U.S. patent number RE36,488 [Application Number 09/082,320] was granted by the patent office on 2000-01-11 for tapping atomic force microscope with phase or frequency detection.
This patent grant is currently assigned to Veeco Instruments Inc.. Invention is credited to Virgil B. Elings, John A. Gurley.
United States Patent |
RE36,488 |
Elings , et al. |
January 11, 2000 |
Tapping atomic force microscope with phase or frequency
detection
Abstract
An atomic force microscope in which a probe tip is oscillated at
a resonant frequency and at amplitude setpoint and scanned across
the surface of a sample, which may include an adsorbed water layer
on its surface, at constant amplitude in intermittent contact with
the sample and changes in phase or in resonant frequency of the
oscillating are measured to determine adhesion between the probe
tip and the sample. The setpoint amplitude of oscillation of the
probe is greater than 10 nm to assure that the energy in the lever
arm is much higher than that lost in each cycle by striking the
sample surface, thereby to avoid sticking of the probe tip to the
sample surface. In one embodiment the probe tip is coated with an
antibody or an antigen to locate corresponding antigens or
antibodies on the sample as a function of detected variation in
phase or frequency. In another embodiment, the frequency of
oscillation of the probe tip is modulated and relative changes in
phase of the oscillating probe tip observed in order to measure the
damping of the oscillation due to the intermittent or constant
tapping of the surface by the tip. In a further embodiment, the
slope of the phase versus frequency curve is determined and
outputted during translating of the oscillating probe. Force
dependent sample characteristics are determined by obtaining data
at different tapping amplitude setpoints and comparing the data
obtained at the different tapping amplitude setpoints.
Inventors: |
Elings; Virgil B. (Santa
Barbara, CA), Gurley; John A. (Santa Barbara, CA) |
Assignee: |
Veeco Instruments Inc.
(Plainview, NY)
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Family
ID: |
27009271 |
Appl.
No.: |
09/082,320 |
Filed: |
May 21, 1998 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
Issue Date |
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926175 |
Aug 7, 1992 |
5412980 |
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Reissue of: |
381159 |
Jan 31, 1995 |
05519212 |
May 21, 1996 |
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Current U.S.
Class: |
250/234; 250/306;
250/307; 73/105; 977/881; 977/863; 977/875 |
Current CPC
Class: |
B82Y
35/00 (20130101); G01Q 60/34 (20130101) |
Current International
Class: |
G12B
21/20 (20060101); G12B 21/08 (20060101); G12B
21/00 (20060101); G01B 007/34 () |
Field of
Search: |
;250/306,307,234,235,201.1,216,559.06,559.22,559.23 ;356/376,377
;73/105,632,579 ;324/754,758,762 |
References Cited
[Referenced By]
U.S. Patent Documents
Foreign Patent Documents
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|
|
1 270 132 |
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Jun 1990 |
|
CA |
|
0 290 647 B1 |
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Nov 1988 |
|
EP |
|
0 397 416 |
|
Nov 1990 |
|
EP |
|
0 587 459 A1 |
|
Mar 1994 |
|
EP |
|
62-130302 |
|
Jun 1987 |
|
JP |
|
63-309802 |
|
Dec 1988 |
|
JP |
|
6-82248 |
|
Mar 1994 |
|
JP |
|
6-201315 |
|
Jul 1994 |
|
JP |
|
6-201371 |
|
Jul 1994 |
|
JP |
|
6-194157 |
|
Jul 1994 |
|
JP |
|
WO 92/12398 |
|
Jul 1992 |
|
WO |
|
Other References
G Binnig et al., "Atomic Force Microscope", Physical Review
Letters, vol. 56, No. 9, pp. 930-933, Mar. 3, 1986. .
L. M. Roylance et al., "A Batch-Fabricated Silicon Accelerometer"
IEEE Transactions on Electron Devices, vol. ED-26, No. 12, pp.
11-1917, Dec. 1979. .
W. A. Ducker et al., "Force measurement using an ac atomic force
microscope", J. Appl. Phys., vol. 67, No. 9, May 1, 1990, pp.
4045-4052. .
A. L. Weisenhorn et al., "Forces in atomic force microscopy in air
and water", Appl. Phys. Lett, vol. 54, No. 26, Jun. 26, 1989, pp.
2651-2653. .
Giambattista et al, "Atomic resolution images of solid-liquid
interfaces", Proc. Natl. Acad. Sci. USA, vol. 84, pp. 4671-4674,
Jul. 1987. .
IBM Technical Disclosure Bulletin, vol. 32, No. 7, Dec. 7, 1989,
New York, U.S., p. 168 "Microprobe-Based CD Measurement Tool".
.
P.C.D. Hobbs, et al., "atomic Force Microscope: Implementations",
SPIE vol. 897 canning Microscopy Technologies and Applications
(1988), pp. 27-30. .
M. Anders et al., "Potentiometry for thin-film structures", J.Vac.
Sci. Technol. A, vol. 8, No. 1, Jan.Feb. 1990, pp. 394-399. .
R. Elandsson et al., "Atomic force microscopy using optical
interferometry", J.Vac. Sci. Technol. A, vol. 6, No. 2, Mar./Apr.
1988, pp. 266-270. .
William A. Ducker et al., "Rapid measurement of static and dynamic
surface forces", Appl. Phys. Lett, vol. 56, No. 24, Jun. 11, 1990,
pp. 2408-2410. .
T.R. Albrecht et al., "Frequency modulation detection using . . .
sensitivity", J. Appl. Phys., vol. 69, No. 2, Jan. 15, 1991, pp.
668673. .
D. Sarid et al., "Review of scanning force microscopy", J. Vac.
Sci. Technol. B vol. 9, No. 2, Mar./Apr. 1991, pp. 431-437. .
D. Rugar et al., "Magnetic force microscopy: General principles . .
. media", J.Appl. Phys., vol. 68, No. 3, Aug. 1, 1990, pp.
1169-1183. .
McGraw-Hill Dictionary of Scientific and Technical Terms, p. 2117.
.
Encyclopedia of Physics, 2.sup.nd Ed. Interatomic and Inermolecular
Forces. p. 545. .
R. Erlandsson et al., "A scanning force microscope designed for . .
. ", Microscopy, Microanalysis, Microstructures 1(1990) pp.
471-480. .
AutoProbe XL2, Multiple Mode Large Sample AFM for Sub-angstrom
Resolution, Park Scientific Instruments, 2 pages. .
Frisbie et al., "Functional Group Imaging by Chemical Force
Microscopy", Science, vol. 265, Sep. 10, 1994, pp. 2071-2073. .
Umemura et al, "High resolution Images of Cell Surface Using a
Tapping-Mode Atomic Force Microscope", Jpn. J. Appl. Phys. vol. 32
(1993) pp. L1711-L1714, Part 2, No. 11B, Nov. 15, 1993. .
Martin et al., "Atomic force microscope-force mapping and profiling
on a sub 100 .ANG. scale", J.Appl. Phys. vol. 61, No. 10, May 15,
1987. .
Umeda et al., "Scanning attractive force microscope using
photothermal vibration", J. Vac. Sci. Technol. B, vol. 9, No. 2,
Mar./Apr. 1991, pp. 1318-1322. .
Marti et al., "Mechanical and thermal effects of laser irradiation
on force microscope cantilevers", Ultramicroscopy 42-44 (1992) pp.
345-350 (published Jul. 1992). .
Nyyssonen et al., "Two-dimensional atomic force microprobe trench
metrology system", J. Vac. Sci. Technol., B9, 3612-3616 (1991)
Nov./Dec. .
Nyyssonen et al., application of a two-dimensional atomic force
microscope system to metrology, Proc. SPIE, vol. 1556, 79-87
(1991). .
Tansock et al., "Force measurement with a piezoelectric cantilever
in a scanning force microscope", Ulramicroscopy 42-44 (1992)
1464-1469. .
Webster's New Dictionary of Synonyms, p. 784-785 and p.
812-813..
|
Primary Examiner: Lee; John R
Attorney, Agent or Firm: Oblon, Spivak, McClelland, Maier
& Neustadt, P.C.
Parent Case Text
CROSS-REFERENCE TO RELATED APPLICATIONS
This application is a continuation-in-part of U.S. application Ser.
No. 07/926,175 filed Aug. 7, 1992 now U.S. Pat. No. 5,412,980.
