U.S. patent number 5,105,305 [Application Number 07/639,629] was granted by the patent office on 1992-04-14 for near-field scanning optical microscope using a fluorescent probe.
This patent grant is currently assigned to AT&T Bell Laboratories. Invention is credited to Robert E. Betzig, Jay K. Trautman.
United States Patent |
5,105,305 |
Betzig , et al. |
April 14, 1992 |
Near-field scanning optical microscope using a fluorescent
probe
Abstract
A novel probe, useful for near-field optical scanning
microscopy, is provided. The probe has a fine tip which includes
fluorescent material. In one embodiment, the invention is an
apparatus which includes such a probe, means for exciting and
detecting fluorescence in the probe tip, means for positioning the
probe tip near the surface of a sample, and means for displacing
the probe tip relative to the sample. In a second embodiment, the
invention is a manufacturing method in which the novel probe is
used to measure a characteristic dimension of a patterned
workpiece.
Inventors: |
Betzig; Robert E. (Chatham,
NJ), Trautman; Jay K. (Bedminster, NJ) |
Assignee: |
AT&T Bell Laboratories
(Murray Hill, NJ)
|
Family
ID: |
24564907 |
Appl.
No.: |
07/639,629 |
Filed: |
January 10, 1991 |
Current U.S.
Class: |
359/368;
250/227.14; 250/458.1; 385/12; 385/125; 850/30; 850/31 |
Current CPC
Class: |
G01Q
60/20 (20130101) |
Current International
Class: |
G12B
21/00 (20060101); G12B 21/06 (20060101); G02B
021/06 () |
Field of
Search: |
;350/96.10,96.15,96.18,96.24-96.26,96.29-96.34 ;128/634
;606/2,15,16 ;250/227.14,227.18,458.1 |
References Cited
[Referenced By]
U.S. Patent Documents
Primary Examiner: Lee; John D.
Attorney, Agent or Firm: Finston; M. I. Pacher; E. E.
Claims
We claim:
1. In a near-field scanning optical microscope, apparatus
comprising:
a probe, the probe having a tip capable of emitting light at least
at a given wavelength;
means for detecting at least a portion of the light emitted by the
probe tip;
means for positioning the probe adjacent a sample such that the
distance between the probe tip and the sample is smaller than about
five times the given wavelength; and
means for displacing the probe tip relative to the sample,
CHARACTERIZED IN THAT
the probe tip comprises fluorescent material capable of emitting
light at least at the given wavelength when the fluorescent
material is appropriately excited; and
the apparatus further comprises means for exciting the fluorescent
material.
2. Apparatus of claim 1, wherein:
the probe comprises a glass pipette having a tapered end and an
aperture in the tapered end; and
the probe tip comprises a fluorescent body adherent to the pipette
adjacent the aperture, the body comprising fluorescent material and
having maximum lateral dimensions smaller than the given
wavelength.
3. Apparatus of claim 1, wherein:
the probe comprises a glass pipette having a tapered end and an
aperture in the tapered end, there being an inner diameter
associated with the aperture; and
the probe tip comprises a spheroidal body having a diameter greater
than the aperture inner diameter, the body adherent to the pipette
such that a first portion of the body lies within the aperture and
a second portion of the body lies outside the aperture, at least
the second portion of the body comprising fluorescent material, and
the body diameter being smaller than the given wavelength.
4. Apparatus of claim 1, wherein:
the probe comprises a single-mode optical fiber having a tapered
terminal portion and an end flat at the end of the tapered portion;
and
the probe tip comprises a fluorescent body adherent to the end
flat, the body comprising fluorescent material and having maximum
lateral dimensions smaller than the given wavelength.
5. Apparatus of claim 1, wherein:
the probe comprises a single-mode optical fiber having a tapered
terminal portion and an end flat at the end of the tapered portion;
and
the probe tip comprises a spheroidal body having a diameter smaller
than the given wavelength, the body adherent to the end flat, at
least a portion of the body comprising fluorescent material.
6. Apparatus of claim 1, wherein:
the probe comprises a single-mode optical fiber having a tapered
terminal portion (to be referred to as the "taper") and an end flat
at the end of the tapered portion, the end flat having a diameter
smaller than the given wavelength;
the probe tip coincides with a terminal portion of the taper,
including the end flat; and
the probe tip comprises implanted, fluorescent ions.
