JPH04291310A - Short distance field scanning optics microscope and its use - Google Patents

Abstract

PURPOSE: To provide an optical system provided with high resolution and reliability capable of efficiently transmitting light. CONSTITUTION: The light emitted from a light source is radiated through the optical opening of a probe 20 to a target object and detected. The probe 20 is composed of a part of an optical fiber having a core and a clad and is provided with a tapered area tapered in terms of heat insulation. A clad outer surface inside the tapered area is smoothed and a part thereof is coated with metal and forms a metal waveguide part. The diameter of the terminating surface of the tapered area is determined to be equal to or less than a metal mode cut-off diameter.

Description

[Detailed description of the invention]

0001

FIELD OF INDUSTRIAL APPLICATION This invention relates to a method in which a small aperture, typically smaller than one optical wavelength, is located in the optical near field of the sample, that is, within approximately one optical wavelength of the sample; The present invention relates to an optical microscope that is scanned raster-wise across the surface of the sample to produce a time-varying optical signal, which time-varying optical signal is detected and reproduced to produce 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 drawbacks of conventional optical imaging systems. That is, so-called near-field scanning optical microscopy (near-field scanning optical microscopy)
anning optical microscopy
, NSOM), an aperture with a diameter smaller than one optical wavelength is positioned close to the surface of the sample and scanned through this surface. In some schemes, the sample is illuminated either reflectively or transparently by an external source. A portion of the reflected or transmitted light is collected by the aperture and relayed to a photodetector via, for example, an optical fiber. In another scheme, light is relayed by an optical fiber to an aperture, which itself functions 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 either case, the detected optical signal is reproduced to obtain image information.

For example, on August 5, 1986, W. D
．． U.S. Patent Nos. 4 and 6 issued to Pohl
No. 04,520 describes a NSOM system that uses a probe made from a pyramidal optically transparent crystal, with an opaque metal coating applied to the crystal. At the apex of the crystal, both the tip of the crystal and the metal coating on the tip are removed to form an essentially square opening with sides less than 100 nm long. Another aperture described in US Pat. No. 4,604,520 is fabricated from single mode optical fiber. One flat end of the fiber is metalized and a coaxial hole is formed in the coating to expose only the core of the fiber.

As of April 17, 1990, A. L. U.S. Patent Nos. 4 and 9 issued to Lewis et al.
In a somewhat different approach described in No. 17,462, the probe is formed from a pipette. That is, a glass tube is drawn into thin chips and coated with an opaque metal layer. After drawing, the pipette leaves a hollow hole that appears through both the glass and the upper metal layer at the tip. The resulting metal ring defines this aperture. This opening is smaller than the hole defined in the glass as a result of radially inward growth of the metal layer. One drawback of the method described above 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 made larger to compensate for the lower signal level. This measure is
This is not desirable because the resolution will drop as a result. for example,
When light is sent through an uncoated pipette from a source to an aperture, the optical field has a fairly large amplitude at the outside wall of the pipette. It is necessary to coat the walls with metal to constrain the radiation. However, attenuation occurs as a result of absorption within the metal coating. moreover,
Metallic coats have a tendency to create defects, such as pinholes, that allow optical leakage. Increasing the thickness to counter this tendency also increases the length (ie, the radial thickness of the pipette) and the outer diameter of the metal ring defining the aperture. As a result, optical losses due to absorption and dissipation within the metal ring increase, and the size of the chip also increases. As the tip becomes larger, it becomes more difficult to scan narrow, recessed topographical features of the sample while maintaining close proximity to the sample's surface. (Importantly, the problem of too large a tip size due to metal deposition is a problem with fixed-diameter fiber optic probes of the type with an aperture defined by a hole in the metal layer that coats the end of the fiber.) (also applies to)

[0005]

[Problem to be solved by the invention] If the probe (for example,
The problem that arises when single-mode fibers with a sharpened conical tip (by etching) and no metal coating, e.g. Girard
and M. Applied Optics by Spajer
, 29 (1990), pages 3726-3733.
odel for reflection near
field optical microscopy)
”. One problem is that a portion of the light passing through the fiber toward the tip is reflected by the sides of the conical taper and then transmitted through it. A second problem is that the sides of the taper capture unwanted optical signals that can propagate through the fiber and consequently increase the noise level at the detector. From the above discussion, to date, efficient transmission of light (i.e., relatively low attenuation due to optical interaction with the probe wall), relatively small tip diameter, high resolution, and high reliability NSO with both genders
It can be seen that there has been no success in providing M probes.

[0006]

SUMMARY OF THE INVENTION In one aspect, the present invention relates to an optical system that includes a probe, at least a portion of which is optically transparent at at least some wavelengths. , this probe also has one distal end. The optical system further includes an optical aperture at the distal end, the aperture having a diameter smaller than the certain wavelength. The optical system further includes a light source as a source, at least some of the light emitted by the light source entering the probe through the aperture at at least the certain wavelength;
or means for optically coupling out of the probe and means for positioning the probe relative to the target. As a feature of the present invention, (a) this probe:
consisting of a portion of a single mode optical fiber having a core and a cladding, the cladding having an outer surface with which a guided dielectric mode is associated;
(b) the fiber has an adiabatically tapered region, at least a portion of which is capable of guiding at least the certain wavelength of light;
c) the tapered region terminates in a substantially flat end surface oriented in a plane substantially perpendicular to the fiber; (d)
the cladding outer surface within the tapered region is substantially smooth; (e) at least a portion of the cladding outer surface is coated with a metal as a blocking material within the tapered region, thereby having the ability to guide metallic modes; A metal waveguide portion is defined and a cutoff diameter for the metal mode is further defined, and (f) the diameter of the end face is less than or equal to the cutoff diameter.

