Shakeel Ashraf Thesis - DiVA portal427144/FULLTEXT01.pdf · Shakeel Ashraf List of Figures 2011-06-27 ix Figure 25: Simulation and FTIR results for λ/4 structure with Ti= 60Å..34

Mid Sweden University The Department of Information Technology and Media (ITM) Author: Shakeel Ashraf E-mail address:[email protected] Study programme: Master in Electronics Design, 120 Hp Examiner: Göran Thungström, [email protected] Tutors: Claes Mattsson, [email protected] Scope: 9228 words inclusive of appendices Date: 2011-06-27

M.Sc. Thesis report within Electrical Engineering, course, 30 points

Evaluation of IR Absorbers Evolution of IR Absorber for Integration in an IR

Sensitive CO2 Detector

Shakeel Ashraf

Evaluation of IR Absorbers Shakeel Ashraf

Abstract 2011-06-27

ii

Abstract Keywords: Interferometric structure, Thermopile, Thermal detector, SU-8 2002, Lead Selenide, AFM, FTIR, Ultimate Specular Reflectance Acces-sory, Thin metal films.


Acknowledgements 2011-06-27

iii

Acknowledgements I would like to thanks my supervisors Dr. Cleas Mattsson for his guid-ance, ideas, suggestion and helping both in and out of the cleanroom throughout the thesis. I would like to thanks for Dr. Göran Thungström, for his time and discussions.

I would also like to acknowledge Mid Sweden University and the fac-ulty of Science, Technology and Media (ITM) for providing good study environment and lab equipment which make me possible to complete this work.

I would like to thanks for Muhammad Asif Javeed for his help in my report writing.

Lastly, I would like to thank my mother, father, brothers and sister for their encouragement. Finally, but most important I would like to thanks my wife and my son Muhammad Abdul Wasiu for their love and care through this time.

Thank You All!

Shakeel Ashraf Sundsvall, June 2011


Table of Contents

iv

Table of Contents

Abstract ............................................................................................................. ii

Acknowledgements ....................................................................................... iii

Table of Contents ........................................................................................... iv

Terminology .................................................................................................... viAcronyms viMathematical notation .................................................................................. vii

List of Figures ............................................................................................... viii

List of Tables ..................................................................................................... x

1 Introduction ............................................................................................ 11.1 Problem Motivation .................................................................... 21.2 Objective ....................................................................................... 21.3 Task descriptions ......................................................................... 4

2 Theory ...................................................................................................... 52.1 Infrared Radiation ....................................................................... 62.2 Thermal detectors ........................................................................ 82.2.1 Different thermal detectors ............................................. 92.3 Photon-Detectors ....................................................................... 102.3.1 Different photo-detectors .............................................. 112.4 Infrared absorption ................................................................... 132.4.1 Quarter-wave length structure (λ/4) ............................ 142.4.1.1 SU-8 2002 (Dielectric material) ..................................... 162.4.2 Lead Selenite (PbSe) ....................................................... 172.4.3 Black Paint ....................................................................... 182.5 Measuring Techniques .............................................................. 192.5.1 Fourier transform infrared spectroscopy (FTIR) ........ 192.5.1.1 VeeMAXTM II (The Ultimate Specular Reflectance

Accessory) ....................................................................... 202.5.2 Atomic Force Microscopy (AFM) ................................. 212.5.3 Energy-dispersive X-ray spectroscopy (SEM EDX) ... 21

3 Design & Fabrication .......................................................................... 223.1 Interferometric absorption structure layer design ............... 223.1.1 Thin Metals Multi-layer theory: ................................... 22


Table of Contents

v

3.2 Fabrication process .................................................................... 253.2.1 Fabrication of the interferometric structure ............... 253.2.2 Fabrication of Lead selenide samples .......................... 26

4 Results and discussion ....................................................................... 274.1 Titanium sheet resistance ......................................................... 274.2 FTIR results for λ/4 structure ................................................... 284.2.1 AFM characterization (λ/4 structure) .......................... 304.3 FTIR results for lead selenide .................................................. 364.3.1 AFM characterization (PbSe) ........................................ 374.3.2 SEM EDX characterization ............................................ 39

5 Summary ............................................................................................... 40

6 Conclusions .......................................................................................... 41

7 Future work .......................................................................................... 42

References ........................................................................................................ 43


Terminology 2011-06-27

vi

Terminology A possible list of terms, abbreviations and variable names are given below.

Acronyms SAR synthetic aperture radar

HVAC Heating, Ventilation and Air Conditioning

ASHRAC American Society of Heating, Refrigerating, and Air Conditioning Engineers

NDIR Non-dispersive infrared

CO2 carbon dioxide

CO carbon monoxide

MEMS Micro Electro Mechanical System

Al Aluminium

Ti Titanium

PbSe Lead Selenide

FTIR Fourier Transformation Infrared Spectroscopy

EMR Electromagnetic radiation

TCR Temperature coefficient of resistance

PVD Physical vapour deposition

CVD Chemical vapour deposition

Si Silicon

GaAs Gallium Arsenide

InAs Indium arsenide

SEM EDX Energy-dispersive X-ray spectroscopy


Terminology 2011-06-27

vii

SiO2 Silicon dioxide

AFM Atomic Force Microscope

Pb Lead

Se Selenium

C Carbon

Cl Chlorine

O Oxygen

Si3N4 Silicon Nitride

Mathematical notation Symbol Description

λ Wavelength


List of Figures 2011-06-27

viii

List of Figures

Figure 1: Spider mite on mirror assembly [6] ............................................... 2Figure 2: Interferometric absorption structure ............................................. 3Figure 3: Radiation Spectrum [8] .................................................................... 5Figure 4: Herschel’s experiment setup. .......................................................... 6Figure 5: Black body radiation at different temperature and the function of wavelength. [12] ............................................................................................. 7Figure 6: A general thermopile working principle. ..................................... 9Figure 7 : Schematic of Semiconductor Photodiode .................................. 12Figure 8 : Absorption vs. wavelength for different dielectric medium. .. 15Figure 9: Schematic for Interferometric structure with resistive film, dielectric layer and a perfect reflector. ......................................................... 16Figure 10: The SU-8 epoxy molecule structure [28] ................................... 17Figure 11: The working principle of FTIR spectroscopy along with VeeMAXTM II .................................................................................................... 19Figure 12 : FTIR spectroscopy along with VeeMAXTM II accessory ......... 21Figure 13 : Interferometric structure with SU-8 2002 is placed between semitransparent metal and reflector. ........................................................... 23Figure 14: Simulation results for quarter-wavelength structure at normal. .............................................................................................................. 24Figure 15: A schematic fabrication process: (A) formation of SiO2 layer, (B) formation aluminum layer, (C) formation of SU8-2002 layer, (D) deposition of Ti layers .................................................................................... 25Figure 16: Schematic view of lead selenide sample for different deposition times. ............................................................................................. 26Figure 17:(A) Sheet resistance of evaporated Ti layer on a silicon wafer, (B)Sheet resistance of Ti layer deposited on SU8-2002. ............................. 27Figure 18: Sheet resistance of Ti layer after fabrication of λ/4 structure. 27Figure 19: FTIR absorption results for quarter wavelength structure. .... 29Figure 20 : FTIR resutls along with simulation results. ............................. 30Figure 21: SU8-2002 sample for AFM measurements ................................ 30Figure 22: AFM topography images a(50umX50um) and b(57.4umX57.4um) for SU8-2002. ................................................................. 31Figure 23: FTIR results and Simulation results @3um ............................... 32Figure 24: FTIR results and Simulation results with 3um of thickness of SU8-2002 and reflective index value 2 ......................................................... 33


List of Figures 2011-06-27

ix

Figure 25: Simulation and FTIR results for λ/4 structure with Ti= 60Å .. 34Figure 26: Simulation results and FTIR results with different Ti thicknesses ........................................................................................................ 35Figure 27: Lead selenide absorption (%A) spectra ..................................... 36Figure 28: Lead Selenide layer thickness vs. deposition time. ................. 37Figure 29: AFM samples design for PbSe with different thickness ......... 37Figure 30: AFM topography image (2D&3D) for 20 min of deposition of PbSe. .................................................................................................................. 38Figure 31: The AFM studies results at different thickness. ....................... 38Figure 32: Quantitative results for 10 min and 20 min deposited PbSe. . 39