Claims
What is claimed as new and desired to be secured by Letters Patent
of the United States is:
1. In a method of operating an atomic force microscope including a
probe including a probe tip mounted on one end of a lever arm and
wherein the probe tip is scanned across the surface of a sample and
data representative of the surface of the sample is gathered in
relation to the positioning of the lever arm as the probe tip is
scanned, the improvement comprising:
a) oscillating the probe, including oscillating the probe tip at or
near a resonant frequency of the probe or a harmonic of said
resonant frequency and with a free oscillation amplitude A.sub.o
sufficiently great so that the oscillating probe tip does not stick
to the surface of the sample when the oscillating probe tip
contacts the surface of the sample;
b) positioning the oscillating probe tip so that the oscillating
probe tip repeatedly taps the surface of the sample with the probe
tip repeatedly contacting and breaking contact with the surface of
the sample without sticking to the surface of the sample;
c) translating the oscillating probe tip across the surface of the
sample with the oscillating probe tip repeatedly tapping the
surface of the sample;
d) controlling the distance between an opposite end of the lever
arm opposite the probe tip and the sample so that the amplitude of
oscillation of the probe tip is maintained essentially constant at
an amplitude setpoint during said translating step;
e) detecting changes in phase in the oscillation of the probe tip
during translating of the probe tip while the oscillation of the
probe tip is maintained at essentially constant amplitude; and
f) producing a signal indicative of changes in phase detected in
said detecting step.
2. The method according to claim 1, wherein said detecting step
comprises:
detecting deflection of said oscillating probe; and
measuring a relative phase between a drive signal causing
oscillation of said probe and detected deflection of said
oscillating probe.
3. The method according to claim 1, comprising:
modulating the oscillating frequency of the oscillating probe
during translating at essentially constant amplitude; and
said detecting step comprising determining corresponding changes in
phase of the oscillating probe tip during translating of said
probe.
4. The method according to claim 1, comprising:
providing a probe including a substance selected to interact with a
corresponding substance on said sample.
5. The method according to claim 4, wherein said step of providing
a probe comprises:
providing a probe coated with an antibody or an antigen.
6. The method according to claim 1 wherein said oscillating step
comprises:
oscillating said probe tip with a free oscillation amplitude
greater than 10 nm.
7. The method according to claim 1, wherein said oscillating step
comprises:
oscillating said probe tip with a free oscillation amplitude
greater than 20 nm.
8. The method according to claim 1, comprising:
changing the amplitude setpoint and repeating said steps a) through
f) while maintaining the amplitude of oscillation of the probe tip
essentially constant at the changed amplitude setpoint.
9. The method according to claim 8, comprising:
comparing signals produced in repeated of steps f) to discriminate
a force dependent characteristic of the sample.
10. The method according to claim 3, comprising:
changing the amplitude setpoint and repeating said steps a) through
f) while maintaining the amplitude of oscillation of the probe tip
essentially constant at the changed amplitude setpoint.
11. The method according to claim 10, comprising:
comparing signals produced in repeated of steps f) to discriminate
a force dependent characteristic of the sample.
12. In a method of operating an atomic force microscope including a
probe including a probe tip mounted on one end of a lever arm and
wherein the probe tip is scanned across the surface of a sample and
data representative of the surface of the sample is gathered in
relation to the positioning of the lever arm as the probe tip is
scanned, the improvement comprising:
a) oscillating the probe, including oscillating the probe tip at or
near a resonant frequency of the probe or a harmonic of said
resonant frequency and with a free oscillation amplitude A.sub.o
sufficiently great so that the oscillating probe tip does not stick
to the surface of the sample when the oscillating probe tip
contacts the surface of the sample;
b) positioning the oscillating probe tip so that the oscillating
probe tip repeatedly taps the surface of the sample with the probe
tip repeatedly contacting and breaking contact with the surface of
the sample without sticking to the surface of the sample;
c) translating the oscillating probe tip across the surface of the
sample with the oscillating probe tip repeatedly tapping the
surface of the sample;
d) controlling the distance between an opposite end of the lever
arm opposite the probe tip and the sample so that the amplitude of
oscillation of the probe tip is maintained essentially constant at
an amplitude setpoint during said translating step;
e) detecting a relative phase between a drive signal causing
oscillation of said probe and deflection of said probe; and
f) controlling the frequency of the oscillation of the probe so
that the relative phase detected in said detecting step is kept
essentially constant during scanning.
13. The method according to claim 12, comprising:
g) producing a signal indicative of variations in the frequency of
oscillation of the probe as a function of position during
translating.
14. The method according to claim 13, comprising:
providing a probe including a substance selected to interact with a
corresponding substance on said sample.
15. The method according to claim 14, wherein said step of
providing a probe comprises:
providing a probe coated with an antibody or an antigen.
16. The method according to claim 13, wherein said oscillating step
comprises:
oscillating said probe tip with a free oscillation amplitude
greater than 10 nm.
17. The method according to claim 13, wherein said oscillating step
comprises:
oscillating said probe tip with a free oscillation amplitude
greater than 20 nm.
18. The method according to claim 13, comprising:
modulating the frequency of oscillation of the probe about the
controlled frequency of step f) and determining corresponding
changes in the phase of the oscillating probe tip during
translating of said probe.
19. The method according to claim 12, comprising:
changing the amplitude setpoint and repeating said steps a) through
f) while maintaining the amplitude of oscillation of the probe tip
essentially constant at the changed amplitude setpoint.
20. The method according to claim 13, comprising:
changing the amplitude setpoint and repeating said steps a) through
g) while maintaining the amplitude of oscillation of the probe tip
essentially constant at the changed amplitude setpoint.
21. The method according to claim 20, comprising:
comparing signals produced in repeated of steps f) to discriminate
a force dependent characteristic of the sample.
22. The method according to claim 18, comprising:
changing the amplitude setpoint and repeating said steps a) through
g) while maintaining the amplitude of oscillation of the probe tip
essentially constant at the changed amplitude setpoint.
23. The method according to claim 22, comprising:
comparing signals produced in repeated of steps f) to discriminate
a force dependent characteristic of the sample.
24. In a method of operating an atomic force microscope including a
probe including a probe tip mounted on one end of a lever arm and
wherein the probe tip is scanned across the surface of a sample and
data representative of the surface of the sample is gathered in
relation to the positioning of the lever arm as the probe tip is
scanned, the improvement comprising:
a) oscillating the probe, including oscillating the probe tip at or
near a resonant frequency of the probe or a harmonic of said
resonant frequency and with a free oscillation amplitude A.sub.o
sufficiently great so that the oscillating probe tip does not stick
to the surface of the sample when the oscillating probe tip
contacts the surface of the sample;
b) positioning the oscillating probe tip so that the oscillating
probe tip repeatedly taps the surface of the sample with the probe
tip repeatedly contacting and breaking contact with the surface of
the sample without sticking to the surface of the sample;
c) translating the oscillating probe tip across the surface of the
sample with the oscillating probe tip repeatedly tapping the
surface of the sample;
d) controlling the distance between an opposite end of the lever
arm opposite the probe tip and the sample so that the amplitude of
oscillation of the probe tip is maintained essentially constant at
an amplitude setpoint during said translating step;
e) detecting a relative phase between a drive signal causing
oscillation of said probe and deflection of said probe; and
f) controlling the frequency of the oscillation of the probe so
that the relative phase detected in said detecting step is kept
essentially constant during scanning;
g) varying the frequency of the oscillation of the probe by an
amount .DELTA.f around the controlled frequency of step f) and
determining a corresponding change .DELTA.p in said relative phase;
and
h) producing a signal indicative of the ratio of
.DELTA.p/.DELTA.f.
25. The method according to claim 24, comprising:
providing a probe including a substance selected to interact with a
corresponding substance on said sample.
26. The method according to claim 25, wherein said step of
providing a probe comprises:
providing a probe coated with an antibody or an antigen.
27. The method according to claim 24, wherein said oscillating step
comprises:
oscillating said probe tip with a free oscillation amplitude
greater than 10 nm.
28. The method according to claim 24, wherein said oscillating step
comprises:
oscillating said probe tip with a free oscillation amplitude
greater than 20 nm.
29. The method according to claim 24, comprising:
changing the amplitude setpoint and repeating said steps a) through
g) while maintaining the amplitude of oscillation of the probe tip
essentially constant at the changed amplitude setpoint.
30. The method according to claim 29, comprising:
comparing signals produced in repeated of steps g) to discriminate
a force dependent characteristic of the sample. .Iadd.