[0007]

DESCRIPTION OF THE PREFERRED EMBODIMENTS In one embodiment, the present invention relates to an optical system. As shown in FIG. 1, this optical system includes a light source 10, a probe 20, and a moving means (
displacement means) 30. The moving means 30 moves the probe, for example, as a probe.
The chip 60 moves relative to the target 40 arranged on the stage 50 adjacent to the chip 60 . The optical system further includes means for optically coupling the light source 10 to the probe 20. In the example shown in FIG. 1, this optical coupling is provided by a single mode optical fiber 70 extending between light source 10 and probe 20. (
Fiber 70 is actually integrated with probe 20. ) The light source 10 is, for example, a laser. Light from the light source 10 can simply be injected into an optical fiber, for example via a single mode coupler 80 that includes a microscope objective 90 and a fiber positioner 100. Also, as an option, mode stripper 1
10 is used to ensure that only a single mode in the core propagates to the probe and no other modes in the cladding. As the movement means 30, for example a piezoelectric tube is used, which is designed to move the probe vertically as well as in two orthogonal lateral directions. Alternatively, the moving means may be mechanical or piezoelectric moving means for moving the stage instead of the probe, or a combination of stage movement and probe movement.

One possible application 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 is exposed to light emitted from the light source. An exposure pattern is created by moving the probe relative to the sample within this photosensitive layer while simultaneously or continuously or intermittently irradiating the probe tip with light from a light source. A second possible application of the optical system as described is the imaging of 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 below the stage (as shown in Figure 1). (Although a pass-through mode is shown, a reflect-back mode is also possible.) The thus collected light is directed to a detector 130, which includes, by way of example, an optical multiplexer.
tube is used. To allow visual positioning of the probe, it is necessary to use a beam splitter 140, which directs a portion of the collected light to an eyepiece 150. Importantly, if the sample is scanned by a raster-like movement of the probe, the detector 1
30 can be reproduced to produce an image of the scanned sample portion.

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

Furthermore, a third possible application of the described optical system is shown in FIG. In the configuration of FIG. 2, the probe tip functions as a collector of light rather than as an emitter. This configuration supports NSO in so-called "collection" mode.
Effective for M imaging. (Although a collection-reflection mode is shown, a collection-passing mode is equally possible.) Light from the light source 10 is directed through a tilted mirror 160 and a tilted annular mirror 170 to an annular objective lens 180. . Lens 180 focuses this light onto the sample surface. Light reflected or emitted from the surface is collected by the probe tip and directed through fiber 70 and objective lens 120 to detector 130 . For reflection mode NSOMs, see, for example, U.S. Pat. No. 4,917,46, cited above.
It is explained in No. 2.

A detector (or, more generally, a converter) 130 converts the thus detected light into an electrical signal. These signals can easily be used to generate two-dimensional images on video display devices such as cathode ray tubes. For such purposes, a scan generator controls movement of the probe position relative to the target;
It is also used to provide a reference signal for constructing the displayed image. The electrical signal produced by converter 130 is typically an analog signal. These are selectively converted to digital signals before being displayed. In such cases, 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., image processing). provided as an option.

One possible type of probe 20 is a single mode optical fiber. Fiber optic probes have indeed been disclosed in the prior art. Figures 3 and 4 show examples of such fiber probes. FIG. 3 shows an untapered single mode optical fiber 190 with an annular cap of opaque material, such as metal, deposited over the chip. The aperture 210 at the center of this ring defines the optical aperture of the probe. FIG. 4 shows a bare optical fiber 220 that has been tapered, for example by chemical etching. We have discovered that the improved probe 230 can be easily fabricated by heating a single mode optical fiber to soften it 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 tip 240 of the fiber thus drawn is tapered at an angle β and terminates in a terminal flat 250.

This optical fiber has a cladding 260 and a core 270. Although the specific cladding and core compositions are not critical to the invention, an exemplary cladding composition is fused silica and an exemplary core composition is doped fused silica with a higher refractive index than the cladding. be. Although the specific cladding and core dimensions are also not critical to the invention, an example core diameter is approximately 3
um, and the outer diameter of the cladding as an example is approximately 125u
It is m. One guided mode, the fundamental or HE11 mode, is associated with the corresponding untapered single mode fiber. Such modes are characteristic of cylindrical dielectric waveguides and are for this reason referred to hereinafter as dielectric modes.

[0014] When the optical fiber is heated and drawn,
Both the core diameter and the cladding outer diameter are reduced. The fractional change in core diameter is approximately equal to the fractional change in the outer diameter of the cladding. (In other words, the cross section of the fiber is
It changes only in its proportion. ) Importantly, the angle at which the core is tapered is significantly less than β. For example, in a linearly tapered fiber with the dimensions shown as an example, the tangent of the core taper angle (
tan) is approximately 3/125 times the taper angle β, or only 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 explained below.

