Layer uniformity obtained by vacuum evaporation:application to Fabry-Perot filters
C. Grezes-Besset, R. Richier, and Emile Pelletier
We show how we can measure with accuracy the distribution law of thicknesses deposited inside a vacuumchamber. These measurement techniques are applied to the simultaneous production of high rejectionnarrowband multiple halfwave Fabry-Perot filters. To prevent any alteration of the filters' optical proper-ties, we must control the variations vs time of the evaporant distribution.
1. Introduction
Through a collaboration with the Centre Nationald'Etudes Spatiales, the optical laboratory of Mar-seilles is interested in wavelength multidemultiplexingsystems between several satellite optical telecom-munication tracks.1 To separate and isolate eachchannel with specific rates (the degree of global isola-tion must be <10-10), it is necessary to use multiplecavity Fabry-Perot filters because of their high perfor-mance. The passband may be <10 nm, and the cros-stalk levels are easily -10-4. Production of these fil-ters usually does not pose a serious problem providedthat accurate direct optical monitoring is used.2 Onthe other hand, the simultaneous coating of severalhigh performance Fabry-Perot filters raises many dif-ficulties due to layer uniformity and time dependenceof the evaporant distribution, which we try to clear upin this paper.
11. Monitoring Techniques: Recalls
With materials such as zinc sulfide and cryolite, therealization of quarterwave multilayer stacks does notraise a serious problem other than thickness monitor-ing. In our laboratory, monitoring techniques3 havebeen developed (Fig. 1) which meet the design require-ments. During deposition, the transmittance is mea-sured for 256 different wavelengths in an adequate 60-
The authors are with Ecole Nationale Superieure de Physique deMarseille, Laboratoire d'Optique des Surfaces et des CouchesMinces, CNRS U.A. 1120, Domaine Universitaire de St Jerome,13397 Marseille CEDEX 13, France.
nm wide spectral range (these measurements areperformed each 0.57 s). Thus we record the evolutionof the filter optical properties during deposition as wellas the variations of the two signals DTE and DTL,which are the derivatives (T/Ot)X 0 and (T/aX)X 0 oftransmittance T vs time t and vs wavelength X. Quar-terwave layer thickness monitoring is based on detec-tion of the zeros of these derivatives. The great advan-tage of this technique is that it allows self-correction ofthickness errors during deposition.2 In the case oftriple halfwave (THW) Fabry-Perot filters with a pass -band of <10 nm, tolerances are extremely severe anddirect optical monitoring must be accurate enough toprevent random errors on thicknesses >0.02XO/4 (X0being the filter peak wavelength).4
In fact, accuracy on the thickness of a layer dependsnot only on the monitoring criterion (DTE or DTL)which is used but also on which specific layer in thestack is considered. It is, therefore, interesting toemphasize using numerical calculation4 which of thetwo methods (DTE or DTL) is best suited to a specificlayer of a particular coating. Two concepts must betaken into account for this choice5: sensitivity of mon-itoring to point out the accuracy on thickness measure-ment and stability of the monitoring process that isconnected to the successive errors committed on layersduring the realization of a multilayer stack. For thispurpose, the slope of DTE or DTL signals near theirzero values gives us some information on the sensitiv-ity of the methods. When this slope is gentle, a poorSNR prevents an accurate detection of zero even if thesignal is filtered. In the contrary case, the detection ofzero does not raise difficulties; however, if the slope istoo steep, the signal variations may be too rapid and itsometimes happens that the calculation of predictionof zero is insufficient. When realization errors do notcross a given threshold, the evolution of the DTL signalis very close to calculation, and for some layers, be-
Fig. 2. Example of signals DTE and DTL recorded during evapora-tion. The figure concerns the deposition of a 2H layer which isadded on a multilayer whose design is: substrate/H L H L 4H, Xo =842 nm, where H is a quarterwave high index layer and L is aquarterwave low index layer. At the end of the deposition, wetheoretically reach an optical thickness of 6H. The endpoint depo-sition is based on the prediction of the zero of DTE, and a mask isused to protect the substrates from the evaporant material. We canobserve that both DTE and DTL are close to zero. This is proof thatthe multilayer is correctly realized. Because of an automatic change
of scale, one can observe some discontinuities in the DTL curve.
cause its sensitivity is much better, it can be preferableto use DTL = 0 criterion rather than DTE = 0. In fact,it is interesting to have simultaneous measurement ofthe two derivatives and to verify that they are bothequal to zero nearly at the same time.6 We have plot-ted in Fig. 2 the measured variations of DTE and DTLsignals during production of a particular layer of aFabry-Perot filter. Numerical calculation does notalways enable us to predict the measured DTE andDTL values since production errors can lead to unex-pected variations of these signals.