7. Apparatus of claim 1, wherein:
the probe comprises a single-mode optical fiber having a tapered
terminal portion (to be referred to as the "taper") and an end flat
at the end of the tapered portion, a borehole being defined in the
end flat, a diameter being associated with the borehole; and
the probe tip comprises a spheroidal body having a diameter greater
than the borehole diameter, the body adherent to the fiber such
that a first portion of the body lies within the borehole and a
second portion of the body lies outside the borehole, at least the
second portion of the body comprising fluorescent material, and the
body diameter being smaller than the given wavelength.
8. Apparatus of claim 1, wherein the exciting means comprise a
light source optically coupled to the fluorescent material.
9. Apparatus of claim 8, wherein the exciting means further
comprise means for transmitting electromagnetic radiation from the
light source, through the sample, to the fluorescent material.
10. Apparatus of claim 8, further comprising means for transmitting
light emitted by the fluorescent material through the sample to the
detecting means.
11. Method for manufacturing an article, comprising the steps
of:
a) providing a multiplicity of semiconductor wafers, each wafer
having a surface to be patterned by means of a patterning
process;
b) setting at least one parameter of the patterning process;
c) processing at least a first wafer according to the process
parameter, such that a pattern is formed on the surface of the
wafer, the pattern having a characteristic dimension;
d) measuring the characteristic dimension in at least one of the
multiplicity of semiconductor wafers;
e) comparing the characteristic dimension to a predetermined range
of values;
f) if the characteristic dimension lies outside the predetermined
range of values, changing the process parameter to bring the
characteristic dimension within the predetermined range of
values;
g) after (f), processing at least a second wafer according to the
process parameter; and
h) performing, on at least the second wafer, at least one
additional step toward completion of the article,
CHARACTERIZED IN THAT
the measuring step comprises:
i) providing a probe having a tip, the probe tip comprising
fluorescent material capable of emitting light at least at a given
wavelength when the fluorescent material is appropriately
excited;
j) positioning the probe relative to the wafer such that the
distance between the probe tip and at least a portion of the
pattern is smaller than the given wavelength;
k) displacing the probe tip relative to the wafer;
l) during the displacing step, exciting fluorescence in the probe
tip; and
m) detecting fluorescent light emitted by the probe tip.
Description
BACKGROUND OF THE INVENTION
This invention relates generally to the field of optical
microscopy, and more particularly to near-field scanning optical
microscopy for high-resolution imaging.
Microscopes employing conventional optical imaging systems cannot
resolve features substantially smaller than about one-half an
optical wavelength, because of diffractive effects. However,
near-field scanning optical microscopy (NSOM) can be employed to
achieve finer resolution in optical imaging. In NSOM, an aperture
having a diameter that is smaller than an optical wavelength is
positioned in close proximity (i.e., within less than one
wavelength) to the surface of a specimen and scanned over the
surface. Light may be either emitted or collected by such an
aperture. The aperture is defined in the end of a probe. Mechanical
or piezoelectric means are provided for moving the probe relative
to the sample. Light that has interacted with the sample is
collected and detected by, e.g., a photomultiplier tube. The
strength of the detected light signal is typically stored, in the
form of digital data, as a function of the probe position relative
to the sample. The stored data can be displayed on, e.g., a
cathode-ray tube as an image of the scanned surface.
One approach to the design of NSOM probes has been described in
U.S. Pat. No. 4,917,462, issued to A. Lewis, et al. on Apr. 17,
1990. According to that approach, the probe is a highly tapered,
glass pipette. The optical aperture is defined at the narrow end of
the pipette, where the capillary bore forms an orifice. The outer
surface of the pipette is coated with metal, typically aluminum, in
order to increase the opacity of the glass wall. The aperture is
defined by metallizing the annular region at the very end of the
pipette, surrounding the orifice. The resulting tip aperture is
readily made less than 1000 .ANG. in diameter, or even smaller.
The pipette behaves approximately like a classical metallic
waveguide. Light is transmitted through the pipette in a
propagating mode or combination of modes. A cutoff diameter is
associated with each such mode. Generally, the outer diameter of
the glass wall of the pipette is everywhere greater than the cutoff
diameter of the lowest desired mode. The cutoff threshold of the
lowest mode is reached only at the thin metallized region at the
tip of the pipette.