For untapered fibers, the electric field of the dielectric mode is mostly confined to the core, which has a very small amplitude near the outer surface of the cladding, typically less than 10 −10 of the peak amplitude. The amplitude falls to . This is not necessarily the case with tapered fibers. As the guided light wave propagates into the tapered region, it encounters an increasingly narrow core. Eventually, the core becomes too small to substantially constrain the guided modes. The light wave is
Rather than being guided by the core, it is guided by the interface between the cladding and the surrounding material, for example air or a metal such as aluminum. As explained above, the core is generally tapered at a small enough angle that this mode is adiabatic. Adiabatic means that substantially all of the energy in the initial HE11 mode remains concentrated in a single mode and is not coupled to other modes, particularly radiation modes that cause optical losses due to radiation.

Initially, the guide mode to escape from the core is H
It retains the characteristics of E11 mode considerably. More specifically, the amplitude of the guided electromagnetic field is relatively small at the outer surface of the cladding. However, as the diameter of the fiber decreases further, the amplitude of the electromagnetic field at the outer surface of the cladding increases relative to the peak amplitude of the fiber. Ultimately, this electromagnetic field acquires a relatively large amplitude at the outer surface of the cladding. Such modes can also 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 well 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 or from the probe tip. Additionally, light leaking through the cladding wall can end up in parts of the sample located relatively far from the probe tip;
The result is unintended exposure of the sample or increased background level at the detector. moreover,
If the outer wall of the cladding is surrounded only by air, stray light will enter the fiber and, again, result in a detector (e.g. when a probe is used to collect light from a sample). This will increase the background level at the location.

For the reasons stated above, it is desirable to coat at least the end portion of the taper from which a large proportion of optical leakage occurs with an opaque material, for example a metal such as aluminum. Importantly, modes guided in such metallized parts have properties typical of modes guided in metals rather than dielectric waveguides. Thus, for example, an initial HE11 mode is converted into a TE11 metal mode as it approaches and enters this metallization region. Importantly, in a waveguide with an adiabatic tapered core, the HE11 mode is
It can be coupled with the TE11 mode with relatively high efficiency (typically 10% or higher efficiency). When the fiber is bare, the portion of the fiber in which the guided modes have large amplitudes at the outer surface of the fiber, but retains significant HE11 characteristics, is referred to herein as the "transition region." In view of the above description, it is clear that it is desirable (though not absolutely required) to metal coat the fiber through this transition region and from this transition region to the distal end of the fiber. In this context, the presence of a significant amount of optical energy in the radiation mode substantially reduces the possibility of pinhole leakage and the possibility of penetration of metals by electromagnetic fields in view of the finite conductivity of the actual coating. Relatively thick metal coatings are required to eliminate This is because the thickness of this coating impedes very close access of the probe tip to the sample, and very thick metal coatings also develop roughness, which promotes rather than prevents the occurrence of pinhole leakage. Having a tendency is undesirable for both reasons. In contrast, here relatively little energy is coupled into the radiation mode and therefore a relatively thin metal coating, typically 750-1500 Å, preferably no more than about 1250 Å, is sufficient. it is conceivable that.

Generally, the guided metal mode is initially a propagating mode. However, as the fiber diameter decreases, this guided mode is gradually converted into a vanishing mode that exhibits relatively strong attenuation in the direction of propagation. Such a transition is defined here as the “cutoff diameter (c
The cutoff diameter is the cutoff diameter, which means that the transition from a propagating mode to an evanescent mode in any tapered waveguide occurs when the waveguide is an infinitely conductive metal. It is the outer diameter of the cladding that occurs when coated with
For the TE11 mode, this cutoff diameter is approximately equal to 1/2 of the guided wavelength. This portion of the fiber extending from the cutoff diameter to the probe tip is here referred to as the "evanescent region."
region)”.

For example, the cutoff diameter of the TE11 mode in a circular cylindrical waveguide is described in, for example, J. D. ``Classical Electrodynamism'' by Jackson.
ics), 2nd edition, John Wiley and Sons, I
nc. ), New York, 1975, page 3
This can be easily predicted from the discussion of metal waveguides, as discussed in 56. This is equal to the quantity χ/k0 n. where k0 is the free-space wavenumber of the guided light, n is the refractive index of the waveguide (in relation to the guided mode and wavelength), and χ is associated with the particular guided mode. It's the amount. For TE11 mode, χ is equal to 1.841. Values of χ corresponding to other guided modes are readily known to those skilled in the art.

In this context, the conversion from the fundamental dielectric mode to the TE11 metallic mode is incomplete. A finite portion of the energy of the dielectric mode is coupled to metal modes other than the TE11 metal mode. However, the TE11 mode generally experiences less damping in the vanishing region than any other metallic mode. For this reason, the light wave that traverses the vanishing region contains substantially only TE11 metal modes. (Moreover, due to the finite conductivity of real metal coatings, a perturbed TE11 mode is predicted; i.e., a mode in which the electric field has a small longitudinal component.)

Since large attenuation occurs within the vanishing region, it is required to make this region as short as possible. However, different erasure lengths may be required in applications with different factors as described below.

The outer cladding surface within the tapered portion of the fiber is provided with a relatively thin (preferably no more than about 1500 Å thick) metal coating to reduce scattering of light from the HE11 mode near the outer cladding surface. It is required that the surface be substantially smooth in order to substantially eliminate imperfections that would cause leakage of optical radiation. The cladding surface was examined here using scanning electron microscopy (SE
A surface is considered to be substantially smooth if no surface texture appears on a scale of about 50 Å or greater when observed at M). A surface with such desirable smoothness can be easily produced by heating and drawing the fiber.

