Atomic force microscopy

Abstract: An atomic force microscope includes a tip (12) mounted on a micromachined cantilever (10). As the tip scans a surface (24) to be investigated, interatomic forces between the tip and the surface induce displacement of the tip. A laser beam is transmitted to and reflected from the cantilever for measuring the cantilever orientation. In a preferred embodiment the laser beam has an elliptical shape. The reflected laser beam is detected with a position-sensitive detector (22), preferably a bicell. The output of the bicell is provided to a computer for processing of the data for providing a topographical image of the surface with atomic resolution.

Description: 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 Microscropy: 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, 15 May 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 developement 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. In an article of D.Rugar et al, "Improved fiber-optic interferometer for atomic force microscopy" Applied Physics Letters (1989), 18 December, No.25, New York, US, pp. 2588-2590 an interferometer is disclosed for measuring the displacement of the tip, caused by the force between the surface atoms and the tip of an atomic force microscope. Because of the high accuracy of interferometric measurements, the position is determined with high sensitivity. The authors claim, that this interferometer can be used even together with micromachined thin-film cantilevers. However the sensitivity to determine the atomic force between the tip and the atoms in the surface, as known from interferometry, is critical dependent on the wavelength, because for an accurate measurement the displacement must be larger than the wavelength. This implies that the length of the cantilever beam cannot be chosen below a given limit, to generate a suitable angular momentum resulting in a large enough displacement for interferometric measurements of that displacement. This limits the application of micromachined cantilever beams in atomic force microscopy together with such an interferometer. In G. Meyer and N. Amer; Applied Physics Letters, 53(12), 19 September 1988, p. 1045 a sensitive and simple optical method for detecting the cantilever deflection in atomic force microscopy is described. The method as incorporated in an atomic force microscope, and imaging and force measurements in ultrahigh vacuum, were successfully performed. The atomic force microscope is suitable for generating a topographical image of a surface of a workpiece and comprises: a tip fixed to one end of the front side of a micromachined cantilever beam adapted for being positioned in proximity to the surface of a workpiece where the forces between the atoms of said tip and the surface deflect the cantilever beam, laser means for transmitting a laser beam to the back of the cantilever beam and position-sensitive detection means for receiving said laser beam after being reflected from the cantilever beam and for converting the reflected beam into an output signal, the output signal of the position sensitive detection means being indicative of the position where the reflected beam is impinging on the position sensitive detection means. The object of the present invention is the provision of a new and improved type of atomic force microscope. According to the present invention as claimed, a macromachined cantilever beam is used for supporting the tip. The cantilever beam is scanned by a piezoelectric tube in a preferred embodiment. Instead of measuring the position of the tip relative to the surface, the orientation of the cantilever beam is measured, that is influenced by the atomic force acting on 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 ellipticity 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. For a better understanding of the present invention, together with further objects and advantages, preferred embodiments of the invention are described in the following with reference to the accompanying drawings, in which: Figure 1 is a schematic representation of a portion of an atomic force microscope; Figure 2 is a schematic diagram of a portion of an atomic force microscope comprising the present invention; Figure 3 is a schematic diagram of a preferred position-sensitive detector useful for practicing the present invention; Figure 4 is an illustration of an elliptical laser beam spot focussed on a micromachined cantilever beam forming a part of an atomic force microscope; and Figure 5 is an illustration of an elliptical laser beam spot received at a position-sensitive detector. Atomic force microscopes are known in the art as described, for example, in U.S. Patent No. 4,724,318 issued to G. Binnig and assigned to the same assignee or the present invention, which patent is incorporated herein by reference. While the Binnig patent describes a method of measuring the tip to surface distance by means of monitoring the tunneling current, the present invention measures the tip orientation by optical-beam-deflection, as will be described hereinafter. The present invention is most advantageous for operation in an inaccessible environment, such as in a vacuum or an ultrahigh vacuum, due to the provision of a remotely positionable position-sensitive detector. Referring now to the figures and to Figure 1 in particular, there is shown a schematic representation of a cantilever beam deflection detection scheme. A stylus-cantilever system includes a cantilever beam 10 made of, e.g., silicon or silicon nitride, having a tip 12 of a length in the range between 1 and 10 microns, and preferably 5 micrometers in length disposed at the end of a pair of supporting arms 14. Alternatively, the tip 12 can be disposed at the end of single supporting arm extending from end of cantilever beam 10. A laser 18 transmits