Model 2010/M Basic Description


 

The Metricon Model 2010/M Prism Coupler utilizes advanced optical waveguiding techniques to rapidly and accurately measure both the thickness and the refractive index/birefringence of dielectric and polymer films. For many thin film and optical waveguide applications, the Model 2010/M offers unique advantages over conventional instruments based on ellipsometry or spectrophotometry:

The Model 2010/M represents the third generation in Metricon's family of prism coupling instruments. Its distinguishing feature is a PC-based controller which offers dramatic improvements in automation and user-friendliness compared to the 2010/M's predecessors, the PC-200 and PC-2000.

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THEORY OF MEASUREMENT

The sample to be measured (Fig. 1) is brought into contact with the base of a prism by means of a pneumatically-operated coupling head, creating a small air gap between the film and the prism. A laser beam strikes the base of the prism and is normally totally reflected at the prism base onto a photodetector. At certain discrete values of the incident angle , called mode angles, photons can tunnel across the air gap into the film and enter into a guided optical propagation mode, causing a sharp drop in the intensity of light reaching the detector (Fig. 2).



To a rough approximation, the angular location of the first mode determines film index, while the angular difference between the modes determines the thickness, allowing thickness and index to be measured completely independently.

Measurements are made using a computer-driven rotary table which varies the incident angle , and locates each of the film propagation modes. As soon as two of the mode angles are found, film thickness and index can be calculated. The entire measurement process is fully automated and requires approximately twenty seconds.

The number of modes supported by a film of given index increases with film thickness. For most film/substrate combinations, a thickness of 1000-2000 Å is required to support the first mode, while films in the one-micron range can support as many as four or five modes. If the film is thick enough to support two or more propagation modes (typically 3000- 4800 Å), the Model 2010/M calculates thickness and index for each pair of modes, and displays the average and standard deviation of these multiple estimates.

The standard deviation calculation, unique to the prism coupling technique, is an indication of measurement self-consistency and a powerful means of confirming the validity of the measurement.

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MODEL 2010/M ADVANTAGES

Prism coupling offers several other advantages compared to existing methods of film characterization:

Generality: The Model 2010/M data analysis software is completely general, permitting measurements of films from the most mundane to the most exotic without reliance on internal fudge factors or menu-based calibration curves.

Insensitivity to index variation with wavelength: The refractive index of every film varies with wavelength. In techniques which rely on spectrophotometry (interference vs. wavelength), if the refractive index variation of the film over the full wavelength range is not accurately known, substantial errors will result. Moreover, the index vs. wavelength curves of some variable-index films such as plasma silicon nitride are highly dependent on film deposition conditions. For such films, the monochromatic measurement of the Model 2010/M offers a clear advantage.

No advance knowledge required: Since ellipsometer data is periodic with film thickness, ellipsometry requires advance knowledge of film thickness to an accuracy of ±750 to ±1250 Å, depending on film index. The Model 2010/M, which makes direct thickness measurements, does not require advance knowledge of film thickness.

No thickness ranges where index measurement impossible: At certain periodic thickness ranges, index measurements with ellipsometry are impossible. With the Model 2010/M, full-accuracy index measurements are obtained once film thickness exceeds a certain minimum threshold value (typically 3000-4800 Å, depending on film/substrate type).

Internal self-consistency check: If film thickness exceeds 5000-7500 Å, multiple independent estimates of film thickness and index are made, and the standard deviation of these multiple estimates is displayed. As long as measurement standard deviation is low (typically 0.3% for thickness and .01% for index), there is little chance that an appreciable error has been made. No other technique provides a similar "confidence check" on the validity of each measurement.

Press on chart for larger view

Measurement of oxynitride on silicon and nitride over oxide on silicon.

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MODEL 2010/M LIMITATIONS

For independent measurement of thickness and index with the Model 2010/M, film thickness must be sufficient to support two or more propagation modes. This establishes a lower thickness limit for measurability of 3000 to 4800 Å for films on high index substrates (e.g., silicon or GaAs). For films on lower index substrates, minimum thickness limits can be lower by as much as a factor of two. In addition, if film index is assumed, thickness-only measurements can be made on films as thin as 1000­2000 Å

If film thickness falls below the minimum measurable threshold, erroneous measurements are not made, and the 2010/M simply informs the user that the measurement cannot be made.

These minimum thickness limits are only approximate, and depend in detail on the refractive index of the film and substrate, and on the measuring wavelength. If your application falls close to any of the above limits, please consult Metricon.

An additional limitation is based on the fact that the prism coupling technique involves contact to the surface being measured. As a result, for applications requiring extreme cleanliness, such as IC fabrication, Model 2010/M measurements are usually made on non-product test samples.

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MODEL 2010/M CAPABILITIES

Many Model 2010/M applications revolve around the need to rapidly and accurately measure variable index films which are deposited by plasma, low temperature, or other deposition methods. For these applications, the Model 2010/M offers a thickness measurement fully corrected for any refractive index variation. Moreover, the Model 2010/M's routine ±.0005 index resolution provides a sensitive monitor of film deposition conditions, warning when gas flows, vacuum conditions, or deposition temperatures start to depart from acceptable windows. Compared to conventional techniques, the Model 2010/M offers a number of other unique capabilities for a broad range of production and R&D applications:

Rapid measurements of film doping: Since most dopants (e.g., phosphorus in doped oxide) cause small changes in film index, the 2010/M's sensitive ±.0005 index resolution can provide immediate production feedback on film doping far more rapidly and inexpensively than other techniques.

