What are the precautions in the cleaning process of optical lens substrate?

Cleaning the optical lens substrate is the core process to ensure its surface cleanliness and avoid the subsequent coating and film stripping/imaging deviation. The core requirements are no particle residue, no water stains and oil stains, no surface damage and no secondary pollution. The cleaning environment, reagent consumables, process operation, drying protection and inspection and acceptance should be strictly controlled, and the cleaning schemes of substrates with different materials (such as sapphire and infrared crystal) should be adjusted accordingly. The following are the matters needing attention in the full-dimensional cleaning process that fits the actual production, which are divided into general requirements and special details, and can be directly implemented:

First, before cleaning: the basic control of environment and tooling, to eliminate pollution from the source.

Pre-protection of cleaning is the key to avoid secondary pollution. Small environmental dust and impurities in tooling will directly lead to the failure of cleaning, so all aspects of pretreatment should be done:

Cleaning environment: high-grade dust-free+constant temperature and humidity

The cleaning process should be carried out in a dust-free workshop with a grade of 1,000 or above (a local dust-free area with a grade of 100 is required for fine cleaning after polishing), equipped with fresh air high efficiency filtration (HEPA), and the ambient temperature should be controlled at 20 2℃ and the humidity should be 45% ~ 60%; Too high humidity is easy to produce water stains and electrostatic adsorption dust, too low humidity will lead to static electricity on the surface of the substrate, which will also adsorb particles; Dust and oil pollution operations are prohibited in the workshop, which is strictly isolated from the rough machining area.

Tooling: exclusive dustless+antistatic treatment

All fixtures (cleaning baskets, clamps, suction cups and tweezers) that contact the substrate should be dust-free for optical purposes, and the materials should be polytetrafluoroethylene (PTFE), polypropylene (PP) or stainless steel (316L). It is forbidden to use ordinary metal/plastic appliances (which are easy to drop chips and generate scratches).

Pre-cleaning+drying is required before the tooling is used: after ultrasonic cleaning with deionized water and optical cleaning agent, nitrogen gas is blown dry for standby, and special tooling is forbidden to be used across processes (for example, rough grinding tooling is forbidden to enter the cleaning area);

Anti-collision design should be made for the cleaning basket: soft dust-free foam/silica gel pad should be stuck inside, and the distance between substrates should be ≥5mm to avoid mutual friction and scratch during cleaning, and at the same time ensure that the cleaning solution fully contacts the surface of substrates.

Pre-inspection and protection of substrate

Before cleaning, check the substrate on the dust-free testing platform to eliminate the defective products with serious edge collapse/damage (to avoid debris falling off and polluting other substrates during cleaning); Only vacuum sucker or dust-free antistatic tweezers can be used to transport the substrate. Bare hands/ordinary gloves are forbidden to contact (grease and glove fibers will cause oil stain/fiber residue). Handle with care to avoid substrate collision.

Second, in cleaning: the core of reagent and process operation is controlled to ensure the cleaning effect.

The core of the cleaning process is to select the right reagent, control the process parameters and avoid surface damage. The parameters and operation details of different cleaning steps directly determine the cleanliness, and it is necessary to follow the cleaning logic of * * "from coarse to fine, step by step" * *:

Reagent consumables: high purity+special for optics, and substitution is prohibited.

Cleaning water: use high-purity deionized water with resistivity ≥ 18.2 mΩ cm (25℃) in the whole process, and tap water and distilled water (containing minerals and impurities, which will form water stains and residual ions) are prohibited; Deionized water needs to be filtered circularly, and the resistivity should be detected regularly to avoid pollution of the water storage tank.

Cleaning agent: use non-corrosive cleaning agent special for optical glass (such as neutral nonionic surfactant), and prohibit using strong acid, strong alkali or ordinary industrial cleaning agent (which will corrode the surface of the substrate, leave impurities and even produce microcracks); The cleaning agent needs to be diluted in proportion (usually 1%~5%), and it is now used to avoid deterioration after long-term storage.

Auxiliary gas: the nitrogen used for drying should be 99.99% or above high purity, free of oil and water, and equipped with gas filtering device to prevent oil and particles from being entrained in the nitrogen.

Core process operation: accurate parameters+standardized actions to avoid damage and residue.