a laser beam through lens 20 where the beam is focussed directly onto the back of the arms 14 in the region of the tip. In an alternative embodiment (not shown) where the tip extends directly from the cantilever beam, the laser beam is focussed onto the back of the cantilever beam; or, alternatively, onto a reflector 16 attached to the back of the cantilever beam for the purpose of enhancing reflective properties. As used herein, the term "onto the back of the cantilever" will be understood to mean a laser beam transmitted onto the back of the cantilever beam itself, onto the back of a reflector coupled to the back of the cantilever beam, or onto one or more tip supporting arms in the region of the tip. The laser beam is reflected onto a position-sensitive detector 22. The output of the detector 22 is provided as one input to a general purpose computer. The x-axis and y-axis positions of the tip as the tip is scanned over the workpiece surface are also provided as inputs to the computer as is known in the art. The computer, in turn, processes the data in a known manner for providing a topographical image of the surface at atomic resolution. The image can be displayed on a screen or on a strip chart, be in tabular form or otherwise provided in a visual format. In a preferred embodiment, the laser 18 is a compact single-mode diode laser operating preferably in the visible light spectrum, preferably at 670 nm, for ease of alignment. However, a laser operating in the infrared or ultraviolet range will perform equally as well. The preferred position-sensitive detector is a silicon bicell. Generally, an atomic force microscope detects the motion of the tip toward and away from a surface to be inspected 24. The motion of the tip is proportional to the interaction force between the tip and surface of the workpiece w. However, in accordance with the present invention, the orientation of the cantilever beam supporting the tip is measured. The measurements can be performed in a vacuum or ultrahigh vacuum, in an aqueous environment or in air depending upon the application. The general atomic force microscope configuration for each environment is well known to those skilled in the art. Figure 2 illustrates a modification to a conventional atomic force microscope which is most useful when performing measurements in an ultrahigh vacuum environment. However, the microscope will perform in water and in non-vacuum environments equally as well with the modification. As shown in Figure 2, the laser beam from laser 18 is coupled to a single mode optical fiber 26 whose output is focussed via lens 28 to a reflector 30 disposed on the back of a cantilever beam. The laser beam is reflected from the reflector to a position sensitive detector. For reference purposes, the side of the cantilever beam including the tip is referred to as the front side of the cantilever beam and the oppositely disposed side of the cantilever beam containing the reflector is referred to as the back of the cantilever beam. A piezoelectric tube 32 is used as a scanner. The cantilever beam, which is micromachined, has a length in the range between 100 and 200 microns and preferably is 100 microns long, and has a width in the range between 5 and 30 microns and preferably is 20 microns wide. The length and width dimensions are dependent upon the material comprising the cantilever beam and are selected in order to achieve a soft lever configuration of the stylus-cantilever system having a force constant in the range between 0.01 and 100 Newton/meter and preferably having a force constant of 0.1 N/m. The cantilever beam is coupled to the tube scanner 32. Use of a micromachined cantilever beam of small dimension enables imaging at atomic resolution as contrasted with the heretofore employed cantilever beams which were typically on the order of one millimeter in length and were limited in terms of resolution. The described arrangement ensures a high scanning speed and imposes virtually no restriction on the size of a surface to be investigated. The maximum scanning speed is determined by the resonance frequency of the cantilever beam, typically 100 kHz, or by the resonance frequency of the tube scanner, typically 10 kHz. A preferred position sensitive detector is a bicell and preferably a silicon bicell for detecting the angle of deflection of the laser beam. An inertial mover, as shown in Figure 3, ensures the rejection of intensity fluctuations of the light falling on the bicell by remotely positioning the bicell symmetrically with respect to the laser beam deflected from the cantilever beam. The inertial mover includes a piezoelectric bar 36 whose length is varied by the application of a saw-tooth waveform voltage signal to the bar as is known in the art. Mounted on a sapphire plate 38 located near one end of the piezoelectric bar 36 is a bicell 40. In the position shown, the bicell can readily slide responsive to an appropriate saw-tooth waveform voltage signal applied to the piezoelectric bar 36 via conductors (not shown) as is known in the art.The sapphire plate 38 and the bicell 40 coupled thereto will move as the saw-toothed waveform voltage signal is applied to the bar. In this manner, the position of the bicell 40 can be remotely controlled in steps as small as 10 nm. The inertial mover is compact and fully computer-controllable which is particularly advantageous for use in ultrahigh vacuum environments. The use of a scanner and detector of the types described in Figures 2 and 3, namely micron-sized micromachined cantilever beams and laser beam deflection, results in an atomic force microscope apparatus that measures the orientation of the cantilever beam rather than its displacement. That is, a change of the cantilever beam position is transformed into an angular change which change is inversely proportional to the length