Measurements of dual film layers: If upper film index is greater than that of the lower film, thickness and index may be measured for each film in a dual film structure.

Polyimides/photoresists/polymers: Every step (baking, radiation exposure, etching) in the processing of polymer films causes changes in film thickness and index. With its independent thickness and index measurement and unparalleled index resolution, the 2010/M is an ideal tool to monitor the consistency of polymer processing in production, or, for R&D, to study the fundamental behavior of these films. In addition, since the 2010/M can measure index both parallel and perpendicular to the plane of the film, index anisotropy is easily obtained.

Thick film measurements: No other film measurement technique comes close to matching the Model 2010/M's performance in measuring films in the thickness range above two microns. And with the VAMFO option, non-contact thickness measurements are possible on films as thick as 100 microns.

Optical Waveguides: For organic/inorganic thin film and diffused waveguides, the 2010/M provides rapid location of modes and automatic calculation of effective mode indices. The 2010/M is also available with a secondary input port to permit use with user supplied lasers.Measurements on transparent substrates: The Model 2010/M can be used to measure films on transparent substrates, even when the refractive index match between film and substrate is relatively close. Bulk/substrate material index measurement: Index of bulk materials may be measured with a typical speed of five seconds or less, without the need for messy or toxic index matching fluids.

Index measurement of bulk/substrate materials: The 2010/M provides rapid (5 second) measurements of the refractive index of bulk glasses, polymers, plastics, garnets and other materials (both rigid and flexible) with a routine resolution of ±.0003 (±.0001 with a high resolution rotary table). Index measurement range extends from 1.0-2.65 and no matching fluids are required.

Free-standing polymers: In-plane and perpendicular plane index for free-standing polymer films, including flexible films, are easily measured by the Model 2010/M without the use of toxic or corrosive index matching fluids. In-plane refractive index can also be measured along any arbitrary in-plane direction by a simple rotation of the sample about the coupling point. Indices ranging from 1 to 2.6 are easily measured and measurements are free of operator subjectivity.

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ULTIMATE STANDARD PERFORMANCE WITH CONVENIENCE

The Model 2010/M is an instrument which combines the absolute accuracy and reliability of an ultimate standard with the speed, convenience, and versatility of a production tool.

In an automated 20-second measurement, the 2010/M offers a routine index accuracy (±.001) and resolution (±.0005) which have, at best, been approached only by the most costly and cumbersome research instruments. Moreover, the 2010/M measures direct, not incremental, thickness, so there is never any need to predetermine interference order of the film. The 2010/M also offers clear advantages in measurements of thicker films, films on transparent substrates, and there are no periodic thickness ranges where index measurement cannot be made.

The Model 2010/M's control software was designed with the philosophy that the user should be warned if there is any doubt about the validity of a measurement. The fundamentals of the prism coupling technique, which permit independent measurement of both thickness and index and powerful checks of measurement self-consistency, make this philosophy easy to implement.

Model 2010/M users report that while there is often a nagging doubt about measurement reliability with other techniques, their confidence level with prism coupling measurements is high. ln summary, the 2010/M provides routine measurement of thickness and index for a broad range of difficult-to-measure films with an ease, an accuracy, and a confidence in measurement validity unmatched by other techniques.

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APPLICATIONS

The following is a representative, but by no means inclusive, list of film/substrate types which are measurable with the Model 2010/M (free-standing films or bulk samples of any of the below materials are also measurable):

 Films   

 Substrates
silicon nitride  low-k dielectrics silicon
 silicon dioxide  ITO  GaAs
silicon oxynitride  sapphire quartz
 photoresists  zinc sulfide glass
 polyimides  titanium dioxide sapphire
 polymers  epi garnent GGG
silicon carbide  holographic gels lithium niobate

Virtually any film/substrate combination (e.g., silicon nitride on sapphire) is measurable, provided film thickness exceeds a minimum threshold value. For minimum measurable thickness values of common film/substrate combinations, please see the enclosed current application information, or consult Metricon.

Press on chart for larger view

Measurement of polymide over aluminum and polymer over quartz.

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NON-CONTACT (VAMFO) MEASUREMENTS

For films in the 2-100 micron thickness range, the Model 2010/M can function as a VAMFO analyzer, providing accurate, non-contact measurements of thickness only with a measurement speed of 20 seconds or less. The VAMFO option, based on simple interference of monochromatic light as a function of incident angle, can be used with virtually any transparent or semi-transparent film on virtually any polished substrate material, including clear substrates.

The VAMFO sample holder, which supports samples by vacuum from the back, is easily installed on the system, permitting changeover between prism coupling and VAMFO measurements in less than one minute.


FUTURE CAPABILITIES

Metricon Corporation has pioneered the practical application of prism coupling technology to problems of thin film, bulk material, and integrated optics characterization. The Model 2010/M represents a major improvement in power and user-friendliness over its predecessors, the PC-200 (first commercially available prism coupler, introduced in 1980) and the PC-2000. Moreover, software and hardware enhancements are under development to extend the measuring range of the 2010/M to material types and thickness ranges which are currently unmeasurable. If your application falls outside any of our current capabilities, please call to discuss modifications or enhancements that might be available to fit your needs.

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Last revised: April 30, 2002
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