Optical profilometer

 

(57) Abstract:

Usage: the invention relates to measuring equipment. The technical result of the use of the invention is to improve the accuracy of the inventions, as well as the opportunity to study the surfaces of large linear dimensions (of the order of 1 m). The inventive result is achieved that the optical profilometer implemented building on a photosensitive element coordinatinates spectra of the radiation corresponding to the aggregate image of the profile of the surface being investigated, and a comparison of the positions of the maxima of the spectra. The measurement accuracy is limited by the dynamic range of coordinatinates, which provide measurements of the positions of the maxima of the spectra, and is not less than /1000 , and the size of the investigated surface is limited only by the capacity of the used light source. 3 C.p. f-crystals, 1 Il.

The invention relates to measurement techniques, in particular to the class of devices designed for non-contact Metrology software subnanometric technologies, and can be used to control the surface profile of products in various fields of technology nasledovaniya.

Known class of profilometers, which are based on the principle of heterodyne interferometry. The essence of this principle is the formation of two rays that have similar frequencies, using laser radiation, using one of the rays as the reference and the measurement of relative changes of phase modulated signals to obtain a profile of the surface. Known heterodyne profilometers differ in the values of the difference frequency laser beams, as well as the specific implementation of the elements of the optical circuits. So, for example, as acid frequencies in different versions of the interferometers of this type uses a rotating quarter-wave plate, the Bragg cell, rotating with constant speed splitter and Siemanowski splitter for He-Ne-laser. To improve the accuracy of measurements often form the reference beam averaged over the surface, which use different optical scheme of its expansion.

However, despite these measures the accuracy of the heterodyne profilometers limited value /500 where the wavelength of the used radiation, because of the difficulty of maintaining the difference between close frequencies with high Stabi the He-Ne-laser, acousto-optic modulator, means for expanding the reference beam, the polarizing beam splitter, a quarter-wave plate, a focusing lens, a movable table on which is located the sample, along with two photodiode and an electronic unit.

Known another class of profilometers, the basis of which is the method of pasaogullari interferometry. Profilometers of this type include the interferometer, one of the shoulders which has the supporting plate and the other of the investigated sample. For measuring the difference of the interfering beams modulate and convert the interference pattern in the photoelectric signal, then the information about the surface profile is extracted from the phase components of the detected signal. To obtain the interference pattern using monochromatic radiation. Fotomodella profilometers are different types of interferometers (Michelson interferometer, the lens Miro and others ), using different types of modulation (sinusoidal, sawtooth), the specific implementation of individual elements of the optical circuits.

Known profilometer of this type provides a relatively high accuracy (/500-/1000) (Osami Sasak which you installed the sample, and in another reference plate mounted on piezoelectrical element, and CCD matrix for the signal.

The disadvantage of this profilometer and other profilometers this type, as well as profilometers heterodyne type is a high level diffractive spatial noise (speckles), associated with a high spatial coherence of monochromatic radiation, and common to all optical devices using a laser. Furthermore, since the profilometer modulation is carried out by moving either the reference or the analyzed plate with a piezoelectric element, the linear dimension of the surface being examined is limited to the technical characteristics of the piezoelectric element and may not exceed 100 microns.

And finally, the third great class of profilometers this profilometers, which are based on the formation of the radiation beam focused on the sample surface. The measure of deviation from the plane in profilometers of this type is the degree of defocus of the image. These profilometers differ by the type of radiation source (monochromatic and white sources ser, known profilometer (F. Quercioli, et al, Opt. Eng. 1988, v 27, N 2, 135), containing a white light source, aperture, collimator, and a chromatic lens, the focus of which is the analyzed sample. The monitor also includes a monochromator, educated chromatic lens, the dispersion element and the focusing lens, the output of which is equipped with a photodiode array.

Limits of measurement accuracy flatness for profilometers this type are related to the fact that the evaluation of asperities is in intensity reflected from the sample surface radiation, and the measurement result depends on reflecting and scattering properties of individual sections of the sample surface, and therefore the accuracy of measurement does not exceed a few microns.

The closest analogue of the developed optical profilometer together similar essential features of an optical profilometer (U.S. patent N 4,641,971, IPC G 01 B 9/02, 1987), which is difficult to attribute to any of the above classes of optical profilometers. It contains sequentially located on the optical axis of the white light source and the interferometer, capable of forming at least two liters of the plate, and color television (TV) camera. Between the white light sources and the input of the TV cameras installed coherent optical filter. Three inputs of the TV-camera is connected to the processing unit, the output of which is connected to the logger, which is used as a monitor. The Registrar is also linked with the TV camera.

