PeakForce Tapping & QNM

Wednesday, 1 June 2011 Comments (0)  

PeakForce Tapping™ is an AC imaging technique, i.e., the cantilever is oscillated but well below resonance. This results in a continuous series of force-distance curves. In addition to direct force control by keeping the peak force constant, a multitude of material properties can be extracted and quantified from the force-distance curve at each pixel within an image, such as modulus, adhesion force, and deformation depth.

LiftMode

Wednesday, 1 June 2011 Comments (0)  

LiftMode™is not an imaging mode, as it by itself does not measure a new quantity to reveal new information, but rather a technique that enables other modes such as MFM, EFM, and SCM. In LiftMode the sample is scanned first in a regular topographic mode: Contact, TappingMode™ or PeakForce Tapping™.The following line, the “lift line” traces the previously acquired topography back and adds a Z-offset. As the feedback laser is not necessarily needed during the lift line, experiments that would be influenced by laser light can be enabled (DarkLift).

Magnetic Force Microscopy (MFM) uses a combination of TappingMode™, LiftMode™ and a properly prepared tip to gather information about the magnetic field above a sample. Each line of the sample is first scanned in TappingMode operation to obtain the sample topography. The topographic information is stored and retraced with a user selectable height offset in LiftMode, during which the magnetic data are collected. Typical lift heights in MFM range from 20-100 nm.

Surface Potential Microscopy (SPoM) is based on the macroscopic Kelvin method. SPoM is able to measure surface topography and surface potential (VDC) information simultaneously. Topography is acquired using TappingMode™ operation but with a conductive tip. The electrical information is extracted by applying AC and DC voltages to the tip. The total voltage acting on the tip is: V = VDC + VAC sin (Ω t). A dedicated feedback loop adjusts the DC voltage to the tip to zero the contact potential difference between the tip and surface at each pixel, making DC a measure of the surface potential. SPoM can be carried out using LiftMode™ or in a dual-frequency one-pass fashion.

STM is a primary AFM mode. The probe is a metal needle. Detector signal is the tunneling current between the tip and sample when an electrical bias, V, is applied. In feedback mode, output signal usually adjusts the Z position of the scanner to maintain a tunneling current setpoint. STM is the highest resolution AFM.

Torsional Resonance

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During Torsional Resonance (TR) Mode, the tip is actuated parallel rather then vertical with respect to the surface. Forces between tip and sample cause a change in resonance behavior that can be used to track the surface at a constant distance. TR has the advantage that the tip remains at a constant distance to the surface at all times. This can be advantageous in modes like TR-TUNA or nearfield-optical experiments.

TUNA & Conductive AFM

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TUNA and Conductive AFM use contact mode AFM and a conductive probe. Sensor signal is the electric current between tip and sample for an applied DC bias. In feedback mode, the output signal is the DC bias, adjusted to maintain the electric current setpoint. Current range is from fA (TUNA) to µA (CAFM). Applications include studies of dielectric strength and thickness uniformity in thin films, e.g., for semiconductor and data storage devices, and conductive polymers.

The probe in SECPM is a sharp metal needle. Detector signal is electric potential difference between tip and sample (or between tip and a reference electrode) in an ionic or polar liquid, where an electric double-layer exists at the liquid/sample interface. SECPM maps the electric potential profile across the depth of the double-layer (versus Z, the tip-sample distance) at tip XY location. In feedback mode, the output signal usually adjusts the Z position of the scanner.

SSRM uses contact mode AFM and a conductive probe. Sensor Signal is the electric current between the tip and sample for an applied DC bias, VDC. SSRM measures the current by referencing it to an internal resistor, using a logarithmic amplifier, to yield local resistance value. SSRM maps the variation in majority carrier concentration in doped
semiconductors.

SCM uses contact mode AFM and a conductive probe and applies to semiconductor samples with an AC bias (amplitude DV, ~90 kHz) with a DC offset. The capacitance of the metal-oxide-semiconductor (MOS) capacitor at tip-sample contact is a function of majority carrier concentration in the sample. SCM uses an ultra-high-frequency (1 GHz) detector to measure tip-sample capacitance variation, DC, at the bias frequency. Sensor Signal is DC/DV. In feedback mode, output signal is DV, adjusted to maintain a DC/DV Setpoint. SCM maps relative changes of majority carrier concentration in semiconductors.