A Practical Guide to AFM Probes and Sensors
Paul West, Pacific Nanotechnology, Tustin, California

Most atomic force microscopes (AFMs) use a probe mounted at the end of a cantilever for measuring images. Probe/cantilever combinations are available in various sizes and shapes, with and without coatings, and choosing the right one for a particular application is straightforward. Using the wrong sensor can result in distorted or misleading images.

Sensor selection involves choosing a scanning mode (contact or vibration) and defining the application (sample type, image resolution required and the material characteristics of interest). Probe tips are matched to the topography of the sample and the degree of resolution required and range from rounded, to sharp, to ultra-sharp and high-aspect ratio tips, including carbon nanotubes.

An AFM can image many characteristics of a surface, including physical topography, magnetization, electrostatic force and tribological properties such as friction, lubrication and wear. AFMs can also sense and image material compositions in some cases and quantitatively measure surface roughness and the dimensions of surface features.


AFM Fundamentals

An atomic force microscope scans a probe mounted on a cantilever (Figure 1) over a surface; the forces between the probe tip and the sample surface are monitored, and an image of the surface is generated. The most common type of force sensor utilizes the relationship between the resulting motion of the cantilever and the applied force. The motion is measured with the "light lever" method in which a photo-detector measures the light reflected from the back side of the cantilever, and the output of the photo-detector is proportional to the motion of the cantilever. Motions as small as 1 nm are routinely measured with the light lever method.

 

Figure 1: An AFM sensor includes a cantilever with a mounted probe.

This combination of probe and cantilever can be monolithic - manufactured by MEMS techniques from single-crystal silicon - or the probe and cantilever can be of different materials. An example of the latter is a carbon nanotube (CNT) tip mounted on a silicon or silicon nitride cantilever. This probe tip is used when extremely high resolution is required.

Generally, the sharper the probe tip, the better the image will be (Figure 2). However, images are actually a combination of probe tip shape and sample topography and the best probe tip shape is not always a sharp point (Figure 3).When imaging rectangular types of features such as those found in many semiconductor chip applications, a more rectangular shape is required. In this case the quality of tip shape is just as important, but the focus is on edge shape rather than point sharpness. An inappropriate probe tip shape or a damaged tip will not follow the surface contour and will produce a distorted image.

Figure 2: Generally, a sharper probe tip will produce a higher resolution image.

 

Figure 3: The probe tip shape should be appropriate to the surface topography and a sharp point is not always best. Left: steep sidewalls as on a semiconductor chip require a rectangular probe tip. Right: A relatively smooth surface with shallow topography requires the more common, sharp probe tip.

The strongest forces between a particular probe and the surface are mechanical; however, other forces can provide information about surface properties other than topography. Obtaining these kinds of information is a combination of AFM operation protocols and sensor characteristics. Images can show hardness, electrical and magnetic properties, tribology, and other material properties. It is also possible to "sense" the materials at a sample's surface. If a surface is perfectly flat, but has an interface between two different materials, the image can show the change in material composition.

A more extensive tutorial on atomic force microscopy can be found at www.pacificnanotech.com.

Physical Characteristics

Cantilevers are approximately 125 µm to 450 µm long - slightly larger than the width of a human hair. The probe itself is 1000 times smaller. Typical probe tips have radii of less than 10 nm (1 nm = 1 billionth of a meter; a carbon atom is about 0.25 nm in diameter.). In addition to the probe tip diameter, the aspect ratio, which is the ratio of probe tip length to its diameter, is important.

Over the years, industry-standard products have emerged for sensors commonly used in atomic force microscopes. The silicon monolithic design is most common (Figure 4). The cantilever and probe are supported by a 1.6 mm x 3.4 mm single-crystal silicon holder (called the substrate or chip). The chip permits easy replacement of the tip without a major readjustment of the detection system.

Figure 4: The monolithic sensor structure is the most commonly used sensor in AFMs.

The silicon is highly doped for electrical conductivity and has a typical resistivity of 0.01 to 0.025 ohm-cm. The structure has no intrinsic stress and, as a result, the cantilever is absolutely straight. Silicon is chemically inert for most applications. The cantilever itself is rectangular and usually has a tapered free end for visibility during positioning. The cantilever has a trapezoidal cross section with a wide side to reflect the laser beam that detects its motion. Cantilever width is usually given as the average (mean) of the two sides of the trapezoid. Sensors with a reflective (or reflex) coating have a layer of aluminum, typically about 30 nm thick, on the backside of the cantilever to enhance their reflectivity from 2 to 2.5 times. However, corrosive environments can attack the aluminum coating and uncoated sensors are available for these applications.

A typical probe tip height is 10-15 µm with a radius of curvature of less than 10 nm at the probe tip end. The macroscopic half-cone angles are 20-25° viewed along the cantilever axis and 25-30° seen from the side, and virtually zero when viewed from the probe tip end (Fig. 5-1). Since the cantilever is flexed in use, the probe is mounted at a slight angle (Fig. 5-3). Probe tips can be sharper, with a radius less than 5 nm and a half cone angle less than 10° at the final 100 nms. A sharper probe tip gives enhanced resolution of micro-roughness and nanostructures.

