U.S. patent application number 10/261084 was filed with the patent office on 2003-05-22 for carbon nanotube probe tip grown on a small probe.
Invention is credited to Mitchell, Thomas Owen.
Application Number | 20030094035 10/261084 |
Document ID | / |
Family ID | 24637147 |
Filed Date | 2003-05-22 |
United States Patent
Application |
20030094035 |
Kind Code |
A1 |
Mitchell, Thomas Owen |
May 22, 2003 |
Carbon nanotube probe tip grown on a small probe
Abstract
A method of fabricating a carbon nanotube probe tip and the
resultant probe tip, particularly for use in an atomic force
microscope. A moderately sharply peaked support structure has its
tip cut or flattened to have a substantially flat end of size of
about 20 to 200 nm across. The support structure may be formed by
etching a conical end into a silica optical fiber. Nickel or other
catalyzing metal such as iron is directionally sputtered onto the
flat end and the sloped sidewalls of the support structure. The
nickel is anisotropically etched to remove all the nickel from the
sidewalls but leaving at least 15 nm on the flat end to form a
small nickel dot. A carbon nanotube is then grown with the nickel
catalyzing its growth such that only a single nanotube forms on the
nickel dot and its diameter conforms to the size of the nickel
dot.
Inventors: |
Mitchell, Thomas Owen;
(Redwood City, CA) |
Correspondence
Address: |
MICHAEL O. SCHEINBERG
P.O. BOX 164140
AUSTIN
TX
78716-4140
US
|
Family ID: |
24637147 |
Appl. No.: |
10/261084 |
Filed: |
September 30, 2002 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
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10261084 |
Sep 30, 2002 |
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09657428 |
Sep 8, 2000 |
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6457350 |
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Current U.S.
Class: |
73/105 ;
850/55 |
Current CPC
Class: |
Y10S 977/863 20130101;
C01B 2202/36 20130101; Y10S 977/876 20130101; C01B 32/162 20170801;
G01Q 70/12 20130101; Y10S 977/856 20130101 |
Class at
Publication: |
73/105 |
International
Class: |
G01B 005/28 |
Claims
What is claimed is:
1. A method of forming a probe tip, comprising the steps of:
providing a member comprising a shaped tip having sidewalls and
extending along an axis; cutting a flat surface in said shaped tip;
anisotropically depositing a catalytic material onto said flat
surface and onto said sloping sidewalls; directionally etching said
catalytic material to remove said catalytic material from said
sidewalls while leaving a thickness of said catalytic material on
said flat surface; and growing a carbon nanotube on a portion of
said catalytic material remaining on said flat surface in a process
catalyzed by said catalytic material.
2. The method of claim 1, wherein said flat surface has a minimum
lateral size of between 15 and 300 nm.
3. The method of claim 1, wherein said catalytic material comprises
nickel.
4. The method of claim 1, wherein said catalytic material comprises
iron.
5. The method of claim 1, wherein said member comprises silicon
oxide.
6. The method of claim 1, wherein said member is formed from silica
fiber.
7. The method of claim 6, wherein said member has said ends formed
into said fiber and said sidewalls slope from an axis of said
fiber.
8. The method of claim 6, wherein said planar end is cut to be
non-perpendicular to an axis of said fiber.
9. The method of claim 1, wherein said cutting step cuts said
sidewalls into said member to be parallel to each other.
10. The method of claim 1, wherein said member comprises
silicon.
11. The method of claim 1, wherein said shaped tip has a pyramidal
shape.
12. The method of claim 1, wherein said anisotropic coating step
comprises sputtering.
13. The method of claim 1, wherein said cutting step comprises
focused ion beam milling.
14. The method of claim 1, further comprising cutting said carbon
nanotube to reduce its length.
15. The method of claim 14, wherein said cutting said carbon
nanotube comprises focused ion beam milling.
16. A probe tip, comprising: a support including a shaped tip
having a planar end of minimum lateral extent of between 15 and 300
nm and sidewalls sloping from said planar end; a catalyzing layer
of material capable of catalyzing growth of carbon nanotubes formed
on said planar end but not on said sidewalls; and a single carbon
nanotube formed on said catalyzing layer.
