As core components of cutting-edge equipment such as EUV lithography machines, high-end cameras, and LiDAR systems, ultra-precision optical lenses require nanoscale surface figure accuracy (λ/50, where λ = 632.8nm) and surface roughness below 0.5nm RMS. These extreme requirements have driven continuous breakthroughs in ultra-precision machining technologies. This article breaks down the manufacturing process of ultra-precision optical lenses from two major technical systems—"traditional mechanical material removal" and "emerging non-traditional machining"—explains the principles and application scenarios of core technologies, and reveals the complete transformation path from lens blanks to high-precision finished products.
1. Traditional Ultra-Precision Machining Technologies: Mainstream Solutions Based on Mechanical Material Removal
Traditional ultra-precision machining technologies are centered on the core concept of "controllable, minimal material removal". By precisely adjusting machining parameters through computer numerical control (CNC) systems, they gradually correct the form and position errors on the lens surface, making them the mainstream technical path for mass production of ultra-precision optical lenses today.
1.1 Ultra-Precision CNC Grinding and Polishing: The Core Foundation of Lens Machining
Beyond the limitations of traditional "manual grinding", ultra-precision CNC grinding and polishing has evolved into a computer-controlled deterministic shaping process. Its core technology is "Computer Controlled Optical Surfacing (CCOS)", which lays the foundation for achieving high-precision surface figures of lenses.
(1) Principles and Core Advantages of CCOS Technology
The core logic of CCOS technology lies in "small tools and precise quantity control": a polishing tool much smaller than the workpiece is used, and its movement path, dwell time, contact pressure, and other parameters on the workpiece surface are controlled in real time by a CNC system. By adjusting the dwell time of the tool in different areas (longer dwell time in areas with larger surface figure errors), micron-level precise control of material removal is achieved, ultimately correcting the lens surface to the target shape.
Its key advantage is "determinism": before machining, optical testing equipment (such as interferometers) can be used to obtain the surface figure error map of the lens. Combined with a material removal model (e.g., Preston equation), the removal effect of the next machining process can be accurately predicted, enabling rapid convergence of surface figure errors (typically from λ/2 to λ/20). Additionally, small-sized tools can specifically correct mid-to-high frequency surface figure errors (such as astigmatism and coma), avoiding "tool marks" that are prone to occur with traditional large tools and laying the groundwork for subsequent fine correction.
(2) Two Core Polishing Technologies: Magnetorheological Finishing (MRF) and Ion Beam Figuring (IBF)
Within the CCOS technology system, Magnetorheological Finishing (MRF) and Ion Beam Figuring (IBF) are key technologies for achieving "high-precision surface figure correction", corresponding to the two core stages of "mid-to-high frequency error correction" and "atomic-level finishing" respectively.
1.2 Magnetorheological Finishing (MRF): An Efficient Corrector for Mid-to-High Frequency Errors
(1) Principle of MRF Technology
MRF technology is centered on a "magnetically controlled flexible polishing tool": a magnetorheological fluid mixed with magnetic particles (e.g., carbonyl iron particles) and abrasives (e.g., diamond micropowder) is delivered to the surface of a high-speed rotating machining wheel. Under the action of an external magnetic field, the magnetorheological fluid on the wheel surface instantly transforms from a "liquid state" to a "viscous Bingham fluid", forming a "flexible polishing pad" with a certain stiffness. The lens is immersed in this polishing pad at a specific angle (usually 30°-60°), and the high-speed rotating wheel drives the polishing pad to generate shear action with the lens surface. Micro-cutting by the abrasives achieves minimal material removal.
(2) Core Characteristics of MRF Technology
- Ultra-High Determinism: The shape of the polishing pad formed by the magnetorheological fluid is stable, and the material removal function (the relationship between removal amount and machining parameters) can be accurately calculated through models. The correction accuracy can reach λ/20-λ/30, enabling efficient elimination of "tool marks" and mid-to-high frequency surface figure errors remaining from CCOS machining;
- No Subsurface Damage: The machining process involves "flexible shear removal", which avoids the impact of traditional rigid tools on the lens surface and barely produces subsurface cracks (subsurface damage layer depth < 10nm), making it particularly suitable for hard and brittle materials such as glass and sapphire;
- High Machining Efficiency: Compared with traditional manual polishing, MRF improves material removal efficiency by 3-5 times, meeting the mass production needs of medium-to-high precision lenses (e.g., high-end camera lenses).
