A New Approach to Atomic Force Microscopy for Nanometrology Applications

Since the invention of the atomic force microscope (AFM), the use of high-resolution topographic images has become the standard in microscopy and surface science publications. The key step to making the AFM a routine instrument was the addition of the PC for control, data capture, and image analysis. In a modern instrument, the PC is taken for granted and, while the basic operation of the AFM has been fine-tuned over the past decade, its fundamental operation has remained the same.

The use of an AFM has always been very operator-intensive, with manual adjustments of a laser onto the back of a cantilever and subsequent detector alignment dictating the quality of the subsequent image. This paper describes Crystal Scanner ™ technology (Pacific Nanotechnology, Inc., Irvine, CA) in which the operating process has been reduced to a single keystroke and the laser/light lever method of detection has been eliminated.

Instrumentation

Crystal Scanner technology has been implemented in the AFM systems manufactured by Pacific Nanotechnology. To date, nearly all commercial AFM systems have used light lever technology in which the operation of the AFM was dependent on a reflected laser signal focused onto the back of the imaging cantilever. This was often a tedious process for the user, requiring patience and skill to ensure that the resulting image was artifact free and truly representative of the surface being imaged. In many cases, the AFM operator would have to develop fundamental instrument skills to make even the most basic topographic measurement. This is not the case with the Crystal Scanner approach, shown in Figure 1 in the Nano-R™ AFM system (Pacific Nanotechnology).

Figure 1 - Crystal Scanner (center) shown mounted directly onto the Nano-R AFM system.

The Crystal Scanner incorporates substantial developments in nano-Newton force measurements. The force sensor is a small crystal oscillator (Figure 2) that has a sharp probe mounted at the end of the crystal. When the probe (Figure 3) approaches a surface, the oscillations of the probe are dampened. The amount of dampening is dependent on the force between the probe and the sample. Advanced feedback software is applied to optimize the oscillation frequency of the probe and the amount of resultant force between the probe and the sample while the surface of the sample is scanned.

Figure 2 - SEM image of the quartz crystal oscillator used in the Crystal Scanner.

Figure 3 - SEM image of cantilever assembly glued to the quartz crystal oscillator.

A key benefit to this approach is that there are no mechanical adjustments needed for the crystal sensor’s operation. This is in contrast to a conventional AFM that uses a laser, which first must be focused onto a very small area on the back of the cantilever. This reflected signal must then be manually focused onto a photodetector, in which position is critical to the clarity and accuracy of the resulting image. This not only saves time, but also requires less training for a new user, making the system more routine in operation.

The quality of the images produced is also improved. The Crystal Scanner used with the flexure piezo-scanner design combined with real-time scan linearization produces a system with minimal cross-talk between scan axes, with the resultant images showing little or no background bow. This is particularly important when studying large, flat areas such as a silicon wafer or a glass panel. This was the most important design goal for the AFM products—to produce a system that is more than a basic imaging tool, while making it as easy to use as a scanning electron microscope.

System technology

In looking at current AFM users, it is clear that while there are researchers doing fundamental science with the microscope, most users just want rapid results that are quantitative, repeatable, and precise. For example, the operation of an analytical services laboratory where samples may vary day to day but may be studied using standard imaging protocols demands an instrument that is easy to use but that also delivers repeatability on a daily basis. This means that the system must be straightforward to use without the incumbent artifacts associated with the skill-intensive older designs of instrumentation. This is provided with the Point and Scan™ technology employed with the Crystal Scan software. It features on-board video tutorials that coach the operator through the setup, image capture, and analysis procedures. Again, this meets the ease-of-use design criteria. With advanced algorithm libraries built into the software platform to establish probe quality, even the scan parameters may be optimized, removing the uncertainties of the setpoints associated with light lever systems. Together, these benefits provide the reliability of quality images each time the system is used. The system even alerts the operator to change the probe if it is not suitable or out of specification.

Whether users are in an academic research environment or in an industrial analysis laboratory, today’s demand is for routine nanoscale measurements with a key driving requirement for today’s imaging tool being the ability to provide results faster with minimal time required to learn the technique. Analogous to scanning electron microscopy, Crystal Force Microscopy (CFM) provides the user with an instrument for tabletop metrology and imaging.

A nonscientific issue is also becoming more relevant when selecting an instrument: Cost of ownership is an area that is becoming very important in deciding which instrument to choose. It is not merely the price paid to put the instrument onto the bench, but also it may be necessary to hire a highly skilled operator to fully appreciate the light lever design of an AFM. However, with the simplicity yet utility of the Crystal Scanner approach, new users of AFM have access to a cost-effective high-resolution metrology microscope with applications in many areas.

In metrology, the study of optical media such as DVDs has required the ability to measure the physical dimension of features at the nanometer level. The study of masters, stampers, and replicas is now routine using AFMs. Use of Crystal Scanner technology makes this a rapid and simple task with specialized software to calculate and produce customized reports from a single keystroke.

The entire field of nanotechnology is exploding and with it is the need to be able to characterize the materials being used at the nanometer scale. Nanotubes, which are finding use in sensor applications, may be visualized using the Crystal Scanner. They may be quantified in terms of length and diameter as well. Particle & Grain Analysis software (Particle Nanotechnology) may be used to characterize nanoparticles in which uniform size distribution may play an important role in the end user application. The size of grains at the nanoscale will often be a critical component in determining in-use properties. For example, the electrical and mechanical properties of many thin films are directly affected by grain size.

Conclusion

The manufacturer made a conscious decision in the design of its scanning technology for AFMs: to produce a system that makes metrology measurement at the nanoscale a routine task. The AFM has been described by many as one of the “picks and shovels” tools for the enabling of nanotechnology. With research in academia and industry focusing a great deal of development and investigative resources in this area, Crystal Scanner technology is poised to play an important and pioneering role for many years to come.

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The authors are with Pacific Nanotechnology, Inc., Technology Center, 17981 Sky Park Circle, Ste. J, Irvine, CA 92614, U.S.A.; tel.: 949-253-8813; fax: 949-253-8816; e-mail: pwest@pacificnanotech.com.

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