EP1769229A1

EP1769229A1 - Ellipsometry device provided with a resonance platform

Info

Publication number: EP1769229A1
Application number: EP05772604A
Authority: EP
Inventors: Max Wiki; Johannes Edlinger; Matthias Vaupel; Andreas Eing
Original assignee: OC Oerlikon Balzers AG
Current assignee: OC Oerlikon Balzers AG
Priority date: 2004-07-21
Filing date: 2005-07-18
Publication date: 2007-04-04
Also published as: CN101023337A; US20070216901A1; TW200604489A; WO2006008112A1

Abstract

The invention relates to a useful improvement of an ellipsometer-type device. For this purpose, an existing ellipsometer is supplemented by a so-called resonance platform on which surface modes are excitable. Contrary to state of the art of known surface plasmons, the inventive modes are laterally localised. In addition, the resonance platform is not necessarily embodied in the form of a metal sheet. The inventive device also can be embodied in the form of an image-forming device. The inventive method consists in placing measurable samples on the platform surface and, afterwards, are exposed to light, thereby being excited in modes. The resonance position of modes is determined by the absorption behaviour of a measurable substance.

Description

Ellipsometry device with a resonance platform

Area to which the invention relates

The invention relates to a device and a method for measuring the amount, composition, and / or spatial distribution and dynamics of substances on substrates.

State of the art

It is generally known that the presence or the properties of substances on surfaces can be determined by means of optical sensors. In this case, imaging techniques which detect an area of the surface spatially resolved are particularly interesting: Classically, the reflection, transmission, absorption, the scattered light or the phase shift are mapped. This can be done at one wavelength or spectrally resolved. An important parameter that can also be varied is the angle of incidence.

Techniques that exploit the influence of the surface on the polarization of radiated light are particularly sensitive. For example, ellipsometry is a widely used technique for analyzing thin films on surfaces. In this case, the ratio of the change in amplitude (Psi) of the s and p polarization after reflection as well as the relative phase shift (delta) of the polarization components after reflection are determined. For this purpose, an ellipsometer comprises at least means for exposing a sample to polarized light and means for measuring the light reflected or transmitted by the sample, in each case for s and p polarization. From this, the optical properties of the surface to be analyzed can be derived. Ellipsometry has been successfully used to detect the adsorption of proteins or smaller molecules on a surface. In US Pat. 4,508,832 to Carter et al., An ellipsometer was used to measure antibody-antigen binding in an immunoassay on a test surface. The imaging ellipsometry, which combines the possibilities of ellipsometry with the possibilities of microscopy, was demonstrated on the basis of thin transparent layers on silicon substrates. However, the change in the polarization properties with a single reflection is very small, so that a strongly noise signal is detected. In addition, it may be disadvantageous here that the light propagates through the surrounding medium, in particular in applications in which the optical properties of the environment may change during the measurement.

The problem of the influence of the surrounding medium can be largely circumvented if an optically transparent substrate and the principle of Total Internal Reflection (TIR) is used. A measuring device comprising a light source assembly, a total reflection assembly and a detection assembly is known from US 6594011 to Kempen. In this case, the surface with the substance to be measured is formed as a total reflection layer system. This means that the surface on which the substance to be analyzed comes to lie is formed by an interface of two transparent layers, whose relative refractive indices are selected such that light which is coupled into a transparent layer above the so-called critical angle is applied to one the Grenz¬ surfaces, which forms the substance to be analyzed is totally reflected. The mechanisms of total reflection are well known to those skilled in the art. In total reflection, the polarization properties of the incident light change. Now, if the polarization properties of the incident light are known, an analysis of the polarization properties of the reflected light allows conclusions to be drawn about the properties, in particular the mass distribution and / or layer thickness of the substance to be analyzed on the surface. In US 6594011 the interaction of the light is limited to only a total reflection. This ensures that the total resolution of the Messanordung is limited solely by the Auflö¬ sungsvermögen the detector arrangement. Thus, theoretically, the measurement could be carried out spatially arbitrarily with an arbitrarily high-resolution detector array. However falls Here too, the change made in the polarization properties during the single total reflection is very low, so that a strongly noisy signal is detected.

