DEVELOPMENT OF AN INFRARED WIDE PASS BAND GERMANIUM …

Dipartimento di Astronomia e Scienza dello Spazio, Università di Firenze, L.go E. Fermi 2, 50125 Firenze (Italy) Ph.+39 (055) 23.07.51

DIPARTIMENTO DI ASTRONOMIA

E SCIENZA DELLO SPAZIO

DEVELOPMENT OF AN INFRARED WIDE PASS BAND

GERMANIUM FILTER

A. De Sio (a), A. Giannini(a), G. Dionisio(a), L. Gambicorti(a,b), P. Bianchi

(b), M. Ciofini

(b),

L. Mercatelli(b), E. Pace

(a,b)

(a)Dep. Of Astronomy and Space Science - Firenze University (Italy) (b) National Institute of Applied Optics – National Research Council, Firenze (Italy)

Technical Report TR01-2008

Version 1.0

Date: 03 2008

Dipartimento di Astronomia e Scienza dello Spazio, Università di Firenze, L.go E. Fermi 2, 50125 Firenze (Italy) Ph.+39 (055) 23.07.51

DIPARTIMENTO DI ASTRONOMIA

E SCIENZA DELLO SPAZIO

SUMMARY

ABSTRACT.............................................................................................. 3 1. INTRODUCTION................................................................................. 3

2. EXPERIMENTAL.................................................................................. 4 2.1. The thin film optical simulator........................................................ 4

2.2. The optical fabrication facilities ...................................................... 4

2.3. The optical treatments deposition facility......................................... 5

2.4. The optical characterization facilities............................................... 5

3. PROCEDURES, RESULTS AND DISCUSSION........................................... 6

3.1. The theory of the anti-reflex monolayer .......................................... 7

3.2. The Optical simulations ................................................................. 8

3.3. The Optical simulations ................................................................. 9

4. CONCLUSIONS: The IR optical window prototype................................. 12

DEVELOPMENT OF AN INFRARED WIDE PASS BAND GERMANIUM FILTER Technical report

Ref.: DASS-1th_Tech_rep_2008.doc 3

ABSTRACT

Many applications, such as IR astronomical sources observations, Earth observation

from space, environmental monitoring or volcano monitoring, require the development

of imaging systems working in the Infra-Red (IR) spectral range. Such systems

require optics and filters able to cut the visible radiation and to maintain high

transparency in the IR spectral range above 2 µm. The design and the fabrication of

an optical window for such type of imaging systems are described in this report. The

optical window will work in the 7 - 14 µm range and has been fabricated on a

Germanium substrate. Ge was selected for its negligible absorption coefficient in the

spectral range of interest. Unfortunately, its transmissivity is limited to 45%, because

of its high refraction index (n = 4.3 @ 10µm). In order to increase the transmissivity

of the window and to avoid ghosts generated by multiple reflections, a ZnS

antireflection coating has been deposited on both the two window surfaces.

The development phases from the optical simulation through the calibration of the

deposition machine and the fabrication of test prototypes to the manufacturing of the

optical window will be here described.

1. INTRODUCTION

Germanium is a very useful material to develop IR imaging systems. Owing to its

physical characteristics, it is an ideal substrate: it is a robust material with a negligible

absorption coefficient in the wavelength range between 8 µm and 14 µm. Above 14

µm the absorption coefficient increases because of some absorption bands.

Unfortunately, Ge has a high refraction index (4.3 at 10 µm) in the interval 2 µm –

14 µm and consequently reflections occur at the air-Germanium interfaces. Such

reflectivity reduces the efficacy of the Ge window, since they cut a 55% of the IR

transmitted radiation intensity and it produces ghosts generated by multiple

reflections inside the material. Furthermore, if images are taken using a Ge window, a

reflected image of the camera on the first Ge surface is often superimposed to the

picture.



Therefore, a ZnS monolayer coating was selected in order to reduce the reflectivity

and to increase the transmittance at the two interfaces of the Germanium window.

The choice of ZnS was driven by the well know adhesion between Ge and ZnS and

because its refraction index is close to the square root of the refraction index of the

Ge. In such case the reflections on the surfaces can be nullified.

Aim of this technical report is to describe the design, the experimental facilities and

process of optimization to obtain a ZnS monolayer coating to improve the

transmissivity of a Germanium window dedicated to IR imaging system.

