Thin Films For Microelectromechanical Systems Research Articles

Investigating the influence of chromium layer thickness on thermal stability, adhesion and agglomeration of Cr/Ag/Cr sandwich layers in microelectromechanical systems (MEMS)

This study explores the influence of chromium layer thickness on the thermal stability and agglomeration of Cr/Ag/Cr sandwich layers used in MEMS applications. Achieving uniform and consistent deposition of thin films is crucial for optimal device performance. Magnetron sputtering, a technique offering precise control over film properties, is commonly employed for depositing thin films in MEMS. Silver is a popular choice due to its desirable properties, but it tends to agglomerate at high temperatures. The researchers investigated the effect of chromium layer thickness on thermal stability and agglomeration. They deposited chromium layers of varying thicknesses onto silicon substrates, followed by a silver layer and another chromium layer to create a sandwich structure. Annealing was performed at different temperatures to assess thermal stability and prevent silver agglomeration. Thermal stability was evaluated by measuring electrical resistance using a four-point probe method, and surface topography was analyzed using a non-contact atomic force microscope. The goal was to identify the optimal chromium layer thickness to minimize agglomeration and maximize thermal stability. The results showed that a sandwich structure with a 5 nm top chromium layer (Si/Cr (5 nm)/Ag (100 nm)/Cr (5-10-15-20 nm)) exhibited decreased adhesion force with increasing annealing temperatures. The use of a chromium sandwich layer significantly reduced surface roughness, as indicated by reductions in Ra and RMS values. A 15 nm thick chromium layer above and below the silver layer provided the best thermal stability and prevented silver agglomeration, resulting in the highest degree of adhesion. This thickness also yielded optimal surface parameters for the chromium sandwich layers at the annealing temperatures. In conclusion, the study demonstrates that the thickness of the chromium layer influences thermal stability, agglomeration, and surface parameters in MEMS applications and enables better control over thin film deposition.

Sputter Deposition and Characterization of Sm-Doped Pb(Mg1/3, Nb2/3)O₃-PbTiO₃ Epitaxial Thin Film on Si Toward Giant-Piezoelectric Thin Film for MEMS Actuator Application.

To meet the growing demand for better piezoelectric thin films for microelectromechanical systems (MEMSs), we have developed an SM-doped Pb(Mg1/3, Nb2/3)O3-PbTiO3 (Sm-PMN-PT) epitaxial thin film as a next-generation piezoelectric thin film to replace Pb(Zr, Ti)O3 (PZT). The inherent piezoelectricity | e31,f | achieved 20 C/m2, which is greater than those of intrinsic PZT thin films and the best Nb-doped PZT thin film. Besides, the simulation results show that the | e31,f | value of the single Sm-PMN-PT film could be around 26 C/m2. Meanwhile, the breakdown voltage of the as-deposited thin film was higher than 300 kV/cm. These results suggest the high potential of the Sm-PMN-PT epitaxial thin film for piezo-MEMS actuators with large displacement or force.

Characterization and Fatigue of the Converse Piezoelectric Effect in PZT Films for MEMS Applications

A measurement setup for the detailed study of the transverse piezoelectric coefficient e31,f in the converse (actuator) mode was developed. It allows the assessment of the piezoelectric stress in thin films on silicon cantilevers and provides for a correlation of this stress with large and small signal responses to ferroelectric polarization and dielectric response, both as a function of slowly sweeping electric field. This test is important for the understanding of piezoelectric thin films in microelectromechanical systems. The method is illustrated at hand of sol-gel lead-zirconate-titanate (PZT) thin films, and verified also with AlN and AlN-ScN alloy thin films. A 1-μm thick, sol-gel derived PZT(53/47) gradient-free sample showed a response of -18.3 C/m <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sup> at 100-kV/cm electric field. Reliability tests of PZT thin films were carried out with the same setup in an accelerated manner. The piezoelectric activity did not degrade significantly up to 10 <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">9</sup> unipolar pulses at 100 kHz with an amplitude of -150 kV/cm. The increase in leakage toward the end of the cycles was explained by a thermal runaway effect.

Vibration Damping Materials and Their Applications in Nano/Micro-Electro-Mechanical Systems: A Review.

The present review explores an overall view of the vibration damping materials ranging from traditionally used viscoelastic materials for macroscale damping to hybrid thin film heterostructures for micro-electro-mechanical systems (MEMS). Vibration damping materials like rubbers, polymers, metals, metal-matrix composites and smart materials are reviewed in terms of damping capacity, stiffness, mechanical strength and figure of merit. Nanoscale shape memory alloys, piezoelectric materials, carbon nanotubes, their composites and thin films are promising materials for future nanoscale damping devices. The main focus of this article is on our development of new vibration damping approach for MEMS structures comprising of ferroelastic/ferroelastic thin film heterostructures. For the first time, nanoindentation has been explored as an alternative tool to evaluate the damping capability of actual components (e.g., thin films for MEMS) where production of dynamic mechanical analyzer (DMA) test samples is not feasible. A comprehensive insight on the existing vibration damping materials and our new approach would definitely trigger some important applications in nano- and micro-electro-mechanical systems.

