Dependence of interface roughness and diffuseness

JOURNAL OF APPLIED PHYSICS

VOLUME 94, NUMBER 5

1 SEPTEMBER 2003

Dependence of interface roughness and diffuseness of Cu–Co electrodeposited multilayers on electrochemical additives Spyridon Merkourakisa) CECM-CNRS UPR 2801, 15 rue Georges Urbain, 94407 Vitry-sur-Seine, France and LPMDI-CNRS UMR 8108, Universite´ de Marne la Valle´e, 5 boulevard Descartes, 77454 Marne la Valle´e cedex 2, France

Martin J. Hy¨tch, Elisabeth Chassaing, and Michael G. Walls CECM-CNRS UPR 2801, 15 rue Georges Urbain, 94407 Vitry-sur-Seine, France

Yamin Leprince-Wang LPMDI-CNRS UMR 8108, Universite´ de Marne la Valle´e, 5 boulevard Descartes, 77454 Marne la Valle´e cedex 2, France

共Received 14 March 2003; accepted 18 June 2003兲 We examine the effect of two organic additives, sds and saccharin, and also the effect of the solution pH on the interface properties of Cu/Co nanolayers, produced by pulsed electrodeposition from a single aqueous bath. Quantitative Fresnel fringe transmission electron microscopy is applied to cross-sectional samples of the layers. The widths of their respective interfaces as well as the widths of individual Cu and Co layers are determined via comparison with computer simulations. These initial results are further numerically treated to yield information about the separate contributions of interdiffusion and roughness to total interface widths. Conclusions on the behavior of these organic additives are considered in the light of the giant magnetoresistance properties of the multilayers, as reported in previous work. © 2003 American Institute of Physics. 关DOI: 10.1063/1.1599626兴

I. INTRODUCTION

surface morphology. The copper was polished mechanically, then electrochemically in a H3 PO4 , C2 H6 O aqueous bath, thoroughly rinsed in demineralized water, rinsed for a few seconds in dilute H2 SO4 , rinsed again in demineralized water, and then immediately immersed in the electrolyte. The electrolyte was an adaptation of the Watt’s bath for the case of Co and Cu 共Table I兲. The layers were then protected with a 0.5-mm-thick copper layer, galvanostatically deposed from a separate bath. Care was taken to minimize the surface exposure to air in all steps following the electrochemical polishing, as it was found that the surface quality of air-exposed specimens was significantly degraded. In the preliminary phase of our study, we performed control observations in air after each of the abovementioned steps, to ensure a reproducible substrate quality. These consisted of atomic force microscopy scans, using a Topometrix TMX 2010 apparatus, and optical microscopy observations on control substrates. In order to elucidate the influence of pH and additives to multilayer growth, five different baths were used. The first two contained no additives, and had pH values of 1.6 and 3, adjusted by additions of dilute H2 SO4 共we refer to these solutions as NA and NApH3兲. To investigate the influence of additives we chose to work with the fixed pH value of 1.6. The third bath contained saccharin only 共SAC兲, the fourth sds only 共SDS兲 and the last one a mixture of the two 共SDS ⫹SAC兲. The composition of the five electrolytes is summarized in Table II. Sds concentration was 0.15 g/l and saccharin concentration was 1.5 g/l. Electrodeposition was carried out by imposing potentiostatic pulses for fixed periods of time with no agitation. A PAR 263A potentiostat-galvanostat and a classical three electrode cell, consisting of the Cu plate 共the working electrode兲,

Metallic multilayer fabrication on the nanometer scale has been quite extensively studied, at first by physical1 and more recently by electrochemical methods.2 Electrochemistry has proved capable of producing layers of competitive quality.3 The magnetic characteristics of metallic multilayers, and in particular giant magnetoresistance 共GMR兲, are an issue of interest. The dependence of this property on interface morphology and structure has already been studied but results are not always concordant.4 –7 Moreover, the role of the additives frequently used to improve the electrolyte’s performance requires elucidation. In this article, we discuss the effect of electrochemical conditions and composition of the electrochemical bath on interface quality of Cu/Co multilayers. We use Fresnel fringe analysis8 in transmission electron microscopy 共TEM兲 to study the impact of two organic additives, saccharin and sds 共sodium dodecyl sulfate兲9–13 on interface width. In doing so, we deal with the frequently encountered problem of differentiating between interdiffusion and roughness contributions to the total width of the interfaces. Results will be correlated with the GMR measurements, based on previous work. II. EXPERIMENTAL DETAILS

