HAL Id: jpa-00249380
https://hal.archives-ouvertes.fr/jpa-00249380
Submitted on 1 Jan 1995
HAL is a multi-disciplinary open access archive for the deposit and dissemination of sci- entific research documents, whether they are pub- lished or not. The documents may come from teaching and research institutions in France or abroad, or from public or private research centers.
L’archive ouverte pluridisciplinaire HAL, est destinée au dépôt et à la diffusion de documents scientifiques de niveau recherche, publiés ou non, émanant des établissements d’enseignement et de recherche français ou étrangers, des laboratoires publics ou privés.
the Screen Printing Method
Y. Boukennous, Brahim Benyahia, M. Charif, A. Chikouche
To cite this version:
Y. Boukennous, Brahim Benyahia, M. Charif, A. Chikouche. Antireflection Coating of TiO2 Study and Deposition by the Screen Printing Method. Journal de Physique III, EDP Sciences, 1995, 5 (8), pp.1297-1305. �10.1051/jp3:1995191�. �jpa-00249380�
J. Phys. III FFance 5 (1995) 1297-1305 AUGUST 1995, PAGE1297
Classification Physics Abstracts
42.78H 81.20 86.30J
Antireflection Coating of Ti02 Study and Deposition by the Screen
Printing
MethodY. Boukennous, B. Benyahia, M.R. Charif and A. Chikouche
Photovoltaic Cells Laboratory, UDTS, 2 Bd Frantz Fanon, B-P- lo1? Alger Gare, Algiers, Algeria (Received 22 February 1994, rev~sed 22 August 1994, accepted 5 May1995)
Abstract. We
are developing the Screen Printing technique for depositing a single layer quarter wavelength thick antireflection coating of titanium dioxide on silicon substrate. The ink is composed by the titanium ethoxide
as the organometallic compound, terpineol as the solvent and the octyphenoxy polyethoxy as the vehicle. It has been applied to 4 inch polished
silicon wafers, dried then fired and characterized. The objective of our work was to control the
deposition parameters and the ink viscosity to determine their effects on the layer properties.
The thicknesses of the Ti02 films were measured by the stylus technique using a Profilometer.
AES, RBS and X-Ray diffraction are used to analyse the layer and to determine its structure and composition according to firing temperatures. The reflection coefficient is measured as a
function of the wavelength. As
a result, we obtain Ti02 coating thicknesses between 600 and 800 I and
a minimum reflection
near 600nm.
1. Introduction
Screen printing technique became a standard process for solar cell metallization and largely
used for this application [ii but sufficiently for antireflection coating. Because of its low cost and ease of automation, this technique is also used to make N+P junctions by diffusion source
deposition [2]. Extension of this method to ARC allows running the major of solar cells
proces§ing steps by integral screen printing.
The layer thickness and its refractive index depend on the nature and purity of the material and strongly depend on the technology carried out to deposit the ARC (screen printing, spin-on,
vacuum evaporation..) [3,4]. Materials commonly deposited as ARC by screen printing method
are Ti02 and Ta2 OS [2]. Both Ti02 and Ta2 OS ARC's give near zero minimum reflectance on polished C-Si surface and produce a significant reduction compared to the uncoated surface. At shorter wavelengths, the Ti02 layer exhibits high reflectance whereas at wavelengths, beyond
soo nm the reflectance of the Ta20s is higher [2]. Titanium oxide coating onto silicon has
potential interest for photovoltaic application because of its refractive index near 2 and can be easily deposited as a thin film with a resistance to degradation and a good adhesion on
substrate. It can be screen printed before or after metallization step.
© Les Editions de Physique 1995
The process is simple, inexpensive and well suited to automated mass production. Besides its economical aspect, the screen printing technique permits the deposition of ARC, with
thicknesses and refrhctive indices similar to those obtained by other techniques.
We are developing a method which permits deposition of single layer Ti02 ARC using screen printing technology. The control of the films thicknesses is achieved by varying the ink viscosity, printer control parameters and heat treatment.
2. Theory
Due to reflection losses, the incident radiation on polished silicon wafers surfaces is reduced by
more than 30% ~5].
Yetall>c Contact
ADhreflecbve Co»hog
I~T~
Base JP
BSF 3P~
Metalhc Contact
Fig. I. Titanium dioxide antireflection coating on photovoltkic device.
