Atomic force microscopy (AFM) probes are critical components that determine the instrument's performance across various applications. The design and material selection of these probes influence resolution, durability, and suitability for specific measurements. Key considerations include the probe's mechanical properties, tip geometry, and functional coatings, each tailored to optimize interaction with the sample surface.

The most common base materials for AFM probes are silicon and silicon nitride. Silicon probes are widely used due to their excellent mechanical properties, including a high Young's modulus, which ensures minimal bending during scanning. Silicon tips are typically sharp, with radii of curvature ranging from 5 to 10 nanometers, making them suitable for high-resolution imaging of flat or moderately rough surfaces. However, silicon is prone to wear when scanning hard materials, which can degrade image quality over time. Silicon nitride probes, on the other hand, offer greater toughness and are often used in contact mode imaging due to their lower spring constants, which reduce sample damage. These probes are ideal for biological samples or soft materials where excessive force could deform or destroy the specimen.

For applications requiring extreme durability, diamond-coated probes are preferred. Diamond's exceptional hardness minimizes tip wear when scanning abrasive or hard surfaces such as ceramics, metals, or certain composites. The coating thickness varies but is typically in the range of 100 to 500 nanometers. Diamond-coated tips maintain their sharpness over prolonged use, making them suitable for long-duration experiments or industrial quality control. However, the increased stiffness of diamond-coated probes can be a drawback for soft samples, as higher forces may be inadvertently applied.

Conductive AFM probes are essential for electrical characterization techniques such as scanning Kelvin probe microscopy or conductive AFM. These probes are coated with conductive materials like platinum, gold, or doped diamond. Platinum-coated tips offer good conductivity and moderate wear resistance, while gold coatings provide excellent conductivity but are less durable. Doped diamond coatings combine conductivity with extreme hardness, making them ideal for combined topographical and electrical measurements on hard materials. The choice of coating depends on the required balance between electrical performance and mechanical resilience.

Tip geometry plays a crucial role in AFM resolution and measurement accuracy. The sharpness of the tip, defined by its radius of curvature, directly influences lateral resolution. Sharper tips with smaller radii can resolve finer surface features but are more susceptible to wear. High-aspect-ratio tips are necessary for imaging deep trenches or high-roughness surfaces where standard tips might fail to reach the bottom features. These specialized geometries prevent artifacts caused by tip convolution, where the tip shape distorts the measured profile of steep or recessed structures.

The aspect ratio also affects imaging fidelity. Standard pyramidal tips work well for moderately rough surfaces, while needle-like high-aspect-ratio tips are better suited for nanostructures with significant height variations. For example, carbon nanotube-modified tips offer ultra-high aspect ratios and minimal tip-sample adhesion, enabling precise measurements of complex topographies such as semiconductor devices or biological fibrils.

Coatings can further enhance probe functionality beyond conductivity. Magnetic coatings, such as cobalt or iron, enable magnetic force microscopy (MFM), where the probe interacts with magnetic domains on the sample surface. Functionalized tips with specific chemical groups allow chemical force microscopy, probing adhesion or binding forces at the molecular level. These modifications expand AFM capabilities beyond topography into chemical and functional property mapping.

Environmental conditions also influence probe selection. In liquid environments, silicon nitride probes are often used due to their compatibility with aqueous solutions and lower spring constants, which prevent excessive force on hydrated samples. For high-temperature AFM, materials with stable thermal properties, such as certain metal-coated or diamond-like carbon tips, are necessary to avoid degradation or changes in mechanical behavior.

The stiffness of the cantilever, determined by its material and dimensions, must match the measurement mode. Soft cantilevers with spring constants below 1 N/m are ideal for contact mode imaging of delicate samples, while stiffer cantilevers (above 40 N/m) are used for tapping mode or hard samples where higher forces are acceptable. The resonant frequency of the cantilever is another critical parameter, especially in dynamic modes, where higher frequencies enable faster scanning speeds.

Wear resistance is a key consideration for prolonged use. While diamond-coated tips excel in longevity, their higher cost may not be justified for all applications. Silicon tips provide a cost-effective solution for routine imaging but require replacement more frequently. For conductive measurements, the trade-off between coating durability and electrical performance must be evaluated based on the sample properties and measurement duration.

In summary, AFM probe design involves balancing material properties, tip geometry, and functional coatings to achieve optimal performance for specific applications. Silicon and silicon nitride probes serve as versatile options for general use, while diamond-coated and conductive tips address specialized needs. Tip sharpness and aspect ratio dictate resolution and suitability for complex topographies, while coatings expand functionality into electrical, magnetic, or chemical probing. Selecting the appropriate probe requires careful consideration of sample properties, measurement mode, and environmental conditions to ensure accurate and reliable data acquisition.