The mainstream of optical substrates is the combined process of "multi-tank ultrasonic cleaning+immersion cleaning+high pressure washing", and the details of each step should be strictly controlled:

Ultrasonic cleaning: it can be divided into rough cleaning and fine cleaning, and it is forbidden to use mixed tanks; Low-frequency ultrasound of 28~40kHz is used for rough cleaning (removing polishing powder and large particles), and high-frequency ultrasound of 80~120kHz is used for fine cleaning (removing tiny particles and oil stains). Too high frequency will easily lead to microcracks on fragile substrates (such as sapphire). The ultrasonic power is controlled at 100~200W/L, the cleaning time is 5~10min/tank, and the temperature is 40~50℃ (to improve the cleaning efficiency and avoid the decomposition of cleaning agent caused by too high temperature); It is forbidden for the substrate to directly contact the ultrasonic vibrating plate (which will cause surface vibration injury and pitting), and the distance between the cleaning basket and the vibrating plate shall be ≥ 30 mm..

Soaking and cleaning: after ultrasonic treatment, transfer to the soaking tank with deionized water and dilute cleaning agent, soak for 5~8min, gently shake the cleaning basket (to avoid dead zone residue), dissolve the stubborn grease on the surface, and preliminarily rinse with deionized water after soaking.

High-pressure deionize water was: as a key step of cleaning finishing, that surface, edges and chamfer of the substrate (which are the hardest hit areas of particle residues) are washed from multiple angles (up and down, left and right) by using 0.1-0.2 MPa high-pressure deionized water, and the washing time is ≥2min to ensure that all residual reagents and tiny particles on the surface are washed clean; It is forbidden to spray high-pressure water directly on the substrate at a single point (which may easily lead to surface damage), and the distance between the nozzle and the substrate is ≥10cm.

Changing tanks and controlling water: avoiding cross contamination.

When the substrate is transferred from one tank to another, the surface liquid needs to be drained quickly (it is forbidden to drip to the ground/workbench) to avoid the polluting reagent in the previous tank from being brought into the subsequent fine washing tank; Deionized water in each tank body needs to overflow continuously to ensure the water quality in the tank is clean, and the tank liquid should be replaced regularly (usually once every 8~12h, or in time according to the water quality test results).

Third, after cleaning: drying+protection to avoid water stains and secondary pollution.

The surface of the cleaned substrate is in a state of high humidity. If it is not dried in time and the protection is not in place, it is easy to produce water stains and absorb dust. This link is the "finishing key" of the cleaning process, and it needs to be dried quickly and protected from dust throughout:

Drying process: no watermark, no particles, precise temperature control and humidity control.

Transfer the substrate to the drying equipment immediately after cleaning (avoid exposure to air for more than 1min), and the main drying method and matters needing attention:

Hot air+nitrogen combined drying: using an optical special dust-free drying box, firstly introducing clean hot air at 60~80℃ to preliminarily dry the surface moisture, and then using high-pressure and high-purity nitrogen to blow the surface of the substrate from multiple angles (especially the edges and chamfers) until it is completely dried; It is forbidden to dry with high-temperature hot air alone (it is easy to produce water stains and thermal stress).

Vacuum drying/freeze drying: It is suitable for special substrates (such as infrared substrates such as calcium fluoride and germanium chips) which are easy to deliquesce and are afraid of high temperature, so as to avoid water stains. The vacuum degree is controlled at ≤100Pa and the drying temperature is ≤40℃.

After drying, wait until the substrate is naturally cooled to room temperature before taking it out (taking it out at high temperature is easy to absorb water vapor and dust in the air due to temperature difference).

Protection after cleaning: dust-free seal, special tooling for the whole process.

The dried substrate needs to be transferred on a 100-stage dust-free operation table, and only vacuum sucker/dust-free tweezers are used to avoid any hard contact;

Immediately put it into an optical special dust-free sealed box, with dust-free paper/anti-static dust-free foam laid in the box, and the single substrate placed independently (to avoid stacking friction). Desiccant (silica gel desiccant, which needs to be wrapped with dust-free gauze) can be placed in the box, and the material identification is affixed outside the sealed box.

The storage area shall be in a dust-free, dry and light-proof environment. It is forbidden to mix with oil, acid, alkali and dust. Handle with care to avoid violent shaking.

4. Substrates made of different materials: special precautions for targeted cleaning

The materials of optical lens substrates are quite different (common optical glass, sapphire, Shi Ying, infrared crystal, etc.), and their physical and chemical properties are different. Therefore, the cleaning scheme needs to be adjusted differently to avoid damage or unclean cleaning caused by common processes:

Ordinary optical glass (K9, BK7): the material is soft and easy to scratch, the ultrasonic power is appropriately reduced (≤150W/L), and the cleaning basket is more tightly protected to prevent the substrates from rubbing against each other; Conventional water-based cleaning agent and ultrasonic cleaning scheme can be adopted.