of the cantilever beam, hence making full use of the small size dimensions. Another advantage of the present atomic force microscope design is that all required alignments and adjustments are in excess of 10 microns, a range which is easily achieved with simple standard mechanical tools. In certain applications, operation in an inaccessible environment such as a vacuum environment is neither required nor desired and the above described design can be simplified.Since the atomic force microscope components are accessible in either a liquid or gas, the optical fiber 26 can be eliminated in order to provide a more compact design. It is possible to eliminate the inertial mover when operating in a non-vacuum environment. A primary function of the inertial mover is to provide the ability to remotely position the bicell in a vacuum chamber. For example, in situ tip replacement can be incorporated in the atomic force microscope design, a feature that could result in significant misalignment of the optical path requiring repositioning of the bicell. The output of a visible diode laser is an elliptical beam having an aspect ratio in the range of approximately 5 to 7:1. In the prior art, the ellipticity is eliminated by the use of suitable optics. To the contrary, the asymmetric beam shape is an important aspect of an alternative preferred embodiment of the invention. As shown in Figure 4, by focussing a spot 42 of an elliptical shaped laser beam on a cantilever beam 44 so that the major axis of the ellipse is substantially parallel to the longitudinal axis of the cantilever beam, the elliptical beam spot reflects from the cantilever beam in accordance with the same aspect ratio and the resultant reflected spot size is approximately six times smaller in the direction perpendicular to the longitudinal axis of the cantilever beam. The result is a geometry which adds the potential for increased sensitivity of the beam deflection arrangement as well as providing for a simplified alignment procedure. Moreover, the size of the reflected laser beam 46 received at the bicell 40 is 5 to 7 times larger in the direction perpendicular to the deflection direction as shown in Figure 5, thereby enabling the use of higher laser power, without exceeding the saturation limit of the bicell, and correspondingly achieving higher measurement sensitivity. Alternatively, the distance between the cantilever beam and bicell can be decreased, thus resulting in an even more compact microscope.

Claims: An atomic force microscope for generating a topographical image of a surface of a workpiece (24) comprising: a tip (12) fixed to one end of the front side of a micromachined cantilever beam (10) adapted for being positioned in proximity to the surface of a workpiece where the forces between the atoms of said tip and the surface deflect the cantilever beam, laser means (18) for transmitting a laser beam to the back of the cantilever beam, and position-sensitive detection means (22) for receiving said laser beam after being reflected from the cantilever beam and for converting the reflected beam into an output signal, wherein the output signal of the position-sensitive detection means (22) is indicative of the position where the reflected beam is impinging on the position-sensitive detection means, and the laser means (18) and the position-sensitive detection means (22) are arranged in respect to the cantilever beam (10), that said output signal is indicative of the orientation of the cantilever beam (10), characterized in that said laser means (18) transmits a laser beam (42) having an elliptical shape to the back (44) of said cantilever beam (10). An atomic force microscope as set forth in claim 1, wherein said laser means (18) comprises an optical fiber (26) for coupling said laser beam to said cantilever beam (10). An atomic force microscope as set forth in claim 1 or 2, further comprising reflective means (16) coupled to the back of the cantilever beam for reflecting the transmitted laser beam. An atomic force microscope as set forth in claim 1, 2 or 3, wherein said tip (12) is fixed to the micromachined cantilever beam (10) by means of at least one arm (14) and said laser means (18) transmits a laser beam to said at least one arm in the region of said tip. An atomic force microscope as set forth in any one of the preceding claims, wherein said position sensitive detection means (22) comprises a bicell (40), preferably a silicon bicell. An atomic force microscope as set forth in any one of the preceding claims, where said laser means (18) comprises a diode laser or a single-mode diode laser. An atomic force microscope as set forth in any one of the preceding claims, wherein said position-sensitive detection means (22) is remotely positionable, preferably by means of an inertial mover (36). An atomic force microscope as set forth in any one of the preceding claims, wherein said tip (12) and said position-sensitive detection means (22) are disposed in an inaccessible environment like a vacuum or ultrahigh vacuum. An atomic force microscope as set forth in any one of the preceding claims 1 to 8 wherein said laser beam (46) received by said position-sensitive detection means (40) has an elliptical shape. An atomic force microscope as set forth in claim 9 wherein said laser beam (46) received by said position-sensitive detection means (40) has an aspect ratio in the range of approximately 5 to 7:1 at said detection means. Method of generating a topographical image of a surface of a workpiece (24) by using an atomic force microscope as set forth in any one of the preceding claims.

Inventors: AMER NABIL MAHMOUD, MEYER GERHARD, AMER, NABIL MAHMOUD, MEYER, GERHARD
Applicants: IBM, INTERNATIONAL BUSINESS MACHINES CORPORATION
Application year: 1991
Main classification: G01B7
IPC codes: G01B11, G01B21, G01N37, G01B7, G01Q20