1.3 Ion Beam Figuring (IBF): The Ultimate Means for Atomic-Level Finishing
(1) Principle of IBF Technology
IBF technology is currently the most precise surface figure correction technology, based on the principle of "physical sputtering effect": in a high-vacuum environment (vacuum degree > 10⁻⁴Pa), an inert gas (e.g., argon) is ionized into a high-energy ion beam through an ionization source. The ion beam is accelerated by an electric field (acceleration voltage usually 500-2000V) and bombards the lens surface at a specific angle. High-energy ions exchange momentum with atoms on the lens surface, "sputtering" surface atoms one by one to achieve atomic-level material removal.
(2) Core Characteristics and Limitations of IBF Technology
- Non-Contact Machining: There is no mechanical contact between the ion beam and the lens during machining, completely avoiding workpiece deformation (e.g., warping of thin lenses) and subsurface damage caused by mechanical forces. It is the only choice for machining ultra-thin lenses (thickness < 1mm);
- Atomic-Level Precision: The single removal amount can be controlled at the atomic level (0.1-1nm), with surface figure correction accuracy reaching λ/50-λ/100 and surface roughness reduced to below 0.1nm RMS. It is the final finishing technology for "ultra-high precision components" such as EUV lithography machine objectives and laser interferometer standard lenses;
- Wide Material Adaptability: It is not limited by the hardness or brittleness of lens materials and can machine various optical materials such as glass, sapphire, silicon carbide (SiC), and infrared crystals (e.g., germanium, zinc selenide);
2. Emerging Non-Traditional Machining Technologies: Innovative Solutions Breaking Through Material and Structural Limitations
As optical lenses develop toward "special materials" (e.g., silicon carbide, sapphire) and "complex structures" (e.g., micro-nano arrays, free-form surfaces), traditional mechanical machining technologies face bottlenecks such as "low machining efficiency" and "easy damage". Emerging non-traditional machining technologies have emerged, among which femtosecond laser machining and diamond turning are the most representative.
2.1 Femtosecond Laser Machining: A Solution for Hard-Brittle Materials and Micro-Nano Structures
(1) Technical Principle
Femtosecond laser machining utilizes the nonlinear interaction between "ultra-fast, ultra-strong laser pulses" and materials: the laser pulse width is only 10⁻¹⁵ seconds (femtosecond level), with a peak power of up to 10¹²W. Energy is focused on a micro-region of the material (diameter < 1μm) in an extremely short time. Before the material can diffuse the energy through thermal conduction, it is directly ionized, vaporized, and removed (the "cold machining" effect), achieving material removal without thermal damage.
(2) Core Application Scenarios
- Machining of Hard-Brittle Materials: For materials prone to cracking in traditional mechanical machining, such as glass, sapphire, and silicon carbide, femtosecond lasers enable "crack-free cutting" and "high-precision grinding", e.g., edge chamfering of LiDAR lenses;
- Micro-Nano Structure Fabrication: Through laser direct writing and interference lithography, diffractive optical elements (DOEs) and anti-reflection micro-nano arrays (e.g., moth-eye structures) are fabricated on the lens surface, improving the optical performance of the lens (e.g., anti-reflection rate < 0.1%);
- Internal 3D Machining: Lasers can penetrate transparent materials (e.g., quartz glass) and focus on specific depths inside for "3D engraving", used in the fabrication of microfluidic chips and integrated optical waveguide components.
2.2 Diamond Turning: An Efficient Machining Solution for Soft Materials and Complex Surfaces
(1) Technical Principle
Diamond turning relies on "natural single-crystal diamond tools" and is implemented with ultra-precision CNC lathes (motion accuracy < 50nm): the cutting edge radius of natural single-crystal diamond tools can be ground to 5-10nm, with a hardness as high as HV10000 (far exceeding that of metals and infrared crystals). Through high-precision transmission components such as aerostatic guideways and torque motors, the lathe drives the tool to move along a preset trajectory with nanoscale precision, turning the workpiece to directly form complex surfaces.
