junctions the maximum efficiency would achieve a 68% [Birkmire, 2001], far enough ...... Susanne Westenhöfer, Replication technology for optical Microsystems, ...
Antireflective and textured thin films for Solar Cells
Master Thesis in Physical Engineering Year 2010-2011 Author: Miquel Gomez Umbert Director: Sabine Portal Marco
This research has been supported by the EU through the HELATHIS project (Grant agreement no. 241378) and the Spanish Ministry of Science and Innovation and the European Regional Development Fund through the project AMIC (ENE2010-21384-C4-03).
INDEX
1.- Introduction
__________________________________________
1
1.1.- Solar Radiation and World consumption ……………………………………
1
1.2.- Si based Solar PV panels
……………………………………
2
1.3.- Antireflective coatings and uses …………………………………………….
2
1.4.- Optical Confinement
……………………………………………………..
3
1.5.- Objectives
………………………………………………………
4
2.- Experimental _________________________________________
4
2.1.- Sol-gel technique
……………………………………………..………
2.1.1- Preparation method of sols
4
…………………………………..
5
2.2.- Spray coating technique ………………………………………………..……
6
2.3.- HEL technique
………………………………………………………………
6
2.3.1.- Masters used ………………………………………………..……
8
2.3.1.1.- Asahi-U
…………………………………………….
8
2.3.1.2.- Unpolished Silicon wafer ………………………………
8
2.3.1.3.- Etched Silicon wafer
9
2.4.- Morphological characterization
……………………….…
………………………………………….….
2.4.1.- Interferometric Microscope
10
………………………………..……
10
……………………………………………….……………….
10
2.4.3.- Contact angle …………………………………………………….…
10
2.4.2.- AFM
2.5- Transmittance measurements
2.5.1.- Total transmittance 2.5.2.- Diffused light
………………………………………………
12
…………………………………….………
13
………………………………………………….……
13
3.- Results and discussion
3.1.- Morphological results
________________________________
14
………………………………………………………
14
3.1.1.- Thickness of the films ………………………………………………
14
3.1.2.- Silanization technique ………………………………………………
15
3.1.3.- Printing results ……………………………………………………….
15
3.1.3.1.- Asahi-U
……………………………………………..
15
3.1.3.2.- Unpolished Silicon wafer
…….…………………….
20
3.1.3.3.- Etched Silicon wafer
…………………………..
21
……………………………………….………………………
22
3.2.- Optical results
3.2.1.- Total transmittance
………………….……………………….. .
22
3.2.2.- Diffused light
………………………………….……….. .
25
………………………………………………………
27
_________________________________________
29
3.3- Wetting properties
4.- Conclusion
Acknowledgements
…………………………………………………….….….….
29
…………………………………………………………………..……
30
List of figures …………………………………………………………….…..…..…
30
ANNEX
……………………………………………………………………….
31
References
…………………………………………………………………….….
32
List of tables
1- Introduction 1.1- Solar Radiation and world consumption
The Sun constitutes the most important Earth’s source of energy. The star provides around 180W of power per each square meter on the Earth’s surface [Escarré, 2008]. Every year, the Earth receives about 2.9x1024 J of energy incoming from the Sun, which is transferred into different natural phenomenon such as plant growing, the water cycle from the seas to the mountains and the wind movement (wind energy).
Independently of the use, and without considering the solar radiation on the ocean surface, the primary energy potential is in the range of 1000 times the world primary energy consumption. With the actual efficiency of the commercial solar cells (~10%), the world energy demand could be covered using only the 1% of the planet surface.
Annual Solar Energy on continents
0,01 0,16
0,16
Carbon reserves
0,42 Oil reserves Gas reserves Primary Energy consumption
6,6
Fig. 1.1.a- Energy Resources in Billion TOE* (Jordi Andreu Batallé, Lectures of Solar Energy, 2010)
Furthermore, the increasing oil prices and the Green House effect may change old habits in the energy production methods. That is why renewable energies and especially solar cells and the photovoltaic effect must be taken very seriously as an important source of renewable energy to cover the world’s demand.
For that reason an appropriated renewable energy policy, lower costs of production, widespread deployment and the improvements in the conversion efficiency of the solar cells are necessary to develop an important and competitive renewable and clean source of energy for the future.
*1 TOE ≈ 42GJ
1
1.2- Si based Solar PV panels A Si based solar panel uses the incoming radiation of the Sun to create electric energy. It is basically composed of a semiconductor junction where the incoming solar radiation can produce an electron-hole pair. Then this electron is promoted to the conduction band and a little electron current is created. There are different types of commercial Si based solar cells each one with different efficiency coefficients and also different costs of production. The most used (and also most expensive) is the monocrystalline Si (c-Si) with a top efficiency of 23%, polycrystalline (mc-Si) with a 15% and hydrogenated amorphous Si (a-Si:H) with 10%, which demand is increasing due to its lower cost of production compared to the other ones [Escarré, 2008].
Fig. 1.2.a- Si based solar cell scheme (www.hvvsolar.com). The main parts are shown.
Despite that, a solar cell conversion efficiency would never reach the idealistic 100%. For instance, if a photon goes across the cell and is not absorbed in the active part of the cell or the generated electron-hole pair recombines just before the electron can create a current, the efficiency will be necessarily lower than this idealistic model. Furthermore, considering an infinity array of junctions the maximum efficiency would achieve a 68% [Birkmire, 2001], far enough from the ideal model. In order to improve the conversion efficient different techniques are considered, i.e. the use of efficient transparent conductive oxides, TCO’s [Seeber, 1999], improvements in the antireflective coatings, optical light confinement by surface texturing [Mingtao, 2003] or the multilayer cell technology [Jin Young, 2007]. In addition to this, materials with self cleaning properties (super hydrophobic materials) are used to reduce the maintenance costs and the durability of the surface stability. For that the combination of different properties of smart materials is an important way to improve this technology. 1.3- Antireflective coatings and uses
Antireflection coatings are used to reduce reflection from surfaces. Whenever a ray of light moves from one medium to another (such as when light enters a sheet of glass after traveling through air), it undergoes different phenomena like transmission, absorption and reflection depending on the media. In the reflection some portion of the light is reflected from the surface (known as the interface) between the two media.
