Atomic force microscopy

Details Atomic force microscopy Atomic force microscopy

1997-01-27

2001-07-31

Noland, Thomas P. (Department: 2856)

Measuring and testing

Surface and cutting edge testing

Roughness

C250S306000, C356S370000

Reissue Patent

active

RE037299

ABSTRACT:

BACKGROUND OF THE INVENTION
The present invention relates to atomic force microscopy and specifically to an atomic force microscope which employs a micromachined cantilever beam in order to achieve atomic resolution. In addition, the atomic force microscope is capable of operation in vacuum, air or liquid environments, of scanning a large surface area and of providing common mode rejection for improved operation.
Atomic force microscopy is based upon the principle of sensing the forces between a sharp stylus or tip and the surface to be investigated. The interatomic forces induce the displacement of the stylus mounted on the end of a cantilever beam. In its original implementation, a tunneling junction was used to detect the motion of the stylus attached to an electrically conductive cantilever beam. Subsequently, optical interferometry was used to detect cantilever beam deflection.
As described by G. Binnig et al, in Phys. Rev. Lett., vol. 56, No. 9, March 1986, pp. 930-933, a sharply pointed tip is attached to a spring-like cantilever beam to scan the profile 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 tiny deflections of the cantilever beam. The deflection is measured by means of a tunneling microscope. That is, an electrically conductive tunnel tip is disposed within the tunnel distance from the back of the cantilever beam, and the variations of the tunneling current are indicative of the beam deflection. The forces occurring between the tip and the surface under investigation are determined from the measured beam deflection and the characteristics of the cantilever beam.
In articles by 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- Å scale”, J. Appl. Phys., vol. 61, no. 10, May 15, 1987, pp 4723-4729, there is described the use of a laser interferometer to measure tip displacement. The advantages of optical detection over tunneling detection of the cantilever beam deflection are increased reliability and ease of implementation, insensitivity to the roughness of the beam, and a smaller sensitivity to thermal drift.
The atomic force microscope has a promising future in research and development and in manufacturing environments because of its unique capabilities of imaging insulators and measuring minute forces. In order to fulfill the promise, the atomic force microscope should be versatile, i.e., operate in vacuum, air or aqueous environments and be reliable, simple, and compact. Moreover, for certain applications atomic resolution and the ability to scan larger areas are additional requirements.
SUMMARY OF THE INVENTION
According to the present invention, a piezoelectric tube is used for scanning a surface and a micromachined cantilever beam is used for supporting the tip. The micromachined cantilever beam orientation is sensed by reflecting a laser beam from the back of the cantilever beam and detecting the reflected laser beam with a position-sensitive detector, preferably a bicell. The laser beam source is preferably, but not necessarily, a single-mode diode laser operating in the visible range. The laser output is coupled into a single-mode optical fiber whose output is focussed onto the back of the cantilever beam. In an alternative embodiment where the tip is supported by one or more arms extending from the end of the cantilever, the laser beam is focussed onto the arm or arms in the region of the tip. The term focussed onto the back of the cantilever will be understood to encompass both focussed onto the back of the cantilever beam itself or onto the arm or arms in the region of the tip. The angle of deflection of the reflected beam is detected with the bicell. Common mode rejection of intensity fluctuations is achieved by symmetrically positioning the bicell with respect to the incoming beam. In the present invention, the positioning is achieved, remotely, by means of an inertial mover as will be described below. Remote positioning of the bicell in ultrahigh vacuum environments is essential. Alternatively, in cases where deviation from the center position on the bicell are small compared to the laser beam diameter, common mode rejection can be achieved electronically, e.g., by attaching a variable resistance, in series, to each segment of the bicell to equalize the voltage drop across the resistances, thus providing an electronic equivalent of centering the reflected laser beam on the face of the bicell. The output of the bicell is provided to a computer for processing the data for providing an image of the surface to be investigated with atomic resolution.
The present invention relies upon the measurement of the cantilever beam orientation rather than displacement. A change in position is transformed into an angular change which is inversely proportional to the length of the cantilever. In prior art atomic force microscopes the length of the cantilever beam has been on the order of 1 mm. The micromachined cantilever beam employed in the present invention is on the order of 100 microns in length thereby enabling atomic resolution of the surface to be investigated. When practicing the invention in an environment not requiring a vacuum, simplifications to the arrangement are possible. For example, the optical fiber can be eliminated, resulting in a more compact design. Also, the inertial mover is not required since the microscope components are accessible.
Preferably, the output of the visible diode laser is an elliptical beam with an aspect ratio in the range of approximately 5 to 7:1. While such ellipiticity is generally considered undesirable requiring optical correction, the asymmetric beam shape is advantageously used in practicing the present invention. By appropriately focussing the laser beam on a rectangular cantilever beam, increased sensitivity of the laser beam deflection measurement and a simplified alignment procedure are achieved. An additional advantage of the elliptical beam resides in the ability to use a laser with higher laser power, without exceeding the saturation limit of the bicell, and thereby achieve higher measurement sensitivity. It is also possible to reduce the distance between the cantilever beam and the bicell, thus making the atomic force microscope even more compact. In cases where the beam is not inherently elliptical, as in the case of the light output from an optical fiber, a cylindrical lens can be used to achieve the advantageous elliptical shape.
A principal object of the present invention is, therefore, the provision of sensing the orientation of a micromachined cantilever beam of an atomic force microscope with optical-beam-deflection.
An object of the present invention is the provision of an atomic force microscope employing an inertial mover coupled to a position-sensitive detector.
Another object of the present invention is the provision of a method for combining the use of optical-beam-deflection techniques with the use of microfabricated cantilever beams, including the use of optical fibers to implement the optical-beam-deflection technique.
Further and still other objects of the present invention will become more clearly apparent when the following description is read in conjunction with the accompanying drawings.


REFERENCES:
patent: Re. 33387 (1990-10-01), Binnig
patent: 1976337 (1934-10-01), Firestone et al.
patent: 2048154 (1936-07-01), Abbott et al.
patent: 2171433 (1939-08-01), Powers
patent: 2205517 (1940-06-01), Devol
patent: 2686101 (1954-08-01), Davis
patent: 3251135 (1966-05-01), Reason
patent: 3335367 (1967-08-01), Skooglund et al.
patent: 3571579 (1971-03-01), Whitehouse et al.
patent: 3617131 (1971-11-01), Taguchi
patent: 3782205 (1974-01-01), Hand
patent: 3798449 (1974-03-01), Reinheimer et al.
patent: 4102577 (1978-07-01), Suzuki et al.
patent: 4267732 (1981-05-01), Quate
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