FA - R P S A E - projects.au.dk...the etching process, it is possible to make structures of different size [3]. In addition by adding isopropyl alcohol, IPA, to the etching solution

F A B R I C A T I O N O FA N T I - R E F L E C T I V E

P Y R A M I D S T R U C T U R E S I NS I B Y A N I S O T R O P I C

E T C H I N G

D A N I E L B A C H J E N S E N2 0 1 4 0 4 7 1 3

A A R H U S U N I V E R S I T YD E P A R T M E N T O F P H Y S I C S A N D A S T R O N O M Y

D A T E : JANUARY 14, 2017S U P E R V I S O R S : P E T E R B A L L I N GC O - S U P E R V I S O R S : S A N J A Y R A M

Abstract

This report is examining the fabrication of anti-reflective uprightpyramid structures and their optical properties regarding its usein solar cells and for upconversion. The structures were etchedin silicon by a aqueous solution of KOH and IPA which resultedin structures with sizes from below 1 µm up to around 13 µm.The measured reflectance and transmittance of the samples werecompared with respect to the structure size, shape and coverage.In addition the reflectance is compared to theoretical calculationsmade from the geometrical (ray tracing) model and the effectivemedium model. It is found that the pyramid structures enhancesthe effect of conventional solar cells but seems to reduce the effectof upconversion.

Dansk

Denne rapport undersøger fremstillingen af anti-reflekterende opret-stående pyramidestrukturer og deres optiske egenskaber med hen-syn til deres anvendelse i solceller og til opkonvertering. Struktur-erne blev ætset i silicium med en vandig opløsning af KOH og IPA,som resulterede i strukturer med størrelser fra under 1 µm og op tilomkring 13 µm. Den målte reflektans og transmittans fra prøverneblev sammenlignet med hensyn til strukturernes størrelse, formog densitet. Desuden sammenlignes reflektansen med teoretiskeberegninger fra den geometriske (ray tracing) model og "Effektivmedium"-modellen. Det konstateres, at pyramidestrukturerne øgereffekten af konventionelle solceller, men de ser ud til at reducereeffekten af opkonvertering.

i

Acknowledgement

This project was made in collaboration with the SemiconductorGroup at the Institute of Physics and Astronomy regarding theSunTune project. Professor Peter Balling was supervisor on thisproject with Assistant Professor Sanjay Ram as co-supervisor.

During this project I have completed multiple training exercisesin order to operate the necessary equipment. Instruction on how touse the cleanroom at iNANO was given by Laboratory TechnicianPia Bomholt Jensen. The training for SEM, on Nova at the PhysicsDepartment and on Magellan at iNANO, was given by EngineerJacques Chevallier. At last the training for the spectrometer PerkinElmer at iNANO was given by PhD Student Harish Lakhotiya.

Finally I want to thank Pia Bomholt Jensen for her great help inthe laboratory and thank PhD Student Emil Eriksen for theoreticalcalculations and help with the theoretical background.

ii

Contents

Abstract i

Acknowledgement ii

Contents iii

1 Introduction 1

2 Experiment 32.1 The Etching Recipe and Set-up . . . . . . . . . . . . 32.2 Sample Etching . . . . . . . . . . . . . . . . . . . . . 4

2.2.1 A New Recipe . . . . . . . . . . . . . . . . . . 62.3 The Optical Measurements . . . . . . . . . . . . . . . 7

3 Results 93.1 Structural Characteristics . . . . . . . . . . . . . . . . 93.2 SEM Characterization . . . . . . . . . . . . . . . . . . 123.3 Optical Results . . . . . . . . . . . . . . . . . . . . . . 17

4 Discussion 224.1 The Etching Process . . . . . . . . . . . . . . . . . . . 224.2 Optical Behaviour . . . . . . . . . . . . . . . . . . . . 23

5 Conclusion and Perspective 27

6 Summary 28

Bibliography 31

A SEM Pictures 33

iii

B Optical Measurements 50

iv

Chapter 1

Introduction

This project considers the use of upright pyramid structures insilicon for use in solar cells. One of the advantages of these pyramidstructures is that they have an anti-reflective optical behaviour,when the size of the structures is around or below the wavelengthof the incident light. This happens because the small structuresappear to light as if they were an effective medium with a gradualincrease in refractive index [1]. This effect is illustrated in Figure 1.1.

Pyramid structures

SiSi

E�ective medium

Figure 1.1: This figure shows a sketch of the effective mediumapproach for pyramid structures. The left figure shows the pyramidstructures. The right figure shows how the pyramids appear in theeffective medium approach when the size of the structures is aboutor below the wavelength of light.

On the other hand there is a purely geometric approach toexplain the anti-reflective behaviour, the ray tracing approach. Thisapproach fits when the structures are around or bigger than thewavelength of the incident light. This model takes into account therefracted and reflected light for each interface the light hits and thelight absorbed inside the medium. Figure 1.2 shows a sketch of theray tracing approach [2]. Common to the calculations of these two

1

CHAPTER 1. INTRODUCTION 2

methods in this project is that they relay on an assumption that thepyramid structures are periodic and uniformly distributed. One ofthe goals of this project is to compare the measured data to thesetwo models and see if there is any relation.

Figure 1.2: This figure shows a sketch of the ray tracing approachfor pyramid structures [2].

The structures are made by anisotropic etch which gives uprightpyramid structures if no mask is used (not to confuse with invertedpyramid holes which are made using a mask). The structuresappears because potassium hydroxide (KOH) etches faster in the(100)-plane than in the (111)-plane of silicon. This leaves, under theright conditions, upright pyramids on the etched surface of a (100)silicon wafer. By controlling the mixture, time, and temperature ofthe etching process, it is possible to make structures of different size[3]. In addition by adding isopropyl alcohol, IPA, to the etchingsolution the structures become smoother. IPA prevents hydroxidefrom etching thus making a smoother surface on the pyramids [4].

To use these structures for solar cells in the SunTune projectthere are two essential optical characteristics which are importantto optimize. One is to increase the absorption at small wavelengthsto be able to use as much energy as possible in generating thephoto current. In this project the absorption is assumed to bethe part of the light that is not reflected nor transmitted (Abs =100% − Rtot − Ttot). The other essential characteristic is to get avery high transmission of photons with large wavelength. This isimportant in order to upconvert this light and use it to generatephoto current. The technique of upconversion is to use two lowenergy photons to generate one photon with enough energy todrive a photo current in the solar cell [5].

Chapter 2

Experiment

2.1 The Etching Recipe and Set-up

The first part of the project concerned the etching of the structuresinto the samples and refining the recipe. The initial recipe, whichwas written based on previous work done by Sanjay Ram, con-sisted of a solution of 2.5 g KOH (potassium hydroxide), 100 mLdemineralized water, and 33 mL IPA. This solution was heated to80 ◦C where the sample was etched for 20 min. The sample wascleaned with demineralized water in an ultrasonic bath for 2 minand left covered in this water for at least half an hour.

Figure 2.1: This figure shows the set-up for the etching process.

3

CHAPTER 2. EXPERIMENT 4

The set-up for the etching process consisted of a 500 ml beakerwith the solution and a lit (a glass dish), a hotplate, and a thermome-ter measuring the temperature of the solution. Figure 2.1 showsa photograph of the set-up used for etching. The silicon substrateused during this project was float zone silicon (100) wafers, thick-ness of 300 µm and p-type with a resistivity of 1-5 Ωcm. Generallythe samples was cut in pieces of 2×1-2 cm.

