FR2760084A1 - Three dimensional measurement device for measuring thin-film structure - Google Patents

Abstract

The device has a monitoring unit (4) which includes a compact casing enclosing a video camera (12), a wide beam illumination source (14), a narrow beam illumination source (20) and optical components. An operation and control unit (8) includes an optical connector and an electrical mounted on the casing of the monitoring unit in order to connect it, respectively via a fibre optic cable and an electrical cable to the operation and control unit and a table (5) mounted above the treatment chamber. The operation and control unit moves the monitoring unit horizontally along two axes (x,y) to choose a site and provides accurate positioning so that the two beams reflected by the surface layer follow neighbouring optical paths close to the optical axis of the video camera (12). A Wollaston prism (24) is arranged on the optical path of the narrow light beam to obtain two narrow and coherent polarised light beams (25,26) having different directions and orthogonal polarisations. The prism is arranged so that the narrow light beams reflected by the later pass through it. A polariser (27) is arranged on the optical axis of the prism so that the reflects narrow light beam passes through it after its return travel through the prism. The polariser is mounted so that it can rotate with respect to the prism and a detection cell (28).

Description

Device and method for three-dimensional measurements and in situ observation of a surface layer deposited on a stack of thin layers.

 The present invention relates to a device and a method for carrying out three-dimensional geometric measurements, in situ, of a surface layer of a thin film structure in a vacuum chamber, and also for measuring the evolution of said structure during an engraving or deposition process.

 Among the applications of the invention, mention may be made of in situ and real-time control of the manufacture of semiconductor layers with deep etching in silicon, for example for the manufacture of electrical microsystems or for the thinning of micromembranes intended the manufacture of microsensors.

 French patent application No. 2 680 414 (SOFIE) discloses a compact set of observations and simultaneous interferometric measurements by laser enabling interferometric measurements to be carried out in situ on a stack of thin layers placed in a vacuum chamber. The assembly includes an observation camera, the objectives of which are crossed, on the one hand, by a light beam and, on the other hand, by one or two laser beams for interferometric measurements.

 Thanks to this technique, it is possible in particular to position the laser beam on the layer to be studied and to measure the rate of growth or decrease of the thickness of the surface layer of the structure with thin layers. However, the monochromatic lighting light beam and the measurement laser beam do not have exactly the same wavelength, which creates a problem of chromaticity making simultaneous focusing for the two light beams possible only if the objectives are achromatic. In addition, this technique does not allow an absolute measurement of the thickness of the surface layer in the observed area, because it is based on an interferometric differential measurement which is repeated modulo a period close to 2n; X being the observation length and n the refractive index of the surface layer.

 Also known from French patent application No. 2,718,231 (SOFIE) is a method for monitoring the thickness of a localized area of the surface layer of a thin-layer structure capable of measuring the absolute thickness of the layer. surface in a specific analysis area by performing a spectral analysis by a spectrograph of a light beam from a beam of white light reflected by the stack of thin layers. This technique is effective but it only makes it possible to know the thickness of the layer and its speed of variation over a localized area for depths of a few tens of microns at most.

 The object of the present invention is to propose an improved technique capable of carrying out the measurements carried out according to the prior techniques, but also adapted to patterns of great thickness or depth for micromechanical applications. In fact, when the layers are thick or when the patterns are deep, the intensity of the light to be measured becomes very low and conventional techniques are no longer suitable.

 The object of the invention is also to provide a device and a method capable of making an instantaneous measurement of the level difference between a surface chosen as the origin and the engraved or deposited pattern.

The device according to the invention is intended for three-dimensional measurement and for in situ and real-time observation of a surface layer of a structure with thin layers during treatment in a vacuum chamber equipped with a window on a wall. The device comprises a monitoring unit which comprises a compact housing enclosing a video camera, a wide-beam lighting source, a narrow-beam lighting source and optical components, an operating and control unit comprising a connector optic and an electrical connector mounted on the housing of the monitoring unit to connect it via respectively an optical fiber cable and an electrical cable, to the operating and control unit, and a table mounted above the processing chamber to ensure, on the one hand, the horizontal displacement of the monitoring unit along two axes in order to choose a site and, on the other hand, the precise positioning so that the two incident beams and the two beams reflected by the surface layer follows neighboring optical paths close to the optical axis of the video camera. The device includes a prism of
Wollaston arranged on the optical path of the narrow light beam to obtain, at the output of said Wollaston prism, two narrow, coherent polarized light beams, of different directions offset by an angle a and of orthogonal polarizations, the prism of
Wollaston being arranged so as to be crossed by the narrow light beams reflected by the layer, a polarizer arranged on the optical axis of the Wollaston prism so as to be crossed by the narrow light beam reflected after it passes back through said prism of Wollaston, said polarizer being rotatably mounted with respect to said Wollaston prism, and a detection cell.