2. EXPERIMENTAL

2.1. The thin film optical simulator

The reflectivity or transmittance can be calculated at different wavelengths and

plotted by using optical simulators and ray-tracing software. Real materials have

optical constants depending on wavelength, so it is fundamental to know their real

values. Realistic calculations of optical properties must include such variations. The

values of the refraction index and the extinction coefficient as a function of

wavelength for each material are stored in a database that is accessible to the

simulation programs so, any dispersion can be effectively modeled. Material behavior

is not always ideal and the optical constants often depend on the particular coating

machine and on the deposition parameters. The operating conditions affect the

material properties, but they can be also simulated.

2.2. The optical fabrication facilities

The optical fabrication was carried out in the laboratory of CNR-INOA (National

Institute of Applied Optics of Firenze), where basic machines are located (two-axes

raw surfaces process, diamond-wire cutting machine, patinas processing, glass cutter,

etc). Other laboratories are equipped with an optical finishing and polishing machine,

a curvature ray measuring machine, etc. In the third laboratory there are an

apparatus that is used to manufacture optical fibers and an interferometer, a

prototype developped at INOA, to measure the material surface quality through the

optical fringe technique.



The available instrumentation in each laboratory can be divided into three main

classes:

� Instruments/machineries or the raw material and/or patinas processing (sawing

machine, scratch machine, glassblowing, rough cut, decanter, etc.);

� Instruments/machineries for the optical processing (lapping machine, optical

fibers polish/junctions, etc.);

� Testing equipment and optical precision tools (interferometer, curvature ray

measuring device, spherometer, micrometers, etc.).

2.3. The optical treatments deposition facility

Film depositions were carried out at the Dept. of Astronomy and Space Science of the

University of Firenze. The deposition facility is composed by a 500 litres vacuum

chamber, evacuated by a primary scroll pump and a turbo-molecular secondary pump

down to a 5x10-7 mbar pressure. The quality of the vacuum can be monitored by a VG

mass spectrometer.

Inside the vacuum chamber, five different deposition sources are placed: three 1.5

kW heating Joule sources and two 3 kW Electron Beams. The deposition rate is

monitored by a standard quartz microbalance.

A 40-cm diameter steel cap is 50 cm above the source plane; there, many substrates

can be arranged. Such a cup can rotate in order to produce a uniform thickness of the

deposited film on the whole cup area. The temperature of the substrates can be

actively controlled by irradiating them with a set of 3.5 kW halogen lamps. The

current bias of such a lamp system is connected to a thermocouple through a

feedback system that is able to supply the electric current that is necessary to fix the

selected temperature.

2.4. The optical characterization facilities

The transmittance measurements in the wavelength range from 0.18 µm up to 3.2 µm

were carried out at CNR-INOA using a Perkin-Elmer Lambda 900 double ray

spectrophotometer. This instrument allows the measurement of the transmittance of

transparent and semi-transparent samples, the diffuse transmittance of transparent



and semi-transparent samples, and the specular reflectance along with the diffuse

reflectance.

The IR measurements using the refraction and reflection mode in the 1.2 - 20 µm

range were carried out by using a Perkin-Elmer System 2000 Fourier Transform IR

(FTIR) spectrometer. FTIR spectroscopy measurements are more precise and fast

than the measurements carried out with spectrophotometers using standard

dispersive elements. The FTIR technique is based on a Michelson interferometer where

the sample is at the end of the fixed arm. The length of the other arm can be varied

and such a variation can be monitored by using a He-Ne laser. The IR spectrum can

be reconstructed from the interference pattern, generated by varying the length of the

arm, with the spectrum of the IR light source.

3. PROCEDURES, RESULTS AND DISCUSSION

In the following paragraphs will be described the methodologies that we have used to

fabricate the IR optical window. We start with a brief introduction to the monolayer

theory, then we describe the optical simulations to predict the absorption spectra of

the final ZnS coating on both Germanium (in the IR) and optical glass (in the VIS).

Such simulations allow us to calibrate a geometrical parameter that depends on

mutual positions of the ZnS evaporation source, the quartz microbalance and the

substrates. Thus, the deposition facility can be calibrated to obtain the desired

thickness layers. Finally, the prototypes of the IR window have been produced and

tested.

