LOW-COST, SIMPLE MASS PRODUCTION OF LIGHT-GUIDING TIPS

Abstract: The present invention concerns light-guiding tips and a method of making the same. A method of manufacturing a light-guiding element with at least one tip having an aperture at its apex for light to pass through said aperture comprising the steps of: forming a tip-like indent in a substrate; depositing a light-guiding photoresist layer which fills the tip-like indent and covers at least a part of the substrate; and structuring the photoresist layer to form the light-guiding element out of the photoresist.

Description: LOW-COST, SIMPLE MASS PRODUCTION OF LIGHT-GUIDING TIPS TECHNICAL FIELD The present invention concerns light-guiding tips and a method of making the same. These tips are particularly suited for use in scanning probe systems, such as in the scanning near-field optical microscope (SNOM). BACKGROUND OF THE INVENTION A scanning near-field optical microscope (SNOM, also called NSOM) is an instrument which offers the possibility to measure optical sample properties with sub-wavelength resolution and surface topography simultaneously. In order to analyze a sample in the optical near-field, a tip must be brought into close proximity of the sample. The tip has an aperture of an optical fiber and is used to illuminate the sample. The end of a fiber is formed into a taper to provide a tip and the sides of the taper are coated with metal, the very end of the tip being uncoated. Conventional tips are fabricated by two basically different techniques: pulling under heating and chemical etching. The first may be done with a pulling-apparatus by heating and stretching to draw out an optical fiber or a glass-fiber into a fine tip, which means the fiber is pulled to a sharp end. For this an optical fiber is fitted between two ball-bearing sliders and pulled apart. If the sliders move simultaneously in opposite directions, impulses of a CO 2 laser beam focused by a single lens impinge and heat the optical fiber. The overall shape of the tip depends on the pulling and heating parameters. The other way of making a tapered tip uses chemical etching with hydrofluoric acid. The acid etching is stopped by an organic protecting layer. The etching of tips requires a considerable time per tip and a precise control of the process parameters. After having made a tip according to one of the described methods, a small aperture is fabricated by coating the tip with metal in such a way, that the light can get out only on a small spot. Metal coating of the tips is a self-aligning process. While the tips are rotating they are usually coated by an aluminum beam from the side. The methods are described, for example, in the German doctoral thesis by T. Lacoste, "Optische Rasternahfeld-Mikroskopie mit Polarisationskontrast", Inauguraldissertation, University of Basel, Switzerland, 1997, pp. 43 to 47. The tip is the most crucial part of a SNOM since the lateral spatial resolution achieved in scanning near-field optical microscopy depends critically on the size and geometry of the optical tip with aperture used as a source of light or as the collecting sensing element. Commonly used manufacturing methods of tips show some problems. Usually, the tips are manufactured by output per unit in the laboratory environment where they are needed. The produced tips in the different laboratories are seldom comparable with each other, have different fabrication parameters and the serial fabrication process is time consuming. The most important shortcomings are non-reproducibility of fabrication, difficulty of achieving sub 50 nm openings and low light transmission. With the pulling method the diameter of the light-guiding core of the tip is reduced compared to the diameter of the entire tip, which influences the light intensity which can be transmitted through the tip. Disadvantages of the etching method are the work with dangerous acids as well as the non-reproducibility of fabrication. After etching the surface, the tip will not be smooth enough to get a homogeneous metal coating. Every single tip has to be checked with an optical microscope, whereby the output of usable tips is often under 50 %. For measurements of polarization a higher quality is required, thus the output drops to 20 %. It is an object of the present invention to overcome the disadvantages of known approaches. It is another object of the present invention to provide a simple method for the fabrication of light-guiding tips. It is another object of the present invention to provide a method for the fabrication of identical light-guiding tips with reliable quality. It is a further object of the present invention to provide a method for cost efficient fabrication of light-guiding tips. It is a further object of the present invention to provide a method for cost efficient fabrication of light-guiding element arrays. It is still a further object of the present invention to provide a method for mass production of light-guiding tips and of light-guiding element arrays. SUMMARY OF THE INVENTION The present invention provides an efficient and simple method for fabricating light-guiding elements. The method, according to the present invention, comprises the steps of forming a tip-like indent in a substrate to create a master, depositing a light-guiding photoresist layer which fills the tip-like indent and covers at least a part of the substrate, and