Multilayers were grown on Cu polycrystalline substrates 共electrolytic Cu 99.9%, Prolabo兲 with a thickness of the order of 0.5 mm after polishing. A rigorous substrate preparation is necessary in order to obtain a reproducible reference a兲

Author to whom correspondence should be addressed; electronic mail: [email protected]

0021-8979/2003/94(5)/3035/6/$20.00

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© 2003 American Institute of Physics

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TABLE I. The composition of the basic electrolyte. CuSO4 5H2 O CoSO4 7H2 O CoCl2 6H2 O H3 BO3 NaCl

6.4⫻10⫺3 M 0.711 M 0.189 M 0.645 M 0.342 M

a Pt counter electrode and a saturated calomel electrode 共SCE兲 reference were used. The temperature of the cell was maintained at 30 °C and the solution was deaerated before electrodeposition. The pulse duration for Cu was 40 s, and for Co 1 s. The potential was fixed at ⫺400 mV with respect to SCE for Cu deposition and resulted in a current density of 0.3 mA/cm2. The Co deposition potential was fixed at ⫺900 mV with respect to SCE and resulted in current densities of 9 mA/cm2 for the NA electrolyte, 8 mA/cm2 for NApH3 and SAC electrolytes, 16 mA/cm2 for the SDS electrolyte and 6 mA/cm2 for the SAC⫹SDS electrolyte. The resulting layer thicknesses were nominally 3– 4 nm for Cu, assuming a current efficiency of 0.9, and 2–3 nm for Co, assuming a current efficiency of 0.6 for the electrolytes containing saccharin and 0.8 for the other cases3,14 共in fact, the layer thicknesses differed somewhat from these values, as will be shown below兲. We can observe, from these current values, that the additives’ influence is quite significant at high negative potentials. This kind of behavior is in agreement, qualitatively speaking, with previous results.14 The number of bilayers grown was fixed at 30, preliminary work having shown that this is sufficient to study significant changes in the evolution of the growth front, as far as roughness of the layers is concerned. Cross sectional TEM specimens were prepared by the tripod method,15 thinned further by an argon ion milling 共Fischione Instruments 3000 ion-beam thinner兲 at low incidence angle 共⬍12°兲. The morphology and microstructure were verified using a TOPCON EM-002B TEM operating at an acceleration of 200 kV. The Fresnel fringe method16 was used to calculate the width of individual layers 共measured directly from the TEM pictures兲 and the width of the interface region between layers. This method exploits the different ‘‘refractive indices’’ of Cu and Co layers, for fast electrons. Cobalt has a higher mean inner potential than copper. Electrons passing through the Co layers are consequently more strongly accelerated than electrons passing through the Cu layers. When the layers are observed edge on and far away from Bragg diffraction conditions, the resulting contrast is dominated by these refraction effects and is known as Fresnel fringe contrast. A through-focal series of 17 images was recorded over a defocus range of 2850 nm. The defocus step calculated from amorphous zones of our specimens was

FIG. 1. TEM cross section of a multilayer made without additives at pH 1.6 共electrolyte NA兲. The image was taken at a defocus of 1500 nm. The bright fringes are on the Co side of the interface and the dark fringes on the Cu. The opposite is observed on an under-focused image. The Fresnel fringe profiles are obtained by averaging intensity contributions over a 40-nm-long region 共dashed line rectangle兲. The inset shows the shape of one fresnel fringe period corresponding to the area near the arrow. E:end of the growth front.

found to be 22.75⫾0.27 nm. The beam convergence value, calculated as the full width at half maximum of diffraction spot intensities, varied between 0.15 and 0.40 mrad. Fringe contrast was enhanced by using a small objective aperture 共radius 3 mrad兲. The images were recorded on imaging plates. These are about ten times more sensitive to electrons than normal films, and can furnish a linear response to electron intensity with a dynamic range of 105 . This means that we can compare directly measured intensities with simulations. The width of the interface as seen in the TEM images can be due to both roughness and chemical interdiffusion. A recently developed method17 was used to separate the respective contributions of each to the total interface widths. The corresponding image analysis was performed using routines we have incorporated into the Semper18 software package. III. EXPERIMENTAL RESULTS A. Measurement of interfacial widths

Figure 1 shows a cross section of multilayers deposited using a type NA electrolyte. The multilayer period, measured directly from pictures of this type, is shown for the different types of electrolytes in Table III. Since the cupric ion con-

TABLE II. Electrolyte types.

