METHOD FOR ELECTRODEPOSITION OF METALLIC MULTILAYERS

Abstract: An electrodeposition method for the production of multilayer structures comprising substrates provided with layers of thin metal films of Co and Au by means of applying a current between two electrodes having a potential therebetween, immersed in a solution at least partly covering one or more substrate(s), said solution comprising Co ions and Au ions, said method comprising growing the multilayers on the substrate(s) in the solution, characterised in that the method is controlled by modulating either the current density and/or the applied potential and that the concentration of the Au ions is kept much lower than the concentration of the Co ions.

Description: Method for electrodeposition of metallic multilayers Technical field The present invention relates to a method for electrodeposition of metallic multilayers, a multilayer structure produced by the method and an electroplating bath for electrodeposition of thin films. Background Today, magnetic multilayers are important since the discovery of a phenomena called "giant magnetoresistance", GMR. The main interest lies in the ability to detect tiny magnetic fields, which can be exploited in a number of applications for example in magnetic data storage and magnetic sensors. However, the deposition techniques normally used are: sputtering and chemical vapour deposition (CVD), which are expensive, compared to electrodeposition. In general, two types of magnetic materials are used in the computer-memory industry. One type is a hard magnetic material, having a coercive force higher than 100 Oe, and the second type is a soft, or low coercivity material. These binary mag- netic materials are usually based on all transition metal magnetic multilayered system, in which the non- ferromagnetic layer is of 3d, 4d, 5d transition metals, or one of the noble metals such as Ag and Cu having low magnetic susceptibilities. Electrodeposited magnetic multilayers can be engineered as substrates provided with alternating layers of magnetic thin films such as Co, Fe, Ni etc, and nonmagnetic or noble metal layers having low magnetic susceptibility, such as Ag or Pt layers. Electrodeposited Au/Co multilayers have several advantages, properties, and great interest in thin film magnetic devices, magnetic sensor, high-density magnetic stor- age, and computer device technologies. Au/Co multilayer can also be used as temperature sensor with advantage of being free of oxidation at room temperature. An electrodeposition process is an electrochemical process that passes an electrical current between an anode and a cathode through an aqueous or non-aqueous solution containing metal ions. The ions are reduced and deposited on the cathode as a metallic coating. If the anode is made of the metal being deposited, it dissolves to replenish the metal ions in solution. If an insoluble anode is used, periodic additions of metal salts must be made to the solution to maintain the metal ion content. The quantity of metal deposited follows the principle of Faraday's law in that the mass of metal deposited is directly proportional to the current flow, the time and the relative atomic mass of the metal, and inversely proportional to the oxidation state of the metal in solution. The current used for plating does not always have to be applied as a continuous flow. For instance in pulse plating, the current is applied in short bursts of high intensity followed by a period in which no current is applied. The cycles represent the ratio of on time to off time, i. e. the duty cycle, and the frequency. By varying the duty cycle and the frequency, desirable alterations of the characteristics of the deposits can be obtained. The deposition process can be divided into two general techniques, single bath and dual-bath technique. In the single bath technique, the magnetic multilayers are grown in an electrolyte containing ions of both constituents. In dual bath technique, one electrolyte per metal is used. The plated parts are activated and transferred to the first solution, plated, then rinsed, activated again, and subsequently, transferred to the second solution. Thus, a drawback is extensive rinsing and activation. The single bath technique has the advantage that the substrate always remains under the electrolyte, limiting the risk of contamination. However, with this technique, the selected components must be far enough apart in reduction potential to allow a separate deposition of each of the components. The properties of the deposited metals are determined by factors such as electrolyte composition, pH, temperature, agitation, potential and current density. Thus, the electrodeposition process can for instance be controlled by modulating either the potential or the plating current density. Factors like chemical composition, thickness of the individual layers, sharpness of interfaces, intermixing/alloying across interfaces are of importance for the magnetic properties of the magnetic multilayer coatings. Since the electrolyte continuously changes the control of the growth rate of the layer thickness, it is essential to main- tain the desired thickness of spacer