Device for the inspection of wafers.

The invention relates to an apparatus for wafer inspection with the features in the preamble of claim 1 said, generically determined.

A device of this type is known from the scientific publication by Karl L.Harris et al., Solid State Technology, February 1984 page 159 et seq. With the help of the known apparatus is a wafer, ie intended to manufacture highly integrated circuits semiconductor plate, which may have dimensions of about 15x15 cm (6 "x6") typical, enlarged by means of a television image display mappable; Image analysis of the TV picture is carried out electronically with a computer with hoher.Verarbeitungsgeschwindigkeit. In a high-resolution, only in some areas carried out scanning come substantially only linewidths of the so-called The structures, ie the individual functional elements of the finished semiconductor device corresponding regions, these regions comprises at various stages of the manufacturing process, the vapor deposition, etching, oxidation and varnish coating processes, to be examined.

Substantial disadvantages of the known device, at least the folgenen:

The vidicon tube used for recording of the television picture can only distinguish about 30 different gray levels, thereby resulting in limitations on the contrast, sharpness and ultimately resolving power. A focus of the scanning electron beam is only to a certain level of investigation of the wafer possible with the result that the TV picture for this reason necessarily having significant blurring. A change in the level of the sharp representation is at best possible in that the wafer is lifted up or deflected; However, the necessary mechanical devices operate slowly and additionally limit the processing speed. on the achievable means of vidicon scanning image information addition to the known device, no further information winnable.

Object of Erfingung is therefore to provide an apparatus for wafer inspection, which is able to deliver more precise and comprehensive data on the structure and the overall condition of the wafer during the various process steps of producing at least comparable information capacity and -Verarbeitungsgeschwindigkeit.

This object is achieved by the features mentioned in the characterizing part of claim 1.

Hence resultiernde advantages of the device according to the invention, at least the following:

By. optically scanning the wafer surface charges are individual areas of the wafer is avoided, which could lead to a change in the electrical properties of the finished component. The device of the invention allows a much faster scanning of the examination area, which with the help of an electronically controllable active mirror the levels of investigation can be changed virtually without inertia and a variety of layered samples of the object under examination are possible, which can be processed to a precise, high-contrast and high-resolution image.

The use of a laser scanning microscope for focusing the laser scanning light beam allows a narrow limitation of depth (depth) of the examination zone and hence a high-resolution subdivision thereof in individually displayable layers, whereby the evaluation of the recoverable image data a detailed topographic view of the wafer surface allowed. By confocal means of the arrangement of a pinhole with the focal plane of the microscope - achieved the objective delimitation of the detection light on the depth of field is a high signal / noise ratio achieved, which allows scanning of the examination area with a high sampling frequency.

The use of an active mirror for the focus offset of the scanning light gives the advantage that can be dispensed to adjust certain scanning planes to devices for raising and lowering the wafer.

By the features of claim 2 for an arrangement thereof is indicated within the scanning beam path.

The planned according to claim 3 possibility of storing for a variety in small increments of z..B. 0.1 Ztm staggered levels of investigation determined image data makes it possible to produce through a computerized ordering of these image data interface profile representations of the object to be examined in any plane.

To at least partially to increase the image contrast, it is advantageous if, as provided in accordance with claim 4, the intensity of the scanning light current can be specifically increased.

By the features of claim 5, which may be implemented in special design according to claim 6, is sufficiently reliably prevented that a canceling of the wanted signal may occur by accidental interference.

The by the features of claim 7 basic construction to defined and by the features of claims 8-11 unspecified, preferred embodiment of the apparatus according to the invention provides the advantage that from an evaluation of the detected by a second scanning device point spread functions sequentially sampled pixels autofocus of the scanning of the intended image scanning device scanning is possible, which can produce sharp profile views of the wafer surface and can also gain accurate information regarding the layer thicknesses of oxide layers.

Such information can also be obtained in the direction indicated by the features of claim 12 and of claim 13 erfindugnsgemäßen design of the device.

By means of a provided according to claim 14, in a special design by those of claim 15 can be realized luminescence - measuring device can resist residues can be reliably detected on the wafer surface in particular.

When designed according to claim 16 transport device is provided for the wafer, can be dispensed with a galvanometer necessary for Y-beam deflection device within the sample.

