WO2000072076A1

WO2000072076A1 - Probe tip that is transparent to light and method for producing the same

Info

Publication number: WO2000072076A1
Application number: PCT/EP2000/004655
Authority: WO
Inventors: Jürgen BRUGGER; Niek Van Hulst
Original assignee: Brugger Juergen; Niek Van Hulst
Priority date: 1999-05-21
Filing date: 2000-05-22
Publication date: 2000-11-30
Also published as: DE19923444C2; DE19923444A1

Abstract

The invention relates to a probe tip that is transparent to light, for emitting and/or receiving light in a specific manner, for use in an optical analyzing or processing system for examining and/or processing surfaces with an optical resolution in the sub-light wavelength range. The inventive probe tip has a body (5) consisting of a light-conductive material, with a surface that is coated with a light-absorbing layer (2). Said layer is interrupted by at least one through opening (3), through which the light passes. The invention also relates to a method for producing the probe tip. The invention is characterized in that the through opening in the layer is completely filled with the light conductive material and in that said light-conductive material finishes flush with the light-absorbing layer or protrudes beyond said layer. The inventive probe tip can be used in the fields of near field microscopy, optical photolithography and in magnetic/optical memories

Description

Light-transparent probe tip and method for producing one

Technical field

The invention relates to a light-transparent probe tip and to a method for producing one for targeted light emission and / or for receiving light for use in an optical analysis or processing system for examining and / or processing surfaces with an optical resolution in the sub-light wavelength range which has a body made of a light-conducting material and has a surface which is covered by a light-absorbing layer which is interrupted by at least one passage opening through which light passes.

State of the art

The invention of the scanning tunneling microscope (STM) in 1982 led to the rapid development of related scanning sample microscopes, for example the atomic force microscope (AFM) or the near-field optical microscope (SNOM). AFM is now the most widespread because it enables the analysis and processing of any non-conductive material.

The success of the AFM is due, among other things, to the fact that the so-called tips, samples or tips required for this were already available in excellent quality and inexpensively in the early development phase. This has promoted wide and rapid - almost avalanche-like - dissemination of the instrument, and has also triggered new techniques and methods. Near-field optical methods would even be superior to the AFM in some aspects, since in addition to topographical properties, optical properties of surfaces can also be examined. This factor plays an important role especially in biology, since light acts "without power" and the structure is not touched and changed.

The reason why SNOM is not yet as widespread as the AFM is lack of high quality and low cost SNOM tips. Improved and cheaper SNOM tips would improve the state of the art in many areas, for example near-field microscopy in areas such as biology, medicine, materials science, but also methods of magneto-optical data storage and in the process of photoresist structuring in semiconductor production.

Today's most widely used probes for near-field microscopy are drawn or etched glass fibers 1 (see FIG. 1a) with a metal jacket 2 and a manufactured aperture 3, which are typically produced using the following process steps: the shape of the probe tip is drawn from a glass fiber 1 and / or etched (see Figure 1 a step 1). The glass fiber 1 is then vapor-coated with a metal layer, for example aluminum (step 2). Finally, the metal layer 1 is opened at the tip of the probe, as a result of which a through opening or aperture 3 is obtained (step 3). Such NFO samples currently have the highest resolution. (See also: BETZIG E, TRAUTMAN JK, HARRIS TD, WEINER JS, KOSTELAK RL; BREAKING THE DIFFRACTION BARRIER - OPTICAL MICROSCOPY ON A NANOMETRIC SCALE; SCIENCE 251: (5000) 1468-1470 MAR 22 1991).

However, the geometry of the tip is unfavorable in relation to the light intensity that can be achieved through the aperture. The reason for this is the so-called cut-off problem. Cut-off is an exponential loss of the intensity of a wave along the direction of propagation as soon as the lateral dimensions of the conductor become equal to or less than the wavelength, with visible light at approx. 500 nm. The drawn or etched probes are already to the last micrometer very tight, so on this last one Path section in the waveguide 3-4 orders of magnitude "cut off" and limit the amount of light that can be carried out.