17. The probe tip of claim 16, wherein said nanotube is a
multi-wall nanotube.
18. The probe tip of claim 16, wherein said material comprises
metallic nickel.
19. The probe tip of claim 15, wherein said material comprises
nickel oxide.
20. The probe tip of claim 15, wherein said material comprises
metallic iron or iron oxide.
21. The probe tip of claim 15, wherein said sidewalls slope from
said planar end by between 60.degree. and 90.degree..
22. The probe tip of claim 15, wherein said catalyzing layer has a
thickness of at least 15 nm.
23. An atomic force microscope including the probe tip of claim 15
and a vertical actuator, wherein said actuator causes said probe
tip to encounter a surface being probed.
Description
FIELD OF THE INVENTION
[0001] The invention relates generally to mechanical probe tips
such as those used in atomic force microscopy. In particular, the
invention relates to a carbon nanotube grown directly on a pointed
end of a probe.
BACKGROUND ART
[0002] Atomic force microscopes (AFMs) have been recently developed
for mechanically profiling small features, for example, determining
critical dimensions (CDs) of via holes in semiconductor integrated
circuits. Such holes have depths of about 1 .mu.m and widths which
are being pushed to 180 nm and below. For detailed measurement of
the feature, an exceedingly fine probe tip is disposed on the end
of a cantilever overlying the feature. In the pixel mode of
operation, the probe tip is successively positioned at points on a
line above and traversing the feature being probed. The cantilever
lowers the probe tip until it encounters the surface, and both the
horizontal position and the vertical position at which the probe
meets the surface are recorded. A series of such measurements
provide the desired microscopic profile. An example of such an
atomic force microscope is the Stylus Nanoprobe SNP available from
Surface/Interface, Inc. of Sunnyvale, Calif. It employs technology
similar to the rocking balanced beam probe disclosed by Griffith et
al. in U.S. Pat. No. 5,307,693 and by Bryson et al. in U.S. Pat.
No. 5,756,887.
[0003] Such a tool is schematically illustrated in the side view of
FIG. 1. A few more details are found in U.S. patent application
Ser. No. 09/354,528, filed Jul. 15, 1999 and incorporated herein by
reference in its entirety. A wafer 10 or other sample to be is
supported on a support surface 12 supported successively on a tilt
stage 14, an x-slide 16, and a y-slide 18, all of which are movable
along their respective axes so as to provide horizontal
two-dimensional and tilt control of the wafer 10. Although these
mechanical stages provide a relatively great range of motion, their
resolutions are relatively coarse compared to the resolution sought
in the probing. The bottom y-slide 18 rests on a heavy granite slab
20 providing vibrational stability. A gantry 22 is supported on the
granite slab 20. A probe head 24 hangs in the vertical z-direction
from the gantry 22 through an intermediate piezoelectric actuator
26 providing about 10 .mu.m of motion in (x, y, z) by voltages
applied across electrodes attached to the walls of a piezoelectric
tube. A probe assembly with a tiny attached probe tip 28 projects
downwardly from the probe head 24 to selectively engage the probe
tip 28 with the top surface of the wafer 10 and to thereby
determine its vertical and horizontal dimensions.
[0004] Principal parts of the probe head 24 of FIG. 2 are
illustrated in the side view of FIG. 2. A dielectric support 30
fixed to the bottom of the piezoelectric actuator 26 includes on
its top side, with respect to the view of FIG. 1, a magnet 32. On
the bottom of the dielectric support 30 are deposited two isolated
capacitor plates 34, 36 and two interconnected contact pads 38.
[0005] A beam 40 is medially fixed on its two lateral sides and is
also electrically connected to two metallic and ferromagnetic ball
bearings 42. The beam 40 is preferably composed of heavily doped
silicon so as to be electrically conductive, and a thin silver
layer is deposited on it to make good electrical contacts to the
ball bearings. The two ball bearings 42 are placed on respective
ones of the two contact pads 38 and generally between the capacitor
plates 34, 36, and the magnet 32 holds the ferromagnetic bearings
42 and the attached beam 40 to the dielectric support 30. The
attached beam 40 is held in a position generally parallel to the
dielectric support 40 with a balanced vertical gap 46 of about 25
.mu.m between the capacitor plates 34, 36 and the beam 40.
