What is Nano Machining and Its Applications

Whether dealing with IC technology, microsystems, or nanotechnology, the common feature is functional structures with feature sizes in the micrometer or nanometer range. Collectively, these are termed Micro- and Nano technology. The realization of these functional structures and devices relies on advanced nano machining Techniques.

For the past five decades, the progress in nano machining has driven Moore’s Law, enabling the integration density of ICs to double approximately every 18 months. Today, advanced Machining can place hundreds of millions of transistors on a chip smaller than a coin. ICs with minimum feature sizes of 40 nm are in mass production, 22 nm chips are entering industrial production, and 16 nm level ICs are already in the research and development phase.

Nano machining does more than shrink transistors, it can scale down complex mechanical gear systems to invisible dimensions, create single-electron transistors, and even enable the manipulation of individual molecules and atoms. It serves as the essential bridge for humanity to access, understand, and utilize the microscopic world. Therefore, understanding nano machining technology is crucial for comprehending modern high-tech industries supported by micro- and nanotechnology.

Types of Nano Machining

Manufacturing is one of humanity’s core productive activities, transforming raw materials into useful shapes using tools. While traditional mechanical machining (e.g., grinding and polishing) can achieve micro- or nano-scale precision, the nano machining discussed in this article refers to techniques where the components or structures themselves possess micro- or nano-scale dimensions.

Nano machining is a broad and continuously evolving field, encompassing as many as 50 distinct methods identified at international engineering conferences. These diverse methods can be broadly categorized into three main types for a generalized overview: Planar processing, Probe-based processing, and Replication (Molding) processing.

Planar Processing (Semiconductor Model)

Planar processing is fundamentally different from traditional mechanical machining. It is the core technology that enabled the transition from discrete transistors to integrated circuits.

Planar processing relies on Lithography, where a photosensitive material (photoresist) is exposed to radiation according to a designed pattern. Subsequent development leaves the pattern on the substrate. This pattern is then transferred to the underlying substrate material through Material Deposition or Etching. Complex micro- or nano-structures are built up through multiple layers of exposure, etching, or deposition.

• Patterning: Exposure can be achieved via projection using an optical mask or by direct scanning with a laser, electron, or ion beam.
• Etching: Techniques include chemical liquid (wet) etching and various plasma-based (dry) etching methods.
• Deposition: Techniques include thermal evaporation, chemical vapor deposition (CVD), and electroforming.

Characteristics

1. Non-Contact Patterning: Structures are formed by imaging systems (light wavelength, beam diameter) rather than direct tool-material interaction, overcoming the dimensional limitations of physical tools.
2. 2D and Quasi-3D Structures: Planar processing inherently forms 2D structures or quasi-3D structures created by stacking multiple 2D layers.
3. Parallel System Integration: The entire system, including all components and their relationships, is formed simultaneously (in parallel) on the substrate, eliminating the traditional process of machining discrete parts and then assembling them.

Applications

While originating in IC manufacturing with key steps being thin-film deposition, patterning, doping, and thermal processing, planar processing is now widely applied in microsystem technology. This includes fabricating micro-mechanical, microfluidic, and microelectro-optical-mechanical devices. For example, micro-gear systems with diameters under $1 \text{ mm}$ are created using clever combinations of multi-layer polysilicon deposition and etching, forming the entire system in a single, integrated process. Microsystem Machining often employs specialized planar processes, such as thick-resist lithography, electroforming, and deep silicon etching (DRIE).

Probe-Based Processing

Probe-based processing is an extension of traditional mechanical machining, where various micro- and nano-sized probes replace conventional cutting tools. Probes can be solid-state (e.g., Scanning Tunneling Microscopy (STM) or Atomic Force Microscopy (AFM) tips) or non-solid-state (e.g., Focused Ion Beams (FIB), laser beams, or spark discharge micro-probes).

• Solid-State Probes: AFM/STM tips can directly manipulate atoms, form nano-scale oxide layers, or perform electronic exposure on a substrate. They can also transfer polymer materials via liquid transport to form patterned single-molecule layers.
• Non-Solid-State Probes: Focused Ion Beams can achieve beam diameters under 10 nm. The beam’s sputtering or chemical gas-assisted deposition can directly create micro- and nano-structures on various materials. Highly focused laser beams, such as those used in femtosecond laser processing, can directly ablate or photopolymerize organic compounds to form 3D micro- and nano-structures.

The major difference from planar processing is that probe-based processing is a sequential (serial) method, processing one feature at a time, making it less suitable for high-volume manufacturing compared to the parallel nature of planar processing. However, it offers direct material modification without the need for a photoresist mask.

Replication (Molding) Processing

Replication processing utilizes micro- and nano-sized molds (stamps) to reproduce corresponding structures, enabling rapid and low-cost mass production.

• Nanoimprint Lithography (NIL): A stamp containing a nano-pattern is pressed onto a softened organic polymer layer. NIL enables the low-cost, high-volume replication of nano-patterns that may have been created using other, more expensive, nanomachining techniques.
• Molding and Casting: This includes traditional plastic molding and metal casting, extended to the micro- and nano-domains. These techniques are fast and cost-effective, typically used for microfluidics, biochips, and structures larger than the micro-scale, making them suitable for high-volume production.

