Probe for near-field optical microscope, method for manufacturing the same and scanning near-field optical microscope

Abstract: A probe for near-field optical microscope in which a tube is thermally bent in the form of a hook (13); a microscopic aperture (12) of the tube which has been thermally extended and cut away is provided at an end thereof; a part of the tube opposite to the microscopic aperture is removed to form an aperture (14) at the removed portion to thereby provide a structure having no obstacle in the space between said aperture and said microscopic aperture. The invention also provides a near-field optical microscope utilising the probe.

Description: The present invention relates to a scanning near-field optical microscope for observing microscopic features and optical characteristics of the surface of a sample. In general, conventional near-field optical microscopes controlled by an atomic force have employed a probe which is obtained by sharpening an end of an optical propagation element such as an optical fiber, coating regions other than the end with a metal film and forming a microscopic aperture at the end portion. While an optical fiber is primarily used as the optical propagation element, an optical fiber can not be used for observation at wavelengths in the ultraviolet region or infrared region which is important in the evaluation of physical properties because it allows propagation of beams having wavelengths generally about 350 to 1600 nm when used as a base material. Under such circumstances, a probe has been proposed which has a hole that penetrates from an end of a probe made of silicon or silicon nitride for an atomic force microscope through a surface opposite thereto. In this case, since there is no solid substance that absorbs light in the hole connected to the aperture and the opposite surface, it is regarded usable for observation at wavelengths in the ultraviolet region and infrared region. Processes for manufacturing the probe of this type, which is a probe for an atomic force microscope formed with a hole, include a method wherein the hole is formed by etching and a method wherein the hole is formed by focused ion beams. According to the process utilizing etching, the hole is formed when etching proceeds to penetrate through, and etching must be stopped at a certain point in time. It is very difficult to perform etching while monitoring the instance of the formation of such a microscopic aperture and, consequently, it is quite difficult to control the size of the aperture formed. On the other hand, the method wherein the hole is formed using focused ion beams has a problem in that it results in low productivity and high manufacturing cost and another problem in that a positional shift attributable to a drift of the focused ion beam apparatus results in the formation of a hole in a location other than an end of a probe. In this regard, in order to spread near-field observation techniques in the ultraviolet region and infrared region, it is indispensable to provide a probe for a near-field optical microscope having good controllability of the formation of a microscopic aperture. One method for achieving this is a method reported by Lewis et al. and Shalom et al. utilizing a microscopic aperture at an end of a tube which has been thermally extended and cut away (U.S. Pat. 4,917,462 (1990); Rev. Sci. Instrum. 63 (1992) 4061). In this case, STM control or shear force control is employed to control the distance between a probe and a sample. STM control has a problem in that a sample must be conductive. In the case of shear force control, although it can be used on a sample which is not conductive by oscillating the probe in the horizontal direction relative to the sample, it does not allow simultaneous observation of information on the physical properties of the surface of a sample which is possible with AFM control. Here, the information on physical properties implies friction, visco-elasticity, surface potential and the like which can be detected by controlling a probe with a force in the vertical direction relative to a sample. Further, the shear force control method has a problem in that it occupies a greater space on the upper surface of a sample than in the AFM control method. The report of Shalom et al. discloses a tube probe which is bent to be usable as an AFM probe. In this case, however, since light can not be preferably propagated at the bent portion of the tube, it is difficult to provide light in an amount sufficient for measurement from a hole at the end thereof. In order to solve the above-described problems, the inventors have conceived a probe for a near-field optical microscope as defined in claim 1. The use of a glass tube as the above-described tube makes it possible to form a microscopic aperture and to manufacture a probe easily. In this case, when light having a wavelength transmitted by glass is used, it is possible to prevent leakage of light because at least the outside of the tapered portion is coated with a material such as a metal which blocks an electromagnetic wave. Further, when an optical lever method is used for control over the distance between a sample and the probe based on atomic force control, more stable detection can be achieved by forming a mirror surface on the surface of the tube opposite to the aperture. Furthermore, observation in the ultraviolet region and infrared region can be carried out by configuring a scanning near-field optical microscope with at least a light source, a collecting optical system, means for relative movement between a probe and a sample, an optical detector, and the above-described probe for a near-field optical microscope. This apparatus may have a configuration wherein collected light is introduced to said hole from the side of said probe opposite to the aperture and a configuration wherein detection light from the side of said probe opposite to the aperture is collected by an optical system. Embodiments of the invention will now be described by way of further example only and with reference to the accompanying drawings, in which:- Figs. 1A, 1B and 1C are schematic views showing a structure of a probe for a near-field optical microscope according to the present invention; Figs. 2A, 2B, 2C and 2D are views showing steps for manufacturing a probe for a