 There are thus two narrow light beams, one, for example, directed towards a portion of the untreated surface layer and the other, for example, directed towards a portion of the surface layer being treated. The rotation of the polarizer around its optical axis makes it possible to select one or the other of the polarized reflected beams or a combination of the two and to carry out an analysis making it possible to know the difference in level between the two portions of the surface layer.

In one embodiment of the invention, the prism of
Wollaston is movable in translation and is mounted on a rotary frame also supporting the polarizer, so that the rotation of the Wollaston prism causes that of the polarizer, the latter being mounted on a rotary support with respect to said frame. The rotation of the frame of the Wollaston prism allows the rotation of the two specific zones of the surface layer illuminated by the two narrow light beams around the optical axis inside the site illuminated by the wide light beam.

 In one embodiment of the invention, the device comprises a laser beam for generating the narrow light beam, more particularly for the measurement of very deep patterns.

 In another embodiment of the invention, the device comprises a white source for generating the narrow light beam.

 Advantageously, the Wollaston prism is provided with an angular separation of the two beams between 10 and 100 minutes of angle.

 In one embodiment of the invention, the device comprises a diaphragm disposed between the polarizer and the detection cell, the hole of said diaphragm being of diameter corresponding to that of the reflected narrow light beam. The diaphragm thus serves as a spatial filter by reducing the light located outside the path of the reflected narrow light beam. Alternatively, the diaphragm can be replaced by an assembly comprising two diaphragms and an intermediate lens, placed in front of the detection cell.

 In one embodiment of the invention, the device comprises an interference filter disposed in front of the detection cell, the bandwidth of the filter corresponding to the wavelength of the reflected narrow light beam. Advantageous frequency filtering is thus carried out when a laser beam is used.

 The method according to the invention is intended for the measurement and observation in situ and in real time of a surface layer of a thin film structure placed in a vacuum chamber equipped with a window on a wall. .

The process consists
- sending a wide light beam of illumination to a site of the structure to be observed, a first narrow light beam of illumination over a first specific area of the structure to be observed and a second narrow light beam of illumination over a second zone specific to the structure to be observed, the beams taking adjacent optical paths close to the optical axis of a video camera and passing through the window of the processing chamber to reach the site, the first and second narrow light beams being coherent with one another and their directions being offset by an angle a, said first and second narrow light beams being generated by a single light source;
- to send the light beam reflected by the site of the thin-film structure to a matrix sensor of a wide-field video camera and the two light beams reflected by the two specific areas and taking the common optical path towards a means capable of optically combine them, the combined reflected light beams interfering with the output of a polarizer followed by a detection cell for the purpose of measuring the level difference between the two specific zones.

 We can send a narrow light beam obtained from a single source to a Wollaston prism to obtain the first and second incident narrow light beams and we can send the two narrow light beams reflected to the Wollaston prism to combine them on a single optical axis while maintaining the orthogonality of their respective polarizations, the polarizer being movable in rotation by an angle of at least 90 "so as to be able to select the first narrow light beam reflected in a single polarization, the second light beam narrow beam reflected in a single polarization offset by 90 "with respect to the polarization of the first beam and the combination of the two narrow light beams reflected in an angular position offset by the order of 45" with respect to the first.

 Advantageously, the distance between the two specific zones can be adjusted by translation of the Wollaston prism and they are moved in rotation by rotation of the Wollaston prism and of the polarizer.

 The Wollaston prism belongs to the category of polarization state separators. The emerging beams are deflected on either side of the mean direction of the incident ray, but these deviations are not strictly symmetrical.