A second, aperture-based approach to NSOM probes has been described
in U.S. patent application Ser. No. 615,537, filed on Nov. 19,
1990. According to that approach, the probe is made from a tapered,
single-mode optical fiber having a flat end portion. At least a
terminal portion of the tapered fiber is coated with metal on the
outer walls. Optionally, the end flat is also overcoated with
metal. The metal layer overlying the end flat is formed as an
annulus, the bare portion within the annulus defining the optical
aperture of the probe. Analogously to the pipette probe, the
metallized terminal portion of the fiber probe behaves like a
metallic waveguide. The diameter of the fiber falls to the cutoff
diameter of the lowest mode of interest at or near the end
flat.
Although useful, the aperture-based probes described above suffer
several disadvantages.
For example, it has been noted that there are cutoff diameters
associated with the metallic waveguiding properties of the probes.
If apertures are made substantially smaller than the cutoff
diameters for the modes of interest, the attainable signal is
substantially decreased. As a consequence, there are practical
limits on how small the tip diameter can be made while still
providing a useful signal. Furthermore, the thickness of the metal
layer contributes to the overall diameter of the probe tip. Thus,
both the tradeoff between aperture size and signal strength, and
the thickness of the metal layer, impose limits on the smallest
practical tip size.
Signal transmission through small apertures is fraught with other
difficulties in addition to the problem of attenuation in a
below-cutoff waveguide. For example, the maximum signal that can be
collected by an idealized aperture of diameter D in an
infinitesimally thin screen is theoretically proportional to
D.sup.6. Thus, it is clear that the signal drops rapidly in
strength as the aperture is reduced in size.
Yet another disadvantage is that the ultimate achievable resolution
is limited by the finite electrical conductivity of any metal
coating. That is, if the metal coating were a perfect conductor,
the guided electromagnetic field would not penetrate into the
metal. However, because any actual metal used for coating the fiber
has some electrical resistivity, and therefore a finite
conductivity, it is inevitable that the electromagnetic field will
extend some distance into the metal. This is true, in particular,
at the aperture. As a consequence, the effective aperture (for
purposes of image resolution) is somewhat larger than the
metal-free region at the end of the fiber. Instead, the effective
aperture extends into the surrounding metallized region. Because of
this, it is difficult, as a practical matter, to resolve features
substantially smaller than about 100 .ANG. in extent, even with the
finest aperture-based probes.
Practitioners in the field have hitherto been unable to circumvent
the limitations discussed above. As one consequence, efforts to
make extremely high-resolution probes, e.g., probes capable of
resolving features smaller than about 100 .ANG. while providing
signal-to-noise ratios large enough for practical imaging, have
been frustrated.
SUMMARY OF THE INVENTION
We have invented an NSOM probe that circumvents at least some of
the problems inherent in aperture-based probes. The inventive probe
incorporates fluorescent means in the probe tip. The fluorescent
means are relatively small in spatial extent, and, in fact, are
readily made smaller than the cutoff diameters typical of
aperture-based probes. The fluorescent means are typically,
although not necessarily, excited by electromagnetic radiation
(here referred to as "excitation light"). Because the light emitted
during fluorescence (here referred to as "fluorescent light")
generally has a different wavelength than the excitation light, a
ready means is provided for eliminating background optical signals.
That is, spectral cutoff filters are readily used to distinguish
the direct and scattered light emitted by the excitation source
from fluorescent light associated with the probe tip.
Significantly, the maximum signal intensity that can be emitted or
collected by such fluorescent means depends relatively weakly on
the diameter of the fluorescent means. (The theoretical dependence
may be approximated by D.sup.n, where n is close to 2, rather than
6 as in the aperture-dependent case.) As a consequence, the probe
tip can be made quite small without suffering a severe penalty in
decreased signal strength. In use, the probe tip is positioned
adjacent a sample surface. The distance between the probe tip and
the surface should be less than about five times the wavelength of
the fluorescent light, and in many cases is less than one
wavelength.
In one aspect, the invention involves apparatus comprising: a
probe, the probe having a tip capable of emitting light at least at
a given wavelength; means for detecting at least a portion of the
light emitted by the probe tip; means for positioning the probe
adjacent a sample such that the distance between the probe tip and
the sample is smaller than five times the given wavelength; and
means for displacing the probe tip relative to the sample,
characterized in that the probe tip comprises fluorescent material
capable of emitting light at least at the given wavelength when the
fluorescent material is appropriately excited; and the apparatus
further comprises means for exciting the fluorescent material.
