ELECTRODEPOSITED HARD MAGNETIC THIN

Sixth International Symposium on Magnetic Materials, Processes and Devices, Proc. Electrochem. Soc. PV 2000-29 (2000).

ELECTRODEPOSITED HARD MAGNETIC THIN FILMS FOR MEMS APPLICATIONS N. V. Myung1, D. Y. Park1, M. Schwartz1 and K. Nobe1 H. Yang2, C.-K. Yang2, and J. W. Judy2 Department of Chemical Engineering1 Department of Electrical Engineering2 University of California Los Angeles, CA 90095 ABSTRACT Electrodeposition has many advantages over other deposition technologies in fabricating IC-compatible hard magnetic materials. Various electrodeposited alloys (CoNi, CoP, CoNiP, CoW, CoWP, CoMnP and CoPtP) were studied to evaluate their hard magnetic properties. Electrodeposition parameters influencing magnetic properties: 1) solution composition, 2) pH, 3) current density, 4) temperature and 5) agitation. Phosphorus-containing alloys, CoP, CoNiP, CoMnP and CoPtP, show promising hard magnetic properties. Coercivity in the parallel direction (//) decreased in the sequence of CoPtP > CoNiP ≈ CoP > CoMnP > CoW ≈ Co/Cu. In the perpendicular (⊥) direction, coercivity decreased in the sequence of CoPtP > CoNiP ≈ CoMnP > CoP> Co/Cu> CoW. Coercivities of 2620 Oe (//) and 2920 Oe (⊥) were obtained with 41Co58Pt1P deposits. INTRODUCTION The ever-increasing demands for faster, smaller and less expensive electronic systems, such as integrated circuits (ICs), microelectromechanical systems (MEMS) and computer drives, have resulted in the development of cost-effective processes. Hard and soft magnetic materials are used in MEMS including microactuators, sensors, micromotors, and frictionless microgears because electromagnetically-actuated MEMS are more stable for high force and large gap applications, are more robust in harsh environments (dust, humidity), and can be actuated with low cost voltage controllers [16]. Soft magnetic thin films such as 80Ni20Fe, 50Ni50Fe, and CoNiFe are used in recording heads to write onto hard disks [7]. Nanostructured giant magnetoresistive (GMR) spin valves are used in reading heads [8]. Hard magnetic materials such as CoCr ternary and quaternary alloys are used as data storage media [9]. As data storage density increases, reduction of magnetic areal moment density (MRt) and increased coercivity are required. In contrast to thin film hard magnetic recording media, the magnetic layers in MEMS can range from submicron to mm range thicknesses. The deposition processes must be compatible with other MEMS processing operations. The magnetic deposits must have good adhesion, low stress, corrosion resistance and thermal stability at operating temperatures without contaminating ICs. Although not addressed here, deposit stress can be a factor in MEMS devices with thick films, resulting in cracked or deformed deposits.


