Negative filter——A multi-layer film device that allows red light to go and blue light to stay - seemingly just a few micrometers thick glass sheet, but determines the imaging quality of laser radar, medical endoscopes, and stage lighting. Its yield has increased from 70% to 95%, and the key is not the more expensive coating machine, but the process control of "calculating every layer error in advance". Let's talk about the four iron rules behind its high yield rate.1、 The base 'cleansing' does not meet the standard, and the rest is completely useless
After cutting, the B270 glass was first cleaned with RCA-1 for 15 minutes (NH ₄ OH: H ₂ O ₂: H ₂ O=1:1:5, 75 ℃) to reduce organic residues to<0.3 nm. Then, after 10 minutes of plasma bombardment, the surface energy was increased to 72 mN · m ⁻¹, increasing the adhesion of the first layer of Ta ₂ O ₅ from 2.5 N to 5.8 N, and continued to bake at 200 ℃ without peeling.
2、 Stable speed of ion beam is more important than high speed
The coating machine uses a 6 cm RF ion source and PID closed-loop control of a 2-inch crystal oscillator to regulate the deposition rate fluctuation to ± 0.2%. Taking the 532 nm interference cutoff plate as an example, with 21 layers of film and a total thickness of 3.8 µ m, as long as the rate drift is 1%, the center wavelength will shift by 2 nm, and the pass through rate will immediately decrease by 10%. Once the speed stabilizes, the wavelength hit is reduced from ± 3 nm to ± 1 nm, resulting in a 15% increase in first-time pass rate.
3、 Real time extreme value monitoring with "zero lag" error compensation
In the ion beam assisted deposition (IAD) process, the chip level spectrometer collects the transmission extremum every 0.5 seconds. The algorithm calculates the actual refractive index n ± 0.001 of the current layer within 1 second and corrects the electron gun power in reverse. The 7th layer found that n was 0.8% lower, and the system immediately increased the Ta ₂ O ₅ thickness by 1.2 nm and pulled the wavelength back to the design value. Compared to the traditional "plating before testing" method, the rework rate of a single piece has decreased from 8% to below 1%.
4、 Stress balance "sandwich" structure anti cracking
High power laser chips require a damage threshold of 60 J · cm ⁻ ², and the membrane stress must be close to zero. The engineer alternated Ta ₂ O ₅ (+180 MPa) and SiO ₂ (-120 MPa) in a 2:3 thickness ratio, and added a 20 nm MgF ₂ low stress buffer layer on the outermost layer, reducing the overall stress from+350 MPa to+30 MPa. After 24 hours of thermal shock (-40 ℃) ↔ After reaching 85 ℃, there were no cracks, and the environmental test pass rate increased from 85% to 98%.
From this, it can be seen that high yield is not about "fast plating", but about "accurate calculation and stable control", eliminating every nanometer of error before light enters the glass.