In a second aspect, the invention involves a method for
manufacturing an article, comprising the steps of: providing a
multiplicity of semiconductor wafers, each wafer having a surface
to be patterned; setting at least one process parameter; processing
at least a first wafer according to the process parameter such that
a pattern is formed on the surface of the wafer, the pattern having
a characteristic dimension; measuring the characteristic dimension
in at least one of the multiplicity of semiconductor wafers;
comparing the characteristic dimension to a predetermined range of
values; if the characteristic dimension lies outside the
predetermined range of values, changing the process parameter to
bring the characteristic dimension within the predetermined range
of values; after changing or not changing the process parameter,
depending on the outcome of the comparing step, processing at least
a second wafer according to the process parameter; and performing,
on at least the second wafer, at least one additional step toward
completion of the article, characterized in that the measuring step
comprises: providing a probe having a tip, the probe tip comprising
fluorescent material capable of emitting light at least at a given
wavelenght when the fluorescent material is appropriately excited;
positioning the probe relative to the wafer such that the distance
between the probe tip and at least a portion of the pattern is
smaller than the given wavelength; displacing the probe tip
relative to the wafer; during the displacing step, exciting
fluorescence in the probe tip; and detecting fluorescent light
emitted by the probe tip.
BRIEF DESCRIPTION OF THE DRAWINGS
FIG. 1 is a schematic drawing of an optical system according to one
aspect of the invention.
FIG. 2 is a schematic drawing of an alternative optical
arrangement.
FIG. 3 is a schematic representation of a tapered pipette probe
with an adherent particle, according to one aspect of the
invention.
FIG. 4 is a schematic drawing illustrating one method for adhering
particles to the probe of FIG. 3.
FIG. 5 is a schematic drawing of a tapered fiber probe according to
an alternative embodiment of the invention.
FIG. 6 is a flowchart illustrating the steps in a manufacturing
process according to one aspect of the invention.
DETAILED DESCRIPTION
The invention involves an optical system. In one embodiment,
depicted schematically in FIG. 1, such an optical system
exemplarily includes a light source 10, a probe 20, and
displacement means 30 for displacing the probe relative to an
object 40. The object is exemplarily disposed on a stage 50, such
that a portion of the surface of the object lies adjacent the probe
tip 60. The probe tip 60 comprises a fluorescent portion 65 that is
capable of emitting light when it is impinged by light from light
source 10. Generally, the fluorescent light emitted by portion 65
has a different wavelength than the excitation light emitted by
source 10. (It should be understood that "light" in this context
comprises electromagnetic radiation in the infrared, visible, and
ultraviolet regions of the spectrum, and that the excitation light
may also lie in the x-ray region of the spectrum.)
The exemplary optical system further comprises means for optically
coupling light source 10 to probe 20. In the example illustrated in
FIG. 1, the optical coupling is provided by a single-mode optical
fiber 70 extending between light source 10 and probe 20. (Fiber 70
may, in fact, be integral with probe 20 if probe 20 is a
tapered-fiber type probe, to be described below.) Light source 10
is exemplarily a laser. Light from source 10 is readily injected
into the optical fiber by way, e.g., of of a single-mode coupler
80, which includes a microscope objective 90 and a fiber positioner
100. The displacement means 30 may, for example, be a piezoelectric
tube adapted for moving the probe vertically as well as in two
orthogonal lateral dimensions. Alternatively, the displacement
means may be mechanical or piezoelectric means for moving the stage
rather than the probe, or some combination of stage-displacement
and probe-displacement means.
The fluorescent light emitted by portion 65 may be employed for
imaging in several different ways. In a so-called transmission
mode, as illustrated in FIG. 1, the fluorescent light is
transmitted through the sample, collected, e.g., by microscope
objective 120, and detected by, e.g., photomultiplier tube 130.
Beamsplitter 140 is readily used to direct a portion of the
transmitted light into eyepiece 150 for visual alignment. Optical
cutoff filter 155 is readily used to block the (generally, shorter
wavelength) excitation light emitted by source 10 from entering the
eyepiece and photomultiplier tube.
Appropriate mounting systems for the probe and the sample are well
known in the art and need not be described here in detail.
Exemplary mounting systems are described in U.S. Pat. No.
4,917,462, issued to A. Lewis, et al. on Apr. 17, 1990. Details of
the optical coupling between a light source and a single-mode
optical fiber are described in U.S. patent application Ser. No.
615,537, cited above.