photolithographically structuring the photoresist layer to form said light-guiding element with at least one tip out of said photoresist. As outlined, the first step concerns the manufacturing of a master. This step is carried out just once. The master is reusable and a lot of light-guiding elements can be produced using the same master. Only the following steps have to be repeated. As mentioned above, a photoresist layer is spin-coated over the master. This first photoresist layer, e.g. being a negative photoresist, fills the tip-like indent and covers a part of the master. The photoresist layer is then exposed and developed. After that, another photoresist layer together with the first photoresist layer might be deposited, exposed and developed, e.g. to provide a coupling section for a light-guiding fiber at the light-guiding element. In the case of using a positive photoresist it should be taken into consideration that a positive photoresist after exposure and development is soluble and not hardbaked and therefore should be hardened (crosslinked). Finally, the light-guiding element is lifted-off from the master. To improve this step a sacrificial layer might be applied on the master. A coating with aperture might be placed between the sacrificial layer or the master and the photoresist layer. Likewise, the light-guiding elements might be delivered with the master to guarantee for a safe transport. The inventive method replaces optical-fiber-based tips and for the first time enables mass fabrication of light-guiding elements, e.g. as SNOM tips and 2D arrays, with identical tips which is not possible with the pulling method or the etching method. Since photoresist, such as the negative tone SU-8, material is available and the process sequence is shortened the cost of manufacturing of light-guiding elements can be kept low. With the described method no transfer steps are necessary. The method is simple and the fabrication throughput is very high, because the major throughput contributions are only the exposures and developments. Depending on the substrate and the size of the light-guiding element used in production, up to several thousand light-guiding elements could be fabricated on one substrate (master) with high yield and one or two exposures only. It is assumed to fabricate up to about 8000 light-guiding elements on one 4 inch wafer (substrate). The light-guiding element, according to the present invention, has at least one tip with an aperture at its apex for light to pass through the aperture, said light-guiding element comprising a light-guiding photoresist, such as SU-8. The light-guiding element has a coating essentially unpenetrable by light to prevent that light leaks out. The light-guiding element may have a coupling section for mounting a light-guiding fiber, preferably comprising an additional photoresist. This light-guiding fiber might be coupled directly to the light-guiding element, coupled using a glue or a connection technique to provide a flexible supply of light. Furthermore, light might be directly coupled into the light-guiding element, for instance by a laser which can be a vertical cavity surface emitting semiconductor laser (VCSEL), with an active medium consisting of one (or a few) quantum wells sandwiched between two distributed bragg reflectors. Making an array-type of light-guiding elements used in combination with multiple light-guiding fibers or even laser arrays may increase the field of view by keeping a higher resolution or density and provides a better quality of scanning. Also possible is a combination of light-guiding elements as usually used in SNOM systems with AFM cantilevers or other probe systems, i.e. for distance control or feedback. The use of a tripod-like scanning-probe for SNOM systems according to the present invention will adjust parallel scanning and improve obstacle detection. The present invention further relates to an apparatus for batch-manufacturing a light-guiding element with at least one tip which comprises means for depositing a photoresist layer on a substrate with indent defining a tip, means for aligning a mask, means for Uluminating the photoresist layer through at least one opening in the mask, and means for developing the photoresist layer to form said light-guiding element. It might further comprise means for depositing a second photoresist layer, means for aligning another mask, means for illuminating the second photoresist layer through at least one opening in the mask, and means for developing the second photoresist layer, thereby providing a coupling section. It should be taken into consideration that the photoresist material for the photoresist layers should either be a negative photoresist or that the applied photoresist should be crosslinked. The apparatus having means for coating the light-guiding element and means for aligning the light-guiding element and/or means for coupling said light-guiding element with a light-guiding fiber. DESCRIPTION OF THE DRAWINGS The invention is described in detail below with reference to the following schematic drawings. It is to be noted that the Figures are not drawn to scale. FIG. 1 is a cross-section through a substrate with an etched mold as a master, in accordance with the present invention. FIG. 2 is a cross-section through the substrate of the master of FIG. 1 with a structured photoresist layer, which covers the substrate and fills