When a metal film is vapor-deposited, high-quality layers cannot be achieved due to the free-standing tip, as is possible, for example, on planar surfaces. Grains 4 in particular form on the metal layer 2, as a result of which the thickness becomes inhomogeneous. The aperture is opened by means of a focused ion beam V and V _s . There are two ways to do this:

According to FIG. 1 b, the ion beam V _F comes from the front. Here you have no control over the exact depth of the light guide behind the aperture 3 (which in itself is a serious disadvantage). Furthermore, grains 4, which prevent the aperture 3 from approaching a work surface, are not removed.

1c, the beam V _s comes from the side. Although the previous problems have been eliminated here, the aperture size varies depending on the film thickness of the metal film 2 and the approach plane of the beam Vs.

Further disadvantages of the drawn / etched NFO samples can be mentioned: The samples suffer from high light intensity losses, since the light on the way in the waveguide to the aperture already extends over a distance of several wavelengths in the so-called "cut-off area" located. This is associated with an exponential drop in intensity, typical losses of 3-4 orders of magnitude.

(The metal layer is vapor-deposited after glass drawing / etching, and is therefore not of optimal quality (grain formation, narrow .: grains) and not of homogeneous thickness (small radius at the free-standing glass tip)), as already mentioned. The NFO samples have an aperture that is produced at the end of the manufacturing process, for example by ion beam or etching. This is a complex and not 100% reproducible step, since each sample has to be processed individually “by hand”. Furthermore, the samples mostly suffer from poor light Polarization properties because the opening does not have a well-defined geometry. In addition, they have no "flat" end of the probe. This prevents the aperture from being brought sufficiently close to the near field, for example at distances of approximately 10 nm and below. This results in intensity and loss of resolution.

An array-shaped arrangement of multiple tips is eliminated with this technique. The manufacturing costs are generally too high.

An alternative to the aforementioned construction of the probe tip is its manufacture from etched light-conducting material (glass, quartz, silicon dioxide / nitride) using microfabrication methods and subsequent metal vapor deposition. See here: Cantilever probes for SNOM applications with Single and double aperture tips Oesterschulze E, Rudow O, Mihaicea C, Scholz W, Werner SULTRAMICROSCOPY 71: (1-4) 85-92 MAR 1998.

Microfabricated NFO tips made of etched glass, silicon dioxide / nitride, however, also suffer from the geometrically determined 'cut-off phenomenon' defined above. The aperture is defined here by means of etching techniques which only provide insufficient precision in the required range (~ 10nm), due to edge effects, timing, no etching stop, etc.

Hollow NFO samples made of silicon nitride using an impression process (such as standard AFM tips) and a refined aperture are also known, see “Microfabrication of near-field optical probes Ruiter AGT, Moers MHP, vanHulst NF, deBoer MJOURNAL OF VACUUM SCIENCE & TECHNOLOGY B 14: (2) 597-601 MAR-APR 1996.

Hollow NFO samples do not have a waveguiding material with a high refractive index (> 1.45) in which the light is guided, which in turn leads to losses (intensity, diffraction, polarization properties ...). Finally, devices for probe tips or samples for subwavelengths for optical lithography are also known, which are based on optical glass fibers or solid immersion lenses. See also: Surface modification in the optical near field Krausch G, Mlynek J MICROELECTRONIC ENGINEERING 32: (1-4) 219-228 SEP 1996; Near-field photolithography with a solid immersion lens, LPGhislain et al., Applied Physics Letters Vol. 74, Number 4, January 25, 1999.

When using optical glass fibers, the optical intensity is limited by the 'cut-off' problem, which requires slow raster speeds in order to irradiate the photoresist with the necessary dose of light. This in turn is too slow as an alternative to existing methods. Solid immersion lens (SIL) is faster because a much higher light dose is applied to the paint. The technology is currently (and probably in principle) limited to approximately 100 nm lateral resolution. Line widths below 100 nm and even 10 nm will be required in the future.

