The miniaturization of structures and components has become a key trend in modern technology. The development of cost-effective micro-machining techniques plays a crucial role in advancing the field of micro-technology. Currently, industrial micro-fabrication is predominantly used in the semiconductor industry, where it excels in mass production but lacks flexibility for smaller-scale or specialized applications. In contrast, micro-fabrication methods used in the printing plate-making industry have significant limitations in terms of the geometry and materials they can process. Micro-machining, however, offers a more versatile and adaptable solution, making it an emerging and promising area within micro-fabrication.
In the late 1960s, the first micro-machining equipment was developed in the United States, primarily for polishing optical components, which led to advancements in super-finishing technologies. Today, micron and submicron precision with surface roughness in the tens of nanometers are routinely achieved in the processing of optical, electronic, and mechanical parts. By the late 1980s, the Carusle Research Center in Germany pioneered the use of micro-cutting to create fine textures on micro-components, such as micro-heat exchangers. They used single-crystal diamond tools to groove copper or aluminum foils, resulting in highly efficient miniature heat exchangers.
Until the 1990s, micro-cutting mainly focused on non-ferrous metals using diamond tools. As microtechnology expanded, there became a growing need to process a wider range of materials, especially steel and ceramics, which have since become a major focus of micro-cutting research.
In the field of super-finishing, single-crystal diamond tools remain the most practical option. Their low friction coefficient and high thermal conductivity improve the cutting process, while their extreme hardness and sharp edges enable atomic-level precision—critical for micromachining. A sub-micron edge can achieve surface roughness in the nanometer range, reducing cutting forces and allowing for greater precision and less tool rigidity.
Diamond tools are ideal for machining soft metals like aluminum, copper, and brass, offering excellent surface quality. However, they are not suitable for ferrous metals. To overcome this, researchers have introduced ultrasonic vibrations into the cutting process, reducing contact time and temperature, thus preventing diamond from converting into graphite.
Micro-cutting draws from traditional machining techniques such as turning, milling, drilling, and grinding. Among these, ultra-precision turning is the most mature and widely studied. It is commonly used to produce molds for Fresnel lenses or surface roughness samples.
By applying high-frequency vibrations via piezoelectric crystals, non-rotationally symmetrical surfaces can be created, achieving mirror-like finishes. Ultra-precision turning now allows for extremely fine shaft diameters.
Milling is considered one of the most flexible microfabrication methods. Diamond disc cutters can machine complex grooves at various angles, making them ideal for optical grid structures. Commercial disc cutters have minimum widths around 100 μm, while diamond shank cutters with diameters of about 300 μm are also available, suitable for thin separators.
So far, micro-cutting has been mostly limited to non-metallic materials. Processing steel through micro-cutting began in the 1990s in Germany, with carbide tools being the primary choice. These tools offer good performance at lower costs, though they cannot match the surface finish of diamond tools.
To achieve sharp edges, ultrafine-grained cemented carbide is often used, with grain sizes between 0.5 and 1.0 μm. These tools are essential for machining hard materials like steel.
German researchers have tested carbide disc cutters with widths as small as 0.15 mm, successfully cutting hardened steel. Carbide shank cutters are widely used in industry, both coated and uncoated, with minimum diameters as small as 0.1 mm.
When working with hard materials, machine tools must be rigid and dynamic to avoid tool breakage. High-speed cutting with controlled feed rates ensures consistent performance.
Manufacturing carbide micro-millers presents challenges, particularly in achieving sharp edges. Grinding tools with small diameters requires careful control of cutting forces. One solution is to machine parts directly from mold steel with high hardness, achieving surface finishes as fine as 0.5 μm.
Grinding is specifically designed for hard and brittle materials like glass, ceramic, silicon, and hard alloys. Commercially available grinding wheels, typically 0.1 mm wide, use diamond or chrome abrasives. New developments include CVD diamond-coated cemented carbide wheels, capable of creating complex micro-surfaces.
At the Technical University of Braunschweig, researchers developed a CVD diamond drill bit with a diameter of 0.9 mm, successfully drilling 55 blind holes in single-crystal silicon. While electroplated diamond core drills are better for through-holes, issues like chipping along crystal axes still require further study.
Micro-machining extends beyond traditional micro-fabrication, enabling the processing of complex spatial structures when combined with laser etching and other techniques. Compared to lithography-based methods, it requires less equipment and eliminates the need for expensive motherboards. Overall, micro-machining offers significant advantages for economically producing medium-sized micro-components.
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