JP3267992B2 - Near-field scanning optical microscope and its use - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION The present invention relates to a method for positioning a small aperture, typically less than one optical wavelength, in the optical near-field of a sample, ie, within approximately one optical wavelength of the sample. It relates to an optical microscope that is scanned in a raster fashion through the surface of the sample to generate a time-varying optical signal, and the time-varying optical signal is detected and reproduced, producing an image with very high resolution. The invention further relates to a method of using this microscope to inspect a workpiece during a manufacturing process.

[0002]

BACKGROUND OF THE INVENTION Many researchers are investigating the use of optical scanning to overcome the inherent shortcomings of conventional optical imaging systems. In other words, so-called near-field scanning optics
In (l microscopy, NSOM), an aperture having a diameter smaller than one optical wavelength is located close to and scanned through the surface of the sample. In some schemes, the sample is illuminated reflectively or transmissively by an external source. Some of the reflected or transmitted light is collected by the aperture and relayed to a photodetector, for example, via an optical fiber. In another scheme, light is relayed to the aperture by an optical fiber, which itself acts as a miniature light source for reflective or transmissive illumination of the sample. In this case, conventional means are used to collect and detect the selected or transmitted light. In any case, the detected optical signal is reproduced to obtain image information.

For example, on August 5, 1986, W.M.
D. United States Patent No. 4,60 issued to Pohl
No. 4,520 describes an NSOM system that uses a probe made from a pyramidal optically transparent crystal, wherein an opaque metal coating is applied to the crystal. At the top of the crystal, both the crystal tip and the metal coating on this chip are essentially square 1
It is removed to form an opening having a side having a length of 00 nm or less. Another aperture described in US Pat. No. 4,604,520 is made from single mode optical fiber. One flat end of the fiber is metallized and a coaxial hole is formed in the coating to expose only the fiber core.

[0004] On April 17, 1990, A. L. U.S. Pat. No. 4,917, issued to Lewis et al.
In a somewhat different approach, described in 462, the probe is formed from a pipette. That is,
A glass tube is drawn into a thin chip and coated with an opaque metal layer. After drawing, the pipette leaves a hollow hole, which appears at the tip through both the glass and the upper metal layer. The resulting metal ring defines this opening. This opening is smaller than the hole defined in the glass as a result of the radial inward growth of the metal layer. One disadvantage of the above described method is that light is transmitted through the probe with relatively low efficiency. This results in a relatively low signal level. In some cases, the aperture must be increased to compensate for low signal levels. This measure is not desired, as it results in reduced resolution. For example, if light is sent from a source to an aperture through an uncoated pipette, the optical field will have a rather large amplitude at the outer wall of the pipette. To confine the radiation, it is necessary to coat the walls with metal. However, attenuation occurs as a result of absorption in the metal coating. In addition, metal coats have a tendency to create defects that allow optical leakage, for example, pinholes. Increasing the thickness to counter this trend,
The length (ie, the radial thickness of the pipette) and the outer diameter of the metal ring that defines the opening also increase. As a result, optical loss due to absorption and extinction in the metal ring increases, and the chip size also increases. As the tip becomes larger, it becomes more difficult to scan the sample's concave morphological features while maintaining close proximity to the surface of the sample. (Importantly, the problem of too large a chip size due to metal deposition is a constant diameter fiber optic probe of the type with an opening defined by a hole in the metal layer coating the end of the fiber. Also applies.)

[0005]

SUMMARY OF THE INVENTION The probe (for example,
Problems that occur with single-mode fibers having a sharpened conical tip and no metal coating are described, for example, in C.I. Girard and M.W. Applied option by Spajer
Politics (Applied Optics), 29 (1990 ), a modem for the post of paper to page 3726-3733 "reflecting near-field optical microscopy (Model for reflection near
field optical microscopy)]. One problem is that a portion of the light passing through the fiber towards the tip is reflected by the conical tapered side and then transmitted therethrough. A second problem is that the side of the taper captures unwanted optical signals that can propagate through the fiber, thereby increasing the noise level at the detector. From the above explanation, to this day, efficient transmission of light (that is,
It can be seen that NSOM probes have been unsuccessfully provided with a relatively small tip diameter, high resolution, and high reliability combined with relatively little attenuation due to optical interaction with the probe wall.

[0006]

SUMMARY OF THE INVENTION In one aspect, the present invention relates to an optical system, which includes a probe, at least a portion of which is optically transparent at least at certain wavelengths. The probe also has one distal end. The optical system further has an optical aperture at the distal end, the aperture having a diameter smaller than the certain wavelength. The optical system further includes means for optically coupling the light source to and from the light source so that at least some light emitted by the light source enters or exits the probe through the aperture at least at the certain wavelength. And means for positioning the probe with respect to the target. As a feature of the present invention, (a) the probe comprises a portion of a single mode optical fiber having one core and a cladding, the cladding having an outer surface, and the fiber being associated with a guided dielectric mode; (B) the fiber has an adiabatic tapered region, at least a portion of which has the ability to guide at least the light of the certain wavelength; Terminating in a substantially flat termination plane oriented in a substantially perpendicular plane;
(D) the outer cladding surface in the tapered region is substantially smooth; (e) at least a portion of the outer cladding surface is coated with metal as a barrier material in the tapered region, thereby guiding a metal mode. A capable metal waveguide portion is defined, and a cutoff diameter for the metal mode is further defined, and (f) the diameter of the termination surface is less than or equal to the cutoff diameter.

[0007]

DETAILED DESCRIPTION In one embodiment, the invention relates to an optical system. As shown in FIG. 1, the optical system includes a light source 10, a probe 20, and a displacement means 30. The moving means 30
The probe is relatively moved, for example, in relation to a target 40 arranged on a stage 50 adjacent to a probe tip 60. The optical system further comprises a light source 1
A means for optically coupling the probe to the probe. In the example shown in FIG. 1, this optical coupling is provided by a single mode optical fiber 70 extending between the light source 10 and the probe 20. (Fiber 70
Is actually integrated with the probe 20. ) Light source 1
0 is a laser as an example. Light from the light source 10 is transmitted into the optical fiber, for example, by a microscope objective lens 90.
And via a single mode coupler 80 including the fiber positioning device 100. Also optionally, a mode stripper 110 is used to ensure that only a single mode in the core propagates to the probe and no other modes in the cladding propagate. As the moving means 30, for example, a piezoelectric tube designed to move the probe in the vertical direction and in two orthogonal lateral directions is used. Alternatively, the moving means may be a mechanical or piezoelectric moving means for moving the stage in place of the probe, or using both the moving of the stage and the moving of the probe.

