AVS1996 Session MM+TF-TuM: Material Issues in MEMS
Tuesday, October 15, 1996 8:20 AM in Room 204B
MM+TF-TuM-1 MEM SLIGA : Multiple X-ray Mask, Sacrificial Layer LIGA for Micromechanical Actuators
H. Guckel (University of Wisconsin, Madison)
Actuators are devices which transfer energy to their environment. Manufacturing processes; the tool; for this class of components must accommodate the 3-dimensional nature of these devices and the required tolerances and material diversity and, most importantly, must lead to cost effective actuators. It is the last requirement which leads to the consideration of photoresist based tools for actuator manufacturing. Since this type of tool leads to prismatic geometries it follows that high performance actuators; high output force per unit area; require large structural heights with good filling fractions. The tool which can accomplish this involves therefore a thick, larger than 500 micrometer, photoresist procedure in which 3-dimensionality is accommodated by multiple, sequential mask layers. The construction materials are electroplated metals which accommodate electrostatic, magnetic, pneumatic, and hydraulic actuators. The primary problem in thick, high resolution photoresist processes arises from high aspect ratio, mechanically weak photoresist shapes which have to survive surface tension and thermal expansion forces in harsh chemical environments. This issue dominates optical sensitivity and optical resolution behavior which are of primary concern in the microelectronics area. Application techniques such as spin coating which produce large built-in strain in the photoresist must be avoided and have been replaced by solvent bonded cell cast sheet procedures. Low strain photoresist layers of poly methyl methacrylate, PMMA, with thicknesses to 10 cm have been produced. Exposure with high energy, 20,000 eV, photons is very attractive because the absorption length in PMMA is 2 cm and large area masks without diaphragms can be fabricated. The processing tool is therefore LIGA-like. Electroplating rates are pattern dependent because of drift and diffusion phenomena. Re-planarization after electroplating must handle various metals in a PMMA matrix and bring the composite to a uniform, predetermined height. This has been accomplished with diamond lapping and polishing. There are several techniques for alignment between mask and wafer. At least one of these uses the low run-out of this process, less than 0.1 micrometer per 100 micrometer of structural height, in a mechanical alignment procedure for optically opaque masks. This procedure has been used to obtain 4 micrometer alignment tolerances in two mask systems. The processing tool is under constant test via actuator construction.
MM+TF-TuM-3 Materials Issues in Ferroelectric MEMS
D. Polla (University of Minnesota)
Ferroelectric thin films have been integrated into silicon-based microelectromechanical systems (MEMS). Both sensor and actuator applications are possible and several initial system concepts have been demonstrated with PZT-type materials and compatible on-chip signal processing electronics. This paper will present materials integration and processing issues important to ferroelectric MEMS. Several ferroelectric MEMS examples will be described including 1) uncooled infrared imaging array, 2) integrated acoustic sensor, 3) microfluidic control systems, 4) linear variable positioning systems, and 5) cantilever-beam microaccelerometers. Technology opportunities and limitations of micromachining with ferroelectric thin film materials will be described.
MM+TF-TuM-5 Materials and Processes for Micro-Hotplate Gas Sensors
R. Cavicchi, F. DiMeo, Jr., N. Tea, S. Semancik, J. Suehle (National Institute of Standards & Technology); J. Kelliher (University Research Foundation)
A new generation of gas sensors based on micro-machined hotplate structures on silicon is posing new challenges for materials. The generic device form consists of a suspended microbridge with a heater, heat distribution plate/thermometer, and electrical contact pads, all separated by insulating layers. The pads make contact to a sensing film grown on top. These devices test extremes of operating temperature up to 1000 \super o\C and rapid temperature changes with ramp rates up to 10\super 6\ \super o\C/s. Issues such as thermal conductivity, melting, glass softening, thermal stress, formation of intermetallics at metal/metal junctions, oxidation, ohmic contacts, and electromigration affect device performance and reliability. Materials must also be compatible with the processes required to produce the micro-machined structure. We compare results from devices that have aluminum metallization, fabricated using MOSIS (Metal Oxide Semiconductor Implementation Service) chips, with similar structures using tungsten, produced using conventional wafer processing facilities. The active areas of the devices are as small as 40 \mu\m x 40 \mu\m. Failure of aluminum (melting) limits the high temperature use of MOSIS chips to below 550\super o\C. The tungsten devices showed stable operation of the heater and thermometer plate at 850\super o\C. Surprisingly, the high temperature treatment induces a novel instability in the resistance of the polysilicon heater for operation in the lower temperature range 350\super o\C -700\super o\C. By contrast, the tungsten resistance is stable through the entire temperature range 20\super o\C -850\super o\C. Temperature programmed sensor response of ZnO and SnO\sub 2\ films will be presented, illustrating the benefits of an enlarged temperature range.
MM+TF-TuM-6 SiC Grown via Reaction of Fullerenes and Silicon for MEMS
A. Hamza, M. Balooch (Lawrence Livermore National Laboratory)
Free silicon carbide microcomponents are produced on silicon wafers. Silicon carbide is selectively grown by reaction of C60 with the substrate or sublimed silicon at surface temperatures between 900 and 1200 K. Components with thicknesses of up to 1.5 microns have been produced and released. The thickness at release can be controlled by the defect concentration of the substrate and/or the flux of sublimed silicon. The components are epitaxially grown. The mechanical properties of these components have also been determined by atomic force microscopy. The coefficient of sliding friction of the SiC components is one half to one third of the coefficient for the native oxide of silicon. In addition, the high hardness (greater than 25 GPa) and high elastic modulus (greater than 300 GPa) of the components makes these SiC microcomponents ideal for MEMS applications.
