Ultra-precision polishing generally refers to the use of abrasive microparticles with particle sizes of only a few nanometers as polishing abrasives, which are injected into a lapping tool to remove minute amounts of workpiece material, thereby achieving specified geometric accuracy and surface roughness. Such extremely high polishing precision imposes greater challenges on polishing techniques, equipment, and materials.
In modern electronics, ultra-precision polishing is tasked not only with planarizing different materials but also with planarizing multi-layer stacks, enabling a silicon wafer just a few millimeters square to form ultra-large-scale integrated circuits composed of tens of thousands to millions of transistors through "global planarization." For example, the fact that computers have shrunk from dozens of tons to just a few hundred grams would not have been possible without ultra-precision polishing.
Currently, Japan, the United States, and Germany hold leading positions in ultra-precision machine tools. They control the core technologies of ultra-precision polishing processes and firmly command the global market initiative. In contrast, China's development in ultra-precision polishing technology faces certain constraints. Therefore, how to overcome the technical bottlenecks in ultra-precision polishing has become a focal point of concern in China's lapping and polishing field.
01 Equipment – Blockaded
High-precision polishing is typically used in high-end fields such as optics, semiconductors, and aerospace, where extreme form accuracy, dimensional precision, and surface integrity (no or minimal surface damage, including micro-cracks, residual stress, and microstructural changes) are required. This places extremely high demands on polishing equipment.
To ensure polishing precision, ultra-precision polishing equipment also requires very stable mechanical structures. The "polishing platen" – a core component of the equipment – imposes even more stringent requirements on material composition and technology. This platen, made of specialized composite materials, must not only meet nanoscale precision for automated operations but also possess a precisely controlled coefficient of thermal expansion, preventing heat generated by friction during high-speed rotation from causing thermal deformation of the platen, which would otherwise affect the stability of machining accuracy and ultimately the flatness and parallelism of the product. Generally, to achieve sub-micron or even nanometer polishing accuracy, the thermal deformation of the platen must be controlled within a few nanometers.
At present, high-precision polishing platens are mostly custom-made rather than mass-produced, which directly limits replication outside countries like the U.S. and Japan. Thus, these nations firmly hold the production technology for high-precision platens. For a long time, China has faced strict technology embargoes. The questions of what materials and processes can synthesize such platens with low thermal expansion, high wear resistance, and ultra-precision lapping surfaces are technical challenges that Chinese enterprises and research institutes must concentrate on solving.
In addition, for small- to medium-aperture (below 200 mm) optical components, domestic equipment is generally capable. However, in the realm of medium- to large-aperture (above 400 mm) ultra-precision grinding machines – currently, there is no commercially viable domestic equipment in China. This means that for large-diameter mirrors used in astronomical telescopes, laser fusion facilities, and EUV lithography machines, we still cannot pass the grinding stage. What is the international state-of-the-art? The UK's Cranfield OGM2500 has a maximum machining diameter of 2.5 meters. The UK's BOX machine tool, used for processing 1.45-meter mirrors for the European Extremely Large Telescope, achieves surface form accuracy PV < 1 μm, with a single-piece production cycle of under 20 hours. Germany's OptoTech UPG500 has the highest commercial maturity.
02 Materials – Still Dependent on Imports
Taking CMP (Chemical Mechanical Polishing/Planarization) as an example, with the evolution of advanced process nodes and advanced packaging, CMP has gradually broken out of its traditional auxiliary role to become the fourth core process in integrated circuit manufacturing, alongside lithography, etching, and thin-film deposition. In the context of continued scaling to advanced nodes and three-dimensional architectures, CMP has become a critical step affecting wafer global flatness and process yield.
The core materials for CMP include slurries and polishing pads. Among them, the slurry is the key medium determining CMP processing quality and removal rate, while the polishing pad plays a crucial role as a physical medium and stress transfer element during the CMP process. There are many types of slurries, accounting for over 50% of the value of polishing materials, and their consumption increases with wafer output and the number of CMP planarization steps.
CMP polishing materials present extremely high technical barriers. Due to China's late start in this field, core patents have long been monopolized by overseas giants. Slurry formulations are complex, R&D and validation cycles are lengthy, and production processes and process control requirements are rigorous. Moreover, high-purity abrasive particles have long relied on imports, and the upstream core supply chain remains controlled by foreign companies.
Abrasive particles are the core raw material for slurries; mainstream products include ceria, silica, and alumina particles. One researcher noted that for mature 28 nm chip processes, domestic substitution is theoretically achievable on the domestic market, but for more advanced nodes, abrasive supply still faces challenges, which directly affect the control of metal impurities in the slurry.
Here again, we are blockaded.
03 Process Technologies – Still in the Trial Phase
(1) Magnetorheological Finishing (MRF)
Magnetorheological finishing is an advanced optical manufacturing technology developed abroad in the 1980s, and QED (USA) began commercializing it in 1997. This technique uses the rheological properties of magnetorheological fluid in a magnetic field to polish workpieces. When the fluid enters the polishing zone, it transforms into a viscoplastic medium under the magnetic field, forming a "flexible polishing pad." Contact with the optical surface generates significant shear stress, enabling stable material removal for polishing and figuring. It effectively addresses requirements such as high machining accuracy, rapid convergence, good surface quality, low subsurface damage, and controllable mid- and high-spatial-frequency errors. It is widely applicable to spherical lenses, aspheric lenses, prisms, freeform surfaces, etc.
(2) Ion Beam Figuring (IBF)
The trend in large-aperture aspheric optical processing is toward higher deviation, steeper slopes, and higher precision. To meet this, Eastman Kodak (USA) proposed ion beam figuring. This technique, performed in a vacuum chamber, uses an ion beam with defined energy and spatial distribution to bombard the mirror surface. Material removal occurs via non-contact energy deposition, offering a new technological option for high-precision figuring of large aspherics. Owing to its high figuring accuracy and no subsurface damage, it, along with MRF, is widely recognized as one of the two most innovative optical processing technologies developed in the past thirty years.
(3) Chemical Mechanical Polishing/Planarization (CMP)
CMP is currently the only planarization technology that can achieve both global and local surface flatness. It was first proposed by Monsanto (USA) in 1965, but initially only applied to obtaining high-quality glass surfaces, such as for military telescopes. Later, because it uniquely combines chemical corrosion and mechanical abrasion, reducing process variations and producing nanoscale planarized surfaces, companies like IBM, Intel, and Applied Materials gradually adopted it in semiconductor manufacturing.
In the field of ultra-precision polishing, foreign countries – especially Japan, Germany, and the United States – have significant advantages. In terms of equipment and process levels, these nations have already achieved nanometer-level polishing precision, while China still lags behind the international advanced level to a certain extent.