A number of variations on the optical arrangement for
transmission-mode imaging depicted in FIG. 1 will be readily
apparent to the skilled practitioner. For example, the excitation
light may be transmitted from source 10 to portion 65 through the
air instead of through light-guiding means. The excitation light
may be transmitted through the air directly from source 10 to
portion 65, or, e.g., it may first be directed into beamsplitter
140, through objective 120, and through the sample before impinging
on portion 65. In yet another variation, light-guiding means such
as fiber 70 are readily employed, not to convey excitation light to
portion 65, but instead to convey fluorescent light to detector
130. In such a variation, the excitation light is impinged on the
bottom of the sample (as viewed in FIG. 1) by objective 120, and
the excitation light impinges on portion 65 only after passing
through the sample.
As noted, the transmission mode is only one of several imaging
modes. One alternative mode is the reflection mode. An exemplary
optical arrangement for reflection-mode imaging is depicted in FIG.
2. As shown in FIG. 2, excitation light from source 10 is directed
onto portion 65 via beamsplitter 140 and annular lens 200. (Lens
200 is optionally made into an annulus in order to permit the back
portion of the probe, or an optical fiber extending from the back
portion of the probe, to extend along the optical axis up to or
even beyond the axial location of lens 200.) Fluorescent light, in
turn, is collected by annular lens 200 and directed into detector
130 via beamsplitter 140 and cutoff filter 155. In at least some
instances, it may be desirable to block excitation light from
impinging directly on probe 20. That is, probe 20 may comprise
glass which is capable of fluorescing when impinged by the
excitation light. Such impingement is undesirable because the
resulting fluorescence may increase the background level detected
by detector 130 and therefore decrease the signal-to-noise ratio.
It will be readily apparent to the skilled practitioner that in an
alternative arrangement, either the excitation light incident on
portion 65, or fluorescent light collected from portion 65, or even
both excitation and fluorescent light, is readily transmitted
through probe 20 to the source or detector, as appropriate. In
particular, a tapered fiber probe provides very efficient means for
transmitting light to or from portion 65.
Several effects are capable of modulating the fluorescent light
intensity detected at detector 130. One such effect is perturbation
of the optical-frequency electromagnetic field at portion 65 due to
the proximity of the sample surface. A second effect is quenching
of the fluorescence in portion 65. For example, dipole-dipole
interactions, which extend, typically, over distances up to about
100 .ANG., are capable of deactivating optically excited species in
portion 65 and thus decreasing the amount of fluorescent light. The
degree to which a given fluorescent species is quenched depends on
the composition of the nearby material with which it is
interacting. As a consequence, fluorescence quenching provides a
mechanism for imaging not only surface topography (which affects
propinquity of the surface to the probe tip), but also surface
composition.
It should be noted in this regard that specialized dyes can be
provided, that are sensitive to special properties of the sample
surface. For example, dyes will be readily apparent to the skilled
practitioner that are sensitive to the local concentration of
hydrogen ions on the sample surface, thus providing pH-sensitive
imaging. Similarly, dyes will be apparent whose fluorescence is
quenched by the presence of other specific ions. Other dyes will be
apparent, having fluorescence that is sensitive to local electric
fields. Probes incorporating such dyes may, for example, be capable
of mapping electrical potentials on the sample surface.
Additionally, fluorescent species may be provided having spectrally
sharp emission that exhibits a measurable Stark shift in the
presence of electric fields. Probes incorporating such species are
also capable of mapping electrical potentials on a sample
surface.
Probe 20 is exemplarily a tapered glass probe, for example a
tapered pipette or a tapered, single-mode optical fiber. Details of
preparing appropriate pipette probes are described in U.S. Pat. No.
4,917,462, cited above. Briefly, the pipette is formed by heating
and drawing a glass tube to taper the pipette and then further
heating and drawing it until it breaks. By selecting various wall
thicknesses of the glass tube, inner tip diameters of from less
than 1000 .ANG. to 5000 .ANG., with outer tip diameters of
5000-7500 .ANG. are readily and reproducibly generated. If the
probe is to function as an aperture-based probe, a metal coating is
evaporated onto the outer surface of the pipette to increase the
opacity of the glass wall. However, such a metal coating is
generally undersirable for purposes of the inventive probe because
it increases the outer diameter of the probe and may cause
scattering of excitation light. Useful pipettes are readily formed
using, for example, a commercially available, gravity-driven
pipette puller with induction heating.