For example, if one chooses a Wollaston prism of angular separation equal to 30 minutes of angle, one obtains at a distance of 20 cm from the structure specific zones offset by about 450 microns. The displacement in translation of the Wollaston prism in the camera makes it possible to adjust this difference. The rotation of the prism of
Wollaston allows you to move the two specific areas. One can thus easily choose a specific zone on a portion of the surface layer covered by a mask and therefore spared by the treatment and the other specific zone in a portion being etched. It is of course possible to carry out a plurality of measurements on different portions of the site during etching. The site can be changed using the horizontal displacement table.

 If there is a specific area on an edge between a portion being etched and a portion covered by the mask, there will be a diffraction phenomenon that can be observed on the surveillance camera, which will allow moving slightly the specific area to avoid this phenomenon. The mask which covers part of the structure with thin layers is transparent to the wavelengths used with a selectivity with respect to the structure itself of the order of 50. When the selectivity between the etched material and the mask is greater at 10, the etching of the mask does not affect the accuracy of the measurements. The camera is used both to recognize the positioning of the beams and to perform geometric measurements thanks to the digitalization it performs. Furthermore, the etching process may tend to widen the portions not covered by the mask by attacking the edges of the mask or else to dig a masked portion at the edge of a portion being etched by attack under the mask. Such changes are observed, in real time, by means of image processing and interferometry because they result in a change in contrasts over time. Two successive images of the sample can be compared to highlight this possible change in contrasts.

The stability of the mask is checked, in real time, in the horizontal plane along the X, Y and depth axes. We therefore obtain three-dimensional information on the evolution of the patterns in the sample.

The invention will be better understood on studying the detailed description of an embodiment taken by way of nonlimiting example and illustrated by the appended drawings, in which
Figure 1 is an explanatory diagram of the apparatus used for the method of the invention;
Figure 2 is an explanatory diagram of the device of the invention;
Figure 3 is a sectional view of the thin film structure monitored according to the technique of the invention; and
FIG. 4 is a top view corresponding to FIG. 3.

 The applicant has developed a very compact set of observations and simultaneous interferometric measurements by laser, in particular on thin-film structures, a technique described in detail in French patent applications Nos. 2,680,414 and Nos. 2,718,231 to which the reader is invited to refer for more details on the apparatus and basic operation of the device.

 As shown in FIG. 1, a vacuum treatment chamber encloses a sample 2 to be treated, for example a silicon wafer being etched under plasma to obtain a membrane, and has, in its upper wall, a silica window 3 . A monitoring unit 4 is mounted above the treatment chamber 1 on a table 5 with horizontal displacement X-Y. The monitoring unit 4 is connected by an optical fiber 6 and an electric cable 7 to an operating and control unit 8 to which are associated a control keyboard 9 and a display screen 10. The unit 8 is connected with two electric stepping motors (not shown) for controlling the horizontal movement of the monitoring unit 4 on the table 5.

 As seen in more detail in Figure 2, the monitoring unit 4 has a housing 1 1 which encloses a video camera 12 whose adjustable lens 13 can be of the autofocus type, a wide beam light source 14 and a number of optical plates for guiding the light beams along the predetermined optical paths. The video camera 12 comprises a sensor 15 preferably made up of a plurality of charge transfer cells (CCD) arranged in a matrix. The sensor 15 is connected in a manner not shown to an electrical connector 16 for the electrical cable 7 in order to supply a digital video signal to the operating and control unit 8 to be displayed on the screen 10, the electrical connector 16 being mounted on the housing 11.

 The housing 11 includes an interior compartment 1 la in which is located the light source 14 which emits in a spectrum preferably having at least partial overlap with the visible light spectrum. To simplify the following description, we will speak of the white light emitted by the lighting source 14. The interior compartment 11a has a window provided with an optical lens 17 which directs the light beam of illumination towards a semi-transparent plate 18 disposed between the objective 13 and the sensor 15 of the camera 12, so that the lighting light beam takes the optical path of the camera, that is to say the optical axis 13a of the objective 13. A first light trap 19 in the form of a strip is placed behind the semi-transparent strip 18 in order to absorb the part of the light beam having passed through the semi-transparent strip 18 and thus reduce the optical disturbances in the monitoring unit 4.