Fe-B-Nd-Nb metallic glass thin films for microelectromechanical systems

In the present study, we investigate the mechanical properties, residual stress, and microprocessing compatibility of Fe67.5B22.5Nd6.3Nb3.7 metallic glass thin films (Fe-MGTFs). The mechanical properties are measured using a specially designed microtensile tester. The fracture toughness of the Fe-MGTF (6.36 MPa × m1/2) is more than twice that of Si, and the highest among the thin films developed for microelectromechanical systems (MEMS) to this point. In addition, the fabrication of freestanding microcantilevers illustrates the low residual stress and high microprocessing compatibility of Fe-MGTFs. The present study verifies the great potential of Fe-MGTFs for use in MEMS.

Fabrication and Characterization of Double-Layer Pb(Zr,Ti)O3 Thin Films for Micro-Electromechanical Systems

In this paper, we address the fabrication and characterization of bimorph structures with relatively thick double-layered Pb(Zr,Ti)O3 (PZT) thin films. The PZT/PZT layers are deposited by RF magnetron sputtering. Hysteresis loops of polarization and electrical field for the top and bottom PZT thin films revealed good ferroelectric characteristics with remanent polarization at approximately 20 µC/cm2 and a coersive electric field of about 100 kV/cm. The vibration tests of fabricated bimorph cantilevers during electrical voltage application revealed a twofold displacement compared with single layer driving, and the piezoelectric coefficient value d31 is estimated to be 13 pm/V. The residual stress difference between the top and bottom layers after the annealing process is calculated to be -0.32 MPa. For a further thickening of the bimorph structure, 6-µm-thick PZT/PZT is also sputtered. The thicker bimorph has a smaller residual stress difference, -30 MPa, between the two layers prepared without the annealing process. The evaluated results demonstrate that the PZT/PZT bimorph structures are applicable to micro-electromechanical systems (MEMS) devices.

Piezoelectric Microgenerators-Current Status and Challenges

This manuscript reviews the developments made in design and fabrication of piezoelectric microgenerators and presents a method for making a comparative study within various vibration energy harvesting mechanisms. Current generation vibration energy harvesters have power density in the range of 0.8 microW/mm3. The manuscript also reports our results on synthesis of barium titanate (BT) thin films for MEMS (micro-electromechanical systems) based energy harvester. BT sol-gel was synthesized by aqueous process using barium acetate and titanium bis (ammonium lacto) dihydroxide with PVP (Polyvinylpyrrolidone). After optimizing the annealing temperature and time, textured BT films with 600 nm thickness were obtained on (111) Pt/Ti/SiO2 wafer. A MEMS fabrication process flow was designed to produce microcantilever chips from BT films constituting 6 cantilevers connected in series with an interdigital electrode pattern. We also present some concepts for further improvement of the power density of vibration energy harvesters by incorporating 3-D structure, magnetoelectric material, and a multimodal scheme.

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.

Piezoelectric properties of Bi 4Ti 3O 12 thin films annealed in different atmospheres

Bismuth titanate (Bi 4Ti 3O 12—BIT) films were evaluated for use as lead-free piezoelectric thin-films in micro-electromechanical systems. The films were grown by the polymeric precursor method on Pt/Ti/SiO 2/Si (1 0 0) (Pt) bottom electrodes at 700 °C for 2 h in static air and oxygen atmospheres. The domain structure was investigated by piezoresponse force microscopy (PFM). Annealing in static air leads to better ferroelectric properties, higher remanent polarization, lower drive voltages and higher piezoelectric coefficient. On the other hand, oxygen atmosphere favors the imprint phenomenon and reduces the piezoelectric coefficient dramatically. Impedance data, represented by means of Nyquist diagrams, show a dramatic increase in the resistivity for the films annealed in static air atmopshere.