Details of preparing appropriate tapered fiber probes are described
in U.S. patent application Ser. No. 615,537, cited above. Briefly,
a single-mode optical fiber is heated and drawn until it breaks,
analogously with the pipette described above. The tapering is
readily achieved in, for example, a commerically available,
gravity-driven pipette puller. Heating is advantageously provided
by a laser. For purposes of the inventive probe, metallization of
the fiber is generally undesirable for the reasons described above
in connection with pipette probes. However, if the fluorescent
portion is to be formed by ion implantation of the fiber, then
after drawing, at least a portion of the tapered fiber is desirably
coated with a metal such as gold to serve as an implantation mask.
The mask is generally stripped off after ion implantation has been
completed.
Significantly, after drawing, the end portion of the fiber
comprises a taper that terminates in an end flat. If ion
implantation is performed, the ions are implanted in the end
flat.
With reference to FIG. 3, the probe is made, in one currently
preferred embodiment, from a tapered pipette 300. The fluorescent
portion comprises one or more fluorescent particles that adhere to
the tip of the pipette. For example, a single, fluorescent,
spherical particle 310 is readily provided having a diameter
slightly greater than the diameter of the aperture in the tip of
the pipette. Such a particle may be placed partially within the
aperture such that it is centered within the aperture and adheres
to a rim-like portion of the inner wall of the pipette at or near
the aperture. As noted, pipettes are readily fabricated having
aperture diameters of 200-5000 .ANG.. Spherical latex balls having
diameters of 200-5000 .ANG. are readily made, and are available
commercially from, e.g., the Interfacial Dynamics company of
Portland, Oreg. Such latex balls are readily impregnated with
fluorescent dyes. Impregnation of latex balls with fluorescent dye
is performed commercially by, for example, the Molecular Probes
company of Eugene, Oreg.
Even smaller fluorescent particles, as small as about 40 .ANG. in
diameter, can be made from fluorescent semiconductor compounds such
as cadmium sulfide or other II-VI compounds. There are several ways
to use fluorescent particles that are smaller in diameter than the
aperture. For example, a latex ball, which may itself be
non-fluorescent, is readily situated in the aperture as described.
Fluorescence is provided by one or more small particles that are
affixed to the surface of the latex ball distal the aperture.
Alternatively, one or more small particles may be affixed directly
to the glass pipette tip near the aperture.
With reference to FIG. 4, in a currently preferred procedure,
capillary action is used to affix a fluorescent-dye-impregnated
latex ball 320 within the aperture of a pipette probe 300. The ball
diameter is slightly larger than the aperture. For example, a batch
of balls is provided having diameters of about 680 .ANG., and a
corresponding pipette is provided having an aperture diameter of
500 .ANG.. The balls are manufactured such that they include
chemical surface groups, such as amidine surface groups, that
promote adhesion to the surface of the glass. The latex balls are
suspended in distilled water 330. The concentration of the
suspension is preferably about 2.5.times.10.sup.-3 wt. % latex. In
order for the process to have a relatively high yield of acceptable
probes, the balls should be relatively uniform in size, desirably
distributed in size with a variance of not more than about 30%, and
still more desirably less than about 10%. The pipette is dipped
into the suspension just until the pipette tip breaks the meniscus
of the suspension, and then the tip is withdrawn. If the dip is
successful, water is drawn up into the pipette by capillary action.
As a consequence of the influx of water, latex balls are carried
toward the pipette tip. The influx is stopped when an entrained
latex ball lodges in the aperture.
Additionally latex balls may adhere to the glass outside of the
aperture region. Significantly, the ball that adheres to a rim-like
region will be held more tightly than balls that cling by
point-contact to the outside of the aperture region. As a
consequence, balls adhering to the outer sides of the pipette are
readily removed by dipping the pipette tip in an ultrasonically
agitated distilled water bath. A few seconds is generally enough to
remove undesired, adherent balls.
It should be noted in this regard that the influx of water into the
pipette is facilitated if the pipette is provided with an omega
dot. (An "omega dot" is a glass fiber fused to the inner surface of
the pipette and extending substantially parallel to the
longitudinal axis of the pipette. Such a feature is well-known to
facilitate capillary action. Glass tubes for forming pipettes that
include omega dots are commercially available, e.g., from Sutter
Instruments.)