the etched mold of FIG. 1. FIG. 3 is a cross-section through the substrate of the same master with a second structured photoresist layer. FIG. 4 is a cross-section through a substrate with a light-guiding element, which is coupled to a fiber. FIG. 5 is a cross-section through an array-type of light-guiding element with a coupled fiber and a through-passing light beam. FIG. 6 is a cross-section through a light-guiding element with a through-passing laser beam. FIG. 7 is a partial cross-section through an array-type of light-guiding element with a through-passing laser beam emitted by a laser device. FIG. 8 is a schematic 3-dimensional view of an array-type of light-guiding element with fibers and mirrors. GENERAL DESCRIPTION Before embodiments of the present invention are described, the basic elements, in accordance with the present invention, are addressed. The basic principle of scanning probe microscopy (SPM) is the exploitation of a local interaction between a small sensor ("probe") and a sample. As these interactions are strongly distance-dependent, they yield a measure of the height of the sample at a given point. If the probe is moved (raster-scanned) over the sample, an image of the sample topography is collected in a computer which controls the experiment and takes the data. SPMs are widely used in the characterization of surfaces in physics, biology, chemistry, materials science and many other fields. Single atoms can be directly visualized in routine - but high resolution is not the only outstanding characteristics of SPM. The optical form of SPMs - the SNOM - tries to combine the variety of contrast mechanisms of optical microscopy (absorption, polarization, spectroscopy...) with the high resolution of SPM. Therefore, SNOM can overcome the resolution limit which makes it impossible for "classical" optical microscopes to discriminate two objects that are at a smaller distance than about 1/2 wavelength of the light used for illumination. Resolutions of better than 50 nm are proven with SNOM, and there should be no principal limit. Tip: A small sensor also called probe is formed to a tapered tip with an aperture at its apex and is usually used in SNOMs which belong to the family of SPMs. The desired optical tip for such a device is quite complicated. Its overall shape usually is sharply pointed, i.e. conical or pyramidal, and the small light source has to be positioned at the aperture of this structure within nanometer precision. In case of investigation a sample, the shape of the tip and the aperture is directly related to the resolution of the scanning system. There are two basic techniques to produce tapered tips from optical fibers. These conventional techniques called pulling and etching are described in the introductory portion. Photoresist: Photoresist SU-8 is an epoxy-based, transparent, light-guiding, negative tone UV resist system with excellent sensitivity designed specifically for ultra-thick, high aspect-ratio microsystems. The primary applications are microfabricated mechanical structures and micro electro-mechanical systems (MEMS)-types. Examples are sensors, actuators, microfluidic components and molds for electroplating. One of its numerous advantages is the broad range of thicknesses which can be obtained in one spin. With single layer coatings, thicknesses of more than 500 μm can be achieved. Thicker resist layers can be made by applying double coatings or multiple coatings. The resist can be exposed with a standard UV aligner and has an outstanding aspect-ratio, that means thickness to width up to 20. Since SU-8 is a negative tone resist, it allows for multiple exposure and development steps following each other to form 3-dimensional structures. This opens capabilities to form 3-dimensional structures by simple exposure and development steps. The photoresist is especially interesting as SU-8 has attractive mechanical properties. One of the key features of SU-8 is the low absorption in the visible range. Any other resist, negative tone or positive tone can be used instead. It should be taken into consideration that a positive tone needs to be crosslinked before the next layer is applied. A preferred embodiment is directed toward making a light-guiding element, as described with respect to FIGS. 1 to 3 and 6. Some further embodiments according to the present invention are described with respect to FIGS. 4, 5, 7 and 8. FIG. 1 shows a rectangular cross-sectional area through a substrate 10 with an etched mold, whereby the substrate 10, e.g. Silicon, is patterned and KOH-etched to form a tip-like indent 12, here having a <111> defined pyramidal shape to determine the tip of a light-guiding element to be formed. The overall shape is sharply pointed, that means that the shape to determine the tip of a light-guiding element can also be conical. Note that the tip-like indent can have any other suitable sharply pointed shape. Oxidation sharpening of the shape by thermal oxidation might be employed to achieve a much sharper tip-like indent 12. Any other approach to form sharp and deep shapes in the substrate can be used instead. The substrate 10 with tip-like indent 12 serves as reusable mold master 16. Referring now to FIG. 2, which shows a cross-section through the master 16 of FIG. 1 and a photoresist layer, further process details are described. The fabrication of a light-guiding element begins with an optional sacrificial