Figure 3 gives an idea of the perfect repeatability ofour fabrication techniques. For the substrate used fordirect optical monitoring, we can consider that the
823.5 nm 842 nm 878 nmwavelength -
Fig. 3. Measured profiles of triple halfwave filters manufacturedduring five different evaporations. The design is: design 1: sub-strate/(H L H L 6H L H L H) L (H L H L 6H L H L H) L (H L H L 6H
L H L H)/air X0 = 842 nm (with notations of Fig. 2).
Fig. 4. Epicycloidal system designed for seven rotating substrates:six satellites and a central substrate which is used for the optical
monitoring.
problem of manufacturing multiple cavity Fabry-Perot filters is well solved.
Ill. Layer Uniformity: Use with Fabry-Perot Filters
To manufacture several filters simultaneously, it isnecessary that layer uniformity7 inside the plant begood enough to obtain identical optical properties onthe largest possible area of the monitoring plate. Thisproblem is, of course, closely connected to the perfor-mances that we want to obtain. For this purpose, wedeveloped an epicycloidal system (Fig. 4) which com-prises a central rotating substrate on which the opticalmonitoring is performed, and six other rotating satel-lite substrates, with a rotational motion around thecentral substrate.
1. Single LayerTo measure uniformity, it is usual to deposit a single
layer on several substrates distributed inside the evap-oration chamber. Measurement of the optical proper-ties (transmittance and reflectance) of these layersleads to refractive indices, extinction coefficients, andthe thicknesses of deposited layers.8 Thus we obtainthe uniformity variations in the vacuum chamber.For materials such as zinc sulfide and cryolite, one canverify that the refractive index does not appreciablydepend on the position of the sample inside the plant:the optical thickness is then enough to determine thethickness distribution of the deposited layers. How-ever, the drawback of this method is due to its lack ofsensitivity. Indeed, the modulations of transmittanceand reflectance vs wavelength are rather low for asingle layer, and we cannot expect an accuracy muchbetter than 1% on thickness determination.
2. Fabry-Perot FilterFor our use, it is necessary to improve the sensitivity
of this method, which we do by studying multilayerstacks. An easy solution consists in using narrowbandinterferential filters of which the spectral response isknown to be closely connected to layer thicknesses.With the assumption that thicknesses are perfectlyquarterwave (no error committed during deposition),one has only to measure the filter peak wavelength toknow the value of deposited thicknesses. With ourepicycloidal system, optical thicknesses of the centralfilter layers are (ne)i = X,/4, and those of the satellitefilters are (ne)i = X/4 so that uniformity of the satellitesubstrate is given by U = XS/Xt. Obviously, this im-plies that uniformity remains the same for each of thetwo materials and that there is no shift X - X, due toproduction errors. For the central substrate which isused for monitoring, we have seen that our apparatusperformances permit obtaining a very good wave-length positioning (random error distribution of<0.02XO/4). For the other substrates, we show (Fig. 5)that in spite of some errors of <0.02XO/4, the wave-length shift between center and satellites remainspractically unchanged. Moreover, insofar as opticalproperties of satellite filters are not altered, one canconsider that thickness uniformity is the same for thetwo materials. We can conclude that measurement ofcentral filter and satellite filter peak wavelength givesa good value of uniformity inside the plant.
B. Use in Optimal Positioning
As shown in the previous section, we can determineby making a characterization experiment the varia-tions of uniformity for the seven different substratesbeing at the same height and coated with the epicycloi-dal system. In Fig. 6, we can see that the six substratesaround the monitoring plate have quasi-identicalproperties. The shape of the satellite filter response isnot far from the one measured on the monitoring plate.Satellite filters are nearly all centered on the same
us
zI-
.M
z
I-
808.5 rim 827.2 nm 863.3 nmwavelength _
Fig. 5. Simulation of making THW filters (design 1 of Fig. 3): a,perfect monitoring, all the layers are quarterwave: profile obtainedon the monitoring sample (a), profile obtained on a satellite (b); ,defects in the monitoring, a random error of <0.02XO/4 on each layerof the stack: profile obtained on the monitoring sample (c); profileobtained on a satellite (d). The value of uniformity is calculated
from Ref. 7 using surface source model (U = 0.982).
Lu0z
I-I-
(Jnz
I:
823.5 nrm 842 nrm 878 nmwavelength _
Fig. 6. Experimental results on 10-nm band THW filters, satellitesubstrates, and monitoring plate have the same height in the evapo-
ration chamber (design 1 of Fig. 3).
wavelength; the dispersion between the peak wave-lengths does not exceed 2.8 nm, and this is certainlydue to imprecise positioning of the substrates. (Thiscorresponds to a variation of uniformity AU = 3 X10-3, which can be explained with calculation 7 by avariation in the height Ah = 1 mm.) Between themonitoring filter and satellites, the wavelength shift is-AX = Xs - X, = 8.4 nm. The value of uniformity canbe deduced by U = X,/X, = 0.990; this value is some-what different from what can be predicted by calcula-tion 7 in the case of a surface source (U = 0.982), takinginto account the geometrical conditions of our cham-ber.