These three methods are all “Top-down” Machining techniques, where functional structures are carved or deposited from a bulk substrate. In contrast, “Bottom-up” methods, such as molecular self-assembly, rely on biological and chemical processes. However, bottom-up techniques often still require a platform constructed using top-down Machining methods. The two approaches are complementary and inseparable, as nano-scale phenomena usually require micro-scale devices for integration with the macro-world.

Applications and Considerations

The ultimate goal of nano machining is to create functional micro- and nano structures and devices for practical use. When selecting an appropriate Machining technique for a specific application, several key principles must be considered:

Minimum Feature Size

The ability to achieve the desired minimum structure size is paramount.

• Optical Lithography: Contact printing is limited to about 1 μm. Advanced projection lithography, depending on wavelength and resolution-enhancement techniques, currently achieves feature sizes around 40 nm.
• Sub-40 nm: Smaller structures require high-resolution methods like electron beam lithography, nanoimprint lithography, or focused ion beam processing.

Economic Feasibility

Nano machining equipment is extremely expensive (e.g., an IC Machining line can cost over 10 billion dollars). The cost of materials must also be considered; single-crystal silicon, while foundational for ICs, is costly. Therefore, disposable micro-devices for medical or biochemical applications are often fabricated using lower-cost materials like plastics or glass.

Production Volume Requirements

The choice between parallel (high-volume) and sequential (low-volume) processing is critical.

• Parallel: Optical lithography, X-ray lithography, and nanoimprint lithography can form massive numbers of structures simultaneously, making them ideal for mass production. This is why optical lithography remains the core technology for high-volume IC manufacturing, despite not having the highest resolution.
• Sequential: Electron beam and focused ion beam processing are sequential, typically reserved for R&D or low-volume, high-precision applications.

Production and R&D

• Mass Production: Requires the most stable and reliable technology to ensure the highest possible yield, often favoring established techniques over the absolute cutting edge.
• R&D: Has no strict yield requirements, allowing for the adoption of novel and diverse techniques to achieve unique micro- and nano-structures for scientific study.

Material Compatibility

The function of a micro-device is determined by both its structure and its material. Machining techniques must be compatible with the material, such as silicon, III-V semiconductors, metals, glass, ceramics, or polymers.

Future Trends in Nano Machining

Nano machining is the foundation for virtually all modern high-tech growth, including semiconductor ICs, nanoelectronics, high-density magnetic storage, microsystems, biochips, and nanotech research. The most significant future trend is System Integration.

System on Chip (SoC) and System in Package (SiP)

Current systems often rely on multiple, discrete functional units (microprocessors, memory, sensors, actuators) integrated on a printed circuit board. The future goal is to integrate all these distinct functions—including microelectronics, micro-mechanics, micro-sensors, micro-fluidics, and micro-optics—onto a single chip (SoC) or within a single packaged unit (SiP).

This level of integration demands the development of fully compatible, low-cost nano machining technologies. The challenge lies in harmonizing Machining processes that are currently optimized for vastly different materials and functions (e.g., standard CMOS for electronics vs. deep etching for mechanical sensors).

The Post-Optical Era and Economic Factors

As IC feature sizes continue to shrink towards and below the 20 nm barrier, optical lithography is nearing its fundamental limit. Replacement technologies, such as Extreme Ultraviolet (EUV) lithography, nanoimprint, and electron beam projection, are currently under development. However, technological superiority does not guarantee adoption; economic viability is the determining factor. The history of X-ray lithography—a technically brilliant process eventually sidelined by the cost-effectiveness and continuous improvement of optical lithography—serves as a strong precedent.

Standardization of Microsystem Machining

Unlike IC manufacturing, which has evolved into a highly standardized and globalized process (allowing designs to be fabricated at any foundry worldwide with consistent performance), microsystem Machining remains diverse and non-standardized. This lack of standardization is a major barrier to the growth of the microsystems industry, resulting in small batches of diverse products. Developing standardized, yet flexible, Machining processes is key to realizing the full potential of microsystem technology.

High-Throughput Nano Machining

For the eventual commercialization of nanotechnology research, the industry requires high-throughput, low-cost Machining methods capable of producing structures below 10 nm. While state-of-the-art techniques like E-beam lithography and AFM manipulation exist for research, they are not scalable. Nanoimprint technology holds significant promise for meeting the high-volume needs of future nano-device manufacturing, while molecular self-assembly offers immense potential as a radically new production paradigm.

Conclusion

Nano machining technology is a constantly evolving field with infinite development potential. New processes are continually emerging, while clever utilization of existing techniques inspires novel device and system development. It is no exaggeration to state that the coming decade will be the age of exponential growth for micro- and nanotechnology.

As a final note, we recall the famous prophetic challenge delivered by Nobel Laureate Dr. Richard Feynman in 1959: “There’s Plenty of Room at the Bottom.” This statement perfectly encapsulates the boundless frontier that nano machining continues to unlock.