layer. For sake of clarity, this layer is not shown in the figure. The master 16 is coated by a very thin (about 10 nm) sacrificial layer (e.g. Aluminum or Teflon) which can be removed chemically with good selectivity to photoresist SU-8. Aluminum can be removed by diluted KOH without attacking the photoresist SU-8. The master 16 is covered by an oxide from the oxidation sharpening and therefore not etched. After sacrificial-layer coating, a layer (not shown in the figure) for integrated aperture definition might be applied. After that, the master 16 is spin coated with a layer 20 of photoresist SU-8 whose thickness corresponds to the required thickness, preferably between 200 μm and 500 μm. The spin coating of photoresist SU-8 is very conformable, hence the shape forming a tip is completely filled-up by SU-8 resulting in a solid SU-8 tip 22 after lift-off. The layer 20 of photoresist SU-8 is exposed and perhaps developed by a mask (not shown in the figure) which is aligned to the shape of the mold master 16. Note that the developing step can also be done later. The FIG. 2 shows now a cross-section through the master 16 with a simple V-shaped SU-8 light-guiding element 24 with a flat attachment element 26 on the top of the master 16 which is defined by the structuring as aforementioned. The tip 22 of the SU-8-based light-guiding element 24 is situated in the middle of the master 16 and projects into the substrate 10. Additional details are outlined referring to FIG. 3, which shows a cross-section through the master 16 according to FIG. 1 and FIG. 2 and a second layer. A second optional photoresist SU-8 layer 30 is either directly formed on the layer 20 or if the layer 20 already is developed on the attachment element 26 and the substrate 10. Then the layer(s) is/are structured by exposing and developed and define now a large attachment or coupling section 32. This second photoresist SU-8 layer 30 is here much thicker than the first layer 20 of photoresist SU-8, preferably 1 mm. The hollow-cylinder shaped coupling section 32 can be achieved by other methods as well, it is variable in shape and thickness and is at last not absolutely necessary. The flat attachment element 26 of the first photoresist layer is thickened by the cylindrical shape of the coupling section 32 which is formed out of the second photoresist SU-8 layer 30. In this embodiment, the second photoresist SU-8 layer 30 is already removed and therefore being depicted as a dashed line. Both layers form the body of the light-guiding element 34 comprising tip 22 with attachment element 26 and coupling section 32. The coupling section 32 with a rectangular cross-section is used to couple light from an optical fiber or a laser into the tip 22 of the light-guiding element 34 or vis versa. Before using the light-guiding element it has to be lifted-off from the mold master 16 and should be coated by metal to prevent light losses, which will be described further in FIG. 6. Another approach is to apply a coating with aperture for the light-guiding element 34 on the substrate 10 before the photoresist is deposited, like described according to FIG. 2. FIG. 4 illustrates a cross-section through a substrate and a light-guiding element according to FIG. 3 with a fixed fiber. The light-guiding element 34, now consisting of a first and second photoresist SU-8 layer, is not removed from the master 16. The tip 22 of the SU-8-based light-guiding element 34 is situated in the middle of the master 16 and protrudes into the substrate 10. Exactly beyond the base of the tip 22 a light-guiding fiber 40, which can be a glass-fiber, is vertically coupled into the coupling section 32. The light-guiding fiber 40 may be fixed by glue 42 or other fixing methods. It might be advantageous to use a suitable glue between the end of fiber 40 and the coupling section 32 for good refractive index matching, as shown in FIG. 5. Through-passing light from the light-guiding fiber 40 can directly pass through the tip 22 of the light-guiding element 34 or vis versa without any light-losses. After lifting-off from the mold master 16 the light-guiding element 34 should be coated by metal to prevent light losses, which will be described further in FIG. 6. In FIG. 5 a cross-section through an array-type of light-guiding element 50 with a coupled fiber 52 and a through-passing light beam 54 is shown. Hence the fabrication process is the same for single and array-types of light-guiding elements, the inventive method is well suited for making light-guiding element arrays, as will be addressed in the following. An array with n light-guiding elements, each of which has at least one tip, can be made using a master with m tip-like indents. A substrate, e.g. Silicon, is patterned and etched to form m tip-like indents. Oxidation sharpening of the indents by thermal oxidation might be employed to achieve very sharp tip-like indents. The substrate with m tip-like indents serves as reusable mold master. The fabrication of an array with n light-guiding elements begins with an optional sacrificial layer. The master is coated by a very thin (about 10 nm) sacrificial layer which can be removed as outlined. After sacrificial-layer coating, the master is spin-coated with a layer of photoresist SU-8 whose thickness corresponds to the thickness of the desired light-guiding element to be formed. The m tip-like indents of the master are