How to reduce the surface roughness of optical lens substrate?

Reducing the surface roughness of optical lens substrate (such as quartz stone, glass, etc.) is the core link to ensure its optical performance (such as transmittance and imaging quality), which needs to be optimized from multi-dimensional systems such as material characteristics, processing technology, tool selection and environmental control. The following are specific methods and key technical points:

First, optimize the machining process chain: gradient control from rough machining to ultra-precision polishing.

The reduction of surface roughness should be realized by "gradually removing the damaged layer", and each process should lay the foundation for the next one to avoid the amplification of the previous defects.

Cutting/forming stage: reduce initial damage

Diamond wire cutting (instead of traditional grinding wheel cutting) is adopted: the wire diameter is ≤0.3mm, edge collapse and surface cracks are reduced by uniform grinding, and the roughness of the cutting surface should be controlled at Ra≤1μm (to provide a low defect base for subsequent grinding).

Avoid high-speed cutting: linear speed ≤15m/s, feed speed ≤2mm/min, and reduce surface melting or microcracks caused by friction overheating (especially for brittle materials such as quartz stone).

Polishing stage: eliminate macro unevenness and damage layer.

Gradient transition from rough grinding to fine grinding:

Rough grinding: use 80#-200# diamond grinding wheel to remove cutting marks, and grind off the surface by 0.1-0.3mm (which needs to exceed the depth of cutting damage layer by 5-10μm), and reduce the roughness to Ra ≤ 0.1 μ m;

Fine grinding: use 3-10μm diamond micro-powder to match with cast iron grinding disc, and eliminate rough grinding scratches through "micro-cutting", with the roughness controlled at Ra≤0.05μm and flatness ≤1μm/100mm (to avoid uneven subsequent polishing caused by local protrusion).

Matching of pressure and rotating speed: rough grinding pressure is 0.2-0.3MPa and rotating speed is 300-500 rpm; The fine grinding pressure is reduced to 0.1-0.2MPa, and the rotating speed is 100-200rpm, so as to avoid secondary scratches caused by abrasive embedding on the surface.

Ultra-precision polishing stage: realize nano-smooth surface.

This is the key process to reduce the roughness, and the goal is to remove the sub-micron scratches left in fine grinding and make the roughness reach Ra≤0.5nm (optical grade requirement).

Second, the precise control of polishing process parameters

The core of ultra-precision polishing is to realize the atomic removal of materials through "chemical-mechanical synergy", and the following parameters need to be optimized:

Polishing agent selection

Colloidal silicon dioxide (SiO₂) or cerium oxide (CeO₂) polishing solution is preferred: the particle size is 0.01-0.1μm (the particle size distribution needs to be uniform to avoid scratching by large particles), and the concentration is 5%-10% (if the concentration is too high, it will easily lead to particle residue, and if it is too low, the efficiency will be insufficient).

PH value of polishing solution: adjust according to the substrate material (for example, quartz stone is neutral to weakly alkaline, pH=7-9, to enhance chemical dissolution; The glass substrate can be acidic, pH=4-6), which promotes the coordination of chemical stripping and mechanical grinding of surface atoms.

Polishing pressure and rotating speed

Pressure: 0.05-0.15MPa (excessive pressure will cause the substrate to heat and deform, or make the polishing agent particles press into the surface to form scratches; If the pressure is too small, the removal efficiency is low, and it is easy to cause surface oxidation due to too long polishing time).

Rotating speed: 50-150rpm (low speed can reduce the uneven distribution of polishing solution caused by centrifugal force and avoid excessive polishing of edges; At the same time, reduce the friction heat between the substrate and the polishing pad and reduce thermal damage).

The polishing pad adapts to the environment.

Material of polishing pad: polyurethane (PU) or felt pad (hardness Shore A 60-80) shall be selected, and the surface shall be flat and have certain porosity (convenient for storing polishing solution and discharging debris). It shall be soaked with deionized water before use to avoid hard contact scratch in dry state.

Regularly dressing the polishing pad: after processing 10-20 substrates, gently polish the surface of the polishing pad with a diamond dresser to remove residual abrasive chips and aging layers to ensure its uniformity.

Third, cleaning and pollution control: avoid secondary damage

Impurities (such as abrasive chips, dust and metal particles) during machining are important reasons for the increase of surface roughness, which should be strictly controlled:

Inter-process cleaning

Rinse with deionized water (resistivity ≥ 18.2 m Ω cm) immediately after grinding/polishing to remove abrasive and debris attached to the surface;

Adopt ultrasonic cleaning (power 300-500W, frequency 40kHz): clean for 5-10min after fine grinding and 15-20min after polishing, and cooperate with neutral cleaning agent (such as electronic detergent) to completely remove submicron pollutants.

Environmental cleanliness

The polishing process should be carried out in a Class 1000 clean room (particles ≥0.5μm per cubic foot ≤1000). Operators should wear dust-free clothes and latex gloves to avoid skin grease or dust from contacting the surface of the substrate.

Cleaning of equipment and tools: the table top and fixture of polishing machine should be wiped with alcohol, and the polishing solution storage tank should be filtered regularly (with 0.2μm filter membrane) to prevent large particles from mixing.

Four, material characteristics adaptation and equipment stability

Adjust the process according to the substrate material

Brittle materials (such as quartzite and sapphire): it is necessary to reduce the mechanical stress in each process, and give priority to the auxiliary removal of materials by chemical action (such as improving the chemical activity of polishing solution) to reduce the surface unevenness caused by brittle fracture;

Tough materials (such as some optical glasses): the polishing pressure can be appropriately increased and removed by mechanical grinding, but the temperature should be controlled to avoid surface softening and deformation.

Accuracy and stability of equipment

Polishing machine should have constant pressure control (pressure fluctuation ≤±0.01MPa) and rotational speed stability (rotational speed fluctuation ≤±5rpm) to avoid surface fluctuation caused by sudden change of pressure/rotational speed;

Vibration control of equipment: the foundation should be treated with shock-proof treatment (vibration amplitude ≤1μm) to avoid external vibration from being transmitted to the polishing interface, resulting in scratches or uneven roughness.

V. Detection and feedback: real-time monitoring and adjustment of the process.

On-line detection: During the polishing process, the surface roughness is sampled by laser confocal microscope (resolution ≤0.1nm) or atomic force microscope (AFM) every 10-15min, and the polishing time, pressure or polishing solution concentration are adjusted according to the results.

End point judgment: When the surface roughness reaches the target value (such as Ra≤0.5nm) and there are no scratches and pits, stop polishing immediately to avoid dimensional deviation or surface oxidation caused by excessive polishing.

summary

To reduce the surface roughness of optical lens substrate, we should follow the principle of "gradient removal, chemical-mechanical cooperation and full cleaning": initial damage should be reduced by cutting and polishing, atomic removal should be achieved by ultra-precision polishing, and polishing parameters, clean environment and equipment stability should be strictly controlled, so as to finally achieve an ultra-smooth surface that meets the optical performance requirements.