Density

(g/cm3)

Refraction

index

@10.6 µm

Transparency

window

(µm)

Melting

point

Evaporation

temperature

ZnS 4.1 2.3 0.39 14.5 1850 950

Ge 5.4 4.0058 2 20 937 1400

Table 1



3.1. The theory of the anti-reflex monolayer

Whenever light passes from one medium into another medium having different optical

properties, part of the light is reflected and part is transmitted. The intensity ratio

between reflected and transmitted components is primarily a function of the different

refraction indexes and the angle of incidence. The fraction of the intensity of incident

light that is reflected from the interface is given by the reflection coefficient R, and the

fraction refracted by the transmission coefficient T.

The reflection coefficient depends on the refraction index at each wavelength. To

reduce the reflection coefficient, the interference effect of a thin layer on the material

substrate can be used. Assuming that the layer thickness can be controlled precisely

so that it is exactly one-quarter of the wavelength of the light deep (λ/4), forming a

quarter-wave coating. If this is the case, the incident beam I, when reflected from the

second interface will travel exactly half its own wavelength further than the beam

reflected from the first surface. If the intensities of the two beams, R1 and R2, are

exactly equal, then since they are exactly out of phase, they will destructively

interfere and cancel each other. Therefore, there is no reflection from the surface, and

all the energy of the beam must be in the transmitted ray, T. Fig. 4.2.2 shows the

reflectance and transmittance of electric and magnetic field vectors incident at a

boundary between two dielectric media for a AR coating.

Fig. 3.1.1 The reflectance and transmittance of electric and magnetic field vectors incident at

a boundary between two dielectric media for a AR coating.



The film of refractive index n is defined as thin when its optical thickness is of order of

wavelength of radiation impinging, so the interferential effects can be observed

3.2. The Optical simulations

The transmittance of ZnS AR layer on Ge substrate has been simulated. The MacLeod

simulation platform has been used to reproduce the optical performance of a Ge

substrate coated by a λ/4 layer of ZnS, having thickness of 10 µm, forming a quarter-

wave coating, to improve the transmittance and to reduce the reflectivity at the

sample interface. Fig.3.2.1 shows the simulated transmittance in the range 1-20 µm.

0 2000 4000 6000 8000 10000 12000 14000 16000 18000 20000

0

10

20

30

40

50

60

70

80

90

100

Transmittance %

wavelength (nm)

ZnS Ge ZnS

Ge

Figure 3-2-1 Simulated transmittance of a Germanium plate 5 mm of thickness, with each

surface coated with a layer of ZnS of λ/4 of thickness at 10 µm (Red line). In comparison

with the simulated transmittance of a Germanium plate 5 mm of thickness.

The same ZnS coating has been simulated on a optical grade glass layer (BK7 type) (Fig. 3.2.2).

Such a simulation gives information on the spacing of the fringes generated in the visible range by

the ZnS layer that will be deposited on the final Ge window. This simulation will be compared with

the ZnS deposition on glass layer to calibrate the deposition facility through standard glass test

substrates, that are cheaper and easy to get.



500 750 1000 1250 1500 1750 2000 2250 2500

0

10

20

30

40

50

60

70

80

90

100

Transmittance %

wavelength (nm)

ZnS Bk7 ZnS

Figure 3-2-2 Simulated transmittance of a BK7 plate 5 mm of thickness, with each surface

coated with a layer of ZnS of λ/4 of thickness at 10 µm.

3.3. The Optical simulations

Thus, we started depositing an arbitrary thick ZnS layer on a glass sample. By

measuring its transmittance in the visible spectral range we reconstruct, owing to the

simulation, the real optical thickness of the treatment. During this phase, the

substrate temperature has been tweaked in order to achieve the best film adhesion

and compactness.

In fact, it is not easy to produce a ZnS layer: it sublimates, but while heating it

merges in a unique block; thus, explosive phenomena can occur at the ZnS/crucible

interface. In order to avoid such small explosions ejecting material out of the crucible,

a particular molybdenum cup has to be used. Then, the ZnS vapour condensates on

the substrate, but it can produce not uniform layers at room temperature. Thus,

heating the substrates at 150°C solve this last problem because the ZnS molecules

gain energy to form a compact and more adherent layer. At temperature higher than

150°C molecules may be sputtered, changing the effective deposition rate. Therefore,

the substrate temperature has to be carefully measured during the deposition and the

transmittance measurements have to be compared with the optical simulation in order

to understand if sputtering effects have occurred.