There are many different processes to deposit and integrate magnetic materials into MEMS. Electrochemical processes have many advantages, including room temperature operation, low energy requirements, fast deposition rates, fairly uniform deposition over complicated shapes, low cost, simple scale-up and easily maintained equipment. The properties of the deposit can be “tailored” by controlling solution compositions and deposition parameters. Figure 1 shows the interrelationships of the various parameters affecting the composition and structure of electrodeposits and their resulting properties. In this paper, we report studies evaluating these interrelationships. Currently available electrodeposited hard magnetic materials and deposition processes are compared. Processing steps and problems in the integration of electrodeposited magnetic materials in MEMS are discussed. EXPERIMENTAL Electroplating solutions consist of metal ions and supporting electrolyte which stabilize the plating solutions and improve solution conductivity; other chemicals are added to improve deposit quality or incorporate alloying elements into the deposit. For example, CoP, CoW, and CoMn can be electrodeposited by adding H3PO3 or NaH2PO2, NaWO4, and MnSO4 or MnCl2 to Co plating solutions, respectively. There have been numerous investigations on electrodeposition of hard magnetic films [10-30]. A survey of electrodeposited hard magnetic thin films with film compositions and their magnetic properties, including HC, MS, MR or S (MR/MS), is listed in Table 1. CoNi, CoP, CoNiP, CoMnP, CoW, CoPtP and Co/Cu multilayer alloys were electrodeposited from various plating solutions. Table 2 gives the plating solution compositions and the deposition conditions investigated. Solution pH was adjusted with either HCl, H2SO4, H3PO4, NaOH, or NH4OH. H3PO3 and NaH2PO2 were added as solution P sources for the electrodeposition of CoP alloys and their effects on deposit compositions and magnetic properties were studied. Brass and evaporated Ni-on-silicon were used as substrates; a soluble cobalt anode was used, except for the CoPtP plating solutions where platinized titanium was used as an insoluble anode. The deposit P content was estimated colorimetrically, using the molybdate-vanadate method [31]. The deposit (~ 200 mg) was dissolved in 25-50 mL HNO3 and the solution heated to remove NOx fumes. After dilution (~ 100 mL), 25 mL KMnO4 (0.02 M) was added to oxidize the Co component, followed by dropwise addition of 2 % NaNO2, to dissolve MnO2 with further dilution to 250 mL. A 10 mL aliquot was mixed with 25 mL of a molybdate-vanadate solution (20 g (NH4)2MO4 + 1 g NH4VO3/L) and diluted to 100 mL. After 5 minutes, a cuvette was filled and the P content determined colorimetrically (λ = 420 nm). The calibration curve followed Beer's Law. Deposit Co, Ni, Mn and Pt contents were determined by either atomic absorption spectrophotometry or EDX. Magnetic properties of electrodeposited films were measured with a vibrating sample magnetometer (Model 1660 ADE Tech.). RESULTS AND DISCUSSION Magnetic properties of materials involve intrinsic properties (magnetic saturation (MS), and Curie temperature (TC)) which depend on the composition, and structure-


sensitive properties, coercivity (HC), remanence (MR), and permeability (µ), which are affected by crystal structure, grain size, preferred orientation, stress and defects. The four important magnetic properties of hard magnetic materials are remanence, coercivity, Curie temperature and magnetic energy product (BH)max. Electrodeposited hard magnetic materials consist of heterogenous alloys. Generally, hard magnetic alloys are cobaltbased because hcp-structured cobalt has a high magnetocrystalline anisotropy; the theoretical value of HC is 5,000 Oe [10]. Co-based alloys with P, As, Sb, Bi, W, Cr, Mo, Pd, Pt, Cu, Mn, O and H have been electrodeposited. The alloying elements, except Pd and Pt, are reported to concentrate in grain boundaries to produce isolated magnetic particles surrounded by nonmagnetic or weakly magnetic boundaries [11]. Pt and Pd readily alloy with Co or CoNi and increase the magnetocrystalline anisotropy [32]. Luborsky electrodeposited Co and CoNi with P, As, Sb, Bi, W, Mo and Cr with the amount of the alloying element required for maximum deposit coercivity decreasing in the order: P > As > Sb > Bi and W > Mo > Cr [11]. Magnetic saturation (MS) decreased with increasing content of the non-magnetic alloying element. On the other hand, low deposit contents of phosphorus or tungsten have resulted in high coercivity with relatively high magnetic saturation (MS). A) Phosphorus alloys The deposit P content can be controlled by the solution source of P, current density and agitation, where the most important parameters are the solution concentration of the added element (as H3PO3 or NaH2PO2) and solution pH. For H3PO3-containing solutions, low pH (pH < 1) was employed and higher pH for NaH2PO2-containing solutions (weakly acidic or alkaline). In the alkaline baths, citrate was added as complexing agent to prevent precipitation of cobalt. Figure 2a and b show the dependence of deposit P content and coercivity on NaH2PO2 concentration in CoP (bath C) and CoNiP (bath D) plating solutions. Deposit P content sharply increased initially then reached a plateau with increasing NaH2PO2 concentration. In the ternary CoNiP deposits, the deposit P content was higher (6.5 %) than the binary CoP deposit (2.5 %) from solutions containing 0.1 M NaH2PO2. The differences diminished as the NaH2PO2 concentration increased. There have been numerous investigations relating magnetic properties with structure and deposit P content of electrodeposited or electroless deposited CoP; high coercivity was related to high deposit P content, smaller grain size and thinner deposits [18,22-25]. Miksic et al. showed that the coercivity of electroless CoP deposits increased with increasing P content [25]. Figures 3ab show the variability of coercivity of P-containing Co alloys as a result of solution deposition conditions, especially current density and pH, as well as the resulting deposit P content. Electrodeposited phosphorus-containing alloys from NaH2PO2-containing solutions show much higher hard magnetic properties than from H3PO3-containing solutions. Lower P contents in electrodeposited CoNiP were required to achieve higher coercivities. The applied current density greatly influenced the deposit composition, morphology, crystal structure and magnetic properties of deposits. Smoother films are generally electrodeposited at lower current densities. Figure 4 shows the dependence of the coercivity of electrodeposited CoP on temperature and current density (bath C).


