Characterization of Young's modulus and residual stress gradient of MetalMUMPs electroplated nickel film

Metal multi-user MEMS processes (MetalMUMPs) offered by MEMSCAP provide a 20 μm thick electroplated nickel film suitable for constructing micro RF tunable capacitors, RF inductors, relays, switches, etc. Currently the Young's modulus and the residual stress gradient of the MetalMUMPs nickel film have not been characterized. In this paper the resonance method is used to characterize the Young's modulus of the MetalMUMPs nickel film. The characterization results show that the nickel film has a Young's modulus of 155–164 GPa with an average of 159 GPa. A stress gradient induced free beam mechanism is proposed in this paper to characterize the residual stress gradient in the MetalMUMPs nickel film. Characterization results show that the residual stress in the electroplated nickel film has a gradient across the film thickness of −5.49 MPa/μm to −4.30 MPa/μm with the average of −4.72 MPa/μm. The residual stress change from the bottom surface to the top surface of the nickel film is −97.7 MPa. The Young's modulus and residual stress gradient of the MetalMUMPs nickel film obtained in this paper provide MetalMUMPs users an important reference for designing, optimizing and analyzing suspended nickel structures. The stress gradient induced free beam mechanism proposed in this paper provides a method of characterizing negative residual stress gradient in thin films without using trenches or through-wafer holes.     

A directly strain measuring method for electroplated nickel micro-tensile test

In order to avoid the post-processing in micro-tensile tests, a directly strain measuring method is presented in this paper. The nickel tensile test specimen was electroplated in Watts solution under certain conditions. The measured Young’s modulus, ultimate tensile strength and elongation are 171.5 GPa, 2.18 GPa and 13.8%, respectively. By comparing these results to that obtained by the traditional method, this new approach has been proved reliable. Furthermore, the mean grain size of the nickel specimen measured by TEM images is about 160 nm with wide distribution. The XRD results show that nickel thin film in our experiment has no preferred orientation in the growth direction. The dimple rupture with small microvoids has been observed after the fracture of specimen.     

Effect of pulse frequency on the morphology and nanoindentation property of electroplated nickel films

This paper reports the effect of pulse current with different frequencies on the morphology and mechanical properties of nickel (Ni) films deposited by electroplating. The pulse frequency varies from 0 (DC) to 500 Hz while the duty cycle (T _on/T _off) is 1 during electroplating. The average roughness and deposition rate of Ni films decrease with the increasing frequency. The smoothest Ni surface with average roughness of 16.5 nm is achieved at the frequency of 500 Hz while the deposition rate reaches a nearly stable rate of 0.04 μm/min. The surface concentration of ions does not vary with time at a sufficiently high frequency with a long off time for ion to diffuse onto the surface. It will result in much more nuclei formed on the surface of cathode at a limited growth rate to get small grains for smooth morphology. The nanohardness of Ni film initially decreases rapidly from 3.9 to 1.18 GPa at 0 to 10 Hz, respectively, then increases to about 4.87 GPa in the range of 100–200 Hz, and decreases slowly to 4.0 GPa at 500 Hz. The stiffness of Ni films electroplated by pulse current at 100–200 Hz is higher than that by dc electroplating. The compromised Ni film with smooth morphology, good hardness and proper deposition rate is obtained at frequency of 100–200 Hz under the current density of 3 Adm⁻².     

Fabrication of a polymeric tapered HARMs array utilizing a low-cost nickel electroplated mold insert

A simple low-cost technique has been developed to fabricate a mold insert for replicating polymeric tapered high aspect ratio microstructures. A backside exposure technique is used to first obtain a tapered sidewall structure as an electroplating mold in SU-8 photoresist on a glass wafer. Nickel electroplating is utilized to form the mold insert. The lowest average surface roughness of the nickel mold insert on the side that interfaces with the glass wafer during electroplating is measured to be 7.02 nm. A novel technique involving use of titanium putty is introduced here to reduce cost and effort required to fabricate the mold insert. Replication of tapered microstructures in polymeric materials utilizing the fabricated mold insert is demonstrated here in polydimethylsiloxane by a direct molding process and in polymethyl methacrylate by hot embossing. The fabrication details for the mold insert are described. Advantages and disadvantages of the use of titanium putty for achieving superior metal surface finish are given.     