750 1000 1250 1500 1750 2000

70

80

90

Transmittance (%)

Wavelength (nm)

ZnS on Glass 50 A.U.

ZnS on Glass 62.5 A.U.

Fig. 3.3.1 Measurements of two different thickness of ZnS on glass. By comparing these

measurements with the simulation reported in Fig. 3.2.2 it is possible to determine the ZnS

thickness that acts as a λ/4 interferential layer on Ge at a wavelength of 10 µm.

The transmittance in the visible spectral range of two test deposition on glass is

shown in Fig 3.3.1. By comparing these measurements with the optical simulations

reported in Fig. 3.2.2 it is possible to determine the ZnS thickness that acts as a λ/4

interferential layer on Ge at a wavelength of 10 µm. Thus, we started depositing ZnS

on the Germanium test substrates.

0 2 4 6 8 10 12 14 16 18 20

0

10

20

30

40

50

60

70

80

90

100

Trasmittance (%)

Wavelength (micron)

Fig. 3.3.2 Measurements of the transmittance of a 5 mm thick test substrate in the IR

spectral range.



A preliminary transmittance measurement was carried out in the IR spectral range to

verify the optical properties of the material, as shown in Figure 3.3.2. The

transmittance always below 45% and it is almost flat in the region 2 µm 12 µm. The

low transmittance it is not due to material absorption ( k is almost equal to 0 in the

whole 2 µm 12 µm spectral range) but it is due to the high refractive index (n = 4.3)

that imply an high reflectivity.

2 4 6 8 10 12 14 16 18 20

-10

0

10

20

30

40

50

60

70

80

90

100

Trasmittance (%)

Wavelength (micron)

2 4 6 8 10 12 14 16 18 20

-10

0

10

20

30

40

50

60

70

80

90

100

Trasmittance (%)

Wavelength (micron)

Fig. 3.3.3 Measurements of the transmittance of a 5 mm thick test substrate with the optical

treatment. The left picture shows the transmissivity once the deposition was carried out on

only one Ge surface. On the right panel, the transmittance measurement after the ZnS optical

treatment has been carried out on both Ge surfaces is shown.

Then, the same thickness calculated from the tests on glass has been deposited on Ge

test substrates in order to perform a finer calibration. A single deposition has allowed

to calculate the final ideal thickness. The results of the transmittance measurements

carried out on the Ge test substrates are reported in Figure 3.3.3. In the left picture of

Fig 3.3.3 the measurement on a Ge substrate treated on a single face is reported. The

transmittance in the region of interest generally increases due to the wide peak

centred at 10 µm, where its value is 65%. By evaporating the optical coating on both

the surfaces of the test Ge optical window, the transmittance peak become more

evident. In the whole region between 7 µm and 14 µm the transmittance arise from

45% to above 70% with a maximum of 96% at 10 µm.



4. CONCLUSIONS: The IR optical window prototype

The enhanced IR transmittance obtained in a wide IR band with the ZnS AR filter is

sufficient to remove almost all the problem introduced by the high refraction index of

Ge. Thus, a prototype window has been fabricated in order to assess it in real

applications.

The window has been made by using an 8-mm-thick 50 mm germanium disk. The

deposition of the ZnS AR filter has been carried out in the same conditions of the test

substrates. The optical widow is shown in Fig 3.4.1.

Fig. 4.4.1 The Ge optical window. It has a 50 mm diameter and is 8 mm thick.

The spectral transmittance of the window before the optical treatments, after coating

one of the two surfaces and after coating both the surfaces are plotted in Figure 3.4.2.



2 4 6 8 10 12 14 16 18 20

-10

0

10

20

30

40

50

60

70

80

90

100

Trasmittance (%)

Wavelength (micron)

Trasm % Ge Substrate

Trasm % (1 Side treated)

Trasm % (2 Sides treated)

Fig. 4.4.2 Transmittance measurements of the and 8-mm-thick 50 mm Ge substrate (black),

after coating one of the two surfaces (blue), and the final transmittance of the optical window

(red).

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DEVELOPMENT OF AN INFRARED WIDE PASS BAND GERMANIUM …