EP2347216A1

EP2347216A1 - Interferometry method for optically examining coatings

Info

Publication number: EP2347216A1
Application number: EP09748278A
Authority: EP
Inventors: Sven Gerhard Dudeck; Detlef Gerhard; Florian Hirth; Martin Jakobi
Original assignee: Siemens AG
Current assignee: Siemens AG
Priority date: 2008-10-20
Filing date: 2009-10-19
Publication date: 2011-07-27
Also published as: DE102009025562A1; WO2010046340A1

Abstract

The invention relates to an interferometry method for optically examining coatings. This can be carried out as a white light interferometry or spectral thin-film interferometry method, for example. In the process, a measuring spot (18) is generated with a diameter (d), said spot used to optically examine the surface (12) of a measured object (11). According to the invention, the method is carried out on a measured object (11) with velocity (v), wherein the illumination of the measured object is done with light pulses of a duration (t). In the process, the fact that the extent (a) of the coating section to be examined must be greater than d + vt in order to perform a measurement must be taken into account. According to the invention, the parameters (d, v and t) are suitably selected for a pre-determined velocity (v) of the measured object. The measurement method can therefore be advantageously applied in ongoing, continuous production of coated components.

Description

description

Interferometry method for optical examination of layers

The invention relates to an interferometry method for optically examining layers in an inspection device, which comprises the following method steps. A measuring object with the at least on 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 having an extension d is projected onto the surface of the layer. Then, the measurement light reflected by the measurement object is projected onto an optical sensor.

A method for optical examination of the type described above is described for example according to US 2008/0180694 Al. In this method, the measurement object is placed under the inspection device. The measuring device has a free-beam 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, it is superimposed with a reference light whose phase shift is changed over time relative to the measurement light. The resulting interferences can be evaluated in order to obtain information about the properties of the investigated layer. These statements can affect the transparent location on the surface of the layer and possibly refer 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.

As an alternative to white-light interferometry, the method of spectral thin-layer interferometry can also be used. This measuring method is generally known and uses the physical effect that the measuring light is reflected at the boundary layers or the surface of transparent thin layers, with the reflected measuring light interferences that can be evaluated with respect to the layer properties.

With this type of interferometry, it is generally necessary for the measurement light to be directed onto the measurement object and the reflected measurement light to be optically evaluated. Depending on the basic conditions specified by the interferometer used, a specific measurement time must be provided for the measurement in order to generate an evaluable signal on the optical sensor. In the case of white-light interferometry, it must also be taken into account that the measurement signal must be evaluated over a temporal phase shift of the reference signal. The required measurement time therefore limits the application possibilities in the measurement method of interferometry.

The object of the invention is to specify an interferometric method which can be used comparatively universally and, in particular, can also be used in the continuous production of layers.

This object is achieved with the interferometric method mentioned above in that the measurement object is moved laterally below the measurement spot at a speed v and the illumination of the measurement object is performed with light pulses of a duration t. According to the invention, the extent a of the layer in the direction of the lateral movement is predetermined by the design features of the object to be generated. In order to guarantee the measurement result, the relationship a> d + vt is maintained by setting the variable parameters under the parameters d and / or v and / or t. In this way, it is advantageously ensured that, despite the movement of the measurement object at the speed v for the required measurement time, the layer to be examined is located in the measurement spot. This can also be described in such a way that the resolution of the measuring method according to the invention is d + vt and the extent a of the layer on the measuring object must correspond at least to the resolution. For example, the layer may advantageously be a structured layer on a semiconductor, which has to be examined for quality. Features of the layer, ie at least the uppermost transparent layer, which can be determined with the aid of interferometry are, for example, the layer thickness or, for a known layer thickness, the refractive index of the relevant layer. The velocity v can be the mean velocity during the duration t of the light pulse. Preferably, v is a constant velocity.

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. Here too, the re- fleeting behavior of the layer to be examined a role by which the proportion of the reflected light is affected. The duration t of the light pulse can not be shortened arbitrarily due to the physical conditions at the light source. Here, the conditions of the response of the light source must be considered.

Advantageously, spectral interferometry can be used as a method for optical examination. Here, as already explained, the interferences occur after the reflection of the measuring signal on the surface.