2
Fig. 1.3.a- Antireflective coating scheme. In this case the antireflective effect is obtained with a layer thickness of λ/4.
A number of different effects are used to reduce reflection. The simplest one is to use a thin layer of material at the interface, with an index of refraction between those of the two media. The reflection is minimized when
n1 = n o n s
(1)
where n1 is the index of the thin layer, and n0 and nS are the indices of the two media. The optimum refractive indices for multiple coating layers at angles of incidence other than 0° are given by Moreno et al. (2005). Such coatings can reduce the reflection for ordinary glass from about 4% per surface to around 2%. Practical antireflection coatings rely on an intermediate layer not only for its direct reduction of reflection coefficient, but also use the interference effect of a thin layer. If the layer's thickness is controlled precisely such that it is exactly one-quarter of the wavelength of the light (a quarter-wave coating), the reflections from the front and back sides of the thin layer will destructively interfere and cancel each other. Further reduction is possible by using multiple coating layers, designed such that reflections from the surfaces undergo maximum destructive interference. By using two or more layers, broadband antireflection coatings which cover the visible range (400-700 nm) with maximum reflectivities of less than 0.5% are commonly achievable. Reflection in narrower wavelength bands can be as low as 0.1%. Alternatively, a series of layers with small differences in refractive index can be used to create a broadband antireflective coating by means of a refractive index gradient. 1.4- Optical Confinement There are different ways to achieve a light confinement inside a PV cell. The most important ones consist in the deposition of a back reflectors and the use of textured layers. If an incident ray of light passes trough the cell without generating an electron-hole pair, a back reflector could be used to redirect this beam and increase its path inside the cell and then the conversion efficiency. Other useful technique is texturing directly the different layers of the cell. The different roughness increases the diffused light into the cell and allows the light to be confined between the active layers of the cell, acting as a photonic structure and increasing the conversion efficiency.
3
1.5- Objectives The objective of this study is to obtain antireflective and textured coatings for solar cells using a spray coating technique with a sol-gel solution and finally texturing the thin layer with HEL (Hot Embossing Lithography) method. If the conditions are fulfilled then the transmittance will be increased as well as solar panel efficiency and the hydrophobic behavior of the coating for selfcleaning properties. This method is susceptible to be scaled up and used in mass production coating for its cheap costs and large area coverage compared with other techniques (i.e. CVD).
2. Experimental 2.1- Sol gel technique Sol-gel techniques consist in wet chemical synthesis processes of a large variety of materials in different shape (bulk, film, particles…). The process starts with a precursor solution (sol) which undergoes a series of hydrolysis and condensation reactions leading to a macromolecular network (gel). Sol–gel techniques constitute an economical and rapid method to obtain ceramics with engineered properties. Their low-temperature production, good homogeneity and the possibility to chemically tune their physical properties have attracted much interest in the scientific and industry communities. Sol–gel derived silica films prepared by sol–gel technique, possess low optical loss [Fardad, 1999], and the adjustable refractive index have many applications in the field of antireflective coatings [Brinker, 1981], waveguides [Coudray, 1997], power splitters [Fallahi, 1999], optical filters [Keddie, 1990], distributed Bragg reflector [Fardad, 1999], resonator [Chen, 1999], diffraction gratings [Mendoza, 2006], fiber-optic gas sensors [Remillard, 1999] and so on. Also erbium-doped sol–gel planar waveguides for optical amplifiers [Orignac, 1996] were reported. Gel– film-based devices are, therefore, very promising for all-optical telecommunications. The precursors of the sol-gel solution used in this study are TMES (Triethoxymethylsilane CH3Si(OC2H5)3) and TEOS (Tetraethyl orthosilicate Si(OC2H5)4).
Fig. 2.1.a- Scheme of TEOS (left) and TMES (right) molecules (www.amarketplaceofideas.com).
During the sol–gel process, in the mixture of TEOS and TMES alkoxides, TEOS is preferentially hydrolyzed in the early stages of the reaction leading to the condensation reactions and the formation of silica clusters through the following chemical reaction: nSi(OC2H5)4 + 2nH2O nSiO2 + 4nC2H5OH
(1a)
4
At latter stages of the previous reaction (1a), the TMES monomers get hydrolyzed and condensed in the following manner: Hydrolysis: (CH3)3Si(OC2H5) + H2O (CH3)3Si-OH + C2H5OH
(1b)
Alcohol condensation: ≡Si-OH + (OC2H5)Si(CH3)3 ≡Si-O-Si(CH3)3 + C2H5OH
(1c)
Water condensation: ≡Si-OH + HO-Si(CH3)3 ≡Si-O-Si(CH3)3 + H2O
(1d)
| - Si | | O O O–H | | | - O – Si – O – Si – O – Si – O | | | R–O O–H
-H20 SiO2 gel -C2H5OH
Fig. 2.1.b- Scheme of the obtained matrix [Arkles, 1997]. The Si-O-Si bonds are present.
Thus, the Si(CH3)3 groups are attached to the silica clusters through oxygen bonds leading to the formation of a hydrophobic silica film surface. 2.1.1- Preparation method of sols Solutions with TEOS (Fluka 99%), TMES (Fluka 98%), Ethanol (Panreac abs. dry 99.8%), distilled water and HCl (1M) were prepared using a TMES:TEOS proportion of 0:100, 25:75, 50:50, 75:25 and 100:0. TEOS was mixed firstly with Ethanol and HCl. TMES was mixed with Ethanol and water. Both solutions were stirred for 30 minutes at room temperature. One of the fixed parameters is the relation between water and TMES+TEOS,
H 2O ≈ 2 allowing a complete hydrolysis TMES + TEOS
process. Then, both solutions were mixed and the resulting solution was heated in a thermal bath at 65ºC for 3 h and was allowed to cool to room temperature while stirring. The aging time is the time between the preparation of the solution and the deposition of the sample. The solutions were stored in a cool atmosphere between 5 and 10ºC for a maximum of one week. Solutions kept at ambient temperature show a quick deterioration due to a principle of cause and effect between the continuation of the polymerization and the solvent evaporation, the later increasing the first one. The solution viscosity increases rapidly and when they are deposited, the films are highly sticky or present cracks.