Hard brittle substrates (sapphire, Shi Ying, silicon wafer): high hardness but high brittleness, easy to produce microcracks and edge collapse, reduce the ultrasonic frequency (rough cleaning ≤30kHz, fine cleaning ≤80kHz), shorten the ultrasonic time (3~5min/tank), prohibit direct injection of high-pressure water, and reduce the washing pressure to 0.08~0.15MPa.

Infrared optical substrates (calcium fluoride, magnesium fluoride, germanium flake, zinc selenide): water-repellent/deliquescent/corrosive. It is forbidden to clean them with water-based cleaning agents and deionized water. Organic solvents such as anhydrous ethanol and isopropanol should be used for ultrasonic cleaning. After drying, nitrogen sealing and moisture-proof packaging should be done, and the environmental humidity should be controlled at ≤40%.

Substrate before coating: the cleanliness requirement is higher, and the plasma cleaning step should be added after conventional cleaning to remove trace organic pollutants on the surface and improve the adhesion of coating; Plasma cleaning needs to control power and time to avoid damaging the surface of the substrate.

V. Cleaning process: detection and control, real-time control of cleanliness, and elimination of nonconforming products.

The cleaning effect of optical substrates cannot be repaired later, so it is necessary to implement * * "process inspection+complete inspection of finished products" * *, and each batch is randomly sampled. Only after passing the complete inspection can it be transferred to the next process (coating/assembly):

Process inspection: regularly test the resistivity of deionized water, concentration of cleaning agent, ultrasonic power and cleanliness of dust-free workshop to ensure that all parameters meet the requirements; Observe the surface of the cleaned substrate, and there is no obvious water stain, oil stain and particles.

Finished product inspection: conducted on a 100-level dust-free inspection platform. The core inspection items are:

Visual inspection: under the high-brightness dust-free lamp, observe the surface of the substrate from multiple angles, and there are no visible water stains, oil stains, scratches and particles;

Particle detection: using laser particle counter to detect, it is required that the number of particles on the surface of the substrate is more than 0.5μm ≤5/piece, and the number of particles above 1μm ≤1/piece;

Water stain/ion detection: use pure water contact angle tester to detect, and the contact angle is ≤ 10 (the surface is free of oil stain, uniform hydrophilic and no water stain residue); Or detect that residual ions on the surface by an ion chromatograph, wherein the ion content is less than or equal to 10 ng/cm;

Surface damage detection: observed by 50~100 times metallographic microscope, there are no microcracks, pits and vibration injuries caused by cleaning.

Disposal of unqualified products: substrates that are unqualified after cleaning (such as particle residues and water stains) can be washed once again (repeated cleaning is prohibited, which is easy to damage the surface). If they are still unqualified after washing again, they will be scrapped directly and circulation is prohibited.

Six, cleaning process: the core taboo and operation specification, to avoid batch scrap.

Cleaning, drying and testing are prohibited in non-dust-free environment, and dust is the first cause of cleaning failure;

It is forbidden to replace high-purity deionized water with ordinary tap water and distilled water, and it is forbidden to replace optical special cleaning agents with industrial cleaning agents;

It is forbidden for the substrate to directly contact the ultrasonic vibrating plate and the single point of high-pressure water direct injection to avoid surface vibration injury and impact injury;

Bare hands, ordinary gloves and chemical fiber rags are forbidden to touch the surface of the substrate, and it is forbidden to wipe the substrate with rags (easy to drop chips and scratch);

It is forbidden to expose the cleaned substrate in the air for a long time, and it is forbidden to put the uncooled substrate directly into the sealed box (it is easy to produce condensation water stains);

It is forbidden to clean substrates with different materials and different processing procedures to avoid cross-contamination and targeted process failure;

It is forbidden to operate for a long time with ultra-frequency and ultra-power during ultrasonic cleaning, especially for brittle substrates, which is easy to produce irreversible microcracks.

summary

The core logic of optical lens substrate cleaning is * * "Dust-free protection throughout the whole process, high purity of reagent consumables, precise process parameters and zero delay in drying protection" * *, and all operations are centered on "removing all pollutants without causing any surface damage". From the pretreatment of environmental tooling before cleaning, to the control of ultrasonic and washing parameters during cleaning, and then to the drying and sealing after cleaning, every detail directly determines the cleanliness of the substrate, and ultimately affects the coating quality and imaging accuracy of the optical lens.