According to another embodiment of the invention, a white light interferometry can also be used as a method for optical examination. In the context of this invention, white light interferometry is understood to mean a measuring method in which a broad spectral light with a short coherence length is used. This light does not necessarily have to be white, but only needs a sufficiently broad spectrum, so that an evaluation can take place according to the mechanism of white-light interferometry. H. the interference phenomena are generated by superposition of the measurement light with a reference light by temporally changing its phase shift with the measurement light.

According to the invention, it must be provided in this variant of the invention that the region of the phase shift required for the measurement is passed through during the duration t of the light pulse. The required range of phase shift depends on the application. Advantageously, a shift over the entire phase (ie 360 °) should be made at the wavelengths to be examined. For certain measurement purposes but also a lower phase shift may be sufficient. The required range of Phase shift must therefore be performed during the duration t of the light pulse, because then no measuring light for a shift more available. However, the duration t of the light pulse can not be extended arbitrarily, since the already explained condition of a sufficient resolution of the measurement signal must be fulfilled. Therefore, the actuator (for example, a piezoelectric actuator) for generating the phase shift with respect to its characteristics (response time, adjustment, required for the adjustment adjustment time <t) must be designed in a suitable manner.

According to a further embodiment of the invention, it is provided that the reference point of the phase shift can be adjusted by a Hilfsaktor. It is particularly advantageous if the auxiliary actuator can be adjusted automatically via a control or regulation as a function of the level of the surface. For this purpose, a distance sensor can be provided in the inspection device, which can measure the level of the surface of the measurement object with respect to the inspection device. Alternatively, it is also possible to adjust the distance by means of the measuring signal itself without the additional sensor. Advantageously, by activating the auxiliary actuator, the reference point of the phase shift can then be changed by the same amount as the level of the surface changes. The phase shift, which is carried out by the other actuator can advantageously be carried out regardless of the level position of the surface of the measured object.

Furthermore, it is advantageous if the extent d of the measuring spot is varied by the imaging optics. This can be done by a corresponding lens system, also possible is an enlargement or reduction of the measuring spot by a diaphragm. It should be noted that the size of the measuring spot also influences the light intensity which impinges on the surface to be examined. However, a larger aperture setting leads, for example, to a larger measuring spot, but also to a higher light intensity, with the smallest possible measuring spot for a good resolution of the method on the one hand and the highest possible light intensity on the other hand being advantageous. Depending on the application, a compromise has to be chosen here.

As already mentioned, the measuring method can be used particularly advantageously if the layer to be examined has a smaller extent than the measurement object in the direction of the lateral movement, in particular the layer is structured on the measurement object. Such layers are produced continuously, for example by printing technology, wherein the layer regions to be examined can have different nominal thicknesses. In this case, the focusing of the measuring light can be tracked in a suitable manner, wherein due to the sudden change in the layer thickness short adjustment must be realized. The light pulses may 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 that are used in the continuous production of printed electronic components, such. B. RFID components can be realized with semiconductor layers.

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 LED's or even LED arrays can be used. The broad spectral compositions of the measuring light in white-light interferometry can also be generated by composing LEDs of different colors in an array. As a result, a precise tuning of the desired spectral distribution is possible. Of course, sen the other components used in the inspection device 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.

In order to increase the photosensitivity of the optical sensor, advantageously also an optical sensor with a single optical sensor element or several of these sensors can be used. Compared to sensor arrays, sensors with a single optical sensor element have the advantage that they have a higher photosensitivity and can process shorter measuring pulses due to a faster response behavior. This has a positive effect on the minimum duration of the light pulses to be realized with the inspection device. Advantageously, it is also possible to divide a plurality of optical sensors, in particular sensors with a single optical sensor element, into a plurality of spectrally evaluating units which have different spectral recording ranges and / or spectral resolutions. As a result, the measuring range and the measuring resolution of the sensor system can be adapted to the respective application and expanded accordingly if necessary. In addition, in the case of spectral interferometry, an evaluation can also take place with different polarization pickup areas, which can advantageously be used to obtain additional information about the test object. In the choice of the white light interferometry, a high-resolution measurement of the uppermost layer of the layer is possible by different polarized illumination at several wavelengths and the demodulation of the signals on the vector side. So z. B. two different wavelengths are polarized differently such that one of the two wavelengths undergoes no surface reflection. The Signals are separated into wavelengths and recorded. By correlating the two signals, the desired optical parameters of the layer can be determined.

