Patent Publication Number: US-2016231352-A1

Title: System and method of performing atomic force measurements

Description:
CROSS REFERENCE TO RELATED APPLICATION 
     This application is a continuation of U.S. patent application Ser. No. 13/761,987, filed Feb. 7, 2013, which claims the benefit of Australian Patent Application No. 2012900444, filed Feb. 7, 2012, the disclosure of which is hereby incorporated by reference in its entirety. 
    
    
     TECHNICAL FIELD 
     This invention relates generally to a system and method of performing atomic force measurements and in particular to a system and method of performing atomic force measurements using beams and/or cantilevers. 
     BACKGROUND ART 
     As is known in the art, an Atomic Force Microscope (AFM) consists of a cantilever with a pointed tip or probe at its end that is used to scan a sample surface. The cantilever is typically made of silicon or silicon nitride with a tip radius of curvature in the order of nanometers using micro-electromechanical fabrication techniques. When the tip is brought into proximity of the sample surface, forces between the tip and the sample lead to a deflection of the cantilever according to Hooke&#39;s law. 
     SUMMARY OF INVENTION 
     Interatomic forces between the probe tip and the sample surface cause the cantilever to deflect as the sample&#39;s surface topography (or other properties) change as the tip is scanned across the sample. A laser light reflected from the back of the cantilever measures the deflection of the cantilever. This information is fed back to a computer, which generates a map of topography and/or other properties of interest. 
     Various measurements can be made including measuring either the deflection of the cantilever (static mode) or a vibration frequency of the cantilever (dynamic mode). In some applications, the tip is coated with a thin film of ferromagnetic material that reacts to magnetic areas on the sample surface. Some applications include:
         Measuring 3-dimensional topography of an integrated circuit device   Roughness measurements for chemical mechanical polishing   Analysis of microscopic phase distribution in polymers   Mechanical and physical property measurements for thin films   Imaging magnetic domains on digital storage media   Imaging of submicron phases in metals   Defect imaging in IC failure analysis   Microscopic imaging of fragile biological samples   Metrology for compact disk stampers       

     A problem with current AFMs is that the sensitivity is limited by shot noise in the optical detection system. Although Brownian motion of the cantilever is a contributor to the noise, in practice it is not a factor as the shot noise is substantially greater than the noise induced by Brownian motion. While noise induced by Brownian motion may be reduced by cooling the cantilever, this is not practical for current AFMs as it may interfere with the alignment of the optical system. A further problem is that, the process of measuring an entire surface of a sample is time consuming, as the probe tip must make many passes over the sample in order to build up an image. 
     Yet a further problem with current AFMs is that the probe often needs to be replaced, and each time the probe is replaced the optical detection system needs to be re-calibrated, which is a time consuming process. 
     There is therefore a need for an improved system and method of performing atomic force measurements. 
     OBJECT OF THE INVENTION 
     It is an object of some embodiments of the present invention to provide consumers with improvements and advantages over the above described prior art, and/or overcome, and alleviate one or more of the above described disadvantages of the prior art, and/or provide a useful commercial choice. 
     SUMMARY OF THE INVENTION 
     In one form, although not necessarily the only or broadest form, the invention resides in a system for performing atomic force measurements including:
         a sensor including:
           a beam having a first side and a second side, the beam including a tip positioned on a surface of the first side for interacting with a sample; and   a grating structure positioned adjacent the second side of the beam, the grating structure including an interrogating grating coupler configured to direct light towards the beam;   a light source optically coupled to an input of the sensor for inputting light; and an analyser coupled to an output of the sensor; wherein   the beam and the interrogating grating coupler form a resonant cavity, a movement of the beam modulates the light source and the analyser determines a deflection of the beam according to the modulated light.   
               