A comparison of a line-shaped scanning region with a specified desired structure can also be effected by means of a so-called Konvolvers, of a non-linear superposition is conveyed in the opposite direction in a piezoelectric material propagating surface waves, where one of the target structure and on the other hand, the real of the structure is imprinted measured intensity distribution of scanning light corresponding amplitude course, so from a measurement of the integral of the correlation function of these surface waves a statement about the compliance of the above structures is winnable.

Further details and features of the invention will become apparent from the following description of specific Ausführuhgsbeispiele reference to the drawing. Show it:<ul><li>Fig.1. the beam path of a beworzugten embodiment of the device according to the invention,</li><li>Fig.2. a schematic representation of the arithmetic operations for evaluating pixel functions and the therefrom winnable control signals for an intended part of the apparatus of the invention for adjusting the levels of analysis and active mirror</li><li>Figure 3. a special type of scanning of the examination area of ​​a wafer.</li></ul>

The in Fig.1, reference is specifically made to the details thereof, illustrated, device 10 according to the invention is intended for a so-called wafer inspection, ie for production control in the manufacture of highly integrated semiconductor devices, which are manufactured in a variety of process steps. a silicon plate with a diameter of up to 6 inches in vapor deposition, etch and oxidation and diffusion and doping processes generate structures that correspond to an extremely complex arrangement of electronic circuit elements - Here, a so-called wafer 11th With the help of the device 10 that are characteristic of certain stages of the production process structures should be as soon as possible comparable with target structures to detect manufacturing defects reliably and encumbered by a wafer such errors to be eliminated from the production process in time ..

Based on the described below constructional and functional details of the device 10 also procedures will be explained to the purpose used correctly :, to be regarded as belonging also to the subject of the invention.

The device 10 is its basic construction to a so-called laser scanning microscope (laser scanning microscope), which with line by line and point-wise within each line scanning of the examination object or type region and a successful end in principle analogous to the representation of a television picture presentation of the examination region 16 operates, which is indicated in FIG. 1 by the focussing plane of a microscope objective lens 17 schematically, which is specifically the surface of a produced by a particular process step, the wafer structure.

substantially point - - elementary areas of the examination zone 16 illuminated sequentially in said line or dot grid with laser light, and it is for each elementary area, the intensity of light emanating from this reflection and scattered light is measured by means of a photoelectric detector The individual will , specifically a photomultiplier tube 19 is detected separately for each picture element. The thus detected intensity values ​​of the reflected from the object under examination laser light are synchronously input to the light scanning of the examination object in an image memory 21 and there for further processing - image display and, if computational processing of intensity data - kept.

As the light source 22 of the device 10, a He-Cd laser is provided at wavelengths A1 = 325 nm and A2 = 440 nm, that emits in the near UV range and in the violet region of the visible spectrum of light. This laser 13 is a so-called. Cw-laser emitting continuously. The primary beam this .Helium cadmium laser 22 is represented in FIG. 1 by its dot-dashed drawn central beam 23 and .His marginal rays 24 and 26. The primary beam 23, 24, 26 of the He-Cd laser 22 is essentially a parallel beam having a beam cross-section of about 1 mm²,

By means of a designated overall by 27 beam-shaping optics is suitable for further utilization of the laser light beam shape in the sense of an adaptation of the beam cross-section to the existing for further optical processing of optical devices as well as in terms of a set yet in greater detail below for illustrative influencing the convergence of laser light beam , The marginal rays of the light beam output of the beam-shaping optics 27, which at its output 28 a larger cross-section than at the input 29, are 24 'and 26', respectively.

The output light beams 23, 24 ', 26' of the beam-shaping optics 27 is a total of 31 designated scanning (scan) sent to -Einrichtung that mediates one hand, a "horizontal" and on the other hand, a "vertical" beam deflection, the sample referred to in the raster allows the scanning of the examination zone 16 in the X and Y coordinate directions.

As a horizontal deflection element is a so-called. Polygon mirror 32 is provided, the facets 33 define a regular octagon in the illustration in FIG. 1. In a typical implementation, the polygon mirror 32 has, however, 24 facets 33 each other in regelmäßigpolygonaler arrangement.

The polygon mirror 32 is driven in rotation by an electric motor 34, the rotation frequency of the polygon mirror 32 is synchronized in a following further explained manner with the storage of the output signals of the photomultiplier 19 in the image memory 21, in which in a digital format, the image of the examination zone 16 is stored.

