How to manufacture high-precision 99.999% concave reflecting mirrors?

How to Manufacture High-Precision 99.999% Concave Reflecting Mirror (Technical Instructions)

UltraOpto High-Precision 99.999% Concave Reflecting Mirror features ultra-high reflectivity, precise surface curvature, excellent surface quality and environmental stability, and is widely used in deep-ultraviolet lithography, excimer laser systems, space optics, high-precision spectral analysis and other high-end optical scenarios. The 99.999% high reflectivity means that the light loss is less than 0.001%, which puts forward extremely strict requirements on the substrate material, surface processing precision and film system design. The following is a detailed standardized manufacturing process, focusing on the core technology and precision control points to ensure the product meets the high-performance requirements.

I. Core Manufacturing Premise: Substrate Selection and High-Purity Pretreatment

The substrate is the basis of the concave mirror's shape accuracy and stability. For 99.999% high reflectivity requirements, the substrate must have high optical uniformity, low thermal expansion coefficient and excellent surface machinability.

Substrate Material Selection

For deep-ultraviolet bands (such as 193nm), high-purity fused silica (UV grade) is preferred, with a purity of ≥99.999%, low hydroxyl content (≤10ppm), and thermal expansion coefficient ≤0.55×10⁻⁶/℃, avoiding thermal deformation affecting curvature accuracy.

For infrared bands, monocrystalline silicon or germanium is selected, with high thermal conductivity and low infrared absorption, ensuring stable reflectivity in the target band.

The substrate must be free of bubbles, inclusions, stripes and other internal defects, and the optical uniformity Δn ≤ 2×10⁻⁶, which is verified by a laser interferometer.

High-Purity Substrate Pretreatment

Precision Cutting: 

Use a diamond wire saw for low-temperature cutting to reduce internal stress of the substrate, with a cutting size tolerance controlled within ±0.1mm, and the thickness deviation ≤ ±0.05mm.

Stress Relief Annealing: 

Put the cut substrate into an annealing furnace, heat it to 800-900℃ at a rate of 50℃/h, keep it warm for 6-8 hours, and cool it down to room temperature at a rate of 30℃/h to completely release the internal stress caused by cutting and avoid subsequent processing deformation.

Ultra-Cleaning: 

Carry out three-level purification treatment - first, ultrasonic cleaning with neutral high-purity cleaning agent for 30 minutes to remove surface oil and particles; then, plasma cleaning for 15 minutes to remove nanoscale contaminants and activate the substrate surface; finally, blow dry with high-purity nitrogen (99.999%) and send it to a Class 10 clean room for standby, with the surface particle size ≤ 0.1μm.

II. Core Manufacturing Process: Precision Shaping and Super-Smooth Polishing

The concave surface curvature accuracy and surface roughness are the key factors affecting the reflectivity and light focusing effect. This link needs to rely on high-precision equipment and professional technology to achieve nanoscale precision control.

Concave Surface Mold Design and Preparation

According to the design requirements of the concave mirror (radius of curvature R, aperture D, focal length f), use computer-aided design (CAD) to draw the mold 3D model, and use CNC precision machining to process the mold core with high-speed steel or tungsten carbide, with the mold curvature accuracy ≤ ±0.05μm.

The mold surface is polished to a roughness Ra ≤ 0.01μm, and a release agent is sprayed to avoid adhesion between the substrate and the mold during the shaping process.

Precision Shaping of Concave Surface

Adopt the hot pressing shaping process for the pretreated substrate: heat the substrate and mold to the softening temperature of the material (fused silica is about 1200℃) in a vacuum furnace (vacuum degree ≥ 10⁻⁵Pa), apply uniform pressure (5-10MPa), keep it warm for 2-3 hours, and then cool it down slowly to room temperature at a rate of 20℃/h.

Real-time monitoring of the curvature during the hot pressing process by a laser displacement sensor, with the curvature deviation controlled within ±0.1μm to ensure the consistency of the concave surface shape.

For small-aperture concave mirrors, the computer numerical control (CNC) grinding process is adopted, with a five-axis CNC grinder for precise grinding, and the feed rate is controlled at 0.001mm/r to gradually form the concave surface, with the preliminary curvature accuracy reaching ±0.5μm.

Super-Smooth Polishing (Core Precision Process)

Rough Polishing: 

Use a polyurethane polishing pad and a cerium oxide polishing liquid (particle size 0.5μm) for rough polishing, remove the grinding damage layer, and reduce the surface roughness to Ra ≤ 0.1μm, with the curvature accuracy corrected to ±0.2μm.

Fine Polishing: 

In a Class 10 clean room with constant temperature (20±0.1℃) and constant humidity (40±5%RH), use a soft pitch polishing pad and a colloidal silica polishing liquid (particle size 0.05μm) for fine polishing. The polishing pressure is controlled at 0.1-0.3MPa, and the polishing speed is 30-50r/min.

Real-time Precision Detection: 

During polishing, use a phase-shifting interferometer to detect the surface shape and roughness in real time. Adjust the polishing parameters according to the detection results until the surface roughness Ra ≤ 0.01μm, the surface shape accuracy reaches λ/[email protected], and there are no scratches, pits or other surface defects (surface quality reaches 40-20 standard).