The water that enters the pipette as a result of capillary action
will eventually evaporate. If desired, the water can be replaced by
injecting a liquid into the pipette through the end opposite the
tip. The latex sphere will continue to adhere even after all the
water has evaporated. However, the continued presence of an
appropriate liquid in the pipette might prolong the useful
lifetimes of some fluorescent dyes, for example by excluding
oxygen.
With reference to FIG. 5, in a second, currently preferred
embodiment, the probe is made from a tapered, single-mode optical
fiber 500 instead of a pipette. A tapered fiber probe is especially
desirable because it is capable of channeling light to or from the
fluorescent portion with high efficiency. Fluorescent portion 565
is created by ion implanting the end flat of the fiber. As noted,
the outer fiber surface along a terminal portion is first coated
with an appropriate metal, such as gold, to serve as an
implantation mask. The mask is stripped off after implantation.
Thus in this embodiment, portion 565 is integral with the fiber.
The diameter of an end flat for this purpose desirably ranges from
about 100 .ANG. or even less to about 1000 .ANG. or more.
Implanting a region substantially smaller than about 100 .ANG. will
result in a correspondingly small signal. Implanting a region
substantially larger than 1000 .ANG. will result in correspondingly
coarse resolution. Various ions are useful for creating a
fluorescent region. These include, but are not limited to, ions of
cerium, terbium, and europium. The implanted region is desirably as
close to the surface of the end flat as possible, in order to
concentrate the ions within the near field of the sample. (However,
the ions should not be concentrated within so shallow a region that
substantial concentration quenching takes place.) A useful range of
implantation densities is from about 10.sup.14 cm.sup.-2 to about
10.sup.16 cm.sup.-2. A substantially smaller density is
undersirable because a correspondingly small signal will be
obtained. A substantially larger density is undersirable because
concentration quenching may take place.
Instead of forming a fluorescent fiber portion by ion implantation,
a tapered fiber, can, alternatively, be provided with an attached,
fluorescent particle. For example, a fiber may be provided, having
a borehole formed in the end flat and extending substantially
parallel to the longitudinal axis of the fiber. Appropriate methods
for forming such a borehole will be readily apparent to the skilled
practitioner. For example, a fiber having a borehole is readily
provided by initially drawing the fiber from a special preform. The
preform includes a rod of glass that is relatively rapidly etched,
embedded in a collapsed tube of glass that is relatively resistant
to etching. After the fiber is heated, drawn, and broken off to
form a probe, the end of the fiber is exposed to an appropriate
etchant. After a borehole is formed, the fiber is dipped into a
suspension, e.g., of latex balls, in analogy with the treatment of
a pipette probe described above.
It should be pointed out that the fluorescent portion is not
necessarily excited by electromagnetic radiation. For example, the
fluorescent portion may be an LED or other electroluminescent light
source. In such a case, excitation is provided by applying an
appropriate electric field or electric current to the fluorescent
portion. A pair of electrical conductors extending to a probe tip
of suitably small dimensions is readily provided, for example, by
the method disclosed in U.S. Pat. No. 4,747,698, issued to H. K.
Wickramasinghe, et al. on May 31, 1988. As disclosed therein,
electropolishing techniques are used to prepare a tungsten probe
having a tip diameter of about 100 .ANG.. The tungsten is
overcoated first with a dielectric layer, and then with a
conductive layer. In the cited patent, the tungsten core and the
conductive layer form a thermocouple junction. However, the
conductive layer is readily applied (e.g. by evaporative deposition
from the side) in such a way that it does not join the tungsten
core. A two-terminal electroluminescent device can then be mounted
on the probe tip such that one terminal contacts the tungsten core
and the other terminal contacts the conductive layer.
The inventive apparatus is useful, inter alia, for inspecting
processed semiconductor wafers on a manufacturing line.
Accordingly, the invention, in one aspect, is a method for
manufacturing an article, using an optical system which includes
the inventive probe. Such an inventive method comprises the steps
outlined below, and shown in the block diagram of FIG. 6. With
reference to FIG. 6, a multiplicity of semiconductor wafers is
first provided, each wafer having a surface to be patterned at one
or more stages of the manufacturing process (Step A of FIG. 6). The
patterns that are to be formed have characteristic dimensions,
sometimes referred to as "line widths," that must generally be kept
within close tolerances. The inventive apparatus is readily used
for measuring line widths (D), such as the width of metallic
conductors on a wafer, or the length of gates formed in
metal-oxide-semiconductor (MOS) structures on a wafer. After one or
more process parameters, exemplarily lithographic exposure times or
etching times, are initially set (B), one or more initial wafers
are processed according to those parameters, such that patterns are
formed on the wafer surfaces (C). The corresponding line widths are
readily measured using the inventive apparatus, and are then
readily compared (E) with predetermined, desired ranges of values.
