DDEDICATEDEDICATED S SPECTROPHOTOMETERPECTROPHOTOMETER

FFOROR L LOCALIZEDOCALIZED T TRANSMITTANCERANSMITTANCE

AANDND R REFLECTANCEEFLECTANCE M MEASUREMENTSEASUREMENTS

Laetitia ABEL-TIBERINI, Frédéric LEMARQUIS, Michel LEQUIMELaetitia ABEL-TIBERINI, Frédéric LEMARQUIS, Michel LEQUIMEInstitut Fresnel -UMR CNRS 6133 – D.U. St Jérôme- 13397 Marseille cedex 20 –FRANCE

laetitia.abel@fresnel.fr frederic.lemarquis@fresnel.fr michel.lequime@fresnel.fr

• Abstract

A dedicated set up is built to obtain localized transmittance and reflectance spectral measurements. The spatial resolution ranges from 50 µm up to 2 mm and the spectral resolution is about 5 nm. This apparatus can be used to study the index and thickness uniformity on single layers in order to determine and optimize the characteristics of the deposition chamber. It can also be used to measure the optical characteristics of intended non uniform coatings such as linear variable filters.

• Introduction :

• Description of the spectrophotometer

Transmittance and reflectance spectrophotometric measurements are frequently used to control the optical properties of multilayer coatings or to perform a thickness and refractive index characterization on single layers. Most often, such measurements are made with a light beam diameter of several millimeters, which results in a spatial average value for the measured properties. However, for some applications, localized measurements over a much smaller diameter may be required, especially when the deposited thickness must be accurately controlled all over the surface of the components.

This is the case for DWDM filters, for which the highest thickness uniformity is expected over the component diameter which is about 1 mm. Localized measurements can therefore be helpful to characterize and optimize the optical thickness distribution of the deposition chamber.

As an opposite example, intended thickness gradients are used for manufacturing linear variable filters. In that case, localized measurements are then required to characterize the optical properties of the components, the thickness gradient and the geometry of iso-thickness lines.

NUMERICAL DATA

Available apertures stops : 50, 100, 200, 600, 1000 and 2000 µm.Sample positioning accuracy: 3 µm.Beam divergence : 1.25 ° Angle of incidence: 2.8°

Light source: Quartz-halogen.Optical Spectrum Analyser: Ando AQ 6315A

spectral range: 400 – 1700 nmspectral resolution: 5 nm with a 600 µm entrance fiber

(0.05 nm with a monomode fiber)A reference photodiode is used to correct power fluctuations.

Stability standard deviation (light source + OSA) : 10-4.The standard deviation of measurement repeatability: 10-3

(including positioning and measurement of both the reference glass and the sample.)

Acquisition time from 400 to 1700 nm: 1 s for a 2 mm analysis 360 s for a 50 µm

Data acquisition and sample positioning are completely computer controlled.

PRINCIPLE

The system is illuminated by a fibered white light source and spectrophotometric measurements are performed with an Optical Spectrum Analyser.The principle of the apparatus consists in forming the image of a calibrated aperture stop on the sample in order to define a small probing zone. For this purpose, the entrance fiber is imaged on the aperture wheel with a magnification equal to 10. The aperture wheel is imaged on the sample with a unit magnification. At last, the sample is imaged on the exit fiber with a magnification equal to 1/10.Telecentric objectives are used all along the light path so that the incoming light can be entirely collected by the exit fibers. Fiber core diameters are progressively increased to allow greater positioning tolerances. The reflectance or transmittance channel is selected by a mirror translation so that optical fiber never need to be disconnected.

Fibered white light source

Optical Spectrum Analyser

Reference photodiode

X Translation stage

XYtranslation stage

ApertureStops wheel

Neutral densitieswheel

200 µm coreoptical fiber

400 µm coreoptical fibers

600 µm coreoptical fiber

Sample orreference

glass

• Thickness uniformity measurements.

OIC3 sample

25.4 mm

Scanning zone : 12.5 mm x12.5 mm

(36 points)

As an example, we give the thickness uniformity characterization we performed on the single layer of the measurement problem of the OIC 2004 Topical meeting.