 The laser beam source 20 comprises a laser diode and emits a narrow beam projected via a semi-transparent plate 21 onto another semi-transparent plate 22 interposed in the optical path between the objective 13 and the sensor 15 of the camera 12, so that the laser beam also follows the optical path of the camera 12 which coincides with the optical axis 13a of the lens 13. Another light trap 23 in the form of a blade is disposed behind the semi-transparent blade 22.

 As a variant, provision can be made to replace the laser source 20 by a white light source by means of a Xenon arc which can be placed outside the housing 1 1 and connected to the latter by means of 'an optical fiber, not shown.

 On the path of the laser beam emitted by the source 20, between the semi-transparent plate 21 and the semi-transparent plate 22, is disposed a Wollaston prism 24 which has the effect of dividing the beam coming from the laser source 20 in two narrow beams 25 and 26 having an angular separation of 30 minutes of angle and whose polarization states are rectilinear and orthogonal to each other. There are thus two coherent laser beams. For reasons of understanding, the angular offset of the beams 25 and 26 has been deliberately exaggerated in FIG. 2. The rectilinear polarizations are preserved during the journey, provided that the directions of rectilinear polarization at the exit of the Wollaston are indeed respectively parallel and perpendicular to the plane of incidence on the blade 22, otherwise each of the beams takes an elliptical polarization.

 Thus, the compact housing 11 of the surveillance unit 4 encloses the video camera 12 with the light source 14 and the laser beam source 20 to emit a light beam composed of the light light beam emitted by the light source. lighting 14 and narrow laser beams 25 and 26 emitted by the source of laser rays 20 and separated by the Wollaston prism 24 according to an optical path passing through the lens 13 of the camera 12. The combined light beam is sent by the monitoring unit 4 through the objective 13 and the window 3 of the treatment chamber 1 to arrive at the sample 2 in a thin-film structure (FIG. 1).

 The light beam reflected by the sample 2 passes through the objective 13 to penetrate inside the housing 11 of the monitoring unit 4. The semi-transparent blade 22 separates the reflected light beam into two parts. A transmitted part which, after having passed through the semi-transparent plates 22 and 18, arrives at the sensor 15 of the camera 12. A part reflected by the plate 22 passes through the Wollaston prism 24, which causes the combination of the two narrow laser beams reflect in one. The single reflected laser beam at the outlet of the Wollaston prism 24 crosses a polarizer 27 to reach a detection cell 28 connected by an optical fiber 6 to the operating unit 8.

 The reflected beam directed towards the sensor 15 of the camera 12 corresponds to the spectrum of the lighting light beam with two high intensity reflected laser beams. In order to avoid dazzling of the sensor 15 and therefore of the video camera 12 by the effect of the reflected laser beams, there is a filter 29 in the optical path of the camera 12 just in front of the sensor 15.

 The optical filter 29 is transparent for a characteristic wavelength and opaque for the other wavelengths so as to allow only an almost monochromatic light to pass to the sensor 15 of the camera 12. Thus, each CCD cell of the sensor 15 behaves individually as an interferometer representing a pixel of the image plane of the camera 12. The video camera 12 thus behaves like a plurality of interferometers mounted in a matrix and thus provides a video signal which can be viewed on the screen 10 and which corresponds to a monochromatic mapping representing the surface of the illuminated site of the sample 2. Preferably, the characteristic wavelength of the optical filter 29 is chosen which is sufficiently close to the wavelength of the reflected laser beams so as to also display on the screen 10, the two laser spots inside the localized illuminated area without the latter dazzling the emptied camera o 12.

 The interference of the beams is possible thanks to the polarizer 27, on the transmission axis from which the electric field vectors of the two beams project. One thus obtains at the output, collinear and phase-shifted vibrations which can interfere, unlike the electric fields before the polarizer which are orthogonal and therefore incapable of interfering, even if the corresponding rays are strictly combined.

 The polarizer 27 is in the form of a film which comprises dichroic particles which absorb light in a first direction of the electromagnetic field and lets it pass in a second direction orthogonal to the first. As the polarizations of the reflected laser beams are not modified by their passage through the Wollaston prism 24, the rotation of the polarizer 27 makes it possible, in a first angular position, to select one of the reflected laser beams, in a second position offset by 90 "with respect to the first, to select the second reflected laser beam, and in a third position offset by about 45" from the first, to analyze the two reflected laser beams and in particular their interference. The detection cell 28 can be of different types, and in particular a spectrograph.

