A COMPARISON OF PRISM COUPLING AND ELLIPSOMETRY
FOR THIN FILM/BULK MATERIAL CHARACTERIZATION
Although the prism coupler and the ellipsometer can both be used to measure the index of bulk materials and to simultaneously measure film thickness and refractive index, there are major differences in the underlying measurement techniques and in the accuracy and the application range of the two instruments. As a result, ellipsometry and prism coupling can be viewed as complementary techniques, with the strengths of one supplementing the weaknesses of the other, and vice versa.
For any given application, the film thickness range to be measured is the single most important factor determining whether ellipsometry or prism coupling is the preferred technique. In general, ellipsometry is the clear choice for measuring thin films such as gate oxides, silicon nitride, or any film whose thickness is thinner than a few thousand Angstroms. For measuring typical moderate-to-thick films such as doped oxide, silicon nitride (SiN) or oxynitride (SiON), or resists/polymers, the prism coupler is usually superior.
Spectroscopic ellipsometers, which use a continuous spectrum of wavelengths, can overcome some of the limitations of single wavelength ellipsometers mentioned below but the cost of spectroscopic ellipsometers is at least two to three times the cost of a Model 2010/M and the data analysis is much more complex and often slow, requiring extensive pre-modeling of the film as well as complete knowledge of the variation of the substrate index vs wavelength. For reliable convergence, index values at one or two wavelengths must often be known in advance.
Differences between the prism coupling technique and ellipsometry include:
Measurement principle: In all ellipsometers, the intensity and polarization state of monochromatic light reflected from the film or bulk sample yields the measured parameters. Prism coupling measurements are based on techniques developed in the field of integrated optics, treating the thin film to be measured as an optical waveguide. The Model 2010/M works by measuring the angles at which the thin film waveguide will propagate light, and then calculating film thickness and refractive index from the observed mode angles.
Direct vs. incremental thickness: Since ellipsometer raw data is periodic with film thickness, single wavelength ellipsometers cannot measure total thickness directly, providing only an incremental film thickness which must be added to a base thickness known in advance or measured by other means. The repetition period for ellipsometer data ranges from roughly 150 nm at refractive index 2.0, to roughly 250 nm at index 1.46. Consequently, if total film thickness is desired, film thickness must be known a priori or pre-measured using some other technique to an accuracy of ±75 nm to ±125 nm. While this requirement is usually not too limiting for measuring thin films, for films in the 500-1000 nm range and above, advance knowledge of approximate film thickness may not be sufficient to clearly identify which of the multiple ellipsometric solutions for film thickness is correct. The prism coupler, on the other hand, provides a direct and unambiguous measurement, of total, not incremental, thickness.
Thickness measuring range: In order for both thickness and index to be measurable with the prism coupling technique, film thickness must exceed a certain minimum threshold, typically 300-500 nm. In addition, either thickness or index may be measured on films in the 120-200 nm thickness range if the other parameter is assumed. For the ellipsometer, however, accuracy is optimum and the ambiguity caused by the incremental nature of ellipsometer measurements is least significant in the 120-200 nm and below range. Thus, the ellipsometer is usually the instrument of choice for films which are relatively thin. An exception to this rule is the case of a thin film on a transparent substrate (or underlying film) of lower index. For this case, the prism coupler can often be used to measure both thickness and index of films as thin as 50 nm, and the relative ease with which the prism coupler handles measurements on transparent substrates or underlying films (see below) is a further advantage.
Conversely, single wavelength ellipsometers suffer from some major disadvantages when used to measure moderate to thick films, such as the ambiguity caused by the incremental nature of the ellipsometer's thickness measurement described above, its sensitivity to optical absorption in the film, and its periodic loss of index resolution, described below. Thus, the prism coupling technique holds a clear advantage for measurement of thicker films.
