Scanning force microscope with automatic surface engagement

A scanning force microscope for examining a surface of a sample, wherein said scanning force microscope comprises:

• a probe assembly including a probe tip directed toward said surface of a sample, and a cantilever having proximal and distal ends, wherein said probe tip is fastened to said distal end of said cantilever;
• an excitation oscillator producing an excitation signal oscillating at a variable excitation frequency;
• an excitation drive applying vibrating motion, in an engagement direction toward said sample surface and opposite to said engagement direction, to said proximal end of said cantilever, and thereby through said cantilever to said probe tip, wherein said vibrating motion is generated within an excitation actuator at said variable excitation frequency in response to said excitation signal;
• a scanning drive applying relative motion between said probe tip and said sample along a plane perpendicular to said engagement direction;
• a motion detector detecting motion of said probe tip, in said engagement direction and opposite thereto, in a range of frequencies including said variable excitation frequency, with said motion detector providing a tip motion signal corresponding to said motion of said tip;
• a demodulator determining an amplitude of vibration of said tip motion signal;
• a phase angle measuring circuit for determining a phase angle between said excitation signal and said tip motion signal;
• a computing system causing said scanning force microscope to operate in a final approach mode in which said probe tip is moved into proximity with said surface of said sample and in a scanning mode in which said scanning drive is operated so that said surface of said sample is examined by said probe tip, wherein, when said scanning drive is operated in said final approach mode, said variable frequency is reduced in response to an increase in said phase angle;
• a first engagement drive moving said proximal end of said cantilever in said engagement direction, and opposite thereto, in response to variations in said amplitude signal during operation of said scanning force microscope in said scanning mode;
• a second engagement drive moving said proximal end of said cantilever during a predetermined approach function during said final approach mode;
• a dithering oscillator producing a dithering signal oscillating at a dithering frequency, wherein said dithering frequency is substantially lower than said excitation frequency;
• a dithering drive applying dithering vibrating motion, in said engagement direction and opposite thereto, to said proximal end of said cantilever in response to said dithering signal during operation of said scanning force microscope in said final approach mode, wherein said dithering vibration motion is developed within a dithering actuator separate from said excitation actuator;
• signal processing means for determining a change in said amplitude signal occurring in response to application of said vibrating motion by said dithering drive;
• and
• comparison means for terminating said approach mode when said change in said amplitude signal exceeds a pre-determined threshold level therefor, wherein operation in said scanning mode begins with said probe tip operating at an engagement distance established when said final approach mode is terminated.