5
%TMES
TEOS(ml)
TMES(ml)
Ethanol(ml)
HCl(ml)
H20(ml)
0%*
2,50
0,00
10,00
0,08
0,40
25%
5,00
1,48
18,20
0,16
1,06
50%
5,00
4,46
14,70
0,16
1,60
75%
5,00
13,40
4,14
0,16
3,20
85%
10,0
50,40
6,12
0,32
10,80
100%
0,00
5,20
19,62
0,16
0,94 *sample carried out by Sabine Portal
Table 1: List of the components and their proportions used to prepare the solutions
2.2- Spray-coating technique Spray-coating is a high-rate, large-area deposition technique that ensures an ideal homogeneous coating on a variety of surfaces with different morphologies and topographies [Girotto, 2009]. The main advantage of spray-coating is the easy and cheap design of the process compared with other techniques like sputtering or CVD. It is frequently used for industrial coating and in-line deposition processes. In spray-coating systems, a solution is atomized at the nozzle by pressure and then directed toward the substrate by a gas. Others advantages of spray-coating are the efficiency compared to other techniques; only a small amount of the solution is wasted and it can be up-scaled for industrial production. In this work a 5x5 cm2 and 10x10 cm2 glass samples were spray-coated using nitrogen at 1,5 and 2 bar of pressure as a carrier gas. The coating process was done manually, trying to maintain the distance between the nozzle and the glass constant (around 10-15cm). To deposit different thicknesses the spraying times were 30s, 60s, 90s, 120s and 300s. 2.3- Hot embossing lithography (HEL) The HEL is a technique used to transfer a texture under controlled conditions. In that process the master is the name referred to the surface that is wanted to be reproduced. This step of fabrication is important to modulate the value of the refractive index of the coating and reach the conditions of antireflective coating and texture for self-cleaning properties [Gale, 2005]. The first step in the process is to put the master above the sample and heat the system to a temperature (T). After that, a uniform force (F) is applied for a determined time (t), known as impression time. Before retiring the master from the sample it is necessary to wait until the system is cooled down to unmold it without any damages. The sample obtained is textured with the negative pattern of the master. The main parameters that control the quality of the impression process are the temperature to which the system is heated, the applied force and the time this force is applied.
Fig. 2.3.a- Scheme of the HEL patterning technique.
6
The main advantages of the embossing process are the sizable ability, and the possibility to reuse the master. In this work the impression process has been made using the Jenoptik HEL HEX 01 system located in the Nanotechnology Platform of the Barcelona Scientific Park. The temperature range lies between 20 to 220ºC and the force available range is between 50N up to 20 kN. Different values of force and temperature were used to obtain different samples. Temperatures of 100ºC, 150ºC, 180ºC and 200ºC where used with forces of 5kN, 7kN, 10kN and 15kN. The impression time was the same in all the samples. The whole process takes about 30 minutes depending on the temperature. A Polytetrafluoroethylene (PTFE) film was put between the matrice plate and the master in order to homogenize the pressure on the sample surface.
Fig. 2.3.b - Scheme of Hot Embossing procedure (left) and Jenoptik HEL HEX system (right).
%TMES
Temp(ºC)
t1(min)
Force(N)
t2+t3*(min)
100,85,75,50,25
100
3
5000
5
100,75,50,25
150
3
5000
5
100,85
150
3
10000
5
100,75
180
3
10000
3
75
180
3
15000
3
75
180
3
7000
3
100,85,75,50,25
180
3
5000
5 t3* =100 s
Table 2: List of the temperatures and forces used in HEL treatments for each percentage in TMES
7
2.3.1- Masters Used Different masters were used in the HEL process to obtain different sample patterns with different roughness values (from 35 to 500 nm). 2.3.1.1- Asahi-U The Asahi-U is a commercial substrate consisting in fluorine doped tin oxide (SnO2:F) layer deposited using CVD on a glass substrate. This layer has a random texture and a Rq value around 35 nm, optimum to be used in a-Si:H solar cells [Escarré, 2008] 85.14nm
-84.15nm 2
2
Microscope direct image (84 x 63 µm )
Interferometric Image (28 x 28 µm )
2
AFM 3-D Image (5 x 5 µm )
profile
Fig 2.3.1.1.a- Interferometric and AFM images of the Asahi-U master. That corresponds to the master with the lowest roughness value.
2.3.1.2- Unpolished Silicon Wafer A silicon wafer has a polished (Rq 0.80-1.10nm) and unpolished side. The unpolished side has a high roughness (Rq of about 370nm) and can be also used as a master. 1.81µm
-1.08µm 2
Microscope direct image (636 x 477 µm )
2
Interferometric Image 60 x 80 µm (inverted)
Fig 2.3.1.2.a- Microscope and Interferometric images of the Unpolished Silicon wafer.
8
2.3.1.3- Etched Silicon Wafer A pyramidal shape mold was made with a (1 0 0) polished silicon wafer and using a solution of KOH (Panreac 90% in flakes), isopropanol (Panreac 99,5%) and distilled water. As known, the formation of the pyramids on the (1 0 0)-Si wafers is due to an anisotropic etch rate of c-Si crystallographic planes achieving a Rq value of around 500nm [Zubel, 1998].
SEM Image (3 K)
SEM Image (15 K)
2
AFM 3-D Image (20 x 20 µm )
profile
Fig. 2.3.1.3.a- SEM and AFM images of Si-etched (100) Silicon wafer. This master presents the highest roughness value.
To compare the transfer of the roughness from the mold to the sample two parameters can be analyzed the average roughness (Ra) and the root mean square (RMS or Rq). In this study the root mean squared value was used to eliminate errors due to negative roughness. The average of the sum squared of all peaks is considered.