The cleaning of the samples was done by Pia Bomholt Jensenand consisted of the standard cleaning procedures RCA-1 andRCA-2 (Radio Corporation of America), which removes organicmatter and metal from the surface, respectively. Further more theoxide layer of the samples was removed with hydrofluoric acid thesame day before the etching process.

2.2 Sample Etching

The first series of samples was etched according to the above men-tioned recipe except for changing the etching time. The parametersfor each sample are shown in Table 2.1. The set-up of the etchingprocess was tested during the first process (A), where two sampleswas etched at the same time. In addition the ability to control thetemperature was tested.

The second series of samples was made on the basis of scanningelectron microscope, SEM, pictures of the first series (especiallyform sample F), and one of the goals was to produce a more narrowsize distribution. This second series was also made to look forany trend in the range of temperature between 60-70 ◦C and etchtimes between 40-80 min. The exact used parameters are found inTable 2.2.

The third series of samples produced had the aim of showingthe influence of IPA. The first half (sample O and P) is etched withdifferent concentration of IPA along with sample L. The last twosamples (Q and R) was made to see if there would be a remarkablechange in the average etch rate when changing to a fresh solutionafter half the time. In this case the two samples were etched2 × 30 min compared sample M. A remark about sample R is that


Table 2.1: This table shows the experimental parameters of eachsample in the first series during the etching process. Note that A1and A2 were etched together in the same solution, which is alsothe reason for the curly brackets.

Series 1 KOH [g] Temp [◦C] Time [min] A1A2 2.4951 81-82

20:2230:08

B 2.4690 80-82 24:57

C 2.5465 81-83 35:04

D 2.5349 80-82 39:53

E 2.5210 74-76 ∼ 40F 2.6090 68-72 39:52

Table 2.2: This table shows the experimental parameters of eachsample in the second series during the etching process.

Series 2 KOH [g] Temp [◦C] Time [min:sec]

G 2.444 68-72 60:08

H 2.422 69-72 79:55

I 2.5845 64-66 40:02

J 2.640 63-66 60:05

K 2.552 64-67 79:57

L 2.5780 59-61 40:12

M 2.536 59-62 59:59

N 2.574 59-62 79:59


the silicon was not cleaned for organic or metallic impurities onthe surface. Only hydrofluoric acid was used to remove the oxidelayer. The observed parameters that differs from the main recipe ofthis third series is tabulated in Table 2.3.

Table 2.3: This table shows the experimental parameters of eachsample in the third series during the etching process. Note thatsample Q and R were etched 2×30 min, which is the reason for thecurly brackets.

Series 3 KOH [g] IPA [ml] Temp [◦C] Time [min:sec]

O 2.4320 25 60-62 40:05

P 2.5405 40 60-62 40:11

Q

2.42352.5152 33

62-6460-64

31:0829:05

R

2.43702.4510 33

60-6260-62

30:1829:40

2.2.1 A New Recipe

After some time optimizing the initial recipe a paper with a newrecipe was found. In this paper, made by D. Muñoz et.al. [6],they had made very dense packed pyramid structures with sizesbetween 2-15 µm. The solution from the paper consisted of 5 %wtof KOH, 5 %vol of IPA and samples were etched for 60 min at80-90 ◦C. This recipe was studied in yet another series of samples.The parameters used for this series are tabulated in Table 2.4. Theetching procedure was carried out in the same way as the previousrecipe with a beaker on a hotplate.


Table 2.4: This table shows the experimental parameters of eachsample in the fourth series during the etching process.

Series 4 Temp [◦C] Time [min]

A’ 75 10

B’ 75 15

C’ 75 30

D’ 75 45

E’ 75 60

F’ 80 15

G’ 80 30

H’ 80 60

2.3 The Optical Measurements

The optical measurements were carried out by measuring the re-flectance and transmittance with the use of a spectrometer with anintegrating sphere. The instrument has two beams, a sample beamand a reference beam. The reference beam was guided through anaperture, then reflected by two mirrors, to remove any scatteringeffects, and at last reflected into the integrating sphere thus hittingthe detector. The sample beam is also guided through a apertureand reflected by two mirrors, after which it hits the integratingsphere where it reflects back and forth until it hits the detector atthe bottom. The instrument is sketched in Figure 2.2 and Figure 2.3.

To get a complete overview of the optical behaviour of the sam-ples four optical properties were measured. The first two propertieswere the total reflectance (Rtot) and the diffused reflectance (Rdi f ).In both cases the properties were measured by placing the sampleright after the integrating sphere. The difference between measur-ing the two properties is that the diffused reflectance is measuredin a way where the direct reflection is removed by letting it outthrough a hole in the integrating sphere as shown in Figure 2.2b.The direct reflectance is then calculated by subtracting the diffusedreflectance from the total reflectance, Rdir = Rtot − Rdi f . Figure 2.2ashows the set-up to measure the total reflectance.


Sample

Beam dump

Aperture

Mirror

Integrating

sphere

Sample beam E1

Refrence beam E2

Detector

(a) This figure shows a sketch of the opti-cal instrument with settings used to mea-sure the total reflectance (Rtot).

Detector

Sample

Beam dump

Aperture

Mirror

Integrating

sphere

Sample beam E1

Refrence beam E2

(b) This figure shows a sketch of the opti-cal instrument with settings used to mea-sure the diffused reflectance (Rdi f ).

Figure 2.2

The last two properties measured were the total transmittance(Ttot) and the direct transmittance (Tdir). The total transmittancewas measured by placing the sample right in front of the integrat-ing sphere as shown in Figure 2.3a, and the direct transmittancewas measured by placing the sample before the two mirrors asshown in Figure 2.3b. All measured samples were investigated forwavelengths between 250-2500 nm.

Sample

Beam dump

Aperture

Mirror

Integrating

sphere

Sample beam E1

Refrence beam E2

Detector

(a) This figure shows a sketch of the opti-cal instrument with settings used to mea-sure the total transmittance (Ttot).

Sample

Beam dump

Aperture

Mirror

Integrating

sphere

Refrence beam E2

Sample beam E1

Detector

(b) This figure shows a sketch of the opti-cal instrument with settings used to mea-sure the direct transmittance (Tdir).

Figure 2.3

Chapter 3

Results

3.1 Structural Characteristics

In the following chapters the structures will among other things bedescribed according to the structure size and coverage which areboth estimated by the use of SEM. The structure size is measuredfrom one side of the structure to the other as shown in Figure 3.1. Bycomparing the size of about a handful of structures the average sizewas estimated. The average size of each sample was only estimatedqualitatively (no statistical estimations) due to the relatively bigdistribution of sizes. The coverage is an estimate of how close thestructures are sitting together. This is estimated by examining theentire sample thus giving the samples the term "high coverage", forsamples almost completely filled with structures, or otherwise theterm "low coverage".

Figure 3.1: This figure illustrates how the structure size of eachstructure is measured by SEM.

9

CHAPTER 3. RESULTS 10

The samples were characterized by SEM. The procedure wasto observe almost the entire surface at low magnification to finda representative site to take a picture and note any characteristicpatterns of structures. The photographed areas were estimated tohave the average coverage of the individual samples. In the firstthree series some of the general patterns were "lines" and "clusters".

(a) This figure shows a line with more struc-tures than the average coverage of the sam-ple. The picture is from the back side ofsample K at 500 times magnification.

(b) This figure shows a line with less struc-tures than the average coverage of the sam-ple. The picture is from the front side ofsample K at 500 times magnification.

Figure 3.2

When talking about lines it can either be lines of more or lessstructures than the average coverage as shown in Figure 3.2a andFigure 3.2b.

