Isne
Chapter 3: EXPERIMENTAL DETAILS
3.6 Measurement of Thickness of the Thin Films
TIuckness is the single most significant film parameter. Any physical quanllty related to film thickness can in principle be used to measure !he film thickness. h may be measured e1lher by several melhods with varying degrees of accuracy. The methods chosen on the basis of their con\'eruence, simplicity and rehability Several of the common methods are i) During Evaporation, ii) Multiple-Beam Interferometry, iii) Using a HysteresIs graph and other methods used in film-thickness determination with particular reference to their relative merits and accuracies, Muhiple-Beam lnterferometry technique was emplo}ed for the measurement of thickness of the thin films, ThIs technique is described belo".
Multiple-Beam Inferferometry IS)
This method utilize the resulting interference effects when two silvered surfaces are brought close together and are subjected 10 optical radiation This interference technique, which is of great value in studying surface topology in general, may be applied simply and directly to film-thickness determination, When a wedge of small angle is fonned between unsil\'ered glass plates, which are illuminated by monochromatic light, broad fiingcs are seen arising from interference between the light beams reflected from the glass Onthe roro sides of the au wedge.
•• t
oJ • ••
Chapter3: ExperimentalDetails
Where the path difference is an integral and odd number of wavelengths, bright and dark fringes occur. If the glass surfaces of the plates are coated "ilh highly rell(lCting layers, one of which ISpartially transparent, then the reflected fringe system consists
of very fine dark lines againsl a bright background. A schematic diagram of the muillple-beam interferometer along with a typical pattern of Fizeau fringes from a film Slep is shown in Fig.3,5.
Fig.3.5 The schematic diagram of multiple-beam interferometer,
As shown in this figure, the film whose thickness is to be measured is over coated with a silver layer to give a good reflecting surface and a half-silvered microscope slide is laid 00 top of the film whose thickness is to be delermined The thickness of the filmd can !hen be determined by the rellllioo
d=~!:
2,
where
A.
is the wavelength and bla is the fractional disoontinulty idenlified in the figure, In general, the sodnnn light IS used, for ",tlich A. = 5893 AO. In practice, several half-silvered slides of ,'arying thickness and therefore of var~ing•
Chapter3' ExperimentalDetails
transmission are prepared, and one of these is selected for maximum resolution.
Accurllte detenmnatlons of fringe spacmgs are difficult and time oonsuming; but a method of image comparison, ""iJichconsiderably improves the ease, and rapidity of measurement has recently been developed. Altemlltively, a simple film- thickness gauge utilizing Newton's nngs may be developed, which involves no critical adjustment of wedges, etc., and \\hich reduces error lfl film-thickness determinat10n In conclusion, it might be mentioned that the Tolansky method of film"thickness measurement is the most v.idely used and in many respects also the most accurllte and satisfactory one
3.7 Plasma Polymer Thin Film Formation
The electnc field, when IIpplied to the gaseous monomers lit low pressures (0.01 to I mbar), produces active species thai may react to form cross-linked polymer films. In this experiments, lIlr was used as the primary plasma, and the monomer vapor was mjected downstream of the pnmary air glow discharge. [n laboratory PPTEOS thin films ",-ereprepared using a bell-jar type capacitively coupled glow discharge system described earlier, The boiling point ofTEOS is about 438K. So in order to deposit good PPTEOS films the naturall10w of monomer gas during deposition is suitable The condlliotlS to prepare PPTEOS thin !ibm were 40W r.f. power, O.15mbar system pressure, and the monomer flow TlItewas kept20cmJ/min The deposition times for these films were varied from 80 minutes to 95 minutes in order to get films of different thicknesses.
,~ •
;~
j= •
,~
~250-
=L __ "~~~~_
00
Dop",~IO" 11m~.11mIn)
Fig. 3.6. Variation offilm thickness \\'lth deposition time (deposition power'" 40W)
Chapter 3: Expenmental Details
The variation of film thickness with deposition time is shown in Fig. 3.6 for fJ1msof thickness 250nm, 300 nm, 350 nm, 400 nm. From the nature of the graph, It is evident that film thickness increase linearly with increase in deposition time, The grol'v1hrate of film deposition was about 10 mnlmin
3.8 Contact Electrodes for ElectriCll! Measurements 3.8.1 Electrode material
Aluminium (AI) (purity of 4N British Chemical Standard) "''lISused for electrode deposition, Al has been reported to have good adhesion with glass slides [9]. AI film has advantage of easy self-healing burn out of flaws in sandwich structure.