Rq =
1 ∑ z i2 n i
(2)
9
2.4- Morphological characterization 2.4.1- Interferometric microscope Interferometric microscopy is based on the interferences obtained between the incident light and the light reflected on the surface of the sample at different distances between the objective and the surface. This technique allows a rapid 3D morphologic characterization of the sample. We used a Sensofar PLµ Microscope. This characterization method will be privileged when the details of the pattern are micrometric. 2.4.2- AFM Atomic force microscopy is based on the measurement of the Van der Waals forces between a sharp nanometric tip and the sample. When the distance between the tip and the surface is relatively large, the attractive force (between valence electrons and ion cores) becomes dominant. These forces depend very strongly on the distance so a good topography image mapping can be obtained with a resolution of about 1nm, depending on the tip size and the measurement mode. This method will be used when the detail of the pattern is nanometric.
Fig. 2.4.2.a- Force dependence with the distance of the tip. For non-contact cantilever the range is marked in blue.
All images were obtained with the Park System AFM using no-contact cantilever (attractiveregime). Non-contact cantilever detects changes in the phase or the vibration amplitude of the cantilever that are induced by the attractive force between the probe tip and the sample while the cantilever is mechanically oscillating near its resonant frequency. 2.4.3- Contact angle The contact angle is a measure of the ability of a liquid to spread on a surface. The method consists in measuring the angle between the outline tangent of a drop deposited on a solid and the surface of this solid. The contact angle is linked to the surface energy so surface energy can be calculated and discriminated between polar and apolar interactions. This technique is very useful to know the wetting property of the surface of a material. Namely the hydrophobicity (large contact angle > 90º) or hydrophilicity (small angle < 90º) of the surface.
10
Fig. 2.4.3.a- Information scheme of the contact angle measurement, matching the value with the property.
All measurements were made with advancing angle technique, using water as liquid compound and calculating the average angle for all the obtained images.
An advancing contact angle is measured when the sessile drop has the maximum volume allowable for the liquid-solid interfacial area: any addition will make the drop expand and increase the liquid-solid interfacial area. This can be thought of as the "wetting angle" because the drop is ready to wet additional area. The relation between the contact angle and the surface energy is given by Young’s Equation [Kappl, 2003] γsg = γsl + γlg cos Θ
(3)
where γ is the surface tension which corresponds to the energy required to create a unit area of an interface.
If the energy required to create the solid-liquid (sl) interface is greater than that required for creation of a solid-gas (sg) interface, then the critical angle will be greater than 90°. In other wor ds, the liquid will bead up on the surface to minimize the solid-liquid interfacial area.
It is also possible to relate the contact angle measurement with the roughness of a surface. For rough surfaces it is possible to describe the effect of surface roughness by the so-called Wenzel equation [Wenzel, 1936] cos Θapp = Rrough · cos Θ
(4)
Fig 2.4.3.b- Wenzel model. The liquid can penetrate into the structure so the contact angle value is lower.
Here, Θapp is the apparent contact angle which is observed by eye or with an optical microscope. Rrough is the ratio between the actual and projected surface area. Since Rrough is always larger or equal to one (Rrough > 1) surface roughness decreases the contact angle for Θ < 90º, while for poorly wetted surfaces (Θ > 90º) the contact angle increases. If a molecularly hydrophobic
11
surface is rough, the appearance is that of an even more hydrophobic surface. If a hydrophilic surface is roughened it becomes more hydrophilic. In the case of textured hydrophobic surface, the Cassie-Baxter theory describes the relation between the contact angle and the roughness defining the fraction φ for the solid surface in contact with water, given as [Cassie and Baxter, 1944] cos Θapp = φ(cos Θ + 1) – 1
(5)
where φ is the area fraction of the solid that is in contact with the liquid. In Cassie-Baxter regime the liquid is suspended on the tops of microstructures and Θ will change to Θapp.
air gaps
Fig 2.4.3.c- Cassie-Baxter model. The presence of air gaps increase the contact angle value.
2.5- Transmittance measurements The transmittance of the silica based film deposited over glass substrate was measured using a LAMBDA 950 UV/Vis spectrophotometer (Perkin-Elmer Instruments), in the range comprised between 300-2500 nm. Total transmittance and diffused light were measured in normal incidence, comparing each one with the reference sample (as deposited) and the glass itself, to compare the antireflective effect and the optic confinement. The laws of electromagnetism establish that light passing through a transparent sample satisfies the condition, T+R+A=1
(6)
where T is the transmittance, R the reflectance and A the absorption. If the sample was transparent, A should be 0 so the equation can be rewritten as, T + R ≈ 1.
(7)
It is also possible to relate the reflectance measurement with the refractive index of the layer assuming an air-layer-glass system [Hecht, 1987]
n1 (no − ns ) 2 cos 2 β + (no ns − n1 ) 2 sin 2 β R= 2 2 n1 (no + ns ) 2 cos 2 β + (no ns + n1 ) 2 sin 2 β 2
2
(8)
where n0 is the index of the air, n1 the index of the layer, ns the index of the glass, φ1 the angle of incidence inside the layer and
β=
2π
λ
n1d1 cos φ1
(9)
12
Eq (8) can be simplified when β = 1 (2 x + 1)π (where x is an integer and assuming normal
2
incidence). Then defining the optical thickness as
n1d1 =
λ
4
(2 x + 1)π
(10)
Finally the Eq. (8) becomes
(no ns − n12 ) 2 R= (no ns + n12 ) 2
(11)
and is minimized when n1=(n0ns)1/2 (R=0). A coating is antireflective when its optical thickness is an odd multiple of λ/4 and its refractive index fulfills Eq. (1). 2.5.1- Total transmittance The total transmittance accounts the quantity of light that passes trough the film and the substrate for each wavelength. This information is very important to deduce the antireflective properties of the film. It’s possible to separate the direct transmittance of the diffused light assuming that the total transmittance (measured) is the sum of the direct transmittance (the light that passes in a straight line) and the scattered light by the film structure. Ttotal = Tdirect + Tdifussed
(12)
2.5.2- Diffused light Diffuse light is the amount of light that is scattered by the surface of the sample. Knowing the total transmittance and diffused light, is possible to calculate the direct transmittance by, Tdirect = Ttotal - Tdifussed
(13)
Removing the SpectralonTM reference located on the reflectance sample holder the diffuse light can be measured so the integration sphere collects all the scattered light which has passed through the sample.
