JP2014215202A - Surface plasmon sensor - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a surface plasmon sensor solving a problem in a thin metallic film material for excitation of surface plasmon, for which Au, Ag or Cu has been considered to be an advantageous material, while Au or Ag is expensive and Cu is considered to be difficult to use in view of stability, as being generally problematic in diffusivity, oxidizability, adhesion property, and crystallinity.SOLUTION: In a plasmon sensor, Cu, being less expensive than Au or Ag, is used as a thin metallic film material for generating surface plasmon. A thin film made of a nitride of Ta or a nitrooxide is formed on a substrate within a range of an appropriate film thickness, making contact with an upper side and a lower side of a Cu film.

Description

  The present invention relates to a chemical sensor using surface plasmons. In particular, the surface refractive index of the material in contact with the thin film and the change of the surface plasmon of the metal thin film attached to the prism are utilized by utilizing the excitation of the propagating surface plasmon by the combination of the surface plasmon of the metal thin film attached to the prism and the light incident on the prism under total reflection conditions. The present invention relates to a chemical sensor that detects from the plasmon resonance angle.

  Collective vibrations of free electrons on a metal surface are called surface plasmons, and in recent years, application to sensors, devices with nanoscale structures, microscopes, etc. has been studied by exciting surface plasmons. There are three main methods for exciting surface plasmons: a method in which light is incident on a prism attached with a metal thin film under total reflection conditions, a method in which a diffractive structure is created on the metal surface and the diffracted light of the incident light is used, a metal There is a method of exciting surface plasmons that are localized by making particles fine.

  In particular, the method in which light is incident on a prism attached with a metal thin film under the condition of total reflection has the advantage that it is not necessary to form a fine structure on the surface of the metal thin film. Since the change can be detected from the incident angle of incident light when the surface plasmon is excited, application to a chemical sensor is being studied. (Refer nonpatent literature 1).

  As a metal thin film material for exciting surface plasmons, a metal having a negative real part of the complex permittivity and a large value obtained by dividing the real part by the imaginary part (real part / imaginary part) is more advantageous. For such materials, Au, Ag, Cu and the like are advantageous materials.

As shown in FIG. 10, when a metal thin film is formed on the upper surface of the prism, the presence or absence of surface plasmon excitation is confirmed by calculating the reflectance of the reflected light 6 with respect to the incident light 5 with the horizontal axis as the incident angle. be able to. For example, the prism substrate material is alumina (Al ₂ O ₃ ), the metal film material is gold (Au) with a thickness of 40 nm, the incident light is TM polarized light with a wavelength of 633 nm (He—Ne laser), and the material to be measured is water or SiO _2. When the reflectance is calculated with the horizontal axis as the incident angle (θ), it is as shown in FIG. All the calculation methods are based on the RCWA (Rigorous coupling Wave Analysis) method. For RCWA, see Non-Patent Document 1, for example.

Referring to FIG. 11, when the material to be measured is SiO ₂ , the incident angle is around 61 °, and in the case of water, the critical angle at which total reflection starts is around 53 °, and the reflectivity drops in the region beyond the critical angle. In the case of SiO ₂ , it can be seen that the reflectance becomes the bottom at an incident angle of around 77 °, and in the case of water at around 62 °. This means that the wave number of the surface plasmon of the metal thin film and the wave number in the interface direction of the electromagnetic wave of the totally reflecting prism surface coincide and resonate, and the reflected light energy is used to excite the surface plasmon. (Hereinafter, the incident angle at which the reflectance is bottom is appropriately referred to as the resonance angle, and the incident light for exciting the surface plasmon is referred to as excitation light).

  Since the angle of the resonance angle changes depending on the refractive index of the material to be measured, it is possible to know the refractive index of the material to be measured and its change from the position of the angle of the resonance angle, and the chemical sensor based on the information on the refractive index It can be.

Satoshi Kawada, All of Nanooptics and Nanophotonics, §4, Frontier Publishing, 2006 http: // www. rit. edu / kgcoe / microsystems / lithography / thinfilms / thinfilms / thinfilms

W007 / 105771

Au, Ag, Cu, etc. have been considered as advantageous materials for metal thin film materials for exciting surface plasmons. However, Au and Ag are expensive, while Cu is generally diffusive, oxidative, and adhesive. In view of stability, crystallinity, and the like, it has been made difficult to use from the viewpoint of stability.
An object of the present invention is to provide a layer structure using Cu, which is difficult to use, for plasmon excitation.

