Theoretical Model: UV mirror at 0° Angle
Of Incidence (AOI) (solid) and 45° AOI (dashed)
High Quality Optical Coatings using Ion Beam Deposition
- Due to sputtered atoms high energy, Ion beam sputtered films have a density very similar to the bulk density of the materials used.
- This density is extremely reproducible from run to run.
- With care it is possible to deposit many materials in an amorphous or semi-crystalline state. This is often highly advantageous for optical coatings, as it reduces the effect of birefringence.
A major feature of an optical coating is it's surface quality. The quality of the surface determines the performance of the optical device itself. High quality optical coatings start with a smooth super-polished optical substrate with roughness typically of 0.05 nm rms.
A conventionally deposited film will add roughness to the surface of the optical substrate, the degree of the roughness dependant upon the technique used. For example a film deposited by evaporative techniques produces a surface roughness of typically 1 nm rms, wile ion assisted deposition techniques produce a surface roughness of typically 0.4 nm rms. Ion beam sputter deposition produces films with a surface roughness equal to that of the super-polished substrate, 0.05 nm rms.
A high quality optical coating should have low optical loss. Losses inside an optical coating arise from scatter and absorption. Ion beam sputtering produces films with total losses so low that sophisticated devises are needed to measure them. Data for the measurement of mirrors with losses less than 2ppm have been published by two independent groups. Both groups made their mirrors on Oxford Instruments Ion beam sputter deposition systems.
-
Laser Bar Facet
-
High Quality Mirror
-
Dual Wavelength Anti-Reflection
-
Notes
-
Laser Bar Facet Coating
|
The facets form a laser bar’s resonant cavity. The light shall emit from the bar only at one facet, with high efficiency. So it is necessary to confine the light in the bar at one end and encourage the light to emerge from the other, with minimal loss.
This is achieved by the addition a HR (high-reflection) coating to retain the light in the bar on one facet and an AR (anti-reflection) coating on the other facet that efficiently lets the light out. |
|
|
The coatings are different for each side of the bar, so the bars need to be secured in a carrier that will allow each side to be coated, and then turned over for the second coating run on the reverse side, all the
while protecting the first side from further flux.
|
|
|
Ion Beam Deposition
- Filamentless RF source
- Filamentless assist RF source
- Filamentless PBNs
Process control by time or Xtal source shut down < 40 msec
- Al2O3 at > 20 nm/ min
- Si at > 20 nm/ min
- SiO2 at > 20 nm/ min
- Ta2O5 at > 25 nm/ min
at < +/- 2 % uniformity |
Typical ion beam layout
For laser bar coating we use the direct
line deposition geometry to achieve:
- minimum deposition at the side walls
- a uniform thickness and depostion profile |
High Quality Mirror Coatings RIBD using the Ionfab500
|
 |
|
Spectrum of a 36-layer 30deg 633nm SiO2/ Ta2O5 mirror |
Process Specification
- Process Gases: Ar, O2
- Deposition carrier: four 10” platens
- Summary performance data:
| Chamber base pressure |
<2E-6 Torr in less than 90 mins |
| Load lock base pressure |
Typical 1E-7 Torr after overnight pumping |
Process Specification
| Parameter/Process |
Mirror coating |
| SiO2 deposition rate (SiO2 target) |
0.5 up to 6.6nm/min |
| Ta2O5 deposition rate (Ta target) |
1 up to 5.4nm/min |
| TiO2 deposition rate (Ti target) |
0.5 up to 3.3nm/min |
| Uniformity across 10” planet [±%] |
<±2% |
| Uniformity repeatability [±%] |
<±2% |
| Refractive index repeatability [±%] |
<±0.001 R.I. |
| Mirror loss for SiO2/ Ta2O5 mirror2 |
<40ppm |
| Surface roughness increase |
<0.02nm |
| RMS for initial substrates |
<0.07nm RMS |
Notes:
Loss readings are subject to suitable substrate and clean room conditions being of a suitably high quality.
 |
| Loss meter reading of a 50ppm mirror fabricated prior to shipping of an Ionfab500 |
|
Optofab3000
Deposition of Dual Wavelength AR coating using White Light Optical Monitor (WLOM)calibration
Dual Wavelength AR coat spectrum measured using Cary 500 Spectrophotometer (referenced to glass) transmission (%) vs wavelength (nm)
White Light Optical Monitor screenshot of process |
Process details
-
Recipe: 8 layer AR Coat (400mA)
-
Layer materials: Ta2O5 SiO2
-
Process gases Argon, Oxygen
-
Deposition rate >125Å/min (SiO2), 5Å/min (Ta2O5)
-
Uniformity <±0.5% over 75mm thick annulus <0.2% over 10mm thick annulus centred at 5mm substrate radius
-
Oxygen Assist O2, 600W plasma
with aperture.
-
Substrate 1”f fused silica held in 8” carrier plate
AR Performance:
Comments
-
Dep’n rate uniformity specified:
± ((max-min)/(2*mean))*100%
-
Time endpoint for all layers
-
System calibrated using QW deposition by WLOM.
|
Additional Notes:
1. Regular cleaning and conditioning of the chamber is required. This may occasionally involve a mechanical clean of the chamber
2. Standard definitions of uniformity given below:
| Within wafer uniformity is measured in a five point pattern: |
Run-to-run uniformity (reproducibility) is calculated from average wafer uniformity in 5 consecutive runs. |
 |
For refractive index uniformity is calculated as:
For refractive index uniformity is calculated as:
|