layers, which are non-magnetic, to obtain the maximum GMR effect. The standard spacer layers, such as the noble metals Ag and Cu having low magnetic susceptibilities have very small induced magnetic moments. The interface characteristics are also vital for achieving sharp interfaces when going from the magnetic to the non-magnetic component. Recently, a number of scientists have improved pulse potentiostatic deposition to a new technology called "nano material based synthesis", as template for production of single, as well as multilayered nanowires. However, the conditions for nanowire arrays are still a great challenge because of the uncertain electrochemical theory of the macro electrode. Furthermore, there are still a number of drawbacks, such as long deposition time, e. g. 24-48 hours, dependent on wire length, and diffusion problems, since diffusion of electrolytes is changing with time. Diffusion, migration or convection can effect the transport of ions from the bulk solution to the electrode surface. At the limiting current, the concentration at the surface is virtually zero. The structures obtained are either compositionally modulated alloys (CMA), or one- dimensional structures, having wire deposits of metals or alloys in the holes of an insulating matrix, a so-called template. Nanowire arrays can be engineered so that the direction of light magnetisation will be parallel to the wire axis and perpendicu- lar to the film. Electrodeposition of these low dimension structures has been prepared from a single electrolyte containing ions of magnetic and non-magnetic metals. Electrodeposited magnetic multilayer materials are used in the fabrication of com- puter memory elements. For instance electrodeposited Au-Co alloys is an important alloy because of its high wear resistance, which has a wide applicability in plating electrical contacts on printed circuit boards. Recently, electrodeposited Au-AuCo multilayers from a commercial hard gold plating bath having high gold concentration have been reported. However, there is still a need for producing Au-Co multilayered films in an economical way. Summary of the invention According to the invention, it has been found that an important parameter in electrodeposition for obtaining Au-Co multilayer structures is alternated current density; a layer having high Co concentration requires a high current density and a low agitation, and a layer having high Au concentration requires low current density. Moreover, according to the invention, the concentration of ions of the nobler, i. e. the more electropositive metal in the electrolyte should be lower than the concentration of the less noble metal. According to one embodiment of the invention this is obtained by electrodeposition of noble metal/metal multilayers, such as Au-Co multilayers using a single bath containing both noble metal/metal ions, such as Au and Co ions, in which bath the concentration of gold, i. e. the noble metal, in the electrolyte is much lower than the concentration of cobalt, i. e. the less noble metal. Thus, at a more electropositive potential only gold is deposited as a "pure layer" at low current density. Herein, "pure layer" is defined as an Au content in the Co layers, and vice versa, to preferably be around 3 wt % and 1 wt %, respectively. The maximum limit is 5 wt %. The most preferred Co content in the Au layer is at most 1 to 0,1 wt %. Preferably, the multilayer structures can be made either under potentiostatic or gal- vanostatic control. According to one preferred embodiment of the invention, there is provided an elec- trodeposition method for the production of multilayer structures comprising substrates provided with layers of thin films of Co and Au by means of applying a current between two electrodes having a potential therebetween, immersed in an electroplating bath at least partly covering one or more substrate(s), said bath comprising Co ions and Au ions, said method comprising growing the multilayers on the substrate(s) in the bath, wherein the method is controlled by modulating either the current density and/or the applied potential and that the concentration of the Au ions is kept much lower than the concentration of the Co ions. Herein, the term substrate refers to substrates such as plates, foils etc, but also in- eludes any other type of suitable surface, such as rods etc. The bath comprises 0,2 to 0,7 g of Co ions, 0,1 to 0,5 mg/1 of Au ions, an organic acid, the pH in the bath is 2,5 to 5,5 and the concentration of Co ions is about 1000 times higher than the concentration of the Au ions. The Au content is preferably 0,2-0,4 mg/1 and most preferably about 0,3 mg/1. The organic acid is preferably C6Hgθ6 in the amount of 10-200-g/l, but another suitable organic acid could also be used in equivalent amounts. Preferably, the potential is switched from open circuit condition to a potential adapted to deposit the Co ions, followed by another potential adapted to deposit the Au ions, but not the Co ions, followed by open circuit condition again, whereafter the switching cycle was repeated as many times as required. By using open circuits between the deposits, the interfaces between the films may be