List of Tables 2011-06-27

x

List of Tables Table 1: Bandgap value, Density and Static Dielectic Constant for PbSe, PbS, PbTe .......................................................................................................... 18Table 2: Absorption values through Simulation ......................................... 24Table 3: AFM measument values for SU8-2002 .......................................... 31Table 4 : Simulation results and FTIR results ............................................. 34


1 Introduction 2011-06-27

1

1 Introduction The importance, demand and use of different sensors applications are increasing rapidly these days. The word sensor comes from Latin word “Sentire” which means “to sense” something. Presently various types of sensor are available to measure the absolute change in all physical quantities i.e to measure the air pressure, quantity of different gasses in environment, temperature, heat, light and many more. The sensors are categorized into two sub classes, active and passive sensors. The sensors which provide their own source of energy to investigate or analysis the target are known as active sensors. Advantages for this type of sensor include the ability to obtain measurement moment by moment. Examples of active sensors are a laser fluorosensor and synthetic aperture radar (SAR). The sensors which rely on energy, emitted from other source, are called passive sensors. Examples of passive sensor are our own eyes or infrared motion detectors, which can detect radiation emitted from a human body. [1] HVAC (Heating, Ventilation and Air Conditioning) is a perfect example of the combination of different sensors and actuators. HVAC systems are useful in maintaining good indoor air quality through controlled ventilation, sufficient filtration and controlled thermal comfort. In an indoor environment, the level of carbon dioxide must be constant in order to retain an acceptable air quality [2]. Carbon monoxide and carbon dioxide are toxic gasses [3] and in air which is indoors , the CO2 level must be less than 1000 ppm according to ASHRAC standards [4]. Non-dispersive infrared (NDIR) sensors can be used for the detection of these gasses. The NDIR system consists of an infrared source and an infrared detector, in which an optical filter is responsible for the selection of the target gas detection through its absorption line [5]. Thermal and photonic detectors are used in infrared detection. In the early days, thermal detectors were thought to be slow and low in sensitivity. The advancements in (Micro Electro Mechanical System) MEMS and micro-machining techniques have not only reduced the costs and size involved but also have increased the sensitivity in the fabrication of the sensors. These advancements have increased the usability of thermal detectors.



2

Figure 1: Spider mite on mirror assembly [6]

Figure 1 shows a spider mite less than 1mm sitting on a mirror assembly. This figure illustrates the advancement in the size reduction for MEMS structures.

1.1 Problem Motivation When the infrared radiation hits the surface of a thermal detector, the infrared is absorbed by the absorber layer on the detector surface. The absorbed IR heats up the absorption layer and a thermopile can detect this heat change by means of the underlying thermocouples, which convert this heat into an electrical output voltage. The motivation involved in pursuing this thesis work is related to the evaluation and investigation of different absorption layer structures. It should prove to be possible to integrate these structures in a closed SU-8 membrane in order to increase the sensitivity and performance for such a thermopile detector.

1.2 Objective The objective of this thesis work is to investigate different absorption layers that could be used in thermal detectors, such as a thermopile. A commonly used absorption layer for thermal detectors is black paint and this type of absorption layer has been previously used in the ther-mopile sensor targeted in this thesis. Black paints are low cost and can have rather high absorption efficiency, and are rather easy to apply. However, applications of black paint are dirty and constitute a non-



3

accurate process, which is not suitable for accurate cleanroom process-ing. Therefore it was felt to be interesting to find a suitable replacement which could be applied in a controlled manner inside the cleanroom facility. One type of infrared absorber, which has, theoretically, been shown to have an almost 100 % absorption efficiency, is the so-called interferometric absorber. Such a structure is shown in Figure 2

Figure 2: Interferometric absorption structure

In an interferometric absorption structure a dielectric medium (SU-8 epoxy) separates the two metal layers. The bottom layer (Al) should act as a perfect mirror, while the top layer(Ti) should be semi-transparent. If an incident radiation is partially transmitted and partially reflected through the top layer of the metal, then partially transmitted radiation is reflected back from the mirror metal covering λ/2 distance in the dielec-tric medium, which then falls out of phase with the radiation that is reflected partly from the top surface, and these two reflected waves –i.e partially transmitted and partially reflect waves, interfere destruc-tively [40]. Thus the maximum absorption can be achieved at λ wave-length.

Top metal

Metallic mirror

Dielectric layer

d=λ/4



4

The second type of infrared absorber, which will be investigated, is in relation to the use of (PbSe) lead selenide as an infrared absorber. Lead selenide is a semiconductor material, which has a direct bandgap and is often used in infrared detectors within the wavelength region between 1.5-5.2 μm. Carbon dioxide has a strong absorption peak at 4.26μm, which means that it should be possible to use PbSe as an absorption layer in such a detector set-up.

1.3 Task descriptions The task descriptions below are given in the order as they were per-formed.

• Study of the literature of thermal detectors and Infrared absorption layers.

• Simulation in Matlab, applying the multilayer theory in order to perform the theoretical results for λ/4 structure.

• Thermal oxidation of silicon wafers and deposition of a thin film using evaporation– preparation of different samples (Silicon wafer + Ti) of different thicknesses of titanium (Ti) for the calculation of sheet resistance.

• Preparation of different samples of (Silicon wafer + SU-8 2002 + Ti) of different thicknesses of titanium (Ti) metal for the calculation of sheet resistance.

• Preparation of different λ/4 structure samples ( silicon wafer + Aluminum + SU-8 2002 + Ti with different thicknesses)

• Preparation of the PbSe absorption layer, by using sputtering ( silicon wafer + PbSe deposition for different times)

• Evaluation of all the constructed samples using FTIR

• Measurements of thickness of SU82002 and lead selenide through AFM

• Composition of the final report.


2 Theory 2011-06-27

5

2 Theory Radiation is energy, transmitted through space or matter in the form of particle or electromagnetic waves. All objects radiate energy to their surroundings above a temperature of the absolute zero (-273° C). Elec-tromagnetic radiation (EMR) is emitted in discrete units that are known as photons which travel at the speed of light in the form of electromag-netic waves. Moreover electromagnetic energy is classified into sub groups and these are distinguished by increasing energy or decreasing wavelengths such as radio waves, microwaves, infrared, visible light, ultraviolet, x-rays and gamma- rays [7].

Figure 3: Radiation Spectrum [8]

A small portion of electromagnetic radiation, visible through the human eye, is the visible portion from 380 nm to 740 nm which is shown in Figure 3.


2 Theory 2011-06-27

6

2.1 Infrared Radiation The portion of the spectrum for infrared radiation consists of wave-lengths from 700 nm up to 1mm. The infrared radiation region was discovered by Sir Fredrick William Herschel in 1800 [9]. In his experi-ment he directed sunlight through a prism and created a light spectrum and then measured the temperature of each colour with a thermometer and, in addition, in order to provide better absorption, the bulbs of the thermometers were blackened. When he measured the temperature just beyond the red portion, he was surprised to note that this region pro-duced the highest temperature. This region is now known as the infra-red region. The experimental set-up is illustrated in Figure 4.

Figure 4: Herschel’s experiment setup.

The infrared region is subdivided into three regions, which are known as the NIR (Near Infrared, 700 nm – 1400 nm), MWIR (Mid Wavelength Infrared, 1400 nm – 3 μm) and LWIR (Long Wavelength Infrared, 3 μm – 1mm) [10]. Nevertheless, these boundaries are altered in literature relating to astronomy and ISO 20473.

All objects having a temperature above the absolute zero (-273.150 C) emit radiation. Although some solid materials are not yet absolute emitters, their emissivity is less than 1 [11]. For a hot body at a specific temperature, the power of the emitted radiation varies according to the wavelength.


