DE102009025562A1 - Method for optically examining layers - Google Patents

Abstract

The invention relates to an interferometry method for the optical examination of layers. This can be embodied, for example, as a white-light interferometry or as a spectral thin-layer interferometry method. In this case, a measurement spot (18) with a diameter (d) is generated with which the surface (12) of a measurement object (11) is optically examined. According to the invention, it is provided that the method is carried out at a measuring object (11) moved with the speed (v), the illumination of the measuring object being effected with light pulses of a duration (t). It should be noted that the extent (a) of the layer section to be examined must be greater than d + vt, so that a measurement can be made. According to the invention, the parameters (d, v and t) are suitably selected at a predetermined speed (v) of the measurement object. Advantageously, the measurement method can therefore be used in the continuous production of layered components.

Description

The The invention relates to an interferometry method for optical inspection of layers in an inspection device, the following Has method steps. A measuring object with the at least the surface of a transparent layer having layer is stored in the inspection device. Then with a Light source and imaging optics a measurement spot with an extension d projected onto the surface of the layer. Then it will be the measuring light reflected from the measuring object onto an optical sensor projected.

A method for optical examination of the type described above, for example, according to the US 2008/0180694 A1 described. In this method, the measurement object is placed under the inspection device. The measuring device has a free-ray optics, wherein the light of a light source is focused on a point on the surface of the measurement object. The light reflected from this point is diverted via a beam splitter and projected onto an optical sensor surface.

The optical inspection method according to US 2008/0180694 A1 follows the principle of white-light interferometry. To evaluate the measurement light, a reference light is superimposed on it, the phase shift of which is temporally changed with respect to the measurement light. The resulting interferences can be evaluated in order to obtain information about the properties of the investigated layer. These statements may refer to the transparent layer on the surface of the layer and possibly further transparent layers below this transparent layer. As transparent in the context of the invention, layers are considered which are permeable to at least part of the measuring light.

alternative For white light interferometry, the method of spectral thin-layer interferometry are applied. This measuring method is well known and uses the physical Effect that the measuring light at the boundary layers or the surface is reflected by transparent thin films, wherein in the reflected measuring light interferences occur in terms of the layer properties can be evaluated.

at This type of interferometry generally requires that the measuring light is directed at the measuring object and the reflected measuring light is visually evaluated. Depending on the conditions, which are given by the interferometer used must be for the measurement will be provided a certain measuring time to be on the optical Sensor to generate an evaluable signal. In white light interferometry must also be taken into account that the measurement signal over a evaluated temporal phase shift of the reference signal across must become. The required measuring time therefore limits the possible applications in the measurement method of interferometry.

The The object of the invention is an interferometry method indicate which is relatively universally applicable and especially in the continuous production of coatings Can be used.

These Task is inventively with the beginning Interferometrie method mentioned solved that the object to be measured at a speed v lateral below the measuring spot is moved through and the illumination of the measurement object with light pulses of a duration t is made. According to the invention while the extension a of the layer in the direction of the lateral movement by the structural features of the object to be generated specified. To guarantee the measurement result, the relationship becomes a> d + vt by setting the variable parameters under the parameters d and / or v and / or t complied with. This advantageously ensures that despite the movement of the object to be measured at the speed v for the required measuring time is the layer to be examined in Measuring spot is located. This can also be described as the resolution of the measuring method d + vt according to the invention and the extent a of the layer on the measuring object at least the Resolution must match. For example, it may be in the layer advantageous to a structured layer on one Semiconductors that are examined for their quality got to. Features of the layer, d. H. at least the topmost transparent one Location that can be determined using interferometry are, for example, the layer thickness or at a known layer thickness the refractive index of the relevant layer. At the speed v can be the mean speed during the duration t of the light pulse act. Preferably, v is a constant Speed.

