Carbon nanotube probe tip grown on a small probe

Details Carbon nanotube probe tip grown on a small probe Carbon nanotube probe tip grown on a small probe

2000-09-08

2002-10-01

Williams, Hezron (Department: 2856)

Measuring and testing

Surface and cutting edge testing

Roughness

C216S011000

Reexamination Certificate

active

06457350

ABSTRACT:

FIELD OF THE INVENTION
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
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 &mgr;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.
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 examined 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 gm 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.
Principal parts of the probe head
24
of
FIG. 1
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
.
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 die 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 gm between the capacitor plates
34
,
36
and the beam
40
. Unbalancing of the vertical gap allows a rocking motion of about 25 gm. 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.
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.
This and other types of AFMs have control and sensing elements more than adequate for the degree of precision for profiling a 1 80 nm×1 &mgr;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.
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.
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 &mgr;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, Nov. 14, 1996, pp. 147-150.
Typically, nanotub

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