Electrolyte Total width 共nm兲

NA

NapH3

SAC

SDS

SAC⫹SDS

No additives pH 1.6

No additives pH 3.0

Saccharin pH 1.6

Sds pH 1.6

Saccharin⫹ sds pH 1.6

1.00⫾0.10

0.50⫾0.15

0.70⫾0.20

1.00⫾0.20

0.57⫾0.20

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J. Appl. Phys., Vol. 94, No. 5, 1 September 2003

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TABLE III. Electrolyte types and multilayer characteristics.

Electrolyte Total width 共nm兲 Roughness 共nm兲 Interdiffusion 共nm兲 Period 共nm兲

NA

NApH3

SAC

SDS

SAC⫹SDS

No additives pH 1.6

No additives pH 3.0

Saccharin pH 1.6

Sds pH 1.6

Saccharin⫹ sds pH 1.6

1.00⫾0.10

0.50⫾0.15

0.70⫾0.20

1.00⫾0.20

0.57⫾0.20

0.45⫾0.18 0.90⫾0.14

0.46⫾0.19 0.20⫾0.32

0.75⫾0.31 ⭐0.52

0.52⫾0.21 0.85⫾0.27

0.64⫾0.20 ⭐0.50

4.8 ⫾0.1

5.0 ⫾0.1

5.7⫾0.1

7.2 ⫾0.1

4.5⫾0.1

centration is low and there is no agitation, the deposition rate is diffusion controlled and for a given applied potential will decrease at first as t ⫺1/2, reaching a quasistationary value after a few cycles.3 The change in the deposition current over the first few cycles may be attributed to the change in the nature of the deposition electrode as Co layers are included. However, as can be seen in Fig. 2, the current density is stabilized after the first cycle. The resulting charge variation was measured by coulometry and was less than 1 mC/cm2 for all the multilayers. But in general, the effects of the additives used here, namely the depolarizing effect of sds and the inhibiting effect of saccharin, are more important. In Fig. 2 we can also see that the Cu deposition current is significantly lower than that of Co, which means that the amount of co-deposited Cu will be very low. This has been confirmed in preliminary electron energy-loss spectroscopy experiments showing the Cu concentration dropping to below 2% in the Co layers.19 Fresnel fringe intensities were extracted from images of the type shown in Fig. 1 at different defoci by averaging the

FIG. 2. Current density 共full line兲 and charge 共dotted line兲 vs electrodeposition time. Figure B is a zoom on the first two and a half cycles of Fig. A. It can be seen that the current density is stable after the first cycle.

intensity along the length of the fringes in a box 40 nm long. Figure 3 shows the result for electrolyte NA. The experimental fringes are compared to simulated ones 共Fig. 4兲. The simulations are generated using standard multislice image simulation techniques, with the interfaces modeled as a step change in the inner potential value from that of Co to that of Cu. The abruptness of the change can be varied by convolution with a Gaussian whose width is a parameter of the simulation.3,17,20 It is this potential change that generates Fresnel fringes. We have used a quantitative criterion to determine which simulations best fit the experimental data 共a semiquantitative approach has been followed in previous work3兲. While varying the abruptness of the interface and the widths of the individual layers, we seek to minimize the quantity ⌬E⫽ 具 兩 共 e 共 x 兲 ⫺m e 兲 / ␴ e ⫺ 共 s 共 x 兲 ⫺m s 兲 / ␴ s 兩 典

共1兲

with e(x) and s(x) the intensities of the experimental and simulated fringes, m and ␴ their respective average intensity and standard deviation. Unlike other authors21 we have not opted for an automated application of our criterion. It was necessary in our case to retain more direct control on the choice of the best simulations. Figure 5 shows the values of ⌬E obtained in this way, as a function of defocus. In fact, some fringes give smaller ⌬E values only for certain defoci. Each curve in Fig. 5 corresponds to a series of simulated fringes 共see Fig. 4兲 which correspond, in turn, to a multilayer of a specific abruptness

FIG. 3. Experimental Fresnel fringes 共electrolyte NA兲. The fringes are extracted from TEM pictures at different defoci. The contrast is reversed around zero defocus. Observe the presence of a double fringe that remains visible at all defoci 共except zero defocus兲 and disappears at the last negative defocus value.