Which parameters d, v, t can be varied depends on the application. In this case, the physical conditions of the inspection device must be taken into account. For example, the duration t of the light pulse must be selected such that a signal-to-noise ratio which is sufficient for the measurement purposes is established. The shorter the light pulse, the lower is the total energy of the optical signal arriving at the optical sensor. In this case, the re reflow behavior of the layer to be examined also plays a role, by which the proportion of the reflected light is influenced. The duration t of the light pulse can be also due to the physical conditions at the light source does not shorten arbitrarily. Here, the conditions of the response of the light source must be considered.

Advantageous may use spectral interferometry as a method of optical inspection be used. Here, as already explained, advantageous the interferences after the reflection of the measuring signal on the surface on.

According to one Another embodiment of the invention can be used as a method for optical Examine also a white light interferometry are used. Under white light interferometry is related to of this invention to understand a measuring method in which a broad spectral Light with a low coherence length is used. This Light does not necessarily have to be white, but only needs a sufficiently broad spectrum, thus an evaluation according to the mechanism of white light interferometry can be done, d. H. that the interference phenomena by overlay of the measuring light with a reference light by temporal change is generated by the phase shift with the measuring light.

According to the invention be provided in this variant of the invention, that for the measurement required range of phase shift during the duration t of the light pulse is passed through. The required Range of phase shift depends on the application. It should be advantageous for the wavelengths to be examined a shift over the entire phase (i.e., 360 °) become. For certain measurement purposes but can also be a smaller Phase shift be sufficient. The required range of phase shift must therefore go through during the duration t of the light pulse be, because then no measuring light for a Shift is more available. The duration t of the Light pulse can not be extended arbitrarily, since the already explained condition of a sufficient resolution of the measuring signal must be fulfilled. Therefore, the actuator needs (For example, a piezoelectric actuator) for generating the phase shift with regard to its characteristics (response time, adjustment, for the adjustment required adjustment time <t) be designed in a suitable manner.

According to one Another embodiment of the invention provides that the reference point the phase shift can be adjusted by a Hilfsaktor. It is particularly advantageous if the auxiliary actuator over a control or regulation automatically depending can be adjusted from the level of the surface. To this Purpose, a distance sensor provided in the inspection device be the level of the surface of the object to be measured can measure with respect to the inspection device. alternative it is also possible without the additional sensor to adjust the distance by means of the measuring signal itself. Advantageous can then by activation of the auxiliary actuator, the reference point of the Phase shift can be changed by the same amount how the level of the surface changes. The Phase shift made by the other actuator can advantageously thereby regardless of the level position the surface of the object to be measured.

Farther it is advantageous if the extension d of the measuring spot by the Imaging optics is varied. This can be done by a corresponding Lens system done, it is also possible an enlargement or reduction of the measuring spot by an aperture. To be considered is that by the size of the spot too the light intensity is affected, which depends on the examining surface hits.

A larger aperture setting leads, for example to a larger spot but also to one higher light intensity, whereby in itself a possible small spot for a good resolution of the Procedure on the one hand and the highest possible light intensity on the other hand is advantageous. Here is dependent to choose a compromise from the application.

As already mentioned, the measuring method is particularly advantageous applicable, if the layer to be examined in the direction of the lateral Movement has a smaller extent than the measurement object, in particular, the layer is structured on the measurement object. such Layers are continuously produced for example by printing, wherein the layer areas to be examined differ nominally Thicknesses may have. In this case, the focus can be be tracked in a suitable manner the measuring light, due to the sudden change in the layer thickness short Adjustment must be realized. The light pulses can have a duration t advantageously in the nanosecond range. The speed v of the measurement object may advantageously be between 1 and 2 m / s. These are speeds involved in continuous manufacturing printed electronic components, such. B. RFID components realized with semiconductor layers become.