Presentation of the invention

The invention has for its object a light-transparent probe tip for targeted light emission and / or for receiving light for use in an optical analysis or processing system for examining and / or processing surfaces with an optical resolution in the sub-light wavelength range, which is made of a light-conducting material existing body with a surface that is covered by a light-absorbing layer, which is interrupted by at least one passage opening through which light passes, in such a way that the above-mentioned disadvantages are eliminated. Furthermore, an inexpensive manufacturing process is to be described, with which it is possible to obtain probe tips with high precision.

The solution to the problem on which the invention is based is specified in claims 1 and 15. Advantageous embodiments of the invention are the subject of the dependent claims. The light-transparent probe tip according to the invention for targeted light emission and / or for light reception for use in an optical analysis or processing system for examining and / or processing surfaces with an optical resolution in the sub-light wavelength range, which has a body consisting of a light-conducting material with a Surface which is covered by a light-absorbing layer, which is interrupted by at least one passage opening through which light passes, is formed in that the passage opening through the layer is completely filled with the light-conducting material, and in that the light-conducting material is flush with the light-absorbing material Completes the layer or towers over it.

The new NFO structure shows a significantly higher light intensity (-1000 times) due to the geometry significantly reducing the "cut-off", with a constant aperture (resolution) and thus increases the signal quality of optical microscopy and the throughput in photoresist lithography applications.

It shows a well-defined polarization property through a symmetrical geometry of the aperture and the waveguide in the area and in the aperture.

It also shows an NFO structure with a flat front and allows the aperture to be brought closer to the surface to be processed down to the near field (<10nm) with high resolution and high intensity.

The probe tip according to the invention can also be used in high-resolution optical lithography with high resolution and light intensity at the same time and thus increases the throughput. This technology will become increasingly important in future (nano) electronic components with ever smaller dimensions.

The probe tip according to the invention has a novel structure that can be used in high-resolution near-field optical methods beyond the diffraction-related limits of light, for example in microscopy or in optical photolithography. In order to achieve resolution in sub-wavelengths, the light beam is guided through a small aperture, through opening or aperture with an opening width of approx. 10-50 nm, which is filled with light-conducting material. The lateral resolution of the light wave in the near field area (working area) behind the aperture largely corresponds to the size of the through opening. In order for a high light intensity to be achieved behind the narrow passage opening, it is made as short as necessary, ie the layer thickness of the light-absorbing layer is only a few 10 nm in order to shield the light from the side of the diaphragm.

The through opening is just as long, i.e. the layer thickness, at least in the area of the through opening, is so thick that the cut-off problem does not occur. Here, the probe tip is formed shortly before the through opening in such a way that the light-conducting material provides a greater lateral spatial expansion than is specified by the clear width of the through opening itself. This ensures that lateral light wave narrowing takes place exclusively through the geometry of the passage opening itself and not through other means limiting the lateral light wave guidance, as is the case, for example, with conically tapering optical waveguides which are surrounded by a metal jacket.

The probe tip is used in all areas of optical microscopy (life science, biology, single molecular detection, DNA analysis, etc.), in integrated optics, in magnet-optical data storage, and in optical lithography applications (for microelectronic circuits, VLSI, nanoelectronics, Structuring on a molecular scale, ...).

The structure combines the two main requirements for the above applications:

1. high intensity of the light (this is important for a high signal quality in microscopy, and for a high dose in photoresist structuring processes,

2. a small aperture (for a high lateral resolution). Advantageous properties of the probe tip are the following:

- The neighboring region around the through opening is sufficiently flat or flat and thus forms a sufficiently large acceptance angle for the incident light without loss of intensity occurring.

- The structure is produced by a new 'nano-molding' process, the aperture being made beforehand in a suitable light-repellent material and then being conformally filled with waveguiding material.

The process for producing a light-transparent probe tip is characterized by the following process steps:

Forming a shape determining the spatial shape of the light-guiding body,

Depositing a light-absorbing layer on the mold surface,

Introducing through openings into the light-absorbing layer deposited on the mold surface,

Filling out the form with, solidifying material and a light-guiding body

Isolate the solidified light-guiding body with the light-absorbing body

Layer from the mold.

