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AFM CANTILEVER VIBRATION DETECTION WITH A TRANSMITTED ELECTRON BEAM

This invention is a new approach for detecting oscillations of an atomic force microscopy (AFM) cantilever with a focused electron beam. The approach alleviates the inherent minimum cantilever size of several micrometers that is detectable using a conventional laser/photodiode detection system by employing a 1 nm 30 keV electron probe. This is the first demonstration of the direct detection of AFM cantilever oscillations with a stationary high energy particle beam.

An AFM cantilever and piezoelectric actuator are attached to a scanning electron microscopy (SEM) sample holder and placed in an SEM chamber between the electron source and an annular shaped electron sensitive detector. Electrical connections to the piezoelectric actuator are made through an electrical feedthrough vacuum port on the SEM chamber wall. An oscillating drive voltage is applied to the piezoelectric actuator, which oscillates the AFM cantilever and tip at the applied frequency and amplitude. A transmitted dark field SEM image of the oscillating AFM probe is acquired with the SEM. The image intensity at each point on the AFM tip is proportional to the density and thickness.  The intensity increases linearly with distance from the AFM tip due to the increasing thickness of material interacting with the electron beam, which causes more electron scattering to the annular detector.

After acquiring the image, a focused stationary electron beam is positioned on the AFM tip within 100 nm of the end of the tip, and the transmitted electron detector signal is read directly with an oscilloscope. Because the AFM tip is oscillating, the amount of electron scattering is oscillatory with time due to the changing material thickness interacting with the stationary electron beam. To extract the oscillation frequency and amplitude of the AFM tip, the oscillatory electron scattering signal is captured by the transmitted electron detector and enters a lock in amplifier that multiplies the electron signal with the drive voltage signal. This technique has been used to determine resonant frequencies of AFM cantilevers up to drive frequencies of 440 kHz. The technique is limited by the stability of the AFM tip with respect to the electron beam and the bandwidth and sensitivity of the transmitted electron detector for detecting high frequency (> 1 MHz) oscillations.

 

Abstract: 

Cantilever oscillations for dynamic atomic force microscopy (AFM) are conventionally measured with an optical lever system. The speed of AFM cantilevers can be increased by decreasing the size of the cantilever; however, the fastest AFM cantilevers are currently nearing the smallest size that can be detected with the current optical lever approach. To address this problem, we invented an electron detection scheme in a scanning electron microscopy (SEM) for detecting AFM cantilever oscillations. The oscillating AFM tip is positioned perpendicular to the direction of a stationary 1 nm diameter electron probe. The oscillatory change in thickness of the AFM tip interacting with the stationary electron beam induces an oscillation in the transmitted electron current that is detected with a transmitted electron detector positioned below the AFM tip.

Benefits: 

An atomic force microscopy (AFM) unit can be designed to fit into any scanning electron microscopy (SEM) chamber equipped with a transmitted electron detector. Additionally, this AFM-in-SEM unit can be utilized to perform ultrafast dynamic AFM experiments with AFM cantilevers that are too small to be detected with a conventional laser/diode system.

Inventors: 
Robert Keller, Jason Killgore, Ryan Wagner, Taylor Woehl
Patent Number: 
10,060,946
Technology Type(s): 
Manufacturing, Nanometrology, Nanotechnology, Electronics, Laser and Optics, Mechanical
Internal Laboratory Ref #: 
16-008
Patent Issue Date: 
August 28, 2018
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