The term clusters will be used when a lot of structures pack in avery small area as opposed to the average coverage. This type ofpattern is shown at the bottom of Figure 3.3a. Also some sampleshave large areas of different coverage like in Figure 3.3b.

In the initial processing there were no recordings of which sidewas up or down in the etching process except for the samples ofseries three. To correct this the samples were examined by SEMon both sides to see the difference. To distinguish the two sides,each was given a label of either "Front side" or "Back side", where


(a) This figure shows a cluster on a sample,where lots of structures are packing closetogether. The picture is from the front sideof sample E at 500 times magnification.

(b) This figure shows a sample that has twoareas with different coverage. The picture isfrom the front side of sample F at 500 timesmagnification.

Figure 3.3

the label "Front side" represented the side with most structures inreference to the procedure of the optical measurements (the frontside is facing the incoming beam).

In series four the structures had some other characteristics.When examining the samples the shape of structures was found tobe quite different. It seemed like the bigger structures were erodedby the etching process as shown in Figure 3.4a. These eroded struc-tures is referred to as "rough" structures whereas structures witha smooth surface are referred to as "smooth" structures, which isshown in Figure 3.4b.


(a) This figure shows rough structures wheresmall structures are forming on top of thebigger structures. The picture is tilted 45◦

and is from sample G’ at 2000 times magni-fication.

(b) This figure shows some of the verysmooth structures in the fourth series. Thepicture is tilted 45◦ and is from sample C’ at2000 times magnification.

Figure 3.4

3.2 SEM Characterization

The first series showed some samples with very dense packing ofstructures (sample A1, A2, and C). In general this series has a lot ofpatterns like clusters, areas, and lines, but sample C seems to havea relatively uniform distribution of structures. The structures ofsample C is shown in Figure 3.5. Also sample F has areas with highcoverage and some of the smallest structures as shown in Figure 3.6.In the entire series the size of the structures varies from 3.0-6.0 µmon the front side and 1.5-7.0 µm on the back side. An overview ofstructure size and patterns can be found in Table 3.1.


Table 3.1: This table shows the size of the biggest structures onthe front side and back side of each sample in the first series. Thecoverage of each sample is also described in reference to the generalpatterns of the samples.

Series 1Front side Back side

Size [µm] Coverage Size [µm] Coverage

A1 3.5-4.0 areas,close packed

3.5 clusters

A2 5.5-6.0 areas,close packed

5.0 clusters,some lines

B 4.0 clusters 3.5 uniform,some clusters

C 4.0 uniform, areas,close packed

5.0 clusters

D 3.0 clusters, lines 3.0 few structures

E 4.5 clusters 6.5-7.0 areas

F 3.0-3.5 lines, areas 1.5-2.0

Figure 3.5: These pictures show the structures on the front side ofsample C. The picture to the left is taken at 2000 times magnificationand the picture to the right is taken at 10000 times magnification.


Figure 3.6: These pictures show the structures on the front side ofsample F. The picture to the left is taken at 2000 times magnificationand the picture to the right is taken at 10000 times magnification.

The samples of the second series did not have a very highcoverage in general. It is however clear that the size of the pyramidstructures gets bigger when increasing either the etching time orthe temperature. One of the samples with most structures is sampleL which also have very small structures. Some SEM pictures ofsample L is shown in Figure 3.7. The structure size and coverage ofeach sample in this series are described in Table 3.2.

Figure 3.7: These pictures show the structures on the front side ofsample L. The picture to the left is taken at 2000 times magnificationand the picture to the right is taken at 10000 times magnification.


Table 3.2: This table shows the size of the biggest structures onthe front side and back side of each sample in the second series.The coverage of each sample is also described in reference to thegeneral patterns of the samples.



G 5.5 few structures 5.0 some clusters

H 8.5-9.0 some clusters 5 clusters, lines

I 3.0-3.5 few structures,some clusters

1.5 few structures

J 4.0-4.5 3.5-4.0 areas

K 8.0-8.5 lines 5.0 lines

L 1.5-2.0 < 1 few structures

M 3.5-4.0 lines 3.0 clusters, lines

N 3.5-4.0 areas 3.5-4.0 some clusters

At the third series the two samples O and P, etched with20 mL and 40 mL IPA respectively, resulted in very few structuresin relation with sample L from series 2 and the size of the structuresdid not seem to differ a lot. The samples Q and R show structuresof smaller size of the front side compared to sample M from series2, but the structures on the backside are bigger on sample Q and Rcompared to M. Figure 3.8 shows pictures from the front side andback side of sample Q. The coverage of all the samples in series 3 isnot enhanced in relation to the samples in series 2. The structuralcharacteristics of this series are listed in Table 3.3.

The samples in the fourth series was in general covered entirelyand as mentioned before the shape of the pyramids changed insome samples to be quite rough. Sample E’, which is shown inFigure 3.9 is an example of a sample with large structures coveredwith smaller structures making the structures look rough. Anotherspecial sample in this series is sample H’ which has a lot of smallstructures about 1 µm like sample E’, but it also has some verybig smooth structures with sizes above 10 µm. 3.10 shows some


Table 3.3: This table shows the size of the biggest structures on thefront side and back side of each sample in the third series. Thecoverage of each sample is also described in reference to the generalpatterns of the samples.



O 2.5 few structures 3.0-3.5

P 2.0-2.5 few structures 3.5-4.0

Q 3.5 5.0-5.5

R 1.5-2.0 3.5-4.0

Figure 3.8: These pictures show the structures of sample Q. Thepicture to the left is from the front side and the picture to theright is from the back side. Both pictures are taken at 2000 timesmagnification.


pictures of sample H’. The characteristics of the entire series aredescribed in Table 3.4.

Appendix A contains a the entire collection of SEM picturestaken through out the project.

Table 3.4: This table shows the size of the biggest structures on thefront side of each sample in the fourth series. The coverage of eachsample is also described in reference to the general patterns of thesamples. For samples with rough shape the size in brackets is thesize of the small structures which represents the roughness.

Series 4Front side

Size [µm] Coverage

A’ 7-8 almost covered, smooth shape

B’ 5-6 fully covered, more small structures

C’ 9-10 fully covered, smooth shape

D’ 11-12 fully covered, almost smooth shape

E’ 9-12 (∼1) fully covered, rough shapeF’ 6-7 fully covered, smooth shape

G’ 3.5-4 fully covered, almost smooth shape

H’ 10-13 (


Figure 3.9: These pictures show the structures on the front side ofsample E’. The picture to the left is taken at 2000 times magnifica-tion with 45◦ tilt and the picture to the right is taken at 15000 timesmagnification.

Figure 3.10: These pictures show the structures on the front side ofsample H’. The picture to the left is taken at 500 times magnificationand the picture to the right is taken at 6500 times magnification.

The plots in Figure 3.11 is made from optical measurements onsample H, K, C’, and D’ which all have a structure size between8-12 µm and are listed with increasing coverage. The anti-reflectiveeffect of the pyramid structures with high coverage is clear to seeat small wavelengths in Figure 3.11a, where the total reflectance isabout halved. At the plot of the total transmittance in Figure 3.11b,the transmittance is almost zero up to the wavelength that fits theband gap, meaning that all photons are either absorbed or reflected.Beyond the band gap the higher coverage seems to give a lowertransmittance. At Figure 3.12 the plots show that the samples withfull coverage reflect and transmit very diffused light since the direct


reflectance and transmittance is almost zero for the entire spectrum.