The transmission of the sample was measured on 36 points with a 2 mm beam diameter, over the spectral range 400-1350 nm.The refractive index as well as the thickness are determined for several points, in order to fix a mean index dispersion curve. Assuming this index value, the thickness was then determined for each point which allow to draw the thickness variation of the layer.

The maximum thickness variation is about 1.2 nm with a standard deviation equal to 0.28 nm.Since the average thickness of the layer was measured equal to 190 nm, the thickness uniformity is thus about 99.4 % for a typical length of 18 mm.

• Measurements of linear variable filters

A regular decrease of the maximum transmittance of the band pass versus wavelength can be observed, due to the limited spectral and spatial resolutions of the spectrophotometer.

• Conclusion

MEASURED transmittance flux ratio along the thickness gradient Glass / M10 4H M10 / air SiO2 , Ta2O5, DIBS ; size : 20 x 20 mm

probing zone diameter: 200 µm ; displacement step : 2 mm

CALCULATED transmittance flux ratioSpatial integration: 200 µm ; spectral integration : 5 nm

Thickness ratio: 2.18 for 16 mm

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Wavelength (nm)

Tra

ns

mit

tan

ce

(%

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1110 1115 1120 1125 1130 1135 1140 1145 1150

Wavelength (nm)

Tra

ns

mit

tan

ce

(%

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Measured transmittance for several points across the thickness gradient probing zone diameter: 200 µm ; displacement step : 2 mm

Allows to take account of the misalignment of the sample during its measurement and estimate the curvature of iso-thickness lines.

CAPABILITIES: Reflectance and transmittance measurements at normal incidence, from 400 to 1700 nm : spectral resolution 5 nm: minimum spatial resolution 50 µm

APPLICATIONS: Thickness uniformity characterization in order to optimize the thickness distribution of a deposition chamber in order to characterize non uniform coatings such as linear variable filters

( in that case, results may need to be interpreted with calculations that take account of spatial and spectral resolution)

IMPROVEMENTS: Increase the brilliance of the light source (Xenon arc) to reach a higher signal level in order to increase spectral and spatial resolution( < 5 nm and < 50 µm respectively ) with a correct SNR.

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Wavelength (nm)

Tra

nsm

itta

nce

(%

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X/Y (mm) 0 2 4 6 8 10 12 14

0 1422.14 1318.97 1218.91 1127.01 1041.44 960.23 879.39 790.17

2 1424.05 1320.10 1219.95 1128.01 1042.91 961.13 880.20 790.32

4 1424.92 1320.32 1220.07 1128.06 1042.36 961.11 880.17 790.28

6 1425.83 1321.09 1220.23 1128.15 1042.95 961.12 880.19 790.32

8 1427.00 1322.04 1221.02 1128.30 1042.99 961.07 880.05 790.17

10 1427.10 1322.05 1220.96 1128.20 1042.85 960.90 879.33 789.98

12 1427.80 1322.12 1220.95 1128.10 1042.14 960.14 879.07 789.21

14 1426.81 1320.95 1219.17 1126.88 1040.88 958.28 877.25 788.02

X / Y (mm) 0 2 4 6 8 10 12 14

0 0.9960 0.9976 0.9983 0.9989 0.9985 0.9991 0.9991 0.9998

2 0.9974 0.9985 0.9991 0.9997 0.9999 1.0000 1.0000 1.0000

4 0.9980 0.9986 0.9992 0.9998 0.9994 1.0000 1.0000 0.9999

6 0.9986 0.9992 0.9994 0.9999 1.0000 1.0000 1.0000 1.0000

8 0.9994 0.9999 1.0000 1.0000 1.0000 0.9999 0.9998 0.9998

10 0.9995 0.9999 1.0000 0.9999 0.9999 0.9998 0.9990 0.9996

12 1.0000 1.0000 0.9999 0.9998 0.9992 0.9990 0.9987 0.9986

14 0.9993 0.9991 0.9985 0.9987 0.9980 0.9970 0.9966 0.9971

Estimatedwavelengthsafter optimumrealignment(2.5 arcsec rotation)

Corrected uniformity acrossthe thicknessgradient