A low-temperature thin-film electroplated metal vacuum package

This paper presents a packaging technology that employs an electroplated nickel film to vacuum seal a MEMS structure at the wafer level. The package is fabricated in a low-temperature (<250/spl deg/C) 3-mask process by electroplating a 40-/spl mu/m-thick nickel film over an 8-/spl mu/m sacrificial photoresist that is removed prior to package sealing. A large fluidic access port enables an 800/spl times/800 /spl mu/m package to be released in less than three hours. MEMS device release is performed after the formation of the first level package. The maximum fabrication temperature of 250/spl deg/C represents the lowest temperature ever reported for thin film packages (previous low /spl sim/400/spl deg/C). Implementation of electrical feedthroughs in this process requires no planarization. Several mechanisms, based upon localized melting and Pb/Sn solder bumping, for sealing low fluidic resistance feedthroughs have been investigated. This package has been fabricated with an integrated Pirani gauge to further characterize the different sealing technologies. These gauges have been used to establish the hermeticity of the different sealing technologies and have measured a sealing pressure of /spl sim/1.5 torr. Short-term (/spl sim/several weeks) reliability data is also presented.     

Micromechanical characterization of electroplated permalloy films for MEMS

In order to improve the reliability of MEMS designs, evaluating the mechanical properties of soft magnetic materials is needed. In this paper, we present a tensile testing method to characterize the mechanical properties of microscale electroplated permalloy (80 wt% Ni, 20 wt% Fe) films. The gauge section of the specimen is 50 μm wide, 100 μm long and 5 μm thick. The measured Young’s modulus of permalloy films is 96.4 GPa, and the tensile strength is 1.61 GPa. The fracture strain measured by the images of specimens is about 2%.     

电镀镍铁合金中糖精对磁通门噪声的影响

由于糖精有利于减小铁磁薄膜的内应力,而这种内应力在镍铁合金的许多应用中是不利的,故在镍铁镀液中,糖精是常用的添加剂.在镀液其他成份不变的基础上,研究了仅当糖精浓度2.5~20 g/L变化时,内置电镀长条形铁芯磁通门噪声的变化情况.对以上长条形铁芯构成的磁通门,进行了噪声功率谱密度测试,结果表明:当糖精浓度从2.5 g/L增加到20 g/L时,1 Hz噪声功率谱密度从9.387槡nT/ Hz非线性地减小到了1.386槡nT/ Hz;同时也表明为了使磁通门的噪声最小时,最常用的糖精浓度(5 g/L)并不是最好的浓度,而应是更大的糖精浓度.     

Morphology and magnetic properties of electroplated Co-rich Co-Zn thin films

This work explores the microstructure and magnetic properties of electrodeposited Co-Zn thin films. Using pulse-reverse electroplating technique, Co-rich Co-Zn films are deposited 0.4–1.9 μm thick from aqueous sulfate-based baths at low temperature (55°C). The influence of current density (25–100 mA/cm²) and electrolyte Zn concentration (0–0.28 M) on the microstructure and magnetic properties are investigated. All of the Co-Zn films exhibit higher out-of-plane coercivity, as compared to in-plane. With increasing current density, the out-of-plane coercivity decreases from 50 to 40 kA/m (628–500 Oe). The influence of the Zn concentration in the electrolyte is more pronounced, affecting the grain size, film composition, and magnetic properties. The best magnetic properties were obtained from a bath with 0.21 M Zn and an average current density of 25 mA/cm², resulting in a Co₉₇Zn₃ composition and an out-of-plane coercivity of 92 kA/m (1,160 Oe).     

Mechanical and electrical properties of electroplated copper for MR-imaging coils

Mechanical and electrical properties of electroplated copper films were investigated to be used in an active tracking system for the patient-safe localisation of a catheter in intravascular interventions using magnetic resonance imaging. For this system spiral micro coils with high aspect ratio are fabricated on a planar polyimide foil to be wrapped around a catheter tip. Due to this special assembly method the coils and the copper, respectively, have to withstand high mechanical forces. The influence of the electroplating parameters as current density and shape, temperature and sample motion on the mechanical and electrical properties is investigated. These samples were characterised using nanoindentation, tensile tests, laser profiler and four-point prober. Results show that a low constant current density at a temperature of 40°C and sample motion generate flexible and high-Q copper coils.     

Uses of electroplated aluminum for the development ofmicrostructures and micromachining processes

In this paper, electroplated aluminum is explored as both a material for the fabrication of microstructures and use in the development of micromachining processes. A method for the fabrication of aluminum microstructures based on electrodeposition from organic solutions is presented. An extension of this process involving the use of plated aluminum structures as plating molds for subsequent electrodeposition of other materials is also discussed. Maximum structure aspect ratios of 21:1 have been demonstrated using this extended micromolding process. Finally, an aluminum-based process, in which the width of a metallic microstructure or the gap between metallic microstructures is achieved by controlling the plating time, is discussed. Using this process, vertical-gap aspect ratios between metallic microstructures of 25:1 have been demonstrated. Since the width of these features is controlled by the plating time and not by photolithography, gaps between metallic microstructures or widths of electroplated features ranging from submicron to tens of microns can be easily achieved using this process     