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FIG. 6. Internal potential models for electrolytes NA and SDS.

FIG. 4. Simulated fringes. In our model, a multilayer of specific individual layer and interface width would produce fringes with exactly the shape shown in the figure.

and width. The best fit parameters 共width of the interface Gaussian兲 for each multilayer are shown as the total interface width in Table II. The models which served for creating the simulations can give quite a realistic representation of the multilayer morphology. Figure 6 shows two internal potential models corresponding to the optimum values given in Table II for electrolyte types NA and SDS. The SDS electrolyte interface is relatively abrupt, whereas the NA electrolyte interface is

FIG. 5. Evaluation of ⌬E values as a function of defoci. The images close to zero were not used since the fresnel fringe contrast here is weak and the contrast is dominated by residual diffraction effects. The width of the Cu layer was fixed at a value of 3.52 nm. The total interface width varied between 0.7 and 1.2 nm. The fringe series that corresponds to a total interface width of 1 nm is the one that fits the experimental fringes most closely.

wider 共the Gaussians modeling neighboring interfaces overlap significantly—the resulting interface profiles are no longer simple Gaussians兲. We can easily observe that for the case of SDS, the internal potential values that match pure layers 共8.29 V for Co and 10.04 V for Cu3 ) are maintained. This means that far from the interface pure metal regions exist. On the other hand, for the NA electrolyte model, interface widths were such that there may be no pure metal layer inside the multilayer. Note that we have chosen to compare two cases with similar total interface widths. From Fig. 6 it is quite evident that the definition of interface width might be rather arbitrary. It is therefore useful for comparison purposes to establish a standard definition of interface width independent of its form, 共Gaussian or not兲. Following the example of previous literature,17 we have chosen to use the double of the standard deviation 共2␴兲 as the interface width in all cases. B. Roughness measurements

We define apparent roughness 共which is measured directly on TEM images兲 as the root mean square deviation of the interface position 共taken as a line of equal intensity in the image along the interface兲 from a straight line representing its average position. Apparent roughness is so called because it always underestimates real roughness due to an averaging of the interface position through projection along the thickness of the TEM sample. Its knowledge is, however, useful as it can give a straightforward appreciation of the evolution of roughness with the advance of the growth front. The real roughness can be estimated from the apparent roughness by measuring the power spectrum of the apparent twodimensional interface position curve and rotating it, to generate the power spectrum of the real three-dimensional roughness. The procedure has been described in full elsewhere.17 This has been performed here for regions in which the total width of the interface had been measured by the Fresnel fringe method. The contribution of interdiffusion to the total width was then estimated using the formula

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Merkourakis et al.

J. Appl. Phys., Vol. 94, No. 5, 1 September 2003

FIG. 7. Apparent roughness measurements for electrolyte NA. Layers grow homogeneously from the beginning to the end of electrodeposition. B:beginning, M:middle, E:end of the multilayer’s growth front. Defocus:1500 nm.

␴ tot⫽ 冑␴ 2d ⫹ ␴ r2

共2兲

with ␴ tot , ␴ d , and ␴ r the standard deviations of total interface width, interdiffusion, and roughness, respectively. The images used have thus already served for the calculation of total interface width by the Fresnel technique and exist for different defoci. Roughness, unlike fringe intensity, should be independent of defocus. In order to check this, we measured apparent and real roughness at two different defoci 共1500 and 1200 nm兲 for the electrolyte NA specimen. We took five different measures for each of four layers at the two defoci and we found that in each case the variation due to defocus was always lower than the roughness measurement errors. Measurements of apparent roughness were made in three zones: at the beginning, middle, and end of the growth front. To improve the statistics, four layers were used in each zone and three measurements were performed for each layer. The length of interface measured for each layer varied between 80 and 100 nm. The roughness profiles extracted from images similar to Fig. 7 were first ‘‘flattened’’ by performing a straight line fit which is then subtracted. This procedure is necessary in order to minimize low frequency contributions which are due to misorientations of the measurement axis from the true horizontal axis. In cases for which the multilayers undulate, low frequencies will still be present even after this correction procedure. Atomic force microscopy measurements of the copper substrate roughness before electrodeposition yielded values between 0.2 and 0.4 nm over a surface area of 104 nm2 . For every case under consideration the apparent roughness of the first electrodeposited layers is comparable to these values, giving us a supplementary argument regarding the reproducibility of our substrates. For the NA electrolyte type, the average value of apparent roughness was 0.12⫾0.05 nm and did not change noticeably throughout electrodeposition. The same situation was observed for electrolyte-type NApH3 with an average value of apparent roughness of 0.18⫾0.08 nm. When saccharin was added to the bath, the apparent roughness was already higher from the first electrodeposited