The Y displacement of the laser-light beam 23, 24 ', 26' ', 26' seen in the reproduced by the arrow 36 direction of propagation of the laser light beam 23, 24, the X-deflection 32, 34, arranged downstream, and obtained in manner known per se by means of a horizontal axis 37 pivotally driven galvanometer 38th

The facets 33 of the polygon mirror 32 are displayed in a so-called telecentric optical path, which is realized by means of two lenses 39 and 41, the reflecting surface 42 of the galvanometer 38th An arranged between the exit lens 41 of this telecentric beam path and the galvanometer mirror 38 deflecting mirror 43 is provided only for achieving a low-beam geometry.

By means of an analog to the telecentric lens arrangement 39,41 telecentric lens arrangement 44, 46, the reflecting surface 42 of the galvanometer mirror 38 imaged onto the entrance pupil 47 of the microscope objective 17, which in turn that as a parallel bundle or nearly parallel light beam passing through the entrance pupil 47 of laser light in the investigation level 16 focused.

The light reflected at the present in the focusing or the investigation level 16 structures of the examination subject or scattered laser light is reflected in the extent described beam path and by means of a partially transparent mirror 48, usually a half mirror disposed between the polygon mirror 32 and the output 28 of the beam-shaping optics 27 is arranged, coupled in the direction of arrow 49 from the scanning beam path. This outcoupled light beams 23 ', 24', 26 'is focused by means of a further microscope objective 51; inthe focal plane 52. This further microscope objective 51, a pinhole 53 is arranged which conveys a bundle limit To that effect, that only such laser light as being unused for the intensity measurement useful light passing through this pinhole 53, from the depth of field of the microscope objective 17 corresponding region of the examination plane 16 forming focal plane of the microscope objective 17 is derived. In the propagation direction 49 of the coupled out of the illumination beam path laser light seen, the photomultiplier 19 is behind the pinhole 53 is arranged which generates a bright to the intensity of reflected from the object to be examined in the scanning beam laser output proportional.

The per a unit area of ​​the examination plane 16 associated output signals of the photomultiplier 19 are controlled by a synchronizer 54, which conveys the purpose just controlling the rotation of the polygon mirror 32 and the oscillating movements of the galvanometer 38, the latter recorded by a rotary actuator 56, in the image memory 21st

To produce a reproducible means of a television screen image of the examination zone 16 the examination area is divided into 512 x 2048 pixels, analogous to the generation of a standard television image. There are therefore 512 scan lines pass and within each row 2048 pixels. The size, ie the extension of the pixels in "horizontal" and "vertical" direction is determined by the size of the field, divided by the number of pixels per line or image height divided by the number of lines. Depending on the focal length of the microscope objective 17 varies the size of the field between 100 .mu.m and several mm; varied according to the pixel size of 0.05 micron and several microns.

The scanning of the examination zone 16 takes place so rapidly that the image field is scanned in 40 ms, in 20 ms, one field is scanned, one of which the odd and the other contains the even lines. These two fields are then superimposed in a known manner for itself today.

The synchronization of the reading of the photomultiplier output signals in the image memory 21 with the scanning movements of the polygon mirror 32 and the galvanometer mirror 38 in a known manner, such as in scientific publication by check (R. Maintenance, R. Baker, R. Buchroeder, D Hillman, R. Shoemaker, and Bartels PH, the Journal of Histochemistry and Cytochemistry 27th described 153 (1979)).

synchronized with the read-in is also an electro-optical or acousto-optical modulator, by means of which the intensity of reflected in the examination plane 16 laser light can be modulated. This makes it possible, portions of the examination area with a higher intensity to light than the other part of the examination field. This is particularly important when individual areas with high contrast to be imaged,

. To be able to 'scan a plurality of investigation levels 16, 16' and 16 'in rapid succession, a so-called active mirror 57 is provided, which by electrical triggering a - positive or negative - power can be impressed that a change in the position of examination plane 16, as seen perpendicularly to the latter, within a range of + 10 microns, based on a preselected position of the examination plane 16, made possible.

It is believed that this active mirror 57 acts as a plane mirror in the non-triggered state.