An antireflection coating (ARC) is used to increase the transmission of incident light into the solar cell. It consists of one, two or multilayers stack of transparent materials deposited
on the cell surface (Fig. 1). The reflection coefficient R(I) for a multilayer system could be calculated with a matrix formulation [6]:
j~j j) (~03fll ~'ls~li22)~ + l'l0'1s3f12 3f21)~
(n03ftl + ns3f22)~ + (n0ns3f12 + M21)~
Where no is the refractive index of the air, ns is the refractive index of the substrate, and Mii, M12, M21 and M22 are given by
~f~~ =
fill
13f12~
~jf
C°S ~j ~ ~) ~~~~~~ ~~~
j=i iNjsinbj cos/j
Where bj =
~~~~Nj phase thickness of the layer j Nj = nj ikj Icomplex refractive index
nj: refractive index of the layer j kj: absorption index
dj: thickness of the layer j
I: wavelength
m: number of layers
N°8 DEPOSITION OF ANTIREFLECTION COATING BY SCREEN PINTING 1299
Basic Process
~
Ink Gauze and mask F~af1°~
Squeegee
/
II
~/
Subsirale Substrale holder
Ink drawn from open mesh j~~
,
Gauze
~§
-Gap
Fig. 2. Schematic arrangement of the system.
To minimize R(I), optimum parameters which are n (refractive index) and d (thickness layer)
have been determined by a modelling program. This program developed in our laboratory permits to optimize a mono or multilayer antireflection coating as a function of material type
(Ti02, Ta20s, Si02). It determines the optimum values of n and d by isocurves method and draws the variation of reflection coefficient as a function of wavelength [7].
3. Experiment
3.I. SAMPLE PREPARATION. It's important that the substrate is free of impurities which could react with the ARC. The wafers are first cleaned in a hot bath of HN03-HF-18 Mfl H20
to remove the oxide layer (Si02). They are then etched in a hot solution of NH40H-H202-
H2O for decontamination followed by an etch in HCI-H202-18Mfl H20 solution to form heavy
metals complexes as Cu, Au, Cr, then in HF-C2HSOH solution to remove the native oxide [8].
The Titanium oxide ink(~ is screen printed onto a 4 inch polished crystalline silicon wafer.
Upon printing, the samples are left in air for 10 mn to make the liquid film uniform, they are
then dried at 120-140 °C for 5 minutes. This is followed by a firing in ambient air in a belt furnace for 8 minutes at either 550 or 700 °C.
The drying step is important since it allows the evaporation to a certain amount of solvents and organic diluants. Three characteristic regions are recorded for the firing process:
the first is that of the increasing temperature where the remaining solvent burn.
the second is that of the maximum temperature where the sintering occurs.
the third is that of the decreasing temperature where a slow cooling is required to avoid thermal shocks.
3.2. THE BAsic PRINTING PRocEss. The basic printing process illustrated in Figure 2, gives a schdmatic arrangement of the various parts of the process.
The Titanium oxide is screen printed onto the silicon wafers using a variety of screen mesh sizes (200 and 325 stainless cloth). However, the 325 mesh screen with a thin emulsion layer
(~ obtained from GENTEC, Belgium
~
~i
~ ~
2
1
5
2
2 5 IO 2 5
V(RPM) Fig. 3. Viscosity graphs as
a function of rotational speed. 1,2: Graphs delimiting regions of good viscosity 3: experimental graph.
permits to obtain optimized ARC. Different combinations of ink viscosity, squeegee pressure, printing speed and gap are tested. The ink viscosity is measured with a rotational viscometer.
A range of rheograms giving excellent printing results is shown in Figure 3 [9].
Near the minimum value of the viscosity interval (Curve 3) different tests of printing give coating thicknesses between 600 and 800 I. The most important influence comes from depo-
sition conditions. The substrate and screen must be maintained in parallel planes. Squeegee
pressure is calculated as a function of the squeegee length (Fig. 4), and is limited to a max- imum value of 7 Kg to avoid the destruction of the screen. Using print speeds in the 25 75
mm Is range, we obtain thin layers of about 700 I. The
screen gap and the squeegee travel are
determined by relations as indicated in Figure 4 [10]. Variations of all these parameters are
done to adjust the coating thickness.
4. Results and Discussion
4.1. ARC THICKNESS. After screen printing deposition, drying and firing, the layer uni-
formity is firstly observed by a visual control and was good enough to give an even colour to the entire 4 inch wafer surface. The thickness of the deposited layers is measured by the
stylus technique using a Sloan Dektak II A Stepheight profilometer. This equipment is also used to evaluate uniformity by 10 thicknesses measurements on each sample and an average
value is determined by the least square method [12]. The average thickness depends on the temperature parameter, between 680 and 800 I.
Figure 5 shows Titanium, Oxygen and Silicon profiles obtained by Auger Perkin Elmer Spec- troscope. Near the surface, an important concentration of Titanium and Oxygen is observed.
At the Ti02/Si interface, it decreases until near zero, while the concentration of Silicon in-
creases rapidly. Finally, layer thicknesses are deduced and are similar to those obtained by
Sloan Dektak (Tab. 1).