One possible use of an optical system as described is direct writing. That is, the sample surface located close to the probe tip is coated with a photosensitive layer that can be exposed to light emitted from the light source. An exposure pattern is created by moving the probe into the photosensitive layer relative to the sample while simultaneously or continuously or intermittently shining light from the light source from the probe tip. A second possible application of the optical system as described is to image the sample in a so-called "illumination" mode. In this application, light from the probe tip is passed through the sample and collected by a microscope objective under the stage (as shown in FIG. 1). (What is shown is an illumination pass mode, but an illumination reflection mode is also possible.)
The light thus collected is directed to a detector 130, which uses, as an example, an optical multiplexer tube. It is necessary to use a beam splitter 140 so that the probe can be visually positioned, but this will cause some of the collected light to pass through the eyepiece 15.
Turn to zero. Importantly, if the sample is scanned by a raster movement of the probe, the signal from the detector 130 can be reconstructed to produce an image of the scanned sample portion.

Such a scanning method is employed in near-field scanning optical microscopy (NSOM), in which the probe tip is within a very small distance from the sample surface, typically It is located within a distance of one wavelength or less of the light emitted by the light source. NSOM
Also provides very high optical resolution by using very small, typically less than one wavelength, apertures in the probe tip. NSOM devices are well known in the art and are described, for example, in W.S. D. U.S. Pat. No. 4,604,520 issued to Pohl and U.S. Pat. U.S. Patent No. 4, issued to Lewis et al.
917,462.

Further, a third possible use of the described optical system is shown in FIG. In the configuration of FIG. 2, the probe tip functions as a collector rather than as an emitter of light. This arrangement is useful for NSOM imaging in a so-called "collection" mode. (What is shown is the collection reflection mode, but the collection pass mode can be realized as well.) Light from the light source 10 is
It is directed to an annular objective lens 180 via 60 and an inclined annular mirror 170. Lens 180 focuses this light on the sample surface. The light reflected or illuminated from the surface is collected by the probe tip and passed through fiber 70 and objective lens 120 to detector 130
Turned to Reflective mode NSOM is described, for example, in U.S. Pat. No. 4,917,462, referenced above.

A detector (or, more generally, a converter) 130 converts the light thus detected into an electrical signal. These signals can be used to easily generate a two-dimensional image on a video display device such as a cathode ray tube. For such purposes, the scan generator controls the movement of the probe position relative to the target,
It is also used to provide a reference signal for composing a display image. The electrical signal generated by converter 130 is typically an analog signal. These are selectively converted to digital signals before being displayed. In such a case, a digital memory is optionally provided for storing the digitized signals and a digital processor is provided for processing the digitized signals before displaying them (e.g. To be improved).

One possible type of probe 20 is a single mode optical fiber. Fiber optic probes have in fact been disclosed in the prior art. Figures 3 and 4 show examples of such fiber probes. FIG. 3 shows an uncapped single mode optical fiber 190 with an annular cap of opaque material, eg, a metal, deposited to cover the chip. The aperture 210 at the center of the ring defines the optical aperture of the probe. FIG. 4 shows a bare optical fiber 220 tapered, for example, by chemical etching. We have discovered that the improved probe 230 can be easily manufactured by heating a single mode optical fiber to soften and drawing the softened fiber to form a tapered fiber.
After drawing, at least a portion of the tapered fiber is coated with an opaque material, such as a metal, as a barrier material. As shown in FIG. 5, the fiber tip 240 thus drawn is tapered at an angle β and terminates in a termination flat 250.

The optical fiber has a cladding 260 and a core 270. Although the specific cladding and core composition is not critical to the invention, an exemplary cladding composition is silica glass, and an exemplary core composition is doped silica glass having a higher refractive index than the cladding. is there. Although the specific cladding and core dimensions are not critical to the invention, an exemplary core diameter is about 3
um, and the outer diameter of the cladding as an example is about 125 u.
m. One guide is the mode, that is, basic or HE ₁₁ mode is associated with a single-mode fiber corresponding non-tapered. Such a mode is characteristic of a cylindrical dielectric waveguide and for this reason is hereinafter referred to as a dielectric mode.

When the optical fiber is heated and drawn,
Both the core diameter and the cladding outer diameter are reduced. A fractional change in core diameter is approximately equal to a fractional change in cladding outer diameter. (In other words, the cross section of the fiber is
It changes only in that proportion. Importantly, the angle at which the core tapers is significantly less than β. For example, in a linearly tapered fiber having the dimensions shown as an example, the tangent (tan) of the taper angle of the core is only about 3/125 times the taper angle β, or 2.4%. . For this reason, even for relatively large values of β, for example, 30 degrees or more, the core has an adiabatic taper as described below.

For an untapered fiber, the electric field in the dielectric mode is almost confined to the core, which is very small amplitude near the outer surface of the cladding, typically less than 10 ^{-10 of} the peak amplitude. Fall to the amplitude. This is not necessarily the case for tapered fibers. As the guided light wave propagates into the tapered region, it encounters an increasingly narrow core. Eventually, the core will be too small to substantially constrain the guided mode. The light waves are guided by the interface between the cladding and the surrounding material, for example air or a metal such as aluminum, rather than the core. As explained above, the core is generally tapered at an angle small enough for this mode to be adiabatic. The adiabatic stays all initial HE ₁₁ mode energy is concentrated substantially single mode, other modes, in particular, means not coupled to the radiation mode causing optical loss by radiation.

At first, the guide mode that escapes from the core is H
Quite keep the characteristics of the E ₁₁ mode. More specifically,
The amplitude of the guided electromagnetic field is relatively small at the outer surface of the cladding. However, as the fiber diameter further decreases, the amplitude of the electromagnetic field at the outer surface of the cladding increases relative to the peak amplitude of the fiber. Eventually, this field will have a relatively large amplitude at the outer surface of the cladding. Such modes can be guided, for example, by the interface between the cladding and the surrounding air. However, such a configuration is not desirable.