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layers 共0.60⫾0.04 nm兲 and continued to rise as electrodeposition continued. As a result of the fact that layers soon begin to intermix, it is difficult to obtain accurate roughness values from the middle of the multilayer onwards. We estimate that at the end of electrodeposition, roughness was 5– 6 times higher than at the beginning. We have observed the creation of ‘‘bridges’’ between the layers, meaning areas where the continuity of the alternating layers is interrupted. For the SDS electrolyte the average apparent roughness was 0.28 ⫾0.08 nm, which is slightly higher than that for electrolytes NA and NApH3, and the value was stable through electrodeposition. For electrolyte SAC⫹SDS the apparent roughness was 0.11⫾0.02 nm at the beginning, 0.66⫾0.32 nm in the middle, and 2.01⫾0.18 nm at the end. Layers were clearly separated all the way along the electrodeposition front. Apparent roughness values were comparable at first to electrolytes NA and NApH3 and reached values near to those of electrolyte SDS at the end of electrodeposition. We believe that saccharin’s influence is weak at the beginning but increases with the advance of electrodeposition. As explained above, we were able to measure the real surface roughness, and thus to evaluate separately the roughness and interdiffusion contributions to the total interface widths. For this set of measures we have used layers that were examined beforehand with the Fresnel fringe technique and for which the total interface width is already known. The results are presented in Table III. In the two cases in which saccharin was present, the measured roughness value is greater than the total interface width. This is not physically possible, but in both cases the difference between the two values is within the limits of error. In such situations, error analysis can only define an upper limit for the interdiffusion—this is given in the table. The large uncertainties in the value for interdiffusion mean that our conclusions about its variation are less clear than those concerning the total width and the roughness. Nevertheless, we can establish certain trends. We can see that the smallest total interface width is achieved with the NApH3 electrolyte. Comparing with the NA electrolyte, this low value is attained through significantly diminishing interdiffusion, whereas roughness remains essentially the same. Hydrogen emission, which is greater for lower pH values, could be responsible for this behavior. With saccharin addition the total interface width drops too, but to a lesser extent. This time the roughness value rises but the interdiffusion value drops so as to make the overall width smaller. On images of these multilayers 共not shown兲 the augmentation of roughness is evident to the naked eye. For the SDS electrolyte, total width, roughness and interdifussion are, within the error values, similar to those of electrolyte NA. The presence of sds does not significantly affect these properties. When both saccharin and sds are present we obtain values that approach those corresponding to the SAC electrolyte. The influence of saccharin prevails, especially if we accept that, as for the pure SDS electrolyte, sds does not have a visible influence on the properties in question. Previous studies on the GMR properties of electrochemically grown metallic multilayers have shown that GMR is higher for low pH values22 and this effect was attributed to a

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reduction in the roughness. Our results have shown that roughness is stable between the two pH values that we have examined. The total interface width increases due to a greater interdiffusion contribution in the case of the lower pH. In other systems, evidence is presented for the enhancement of GMR with the increase in roughness.4,5 GMR also deteriorates with the addition of additives. We have observed that even though saccharin diminishes the total interface width by a radical diminution of interdiffusion, it increases roughness, especially at low frequencies. As far as sds is concerned, we did not observe a marked influence on the properties under examination. When saccharin and sds are used at the same time the influence of saccharin prevails. IV. CONCLUSIONS

We have investigated the influence of sds, saccharin, and solution pH on the interfacial properties of Cu/Co nanolayers. The total width of the layers was calculated by the Fresnel fringe method. Contributions of interdiffusion and roughness to total interface width were also calculated from the Fresnel data. In a global way, sds does not significantly influence these values. Saccharin diminishes the total width of interfaces as does increasing pH. When saccharin and sds are used together, the effect of saccharin is dominant. Roughness is enhanced by the presence of saccharin but does not change with pH variation. Interdiffusion, on the other hand, is reduced by both saccharin and higher solution pH values. ACKNOWLEDGMENTS

The National Scholarships Foundation is thanked for financing part of the doctoral thesis of Dr. S. M. and

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