Optics: measuring and testing – By light interference – Having light beams of different frequencies
Reexamination Certificate
1999-03-10
2001-07-10
Font, Frank G. (Department: 2877)
Optics: measuring and testing
By light interference
Having light beams of different frequencies
C356S516000
Reexamination Certificate
active
06259530
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a method and device for the measurement of crater bottoms obtained by the bombardment, with a beam of primary ions, of a sample during its physico-chemical analysis.
2. Description of the Prior Art
One of the methods of analysis particularly suited for the performance of these analyses is the method that uses the SIMS or secondary ion mass spectrometer. This method analyzes the mass of secondary ions by spectrometry.
With these devices, it is often necessary to make measurement, during analysis, of the depth of the craters formed by the impact of the ion beam on the surface of the analyzed sample, especially when the sample has several superimposed layers of material as is the case for samples formed by several layers of superimposed semiconductor materials. The craters obtained are generally very small-sized. They are about 100 &mgr;m long and their depth varies during erosion between 1 nm and some &mgr;m for an erosion speed of 0.1 to 10 nm/s. The record of the physico-chemical composition of the sample in its different layers is generally done by drawing up a correspondence, during analysis, between the time of analysis during which the sample is subjected to ion bombardment and the depth of the resulting crater, this parameter being measured by means of a surface profiler. This method of measurement entails constraints because it requires the removal of the sample from the analyzer whenever an in-depth measurement has to be made. It also lacks precision.
To overcome these drawbacks, It has been proposed to use a measurement device applying the principle of optical interferometry. For the implementation of this principle, a beam of monochromatic and coherent light is split at a determined place into two beams. Each beam gets propagated in space, along a trajectory that is proper to it, up to another place where the two beams are recomposed into one by an appropriate optical system. The beam resulting from this recomposition is thus formed by the sum of two beams having travelled different lengths of space. The difference in length is often called a path difference. The beams are phase-shifted with respect to each other. The phase shift thus created gives rise to a system of interference fringes formed by alternations of weak and strong luminous intensities. Depending on the structures involved, the system of fringes may or may not be localized and it may or may not be wide. The relative variation of the path difference is determined by counting the fringes flowing past a given space. In this system, the quality of the measurement depends on the contrast between bright fringes and dark fringes and therefore on the signal-to-noise ratio as well as the interpolation that can be made. A well-known interferometer working according to the above-described principle is the Michelson interferometer, a description of which can be found for example in G. Bruhat and A. Kastler, Cours de Physique Générale, <<Optique>>, Masson et Cie, 120 bd. Saint Germain, Paris VI, page 135. In this interferometer, the period of the pattern of fringes corresponds to a path difference of &lgr; where &lgr; is the wavelength of the light beam. The path difference that results from two to-and-fro trips of the two beams on reflecting targets makes it possible to measure a variation of {fraction (&lgr;/2+L )} in the relative distance of the two targets. For example, if the light source is a helium-neon laser, {fraction (&lgr;/2+L )}=316.5 nm in vacuum. This resolution may be improved to {fraction (&lgr;/2+L )} or {fraction (&lgr;/16+L )} by interpolation, that is, by about 40 nm in exceptional conditions. Indeed, these interferometers remain highly sensitive to the variations of contrast that may be prompted by the possible variations of the reflectivity of one of the two targets and it is very difficult to position them in the analysis chamber of an SIMS ion analyzer because of the 90° orientation of the beam-returning mirrors with respect to one another, one of the mirrors being formed on the surface of the sample itself.
One method of measurement that can be used in SIMS analyzers is described in an article by M. J. Kempf, “On-line Sputter Rate Measurements During SIMS, AES Depth Profiling”, published by A. Benninghoven et al., Springer-Verlag Publications, Berlin-Heidelberg-New York, 1979. A variant of this method is described in the U.S. Pat. No. 4,298,283, “Interferometric measuring method”. This variant uses a laser interferometer whose incident beam is split into two paths by a calcite crystal before being directed, in the analysis chamber of an SIMS analyzer, on to the analyzed sample in a direction normal to the sample. The two reflected beams are redirected to the calcite crystal and then recombined into a single beam and form an interference system that depends on the “path difference” between the two beams.
This method enables the performance of very accurate measurements of depth of about 1 nanometer throughout the time of the analysis. On the other hand, it requires very delicate adjusting of the interferometer because the result of the measurements is highly dependent on the orientation of the sample in relation to the direction of the two beams.
Another method, also described in the above-described patent as well as in the U.S. Pat. No. 4,353,650 entitled “Laser Heterodyne Surface Profiler” implements the known principle of heterodyne interferometry. It is consists not in detecting a difference of luminosity of the fringe system to count the fringes that flow past, but in measuring the phase shift of an information element contained in the system with respect to the same information contained in the light source before the splitting of the beams. This method makes it possible to remove the dependence on the contrast variations so long as the signal-to-noise ratio remains appropriate. The fine measurement of this phase shift may attain
{fraction (1/256+L )} of the temporal period. If the interferometer is such that a phase shift of one temporal period corresponds spatially to a path difference of {fraction (&lgr;/
2+L )}, a resolution of {fraction (&lgr;/512+L )} or 1.25 nm is obtained. The implementation of a heterodyne interferometer requires the use of a light source which is no longer a monochromatic but a bi-frequency source. This source sends two beams that are quite cylindrical. Their difference in frequency may be about 3 MHz or 20 MHz. The two frequency components are polarized in a monoplane and are mutually orthogonal. An output beam fraction is sent to an analyzer, fixedly adjusted at 45° with respect to the two polarization planes, which lets through a fraction of the two components in the same output plane. A photodetector placed behind the analyzer is modulated at the half-sum frequency and at the half-difference frequency. The half-sum frequency is beyond the bandpass of the detector. The half-difference frequency is used as a phase reference. The output signal of the photodetector is shaped as a square-wave signal. The two orthogonally polarized components are then split by an interferometer to form two beams which are sent on two distinct paths, a path called a reference path and a path called a measurement path. After reflection on the target, the two beams are recombined on the same axis with mutually orthogonal plane directions of polarization and the whole resultant beam is applied to a detector which measures the path difference of the two beams. The detector consists of an analyzer fixedly adjusted at 45° with respect to the two directions of polarization received. A photodetector located behind receives the sum signal of the two frequencies and, like the detector located in the source, gives a low frequency square-wave signal. If the two targets are not in relative motion, this signal is at the same frequency as the one given by the detector of a fixed value, but it is simply phase shifted by a fixed value which depends on the lengths of the paths of the two bea
Cameca
Font Frank G.
Lee Andrew H.
Oblon & Spivak, McClelland, Maier & Neustadt P.C.
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