One way to increase the signal and thus sensitivity is to combine ellipsometry with surface plasmon (SPR) chemistry. In the context of conventional SPR technology, the reflected light is measured as a function of the angle of incidence. At the so-called resonance angle, surface plasmons are excited, resulting in a strong evanescent field in the

The minimum in the reflection associated with the excitation of surface plasmons shifts when a substance is adsorbed on the surface, i. forms a layer and increases the layer thickness. The ellipsometer measures the ellipsometric parameters Psi and Delta. Psi is analogous to the measurement signal of the classical SPR method, while delta provides additional information and in particular can show a sharp resonance as a function of the angle of incidence. The disadvantage is that the substrate on the surface of which the plasmons are to be excited is metallic or at least have a metallic film. However, the use of metallic films is often disadvantageous in the application of bioanalyses because these films cause difficulties in production, especially as far as reproducibility is concerned. In particular, the lifetime of metallic films is often very limited.

Since the resolving power of the detector array itself has physical and technical limitations, a total reflection system with multiple reflections can be used. Light is coupled into a highly refractive layer under a minimum angle enabling total reflection, and is totally reflected several times before it emerges from the layer. The distance between light coupling and

Light decoupling is chosen so that the range of multipath Total reflection corresponds to the resolution of the detector array. The layer thickness is chosen such that in the layer the light is repeatedly totally reflected before it emerges from the layer. The configuration of the total reflection system results in a multiple interaction between the evanescent field of the coupled-in light and the layer to be analyzed in a spatial area around each measuring point which corresponds to the resolution capability of the detection arrangement. Such an arrangement is familiar to the person skilled in the art as an asymmetrical waveguide. The coupling into such a waveguide is a typical optical resonance phenomenon. The described multiple total reflection is called waveguiding. Waveguide takes place only above a certain layer thickness and only for certain (resonance) angle. The associated propagation of light is referred to as waveguide modes. If, however, the layer thickness falls below a so-called cut-off value, the asymmetric waveguide no longer undergoes any multiple total reflection, since no more mode is permitted. The path the light travels in the waveguide within a zigzag period corresponds to the inverse of the propagation constants. For a typical waveguide layer with refractive index Ns = 2.2 on glass with refractive index Ng = I.5 and at a wavelength in vacuum of 633nm the cut-off thickness is 200nm. The light propagates just above the total reflection angle between the glass and the layer. In order to obtain twice total reflection at the outer interface, therefore, a propagation of more than 400 nm is needed. At 11 total reflections it is already 4μm and limits the resolution to a very considerable extent.

Overview of the present invention

It is therefore an object of the present invention, a generic device that is based on the spatially resolved detection of changes in the light properties in the range of optical resonance, such that the quality of the detected signal is improved and in particular improved resolution can be achieved er¬ , This object is achieved in that the device according to the invention comprises a resonance platform which is designed such that it enables the excitation of laterally localized resonances.

It is a further object of the present invention to develop a generic method which is based on the spatially resolved detection of the changes in the light properties in the region of optical resonance in such a way that the quality of the detected signal is improved and in particular an improved resolution assets can be achieved.

This object is achieved by the fact that in the process a resonant platform is exposed to light with such parameters, which lead to the excitation of laterally localized resonances.

The invention thus comprises a device for carrying out ellipsometry measurements, with an ellipsometer and a resonance platform, wherein means are provided on the resonance platform, by means of which laterally localized resonant modes can be excited when exposed to light from the ellipsometer.

For example, the means provided in the resonance platform may comprise a resonant grating whose grating period is of the order of the wavelength of the light which emits the light source of the ellipsometer.

The resonance platform with the resonant grating may comprise a transparent substrate which is coated with at least one dielectric layer and the resonant grating is provided in this layer or at at least one of the boundary surface bounding this layer.

