Sensing mode atomic force microscope

Abstract

An atomic force microscope utilizes a pulse release system and improved method of operation to minimize contact forces between a probe tip affixed to a flexible cantilever and a specimen being measured. The pulse release system includes a magnetic particle affixed proximate the probe tip and an electromagnetic coil. When energized, the electromagnetic coil generates a magnetic field which applies a driving force on the magnetic particle sufficient to overcome adhesive forces exhibited between the probe tip and specimen. The atomic force microscope includes two independently displaceable piezo elements operable along a Z-axis. A controller drives the first Z-axis piezo element to provide a controlled approach between the probe tip and specimen up to a point of contact between the probe tip and specimen. The controller then drives the first Z-axis piezo element to withdraw the cantilever from the specimen. The controller also activates the pulse release system which drives the probe tip away from the specimen during withdrawal. Following withdrawal, the controller adjusts the height of the second Z-axis piezo element to maintain a substantially constant approach distance between successive samples.

1 . A microscope for determining the characteristics of the surface of a specimen, the microscope comprising: 
 a flexible cantilever having a first end and a second end;    a probe tip, said probe tip being affixed to said cantilever proximate said first end;    a displaceable cantilever base, said cantilever base being displaceable along a Z axis, said cantilever being affixed to said cantilever base proximate said second end;    a scanner for generating relative motion between said probe tip and the specimen, said scanner being displaceable in an X, a Y and the Z axes;    a deflection detector for measuring an angular deflection of said cantilever;    a probe tip release system for applying a momentary force to said cantilever first end after said probe tip has contacted said specimen, said momentary force being sufficient to overcome an attractive force between said probe tip and the specimen;    a controller, said controller providing control signals to the cantilever base and scanner effecting relative motion between said probe tip and said specimen, said controller determining probe tip contact in response to said deflection detector, effecting Z-axis withdrawal of one of said cantilever base and scanner and activating said probe tip release system.    
     
     
         2 . A microscope as defined by  claim 1 , wherein said probe tip release system comprises: 
 a magnetic particle affixed proximate said cantilever first end; and    an electromagnetic coil, said coil generating a magnetic field in response to a received signal from said controller, the magnetic field providing the momentary force on said cantilever.    
     
     
         3 . A microscope for determining the characteristics of the surface of a specimen, the microscope comprising: 
 a flexible cantilever having a probe tip affixed proximate a free end;    means for effecting relative X,Y and Z motion between said probe tip and the specimen;    means for detecting contact between said probe tip and the specimen; and    means for applying a force to said probe tip sufficient to overcome an adhesive force exhibited between said probe tip and the specimen.    
     
     
         4 . A microscope as defined by  claim 3 , wherein said means for applying a force to said probe tip further comprises: 
 a magnetic particle affixed proximate said cantilever free end; and    an electromagnetic coil, said coil generating a magnetic field in response to a received signal from said detecting means.    
     
     
         5 . A microscope as defined by  claim 4 , wherein said means for effecting relative motion includes: 
 a first piezo-electric element displaceable along a Z axis; and    a second piezo-electric element displaceable along the Z axis, independently of the first piezo-electric element, whereby one of said first and second piezo-electric elements can be displaced to maintain a substantially constant distance between said probe tip and the specimen    
     
     
         6 . A probe tip release system for an atomic force microscope having a flexible cantilever with a probe tip affixed to one end, the probe tip release system comprising: 
 means for detecting contact between said probe tip and a specimen;    means for inducing relative withdrawing motion between said probe tip and the specimen; and    means for applying a force to said probe tip sufficient to overcome an adhesive force exhibited between said probe tip and the specimen.    
     
     
         7 . A probe tip release system as defined by  claim 6 , wherein the force applying means comprises: 
 a magnetic particle affixed proximate said cantilever free end; and    an electromagnetic coil, said coil generating a magnetic field in response to a received signal from said detecting means    
     
     
         8 . A probe tip release system, as defined by  claim 7 , wherein the means for inducing relative withdrawing motion includes at least one piezo-electric element displaceable along an axis substantially perpendicular to a surface of the specimen.  
     
     
         9 . A method of operating an atomic force microscope to perform a surface measurement of a specimen, the atomic force microscope including a flexible cantilever with a probe tip affixed to a free end, the method comprising the steps: 
 advancing one of the cantilever and specimen;    determining contact between the probe tip and specimen;    withdrawing one of the cantilever and specimen; and    applying a momentary force to the cantilever, proximate the probe tip,    sufficient to overcome an adhesive force between the probe tip and specimen.    
     
     
         10 . The method of operating an atomic force microscope as defined by  claim 9 , wherein after said applying step the method further comprises the step of adjusting an initial separation between the probe tip and specimen to achieve a substantially constant advancing distance.  
     
     
         11 . The method of operating an atomic force microscope as defined by  claim 9 , wherein after said applying step the method further comprises the step of applying a second momentary force to the cantilever, said second momentary force driving the cantilever to restore a neutral position of the cantilever.  
     
     
         12 . A method for measuring the surface properties of a specimen using an atomic force microscope, the atomic force microscope including: a flexible cantilever with a probe tip affixed to a free end, a displaceable cantilever base supporting the flexible cantilever and providing relative motion in the Z-axis between the cantilever and the specimen, a scanner providing relative motion in the X, Y and Z axes between the probe tip and specimen, a deflection detector, and a pulse release system, the scanner being driven to effect a scan of the specimen, the scan being defined by a plurality of X-Y coordinates, for each of X-Y coordinate, the method comprising the steps: 
 driving one of the cantilever base and scanner to effect an approach between the specimen and probe tip;    analyzing signals from the deflection detector to determine the point of contact between the probe tip and the specimen;    determining the surface topology at the point of contact;    withdrawing one of the cantilever base and scanner to effect withdrawal of the cantilever base from the specimen;    activating the pulse release system, thereby generating a force on the cantilever free end sufficient to release the probe tip from the specimen;    determining the approach distance; and    adjusting one of the cantilever base and scanner to maintain a substantially constant approach distance.    
     
     
         13 . A method of operating an atomic force microscope to perform a surface measurement of a specimen, said microscope including a cantilever having a movable base portion and a free end that is deflectable relative to the base portion, and having a probe tip affixed to said free end; the method comprising the steps: 
 advancing one of the cantilever and specimen toward one another,    substantially continuously monitoring the position of said cantilever base portion and cantilever deflection,    substantially continuously controlling the position of the cantilever base portion and the cantilever deflection,    determining contact between the probe tip and specimen, and responsive to said contact affecting a withdrawal of the cantilever base portion away from the specimen, and    applying a momentary force to the cantilever sufficient to overcome an adhesive force between the probe tip and specimen.    
     
     
         14 . A method as defined in  claim 13  including the step of controlling the position of the cantilever base portion or the cantilever deflection within about 5 microseconds of said monitoring determining a change of said position or deflection.  
     
     
         15 . A method as defined in  claim 13  including the step of recording and selectively outputting complete force curves for all pixels in a scan of a specimen.