31. A method of operating an atomic force microscope including a
probe tip on one end of a lever arm, comprising:
a) oscillating the probe tip at or near a resonant frequency of
said lever arm and with a free oscillation amplitude (A.sub.o)
sufficiently great so that the probe tip does not stick to the
surface of a sample when the probe tip repeatedly taps the surface
of the sample;
b) producing relative translation between the oscillating probe tip
and the sample by translating either the lever arm or sample with
the oscillating probe tip repeatedly tapping the surface of the
sample and so that the amplitude of oscillation of the probe tip is
affected by the repeated tapping of the sample surface;
c) controlling the distance between an end of the lever arm
opposite the probe tip and the sample so that the amplitude of
oscillation of the probe tip is maintained essentially constant at
an amplitude setpoint during said step of producing
translation;
d) detecting the phase of oscillation of the probe tip during said
step of producing translation while the oscillation of the probe
tip is maintained at essentially constant amplitude; and
e) producing a signal indicative of the phase detected in said
detecting step..Iaddend..Iadd.32. The method according to claim 31
comprising:
sending the produced signal indicative of the phase detected to a
display..Iaddend..Iadd.33. The method according to claim 31,
wherein said step e) comprises:
producing an image having lateral coordinates corresponding to the
relative lateral position between the oscillating probe tip and the
sample..Iaddend..Iadd.34. The method according to claim 31, wherein
said detecting step comprises:
detecting deflection of said oscillating probe; and
measuring a relative phase between a drive signal causing
oscillation of said probe and detected deflection of said
oscillating probe..Iaddend..Iadd.35. The method according to claim
31, comprising:
modulating the oscillating frequency of the oscillating probe
during said producing translation step; and
said detecting step comprising determining changes in phase of the
oscillating probe tip during said modulating
step..Iaddend..Iadd.36. The method according to claim 31,
comprising:
providing a probe including a substance selected to interact with
said sample or a portion of said sample..Iaddend..Iadd.37. The
method according to claim 36, wherein said step of providing a
probe comprises:
providing a probe coated with an antibody or an
antigen..Iaddend..Iadd.38. The method according to claim 31,
wherein said oscillating step comprises:
oscillating said probe tip with a free oscillation amplitude
greater than 20 nm..Iaddend..Iadd.39. The method according to claim
31, comprising:
changing the amplitude setpoint and repeating said steps a) through
e) while maintaining the amplitude of oscillation of the probe tip
essentially constant at the changed amplitude
setpoint..Iaddend..Iadd.40. The method according to claim 39,
comprising:
comparing signals produced in repeated of steps e) to discriminate
a force dependent characteristic of the sample..Iaddend..Iadd.41. A
method of operating an atomic force microscope including a probe
tip on one end of a lever arm, comprising:
a) oscillating the probe tip at or near a resonant frequency of
said lever arm and with a free oscillation amplitude (A.sub.o)
sufficiently great so that the probe tip does not stick to a
surface of the sample when the probe tip repeatedly taps the
surface of the sample;
b) producing relative translation between the oscillating probe tip
and the sample by translating either the lever arm or sample with
the oscillating probe tip repeatedly tapping the surface of the
sample and so that the amplitude of oscillation of the probe tip is
affected by the repeated tapping of the sample surface;
c) controlling the distance between an end of the lever arm
opposite the probe tip and the sample so that the amplitude of
oscillation of the probe tip is maintained essentially constant at
an amplitude setpoint during said step of producing
translation;
d) detecting a relative phase between a drive signal causing
oscillation of said probe and deflection of said probe; and
e) controlling the frequency of the oscillation of the probe so
that the relative phase detected in said detecting step is kept
essentially constant during said step of producing
translation..Iaddend..Iadd.42. The method according to claim 41,
comprising:
f) producing a signal indicative of the frequency of oscillation of
the probe as a function of lateral position of the probe relative
to the sample during said step of producing
translation..Iaddend..Iadd.43. The method according to claim 42,
comprising:
producing a visual image representing the signal indicative of the
frequency of oscillation of the probe as a function of lateral
position of the probe relative to the sample during said step of
producing translation..Iaddend..Iadd.44. The method according to
claim 42, comprising:
providing a probe including a substance selected to interact with
said
sample or a portion of said sample..Iaddend..Iadd.45. The method
according to claim 44, wherein said step of providing a probe
comprises:
providing a probe coated with an antibody or an
antigen..Iaddend..Iadd.46. The method according to claim 42,
wherein said oscillating step comprises:
oscillating said probe tip with a free oscillation amplitude
greater than 20 nm..Iaddend..Iadd.47. The method according to
claims 41 or 42, comprising:
changing the amplitude setpoint and repeating said steps a) through
e) while maintaining the amplitude of oscillation of the probe tip
essentially constant at the changed amplitude
setpoint..Iaddend..Iadd.48. The method according to claim 47,
comprising:
comparing signals produced in repeated of steps e) to discriminate
a force dependent characteristic of the sample..Iaddend..Iadd.49. A
method of operating an atomic force microscope including a probe
tip on one end of a lever arm, comprising:
a) oscillating the probe tip at or near a resonant frequency of
said lever arm and with a free oscillation amplitude (A.sub.o)
sufficiently great so that the probe tip does not stick to a
surface of a sample when the probe tip repeatedly taps the surface
of the sample;
b) producing relative translation between the oscillating probe tip
and the sample by translating either the lever arm or sample with
the oscillating probe tip repeatedly tapping the surface of the
sample and so that the amplitude of oscillation of the probe tip is
affected by the repeated tapping of the sample surface;
c) controlling the distance between an end of the lever arm
opposite the probe tip and the sample so that the amplitude of
oscillation of the probe tip is maintained essentially constant at
an amplitude setpoint during said step producing translation;
d) detecting a relative phase between a drive signal causing
oscillation of said probe and deflection of said probe;
e) controlling the frequency of the oscillation of the probe so
that the relative phase detected in said detecting step is kept
essentially constant during said step of producing translation;
f) varying the frequency of the oscillation of the probe by an
amount around the controlled frequency of step f) and determining a
change .DELTA.p in said relative phase; and
g) producing a signal indicative of said change .DELTA.p in said
relative phase as a function of the relative lateral position
between the probe and the sample during said step of producing
relative
translation..Iaddend..Iadd.50. The method according to claim 49,
comprising:
providing a probe including a substance selected to interact with
said sample or a portion of said sample..Iaddend..Iadd.51. The
method according to claim 50, wherein said step of providing a
probe comprises:
providing a probe coated with an antibody or an
antigen..Iaddend..Iadd.52. The method according to claim 49,
wherein said oscillating step comprises:
oscillating said probe tip with a free oscillation amplitude
greater than 20 nm..Iaddend..Iadd.53. The method according to claim
49, comprising:
changing the amplitude setpoint and repeating said steps a) through
g) while maintaining the amplitude of oscillation of the probe tip
essentially constant at the changed amplitude
setpoint..Iaddend..Iadd.54. The method according to claim 53,
comprising:
comparing signals produced in repeated of steps g) to discriminate
a force
dependent characteristic of the sample..Iaddend..Iadd.55. A method
of operating an atomic force microscope including a probe tip on
one end of a lever arm in order to measure an attractive force
distribution, comprising:
a) oscillating the probe tip at or near a resonant frequency of
said lever arm and with a free oscillation amplitude (A.sub.o)
sufficiently great so that the probe tip does not stick to a
surface of a sample when the probe tip repeatedly taps the surface
of the sample;
b) producing relative translation between the oscillating probe tip
and the sample by translating either the lever arm or sample;
c) controlling the distance between an end of the lever arm
opposite the probe tip and the sample so that the amplitude of
oscillation of the probe tip is maintained essentially constant at
an amplitude setpoint;
d) setting the amplitude setpoint such that the probe tip taps the
surface of the sample in areas of small attractive force and is
lifted above the surface by the controlling in step c) in areas of
large attractive force;
e) detecting the phase of oscillation of the probe tip during said
step of producing translation while the oscillation of the probe
tip is maintained at essentially constant amplitude; and
f) producing a signal indicative of the phase detected in said
detecting step..Iaddend..Iadd.56. The method according to claim 55,
comprising:
providing a probe including a substance selected to interact with
said sample or a portion of said sample..Iaddend..Iadd.57. The
method according to claim 56, wherein said step of providing a
probe comprises:
providing a probe coated with an antibody or
antigen..Iaddend..Iadd.58. The method according to claims 55 or 56,
comprising:
providing a probe including a substance selected to interact
chemically
with said sample or a portion of said sample..Iaddend..Iadd.59. The
method according to claims 55 or 56, comprising:
providing a probe including a substance which produces a magnetic
attractive interaction with said sample or a portion of said
sample..Iaddend..Iadd.60. The method according to claims 55 or 56,
comprising:
providing a probe including a substance which produces an
electrical attractive interaction with said sample or a portion of
said sample..Iaddend..Iadd.61. The method according to claims 55 or
56, comprising:
providing a probe including a substance which produces a capillary
attractive interaction with said sample or a portion of said
sample..Iaddend..Iadd.62. The method according to claims 55 or 56,
comprising:
providing a probe including a substance which produces a Van Der
Waals attractive interaction with said sample..Iaddend..Iadd.63. A
method of operating an atomic force microscope including a probe
tip on one end of a lever arm, comprising:
a) oscillating the probe tip at or near a resonant frequency of
said lever arm and with a free oscillation amplitude (A.sub.o)
greater than 20 nm;
b) producing relative translation between the oscillating probe tip
and the sample by translating either the lever arm or sample with
the oscillating probe tip repeatedly tapping the surface of the
sample and so that the amplitude of oscillation of the probe tip is
affected by the repeated tapping of the sample surface; and
c) detecting the amplitude of oscillation of the probe tip during
said step of producing relative translation..Iaddend..Iadd.64. A
method of operating an atomic force microscope including a probe
tip on one end of a lever arm, comprising:
a) oscillating the probe tip at or near a resonant frequency of
said lever arm and with a free oscillation amplitude (A.sub.o)
sufficiently great so that the probe tip does not stick to a
surface of a sample when the probe tip repeatedly taps the surface
of the sample;
b) producing relative translation between the oscillating probe tip
and the sample by translating either the lever arm or sample with
the oscillating probe tip repeatedly tapping the surface of the
sample and so that the amplitude of oscillation of the probe tip is
affected by the repeated tapping of the sample surface;
c) detecting the amplitude of oscillation of the probe tip during
said step of producing translation; and
d) controlling the distance between an end of the lever arm
opposite the probe tip and the sample so that the detected
amplitude of oscillation is maintained essentially constant at a
predetermined amplitude setpoint during said step of producing
relative translation..Iaddend..Iadd.65. The method according to
claims 64, wherein said free oscillation amplitude (A.sub.o) is
greater than 20 nm..Iaddend..Iadd.66. The method according to
claims 63, 64, or 65, comprising:
producing a signal indicative of the amplitude of oscillation of
the probe tip determined in step c); and
sending the produced signal to a display..Iaddend..Iadd.67. The
method according to claim 66, wherein said producing step
comprises:
producing an image having lateral coordinates corresponding to the
relative lateral position between the oscillating probe tip and the
sample..Iaddend..Iadd.68. The method according to claims 64 or 65
comprising:
e) producing a signal indicative of the controlled distance between
the end of the lever opposite the probe tip and the sample; and
f) displaying the signal produced in step e)..Iaddend..Iadd.69. The
method according to claim 68, wherein the produced signal
indicative of said controlled distance is an
image..Iaddend..Iadd.70. The method according to claims 68,
comprising:
detecting the phase of oscillation of the probe tip during said
step of producing relative translation; and
displaying at least one of the detected controlled distance and the
detected phase of oscillation of the probe tip..Iaddend..Iadd.71.
The method according to claims 69, comprising:
detecting the phase of oscillation of the probe tip during said
step of producing relative translation; and
displaying at least one of the detected controlled distance and the
detected phase of oscillation of the probe tip..Iaddend..Iadd.72.
The method according to claim 68, comprising:
changing the amplitude setpoint and repeating the steps a) through
e) while maintaining the amplitude of oscillation of the probe tip
essentially constant at the changed setpoint..Iaddend..Iadd.73. The
method according to claim 72, comprising:
comparing signals produced in repeated of steps e) to discriminate
a force dependent characteristic of the sample..Iaddend.
Description
BACKGROUND OF THE INVENTION
1. Field of the Invention
This invention relates to an ultra-low force atomic force
microscope, and particularly an improvement to the atomic force
microscope described in related commonly owned U.S. patent
application Ser. No. 08/147,571 and related U.S. Pat. Nos.
5,229,606 and 5,266,801.
2. Discussion of the Background
Atomic Force Microscopes (AFM's) are extremely high resolution
surface measuring instruments. Two types of AFM's have been made in
the past, the contact mode (repulsive mode) AFM and the non-contact
(attractive mode) AFM.
The contact mode AFM is described in detail in U.S. Pat. No.
4,935,634 by Hansma et al, as shown in FIG. 2. This AFM operates by
placing a sharp tip attached to a bendable cantilever directly on a
surface and then scanning the surface laterally. The bending of the
lever in response to surface height variations is monitored by a
detection system. Typically, the height of the fixed end of the
cantilever relative to the sample is adjusted with feedback to
maintain the bending at a predetermined amount during lateral
scanning. The adjustment amount versus lateral position creates a
map of the surface. The deflection detection system is typically an
optical beam system as described by Hansma et al. Using very small
microfabricated cantilevers and piezoelectric positioners as
lateral and vertical scanners, AFM's can have resolution down to
molecular level, and may operate with controllable forces small
enough to image biological substances. Since AFM's are relatively
simple, inexpensive devices compared to other high resolution
techniques and are extremely versatile, they are becoming important
tools in a wide variety of research and high technology
manufacturing applications. The contact mode AFM, in which the tip
is maintained in continuous contact with the sample, is currently
the most common type, and accounts for essentially all the AFM's
sold commercially to date.
The contact AFM has found many applications. However, for samples
that are very soft or interact strongly with the tip, such as
photoresist, some polymers, silicon oxides, many biological
samples, and others, the contact mode has drawbacks. As pointed out
in Hansma et al, the tip may be attracted to the surface by the
thin liquid layer on all surfaces in ambient conditions, thus
increasing the force with which the tip presses on the surface. The
inventors and others have also observed that electrostatic forces
may attract the tip to the surface, particularly for some
tip-sample combinations such as silicon nitride tips on silicon
oxide surfaces. When the tip is scanned laterally under such
conditions, the sample experiences both compressive and shearing
forces. The lateral shearing forces may make the measurement
difficult and for soft samples may damage the sample. Further, a
stick-slip motion may cause poor resolution and distorted images.
Hansma et al's approach to this problem was to immerse the tip,
cantilever, and sample surface in liquid, thus eliminating the
surface layer forces, and for a polar liquid, the electrostatic
forces. This technique works very well, and has the further
advantage that it allows samples that are normally hydrated to be
imaged in their natural state. However for many samples and
applications, immersion in liquid is not of much use. Operating in
liquid requires a fluid cell and increases the complexity of using
the AFM, and for industrial samples such as photoresist and silicon
wafers, immersion is simply not practical.
The non-contact AFM, developed by Martin et al, J. Applied Physics,
61(10), 15 May, 1987, profiles the surface in a different fashion
than the contact AFM. In the non-contact AFM, the tip is scanned
above the surface, and the very weak Van der Waals attractive
forces between the tip and sample are sensed. Typically in
non-contact AFM's, the cantilever is vibrated at a small amplitude
and brought near to the surface such that the force gradient due to
interaction between the tip and surface modifies the spring
constant of the lever and shifts its natural resonant frequency.
The shift in resonance will change the cantilever's response to the
vibration source in a detectable fashion. Thus the amount of change
may be used to track the surface typically by adjusting the probe
surface separation during lateral scanning to maintain a
predetermined shift from resonance. This AC technique provides
greater sensitivity than simply monitoring the DC cantilever
deflection in the presence of the attractive Van der Waals force
due to the weak interaction between the tip and surface. The
frequency shift may be measured directly as proposed by Albrecht et
al. J. Applied Physics, 1991, or indirectly as was done originally
by Martin et al.
The indirect method uses a high Q cantilever, such that damping is
small. The amplitude versus frequency curve of a high Q lever is
very steep around the resonant frequency. Martin et al oscillated
the lever near the resonant frequency and brought the tip close to
the surface. The Van der Waals interaction with the surface shifts
the resonance curve. This has the effect of shifting the resonance
closer or further to the frequency at which the lever is
oscillated, depending on which side of resonance the oscillation is
at. Thus, indirectly, the amplitude of oscillation will either
increase or decrease as a consequence of the resonance shift. The
amplitude change is measurable (AM type detection). This change in
amplitude close to the surface compared to the amplitude far away
from the surface (the free amplitude) can be used as a setpoint to
allow surface tracking. The direct method measures the frequency
shift itself (FM type detection). Both methods are bound by the
same interaction constraints.