2 Theory 2011-06-27

7

Figure 5: Black body radiation at different temperature and the function of wavelength. [12]

At the surface of the sun the temperature is 6000K. This temperature has its peak in the yellow part of the visible spectrum and is the reason for sunlight to appear to be yellow. However, it is the case that a black body absorbs all the radiation which falls on it. For an ideal black body, the emitted radiation is equal to the absorbed radiation. Plank's law de-scribes the spectral radiance from a black body at all temperatures as shown in Figure 5. The normal skin temperature of a human body is 34°, which has its peak emittance in the infrared spectrum at about 10 μm. In the atmosphere water vapour absorbs and emits some radiation which thus enables the earth to remain habitually warm.

A wide variety of infrared detectors can be found in both civil and military applications. Infrared detectors have been widely used during the last two decade. Infrared detectors are categorized into two main groups, namely photo-detectors and thermal detectors. The choice of detector depends on the application specification. In some cases, the requirement is for both speed and a high sensitivity, whereas in other cases a slower detector with a lower response might prove to be suffi-cient. A brief explanation with regards to thermal and photo-detectors is provided below.


2 Theory 2011-06-27

8

2.2 Thermal detectors The incident radiation heats up the surface of thermal detectors and this heating affects the property of the heated material such as its electrical conductivity k (W/K m). The response of a thermal detector is usually independent of the wavelength of the incident radiation. Using this thermal detector characteristic, it becomes possible to utilize this for specific purposes. If a specific wavelength band is required in a specific application, it is possible for the unwanted wavelength to be rejected by using an interference filter at the front of detector. The effective thermal conductivity GR is given by (Ballingall, 1990) Equation 1 [13]:

Equation 1

where σ = 5.67 X10-12 W cm-2 K-4 is the Stefan-Boltzmann constants [14].

T is the detectors temperature and A is the sensitive area of the detec-tor. Thus the detector's limit is given by Equation 2

Equation 2

where k (W m-1 K-1) is the thermal conductivity and G is the limit of GR.

Theoretically, a maximum of D = 1.8 X 1010 Hz½W-1 can be obtained with thermal detectors [13].

𝐺𝑅 = 4𝜎𝑇3𝐴

𝐷 = 𝐴4𝑘𝑇2𝐺


2 Theory 2011-06-27

9

2.2.1 Different thermal detectors Thermoelectric:

Thermocouples convert a thermal signal into an electrical sig-nal. The junction of dissimilar metals generates a voltage po-tential and this generated voltage is directly proportional to the temperature and is described as follows [15] in .

( ).abV S T T∆ = ∆ Equation 3

The operational principal of the thermocouple is based on the Seebeck effect, and Sab is known as the Seebeck coefficient. It is possible to enhance the sensitivity by serially connecting several thermocouple junctions.

Figure 6: A general thermopile working principle.

Using advanced techniques from the semiconductor processing field, multiple junctions of dissimilar metals are connected in series to form the thermopile structures. The re-sponse time of a thermopile is dependent on its thermal mass and thermal capacity.

Bolometer:

I n a bolometer, the resistance of fine wire is changed when infrared radiation interacts with the detector. Bolometers are often connected in a Wheatstone bridge configuration. One of the drawbacks associated with bolometers and other thermal detectors is their relatively slow response time (τ~ 1 to 100 ms), which limits their use to slow varying process [13]. An

Incident radiat ion

ReferanceCold Junct ion

ReferanceHot Junct ion

Metal ‘A’

Metal ‘B’

ΔV


2 Theory 2011-06-27

10

important parameter for bolometers is the temperature coeffi-cient of resistance,TCR, which is denoted by α and is given in equation 4 [16].

1 ddTρα

ρ= Equation 4

where ‘ρ’ is the electrical resistivity of the material and ‘T’ is the temperature. In general the resistivity of metals increases with temperature, which results in positive values for α. Pla-tinum and Nickel are the most commonly used metals and their values for ‘α’ are around 0.005 K-1. In order to provide greater sensitivity, semiconductor materials such as silicon can be used

2.3 Photon-Detectors The energy of photons depends upon their wavelength and intensity. A photon detector detects the quantum interaction among incident pho-tons and the semiconductor material. A photon of sufficient energy which strikes an electron in a valance band could raise the energy of the electron. The excited electron jumps to a conduction band and causes conductivity in the substrate. In semiconductor materials the bandgap energy is less than that for the insulators. When the infrared energy raises the thermal temperature of the semiconductor material, an elec-tron having a greater energy than the bandgap energy can jump into the conduction band. The Planck energy relation is used to calculate the energy of the incident photon.

Equation 5

where ‘h’ is the Planck’s constant and ‘v’ is the frequency of the incident photon.

𝐸 = ℎ ∙ 𝑣


2 Theory 2011-06-27

11

2.3.1 Different photo-detectors Photoconductor

Semiconductors which detect the incident light by means of the change in electrical conductivity when they are exposed to light are known as photoconductors. Automatic light switch-ing control systems in homes and burglar alarms and light meters in cameras are various applications for these detectors. The optical sensitivity of these photoconductors can be eva-luated by means of the optical generation rate gop. The gener-ated change in the photo-conductivity caused by the incident light is given by the equation 6 [16]

Equation 6

.

where, ‘gop’, ‘μ’ and ‘τ’ are the optical generation, carrier life-time and mobility (of electron and holes) respectively.

When a voltage ‘V’ is applied across a semiconductor slab with length ’L’ depth ‘D’ and width ‘W’, there is change in the photo-conductivity and a current is induced Δi in the semi-conductor's slab which is given in equation 7 [17]

Equation 7

Cadmium sulfide (CdS) and cadmium selenide (CdSe) are the both used as low cost visible radiation sensors [29]. They have a high photoconductive gain of about 103 to 104 but they have a poor response time of around 50 ms. Lead sulfide is another well known photoconductor material for the near-IR within the wavelength range from 1μm to 3.4μm.

Photodiode

A photodiode is a p-n junction which operates within low or zero reverse bias voltage. When infrared radiation strikes the surface of a photodiode, the depletion region works to sepa-rate the photo-generated electron-hole pairs. To reduce the transit time, the depletion region is retained at a very thin lev-el for high frequencies operations. The number of electron-hole pairs generated for each incident photon is known as the quantum efficiency. There is always trade-off between the

Δ𝜎 = 𝑞𝑔𝑜𝑝𝜏𝑛𝜇𝑛 + 𝜏𝑝𝜇𝑝

Δ𝑖 =𝑊𝐷𝐿

Δ𝜎𝑉


2 Theory 2011-06-27

12

quantum efficiency and the response speed. Quantum effi-ciency is given by the formula [18].

Equation 8

In the above Equation 8 Ip is the current, generated by the ab-sorption of incident optical power Popt at wavelength ‘λ’.

Figure 7 : Schematic of Semiconductor Photodiode

Materials with high quantum efficiency are Ge, Ga, In and As. The photodiodes are cooled down to 77K because a small change in temperature causes the change in the energy gap. The electrons will jump to the conduction band based on the change in the energy gap. Thus, in order to obtain high-efficiency and to cause a longer wavelength operation, the photodiodes are cooled down.

pn

SiOLight

Metal Contact++ n-type Silicon

2

𝜂 = 𝐼𝑝𝑞 𝑃𝑜𝑝𝑡ℎ𝑣

−1


2 Theory 2011-06-27

13

2.4 Infrared absorption Infrared absorption layers play a key role in the performance of thermal detectors. The absorption in a thermal detector must be high for high sensitivity. An absorption layer absorbs incident infrared radiation and converts it into the heat. When constructing absorption layer there is a trade-off between the mass of the layer and the thermal time constant of the detector. The use of a thicker absorption layer might increase the absorption efficiency but this will also cause an increase in the thermal capacitance of the detector, which, in turn, will increase the time con-stant.

The radiation absorbed by the layer can be detected in different ways. Firstly [20], by using the Seebeck principle (thermopiles and thermo-couples) in which the voltage changes with a change in temperature. Secondly, this can be achieved by measuring changes in the electrical conductivity parameters (bolometers) and finally, can be detected by using pyro-electric detectors which work with the change in electrical polarization. The absorption layers could increase the sensitivity and response time of a thermal detector within the specific wavelength. The optical absorption, reflection and transmission are related to each other by means of Equation 9 [13]

Equation 9

In the above equation, Aop is optical absorption, Rop is reflection and Top is optical transmission.