completely filled-up by SU-8. The layer of photoresist SU-8 is exposed by a suitable mask which has to be well aligned. After that, a second photoresist SU-8 layer can directly formed on the first layer, exposed and developed by another suitable mask to define a coupling section. This second photoresist SU-8 layer should be much thicker than the first layer of photoresist SU-8 to provide a useable coupling section. Finally, the whole array, consisting of at least one photoresist SU-8 layer is lifted-off from the mold master by chemical removal of the sacrificial layer and coated by a metal to prevent light losses. As shown in FIG. 5, the fiber 52 is coupled to an array-type of light-guiding element 50 by transparent or light-guiding glue 56 or optical cement. The light beam 54 passing vertically through the light-guiding fiber 52, the light-guiding glue 56 and the tip 58 of the light-guiding element 50. Three more fibers are mountable onto the light-guiding element 50. FIG. 6 illustrates a cross-section through a light-guiding element 60 with a through-passing laser beam 61. The light-guiding element 60 is made by using the master 16 and the described steps according to FIG. 1 to 3. As shown in FIG. 6, the whole part, now consisting of two photoresist SU-8 layers, is lifted-off (not shown in the figure) from the mold master 16 by chemical removal of the sacrificial layer. Other means of removal can be used instead, which include different adhesion layers for physical removal, pulling, or thermal expansion removal. A coating should be used, whereby the light-guiding element 60 particularly the tip 62 and the sides of the coupling section 63 might be coated. Metal coating of tips is a self-aligning process. This known technique was applied to micropipets before and later also to pulled optical fibers. Details are not shown in the figure. While the tips (tip 62 of light-guiding element 60 and may be many others) are rotating in a stream devise they are coated, i.e. by an aluminum beam from the side. This leaves a shadow at the apex 66 of the tip where an aperture 64 is formed. Any other suitable material can be used instead of aluminum. The light-guiding element 60 has a concave cavity 65 on the top of the coupling section 63. The coupling section 63 is used to incouple light from the laser beam 61 into the tip 62 of the light-guiding element 60 where the light at the apex 66 through the aperture 64 can be positioned with nanometer precision to a sample. It should be noted that the coupling section 63 and particularly the top portion of the coupling section 63 can have any suitable shape for in- or outcoupling light or supplementary means, i.e. a suitable lens to focus the light. Referring to FIG. 7, which shows a partial cross-section through a tip 70 and a coupling section 72 of a light-guiding element 74, an other embodiment is addressed. The light-guiding element 74 with a coupling section 72 is an array-type and has four tips for scanning, but every useful number of tips can be made and used instead. This light-guiding element 74 is fabricated by using the described steps according to FIG. 1 to 3 and 6. Only tip 70 and an adjacent part of coupling section 72 are depicted in cross-section where a vertical laser beam 76 from a laser device 78 is passing through the tip 70. The laser device 78 is situated on the top, which can be a VCSEL or any other light emitting device. Furthermore, the laser device 78 may include a micro-fabricated lens 79, called DOE (diffraction optical element), to focus light into the tip 70, which is indicated in cross-section on the bottom of the laser device 78. The coupling section 72 of the light-guiding element 74 is coupled onto the laser device 78 and is therefore directly situated under the laser device 78. FIG. 8 shows a further embodiment in a schematic 3-dimensional view. A light-guiding element 80 is fabricated by using the described steps according to FIG. 1 to 3 and 6. The array-type of light-guiding element 80 has two tips 81.1 and 81.2 on the bottom side of the light-guiding element 80. On the rectangular top side of the light-guiding element 80 which is formed as a coupling section 82, two ends of light-guiding fibers 83.1 and 83.2 rest one third on top of the coupling section 82. The light-guiding fibers 83.1 and 83.2 may be fixed in V-shaped grooves. The direction of a light beam 84 passing through fiber 83.1 is deflected by a mirror 85 directly into the tip 81.1. The mirror 85 and a further mirror 86 may be controlled to secure the suitable deflection angle. Light-guiding elements can also be batch-manufactured. An apparatus with the following means is applicable. The means for depositing a photoresist and/or a second photoresist layer might be nozzles to spray photoresist on a substrate or on a photoresist layer. A drive rotates the master. Masks can be applied and aligned automatically by a control system. Suitable lamps illuminate the photoresist layers through openings in these masks. Other nozzles spray developer on the photoresist layers. This is followed by a cleaning step with a cleaner. The light-guiding elements can be removed mechanically or by a solvent. Light-guiding fibers should be aligned automatically by a control system for coupling to the light-guiding elements. The light-guiding elements might be delivered in the master to protect the tips for damages during transport and storage.