Depending upon the results of such comparison, the process
parameters are then readily adjusted (H) to bring the measured
dimensions within the desired tolerances, and further wafers are
processed (F). Additional steps (G) toward completion of a
manufactured article are then performed. The measuring step
includes providing a probe having a tip, the probe tip comprising
fluorescent material capable of emitting light at least at a given
wavelength when the fluorescent material is appropriately excited,
positioning the probe relative to the wafer such that the distance
between the probe tip and at least a portion of the pattern is
smaller than the given wavelength (I), displacing the probe tip
relative to the wafer, during the displacing step, exciting
fluorescence in the probe tip (J), and detecting fluorescent light
emitted by the probe tip (K).
EXAMPLE I
An aluminosilicate glass pipette was obtained, having outer and
inner diameters of 1 mm and 0.58 mm, respectively. The pipette
included an omega dot to facilitate capillary action. The pipette
was mounted in a Sutter Instruments Mod. P-87 micropipette puller,
and pulled under the control of an eight-line program. Each of the
first seven lines of the program had the following entries:
cooling-air pressure, 400; heat, 820; hard pull, 0; velocity, 10;
time, 255. The eighth line had the following respective entries:
400; 840; 50; 8; 50. The resulting pipette had, at the tip, an
outer diameter of about 1000 .ANG. or slightly less, and an inner
diameter of about 500 .ANG.. Latex balls were obtained, having
diameters of about 680 .ANG.. As obtained, the balls were
impregnated with fluorescent dye and included amidine surface
groups to promote adhesion to glass. The dyes came from the
commercial supplier in an aqueous suspension, which was diluted
with distilled water to 2.5.times.10.sup.-3 wt. % latex. While
under visual observation in a stereoscopic microscope, a pipette
was lowered into the latex ball suspension until the pipette tip
barely broke the meniscus at the surface of the suspension. As a
result of this step, a latex ball lodged in the aperture of the
pipette, and additional balls attached to the outer wall of the
pipette. The pipette tip was then withdrawn and subjected to
ultrasonic agitation in a container of distilled water. The
agitation removed the excess latex balls. The pipette tip was then
visually inspected under a microscope with excitation light
impinging on the latex ball in the aperture such that fluorescent
light visibly emanated from the ball. The pipette was then mounted
in a scanning device and illuminated with laser light at a
wavelength of 488 nm. As a result of the laser illumination, the
ball fluoresced at a peak wavelength lying in the range 530-550
nm.
The pipette was used to scan, in a transmission-mode geometry, a
sample consisting of an aluminum grating on a glass substrate. The
illumination geometry that was used is conveniently described with
reference to FIG. 1. As viewed in the figure, the probe was
positioned above the sample, and an objective (objective 20 of the
figure) was positioned below the sample. The same objective was
used both to concentrate excitation light on the probe tip, and to
collect fluorescent light from the probe tip. Thus, both excitation
and fluorescent light passed through the sample. Features smaller
than 0.25 .mu.m were resolved by this means.
EXAMPLE II
A single-mode optical fiber was drawn in a Sutter Instruments Mod.
P-87 micropipette puller to make a probe. During drawing of the
fiber, the fiber was heated with a continuous, non-focused 25-watt
beam from a carbon dioxide laser. The control settings for the
pipette puller (in a one-line program) were: pull, 75; velocity, 4;
time, 1. After drawing, a terminal portion of the fiber was
evaporatively coated with metal to serve as an implantation shield
for subsequent ion implantation. First, 50 .ANG.-100 .ANG. of
chromium was deposited, and over that, about 750 .ANG. of gold was
deposited. Ions were implanted head-on toward the end flat of the
tapered fiber. Exemplary ions used were ions of cerium, europium,
and terbium. In each case, the implantation energy was about 100
keV, and the implantation density was from about 10.sup.14
cm.sup.-2 to about 10.sup.16 cm.sup.-2. After implantation, the
gold was stripped in a potassium iodide etchant. The chromium was
then stripped in a ceric ammonium nitrate etchant. Remaining traces
of chromium were volatilized by flash heating in a roughly 10-watt
carbon dioxide laser beam. The fiber was then mounted in a scanning
device.
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