 In front of the polarizer 27, provision may be made to mount a diaphragm 30 the hole of which has a diameter corresponding to that of the reflected laser beams, so as to limit the detection by the detection cell 28 of the light coming from the source 14. On may also provide an interference filter, not shown, disposed between the polarizer 27 and the detection cell 28 in order to avoid an action of the filter on the polarization and whose bandwidth corresponds to the wavelength of the source 20 so to reduce the light of other colors also coming incidentally or reflected from the source 14. This improves the signal to noise ratio of the detection cell 28, in particular when using a rich Xenon arc in the blue domain while the source 14 is rich in the red domain and an analysis is carried out in the latter domain.

 Figures 3 and 4 schematically show the representative images of the method of the invention. The sample to be treated 2 is a structure with thin layers used for the fabrication by etching of membranes. The sample 2 comprises a silicon substrate 31 of which certain parts are protected by a mask 32. The area 31a of the substrate 31 not protected by the mask 32 is attacked by a plasma process, known per se, to a predetermined thickness .

 The incident light beam emitted by the monitoring unit 4 illuminates a site 2a of the upper surface of the sample 2. The site 2a is delimited by the white light lighting beam 33. The first laser beam 25 reaches 1 sample 2 on an area protected by the mask 32. As the mask 32 is transparent to light, the laser beam 25 is reflected on the surface of the substrate 31 disposed under the mask 32. The second laser beam 26 is reflected on the area 31a of substrate 31 during etching. On the screen of the monitoring unit 4, the position of the laser beams 25 and 26 is visualized and can thus be moved so that one is reflected on an area protected by the mask 32 and the other is reflected. on a trench being etched in the substrate 31.

The movement of the laser beams 25 and 26 relative to the sample 2 is carried out, on the one hand, thanks to the movement of the table 5 in a horizontal plane and, on the other hand, thanks to the movement of the Wollaston prism 24 The Wollaston 24 prism is movable in translation along its optical axis, which makes it possible to adjust the spacing between the laser beams 25 and 26 on the sample 2. The Wollaston 24 prism is mounted on a rotating frame , not shown, which makes it possible to move the laser beams 25 and 26 by rotation about their original axis 34. The polarizer 27 is also mounted on the rotating frame of the Wollaston prism 24. The rotation of the prism of
Wollaston 24 causes that of the polarizer 27 so that a displacement of the laser beams 25 and 26 does not modify the setting of the polarizer 27. The polarizer 27 is mounted on a support, not shown, rotatable relative to the frame of the prism of Wollaston 24 The polarizer 27 is thus adjusted independently of the Wollaston prism 24.

Due to the difference in the lengths of the paths taken by the two reflected laser beams due to the etching depth of the exposed area 31a of the substrate 31 of sample 2, the intensity of the two reflected laser beams, after passing through the prism of
Wollaston 24, is not absolutely equal, which decreases the contrast of the interference fringes. When it is desired to study the interference between the two reflected laser beams, the polarizer 27 is then adjusted at an angle which can be very slightly different from 45 "so as to balance in intensity the components originating respectively from the two reflected laser beams. The difference in intensity of the reflected laser beams can also be due, in part, to the reflection on the plate 22 during which the polarization parallel to the plane of incidence on the plate 22 is favored.

 The phase shift between the beams comes on the one hand from the reflection on sample 2, it is the useful phase shift which makes it possible to measure a layer thickness or an etching depth, but on the other hand, from the reflection and the transmission through the blades 21 and 22 and the prism of Wollaston 24, especially if the latter is not well centered. In monochromatic light (laser), a calibration can be carried out by directing the two beams on a single surface of the sample 2.

A polarization cube or a second Wollaston prism can be placed on the optical path of the reflected beam after passing back through the Wollaston prism 24 to spatially separate the beams according to their polarization, measure their amplitudes separately
Rx and Ry and calculate their ratio. A polarized cube allows a spatial separation of an angle of 90 ". A Wollaston prism made of calcite allows a spatial separation of an angle of several degrees.

 This gives information relating to the modification of the surface states of the sample.