For measurement, it may be advantageous if the resonance platform in the device about an axis which is perpendicular to the surface of the resonance platform, relative to the plane of incidence of the light of the Eilipsometers is rotatably mounted and can be fixed in a rotated position.

The device of the type described above may comprise an imaging ellipsometer.

According to the invention, the following procedure can be used to measure the adsorption or desorption of substances on a surface:

Providing an ellipsometer; providing a substrate having resonant surface for laterally located modes

- Contacting the surface with a substance to um¬ measuring medium

- Excitation of laterally localized modes by exposure of the resonance platform with polarized light

- Determining the position of the resonance curve as a function of at least one excitation parameter.

In addition, the position of the resonance curve on contact with the surface without the substance to be measured can be determined as a reference.

If an imaging method is used, the location of the reference curve can be determined locally. This opens up the possibility of processing the determined spatially resolved position data into an image.

It is advantageous if the parameters PSI and / or DELTA customary in ellipsometry are determined, wherein PSI indicates the ratio of the amplitude change of the s and p polarization after reflection and DELTA the relative phase shift (delta) of the polarization components after reflection.

The method can be optimized by applying light to the excitation of latently localized modes. The device according to the invention and the method according to the invention will be discussed together below.

For the purposes of this application, surface plasmons (SPRs) do not count as laterally located resonances because their propagation typically extends over several microns. The possible excitation of resonances lying close to each other, which is necessary for a high resolution, is therefore not possible.

In this application, as laterally localized resonance, any optical resonance is referred to which is not a plasmon resonance and which limits the resonant interaction with the surface essentially to a maximum of the order of magnitude of the classic total reflection associated with those skilled in the art - common Goos-Hänchen effect lies.

The laterally localized resonance phenomena include, among others, the so-called evanescent resonance, which can be achieved, for example, by resonant gratings. For resonant gratings, the grating period is on the order of the light used to excite the resonance. The structures referred to in the English language literature as photonic band gap structures lead to extremely lossy and therefore localized modes in the propagation which have a high field at the surface, as described in detail in "Localization of One Photon States ¹ " by C. Adlard , ER Pike Sarkar in Physical Review Letters, Vol. 79, No. 9, pages 1585-87 (1997). Some of the evanescent resonators exhibit abnormally high reflectance or transmittance at resonance. Such a structure with an abnormally high reflection is disclosed in patent application WO2001002839. It should be specifically noted that this structure is successfully used in the field of fluorescence analysis. Compared with the traditional waveguide structures, this lattice structure has the advantage that the light applied does not propagate near the high-index layer. The structure discussed here by way of example comprises a glass substrate (it is also possible to use plastic) with refractive index ns = 1.52 on one surface of which a periodic surface grating (grating period p = 360 nm, grating depth 40 nm, web to trench ratio 1: 1) is realized. This is coated with a 150 nm thick Ta 2 O 5 layer (refractive index nf = 2.1) (other layer materials such as TiO 2, Nb 2 O 3 and others can be used with appropriately adapted parameters). In this case, one is by no means limited to high-index materials, for example if a layer system is built up). The grating of the glass surface reproduces on the surface of the Ta2O5 layer again. The medium adjacent to the layer is water (nc = 1.33). If this structure is exposed to light of wavelength 633 nm with angles of incidence from 0 ° to 6 °, the plane of incidence being spanned by the grid vector and the normal to the surface (non-conical loading), the result is a dashed line in FIG Reflection shown for TE polarization (electric field vector perpendicular to Ein¬ fall plane) and shown with a solid line reflection for TM polarization (electric field vector perpendicular to plane of incidence). For both counts the left Y-axis (reflection 1 corresponds to 100%). The phase difference of the phase shift in reflection, which corresponds to the large delta known from ellipsometry (crosses), is shown in addition. Here counts the right Y-axis. It is striking that there is a strong change in the range of 4 ° to 4.6 °, both for the reflection TM and for delta. In particular, the steepness of delta as a function of the angle of incidence is interesting because this steepness for traditional ellipsometry determines the resolution of the measuring system, ie determines the smallest measurable layer thickness change. In conventional ellipsometry, there is a functional dependence which is not valid for resonant grating structures. However, the inventors have recognized that an assignment of the displacement of the delta curve can be determined by means of theoretical simulation, which then permits a very accurate determination of the layer thickness change on the basis of the delta curve. For the biological substances considered in the context of this application, whose adsorption on the surface is to be determined, it will be assumed in the first example for the sake of simplicity that their refractive index corresponds to that of the coating. Deposition of biologically On the surface, logy then essentially corresponds to an increase in the layer thickness. On closer consideration, an index adapted to the biology can be adopted and the occupancy of the web walls can be taken into account. In principle, this does not change the effect. FIG. 2 shows, in the range from 3.8 ° to 4.3 ° for 2 different thicknesses (10 nm and 100 nm), the sizes Psi = tan (R _TM / R _TE ) and delta customary in ellipsometry. It becomes clear that a much finer resolution than the deposit can be measured. From the drawing ab¬ is to estimate that a resolution of O.lnm is quite to measure.