In this thesis work, an evaluation of different absorption structures and layers will be discussed. Firstly, an interferometric structure, in which 100% absorption can be achieved at the desired wavelength, will be investigated. The quarter wavelength structure consists of a perfect metal reflector, a thin metal film on top and dielectric layers in-between. Secondly, the uses of lead chalcogenide semiconductor materials are well known in IR detectors. In this thesis, only the use of lead selenide PbSe as an absorption layer will be discussed and evaluated. The use of PbSe as an infrared absorber is suitable between 3 to 7μm wavelength [21].

opopop TRA −−=1


2 Theory 2011-06-27

14

2.4.1 Quarter-wave length structure (λ/4) According to classical electrodynamics theory”The transmission occurs through the thin metal films and thick metals films act as perfect reflec-tors“ [19]. The reason behind this reflection from thick metal films is the high mobility of an electron in the film. The reflection from thick metal films will thus be a consequence of the high mobility of electrons and the absence of electric field strength i.e. electric fields power parallel to surface. The transmission through thin metal films is the result of insuf-ficient electrons currently existing in order to interact with the incident wave [19]. Thin metals films of sheet resistance 189 Ω/ would absorb 50% of the incident resistance [22]. Using a structure which consists of a thin metal film in a dielectric layer and a perfect reflector, could, in theory, absorb 100% of incident radiation at a desired wavelength [23]. In this structure, the resistance of the thin metal should be equal to the impedance of the free space i.e. 377 Ω/. The thickness of the dielectric layer is determined by Equation 10

Equation 10

where n is the reflective index of the dielectric layer and d is the thick-ness of the dielectric layer. [20]

According to [24], the spectral absorption α(λ) can be calculated accord-ing to Equation 11. In this equation Rr and Rs are the sheet resistance of the reflector layer and the thin metal films, respectively.

( ) ( )2

2 22 2

14( ) sin coss rr r s

f ff f f

Dn nα λ θ θ

+ = + + +

Equation 4

( )( ) 22 2

2 2

1 1 21 sin cos

120

120

2

r s r s

rr

ss

where

f f f fDn n

fR

fRnd

θ θ

π

π

πθλ

+ + + + = + +

=

=

=

......3,2,1,0,4

)12(=

+= m

nmmd


2 Theory 2011-06-27

15

If the sheet resistance of the reflecting layer is zero, the above formula could be derived as [22].

( )2 2 2

4( )1 cot

s

s

ff n

α λθ

=+ +

Equation 5

where Rs = 377 Ω/ and d is given by the above Equation 10.

Figure 8 : Absorption vs. wavelength for different dielectric medium.

Figure 8, shows the variation of absorption vs. wavelength for two interferometric absorber structures for different values of the refractive index. In this case, the refractive index of the dielectric medium for air is about 1 and for the SU8-2002 is about 1.575 [25]. The refractive index of the SU-8 is given for a wavelength of 1.56μm. However, in this simula-tion, this value is assumed to be constant and this is also the case for longer wavelengths. Equation 12, further assumes that the absorber structure is located in vacuum. Figure 9 shows a schematic image of the quarter wavelength absorber structure.


2 Theory 2011-06-27

16

Figure 9: Schematic for Interferometric structure with resistive film, dielectric layer and a perfect reflector.

2.4.1.1 SU-8 2002 (Dielectric material) The class of material [25], which have many exclusive characteristics such as high optical clearness, low attenuation and outstanding survival capability in any weather, are known as polymers. Polymer materials are used in many electronic and photonic measurement systems and possess a large number of different properties. SU-8 2000 is a negative epoxy based photoresist fabricated by MicroChem. and has been widely used in MEMS application for many years [26]. SU-8 has an optical transmission of more than 92% for wavelengths above 415nm. The material can be applied in thicknesses ranging from less than 1 μm to more than 100μm [27].

SU-8 2000 is applied through standard photolithographic processing steps, which means spin coating and UV lithography. These types of equipment are available in all cleanroom facilities. SU-8 has a refractive index value of 1.575 @ 1550nm [25]. SU-8 epoxy resins contain one or several epoxy groups, as shown in Figure 10, which contains eight such groups [28]


2 Theory 2011-06-27

17

Figure 10: The SU-8 epoxy molecule structure [28]

The thermal conductivity of SU-8 is 0.3 (W/m OK) [26]. This is very low compared to silicon and silicon nitride Si3N4, which have thermal con-ductivities of 150 W/m K and 2.3-30 W/m K, respectively [29]. Due to these properties they are suitable for use in applications requiring low thermal conductivity, such as in thin membranes in thermal detectors.

In this thesis, a quarter wavelength structure, as shown in Figure 9, has been fabricated utilizing SU-8 as dielectric layer.

2.4.2 Lead Selenite (PbSe) Lead selenide is a narrow bandgap semiconductor material and this material has a number of applications in infrared detectors, laser and many other electronic devices.

PbSe has a direct bandgap at room temperature of 0.27 eV, but its per-formance can be increased by decreasing the temperature. Several methods could be used for the deposition of a PbSe thin film. Examples of these include, thermal evaporation, molecular beam epitaxial growth method, physical vapour deposition (PVD) and chemical vapour depo-sition (CVD) [38]. From this list, thermal evaporation is the most com-monly used method. Table 1 shows the material properties of PbSe in comparison to Lead Sulphide and Lead Telluride.


2 Theory 2011-06-27

18

Table 1: Bandgap value, Density and Static Dielectic Constant for PbSe, PbS, PbTe

PbS PbSe PbTe

Static dielectric Constant 161 280 360

Density(g cm-3) 7.61 8.15 8.16 Bandgap at 77K(eV) 0.307 0.176 0.217

Bandgap at 300K(eV) 0.41 0.27 0.31

Bandgap at 373K(eV) 0.44 0.31 0.34

As can be seen in table 1, the bandgap of PbSe is lower at a low tempera-ture as compared to higher temperatures. The bandgap value for PbSe is also comparatively small when it is compared to other lead chalco-genides such as PbS and PbTe.

The static dielectric is defines as” It is the function of temperature and frequency at which the alternation electric field varies” [39].The value of the static dielectric constant of PbSe is 280 as a function of temperature, which is comparatively high as compared to other semiconductor materials such as Si = 11.8, GaAs = 13.2 and 0InAs = 14.6 [21]. The refrac-tive index value for PbSe is reported in [31] within the wavelength range 5.4–8μm is 4.8636 – 4.7789 at room temperature.

2.4.3 Black Paint Black paints are well known infrared radiation absorbers. They could have high absorption in specific wavelength.

Black paints have shown good absorption results when used as the absorption layer in thermal detectors. However, the application method often makes it difficult to control the surface roughness, layer thickness and area. Thus it becomes more interesting to develop absorption struc-tures which can be manufactured with greater precision.


2 Theory 2011-06-27

19

2.5 Measuring Techniques 2.5.1 Fourier transform infrared spectroscopy (FTIR)

FTIR spectroscopy has been used for the analysis of different composi-tion and materials. The word FTIR stands for Fourier Transform Infra-red. In the FTIR spectroscopy, infrared radiation passes through a sample and here some of the infrared radiations are absorbed by the sample and some are transmitted through the sample. The working principle for an FTIR spectrometer set-up has been illustrated in Figure 11

Figure 11: The working principle of FTIR spectroscopy along with VeeMAXTM II

In transmission mode, the infrared transmission through the sample is measured by an infrared detector. However, in figure 11, the sample compartment is equipped with a VeeMAXTM specular reflectance acces-sory, which makes it possible to measure the reflectance rather than the transmittance.

Some significant benefits and uses for FTIR are listed below;

• It could be used to identify the different objects.

• It is utilized to determine the quality or consistency of a given sample.