The terminal flat is required to be substantially flat and substantially perpendicular to the axial direction of the fiber. A termination flat is herein considered to be flat if inspection by SEM shows that no lateral feature extent across the surface of the termination flat deviates from flatness by more than about 100 Å.

Desirably, the edges of the terminal flat are relatively sharply defined. An edge is herein considered to be sharply defined if the average curvature at the edge is less than or equal to about 100 Å as examined by SEM. Termination flats with such desired flatness and with such desired sharpness can also be easily produced by heating and drawing the fiber.

One method of metal coating the terminal portion of the fiber (preferably including at least the transition region, as discussed above) uses, for example, aluminum as a deposition source. As shown in FIG. 6, the fiber 230 has an end connected to the source 29 during aluminum deposition.
Not directed towards 0. The probe end of the fiber is oriented away from the source so that the end flat is in shadow with respect to the direction of incidence of the deposited metal. Typically,
The fiber axis is tilted at an angle θ of approximately 75 degrees with respect to a line drawn from the source to the fiber tip. During deposition, the fiber is rotated about its own axis so that all sides of the fiber are evenly coated. When such a method is employed, a coating is produced that smoothly covers the outer surface of the cladding, but leaves the terminal flats 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. Since the spatial resolution of the probe is primarily determined by the diameter of the aperture, it is desirable for high resolution applications to have a very small diameter of the end flat. For this reason, the diameter of the end flat in such probes is generally smaller than the cutoff diameter. For example, the cutoff diameter for 5145 Å light from an argon ion laser is typically about 2000 Å for fibers with the exemplary dimensions described above. The diameter of a typical corresponding end flat is approximately 5
00 to 1000 Å, and small end flats of about 200 Å or less can be easily produced by heating and drawing the fiber.

Generally, the average thickness of the aluminum layer is:
Preferably, the thickness is not less than about 750 Å. This is because layers much thinner than this are prone to excessive optical leakage. (Even smaller thicknesses are possible if the probe is used for both purposes of illuminating the target and collecting light from the target.) The thickness preferably should not exceed about 1500 Å. This is to minimize the total 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 end flat and the thickness of the metal restraining the end flat in diametrically opposed positions. This total diameter is made as small as possible to provide a probe tip that can be inserted into relatively narrow cavities or crevices in the sample surface, or more generally, penetrate deeper into the near field of the sample. It is desirable to do so.

Importantly, probes of the type described above have a vanishing region extending from the cutoff diameter to the terminal flat. This vanishing area is considerably longer compared to the alternative designs described below. For example, for a cutoff diameter of 2000 Å, a terminal flat diameter of 500 Å, and a taper angle of 15 degrees, this vanishing length would be 5600 Å. With such long distances, a significant amount of attenuation occurs. Considering this attenuation, the solution is to make the taper angle as large as possible to make the vanishing length as short as possible. However, it must also be taken into account that a smaller taper angle allows for easier penetration into cracks and easier access to stepped surface features.

In one preferred method for tapering fibers, the fibers are first loaded into a commercially available pipette puller and the fibers are heated before and during drawing. An example 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. In general, the fiber will eventually undergo a "hard pull" step, which is a pulling step accomplished by rapidly increasing the force.
)”. The velocity of the fiber end at the moment when a strong pull is applied can also be controlled, as can the time delay between cessation of heating and the application of a 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, reducing the pulling force, pulling in multiple steps, or reducing the extent of the heated area. The diameter of the end flat can be simply increased by increasing the heating rate and/or by increasing the pulling force. Additionally, by using fibers with higher temperature glass compositions, larger taper angles and smaller tips are obtained. The required combination of process parameters will be apparent to those skilled in the art after a little research.

Another type of probe is shown in FIG. In this type of probe, a metal coating is placed on the terminal flat as well as on the outer surface of the cladding. As discussed above, the thickness of the metal on the outer surface of the cladding is preferably about 750-1500 Å thick. The metal layer on the termination flat is preferably about 250-500 Å thick, as it is required to be thick enough to exclude stray light, but as thin as possible to reduce the extinction length. be made into In this case, the optical aperture 300 corresponds to only a portion of the end flat, rather than the entire end flat. The metal layer on the end flat is formed into an annular shape with an aperture defining an optical aperture. Importantly, this aperture can be centrally located in the terminal flat or offset from the center of the flat. Openings that are not centered are
Required when multiple modes are expected to exist within the chip area. In such cases, a non-centered aperture provides more efficient output coupling since not all modes are efficiently coupled to the centrally located aperture. More specifically, a non-centered aperture is expected to provide optimal optical coupling for the TE11 mode.

[0032] This central opening is first formed, for example, by providing the fiber with an embedded glass rod that has a faster etch rate than the fiber's host glass. (One example is borosilicate glass embedded within fused silica glass.) After the fiber ends are drawn, but before the fibers are metallized, they are exposed to a chemical etchant. This etchant forms cavities within the termination flats. This cavity is shadowed during the metallization of the end flat, resulting in the formation of an opening in the metal layer.

Typical apertures formed by this method have a diameter of about 200-2000 Å, although larger or smaller apertures can be formed using the same method.