The structure can be produced inexpensively, in large quantities, and reproducibly with the above microtechnical method, since the shape is reusable. The method is also suitable for the production of 'arrays' which can be used in parallel optical processing / analysis of surfaces. A high throughput is achieved with constant maximum resolution (wafer lithography).

Brief description of the invention

The invention is described below by way of example without limitation of the general inventive concept using exemplary embodiments with reference to the drawing. Show it: Fig. 1 ac light transparent probe tip according to the prior art

2 a-d alternative embodiments of the light-transparent probe tip according to the invention,

Fig. 3 light transparent probe tip on flexible bars and

Fig. 4 sequence pictures of the manufacturing process

WAYS OF CARRYING OUT THE INVENTION, INDUSTRIAL APPLICABILITY

With regard to FIG. 1, reference is made to the above in the introduction to the description in order to appreciate the prior art.

FIG. 2a shows a probe tip which essentially consists of a body 5 which has a flat surface 7 and which is made of light-conducting material, for example PDMS, SU8, glass or the like. Furthermore, light-emitting polymers (OLED = organic light-emitting diodes) are conceivable as active light sources which emit the light through the diaphragm.

A metal layer 2 is applied to the flat surface and provides a through opening 3, the so-called aperture. The passage opening 3 is completely filled with the light-conducting material. The surface of the metal layer 2 and the through opening are made flush with the surface 6.

The through opening has a diameter A of approximately between 10 and 50 nm. The thickness of the metal layer is D -10-30 nm. The aperture of the through opening has a large opening angle of approximately 90 degrees for incident light. The metal layer 2 and the light-conducting material in the passage opening are on the same level on the side facing the work surface 6; therefore, the light guide can be approximated to the surface to be processed in the near field (<10nm). The structure thus combines high intensity with extremely high lateral resolution. The "cut-off" problem practically no longer occurs here.

A variation of the probe tip is shown in Fig. 2b. It has the shape of an inverted inverted pyramid, or a truncated cone. This ensures access to rough surfaces (such as AFM). In this case, the area is kept to a minimum. The light-conducting material of the body 5 has been filled conformally into the metal layer 2 up to the through opening 3.

Another variation is shown in Fig. 2c. Here, the light-guiding body 5 is semicircular.

In FIG. 2d, the body 5 and the metal layer 2 have a plurality of through openings 3, which can also be arranged in an array.

Another variation of the probe tip is shown in FIG. 3. The probe tip is located on a flexible membrane 7 or bending beam (such as AFM) to compensate for height differences.

The manufacturing process is shown in FIG. 4. It is characterized by the following steps:

Step 1: etching a well with the (negative) shape of the later structure Step 2: deposition of the metal film 2; the area to be vaporized is planar

(in contrast to etched tips). So here is an improved one

Quality achieved with a well-defined layer thickness. Step 3: opening aperture 3 (lithography, lift-off, focused ion beam) Step 4: conformally filling aperture 3 with waveguiding material 5 (polymer, PDMS, SU8, glass ...); these materials conform geometrically to geometries down to the nanometer range. Step 5: Polymerization / hardening of the material and removal of the structure from the well, reusing the well / mold.

A variation uses a sacrificial layer between the metal and the trough, which is selectively removed at the end. This process is preferably carried out between steps 3 and 4.

The removal of the light-guiding body with the light-absorbing layer is facilitated by providing a corresponding sacrificial layer, in that a so-called 'de-adhesion layer' is introduced between the working surface and the double layer of light-absorbing and light-guiding material to be removed. This is e.g. by means of self-organizing organic molecules, so-called “self-assembled monolayers”. The thickness depends on the material and the process, and sub-nanometer thicknesses are possible. This layer forms a kind of carpet on which the following layers can be built Due to the extremely thin layer thicknesses, surface structures are mapped with nanometers, which is important here in order not to 'round off' the probe geometry.

