Sensing mode atomic force microscope

Details Sensing mode atomic force microscope Sensing mode atomic force microscope

1998-04-03

2003-02-11

Lee, John R. (Department: 2881)

Radiant energy

Inspection of solids or liquids by charged particles

C250S307000, C073S105000

Reexamination Certificate

active

06518570

ABSTRACT:

BACKGROUND OF THE INVENTION
The present invention relates generally to atomic force microscopy and more particularly relates to an atomic force microscope and controller which minimize contact forces between a probe tip and a specimen and is well suited for the study of biological specimens.
In the study of biology, it is desirable to observe biological specimens under very high magnification in a native environment. Such observations allow scientists to monitor, in real time, biological processes at the molecular and sub-molecular level. Such processes include the interaction of proteins with DNA and with each other. Currently, these processes cannot be observed in real time with electron microscopes or x-ray crystallography techniques which are known in the art, as the specimens are not in their native environment when using these apparatuses. Accordingly, scientists have sought alternate methods to observe biological specimens. One such alternative is known as the atomic force microscope.
Atomic force microscopes (AFM), which are generally known in the art, physically probe a specimen to create an image of the specimen's surface.
FIG. 1
illustrates a typical embodiment of an AFM known in the art. The AFM has two primary components, a scanner
10
and a flexible cantilever
12
having a probe tip
14
on a free end. The scanner
10
has a top surface
16
on which a specimen
18
to be imaged is placed. The scanner
10
typically employs three piezoelectric elements
20
,
22
,
24
to move the specimen
18
in three dimensions, X, Y and Z, relative to the position of the probe tip
14
. The probe tip
14
is affixed to the free end of the flexible cantilever
12
and contacts the specimen
18
. The AFM includes a laser
26
directed onto the cantilever
12
and a photo detector
28
which is responsive to laser light to measure the deflection of the cantilever
12
. As the degree of cantilever deflection is proportional to the contacting force between the probe tip
14
and the specimen
18
, such force can accurately be calculated based on the angle of cantilever deflection.
To create an image of a specimen, the scanner
10
directs the specimen
18
in a raster-scan fashion in the X-Y direction while continuously sampling the contour of the specimen
18
in the Z direction. The sampling is generally performed using one of two techniques known in the art, namely contact mode or tapping mode®. (Tapping mode is a registered trademark of Digital Instruments, Inc. of Santa Barbara, Calif.)
In contact mode, the scanner
10
is controlled in the Z direction such that the contacting force between the probe tip
14
and the specimen
18
is substantially constant. As the contour of the specimen changes, the deflection of the cantilever
12
also changes and a servo system driving the scanner
10
adjusts the Z coordinate of the scanner
10
to restore the desired constant force. At each specimen point, the coordinate of the Z axis is indicative of the specimen contour. Because the probe is constantly contacting the surface of the specimen during the X-Y raster scan, significant lateral forces are applied to both the specimen
18
and the probe tip
14
. The probe tip
14
, which is typically 200-300 angstroms (Å) in diameter is subject to rapid wear and breakage under these forces. Also, when used on soft specimens, such as biological specimens, the probe tip is likely to destroy the surface of the specimen, making accurate and repeatable measurements impossible.
In tapping mode®, the cantilever
12
is driven in an oscillatory fashion at the resonant frequency of the cantilever. This may be achieved by affixing the cantilever to a piezoelectric element
30
and driving the piezoelectric element
30
with a voltage signal at the resonant frequency of the cantilever. To determine the contour of the specimen in tapping mode®, the scanner
10
moves the specimen in the Z direction until a predetermined reduction in oscillation amplitude is detected. The reduction in oscillation amplitude is the result of the probe tip
14
contacting the surface of the specimen
18
during each cycle of oscillation. Because the probe tip
14
only momentarily contacts the specimen
18
during the X-Y raster scan, the lateral force present during contact mode is substantially reduced. However, because the probe tip
14
is moving rapidly on arrival at the specimen surface, the contacting force, while short in duration, is large in magnitude. The force that results from tapping modes tends to be destructive to biological specimens. Thus tapping modes is most useful in sampling hard surfaces, such as those found in integrated circuit manufacturing processes and the like. Also, tapping mode® is difficult to use when measuring a fluid based specimen. When the cantilever assembly is submerged into a fluid environment, the desired oscillation of the cantilever can be dampened and additional resonances are developed which can adversely affect operation and accuracy. Also, fluid flow induced by the tapping oscillation tends to erode the specimen. Because biological specimens tend to reside in a fluid environment, tapping mode is not well suited for measuring these specimens.
An alternative operating mode to both contact mode and tapping mode® is described in U.S. Pat. No. 5,229,606 to Elings et al. Elings et al. describe what the inventors refer to as “jump scanning.” In jump scanning, the probe is momentarily brought into contact with the surface to be measured. The probe is then lifted away from the surface as the specimen is moved in the X direction and the probe tip is then brought back down into contact to take the next specimen. By jumping over the surface of the specimen, Elings et al. teach a method of increasing scanning speed with reduced risk of probe damage. However, when the probe tip and specimen contact one another, an attractive force tends to hold the probe tip in contact with the specimen. To ensure that the probe tip is able to release, the cantilever
12
must be formed with a sufficient spring constant to overcome this attractive force. Unfortunately, increasing the spring constant of the cantilever
12
increases the magnitude of the contact force between the probe tip
14
and specimen
18
which is required to achieve a measurable cantilever
12
deflection. Such stiff cantilevers, i.e., in the range greater than 0.1 Newtons per meter (N/M), which are required for jump mode, are incompatible with the more sensitive biological specimens which are easily damaged under the application of such forces.
The problem of overcoming the attractive forces between an AFM probe tip
14
and specimen surface was addressed in U.S. Pat. No. 5,515,719 to Lindsay. Lindsay recognized that when soft (low spring constant) cantilevers are used, the adhesive interaction between the specimen
18
and probe tip
14
tends to draw the probe tip in and the probe tip
14
will stick to the surface until enough force is applied to the cantilever base to release the probe tip
14
. To address this problem, Lindsay teaches the addition of a magnetic particle attached to the cantilever in combination with a magnetic solenoid located proximate to the cantilever. The solenoid generates a magnetic field which is variable and precisely regulated by a servo circuit. The servo circuit monitors the deflection of the cantilever and continuously adjusts the magnetic field such that the attractive force between the probe tip
14
and specimen
18
is substantially neutralized. In this way, the probe tip
14
, as taught by Lindsay, never makes adhesive contact with the specimen. However, in order to operate in a stable fashion, the servo circuit taught by Lindsay must precisely neutralize the attractive force, otherwise instability may result.
The use of a magnetic particle affixed to a flexible cantilever and controlled by a magnetic coil as used by Lindsay was first disclosed in an article by Florin et al., entitled “Atomic Force Microscope with Magnetic Force Modulation”, published in the Review of Scientific I

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