[0034] The metal coating is deposited, by way of example, from the vapor deposition source described above in connection with a bare tip probe. However, two separate deposition steps are used. That is, in a first step, the outer surface of the cladding is coated as explained above, and in a second step, the fiber orientation is changed and the end flat is coated.

Since in this type of probe the aperture is not determined by the diameter of the end flat, the diameter of the end flat typically does not need to be smaller than the cutoff diameter, even in high resolution applications. Therefore, to minimize damping, it is desirable that the diameter of the terminal flat be approximately at least the same as the cutoff diameter. However, as explained above, it is desirable that the total diameter of the probe tip be as small as possible to allow for penetration into crevices. For this reason, the diameter of the end flat is preferably not made much larger than the cut-off diameter.

In this context, the diameter of the aperture is usually smaller than the diameter of the cutoff, so that a 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 accomplish this. However, because the coating typically has a thickness of about 500 Å or less, this vanishing length is much smaller than for the previously described bare tip probes. Obviously, keeping the damping low requires that the metal coating on the end flat be as thin as possible. The acceptable range for the metal thickness on the termination flat is about 250-500 Å, typically 500 Å as mentioned above.

We have calculated the expected ideal power attenuation within the vanishing portion of the bare tip probe. The corresponding power transmission coefficient may conveniently be expressed using χ, which is a property of the guided mode as explained above, the taper angle β, and dimensionless parameters defined by the following equation: Can be done. α=χ/nk0 a where a is the diameter of the aperture as explained above, k0 is the free space wavenumber of the guided light, and n is the appropriate value of the waveguide index of refraction. . It is helpful to know that α is equal to the ratio of the cutoff diameter to the aperture diameter. When expressed in decibels, the power transmission coefficient corresponding to the ideal extinction attenuation is given by:

[Equation 1] We have determined that the overall measured power transmission coefficient is 1 than Tev.
We have successfully manufactured probes with various values of α in the range between 1 and 10 and various values of β in the range between 5 and 20 degrees with no drop of more than 0 dB. At least for α in the range of 2 to 8, Tev can be roughly approximated by -2χα/tan βdB. Thus, we have succeeded in manufacturing a probe whose total measured power transmission coefficient, denoted by T, satisfies the following: T(db)>-(2χα/tan β)-10

003
8] The optical system according to the invention is particularly useful as a manufacturing tool. For example, the probe can be easily used to perform actinic radiation from a light source onto a small area of the surface of a workpiece located adjacent to the optical aperture of the probe. For example, if the surface of the workpiece is coated with a photosensitive layer, e.g. produced by pouring it over, thereby exposing this layer. Additional steps are performed, thereby completing the industrial product, for example. An example industrial product is a semiconductor integrated circuit, and these additional steps include: developing a photoresist;
and exposing the resist-coated surface to an etchant to pattern the underlying layer of photoresist. Importantly, not only semiconductor wafers, but also glass plates used for photolithographic masks can be easily patterned in this manner.

In another process, actinic radiation from a probe is directed onto the surface of a workpiece;
This causes material to be deposited on this surface. for example,
The surface is exposed to an organometallic vapor or solution. Actinic radiation impinging on the surface causes localized decomposition of one or more components of the vapor or solution, eg, producing metal residues that adhere to the surface.

The optical system according to the invention is also useful as an inspection device on a production line. For example, the 9th
As shown, in the manufacture of semiconductor integrated circuits, a semiconductor wafer is often provided with a surface that is patterned at one or more stages of the manufacturing process (Step A). The patterns thus formed generally have a characteristic dimension, commonly referred to as "line width," which is required to be kept within some narrow tolerance. The optical system according to the invention is suitable for measuring line widths, for example the width of metal conductors on a wafer or the length of gates formed in metal-oxide semiconductor (MOS) structures on a wafer (step D). Easy to use. These line widths are compared to desired values (step E). Process parameters, such as lithographic exposure or etch times, are initially set (Step B) but are adjusted (Step H) so that they bring the measured dimensions within desired tolerances. Next, an additional step (Step G) is performed to complete the product. The measuring step includes positioning the probe adjacent to the patterned surface (Step I).
, by irradiating light thereon (step J), and in substantially the same manner as described above, the light source and the detector are optically coupled via the probe according to the invention. A step of detecting light from the surface (step K) is included.

The optical system according to the invention is also useful for examining bit patterns in magnetic digital storage media, such as magnetic disks. Magnetic storage media generally include a bit pattern characterized by a modulated direction of magnetization to exhibit a Faraday rotation of polarization.
For example, it can be easily visualized by inspection using crossed polarizers. Thus, for example, the optical system according to the invention can detect the reflection modes of this medium by using a polarizing light source and by using a polarizing filter before the detector.
Can be used for imaging. In manufacturing processes that involve impressing such media with bit patterns having predetermined characteristics, 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 a linearly polarizing film. Before the light is coupled into 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 within the optical fiber. (Polarization-preserving fibers can also be used, although this is not necessary.) The linearly polarized component of the light exiting the fiber is e.g. Optimized visually by adjusting the wave plate.

