Spectral Selective High Emissivity Pattern for Applications in near

Infrared

Enrique Carretero, Rafael Alonso and Cristina Pelayo

Department of Applied Physics, University of Zaragoza, C/Pedro Cerbuna, 12, 50009 Zaragoza, Spain

Keywords: Spectral Selective, Emissivity, Thin Films, Sputtering.

Abstract: In this work, we develop some high emissivity patterns for the near infrared range, between 1200 and

2500nm. These patterns were made by means of the magnetron sputtering technique, and they achieve their

functionality by using the optical interference phenomenon so that their superficial reflectance is

diminished. This is how we manage to produce surfaces that have a very low reflectance and high spectral

emissivity in the abovementioned range. Such patterns can be used for the calibration of temperature-

measuring systems based on photodiodes detecting near infrared radiation.

1 INTRODUCTION

It is well known that all bodies emit radiation, as the

black body theory describes. The total hemispherical

spectral emissive power or radiance of a surface

(DeWitt and Nutter, 1988; Howell et al., 2015),

namely, E

(λ, T), is the power density distribution

emitted into the hemispherical solid per unit of

surface area whose dependence is provided by the

following equation:



hc kT











(1)

where T is the absolute temperature,



is the

wavelength of the radiation, h is the Planck constant,

c is the speed of light in vacuum and k is the

Boltzmann constant. But real bodies do not emit

radiation in the same way as a black body would do,

because they are not really “black”. The concept of

“emissivity” is introduced to take this into account.

The emissivity of an object is determined by its

absorption, so it can be measured with a

spectrophotometer, using the following expression:







1 RT





 

(2)

where ε(λ) is the spectral emissivity of the object,

R(λ) is its spectral reflectance and T(λ) is its spectral

transmittance. In this way, the total hemispherical

spectral emissive power or radiance of a real surface

is:



hc kT













(3)

Emissivity is very important for a lot of

applications, for instance, in the field of

concentrated solar power. The receiver tubes where

solar radiation must be concentrated on need to be

highly absorbent, that is, they need to have a high

emissivity in the spectral range where the sun emits

its maximum radiation (black body radiation at

5700K), between 300 and 2500nm (Setien-

Fernandez et al., 2013). At the same time, these

tubes must keep at a minimum the losses due to

emitted radiation when heated to 673K, so they must

have a low emissivity in the spectrum range of a

black body at 673K, between 2.5 and 30 μm. In

order to fulfil these requirements, selective coatings

are deposited by PVD (Physical Vapour Deposition)

techniques (Cao et al., 2015; Céspedes et al., 2014;

Hernandez-Pinilla et al., 2016; H.J.Gläser, 2000).

Nowadays, the improved performance and

lowered prices of InGaAs PIN photodiodes has

allowed its use for small signal detection in near

infrared for domestic appliances, temperature

measurement in cooking vessels from cooktops

being one example of this. This system is based on

the measurement of the black body radiation emitted

by a pot when cooking at temperatures between 100

and 200ºC (Imaz et al., 2014; Lasobras et al., 2014).

The photodiode is located inside the cooktop, under

the ceramic glass surface, so light detection must be

carried out in a wavelength range where the ceramic

glass is transparent. Such requirement rules out other

260

Carretero E., Alonso R. and Pelayo C.

Spectral Selective High Emissivity Pattern for Applications in near Infrared.

DOI: 10.5220/0006167102600264

In Proceedings of the 5th International Conference on Photonics, Optics and Laser Technology (PHOTOPTICS 2017), pages 260-264

ISBN: 978-989-758-223-3

options measuring in wavelength ranges above 3μm,

although pots and pans in the abovementioned

cooking temperature range emit most of the

radiation above 3μm. InGaAs photodiodes detect

radiation between 600 and 2500nm. A surface

heated to 100ºC emits very little power in this

wavelength range, but this type of photodiodes is

sensitive enough to measure it.

One important issue with these detectors is its

calibration. They need to be calibrated with very low

emissive patterns and also with very high emissive

patterns. Low emissive patterns are very easy to

produce, because there is a great number of metals

which are low emissive in near infrared, as gold. On

the other hand, it’s easy to find a high emissive

surface with values around ε=0.90 in the detection

range of InGaAs PIN photodiodes, such as “burnt-

out” surfaces, but we have been able to produce a

pattern which has a higher emissivity (ε>0.97) than

traditional surfaces in this detection range. Such

patterns permit us to obtain a greater signal in the

temperature sensor of a cooktop.

Highly emissive patterns were produce by means

of the magnetron sputtering technique. These

patterns can be obtained by combining metallic and

dielectric layers (Sergeant et al., 2009). A coating

was deposited comprised of 4 layers, using both an

absorbent metallic material (Stainless Steel 316) and

a transparent dielectric material (SiAlN

) in a way

that the coating results opaque (T=0) and has a

minimal superficial reflectance between 1200 and

2500nm, using interferential coatings.