MM+TF-TuM-7 Detail Studies of PMMA Development Rate in LIGA Process
M. Tan, M. Bankert (Sandia National Laboratories); K. Jackson (Lawrence Berkeley National Laboratory)
One key process in LIGA is the development of PMMA after synchrotron exposure. GG developer has been widely used in this process. It is known that the developer reacts with air slowly, and changes from a clear solution to a yellow solution. In this talk, we present a detailed study of the development process of PMMA. The chemical composition of the developer was monitored with Gas Chromatography. The development rate vs. exposure relationships for the Good Fellow CQ and non-CQ grade, and PLEX-Glass G PMMA were obtained. The effect of air oxidation of the GG developer to the development curves was studied in detail. The development curves of the PMMAs under nitrogen atmosphere will be compared to the ones obtained in air. The temperature effect and other developer compositions will also be presented.
MM+TF-TuM-8 Dry Etching of Deep Si Trenches for Released Resonators in a Cl\sub 2\ Plasma
J. Weigold, S. Pang (University of Michigan)
High aspect ratio structures in Si can be made into mechanical elements for use in many applications such as sensing or actuation. These structures can be formed by etching deep trenches in Si, heavily doping the silicon structures with boron, and then releasing them in ethylenediamine pyrocatechol (EDP). This new technique allows released high aspect ratio structures that are impractical with other technologies to be fabricated with a deep etch and subsequent shallow boron diffusion. An electron cyclotron resonance (ECR) source has been used to generate a Cl\sub 2\ plasma and etch trenches greater than 100 micrometers deep in Si. High microwave power was used to achieve etch rates of 1.0 micrometers/min. Etching at high pressure increased the anisotropy of the etch, due to sidewall passivation. In addition, etch rates were faster at high pressures. Features with aspect ratios greater than 30 have been fabricated under these conditions. No cooling of the wafer is necessary to obtain a vertical profile, and no polymer deposition occurs in a Cl\sub 2\ plasma. A selectivity greater than 250 over a Ni mask was achieved. This allowed the use of an evaporated Ni mask for trench depths greater than 50 micrometers. It was also found that the addition of SF\sub 6\ increased the etch rate significantly while keeping the verticle profile and smooth surface. Etch rate was found to depend on the aspect ratio of the trenches being etched. Techniques to obtain uniform etching for trenches with different aspect ratios will be investigated. In-situ diagnostics, including mass spectrometry and optical emission spectroscopy, will be used to monitor the etch products and the etch species. Their effects on etch rate, profile, surface morphology, and microloading effect will be studied.
MM+TF-TuM-9 Effect of Stress Concentrations on the Mechanical Properties of MEMS Materials
W. Sharpe, Jr., R. Vaidyanathan, B. Yuan, G. Bao, R. Edwards (Johns Hopkins University)
Researchers at Johns Hopkins University have developed a new approach to determine the elastic and strength properties of very thin MEMS materials by imposition of a uniaxial stress field and measurement of strain directly on the specimen. This method has been used to study the effect of stress concentrations on thin MEMS materials. The material is phosphorus doped, LPCVD polysilicon deposited at the MCNC from the MUMPS Process. The specimen is 3.5 \mu\m thick and 0.6 mm wide at its narrowest point. Anisotropic etching from the back of the die leaves the tensile portion suspended between two grip ends that are connected by support strips. The specimen ends are glued in place in the two grips of a test machine, and support strips are cut away, freeing the tensile specimen. An air bearing supports the movable grip to avoid friction. A 1-pound load cell connects the movable grip to a computer-controlled piezoelectric actuator. Interference patterns are produced by a 10 milliwatt He-Ne laser shining on two gold lines on the central portion of the tensile specimen and the fringe motions are monitored to compute the strain on a real-time basis. The effect of stress concentrations in this material was studied by comparing the mechanical properties of specimens with a series of 5 \mu\m diameter holes spaced 30 \mu\m apart, to that of specimens without such holes. The average strength of specimens without holes was ~ 2.1 times the average strength of specimens with holes. This factor was ~ 3 according to Peterson's Stress Concentration Factor Tables. Finite Element Modeling (FEM) results as well as results of SEM examinations will be presented.
MM+TF-TuM-10 The Influence of Strengthening Mechanisms on Stress Relaxation in Thin Film Metallization
J. Gorrell, P. Holloway (University of Florida, Gainesville); H. Jerman (EG&G IC Sensors)
With the development of thermally activated microelectromechanical systems (MEMS) there is a need for stronger metal thin films that resist stress relaxation. A number of strengthening mechanisms are used extensively for bulk metal alloys, but very few have been used to improve the performance of thin films. For example, pure aluminum is soft and relaxes stress easily at room temperature. The standard alloy used in microelectronics (Al-2%Cu-1%Si) can be age hardened by precipitation of metastable Al-Cu phases, but under normal processing conditions the alloy overages and therefore exhibits low strength and high stress relaxation rates. In the present study, aluminum based alloys T201 (Al-4.6%Cu-0.6%Ag-0.4%Mn-0.3%Mg-0.3%Ti), 2090 (Al-2.6%Cu-2.1%Li-0.1%Zr), 5052 (Al-2.5%Mg-0.25%Cr), along with manganese nickel copper alloys, and copper gold intermetallics have been electron beam evaporated or sputter deposited onto (100) silicon substrates. Stress versus temperature and stress relaxation were measured using substrate curvature measured by laser reflectometry. Grain morphology and phase compositions were examined with TEM and EDX. These data demonstrate that strengthening mechanisms such as solid solution hardening and age hardening can be used in thin films to strengthen and reduce stress relaxation.