ATOMIC FORCE MICRSCOPY - AFM
fundamentals

:.........................................................................................

The Atomic Force Microscope (AFM) was an offshoot of the Scanning Tunneling Microscope (STM) designed to measure the topography of a nonconductive sample. The AFM has undergone several enhancements over the years, allowing studies far beyond the limitations of conventional optics. Nowadays, AFM has become the method of choice in a wide field ranging from biological applications to material characterization. AFM is an extremely accurate and versatile instrument enabling investigations of surface topography, tip-sample interaction forces, and magnetic surface phenomena (MFM).

The simplest operational mode of every AFM is referred to as ‚contact mode‘. A very fine tip mounted to the end of a small deflecting spring – known as cantilever – is brought into contact with the sample surface. The tip is then moved across the surface in numerous line scans. Any vertical deflection of the tip due to short-range repulsive interaction forces with the sample can be measured and recorded with extremely high accuracy. Over the years, more sophisticated AFM modes have evolved such as non-contact mode (nc-AFM) and frequency modulated non-contact mode (FM-AFM). In contrast to contact mode, the nc-AFM mode is sensitive to long range attractive forces such as caused by van-der-Waals, electrostatic, and magnetic interaction. The latter resulted in the development of the magnetic force microscope (MFM), an instrument which is nowadays widely used in applications such as vortex imaging and magnetic thin film analysis.
attocube systems‘ attoAFMs are designed particularly to be used at extreme environmental conditions such as ultra low temperature, high magnetic field, and high vacuum. Reliable functionality in these extreme environments is provided by implementing the outstanding attocube systems nanopositioning modules.To perform low temperature microscopy, the
attoAFMs are cooled by a controlled exchange gas atmosphere in a vacuum shielded liquid Helium bath cryostat. Furthermore, the
attoAFMs are now also available in combination with a He3 insert allowing measurements below 350 mK. For applications where liquid Helium is not available or desired, the attoAFMs can be combined with cryogen-free pulse-tube based coolers.

Interferometric Sensor

The deflection detection scheme for the
attoAFM I microscope systems is based on an all fiber low coherence interferometer. The schematic drawing to the right describes the setup. A laser diode beam coupled into a single mode fiber (port 1) is used to illuminate an interferometer based on a fiber coupler. At the end of the second interferometer arm (port 2) the light is transmitted and partially reflected at the AFM cantilever. Therefore, the tip interface and the fiber end face form a Fabry-Perot interferometer. A large part of the light reflected in this structure is coupled back into the optical fiber and detected with detector 1. Detector 2 mounted on arm 3 can be used to monitor the intensity emitted by the laser (optional). Monitoring the intensity of the interference fringes allows to measure the tip movement.
As this deflection detection mode is compatible with commercially available cantilvers, it is perfectly compatible with standard imaging modes such as Magnetic Force Microscopy (MFM), Electric Force Microscope (EFM), etc.

Tuning Fork Sensor

The attoAFM III uses a tuning fork sensor as detection mechanism for the tip-sample distance, allowing high resolution non-contact mode imaging without the need for any optical detection techniques. In general, an AFM tip is glued onto one leg of a small quartz tuning fork and forced to oscillate in horizontal direction with an amplitue of typically 50 pm. Damping of the amplitude by tip-sample interaction forces is monitored and/or used as a feedback signal. The force resolution of this technique is typically 0.1 pN. In addition to glued tips, this system is fully compatible with the commercially available Akiyama probe.

As this deflection detection mechanism is non-optical, it is perfectly suited for e.g. Scanning Gate Microscopy (SGM) on 2-dimensional electron gases.

attocube systems AFMs

Two different AFM setups optimized to meet various customer requirements are the result of a decade of experience in cryogenic scanning probe microscopy. All attocube microscope systems are compatible with cryogenic and vacuum environments as well as high magnetic fields.

attoAFM I:
This cantilever-based, compact system ensures outstanding stability enabling ultra high resolution imaging. The adjustment of the cantilever is performed outside of the cryostat prior to cooling down the microscope. Compatible with commercially available cantilevers, this system is ideally suited for imaging modes as MFM, EFM, etc.

attoAFM III:
Based on a non-optical tuning fork detection technique, this system is ideally suited for applications where input of light is problematic. A typical application is Scanning Gate Microscopy (SGM) on semiconductor structures.