2 Theory 2011-06-27

20

• It is employed to determine the amount of components in a mix-ture.

• It is a non-destructive technique which provides a precise mea-surement method that requires no external calibration.

• It increases the speed for the collection in a scan to every second.

• It also increases sensitivity – one second scans can be co-added together in order to overcome random noise.

• It is mechanically simple as it only consists of one moving part and also has a greater optical output.

In this thesis, FTIR in specular reflectance mode has been used for the analysis of different samples. A brief description of the the VeeMAXTM II accessory is given below [33] [34].

2.5.1.1 VeeMAXTM II (The Ultimate Specular Reflectance Accessory) VeeMAXTM II is the product of PIKE Technologies. The VeeMAXTM has the ability to measure a high specular reflectance for a wide range of samples. VeeMAXTM II also has the ability to change the incident angle between 30°–80°. The sample is placed on the top with the sampling surface facing downwards. Figure 12 shows a schematic description of the VeeMAXTM II.

The infrared beam from the spectrometer is incident on the input mir-ror, which collimates and reflects the beam to other reflecting mirrors. After this, the beam hits one side of an upward adjustable sliding mir-ror, which directs the beam towards the big parabolic mirror. As the beam is collimated, the big parabolic mirror produces a focused spot at the sample position. As mentioned previously, the incident angle can be varied from 30°–80° using a sliding mirror. The reflected beam from the sample passes through the same set of mirrors on the output side so as to be detected by the FTIR IR detector.


2 Theory 2011-06-27

21

Figure 12 : FTIR spectroscopy along with VeeMAXTM II accessory

In this thesis work, all samples have been analyzed with the FTIR to-gether with VeeMAXTM II accessory. For all samples, reflectance, trans-mittance and absorption have been measured.

2.5.2 Atomic Force Microscopy (AFM) The thickness of the SU8-2002 layer and lead selenide layer are analyzed through a Atomic Force Microscopy.

2.5.3 Energy-dispersive X-ray spectroscopy (SEM EDX) The composition of lead selenide PbSe is analyzed through an Energy-dispersive X-ray spectroscopy (SEM-EDX)

Sample FT

-IR R

aditi

on

Det

ecto

r

FT-IR

Rad

ition

In

put


3 Design & Fabrication 2011-06-27

22

3 Design & Fabrication Design and Fabrication of infrared absorption structures

3.1 Interferometric absorption structure layer design 3.1.1 Thin Metals Multi-layer theory:

Multilayer theories are appropriate approaches in order to explain the wave propagation in multilayer structures in which optical properties of the medium vary in one direction. Consider a single layer of a dielectric having no and nT indices and a thickness l between two media. The amplitude of the incident electric field vector is EO, ER for the reflected beam and ET for the transmitted. The boundary condition can then be expressed in matrix form as shown below [35].

1 1 1

o o T

r M tn n n

+ = −

where 'o

o

ErE

= , T

o

EtE

= ,

M is known as the transfer matrix 1

1

cos sin

sin cos

ikl klnM

in kl kl

− = −

For N multilayers, the above equation can written as

1 2 3 4

1 1 1 1.......... N

o o T T

r M M M M M t M tn n n n

+ = = −

The transfer matrix M is the overall product of all transfer matrixes, M1M2.M3.M4.M5……..MN =M. Now suppose that A, B, C and D are the elements of the transfer matrix as:

1 2 3 4.......... N

A BM M M M M M

C D

= =

By solving the above equation for ‘r’ and ‘t’, we obtain



23

2

o T o T

o T o T

T

o T o T

An Bn n C DnrAn Bn n C Dn

ntAn Bn n C Dn

+ − −=

+ + +

=+ + +

The reflectance R and transmittance T are calculated by taking the square of the absolute value of r and t as shown below.

2

2

| || |

R rT t=

=

The absorption can be calculated by the relation given below.

A = 1 – R – T Equation 6

where, A= Absorption, R= Reflectance, T= Transmittance

The proposed interferometric absorption layer structure is illustrated in Figure 13. In this structure, the top semitransparent metal consists of a 50Å titanium layer, 2μm SU-8 2002 as the dielectric layer and a 2000Å thick aluminum mirror.

Figure 13 : Interferometric structure with SU-8 2002 is placed between semitransparent metal and reflector.

Aluminum has a lower density and electrical resistivity when compared to titanium. Because of the higher electrical resistivity of titanium, it is more suitable as a semitransparent top layer. Aluminum has reflectivity >90 % within the wavelength range 0.2–10 μm at T= 295 K [36], which makes it a perfect choice as a backside mirror material.



24

Absorption in a quarter wavelength structure consisting of a 60 Å tita-nium layer, 2μm SU-8 and a 2000Å aluminum layer was simulated and the results from this simulation are shown in Figure 14.

Figure 14: Simulation results for quarter-wavelength structure at normal.

The results show that 97% absorption has been noted at wave number 1106.19, which corresponds to a wavelength of 9.04μm. The simulation results are summarized in table 2

Table 2: Absorption values through Simulation

Wavenumber 1106.19 2531.6 4048.5 λ in μm 9.04 3.95 2.47

Absorption in % 97.0782 84.74 79.8537



25

3.2 Fabrication process 3.2.1 Fabrication of the interferometric structure

A schematic description of the fabrication process is shown in figure 15. In the process, a 4” silicon wafer of thickness 525 μm was used. Initial-ly the silicon wafer was cleaned using a standard cleaning step. In the next stage, the silicon wafers were thermally oxidized to form 1.1 μm SiO2 layer on the silicon wafer as shown in Figure 15 (A). After this, a layer of aluminum Al of 2000Å was deposited on the substrate using a thermal evaporation process. As previously mentioned this layer will act as a reflector as shown in Figure 15 (B). After completing the thermal evaporation process, a layer of 2μm thick SU8-2002 epoxy was formed on the substrate using a photo-resist spinner rotating at 3000 rpm [26]. The resulting layer is shown in Figure 15 (C) This SU8-2002 layer will act as the dielectric medium. In the final stage, the titanium layer was evaporated on the top of the wafer as shown in Figure 15 (D)

Figure 15: A schematic fabrication process: (A) formation of SiO2 layer, (B) formation aluminum layer, (C) formation of SU8-2002 layer, (D) deposition of Ti layers



26

3.2.2 Fabrication of Lead selenide samples The schematic view of the fabrication of the PbSe samples is shown in Figure 16 and for this fabrication process the same type of wafer was used. Initially, the wafer was cut down into four pieces. Then a thin film of lead selenide was deposited on each piece for different times using argon plasma sputtering. By using different deposition times, this resulted in samples with different thicknesses. The PbSe deposition was followed by a thermal annealing at 400°C in order to make the PbSe infrared active. The thickness of the PbSe layer was estimated by using the mass of the sample before and after deposition, and also by meas-urements using an atomic force microscope (AFM). The calculation has been given in the results section. After annealing the samples, the ab-sorption has been measured through FTIR spectroscopy. The results have been given in the results section.

Figure 16: Schematic view of lead selenide sample for different deposition times.


4 Results and discussion 2011-06-27

27

4 Results and discussion 4.1 Titanium sheet resistance

The sheet resistance of titanium Ti layer was measured with the help of a four point probe for all layer thicknesses. The results from the mea-surement are presented in Figure 17.

Figure 17:(A) Sheet resistance of evaporated Ti layer on a silicon wafer, (B)Sheet resistance of Ti layer deposited on SU8-2002.

Figure 18: Sheet resistance of Ti layer after fabrication of λ/4 structure.