Optimal uniformity requires detailed study of thewavelength shift - X vs sample position in thechamber; for this, we heightened the central substrate
Fig. 7. Measured and calculated variations of wavelength shift vscentral sample height in the chamber. The calculation is performedusing a surface source model.7 Relative height is the difference
between central substrate and satellite substrates.
LU
O*zisl-
1'-
z
'-
or823.5 nrm 842 nrm 878 nm
wavelength _Fig. 8. Experimental results after adjustment of the height of the
different substrates (design 1 of Fig. 3).
with wedges of different thicknesses. As shown in Fig.7, variations of phenomena are quasilinear, and ex-trapolation of the experimental curve leads to optimalpositioning of the substrates (5-mm thick wedge). Wecan verify (Fig. 8) that in these conditions the wave-length shift is very low (0.9 nm). It is interesting tonotice that experimental and theoretical7 results arequite different. However, calculation gives a goodidea of the sensitivity of phenomena. (The two curveshave similar slopes.)
IV. Limitations due to Time Dependence of theEvaporant Distribution
If more resolving filters (6-nm band THW Fabry-Perot filters) are required, it becomes necessary to takeinto account tolerances which are much more severe;but above all, because of the great amount of materialto evaporate, we have to control variations vs time ofthe evaporant distribution. Indeed the depositionprocess involves a high number of parameters forwhich it is quasi-impossible to guarantee a constantvalue during experiment. Moreover, from one experi-ment to another, variations of these parameters vs
858 nmwavelength-
Fig. 9. With the optical monitoring process used, the filter ob-tained on the monitoring plate has correct optical properties. Onthe satellites, the profiles are strongly affected. One must take intoaccount the dependence with time of the layer uniformity to explainthese discrepancies (see Fig. 10). Design 2 with X = 824 nm:substrate/(H L HL H 6L H LHL H) L (H L HLH 6L HL HL H) L
(H L H L H 6L H L H L H)/air (with notations of Fig. 2).
LU
z
-
(n
z
nmwavelength -
Fig. 10. To explain experimental results obtained on satellite fil-ters (Fig. 9), we assume that the uniformity is slightly affected forsome layers in the stack (design 2 - X0 = 824 nm). Some defects of
-1 nm are sufficient (see Table I).
time are not the same. We have to take great care ofthe electron-beam sweep process, crucible size andshape, initial state of materials, and so on. All thesephenomena cause the layer uniformity to be depen-dent on each specific layer in the stack. In this case,independent errors (dynamic errors9) may occur whichlead to a serious alteration of the filter optical proper-ties.
For example, Fig. 9 shows an experiment for whichoptical properties of the central filter are quasiperfect,while satellite filters are severely altered. It is obviousthat in such a case no valuable information on unifor-mity can be extracted from measurement of the wave-length shift - X. Such alteration of optical proper-ties can be explained by numerical calculation asshown in Fig. 10. For this calculation, we assumed
Filter with errors Perfect filterLayer thickness (nm) thickness (nm)
18 6L 952.6 950.830 6L 947.6 950.831 H 88.9 90.132 L 157.3 158.533 H 88.9 90.134 L 157.3 158.535 H 88.9 90.1
a Layers 18 and 30-35 are affected by thickness errors of -1 nm,and the calculated spectral profile (Fig. 10) is very close to theexperimental result of Fig. 9.
- monitoring filter - --tellite filters
8 I
C,z
a,
cc
0823.5 nim 878 nin
wavelength -P-
Fig. 11, Problems due to the time dependence of uniformity arepartially eliminated by using greater crucibles (design 2 with Xo
846 nm).
Lu
z-
z
l-_
0
wavelength -
Fig. 12. Filters obtained on the satellites are identical to the moni-toring plate except for a wavelength shift. This result is obtainedwith the help of several experimental adjustments, such as systemat-ic filling of the crucibles at different stages of the realization of thestack. We thus eliminate the variation of uniformity law with time
(design 2 with Xo = 846 nm).
that spacer layer thicknesses are different and that lessand less material was deposited at the end of the ex-periment on the satellite substrates. Moreover, weobserved that the surface of evaporant had been great-ly modified during this experiment. Then a solutionconsists of using greater crucibles and adjusting theoscillatory movement of the crucible to keep the eva-
porant surface as uniform as possible. As shown inFig. 11, the consequence is an improvement in opticalproperties. Better results are obtained (Fig. 12) if weopen the chamber after each Fabry-Perot filter of theTHW stack to make the evaporant surface uniform.At last, with the method previously described, we suc-ceeded in obtaining optimal positioning for which allthe filters are similarly centered.
V. Conclusion
We have shown how it is possible to measure thethickness distribution inside the evaporation chamberwith high accuracy. Such techniques have been usedin making several narrowband THW filters. If moreresolving filters are required, we have shown that themain problem is dynamic error due to the time varia-tion of the evaporant distribution. A detailed experi-mental study of most parameters that interfere duringdeposition led to optimal conditions and permitted usto manufacture simultaneously several samples of 6-nm band THW filters.
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