FIG. 5 illustrates this non-contact operation. The tip is driven at
a known amplitude and frequency of oscillation, which is typically
near a cantilever resonance. The amplitude of this oscillation is
detected by a deflection detector, which can be of various types
described in the references. When the tip is sufficiently far away
from the surface, it will oscillate at the free amplitudes A.sub.o,
as shown in FIG. 5. As shown in FIG. 5, when the tip is brought
closer to the surface, the Van der Waals interaction will shift the
resonant oscillatory frequency slightly. This shift causes either
an increased or decreased amplitude. A.sub.s, or the frequency
shift may be measured directly. This modified amplitude value may
be used as a setpoint in the manner of other above described SPM's,
such that as the tip is scanned laterally, the tip height may be
adjusted with feedback to keep setpoint, A.sub.s, at a constant
value. Thus an image of the surface may be generated without
surface contact, and without electrical interaction as needed by a
scanning tunnelling microscope STM. The resonant shift may also be
caused by other force interactions, such as magnetic field
interaction with a magnetic tip. Thus this type of AFM may in
theory be easily configured to map a variety of parameters using
the same or similar construction.
The Van der Waals force is very weak, and decreases rapidly with
separation, so the practical furthest distance for measurable
interaction is 10 nm above the surface, as shown in FIG. 1, taken
from Sarid, Scanning Force Microscopy, Oxford University Press,
1991. To shift the resonance of the lever, the lever must oscillate
within this envelope of measurable force gradient. If just a small
portion of the oscillation is within the envelope, the resonance
will not be appreciably affected. Thus the oscillation amplitude
must be small. A compendium of all non-contact AFM research can be
found in Scanning Force Microscopy by Sarid, above noted, no
researcher was able to operate a non-contact AFM with a free
oscillation amplitude of greater than 10 nm. This limitation as
will be shown limits the usefulness of the non-contact method.
Although developed at essentially the same time as the contact AFM,
the non-contact AFM has rarely been used outside the research
environment due to problems associated with the above constraints.
The tip must be operated with low oscillation amplitude very near
the surface. These operating conditions make the possibility very
likely of the tip becoming trapped in the surface fluid layer
described by Hansma et al. This effect is illustrated in FIG. 6, an
amplitude versus displacement curve. A cantilever with probe is
oscillated at a free amplitude A.sub.o, and the vertical position
of the fixed end of the lever is varied from a height where the
probe is not affected by the surface to a point where the probe is
captured by the surface and oscillation ceases. The curve is
typical for oscillation amplitudes of 10 nm or less. Such curves
have been measured by the inventors, and were also described by
Martin et al, and also by Ducker et al, in "Force Measurement Using
an AC Atomic Force Microscope", J. of Applied Physics, 67(9), 1 May
1990. As the curve clearly shows, when the tip is brought near the
surface there is a narrow region where the amplitude is affected by
the Van der Waals interaction before it becomes abruptly captured
by the surface fluid layer, and oscillation becomes very small It
is this narrow region in which the non-contact AFM must operate As
a surface is scanned, any variations in the surface topography may
cause the tip to become captured if the feedback cannot perfectly
respond to the topography variations. If the tip does become
captured, the control system will lift the fixed end of the lever
until the tip breaks free, and then re-establish the setpoint. As
can be seen from FIG. 6, there is significant hysteresis in the
withdraw process, which will cause serious instability in the image
data. Thus non-contact microscopes must scan very slowly so the
feedback loop has sufficient time to prevent the tip becoming stuck
to the surface. Moreover, because the tip must be operated above
the fluid layer, the lateral resolution is inferior to the contact
mode. Typically, the noncontact AFM must operate with the tip 5-10
nm above the surface, which limits the lateral resolution to 5-10
nm. Contact mode AFM's typically have lateral resolution of better
than 1 nm.
For measuring the frequency shift using amplitude detection, the
sensitivity depends on the cantilever having a very sharp resonance
peak, which in turn gives a very slow response time because
undamped systems require a long time to recover from a
perturbation. Thus, sensitivity and response time are inversely
coupled. The high Q requirement also places restrictions on the
design of the lever to minimize the effect of air as a damping
agent. One could improve the time response by using cantilevers
which may be operated at a higher frequency, but such levers are
stiffer and therefore have reduced sensitivity to the Van der Waals
interaction. Thus it can be seen that high sensitivity and fast
response are very difficult to achieve with a non-contact AFM.
Furthermore, the weak force interaction places restrictions on the
height at which the tip may be operated and the amplitude of
oscillation. The presence of the fluid layer near this height makes
capture of a lever with a small oscillation likely, so slow time
response is a serious stability problem. For these reasons, despite
their many potential advantages, non-contact AFM's have yet to be
successful commercially.
The non-contact AFM has been used successfully in the measurement
of magnetic fields on objects such as magnetic storage media. With
a tip of, or coated with, magnetic material, the force interaction
between the tip and magnetic sample is much stronger than the Van
der Waals interactions, and is longer range. Thus, the non-contact
FM (also called magnetic force microscope, MFM) may be operated
without the need for ultra-high sensitivity, as required for
surface profiling. However since magnetic fields are seldom
continuous, some interaction is necessary to guide the tip over the
surface between magnetic regions. Rugar et al, (Magnetic Force
Microscopy, IBM Research Report, Almaden Research Center, Dec. 12,
1990) found that applying an electric field between the tip and
sample would produce a larger effect than the Van der Waals force,
so the hard disks could be scanned without the probe sticking to
the surface. This method limits the technique to conductive
surfaces.
SUMMARY OF THE INVENTION
Accordingly, one object of this invention is to provide a novel AFM
that does not produce shear forces during scanning and does not
have the operability limitations of the non-contact AFM.
A further object of the invention is to provide a novel AFM and
method to profile surfaces, including soft or sticky surfaces, at
high resolution with high sensitivity and fast time response, thus
overcoming the drawbacks of prior art contact and non-contact mode
AFM's.
It is a further object of this invention to provide an AFM that may
map magnetic or other force distributions while retaining the
ability to track topography without other force components.
These and other objects are achieved according to the present
invention by providing a new and improved AFM and method of
operating an AFM, wherein the probe is oscillated at or near
resonance or a resonant harmonic to strike the surface of the
sample, so that the tip has minimal lateral motion while in contact
with the surface, thus eliminating scraping and tearing. The
cantilever probe is oscillated at a large amplitude, greater than
10 nm, preferably greater than 20 nm, and typically on the order of
100-200 nm, so that the energy in the lever is large enough, much
higher than that lost in each oscillation cycle due to, for
example, damping upon striking the same surface, so that the tip
will not become stuck to the surface. The oscillation amplitude is
affected by the tip striking the surface in a measurable fashion,
and this limited amplitude is a direct measure of the topography of
the surface. Alternatively, a feedback control can be employed to
maintain the oscillation amplitude constant, and then a feedback
control signal can be used to measure surface topography. The
striking interaction is strong, so the sensitivity is high. The
resolution approaches the contact mode microscope because the tip
touches the surface. The technique can use high frequency jumps
with no loss in sensitivity since the measurement of the amplitude
change does not depend on frequency.
The invention may be employed in the measure of magnetic or other
force distributions in conjunction with the non-contact method, to
track the surface in regions where there is no other force.
In further embodiments, the probe is oscillated and translated
across the surface of the sample at constant amplitude and changes
in phase are detected and corresponding output signals produced. In
another variation, a relative phase between a drive signal causing
oscillation of the probe and deflection of the probe is detected
and the frequency of oscillation of the probe varied so that the
relative phase is kept essentially constant during scanning. An
output signal is then produced indicative of variations in the
frequency of oscillation of the probe as a function of position
during scanning. According to a further embodiment, oscillation
frequency is modulated by an amount .DELTA.f, changes in relative
phase .DELTA.p detected, and signals indicative of the slope
.DELTA.p/.DELTA.f output. In a further embodiment, data is obtained
by operating at different tapping amplitude setpoints and then
compared to discriminate a force dependent sample
characteristic.