improved. According to another preferred embodiment of the invention, the switching cycle is repeated up to 10-5000 times, preferably up to 200 times. Suitable, the reduction potential of the components is far enough apart to allow a separate electrodeposition of each of the components. The Co layers are first deposited at a potential of -1,0 - -1,3 V, whereafter the potential is switched to a higher voltage, for instance -0,5 to -0,8 V, whereafter the potential is switched to open circuit. The new method according to the invention makes it possible to operate at a temperature of 20°C to 30°C. Another object of the invention is to provide a multilayer structure comprising substrates provided with layers of thin films of Co and Au, the Au content in the Co layers is at most 5 weight%, preferably 3 weight% and the Co content in the Au lay- ers is at most 5 weight%, preferably 1 weight%, wherein the multilayer is produced by an electrodeposition method by means of applying a current between two electrodes having a potential there between, immersed in an electroplating bath at least partly covering one or more substrate(s), said bath comprising Co ions and Au ions, said method comprising growing the multilayers on the substrate(s) in the bath, wherein the method is controlled by modulating either the current density and/or the applied potential and that the concentration of the Au ions is kept much lower than the concentration of the Co ions. A multilayer structure of Co films and Au films which are so pure as obtained according to the invention has not been able to produce by a single bath electrodeposition method before. This is a great advantage, since this method is less expensive compared with for example sputtering. It is also possible to produce a thick layer in a short time. It is even possible to obtain Au layers, wherein the Co content is at most about 1 ,0 to 0,1 wt %. The multilayer is produced according to the method described above. Further, an object with the invention is to provide an electroplating bath for electrodeposition of thin films of Co and Au, wherein the bath comprises 0.2 to 0.7 g/1 of Co ions, 0.1 to 0.5 mg/1 of Au ions, an organic acid, the pH in the bath is 2.5 to 5.5 and the concentration of Co ions is about 1000 times higher than the concentration of Au ions. The organic acid is preferably CβHgOό. The Au is preferably present in the bath as a KAu(CN)2 complex and the concentration of the complex is about 0,01 to 0,5 g/1. The purpose with the complex formation is to ensure the solubility of Au and reduce the potential difference between Co and Au to a level wherein Co is not dissolved in an uncontrolled manner when Au is electrodeposited. The Co is present in the bath as a salt of for example sulphates, sulfanmates, pyrophosphates, or chlorides dissolved in the bath. The Co is preferably added as C0SO4 x 7H2O and the concentration is about 4 to 100 g/1 of C0SO4 x 7H2O. The acid is used as a pH-buffert, while the pH is adjusted by hydroxide or carbonate and a preferred pH interval is 3,5 to 4,5. A preferred electroplating bath comprises 0,2 to 0,7 g/1 of Co ions, 0,1 to 0,5 mg/1 of Au ions, 10 to 200 g/1 of C6Hgθ6, around 120 g/1 of KOH and the pH in the bath is 3,5 to 4,5. There is no known electrolyte which is available to produce Au/Co multilayers as pure as according to the claimed method and with the claimed electrodeposition bath. In the electrolyte is a very low amount of the Au ions used, the KAu(CN)2, which is advantageous since this is a very expensive component of the electrolyte. The method can be used for producing magnetic sensors and recording devices for GMR applications. Furthermore, the method can be used for the production of single metal as well as multilayer nanowires in very short time. The method can also be used for the production of gas sensors and different kinds of biocompatible materials in nanoscale, such as electronic tongues and micro-muscles. Brief description of the drawings Fig. 1 illustrates a rotating cylinder set-up. Fig. 2 illustrates a rod provided with alternating layers of Au and Co. Detailed description of preferred embodiments In Fig. 1 is illustrated a rotating cylinder set-up 100 for electrodeposition. An anode 10 and a rotating cathode 20 are provided to conduct a current through a solution 30 containing metal and noble metal ions. A substrate 40 is immersed in the solution 30. The set-up will not be described in more detail, since devices for electrodeposition are well known. Instead, the invention will now be described in more detail by means of illustrating examples. The invention is not limited to the examples, which are only intended to explain the invention. For instance the concentration of the components in the electrolyte could be changed if the ratio between Co and Au concentration is not altered, without de- parting from the claimed invention. Also influence from pH effect on bath stability because of complex formation is considered to be obvious for a person skilled in the art, without departing from the scope of the invention. Agitation can be varied from no agitation to 1000 rpm (corresponding to surface speed of about 100 cms ~ l. Example 