It is particularly advantageous if the light source has one or more LEDs. As a result, LED-based light sources can advantageously be adapted to the respective application. Single LEDs or LED arrays can be used. The broad spectral compositions of the measuring light in the white light interferometry can also by composition of different colors LED's are generated in an array. As a result, a precise tuning of the desired spectral distribution is possible. Of course, the other components of the inspection device used must also be adapted to the spectral distribution of the light source used. In particular, the optical sensor used must be sensitive to the desired spectrum of the measuring light. Spectral distributions in the UV, visual and IR ranges can be realized.

Around can increase the photosensitivity of the optical sensor can advantageous also an optical sensor with a single optical Sensor element or more of these sensors are used. Across from Sensor arrays have sensors with a single optical sensor element the advantage that they have a higher photosensitivity and shorter because of faster response Can process measuring pulses. This has a positive effect the minimum to be realized with the inspection device duration t of the light pulses. Advantageously, it is also possible several optical sensors, in particular sensors with a single optical sensor element, in several spectrally evaluating units to divide the different spectral recording areas and / or have spectral resolutions. This is possible the measuring range and the measuring resolution of the sensor system adapt to the particular application and, if necessary, accordingly expand. Also, in spectral interferometry also an evaluation with different polarization recording areas take place, which advantageously provides additional information about let the measurement object win. In the choice of white light interferometry is due to different polarized lighting at several Wavelengths and the demodulation of the signals on the vector side High-resolution measurement of the uppermost layer of the layer possible. So z. B. two different wavelengths be polarized so differently that one of the two Wavelengths no surface reflex undergoes. The signals are separated into wavelengths and recorded. By correlation of the two signals can be the desired determine the optical parameters of the layer.

All in all the evaluation of the measurement result is done by comparing the measured data or the characteristics of the layer derived therefrom, which results in a feature space. For comparison, appropriate Data or characteristics used as nominal values must result. The nominal values of the features can be detected, for example, by a measurement on one produce high-quality measurement object. But it can be one so-called fit-based method are used. Fit-based means the following (Fit = English fit): It calculates with help physical models of the layer sequence and the measuring apparatus a spectrum as it has flowed into the model Layer thicknesses and optical layer properties are measured would. This simulated spectrum comes with the actual measured compared. The parameters of the simulation will be so long adjusted until the simulation and the real measured value match.

hereby is advantageous a quality assurance over a longer period of manufacture of the relevant layers possible. The characteristics, which are evaluated here, include u. a. the measured spectra, the parameters of the light source, the light duration t of the light pulses and possibly polarization information with regard to the measuring light and its changes.

to Improving the measurement results, it is also beneficial during the illumination of the measurement object with the light pulses or during the generation of separate light pulses that are not for measurement on the measurement object be used, the reflectivity of the surface to be determined by the relevant light pulse as a reference signal is evaluated. Under reflectivity of the surface their reflection behavior is to be understood in terms of how big the Proportion of reflected light in relation to the the surface is led light. The reference measurement the light source before or during the measurement is advantageous, to determine the reflectivity of the measurement object in detail. This reference measurement can, for example, via a second reference beam path be enabled. Is the reference measurement during the generation of the light pulses for the actual measurement made, so must for the reference measurement a separate Reference sensor system may be present in the inspection device. If the reference measurement by generating separate light pulses made, so can also be advantageous for the measurement otherwise used optical sensor find use.

Further Details of the invention are described below with reference to the drawing described. Same or corresponding drawing elements are each provided with the same reference numerals and will only in so far explained several times, how are differences between the individual figures. Show it:

1 the geometric relationships in an embodiment of the measuring method according to the invention, in which the measuring object is moved at the speed v,

2 An embodiment of an inspection device, with an Ausführungsbei game of the measuring method according to the invention is carried out as white light interferometry and

3 and 4 Exemplary embodiments of inspection devices with which exemplary embodiments of the method according to the invention are carried out according to the principle of spectral thin-layer interferometry.