near-field optical microscope according to the present invention; Figs. 3A, 3B, 3C, 3D and 3E are schematic views showing configurations of near-field optical microscopes according to the present invention; Figs. 4A and 4B are views showing examples of means for detecting the displacement of a probe for a near-field optical microscope according to the present invention. Figs. 5A, 5B, 5C, 5D, 5E and 5F are views showing steps for manufacturing a probe for a near-field optical microscope according to the present invention. Embodiments of the present invention will now be described in detail using the drawings. Figs. 1A, 1B and 1C schematically show a probe for a near-field optical microscope according to the present invention. In Fig. 1A, the probe is a tube which has been thermally extended and cut away and has a tapered portion 11 and a microscopic aperture 12, which-portion is configured as an end portion of a thermally bent hook-shaped portion 13 and has a structure wherein a part of the tube opposite to the aperture of the bent portion is removed until the hollow of the tube is reached such that no obstacle exists in the space between an aperture 14 thus formed and said aperture 12 at the end. Fig. 1B shows a sectional view and shows an example in which a metal coating film 16 is formed around the tube 15. While the metal coating film is formed on the outer surface of the tapered portion in Fig. 1B, a film such as a metal coating which blocks an electromagnetic wave may be formed on the inner surface of the tapered portion as shown in Fig. 1C. This is effective when infrared light is used which is subjected to significant absorption by glass. Next, Figs. 2A, 2B, 2C and 2D show an example of steps for fabricating a probe according to the present invention. In Fig. 2A, a part of a glass tube 20 is heated by, for example, laser beams 21 to apply tension 22 across the tube, thereby sharpening an end of the glass tube and forming a tapered configuration from the end. At this time, a microscopic aperture 12 that reflects the shape of the tube is formed at the end and, as is already known, the controllability of the diameter of the aperture is quite good. Means usable for heating at this time include methods such as irradiation with carbon dioxide gas laser, heating with a coil heater and heating with a burner. Next, as shown in Fig. 2B, carbon dioxide gas laser beams 21 are applied to a part of a cut away tube 23 which is about 0.1 to 1 mm apart from an end 24 on a side thereof in one direction. As a result, the one side is heated more significantly than the other side and, when softening begins, a bent configuration can be formed on the heated side by surface tension. Subsequently, as shown in Fig. 2C, a part of the tube at the bent portion opposite to the aperture 12 at the bent portion 25 is removed until the hollow in the tube is reached. This step of removal can be carried out using a grinding plate 26. As an alternative method, focused ion beams may be used. Thereafter, as shown in Fig. 2D, when the optical lever method is used to control the distance between the sample and probe based on atomic force control, grinding is performed to form a flat portion 15 to serve as a mirror on the surface of the tube opposite to the aperture. Further, when light having a wavelength transmitted by glass is used, the outside of the tapered portion is coated with a material such as a metal which blocks an electromagnetic wave. Vacuum deposition, sputtering, CVD or electroless plating may be used to form the metal film. When the coating film is formed on the inner surface, electroless plating or CVD may be used. A second example of steps for fabricating a probe according to the present invention is shown in Figs. 5A, 5B, 5C, 5D, 5E and 5F. In Fig. 5A, a part of a glass tube 20 is heated by, for example, laser beams 21 to apply tension 22 across the tube, thereby sharpening an end of the tube and forming a tapered configuration from the end. Next, as shown in Fig. 5B, carbon dioxide gas laser beams 21 are applied to a part of a cut away tube 23 which is about 0.1 to 1 mm apart from an end 24 thereof on a side thereof in one direction. As a result, the one side is heated more significantly than the other side and, when softening begins, a bent configuration can be formed on the heated side by surface tension. When thet optical lever method is used to control the distance between the sample and probe based on atomic force control, as shown in Fig. 5C, grinding is performed to form a flat portion 15 to serve as a mirror on the surface of the tube opposite to the aperture. Thereafter, the tube is coated with a protective thin film 60 made of gold or the like which is not corroded by an etchant such as hydrogen fluoride water (Fig. 5D). Subsequently, as shown in Fig. 5E, the coated portion of a part of the tube opposite to the aperture 12 is removed at the bent portion 25, and the removal proceeds short of the hollow of the tube. The step of removal may be carried out using a grinding plate 26. As an alternative method, focused ion beams may be used. Thereafter, the portion from which the coating has been removed is removed by etching using an etchant such as hydrogen fluoride water to perform etching until it penetrates to the hollow. At this time, as shown in Fig. 5F, only the portion to be etched may be put in contact with the hydrogen fluoride water in a liquid having two phases of hydrogen fluoride water 61 and an organic solvent 62 to prevent other portions from being corroded accidentally. When gold is used as the-protective thin film, it also functions as a material for blocking an electromagnetic wave. In order to improve the adhesion of gold, a thin film of titanium, chromium or the like may be deposited between the glass and gold. While grinding particles can enter the microscopic aperture at the step of removing the part of the tube opposite to the aperture 12 at the bent portion 25 until it penetrates in the first example of steps, the second example of steps is advantageous in that this problem does not occur, although the number of steps is increased. Referring specifically to the size of the probe, for example, a glass tube having an outer diameter of 210 micron and an inner diameter of 127 micron may be used. An optical fiber having an outer diameter of 125 micron may be