Since the electric field extends far outside the cladding, a significant amount of optical leakage is expected. this is,
This is undesirable because it reduces the efficiency with which light is channeled to and from the probe tip. In addition, light leaking from the cladding walls can eventually reach the portion of the sample located relatively far from the probe tip,
The result is unintentional exposure of the sample, or an increase in background level at the detector. further,
If the outer wall of the cladding is only surrounded by air, stray light enters the fiber, again resulting in a detector (eg, when a probe is used to collect light from the sample). The background level at the point will be increased.

For the reasons mentioned above, it is desirable to coat at least the end portion of the taper, where optical leakage therefrom accounts for a large proportion, with an opaque material, for example a metal such as aluminum. Importantly, such a mode guided in a metallized portion has properties typical of a mode guided in a metal rather than a dielectric waveguide. Thus, for example, the initial HE ₁₁ mode is converted to the TE ₁₁ metal mode as it approaches and enters this metallized region. Importantly, in waveguides with adiabatic tapered cores, the HE ₁₁ mode has a relatively high efficiency (typically 10% or more) of TE _11.
Can be combined with mode. If the fiber is bare, but guided mode has a large amplitude at the outer surface of the fiber, however, where a portion of the fiber keep considerable HE ₁₁ characteristics is referred to as a "transition zone". In view of the above description, it is apparent that it is desirable (but not absolutely necessary) to metal coat the fiber through this transition region and from this transition region to the far end of the fiber. In this context, if there is a significant amount of optical energy in the radiation mode,
Relatively thick metal coatings are required to substantially eliminate the possibility of metal penetration by electromagnetic fields in view of the potential for pinhole leakage and the finite conductivity of the actual coating. This prevents the thickness of this coating from very close proximity of the probe tip to the sample, and the very thick metal coating can be grainy, which may even help prevent pinhole leakage from occurring. The tendency is undesirable for both reasons. In contrast, here, a relatively small amount of energy is coupled into the radiation mode, and therefore a relatively thin metal coating, typically 750-1500 °, preferably
A coating of about 1250 ° or less is considered sufficient.

In general, the guided metal mode is initially a propagation mode. However, as the fiber diameter decreases, this guided mode is gradually converted to a vanishing mode that exhibits relatively strong attenuation in the direction of propagation. Such a transition is now referred to as "cut-off diameter (cu
toff diameter)).
This cut-off diameter is defined as the transition from the propagation mode to the evanescent mode of any tapered waveguide, outside the cladding where the waveguide is infinitely coated with conductive metal. Is the diameter. Normal,
Against TE ₁₁ mode, the cut-off diameter approximately equal to half of the wavelength guided. This part of the fiber that extends from the cut-off diameter to the probe tip is here called the "evanescent region"
Called.

For example, the cut-off diameter of the TE ₁₁ mode in a circular cylindrical waveguide is described, for example, in J. Mol. D. By Jackson, "Classical Electron Dynamics (Cl
assical Electrodynamics), 2nd edition, John Wiley and Sons, Inc.
), New York, published 1975, page 356, and can be easily predicted from the metal waveguide discussion. This is equal to the quantity χ / k ₀ n. Where k ₀ is the free space wave number of the guided light, n is the index of refraction of the waveguide (in relation to the guided mode and wavelength), and χ is associated with the particular mode to be guided. Is the amount to do. For the TE ₁₁ mode, χ is equal to 1.841. The value of 対 応 corresponding to the other guided modes is readily known to those skilled in the art.

In this context, the conversion from the fundamental dielectric mode to the TE ₁₁ metal mode is incomplete. A finite portion of the energy of the dielectric mode is coupled to metal modes other than the TE ₁₁ metal mode. However, the TE ₁₁ mode generally suffers less attenuation in the vanishing region than any other metal mode. For this reason, light waves traversing the disappearance region comprises substantially only TE ₁₁ metal mode. (In addition, due to the finite conductivity of the actual metal coating, a disturbed TE ₁₁ mode is expected.
A mode in which the electric field has a small longitudinal component is expected. )

In the vanishing region, large attenuation occurs, so that this region needs to be made as short as possible. However, in applications where other factors described below are different, different erasure lengths are required.

The cladding outer surface of the tapered portion of the fiber, to reduce the scattering of light from the HE ₁₁ mode in the vicinity of the cladding outer surface, a relatively thin (preferably, about 1500Å thick or less) metal Substantially smooth is required to receive the coating and to substantially eliminate defects that cause leakage of optical radiation. In this case, the cladding surface is a scanning electron microscope (SE
If no surface texture appears at a scale of about 50 ° or more when observed in M), it is considered substantially smooth. A surface having such a desired smoothness can be easily manufactured by heating and drawing the fiber.

The terminating flat is required to be substantially flat and substantially perpendicular to the axial direction of the fiber. A termination flat is considered flat here if inspection by the SEM indicates that any lateral feature area does not deviate from the flatness by more than about 100 ° through the surface of the termination flat.

The edge of the terminal flat is preferably defined relatively sharp. An edge is considered here to be sharply defined if the average curvature at the edge as determined by SEM is less than about 100 °.
A terminal flat having such a desired flatness and such a desired sharpness can also be easily manufactured by heating and drawing the fiber.

One method of metallizing the end portion of the fiber (preferably, including at least the transition region, as described above) uses, for example, aluminum as a deposition source. As shown in FIG. 6, the fiber 230 has a source 29 at the end during the deposition of aluminum.
Not aimed at zero. The probe end of the fiber is oriented away from the source so that the termination flat is shadowed with respect to the direction of incidence of the deposited metal. Typically,
The fiber axis is tilted to have an angle θ of about 75 degrees with respect to a line drawn from the source toward the fiber tip. During deposition, the fiber is rotated about its own axis such that all sides of the fiber are uniformly coated. When such a method is employed, a coating is created that smoothly covers the outer surface of the cladding, but leaves the terminal flat substantially free of metal. As shown in FIG. 7, the method described above is effective for manufacturing a probe in which the optical aperture 300 corresponds to the entire surface of the termination flat. Because the spatial resolution of the probe is primarily determined by the diameter of the aperture, for high resolution applications it is desirable to have a very small diameter of the termination flat. For this reason, the diameter of the terminal flat in such a probe is generally made smaller than the cutoff diameter. For example, the cutoff diameter for 5145 ° light from an argon ion laser is typically about 2000 ° for fibers having the exemplary dimensions described above. A typical corresponding end flat diameter is about 5
Small termination flats, ranging from 00 to 1000 ° and about 200 ° or less, can be easily manufactured by heating and drawing the fiber.