2 MATERIAL AND METHODS

Spectral Selective High Emissivity Patterns were

deposited in a semi-industrial high vacuum

magnetron sputtering system by the DC pulsed

technique using rectangular targets with dimensions

600x100mm and 12mm thick. Substrates were

ceramic glass pieces of 100x100mm and 4mm thick.

Ceramic glass has low thermal expansion coefficient

which makes it resistant to high temperatures

gradients without breaking. Substrates were cleaned

with a detergent solution (ACEDET 5509) and

finally rinsed with distilled water. A Stainless Steel

disc was also used to simulate a rough metallic

surface.

Thin films were grown with a base pressure of

2.0·10

-6

mbar and working pressure in the range of

-3

mbar. Ar and O

(both 99.99%) flows were

introduced into the process chamber and controlled

via mass flow controllers. The substrate was

maintained at room temperature during deposition.

Thin films of Stainless Steel (Sst) were deposited

from a Sst 316 target (99.99% pure). Applied power

was 2000W, equivalent to a power density of

3.33W/cm

. The Ar flow was fixed at 200sccm,

equivalent to a pressure of 1.5·10

-3

mbar in our

deposition system. Thin films of SiAlN

were

deposited by reactive sputtering from a Si-Al target

(90% Si and 10% Al, 99.99% pure). Applied power

was 2500W, equivalent to a power density of

4.17W/cm

. The Ar flow was fixed at 100sccm and

the N

flow at 100sccm.

Spectrophotometric measurements were

performed with a home-made spectrophotomer

(designed and built by some of the authors) in the

near-infrared region of the electromagnetic

spectrum, between 1100nm and 2500nm with 50nm

intervals, at an angle of incidence of 8º. The specular

reflectance of the patterns deposited over smooth

surfaces was measured without an integrating

sphere, because the low roughness of the substrate

minimizes the scattered component to negligible

values and because the specular measurements are

more precise. For patterns deposited over a rough

surface, reflectance measurements were carried out

through an integrating sphere. Finally, a

transmittance measurement was taken to verify the

low transmittance of the patterns.

3 RESULTS AND DISCUSSION

There are diverse types of InGaAs photodiodes

having different sensitive ranges. The most common

ones are those capable of detection radiation

between 600 and 1700nm, but there are extended

sensitivity models which can detect radiation

between 800 and 2600nm. On the other hand,

temperature sensors must be calibrated between 100

and 250ºC, and in this temperature range the black

body radiation below 1200nm is negligible. So the

objective of this work is to minimize reflectance in

the range between 1200 and 2500nm. For this

purpose, we designed an interference multilayer

comprised of 4 layers, fig.1:

A first “thick” metallic layer (over 200nm)

directly deposited on the substrate. This layer has a

double functionality: for transparent substrates, this

layer has a high optical absorption, so the pattern

can be opaque (T=0), and furthermore, it will act as

a basic layer to calculate the interferential coating

which minimizes reflectance between 1200 and

2500nm.

The thicknesses of the three subsequent layers

must be adjusted very precisely, because they

Spectral Selective High Emissivity Pattern for Applications in near Infrared

261

determine the reflectance of the pattern. For this

purpose, the coating was simulated with the aid of

existing formalisms for the calculation of the optical

properties of interferential coatings (J. A.

Dobrowolski, 1995; Macleod, 2010; Thelen, 1989).

Values minimizing reflectance as required are

shown in table 1.

Figure 1: Multilayer structure of high emissivity patterns.

Table 1: Multilayer structure and layer thicknesses of

produced patterns.

Material Thickness (nm)

Substrate

Sst >200

SiAlN

186

Sst 13

SiAlN

204

Thus, we proceeded with the experimental

making of the patterns, both on ceramic glass and on

metallic discs, as depicted in Fig.2. So two patterns

were made: a specular pattern and a non-specular

one, the last one owing its feature to the roughness

of the substrate surface. These patterns visually

appear as purple coloured, because interferential

coatings usually produce striking colours. The fact

that they are (almost) black bodies in the 1200-

2500nm range is remarkable, considering this is not

true for the visible part of the spectrum, as they

indeed have a high reflectance at some particular

wavelengths.

Figure 3 shows the reflectance curves of the two

produced patterns. In both cases we find similar

values of reflectance in spite of the differences over

surface texture of both substrates. Additionally,

measured values satisfactorily agree with simulated

values. On behalf of these results and knowing the

sensibility of the photodiodes provided by the

manufacturer, which in this particular case is

Teledyne Judson Technologies LLC, as we have

used models J22-18I-R01M and J2318I-R01M-2.6

(detecting respectively up to 1700 and 2600nm), we

can calculate the integrated apparent emissivity of

our patterns with this expression:



1700

()

SET Rd

SETd

 















(4)

Where S is the sensitivity of the J22-18I-R01M

photodiode, R is the spectral reflectance of the

pattern, ε is the integrated apparent emissivity at the

temperature T of the pattern as calculated for the

photodiode which detects up to 1700nm. This last

magnitude has a value of ε

1700

(473K)=0.987 for a

temperature of 473K.