500 1000 1500 2000 2500

Wavelength [nm]

0

10

20

30

40

50

60

70

80

% R

C'

D'

K

H

(a) Total reflectance

500 1000 1500 2000 2500

Wavelength [nm]

0

10

20

30

40

50

60

70

80

% T

C'

D'

K

H

(b) Total transmittance

Figure 3.11: These two plots show the total reflectance and totaltransmittance of the samples H, K, C’, and D’ with a structure sizebetween 8-12 µm. The four samples are listed in order of increasingcoverage with sample H having the lowest coverage.

500 1000 1500 2000 2500

Wavelength [nm]

0

5

10

15

20

25

30

35

40

45

50

55

% R

C'

D'

K

H

(a) Direct reflectance

500 1000 1500 2000 2500

Wavelength [nm]

0

5

10

15

20

25

% T

C'

D'

K

H

(b) Direct transmittance

Figure 3.12: These two plots show the direct reflectance and directtransmittance of the samples H, K, C’, and D’ with a structure sizebetween 8-12 µm. The four samples are listed in order of increasingcoverage with sample H having the lowest coverage.

The next two series of plots were used to investigate any opticaldependence for the structure size. The first series consists of the


samples L, F, M, B, and K all with relatively low coverage and listedin order of increasing structure size between 1.5-8.5 µm (the plotsis shown in Figure 3.13). The other series consists of the samplesG’, B’, F’, A’, C’, and D’ all with a high coverage and listed inorder of increasing structure size between 3.5-12 µm (the plots isshown in Figure 3.14). Both series of optical measurements showsthe characteristic band gap of crystalline silicon, but there is noindication that the size have a significant influence in these opticalmeasurements.

500 1000 1500 2000 2500

Wavelength [nm]

0

10

20

30

40

50

60

70

80

% R

K

B

M

F

L


500 1000 1500 2000 2500

Wavelength [nm]

0

10

20

30

40

50

60

70

80

% T

K

B

M

F

L


Figure 3.13: These two plots show the total reflectance and totaltransmittance of the samples L, F, M, B, and K all with low coverage.The five samples are listed in order of increasing structure size withsample L having the smallest structures.

To determine if the shape (roughness) of the sample has anyinfluence on optical properties, the optical measurements of thesamples H’, E’, D’, and C’ are compared in Figure 3.15. Thesesamples are listed in order of increasing roughness and they allhave a structure size of 9-13 µm and a high coverage. In Figure 3.15aat small wavelengths it seems that increasing roughness reduces thetotal reflectance. In other words it seems like the small structures,representing the roughness, on the bigger structures enhance theanti-reflective effect at small wavelengths.

Appendix B contains the complete collection plots made fromall the optical measurements produced during this project.


500 1000 1500 2000 2500

Wavelength [nm]

0

10

20

30

40

50

60

70

80

% R

C'

A'

F'

B'


500 1000 1500 2000 2500

Wavelength [nm]

0

10

20

30

40

50

60

70

80

% T

C'

A'

F'

B'


Figure 3.14: These two plots show the total reflectance and totaltransmittance of the samples G’, B’, F’, A’, C’, and D’ all with highcoverage and smooth structures. The six samples are listed in orderof increasing structure size with sample G’ having the smalleststructures.

500 1000 1500 2000 2500

Wavelength [nm]

0

10

20

30

40

50

60

70

80

% R

C'

D'

E'

H'


500 1000 1500 2000 2500

Wavelength [nm]

0

10

20

30

40

50

60

70

80

% T

C'

D'

E'

H'


Figure 3.15: These two plots show the total reflectance and totaltransmittance of the samples H’, E’, D’, and C’ all with high cov-erage and structure size between 9-13 µm. The four samples arelisted in order of increasing roughness with sample H’ having thesmoothest structures.

Chapter 4

Discussion

4.1 The Etching Process

The first thing to mention is the etching procedure. From all theproduced samples it is clear that a longer etching time or a highertemperature gives larger structures, which is also well documentedin terms of the etch rate [3]. In the etching process it is important tonote that the temperature was measured in the middle of the liquidwhile the sample was lying at the bottom of the beaker close to thehotplate. This means that the actual temperature might be a bithigher than what is recorded. Another problem is that the samplesmight etch more at certain areas. The reason for this concern is thatthe samples were lying on the bottom of the beaker and becausethe etching solution was not stirred thus etching differently on thetop relative to the bottom.

It is also found that the amount of IPA has a big influence onboth the shape and the coverage of the structures. When comparingthe samples of series 4 (and sample A1, A2, B, C, and D from series1), made with the "new recipe", to the samples of the other seriesit is clear that the coverage is much greater with full coverage inalmost all samples. This difference is mainly because of the IPA. Itseems that the higher concentration of IPA prevents the hydroxidefrom etching the silicon, which is proclaimed in various papers [4].

The reason why sample A1, A2, B, C, and D from series 1 alsohave a relatively great coverage might be because of the IPA evap-

22

CHAPTER 4. DISCUSSION 23

orating from the system thus reducing the concentration. Thesamples were etched at about 80 ◦C for between 20-40 min whichseems to be enough to reduce the concentration of IPA significantlyin comparison to the samples of the second series. This is sup-ported by the data sheet for IPA which state that it has a boilingtemperature of 81-83 ◦C [8].

Another thing about the IPA is that it enhances the smoothnessof the structures. In series 4 it is found that the structures suddenlychange from smooth structures to very rough structures whenetching for around 45-60 min at 75 ◦C or 80 ◦C. This is probablyalso because of the IPA evaporating and this effect is also wellexplained in the literature [4].

One of the most extraordinary samples is sample H’ from series4 which has both very small structures, like the roughness of othersamples in series 4, but also big smooth structures with sizes above10 µm. This is very different from the other samples in this projectand might have something to do with the IPA acting as a mask atthe tip of the pyramid which is also explained in the literature [3].

4.2 Optical Behaviour

To investigate the potential of each sample used in a solar cell thetotal reflectance, total transmittance, and the absorption is com-pared in Figure 4.1a and Figure 4.1b. The two plots show theaverage reflectance, transmittance, and absorption for a certainrange of wavelength, at Figure 4.1a the range is 500-800 nm and atFigure 4.1b the range is 1300-2000 nm. The two areas are chosenbecause of minimal noise and that they give an indication of theoptical properties in the high energy range, the main source ofenergy in conventional solar cells, and the low energy range whichhas potential for upconvertion. Term "high energy range" refers towavelengths below the band edge and the term "low energy range"then refers to wavelengths above the band edge.

At the high energy range shown in Figure 4.1a there is a bigdifference in the total reflectance when comparing the samples oflow coverage (sample F-Q) to the samples of high coverage (sample


F H K L M N B Q A' B' C' D' E' F' G' H'

Measured samples

0

10

20

30

40

50

60

70

80

90

100

%T

& %

Ab

s &

%R

Ttot,av

Rtot,av

Absav

(a) The high energy range with wavelengthbetween 500-800 nm

F H K L M N B Q A' B' C' D' E' F' G' H'

Measured samples

0

10

20

30

40

50

60

70

80

90

100

%T

& %

Ab

s &

%R

Ttot,av

Rtot,av

Absav

(b) The low energy range with wavelengthbetween 1300-2000 nm

Figure 4.1: These plots show the total reflectance, the total trans-mittance, and the absorption of each measured sample. The pointsreprecents the mean value of the given parameter in a certain rangeof wavelength, either the high energy range between 500-800 nmor the low energy range between 1300-2000 nm. The samples fromF-Q have low coverage and the samples A’-H’ have high coverage.

A’-H’). This indicates for the samples produced in this project,as mentioned previously in section 3.3, that the coverage seemsto have a greater influence on the anti-reflective effect than thestructure size. The plot also shows that, however the transmittancereduces a bit for samples with higher coverage, the absorptionmainly depends on the reflectance.