Unbalancing of the vertical gap allows a rocking motion of about 25
.mu.m. The beam 40 holds on its distal end a glass tab 48 to which
is fixed a stylus 50 having the probe tip 52 projecting downwardly
to selectively engage the top of the wafer 10 being probed.
[0006] Two capacitors are formed between the respective capacitor
plates 34, 36 and the conductive beam 40. The capacitor plates 34,
36 and the two contact pads 38, commonly electrically connected to
the conductive beam 40, are separately connected by three
unillustrated electrical lines to three terminals of external
measurement and control circuitry This servo system both measures
the two capacitances and applies differential voltage to the two
capacitor plates 34, 36 to keep them in the balanced position. When
the piezoelectric actuator 26 lowers the stylus 50 to the point
that it encounters the feature being probed, the beam 40 rocks upon
contact of the probe tip 52 with the wafer 10. The difference in
capacitance between the plates 34, 36 is detected, and the servo
circuit attempts to rebalance the beam 40 by applying different
voltages across the two capacitors, which amounts to a net force
that the stylus 50 is applying to the wafer 10. When the force
exceeds a threshold, the vertical position of the piezoelectric
actuator 26 is used as an indication of the depth or height of the
feature.
[0007] This and other types of AFMs have control and sensing
elements more than adequate for the degree of precision for
profiling a 1180 nm.times.1 .mu.m hole. However, the probe tip
presents a challenge for profiling the highly anisotropic holes
desired in semiconductor fabrication as well as for other uses such
as measuring DNA strands and the like. The probe tip needs to be
long, narrow, and stiff. Its length needs to at least equal the
depth of the hole being probed, and its width throughout this
length needs to be less than the width of the hole. A fairly stiff
probe tip reduces the biasing introduced by probe tips being
deflected by a sloping surface.
[0008] One popular type of probe tip is a shaped silica tip, such
as disclosed by Marchman in U.S. Pat. Nos. 5,395,741 and 5,480,049
and by Filas and Marchman in U.S. Pat. No. 5,703,979. A thin silica
fiber has its end projecting downwardly into an etching solution.
The etching forms a tapered portion near the surface of the fiber,
and, with careful timing, the deeper portion of the fiber is etched
to a cylinder of a much smaller diameter. The tip manufacturing is
relatively straightforward, and the larger fiber away from the tip
provides good mechanical support for the small tip. However, it is
difficult to obtain the more desirable cylindrical probe tip by the
progressive etching method rather than the tapered portion alone.
Furthermore, silica is relatively soft so that its lifetime is
limited because it is continually being forced against a relatively
hard substrate.
[0009] One promising technology for AFM probe tips involves carbon
nanotubes which can be made to spontaneously grow normal to a
surface of an insulator such as glass covered with a thin layer of
a catalyzing metal such as nickel. Carbon nanotubes can be grown to
diameters ranging down to 5 to 20 nm and with lengths of
significantly more than 1 .mu.m. Nanotubes can form as single-wall
nanotubes or as multiple-wall nanotubes. A single wall is an
cylindrically shaped atomically thin sheet of carbon atoms arranged
in an hexagonal crystalline structure with a graphitic type of
bonding. Multiple walls bond to each other with a tetrahedral
bonding structure, which is exceedingly robust. The modulus of
elasticity for carbon nanotubes is significantly greater than that
for silica. Thus, nanotubes offer a very stiff and very narrow
probe tip well suited for atomic force. microscopy. Furthermore,
carbon nanotubes are electrically conductive so that they are well
suited for scanning tunneling microscopy and other forms of probing
relying upon passing a current through the probe tip. Dai et al.
describe the manual fabrication of a nanotube probe tip in
"Nanotubes as nanoprobes in scanning probe microscopy," Nature,
vol. 384, 14 November 1996, pp. 147-150.