Maximum coercivities were obtained between room temperature and 34 °C and between 5 and 10 mA cm-2, respectively. Myung et al. showed that the solution pH influenced the crystal structure of cobalt deposits, low pH promoting a metastable fcc phase and high pH promoting the hcp phase [29]. Mirzamaani et al. stated that solution pH is one of the important parameters to obtain hard magnetic materials because hard magnetic properties are associated with hcp structures [19]. Figure 5 shows the dependence of coercivity on pH. CoP and CoNiP alloys deposited from low pH solutions (pH< 2) exhibited soft magnetic properties, whereas deposits from higher pH solutions (pH > 2) show hard magnetic behavior. For the latter, the optimum solution pH was between 3 and 5. CoP deposits exhibit maximum coercivities at solution pH 3.5 (∼ 750 Oe) and pH 5.0 (∼ 1000 Oe) for // and ⊥ directions, respectively (Figure 5a). CoNiP deposits reached maximum coercivities ( HC,// and HC, ⊥) at solution pH 2.5 and remained constant in the pH range 2.5 – 6.0. Magnetic properties are also strongly dependent on film thickness. Miksic et al. observed that the coercivity of electroless CoP deposits increased with increasing film thickness when P < 2 wt. % and decreased with increasing film thickness when P > 2 wt. %. X-ray diffraction studies indicated that the c-axis is parallel to the substrate when P > 2 wt. %, and normal to the plane when P < 2 wt. % [25]. Similarly, it has been shown that crystal orientation of electrodeposited Co changed with increasing film thickness [29]. Figures 6ab show the dependence of coercivity on film thickness of electrodeposited CoP (bath C) at 2.5 and 5 mA cm-2, respectively. At 2.5 mA cm-2, coercivity increased with increasing film thickness, whereas coercivity in both parallel and perpendicular directions reached a maximum at a film thickness of 1 micron (equivalent to 20 coulombs) at 5 mA cm-2. Figure 7 shows hysteresis loops of electrodeposited CoP in the parallel direction for different deposit thicknesses; coercivity and squareness increasing with increasing deposit thickness. B. Tungsten Alloys CoW films are of interest because of their magnetic properties, high Curie temperature, good wear and corrosion resistances. CoW can be electrodeposited from both acid and alkaline baths using complexing agents (e.g. citrate or tartrate). CoW films have been electrodeposited from a citrate bath (bath F) at various current densities and temperatures to evaluate their effects on deposit content and magnetic properties. Deposit W content increased with increasing temperature and decreased with increasing current density (Figure 8a). At room temperature, both parallel and perpendicular coercivities decreased monotonically with increasing current density. At 70°C, a maximum coercivity was obtained at 20 mA cm-2 (Figure 8b). X-ray diffraction (Figure 9) indicated intermetallic Co3W compound (hcp) was electrodeposited at 10 mA cm-2 while fcc solid solution CoW was deposited at 100 mA cm-2. At 25 and 50 mA cm-2, mixed Co3W and CoW phases were deposited. The observed changes in magnetic properties are probably caused by crystal structure changes, where hcp Co3W phases have hard magnetic properties and fcc CoW phases have soft magnetic properties. Polukarov et al. also showed that deposit W content and solution operating temperatures greatly influenced the crystal structure and magnetic properties of deposits, with the crystal structure of hard magnetic CoW films dominated by hcp intermetallic Co3W [30]. Figure 10 shows the hysteresis loops of CoW electrodeposited at different current densities. Based on the effect of current density on the phases of the CoW deposits,