Generally, the average thickness of the aluminum layer is:
Preferably, it should not be less than about 750 °.
This is because much thinner layers are prone to excessive optical leakage. (If the probe is used both to illuminate the target and to collect light from the target, even smaller thicknesses are possible.) The thickness should preferably not exceed about 1500 °. This is to minimize the overall diameter of the probe tip and to make the coating as smooth as possible. This total diameter is the sum of the diameter of the terminal flat and the thickness of the metal that restrains the terminal flat at diametrically opposite locations. This total diameter should be as small as possible to provide a probe tip that can be inserted into a relatively narrow cavity or breach in the sample surface, or more generally, can penetrate deeper into the near field of the sample. It is desirable to do.

Importantly, probes of the type described above have a vanishing area that extends from the cutoff diameter to a terminal flat. This vanishing area is considerably longer compared to the alternative designs described below. For example, for a cutoff diameter of 2000 °, a terminal flat diameter of 500 °, and a taper angle of 15 degrees, this vanishing length would be 5600 °.
With such long distances, a considerable amount of attenuation occurs. In view of this attenuation, the solution is to increase the taper angle as much as possible in order to minimize the disappearance length. However, if you make the taper angle smaller,
It must also be taken into account that the penetration into the cracks is facilitated and that the stepped surface can be easily approached.

In one preferred method for tapering the fiber, the fiber is first loaded into a commercially available pipette puller and the fiber is heated before and during drawing. An exemplary heat source is a carbon dioxide laser. Controllable parameters include the intensity of the incident light (which determines the heating rate), the pulling force applied to the end of the fiber, and the number of individual pulling steps. Generally, the fiber is ultimately broken by a "hard pull", a pulling step performed by suddenly increasing the force. The speed of the fiber end at the moment the strong pull is applied can be controlled as well as the time delay between the cessation of heating and the application of the strong pull.

We have discovered that by using any given set of parameters, probes with highly reproducible properties can be produced. The taper angle can be easily increased by a selection or combination of slowing the heating rate, weakening the pulling force, pulling in multiple steps, or reducing the extent of the area to be heated. The diameter of the terminal flat can be easily increased by increasing the heating rate and / or increasing the pulling force.
In addition to this, using a fiber with a higher temperature glass composition results in a larger taper angle and a smaller tip. The required combination of process parameters will be apparent to those of ordinary skill in the art after a short review.

Another type of probe is shown in FIG.
In this type of probe, a metal coating is applied over the termination flat as well as on the outer surface of the cladding. As described above, the thickness of the metal on the outer surface of the cladding is preferably about 750-1500 °. The metal layer on the termination flat is preferably thick enough to eliminate stray light, but is required to be as thin as possible to reduce the extinction length, so that it has a thickness of about 250-500 °. To be. In this case, the optical aperture 300 does not cover the entire terminal flat, but only a portion of the terminal flat. The metal layer on the termination flat is formed in an annular shape with an opening defining an optical opening. Importantly, this opening can be centered in the termination flat or offset from the center of the flat. Non-central openings
Required when multiple modes are expected in the chip area. In such cases, a non-centered aperture provides more efficient out-coupling because not all modes are efficiently coupled to the centrally-located aperture. More specifically, the opening is not located in the center is expected to provide optimal optical coupling with respect to TE ₁₁ mode.

This central aperture is formed, for example, by first providing a fiber with a buried glass rod with a faster etch rate than the fiber's host glass. (By way of example, borosilicate glass embedded in quartz glass is used.) After the end of the fiber is drawn, it is exposed to a chemical etchant before the fiber is metallized. This etchant forms a cavity in the termination flat. This cavity is shadowed during the metallization of the termination flat, resulting in an opening in the metal layer.

A typical opening formed by this method has a diameter of about 200-2000 °, but the same method can be used to form larger or smaller openings.

The metal coating is deposited, by way of example, from the evaporation sources described above in connection with the probe having a bare tip. However, two separate deposition steps are used. That is, in the first step, the outer surface of the cladding is coated, as described above, and in the second step, the orientation of the fiber is changed and the termination flat is coated.

In this type of probe, the diameter of the termination flat is usually not required to be smaller than the cutoff diameter, even in high resolution applications, since the aperture is not determined by the diameter of the termination flat. Therefore, in order to minimize damping, it is desirable that the diameter of the terminal flat be generally at least equal to the cutoff diameter. However, as explained above, it is desirable that the overall diameter of the probe tip be as small as possible so that it can penetrate the fracture. For this reason,
The diameter of the terminal flat is preferably not much larger than the cut-off diameter.

In this context, the diameter of the aperture is usually smaller than the diameter of the cut-off, so that the central aperture in the metal layer over the termination flat defines a metal waveguide in which the optical field disappears. Care must be taken to achieve this. However, since the coating typically has a thickness of about 500 ° or less, this extinction length is much smaller than in the previously described probe with a bare tip. Obviously, low attenuation requires the metal coating on the termination flat to be as thin as possible. An acceptable range for the thickness of the metal on the termination flat is about 250-500 °, typically 500 °, as discussed above.