1 (Copper foils substrates) A rotating cylinder electrode set-up illustrated in Fig. 1 was used for electrodeposition. Copper foils (35 μm thick) were used as substrates. The foils were cleaned by ultrasonic rinsing in 4 % Neutracon solution. Prior to deposition, the foils were electropolished in 775 ml/1 phosphoric acid solution and 225 ml/1 propylene glycol at 350 mA/cm2 for 15 s. The foils were subsequently activated in 10 % H2SO4 for 30 s. The electrolyte used had the following composition: C6H 8 0 6 140 g/1 KAu(CN)2 0,08 g/1 KOH 120 g/1 pH 3,5-4 Some other agent than C0SO4X7H2O can also be used if suitable, if the concentra- tion of Co is not altered. Thus, the invention is not limited to this agent. Suitable agents can be for instance persulphates. The C0SO4 x 7 H2O content in Example 1 corresponds to 0,285 g/1 of Co ions. The KAu(CN)2 content in the Example corresponds to 0,3 mg/1 of Au ion which is preferably used. However, 0,01-0,5 g/1 of KAu(CN)2 could be used. Also some other addition agent than C H^O having the same "characteristics" could be used, for example oxalic acid. The current density can be varied in a range, preferably from 0,1 to 50 mAcm ~ 2. Deposits were prepared at room temperature. Electrodeposition of multilayers was performed using a computer-aided pulse plating (CAPP) system provided with a rectifier from Axel Akerman A S. The anode was made of inert platinised titanium mesh. The moφhology and composition of the deposits were studied by scanning electron microscopy (SEM) and energy dispersive X-ray spectroscopy (EDS) at 20 kV. X- ray diffraction (XRD) analysis was performed at high and low angles by means of a Philips wide-range diffractometer provided with a Cu tube. The multilayer structures were examined by cross-sectional transmission electron microscopy (TEM). Cross sectional sample preparation for TEM analysis was performed by gluing two pieces of about 0,5X1X2,5 mm of the Cu foils having the multilayer coatings face to face in a Ti grid, acting as a support for the sample. Prior to the ion-milling process, the slice is thinned to a thickness of approximately 50 um. Finally, an electron transparent sample was obtained using low angle Ar ion beam milling with a Bal-Tec instrument operated at 10 kV. In order to investigate the quality of the multilayers and the interface roughness between the Co and the Au layers, three multilayer depositions having different thickness were made. The different multilayers, 10, 100, 150 nm were characterised by X-ray diffraction. The 10 nm multilayer consisted of a 6,1 nm Au layer and a 4,9 nm Co layer, the 100 nm multilayer consisted of a 50 nm Au layer and a 50 nm Co layer and the 150 nm multilayer of consisted of 100 nm layer of Au and a 50 nm layer of Co. All the peaks accounted for either Au, Co or Cu peaks. The Au grain size is estimated to be 7,3 nm in the Co 50 nm/Au 100 nm multilayer, 5, 8nm in the Co 50 nm/Au 50 nm multilayer. Low angle XRD measurement showed no specific X-ray diffraction characteristics as would be expected for a chemical composition modulation. The surface moφhology of the multilayers showed a compact but rough surface with well-defined grain boundaries. Different cobalt sources are possible. However, the most preferred complexing agent is cobalt pyrophosphate. Example 2 (Production of nanowires) Au-Co multilayered deposition was performed from a single bath containing both Co and Au ions in the same way as described in Example 1. The more noble metal ions (Au) were kept at a low concentration. Thereby, the concentration of the more noble metal ions in the less noble metal layer can be minimised because reduction rate is limited by mass transport. As a result, multilayers comprising 96 wt % Co and 99 wt % Au were successfully obtained. High-speed potentiostatic deposition was performed for electrodeposition of nanowire multilayers on 20 μm thick membranes having a pore diameter between 150 and 200 nm, and a pore density of 10^-10^ pores/cm^. This is illustrated in Fig. 2 which shows a membrane (rod) provided with alternating layers of Au and Co. The electrodeposition time was two hours. The resulting nanowires comprised al- most 4 nm pure Co layer alternated with 2 nm pure Au layer, provided on the membrane (See Fig. 2). At open circuit, Au deposited because of autocatalytic reaction. Then the potential was switched to -1,2 V, whereby Co layers deposited for 8 ms, whereafter the po- tential was switched to -0,8 V, corresponding to the potential for pure Au deposition and finally to open circuit again. The cycling procedure was repeated up to 200 times. This rapid cycling is very important because during open circuit conditions and deposition at -0,800 V, the noble metal ions having low concentration is rapidly depleted at the cathode surface. Consequently, it is possible to solve the problems with passivation or dissolution of layers during the sweep from negative potential to the more positive. A very pure Au/Co multilayer is possible to obtain according to the invention, which is very useful since such a multilayer has inter alia good GMR effects. Ac- cording to the method this is achieved in a cheap way, and on a short time.