N°8 DEPOSITION OF ANTIREFLECTION COATING BY SCREEN PINTING 1301
,',
~l~
~
'~
j----
/
/ i
II
,
c
G L
Fig. 4. Geometrical recommendations depending on the image size [iii. K, L:
screen dimension
K = 3R; L
= 2C; R: squeegee length, R > 3+o.5"; C: squeegee travel, C > 3+1"; G: Gap
o.oo2 < G/K < o.oos 3: wafer diameter
= 4 inch.
' '
90
' '
80 '
' '
70 ' '
~
60 ' ' '
~
~ ~~
' ' j '
~o ' '
30 ' '
'
20 '
' f ,
lo '
0 ' '
2 4 6 8 lo 12 14
SPUrWR UME (Mh1)
Fig. 5. Typical profde of Ti02 layer by Auger Spectroscopy.
4.2. RUTHERFORD BACK SCATTERING ANALYSIS. Figure 6 shows the RBS typical spec-
trum of Ti~oy/Si deposited layer, it indicates the existence of the three essential elements in the case of firing temperature of 550 and 700 °C.
The steochiometric ratio between Oxygen and Titanium are determined using a calculator software [13]. Results are indicated in Table II.
The contribution of Titanium in the Ti02 film decreases with increasing firing temperature.
Table I. Results of thickness measurements.
Tp(°C) Thickness Thickness
550 800 780
700 680 720
Energy (MeV)
~~~
0.6 0,6 1-o I-Z I.4 1.6
3000
,
'He+, E-2 MeV
s 1+--
2500 _o
,rj ~~
~~e
2
3 2000
~q.~
I '~~§
1500 '[
~
1000 :sl /)
l~
500
o
loo 200 300 400 500
Channel Fig. 6. He+ ions R-B-S spectrum.
4.3. X-RAY DIFFRACTION ANALYSIS AND ELLIPSOMETRIC MEASUREMENTS. X-Ray
diffraction technique is used to determine the coating structure and the preferential crys- tallographic orientation.
The analysis are undertaken with the Philips Powder diffractometer and the ASTM cards which help us to identify the different compounds and their phases (Tab. III).
Table II. Steochiometric ratio of Ti~ Oy layers. (a~ay are the errors on z and y respectively).
~
X Y ax °y
550 1.008 2.004 0.005 0.011
700 0.610 2.000 0.006 0.002
N°8 DEPOSITION OF ANTIREFLECTION COATING BY SCREEN PINTING 1303
Table III. Identification results of the Ti02 phases according to firing temperatures.
Peaks Peaks
recorded Phase recorded Phase
at at 700°C
550 °C
51.845 Brookite 25.23 Anatase
61.750 Brookite 46.82 Brookite
65.910 Brookite 54.485 Ruffle
68.845 Anatase 65.790 Brookite
68,695 Brookite
83.82 Rutile
87.43 Rutile
Table IV. Results of ellipsometric measurements in, d) for two firing temperatures.
Tp (°C) n llfickness
550 2.23 685
700 2.31 665
The refractive index is determined with an ellipsometric method (Tab. IV).
The films fired at 550 °C present a polycrystalline phase and the peaks observed indicate the contribution of the Brookite and Anatase phases. For firing temperature of about 700 °C,
a slight peak for 29
= 25.2° is recorded which confirms the existence of the Anatase phase.
However, the Brookite and the Rutile phases predominate.
Possible effects of temperature variation on the refractive index and the coating thickness
are structural changes in the film itself. The contribution of Rutile in the film should cause
an increase in the refractive index and a decrease in the film thickness. This correspondence
between structure, n and d is also observed in [14].
Thickness and refracrive index are plotted in Figure 7 for different location on the sample.
The refractive index remains unchanged throughout the sample. It increases slightly as the
firing temperature is changed from 550 to 700 °C while the coating thickness decreases.
According to [14] our screen printed Ti02 films possess values of n and d similar to those obtained by Szulfick et al.
4.4. REFLECTION CHARACTERISTIC MEASUREMENTS. The reflection coefficient as a func-
tion of wavelength is a technique commonly used to characterize the ARC. Results are shown in
Figure 8 for films fired at 500-700 °C. Both Curves indicate a minimum of reflection coefficient.
The films fired at 550 °C exhibit a minimal reflection coefficient of 2% at 600 nm, while at 700 °C, the minimal reflection coefficient (3%) correspond to 639 nm.
The weak red shift of the minimal reflection coefficient could be caused by the increasing of refractive index (2.2 to 2.3) and the decreasing of thickness due to the change of firing
temperature from 500 to 700 °C.