In 4.1b, which shows the high energy range, there is not muchchange in either total reflectance nor total transmittance. By takinga closer look at the plot there could be a tendency that the totalreflectance and the absorption increases for the samples with highercoverage while the total transmittance reduces, but it is a very smalleffect.

From the statements above there seems to be a trade-off betweenabsorbing light in the high energy range and transmitting light inthe low energy range for upconversion when using these pyramidstructures. Figure 4.2 shows the two representative samples N andC’ which have the highest transmittance and absorption respec-


tively. This plot also shows the trade-off between the transmittanceand the absorption.

0 500 1000 1500 2000 2500

Wavelength [nm]

-10

0

10

20

30

40

50

60

70

80

90%

T &

%A

bs

Ttot

N

Ttot

C'

Abs N

Abs C'

Figure 4.2: This figure shows the total transmittance and absorptionof the two samples N and C’ each representing the samples withlow and high coverage respectively.

The two samples N and C’ are also used to compare the mea-sured data to theoretical calculations. In Figure 4.3 the total re-flectance of the two samples are plotted along with the total re-flectance measurements of a clean sample and the theoretical cal-culated reflectance of a flat piece of silicon and the geometricalmodel of the pyramid structures. The theoretical calculated curveswere produced by PhD Emil Eriksen. This plot shows that thereis almost no anti-reflective effect of sample N with low coveragesince it follows both the measured clean sample and the theoreticalcalculation of a flat sample.

The two red curves also show a connection between the struc-tures of sample C’ and the geometrical model in the high energyrange. This also means that the structures are not small enough,relative to the wavelength of the light, to fit the effective mediumapproach. This is however reasonable since the structure size ofsample C’ are 9-10 µm.

In the low energy range the total reflectance of sample C’ differsquite a lot from the geometrical model. This is however not very


surprising since this geometrical model breaks down for wave-lengths above the band edge. The reason for this is that the bandgap of silicon is larger than the energy of the photons which givesa dramatic drop in absorption around the band edge. The reducedabsorption increases the amount of photons hitting the back sideof the sample thus reflecting some of the light. The effect of thereflected photons from inside the material is not a part of the cal-culation illustrated in Figure 4.3, which is part of the explanation.The odd thing however is that the total reflectance of sample C’ liesvery near the total reflectance of a flat sample, both measured andcalculated reflectance. This is odd since the structures should ap-proach the effective medium model as the wavelength gets biggerinstead of approaching the flat model. For further investigationbeyond this project it would be interesting to see if it is the mainlythe back side which is responsible for this effect.

0 500 1000 1500 2000 2500

Wavelength [nm]

0

10

20

30

40

50

60

70

80

%R

Rtot

(N)

Rtot

(C')

Rtot

clean

CalcFlat

CalcGeometric

Figure 4.3: This plot shows the total reflectance both measuredand calculated. The two full lines are the total reflectance frommeasurements on sample N (blue) and C’ (red) each representingthe samples with low and high coverage. The black dotted line isthe total reflectance of a clean sample without structures. The twodashed lines show the calculated reflectance of a flat sample (blue)and a sample with structures using the geometric model (red).

Chapter 5

Conclusion and Perspective

From the data produced in this project it looks like the pyramidstructures are not very good at transmitting the part of the lightthat could be used for upconversion, but there may be ways toincrease this transmission. For instance it might be possible toincrease the transmission by covering the front of the samples withan anti-reflective coating.

If instead it is the structures at the back side of the sampleswhich are the reason for the reduced transmission then an improve-ment might also be to glue some other material with a differentrefractive index to the back side. Another solution to this problemmight also be to cover the back side during the etching process.This could be done by coating the backside with some kind of maskor maybe just not remove the oxide layer of the back side duringthe cleaning procedure.

One of the things that made the etching process very hard tocontrol was the evaporation of IPA. A suggestion to improve thiswould be to install a reflux condenser to prevent the IPA fromleaving the system, which has been done by D. Muñoz et.al. [6].This would make the system more controllable especially at hightemperatures.

27

Chapter 6

Summary

The goal of this project was to fabricate anti-reflective pyramidstructures in silicon and measure the optical properties regardingits use in solar cells and especially for the purpose of upconversionin solar cells. To do this the silicon was etched in an aqueoussolution of KOH with IPA. It was found that an etching solution of5 %wt KOH and 5 %vol IPA at a temperature of 75-80 ◦C gave thebest coverage of structures. The structure sizes varied from below1 µm up to around 13 µm.

Each sample was examined by SEM and characterized in termsof the structure size and the coverage. Further more the reflectanceand the transmittance were measured for some of the samples.From these measurements the reflectance and transmittance ofthe different samples were compared according to structure size,coverage, and shape. It was found that a high coverage and roughstructures enhanced the anti-reflective effect in the high energyrange while the structure size did not seem to conclude any trend.

By comparing the reflectance measurements to theoretical cal-culations it was found that samples with low coverage followedthe calculations of a flat sample without structures thus havingalmost no anti-reflective effect. The samples with high coveragehowever followed the geometric model at the high energy rangebut at the low energy range it followed the calculations of a flat sam-ple. There was no indications of any samples fitting the effectivemedium model.

For the use of pyramid structures in solar cells and for upconver-

28

CHAPTER 6. SUMMARY 29

sion it was found that there might be a trade-off between absorbingphotons in the high energy range and transmitting photons in thelow energy range. In other words the pyramid structures enhancethe effect of conventional solar cells but seem to reduce the effectof upconversion.

Resume

Målet med dette projekt var at fabrikere anti-reflekterende pyra-midestrukturer i silicium og måle de optiske egenskaber med hen-blik på anvendelse i solceller og specielt til opkonvertering i sol-celler. For at gøre dette blev silicium ætset i en vandig opløsningaf KOH med IPA. Det blev konstateret, at en ætsningsopløsningbestående af 5 %wt KOH og 5 %vol IPA ved en temperatur på75-80 ◦C gav den bedste densitet af strukturer. Strukturstørrelsernevarierede fra under 1 µm op til omkring 13 µm.

Hver prøve blev undersøgt med SEM og karakteriseret medhensyn til strukturernes størrelse og densitet. Foruden blev re-flektansen og transmittansen målt for nogle af prøverne. Ud fradisse målinger blev reflektansen og transmittansen af de forskel-lige prøver sammenlignet efter strukturernes størrelse, densitet ogform. Det blev konstateret, at en høj densitet af strukturer og rustrukturer forbedrer den anti-reflektive effekt i højenergiområdet,mens struktur størrelsen ikke synes at vise nogen tendens.

Ved at sammenligne reflektansmålinger med teoretiske bereg-ninger blev det konstateret, at prøver med lav densitet fulgte bereg-ningerne for en flad prøve uden strukturer, hvilket betyder atden næsten ingen anti-reflekterende effekt har. Tværtimod ful-gte prøverne med høj densitet af strukturer den geometriske modeli højenergiområdet, mens at de i lavenergiområdet fulgte bereg-ningerne for en flad prøve. Der var ingen indikationer på prøver,som passede på "Effektiv medium"-modellen.

I forbindelse med at bruge pyramidestrukturerne i solceller ogtil opkonvertering blev det konstateret, at der kan være en balancemellem at absorbere fotoner i højenergiområdet og at transmiterefotoner i lavenergiområdet. Med andre ord øger pyramidestruk-

CHAPTER 6. SUMMARY 30

turerne effekten af konventionelle solceller, men de lader til atreducere effekten af opkonvertering.