Characterization of RF sputtered thin film potassium sodium niobate (KNN) with silicon and nickel electrodes

A low cost recipe for thin film deposition of Potassium Sodium Niobate, (Na,K)NbO₃ (KNN) is pursued. The use of expensive noble metals as electrodes was avoided and instead highly doped silicon was used for both the structural layer and the bottom electrode. Nickel was used for the top electrode. In order to evaluate the outcome, the films were studied in terms of stoichiometry, crystal structure and leakage current density. RF sputtering of thin films of KNN at room temperature was successfully done. Proper crystal structure (Perovskite structure) was achieved after post deposition annealing. Though the leakage current density exhibited high dependency on the polarity of the applied voltage, a leakage current density of 1 × 10⁻⁶ A/cm² at 100 kV/cm was measured. A stoichiometry study revealed that the relative ratio of the volatile elements (Na and K) in the samples was within the acceptable range, however, a total loss of about 25–33 % was observed.

     

3D electroplated microstructures fabricated by a novel height control method

 A 3D electroplating process by means of pixel-wise, step height control of selective electroplating has been successfully demonstrated. A total of eight rectangular pixels of 80 × 20 μm² in area and 1.1–9.3 μm in height have been fabricated with a height difference of 1 μm in adjacent pixels. The process requires only two masks in a single electroplating process. Nickel electroplating is selected as the demonstrating material and is performed at 50 °C with current density of 400 A/m² for 30 min. A theoretical model in electroplating process is discussed to assist the design of 3D microstructures and verified with the experimental data. As such, this process has potential applications in making pixel-wise, 3D microstructures with precise height control. Received: 10 August 2001/Accepted: 24 September 2001     

RMS voltage sensor based on a variable parallel-plate capacitor made of electroplated copper

We present an advanced RMS voltage sensor based on a variable parallel-plate capacitor using the principle of electrostatic force. The device is fabricated in a micromechanical surface process with a high-aspect ratio actuator, reinforced by copper electroplating employing a sacrificial photo-resist layer. Another copper layer with a coplanar waveguide below the actuator provides separated excitation and sensing electrodes. Flip-chip technology is employed for low-loss electrical connectivity. The presented design has a plate area of up to 3 × 3 mm² and an initial gap distance of only 1.5 μm. We present results achieving a pull-in voltage below 1 V at frequencies from DC up to 1 GHz and sensitivities up to 1 fF/mV.     

镍和氧化镍膜修饰的过氧化氢传感器的研究

该文研究了一种新的过氧化氢电化学传感器的制备方法.在含镍离子的溶液中用电化学方法在玻碳电极(GCE)表面修饰一层镍和氧化镍膜(Ni/Ni2O3),在一定电位条件下,该传感器能催化氧化过氧化氢,其响应电流与过氧化氢的浓度在1.5×10-7～3.8×10-3 mol/L范围内呈线性关系.研究了各种实验条件对过氧化氢传感器性能的影响.该传感器制作简单,使用寿命长,在实际试样的回收率测定中,结果满意.     

Design and development of sub-micron scale specimens with electroplated structures for the microtensile testing of thin films

A novel designed microtensile specimen with electroplated structures is described here. It can be fit into a specially designed microtensile apparatus, which is capable of carrying out a series of tests on sub-micron scale freestanding thin films. Several thin films for microelectromechanical systems (MEMS) applications has been tested here including sputtered copper, gold, gold-chrome and tantalum nitride. All the metal specimens were fabricated by sputtering. For the tantalum nitride film samples, nitrogen gas was introduced into the chamber during the process of sputtering tantalum films on the silicon wafer. We have used copper, gold, 5% gold-chrome alloys and tantalum nitride thin films with thickness of 200–800 nm. The E values of the thin films tested here are consistent with the results from other measurement methods. The test results of metal specimens show the similar trend of the Hall-Petch prediction. However, the values of tantalum nitride thin films do not exhibit any systematic variation with respect to the thickness.     

Fabrication of a freestanding micro mechanical structure using electroplated thick metal with a HAR SU-8 mold

In this thesis, fabrication technology of a freestanding micro mechanical structure using electroplated thick metal with a high-aspect-ratio SU-8 mold was studied. A cost-effective fabrication process using electroplating with the SU-8 mold was developed without expensive equipment and materials such as deep reactive-ion etching (DRIE) or a silicon-on-insulator (SOI) wafer. The process factors and methods for the removal of SU-8 were studied as a key technique of the thick metal micro mechanical structure. A novel method that removes cross-linked SU-8 completely without leaving remnants of the resist or altering the electroplated microstructure was utilized. The experimental data pertaining to the relationship between the geometric features and the parameters of the removal process are summarized. Based on the established SU-8 removal process, an electroplated nickel comb structure with high-aspect-ratio SU-8 mold was fabricated in a cost-effective manner. In addition, a freestanding micro mechanical structure without a sacrificial layer was successfully realized. The in-plane free movements of the released freestanding structure are demonstrated by electromagnetic actuation. This research implies that various types of MEMS devices can be developed at a low-cost with design flexibility.     

