DE20213343U1 - Arrangement for determining changes in layer thickness - Google Patents

Description

Patent Attorney^ .·

GEYER, FEHNERS & PARTNER (G.b.R.)

European Patent and Trademark Attorneys
MUNICH-JENA

Office Munich /Munich Offices: Perhamerstraße 31 · D-80687 Munich
Telephone: (089) 5 4615 20 -Fax: (089) 5 46 03 92 · Telegrams: gefepat muenchen

Office Jena/Jena Offices: Sellierstraße 1 · D-07745 Jena ■ Telephone: (03641) 2 91 50 ■ Fax: (03641) 2915 21

Analytik Jena AG Jena, August 28, 2002

o.Z.: GM 9036/8 DE

Arrangement for determining layer thickness changes

The invention relates to an arrangement for determining layer thickness changes of a multilayer sample by means of reflectrometric interference spectroscopy, comprising a light source for illuminating the sample, a sample holder for fixing the sample and an evaluation unit for analyzing the light reflected by the sample.

Reflectometric interference spectroscopy (RIFS) has been widely used for more than ten years to investigate the change in the layer thickness of a sample, for example in optical sensors, where thin polymer films are used as sensors for hydrocarbons. The thickness of such a polymer film changes as a result of physical or chemical reactions, and this change is detected using RIFS. This method is also increasingly used in biochemical analysis, as it enables the direct investigation of the interaction between biomolecules, for example in antibody/antigen reactions. For this purpose, one reaction partner (ligant) is immobilized on the surface of the sample, and the other reaction partner (analyte) is passed over the sample in solution. The interaction can be directly detected as a change in layer thickness; no labeling of the reaction partners is necessary, as is the case with radioimmunoassay (RIA) or enzyme immunoassay (ELISA).

The method is based on the interference of white light through reflection at the interfaces of thin, transparent layers. If the reflected light is spectrally broken down, interference spectra are obtained in which the areas of constructive superposition (maxima) and destructive superposition (minima) of the partial beams are included.

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The position of the minima and maxima in the spectrum is characteristic of the layer system and depends on the thickness of the individual layers. In principle, the thickness of the individual layers of the system can be deduced from the interference spectrum using mathematical formulas. A change in the thickness of a layer, e.g. through one of the processes mentioned above, also results in a change in the interference spectrum.

In optical sensor technology, changes in the thickness of thin layers can be tracked, for example, using arrangements such as those described in the article "Thin polymer films as sensors for hydrocarbons" by W. Nahm and G. Gauglitz in GIT Fachz. Lab. 7/90 on pages 889 - 893 or in the patent specification DE 42 00 088 C2. Light from a white light source is coupled into the first arm of a Y-light guide and, when it exits, is emitted via the light exit surface onto the sensitive layer fixed to a transparent holder - this system consisting of holder and sensitive layer is referred to below as the "sensor chip". The light exit surface is simultaneously also the light entry surface for the light reflected by the sensor chip, which is then guided via a splitter through the second arm of the Y-light guide to a spectrometer. In the spectrometer, the light is spectrally broken down and the interference spectrum is recorded. This can then be evaluated using methods such as those described in the above-mentioned articles by W. Nahm and G. Gauglitz.

However, this arrangement has some disadvantages. For example, light reflections occur on the front surface of the Y-waveguide that is directed towards the sensor chip and is intended for simultaneous light entry and exit. These reflections are superimposed on the actual measurement signal from the surface of the sensor chip. As a result, the interference spectrum is only very weak. For this reason, changes in the interference spectrum that accompany a change in the layer thickness are in principle difficult and imprecise to detect. In the current state of the art, attempts are therefore made to suppress these light reflections using further, complex measures: If the holder for the sensitive layer is made of optically transparent glass with a known refractive index, the disturbing reflections can be prevented by bringing the back of the sensor chip - i.e. the side of the transparent holder facing the lightguide - into contact with the front surface of the Y-waveguide using a liquid medium such as glycerine, which is well suited due to its refractive index. This leads to the desired increase in contrast in the interference spectrum.

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The disadvantage of this solution is that the overall intensity is reduced because the reflections from the front surface of the light guide and the back of the sensor chip are suppressed. This can extend the measurement times. Replacing the sensor chip is also complicated - it is essential to prevent traces of the liquid medium from reaching the sensitive and reactive layers of the sensor chip, where they can impair the sensor function.

