What should be paid attention to when processing optical lens substrate?

Optical lens substrates (mainly made of glass, such as Shi Ying, K9, sapphire, etc.) are the core basic parts of optical lenses. The processing core requirements are ultra-high surface accuracy, no surface damage, accurate geometric dimensions, and low stress deformation. The processing process is prone to problems such as scratches, edge chipping, surface shape error out of tolerance, internal stress cracking, etc., which need to focus on five cores: raw material protection, processing environment, process control, precision detection, and cleaning protection. The following are the key points for attention in the whole process of actual processing and production * *, covering core processes such as rough machining, finishing, polishing, cleaning and testing, which can be directly implemented:

First, basic control before processing: environment+raw materials, avoiding defects from the source.

Optical substrates have strict requirements on the processing environment and the initial state of raw materials, and tiny environmental impurities and bumps of raw materials will lead to subsequent processing and scrapping. Pre-control is the basis:

Dust-free and clean environment requirements

The processing area shall meet the standards of a thousand-class/ten-thousand-class clean workshop (polishing/fine grinding process shall be more than a thousand-class), equipped with fresh air filtration and constant temperature and humidity systems, and the ambient temperature shall be controlled at 20 2℃ and the humidity shall be 45%~65% (large temperature fluctuation will easily lead to thermal deformation of the substrate, high humidity will easily lead to water stains and low humidity will easily lead to electrostatic adsorption dust);

The floor and wall of the workshop are made of antistatic and easy-to-clean materials, and the processing equipment is sealed and dustproof to avoid dust and particles adhering to the surface of the substrate and causing processing scratches.

Protection of raw materials and tooling

The raw materials of the substrate need to be independently packed with anti-static (pearl cotton+dust-free sealed bag) after storage, and should be handled with care. Stacking and bumping are prohibited. Special vacuum suction cups or anti-static rubber claws should be used during handling to avoid direct contact with the surface of the substrate (grease and sweat on the hands will pollute the surface, which will be difficult to clean later and affect the processing accuracy);

The tooling (grinding disc, polishing pad, fixture and cutter) used for processing should be cleaned in advance and treated with anti-static. Soft anti-static rubber/foam should be pasted at the contact part between the fixture and the substrate to avoid pinching and crushing the edge of the substrate, and the positioning accuracy of the fixture should be ≤ 0.005 mm..

Equipment calibration

Before processing, all equipment (cutting machine, grinder, polisher and centering edger) shall be calibrated for accuracy, including spindle runout, table flatness and feed accuracy. The radial runout of spindle shall be ≤0.001mm, and the table flatness shall be ≤0.002mm/100mm, so as to avoid out-of-tolerance of geometric size and surface shape errors of the substrate caused by equipment accuracy deviation.

Second, the rough machining process: sizing+shape modification, focusing on preventing edge collapse and breakage.

Rough machining mainly includes slicing, edge grinding and rough grinding. The core is to process the glass blank to the geometric size close to the finished product. This process is prone to edge collapse, corner damage and deep scratches on the surface. The key points of control are micro-force cutting, accurate positioning and tool adaptation:

Slicing process (blank cutting)

Adopt diamond wire cutting machine/internal circle cutting machine (ordinary grinding wheel cutting machine is prohibited, which is easy to cause edge collapse and deep scratches), the cutting speed is adjusted according to the glass material (for example, the cutting line speed of K9 glass is ≤5m/s, and sapphire is ≤3m/s), and the feed rate is micro (0.01~0.05mm/time);

When cutting, use special cooling liquid (special cutting fluid for optical glass, without impurities and corrosiveness), and fully cool down to avoid microcracking of glass caused by high temperature during cutting; After cutting, the edge chipping of the substrate should be controlled at **≤0.05mm**, and the surface scratch depth should be ≤ 0.02mm..

Edge grinding process (profile/centering edge grinding)

A diamond grinding wheel centering edging machine is adopted to precisely edge the substrate according to its designed shape (round, square and irregular), and a two-step method of rough grinding and fine grinding is adopted during edging, and the rough grinding is used to remove the blank allowance, and the edge precision is finely trimmed, and the tolerance of the outer diameter/side length of the substrate is controlled to be less than or equal to ≤0.005mm and the concentricity is less than or equal to ≤0.003mm；;

When edging, the cooling liquid continuously washes the edging part to take away the abrasive chips, so as to avoid the abrasive chips from scratching the substrate for the second time. The edge chamfer is processed according to the design requirements (usually C0.1~C0.3mm) to prevent the corner from cracking in the subsequent process.

Rough grinding process (surface thinning)

Using a double-sided grinder, diamond abrasive (particle size W40~W20) is used for double-sided synchronous rough grinding, and the substrate is ground to the nominal thickness of the finished product of+/-±0.01mm, and the grinding pressure (0.1~0.3MPa) is controlled during grinding, so that the pressure is even without unbalanced load (unbalanced load will lead to uneven thickness of the substrate);

After rough grinding, the surface of the substrate should be free of deep scratches and pits, with flatness ≤0.01mm and parallelism ≤ 0.005 mm.