Fig. 2.5.2.a- Perkin Elmer integrating sphere scheme. The beam of light enters trough the textured layer located in the sample holder.
13
3.- Results and discussion 3.1- Morphological results The surface characterization of the samples was made in the nanometric and micrometric scale, depending on the size of the pattern features, using AFM and Interferometric microscopy. These techniques are very useful in the range between 20nm and 90µm for AFM (depending on the tip) and from 1 and 500µm for interferometric microscope in order to analyze the nano-micro texture of the films. 3.1.1- Thickness of the films
The thickness of the films were measured before and after the embossing process to obtain firstly the deposition rate of the coating and secondly the effect of the temperature and pressure over the layer. This thickness was determined by a simple method: the glass substrates were marked with ink and different spray times were used. Then once the layers were dry, the samples were immersed in a bath of ethanol for 10 seconds to remove the ink and the above deposited material. The underlying substrate got uncovered and the step between the substrate and the top of the layer was revealed. Interferometric microscope in profile mode was used to estimate the film thickness.
Dep. time (min)
M1 (±1µm)
M2 (±1µm) M3 (±1µm) Av (±2µm)
1
5
6
6
6
2
10
12
15
12
3
20
15
20
18
5
30
25
24
26
Table 3: Measurements of the thickness of the films as a function of the sprayed time 30
av. thickness (µm)
25 20 15
y = 5,5x R2 = 0,98
10 5 0 0
1
2
3
4
5
dep. time (min)
Fig. 3.1.1.a- Fitting of the thickness of the deposited films as function of time. The deposition rate results in a 5.5±0.2 µm per minute. Note: The composition was 85%TMES, the gas carrier pressure was 1,5 bars and the distance between the nozzle and the sample was between 10-15 cm.
14
3.1.2- Silanization technique It has been found that stickiness increases with the TMES molar ratio. In order to prevent stickiness, silanization was done for all the samples and molds. Silanization consists in the application of a thin layer of Trichloro(1H,1H,2H,2H-perfluorooctyl)silane (C8H4Cl3F13Si, 97% Aldrich) (TCS) over the samples. The deposition of this layer was done by evaporation of TCS: a drop of TCS was deposited on a small glass inside a desiccator previously filled with the samples and the air inside the desiccator was evacuated by a mechanical pump creating an ambient saturated with the Trichloro(1H,1H,2H,2H-perfluorooctyl)silane vapor. The silanization process lasts about 30 minutes. After it an optional heat treatment can be realized in an oven at 80º for a period of about 1h. Trichloro(1H,1H,2H,2H-perfluorooctyl)silane F F F F F F F Cl
F F F F
F F
Si
Cl
Cl Fig. 3.1.2.a- Scheme of the silanization process (J. Escarré, 2008)
3.1.3- Printing results After HEL, treatments between 5000N and 10000N lead to a reduction of around 20% in the film thickness (that means reduction in thickness from 5µm to 4µm for 1 minute of deposition and a reduction from 10µm to 8µm for 2 minutes of deposition). A summary of the conditions of deposition, film composition and the results is given in ANNEX. 3.1.3.1- Mold: Asahi-U Next images show the dependence of the textured area coverage with the force for the samples with 100% of TMES and the Asahi-U master. 100% TMES (1 minute of deposition)
Textured
No texture Textured 2
T=150ºC F=5000N (636 x 477 µm )
2
T=150ºC F=10000N (636 x 477 µm )
Fig. 3.1.3.1.a- Images of the samples showing the uniformity of the textured areas as a function of the force for 100% TMES samples.
15
2
2
T=150ºC F=5000N AFM 3-D Image (5 x 5 µm )
T=150ºC F=10000N AFM 3-D Image (5 x 5 µm )
profile
profile
Fig. 3.1.3.1.b- AFM images of the 100% TMES samples. The dependence with the force reveals different roughness.
Sample with 5000N treatment had a Rq value around 38nm and the sample with 10000N treatment had a Rq value of 32nm. Compared with the Asahi-U master (Rq 35nm), that means a relative difference of about 10%. This result shows a good patterning transfer between the mold and the sample. We noticed that some of the 100% of TMES samples presented a very sticky behavior, even silanized.
2
AFM 3-D Asahi-U Image (5 x 5 µm )
profile
Fig 3.1.3.1.c- AFM images of the Asahi-U master. The good transfer of the roughness is revealed by comparing the profiles of the samples and the master.
16
85% TMES (1 minute of deposition)
Textured
No texture Fuck you ralos
2
Interferometric image(28 x 28 µm )
2
profile
2
profile
T=180ºC F=5000N (636 x 477 µm )
AFM 3-D Sample Image (5 x 5 µm )
AFM 3-D Asahi-U Image (5 x 5 µm )
2
Fig. 3.1.3.1.d- Large and AFM images of the 85% TMES sample and the Asahi-U master. In this case both profiles present some differences.
Sample with 85% of TMES embossed at 180ºC and 5000N showed large areas of continuous texturing although the sample was not totally covered. The Rq value was around 27 nm. This value corresponds to a relative difference around 20% with the master so that means a smoother patterning compared with the 100% of TMES samples due to the lower proportion of the organic compound.
17
75% TMES (1 minute of deposition)
Textured
No texture 2
Interferometric image (28 x 28 µm )
2
profile
2
profile
T=180ºC F=10000N (636 x 477 µm )
AFM 3-D Sample Image (5 x 5 µm )
AFM 3-D Asahi-U Image (5 x 5 µm )
2
Fig. 3.1.3.1.e - Large and AFM images of 75% TMES sample and Asahi-U master. In this case both profiles show close similarity, revealing the good transfer but more untextured regions.
