Optical techniques

Theoretical background

Figure 3.8: Description of the behaviour of the light at the interface between two media. Let us consider the interface between two media (labelled 1 and 2) with different absolute refractive indexes (respectively n1 and n2), e.g. optical properties (see Figure 3.8). When an

electromagnetic wave propagating in media 1 encounters this interface with an incident angle θi, it is split in a reflected and transmitted part where the angle of reflection θris equal to θi.

According to the Snell’s law,

n1sinθi= n2sinθt (3.2)

where θtrepresents the transmission angle. The reflectance R represents the ratio between the

intensity of the reflected light to the intensity of the incident light. Terminologically speaking, the reflectivity is defined for “thick” reflecting objects while the reflectance term also applies for thin layers and its value can vary with layer thickness. For thick films, reflectivity and reflectance are equivalent. The reflectivity at a certain wavelength depends on several parameters such as angle of incidence θi, materials properties (contained in n1 and n2) and polarization of the

light. Regarding the light polarization, two cases can be distinguished namely s and p-polarized light (perpendicular and parallel to the incident/transmission/reflection plane, respectively). Assuming that the two media are linear, homogeneous and isotropic, one can apply Fresnel equations and evaluate the complex Fresnel coefficients rs and rp for the reflected beam for s

and p-polarized light, and thus the reflectivity Rs and Rp. For a natural or unpolarized light

(containing as much s and p-polarization), R is given by: R = Rs+ Rp

2 (3.3)

The description above only holds for a perfect surface of medium 2. If the surface roughness is comparable to the wavelength of the incident light, part of the reflected light is scattered in all directions and is called diffuse reflectivity. The specular reflectivity, also named mirror-like reflectivity denotes the ability of a surface to reflect the light in a certain direction where, as mentioned earlier, θi= θr. The diffuse (resp. specular) reflectivity is defined as the ratio between

3.2. Sample characterization

light. Finally, the total reflectivity (Rtot) is the sum of the specular and diffuse reflectivities

(resp. Rspecand Rdif f)

Rtot= Rspec+ Rdif f (3.4)

A relation between the surface roughness of a material and the specular reflectivity at normal incidence was given by Bennett in 1961 [78]:

Rspec(λ) = R0(λ)e−(4πRrms)

2_/λ2

(3.5) where Rrms is the surface root-mean-square roughness, R0 is the reflectivity of the same

material ideally smooth and λ is the wavelength of the incident light. From this formulation, one can clearly set a limit for surface roughening for FMs. If the limit in specular reflectivity loss of FMs is set to 10 %, their roughness should be kept below λ/40. For diagnostics working in UV, for example at 250 nm, this implies that the roughness should not exceed 7 nm. Ex situ reflectivity measurements

Most of ex situ UV-VIS-NIR specular and diffuse reflectivity measurements were measured with a Varian Cary 5 spectrophotometer (250–2500 nm) in Basel. A calibrated PolyTetraFlu- orEthylene (PTFE) probe is used as a reference to determine the absolute reflectivity of each sample. Therefore intensity measurements of the background, made by blocking the incident light (Ibackground), and of the PTFE reference (Iref erence) are necessary to calibrate the spec-

trophotometer. The absolute reflectance is calculated with the formula: R(λ) = C(λ)_× Isample− Ibackground

Iref erence− Ibackground

(3.6) where C(λ) is the absolute reflectance of the PTFE sample at a certain wavelength. The reflected light (total or diffuse) is measured with an 110 mm diameter integrating sphere (also called Ulbricht’s sphere) coated with PTFE. The specific geometry of the integrating sphere allows collecting either the total or the diffuse reflectivity. When the incident light arrives on the sample with an incidence angle of approx 3◦, the total reflected light is collected by the sphere. With another adjustment, the incident light hits the sample at normal incidence so that the direct reflected beam Rspec leaves the sample with the same normal incidence out of

the sphere without being measured. In this case, only the diffuse component is measured. The specular reflectivity is obtained by subtracting the diffuse from the total reflectivity.

In some cases, reflectivity measurements were performed in the JET-BeHF for Be-related experiments. The set-up comprised an integrating sphere (PTFE coated), a PTFE reference sample and a spectrophotometer measuring from 400 to 1600 nm and allowed only Rtot mea-

surements.

In situ reflectivity measurements

To follow changes of reflectivity during deposition or cleaning processes, an in situ reflectometry system was developed in Basel [79]. The reflectometer measurement part is composed of two spectrometers with a usable wavelength range going from 200 to 1100 nm. A 150 W tungsten halogen light source is providing the light beam to the beam sampler. Thus a small part of the beam (4 %) is reflected back to one of the two spectrometers. This reference signal, which did not travel through the vacuum chamber, is used to correct the reflected signal if the intensity of the light source changes during one experiment. At this point, it is important to notice that the reference signal is only used to account for intensity variations taking place before the entry of the light source into the plasma chamber. Therefore, if the transmittance of the vacuum chamber’s windows change during one experiment (due to the coating of the windows), it will not be taken in account in the reference signal.

The main part of the beam is focused onto a 10 mm spot on the sample surface, where it is reflected and exits the vacuum chamber. The reflected beam is collected with an integrating

sphere and measured with the second spectrometer. Thanks to the high power light source, the plasma light has no influence on the reflectivity measurements. In a typical measurement, the dark current of the spectrophotometer is 200–300 counts at room temperature, while the reflected signal is 55000 counts: the dark current can be neglected. The actual light source only permits measurements between 400 and 800 nm.

The system has not been calibrated and is thus not measuring an absolute reflectivity but the evolution of the reflectivity spectrum relative to the first time slice. The evolution of the relative reflectivity ( ˇR) for a given wavelength and corrected with the evolution of the reference signal is given by:

ˇ R(λ, t) = Is(λ, t) Is(λ, 0)× Ir(λ, t) Ir(λ, 0) (3.7) where Is(λ, t) is the number of counts at time t and wavelength λ measured by the spectrom-

eter sampling the reflected beam. Is(λ, 0) is the first time slice taken prior plasma start. Ir(λ, t)

is the number of counts measured by the reference spectrometer and Ir(λ, 0) is the average of

a number of time slices taken before exposure start. For data acquisition, a LabVIEW program is used.

The in situ reflectometry system was installed on ESCA-1 facility (3.1.1 see Figure 3.1). The primary light beam enters the experiment chamber through a glass viewport, hits the sample surface and gets reflected, and finally exits from another viewport. Impinging and reflected light beams had an angle of 104◦between them, and therefore the actual quantity which is measured was the relative reflectivity at θi = 52◦.

The same system was transferred to JET and mounted on the Be chamber (3.1.2). Due to the geometry of the chamber, the angle between the incident and reflected light beam was approx. 30◦.