Refractive index resolution/accuracy: The Model 2010/M offers a routine refractive index resolution of ±.0003 and an absolute accuracy of ±.0005, an order of magnitude better than obtainable with single wavelength ellipsometry, even when the ellipsometer is working in its most sensitive thickness range. In addition, refractive index resolution with single wavelength ellipsometry is compromised significantly during an appreciable fraction of every cycle thickness, so that index measurement is not even feasible on a significant fraction of samples. Finally, for many applications an optional high resolution rotary table can improve index resolution of the Model 2010/M to ±.00005, and use of index calibration standards available from Metricon can improve absolute accuracy to ±.0001.
Sensitivity to optical absorption: Single wavelength ellipsometers are first-order sensitive to optical absorption in measured films, and if the usual thickness-index measurement is desired, inaccuracies in measured index (and thus cycle thickness) will result if the film has even a small amount of optical absorption. This problem is compounded with thicker films, since total absorption across the full film thickness increases and the measured cycle thickness becomes even more inaccurate. Since the total film thickness is an integer multiple of the cycle thickness plus the measured incremental thickness, errors in the measured cycle thickness are multiplied by the integer order of interference, and absorption errors build up quickly in thicker films.
The prism coupling technique, on the other hand, can handle fairly absorbing films without appreciable loss of accuracy. In the limiting case, when films become highly absorbing, film propagation modes simply vanish, and the Model 2010/M's software informs the user that the measurement cannot be made.
Insensitivity to substrate materials: All ellipsometers require accurate advance knowledge of the real and imaginary parts of the substrate refractive index. If these parameters are not known, or if they vary due to roughness or other surface conditions, appreciable errors can result. The prism coupling technique is only weakly sensitive to substrate optical parameters. For the case of a low index film over a higher index substrate (e.g. silicon, GaAs, metals) it is only important for the Model 2010/M to know that the index is higher -- the exact index is unimportant. For the case of a film over a lower index substrate, thickness and index as measured by prism coupling are weakly dependent on the real part of the substrate index only. For low index substrates where index is not known in advance, the bulk material index measurement of the 2010/M offers a rapid and accurate method of determining substrate index.
An example of the insensitivity of prism coupling measurements to optical parameters of substrate materials is measurement of polymer or dielectric films on metal substrates or underlying films of metal. Such measurements are often difficult with ellipsometry because of variability in the roughness and reflectivity of the metallic materials, whereas prism coupling measurements on all but the roughest metal surfaces are usually quite simple.
Measurements of films on transparent substrates: The Model 2010/M offers major advantages when film measurements must be performed on transparent substrates, since, unlike ellipsometry, the prism coupling technique is largely insensitive to reflections of light from the back side of the transparent substrate material.
Measurements of dual films: For the case of a high index film over a lower index film, the Model 2010/M can measure thickness and index of one or both films, a total of four measured parameters. Four-parameter measurement of dual films with single wavelength ellipsometers is not possible -- at most two parameters can be measured, and these can be accurately measured only if the other two parameters are accurately known in advance.
Birefringence/index anisotropy: The Model 2010/M can measure refractive index along x, y, and z directions for both thin films and bulk materials. Measurements of perpendicular index for films on high index substrates such as silicon, however, sometimes require minimum film thicknesses of 1.5-2 microns.
Measurements of film dispersion (index vs wavelength): The Model 2010/M can be configured with multiple lasers to permit measurement of index vs wavelength. If index is measured at three or more wavelengths, a built-in Cauchy fit routine in the control software for the 2010/M will provide continuous index vs wavelength data over the wavelength range measured (the Cauchy fit dispersion equation can be generated and plotted with only two Mouse clicks after the three index values are obtained). For example, a 2010/M system with 405, 532 and 780 nm lasers can provide a complete dispersion curve across the visible spectrum in essentially the time it takes to make three individual index measurements (less than two minutes). Spectroscopic ellipsometers can provide index measurements at a continuum of wavelengths over a similar wavelength range, but cost is typically at least twice as high as a Model 2010/M with three lasers and data analysis is often quite complex and slow and requires detailed knowledge of the substrate index as well as knowledge of the film index at one or two wavelengths (see above).
Please do not hesitate to call if you have any additional questions concerning the prism coupling technique. We welcome the opportunity to discuss your application.
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