High-aspect-ratio probe tips are available for measurements on samples with sidewall angles approaching 90° such as deep, narrow trenches on semiconductor structures (Fig. 5-2). They have an overall height of 10-15 µm to allow measurements in deep crevices and corrugations. The last few µm have a symmetrical high aspect ratio with a radius of about 10-15 nm. The high aspect ratio portion of the probe tip is 1.5 µm to 2.0 µm long. The length/diameter aspect ratios can be as great as 12:1 resulting in half cone angles as small as 2.8°.

To compensate for the tilt angle of the cantilever when mounted on the AFM head, the high-aspect-ratio probe can be "tilt compensated," that is, mounted so that the high aspect portion of the probe tip is tilted about 13 ° with respect to their center axis (Figure 6). This allows symmetrical imaging and accurate slope measurement of near-vertical sidewalls.

Figure 5: These are scanning electron microscope (SEM) photos of typical silicon probes and probe tips. (1). Standard pyramidal probe tip. (2). High-aspect-ratio probe tip. (3). Probe at end of cantilever. (4). View of a standard probe from the end of the cantilever.

 

Figure 6: Tilt-compensated high-aspect-ratio probe tips deal with the tilt angle of the cantilever when mounted on the AFM head.

Generally, smaller (sharper) probe tips and probe tips with higher aspect ratios are more expensive.

Image Resolution

In-plane image resolution depends on the relation of the probe tip geometry - the sharpness, shape, and aspect ratio - to the topography of the surface. The bottom of the probe tip must be able to follow the surface contours for an accurate image to be produced. Figure 7 shows a perovskite surface scanned with a 5 nm diameter probe tip and a different, but similar, region on the same sample scanned with a 40nm diameter probe tip.

Figure 7: AFM images of perovskite surface. A probe tip with a 40nm diameter was used to measure the left image and a probe tip with 5 nm diameter was used to measure the probe at the right side.

Scanning Modes

The sensor must be matched to the scanning mode and there are two principal scanning methods: contact (or deflection) mode and non-contact (or vibration) mode (Figure 8). In contact mode, the probe is scanned over the surface with a fixed cantilever deflection. The force on the surface is directly proportional to the deflection and is extremely small, often less than a nano-newton. The probe tip is minimally touching the surface. In the vibration mode (also called close-contact, or intermittent contact) the cantilever oscillates at its resonant frequency during scanning. The resonance changes as the probe tip approaches the surface and the vibration amplitude and phase are measured. Although the non-contact mode is often called "tapping", it is important that the probe tip not "tap" the sample or it may be broken or the sample damaged.

Figure 8: The two common scanning modes are contact mode (left) and vibration or non-contact mode (right).

Probe tips used in contact mode are more subject to wear-out, and wear-resistant diamond coatings are available. All sensors are fragile, but in general their fragility increases with the sharpness and aspect ratio of the probe tip.

Vibration mode is used with very soft samples. Also, in combination with very sharp probe tips, vibration mode is preferred to obtain high image resolutions (<10nm).
In vibration mode, the magnitude of amplitude damping and the amount of phase change of the cantilever depends on the surface chemical composition and the physical properties of the surface. On an inhomogeneous sample, contrast can be observed between regions of varying chemical composition. This technique has many names including phase mode, phase detection and force modulated microscopy.

Coatings

Probes and cantilevers are available with several coatings for various applications. The most commonly used coating is the reflex coating for improved image quality, but it can't be used in a corrosive environment. Other coatings are required to image some sample characteristics other than topography and/or deal with sample hardness. The common coatings encountered in choosing the best AFM sensor are described below:

Reflex Coatings for Image Quality - A 30-nm thick, stress-free aluminum reflex (reflective) coating on the detector side of the cantilever can improve the reflectivity of the laser beam by up to 2-½ times and increase the signal/noise of the AFM. The coating prevents light interference, which degrades the reflected signal. However, reflex coatings cannot be used in corrosive environments that react with the aluminum. An uncoated cantilever is nearly inert.

PrIr5 Coating for Enhanced Electrical Contact and Conductivity - The PrIr5 coating is an approximately 23 nm thick double layer of chromium and platinum iridium5 on both sides of the cantilever. The probe-side coating enhances the conductivity of the probe and allows electrical contact. The detector-side coating also enhances the reflectivity of the laser beam by a factor of 2 and is corrosion resistant. The PtIr5 layer is stress compensated and wear resistant; cantilever bending due to stress is typically less than 2°. Electrostatic Force Microscopy (EFM) applications would always use this coating.

Diamond Coating for Wear Resistance - For applications that require hard contact between the probe tip and sample, a 100 nm thick, diamond probe-side coating provides extremely high wear resistance. Applications include friction measurements, the measurement of elastic properties of samples that are not too soft, and wear measurements.