Overall, the evaluation of the measurement result is carried out by comparing the measured data or the characteristics obtained therefrom of the layer, from which results a feature space. For comparison, appropriate data or characteristics are used, which must be determined as setpoints. The nominal values of the features can be generated, for example, by a measurement on a proven high-quality measurement object. However, it can be used from a so-called fit-based method. Fit-based means the following (Fit = English fit): With the help of physical models of the layer sequence as well as the measuring apparatus, a spectrum is calculated, as it would have to be measured for the layer thicknesses and optical layer properties incorporated into the model. This simulated spectrum is compared with the actually measured. The parameters of the simulation are adjusted until the simulation and the real measured value match.

As a result, a quality assurance over a longer period of production of the respective layers is advantageously possible. The features that 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 improve the measurement results, it is also advantageous during the illumination of the measurement object with the light pulses or during the generation of separate light pulses which are not used for the measurement on the measurement object, the reflection Surface of the surface to be determined by the relevant light pulse is evaluated as a reference signal. By reflectivity of the surface, its reflection behavior is to be understood as meaning the proportion of reflected light in relation to the light conducted onto the surface. The reference measurement of the light source before or during the measurement is advantageous in order to determine the reflectivity of the measurement object in detail. This reference measurement can be made, for example, via a second reference beam path. If the reference measurement is made during the generation of the light pulses for the actual measurement, a separate reference sensor system must be present in the inspection device for the reference measurement. If the reference measurement is carried out by generating separate light pulses, advantageously the optical sensor otherwise used for the measurement can also be used.

Further details of the invention are described below with reference to the drawing. Identical or corresponding drawing elements are each provided with the same reference numerals and will only be explained several times as far as there are differences between the individual figures. Show it :

Figure 1 shows the geometric relationships in a

Embodiment of the measuring method according to the invention, in which the measuring object is moved at the speed v,

Figure 2 shows an embodiment of a

Inspection device, with which an embodiment of the measuring method according to the invention is performed as a white light interferometry and FIGS. 3 and 4 are 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 FIG. 1, a measurement object 11 with a layer 13 forming the surface 12 for examination is optically examined with a measurement beam 14. The layer 12 has an uppermost transparent layer 15 and a further transparent layer 16. These layers may be, for example, semiconductor layers, which are used to produce a complex electronic component, such. B. a TFT display that nen. On the surface 12, for example, printed conductors 17 are further applied, whereby a uniform examination of the layer 13 is not possible. On the contrary, between adjacent interconnects 17, as shown in FIG. 1, a distance a is created within which an optical examination of the layer 13 must take place. This is at a

Diameter d of the light spot 18, which is formed by the measuring beam 14 on the surface 12, in principle possible.

Furthermore, it should be noted, however, that according to the invention, the measurement on the surface 12 is to take place during a continuous production of the measurement object 11, which is moved for the purpose of production at the speed v by a production device, not shown. During this production, the measurement with the

Measuring beam 14 take place, then the section 12 defined by the distance a of the surface 12 is moved under the measuring beam 14 during the measurement. For this purpose, taking into account the diameter d of the measuring beam 14 is only one Route 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. In the present problem, therefore, the light source must be selected such that the light intensity of the measurement signal in the available duration t is sufficient to enable 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-mentioned parameters, which can be realized with reasonable technical effort, can be stated as follows. If the light source is realized as an LED array, then pulse durations t of 300 ns can be realized. The optical measuring spot may have a diameter d of

40 to 100 microns received. A typical speed for the continuous production of layer-bearing semiconductor devices is 2 m / s. However, with such short durations of the measuring pulses, very sensitive optical sensors are necessary, since only comparatively little measuring light is available. For larger area sections, therefore, longer pulse durations t of, for example, 4 μs should be selected. This has the advantage that an inspection device designed for this purpose can be equipped with less expensive components. In this case, with a spot size d of 40 μm, surface sections with a length a of approximately 50 μm can be examined.

2 shows an inspection device for carrying out the method of white-light interferometry. For this purpose, a light source 19 is provided, whose light is directed by means of a schematically illustrated imaging optics 20 in the form of the measuring beam 14 on the surface 12 of the measuring object 11. The measuring beam 14 passes through a Beam splitter 21, which divides a reference beam 22 from the light of the light source 19. This is reflected by a mirror 23.