1000; 7,78500; 17,22100;

119,20

50; 371,67

40; 594,82

30; 980,54

0,00

150,00

300,00

450,00

600,00

750,00

900,00

0 150 300 450 600 750 900 1050

Shee

t Res

ista

ce Ω

/

Ti Metal Sheet thickness in Å

Sheet Resistace Ω/ vs Thickness of Ti with SU-8 2002

1000; 6,88500; 16,53100; 89,96

50; 259,20

40; 367,38

30; 585,37

0,00

100,00

200,00

300,00

400,00

500,00

600,00

0 150 300 450 600 750 900 1050

Shee

t Res

ista

ce Ω

/

Ti Metal Sheet thickness in Å

Ti Thickness vs Sheet resistance Ω/

A B

40; 710,90

50; 468,0060; 345,15

0,00

150,00

300,00

450,00

600,00

750,00

0 10 20 30 40 50 60 70

Shee

t res

ista

nce

Ohm

/squ

re

Ti Metal Sheet thinkness in Å unit

Sheet Resistance vs Thinkness of Ti(@ λ/4 Structure )



28

The Ti has a low sheet resistance for thick layers. The sheet resistance gradually increases as the thickness gradually decreases. The resistivity for thin films is known to be higher as compared to that for the bulk resistivities. This can be seen in Figures 17 A and B, where the sheet resistance rapidly increases below a thickness of about 100 Å. It has been noted that at 40 Å the sheet resistance is 367 Ω/ for a Ti layer deposited on an oxidized silicon wafer sample. When the titanium is deposited on the SU-8, a value of 371 Ω/ ,for a layer thickness of 50 Å, has been measured. Finally, after completing the whole quarter wave-length structure, a similar sheet resistance is observed at a Ti thickness of about 60Å. A possible explanation for this might be that the addition-al layers increase or decrease the stress of the metal film. Electrical resistivity is known to be affected by stress, which means that the sheet resistance for a certain film thickness may change because of the stress. This means that for the quarter wavelength structure a Ti thickness of 60 Å is closer to 377 Ω/ , which according to the theory is the optimum resistance value for the semitransparent layer. However, for these film thicknesses, the accuracy of both the four-point probe measurement and the layer thickness given by the evaporator introduces uncertainties in the measured values, which might affect the results.

4.2 FTIR results for λ/4 structure In FTIR spectroscopy, the detector of the FTIR detects the transmitted radiation through the sample and calculates the absorption with respect to those values. In this case, an additional component has been used with the FTIR called the VeeMAXTM II. As mentioned previously, with this accessory, the radiation reaching the detector is the radiation re-flected from the structure rather than the radiation transmitted through the sample. However, the FTIR still regards the radiation that reaches the detector as the transmittance value and therefore calculates the absorption according to equation 14. Because of the thick aluminium mirror on the back side of the structure, the transmission through the sample is considered to be zero. This means that the absorption value given by the FTIR could be used to calculate the absorption by combin-ing Equation 14 and Equation 15 [37].



29

10log 10 absorbanceAabsorbance transmittance transmittanceA T T −= − ⇒ = Equation 7

1absorption transmittance reflectanceA T R= − − [13] , 0reflectanceif R =

1absorptance transmittanceA T= − Using Equation 9

In Figure 19, the absorption values calculated from the FTIR measure-ment are given. In relation to this measurement the incident angle was 30 and the titanium thickness was 60Å. It can be seen that about 98% of incident radiation has been absorbed at a specific wavelength, and only around 2% is reflected. As shown in Figure 19, noise occurs specifically around the 3000 and 500–1500 wavenumbers. This noise comes from the specific absorption peaks in the SU8-2002 epoxy resin. This has been verified using the build material library in the FTIR equipment.

Figure 19: FTIR absorption results for quarter wavelength structure.

0 500 1000 1500 2000 2500 3000 3500 4000 45000

10

20

30

40

50

60

70

80

90

100

wavenumber cm-1

% A

bsor

ptio

n

% Absorption vs. wavenumber cm-1



30

Figure 20 : FTIR resutls along with simulation results.

In Figure 20, both the measurment and the simulations results are plotted. As can be seen, the simulation results has three absorption peaks, but the FTIR results has five absorption peaks. If the incident radiation has an angled incidence, as shown in Figure 20, the absorption peaks shift towards the shorter wavelengths as compared to theabsorption spectra for the simulation in Figure 14. Moreover FTIR results show comparatively low values for the absorption peaks.

4.2.1 AFM characterization (λ/4 structure) To analyze the problem, the thickness of SU8-2002 has been characterized through AFM.

Figure 21: SU8-2002 sample for AFM measurements

0 500 1000 1500 2000 2500 3000 3500 4000 45000

10

20

30

40

50

60

70

80

90

100

wavenumber cm-1

% A

bsor

ptio

n% Absorption vs. wavenumber cm-1

Simulation results for AbsorptionFTIR Absorption resutls

SU8-2002 Bulk material

SU8 thickness



31

For this purpose steps of SU8-2002 have been fabricted on the silicon wafer in order to measure the thickness as shown in Figure 21

The AFM results for SU8-2002 the thickness are given in table 3

SU82002-@3000RPM

No of Observation

No of Samples 1st 2nd 3rd AVG Original %Error

1 2.812 3.081 2.982 2.958333 2 32.39437

2 3.013 2.871 3.103 2.995667 2 33.2369

3 3.1581 3.216 2.812 3.062033 2 34.68392

4 2.779 3.103 2.9055 2.929167 2 31.72119

5 2.955 2.901 3.13 2.995333 2 33.22947

Table 3: AFM measument values for SU8-2002

The AFM topography images are shown in Figure 22 and here the black lines show the steps in the SU8-2002.

Figure 22: AFM topography images a(50umX50um) and b(57.4umX57.4um) for SU8-2002.

The results from above Table:3 clearly show that the thickness of the SU8-2002 is around 3μm instead of 2um. The new simulation results together with the FTIR results are shown in Figure 23



32

Figure 23: FTIR results and Simulation results @3um

As seen in Figure 23, the simulation shows four absorption peaks in-stead of three. However, there is still some difference between the simulation and the measurement results. An important parameter is the refractive index of the dielectric material. For the entire wavelength range, the refractive index value of 1.575 for SU8-2002 has been used in the simulation. The variations in the results may be possibly caused by changing the value of the refractive index. If it is assumed that this value is, instead, equal to 2 it can be seen in Figure 24 that this has a consider-able effect on the result.

0 500 1000 1500 2000 2500 3000 3500 4000 45000

10

20

30

40

50

60

70

80

90

100

wavenumber cm-1

% A

bsor

ptio





33

Figure 24: FTIR results and Simulation results with 3um of thickness of SU8-2002 and reflective index value 2

As seen in Figure 24, the number of peaks is now equal, but the positions of the peaks are not in the same position. By increasing the refractive index even further, as shown in Figure 25, it is possible to achieve a higher degree of similarity between the measurement and simulation results. It should be noted that the used SU-8 has passed it expiration date, and this may have affected the thickness and the material parameters.

0 500 1000 1500 2000 2500 3000 3500 4000 45000

10

20

30

40

50

60

70

80

90

100

wavenumber cm-1

% A

bsor

ptio





34

Figure 25: Simulation and FTIR results for λ/4 structure with Ti= 60Å

Simulation Results

1st peak 2nd peak 3rd peak 4th peak 5th peak

λ in μm 9.1701 5.6402 4.04 3.13001 2.55

Wavenumber 1090.5 1773 2475 3194.88 3921.5

% Absorption 87.64 77.32 72 69 67.43

FTIR Results

1st peak 2nd peak 3rd peak 4th peak 5th peak

λ in μm 9.4797 5.5998 4.064 3.15222 2.58

Wavenumber 1054.888 1785.785 2460.76 3172.37 3868.565

% Absorption 98.82 98.4677 98.29 98.29 97.97

Table 4 : Simulation results and FTIR results

Thus, by considering a reflective index value of 2.4 for the SU8-2002, the simulation and FTIR results produce a good agreement. In Figure 25 for the FTIR results at a wavelength of 9.47μm that 98.82% absorption has been noted. With a decrease in wavelength, 98.46% absorption has been noted at a wavelength of 5.6μm. These values for the absorption have been taken for a titanium thickness of 60Å.

0 500 1000 1500 2000 2500 3000 3500 4000 45000

10

20

30

40

50

60

70

80

90

100

wavenumber cm-1

% A

bsor

ptio





35

.