Based on this prior art, the invention is therefore based on the object of developing an arrangement for reflectrometric interference spectroscopy which overcomes these disadvantages of the prior art.

According to the invention, this object is achieved in that, in an arrangement of the type described above, a first light guide with a light exit surface is provided for transmitting the light from the light source to the sample and a second light guide with a light entry surface is provided for transmitting the light reflected from the sample to the evaluation unit.

By using two separate light guides for light irradiation and light registration, the disturbing reflections that occur on the light exit surface of the first light guide can no longer overlap with the reflected light that enters the light entry surface of the second light guide and is transmitted to the evaluation unit. In this way, contact between the sample and a liquid medium such as glycerine can also be avoided. This makes handling much easier.

Since the optical fibers no longer have to be in contact with the sensor chip in the arrangement according to the invention, there are several degrees of freedom for the arrangement of the optical fibers in relation to the surface of the sample. The light entry and exit surfaces of both optical fibers are generally chosen as cross-sectional areas in relation to the longitudinal axes of the optical fibers, but this does not exclude the possibility of using other cutting surfaces.

In an advantageous embodiment of the invention, the normals of the light exit surface of the first light guide and the light entry surface of the second light guide form essentially the same angle with the surface normal of the sample. The light entry and exit surfaces can be arranged in two different ways.

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MUNICH-JENA

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Firstly, in such a way that the normals together enclose an angle of approximately twice the angle with the surface normal. In this case, the angle corresponds to the angle of incidence and reflection of the light. This angle is preferably variable. If the distance between the light exit surface of the first light guide and the light entry surface of the second light guide is also designed to be variable, then such an arrangement can be used to very effectively optimize the signal reflected by the sensor chip in terms of contrast and intensity.

In a further preferred variant, the included angle is approximately 0°. The light exit surface of the first light guide and the light entry surface of the second light guide are arranged essentially parallel to the sample surface. Although the recorded intensity is somewhat lower than in the previously described variant, since only rays from the edge area of the aperture of the optical fibers contribute to the signal, the technical design is simplified in this way. Preferably, the light exit surface of the first light guide and the light entry surface of the second light guide are arranged at essentially the same distance from the sample surface, which enables simple production.

In a particularly preferred embodiment, the second light guide is composed of several optical fibers with light entry surfaces, and the light entry surfaces of the optical fibers are arranged essentially concentrically around the light exit surface of the first light guide, with all surfaces lying approximately in one plane. The optical fibers that make up the second light guide can then be combined to form a fiber bundle at the input of the evaluation unit. This arrangement can increase the intensity of the signal. As long as the aperture of the spectrometer, which ultimately registers the light, is not exceeded, the signal intensity increases proportionally to the number of optical fibers. Of course, the first light guide can also be composed of several optical fibers, and other arrangements of the individual fibers of both light guides are possible - for example, the first light guide can also consist of a fiber bundle, and the fibers can be arranged so that a fiber of the first light guide is only surrounded by fibers of the second light guide, and vice versa.

In a further advantageous embodiment of the invention, the distance between the sample and the first and second light guides can be varied. In this way, the ratio between the intensity of the signal and its modulation can be varied, since

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As the distance increases, the intensity of the signal increases, while the modulation decreases, due to the increasing detection of reflections from the back of the sensor chip. This makes it very easy to adapt to different measurement tasks. For quick overview measurements, for example, the focus is on maximum intensity. On the other hand, if very precise measurements are required, maximum modulation of the signal is advantageous.

The invention will be explained below using individual examples. The accompanying drawings show

Fig. 1 shows an arrangement as known from the prior art,

Fig.2 the normalized intensity of the interference spectrum as a function of the

Wavelength for an arrangement as shown in Fig.1,
Fig.3 shows an arrangement according to the invention with a first and a second light guide and

Fig.4 shows an arrangement according to the invention in which the second light guide is composed of several optical fibers.