Figure 2: Images of the produced patterns on ceramic

glass (top) and on a rough metallic disc (bottom).

We can repeat this procedure for the extended-

range photodiode that detects up to 2600nm,

modifying only the value of S

1700

(λ) for S

2600

(λ) in

expression (4), and we obtain an integrated apparent

emissivity of ε

2600

(473K)=0.976 Finally, we were

able to verify that our patterns produced the greatest

measured signals over all surfaces that were

measured with the temperature sensor (Imaz et al.,

2014; Lasobras et al., 2014).

PHOTOPTICS 2017 - 5th International Conference on Photonics, Optics and Laser Technology

262

One must keep in mind that the integrated

apparent emissivity is a magnitude taking into

account the spectral width between 1200 and

2600nm. Concerning spectral emissivity, values as

high as ε(1850nm)=0.994 were reached.

Nevertheless, if the final aim of the pattern was to

maximize emissivity at a particular wavelength

(instead of a wide range), it could be possible to

obtain values closer to 1.

Figure 3: Reflectance curves measured and simulated for

the highly emissive produced patterns, and normalized

black body emission at 473K.

One very significant issue is the tolerance in the

thickness of the deposited layers, because any

variation in these thicknesses can affect the

reflection spectrum of the pattern. Figure 4 shows

the changes in reflection spectra for coatings in

which some of its layers have a thickness deviated

from the values given in Table 1. One can appreciate

in this figure how an increment in the thickness of

the outer layer increases reflectance in the 1200-

1800nm range and decreases it in the 2100-2500nm

range. On the other hand, a 10% increment in both

dielectric layers shifts the low reflectance zone

towards higher wavelengths, whereas a 10%

increment in all the coating layers produces a slight

displacement and narrowing of the low reflectance

zone. If the thin metallic film has a 30% thinner

thickness, the wide part of the spectrum with low

reflectance disappears and two reflectance

minimums appear at 1250 and 2300nm. If that same

film has a 30% thicker thickness, the low reflectance

zone is narrowed but very low reflectance values can

be achieved at a specific wavelength (1780nm for

the studied case).

Figure 4: Simulated reflectance spectra for coatings with

different thicknesses: multilayer structure as described in

Table 1 (black line), (a) Outer transparent dielectric layer

with a 10% thicker thickness than in Table 1, (b) both

dielectric layers with a 10% thicker thickness, (c) All

layers with a 10% thicker thickness, (d) Thin metallic film

with a 30% thinner thickness, (e) Thin metallic film with a

30% thicker thickness.

So it can be concluded that layer thicknesses

must be adjusted with a tolerance lower than 10%

and preferably lower than 3%. Nevertheless, coating

adjustment is an iterative process where a coating is

deposited with thickness values near those exposed

in the table, then the reflectance of the coating is

measured and the experimentally obtained results

are adjusted by means of a simulation software in

such a way that we can know which thicknesses

must be modified for a better adjustment of optical

properties. In this way, these results can be

reproduced in other deposition systems, although

small further tuning of thicknesses might be

required.

It is remarkable that this kind of patterns can be

produced using different materials. Other dielectric

materials such as oxides and nitrides (SnO

…) can be used, although they may require

thicknesses to be recalculated because of their

different index of refraction. SSt can also be

replaced by other metals commonly employed for

the fabrication of selective coatings in the solar

energy field, such as Mo, W…

4 CONCLUSIONS

Highly emissive patterns can be used to calibrate

temperature sensors based on infrared radiation

detection. We have made two highly emissive

patterns in the 1200-2500nm range, one of them

being specular and the other diffuse on a rough

Spectral Selective High Emissivity Pattern for Applications in near Infrared

263

surface, and both reaching values over 0.97.

Moreover, we have proven the feasibility of

reaching spectral emissivities over 0.99 at particular

wavelengths, by means of interferential multilayer

deposition. These patterns were used to calibrate a

temperature sensor. Finally, a good agreement

between simulated and measured values of

reflectance has been verified for these coatings.

ACKNOWLEDGEMENTS

We thank Carmen Cosculluela for her valuable help.

This work was partly supported by the Spanish

MINECO under grant RTC-2014-1847-6, in part by

the Diputación General de Aragón / Fondo Social

Europeo through the funding for the Photonics

Technologies Group (GTF), in part by the

Diputación General de Aragón under FPI

programme B143/12 and in part by the BSH Home

Appliances Group.

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