Samples embossed at 180ºC and 10000N showed the largest textured areas for this percentage of TMES. With a Rq value of 33nm, the roughness of the sample is very close to the Asahi-U master (Rq 35nm), with relative difference of less than a 6%, indicating a good textured surface. It can be seen that the textured area decreases with lower TMES ratio, because the material is becoming more rigid. Using less TMES concentration leads to a higher concentration of cracks and detachment of the layer. The worst cases of these effects were the samples with 0% and 25% of TMES, where practically no texture was achieved.
18
Next figure shows the dependence of the force (tested in the 75% of TMES samples). The best force parameter is achieved with 10000N where the sample shows the largest areas textured. However, increasing the force to values over 15000N promote the adhesion of the film to the master, even after silanization.
Textured areas T=180ºC F=7000N
T=180ºC F=10000N
No textured areas
T=180ºC F=15000N Fig 3.1.3.1.f- Large images of 75% TMES showing the dependence of the texture homogeneity with the force.
19
3.1.3.2- Mold: Unpolished Silicon wafer Samples with 75% of TMES solution deposited in 1 minute were used as a trial with the unpolished side of a silicon wafer as a master. Next figures show the uniformity dependence of the temperature of the samples embossed at 150ºC and 180ºC. Both samples were embossed at 5000N. No silanization was realized.
No texture No texture
Textured
Textured 2
2
T = 150ºC F = 5000N (636 x 477 µm )
T = 180ºC F = 5000N (636 x 477 µm )
Fig. 3.1.3.2.a- Direct images of the samples. Sample embossed at 150ºC show better uniformity than the sample embossed at 180ºC.
1.94µm
2.01µm
-0.91µm
-1.15µm 2
T=150ºC F=5000N (60 x 80 µm )
2
T=180ºC F=5000N (60 x 80 µm )
1.81µm
-1.08µm 2
silicon master (inverted, 60 x 80 µm ) Fig. 3.1.3.2.b- Images with interferometric microscope of the samples and the master. The close similarity of the surface roughness reveals a good transfer of the pattern over the samples.
Samples embossed at 150ºC and 180ºC had a Rq value of around 470nm, and the master a value of aprox. 370nm. That means a relative difference of around 25% so it can be considered a good transfer of the texture of the mold. The HEL process at 150ºC gives better uniformity than at higher temperature. The transfer can be improved using the silane agent. A lower roughness was expected for all the samples. However, the higher roughness of the samples compared with the master are due to the elasticity of the layer. When the mold is removed from the sample, the stickiness of the coating produces an elastic deformation of the samples.
20
3.1.3.3- Etched Silicon wafer 100% TMES (1 minute of deposition)
2
2
AFM (90 x 90 µm )
AFM 3-D Image (20 x 20 µm )
2
Inverted sample image (20 x 20 µm )
2
Mold image (20 x 20 µm )
2
Inverted sample profile (20 x 20 µm )
2
Mold profile (20 x 20 µm )
Fig. 3.1.3.3.a- AFM Images of the 100% TMES treated with Si - etched wafer. The transfer of the master roughness is evident comparing the image of the mold and the inverted image of the sample.
Samples with 100% of TMES embossed at 150ºC and 10000N showed the large textured areas. With an Rq value around 400nm, the roughness of the sample is lower than the Si-etched mold (Rq ≈ 500nm), with relative difference of 20%. Despite this difference Fig. 3.1.3.3.a shows close similarity of the master and the inverted shape of the sample.
21
3.2- Optical results The Optical properties of the samples were studied by transmittance spectrometry at a larger scale (in the range of a few cm2) than morphological studies (cf. 3.1). We should take into account this difference of scale when we compare both characterization results. 3.2.1- Total Transmittance Total transmittance was measured for all the samples. All measurements were normalized with the transmittance of the glass to analyze the antireflective effect. Assuming an average value of 92% of transmittance for the glass, the idealistic value of this comparison will be
100
92
≈ 1,08 that means a maximum improvement of an 8%. Next figures show the improvement in
the best obtained samples, indicating the samples without HEL treatment (Ref) and embossed samples with their respective temperature treatment. Samples with best antireflective effect were achieved using the Asahi-U master.
100% TMES (1 minute of deposition) Samples of 100% of TMES embossed with Asahi-U master at 150ºC and 5000N showed the best antireflective effect. As shown in the next figure, the improvement achieves a value between 2% and 3% in the visible range (400-800nm). This composition makes the coating very sticky and had influence on the reproducibility of the process. Despite that, several attempts to repeat the sample failed due to the stickiness of the samples to the master. 1.04 1.03 1.02
T/Tglass
1.01 1 0.99 0.98 Ref 100C 150C 180C
0.97 0.96 300
400
500
600
700
800
900
1000
wavelenght(nm) (nm) wavelength Fig.3.2.1.a- Normalized transmittance of the samples with different treatments.
22
85% TMES (1 minute of deposition)
1.04 1.03 1.02
T/Tglass
1.01 1 0.99 0.98 Ref 100C 150C 180C
0.97 0.96 300
400
500
600
700
800
900
1000
wavelenght (nm) wavelength (nm)
1.01 1.008
T/Tglass
1.006 1.004 1.002 1 0.998
Ref 100C 150C 180C
0.996 400
450
500
550
600
650
700
750
800
wavelenght (nm) wavelength (nm) Fig.3.2.1.b- Normalized transmittance of the samples with different treatments.
Samples with 85% of TMES embossed at 10000N have a more stable behavior (no sticking), but low antireflection effect. As shown in the previous figure, improvement less than a 1% were achieved. Paradoxically the antireflective behavior of the sample without HEL treatment was better. This fact may be due to the good homogeneity of the layer itself and the presence of voids that increase the transmittance. When the layer is heated in HEL, these voids disappear and the refractive index could have changed producing a reduction of the transmittance.