Corrosion behavior of a structural nickel electrodeposit

The corrosion behavior of electrodeposited structural Ni (LIGA Ni) was assessed under aqueous and atmospheric conditions. Corrosion rates were measured and damage morphology was documented for comparison with wrought materials such as commercially available Ni 201 and high purity (99.999) Ni. As-fabricated LIGA Ni showed excellent corrosion resistance in aqueous NaCl solution even when galvanically coupled to a more noble material such as gold. Mechanically polished LIGA Ni showed corrosion resistance comparable to that of Ni 201, whereas polished 99.999 Ni had the best overall corrosion performance. Exposure to an aggressive atmospheric environment resulted in a higher density of corrosion sites on the LIGA material than on the control materials; however, the composition of the corrosion products (nickel-chloride) and the morphology of the attack was nominally the same on all tested samples. Overall, no new corrosion modes or susceptibilities were identified for LIGA Ni compared to wrought Ni materials. The superior corrosion resistance of the as-fabricated LIGA compared to mechanically polished LIGA, is discussed in terms of surface property modification during a chemical processing step.The authors would like to thank Sam Lucero for carrying out the atmospheric exposure testing and Steve Goods for assisting with sample procurement. Sandia is a multiprogram laboratory operated by Sandia Corporation, a Lockheed Martin Company, for the U.S. Dept. of Energy under Contract DE-AC04-94AL85000.     

Micromechanical switches fabricated using nickel surfacemicromachining

Micromechanical switches have been fabricated in electroplated nickel using a four-level surface micromachining process. The simplest devices are configured with three terminals, a source, a drain, and a gate and are 30 μm wide, 1 μm thick, and 65 μm long. A voltage applied between the gate and source closes the switch, connecting the source to the drain. Devices switch more than 10⁹ cycles before failure and exhibit long-lifetime hot switching currents up to 5 mA. The initial contact resistance is less than 50 mΩ. The breakdown (stand-off) voltage between the source and the drain is greater than 100 V and the off-current is less than 20 fA at 100 V     

Investigation on nickel/alumina-nanoparticles co-deposition

Ni/Al₂O₃-nanoparticles composite microcomponents with thickness of 300 μ are fabricated for micromechanical workpieces by using a combination of UV-lithography and micro electroforming. An effective and low cost process for electroplating the composite microcomponents using a spraying assistance is developed in this paper to disperse the alumina nanoparticles uniformly. The dispersing state is simulated presumably by using hydrokinetics finite element analysis. The simulated results show that the mass of tiny laminar flows and whirlpools caused by spraying riptides would be helpful to prevent the nanoparticles from reuniting and maintain the suspended state of nanoparticles around the cathode surface. The experimental results show that the alumina nanoparticles are uniformly dispersed in the microcomponents during the co-depositing process with the spraying assistance. The spraying-deposition process not only improves the dispersing condition of alumina nanoparticles but also promotes them to be adsorbed and deposited uniformly onto the cathode surface. The micromechanical properties of the composite components are also tested. The data shows that the mechanical properties of the components are significantly improved compared with the pure nickel components.     

Surface studies of the Medicus nickel matrix cathode

It is shown that activation of the Medicus nickel matrix cathode results in the development of a surface microstructure consisting of alkaline earth oxide-rich regions, 20–40 μm in diameter, distributed over the nickel-rich cathode coating. An in situ correlation between thermionic emission current and surface composition indicates a primary dependence upon the relative Ba to Ni concentration. A time dependent increase in the average calcium concentration at the surface has also been observed; compositional analyses of material evaporating from the cathode revealed that this calcium may accumulate at the surface due to the preferential release of barium and strontium. The accumulation of calcium at the surface, as well as the loss of barium, are believed to contribute to emission degradation during cathode life. Ion sputtering studies suggest the dispensing of barium and the dependence of the surface microstructure upon the initial thermal/ electrolytic activation. Although barium concentrations in excess of the oxide stoichiometry are measured, the presence of elemental barium monolayers has not been verified. It is concluded that owing to these chemical, compositional and morphological effects, the nickel matrix cathode should not be viewed simply as an oxide cathode physically supported by a metal matrix.