The active mirror 57 is 17 corresponding pupil plane 47 arranged in a to the entrance pupil plane 47 of the microscope objective '. He is a partially transparent - 58, the beam-shaping optics 27 illuminated mirror on which the means of a telecentric lens arrangement 59.61, the two converging lenses comprising 59 and 61 of different focal lengths, expanded output beam 23,24,26 of the laser 22 to - semipermeable active mirror 57 is deflected out. The reflected from the active mirror 57, by the marginal rays 26 'and 24' 'represented, is through the partial reflection mirror 58 passing luminous flux by means of another telecentric arrangement of lenses 62 and 63, the beam-shaping optics 27 on the other, already explain exploitation the laser light flux brought suitable cross-section, with which the laser light beam 23, 24 ', 26', at the output 28 of the beam-shaping optics emerges 27th

The extent explained device 10 can thus by functional activation of the active mirror 57., possibly programmatically, quickly and accurately to the sampling of various, eg equidistant staggered investigation levels are 16, 16 'and 16' set ', whose images, a sufficient capacity of the image memory 21 provided in its entirety can be saved. The content of the image memory 21 can be exploited to display arbitrary cutting planes by the total detected examination area that can extend obliquely to the scanned examination planes or perpendicular to them.

Due to the described type of scanning of the examination regions 16, 16 'and 16' ', the recoverable pictures of these areas of study inevitably entail blurring that come about in that areas that are upstream and downstream of the plane with optimum sharpness, the intensity distribution of the for detecting unused laser bright contribute. However, these factors can be taken into account and kpmpensiert mathematically by successive tomograms and their characterized detectable changes so sharp and contrasty images of the examined structures can be erzeilt from a mathematical post-processing of memory contents.

This is expediently carried out by taking advantage of a known voraussetzbaren - dimensional transfer function of the microscope objective 17. The mentioned computational correction of sharpness based on a series of tomographic images for different levels of investigation also includes the ability to produce a sharp spatial image of the object with quasi infinite depth of field, since the mentioned computational correction can of course be done for each layer plane.

It can have a variety of sectional profiles of the structures applied to the wafer - etched photo-lacquer structures, conductive metal structures and insulating SiO₂ be captured and displayed a characteristic feature of the device 10 according to Figure 1 is, as already mentioned, the confocal arrangement of the pinhole 53 to the investigating plane marked focal plane 16 of the microscope objective 17, with the result that only light passing through the pinhole - strips can, is able to pass out of the depth of field of a entwerfbaren through the microscope objective 53 image. The other areas, which - seen in the propagation direction of the laser light - are ahead of or behind the focal plane 16 of Mikroskopobjrktivs, reflected or getreuten light streams are thrown by the confocal optical path on the edge of the aperture 53 and thus shaded from the photomultiplier 19th

This is particularly important to ensure that the the depth of field of the microscope objective 17 corresponding thickness of the examination zone 16 is low. She is in typical cases between 0.05 microns and 1pm.

In typical design of the unit 10, the microscope objective 17 has a focal length of 5 mm and a numerical aperture of 0.9. The diameter of the laser light beam 23, 24 ', 26', the fokussiert.wird through the microscope objective 17 in the plane of examination 16, is up to 6 mm.

Depending on the choice of the microscope objective 17 and of said beam cross-section, a resolution of less than 0.2 / .mu.m are achieved.

In addition to the sampling device 31 further, generally designated 64 scanning device is provided which, as explained above in principle the same way, using the scanning device 31, a partial or complete scanning of Untersuchungsbildfeldes'ermöglicht. This scanning device 64 comprises a sensing a first galvanometer mirror 66 which is pivotable about a horizontal, ie parallel to the Ausgreitungsebene of the scan light axis 67 and conveys the Y deflection of the scanning light, and a second galvanometer mirror 68 to a perpendicular propagation plane of the scanning light extending axis 69 is pivoted and accordingly the X-deflection, ie the line deflection imparted.

When scanning is a diverted from the output luminous flux 23, 24, 26, the He-Cd laser 22 partial luminous flux 23 '', 24 ", 26",. utilized. This partial luminous flux 23 '', 24 ", 26" is in the position shown in FIG. 1 via a first geometry. partially transmitting deflecting mirror 71 and a second partially transmitting deflecting mirror 72 and on the other scanning device 64 and another partially reflective mirror 73 is coupled into the provided for the imaging of the examination zone 16 beam path, said coupling further sample luminous flux between the galvanometer mirror 38 of the first sample -Einrichtung 31 and a lens 44 of the telecentric lens assembly 44, 46, takes place, which is arranged in front of the entrance pupil 47 of the microscope objective 17th

The intensity of the exploited for additional sampling partial light stream 23 ", 24", 26 '' is only about 10% of the intensity of the output light flux of He-Cd laser.

