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Real-time optical monitoring of spin coating
F. Horowitz, E. Yeatman, E. Dawnay, A. Fardad
To cite this version:
F. Horowitz, E. Yeatman, E. Dawnay, A. Fardad. Real-time optical monitoring of spin coating. Journal de Physique III, EDP Sciences, 1993, 3 (11), pp.2059-2063. �10.1051/jp3:1993253�. �jpa-00249065�
Classification Physics Abstracts
47.80 68.15 81.15L
Short Communication
Real-time optical monitoring of spin coating
F. Horowitz (*), E. Yeatman, E. Dawnay and A. Fardad
Department of Electrical and Electronic Engineering, Imperial College of Science, Technology
and Medicine, London SW? 2BT, Great-Britain
(Received 29 June 1993, accepted 28 September 1993)
Rdsumd Nous montrons qu'une nouvelle application de l'interf4rom4trie optique, le contr61e du ddp6t I la tournette, fournie des informations fondamentales pour la compr4hension de la
dynamique du d6p6t. Cette comprdhension pourrait nous permettre d'am61iorer la production I la toumette de couches de caract4ristiques prdcises. L'dvolution temporelle du d6p6t de couches de verre, aussi bien en atmosphkre ouverte que satur4e en solvant, est ddtermin4e pour des
dur6es de centrifugation variant de 10 1 60 s.
Abstract A novel application ofoptical interferometry, monitoring ofspin coating, is shown to provide valuable information for the basic understanding of the process dynamics. This could increase the potential of spin coating for the production of films with required properties.
Temporal evolution of spin-on glass films, in open air and saturated solvent atmospheres, is determined for 10-60
s spinning times.
1 Introduction.
Although spin coating has been known for several decades, with extensive use in the semi- conductor industry, still today limitations in understanding its underlying physics can be at- tributed to the lack of experimental data for changing parameters during the process ill.
Several models have been proposed for spin coating. It was initially viewed as flow of
a Newtonian liquid of constant viscosity [2], or under assumption of steady-state flow, for Newtonian and non-Newtonian liquids [3]. Later, solvent evaporation was introduced as a
post-spin off stage and at a constant rate [4], by assunling equilibrium in solvent concentration [5], or a linear mass transfer coefficient for the solvent transport [5], between film surface and
(*) Permanent address: Instituto de Fisica, UFRGS, Campus do Vale C-P. 15051, 91501-970
Porto Alegre, RS Brasil.
2060 JOURNAL DE PHYSIQUE III N°11
atmosphere. Further analytical refinement was made possible by approximating liquid viscosity
as a function of film thickness [6], but also a finite-element approach to the coupled flow and
mass transfer equations has been reported [7]. Conflicts on the basic assunlptions used in these models remain unresolved, as most model predictions are still experimentally unsupported.
Empirical relationships for final fi1nl thicknesses have been proposed for electron resists with
a variety of solvents on glass [9], for polymers with different viscosities on silica [10], and for several solvent /polynler/speedlatnlosphere conlbinations, on glass [11]. Such outconles have been attained basically through post-spinning nleasurenlents, and comparison with existing
model predictions is partial and qualitative at best.
In this conlnlunication, for the first t1nle to the authors' best knowledge, an exper1nlental
way is shown to capture quantitatively essential features of the temporal dynamics of spin coating. This is achieved through the real-t1nle, in situ, interferometric monitoring of the
process.
Although the technique described here is general to spin-coating, the particular process investigated was fabrication of silica spin-on glasses by sol-gel. A general description of sol-gel processing can be found in reference [1].
2 Experimental.
The experimental arrangement, as outlined in figure I, is set for the analysis of the HeNe laser beam reflected by the sol-gel film on a horizontal silicon wafer, which spins at 700-3000 rpm.
The ratio between the sample and the reference signals is computed after processing by an AID converter, and, at present, readings can be taken every 0.01 s. Alignment of the system is critical, to insure that the illuminated spot remains at the centre of the wafer at all spinning
times.
Spinning t1nles are typically up to 20 s in open air, depending on process paranleters, such as
speed of rotation and conlposition of the sol (varying degrees of aging and dilution in ethanol, pH 1.0-2.2, giving silica and silica/titania coatings). Under saturated solvent atnlospheres,
spinning times up to 60 s are chosen to capture most significant reflectance changes.
-i~'
"HeNe laser
~~ L2
fir fi
ch.1 ch.2
e~i
~3wafer
~
SP'""~~
Fig. I. Double-beam experimental set-up with a HeNe laser followed by Att. attenuator, M. mirror, BS. beam splitter, L 1-3. condenser lenses, ch.1,2. reference and sample detection channels.