3. Finishing process: fine grinding+polishing, with core accuracy and no surface damage.

Finishing is the key to determine the surface quality of optical substrates, including fine grinding, polishing, centering and edge trimming. The core requirements are ultra-high surface accuracy, no subsurface damage and surface shape error reaching the standard, which is the top priority of processing control:

Fine grinding process (removing coarse wear layer)

Following the rough grinding process, fine-grained diamond abrasives (W14~W5) are used for double-sided fine grinding, the core purpose of which is to remove the surface/subsurface damage layer caused by rough grinding (the depth of the damage layer should be controlled at ≤ 0.001 mm);

During fine grinding, the grinding pressure is reduced (0.05~0.15MPa), the rotating speed of the grinding disc is increased (30~50r/min), and the cooling liquid is high-purity deionized water and optical grinding liquid, so as to ensure the uniform dispersion of the abrasive. After fine grinding, the surface roughness Ra of the substrate is less than or equal to 0.02 μ m, the flatness is less than or equal to ≤0.003mm, and the thickness tolerance is less than or equal to 0.002 mm..

Polishing process (core ultra-precision machining)

Double-sided polisher/single-sided polisher (single-sided polishing for high-precision substrates) is adopted, the polishing pad is made of soft wear-resistant materials such as polyurethane and felt, and the polishing solution is made of optical special polishing powder (particle size W3.5~W0.5, high purity ≥99.9%) such as cerium oxide (CeO₂) and silicon dioxide (SiO₂), and the hardness of the polishing powder is accurately matched according to the glass material;

When polishing, control the polishing pressure (0.02~0.08MPa), rotating speed (20~40r/min) and slurry flow, and apply the pressure evenly to avoid the distortion of the substrate surface caused by excessive local pressure; Monitor the surface quality in real time during polishing, and prohibit excessive polishing (which will lead to out-of-tolerance of surface shape error);

After polishing, the substrate should meet the core indicators: surface roughness Ra≤0.001μm (mirror effect), no scratches, pits and fog spots, surface shape error (aperture N)≤1, local aperture δ n ≤ 0.5, and no subsurface damage.

Secondary centering edging

After polishing, the substrate is subjected to high-precision centering, edging and trimming, and the tiny shape deviation generated in the polishing process is corrected to ensure that the center thickness deviation of the substrate is less than or equal to ≤0.001mm, the concentricity is less than or equal to ≤0.002mm, and the edge is free from edge collapse and burr, so as to prepare for subsequent coating and assembly.

Four, cleaning and drying process: no residue, no water stains, to avoid surface pollution.

Impurities such as polishing powder, cooling liquid, grease, etc. will be attached to the polished surface of the optical substrate. If it is not thoroughly cleaned, it will lead to subsequent coating and demoulding, and the image quality will be degraded. The core of cleaning and drying is no residue, no water stain and no secondary pollution:

Multi-step cleaning process

Adopt * * "ultrasonic cleaning+immersion cleaning+high-pressure deionized water washing" * * three-step method, use high-purity deionized water (resistivity ≥ 18m Ω cm) in the whole process, and tap water (containing minerals, which will produce water stains) is prohibited;

(1) ultrasonic cleaning: divided into 2~3 tanks, the frequency is 28kHz (coarse cleaning, removing large particles of polishing powder) +80kHz (fine cleaning, removing small particles), the cleaning liquid is an optical cleaning agent (non-corrosive, easy to degrade), the cleaning time is 5~10min/tank, and the temperature is 40~50℃ (to improve the cleaning efficiency);

(2) soaking and cleaning: soaking in deionized water and a small amount of cleaning agent for 5-8min to dissolve the residual grease on the surface;

③ High-pressure washing: washing the surface/edge of the substrate from multiple angles with 0.1~0.2MPa of high-pressure deionized water to take away the residual tiny impurities.

Drying process

Immediately after cleaning, the substrate is dried in a dust-free drying box, and the combined mode of hot air drying (60-80 DEG C) and nitrogen air drying is adopted, so that the nitrogen purity is more than or equal to 99.99%, and dust in the air is prevented from being adsorbed on the surface of the substrate during the drying process;

After drying, the surface of the substrate should be free of watermarks, stains and residual particles, which can be detected by laser particle counter. The number of particles on the surface of 0.5μm or more is ≤5/piece.

Protection after cleaning

The dried substrate should be immediately transferred to a dust-free sealed package, and stored independently in an anti-static dust-free box. Dust-free paper/anti-static foam is laid in the box, and bare hands are forbidden to touch it. Special vacuum sucker tooling is used for handling.