Bibliography

[1] Alexei Deinega, Ilya Valuev, Boris Potapkin, and Yurii Lozovik.Antireflective properties of pyramidally textured surfaces. Opt.Lett., 35(2):106–108, Jan 2010.

[2] Alexei Deinega, Ilya Valuev, Boris Potapkin, and Yurii Lozovik.Minimizing light reflection from dielectric textured surfaces. J.Opt. Soc. Am. A, 28(5):770–777, May 2011.

[3] I. Zubel and M. Kramkowska. Etch rates and morphology ofsilicon (h k l) surfaces etched in {KOH} and {KOH} saturatedwith isopropanol solutions. Sensors and Actuators A: Physical,115(2–3):549 – 556, 2004. The 17th European Conference onSolid-State Transducers.

[4] Irena Zubel and Małgorzata Kramkowska. The effect of iso-propyl alcohol on etching rate and roughness of (1 0 0) si surfaceetched in {KOH} and {TMAH} solutions. Sensors and ActuatorsA: Physical, 93(2):138 – 147, 2001.

[5] Wilfried GJHM van Sark, Jessica de Wild, Jatin K. Rath, AndriesMeijerink, and Ruud EI Schropp. Upconversion in solar cells.Nanoscale Research Letters, 8(1):81, 2013.

[6] D. Muñoz, P. Carreras, J. Escarré, D. Ibarz, S. Martín de Nicolás,C. Voz, J.M. Asensi, and J. Bertomeu. Optimization of {KOH}etching process to obtain textured substrates suitable for het-erojunction solar cells fabricated by {HWCVD}. Thin Solid Films,517(12):3578 – 3580, 2009. Proceedings of the Fifth InternationalConference on Hot-Wire {CVD} (Cat-CVD) Process.

31

BIBLIOGRAPHY 32

[7] Charles Kittel. Introduction to Solid State Physics. Wiley, 8. edi-tion, 2005.

[8] Sigma-Aldrich. Safty data sheet: Isopropyl alcohol, natural,>=98%, fg, 2017.

Appendix A

SEM Pictures

This chapter contains a the entire collection of SEM pictures takenthrough out the project. Below is a list of the figures which comparethe different pictures.

Figure Samples Etch parameters Side Magnification

A.1 A1, A2, B, C, D 20-40 min, 80 ◦C Front side 500-10000x

A.2 A1, A2, B, C, D 20-40 min, 80 ◦C Back side 500-10000x

A.3 D, E, F 40 min, 70-80 ◦C Front side 500-10000x

A.4 D, E, F 40 min, 70-80 ◦C Back side 500-10000x

A.5 F, G, H, I, J, L, M, N 40-80 min, 60-70 ◦C Front side 500x



A.8 F, G, H, I, J, L, M, N 40-80 min, 60-70 ◦C Back side 500x



A.11 L, O, P 25-40 mL IPA, 40 min, 60 ◦C Top side 500-10000x

A.12 L, O, P 25-40 mL IPA, 40 min, 60 ◦C Bottom side 500-10000x

A.13 M, Q, R 2x30 min and 60 min, 60 ◦C Top side 500-10000x

A.14 M, Q, R 2x30 min and 60 min, 60 ◦C Bottom side 500-10000x

A.15 A’, B’, C’, D’, E’ New recipe, 10-60 min, 75 ◦C Front side 2000x

A.16 F’, G’, H’ New recipe, 15-60 min, 80 ◦C Front side 2000x

33

APPENDIX A. SEM PICTURES 34

20

30

40

Fro

nt

sid

e

Tim

e [

min

]

25

35

x1

00

00

x2

00

0

x5

00

DC

A2

BA

1

DC

A2

BA

1

DC

A2

BA

1

Magni

cati

on

Tem

p 8

0 O

C

Figu

reA

.1:T

his

figur

esh

ows

the

fron

tsid

eof

sam

ples

A1,

A2,

B,C

,and

Dfr

omse

ries

1.T

heSE

Mpi

ctur

esar

eor

dere

dw

ith

resp

ectt

oth

em

agni

ficat

ion

and

the

etch

ing

tim

e.A

llth

esa

mpl

esar

eet

ched

at80

◦ C.


20

30

40

Back s

ide

Tim

e [

min

]

25

35

E

x1

00

00

x2

00

0

x5

00

DC

A2

BA

1

A1

BA

2C

DDC

A2

BA

1

Magni

cati

on

Tem

p 8

0 O

C

Figu

reA

.2:T

his

figu

resh

ows

the

back

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eof

sam

ples

A1,

A2,

B,C

,and

Dfr

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1.T

heSE

Mpi

ctur

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dere

dw

ith

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oth

em

agni

ficat

ion

and

the

etch

ing

tim

e.A

llth

esa

mpl

esar

eet

ched

at80

◦ C.

APPENDIX A. SEM PICTURES 36M

agni

cati

on

x1

00

00

x2

00

0

x5

00

70

75

80

Tem

p [

OC

]

Fro

nt

sid

e

LDDD

EEE

FFF

Tim

e 4

0 m

in

Figu

reA

.3:T

his

figur

esh

ows

the

fron

tsid

eof

sam

ples

D,E

,and

Ffr

omse

ries

1.T

heSE

Mpi

ctur

esar

eor

der

edw

ith

resp

ectt

oth

em

agni

ficat

ion

and

the

tem

pera

ture

duri

nget

chin

g.A

llth

esa

mpl

esar

eet

ched

for

40m

in.


agni

cati

on

x1

00

00

x2

00

0

x5

00

70

75

80

Tem

p [

OC

]

Back s

ide

LDDD

EEE

FFF

Tim

e 4

0 m

in

Figu

reA

.4:T

his

figu

resh

ows

the

back

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eof

sam

ples

D,E

,and

Ffr

omse

ries

1.T

heSE

Mpi

ctur

esar

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der

edw

ith

resp

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oth

em

agni

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ion

and

the

tem

pera

ture

duri

nget

chin

g.A

llth

esa

mpl

esar

eet

ched

for

40m

in.

APPENDIX A. SEM PICTURES 38Tim

e [

min

]

80

60

40

60

65

70

Tem

p [

OC

]

Fro

nt

sid

ex5

00

LMN

IJK

FGH

Figu

reA

.5:T

his

figur

esh

ows

the

fron

tsid

eof

sam

ples

Ffr

omse

ries

1an

dsa

mpl

eG

,H,I

,J,K

,L,M

,and

Nfr

omse

ries

2.Th

eSE

Mpi

ctur

esar

eor

dere

dw

ithre

spec

tto

the

etch

ing

time

and

tem

pera

ture

.All

pict

ures

are

take

nat

500

tim

esm

agni

ficat

ion.


e [

min

]

80

60

40

60

65

70

Tem

p [

OC

]

Fro

nt

sid

ex2

00

0

LMN

IJK

FGH

Figu

reA

.6:T

his

figur

esh

ows

the

fron

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sam

ples

Ffr

omse

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1an

dsa

mpl

eG

,H,I

,J,K

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,and

Nfr

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2.Th

eSE

Mpi

ctur

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dere

dw

ithre

spec

tto

the

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ing

time

and

tem

pera

ture

.All

pict

ures

are

take

nat

2000

tim

esm

agni

ficat

ion.


e [

min

]

80

60

40

60

65

70

Tem

p [

OC

]

Fro

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sid

ex1

00

00

FGH

IJK

LMN

Figu

reA

.7:T

his

figur

esh

ows

the

fron

tsid

eof

sam

ples

Ffr

omse

ries

1an

dsa

mpl

eG

,H,I

,J,K

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,and

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2.Th

eSE

Mpi

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dere

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the

etch

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time

and

tem

pera

ture

.All

pict

ures

are

take

nat

1000

0ti

mes

mag

nific

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n.


e [

min

]

80

60

40

60

65

70

Tem

p [

OC

]

Back s

ide

x5

00

FGH

IJK

LMN

Figu

reA

.8:T

his

figur

esh

ows

the

back

side

ofsa

mpl

esF

from

seri

es1

and

sam

ple

G,H

,I,J

,K,L

,M,a

ndN

from

seri

es2.

