually exhibit more than two interference minima, and for these films the VAMFO analysis software provides multiple measurements of film thickness (one for each pair of minima) and displays the average and standard deviation of these multiple estimates. The multiple estimates of film thickness made by the VAMFO technique are highly self-consistent, providing a powerful check of measurement self-consistency and accuracy, a feature lacking in other techniques.

For thicker films, the VAMFO option complements the capabilities of the prism coupling technique. Prism coupling thickness measurements start to lose accuracy when film thickness reaches 15-20 microns, although accurate index-only measurements can still be made on thick films using the Model 2010/M's bulk index measurement. Thus, for films thicker than 15-20 microns, the VAMFO measurement can be used for accurate thickness measurement, with the prism coupling measurement providing index via the bulk index measurement. In this way, thickness and index measurements are possible on films in excess of 100 microns in thickness.

VAMFO measurements are ideal for production applications where the speed, simplicity, and non-contact nature of the technique are most important. For most production applications, routine measurements are made using the VAMFO technique, with occasional checks of refractive index provided by operation of the system in the prism coupling mode.

SPECIFICATIONS -- OPTION #2010/M-VO (VAMFO MEASUREMENT)

Operating wavelength: He-Ne laser (632.8 nm), 1.0 mw max. (Class II).

Film/substrate types measurable: Oxides, nitrides, dielectrics, polymers, or films of virtually any transparent or semi-transparent material are measurable on substrates of silicon, quartz, glass, or virtually any polished material with index differing from film index by approximately .05 or more.

Thickness measuring range: Aproximately 2 to 150 microns. Exact thickness measuring range depends on film/substrate type and can often be extended at the high or low end. Please consult Metricon for details.

Accuracy and resolution: ± 1% accuracy, 0.5% resolution.

Measurement time: 20 seconds.

Measurement area: 1 mm.

Maximum substrate size: 8" x 8" (20cm by 20cm). Larger sizes optional.

Dimensions, weight, and electrical services for systems with the VAMFO option are the same as those for the standard Model 2010/M (see 2010/M specs). In addition, the VAMFO option requires a low quality vacuum line.

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Last revised: 11/8/06