 Thanks to the invention, it is possible to monitor deep etching operations of semiconductor layers with two coherent laser beams due to their origin from a single source. An accurate measurement of the etching depth is thus carried out. Before the start of the etching, several patterns of the sample can be observed by positioning themselves successively on each of them to deduce an interesting model of the sample for an industrial etching process in large series.

Claims (10)

 1. Device for three-dimensional measurements and observation in situ and in real time of a surface layer of a thin-film structure during treatment in a vacuum chamber (1) equipped with a window on a wall comprising a surveillance unit (4) which comprises a compact housing enclosing a video camera (12), a wide beam light source (14), a narrow beam light source (20) and optical components, a light unit operation and control (8) comprising an optical connector and an electrical connector mounted on the housing of the monitoring unit to connect it via respectively an optical fiber cable and an electrical cable, to the operating and control, and a table (5) mounted above the treatment chamber to ensure, on the one hand, the horizontal movement of the monitoring unit along two axes (X, Y) in order to choose a site and, d on the other hand, the pre positioning cis so that the two incident beams and the two beams reflected by the surface layer take adjacent optical paths close to the optical axis of the video camera, characterized in that it comprises a prism of Wollaston (24) arranged on the optical path of the narrow light beam to obtain, at the output of said Wollaston prism, two narrow, coherent polarized light beams (25, 26) of different directions offset by an angle a and orthogonal polarizations, the prism of Wollaston being arranged so as to be crossed by the narrow light beams reflected by the layer, a polarizer (27) disposed on the optical axis of the Wollaston prism so as to be crossed by the narrow light beam reflected after it passes back in said Wollaston prism, said polarizer being rotatably mounted relative to said Wollaston prism, and a detection cell (28).  2. Device according to claim l, characterized in that the Wollaston prism is movable in translation and is mounted on a rotary frame also supporting the polarizer so that the rotation of the Wollaston prism causes that of the polarizer, the latter being mounted on a support rotatable relative to said frame.  3. Device according to claim 1 or 2, characterized in that it comprises a laser beam for generating the narrow light beam.  4. Device according to claim 1 or 2, characterized in that it comprises a white source for generating the narrow light beam.  5. Device according to any one of the preceding claims, characterized in that it comprises a diaphragm (29) disposed between the Wollaston prism and the polarizer, the hole of which has a diameter corresponding to that of the narrow reflected light beam.  6. Device according to any one of claims 1 to 4, characterized in that it comprises an assembly provided with two diaphragms and an intermediate lens, placed in front of the detection cell.  7. Device according to any one of the preceding claims, characterized in that it comprises an interference filter disposed between the polarizer and the detection cell, the pass band of the filter corresponding to the frequency of the reflected narrow light beam.  8. Method for in situ measurement and observation in real time of a surface layer of a thin-film structure placed in a vacuum chamber equipped with a window on a wall, characterized in that it consists of  - send a wide light beam of illumination to a site of the structure to be observed, a first narrow light beam of lighting over a first specific area of the structure to be observed and a second narrow light beam of lighting over a second specific area of the structure to be observed, the beams taking adjacent optical paths close to the optical axis of a video camera and crossing the window of the processing chamber to reach the site, the first and second narrow light beams being coherent with each other and their directions being offset by an angle a, said first and second narrow light beams being generated by a single light source,  - send the light beam reflected by the site of the thin-film structure to a matrix sensor of a wide field video camera and the two light beams reflected by the two specific zones and taking the common optical path towards a means capable of optically combine, the combined reflected light beams interfering with the output of a polarizer followed by a detection cell for the purpose of measuring the level difference between the two specific zones.  9. Method according to claim 8, characterized in that a narrow light beam obtained from a single source is sent to a Wollaston prism to obtain the first and second incident narrow light beams and that the two narrow light beams reflected towards the Wollaston prism to combine them on a single optical axis while maintaining the orthogonality of their respective polarizations, the polarizer being movable in rotation by an angle of at least 90 "so as to be able to select the first narrow light beam reflected in a single polarization, the second narrow light beam reflected in a single polarization offset by 90 "from the polarization of the first beam and the combination of the two narrow light beams reflected in an angular position offset by order of 45 "compared to the first.  10. Method according to claim 9, characterized in that the distance between the two specific zones is adjusted by translation of the Wollaston prism and they are moved in rotation by rotation of the Wollaston prism and the polarizer.