In the method according to 1 becomes a measurement object 11 with a the surface 12 for investigation forming layer 13 with a measuring beam 14 visually examined. The layer 12 has a top transparent layer 15 and another transparent location 16 on. These layers may be, for example, semiconductor layers, which are used to produce a complex electronic component, such. As a TFT display serve. On the surface 12 continue to be, for example, tracks 17 applied, creating a uniform examination of the layer 13 not possible. Rather, arises between adjacent interconnects 17 , as in 1 represented a distance a, within which an optical examination of the layer 13 must be done. This is at a diameter d of the light spot 18 by the measuring beam 14 on the surface 12 is formed, in principle possible.

Furthermore, however, it should be noted that according to the invention, the measurement on the surface 12 during a continuous production of the measurement object 11 should be done, which is moved for the purpose of manufacturing at the speed v by a manufacturing device, not shown. During this production the measurement with the measuring beam should be done at the same time 14 take place during the measurement of the defined by the distance a section of the surface 12 under the measuring beam 14 moved through. This is taking into account the diameter d of the measuring beam 14 only a section s available, which is defined by a - d. Taking into account the velocity v and the relationship s = vt, it follows that the duration t of the measuring pulse may be at most s / v. The light source must therefore be selected in the present problem such that the light intensity of the measurement signal in the available duration t is sufficient to allow an evaluation by an optical sensor, not shown. Here, the signal-to-noise ratio of the sensor is taken into account.

typical Values for the above parameters that are reasonable technical effort can be realized, as follows be specified. If the light source realized as an LED array, so pulse durations t of 300 ns can be realized. The optical measuring spot can have a diameter d of 40 to 100 microns. A typical speed for continuous production of layer-bearing semiconductor devices is 2 m / s. Indeed are very sensitive optical at such short durations of the measuring pulses Sensors necessary, since only comparatively little measuring light is available. For larger surface sections should therefore possible longer pulse durations t of example 4 μs can be selected. This has the advantage that a designed thereon inspection device with cheaper Components can be equipped. Here can at a spot size d of 40 μm area sections be examined with a length a of about 50 microns.

According to 2 an inspection device for carrying out the method of white light interferometry is shown. For this purpose is a light source 19 provided, the light by means of a schematically illustrated imaging optics 20 in the form of the measuring beam 14 on the surface 12 of the measured object 11 is directed. The measuring beam 14 goes through a beam splitter 21 that is from the light of the light source 19 a reference beam 22 divides. This one is at a mirror 23 reflected.

By the reflection of the reference beam 22 at the mirror 23 as well as by reflections on the surface 12 Reflected measuring light passes after reflection in the beam splitter 21 and reference light after passing through the beam splitter 21 to an optical sensor 24 , wherein the superposition of the reference light and the reflected measuring light, the measurement result beam 25 forms. In this case, during the pulse duration of the light pulse t, the mirror 23 by means of a piezoactuator 26 shifted in its phase shift with respect to the reflected measurement light and the measurement result beam 25 over the entire pulse length through the sensor 24 recorded.

To stages 27 in the layer 13 to balance is the mirror 23 indirectly via a likewise acting on the piezo principle auxiliary actuator 28 stored. Through the stage 27 shifts the phase position of the reflected measuring light leaps and bounds 14 , wherein the phase position of the reference light 22 by means of the auxiliary actuator 28 can be adjusted.

According to 3 is a measuring device for a spectral thin-layer interferometry through optical fibers 32 . 33 . 34 realized. In this case, the light is from the light source 19 about an imaging optics 20 coupled into a light guide. About a coupler, the light is divided, with one end of the light guide 34 to a measuring head 29 with another imaging optics 30 leads and the end of the light guide 33 directly into a spectral evaluation unit with the sensor 24 to be led. The measuring beam 14 at the measuring head 29 gets off the surface 12 the layer 13 reflected and into the light guide 32 fed while over the other light conductor 33 the sensor 24 a reference beam can be supplied. Through an optical switch 31 can either reflect the reflected light over the light guide 32 or the reference light via the light guide 33 be evaluated.