Ferroelectric and piezoelectric properties of bismuth titanate thin films grown on different bottom electrodes by soft chemical solution and microwave annealing

Bismuth titanate (Bi 4Ti 3O 12, BIT) films were evaluated for use as lead-free piezoelectric thin films in micro-electromechanical systems. The films were grown by the polymeric precursor method on LaNiO 3/SiO 2/Si (1 0 0) (LNO), RuO 2/SiO 2/Si (1 0 0) (RuO 2) and Pt/Ti/SiO 2/Si (1 0 0) (Pt) bottom electrodes in a microwave furnace at 700 °C for 10 min. The domain structure was investigated by piezoresponse force microscopy (PFM). Although the converse piezoelectric coefficient, d 33, regardless of bottom electrode is around (∼40 pm/V), those over RuO 2 and LNO exhibit better ferroelectric properties, higher remanent polarization (15 and 10 μC/cm 2), lower drive voltages (2.6 and 1.3 V) and are fatigue-free. The experimental results demonstrated that the combination of the polymeric precursor method assisted with a microwave furnace is a promising technique to obtain films with good qualities for applications in ferroelectric and piezoelectric devices.

Crystallization and phase transformations in amorphous NiTi thin films for microelectromechanical systems

Amorphous sputtered nickel–titanium thin films were deposited onto micromachined silicon-nitride membranes and subjected to heating and cooling conditions. Their associated microstructure was monitored directly and simultaneously with in situ transmission electron microscopy. These electron-transparent membranes constrained the NiTi films and rendered it possible for observation of the complete transformation cycle, which includes: the crystallization of the amorphous phase to austenite phase (cubic B2 structure) with heating; and the conversion of austenite (B2) to martensite (monoclinic B19′ structure) with cooling. Electron micrographs show the nucleation and growth of grains occurs at a temperature of 470°C and at a rate that indicates a polymorphic transformation. The onset of martensitic transformation occurs between 25 and 35°C. Calorimetric measurements are consistent with the observed crystallization.

Shape-Memory Polymers for Microelectromechanical Systems

This paper investigates the use of shape-memory polymer thin films in microelectromechanical systems (MEMS). shape-memory polymers possess the capacity to recover large-strain deformations by the application of heat and are candidates for small-scale transduction. The key advantages of shape-memory polymers are their low material/fabrication cost coupled with their simplicity of integration/operation. In the present study, shape-memory polymers are spin coated onto a standard Si wafer and polymerized by thermal annealing. The thermomechanics of strain storage and recovery in the polymer films are studied using instrumented microindentation. The sharp microindents demonstrate full recovery at all load levels, establishing the feasibility of microscale actuation. The microindentation response of the polymer film is shown to depend on temperature and the cooling cycle during indentation. In turn, the subsequent recovery behavior of an indent depends on the thermal history during indentation. Indents performed at higher temperatures are larger in size, but have smaller stored strain energy compared to indents performed at low temperature. The larger stored strain energy in low temperature indents results in lower shape recovery temperatures. The effects of indentation temperature and load are systematically investigated to provide a framework for the use of shape-memory polymers in microsystems. Application of shape-memory polymers is demonstrated through the development of an active microfluidic reservoir. The reservoir was created by indentation at the end of a microfluidic channel and was activated by local heating. The collapse of the filled reservoir caused the motion of fluid down the microfluidic channel.

Ion irradiation effects on the mechanical stability of compressed metallic thin films

Residual stresses in materials and components such as thin films in microelectromechanical systems are developed during the production process and may drastically limit their functionality and lifetime; their minimization is thus of utmost technical importance. Ion beam irradiation in polycrystalline Mo thin films sputter-deposited on (100) silicon wafer at room temperature has been used to reduce the residual stresses present in the layer and thus to improve the mechanical stability of the film/substrate set. Although a strong reduction of residual stresses in the whole film is evidenced, irradiation affects the film-substrate bond strength; defect creation at the film-substrate interface due to ion implantation induces thin-film delamination.

Tailoring of the Elastic Properties by Texture Control in Ferroelectric Thin Films for MEMS

The use of piezoelectric thin films in advanced devices such as microelectromechanical systems (MEMS), not only rests on high piezoelectric coefficients, as the ones achieved in some ferroelectric lead titanate derived compositions, but also on their mechanical properties. This makes the study of the correlation between texture and elastic properties of high interest for the processing of piezoelectric thin films with tailored properties, regardless of which, not much related work has been reported up to now. In this paper we present the analysis of the variations observed in the elastic tensor of a series of modified lead titanate thin films with different orientations. The different textures were induced either by deposition on modified substrates or by changes of the annealing scheme. Orientation distribution functions (ODF) for all films were obtained by an advanced texture quantitative analysis of x-ray diffraction data of several pole figures. From the ODF and the single crystal tensor, we are able to calculate the effective elastic properties of the films by a geometric averaging procedure. This is a promising result in the tailoring of the mechanical properties of these piezoelectric thin films for applications.