We have calculated for the ideal power decay expected within the missing part of the bare-tip probe. The corresponding power transmission coefficient is conveniently expressed using the parameters of the mode guided as described above, χ, the taper angle β, and the dimensionless parameters defined by the following equation: Can be. α = χ / nk ₀ a where a is the diameter of the aperture as described above, k ₀ is the free-space wavenumber of the guided light, and n is the appropriate value of the waveguide index. It is. It is useful to know that α is equal to the ratio of the cut-off diameter to the aperture diameter. When expressed in decibels, the power transmission coefficient corresponding to ideal extinction attenuation is given by:

(Equation 1) We have found that the overall measured power transmission coefficient is greater than T _ev by 10
Probes with varying values of α in the range between 1 and 10 that do not fall by more than dB and varying values of β in the range between 5 and 20 degrees have been successfully manufactured. at least,
For α in the range between 2 and 8, T _ev is approximately -2
It can be approximated by χα / tan βdB. Thus, we have successfully manufactured a probe whose total measured power transmission coefficient, denoted by T, satisfies: T (db)> − (2χα / tan β) −10

The optical system according to the invention is, inter alia,
Effective as a manufacturing tool. For example, the probe can be easily used to perform actinic radiation from a light source to a small area on the surface of a workpiece located adjacent to the optical aperture of the probe. For example, the surface of the workpiece is coated with a photosensitive layer, for example, a photoresist,
While the pattern in the layer moves the probe relative to the probe, light exits the aperture through the probe and pours over the layer, thereby creating the layer by exposing it. Additional steps are performed, whereby, for example, an industrial product is completed. An industrial product as an example is a semiconductor integrated circuit,
These additional steps include developing the photoresist and exposing the resist-coated surface to an etchant to pattern a layer under the photoresist. Importantly, not only semiconductor wafers, but also glass plates used for photolithographic masks can be easily patterned in this way.

In another process, actinic radiation from the probe is directed onto the surface of the workpiece,
This deposits material on this surface. For example,
The surface is exposed to an organometallic vapor or solution. Actinic radiation impinging on this surface causes one or more components of the vapor or solution to promote local decomposition, for example, producing a metal residue that adheres to the surface.

The optical system according to the present invention is also effective as an inspection device on a production line. For example, ninth
As shown, in the manufacture of semiconductor integrated circuits, a semiconductor wafer having a surface to be patterned at one or more stages of the manufacturing process is often provided (Step A). The pattern formed in this way generally has a characteristic dimension, usually called "line width", which is required to be kept within certain narrow tolerances. The optical system according to the invention measures the line width, for example the width of a metal conductor on a wafer, or the length of a gate formed in a metal-oxide semiconductor (MOS) structure on a wafer (step D). Easy to use. These line widths are compared with desired values (step E). Process parameters, such as lithographic exposure time or etching time, are initially set (step B), but are adjusted (step H) so that they bring the measured dimensions within the desired tolerances. Next, an additional step to complete the product (Step G) is performed. The measuring step includes positioning the probe adjacent to the patterned surface (Step I), irradiating light thereon (Step J),
And detecting light from the surface by making the light source and the detector optically coupled via the probe according to the invention (step K) in substantially the same manner as described above. It is.

The optical system according to the present invention is also useful for examining bit patterns in magnetic digital storage media, for example, magnetic disks. Magnetic storage media typically include a bit pattern characterized by the direction of the modulated magnetization to indicate Faraday rotation of the polarization.
For example, it can be easily visualized by inspection using a cross polarizer. Thus, for example, the optical system according to the invention uses a light source for polarization and a polarizing filter in front of the detector, so that the reflection mode
Can be used for imaging. In a manufacturing process that involves impressing such media with a bit pattern having predetermined characteristics, the relevant process parameters are adjusted so that the detected bit pattern matches the desired pattern.

In at least some cases, the laser light source is itself a source of polarized light. In some other cases, it is desirable to pass light from the source through the linear polarizing film. Before the light is coupled into the optical fiber 70 (see FIG. 1), it is typically passed through a half-wave plate and then through a quarter-wave plate. The orientation of the plates can be easily adjusted to compensate for birefringence in the fiber. (Although not necessary, a polarization preserving fiber can be used.) The linear polarization component of the light exiting the fiber can be, for example, half-wave and quarter-wave while detecting this through a second linear polarization film. Visually optimized by adjusting the wave plate.

Manufacturing and Inspection of Optical System According to the Invention
To those of ordinary skill in other applications for
It can be done easily. . For example, according to the present invention,
Optical systems also provide digital data for storage.
For imprinting on optical or magnetic storage media
Or store the data stored in this way
It is also effective for reading from the body. For example, data
Of surrounding film part on magnetizable metal film
Spots with local magnetization in a direction different from the direction of magnetization
It is well known that you can record in the form of patterns
You. This data storage technology, for example,MRS Publication 15
(MRS Bulletin15), April 1990, page 20-
R.24 J. Arguments Posted by Gambino
Statement "Optical Storage Disk Tech
nology)], andMRS Publication 15(MRS Bulletin 15
 ), April 1990, pages 31-39. J.
A. M. Greidanus and W.C. B. Gee
A paper published by Zeper, “Magneto-optical storage material.
(Magneto-Optical Storage Materials)]
Has been stated.

A typical magnetic storage medium is a layer composed of an amorphous alloy of one or more rare earths and one or more transition metals. (Alternative magnetic storage materials include cobalt platinum or cobalt palladium multilayers and magnetic oxide materials such as ferrite and garnet.) For example, spots representing bits of digital data may Exposed to a magnetic field,
The spot is written by optically heating it above the Curie temperature or the compensation point of the medium. (In some cases, local reversal of magnetization is possible only by applying an internal demagnetizing field to the medium, without the need to apply an external magnetic field.) Such spots are customary Has a diameter of 1 um. However, with the optical system according to the present invention, smaller spots, for example very small spots of about 0.2-0.5 um and even less than 0.06 um, can easily be created. Such small spots can also be read by the optical system of the present invention.

In one presently preferred embodiment, the magnetic storage medium is a thin amorphous film of an alloy comprising at least one rare earth and at least one transition metal. One such alloy is terbium-iron. The aperture of the near-field probe is located within about one illumination wavelength from the surface of the medium. (If a larger spot is required, the probe can be easily placed at one or more wavelengths from the medium.)
The medium is selected to provide sufficient heating. For example,
The terbium-iron film can be easily heated by a dye laser pumped by a YAG laser and emitting approximately nanosecond pulses at about 600 nm. Light from a laser is directed through an optical fiber to a probe according to the invention and is directed from a probe tip onto a storage medium. A typical local temperature change required to write a spot is about 150 degrees Celsius. Writing can be accomplished using either continuous or pulsed lasers for illumination, but pulsed lasers are preferred because they reduce the average power requirements of the laser and heat relatively small spots.