Edge Chamfering: 

Use a diamond chamfering wheel for precision chamfering, with a chamfer size of 0.2-0.5mm×45°, to remove sharp edges, avoid edge chipping and reduce light scattering at the edge.

III. Key Process for High Reflectivity: Ultra-High Precision Film System Deposition

The 99.999% high reflectivity is mainly achieved by the design and deposition of the multi-layer dielectric film system. The film layer thickness and uniformity directly determine the reflectivity effect.

Customized Design of High-Reflectivity Film System

Adopt the multi-layer alternating film system of high-refractive index material (H) and low-refractive index material (L), such as TiO₂/SiO₂ for ultraviolet band and Ta₂O₅/SiO₂ for visible light band.

Use TFCalc optical simulation software to optimize the film system structure. For 99.999% reflectivity, the number of film layers is usually 50-100 layers. By adjusting the optical thickness of each layer (usually λ/4), the constructive interference of the target band light is realized, and the reflectivity is maximized.

Simulate the influence of film thickness deviation on reflectivity, and preset the compensation parameters during deposition to ensure that the film thickness deviation is ≤ ±0.3nm.

Ultra-High Precision Vacuum Deposition

Vacuum Environment Construction: Put the polished concave mirror substrate into an ultra-high vacuum coating machine, and pump the vacuum degree to ≥ 10⁻⁷Pa. Use a cryogenic cold trap to remove residual water vapor and oxygen, with the residual gas content ≤ 1×10⁻⁸Pa, to avoid film oxidation.

Precision Deposition Control: 

Adopt ion beam assisted deposition (IAD) technology, with high-purity target materials (purity ≥ 99.999%), and control the deposition rate at 0.1-0.2nm/s. Use a quartz crystal oscillator and an ellipsometer for real-time monitoring of the film thickness, with the thickness accuracy controlled within ±0.2nm.

Uniformity Control: 

Use a planetary rotation mechanism to rotate the substrate during deposition, with the rotation speed controlled at 5-10r/min, to ensure that the film thickness uniformity of the entire concave surface is ≤ ±1%. For large-aperture concave mirrors, use a mask to adjust the deposition rate distribution and improve the uniformity.

Film System Annealing and Stabilization Treatment

After deposition, put the concave mirror into an annealing furnace for heat treatment, heat it to 150-200℃ at a rate of 20℃/h, keep it warm for 4-6 hours, and cool it down to room temperature slowly to release the film stress, improve the film adhesion (≥ 5B level according to ISO 15184 standard), and avoid film peeling and cracking.

IV. Full-Process Quality Control and Precision Detection

To ensure that each concave mirror meets the 99.999% high reflectivity standard, a full-process quality control system is implemented, with 100% inspection of finished products.

Key Performance Detection Items and Methods

Reflectivity Detection: Use a laser reflectometer to detect the reflectivity of the target band, with the detection accuracy ≤ ±0.0001%. Ensure that the reflectivity reaches 99.999% and the reflectivity uniformity of the entire surface is ≤ ±0.0005%.

Surface Shape and Roughness Detection: Use a phase-shifting interferometer to detect the surface shape accuracy, and an atomic force microscope (AFM) to detect the surface roughness, ensuring that the indicators meet the design requirements.

Environmental Reliability Detection: 

Sample detection is carried out, including high-low temperature cycle test (-40℃~85℃, 500 cycles), humidity test (85%RH, 500h), and laser damage threshold test (≥ 500mJ/cm²@193nm, 10ns pulse). After the test, the reflectivity attenuation is ≤ ±0.0002%, and there is no film damage.

Geometric Size Detection: Use a coordinate measuring machine (CMM) to detect the aperture, thickness, radius of curvature and other dimensions, with the tolerance controlled within ±0.05mm.

Unqualified Product Handling and Archiving

Unqualified products (such as reflectivity not up to standard, surface defects) are isolated and analyzed for the cause. Repairable products are re-polished and re-coated, and unrepaired products are scrapped.

For qualified products, a detailed test report is issued, including reflectivity curve, surface shape detection map, size parameters, etc. The test data and production process records are archived for ≥ 3 years to ensure product quality traceability.

V. UltraOpto Manufacturing Advantages

Ultra-High Precision Control: The surface shape accuracy reaches λ/[email protected], the film thickness deviation is ≤ ±0.2nm, and the reflectivity is stably up to 99.999%, meeting the requirements of high-precision optical systems.

High-Purity Material Guarantee: The substrate and target material purity are both ≥ 99.999%, avoiding impurity interference and ensuring the long-term stability of reflectivity.

Full-Process Quality Control: Implement 100% inspection of key processes and finished products, with strict detection standards and complete data archiving, ensuring product consistency.

Customized Service: According to the customer's target band, aperture, radius of curvature and other requirements, customize the film system and shape design to meet the specific needs of different high-end optical scenarios.

UltraOpto relies on advanced precision machining equipment, professional film system design technology and strict quality control system to break through the technical difficulties of 99.999% high reflectivity concave mirror manufacturing, providing high-precision and high-stability optical components for global high-end optical equipment manufacturers