Radiant energy – Inspection of solids or liquids by charged particles
Reexamination Certificate
2000-01-25
2003-11-04
Lee, John R. (Department: 2881)
Radiant energy
Inspection of solids or liquids by charged particles
C250S307000, C250S368000
Reexamination Certificate
active
06642517
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention pertains to atomic force microscopy, and specifically, to an apparatus and method for sensing the deflection of a cantilever of an atomic force microscope.
2. Description of the Prior Art
Atomic force microscopy is directed to sensing the forces between a sharp stylus or tip of, for example, a probe and the surface of a sample to be investigated. The interatomic forces between the two cause a displacement of the tip mounted on the end of a cantilever, and this displacement is indicative of surface characteristics, e.g., topography, of the sample.
An atomic force microscope (AFM) is an apparatus capable of observing these surface characteristics of the sample, which can be on the atomic scale. In one mode of AFM operation, the probe is moved close to the sample surface, and a Van der Waals attractive force acts between an atom at the tip of the probe and an atom on the sample surface. If both atoms move close to each other, so as to nearly contact, a repulsive force occurs therebetween due to the Pauli exclusion principle. The attractive and repulsive forces between the atoms are very weak, typically about 10
−7
to 10
−12
N, and thus can be difficult to detect.
In general, the AFM probe is positioned a distance from the sample surface that is within a range in which the cantilever is deflected by the inter-atomic force on the probe tip. Then when the probe is scanned along the sample surface, the distance between the probe and the sample varies depending upon the configuration of the sample surface and, accordingly, the amount of deflection of the cantilever varies. Typically, this movement of the probe tip follows the topography of the sample surface. This variation in deflection of the cantilever is detected, and feedback control is effected by use of a fine movement element, such as a piezoelectric element, to return the amount of deflection of the cantilever to an initial, set point value. Based on the voltage applied to the piezoelectric element by the feedback system in response to the displacement of the probe tip, an image of the surface configuration of the sample can be obtained.
Typically, the cantilever employed in the AFM, or magnetic force microscope (MFM), or other scanning probe microscope (SPM) or profiler is a beam that is fixed at one end and free to bend at the other end. The dimensions (length, width, thickness) and Young's modulus determine the spring constant which may be selected to ensure that the cantilever exhibits high responsiveness to weak inter-atomic or magnetic forces such that the system is sensitive to cantilever deflection. Notably, various techniques have been employed to detect this cantilever deflection. In particular, various tip sensors are known in the prior art and include those that utilize tunneling currents, optical interferometry, or optical lever, etc. An AFM implemented with a tunneling sensor includes a probe having a sharply pointed tip that is attached to a spring-like cantilever beam to scan the topography of a surface to be investigated. The attractive or repulsive forces occurring between the atoms at the apex of the tip and those of the surface result in small deflections of the cantilever beam, as described above. The deflection may be measured by a tunneling microscope, which includes an electrically conductive tunnel tip that is disposed from the sample surface a particular distance. In operation, a tunneling current is measured as the tunnel tip is scanned over the sample surface. Variations in the tunneling current are then monitored, with the variations being indicative of cantilever beam deflection. Using these beam deflection measurements in conjunction with known characteristics of the cantilever, the tunneling sensor determines the forces between the tip and the surface under investigation. An AFM implemented with a tunneling sensor is described by G. Binnig et al, in Phys. Rev. Lett., vol. 56, No. 9, March 1986, pp. 930-933.
Alternatively, optical methods may be implemented. For example an AFM implemented with a laser interferometer can be used to measure the tip displacement. See, generally, G. McClelland et al., entitled “Atomic Force Microscopy: General Principles and a New Implementation”, Rev. Progr. Quart. Non-destr. Eval., Vol. 6, 1987, p. 1307, and Y. Martin et al., entitled “Atomic Force Microscope-Force Mapping and Profiling on a Sub 100-A scale”, J. Appl. Phys., vol. 61, no. 10, May 15, 1987, pp. 4723-4729. Laser interferometers utilize optical beam splitters to separate a laser beam into a plurality of beam components which includes the primary beam components of interest, and a photosensor to detect an interfering component of the primary beam components. This interfering component is indicative of cantilever displacement. The advantages of optical detection over tunneling detection include increased reliability and ease of implementation, insensitivity to the roughness of the beam, and a smaller sensitivity to thermal drift.
Another optical deflection method includes using an optical lever in conjunction with a laser beam directed towards the back of the cantilever. Notably, to achieve good sensitivity with the optical lever, the incident angle of light on the reflective surface of the probe with respect to the longitudinal axis of the cantilever should be large, with the best sensitivity theoretically occurring when the angle of incidence is 90°. The sensitivity falls off with the sine of the angle of incidence, and drops to zero when the angle of incidence is zero. Importantly, for known optical lever systems which reflect the light beam off the back of the cantilever, it is necessary to place the laser directly above the cantilever. Such an arrangement is disclosed in U.S. Pat. No. 5,497,656.
FIG. 1
shows a schematic representation of an AFM
1
having a conventional cantilever beam deflection apparatus
2
for detecting the deflection of a probe assembly
3
as a tip
4
of the probe assembly interacts with a surface
5
of a sample
6
. Notably, sample
6
is mounted on a scanner
7
which moves sample
6
to allow AFM
1
to scan surface
5
. Probe assembly
3
includes a substrate
8
having a cantilever
9
extending therefrom, while tip
4
extends from the free end of cantilever
9
. Deflection apparatus
2
includes a laser
10
for directing a beam of light downwardly towards the top surface of cantilever
9
in a direction generally perpendicular to sample surface
5
. During operation, cantilever
9
reflects the laser beam towards a mirror
12
which directs the beam through a collecting lens
13
and towards a position sensing detector
14
. Optical beam deflection detection apparatus
2
then measures the position of the deflected light beam which is indicative of the deflection of the cantilever. The deflection of cantilever
9
is a measure of the interaction force between tip
4
and surface
5
of sample
6
. Although this straight-forward arrangement is useful in some applications, in many applications (for example, near-field scanning optical microscopy, discussed below), the arrangement shown in
FIG. 1
has significant disadvantages.
For example, it is known to combine an AFM with a conventional optical microscope to aid in alignment of the laser beam on the back of the cantilever and to provide a view of the surface features of the sample. Notably, high performance microscope objectives have a short working distance and must be positioned close to the sample surface. High resolution optical imaging is therefore difficult to implement in combination with the optical lever (or an interferometer) because there is not enough room for both the laser source and/or beam and a high-performance microscope objective. A similar problem exists with near-field scanning optical microscopy when using a solid immersion lens and a microscope objective. See, for example, U.S. Pat. No. 5,939,709 to Ghislain et al. and entitled “Scanning Probe Optical Microscope Using a Solid Immersion Lens,” whic
Elings Virgil B.
Ghislain Lucien P.
Lee John R.
Patterson Thuente Skaar & Christensen P.A.
Quash Anthony
Veeco Instruments Inc.
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