The disadvantage of this monitor is its low accuracy, due to the fact that his work is based on comparing the intensity of the interference pattern in the three portions of the spectrum with three different receivers, using the standard TV camera. Therefore, the accuracy of the monitor is probably achievable is limited by the intensity of the amplitude characteristics of the channels of the receivers, which have a modern TV cameras does not exceed 1% in Addition, as is known, the maximum dynamic range of the existing color TV camera is 252 level. It is obvious, therefore, that limit the accuracy of this monitor cannot exceed /200.

However, the problems of measurement and control with a much higher accuracy, namely with accuracy comparable to the size of atoms and molecules characteristic of many modern technological processes. First of all, these control methods required the permission. Particularly acute these problems occur when creating devices for optical remote sensing, in particular optical sensing of astronomical objects. Currently, the sensitivity of the photodetectors with a score of photons allows to detect effects which cause changes in the characteristics of light emission of the order of 10-410-6from their nominal values. Simple estimates show that the implementation of these capabilities is the need to create elements that have the same degree of homogeneity and stability. According to experts, the establishment of the necessary technologies mainly hindered by the lack of appropriate means of measurement and control. The field of application subnanometric measuring devices currently covered not only scientific, but also a large number of practice areas. The most obvious example of the use of such devices for industrial purposes is in the area of microelectronics. With the advent of these devices link the creation of microelectronic devices of last generation, which should make, according to scientists, the revolution in electronic technology.

Thus, the problem to solve which napravlyalysya ensure subnanometric technologies.

The invention consists in that the optical profilometer as is known, contains located on the optical axis of the white light source and the interferometer, including the output lens and capable of forming at least two beams of white light, and the Registrar, a control input connected to the synchronizer. One of the plates of the interferometer is investigated sample, and the other of the reference plate.

New developed optical profilometer is that it additionally imposed consistently on the optical axis behind the interferometer spectrograph and coordinatometer. The interferometer and the spectrograph mounted for movement relative to each other by using the actuator of the imaging plane of the surface being examined and the entrance of the spectrograph in the specified plane. The entrance of the spectrograph optically connected with the sensor element of coordinatinates, the output of which is connected to the monitor. The control input of the actuator is connected to the synchronizer.

In the particular case of the interferometer is made in the form of a Fabry-Perot interferometer, at the entrance of which ustanovit> In another particular case, the spectrograph includes sequentially installed on the optical axis of the light pipe, the inlet of which is the entrance of the spectrograph, the second collimator, a dispersing element and a focusing lens, a focal plane which is the output of the spectrograph.

Optical profilometer can include coherent optical filter positioned between the source of white light and coordinatesystem.

In the developed optical profilometer introduction arranged in series on the optical axis behind the white light interferometer spectrograph and coordinatinates can move with the actuator relative to each other the image plane of the surface being examined and the entrance of the spectrograph in the specified plane provides a photosensitive element coordinatinates spectra of the radiation corresponding to the aggregate image of the profile of the surface. Coordinatometer, the dynamic range is 106provides measurement of the positions of the maxima of the spectra image interferogram of the surface being examined, and the position of the maxima posterie of coordinatinates. Since the preview image the sample surface when the change in the spectral composition of the radiation changes the spatial position of the energy center of the light spot on the photosensitive element coordinatinates corresponding change in the voltage at the output of coordinatinates recorded synchronously with mutual relative movement of the image plane of the surface being examined and the entrance of the spectrograph in the specified plane, corresponds to the profile of the surface. This and achieving the required technical result of the improved accuracy of the measurements to values not less than /1000. Other achievable technical result is the ability to study surfaces substantially large size (about 1 m) than allow other known profilometers, as the size of the investigated surface is limited only by the capacity of the used light source. This allows the use of optical profilometer for non-contact measurements in the Metrology support subnanometric technologies.

The drawing shows a structural diagram of the developed optical profilometer.

Optiformer 2, including on the exit lens 3 and provides for the formation of at least two beams of white light, with one of the plates of the interferometer 2 is the sample 4, and the other reference plate 5.

As the interferometer 2 can be used any interferometer that meets the above requirements. In the specific implementation shown in Fig. 1, the interferometer 2 is made in the form of a Fabry-Perot interferometer, the input is set to the first collimator 6, and in the focal plane of the collimator 6 is installed a rotary mirror 7.

The output of the interferometer 2 is optically connected with the spectrograph 8, while the interferometer 2 and the spectrograph 8 mounted for movement with the actuator 9 from each other on the image plane of the surface being examined and the entrance of the spectrograph 8 in the specified plane. The output of the spectrograph 8 optically connected with coordinatesystem 10, the output of which is connected to the recorder 11, and the control input of the actuator 9 is connected to the synchronizer 12.

As the spectrograph 8 can be used spectrograph of any type. In a specific implementation (Fig. 1) the spectrograph 8 contains the Torah collimator 14, dispersion element 15 and the focusing lens 16, a focal plane which is the output of the spectrograph 8.