     Preferably, the beam is a cantilever beam. Alternatively, the beam is fixed at opposite ends and includes a flexible portion between the ends. Preferably, the tip is positioned between the two ends of the beam. 
     Preferably, the modulated light is amplitude modulated. Alternatively or additionally, the modulated light is frequency modulated. 
     Preferably, the system includes a plurality of sensors. 
     Preferably, the system further includes a de-multiplexer wherein an input of the de-multiplexer is optically connected to the light source and each output of a plurality of outputs of the de-multiplexer is optically connected to a respective input of a grating structure of a respective sensor. 
     Preferably, the system further includes a multiplexer wherein each output of the plurality of grating structures of a respective sensor is optically connected to an input of the multiplexer, and the output of the multiplexer is connected to the analyser. 
     Preferably the de-multiplexer is a wavelength division de-multiplexer. 
     Preferably, light input into the multiplexer is separated into a plurality of discrete wavelengths and/or wavelength bands. 
     Preferably, each wavelength of the plurality of discrete wavelengths is modulated by a respective sensor. 
     In another form the invention resides in a method of performing atomic force measurements on a sample, the method including the steps of:
         inputting light into a resonant cavity formed between a beam and a grating structure of a sensor;   receiving at an analyser light modulated by a movement of the beam; and   analysing the modulated light to determine a characteristic of the sample.   Preferably, the characteristic is a topography of the sample.       

    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       To assist in understanding the invention and to enable a person skilled in the art to put the invention into practical effect, an embodiment of the invention is described with reference to the accompanying drawings in which: 
         FIG. 1  is a bottom perspective view of a system for performing atomic force measurements; 
         FIG. 2  is a cross-sectional end view of a sensor of  FIG. 1 ; 
         FIG. 3  is a block diagram illustrating a system of an array of the sensors shown in  FIGS. 1 and 2  for performing atomic force measurements; and 
         FIG. 4  illustrates a method of performing atomic force measurements on an object, according to an embodiment of the present invention. 
     