The inspection device according to 4 is designed as a free-beam optic. The structure is similar to the inspection device according to 2 on, but with no reference beam superimposed on the measuring light is formed, but the reference beam from that of the light source 19 outgoing measuring light via the beam splitter 21 separated and an evaluation unit with an additional optical sensor 35 is supplied. How to 3 described is a measuring head 29 provided by the measuring beam 14 is generated and by the also the reflected measuring light the beam splitter 21 is fed again. About the beam splitter 21 the reflected measurement light passes as a measurement result beam 25 to the light sensor 24 , The control of the light source 19 as well as the recording of the measurement data from the optical sensors 24 . 35 is by a central processing unit 36 ensured (this can be done in a manner not shown in the inspection devices according to 2 and 3 be solved analogously). To the reference beam 22 and the measurement result beam 25 to bundle, are more optics 37 intended.

Alternatively to the in 4 shown structure may instead of the optics 37 and the light sensor 35 Also, a mirror can be arranged, which reflects the reference beam and through the beam splitter through the light sensor 24 feeds (not shown). This has the advantage that the cost of components is reduced and the inspection device is thus less expensive to produce. However, the recording of reference measurements can only take place if no measurement is currently being performed on the measurement object.

QUOTES INCLUDE IN THE DESCRIPTION

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Cited patent literature

• US 2008/0180694 A1 [0002, 0003]

Claims (12)

Interferometry method for optical examination of layers ( 13 ) in an inspection device, comprising the method steps that • a measurement object ( 11 ) with the at least on the surface ( 12 ) a transparent situation ( 15 ) layer ( 13 ) is stored in the inspection device, with a light source ( 19 ) and an imaging optics ( 20 ) a measuring spot ( 18 ) with an extension d on the surface ( 12 ) of the layer ( 13 ) and • that of the measurement object ( 11 ) reflected measuring light on an optical sensor ( 24 ) is projected, characterized in that • the measurement object ( 11 ) at a velocity v lateral below the measuring spot ( 18 ) is moved through and • the illumination of the measurement object ( 11 ) is performed with light pulses of a duration t, the extent a of the layer ( 15 ) in the direction of the lateral movement and the relationship: a> d + vt is maintained by setting the variable parameters under the parameters d and / or v and / or t. Method according to claim 1, characterized in that that as a method for optical examination, a spectral interferometry is used. A method according to claim 1, characterized in that as a method for optical examination a white light interferometry is used, in which with an actuator ( 26 ) a reference beam with time-varying Phasenver shift is generated and with the by the on the surface ( 12 ) of the layer ( 13 ) Reflected measurement light formed measurement result beam ( 25 ) is superimposed, wherein the time required for the measurement range of the phase shift during the duration t of the light pulse is traversed. A method according to claim 3, characterized in that the reference point of the phase shift by a Hilfsaktuator ( 28 ) can be adjusted. Method according to claim 4, characterized in that the auxiliary actuator ( 28 ) is adjusted automatically via a control or regulation as a function of the level of the surface. Method according to one of the preceding claims, characterized in that the extension d of the measuring spot ( 11 ) can be varied by optics. Method according to one of the preceding claims, characterized in that layer ( 13 ) in the direction of the lateral movement a smaller extent than the measuring object ( 11 ), in particular the layer is structured on the measurement object. Method according to one of the preceding claims, characterized in that the light pulses have a duration t in the ns range exhibit. Method according to one of the preceding claims, characterized in that the speed v of the measurement object between 1 and 2 m / s. Method according to one of the preceding claims, characterized in that the light source ( 19 ) has one or more LEDs. Method according to one of the preceding claims, characterized in that during the illumination of the DUT with the light pulses or during generation separate light pulses the reflectivity of the surface is determined by the relevant light pulse evaluated as a reference signal becomes. Method according to one of the preceding claims, characterized in that an optical sensor ( 24 ) with a single optical sensor element or several of these sensors ( 24 ) are used.