Diamond Coating for Conductivity - Special conductive diamond-coated probes are available when there will be hard contact between the probe tip and sample and electrical conductance is also required. Examples of typical applications include Scanning Spreading Resistance Microscopy (SSRM), Tunneling AFM, and Scanning Capacitance Microscopy (SCM).

Hard Magnetic Coating for Magnetic Force Microscopy (MFM) - Investigating the magnetic properties of surfaces on the nanometer scale requires a hard magnetic coating on the probe. Typically, about 40 nm of cobalt alloy provides a permanent magnetic field at the probe tip. An example is the measurement of magnetic domains on magnetic recording media. The probe must be magnetized by means of an external strong magnet. Soft magnetic samples may be influenced by the probe tip magnetization.

Other Special Sensors

Some applications require that the cantilever have special characteristics. For example, Lateral Force Microscopy (LFM) sensors have an extremely soft, thin cantilever to provide high sensitivity to lateral or friction forces. Force Modulation Mode (FMM) sensors have force constants tailored to their application.

Sensors for special and research applications are available including cantilevers without probes, rounded probe tips having radii from 50 nm to 500 nm, and plateau probe tips with a flattened end from 1 µm to 8 µm wide.

Force Constant and Resonant Frequency

Cantilevers are characterized by their force constant and resonant frequency and are matched to the operating mode and AFM instrument. The longer the cantilever, the lower the resonance frequency. Typical characteristics and operating modes are listed in Table I.

Table I. Typical Cantilever Sensor Characteristics

Mode Force Constant Resonant Frequency Contact Mode 0.2 N/m 1.6 N/m 13-14 kHz 27 kHz Non-Contact 42 N/m 48 N/m 15 N/m 27 N/m 330 kHz 190 kHz 130 kHz Force Modulation 2.8 N/m 75 kHz Lateral Force (Torsion) 0.2 N/m 25 kHz Electrostatic or Magnetic Force 2.8 N/m 75 kHz

Some AFM microscopes require a minimum cantilever length (> 125 µm) and/or do not accept high frequencies. Sensors can be obtained in a choice of lengths and resonant frequencies.

Sensor Management in the Microscope

Sensors can be purchased in small packages, typically 10 to 50 per package or as silicon wafers containing several hundred cantilevers with integral probes. Usually, yield from the wafer is guaranteed to be a fixed number or 80-90% of the total. Purchasing unmounted sensors in wafer form saves money but adds labor in terms of handling and mounting.

AFM sensors are very small and hard to pick up and put in the microscope. The sensor chip on which the cantilever is mounted typically measures 1 x 2-4 mm and can be easily dropped. To simplify mounting and prevent damage to sensors, Pacific Nanotechnology offers sensors mounted on a larger metal substrate that is held in the microscope by magnets. If it is necessary to use un-mounted sensors, the Pacific Nanotechnology mounting tool may be used for mounting AFM cantilevers to a clip holder. (Figure 9) To operate the probe fixture tool, the probe clip holder is placed into the fixture. Then the probe is placed onto the fixture; finally the probe can easily slide into the probe clip holder.

Sensor lifetime is an important issue in any AFM application, and how long a probe tip lasts is related to the skill of the microscope operator. Lifetimes increase with careful probe tip approach to the surface, by preventing the probe tip from tapping the surface with excess force in vibration mode and by scanning at speeds appropriate to the sensor's physical characteristics (i.e. not too fast). Probe tips can last a long time with careful use. More expensive, special-application sensors can be removed and reinserted for use only when required by the application.

Some AFMs are used for one type of application in dedicated research environments or with only one or two users doing routine examination of similar samples. Other AFMs see many users and perform many different applications in a single day. Most fall somewhere in between.

The challenge for instruments used for a wide variety of surfaces is to minimize sensor changes and sensor inventory. It may be feasible, now that AFMs are becoming more affordable, to have more than more instrument and dedicate one to advanced applications and one to routine imaging.

Figure 9: This probe exchange tool, opened with probe clip holder sitting in front is an example of a tip mounting tool provided by Pacific Nanotechnology.

Flatness standards, height standards, and several standards with 100, 200, or 300 pitch for precise x-y calibration of the microscope's scanning mechanism are readily available.

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Glossary of Terms Relating to AFM Sensors

AFM - Atomic Force Microscopy or Atomic Force Microscope - the technique or microscope used to image topography and many other material characteristics of a surface on the nanometer to micrometer scale.

Sensor - The combination of a probe/probe tip, cantilever, and substrate (chip) used to measure the forces from which an AFM image is developed.

Probe - The structure, mounted on the end of an AFM cantilever, that interacts with the sample surface being imaged. The term AFM probe refers to the entire structure including the probe tip.

Probe Tip - The outer end of the AFM probe that interacts directly with the sample surface during imaging. Its shape is the most critical aspect in choosing the best sensor for a particular application. Sometimes also referred to as a sensor tip.

Cantilever - The part of an AFM Sensor on which the probe is mounted. The motion of the cantilever is the metric used to measure the forces between the probe tip and the sample surface.

Substrate or Chip - Terms used interchangeably to refer to the support on which a cantilever and probe and probe tip are mounted.