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
     Elements of the invention are illustrated in concise outline form in the drawings, showing only those specific details that are necessary to understanding the embodiments of the present invention, but so as not to clutter the disclosure with excessive detail that will be obvious to those of ordinary skill in the art in light of the present description. 
     In this patent specification, adjectives such as first and second, left and right, front and back, top and bottom, etc., are used solely to define one element from another element without necessarily requiring a specific relative position or sequence that is described by the adjectives. Words such as “comprises” or “includes” are not used to define an exclusive set of elements or method steps. Rather, such words merely define a minimum set of elements or method steps included in a particular embodiment of the present invention. It will be appreciated that the invention may be implemented in a variety of ways, and that this description is given by way of example only. 
       FIG. 1  is a bottom perspective view of a system  100  for measuring Atomic Force according to a first embodiment of the present invention, and  FIG. 2  is a cross-sectional end view through A-A of  FIG. 1 . Referring to  FIGS. 1 and 2 , the system  100  includes a sensor  200 , a light source  300  and an analyser  400 . The light source  300  is connected to an input of the sensor  200  by an optical waveguide  700 A such as an optical fibre. An output of the sensor  200  is connected to the analyser  400  by an optical waveguide  700 B such as an optical fibre. 
     In one embodiment, the sensor  200  is made using micro-electro-mechanical systems (MEMS) technology and includes a beam in the form of a cantilever beam  210  and a grating structure  220  positioned adjacent the cantilever beam  210 . The cantilever beam  210  is planar and includes a tip  211  which is pointed. The cantilever beam  210  includes a first side  214  and a second side  215 . The tip  211  is positioned on the first side  214  of the cantilever beam  210  and towards a distal end  213  of the cantilever beam  210 . The tip  211  extends away from the cantilever beam  210  towards a sample  800  to be measured. A proximal end  212  of the cantilever beam  210  is fixed allowing the distal end  213  to flex as the tip  211  is moved over the sample  800 . 
     In another embodiment (not shown), the beam is fixed at both the distal end and the proximal end but allowing the beam to flex. A tip is positioned in between the distal end and the proximal end. Preferably the tip is positioned mid-way between the distal end and the proximal end. However it should be appreciated that the tip may be positioned anywhere between the distal end and the proximal end. By fixing the beam at both ends, Brownian motion is reduced, and sensitivity of a measurement is increased. 
     In one embodiment, the grating structure  220  uses Silicon on Insulator (SOI) technology and includes a substrate  221 , a buried oxide layer  222  and a waveguide layer  223 . Furthermore, the substrate  221  and the waveguide layer  223  are made from silicon. The buried oxide layer  222  is formed on the substrate  221  and the waveguide layer  223  is formed on the buried oxide layer  222 . The waveguide layer  223  includes grooves to form an interrogating grating coupler  224  and the interrogating grating coupler  224  is positioned adjacent the second side  215  of the cantilever beam  210 . In one embodiment, the waveguide layer  223  is 220 nm thick fabricated over a 2000 nm buried oxide layer  222  using an infra-red light source  300 . However it should be appreciated that other thicknesses may be used, for example between 100 nm and 2000 nm. 
     Although the grating structure  220  has been described in relation to SOI technology, a person skilled in the art will appreciate that the grating structure  220 , including the waveguide layer  223  and the buried oxide layer  222 , may be made from many other materials. The main requirement is that the waveguide layer  223  has a higher refractive index than the buried oxide layer  222  so as to get the total internal reflections in the waveguide. For example, the waveguide layer  223  may also be made from, but is not limited to, Germanium (Ge) and Silicon Oxy Nitride and the buried oxide layer  222  may be made from SU-8, Silicon dioxide (SiO2) or Magnesium Oxide (MgO). In addition, the thicknesses used to fabricate the waveguide layer  223  and the buried oxide layer  222  depend on the materials used and the wavelength of the light source  300 . 
     Typically a gap between the interrogating grating coupler  224  and the cantilever beam  210  is between 0.05 and 10 μm. However it should be appreciated that other distances may be used depending on the wavelength of the light source  300  and the types of materials used for the grating structure  220 . 
     Although in the example above the sensor  200  has been designed using a light source  300  in the infra red band (with a wavelength of between 0.74 μm to 30 μm), it should be appreciated that the light source  300  may produce light in the visible band (with a wavelength between 390 nm to 750 nm) or the ultra-violet band (with a wavelength between 10 nm to 400 nm). 
     A pattern or shape of the interrogating grating coupler  224 , for example dimensions of grooves of the interrogating grating coupler  224 , determines a resonance of light resonating between the interrogating grating coupler  224  and the cantilever beam  210 . 
     In use, the unmodulated light  500  is input to the sensor  200 . The unmodulated light  500  propagates along the silicon waveguide layer  223  until the unmodulated light  500  exits the waveguide layer  223  towards the cantilever beam  210  at the interrogating grating coupler  224 . The interrogating grating coupler  224  couples and directs light out of the waveguide  223  towards the cantilever beam  210  and couples light reflected from the second side  215  of the cantilever beam  210  back into the waveguide thereby forming a resonant cavity with the cantilever beam  210 . As the cantilever beam  210  moves towards and away from the interrogating grating coupler  224 , an intensity and/or frequency of light output to the analyser  400  is modulated as a function of the separation between the interrogating grating coupler  224  and the cantilever beam  210 . From the modulation, the analyser  400  may determine a displacement of the cantilever beam  210  in order, for example, to determine a topography. In some embodiments the second side  215  of the cantilever beam  210 , is coated with a reflective material such as gold in order to increase the reflectivity. 
     Although referred to as unmodulated light, it should be appreciated that the light input to the sensor  200  may be modulated with a first modulation. As the first modulated light passes through the sensor  200  it is modulated by a second modulation. The second modulation may then be analysed by the analyser  400  in order to determine a displacement of the cantilever beam  210 . 