The

SEM

pict

ures

are

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with

resp

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dte

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re.A

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]

80

60

40

60

65

70

Tem

p [

OC

]

Back s

ide

x2

00

0

FGH

IJK

LMN

Figu

reA

.9:T

his

figur

esh

ows

the

back

side

ofsa

mpl

esF

from

seri

es1

and

sam

ple

G,H

,I,J

,K,L

,M,a

ndN

from

seri

es2.

The

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pict

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are

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with

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chin

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ean

dte

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ratu

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llpi

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at20

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mag

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e [

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]

80

60

40

60

65

70

Tem

p [

OC

]

Back s

ide

x1

00

00

FGH

IJK

LMN

Figu

reA

.10:

This

figur

esh

ows

the

back

side

ofsa

mpl

esF

from

seri

es1

and

sam

ple

G,H

,I,J

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from

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The

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are

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at10

000

tim

esm

agni

ficat

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agni

cati

on

x10000

x2000

x500

25

33

40

IPA

[m

L]

Top s

ide

LPPP

LLL

OOO

Tem

p 6

0 O

C

Tim

e 4

0 m

in

Figu

reA

.11:

This

figur

esh

ows

the

top/

fron

tsid

eof

sam

ple

Lfr

omse

ries

2an

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agni

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x2000

x500

25

33

40

IPA

[m

L]

Bott

om

sid

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LPPP

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p 6

0 O

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Tim

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0 m

in

Figu

reA

.12:

This

figur

esh

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the

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ack

side

ofsa

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eL

from

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nific

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nan

dth

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Aus

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the

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ess.

All

the

sam

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are

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edfo

r40

min

at60

◦ C.


agni

cati

on

x10000

x2000

x500

2x30

60

2x30

Tim

e [

min

]

Top s

ide

LRRR

MMM

QQQ

Tem

p 6

0 O

C

Figu

reA

.13:

Thi

sfi

gure

show

sth

eto

p/fr

onts

ide

ofsa

mpl

eM

from

seri

es2

and

sam

ple

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dR

from

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es3.

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pict

ures

are

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red

with

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em

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ficat

ion

and

the

etch

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ess

(one

ortw

oso

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ra

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lof6

0m

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All

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sam

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are

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edat

60◦ C

.


agni

cati

on

x10000

x2000

x500

2x30

60

2x30

Tim

e [

min

]

Bott

om

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e

LRRR

MMM

QQQ

Tem

p 6

0 O

C

Figu

reA

.14:

This

figur

esh

ows

the

bott

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ack

side

ofsa

mpl

eM

from

seri

es2

and

sam

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dR

from

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pict

ures

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with

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proc

ess

(one

ortw

oso

lutio

nsfo

ra

tota

lof6

0m

in).

All

the

sam

ples

are

etch

edat

60◦ C

.


10

30

60

Fro

nt

sid

ex2000

Tim

e [

min

]

15

45

A'

B'

C'

D'

E'

E'

D'

C'

B'

A'

Tem

p 7

5 O

C

Figu

reA

.15:

Thi

sfig

ure

show

sth

efr

onts

ide

ofsa

mpl

eA

’,B

’,C

’,D

’,an

dE

’fro

mse

ries

4.T

heSE

Mpi

ctur

esar

eor

der

edw

ith

resp

ectt

oth

eet

chin

gti

me.

All

the

sam

ples

are

etch

edat

75◦ C

and

the

pict

ure

sar

eta

ken

at20

00ti

mes

mag

nific

atio

n.


15

30

60

Tim

e [

min

]

Fro

nt

sid

ex2000

F'

G'

H'

H'

G'

F'

Tem

p 8

0 O

C

Figu

reA

.16:

This

figur

esh

ows

the

fron

tsid

eof

sam

ple

F’,G

’,an

dH

’fro

mse

ries

4.Th

eSE

Mpi

ctur

esar

eor

dere

dw

ith

resp

ect

toth

eet

chin

gti

me.

All

the

sam

ple

sar

eet

ched

at80

◦ Can

dth

ep

ictu

res

are

take

nat

2000

tim

esm

agni

ficat

ion.

Appendix B

Optical Measurements

This chapter contains the complete collection plots made from allthe optical measurements produced during this project. Below is alist of the different plots.

Figure Samples Order of samples R & T Structure parameters

B.1 Q, N, M, F Coverage Total 3-4 µm

B.2 Q, N, M, F Coverage Direct 3-4 µm

B.3 H, K, C’, D’ Coverage Total 8-12 µm

B.4 H, K, C’, D’ Coverage Direct 8-12 µm

B.5 L, F, M, B, K Structure size Total low coverage

B.6 L, F, M, B, K Structure size Direct low coverage

B.7 G’, B’, F’, A’, C’, D’ Structure size Total high coverage

B.8 G’, B’, F’, A’, C’, D’ Structure size Direct high coverage

B.9 H’, E’, D’, C’ Shape Total 9-13 µm, high coverage

B.10 H’, E’, D’, C’ Shape Direct 9-13 µm, high coverage

50

APPENDIX B. OPTICAL MEASUREMENTS 51

500

1000

1500

2000

2500

Wa

ve

len

gth

[

nm

]

0

10

20

30

40

50

60

70

80

% RF M N Q

(a)T

otal

refle

ctan

ce

500

1000

1500

2000

2500

Wa

ve

len

gth

[

nm

]

0

10

20

30

40

50

60

70

80

% T

F M N Q

(b)T

otal

tran

smit

tanc

e

Figu

reB.

1:Th

ese

two

plot

ssh

ows

the

tota

lrefl

ecta

nce

and

tota

ltra

nsm

ittan

ceof

the

sam

ples

Q,N

,M,a

ndF

with

ast

ruct

ure

size

betw

een

3-4

µm

.The

four

sam

ples

are

liste

din

orde

rof

incr

easi

ngco

vera

gew

ithsa

mpl

eQ

havi

ngth

elo

wes

tcov

erag

e.


500

1000

1500

2000

2500

Wa

ve

len

gth

[

nm

]

20

25

30

35

40

45

50

55

60

65

70

75

% RF M N Q

(a)D

irec

trefl

ecta

nce

500

1000

1500

2000

2500

Wa

ve

len

gth

[

nm

]

05

10

15

20

25

30

35

40

% T

F M N Q

(b)D

irec

ttra

nsm

itta

nce

Figu

reB

.2:T

hese

two

plot

ssh

ows

the

dir

ectr

eflec

tanc

ean

dd

irec

ttra

nsm

itta

nce

ofth

esa

mpl

esQ

,N,M

,and

Fw

ith

ast

ruct

ure

size

betw

een

3-4

µm

.The

four

sam

ples

are

liste

din

orde

rof

incr

easi

ngco

vera

gew

ith

sam

ple

Qha

ving

the

low

estc

over

age.


