Background: Asylum MFP-3D AFM Overview


Using the Union MFP3D Atomic Force Microscope, this project aims to explore potential applications of applying an impulse to the AFM tip. An AFM (atomic force microscope) can achieve resolution on an atomic scale (down to fractions of a nanometer), and achieves this by using a probe to scan the material surface. See Figure 1 below (Eaton, West, 2010) for a comparison of different scale lengths achievable with various microscopes.

Figure 1: AFM Resolution compared to various microscopes



A problem often faced by users of AFM technology is the recurring breaking of probes. Resolution of data depends on spatial locality of the probe to the material. Thus, the closer the probe can get to the material surface, the more resolution is available. Since material surfaces are not uniform, this close interrogation of the material often leads to the atom-sharp probe hitting the surface and breaking.

It is the goal of this project to study the impulse response of the tip as a function of height with respect to the surface. As no current modules exist within the Asylum codebase that allow for customizable probe tip input, the first problem to be addressed is the implementation of a new module built on top of current AFM code that will allow the user to write new commands to test the tip’s response at different heights to various inputs. As such, this problem begins with determine the capabilities and limitations of the Asylum AFM-3D open source code environment. Incorporated with this will be the learning of basic AFM operation as well as working with user forums to gain insight for how to work with Asylum Research’s open sourced code.

The basic operation of the AFM can be boiled down to the three main modes of instrumentation: the microscope stage, the control electronics, and the computer. The stage contains the scanner, sample holder, and force sensor. Some AFM machines also contain an optical microscope to view the sample and tip. See Figure 2 below for a labeled picture of the AFM stage (Eaton, West, 2010).


Figure 2: AFM Stage

After the stage, the second main part of the instrumentation are the control electronics. The role of these electronics is to interface the microscope stage and computer. It generates and digitizes the forces read from the force sensor and to drive the sensor. There is a feedback system that works according to the force reading which corresponds to the tip’s relative position to the material. Figure 3 below (Eaton, West, 2010) shows the basic operation of the force sensor, probe, and signal that is received by the control electronics. Figure 4 below (Eaton, West, 2010) shows a visual of how the feedback control system reacts to changing material surface structure. The feedback loop senses a higher or lower force as the tip moves horizontally across changing surface height, and reacts by moving it in the z-direction back to a force equilibria.

Figure 3: Force Transducer measurement visual


Figure 4: Feedback reaction of changing surface height

As the machine records the reaction of a laser probe to changes in the feedback system (normally done with a laser and sensitive photodiode), the computer system records an image over time as the tip moves across the sample in a raster pattern. An important aspect of this study will be in testing the impulse response of the tip in both repulsive and attractive regimes. The force measurement recorded by the force transducer is much higher when the tip is in direct, or hard contact with the surface of the material- this is called the repulsive regime. As the tip moves gradually away in the z-direction, the measurement of force follows the curve below in Figure 5 (Rabe, Turner 1997) until it enters the attractive regime. Figure 6 below (Eaton, West, 2010) shows a block diagram of AFM operation, with the stage components and electronics as a single system. The x, y, and z directions of the tip are controlled by piezoelectronics, allowing for very precise movements in any direction.


Figure 5: Attractive and repulsive regime force curve


Figure 6: Block diagram of AFM operation

The third most important system of instrumentation in the AFM is the computer. This controls system software that interfaces with the equipment. This also interfaces the machine with the user. Here, in the intricate and nanoscale-precise instrument, is where my project takes place. It is suspected that if an impulse were to be applied to the tip of the machine, the impulse response would be nonlinear. According to Rabe, Turner 1997:

“Acoustical vibrations of atomic force microscope cantilevers can be excited either by insonification of the sample or by vibration of the clamped cantilever end. The resulting dynamical system is complex and highly nonlinear.”

To investigate this suspected complex and potentially interesting issue, I will design a module which permits an AFM user to monitor the non-linear impulse response of an AFM tip as a function of height above the sample through user-defined control of the AFM piezos. With this module in place, it will be possible to study the characteristics of the impulse response as a function of height above the sample, comparing the response in the attractive and repulsive regimes. This problem may lead to a new or alternative system of probe/sample position interrogation. In the words of Eaton, West, 2010:

“One of the major challenges in AFM design is making a motion control system that permits the approach of the probe to the surface before scanning. This must be done such that the probe does not crash into the surface and break. An analogous engineering challenge would be to fly from the earth to the moon in 60 seconds and stop 38 meters from the surface without overshooting or crashing.

It is the hope of this study to gain new insights into the approach and reaction of the tip of the AFM to different signals. With the ability to develop new software that can interface with the machine, new doors may be unlocked in studying components of the AFM. At a nanoscale level, this sounds daunting. However, this problem of flying from the earth to the moon in 60 seconds and stopping 38 meters from the surface sounds like a worthy problem to address.