  In order to solve the above-mentioned problem, the present invention according to claim 1 is a surface plasmon sensor having a prism substrate and a metal thin film made of Cu for generating surface plasmons, wherein the surface plasmon sensor is provided between the prism substrate and the Cu thin film. The surface plasmon sensor is characterized by having a barrier film for preventing diffusion of Cu and a protective film for preventing deterioration of the Cu thin film on the surface of the Cu thin film in contact with the object to be measured.

  The surface plasmon sensor according to claim 1, wherein at least one of the barrier film and the protective film material is a thin film containing Ta and nitrogen as components.

  According to a third aspect of the present invention, in the surface of the second aspect, the film thickness of at least one of the barrier film and the protective film containing Ta and nitrogen as components is in the range of 10 nm to 25 nm. It is a plasmon sensor.

  The present invention described in claim 4 is the surface plasmoncene according to claim 1, wherein at least one of the barrier film and the protective film material is a thin film containing Ta, nitrogen, and oxygen as components.

  The present invention described in claim 4 is characterized in that the film thickness of at least one of the barrier film and the protective film containing Ta, nitrogen and oxygen is in the range of 10 nm to 40 nm. The surface plasmon sensor.

  In the present invention, Cu, which is cheaper and easier to obtain than Au and Ag, is used as the metal thin film material for generating surface plasmons, so that the manufacturing cost can be reduced. Furthermore, because it has a thin film made of Ta nitride or nitride oxide in contact with the upper and lower sides of the Cu film, diffusion of Cu, which has been a problem in the past, into the prism substrate and the material to be measured, and oxidation of the Cu film itself Deterioration can be suppressed, and adhesion to the prism substrate can be improved.

It is a cross-sectional schematic diagram which shows the part containing the detection part of the surface plasmon sensor of this invention. The surface plasmon sensor of the present invention shows the result of calculating the reflectivity with respect to the incident angle of excitation light with GaP as the substrate material, TaN, Ta ₂ O ₅ as the barrier film and protective film material, and the film thickness as 10 nm. FIG. In the surface plasmon sensor of the present invention, GaP was used as a substrate material, TaN, Ta ₂ O ₅ was used as a barrier film and a protective film material, and the thickness of the protective film was 10 nm. It is a characteristic view which shows a result. In the surface plasmon sensor of the present invention, GaP was used as a substrate material, TaN, Ta ₂ O ₅ were used as a barrier film and a protective film material, the thickness of the barrier film was 10 nm, and the reflectance was calculated with respect to the incident angle of excitation light. It is a characteristic view which shows a result. In the surface plasmon sensor of the present invention, Al ₂ O ₃ is used as a substrate material, TaN, Ta ₂ O ₅ is used as a barrier film and a protective film material, the film thickness is 10 nm, and the reflectance is calculated with respect to the incident angle of excitation light. It is a characteristic view which shows a result. In the surface plasmon sensor of the present invention, Al ₂ O ₃ is used as a substrate material, TaN, Ta ₂ O ₅ is used as a barrier film and a protective film material, and the protective film has a thickness of 10 nm. It is a characteristic view which shows the result of having calculated. In the surface plasmon sensor of the present invention, Al ₂ O ₃ is used as a substrate material, TaN, Ta ₂ O ₅ is used as a barrier film and a protective film material, the thickness of the barrier film is 10 nm, and the reflectivity with respect to the incident angle of excitation light It is a characteristic view which shows the result of having calculated. It is a table | surface which shows the result of (FIG. 2)-(FIG. 7) collectively. It is a table | surface which shows collectively the optical constant of the material relevant to calculation and description of the surface plasmon sensor of this invention. It is a cross-sectional schematic diagram which shows the part containing the detection part of the surface plasmon sensor of conventional structure. It is a characteristic view which shows the result of having calculated the reflectance with respect to the incident angle of excitation light, using Al ₂ O ₃ as a substrate material and Au as a metal thin film material in a surface plasmon sensor having a conventional structure.

  As shown in FIG. 1, the surface plasmon sensor according to the embodiment of the present invention has a prism substrate 1 and a metal thin film 2 made of copper (Cu) for generating surface plasmons, and further includes the prism substrate 1 and the metal. A barrier film 3 for preventing diffusion of Cu between the thin films 2 and a protective film 4 for preventing alteration of the metal thin film 2 are provided on the surface of the metal thin film 2 in contact with the object to be measured.

  Preferably, at least one of the barrier film and the protective film material is a thin film containing Ta and nitrogen as main components (hereinafter referred to as TaN), or a thin film containing Ta, nitrogen and oxygen as main components (hereinafter referred to as TaNO). It is.

  More preferably, the film thickness of at least one of the barrier film made of TaN and the protective film is in the range of 10 nm to 25 nm, or the film thickness of at least one thin film of the barrier film made of TaNO and the protective film is from 10 nm. It is a thin film in the range of 40 nm.