80
d=(68,5+0.3)nm
~~~~'
70 7 ~
60 d=(66 5+0.5)nm 6
50 5
40 4
~~
n=2.311+0.001
~
B 0 0 0 0 0 0 0 0 0 a n
~~ ~~~'~~~~°'°°~
C~ echant.Ajl j550°Cj
~
~+%~ echant.A 2 700°C
~~0 5 lo 15 20 25 3i
Linescan x/mm
Fig. 7. Refractive index and film thickness given by ellipsometric measurements.
Table V. Ellipsometric results comparison.
Tp (°C) Refractive index Refractive index this work
550 2.23 2,16
700 2.31 2.25
T=700°C, ~rnin= 639 nrn, Ri~in= 306 ~
l=55C°C, ~mi~= 620 nrn, Rm<n= 202 ~
i
w
~ i
c c
~ l
~ l
~ ~, ,'
~
'
f l
/~
~ ~' A'
'
_/~
' ,,,"'
' ,-~"""
' ,,"'~
,'~
~~
loo zoo 400 600 mo 900 taco iioo izoo
Wavelength [nm)
Fig. 8. Total reflectance as a function of wavelength for Ti02 and Ta205.
N°8 DEPOSITION OF ANTIREFLECTION COATING BY SCREEN PINTING 1305
5. Conclusion
The properties of screen printed Ti02 ARC on polished silicon wafers have been examined for two firing temperatures, 550 and 700 °C. The later was selected because it could be used
as metallization firing temperature, either if the front contacts are deposited before or after the ARC- As shown, the final thickness and structure of the obtained layer depend on the
firing temperature. Anatase and Brookite phases are converted to Rutile one by heating above 700 °C. The obtained refractive index, ranged from 2.2 at 550 °C to 2.3 at 700 °C, also depends
on the firing conditions. The properties of the Ti02 layer are easy to control by varying the
firing and deposition conditions. Refractive index and thickness correspond to optical and
geometrical properties of antireflective coating deposited on solar cells. These layers present a minimal reflection coefficient value of 2% at 600 nm, that is the optimum wavelength in case
of solar cell ARC's,
Acknowledgments
The authors would like to express their gratitude to the valuable technical assistance of Mrs K.
Melhani, Chemistry Engineer, Mr D. Bouhafs, Researcher, the technical staff of RBS, X-Ray diffractometer and Auger Spectroscopy laboratory, at the Haut Commissariat h la Recherche
(Algiers).
References
[ii Van Overstraeten R., Technologie de la s6rigraphie appliqu6e h la fabrication des cellules solaires
au silicium, L'6nergie photovoltkique, Etat de la recherche (1984).
[2] Thomas R-E- et al., Screen printed Ta205 and Ti02 antireflection coating for crystalline and
polycrystalline silicon solar cells, Can. J. Phys. 67 (1989) 430.
[3] Kern W. and Tracy E., Titanium dioxide antireflection coating for silicon solar cells by spray deposition, RCA Review 41 (1980) 133-180.
[4] Yoldas B-E- and O'Keeffe T-W-, Antireflective coatings applied from metal organic derived liquid
precursors, Appl. Opt. 18 (1979) 3133.
[5] Jellison Jr G-E- and Wood R-F-, Antireflection coatings for planar silicon solar cells, Sol. Cells 18 (1986) 93-l14.
[6] Mouchart J., Thin film optical coatings, Appl. Opt. 16 (1977) 2722.
[7] Bouhafs D., Mod61isation, simulation, conception et r6alisation des couches antireflet, Th6se Mag- ister, UDTS (1993).
[8] Chikouche A., Boukennous Y., Benyahia B. and Charif M-R-, D6p6t de grilles m6talliques et couches antireflet par s6rigraphie, Rapport interne LCP/UDTS (1993).
[9] Baudry H. and Franconville F., Encres sdrigraphiables pour haute d6finition rh6010gie et impres- sion, Acta Electron. 2 4 (1978) 283-295.
[10] Herbst M. and Jacobs M., How to control variables when setting up a screen printer, Hybrid
Circuit Technology, December (1989) 51.
[iii Bopp-SD, Toile de s6rigraphie en acier inoxydable, Technical Brochure.
[12] Wolf S. and Tauber R-N-, Silicon Processing for the VLSI era Vol. I Process Technology, Lattice press.
[13] Chu W-K- et al., Principles and application of ions beam technique for the analysis of solids and thin films, Thin Sol. Fi. 17 (1973) 1-40.
[14] Szulfick J. et al., Ti02 Antireflection coating for silicon solar cells, Sol. Energy Mat. 18 (1989)
241-252.
© Les Edtions de Phy@quo 1995 Directdce de ~ Publication : Jeanne BERGER
SaisieT~X-LAT#X:Les Editions de Ph~sique impression JOUVE, 18, nJe Saint.Dents, 750Ol PARIS
N° 229044F. Dbpbt ldgd Ao0t1995