It is advantageous if sufficient reflection signal is present in the angular range relevant for the measurement for both polarizations (greater than or equal to 1% of the irradiated intensity). Then, the quotients necessary for calculating the quantities delta and psi can be well formed without disturbing any possible, for example, electronic background. In the example considered here, the reflection for both TM and TE polarization in the angular range of 3.9 ° to 4.3 ° values is greater than 1%. The formation of the quotients TM / TE ensures that, for example, fluctuations in the light intensity are found out. However, special emphasis is placed on controlling the polarization.

FIG. 3 again shows the same functions Psi and Delta for 4 different layer thicknesses (150 nm 150 nm and 150 nm and 151 nm). The thick lines correspond to the values for 150.lnm. As you can see a resolution of O.lnm and below is easily possible.

As already mentioned above, in a somewhat more precise consideration, the exact index of the biology (nb = 1.48) and the way in which the material is adsorbed (also covering the sidewalls of the grid) can be included in the calculation. Figure 4 shows Psi and Delta as a function of angle of incidence for an Inm Biology-covered structure (solid lines) and values without this coverage, ie, the starting structure with 150nm layer thickness (broken lines). Here, too, it becomes clear that a change in layer thickness can be measured with considerable accuracy.

We could also have considered the quotient of the reflections in the angle range from 0 ° to 2 °. In this case, however, the reflection signal of the TM polarization is approximately 0.35% and the quotient formation becomes rather critical. In that case, one better approaches the absolute signal of the TE polarization. DELTA also shows a characteristic which shifts with increasing layer thickness in this angular range. Both are not so steep and do not deliver the resolution.

With such a method very well-imaging ellipsometry can be operated. A substrate according to the invention as described above can hereby be investigated in a spatially resolved and label-free manner with respect to adsorbed material. Since this is possible without contact, it is also possible to make dynamic measurements.

As described above, in principle any type of optic resonance phenomenon which occurs laterally localized is suitable. Of the

The scope of the invention should therefore not be limited to the resonant gratings discussed in detail, which have a reflection anomaly.

So far, the discussion has been limited to the case of non-conical lighting. Non-conical illumination denotes the exposure to light in which the plane of incidence is perpendicular to the bars, ie the grid vector lies in the plane of incidence. If the grating vector is turned out of the plane of incidence, it is called conical illumination. The inventors have found that the conical illumination can be used advantageously in the context of the invention. It should be noted that the resonance angle changes with the amount of the grating rotation angle, ie the angle amount by which the grating vector is rotated out of the plane of incidence. As already discussed above, the Del ta gradient is limiting factor for the measurement accuracy. The following The table below gives the measured dependence of the delta slope on the lattice rotation angle. In the last column, this parameter translates into the sensitivity.