[0010] Typically, nanotubes suffer from the disadvantage that a
large number of them simultaneously form on a surface producing
either a tangle or a forest of such tubes, as is clearly
illustrated by Ren et al. in "Synthesis of large arrays of
well-aligned carbon nanotubes on glass," Science, vol. 282, 6
November 1998, pp. 1105-1107. The task then remains to affix one
nanotube to a somewhat small probe tip support. Dai et al. disclose
an assembly method in which they coat the apex of a silicon pyramid
at the probe end with adhesive. The coated silicon tip was then
brushed against a bundle of nanotubes, and a single nanotube can be
pulled from the bundle. This method is nonetheless considered
expensive and tedious requiring both optical and electron
microscopes. Additionally, there is little control over the final
orientation of the nanotube, certainly not to the precision needed
to analyze semiconductor features. Cheung et al. describe another
method of growing and transferring nanotubes in "Growth and
fabrication with single-walled carbon nanotube probe microscopy
tips," Applied Physics Letters, vol. 76, no. 21, 22 May 2000, pp.
3136-3138. However, they either produce poor directional control
with a very narrow, single nanotube or require a complex transfer
mechanism with nanotube bundles.
[0011] Ren et al. describe a method of growing isolated nanotubes
in "Growth of a single freestanding multiwall carbon nanotube on
each nanonickel dot," Applied Physics Letters, vol. 75, no. 8, 23
August 1999, pp. 1086-1088. They deposit 15 nm of nickel on silicon
and pattern it into a grid of nickel dots having sizes of somewhat
more than 100 nm. Plasma-enhanced chemical vapor deposition using
acetylene and ammonia produces a single nanotube on each dot having
an obelisk shape with a base diameter of about 150 nm and a
sharpened tip. However, Ren et al. do not address the difficult
problem of transferring such a nanotube, which they describe as
being tightly bonded to the nickel, from the nickel-plated
substrate to a probe end.
[0012] Cheung et al. disclose another method of growing isolated
nanotubes in "Carbon nanotube atomic force microscopy tips: Direct
growth by chemical vapor deposition and appliaction to
high-resolution imaging," Proceedings of the National Academy of
Sciences, vol. 97, no. 8, 11 April 2000, pp. 3809-3813. They etch
aniostropic holes in a silicon tip and deposit the catalyzing iron
or iron oxide in the bottom of the holes. The carbon nanotubes grow
out of the holes. However, growth in such restricted geometries is
considered to be disadvantageous and to favor single-wall rather
than multiple-wall nanotubes. Further, this method provides only
limited control over the number and size of the nanotubes being
grown.
[0013] Accordingly, a more efficient method is desired for forming
a probe tip having a single carbon nanotube. Furthermore, the
structure of the probe end and probe tip should facilitate assembly
of the probe and contribute to its robustness.
SUMMARY OF THE INVENTION
[0014] A probe end is shaped to have sloping sides and a generally
flat end, that is, in the shape of sloping mesa. The diameter of
the mesa top is preferably in the range of 20 to 300 nm. Nickel or
other material that catalyzes the growth of carbon nanotubes is
directionally deposited onto the probe end. Because of the
geometry, the thickness of the deposited nickel, as measured from
the underlying surface, is greater on the mesa top than on the mesa
sides. The nickel is then isotropically etched for a time
sufficient to remove the nickel from the mesa sides but to leave
sufficient nickel on the mesa top to catalyze the growth of a
single carbon nanotube. Typically, the nanotube grows with a bottom
diameter approximately equal to that of the nickel dot on top of
the mesa.
BRIEF DESCRIPTION OF THE DRAWINGS
[0015] FIG. 1 is a schematic elevational view of a rocking beam
atomic force microscope.
[0016] FIG. 2 is a cross-sectional side view of a portion of the
atomic force microscope of FIG. 1.
[0017] FIG. 3 is a cross-sectional side view of a probe end having
a tapered tip and available in the prior art, the figure including
an exploded view of the sharp probe end.
[0018] FIG. 4 is a cross-sectional view showing the position of a
sectioning of the probe end of FIG. 2.
[0019] FIG. 5 is a cross-sectional view showing the sectioned probe
end.
[0020] FIG. 6 is a cross-sectional view showing the directional
deposition of nickel or other catalyzing material.
[0021] FIG. 7 is a cross-sectional view showing a nickel dot formed
only on the sectioned end of the probe end.