BRIEF DESCRIPTION OF THE DRAWINGS
A more complete appreciation of the invention and many of the
attendant advantages thereof will be readily obtained as the same
becomes better understood by reference to the following detailed
description when considered in connection with the accompanying
drawings, wherein:
FIG. 1 is a graph showing the Van der Waals force as a function of
height above a surface, and where a non-contact mode microscope
must operate;
FIG. 2 is a simplified functional block diagram of the probe
positioning apparatus of a prior art contact mode atomic force
microscope;
FIG. 3 is a simplified functional block diagram of the probe
positioning apparatus of an atomic force microscope incorporating
the present invention;
FIG. 4 is a block diagram of another type of AFM which may use the
present invention;
FIG. 5 is an illustration of the operation of a vibrating lever
brought close to a surface in the prior art non-contact mode;
FIG. 6 is a graph of an amplitude vs. position curve that
illustrates the behavior of the probe oscillation in a prior art
non-contact mode AFM as a function of probe height above a
surface;
FIG. 7 is an illustration of the operation of a vibrating lever
brought close to a surface in a preferred embodiment of the present
invention;
FIG. 8 is a graph of an amplitude vs. position curve that
illustrates the behavior of the probe oscillation in a preferred
embodiment of the present invention as a function of probe height
above a surface;
FIG. 9A is an illustration of how the present invention may be used
to achieve improved performance when measuring surfaces with steep
walls and trenches, and
FIG. 9B is a graph illustrating oscillation amplitude as a function
of scan position in the trench of FIG. 9A;
FIG. 10 is an illustration of an alternative approach where the
present invention may be used to achieve improved performance when
measuring surfaces with steep walls and trenches;
FIG. 11 is a graph of an amplitude-distance curve that illustrates
the behavior of the probe oscillation in a preferred embodiment of
the present invention as a function of probe height above a surface
in a mode where the probe oscillates within the surface fluid
layer;
FIG. 12 is a graph of an amplitude-distance curve that stops before
the amplitude decreases below a predetermined point;
FIG. 13 is an illustration of how a probe is pulled into a steep
wall in the prior art contact AFM; and
FIG. 14a is an illustration of scanning a probe tip in a trench
while oscillating the probe tip, and FIG. 14b is a graph
illustrating the observed oscillation amplitude resulting from the
scanning of FIG. 14a;
FIG. 15 is a block diagram of a further embodiment of the tapping
AFM of the present invention, in which probe tip oscillation phase
is detected as a function of lateral position during scanning;
FIG. 16 is a block diagram illustrating a further embodiment of a
tapping AFM according to the present invention, in which a change
in resonant frequency of the oscillating probe tip is detected
during tapping at constant amplitude; and
FIGS. 17a and 17c are graphs showing the effect of damping on
oscillation amplitude as a function of frequency, and FIGS. 17b and
17d are graphs showing the effect of damping on the slope of the
phase vs. frequency curve.
DESCRIPTION OF THE PREFERRED EMBODIMENT
The present invention utilizes the inventors' discovery that if the
probe is oscillated at or near one of the resonant frequencies of
the lever, that in fact the probe tip has much less of a tendency
to stick to the surface because a resonant system tends to remain
in stable oscillation even if some damping exists. Thus the
preferred embodiment of the present invention utilizes a resonant
oscillation of the cantilever at sufficient oscillation amplitude
to achieve the advantages described above without the probe
becoming stuck to the surface. This preferred embodiment also
provides many of the benefits of the non-contact AFM as described
above.
Existing development of AFM's using oscillation of the probes has
been directed at avoiding surface contact, as described above, and
as such is limited in practicality despite the potential advantages
of the technique. For applications where the non-contact mode is
desired, the inventors have found that the amplitude-distance curve
of FIG. 6 can of be of great aid in establishing the setpoint for
non-contact mode operation. Using the amplitude-distance curve, one
can optimize the operating frequency, free amplitude and setpoint
to achieve the most suitable operating characteristics for a
particular sample and cantilever combination. This novel
application of the curve was clearly not anticipated by Ducker or
Martin. Using the computational and display capabilities of typical
scanning probe microscopes, SPM's, the lever may be oscillated and
the lever vertical position varied, while the curve is displayed on
a terminal. The various parameters may be varied, such that desired
operating conditions may be determined, and used when the SPM is in
the imaging mode.
The AFM of the present invention does not avoid contact with the
surface. Thus, the invention is not limited in the amplitude of
oscillation, and in fact as will be shown, very large amplitudes
compared to the non-contact mode are advantageous. In FIG. 6, it is
shown that for small oscillation amplitudes as the tip is brought
near the surface, it becomes trapped by the fluid layer and
oscillation ceases abruptly. If the oscillation amplitude is
larger, greater than 10 nm, preferably greater than 20 nm and
typically 100-200 nm, then the energy in the oscillation may be
sufficient in most cases to overcome the stickiness of the surface
for a wide range of vertical positions of the lever.
FIG. 7 shows that for a large free amplitude, A.sub.o, the lever
may be brought down to where the tip strikes the surface. The
energy lost by striking the surface and overcoming the fluid layer
attraction limits the oscillation to a reduced value, A, but does
not stop the oscillation as happens for low drive oscillation
amplitudes. The difference in behavior for higher amplitude
oscillations is illustrated in FIG. 8 where the curve of FIG. 6 is
duplicated for a free amplitude, A.sub.o, of greater than 10 nm. As
can be seen, there is wide range of limited amplitudes, with the
probe striking the surface, that could be used as an operating
point for a feedback loop. Abrupt capture of the probe does not
take place, so stable operation is possible. As the curve shows,
the lever may be further lowered such that oscillation is stopped.
The withdraw characteristics are similar to the low amplitude case
in that the amplitude increases gradually until a point is reached
where the cantilever breaks free and resumes oscillation at the
free amplitude.
According to the present invention, the AFM is then operated at or
near a cantilever resonance with sufficient amplitude that upon the
probe striking the surface, the amplitude of oscillation of the
probe is affected and the probe does not stick to the surface. A
preferred version of this invention can be practiced on the AFM of
FIG. 3. In FIG. 3, the tip is driven at a known amplitude and
frequency oscillation, which is typically near a cantilever
resonance. The amplitude, A.sub.o, of this oscillation is detected
by the deflection detector of FIG. 3, which is of the type shown in
FIG. 2 and described by Hansma et al. When the tip is sufficiently
far away from the surface, it will oscillate at the free amplitude,
A.sub.o, as shown in FIG. 7. The amplitude is measured in the AFM
of FIG. 3 as an RMS value of the AC deflection detector signal. As
shown in FIG. 7, when the tip is brought closer to the surface,
striking the surface will limit, typically due to damping, the
oscillatory motion. The amount of change is measurable as a
decreased RMS value, A.sub.s. This modified amplitude value may be
used as a setpoint in the manner of other above described SPM's,
such that as the tip is scanned laterally, the cantilever height
may be adjusted with feedback to keep the RMS setpoint, A.sub.s, at
a constant value. Alternatively, changes in the amplitude of
oscillation themselves can be used as a direct measure of surface
topography. Thus, an image of the surface may be generated. The
preferred embodiment uses a digital processor to provide the servo
control by means of feedback programs executed by the processor. An
analog feedback system is also possible. Strain gauges, such as
resistive or piezo-resistive strain gauges or piezoelectric
elements built into the cantilever arm, may be employed in place of
the optical deflector detector shown.
As shown in FIG. 4, this version of the invention may also be
implemented with other types of AFM's. For instance, the Compact
AFM, disclosed in U.S. Pat. No. 5,189,906, describes an AFM where
the probe is scanned rather than the sample. This AFM has provision
to attach the probe to a separate positioner, which may be used to
impart the oscillation, such that a setpoint may be established for
contact with the surface.
This preferred embodiment of the invention has several advantages.