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

To compensate for steps 27 in layer 13, the mirror is

23 mounted indirectly via a likewise functioning on the piezo principle auxiliary actuator 28. Through the step 27, the phase position of the reflected phase jumps abruptly

Measuring light 14, wherein the phase angle of the reference light 22 can be adjusted by means of the Hilfsaktors 28.

According to FIG. 3, a measuring device for a spectral thin-layer interferometry is realized by light guides 32, 33, 34. In this case, the light from the light source 19 is coupled via an imaging optical system 20 in an optical waveguide. The light is split by way of a coupler, one end of the light guide 34 leading to a measuring head 29 with a further imaging optical unit 30 and the end of the light guide 33 directly into a spectral evaluation unit with the sensor

24 is guided. The measuring beam 14 on the measuring head 29 is reflected by the surface 12 of the layer 13 and fed into the optical waveguide 32, while above the other light source. conductor 33, the sensor 24, a reference beam can be supplied. By means of an optical switch 31, either the reflected measuring light via the optical waveguide 32 or the reference light via the optical waveguide 33 can be evaluated.

The inspection device according to FIG. 4 is designed as a free-form optic. The structure is similar to the inspection device according to FIG. 2, although no reference beam is superimposed on the measurement light, but the reference beam is separated from the measurement light emanating from the light source 19 via the beam splitter 21 and fed to an evaluation unit with an additional optical sensor 35. As described for Figure 3, a measuring head 29 is provided, through which the measuring beam 14 is generated and through which the reflected measuring light the

Beam splitter 21 is supplied again. Via 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 and the acquisition of the measurement data from the optical sensors 24, 35 is ensured by a central processing unit 36 (this can be done in the inspection devices according to FIG Figure 2 and 3 to be solved analogously). In order to focus the reference beam 22 and the measurement result beam 25, further optics 37 are provided.

As an alternative to the construction shown in FIG. 4, instead of the optics 37 and the light sensor 35, a mirror can also be arranged which reflects the reference beam and passes it through the beam splitter to the light sensor 24 (not shown in detail). This has the advantage that the

Cost of components is reduced and the inspection device is thus cheaper to produce. However, the recording of reference measurements can only take place if no measurement is currently being performed on the measurement object.

Claims

claims

1. Interferometry method for optically examining layers (13) in an inspection device, comprising the method steps that

• a measuring object (11) with at least the surface

(12) a transparent layer (15) having 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 is projected onto the surface (12) of the layer (13) and

• the measuring light reflected by the measuring object (11) is projected onto an optical sensor (24), that • the measuring object (11) is moved laterally below the measuring spot (18) at a speed v, and

The illumination of the object to be measured (11) is performed with light pulses of a duration t, the extent a of the layer (15) being predetermined in the direction of the lateral movement and the relationship: a> d + vt by setting the variable parameters under the parameters d and / or v and / or t is maintained.

2. The method according to claim 1, wherein a spectral interferometry is used as the method for optical examination.

3. The 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 temporally variable Phasenver- Shift is generated and is superimposed with the measurement result beam (25) formed by the measurement light reflected on the surface (12) of the layer (13), wherein the required for the measurement range of the phase shift during the duration t of the light pulse is traversed.

4. The method according to claim 3, wherein a reference point of the phase shift can be adjusted by an auxiliary actuator (28).

5. The method as claimed in claim 4, wherein the auxiliary actuator is automatically adjusted as a function of the level of the surface via a controller.

6. Method according to one of the preceding claims, characterized in that the extent d of the measuring spot (11) can be varied by an optical system.

7. Method according to one of the preceding claims, characterized in that layer (13) in the direction of the lateral movement has a smaller extent than the measurement object (11), in particular the layer is patterned on the measurement object.

8. Method according to one of the preceding claims, characterized in that the light pulses have a duration t in the ns range.

9. The method according to any one of the preceding claims, characterized in that the velocity v of the measured object is between 1 and 2 m / s.

10. Method according to one of the preceding claims, characterized in that the light source (19) has one or more LED's.

11. The method according to claim 1, wherein during the illumination of the measurement object with the light pulses or during the generation of separate light pulses, the reflectivity of the surface is determined by evaluating the relevant light pulse as the reference signal.

12. The method according to claim 1, wherein an optical sensor (24) having a single optical sensor element or a plurality of these sensors (24) is used.