Other applications for manufacturing and testing optical systems according to the invention will be readily apparent to those skilled in the art. . For example, the optical system according to the invention is also useful for impressing digital data onto an optical or magnetic storage medium for storage, or for reading data thus stored from a storage medium. . For example, it is well known that data can be recorded on a magnetizable metal film in the form of a pattern of spots having a local magnetization in a direction different from that of the surrounding film portions. This data storage technique is, for example, MRS Publication 15 (MRS Publication 15)
R.S Bulletin 15), April 1990 issue, pages 20-24. J. Gambino
The paper “Optical Storage Disk Technology (Opti)” published by
calStorage Disk Technology
y ), and MRS Bulletin 15 (MRS Bulletin
in 15), April 1990 issue, pages 31-39
to F. J. A. M. Greidanus
) and W. B. The paper “Magneto-Opt Storage Materials” published by Zeper
ical Storage Materials)”
It is explained in.

A typical magnetic storage medium is a layer 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 multilayer films, and magnetic oxide materials such as ferrite and garnet.) For example, spots representing bits of digital data may It is written by exposing it to a magnetic field and optically heating this spot 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.) For this purpose, the diameter is 1 um. however,
Using the optical system according to the invention, smaller spots can easily be produced, for example very small spots of about 0.2-0.5 um and even 0.06 um or less. Such small spots can also be read out by the optical system of the invention.

In one currently preferred embodiment, the magnetic storage medium is a thin amorphous film of an alloy containing at least one rare earth and at least one transition metal. An example of such an 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, this probe can easily be placed more than one wavelength away from the medium.) The illumination wavelength is
selected to provide sufficient heating to the medium. for example,
Terbium-iron films can be easily heated by a dye laser pumped by a YAG laser and emitting approximately nanosecond pulses at about 600 nm. Light from the laser is directed to the probe according to the invention via an optical fiber and is directed from the probe tip onto the storage medium. The 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 irradiation, but pulsed lasers are preferred because they reduce the average power requirements of the laser and heat relatively small spots.

Importantly, the present invention encompasses not only magnetic film storage media, but also other media that can be written with a radiation source. Examples of such media include:
A polycrystalline film (e.g., a film of indium antimonide doped with terium) is included, which is locally heated, e.g. by a laser pulse, to a temperature above the melting point of the film and rapidly cooled to an amorphous state. Ru. This cooling rate can be controlled, for example, by suitably shaping the time dependence of the laser pulse.

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

In one presently preferred method for reading a pattern impressed on a magnetic film storage medium, linearly polarized light emitted by a probe according to the invention is transmitted through the storage medium and conventional means 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 maximum optical response (ie, the rotation in the direction of the light's polarization is greatest 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 onto the surface of the medium and to collect the portion of this light reflected from the surface. passed through a magnetized spot in the medium,
Or as a result reflected from this, typically
A polarization rotation of approximately 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 within the medium is reproduced. As is well known, such information
For example, it can be recorded sound, images, text, or digital data.

For example, in another embodiment involving a phase change, modulation is typically achieved by a change in reflectance rather than a rotation of polarization. This change can also be easily detected by collecting the reflected light using a 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 invention overcomes, at least in some cases, the low signal level problem that often undermines the effectiveness of prior art NSOM systems for medical and biological imaging. Thus, for example, a microscope according to the invention can be used to image sectioned samples of biological tissue and discover and identify physical pathology within that tissue. By a similar method, the microscope according to the invention can easily be used to examine the distribution of materials detectable by their specific appearance or fluorescence within sectioned tissues, as well as materials labeled with e.g. 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 in, for example, metaphase. More specifically, chromosomes or portions of chromosomes labeled with fluorescent substances can be easily identified using the microscope according to the present invention. Methods of fluorescent labeling, which generally involve the process of reacting the nuclear material of a cell with a phosphorus-containing material, are well known in the art and need not be described in detail here.

Imaging using fluorescence can be achieved by using the microscope according to the present invention in irradiation mode (FIG. 1) or condensing mode (FIG. 2).
This can be easily achieved by using either mode. In the former, electromagnetic radiation capable of stimulating the sample to emit fluorescence is applied to the sample from a probe, and the resulting fluorescence is collected, for example, by a conventional microscope objective. In the latter, excitation radiation is applied to the sample according to conventional methods and fluorescence is collected by a probe.

As mentioned above, the preferred probe 20 (FIG.
) are manufactured by tapering a single mode optical fiber. In at least some cases, useful probes can also be produced by tapering and coating multimode fibers. Depending on the dimensions (relative to the guided wavelength) of the multimode fiber, more or fewer modes can be guided. In general, the fewer the number of guided modes (ie, the closer the fiber is to a single mode fiber), the greater the signal-to-noise ratio achieved by any given aperture.

As also previously mentioned, an exemplary opaque coating for the probe is a metal such as aluminum. More generally, suitable coatings are those consisting of a material in which the guided radiation has a low degree of penetration. Aluminum is
One preferred material is when the radiation is, for example, in the visible spectrum, but a semiconductor such as silicon is preferred when, for example, infrared radiation is to be guided.