  By the way, aluminum (Al) is the most common wiring material for semiconductor elements, but Cu wiring has also been studied as a material having a smaller specific resistance. However, problems such as diffusion prevention, oxidation prevention, and workability have been pointed out for practical use of Cu wiring, and TiN, WN, TiW, TaN, etc. have been proposed as barrier metals for preventing diffusion.

  The surface plasmon sensor of the present invention uses Cu, which is less expensive than Au or Ag, as a metal thin film for generating surface plasmons, and is relatively transparent as a barrier film or protective film material. TaN or TaNO, which has a small influence on the generation of plasmons and can maintain the performance as a surface plasmon sensor, is used.

In the following, based on the present invention, as shown in FIG. 1, a Cu thin film exists on a prism substrate, and a barrier film made of TaN or Ta ₂ O ₅ is further interposed between the prism substrate and the Cu thin film, and the Cu thin film is measured. In the case where a protective film made of TaN or Ta ₂ O ₅ is present on the surface in contact with the object, the change in the reflectance of the reflected light 6 with respect to the incident light 5 is calculated with the horizontal axis as the incident angle, and the surface plasmon excitation is used as a sensor. The result of having examined the range of the film thickness of the applicable barrier film and protective film is shown. Here, the reason why Ta ₂ O ₅ is used instead of TaNO is that the refractive index differs depending on the composition ratio in the case of TaNO, but the refractive index of Ta ₂ O ₅ is known. _{This is} because an intermediate result of ₂ O ₅ results. The incident light is TM polarized light having a wavelength of 633 nm (He—Ne laser), and the material to be measured is water or SiO ₂ as an example. FIG. 9 shows a table summarizing optical constants (refractive index and extinction coefficient) of each material used in the calculation at a wavelength of 633 nm (see Non-Patent Document 2).

Example 1
Example 1 shows a case where gallium phosphide (GaP) is used as the prism substrate material. FIG. 2 shows a case where the thickness of both the barrier film and the protective film is 10 nm. The reason why 10 nm is selected here is that a film that is too thin generally has many pinhole-like defects, and it is considered that a film thickness of at least 10 nm is necessary to obtain stability. As can be seen from FIG. 2, even when either the TaN film or the Ta ₂ O ₅ film is used as a barrier film or a protective film, a resonance angle at which the reflectance drops sharply in an incident angle region slightly larger than the critical angle of total reflection appears. It can be seen that plasmon excitation occurs.

Next, the thickness of the protective film is fixed to a minimum of 10 nm, the barrier film thickness is changed, the same calculation as in FIG. 2 is performed, surface plasmon excitation occurs, and the range of the barrier film thickness applicable as a sensor I investigated. FIG. 3 shows a calculation result when the film thickness of the barrier film is considered to be near the upper limit applicable as a sensor. It can be seen that when the Ta ₂ O ₅ film is used as a barrier film, a considerably thicker film can be used than when the TaN film is used as a barrier film. It can also be seen that the Ta ₂ O ₅ protective film has a wider applicable film thickness range than the TaN protective film.

Next, the barrier film thickness is fixed to a minimum of 10 nm, the protective film thickness is changed, and the same calculation as in FIG. 2 is performed. Surface plasmon excitation occurs, and the range of the protective film thickness applicable as a sensor I investigated. FIG. 4 shows a calculation result when the thickness of the protective film is considered to be near the upper limit applicable as a sensor. Regardless of whether the barrier film is TaN or Ta ₂ O ₅ , it can be seen that when the Ta ₂ O ₅ film is used as a protective film, a thicker film can be used than when the TaN film is used as a protective film.

(Example 2)
In Example 2, the case where alumina (Al ₂ O ₃ ) is used as the prism substrate material is shown. FIG. 5 shows a case where the thickness of both the barrier film and the protective film is 10 nm. The reason why 10 nm is selected here is that a film that is too thin generally has many pinhole-like defects, and it is considered that a film thickness of at least 10 nm is necessary to obtain stability. As can be seen from FIG. 5, even when either the TaN film or the Ta ₂ O ₅ film is used as a barrier film or a protective film, a resonance angle at which the reflectance drops sharply in an incident angle region slightly larger than the critical angle of total reflection appears. It can be seen that plasmon excitation occurs. Further, both the critical angle and the resonance angle are larger than when GaP is used as the prism substrate material.