Claims: Claims 1. A multilayer structure comprising a substrate provided with layers of thin films of Co and Au, the Au content in the Co layers is at most 5 weight%, preferably 3 weight% and the Co content in the Au layers is at most 5 weight%, preferably 1 weight%, wherein the multilayer is produced by an electrodeposition method by means of applying a current between two electrodes having a potential there between, immersed in an electroplating bath at least partly covering one or more substrate(s), said bath comprising Co ions and Au ions, said method comprising growing the multilayers on the substrate(s) in the bath, wherein the method is controlled by modulating either the current density and or the applied potential and that the concentration of the Au ions is kept much lower than the concentration of the Co ions. 2. A multilayer according to claim 1, characterised in that the bath comprises 0.2 to 0.7 g/1 of Co ions, 0.1 to 0.5 mg/1 of Au ions, an organic acid, the pH in the bath is 2.5 to 5.5, and the concentration of Co ions is about 1000 times higher than the concentration of Au ions. 3. A multilayer according to claim 2, characterised in that the potential during a certain time, is switched from open circuit condition to a potential adapted to deposit the Co ions, followed by another potential adapted to deposit the Au ions, but not the Co ions, followed by open circuit condition again, whereafter the switching cycle was repeated as many times as required. 4. A multilayer according to claim 3, characterised in that the switching cycle is repeated up to 10-5000 times, preferably up to 200 times. 5. A multilayer according to any of the preceding claims, characterised in that the reduction potential of the components is far enough apart to allow a separate electrodeposition of each of the components. 6. A multilayer according to any one of the claims 1-5, characterised in that Co layers is first deposited at a potential of -1,0 - -1,3 V, whereafter the potential is switched to a higher voltage, for instance -0,5 - -0,8 V, whereafter the potential is switched to open circuit. 7. A multilayer according to any of claims 2-6, characterised in that the organic acid is CβHgOβ in the amount of 10-200 g/1. 8. A multilayer according to any of the preceding claims, characterised in that the Co content in the Au layer is at most 1-0,1 wt %. 9. A multilayer according to any of the preceding claims, characterised in that the multilayer structure is a nanowire. 10. An electrodeposition method for the production of multilayer structures comprising substrates provided with layers of thin films of Co and Au by means of applying a current between two electrodes having a potential there between, im- mersed in an electroplating bath at least partly covering one or more substrate(s), said bath comprising Co ions and Au ions, said method comprising growing the multilayers on the substrate(s) in the bath, characterised in that the method is controlled by modulating either the current density and/or the applied potential and that the concentration of the Au ions is kept much lower than the concentration of the Co ions. 11. An electrodeposition method according to claim 10, characterised in that the bath comprises 0.2 to 0.7 g/1 of Co ions, 0.1 to 0.5 mg/1 of Au ions, an organic acid, the pH in the bath is 2.5 to 5.5 and the concentration of Co ions is about 1000 times higher than the concentration of Au ions. 12. An electrodeposition method according to claim 11, characterised in that the organic acid is CgH O in the amount of 10-200 g/1. 13. An electrodeposition method according to claim 12, wherein the potential during a certain time, is switched from open circuit condition to a potential adapted to deposit the Co ions, followed by another potential adapted to deposit the Au ions, but not the Co ions, followed by open circuit condition again, whereafter the switching cycle was repeated as many times as required. 14. An electrodeposition method according to claim 13, characterised in that the switching cycle is repeated up to 10-5000 times, preferably up to 200 times. 15. An electrodeposition method according to any of claims 10-14, characterised in that the reduction potential of the components is far enough apart to allow a separate electrodeposition of each of the components. 16. An electrodeposition method according to any one of the claims 10-15, wherein Co layers is first deposited at a potential of -1,0 - -1,3 V, whereafter the potential is switched to a higher voltage, for instance -0,5 - -0,8 V, whereafter the potential is switched to open circuit. 17. An electrodeposition method according to any of claims 10-16, characterised in that the operating temperature is 20 to 30 °C. 18. An electroplating bath for electrodeposition of thin films of Co and Au, wherein the bath comprises 0.2 to 0.7 g/1 of Co ions, 0.1 to 0.5 mg/1 of Au ions an or- ganic acid, the pH in the bath is 2.5 to 5.5 and the concentration of Co ions is about 1000 times higher than the concentration of Au ions. 19. An electroplating bath according to claim 18, characterised in that the organic acid is C^HgOβ in the amount of 10-200 g/1. 20. An electroplating bath according to claim 19, characterised in that the Au is present in the bath as a KAu(CN)2 complex. 21. An electroplating bath according to claim 19, characterised in that the Co is present in the bath as a salt of for example sulphates, sulfamates, pyrophosphates or chlorides dissolved in the bath. 22. An electroplating bath according to claim 21, characterised in that the Co is present in the bath as C0SO4 x 7H2O. 23. An electroplating bath according to any of claims 18-22, characterised in that the pH is adjusted by hydroxid or carbonate. 24. An electroplating bath according to any of claim 18 to 23, characterised in that the pH is 3.5 to 4.5

Inventors: VALIZADEH SIMA, VALIZADEH, SIMA
Applicants: RES INST ACREO AB, VALIZADEH SIMA, RESEARCH INSTITUTE ACREO AB, VALIZADEH, SIMA
Application year: 2000
Main classification: C25D5
IPC codes: C22C5, C22C19, C25D3, C25D5, C25D7