Figure 26: Simulation results and FTIR results with different Ti thicknesses

It has been determined from the FTIR results and the simulation results, that the maximum absorption has been noted as 99% for the λ/4 struc-ture with a Ti thickness of 60Å. In Figure 26, the absorption for the three different titanium thicknesses, 40, 50 and 60Å the maximum absorption has been determined as being 97-99% for 60Å and 99.5%–86.26% for 40 Å at 30 degree.

0 500 1000 1500 2000 2500 3000 3500 4000 45000

10

20

30

40

50

60

70

80

90

100

wavenumber cm-1

% A

bsor

ptio

n

% Absorption vs. wavenumber cm-1 at different thickness of Ti


40Å 50 Å 60 Å



36

4.3 FTIR results for lead selenide It has been mentioned in above chapters that VeeMAXTM II has been used together with the FTIR spectroscopy. Thus, using the setup given in Figure 11 both the transmission and the reflection modes have been measured. The samples have been for the measurements using the FTIR before annealing. The absorption has been calculated from the FTIR results using the relation given below.

1absorptance transmittance reflectanceA T R= − − [13] Using Equation 9

The calculated absorption results for lead selenide at different deposi-tion times have been plotted in the Figure 27.

Figure 27: Lead selenide absorption (%A) spectra

The absorption spectra have been determined at room temperature for all samples. The sample which was annealed for 20 min, showed an absorption of 30% at a wavelength of 6.67 μm. However, it rapidly changed to nearly 48% at a 3.33 μm wavelength. There is a great deal of variation in the results and it is thus difficult for any definite conclu-sions to be drawn in relation to the presented results.

0 500 1000 1500 2000 2500 3000 3500 40005

10

15

20

25

30

35

40

45

50

55

Wavenumber cm-1

% A

bsor

ptio

n

% Absorption vs. wavenumber cm-1 for PbSe

No-anneling @5-minAnneling @5-minNo-anneling @10-minAnneling @10-minNo-anneling @20-minAnneling @20-min



37

The thickness of PbSe layer has been calculated in such a way that the mass of the sample is determined before and after the PbSe deposition. At a later stage the density formula given in equation15 was used.

density area

mthicknessAρ

=×

Equation 8

Thus, the thickness has been calculated for each sample and plotted as shown in Figure 28

Figure 28: Lead Selenide layer thickness vs. deposition time.

4.3.1 AFM characterization (PbSe) For the AFM studies, a step of PbSe has been made on a silicon substrate for different thicknesses as shown in Figure 29

Figure 29: AFM samples design for PbSe with different thickness

Figure 30 shows the AFM images (2D and 3D) of the PbSe thin film for 20 min. and the step is clearly shown in the 3D image. The measure-

5; 86,6574510; 217,6749967

20; 423,8934229

40; 870,6999868

60; 1306,04998

y = 22,04x - 14,10

0

200

400

600

800

1000

1200

1400

0 10 20 30 40 50 60 70

Thic

knes

s (n

m)

Deposition Time(min.)

PbSe Thickness Vs. Deposition time



38

ments have been conducted for 1min, 5min, 10min and 20min samples and these are plotted in figure31. The difference between the calculated thickness and the measured thickness is about 5%–7%.

Figure 30: AFM topography image (2D&3D) for 20 min of deposition of PbSe.

Figure 31: The AFM studies results at different thickness.

1; 51,65; 82,666

10; 234,666

20; 410,333

050

100150200250300350400450

0 5 10 15 20 25

Thic

knes

s (n

m)

Deposition time (min)

AFM restuls for PbSe thickness



39

4.3.2 SEM EDX characterization The composition of the PbSe material on the silicon wafer has been characterized through SEM EDX. The qualitative results for a 10 min deposition and a 20 min deposition of PbSe, are shown in Figure 32

Figure 32: Quantitative results for 10 min and 20 min deposited PbSe.

The presence of the carbon C in the PbSe composition was probably due to the standard used being coated with carbon. As this test has been performed in the vacuum, the oxygen probably comes from the silicon dioxide present on the sample and chlorine Cl may be add while touch-ing with hands. The results show that lead formed 30–40% in the sam-ples and that selenium formed 10–12%.

Quantitative results

Wei

ght%

0

10

20

30

40

50

C O Si Cl Se Pb

Quantitative results

Wei

ght%

0

10

20

30

40

O Si Se Pb

10 min

20 min


5 Summary 2011-06-27

40

5 Summary The infrared absorber is the one of the most essential parts of the IR detectors, which improves the sensitivity and absorption rate within the specific wavelength range without increasing the thermal mass of the detector. In this thesis, the theory associated with two absorptions layers, λ/4 structure and lead selenide, for a thermal IR detector have been described. The λ/4 structure is fabricated by means of sandwiching a dielectric layer between two metals layers, one reflective and one semitransparent. Aluminum with a thickness of 2000Å acts as a reflec-tor. The dielectric layer was formed using the SU8-2002, which has almost 95% transmission above 400 nm and the titanium acts as the semitransparent layer. The titanium layer has a sheet resistance of 345 Ω/ at 60Å, 468Ω/ a 50Å and 700Ω/ at 40Å. The deposition of layers has been conducted by means of a thermal evaporation process. The thickness of the SU8-2002 has been measured using an atomic force microscopy. The fabricated structure has been characterized using a Fourier transform infrared spectroscopy and a specular reflectance accessory within the wavelength region of 2 – 20μm. The absorption has been determined to be 80– 99% at a specific wavelength. A theoretical simulation has been performed by means of transfer matrix theory. The simulated absorption shows comparatively less absorption than that for the experimental absorption for the wavelength range of 2.5–20μm. It is has been found that by changing the reflective index values of the SU8-2002 to about 2.40 that this provided a better agreement with the expe-rimental results.

Lead selenide has been studied as an infrared absorber. It has been deposited for different deposition times using argon-plasma sputtering. The samples are characterized by using a FTIR and a specular reflec-tance accessory. The thickness of the PbSe layers has been calculated and also verified through AFM. The PbSe absorption spectra show 30%–50% absorption for the wavelength region 2.5 – 6.67μm.


6 Conclusions 2011-06-27

41

6 Conclusions An interferometric absorption structure has been fabricated through standard cleanroom processing and has been characterized using a Fourier transformation infrared spectroscopy and an ultimate specular reflectance accessory. A λ/4 structure including a reflector layer of 2000Å thick Al, 2000Å thick SU8-2002 and a thin Ti film was fabricated. The absorption for the λ/4 structure has been determined to be 80–99% for a titanium sheet resistance of 345–700Ω/. It has been found by using AFM, that the thickness of the SU-8 layer was 3μm instead 2μm. In addition, by using the refractive index used in the simulations from 1.575 to 2.4 for the SU8-2002 that this provided a better agreement between the simulation and the measurement results. A lead selenide thin film was deposited for different times and the absorption spectra were studied by means of the FTIR. The deposition rate of the PbSe sputtering has been determined by calculating the layer thickness. These calculations have been verified by also measuring the thickness using the AFM. The absorption spectrum for the PbSe has shown varying results, which has made it difficult to draw conclusions in relation to the absorption at this point.


7 Future work 2011-06-27

42

7 Future work By using a good SU8-2002 material, it may possible to produce better results. The deposition of lead selenide by means of other standard cleanroom processing may possibly lead to good results


References 2011-06-27

43

References

[1] “Active and passive sensors”

http://seis.bris.ac.uk/~ggjlb/teaching/ccrs_tutorial/tutorial/chap1/c1p6e.html (accessed on March 25, 2011

[2]

)

http://www.epa.gov/iaq/schooldesign/hvac.html(accessed on March 26, 2011

[3]

)

http://www.analox.net/carbon-dioxide-dangers.php(accessed on March 27, 2011

[4]

)

http://www.engineeringtoolbox.com/co2-comfort-level-d_1024.html(accessed on March 28, 2011

[5] D. Rossberg,“Silicon micromachined infrared sensor with tunable wave-length selectivity for application in infrared spectroscopy”, Sen-sor and Actuators A 46-47 (1995) 413-416.