Importantly, the invention encompasses not only magnetic film storage media, but also other media that can be written to by an illumination source. As such a medium, for example,
Includes a polycrystalline film (eg, a film of indium antimonide doped with terium), which is locally heated to a temperature above the melting point of the film, eg, by a laser pulse, and rapidly cooled to an amorphous state. You. This cooling rate can be controlled, for example, by appropriately shaping the time dependence of the laser pulse.

A spot representing the stored data is typically written to a track in the storage medium. This track extends, for example, circumferentially on a rotating disk. The diameter of a spot written according to the invention is typically much smaller than the width of such a track. Thus, advantageously, a plurality of such spots are written in a band extending transversely across the track. One advantage of the present invention is that these spots in such a band can be read out simultaneously by the near-field probe of the linear array according to the present invention.

In one presently preferred method for reading an impressed pattern on a magnetic film storage medium, linearly polarized light illuminated by a probe according to the present invention is transmitted through this storage medium, Is used to collect a portion of the light thus transmitted, which is polarimetrically analyzed. For readout, the preferred wavelength is the wavelength that has the greatest optical response (i.e., its rotation in the direction of polarization of the light as it traverses the magnetic medium). For transition metal rare earth media,
Such wavelengths are typically in the near infrared or visible spectrum.

In another embodiment, the probe is used both to direct polarized light to the surface of the medium and to collect the portion of the light reflected from the surface. Passed through a magnetized spot in the medium,
Or as a result of reflection from it, typically
A polarization rotation of about 0.5 degrees occurs. As a result of this rotation,
The intensity of the light transmitted through the analyzer and subsequently detected is modulated. This modulation is decoded, and the information recorded in the medium is reproduced. As we all know, such information
For example, it may be recorded sound, image, text, or digital data.

For example, in another embodiment involving a phase change, the modulation is typically achieved by a change in reflectance rather than a rotation of the polarization. This change can also be easily detected by collecting the reflected light using the probe according to the invention.

The microscope according to the invention is also useful for imaging applications in biological research and clinical medicine. More specifically, the microscope according to the present invention overcomes the problem of low signal levels, at least in some cases, which often impairs the effectiveness of prior art NSOM systems for medical and biological imaging.
Thus, for example, a microscope according to the present invention can be used to image a sectioned sample of biological tissue and to discover and identify physical pathologies within that tissue. In a similar manner, the microscope according to the invention can easily be used to examine the distribution of materials which are detectable by their unique appearance or fluorescence in sectioned tissues, as well as, for example, those which are labeled with fluorescent dyes. Can be used for

The microscope according to the invention is also useful in genetic pathology and research applications for imaging chromosomes when they are, for example, in metaphase. More specifically, chromosomes or parts of chromosomes labeled with a fluorescent substance can be easily identified using the microscope according to the present invention. Generally, methods of fluorescent labeling, including the process of reacting cellular nuclear material with a phosphorus-containing material, are well known in the art and need not be described in detail here.

Imaging by fluorescence can be performed by irradiating the microscope according to the present invention with an irradiation mode (FIG. 1) or a light collecting mode (FIG. 2).
It can be easily achieved by using any of the modes. In the former, electromagnetic radiation having the ability to encourage the sample to emit fluorescence is applied from the probe to the sample, and the resulting fluorescence is collected, for example, by a conventional microscope objective. In the latter, the excitation radiation is applied to the sample according to conventional methods, and the fluorescence is collected by the probe.

As described above, the preferred probe 20 (FIG. 1)
Is manufactured by tapering a single mode optical fiber. In at least some cases, useful probes can also be made by tapering and coating multimode fibers. More or fewer modes are guided by the dimensions (for the guided wavelength) of the multimode fiber. In general, the smaller the number of guided modes (ie, the closer this fiber is to a single mode fiber), the greater the S / N ratio achieved by any aperture.

As also mentioned earlier, a metal such as aluminum is used as an exemplary opaque coating for the probe. More generally, suitable coatings are coatings composed of a material whose guided radiation has a low penetration. Aluminum is
One preferred material is when the radiation is, for example, in the visible spectrum, except for semiconductors such as silicon where, for example, infrared radiation is to be guided.

The following is an example. 3-um single mode fiber (FS-
VS-2211) is the Sutter Instrument (Su)
tter Instruments). The fiber was heated and drawn from a 25 watt carbon dioxide laser at a 25 watt 3 mm spot in a P-87 micropipette puller. Micropipette puller with hard pull at 75 settings (range 0-255), 4 settings (range 0-255)
"Velocity at pull",
And a time delay of 1 (range 0-255). 12 degree taper angle, 670 °
Chip properties with a flat end diameter and a value for α of about 3 were obtained. One end of the fiber was placed in a rotor and vapor deposited with about 1260 ° of aluminum at a base pressure of about 10 ^-6 Torr. The tapered end was then mounted in a piezoelectric tube in the optical device of FIG. One milliwatt of 514.5 nm light from an argon ion laser was coupled into the fiber.
The optical power output at the fiber tip is about 1.1
Nanowatts, which is approximately -60 d
B corresponds to the total power transmission coefficient. When used to form an image of the sample surface, this probe provided a spatial resolution of about 25 nm.

[Brief description of the drawings]

FIG. 1 is a schematic diagram of an exemplary optical system useful for a near-field scanning optical microscope.

FIG. 2 is a schematic diagram of another example optical system useful for a near-field scanning optical microscope.

FIG. 3 is a schematic diagram of a fiber optic probe according to the prior art.

FIG. 4 is a schematic diagram of a fiber optic probe according to the prior art.

FIG. 5 illustrates an optical fiber according to one embodiment of the present invention.
4 is a schematic diagram of a probe.

FIG. 6 illustrates an exemplary method for metallizing an optical fiber probe.

FIG. 7 is a schematic diagram of an optical aperture according to one embodiment of the present invention.

FIG. 8 is a schematic diagram of another optical aperture.

FIG. 9 is a flowchart illustrating a manufacturing process according to one embodiment of the present invention.