3.8.2 Electrode deposition
Electrodes were deposited using an Edward coating unit E-306A (Edward, UK). The system was evacuated by an oil diffusion pump backed by an oil rotary pump. The chamber could be evacuated to a pressure less than 10.1 Torr. The glass substrate
",-ere masked \vith 0,08mXO.08mX O,OOlmengraved brass sheet for the electrode deposition. The eleclrode assembly used in the study is shown in fig,3.7. The glass substrates with mask ",-ere supported by a metal rod 0.1 m above the tungslen filamenL For the electrode deposllion Al was kepI on the tungsten filament. The filament was heated by low-tension power supply of the coating unit. The low- tension power supply was able to produce 100 A currenl al a potential drop of 10 V.
Dunng evacuation of the chamber by diffusion pump, ilie diffusion unit ",as cooled by the flow of chilled waler and its outlet temperature was nOIallowed to rise above 305K. When Ihe penrung gauge reads about 10-' Torr, the AI on tungsten filament was heated by low-tension power supply until II was meltoo,
The AI was evaporated, thus lower electrode onto the glass slide was deposited. AI coated glass substrates were taken out from the vacuum coating unit and were placed on the middle of the lower electrode of the plasma deposition chamber for TEOS thm film deposition under optimwn condition, The top Al electrode was also prepared on the PPTEOS film as described above [10-11].
ClIIlpl" 3:ExperimentalDetails
"-
" "
, .. 1
'-
0
' o.1''-
t ••••••_ .•••
.s..
'-
Fig.3.1 Theelectrode =bly.
- :.r
-~•
•
. , I '4'
, -
,~.
Fig.3.g Photograph ora PP'TEOS thin film nnd e1ectriall 8S5embly.
"
•
Chapter3, ExperimcnllliDetails
3.9 Samples for Different Measurements
PPTEOS thin films were deposited on 10 chemically cleaned microscope glass substrates for UV-Visible spectroscopy analyses. For EA, DTA, TGA sludy and IR spectroscopy the films were scraped ofT from the substrate. MetaJJ PPTEOSlMetal sandwich structures, shown In Fig 3,8, were prepared for all shorts of electrical imestigatlOns.
References
L Ii Yasuda, "Plasma Polymenzatlon" Academic Press, Inc, Tokyo, 1985 2. F,-U-Z. Chowdhul)', AB,M,O. Islam, Ali Blruiyan, "Chemical analysIs of
plasma- polymerized diphenyl thin films", Vacuurn 57 (2000) 43-50.
3, S.D. Angllel, T. FrenlJu, E.A. Cordos, A Simon, A Popescu:"Atmospheric pressure capacitively coupled plasma source for the direct analysis of non- conducting soM sampies", J Anal. At. Spectrom, 14(1999) 541-545.
4, N, Inagaki, "Plasma Surface Modification and Plasma Polymerization", Teclmomic Pubilshing Co Inc., New York, 19'J<i,
5. F.F. Chen, "Introduction to Plasma Physics", Plenum Press, New York, 1')74.
6. M,A. Liehcrmllll, AI Lichtenberg, "Principles of Plasma Discharges and Materials Processmg", Wiley, New York, 19')4.
7. A Bogaerts, E. Neyls "Gas discharge plasma and their apphcations".
Spectrochtmica Acta Part B 57 (2002) 609-658,
8. S, Tolansky, "Multiple Beam Interferometry of surfaces and films", Clarendon Press, Oxford, 1948.
'J. AB.M Shah Ialal, S. Ahmed, AH. Bh\llyan and M. Ibrahim, "On the conduction mechanism in plasma polymeriled m-xylene thin films", Thin Solid Films 295 (1997) 125-130,
10, S, D. Phadke, K Sathianandan and R. N, Karekar, "Electrical conduction in pol)ferroceoe thin films", Thin Soltd Films 51 (177H) 9-11,
II RD. Gould and TS. Shafai, "Condetion in lead phthalocyanine films with aluminim electrodes", Thin Solid Films 373 (1-2) (2000) 89-93.