This AFM can be operated with extremely light tapping forces. In
general, the inventors have found that even using relatively stiff
levers, on the order of 10's of newtons per meter in order to give
high frequency oscillations such as up to 2 MHz, the forces on the
sample are still extremely light. For instance, it is easy to
establish a setpoint that is 10 nm less than the free oscillation
amplitude, which may be on the order of 100 nm. Thus the energy in
the lever oscillation is much higher than that lost in each cycle
by striking the surface. A conservative estimate of the actual
force imparted to the surface is to assume the contact is inelastic
and therefore the bending of the lever due to surface contact is
just the amplitude gained in one cycle, approximately (A.sub.o
-A.sub.s)/Q, where Q is the quality factor of the lever. Typical
silicon levers have Q's of 100 to 1000, so for a setpoint 10 nm
below the free amplitude of 100 nm the force per strike is 0.1 to 1
nanonewtons for a cantilever with a force constant of 10
newtons/meter. A contact mode AFM is limited to a contact force of
about 50 nN in air due to the fluid layer attraction, and this can
be reduced to about 1 nN operating in a liquid cell. So the present
invention has extremely light contact forces and no shear forces at
all. Thus, this technique is comparable to the non-contact mode in
terms of surface damage, and operates much more stably and
reliably. This mode effectively eliminates the effect of the
surface fluid layer which limits the utility of both prior art
contact and noncontact mode systems, and the advantage of operation
under liquid disappears for many applications.
The invention also can achieve very high resolution. The
oscillation can potentially be at very high frequency since only a
very small lever must be driven, and because the low striking
forces allow stiff levers to be used. This mode does not require
resonant operation per se, but requires sufficient energy in the
oscillation to overcome the stickiness of the surface. Thus in
practice, resonant operation is necessary, but the higher harmonies
of the resonant frequency may be usable. The inventors have
successfully operated the invention at oscillation frequencies of
up to 2 MHz. It is straightforward to oscillate at a frequency that
for typical scan sizes and rates will cause the tip to strike the
surface many times before it has displaced laterally by one tip
diameter. Thus the lateral resolution is only limited by the tip
size, which is much better resolution than achieved by prior art
non-contact AFM's, whose resolution is determined by the height of
the tip above the surface. Since the vertical deflection detector
for a typical AFM has sub-nanometer resolution, the invention will
also be able to maintain the setpoint to sub-nanometer accuracy.
The invention has been successfully used to measure surface
roughness on polished silicon of under 1 angstrom RMS. The
invention does not depend on Van der Waals interactions like the
non-contact AFM to sense the surface, so it can operate under
fluids for samples that need to be hydrated.
As can be seen from FIG. 8, the region of stable operation for the
invention is forgiving, but if the probe does become stuck to the
surface there is significant hysteresis in the recovery process.
The probe has to be pulled away a relatively large distance, then
it will abruptly restart the free oscillation. It then has to be
brought back to the operating height. If the setpoint amplitude
A.sub.s is too low, the tip may remain oscillating in the surface
fluid layer with the feedback system not trying to pull the tip
loose. The inventors have found that this type of scanning is
stable, but gives low resolution similar to the non-contact method.
The amplitude-distance curve of FIGS. 6 and 8 is of great utility
in evaluating the stickiness of a surface and choosing an A.sub.o
and an A.sub.s that will produce satisfactory operation.
The preferred method of operation is to select a free amplitude
A.sub.o such that the amplitude versus approach curve of FIG. 8
gives a continuous, stable decrease of amplitude as the lever is
brought closer to the surface. The amplitude will depend on the tip
sharpness, cantilever stiffness, and the sample surface, as well as
current environmental conditions such as humidity. Preferably, in
an SPM where this curve may be produced, displayed and adjusted in
real time, the operator can determine a suitable free amplitude by
starting with a low amplitude and increasing it until the curve
changes shape from a curve like FIG. 6 to a curve like FIG. 8. The
preferred setpoint for high resolution imaging topography is an
A.sub.s that is greater than the amplitude at which the cantilever
breaks free on the withdraw portion of FIG. 8. This point is
labeled A.sub.p. If one operates at a setpoint above A.sub.p, then
if the tip does become stuck to the surface, the feedback will
return to a condition where the lever is free of the surface liquid
layer.
Using the amplitude-distance curve to evaluate and select operating
parameters is important because the actual behavior of the lever is
sample and condition dependent. For most samples, for some value of
A.sub.o, a curve such as FIG. 8 may be produced, and if an A.sub.s
is selected greater than A.sub.p, stable operation will result in
the imaging mode. However, some combinations of sample and
conditions will not allow an amplitude-distance curve such as shown
in FIG. 8. Either the pull-off curve or the approach curve may not
follow the same pattern. For instance, the pull-off may be abrupt,
indicating that the lever is affected little by the fluid layer on
pull-off. This condition does not usually affect stable operation.
Another condition may be an abrupt capture on approach, similar to
small oscillation (non-contact) operation. For this case, it is
vital to adjust the parameters to achieve as wide an operating
region as possible. Moreover, the amplitude-distance curve may be
used to set operating parameters that result in interesting and
potentially useful modes. If A.sub.s is set lower than A.sub.p for
a sample that has an amplitude-distance curve like FIG. 8, then if
the lever can be initially pushed into the sticking position, the
feedback loop will then maintain an imaging mode where the tip is
oscillating within the fluid layer, and not actually in contact
with the surface. This is a very stable non-contact mode. In this
mode the instrument is essentially imaging the surface fluid layer,
and the topography of this layer compared to the topography of the
surface may provide useful information. The resolution is lower
than when striking occurs, but is comparable to the less stable
prior art non-contact mode where the tip is above the layer and the
free amplitude oscillation is small. Another potential mode occurs
if the lever is oscillated below resonance when the tip is far from
the surface. When the tip is withdrawn, the fluid layer attached to
the lever will lower the resonance such that the oscillation
response to the drive signal may be actually larger than the free
amplitude because the drive signal is now closer to resonance. This
case is illustrated in FIG. 11. The inventors have noticed that
this mode is highly sample dependent, and thus may contain useful
information about the surface fluid layer. Although both of these
modes should be avoided for reliable topographic measurements, they
illustrate the usefulness of the amplitude-distance curve to
intelligently select operating parameters, and the potential
diversity of applications for the invention.
Because stiff levers are typically used for the invention for high
frequency operation, the inventors have found it is often useful
when generating amplitude-distance curves to limit the distance
over which the curve is obtained. If the lever is pushed
sufficiently far into the surface that oscillation has ceased, then
the force on the sample is simply the displacement of the lever
times the spring constant. Typical levers suitable for this
invention have spring constants many times those used in
conventional contact AFM's, so the force exerted in an
amplitude-displacement curve at the end where oscillation ceases
can be very large. The sample and the cantilever may be damaged by
the operation of obtaining the curve. Thus, the amplitude may be
monitored during the distance modulation, and the lever only pushed
into the sample until a predetermined decrease in amplitude is
observed, and then pulled away. For instance, the digital
controller that operates the scanner can be programmed to move the
sample toward the cantilever until a certain amplitude or fraction
of the free amplitude is reached and then caused the sample to be
retracted. The user could enter the target amplitude into the
controller. Such a curve is shown in FIG. 12. The probe is pushed
into the sample until an amplitude A.sub.min is observed, and no
further. The setpoint can be picked on the sloped part of the curve
A.sub.min. This technique will prevent damage to tip and sample
during the amplitude-distance curve operation.
The invention can also be used to measure attractive force
distributions such as magnetic fields, in a mixed mode operation,
using both the invention and the prior art non-contact mode. For
instance, if a magnetic tip is used, and a surface is scanned with
alternating magnetic and non-magnetic regions, the reduction in
amplitude will be affected not only by the tip striking the surface
but also by the effect of the magnetic force gradient on the
resonant frequency of the cantilever, as described above (if the
driving frequency is slightly above the resonant frequency). Over
areas where the magnetic field decreases the oscillation amplitude
below the setpoint, the feedback will raise the lever to maintain
the setpoint, such that the magnetic areas will appear in the image
as regions of increased height. If the magnetic field interaction
is strong enough, the tip will no longer be striking the surface
over the magnetic regions. In regions with no magnetic field, the
feedback will return the tip to the striking mode to maintain the
setpoint. This same technique is potentially useful to measure
other parameters such as electric field, which with appropriate
tips and electronics may also exert force interactions on
oscillating probes.
Another problem with contact mode prior art AFM's is that they have
poor performance when measuring steep sample features, such as
trenches on integrated circuits. There are at least two problems
associated with steep features. The first is that when the tip
scans into the feature it twists laterally, and this lateral twist
is not detected or interpreted properly by the vertical deflection
detector. The second problem is that suitable tips need to be long
and narrow to get into the grooves so these tips are not very stiff
laterally. As the tip approaches the sidewall, attractive forces
tend to pull the tip toward the surface as shown in FIG. 13, and
the tip tends to stick to the surface because of surface tension as
the scan moves away from a wall. Both of these effects distort
measurements of linewidth or step height that need to be made to
accuracies of fractions of a percent. Since the sticking may depend
on ambient conditions, such as humidity, the measurements can vary
from day to day. A modification of the invented technique can be
used for such measurements.