For intensity control Abtastlichtströme a partially transparent mirror 70 are in this case 2 modulators 56 'and 56' 'in the manner shown in FIG. 1 arrangement with respect provided.

The other pickup 64 is asynchronous with the sampling device 31 and operated at a much lower sampling frequency. This reflected by the object to be examined by reflection or scattering in the beam path of another scanning 66 light array photoelectric receiver, preferably a diode matrix 73, recorded with a two-dimensional (2 D) with respect to its two-dimensional spatial intensity distribution, the intensity distribution of each of measured an illuminated pixel reflected radiation and stored for further processing, that is, the so-called point spread function. (point spread function - PS function) recorded. may be selected from the thus detected PS function. using known algorithms, the main lines of the following for a better understanding will be explained shortly, a deviation of the gradient of the wavefront of the light passing through parts of the object scanning light are calculated from the expected to the geometrical optical constraints ideal course. Particularly interesting is the determination of the PS function is because it can be concluded from the spatial intensity distribution of the point image on structures whose dimensions are smaller than those characteristic of the illuminated picture element as a sampling point. It can in this way edge structures accurately detected werder, and the smallest particles of dirt are erkannnt.

In the inventive device 10 measured by means of the further scanning device 64 point spread function is also utilized to a pointwise auto focusing of the scanning light beam on the aufgerachte on the wafer 11 structure. To this end, a comparison of the measured intensity distribution with a characteristic for accurate focussing of the scanning light beam on the scanned area of ​​a computer 74 - carried out point-spread-function - ideal. From this comparison, control signals for a compensatory control of the active mirror 57 are generated such that, as measured by the scanning device 64 further intensity distribution is "adjusted" to a good approximation to the ideal point image function.

The control signals necessary dadür can be evaluated in units of a distance from a reference plane of magnitude and change sense. Thus, the layer thicknesses or heights can be detected continuously by surface profiles in the result.

The surface structure of the wafer 11 is thus in all coordinate directions (X- and Y-directions by the scan, the Z-direction by analyzing the point spread function) darstllbar.

For a simplified explanation of the calculation method by which the computer 74 processes the acquired with the farther sample einrichtung64 PS-Funktionsdater, is now referred to Figure 2, in this calculation method is illustrated schematically: FIG.

This method consists in carrying out an iteration, according to which, taking account of the diode matrix 73 erfaßtenIntensitätsverteilung (the point spread or the PS-function) In (u, v) (u = lines Meanwhile, v = column index of the diode array), a shape of the wavefront of the scanning light at the location of the entrance pupil 47 of the Mikroskopobjrktivs is computed 17, which with the measured intensity distribution I_m (U, v) is consistent.

The starting point is in this case from the plausible assumption that the light field at the location (x, y) of the entrance pupil 47 of the _Mikroskopobjektivs 17 a φ by a phase factor_O(X, y) have characterizable plane wavefront. Taking into account the transfer function of the optical system by folding the wavefront R (x, y) expikφ_O with the transfer function F of this system - the profile of the light field √I (u, v) expi (u, v) is in the focal plane - calculates the microscope objective 17 - the examination plane sixteenth By replacing the amplitude factors √I (u, v), the recoverable values ​​from the measurements √I_m(Uv) and applying the inverse transfer function F ¹ angesetzen.Lichtfeldverlauf on this for the focal plane is now a consistent with this field configuration shape of the wavefront at the entrance pupil 47 of the microscope objective 17, ie, a so consistent phase factor φ₁ (X, y) is calculated. In the first iteration of this phase factor is now assumed for the description of the light field at the location of the entrance pupil and again, as described above, the wave field is calculated in the focal plane of the microscope objective 17; Furthermore, the amplitude factors can be replaced by the roots of the measured intensity values ​​and the new phase factor φ₂(X, y) is calculated. Such iterations are repeated until the measured intensities I_m (U, v) correspond within predeterminable limits with the calculated intensity values ​​and to this _Weise a wavefront at the location of the entrance pupil 47 of the microscope objective 17 determined with the measured intensity values ​​are consistent.