nanostructured hard/soft (Co3W/CoW) magnetic multilayers were electrodeposited by alternating the current densities between 10 and 100 mAcm-2 and by varying the hard magnetic component (Co3W). Figure 11 shows hysteresis loops of Co3W/CoW multilayers with the CoW soft magnetic layer thickness fixed at 26 nm. The magnetic properties of Co3W/CoW multilayers were strongly dependent on the layer thickness and volume fraction of the hard magnetic layers. Co3W/CoW multilayers shifted from soft magnetics (HC,// = 3 Oe) to hard magnetics (HC,// = 250 Oe) as the Co3W layer thickness increased. Electrodeposition of CoW alloys provides the flexibility of producing soft and hard magnetic films from the same solution by varying and controlling the applied current density. C. Platinum Alloys CoPt and FePt are promising hard magnetic thin film materials due to their high magnetocrystalline anisotropy and magnetic saturation [32]. Specifically, tetragonal L1O ordered phase materials (Co50Pt50 and Fe50Pt50) show very high coercivities (> 10,000 Oe) [33]. Most investigations of CoPt and FePt deposits were conducted using vacuum processes (MBE [34] and sputtering [33,35,36]), where CoPt and FePt were deposited in multilayered structures, then annealed to produce ordered phases. A major disadvantage of these deposition methods for magnetic MEMS is the requirement for post annealing temperatures (500 to 700°C); integrated circuits (IC) would not survive these temperatures. Farrow and Marks report that the annealing process can be reduced by electrolytically charging the transition metal alloy with hydrogen [36]. Nevertheless, this is still a problem. CoPt and Co/Pt multilayers can be electrodeposited from various plating solutions at near room temperature [16,20,26-28]. The best reported electrodeposited hard magnetic platinum alloys have been obtained by Cavalloti et al. [14]. The plating solution contained 0.1 M Co(NH2SO3)2, 0.01 M Pt P-salt {Pt(NH3)2(NO2)2}, 0.01 M (NH4)2C6H6O7, 0.1 M NH2CH2COOH, and (0.05 – 0.5 M NaH2PO2). The coercivity of the electrodeposited CoPtP was reduced from 4000 to 2000 Oe as the film thickness increased from 50 nm to 10 µm. We electrodeposited CoPtP films using a plating solution (bath G, table 2) containing chloroplatinic acid, cobalt pyrophosphate and sodium hypophosphite. Figure 12 shows the dependence of coercivity and squareness of electrodeposited CoPtP films on current density. Figure 13 shows the dependence of coercivity on film thickness deposited at 5 mA cm-2. Both parallel and perpendicular coercivities increased initially and then reached a plateau at approximately 0.1 µm, conducive for MEMS applications because various film thicknesses can be obtained without affecting coercivities. Figure 14 shows the hysteresis loop of a 1 µm thick 41Co58Pt1P deposit with the MR and HC is reasonably close in both // and ⊥ directions. D. Nanostructured Multilayers Electrodeposited nanostructured magnetic materials elicit considerable interest because such films can be “tailored”. Giant magnetoresistive (GMR) materials are an example where the change in electrical resistivity in the presence of magnetic fields is dependent on layer thicknesses [37-41]. In addition to the GMR effect, other magnetic properties can altered by varying layer thicknesses. For example, Figure 15 shows the