An example is shown below. 3-um single mode fiber (FS-
VS-2211) is a Sutter Instrument (S
Mod. manufactured by Utter Instruments). The fiber was drawn in a P-87 micropipette puller by heating with a 25 watt 3 mm spot from a 25 watt carbon dioxide laser. The micropipette puller is set to 75 (range 0-
Hard pull at 255), Velocity at pull at setting 4 (range 0-255)
ocity at pull)”, and a time delay of 1 (
range 0-255). Chip properties were obtained with a taper angle of 12 degrees, a flat end of 670 Å diameter, and a value for α of approximately 3. One end of the fiber was placed in a rotor and vapor coated with about 1260 Å of aluminum at a base pressure of about 10 −6 Torr. The tapered end is then shown in Figure 1.
mounted inside a piezoelectric tube inside an optical device. 514.5n of 1 milliwatt from an argon ion laser
m light was coupled into the fiber. The optical power output at the fiber tip is approximately 1.1
nanowatts, which is about -60d
B corresponds to the total power transfer coefficient. When used to image the sample surface, this probe provided a spatial resolution of approximately 25 nm.

[Brief explanation of drawings]

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

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

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 shows an optical fiber according to one embodiment of the present invention.
Figure 2 is a schematic diagram of the probe.

FIG. 6 illustrates an example method for metallizing a fiber optic probe.

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

Claims (27)