Optical profilometer can include coherent optical filter 17, located between the white light source 1 and coordinatesystem 10. In a specific implementation according to Fig. 1 coherent optical filter 17 is installed in the interferometer 2 in front of the lens 3.

The distance between the plates 4, 5 of the interferometer 2 when coherent optical filter 17 in the optical profilometer is missing, set the order of the wavelength of the Central part of the visible spectrum.

With the introduction of a coherent optical filter 17 in the optical profilometer specified limit on the distance between the plates 4, 5 are missing, however, bandwidth coherent optical filter 17 must be coordinated with the distance between the plates 4, 5 of the interferometer 2. Coherent optical filter 17 may be made in the form of an interference filter.

The actuator 9 in a particular implementation is connected to the input of the optical fiber 13 (not shown), and must provide the movement of the input optical fiber 13 in the image plane islegal used, for example, the actuation of any two-coordinate recorder.

As coordinatinates 10 can be used coordinatometer energy center of the light spot (ed.St. USSR N 1106425, 1965). Photosensitive element coordinatinates 10 must be located in the output plane of the spectrograph 8.

The synchronizer 12 is designed to provide synchronous operation of the actuator 9 and 11 Registrar. As a synchronizer 12 can be used, for example, the generator type G6-15.

Dispersion element 15 may be made in the form of a prism.

Optical profilometer works as follows.

Diverging radiation beam from the white light source 1 impinges on the collimator 6 of the interferometer 2. A parallel beam of radiation formed by the collimator 6, light plates 4, 5 of the interferometer 2. The interferometer 2 forms an interference picture using the first collimator 6 focuses on the rotary mirror 7 of the interferometer 2.

When coherent optical filter 17 is not present, the radiation, the spectrum of which corresponds to the selected wavelength, enters the rotary is their beams at the entrance of the spectrograph 8.

In the particular case when the optical profilometer contains coherent optical filter 17, the latter allocates the selected part of the spectrum of the radiation coming from the rotary mirror 7 and the lens 3 as in the previous case, constructs an image of the investigated surface plate 4 in the interfering beams at the entrance of the spectrograph 8.

Because the wavelengths corresponding to the maxima of the interference pattern is uniquely associated with the distance between the plates 4, 5 of the interferometer 2, the spectral composition of radiation at each point of the image constructed by the lens 3, is also uniquely determined by the surface.

When moving with the actuator 9 of the input optical fiber 13 in the image plane of the investigated surface of the light guide 13 transmits the radiation corresponding to the spectral content of the input of the second collimator 14. The second collimator 14 provides the formation of a parallel beam of radiation coming from the output of the light guide 13. Dispersion element 15 provides angular splitting of the wavelength of the radiation coming from the second collimator 14. A focusing lens 16 converts the angular distribution of radiation in space is the first element of coordinatinates 10. Coordinatometer 10 provides for the measurement of the position of the maximum of the constructed spectrum, which corresponds to the position of the energy center of the light spot on the photosensitive element coordinatinates 10. If you move with the actuator 9 of the input optical fiber 13 in the image plane of the surface being investigated changes in the spectral composition of the radiation is changed and the spatial position of the energy center of the light spot on the photosensitive element coordinatinates 10. The voltage generated at the output of coordinatinates 10 corresponds to a spatial position of the energy center of the light spot on the photosensitive element, so the last change causes a change in the specified voltage range. The voltage output of coordinatinates 10 served on a synchronized by the synchronizer 12 actuator 9 11 Registrar, the scale of which pre appropriately calibrated. Thus, the change in the voltage registered by the Registrar 11, corresponds to the profile of the surface.

1. Optical profilometer, containing a white light source and located on the th formation of at least two beams of white light and includes at the output of the lens, and the synchronizer and the Registrar, the control input of which is connected to the synchronizer, characterized in that it has installed sequentially on the optical axis behind the interferometer spectrograph entrance which is optically conjugate with the image plane of the surface being examined, and coordinatesystem, the interferometer and spectograph made with the possibility of relative displacement of the image plane of the surface being examined and the entrance of the spectrograph using the actuator, the output of the spectrograph optically connected with coordinatesystem, the output of which is connected to the logger, and the control input of the actuator is connected to the synchronizer.

2. The profilometer under item 1, characterized in that the interferometer is made in the form of a Fabry-Perot interferometer, the input is set to the first collimator and the focal plane of the first collimator mounted swivel mirror.

3. The profilometer under item 1 or 2, characterized in that the spectrograph is made in the form of a series set on the optical axis of the second collimator, the dispersion element and a focusing lens, a focal plane which is the output of the spectrograph, and the second collegecourses fact, that it introduced a coherent optical filter positioned between the source of white light and coordinatesystem.

 

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