Five, the whole process of inspection and control: real-time inspection, layer by layer to prevent the flow of unqualified products.

The accuracy and surface quality of optical substrates cannot be improved by post-repair, so the whole process of * * "process self-inspection+mutual inspection+special inspection" is required, and each process can be transferred to the next one only after passing the inspection. The core inspection items and requirements are as follows:

Geometric dimension detection: detect the thickness, outer diameter/side length, concentricity and parallelism of the substrate with a high-precision thickness gauge, a two-dimensional imager and a concentricity measuring instrument, and all dimensions must meet the design tolerance (usually the tolerance is ≤ 0.005 mm);

Surface quality inspection: on a 100-level dust-free testing platform, use a high-magnification metallographic microscope (50-200 times) and a laser interferometer to inspect the surface scratches, pits and fog spots. It is required that there are no visible scratches, pits and subsurface damage;

Surface shape accuracy detection: use Fizeau interferometer and Tyman interferometer to detect the flatness, aperture and local aperture of the substrate, and the surface shape error should meet the optical design requirements (for example, the aperture n of the SLR lens substrate is less than or equal to 0.5);

Cleanliness detection: use laser particle counter and surface cleanliness detector to detect the surface particle number and stain residue to ensure the cleanliness reaches the standard;

Internal stress detection: the internal stress of the substrate is detected by a polarizing stress meter, and the internal stress is required to be ≤5nm/cm (excessive internal stress will lead to the later cracking and surface deformation of the substrate, which will affect the optical performance).

Six, special material substrate processing matters needing attention

The hardness, brittleness and thermal expansion coefficient of different optical glass materials (such as sapphire, Shi Ying, calcium fluoride, K9) are quite different, so it is necessary to adjust the process in processing to avoid defects caused by common processes:

Hard and brittle substrates (sapphire, Shi Ying): high hardness, high brittleness, easy edge collapse and microcracks. When cutting/grinding, it is necessary to reduce the feed rate and increase the coolant flow. When polishing, soft polishing pad and fine-grained polishing powder are selected, and the grinding/polishing pressure needs to be lower (≤ 0.05 MPa);

Substrates with low expansion coefficient (Shi Ying, glass-ceramics): The thermal expansion coefficient is very small, so the temperature should be strictly controlled during processing to avoid thermal stress cracking of the substrate caused by sudden change of coolant temperature, and the temperature should be slowly lowered after polishing;

Soft optical glass (K9, BK7): the hardness is low, and it is easy to produce surface scratches. When processing, it is necessary to choose finer abrasive/polishing powder, and the contact part between the tooling and the substrate should be soft protected to avoid hard contact scratches;

Infrared optical substrates (calcium fluoride and germanium chips): easy to deliquesce and scratch, and the processing environment needs lower humidity (≤40%). When cleaning, water-based cleaning agents are prohibited, and organic solvents such as anhydrous ethanol are used for cleaning. After drying, moisture-proof sealing treatment is required.

7. Core Taboo: These operations are absolutely forbidden to avoid batch scrapping.

It is forbidden to carry out fine grinding, polishing, cleaning and testing in a non-dust-free environment, because dust is the core inducement of surface scratches and particle residues;

It is forbidden for bare hands to directly touch the surface of the substrate. Whether gloves are worn or not, special tools (vacuum suction cups and anti-static jaws) are required for operation.

It is forbidden to use ordinary grinding wheels and cutters to process optical substrates. Special diamond cutters/abrasives must be used to avoid deep scratches and edge chipping.

It is forbidden to use tap water and ordinary cleaning agent to clean the substrate. High-purity deionized water and optical cleaning agent must be used to avoid water stains and impurity residues.

It is forbidden to adjust the equipment parameters (such as grinding pressure and polishing speed) at will during machining, and the parameter adjustment needs to be verified by professionals to avoid out-of-tolerance accuracy;

Non-conforming products are prohibited from circulating. Defective products detected in any process (such as scratches, edge chipping and out-of-tolerance surface errors) should be isolated immediately, and rework is prohibited (optical substrate defects cannot be completely repaired, and optical performance will still be affected after rework).

summary

The core of optical lens substrate processing is * * "extreme precision control+non-damaged surface processing+dust-free protection in the whole process" * *, and all operations need to be carried out around "avoiding surface damage, controlling surface shape accuracy, reducing internal stress and ensuring cleanliness". From the cleanness of the processing environment, to the micro-force and precise processing of each process, to the layer-by-layer inspection of the whole process, every detail directly determines the quality of the substrate, and ultimately affects the imaging accuracy and performance of the optical lens.

I can help you sort out a checklist for the whole process of optical lens substrate processing, classify it according to rough machining, fine machining, cleaning and testing, mark the core requirements and testing standards of each process, and use it directly for on-site processing control. Do you need it?