dependence of coercivity on the copper layer thickness of Co/Cu multilayer deposits, with the cobalt layer thicknesses fixed at 4.5 and 9 nm. Compared to electrodeposited cobalt films (HC of 30 to 150 Oe), the coercivities of nanostructured Co/Cu multilayers are greater. Further, the coercivity of the multilayers can be increased or decreased with increasing copper layer thickness depending on the thickness the cobalt layer. E. Integration of Electrodeposited Hard Magnetic Films on MEMS Devices To integrate electrodeposited magnetic materials successfully with ICs and MEMS, the electrodeposited materials must have a) low deposit stress to prevent film cracking or structure deformation, b) corrosion resistance to HF and other strong acids which are frequently employed in MEMS to release and etch micromechanical structures, and c) good adhesion between the seed layer and the electrodeposited layers. Deposit stress can be controlled with solution additives (e.g. saccharin) and operation at elevated temperatures. Adhesion between electrodeposited hard magnetic materials and nickel seed layers can been improved by activating nickel surface with 20% HCl and then employing a Co or Ni “strike” solution such as baths I or J (Table 2). Corrosion resistance can be improved by producing denser deposits. Based on a micromechanical compass with integrated electrodeposited hard magnetic films and differential capacitive position-sensing electrodes, shock-resistant low-power high-sensitive MEMS magnetometers are currently being developed at UCLA. This MEMS magnetometer detects the deflection of the microcompass capacitively, just as the motion of a mass is detected by inertial microsensors. Utilizing this concept, MEMS magnetometers with electrodeposited hard magnetic materials can detect covertly ferrous objects (small arms, jeeps, tanks, submarines, etc) at significant distances (>100 m) in adverse conditions (rain, fog, snow, etc.). Figure 16 shows the first generation UCLA MEMS magnetometer. CONCLUSION Various electrodeposited hard magnetic films have been studied to elucidate the relationship between electrodeposition conditions and resulting deposit magnetic properties. In addition, processing steps to integrate electrodeposited magnetic materials in MEMS devices are described. Electrodeposition parameters which affect deposit magnetic properties are solution composition, pH, current density and temperature. Magnetic properties can also be engineered by nanostructuring multilayered deposits. The phosphorus-containing alloys, CoP, CoNiP, CoMnP and CoPtP, show promising hard magnetic properties. In the parallel direction (//), maxima in coercivity decreased in the sequence, CoPtP > CoNiP ≈ CoP > CoMnP > CoW ≈ Co/Cu. In the perpendicular direction (⊥), maxima in coercivity decreased in the sequence, CoPtP > CoNiP ≈ CoMnP > CoP> Co/Cu> CoW. Coercivities of 2620 Oe (//) and 2920 Oe (⊥) were obtained with 41Co58Pt1P. These results indicate the desirability for additional studies on the electrodeposition of iron group (Fe, Co, Ni)-Pt group (Pt, Pd, Os, Ru, Ir, Rh) alloys to provide improved, high-performance hard magnetic films. Electrodeposition provides a low cost and low temperature method for fabricating thin hard magnetic films in MEMS and magnetic storage devices. ACKNOWLEDGEMENT


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Table 1. Hard magnetic properties of electrodeposited alloys Electrodeposited Alloys

Alloying Elements (wt. %)

Hc (Oe) [Thickness]

Ms (T)

S

Reference

CoNi CoNi CoP CoP

20-40 Ni 20-40 Ni 2.3 P 4 P

1.4-1.6 1.4-1.6 1.61 1.00

0.7-0.8 0.6-0.775

[10] This work [11] [17]

CoP (Electroless) CoP

5P 2-4 P

1.5-1.6

CoNiP

10-20 Ni, 3-4 P

300-500 100 (//), [2 µm] 700 [0.65 µm] 470 [0.4 µm], 320 [1.0 µm] 900 [0.04 µm] 1400 (//), 1300 (⊥ ⊥), [2 µm] 700-800

0.2-0.5 (//) 0.1-0.3 (⊥ ⊥) 0.5-0.6

CoNiP CoNiP (Electroless) CoNiP

CoMnP

18-37 Ni, 1-3 P 1 Mn, 5P < 3 P, < 1Mn

CoMnP

2-4 P,