──────────────────────────────────────────────────の Continued on the front page (51) Int.Cl. ⁷ Identification symbol FI H01S 3/00 G12B 1/00 601C (72) Inventor Ernst Michael Georgy United States 07940 New Jersey, Madison, Woodcliff Drive 6 (72 Inventor Francis Hermann United States 92130 California, San Diego, Maritime Place 13014 (72) Inventor Jay Kenneth Troutman United States 07921 New Jersey, Bedminster, Morgan Court 13 (72) Inventor Raymond Wolf United States 07974 New Jer See, New Providence, Walker Drive 21 (56) Reference US Patent 4,917,462 (US, A) WO 90/4753 ( O, A1) R. C. Reddick, R.A. J. Warmack, T.W. L. Ferrel, "New form of scanning optical microscopy", PHYSICAL REVIEW B / CONDENSED MATTER, United States, The American Physical, January 1, 1989, January 1, 1989. 39, No. 1, pp. 767-770 E.P. Betzig, M .; Isaacs on, A .; Lewis, “Collection mode near-field scanning optical microscopy”, Applied Physics Letters, United States, American Institute of Physics, 1987, 1987. 59, No. 10, p. 2088-2090 U.S.A. Durig, D.C. W. Phol, F.C. Rohner, "Near-Field optical-scanning microscopy", Journal of APPLIED PHY SICS, USA, American Institute of Physics, March 15, 1986, Vol. 59, No. 10, pp. 3318-3327 A. Harootunican, E. Betzig, M.E. Isaacson, A. Lewis, "Super-resolution solution near-field scanning optical microscopic", Applied Physics Letters, American Institute of Technology, September 15, 1986, Applied Physics Letters, United States. 49, no. 11, pp. 674 −676 (58) Fields investigated (Int.Cl. ⁷ , DB name) G01N 13/10-13/24 G12B 21/00-21/24 G02B 6/00 G02B 6/10 G02B 21/00 H01S 3 / 00 JICST file (JOIS)

Claims (27)