Different grid geometries, which have resonances, can be used. Cross gratings with, for example, different grating periods in the x and y directions can also be used. Of particular interest are those structures which have a resonance at very small angles of incidence. Here, for example, an expansion of the otherwise customary fluorescence scanners to the ellipsometric method would be conceivable. However, there are already fluorescence scanners that allow a choice of the angle of incidence (example Fa. Tecan). Again, an extension to the above-described label-free method would be conceivable.

In summary, it can be said that the sensitivity achieved with the present invention is equal to that of a full SPR measurement. As already stated, it is possible to set the sensitivity to a desired value by grid rotation. For this purpose, no complicated aids, such as applying an electric field in the case of the tunable SPR sensor [Patent DE 100 19 359] are necessary. Unlike the SPR, it is not necessary here to couple the light through a prism using immersion oil. Likewise, the use of a metal film is avoided. The disadvantage of metal films is that thiol must be used as an activator for the biological binding partners, and that the optical properties of gold films are not well reproducible. Handling is much simpler and faster without metal and without immersion oil. To quantitatively determine the thickness of a reaction layer on an SPR sensor, the parameters thickness, index, and extinction of the gold film on each sensor must be determined prior to kinetics, as these parameters generally vary across production batches. This problem does not exist with the resonant platform ; In comparison with the SPR, the resonance platform is therefore characterized by the fact that layer thicknesses on the Ta 2 O 5, for example in a reaction kinetics, are measured with less measurement effort and with higher accuracy.

The recording of reaction kinetics in many channels simultaneously can be performed on the grating coupler as well as with imaging SPR on a SPR sensor. The change in delta during a change in layer thickness is recorded at a constant angle of incidence, and the layer thickness kinetics is determined therefrom.

Claims

Claims:

1. An apparatus for performing ellipsometry measurements, with ei¬ nem Eilipsometer and a resonance platform, wherein on the resonance platform means are provided by the laterally located upon exposure to light of Eilipsometers located resonant modes can be ange¬ excited.

2. Apparatus according to claim 1, characterized in that the said means provided comprise a resonant grating whose

Grating period in the order of the wavelength of the light is that emits the light source of the ellipsometer.

3. A device according to claim 2, characterized in that the resonant plates comprise a transparent substrate which is coated with at least one dielectric layer and the resonant grid is provided in this layer or at least one of the interface defining this layer.

4. Device according to one of claims 1 to 3, characterized gekenn¬ characterized in that the resonance platform in the device about a Ach¬ se, which is perpendicular to the surface of the resonance platform, relative to the plane of incidence of the light of the ellipsometer rotatably ge is stored ge and can be fixed in the rotated position.

5. Device according to one of the preceding claims, characterized in that the ellipsometer is an imaging ellipsometer.

6. A method of measuring the adsorption or desorption of substances on a surface comprising the steps of:

- Provide an ellipsometer

Providing a substrate with resonant surface for laterally localized modes

- contacting the surface with a medium comprising the substance to be measured - Excitation laterally localized modes by applying the Reε sonanzplattform with polarized light

7. The method according to claim 6, characterized in that the position of the resonance curve when contacting the surface with the medium without the substance to be measured is determined as Refe¬ renz.

8. The method according to any one of claims 6 and 7, characterized gekennzeich¬ net, that the determination of the position of the reference curve takes place dissolved locally.

9. The method according to claim 8, characterized in that the determined spatially resolved position data are processed into an image

10. The method according to any one of claims 6 to 9 characterized gekennzeich¬ net, that the usual in ellipsometry parameters PSI and / or DELTA are determined, PSI the ratio of the amplitude change of the s and p polarization after reflection and DELTA the Relati¬ ven Phasenschift (delta) of the polarization components indicates after reflection.

11. The method according to any one of claims 6 to 10, characterized gekenn¬ characterized in that is applied to excite laterally localized modes conically with light.