inserted in the same. When such a glass tube having an outer diameter of 210 micron is used, it will provide a resonance frequency of about 5 to 20 kHz when fixed at a distance of 1 to 2 mm from an end thereof. Alternatively, for example, a glass tube having an outer diameter of about 120 micron and an inner diameter of about 75 micron may be used. The size of the microscopic aperture may be processed into a size of about 50 nm to 1 micron when a glass tube is used. The metal film as a material for blocking an electromagnetic wave must have a thickness of about 100 to 250 nm. A configuration of a scanning near-field optical microscope utilizing this probe will now be described. In Fig. 3A, a probe 31 according to the present invention is provided with the microscopic aperture 12 located close to the surface of a sample plate 32; the aperture 14 formed by grinding is located in a direction perpendicular to the surface of the sample from the microscopic aperture; and a primary spring element of the probe 31 is provided perpendicularly to the surface of the sample. A light source 33 and an optical system consisting of a lens 34, a mirror 35 and the like are provided above the aperture 14 to collect and introduce light into said aperture 14. As a result, the surface of the sample is irradiated with light from the microscopic aperture 12. In the example of Fig. 3A, a collecting optical system 36 is provided under the sample plate 32 to collect light transmitted through the sample, and an optical detector 37 detects the intensity of the light. In this example, the distance between the end of the probe and the surface of the sample is controlled using the optical lever method in which a beam is projected on the mirror surface 15 from a light source 38 and the displacement of reflected light is detected by a divided optical detector 39 as the displacement of the probe. The sample plate 32 on a sample table 41 can be moved in X-, Y- and Z-directions by means 40 for relative movement between the probe and sample, and a controller 42 performs scanning on the XY plane and automatically performs control over the distance in the Z-direction along the configuration of the surface of the sample. The acquisition of the detected optical signal is carried out simultaneously with the control of the distance. As the means for relative movement, a micrometer or stepping motor is used as a coarse movement mechanism and a piezoelectric element is used as a fine movement mechanism. Further, when the probe is controlled using oscillation, a piezoelectric diaphragm 44 may be provided between probe fixing means 43 and the probe 31, and an AC power source 45 may be used to oscillate the probe 31. Next, Fig. 3B is a view showing a part of an apparatus as a variation of the scanning near-field optical microscope according to the present invention. This example has a configuration in which when the surface of the sample plate 32 is irradiated from the microscopic aperture 12, light in the direction of reflection from the surface is collected by the collecting optical system 36. Fig. 3C shows a configuration in which the surface of a sample is directly irradiated with light, and light which has entered the probe through the microscopic aperture 12 is collected by the collecting optical system 36 through the aperture 14. Fig. 3D shows a configuration in which the surface of the sample plate 32 is irradiated with light from the microscopic aperture 12 through the aperture 14 of the probe and in which light from the surface of the sample is collected by the collecting optical system 36 through the microscopic aperture 12 and aperture 14 to detect light transmitted through a half mirror or dichroic mirror 46. Fig. 3E shows a configuration in which light from the light source 33 is caused to impinge upon the lower surface of the sample plate 32 directly upward or at an angle which results in total reflection at the surface of the sample using an optical component 47 such as a prism and in which light from the sample surface which has passed through the aperture 14 through the microscopic aperture 12 is collected. As the collecting optical system used in the above examples of configuration, not only a lens or the like formed of glass or crystal but also a reflection type lens (Cassegrain lens) may be used. Therefore, the above-described system may be used in the ultraviolet region or infrared region taking advantage of a probe according to the present invention. Further, while the optical lever method is used in the above examples as a method for detecting the displacement of a probe, other methods for detecting displacement may be used. For example, as shown in Fig. 4A, displacement may be detected also by collecting light from the light source 38 with a lens 51, projecting the same on the probe 31 on a side thereof and detecting interference light with the divided optical detector 39. Further, as shown in Fig. 4B, the displacement of the probe may be detected by providing a piezoelectric element 52 for the detection of displacement separately from the piezoelectric diaphragm 44 and monitoring changes in electrical characteristics of the piezoelectric element. An apparatus having the above-described configuration makes it possible to carry out observation of optical resolution of optical information in the ultraviolet region and infrared region and observation of images of features at the same time. The use of a detection element, modulation circuit, lock-in amplifier or the like depending on each measurement makes it possible to observe physical properties such as friction, visco-elasticity and surface potential at the same time. For example, the measurement of friction requires a four-division optical detector for detecting torsional components of the probe. The probe for a near-field optical microscope and the near-field optical microscope according to the present invention have made it possible to form microscopic apertures with high reproducibility to allow practical use of the same in the art of the near-field optical microscopes for performing observation of optical resolution of optical information in the ultraviolet region and infrared region, observation of images of features and observation of physical properties such as friction, visco-elasticity and surface potential at the same time.