23
50% TMES (1 minute of deposition) 1.04 1.03 1.02
T/Tglass
1.01 1 0.99 0.98 Ref 100C 150C 180C
0.97 0.96 300
400
500
600
700
800
900
1000
wavelenght(nm) (nm) wavelength 1.01 1.008
T/Tglass
1.006 1.004 1.002 1 0.998
Ref 100C 150C 180C
0.996 400
450
500
550
600
650
700
750
800
wavelenght (nm) (nm) wavelength Fig.3.2.1.c - Normalized transmittance of the samples with different treatments.
Samples with 50% of TMES embossed at 180ºC and 5000N presents the best antireflective effect for this concentration with an improvement of about 0.7%. The same effect with the sample without treatment is also show.
24
One of the critical points of the samples were, on the one hand, the ability to transfer a texture and on the other hand the achievement of an antireflective effect. The production of coating showing a good replication of the master but good antireflective properties were hindered by the spray coating process. In general samples showing the best patterning transfer had low antireflective effect while samples with low or no pattern were characterized by high antireflective effect. The production of samples is strongly conditioned by this behavior and the result of the texturing and the optical properties of the layer are counterbalanced. This paradoxical effect may be due to the spray coating set-up process (viscosity change), so reproducibility of the samples sprayed with same conditions were difficult to achieve. We noticed a change of the transparency of the solution in excess in the gun recipient after the spray coating. The modification of the sol-gel chemistry would explain the non-reproducibility of the experiments. Different spray directions (sample horizontally or vertically oriented) were tried to avoid that effect with no difference in results.
3.2.2- Diffused light The diffused light was also measured to analyze the scattering properties of the textured layers. The results were normalized with the diffused light of the glass to obtain the scattering improvement (optical confinement) of the layer. 75% TMES (Unpolished silicon wafer, 1 minute of deposition) 25
Ref 150C 180C
Td/Tdglass
20
15
10
5
0 300
400
500
600
700
800
900
1000
wavelength (nm) wavelength (nm) Fig. 3.2.2.a- Normalized diffused light of the samples with different treatments for 75% TMES and Unpolished Si master (F=5000N). This is the sample with the highest scattering properties.
25
100% TMES (Asahi-U master, 1 minute of deposition) 25
Ref 100C 150C 180C
Td/Tdglass
20
15
10
5
0 300
400
500
600
700
800
900
1000
wavelength (nm) wavelength (nm)
5
Ref 100C 150C 180C
Td/Tdglass
4
3
2
1
0 300
400
500
600
700
800
wavelength (nm) wavelength (nm) Fig. 3.2.2.b- Normalized diffused light of the samples with different treatments for 100% TMES and Asahi-U master (F=5000N). In this case the lower roughness of the sample implies less scattered light.
Previous graphics shows the effect of the patterning versus the antireflective effect. 100% of TMES have a lower diffused light due to a more homogeneous layer and a good transparency. However 75% TMES sample embossed with the unpolished Silicon side, shows very high rates of scattered light despite their low antireflective effect. The patterning of the film and the presence of cracks in the film are responsible for light scattering in all directions and consequently for the huge ratio of diffused transmitted light.
26
3.3- Wetting properties In table nº 4 and 5 the average values of the contact angle values of samples with different compositions before and after HEL process are listed. The general trend is the increment of the contact angle value after the lithography due to the increment of the roughness by nanopatterning. Master
Θ1
Θ2
Θav.
Asahi
123±1
122±2
122±2
Si-etched
134±2
131±4
132±3
Si-unpolished
49±3
49±2
49±3
glass
27±1
25±1
26±1
Table 4: List of the contact angle values with the standard deviation of the masters after silanization
Glass
100%TMES-AS
S1
S2
S5
Fig. 3.3.a- Images of the droplets over the surface. Different sample roughness produces different hydrophobic behavior.
In figure 3.3.a, the effect of the force on the pattern transfer quality is evidenced by the increase of the contact angle value, comparing the value of the layer as deposited (AS) with the embossed samples. Sample
Av. ΘAS
S1
Av. Θ HEL 101±1
S2 = 100%TMES-Asahi-U-150ºC-10000N
81±2 S2
S1 =100%TMES-Asahi-U-150ºC-5000N
107±2
S3 = 85%TMES-Asahi-U-180ºC-5000N
S3
90±2
96±3
S4 = 75%TMES-Si-Unpolished-150ºC-5000N
S4
97±2
106±3
S5 = 100%TMES- Si-KOH etched-150ºC-1000N
S5
98±3
120±2
Table 5: List of the contact angle measurements for each sample before and after embossing
27
Previous figures show the good hydrophobic behavior for all the samples, achieving the high contact angles for samples with 100%TMES embossed at 150ºC and 10000N. Also samples with 75% TMES shows good hydrophobic effect due to the high textured areas. Sample with 100% of TMES embossed at 150ºC and 5000N have an improvement of the 25% in the hydrophobycity while the 100% based films after embossing at 150ºC and 10000N reaches around 30%. This behavior demonstrates the good patterning transfer on the samples surface. Indeed nano or micro structuring of the film surfaces increases the original roughness of the film and this results in an increase of the contact angle whether the surface was originally hydrophobic In addition to this, best hydrophobic results are shown in the samples with 100% TMES and treated with the Si-etched master, achieving high values of around 120º for the contact angle. Sample with 85% TMES embossed at 180ºC and 5000N had a small improvement of around 6%. This fact is due to the non uniform patterning of the film. Sample with 75% TMES embossed at 150ºC and 5000N showed a better result than samples with 100% of TMES, but the less textured areas decrease this gain to 8%.