Fig. 1 shows the principle of an arrangement for determining layer thickness changes in a sample using reflectrometric interference spectroscopy, as is known in the art. Light is emitted from a light source 1, for example a white light source, via a Y-light guide 2 through a splitter 3 onto a sample 4, which is located on a sample holder 5. The light reflected from the sample is received by the same Y-light guide 2 and guided via the splitter 3 into the other arm of the Y-light guide 2, which is connected to the evaluation unit 6. In the evaluation unit 6, the light is spectrally broken down and further analyzed.

A typical interference pattern, as obtained with such an arrangement, is shown as a solid line in Fig. 2. The sample used here was an interference layer system consisting of a tantalum pentoxide layer 10 nm thick and a silicon dioxide layer 330 nm thick on a 1 mm thick glass pane made of D263 from Schott. Due to the disturbing light reflections on the light exit surface of the Y-light guide 2 at the end facing the sample, the variations in the intensity with the wavelength, which is standardized with respect to an uncoated glass pane, are only very weak.

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MUNICH-JENA

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The disadvantages of such an arrangement described at the beginning can be avoided with the arrangement according to the invention. Such an arrangement is shown in Fig. 3. Instead of the Y-light guide 2, a first light guide 7 and a second light guide 8 are now provided. The first light guide 7 transmits the light from the light source 1 to the sample 4, the second light guide 8 transmits the light reflected from the sample 4 to the evaluation unit 6. This results in an interference figure with significantly higher contrast, as shown in Fig. 2 by the dashed line.

An arrangement with which the intensity of the interference signal can be further increased is shown schematically in Fig. 4. The second light guide 8 is here composed of several optical fibers that converge at the entrance of the analysis unit 6. At the ends facing the sample, the optical fibers are arranged around the first light guide 7 in such a way that the light entry surfaces 11 of the optical fibers are arranged essentially concentrically around the light exit surface 9 of the first light guide 7. In this way, an intensity six times higher is obtained if the aperture of the spectrometer used in the evaluation unit 6 is not exceeded.

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Analytik Jena AG currently: GM 9036/8 DE Jena, August 28, 2002

List of reference symbols

1 Light source 2 Y-light guide 3 Splitter 4 sample 5 Sample holder 6 Evaluation unit 7 first light guide 8th second light guide 9 Light exit surface 10 Light entry surface 11 Light entry surface

Claims (7)

1. Arrangement for determining layer thickness changes of a sample ( 4 ) by means of reflectrometric interference spectroscopy, comprising a light source ( 1 ) for illuminating the sample ( 4 ), a sample holder ( 5 ) for fixing the sample ( 4 ) and an evaluation unit ( 6 ) for analyzing the light reflected from the sample ( 4 ), characterized in that a first light guide ( 7 ) with a light exit surface ( 9 ) is provided for transmitting the light from the light source ( 1 ) to the sample ( 4 ), and a second light guide ( 8 ) with a light entry surface ( 10 ) is provided for transmitting the light reflected from the sample ( 4 ) to the evaluation unit ( 6 ). 2. Arrangement according to claim 1, characterized in that the normals of the light exit surface ( 9 ) of the first light guide ( 7 ) and the light entry surface ( 10 ) of the second light guide ( 8 ) enclose substantially the same angle with the surface normal of the sample ( 4 ). 3. Arrangement according to one of the preceding claims, characterized in that the angle between the normals of the light exit surface ( 9 ) of the first light guide ( 7 ) and the light entry surface ( 10 ) of the second light guide ( 8 ) can be varied with the surface normal of the sample. 4. Arrangement according to one of the preceding claims, characterized in that the light exit surface ( 9 ) of the first light guide ( 7 ) and the light entry surface ( 10 ) of the second light guide ( 8 ) are arranged substantially parallel to the sample surface. 5. Arrangement according to one of the preceding claims, characterized in that the light exit surface ( 9 ) of the first light guide ( 7 ) and the light entry surface ( 10 ) of the second light guide ( 8 ) are arranged at a substantially equal distance from the sample surface. 6. Arrangement according to one of the preceding claims, characterized in that the second light guide ( 8 ) is composed of several optical fibers with light entry surfaces ( 11 ), and the light entry surfaces ( 11 ) of the optical fibers are arranged essentially concentrically around the light exit surface ( 9 ) of the first light guide ( 7 ), all surfaces lying approximately in one plane. 7. Arrangement according to one of the preceding claims, characterized in that the distance between the sample ( 4 ) and the two light guides ( 7 , 8 ) is variable.