The miniaturization of components and structures has become a key trend in modern technology. Developing cost-effective micro-machining techniques is crucial for advancing micro-technology, as it enables the production of smaller, more precise parts. Currently, industrial micro-fabrication is largely confined to the semiconductor industry, where it's most efficient for mass production. In contrast, the printing plate-making industry faces significant limitations in terms of geometry and material compatibility. Micro-machining, however, offers a more versatile solution, addressing these shortcomings and emerging as a promising field in micro-fabrication.
In the late 1960s, the first micro-machining equipment was developed in the U.S., primarily used for polishing optical surfaces, which led to the advancement of super-finishing techniques. Today, micrometer and sub-micrometer precision with surface roughness in the tens of nanometers is achievable in the processing of optical, electronic, and mechanical components. By the late 1980s, the Carusle Research Center in Germany used micro-cutting to create fine textures on micro-components, leading to the development of miniature heat exchangers. They utilized single-crystal diamond tools to groove copper or aluminum foils, resulting in highly efficient micro-heat exchangers.
By the 1990s, micro-cutting was mainly applied to non-ferrous metals using diamond tools. As microtechnology expanded, there became a need to process a wider range of materials, especially steel and ceramics, which now represent a major direction in micro-cutting research.
In super-finishing, single-crystal diamond tools are nearly the only practical option. Their low friction coefficient and high thermal conductivity enhance the cutting process, while their extreme hardness and sharp edges allow for atomic-level precision, essential in micro-machining. A sub-micron edge can produce surface roughness on the order of a few nanometers, reducing cutting forces and improving machining accuracy.
Diamond tools are ideal for soft metals like aluminum, copper, and brass, offering excellent surface quality. However, they are unsuitable for ferrous metals. To address this, ultrasonic-assisted cutting systems have been developed, reducing tool contact time and preventing diamond from converting into graphite during steel machining.
Micro-cutting encompasses various traditional machining methods, including turning, milling, drilling, and grinding. Among these, ultra-precision turning is the most researched and mature technique. It’s commonly used for producing molds for Fresnel lenses or surface roughness samples.
By integrating high-frequency vibrations from piezoelectric crystals into the feed mechanism, non-rotationally symmetrical surfaces can be achieved, resulting in mirror-like finishes. Ultra-precision turning can now machine extremely fine shaft diameters.
Milling is considered one of the most flexible microfabrication techniques. Single-tooth diamond disc cutters can machine complex grooves at various angles, making them suitable for optical grid molds. Commercial disc cutters have a minimum width of around 100μm, while diamond shank cutters with a diameter of 300μm are also available, ideal for thin separators.
Currently, micro-cutting is mostly limited to silicon and non-metallic materials. However, research into steel cutting began in Germany in the 1990s, focusing on tool and die applications. Carbide tools, rather than diamond, are typically used for steel due to cost and performance considerations.
To achieve sharp edges, ultrafine-grained tungsten-cobalt carbide is often employed. These tools have grain sizes between 0.5–1.0 μm and cutting edge radii of a few micrometers.
Carbide micro-milling cutters are widely used in industry, both coated and uncoated, with minimum diameters as small as 0.1 mm. When machining hard materials like steel, ensuring machine rigidity and smooth operation is critical to avoid tool breakage or wear.
Manufacturing carbide micro-milling cutters presents challenges, particularly in achieving sharp edges on uneven materials. Specialized techniques, such as direct milling on hardened mold steel, are being explored to improve efficiency and surface quality.
Grinding is specifically designed for hard and brittle materials like glass, ceramics, and silicon. Diamond-coated grinding wheels, some as narrow as a fraction of a millimeter, are commercially available. New developments include CVD diamond-coated cemented carbide wheels, capable of creating intricate micro-surface geometries.
At the Technical University of Braunschweig, researchers have developed a CVD diamond drill bit with a 0.9mm diameter, successfully drilling 55 blind holes in single-crystal silicon. While electroplated diamond core drills are better suited for through-holes, issues like chipping along crystal axes remain a challenge.
Micro-machining complements other techniques like laser etching, allowing for the fabrication of complex spatial structures across various materials. Compared to lithography-based methods, it requires less equipment and eliminates the need for expensive motherboard manufacturing. Overall, micro-machining provides a cost-effective solution for producing medium-sized micro-components.
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