[0022] FIG. 8 is a cross-sectional view showing a nanotube grown on
the nickel dot.
[0023] FIG. 9 is a cross-sectional view showing a second embodiment
of the invention.
DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS
[0024] The invention allows the fabrication of a single carbon
nanotube on a narrow support structure well suited for easy
attachment to a probe of an atomic force microscope (AFM) or other
type of microprobe.
[0025] A support structure 60, illustrated in side view in FIG. 3,
is formed having a relatively massive upper portion 62 and a shaped
tip 64 with a sharp point 66 having a curvature of less than about
50 nm. On the scale of probe tips, the upper portion 62 and the
shaped tip 64 have a common longitudinal axis. The support
structure 60 is illustrated with the orientation of its intended
final use in a microscope overlying a sample being probed. The
support structure 60 may be the quartz (silica) fiber of Marchman
in which the shaped tip 64 is formed by placing an end of a 125
.mu.m fiber in a bath of hydrofluoric acid (HF) overlaid by a layer
of oil and leaving it in that position for a sufficiently long
period that the fiber end is etched to a point. That is, the
etching continues to completely etch away the cylinder of the
Marchman tip. The point at which the HF completely dissolves the
fiber defines the sharp point 66. Alternatively, the shaped tip can
be defined by polishing and grinding, particularly for sapphire
fiber. The shaped tip 64 need not have a strictly conical shape,
but it is advantageous that there be an sloping portion between the
sharp point 66 and the relatively massive fiber 62 to provide
mechanical stability in the finally assembled probe.
[0026] The support structure 60 is then subjected to focused ion
beam (FIB) milling along a line 68, illustrated in the
cross-sectional view of FIG. 4, that in this embodiment is
transverse to the axis of the support structure and passes through
a predetermined width of the shaped tip 64. The predetermined width
closely corresponds to the width of the final carbon nanotube and
may be, for example, 100 nm. FIB milling is a well known technique
for micromicromaching and relies upon a focused beam typically of
gallium ions to mill structures with a resolution down to about 5
nm. Such a system is the FIB 200TEM available from FEI Company of
Hillsboro, Oreg. Other milling techniques could be used, but FIB
milling is effective and economical.
[0027] The milling produces a shaped tip 64', illustrated in the
cross-sectional view of FIG. 5, having a flat end 70 and sloping
sidewalls 72. Then, as illustrated in the cross-sectional view of
FIG. 6, a film 76 of nickel or other catalyzing metal is then
directionally deposited onto the probe tip 64', preferably by
sputtering metal atoms along the longitudinal axis of the shaped
tip 64'. The thickness of the deposition, as measured along the
longitudinal axis, is substantially constant between the area of
the flat end 70 and the sloping sidewalls 72 of the shaped tip 64'.
However, the thickness, as measured at a perpendicular to the
underlying surface, is substantially thicker in the area overlying
the flat 70 than in the areas overlying the sloping sidewalls 72.
The effect is primarily geometric. If the probe tip has a tip angle
2.theta. and the deposition is totally aniostropic, then the
sidewall thickness is sin.theta. times the end thickness. For
example, if 2.theta.=31.3.degree., then the sidewall thickness is
27% of the end thickness. The sputtering may be performed in an ion
sputtering system using a nickel target. Such a system is the Model
681 High Resolution Ion Coater from Gatan of Pleasanton, Calif.
Other types of deposition are possible, such as molecular beam
techniques usually associated with molecular beam epitaxy.
[0028] As illustrated in the cross-sectional view of FIG. 7, the
nickel-plated shaped tip 64' is subjected to isotropic etching of
the nickel for a time just sufficient to remove the nickel from the
tip sidewalls 72 but leaving a nickel dot 80 over the tip end 70. A
minimum thickness of approximately 15 to 20, preferably 30 to 40
nm, of nickel is desired in the area of the nickel dot 80 to
catalyze the nanotube growth. Assuming the lower value of 15 nm and
a tip angle 2.theta. of 31.3.degree., about 27 nm of nickel needs
to be anisotropically deposited over the area of the tip end 70 to
account for the end nickel being thinned during removal of the
sidewall nickel, which has an initial thickness of 7 nm. The
etching time obviously needs to be controlled so that it continues
long enough to remove the sidewall nickel while leaving sufficient
of the end nickel.