)

[6] Bug on MEMS Abailable:http://mems.sandia.gov/gallery/images (accessed on April 01,2011)

[7] “Electromagnetic-Radiation” http://en.wikipedia.org/wiki/Electromagnetic_radiation(accessed on April 04,2011)

[8] http://www.earthtym.net/ref-far-infared.htm#1 (accessed on April 04,2011)

[9] “Herchel Discover Infrared Light”

http://coolcosmos.ipac.caltech.edu/cosmic_classroom/classroom_activities/herschel_bio.html(

[10] International Commission of Illumination (CIE)

accessed on April 05,2011)

http://www.cie.co.at/index_ie.html (accessed April 05, 2011)

http://seis.bris.ac.uk/~ggjlb/teaching/ccrs_tutorial/tutorial/chap1/c1p6e.html

http://seis.bris.ac.uk/~ggjlb/teaching/ccrs_tutorial/tutorial/chap1/c1p6e.html

http://www.epa.gov/iaq/schooldesign/hvac.html

http://www.analox.net/carbon-dioxide-dangers.php

http://www.engineeringtoolbox.com/co2-comfort-level-d_1024.html

http://www.engineeringtoolbox.com/co2-comfort-level-d_1024.html

http://mems.sandia.gov/gallery/images

http://en.wikipedia.org/wiki/Electromagnetic_radiation

http://www.earthtym.net/ref-far-infared.htm#1

http://coolcosmos.ipac.caltech.edu/cosmic_classroom/classroom_activities/herschel_bio.html

http://coolcosmos.ipac.caltech.edu/cosmic_classroom/classroom_activities/herschel_bio.html

http://www.cie.co.at/index_ie.html



44

[11] http://www.weatherquestions.com/What_is_infrared_radiation.htm(accessed on April 07,2011

[12]

)

http://upload.wikimedia.org/wikipedia/commons/f/ff/BlackbodySpectrum_loglog_150dpi_en.png(

[13] Theory and Practice Of Infrared Technology For Non-destructive Testing, Xavier P. V. Maldague, Wilyes Series in microwave and optical eng. 2001, ISBN 0-471-18190-0.

accessed on April 10,2011)

[14] “Stefan-Boltzmann” http://en.wikipedia.org/wiki/Stefan%E2%80%93Boltzmann_law(accessed on April 10,2011)

[15]

[16] Solid State Electronic Devices, Pearson Prentice Hall, 2006, ISBN 0-13-149726-X, pp. 132-134.

A. Graf, M. Arndt, M. Sauer, G. Gerlach,”Review of mircoma-chined thermopiles for infrared detectors”, Meas. Sci. Technol., vol 18, pp. 59-75, 2007.

[17] Optoelectronics-An Introduction, J. Wilson, J. Hawkes, Prentice Hall, 1998, ISBN 0-13-103961-X.

[18] SEMICONDUCTOR DEVICES Physics and Technology, S.M. SZE, JOHN WILEY & SONS,1985, ISBN 0-471-87424-8

[19] Walter Lang, Karl Kuhl and Hermann Sandmaier “Absorbing layers for thermal infrared detector”, Sensors and Actuators A,34(1992) 243-248

[20] M. P. Thompson, J. R. Troxell, M. E. Murray, C. M. Thrush and J. V. Mantese “Infrared absorber for pyro-electric detectors”

[21] Heini Saloniemi “Electrodeposition of PbS, PbSe and PbTe thin films”ISBN 951-38-5588-0, VTT elektroniikka, Mikroelektroniik-ka, Finland.

[22] K. C. Liddiard. “Application of interferometric enhancement to self-absorbing thin film thermal IR Detectors”, Infrared Phys. Vol. 34, No, 4,pp. 379-387.

[23] Pontus Eriksson, Jan Y. Andersson and Göran Stemme “Theoreti-cal and Experimental Investigation of Interferometric Absorbers for

http://www.weatherquestions.com/What_is_infrared_radiation.htm

http://www.weatherquestions.com/What_is_infrared_radiation.htm

http://upload.wikimedia.org/wikipedia/commons/f/ff/BlackbodySpectrum_loglog_150dpi_en.png

http://upload.wikimedia.org/wikipedia/commons/f/ff/BlackbodySpectrum_loglog_150dpi_en.png

http://en.wikipedia.org/wiki/Stefan%E2%80%93Boltzmann_law



45

Thermal Infrared Detector”, Sensor and Materials, Vol 9, No. 2(1997) 117-130 MYU Tokyo.

[24] A. Roncaglia, F. Mancarella, G.C. Cardinali “CMOS-compatible fabrication of thermopiles with high sensitivity in the 3–5m atmospheric window” Sensor and Actuators B 125 (2007 )214-223

[25] “Investigations on SU-8 Waveguides”, http://scholarbank.nus.edu.sg/bitstream/handle/10635/14412/05Chap_SumTC.pdf?sequence=5(Accesed 30 May. 2011)

[26] SU-8 2000 Processing Guidelines, Available: http://www.microchem.com/ (Accessed 30 May. 2011)

[27] Yun-Ju Chuang, Fan-Gang Tseng, Jen-Hau Cheng, Wei-Keng Lin“A novel Fabrication method of embedded micro-channels by using SU-8 thick-film photoresists”, Sensor and Actuators A 103 (2003) 64-69.

[28] M.J. Key, V. Cindro , M. Lozano “On the radiation tolerance of SU-8, a new material for gaseous microstructure radiation detector fabrica-tion”, Radiation Physics and Chemistry 71 (2004) 1003-1007.

[29] Claes Mattsson,“Design, Fabrication and Optimization of Thermal Radiation Detectors Based on Thin Polymer Membranes”, ISBN 978-91-86073-46-6.

[30] Sushil Kumar, Zishan H. Khan, M.A Majeed Khan, M. Husain,“Studies on thin films of lead chalcogenides ” Current Ap-plied Physics 5 (2005) 561-566.

[31] Barbara Jensen and Ahmad Torabi,“The Refractive index of Com-pounds PbTe, PbSe and PbS”, IEEE Journal of quantum electron-ics, Vol. QE-20, No 6, June 1984.

[32] Anna Vila, Nuria Ferrer, Jose F. Garcia,“Chemical composition of contemporary black printing inks based on infrared spectroscopy”, Analytica Chimica Acta 591 (2007) 97-105.

[33] Product-documentation, http://www.piketech.com/products/product-documentation-pdfs/VeeMAX_PDS.pdf (Accessed on June 01, 2011)

http://scholarbank.nus.edu.sg/bitstream/handle/10635/14412/05Chap_SumTC.pdf?sequence=5(Accesed

http://scholarbank.nus.edu.sg/bitstream/handle/10635/14412/05Chap_SumTC.pdf?sequence=5(Accesed



46

[34] User-manuals,http://www.piketech.com/technical/user-manuals/VeeMAXII-MAN.pdf#zoom=100%] (Accessed on June 01, 2011)

[35] Introduction to Modern Optics, Grant R. Fowles, II ed. ISBN 0-486-65957-7

[36] J. Bartl, M. Baranek ,“Emissivity of aluminium and its importance for radiometric measurement”, Measurement of Physical Quantities.

[37] http://en.wikipedia.org/wiki/Transmittance (Accessed on 2011-06-11)

[38] V. Arivazhagan, S. Rajesh ,“Effect of cryo substrate on thermally evaporated PbSe multilayer thin films”, Chalcogenide Letters Vol. 7, No. 7, July 2010, p. 465-470.

[39] [http://202.127.145.151/siocl/cdbank/WebHelp/Beilstein/brefhtml/sdic.htm]( Accessed on 2011-06-20)

[40] Handbook of Optics, 3rd Edition. ISBN 978-0-07-163604

http://202.127.145.151/siocl/cdbank/WebHelp/Beilstein/brefhtml/sdic.htm

http://202.127.145.151/siocl/cdbank/WebHelp/Beilstein/brefhtml/sdic.htm

Documents

Shakeel Ashraf Thesis - DiVA portal427144/FULLTEXT01.pdf · Shakeel Ashraf List of Figures 2011-06-27 ix Figure 25: Simulation and FTIR results for λ/4 structure with Ti= 60Å..34