     It should be appreciated that in order to perform a scan, the sample  800  may be fixed and the sensor  200  moved across the sample  300  under the control of the analyser  400 . Alternatively, the sample  800  may be moved under the control of the analyser and the sensor  200  may be stationary. 
     An electrostatic element may be used to control an initial deflection of the beam so as to tune the resonance of the optical cavity to its most sensitive position. An electrode is placed underneath the beam or cantilever beam  210 , but not over the grating structure  220 . The voltage between the electrode and the metal on the underside of the beam is then controlled to attract or repel the beam as necessary. 
     The sensor  200  of the present invention may be used in an array in order to measure larger sections of the sample  800 .  FIG. 3  is a block diagram illustrating a system of an array of sensors  200  shown in  FIGS. 1 and 2  for performing atomic force measurements according to an embodiment of the present invention. The system  900  includes a plurality of sensors  200 A,  200 B,  200 C formed in a row. However it should be appreciated that the sensors  200 A,  200 B,  200 C may be positioned in any suitable arrangement. 
     In this embodiment, the light source  300  is connected to a wavelength division de-multiplexer  910  via a single optical waveguide  700 A. The wavelength division de-multiplexer  910  separates the light source  300  into a plurality of discrete wavelengths or wavelength bands λ 1 , λ 2  and λ 3 . Each output of the wavelength division de-multiplexer  910  is connected to a respective sensor  200 A,  200 B,  200 C by a respective optical waveguide  700 C,  700 D,  700 E in order to couple the light at each wavelength λ 1 , λ 2 , λ 3  to a respective grating structure  220  of a respective cantilever sensor  200 A,  200 B,  200 C. As each cantilever beam  210  of a respective sensor  220 A,  220 B,  220 C moves it modulates the light at the respective wavelength or wavelength band λ 1 , λ 2 , λ 3 . 
     The modulated light  600  at each wavelength λ 1 , λ 2 , λ 3  is then multiplexed by a multiplexer  920 . Each sensor  200 A,  200 B,  200 C is connected to the multiplexer  920  by a respective optical waveguide  700 F,  700 G,  700 H such as an optical fibre. An output of the multiplexer  920  is connected to the analyser  400  by optical waveguide  700 B and the modulated light at each wavelength λ 1 , λ 2 , λ 3  is passed to the analyser  400 . The analyser  400  analyses the modulated light  600  at each discrete wavelength or wavelength band λ 1 , λ 2 , λ 3  to determine a movement of each sensor  200 A,  200 B,  200 C and accordingly determine a characteristic of the sample  800 . 
     In another embodiment, the light from the light source  300  may not be de-multiplexed into separate wavelengths; rather each sensor  200  in the array may be supplied from its own light source  300  or with a same wavelength of light from a same light source  300 . Furthermore, an output from each sensor  200  may connect to a separate analyser  400 , and each output analysed using a computer for example. 
     According to certain embodiments, the system  100  includes a movement sensor (not shown), to determine the relative motion between the sensor  200  and the sample  800 . This enables the determination of a contour of a sample irrespective of the rate of movement of the sample. 
       FIG. 4  illustrates a method  1000  of performing atomic force measurements on an object, according to an embodiment of the present invention. 
     At step  1005 , light is input into a resonant cavity formed between a beam and a grating structure of the sensor. A tip of the beam is positioned adjacent to and in contact with the object, such that the beam moves according to a contour of the sample. 
     At step  1010 , the light from the resonant cavity is received at an analyser, the light modulated according to a position of the beam. 
     At step  1015 , the modulated light is analysed to determine a contour of the sample. 
     Steps  1005 - 1015  are advantageously performed on multiple points of the object, either sequentially, for example through movement of the beam across the object, in parallel, for example through the use of several beams and resonance cavities, or through a combination of series and parallel. 
     It should be appreciated that the present invention may be used in a variety of modes such as a static mode (where the beam flexes) and a dynamic mode (where the cantilever beam oscillates) in order to perform a variety of measurements. 
     For example the invention may be used in a contact mode where the sensor is scanned at a constant force between the sensor and a sample surface to obtain a 3D topographical map. 
     In an Intermittent Contact (Tapping Mode) the cantilever beam is oscillated at or near its resonant frequency. The oscillating tip is then scanned at a height where it barely touches or “taps” the sample surface. The analyser monitors the sensor position and a vibrational amplitude to obtain topographical and other property information allowing topographical information can be obtained even for fragile surfaces. 
     An advantage of the present invention is that the optical readout of the grating structure  220  leads to increased sensitivity over existing free space optical monitoring. The present invention uses an optical resonant cavity formed between the grating structure  220  and the cantilever beam  210 , or doubly clamped beam, coupled to a waveguide to increase an amplitude of a signal output from the sensor  200  to levels significantly above the shot noise and thereby increasing the signal to noise ratio. 
     Another advantage is that the necessity to align the optics of an AFM whenever the probe is replaced is effectively eliminated due to the close coupling of the optical cavity to the waveguide. This is because the sensor  200  and the AFM may be fabricated such that when installed, the waveguide layer  223  aligns with the light source  300  in the AFM. 
     In addition Brownian motion noise may be reduced by clamping the beam at each end and a further reduction in Brownian noise may be made by cooling the sensor  200 . 
     Finally, miniaturization of the AFM may be achieved allowing multiple beams and AFM tips to form an array and to be integrated in the one structure, effectively increasing the scan rate. 
     The above description of various embodiments of the present invention is provided for purposes of description to one of ordinary skill in the related art. It is not intended to be exhaustive or to limit the invention to a single disclosed embodiment. As mentioned above, numerous alternatives and variations to the present invention will be apparent to those skilled in the art of the above teaching. Accordingly, while some alternative embodiments have been discussed specifically, other embodiments will be apparent or relatively easily developed by those of ordinary skill in the art. Accordingly, this patent specification is intended to embrace all alternatives, modifications and variations of the present invention that have been discussed herein, and other embodiments that fall within the spirit and scope of the above described invention.