(2) Core Applications and Limitations
- Machining of Soft Materials: Suitable for non-ferrous metals such as oxygen-free copper and aluminum alloys, as well as infrared crystals such as germanium (Ge), silicon (Si), and zinc sulfide (ZnS). It can efficiently machine LiDAR infrared lenses and infrared thermal imaging lenses;
- Complex Surface Forming: It can directly machine aspheric and free-form surfaces (e.g., quadratic surfaces, high-order surfaces) without subsequent polishing, with machining efficiency 10-20 times higher than traditional grinding. It is the core technology for mass production of infrared optical components;
- Limitations: It cannot machine hard-brittle materials such as glass and sapphire (tool wear rate > 1μm per minute), and the machined surface has "turning textures", which require subsequent polishing to meet high-precision optical requirements.
3. Complete Machining Process of Ultra-Precision Optical Lenses: A Closed-Loop System from Blank to Finished Product
The manufacturing of ultra-precision optical lenses is not the application of a single technology, but a closed-loop process of "multi-technology collaboration and multi-stage progression". Each stage is closely linked, ultimately realizing the transformation from lens blanks to nanoscale precision finished products. The specific process is as follows:
3.1 Rough Shaping: Preliminary Machining to Approach the Target Shape
Using blanks of materials such as optical glass and sapphire as raw materials, conventional CNC milling machines or grinding wheel grinders are used to remove most of the excess material, processing the lens into a state "approaching the target shape". For example, a circular blank is processed to the required diameter (e.g., φ50mm), with surface figure error controlled within 10-50μm, and a machining allowance of 5-10μm reserved for subsequent fine grinding. The core goal of this stage is to "efficiently remove excess material" without pursuing high precision.
3.2 Fine Grinding: Surface Figure Correction Laying the Foundation for Polishing
An ultra-precision CNC grinder (e.g., spindle speed > 10,000rpm) equipped with a diamond grinding wheel (grain size 800-2000#) is used for fine grinding of the roughly shaped lens. Through the CNC system controlling the movement trajectory and pressure of the grinding wheel, the surface figure error is gradually reduced to 1-3μm, and the surface roughness is reduced to Ra 0.1-0.5μm. At the same time, the subsurface damage layer (depth > 1μm) caused by rough grinding is removed, providing a flat, low-damage surface for the subsequent polishing process.
3.3 Polishing and Surface Figure Correction: The Core Stage of Precision Improvement
This stage is divided into two steps: "rapid damage removal" and "mid-to-high frequency error correction":
- Step 1 (CCOS Polishing): CCOS technology is used, combined with a polyurethane polishing pad and cerium oxide polishing fluid, to quickly remove the subsurface damage layer remaining from fine grinding. The surface figure accuracy is improved to λ/10-λ/20 (λ = 632.8nm), and the surface roughness is reduced to Ra 0.01-0.05μm;
- Step 2 (MRF Correction): Aiming at mid-to-high frequency surface figure errors (e.g., astigmatism, local depressions) remaining after CCOS machining, MRF technology is used for fine correction. The surface figure accuracy is further improved to λ/20-λ/30, ensuring the "mid-frequency smoothness" of the lens surface (error < λ/50 in the spatial frequency range of 10-100lp/mm).
3.4 Final Finishing: The Ultimate Breakthrough in Atomic-Level Precision
For components with "highest precision requirements" such as EUV lithography machine objectives and high-precision interferometer standard lenses, atomic-level final finishing is required:
- IBF technology is used to perform atomic-level material removal on the lens surface in a high-vacuum environment, improving the surface figure accuracy to λ/50 or higher (some EUV lenses can reach λ/100), and controlling the surface roughness to < 0.5nm RMS;
- After finishing, detection is performed using equipment such as atomic force microscopes (AFM) and white light interferometers to ensure that the surface figure accuracy and surface roughness of the lens fully meet the design requirements, ultimately completing the manufacturing of ultra-precision optical lenses.
Conclusion
The manufacturing of ultra-precision optical lenses is the product of interdisciplinary integration of "mechanical engineering", "optical engineering", and "materials science". Traditional mechanical machining technologies (CCOS, MRF, IBF) ensure mass production precision and efficiency, while emerging non-traditional machining technologies (femtosecond laser, diamond turning) break through the limitations of materials and structures. From micron-level rough grinding to atomic-level finishing, each technological advancement drives optical lenses toward "higher precision, more complex structures, and wider material adaptability", providing core support for performance breakthroughs in cutting-edge equipment. In the future, with the integration of new technologies such as quantum dot lasers and nanoimprinting, the machining precision of ultra-precision optical lenses is expected to reach a new "sub-nanoscale" height, further expanding the boundaries of optical applications.