28
4- Conclusion - The spray coating technique was studied as a mechanism of thin film deposition on large area of sol-gel Si based solutions. These solutions were composed of a mixture of TEOS and TMES. - Hot embossing lithography was carried out to obtain a patterning over a polymer based thin films. - Samples with higher ratio of TMES (organic compound) showed better patterning, due to –CH3 bounds which decreases the temperature of transition of silica. Concentrations over 75% of TMES were necessary to obtain a good patterning whilst concentrations below 50% TMES were insufficient. - Temperatures between 150ºC and 180ºC and forces between 5000N and 10000N showed the best patterning results in a reduction of the film thickness around a 20%. For temperatures below 150ºC, very low patterning effect were achieved, while forces over 15000N increases the sticking of the sample to the master, destroying partially the roughness of the sample. - 100% TMES and 85% TMES were the samples with best antireflective effect, achieving an average improvement of a 1% in the transmittance compared with bare glass. Best sample reached a maximum value in total transmittance of 93.5%. Several attempts to repeat the sample with the same conditions failed because of spray coating process not optimal deposition conditions. - The compromise between good patterning and good antireflective effect was very difficult to achieve, so good patterned samples show low transmittance while low textured samples have a good antireflective improvement. - Patterned samples have an improvement in the diffuse light in the range of 4-25 times of the glass. The good patterned samples are characterized by a high roughness so the light is scattered mainly by the nano and micro texture. - High rates of TMES added to the good patterning of the layer contributes to increase the hydrophobic behavior of the layer, achieving contact angle values around 120º for 100% of TMES embossed with KOH etched Silicon wafer.
Acknowledgements I want to thank GES-UB and FEMAN groups for the equipment provided, the Nanontechnology Platform of the PCB and Yolanda Atienza for the support. Finally I want to dedicate some words to my family, my girlfriend and of course Dr. Sabine Portal for the huge amount of help she provided me.
29
List of Tables Table 1: List of the components and their proportions used to prepare the solutions. Table 2: List of the temperatures and forces used in HEL treatments for each percentage in TMES Table 3: Measurements of the thickness of the films as a function of the sprayed time. Table 4: List of the contact angle measurements with the standard deviation of the masters after silanization. Table 5: List of the contact angle measurements for each sample after and before embossing. Table 6 (Annex): Description of the patterning results for each sample showing the aging time of the solution, the sprayed time and the embossing temperature and force.
List of Figures Fig. 1.1.a- Energy Resources in Billion TOE Fig. 1.2.a- Solar Cell scheme (www.hvvsolar.com) Fig. 1.3.a- Antireflective coating scheme Fig. 2.1.a- Scheme of TEOS and TMES molecules (www.amarketplaceofideas.com) Fig. 2.1.b- Scheme of the obtained matrix Fig. 2.3.a- Scheme of the HEL patterning technique Fig. 2.3.b - Scheme of Hot Embossing procedure and Jenoptik HEL HEX system Fig. 2.3.1.1.a- Interferometric and AFM images of the Asahi-U master Fig. 2.3.1.2.a- Interferometric and AFM images of the Unpolished silicon wafer Fig. 2.3.1.3.a- SEM and AFM images of Si-etched (100) Silicon wafer Fig. 2.4.2.a- Force dependence with the distance Fig. 2.4.3.a- Information scheme of the contact angle measurement Fig. 2.4.3.b- Wenzel model Fig. 2.4.3.c- Casie-Baxter model Fig. 2.5.2.a- Perkin Elmer 150mm integrating sphere scheme Fig. 3.1.1.a- Fitting of the av. thickness of the deposited films (85%TMES) as function of time Fig. 3.1.2.a- Scheme of the silanization process Fig. 3.1.3.1.a- Large and images of the 100% TMES samples Fig. 3.1.3.1.b- AFM images of the 100% TMES samples Fig. 3.1.3.1.c- AFM images of the Asahi-U master Fig. 3.1.3.1.d - Large and AFM images of the 85% sample Fig. 3.1.3.1.e- Large and AFM images of 75% TMES sample Fig. 3.1.3.1.f- Large images of 75% TMES showing the homogeneity of the texture with the force Fig. 3.1.3.2.a- Direct image of the 75% TMES samples. The HEL process at 150º gives better uniformity than at higher temperature. The transfer can be improved using the silane agent Fig. 3.1.3.2.b- Images with interferometric microscope of the samples and the master
30
Fig. 3.1.3.3.a- AFM Images of the 100% TMES treated with Si - etched wafer Fig. 3.2.1.a- Normalized transmittance of the samples with different treatments Fig. 3.2.1.b- Normalized transmittance of the samples with different treatments Fig. 3.2.1.c- Normalized transmittance of the samples with different treatments Fig. 3.2.2.a- Normalized diffused light of the samples with different treatments Fig. 3.2.2.b- Normalized diffused light of the samples with different treatments Fig. 3.3.a- Images of the droplets over the surface
ANNEX
%TMES
Master
Aging
Dep.
time
Time
(days)
(min)
Force (N)
100º C
150º C
180º C
100
Si-etched
5
1
10000
-
Good textured
-
100
Asahi-U
9
1
10000
-
Good textured
Little textured
100
Asahi-U
9
1
5000
-
Good textured
No textured
100
Asahi-U
7
1
5000
Good textured
Little textured
Cracked
85
Si-etched
5
1
10000
-
Little textured, cracks
-
85
Asahi-U
1
2
5000
Good textured
-
Good textured
85
Asahi-U
9
5
10000
No textured
No textured
No textured
85
Asahi-U
1
2
10000
-
Good textured
-
85
Asahi-U
14
2
10000
-
Little textured, cracks
-
75
Si-etched
5
1
10000
-
Little textured, cracks
-
75
Asahi-U
6
1
5000
No textured
Little textured
Good textured
75
Asahi-U
20
1
7000
-
-
Good textured
75
Asahi-U
20
1
10000
-
-
Good textured
75
Asahi-U
20
1
15000
-
-
No text
8
1
5000
-
Good textured
Good textured
75
Unpolished Silicon wafer
50
Asahi-U
1
2
5000
Little textured
Little textured
Little textured
25
Asahi-U
7
1,5
5000
Little textured
Little textured
No textured
25
Asahi-U
13
1,5
5000
Crack
Crack
Crack
Table 6 (Annex): Description of the patterning results for each sample showing the aging time of the solution, the sprayed time and the embossing temperature and force.
31
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