From the comparison of arising from the iterative wave front with the assumed for the ideal case of field distribution at the entrance pupil 47 of the microscope objective 17 can use the Rechners74 and the actuating device 76 control signals for controlling the active mirror 57 are produced, such that the measured intensity distribution with which 47 resulting intensity distribution matches from an ideal field distribution at the location of the entrance pupil in the focal plane.

In this way can be achieved in the surface of the object during the scanning of the examination area a continuous, automatic tracking of the focus 16 of the sample-Strahlengenges. It is understood that this tracking can not be performed for each sample pixel, but, because the computational processing of the point spread function takes a certain time, for example only with 1/100 - 1/1000 of the sampling frequency.

As part of the device 10 according to the invention is further provided that in the beam path of the further scanning device 64 einkoppelbarist also test light with broadband spectral distribution. Thereby the possibility is created to identify certain areas in the field of observation, in particular in those regions which are analyzed using the "fast" image-pickup device 21, and the optical thicknesses of the layer structures. The thickness of these structures is thereby pointwise detected with up to 1 000 coating thickness values ​​can be measured per second. The layer thickness measurement takes place here after Prinzpip the so-called. White light interference, and for determining the wavelengths of constructive interference, a diode-array spectrometer 78 is provided, on the entrance slit 79, the back reflected by the object under examination light is coupled via a partially transparent mirror.

As white light source a XBO lamp (high-pressure xenon lamp) is advantageously used.

The device 10 can be used thanks to the use of the He-Cd laser 22 as a light source and for recording of the spatial distribution in the examination zone existing luminescent molecules. It is sufficient if between the partially reflecting mirror 48, through which the exploited for the detection light is coupled out of the scanning beam path, and the photomultiplier 19, a filter is arranged which expediently also for stimulating UV radiation of the laser 22 and which emitted in the visible spectral radiation impermeable. The measurement of the luminescence radiation is of particular interest for the determination of the distribution of photoresist residues on the wafer 11. A suitable filter 82 for the measurement of luminescence is suitably, as shown in Fig. ' indicated by dashed lines, between the partially reflecting mirror 48 and the lens 51 is located, the pinhole is in the focal plane 52 53rd

The scanning of the examination field as an alternative to the described with reference to FIG. 1 also take place as s.chematisch shown in FIG. 3 that the wafer, for example, on an X, Y coordinate carriage in X- and Y-directions is movable and is moved along a meandering or rectangular-wave-shaped path. In the X direction seen, ie in the direction in which the polygon mirror 32, the beam deflection imparted, the pitch of the wafer movement is chosen equal to the amplitude of Strahauslenkung. In the Y-direction of the stroke of movement of the wafer 11 corresponds to the extension of the selected scan range. In this way, by means of scanning each adjacent, parallel strip-shaped portions 11 ', 11' 'and 11' '' sampled. This configuration of the erfirigungsgemäßen device has the advantage that under the scanner 31 according to Figure 1 in place of the galvanometer mirror 38, a fixed mirror can be used. In addition, a faster scanning of the examination field is possible, because the wafer 11 may be transported continuously in the Y direction. It is understood that such an X, Y transfer device shown in combination with a device 10, as shown in Fig. 1, can be used, wherein the transport device can be selectively shut down for scanning a small selected portion of the wafer 11.

A time very quickly feasible comparison of a line-shaped scanning region with a predetermined desired structure can be made of the same also by means of a so-called. Konvolvers. Fie function of such a construction element is based on a non-linear superposition of surface acoustic waves, which an amplitude response can be impressed, corresponding to the detected within a scan line intensity distribution of the reflected light on the detector of the scanning device. In the convolver traveling acoustic waves are generated in the opposite direction, one of which corresponds in terms of its amplitude profile of the target structure and the other of the measured intensity course. Do this in the opposite direction propagating surface wave the same amplitude response, so is the integral of costs resulting from non-linear superposition these acoustic waves correlation function - can be detected by a tapped at a top electrode of the existing piezoelectric material Konvolvers voltage - max. the correspondence of the scanned structure with the desired structure can thus be determined from the thus measured value of the integral of Korrelationsfunkion of the two surface waves.

Such a convolver device can not be used only for the purpose of detection of structural defects, but also for the purpose of the above-explained analog auto-focusing of the scanning beam on the surface of the structure under test.