MEMS 박막의 푸와송 비 측정을 위한 미소굽힘기법

MEMS(미소전기기계시스템) 박막의 푸와송비 측정을 위한 미소굽힙기법이 제안되었다. 푸와송비 측정에 민감한 쌍원시편(두 개의 원모양)을 설계하고 표면미세가공 공정을 사용하여 제작하였다. 미소압입기로 하중을 가한 쌍원시편의 하중-변위 곡선을 분석하여 푸와송비를 측정할 수 있었다. 제안도니 미소굽힘기법은 표면미세가공에 적합하여 소자제작과정에서의 동시측정이 가능하고(in-situ measurement), 소자가 위치해 있는 작은 영영에서의 물성을 국부적으로 측정할 수 있는 장점이 있다. 제안된 기법을 검증하기 위하여 저압화학기상증착법에 의하여 증착된 2.3㎛ 다결정실리콘(Poly-silicon)의 푸와송비를 측정하였다. 실험에 사용된 다결정실리콘막의 푸와송비는 0.2569 이고 쌍원시편의 강성에 대한 측정표준편차는 2.66% 이었다. Nano-bending method is presented to measure the Poisson's ratio of thinfilms for MEMS (Micro-Electro-Mechanical Systems) applicaiton. The douvle-ring specimen is designed and fabricated based on the surface micromachining process to facilitate the measurement of the Poisson's ratio. The Poisson's ratio can be obtained through analyzing the linear load-displacement relationship of the double ring specimen subjected to nano-indenter loading. The Present nano-bending mehod is an in-situ measurement approach due to the compatibility to the surface micromachining process. The Poisson's ratio is locally obtained at the location of the double ring specimen with micro dimension. To validate the nano-bending method, the Poisson's ratio of LPCVD (Low Pressure Chemical Vapor Deposition) poly-silicon with thickness of 2.3㎛ is investigated. Experimental results reveal that the Poisson's ratio of the poly-silicon film is 0.2569. The standard deviation of the nano-bending measurement for the stiffness of double ring specimens is 2.66%.

Stress in Sputtered Films of Near-Equiatomic TiNiX on (100) Si: Intrinsic and Extrinsic Stresses and Their Modification by Thermally Activated Mechanisms

The development of intrinsic and extrinsic stresses in TiNi and TiNiHf thin films, and their relaxation during isochronal and isothermal annealing, has been studied in both crystalline and amorphous materials sputtered onto (100) silicon. The development of intrinsic compressive stress was sensitive to both deposition pressure and temperature, and to film thickness, and was thermally stable below the deposition temperature. Kinetic parameters for isothermal stress relaxation and isothermal crystallization were determined. A two-stage process for both isochronal and isothermal stress relaxation was identified, with an initial stress change governed by a structural modification mechanism characterized by large activation volume and energy, giving way to a slower, diffusion-controlled relaxation mechanism. Tensile stresses developed during isothermal crystallization were found to relax at rates that depended on deviation of the film composition from stoichiomelry. The results are important with respect to the deployment of shape-memory thin films in microelectro-mechanical systems (MEMS), in which large extrinsic tensile stresses can be exploited to enable reversible high-energy mechanical actuation.

Mechanical Properties and Adhesion of PZT Thin Films for MEMS

AbstractPiezoelectric films are attractive materials for use in microelectromechanical systems (MEMS) due to their ability to act as both sensors and actuators. One of the primary modes of deformation is the deflection of lead zirconate titantate (PZT) beams and membranes, where the adhesion of the film is critical for the reliability of the device. Thin films of PZT between 250 and 750 nm have been grown via solution deposition routes onto platinized silicon substrates. The films have been tested using nanoindentation techniques. Two failure mechanism in these films have been observed Indentation induced delamination at the PZT-Pt interface occurs after the indenter tip is removed from the film when loads between 1 and 10 mN are applied to the sample, and at large loads (&gt;75 mN) failure can be generated between the underlying oxide film and the silicon substrate while the tip is still engaged with the sample. Since each of these failure modes has a different mechanics solution, the results are compared to determine adhesion energy of the films. Fracture around the delaminated regions has been examined using scanning probe and electron microscopy. Freestanding PZT membranes above micromachined cavities have been mechanically deformed to examine the mechanical response and failure modes in these structures. The adhesion of the PZT improves with increased percent crystallization due to the introduction of residual tensile stresses. Processing, mechanical properties, and failure modes in these devices will be discussed.

Patterned eutectic bonding with Al/Ge thin films for microelectromechanical systems

In this article, we report our results using the aluminum/germanium eutectic to create high quality patterned bonds between two silicon dice. The bonds are formed using thin metal layers and with essentially no pressure applied. We have measured bond strength by fabricating and bonding patterned dice. Pull tests were conducted and the force required to separate the bonds was measured and found to be about 1.6×107 Pa. When bonds break, portions of the substrate are removed. Testing of the hermiticity of the bond demonstrated that leak rates below the detection limit of the leak tester (10−9 sccs) are possible.