Next, the thickness of the protective film is fixed to a minimum of 10 nm, the barrier film thickness is changed, the same calculation as in FIG. 5 is performed, surface plasmon excitation occurs, and the range of the barrier film thickness applicable as a sensor I investigated. FIG. 6 shows the calculation results when the barrier film thickness is considered to be near the upper limit applicable as a sensor. It can be seen that when the Ta ₂ O ₅ film is used as a barrier film, a thicker film can be used than when the TaN film is used as a barrier film. It can also be seen that the Ta ₂ O ₅ protective film has a wider applicable film thickness range than the TaN protective film.

Next, the barrier film thickness is fixed to a minimum of 10 nm, the protective film thickness is changed, the same calculation as in FIG. 5 is performed, surface plasmon excitation occurs, and the protective film thickness range applicable as a sensor I investigated. FIG. 7 shows the calculation result when the thickness of the protective film is considered to be near the upper limit applicable as a sensor. Regardless of whether the barrier film is TaN or Ta ₂ O ₅ , it can be seen that when the Ta ₂ O ₅ film is used as a protective film, a thicker film can be used than when the TaN film is used as a protective film.

  A table summarizing the results from FIG. 2 to FIG. 7 is shown in FIG. The parentheses in FIG. 8 indicate the upper limit (maximum thickness) of the other film applicable as a sensor because surface plasmon excitation occurs when the thickness of one of the barrier film and the protective film is fixed at 10 nm. In order to increase the barrier properties of the barrier film and the protective function of the protective film against Cu, it is preferable to make these films as thick as possible. However, if it is too thick, it becomes difficult to use surface plasmon excitation as a sensor as described above.

Furthermore, as described above, Ta ₂ O ₅ can be used up to a range thicker than TaN when it is used as both a barrier film and a protective film. This is because the transparency of Ta ₂ O ₅ is higher than that of TaN. However, since Ta ₂ O ₅ is an oxide, there is a concern that Cu may be oxidized. Therefore, only TaN is used as a barrier film or protective film of Cu, or a TaNO film in which oxygen (O) is appropriately added to TaN is used in balance with a sensor function using surface plasmon excitation. It is desirable to do.

From FIG. 8, it is possible to know an appropriate range of film thickness when the barrier film and the protective film are TaN or Ta ₂ O ₅ , respectively. For TaN, the range is considered to be in the range of 10 nm to approximately 25 nm. An appropriate range of the film thickness when TaNO in which O is appropriately added to TaN can be estimated from the results of FIG. 8 and the above-mentioned consideration, and is considered to be a range of 10 nm to about 40 nm.

Further, FIG. 8 shows that the applicable film thickness range of the protective film and the barrier film becomes wider as the substrate material having a higher refractive index is used. In Examples 1 and 2, the prism substrate material is GaP or Al ₂ O ₃ as the material for the prism substrate. However, other transparent substrates include silicon nitride (Si ₃ N ₄ ), zirconium oxide (ZrO). ₂ ), zircon (ZrSiO ₄ ), etc. are conceivable, but the refractive index of these materials is between GaP and Al ₂ O ₃ . Therefore, the calculation result of the reflectance with respect to the incident angle is an intermediate result between GaP (FIGS. 2 to 4) and Al ₂ O ₃ (FIGS. 5 to 7). Therefore, the proper ranges of the thickness of the TaN film and the TaNO film when these materials are used as the substrate are within the range of 10 to 25 nm for TaN and 10 to 40 nm for TaNO.

  Since the present invention can detect the refractive index of a substance such as liquid or gas and its change from information on the incident angle (resonance angle) of incident light when exciting surface plasmons using a prism substrate, Expected to be used as a chemical sensor based on rate.

DESCRIPTION OF SYMBOLS 1 ... Transparent substrate 2 ... Cu film | membrane 22 ... Metal thin film 3 ... Barrier film 4 ... Protective film 5 ... Incident light 6 ... Reflected light

Claims (5)

  A surface plasmon sensor having a prism substrate and a metal thin film made of Cu for generating surface plasmons, a barrier film for preventing diffusion of Cu between the prism substrate and the Cu thin film, and a measured object of the Cu thin film A surface plasmon sensor characterized by having a protective film for preventing deterioration of a Cu thin film on a surface in contact therewith.   2. The surface plasmon sensor according to claim 1, wherein at least one of the barrier film and the protective film material is a thin film containing Ta and nitrogen as components.   The surface plasmon sensor according to claim 2, wherein the thickness of at least one of the barrier film and the protective film containing Ta and nitrogen as components is in the range of 10 nm to 25 nm.   2. The surface plasmon sensor according to claim 1, wherein at least one of the barrier film and the protective film material is a thin film containing Ta, nitrogen, and oxygen as components.   5. The surface plasmon sensor according to claim 4, wherein the thickness of at least one of the barrier film and the protective film containing Ta, nitrogen and oxygen as components is in the range of 10 nm to 40 nm.

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