Claims: CLALMS 1. A method of manufacturing a light-guiding element (24, 34, 50, 60, 72, 80) with at least one tip (22, 58, 62, 70, 81.1, 81.2) having an aperture (64) at its apex (66) for light to pass through said aperture (64) comprising the steps of: a) forming a tip-like indent (12) in a substrate (10); b) depositing a light-guiding photoresist layer (20) which fills said tip-like indent and covers at least a part of said substrate (10); and c) structuring said photoresist layer (20) to form said light-guiding element (24, 34, 50, 60, 72, 80) out of said photoresist. 2. The method according to claim 1, whereby a layer, preferably aluminum or teflon, is applied before depositing said photoresist layer (20) to improve a step of separating said light-guiding element from said substrate. 3. The method according to claim 1, whereby said tip-like (12) indent is formed by means of etching and/or sharpened by means of an oxidation technique. 4. The method according to claim 1, whereby said substrate (10) is a reusable master (16). 5. The method according to claim 1, whereby a second photoresist (30) layer is deposited and structured, thereby providing a coupling section (32, 63, 74, 82). 6. The method according to claim 1, whereby a coating essentially unpenetrable by light is applied before depositing said light-guiding photoresist layer (20) to provide said aperture (64). 7. A Hght-guiding element (24, 34, 50, 60, 72, 80) with at least one tip (22, 58, 62, 70, 81.1, 81.2) having an aperture (64) at its apex (66) for Hght to pass through said aperture (64), said Hght-guiding element (24, 34, 50, 60, 72, 80) comprising a Hght-guiding photoresist, such as SU-8. 8. The Hght-guiding element according to claim 7, whereby said Hght-guiding element (24, 34, 50, 60, 72, 80) has a coating essentiaUy unpenetrable by Hght. 9. The Hght-guiding element according to claim 7 or 8, whereby said Hght-guiding element has a coupling section (32, 63, 74, 82), preferably comprising an additional photoresist (30). 10. Apparatus for batch-manufacturing a Hght-guiding element (24, 34, 50, 60, 72, 80) with at least one tip (22, 58, 62, 70, 81.1, 81.2) having an aperture (64) at its apex (66) for light to pass through said aperture (64), said apparatus having: • means for depositing a photoresist layer on a substrate with indent defining said tip; • means for aligning mask; • means for Ruminating said photoresist layer through at least one opening in said mask; and • means for developing said photoresist layer to form said light-guiding element. 11. The apparatus of claim 10 having: • means for depositing a second photoresist layer; • means for aligning mask; • means for Ruminating said second photoresist layer through at least one opening in said mask; and • means for developing said second photoresist layer, thereby providing a coupling section. 12. The apparatus of claim 10 or 11 having means for coating the Hght-guiding element. 13. The apparatus of one of the claims 10 to 12 having means for aHgning the Hght-guiding element and/or means for coupling said Hght-guiding element with a Hght-guiding fiber. H= * *

Inventors: ANDREOLI CORNEL, BRUGGER JUERGEN P, DRECHSLER UTE, VETTIGER PETER, ANDREOLI, CORNEL, BRUGGER, JUERGEN, P., DRECHSLER, UTE, VETTIGER, PETER
Applicants: ZEISS CARL JENA GMBH, CARL ZEISS JENA GMBH
Application year: 1999
Main classification: G03F7
IPC codes: G01N13, G03F7, G12B21