An extensive set of measurenlents (more than lo0) was obtained. This corroborated previous reports that repeatability in final film properties is achievable in spite of slight variations in the deposition and spin-up stages. In our conditions, this is found particularly true for speeds of1000 rpnl or higher, when the initial liquid sol layer is not too thin. As a consequence, and in order to concentrate on the spin-off stage already pointed out as the key to spin coating [12] we have opted to flood entirely the substrate before spinning.
3. Results and discussion.
As an illustration, typical curves of modulated reflectance versus spinning t1nles, from now
on referred to as "optospinograms", are shown in figure 2 for a saturated solvent atmosphere (a), and for open air (b) with otherwise the saute initial conditions (TEOS precursor, I:I vol. dilution in ethanol, pH = I-o, 12-day old sol) for production of silica films on silicon at
2000 rpm.
Stage:I II III Stage: I II III
75 36
65
j jf 30
~ 55 g
( I 23
45
~
~§ ~
/j °~ 17
25 10
5 0 5 10 IS 20 25 30 2.5 0 2.5 5 7,5
Time (s) Time (s)
a) b)
Fig. 2. a) Optospinogram for a saturated solvent atmosphere, which was enclosed by a glass lid;
b) optospinogram for open air, where reflectance values are as calibrated from that of the bare silicon substrate, for which a refractive index n
= 3.858 +10.018 at wavelength 632.8 nm was taken.
The optospinogram in figure 2a can be divided into three main stages in time, where zero
time refers to the start of spinning. For negative times, stage I, low frequency oscillations in the
liquid are detected mainly as a result of slight beam deflections. Here signal modulation cannot be attributed to interference alone, since in that case with the liquid thickness much smaller than the coherence length of the laser light, and variations slow to the detection system one
would obtain reflectance values scanning the whole range allowed by the boundary conditions
[see also Fig. 2b].
From zero to around two seconds, stage II of intense convective mass flow is identified, where the t1nle for a full reflectance cycle is too small, as conlpared to the sampling period,
for detailed observation.
As the process gradually slows down in stage III, sequential interference minima and maxima
corresponding to the antireflection and absentee-layer conditions can be distinguished
2062 JOURNAL DE PHYSIQUE III N°11
unambiguously. After spinning and under fluorescence light illumination, exposure of the film to open air induces changes in its colour almost immediately. This attests that a significant
amount of solvent is present throughout the spinning process.
The optospinogranl in figure 2b presents the temporal behaviour observed for spinning in open air. Here stages I and II are similar to those in a saturated solvent atmosphere. This,
as expected, indicates that thickness changes are entirely dominated by convective mass flow in the first two to three seconds after spinning has started. Later on in stage III, on the contrary, there is a distinct behaviour from that in figure 2. This clearly shows that convection and evaporation now become increasingly intertwined with comparable contributions to mass
loss, until a fourth stage is reached with a significant decrease in the rate of change. Here convection has dropped to negligible values, while limited evaporation still remains, I-e. the sol has gelified. This is also supported by detailed observation of post-deposition drying, at rest, when the same slow-change behaviour is observed.
From both optospinograms, variation in optical thickness is determined by counting a full quaterwave every time reflectance evolves between two successive extremes. The resulting
data are shown in figure 3. In open air, the role of evaporation in spin coating cannot be
disregarded as in the EBP nlodel [2], since a pronounced departure of the slope from values at a saturated solvent atmosphere is present after the first few seconds. Also the measured
evidence is against the assunlption of an intermediate stage of stready-state flow, where film thickness would not change with time [3]. Furthermore, the assumption in reference [4] of a
separate solvent evaporation staje, after convection has fully ceased, is not supported by the experimental data.
E~ 40
~ .
~ .
~ x .
~ .,
~ . .
i 30 I >Atm°SPhere~ rPm°
~ . .
t I I
o sat solv 2000
~ j .,
i ~. ~$~~~ ~~
~ 20 : °. 1000
~ 4 . .
- , o ,
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~
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.
°,
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~ iQ o ,
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o to 20 30 40 SO 60
Tlme (s)
Fig. 3. Optical thickness variation with time in quaterwave units for spin coating at three different sets of process parameters. Counting of quaterwaves starts from the last reflectance extreme in each
optospinogram.
These are first conclusions from this investigation, and should be augmented as work pro- ceeds in our laboratory and other model predictions are tested. A third curve was added to
figure 3, corresponding to loco rpm in a saturated solvent atmosphere, to illustrate further the
sensitivity of the technique to a change in a process parameter. The results above indicate that
optospinography might have an important role in clearing the way to the ultimate setting of
specific coating protocols regarding desired microstructures, mechanical or optical properties, under a greater degree of control and a variety of compositions.
Acknowledgements.
This work is partially supported by CAPES and the CEC. We are grateful to Professor Mino Green for helpful discussions.
References
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