Claims: A probe for a near-field optical microscope, said probe comprising a thermally extended and cut away hollow tube, the tube having a thermally bent portion in the form of a hook (13), a tapered portion (11) and a microscopic aperture (12) at the end of the tube, the tapered portion (11) being configured as an end portion of said hook-shaped portion (13), the probe having a structure wherein a part of the tube opposite to the microscopic aperture (12) is removed to form an aperture (14) at the removed portion and wherein there is no obstacle in the space between said aperture (14) and said microscopic aperture (12). A probe for a near-field optical microscope according to Claim 1, wherein said tube is a glass tube. A probe for a near-field optical microscope according to Claim 1, wherein at least the outside of said tapered portion is coated with a material which blocks an electromagnetic wave. A probe for a near-field optical microscope according to Claim 1, wherein at least the inside of said tapered portion is coated with a material which blocks an electromagnetic wave. A probe for a near-field optical microscope according to Claim 1, wherein a mirror is provided on the surface of the tube opposite to said microscopic aperture. A scanning near-field optical microscope comprising at least: a light source (33); an optical system; a probe for a near-field optical microscope; means for relative movement between the probe and a sample; an optical detector (37); and wherein said probe is configured according to claim 1. A scanning near-field optical microscope according to Claim 6, which has a configuration; wherein light from said light source is applied to a sample through said microscopic aperture and through the aperture at said removed portion. A scanning near-field optical microscope according to Claim 6, which has a configuration wherein light from the surface of a sample is introduced to said detector through said microscopic aperture and through the aperture at said removed portion. A method for manufacturing a probe for a near-field optical microscope comprising at least the steps of: sharpening an end of a hollow glass tube (20) and forming a tapered configuration at said end and forming a first aperture (12) at said end; forming a bent configuration of the tube; forming a further aperture in the tube by removing a part of the tube in the bent portion until the hollow in the tube is reached; and coating the outside of the tapered portion with a material which blocks an electromagnetic wave. A method for manufacturing a probe for a near-field optical microscope according to Claim 9; wherein it includes a step of forming a flat portion to serve as a mirror on the surface of the tube in the bent portion between said step of forming a bent configuration and said step of coating the outside of the tapered portion with a material which blocks an electromagnetic wave. A method for manufacturing a probe for a near-field optical microscope comprising at least the steps of: sharpening an end of a hollow glass tube (20) and forming a tapered configuration at said end and forming a first aperture (12) at said end; forming a bent configuration of the tube; coating the tube with an anti-etching thin film; removing the coated portion of a part of the tube at the bent portion and proceeding with removing it up to a position short of the hollow in the tube; and performing etching with an etchant such that the etching removes the portion from which the coating has been removed and penetrates to the hollow, thereby forming a further aperture in the tube. A method for manufacturing a probe for a near-field optical microscope according to Claim 11; wherein it includes a step of forming a flat portion to serve as a mirror on the surface of the tube between said step of forming a bent configuration: and said step of coating the tube with an anti-etching thin film. A method for manufacturing a probe for a near-field optical microscope according to Claim 9 or 11, wherein it includes a step of forming a coating film on an inner surface.

Inventors: MURAMATSU HIROSHI, MURAMATSU, HIROSHI
Applicants: SEIKO INSTR INC, SEIKO INSTRUMENTS INC.
Application year: 1999
Main classification: G02B21
IPC codes: G01B21, G01B11, G01N37, G01Q70, G01Q60