Dr Sang-Joon Cho | 3D Atomic Force Microscopy: Overcoming Challenges in Nano-Scale Measurement

Nano-scale imaging and measurement are crucial for the development of new gadgets – from the latest phones to advanced drug discovery technologies. Dr Sang-Joon Cho and his team at Park Systems Corporation have developed a new approach to measuring and characterising microscopic components, offering exciting potential to accelerate advancements in the technologies essential to the modern world.

Advancements in Semiconductor Manufacturing

Innovative electronics are at the forefront of global development and underpin the future of a wide range of industries. From agriculture to medicine, the energy industry to environmental protection, the latest generation of electronics is critical in meeting global challenges and consumer appetites. In response to this pressure, electronic devices are becoming smaller and more complex, and the components needed to manufacture them are increasingly sophisticated.

Central to the development of innovative electronics are advancements in semiconductor manufacturing. Semiconductors are a select range of metals, including silicon and germanium, whose properties can be manipulated to alter how they conduct electricity. They are the materials used to make diodes, transistors, and microchips, among other electronic components. The physical shape and structure of the semiconductors in these components are critical to their function.

The Need to Measure at the Smallest of Scales

Nano-technology operates at the smallest of scales – a nanometer being a millionth of a millimetre. This is the scale of individual molecules. At this scale, some materials take on unique physical and chemical properties such as increased strength or enhanced electrical conductivity. In the case of semiconductors, precision engineering at the nano size allows for massive increases in the efficiency of components such as advanced transistors with greater performance at a much smaller size.

Design and development of nano-technology, whether in electronics, materials science, or biotechnology, rely on our ability to measure the very, very small accurately. Traditional methods of measuring at the nano-scale, such as conventional atomic force microscopy (AFM) and scanning electron microscopy, are limited in their resolution when measuring complex shapes or sometimes destroy the sample, making them inappropriate for developing smaller and more complex technology.

The Limits of Atomic Force Microscopy

Dr Sang-Joon Cho and his team from Park Systems Corporation anticipated the increasing demand for nano-scale measurements in rapidly advancing high-tech industries, and identified the need to address the limitations of conventional AFM. They aimed to demonstrate that AFM could provide high-resolution, accurate nano-scale measurements with reproducibility and repeatability.

AFM is a technique that scans an object’s surface with a tiny tip on the end of an arm (or cantilever) that amplifies the signal produced, resembling a microscopic record player needle. Conventional AFM methods are limited in accuracy due to orthogonal cross-talk when scanning complex shapes. This occurs when the scanning motion in the X, Y, and Z directions are interdependent, causing image distortion when scanning ridges, indentations, or overhanging sections of complex shapes. The team overcame this by decoupling the scanners in the XY and Z planes and using mechanical designs to complement the movements. Additionally, they developed a non-contact scanning method to eliminate various measurement errors and prevent damage to the probe and sample surface.

Overcoming New Limits of Atomic Force Microscopy

The traditional AFM scanning method, which requires the probe to meet the surface perpendicularly, is not able to accurately measure the roughness of undercut shapes such as ridges or recesses in modern transistors and photonic devices like solar cells and LEDs. Dr Cho’s team saw the trend of increasingly complex 3D components and identified the need for a new technique to accurately assess the nano-scale shapes and roughness of undercut surfaces. Without such a tool, it would be challenging to accurately evaluate the quality of new generations of electronic components and develop more intricate designs.

With this in mind, the team set out to create a three-dimensional atomic force microscope (3D-AFM) capable of measuring sidewall surfaces with nanometer precision. They began designing a new scanner that could reach difficult-to-measure areas while maintaining the advantages of improved AFM and overcoming its limitations.

Surface Sidewall Roughness

One of the major challenges in developing electronic products is controlling the roughness of various surfaces during semiconductor manufacturing. Among them, the sidewall surface roughness of advanced 3D semiconductors is a critical parameter that determines the performance and reliability of the semiconductor. These surface features significantly influence how electricity flows through the device, affecting its efficiency and speed. If the sidewalls are rougher than expected, unwanted resistance can occur, generating heat and potentially causing device failure. Conversely, smoother surfaces can enhance electrical performance and improve the overall functionality of the device. Controlling this parameter is crucial for engineers to ensure that electronic devices operate optimally, but there is no universal technology to measure and evaluate this parameter. As various forms and sizes of 3D nano-devices are developed with increasingly complex shapes, the need to measure every nook and cranny of components, including the sides, will continue to grow.

Developing 3D Atomic Force Microscopy

Dr Cho’s team devised a modified AFM system that allowed the probe to tilt up to 40 degrees and included a new type of cantilever to avoid interference during scanning. They then developed an algorithm that could merge multiple scans of the same area at varying tilt angles, resulting in a high-resolution scan of all sides of an object. The team tested their new design on a sample and successfully demonstrated that their prototype 3D-AFM could produce a high-resolution scan of all sides of a wall-shaped object, crucially with high-resolution roughness of the sidewalls. This confirmed that 3D-AFM could accurately characterise undercut sidewalls in real-world situations, revealing details that previous techniques could not capture. The new method allowed for precise measurements of sidewall roughness and the acute critical angles of undercut structures, which are crucial for understanding and improving semiconductor devices.

Wide-reaching Implications

This innovative approach has resulted in a powerful tool that significantly improves measurement accuracy in this field, addressing a critical need for better imaging techniques as device components continue to shrink.

Dr Cho’s work has wide-reaching implications, as 3D-AFM has great potential to enhance nano-scale imaging across a range of fields. This spans from improving semiconductor manufacturing processes, such as quality checking, to supporting the development of new technologies reliant on more complex topographies. In addition to the 3D AFM introduced here, Park Systems is developing various atomic force microscopy technologies that can provide solutions to various problems arising in advanced science and industry. Automation that maximises user convenience and offers enhanced environmental monitoring sensors, and integration with other advanced measurement technologies can provide solutions in various fields, including semiconductors, energy, biotechnology, and future materials. Dr Cho continues to work towards broadening the reach of AFM and realising its potential to support technological advancement.