(57) [Claims] 1. An optical system, comprising:
A probe having at least a portion optically transparent at least at a predetermined wavelength and having a single distal end; an optic provided within the distal end and having a diameter smaller than the predetermined wavelength. An aperture; and means for positioning the probe with respect to a target, comprising: a) the probe comprises a portion of an optical fiber having a core and a cladding, the cladding having an outer surface; B) the fiber has an adiabatic tapered region, wherein at least a portion of the tapered region transmits at least the predetermined wavelength of light. C) the tapered region terminates in a substantially flat termination plane oriented in a plane substantially perpendicular to the fiber; d) in the tapered region. E) the cladding outer surface is substantially smooth; and e) at least a portion of the cladding outer surface in the tapered region has a relatively low penetration for the predetermined wavelength of electromagnetic radiation. An optical system characterized in that it defines a metal waveguide portion that can be applied to the substrate to guide the metal mode. 2. A means for optically coupling the light source to the probe such that at least some light emitted by the light source enters or exits the probe through the aperture at least at the predetermined wavelength. The optical system according to claim 1, further comprising: 3. A source of electromagnetic radiation as a light source; a scan generator for driving a moving means such that the probe is moved into a raster pattern adjacent to a part of the surface of the sample; Conversion means for detecting at least a portion of the light from the incoming or outgoing light source and generating an electrical signal in response to the detected light; and at least a portion of the probe and at least a portion of the target. A far-field microscope means for viewing; and a video signal having a signal receiving relationship with the transducer for displaying a two-dimensional image relating to the amount of light detected at least at some displacement of the probe relative to the sample. 3. The optical system of claim 2, further comprising display means, wherein said displacement is part of a raster pattern. 4. The conversion means for generating an analog electrical signal, wherein the system further converts the analog electrical signal to a digital signal and transmits the digital signal to the video display means. 4. The optical system of claim 3, including means. 5. The optical system according to claim 4, further comprising storage means for digitally recording at least a part of said digital signal. 6. A digital signal processing means for digitally processing said digital signal, said digital processing means being in receiving relation to said analog to digital converting means and transmitting to said video display means. 6. The optical system of claim 5, wherein said optical system is in a relationship. 7. An electromagnetic radiation source capable of stimulating at least a portion of the target to emit fluorescence, wherein the radiation source is coupled to the target such that radiation is transmitted from the source through the probe to the target. 2. The method of claim 1, further comprising means for optically coupling to the probe; and means for detecting at least a portion of the fluorescence emitted by the target.
Optical system. 8. An electromagnetic radiation source capable of targeting and stimulating to emit fluorescence at at least the predetermined wavelength;
The target and the radiation source such that at least a portion of the radiation emitted by the source strikes the target and at least a portion of the fluorescent light emitted by the target is incident on the probe through the optical aperture. The optical system of claim 1, further comprising means for supporting the probe. 9. The optical system of claim 1, wherein said termination surface has a diameter less than or equal to a cutoff diameter for said metal mode. 10. The fiber of claim 1 wherein said termination surface is substantially uncoated with a barrier material; said fibers have a cladding outer diameter at individual axial locations; 10. The device of claim 9, wherein the cladding outer diameter is less than the cut-off diameter, at least in part, and the opening substantially coincides with the termination surface.
Optical system. 11. The probe of claim 11, wherein the diameter of the termination surface is approximately equal to the cutoff diameter; the probe further includes an annular layer of barrier material coated on the termination surface, the annular layer surrounding the opening, thereby The optical system of claim 9, wherein an aperture is defined. 12. The ratio of the cutoff diameter to the aperture diameter is represented by α; the metal mode is a TE ₁₁ mode; the taper angle is represented by β; And about 8; and when the probe is expressed in decibels, the following relationship, T>-
The optical system according to claim 9, wherein the optical system has a transmission coefficient represented by T that satisfies (3.68α / tan β) -10. 13. The optical system of claim 11, wherein said opening has one center, said terminal surface has one center, and the center of said opening is offset from the center of said terminal surface. . 14. A method for examining a sample, the method comprising: producing an enlarged image of at least a portion of the sample; and visually examining the image, wherein the image producing step comprises: A probe is positioned adjacent to the first surface of the sample and at a distance from the first surface no more than a predetermined wavelength, directs electromagnetic radiation onto the sample, and emits electromagnetic radiation from the first surface. Collecting radiation and detecting at least a portion of the collected radiation, wherein radiation applied on the sample is emitted from the probe and has at least the predetermined wavelength;
Alternatively, the collected radiation is collected by the probe and has at least the predetermined wavelength: a) the probe comprises at least a portion of an optical fiber having a core and a cladding, wherein the cladding has one outer surface; Associated with the fiber at least one guide dielectric mode for radiation of a predetermined wavelength; b) the fiber has an adiabatic tapered region, at least a portion of the tapered region being at least the predetermined C) the tapered region terminates in a substantially flat termination surface oriented in a plane substantially perpendicular to the fiber;
An opening is provided in the terminal surface; d) the outer cladding surface in the tapered region is substantially smooth; and e) at least a portion of the outer cladding surface in the tapered region is the predetermined outer surface. A method characterized in that it is coated with a blocking material having a relatively low penetration of electromagnetic radiation of a wavelength, thereby defining a metal waveguide part capable of guiding a metal mode. 15. The sample having a second surface, wherein applying electromagnetic radiation to the sample comprises applying radiation to the second surface, wherein the radiation emitted by the first surface comprises applying the radiation to the sample. 15. The method of claim 14, wherein the radiation is transmitted through. 16. The method of claim 14, wherein the step of directing radiation comprises directing radiation to the first surface, and the step of collecting comprises the step of collecting reflected radiation. 17. The step of directing electromagnetic radiation comprises directing radiation on the first surface, wherein the applied radiation has at least one wavelength of radiation capable of exciting fluorescence in at least a portion of the sample. 15. The method of claim 14, comprising: collecting the fluorescence. 18. A method for patterning a surface, comprising:
The method includes: a) providing a workpiece having a surface to be patterned; and b) at least a portion of light illuminated at least one predetermined wavelength on the surface from an optical aperture. Projecting or reflecting or illuminating so as to be transmitted therethrough, the step of illuminating further comprising: c) placing a probe having an end surface and an optical aperture provided in the end surface adjacent the surface. And wherein the distance between the aperture and the surface is at most about one predetermined wavelength; and d) the probe comprises a portion of an optical fiber including a core and a cladding. Wherein said cladding has one outer surface and said fiber is associated with at least one guided dielectric mode for radiation of said predetermined wavelength; A tapered region, wherein at least a portion of the tapered region is capable of guiding at least light of the predetermined wavelength; f) the tapered region is substantially Terminating in a substantially flat termination surface oriented in a substantially vertical plane, wherein the opening is defined in the termination surface; g) the cladding outer surface in the tapered region is substantially smooth; And h) at least a portion of the outer cladding surface in the tapered region is coated with a barrier material having a relatively low penetration for the predetermined wavelength of electromagnetic radiation, thereby guiding a metallic mode. A method wherein a possible metal waveguide portion is defined. 19. The surface to be patterned comprises a recording medium onto which a bit pattern can be impressed; in the step of applying radiation, light is applied from the optical aperture onto the medium. Applying the light causes a local physical change in the medium such that the optical signal is modulated by the physical change, the modulation carrying at least one bit of information. 19. The method of claim 18, wherein: 20. The medium, wherein the medium is a magnetic medium and the step of directing the radiation heats the medium above its Curie temperature, or above its compensation point, resulting in the formation of a spot and the local magnetization of the medium. 20. The method of claim 19, wherein the magnetization of the surrounding adjacent region is different. 21. The method of claim 20, wherein the step of applying radiation is performed in the presence of an external magnetic field.
the method of. 22. The method of claim 20, wherein said medium is a magnetic alloy comprising at least one rare earth and at least one transition metal. 23. A spot that is substantially polycrystalline, wherein a portion of the medium is rapidly cooled after being locally heated as a result of the step of directing radiation, and is locally substantially amorphous. 20. The method of claim 19, wherein is formed. 24. The step of providing the workpiece comprises: providing a plurality of semiconductor wafers, each wafer having a surface to be patterned; and the method further comprises: a) at least one process Setting parameters; b) processing at least one first wafer according to the processing parameters such that a pattern having a particular dimension is formed on a surface of the wafer; c) on the first wafer. Measuring the specific dimension; d) comparing the specific dimension to a value in a predetermined range; e) if the specific dimension is outside the value in the predetermined range, changing the processing parameter to the predetermined range. F) modifying at least one second wafer according to the processing parameters after step e). G) performing at least one additional step to complete an object with respect to the at least second wafer; and h) applying the radiation comprises: Positioning a probe adjacent to the patterned surface of the first wafer and directing light onto the surface such that at least a portion of the light is reflected from the surface; 19. The method of claim 18, wherein a portion of the light projected from or reflected by the probe is collected by the probe, and further comprising detecting at least a portion of the reflected light. 25. The step of providing the workpiece further comprises providing a plurality of substrates each having a surface layer of magnetic material to be impressed in a bit pattern; the method further comprises: a) at least one Setting two processing parameters; b) processing at least a first substrate such that a bit pattern is formed in a surface of the substrate according to the processing parameters; c) at least the bits of the first substrate Detecting a pattern; d) comparing the detected bit pattern with a predetermined bit pattern; e) if the detected bit pattern is different from the predetermined bit pattern, changing the processing parameter. Changing the detected bit pattern to match the predetermined bit pattern; f. A) processing at least a second substrate according to the processing parameters after step e); and g) performing at least one additional step to complete an object with respect to the at least second substrate. H) directing the radiation comprises, during the detecting step, positioning a probe adjacent to the patterned surface of the first wafer and disposing light on the surface at least from the light; Illuminating a portion so that it is reflected, wherein the applied light is projected from the probe, or a portion of the reflected light is collected by the probe, and at least one of the reflected light. 19. The method of claim 18, including the step of detecting a part. 26. The method according to claim 26, wherein the surface comprises a photoresist; and the step of shining the light comprises applying electromagnetic radiation having photochemical effects from the optical aperture such that a latent image is formed in the photoresist on the surface. 19. The method of claim 18, further comprising the steps of: developing the photoresist; and exposing the workpiece to an etchant such that a pattern is formed in the surface.
the method of. 27. The method according to claim 27, further comprising exposing the surface to a chemically active liquid or vapor, wherein the step of applying the radiation comprises applying electromagnetic radiation having a photochemical effect to the surface from the optical aperture. 19. The method of claim 18, comprising applying the material to form by chemical degradation, the material depositing and sticking on the surface, thereby patterning the surface.