As shown in FIG. 9A, the oscillation can be done in a horizontal
direction, using a probe with a suitable shape in a trench or near
any step. If the probe is scanned laterally in the trench or near a
step while oscillating laterally, the free oscillation amplitude
will decrease when the wall is touched, as shown in FIG. 9B. Thus a
setpoint for horizontal motion may be established that causes the
probe to contact a trench or step sidewall with a series of taps so
that measurement accuracy will be unaffected by attractive forces
or sticking. This setpoint can be used to maintain the probe at a
sidewall with low force and no sticking. The probe could then be
scanned vertically and served laterally to maintain the setpoint in
order to profile the wall. Alternatively, linewidth could be
measured if the probe is scanned back and forth across a trench,
such that the lateral motion reverses direction when a wall is
encountered, as determined by the oscillation amplitude reducing to
a setpoint at the walls. This process could be repeated as the
probe is scanned along the trench to measure variations in
linewidth, or a combination of lateral and vertical profiling could
be used.
Yet another technique to measure steep walls is shown in FIG. 10.
If the lever is mounted at an angle to the surface, preferably
greater than the half angle of the probe, and the probe is
oscillated perpendicular to the lever then the probe will scan
along the surface until the wall is encountered, and will follow
the wall up, because the oscillation setpoint will be affected both
by the floor and the wall. Thus an x-z profile of the wall and
floor will be produced. This approach will even work for undercut
walls depending on the shape of the tip and nature of the
undercut.
Another mode is to increase the amplitude of oscillation in a
trench until the tip hits both sides. The tip could then be scanned
laterally across the trench as shown in FIG. 14a so that the point
of maximum oscillation is found. This maximum oscillation will be
where the tip is in the center of the groove, hitting each side
equally. This amplitude, A.sub.max, shown in FIG. 14b, combined
with the width of the tip, will give a measure of the width of the
groove. One could then trace this cut vertically.
Further embodiments of the present invention utilize the phenomenon
that as the probe taps the surface of the sample, the attractive
force caused by the capillary forces on the tip and any momentary
adhesion between the tip and sample material affect both the
resonant frequency of the cantilever and the phase between the
signal driving the cantilever oscillation and the actual
oscillation of the free end of the cantilever. Although the
invention relies on the probe oscillation having sufficient energy
to prevent any long term, i.e., more than a small fraction of an
oscillation cycle, sticking of the probe tip to the surface, the
short period in each cycle where the probe strikes the surface will
result in some adhesion. This momentary adhesion may depend on the
elasticity or damping of the probe-surface contact and will
measurably affect the resonant frequency and phase of the
cantilever oscillation.
This measurement of phase or resonant frequency during tapping of
the probe against the surface is typically used as a chemical force
microscope where the tip is made from a particular material or
coated with a particular material so that the adhesion of the tip
could be tailored to be specific to a certain substance on the
surface. The phase or frequency measurement is then used to find
and image where the specific substance is on the surface. An
example of this is an antibody antigen interaction, where a
specific antibody or antigen is placed on the tip in order to find
its corresponding antigen or antibody on the surface. A prior art
chemical force microscope is described by Frisbie, et al., SCIENCE
265:2071-2074, 1994, where a frictional force measured by the tip
dragging across the surface is used to image the position of a
specific chemical on the surface with different chemicals placed on
the tip. As pointed out earlier, though, this dragging of the tip
across the surface has adverse affects and should be avoided. For
instance, the dragging might remove the material coated to the tip.
According to the present invention, the tapping while measuring
phase or resonant frequency is used as a measure of adhesion or
chemical interaction
In a preferred embodiment, the cantilever is oscillated and
feedback is used to control the vertical position of the fixed end
of the cantilever so that the oscillation of the cantilever is at a
constant amplitude as the tip taps the surface, and changes in
phase, from as small as 0.01 degrees to several degrees, are
recorded as a function of the tip lateral position during scanning.
An image is then made of the phase measurement as a function of
lateral position, i.e., X and Y. Such a device is shown in FIG. 15,
where a phase detector is included, with the detected phase being
sent to the processor for storage and display.
In another embodiment, the resonant frequency of the cantilever,
which is typically 100-300 KHz, normally 150 KHz, is tracked during
tapping at constant amplitude, the change in frequency giving an
indication of a change in momentary adhesion between the tip and
surface. This is done with a second feedback loop operating to keep
the phase constant by varying the drive frequency of the
cantilever. If the phase is kept constant at, for instance,
90.degree. then the drive frequency tracks the resonant frequency
of the system, and gives a measure of the adhesion between the tip
and sample. Typically, the resonant frequency varies in a range of
0.1 to 100 Hz. In this embodiment, one always drives the cantilever
at the same point on the resonance curve, so the force of tapping
would remain constant during imaging. This regulation of the
frequency to follow the resonant frequency is also useful in cases
where one wants to use a very light tapping force, which requires
that the tapping force be well regulated, i.e., that the free
amplitude A.sub.o does not vary as it would if the resonant
frequency varies and the drive frequency stays fixed. Also, the
frequency of resonance is more related to physical parameters of
the oscillator than a phase measurement and allows a calculation of
the adhesive force knowing other parameters of the cantilever. This
frequency tracking embodiment is shown in FIG. 16.
One can gain more information about the interaction between the tip
and surface during tapping by modulating the cantilever drive
frequency, typically by an amount of 1-5 KHz, and observing the
corresponding change in phase, i.e., determining the slope of the
phase vs. frequency curve during tapping. For a Lorenzian shaped
curve, which is the resonance curve of amplitude (A) vs. frequency
(f) for an harmonic oscillator, as shown in FIGS. 17a and 17c, this
slope as shown in FIGS. 17b and 17d is related to the width of the
curve and therefore is a measure of the damping forces on the
oscillator. As shown in FIG. 17c in relation to FIG. 17a, higher
damping results in a broader cantilever resonance curve
(.gamma..sub.1 <.gamma..sub.2 at the same amplitude), which, as
shown in FIGS. 17b and 17d, decreases the slope (.DELTA.p/.DELTA.f,
where .DELTA.p is a change in phase produced by a change .DELTA.f
in frequency) of the phase curve in the region around the
resonance. Thus, modulating the cantilever drive frequency by an
amount .DELTA.f, monitoring the ratio .DELTA.p/.DELTA.f, and
computing the slope .DELTA.p/.DELTA.f provides a measure of the
probe-sample interaction. In another embodiment of this invention,
this damping is measured as a function of X and Y to produce an
image of damping forces over the scanned surface. This is done in
the frequency feedback method where the modulation is done while
the average frequency is controlled to keep the average phase
constant, i.e., the frequency is modulated about the resonant
frequency. This will produce a modulation .DELTA.p of the phase
about the phase setpoint.
In the above described embodiments, feedback control of the
amplitude and/or phase servos and/or frequency modulation are
usually performed digitally by means of a digital computer, so that
feedback control and modulation control can be programmed so that
they do not conflict.
The adhesion and damping between the sample and probe may be
dependent on the tapping force, which is controlled by the setpoint
amplitude, the lower the setpoint the higher the force. Thus, the
previous described measurements of phase, frequency, or damping
could be made as a function of the tapping amplitude setpoint, by
varying the tapping amplitude setpoint at each data point, each
scanline, or each complete image. Comparison of the data obtained
at different tapping amplitude setpoints can be made either
qualitatively (visually) or quantitatively (arithmetically),
according to this embodiment, and is useful to discriminate force
dependent sample characteristics.
This measurement of phase or resonant frequency while tapping also
provides information about surface coverage of substances put down
on the surface of the sample, such as oil or lubricating layers on
the sample, or monomolecular films applied to the surface.
The phase of the cantilever oscillation will not only be sensitive
to adhesion, but also may be affected by long range forces such as
electric and magnetic forces. These forces are measured more
sensitively by lifting the cantilever off the surface so that phase
shifts due to adhesion are avoided. This method of measuring long
range forces is described in U.S. Pat. No. 5,308,974 by the same
inventors.
Obviously, numerous modifications and variations of the present
invention are possible in light of the above teachings. It is
therefore to be understood that, within the scope of the appended
claims, the invention may be practiced otherwise than as
specifically described herein.
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