[0029] It may be advantageous to oxidize the nickel prior to
etching, and in any case nickel will typically have an oxidized
surface layer upon any exposure to air. Any number of isotropic wet
etchants for nickel and nickel oxide are known, as tabulated in CRC
Handbook of Metal Etchants, eds. Walker et al., CRC Press, 1991,
pp. 857-875 and include dilute nitric and sulfuric acids for nickel
and ammonium hydroxide for nickel oxide.
[0030] The nickel dot 80 provides a small catalyzing area for the
growth of a single carbon nanotube 84 illustrated in
cross-sectional view in FIG. 8. Ren et al. and Cheung et al.
describe the process for selective growth of nanotubes in the above
cited references. The diameter of the nanotube 34 corresponds
generally to the diameter or average lateral extent of the
flattened end 70 of the shaped tip 64'. For a non-circular end 80,
the nanotube diameter is approximately equal to the minimum lateral
extent of the end.
[0031] It has proven difficult to control the length to which
nanotubes grow. Accordingly, it may be necessary to perform an
additional step of cutting the carbon nanotube to a prescribed
length, for example, by FIB milling.
[0032] The probe structure illustrated in FIG. 8 includes a
relatively rugged support structure 62, illustrated in FIG. 3,
which is ready to be mounted onto the probe of the AFM or other
microscope using a stylus.
[0033] The method described above requires that the sidewalls of
the shaped tip slope away from the tip end. To achieve the required
differential but isotropic etching, the slope is preferably at
least 60.degree. from the plane. The differential coating works
even with a slope of 90.degree., that is, vertical sidewalls. Such
a shape may be produced by FIB milling, for example, a cylinder
having a diameter of 100 nm or a similarly sized rectangular post
into the tip 66 at the end of the conical tip 64 prior to nickel
deposition.
[0034] The embodiment described above produces a carbon nanotube
extending along the axis of the conical tip. However, in another
embodiment illustrated in the cross-sectional view of FIG. 9, a
shaped tip 64" is formed by milling the conical tip 64 to have an
inclined flat end 70'. That is, the flat end 70' is not
perpendicular to the axis of the fiber or the shaped tip 64". The
deposition of the nickel 80 and the growth of the carbon nanotube
84 are then performed as described above. This configuration is
particularly useful for probing very narrow features at the bottom
edges of somewhat wider holes, for example, punch through occurring
at the corner of a narrow trench, which occurs in semiconductor
processing.
[0035] Although the above description includes a support structure
formed from a quartz fiber, it is known that the preferential
etching of <111> planes of <001>-oriented silicon can
form pyramids having an apex angle of 2.theta.=70.5.degree., which
is equivalent to a slope of 54.74.degree. from the plane. Often a
thin layer of silicon nitride is coated on the silicon pyramid.
After cutting, a square end surface is formed. The underlying
silicon is very easily mounted to the AFM probe. Cheung et al.
describe a method of cutting a flat surface at the pyramid apex by
dragging the pyramid across a hard surface. Such a surface may not
be completely flat but most probably deviates by less than
10.degree. from a planar surface. The support structure may be
composed of other materials.
[0036] Nickel is not the only possible material for catalyzing
nanotube growth. Iron and iron oxide have been used. Cobalt has
been suggested as a catalyst. All these catalyzing materials can be
used with the process described above.
[0037] None of the steps described above are particularly difficult
or problematic. FIB milling has been shown to be easily and
reliably performed. Thereby, probe tips produced according to the
invention are relatively economical. Further, the sputter coating
and isotropic etching can be simultaneously performed upon a large
number of probe tips mounted on a common tip holder, thereby
further improving the efficiency of the fabrication method of the
invention.
[0038] The carbon nanotubes produced according to the invention are
grown on substantially planar and well defined areas of nickel or
other catalyzing material. Thereby, the tip diameter and
orientation are well controlled. Carbon nanotube tips have the well
known characteristics of high stiffness and toughness to wear under
continued use.
* * * * *