[Claims] 1. An optical system comprising: a probe, at least a portion of which is optically transparent at at least a predetermined wavelength, and having a distal end; an optical aperture having a diameter smaller than the predetermined wavelength; and means for positioning the probe relative to a target; a) the probe comprises a portion of an optical fiber having a core and a cladding; a cladding has an outer surface and at least one guiding dielectric mode is associated with the fiber for radiation at the predetermined wavelength; b) the fiber has an adiabatically tapered region, the tapered region at least a portion of the fiber is capable of guiding light of at least the predetermined wavelength; c) the tapered region terminates in a substantially flat end face oriented in a plane substantially perpendicular to the fiber; d) the the cladding outer surface within the tapered region is substantially smooth; and e) at least a portion of the cladding outer surface within the tapered region has a relatively low penetration to electromagnetic radiation of the predetermined wavelength. An optical system characterized in that it is coated with a blocking material, thereby defining a metallic waveguide section capable of guiding metallic modes. 2. 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. 2. The optical system of claim 1, further comprising: 3. A source of electromagnetic radiation as a light source; a scanning generator for driving a moving means such that the probe is moved in a raster pattern adjacent a portion of the surface of the sample; converting means for detecting at least a portion of light from a light source entering or exiting the light source and generating an electrical signal in response to the detected light; far-field microscopy means for viewing; and a video camera having a signal receiving relationship with the transducer for displaying a two-dimensional image related to the amount of light detected at at least some displacements of the probe relative to the sample. 3. The optical system of claim 2 further comprising display means and wherein said displacement is part of a raster pattern. 4. The converting means is adapted to generate an analog electrical signal, and the system further includes a system for converting the analog electrical signal into a digital signal and transmitting the digital signal to the video display means. 4. The optical system of claim 3, further comprising means. 5. The optical system of claim 4 further comprising storage means for digitally recording at least a portion of the digital signal. 6. further comprising means for digitally processing the digital signal, the digital processing means being in a receiving relationship with the analog-to-digital conversion means;
6. The optical system of claim 5 in transmitting relation to said video display means. 7. Further comprising an electromagnetic radiation source capable of stimulating at least a portion of the target to emit fluorescence;
the radiation source being optically coupled to the probe such that radiation is transmitted from the radiation source through the probe to the target; and further including means for detecting at least a portion of the fluorescence emitted by the target. The optical system of claim 1, characterized in that: 8. An electromagnetic radiation source capable of stimulating a target to emit fluorescence at least at the predetermined wavelength; and at least a portion of the radiation emitted by the radiation source impinges on the target; further comprising means for supporting the target relative to the radiation source and the probe such that at least a portion of the fluorescence emitted by the target is incident on the probe through the optical aperture. The optical system of claim 1. 9. The end face has a diameter less than or equal to the cutoff diameter for the metal mode.
optical system. 10. The end face is substantially uncoated with blocking material; the fiber has one cladding outer diameter at each axial location; 10. The optical system of claim 9, wherein, at least in part, the cladding outer diameter is smaller than the cutoff diameter and the aperture substantially coincides with the end surface. 11. The diameter of the end face is approximately equal to the cutoff diameter; the probe further includes an annular layer of blocking material coated on the end face, the annular layer surrounding the aperture, thereby 10. 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 TE11
mode; the taper angle is represented by β;
α is at least about 2 and at most about 8; and when the probe is expressed in decibels, the following relation: T
10. The optical system according to claim 9, having a transmission coefficient expressed by T satisfying >-(3.68α/tan β)-10. 13. The optical system of claim 11, wherein the aperture has a center, the end surface has a center, and the center of the aperture is offset from the center of the end surface. . 14. A method for examining a sample, the method comprising: generating an enlarged image of at least a portion of the sample; and visually examining the image, the image generating step comprising:
a probe is positioned adjacent to a first surface of the sample and at a distance no greater than a predetermined wavelength from the first surface, and directs electromagnetic radiation onto the sample, emitting electromagnetic radiation from the first surface. collecting radiation and detecting at least a portion of the collected radiation, wherein the radiation impinged onto the sample is emitted from 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, the cladding having an outer surface and having at least the predetermined wavelength of radiation collected by the probe; at least one guided dielectric mode is associated with the fiber; b) the fiber has an adiabatically tapered region, and at least a portion of the tapered region guides at least light of the predetermined wavelength; c) the tapered region terminates in a substantially flat end surface oriented in a plane substantially perpendicular to the fiber;
an opening is provided in the end face; d) the cladding outer surface within the tapered region is substantially smooth; and e) at least a portion of the cladding outer surface within the tapered region has the predetermined shape. A method characterized in that a metal waveguide section is defined by being coated with a blocking material that has a relatively small penetration for electromagnetic radiation at wavelengths, thereby defining a metal waveguide section that is capable of guiding metal modes. 15. The sample has a second surface, and applying electromagnetic radiation to the sample comprises applying radiation to the second surface, and wherein the radiation emitted by the first surface is applied to the sample. 15. The method of claim 14, wherein the radiation is transmitted through. 16. The method of claim 14, wherein said step of applying radiation comprises applying radiation to said first surface and said collecting step comprises collecting reflected radiation. 17. Applying the electromagnetic radiation comprises applying radiation onto the first surface, the applied radiation having at least one wavelength of radiation capable of exciting fluorescence in at least a portion of the sample. 15. The method of claim 14, wherein said collecting step comprises collecting fluorescent light. 18. A method of patterning a surface, the method comprising: a) providing a workpiece having a surface to be patterned; and b) applying at least one predetermined wavelength of light on the surface. c) such that at least a portion of the applied light is projected, reflected, or transmitted through the optical aperture;
positioning a probe having a termination surface and an optical aperture provided in the termination surface adjacent to the surface such that the distance between the aperture and the surface is at most about one predetermined wavelength; d) where the probe is comprised of a portion of an optical fiber comprising a core and a cladding, the cladding having an outer surface and at least one a guided dielectric mode is associated; e) the fiber has an adiabatically tapered region such that at least a portion of the tapered region is capable of guiding light of at least the predetermined wavelength; f) the tapered region terminates in a substantially flat end surface oriented in a plane substantially perpendicular to the fiber, and the aperture is defined within the end surface; g) within the tapered region the cladding outer surface of the cladding is substantially smooth; and h) at least a portion of the cladding outer surface within the tapered region is of a blocking material having a relatively small penetration to electromagnetic radiation of the predetermined wavelength. A method characterized in that a metal waveguide portion is defined by which a metal waveguide portion is able to guide metal modes. 19. The surface to be patterned comprises a recording medium onto which a bit pattern can be impressed; in the step of applying the radiation,
light is directed from the optical aperture onto the medium; the step of directing the light causes a localized physical change within the medium such that the physical change modulates an optical signal; 19. The method of claim 18, wherein the modulation carries at least one bit of information. 20. The medium is a magnetic medium, and the step of applying the radiation causes the medium to have its Curie temperature,
20. A method as claimed in claim 19, characterized in that it is heated or above its compensation point, resulting in the formation of a spot whose local magnetization is different from the magnetization of an adjacent region around the spot. 21. Claim 2, wherein the step of applying radiation is performed in the presence of an external magnetic field.
0 method. 22. The method of claim 20, wherein the medium is a magnetic alloy containing at least one rare earth and at least one transition metal. 23. The medium is substantially polycrystalline, and as a result of the step of applying the radiation, a portion of the medium is locally heated and then rapidly cooled to form a locally substantially amorphous spot. 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;
The method further includes: a) setting at least one processing parameter; b) processing at least one first wafer in accordance with the processing parameter such that a pattern having specific dimensions is formed on the surface of the wafer. c) measuring the particular dimension on the first wafer; d) comparing the particular dimension to a predetermined range of values;
e) if the particular dimension is outside the predetermined range of values, modifying the processing parameter to fall within the predetermined range of values; f) after step e), at least according to the processing parameter; processing one second wafer; and g) performing, with respect to the at least second wafer, at least one additional step to complete the article; h) applying the radiation, during the measuring step, positioning a probe adjacent the patterned surface of the first wafer and applying light onto the surface such that at least a portion of the light is reflected from the surface; further comprising the step of detecting at least a portion of the reflected light, wherein relevant light is projected from the probe or a portion of the reflected light is collected by the probe. 20. The method of claim 18, wherein: 25. The method further comprises: a) providing at least one substrate, each having a surface layer of magnetic material to be impressed with a bit pattern; 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) the bits of at least the first substrate; - detecting a pattern; d) comparing the detected bit pattern with a predetermined bit pattern; e) if the detected bit pattern differs from the predetermined bit pattern, adjusting the processing parameters; altering the detected bit pattern and the predetermined bit pattern to match; f) after step e), processing at least a second substrate according to the processing parameters; and g) the at least performing at least one additional step with respect to the second substrate to complete the object; h) the applying the radiation includes the patterning of the first wafer during the detecting step; positioning a probe adjacent to a surface where the applied light is projected from the probe, and applying 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 reflected light is collected by the probe and further comprising detecting at least a portion of the reflected light. 26. The surface comprises a photoresist; and the step of applying light comprises applying actinic radiation, electromagnetic radiation, from the optical aperture onto the surface such that a latent image is formed in the photoresist. 20. The method of claim 18, wherein the method further comprises: developing the photoresist; and exposing the workpiece to an etchant such that a pattern is formed in the surface. 27. The step of exposing the surface to a chemically active liquid or vapor, the step of applying radiation directing actinic or electromagnetic radiation from the optical aperture onto the surface of the chemically active liquid or vapor. 19. The method of claim 18, comprising the step of applying a material to form by decomposition, the material depositing and adhering to the surface, thereby patterning the surface.