Another variation uses an already free-standing metal aperture and fills it with light-conducting material; surface chemical effects are used (surface tension of polar materials and capillary effects) to optimally fill the aperture)

Claims

claims

1. Light-transparent probe tip for targeted light emission and / or for receiving light for use in an optical analysis or processing system for examining and / or processing surfaces with an optical resolution in the sub-light wavelength range, which has a body consisting of a light-conducting material with a Surface covered with a light-absorbing layer which is interrupted by at least one passage opening through which light passes, characterized in that the passage opening through the layer is completely filled with the light-conducting material, and in that the light-conducting material is flush with the light-absorbing layer completes or towers over them.

2. Light-transparent probe tip according to claim 1, characterized in that a region of the layer surface immediately adjacent to the passage opening is flat.

3. Light-transparent probe tip according to claim 1 or 2, characterized in that the passage opening has a circular cross section with a diameter between 10 and 50 nm.

4. Light-transparent probe tip according to one of claims 1 to 3, characterized in that the light-absorbing layer has a layer thickness of about 10 - 30 nm.

5. Light-transparent probe tip according to one of claims 1 to 4, characterized in that the area just formed around the passage opening is at least one 100 nm wide.

6. Light-transparent probe tip according to one of claims 1 to 5, characterized in that the light-absorbing layer is designed as a metal film.

7. Light-transparent probe tip according to one of claims 1 to 6, characterized in that the passage opening has an opening angle of 90 ° for light passing through the passage opening.

8. Light-transparent probe tip according to one of claims 1 to 7, characterized in that the light-conducting body is frustoconical and provides a truncated cone surface in which at least one passage opening is provided through the light-absorbing layer.

9. Light transparent probe tip according to claim 8, characterized in that the passage opening is arranged centrally in the truncated cone surface.

10. Light-transparent probe tip according to one of claims 1 to 7, characterized in that the light-guiding body is semicircular or semicircular, and that the light-absorbing layer has at least one passage opening in the center of the semicircular or semicircular shape.

11. Light-transparent probe tip according to claim 10, characterized in that the curvature of the semicircular or semicircular shape runs flat at least in the region of the passage opening.

12. Light-transparent probe tip according to one of claims 1 to 7, characterized in that the light-transparent body has a flat surface, and that the light-absorbing layer provides a plurality of side openings, preferably arrayed, through openings.

13. Light-transparent probe tip according to one of claims 1 to 12, characterized in that the light-transparent body is attached to a flexible bending beam and thus images topographies of the surface like an AFM.

14. Light-transparent probe tip according to one of claims 1 to 13, characterized in that the light-conducting material in front of the passage opening has a larger lateral spatial extent than the clear width of the opening diameter of the passage opening.

15. A method for producing a light-transparent probe tip according to one of claims 1 to 14, characterized by the following method steps:

Forming a shape determining the spatial shape of the light-guiding body,

Depositing a light-absorbing layer on the mold surface,

Filling out the form with material that solidifies into a light-guiding body

and

Isolate the solidified light-guiding body with the light-absorbing layer from the mold.

16. The method according to claim 15, characterized in that the mold surface is produced by means of an etching process.

17. The method according to claim 15 or 16, characterized in that metal is deposited as a light-absorbing layer in a deposition process on the mold surface with a homogeneous distribution.

18. The method according to claim 17, characterized in that metal is applied in the course of a vapor deposition or sputtering process.

19. The method according to any one of claims 15 to 18, characterized in that the through openings are introduced by means of the etching method.

20. The method according to any one of claims 15 to 19, characterized in that an intermediate layer is introduced between the light-absorbing layer and the mold surface, which is removed from the mold after the insulation of the light-conducting body from the light-absorbing surface.

21. The method according to claim 20, characterized in that the intermediate layer is a layer of self-organizing organic molecules which deposit in the manner of a mono- or multilayer layer with a layer thickness of up to a few nanometers on the mold surface.

22. Use of the light-transparent probe tip in scanning near-field optical microscopy or photoresist lithography.