500

1000

1500

2000

2500

Wa

ve

len

gth

[

nm

]

0

10

20

30

40

50

60

70

80

% RC

'

D'

K H

(a)T

otal

refle

ctan

ce

500

1000

1500

2000

2500

Wa

ve

len

gth

[

nm

]

0

10

20

30

40

50

60

70

80

% T

C'

D'

K H

(b)T

otal

tran

smit

tanc

e

Figu

reB

.3:T

hese

two

plo

tssh

ows

the

tota

lrefl

ecta

nce

and

tota

ltra

nsm

itta

nce

ofth

esa

mp

les

H,K

,C’,

and

D’

with

ast

ruct

ure

size

betw

een

8-12

µm

.The

four

sam

ples

are

liste

din

orde

rof

incr

easi

ngco

vera

gew

ithsa

mpl

eH

havi

ngth

elo

wes

tcov

erag

e.


500

1000

1500

2000

2500

Wa

ve

len

gth

[

nm

]

05

10

15

20

25

30

35

40

45

50

55

% RC

'

D'

K H

(a)D

irec

trefl

ecta

nce

500

1000

1500

2000

2500

Wa

ve

len

gth

[

nm

]

05

10

15

20

25

% T

C'

D'

K H

(b)D

irec

ttra

nsm

itta

nce

Figu

reB.

4:T

hese

two

plot

ssh

ows

the

dire

ctre

flect

ance

and

dire

cttr

ansm

itta

nce

ofth

esa

mpl

esH

,K,C

’,an

dD

’w

itha

stru

ctur

esi

zebe

twee

n8-

12µ

m.T

hefo

ursa

mpl

esar

elis

ted

inor

der

ofin

crea

sing

cove

rage

with

sam

ple

Hha

ving

the

low

estc

over

age.


500

1000

1500

2000

2500

Wa

ve

len

gth

[

nm

]

0

10

20

30

40

50

60

70

80

% RK B M F L

(a)T

otal

refle

ctan

ce

500

1000

1500

2000

2500

Wa

ve

len

gth

[

nm

]

0

10

20

30

40

50

60

70

80

% T

K B M F L

(b)T

otal

tran

smit

tanc

e

Figu

reB

.5:T

hese

two

plot

ssh

ows

the

tota

lrefl

ecta

nce

and

tota

ltra

nsm

itta

nce

ofth

esa

mpl

esL

,F,M

,B,a

ndK

allw

ith

low

cove

rage

.The

five

sam

ples

are

liste

din

ord

erof

incr

easi

ngst

ruct

ure

size

wit

hsa

mpl

eL

havi

ngth

esm

alle

stst

ruct

ures

.


500

1000

1500

2000

2500

Wa

ve

len

gth

[

nm

]

10

15

20

25

30

35

40

45

50

55

60

% RK B M F L

(a)D

irec

trefl

ecta

nce

500

1000

1500

2000

2500

Wa

ve

len

gth

[

nm

]

05

10

15

20

25

30

35

40

% T

K B M F L

(b)D

irec

ttra

nsm

itta

nce

Figu

reB.

6:Th

ese

two

plot

ssh

ows

the

dire

ctre

flect

ance

and

dire

cttr

ansm

itta

nce

ofth

esa

mpl

esL,

F,M

,B,a

ndK

allw

ith

low

cove

rage

.The

five

sam

ples

are

liste

din

ord

erof

incr

easi

ngst

ruct

ure

size

wit

hsa

mpl

eL

havi

ngth

esm

alle

stst

ruct

ures

.


500

1000

1500

2000

2500

Wa

ve

len

gth

[

nm

]

0

10

20

30

40

50

60

70

80

% RC

'

A'

F'

B'

(a)T

otal

refle

ctan

ce

500

1000

1500

2000

2500

Wa

ve

len

gth

[

nm

]

0

10

20

30

40

50

60

70

80

% T

C'

A'

F'

B'

(b)T

otal

tran

smit

tanc

e

Figu

reB.

7:Th

ese

two

plot

ssh

ows

the

tota

lrefl

ecta

nce

and

tota

ltra

nsm

ittan

ceof

the

sam

ples

G’,

B’,F

’,A

’,C

’,an

dD

’all

wit

hhi

ghco

vera

ge.T

hesi

xsa

mpl

esar

elis

ted

inor

der

ofin

crea

sing

stru

ctur

esi

zew

ith

sam

ple

G’h

avin

gth

esm

alle

stst

ruct

ures

.


500

1000

1500

2000

2500

Wa

ve

len

gth

[

nm

]

0

0.51

1.52

2.53

3.54

4.55

% RC

'

A'

F'

B'

(a)D

irec

trefl

ecta

nce

500

1000

1500

2000

2500

Wa

ve

len

gth

[

nm

]

0

0.51

1.52

2.53

3.54

4.55

% T

C'

A'

F'

B'

(b)D

irec

ttra

nsm

itta

nce

Figu

reB.

8:T

hese

two

plot

ssh

ows

the

dire

ctre

flect

ance

and

dire

cttr

ansm

itta

nce

ofth

esa

mpl

esG

’,B’

,F’,

A’,

C’,

and

D’a

llw

ith

high

cove

rage

.T

hesi

xsa

mp

les

are

liste

din

ord

erof

incr

easi

ngst

ruct

ure

size

wit

hsa

mp

leG

’ha

ving

the

smal

lest

stru

ctur

es.


500

1000

1500

2000

2500

Wa

ve

len

gth

[

nm

]

0

10

20

30

40

50

60

70

80

% RC

'

D'

E'

H'

(a)T

otal

refle

ctan

ce

500

1000

1500

2000

2500

Wa

ve

len

gth

[

nm

]

0

10

20

30

40

50

60

70

80

% T

C'

D'

E'

H'

(b)T

otal

tran

smit

tanc

e

Figu

reB

.9:T

hese

two

plot

ssh

ows

the

tota

lrefl

ecta

nce

and

tota

ltra

nsm

itta

nce

ofth

esa

mpl

esH

’,E

’,D

’,an

dC

’w

ith

ast

ruct

ure

size

betw

een

9-13

µm

and

ahi

ghco

vera

ge.

The

fou

rsa

mp

les

are

liste

din

ord

erof

incr

easi

ngro

ughn

ess

wit

hsa

mpl

eH

’hav

ing

the

smoo

thes

tstr

uctu

res.


500

1000

1500

2000

2500

Wa

ve

len

gth

[

nm

]

0

0.51

1.52

2.53

3.54

4.55

% RC

'

D'

E'

H'

(a)D

irec

trefl

ecta

nce

500

1000

1500

2000

2500

Wa

ve

len

gth

[

nm

]

0

0.51

1.52

2.53

3.54

4.55

% T

C'

D'

E'

H'

(b)D

irec

ttra

nsm

itta

nce

Figu

reB.

10:T

hese

two

plot

ssh

ows

the

dire

ctre

flect

ance

and

dire

cttr

ansm

ittan

ceof

the

sam

ples

H’,

E’,D

’,an

dC

’w

ith

ast

ruct

ure

size

betw

een

9-13

µm

and

ahi

ghco

vera

ge.

The

fou

rsa

mp

les

are

liste

din

ord

erof

incr

easi

ngro

ughn

ess

wit

hsa

mpl

eH

’hav

ing

the

smoo

thes

tstr

uctu

res.

AbstractAcknowledgementContentsIntroductionExperimentThe Etching Recipe and Set-upSample EtchingA New Recipe

The Optical Measurements

ResultsStructural CharacteristicsSEM CharacterizationOptical Results

DiscussionThe Etching ProcessOptical Behaviour

Conclusion and PerspectiveSummaryBibliographySEM PicturesOptical Measurements

Documents

FA - R P S A E - projects.au.dk...the etching process, it is possible to make structures of different size [3]. In addition by adding isopropyl alcohol, IPA, to the etching solution