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Probing MEMS Materials
Enter the world of Professors Kevin Hemker and William Sharpe, and you step into an incredible shrinking universe. Imagine a sensor so small that it could be placed on the end of a catheter to measure blood pressure intravenously. Inside the doors of a new car, tiny accelerometers stand ready to deploy the side airbags in a crash. These and other similar devices, known as MicroElectroMechanical Systems, or MEMS, consist of tiny mechanical systems, often bundled together with electro
nic proscessing circuitry, all on a silicon chip the size of your fingernail or smaller. Driven by the possibility of exciting commercial applications and encouraged by a manufacturing technology already in place thanks to the prevalence of microchips, scientists have unleashed a virtual flood of MEMS research in the past decade. As a result, nanotechnology is quickly moving from the realm of science fiction into everyday life. Mini-robots, mini-tweezers, mini-gyroscopes, and any number of millimeter-size devices may soon be manufactured at very low cost and employed in many aspects of our lives.
To imagine things this small, relative scales help—the trip from 10 m to the atomic scale (nanometers) spans the same orders of magnitude as going from 10 m to the solar system scale. A micrometer is about 100 times smaller than the width of a hair. Tiny pieces of a material at the microscale behave very differently than large hunks of the same stuff at the macroscale. Gravity, weight, and inertial forces are overshadowed by frictional forces, surface tension, and electrostatics. Understanding the mechanical properties of materials at this tiny scale and predicting the materials’ behavior are fundamental to improving MEMS technology. Professor Kevin Hemker explores how individual grains of a material will behave under various conditions, testing microsamples for strength and other mechanical properties, and watching defects form and spread in single crystals of a material.
Atoms in a material line up in a particular way, into “grains” that form a crystalline structure. By altering the processing parameters slightly, it is possible to “tune” the underlining structure to optimize the mechanical properties of the material, such as its strength. Strength is governed by how defects spread, both within a grain and from one grain to the next throughout the sample. The smaller the grains within a structure, the more boundaries there are to stop a dislocation from 
spreading, and thus the stronger the material. In a related project, Prof. Hemker and PhD student John Balk have employed Transmission Electron Microscopy, or TEM, to characterize the atomic structure of defects in a sample of single-crystal gold, about 10 nm thick (25-30 atoms high). They found that the defects always spread on a specific plane, about 6 atoms wide, and showed that defects in iridium have the same atomic structure as in gold. This similarity was predicted by colleagues at Northwestern University, and the TEM observations are now being used as benchmarks for more detailed calculations.
When he’s not peering at TEM images of atoms, Prof. Hemker is testing microsamples of materials from other laboratories. The samples, which look like flea-size dog bones, are destined for use in a variety of MEMS applications, and benchmarks for their mechanical properties are needed for proper design of MEMS devices. With special tools, he stretches and examines the samples, discovering when they behave elastically (i.e., whether they return to normal after stretching), deform, or break.
Tensile tests on materials used in MEMS technology are not commonplace; in fact the JHU facility is the only place in the United States that conducts tensile tests on MEMS materials. Professor William Sharpe laid the foundations for this strain testing measurement method back when he was working as a graduate student at Hopkins in the 1960s. He spent the last 35 years perfecting the technique, coming full circle in the process, back to Hopkins, where he has been a member of the ME faculty since 1983, and chaired the department from 1983 to 1988 and from 1991 to 1997.
Some of the materials sent by MEMS manufacturers have seen use in microelectronics, but their mechanical properties have never been considered. Polysilicon, for example, is a ceramic widely used in integrated circuits as an electrical material, but in MEMS technology it is often used structurally. Its strength and other mechanical properties therefore become the critical factors. Prof. Sharpe uses an Interferometric Strain Displacement gauge to assess the strength, modulus of elasticity, and Poisson’s ratio of the material. In this technique, each specimen, about a micron thick and 50 microns wide, is “marked” in the middle with two tiny gold lines. The tester then shines a laser beam across the sample, and the beam is diffracted by the lines, setting up an interference pattern. As the material is stretched, the lines move relative to one another, and this change is picked up in the interference pattern. This technique can detect changes as small as one or two nanometers. The way a material deforms under a given strain reveals its mechanical properties. Their strain measurements of polysilicon and silicone nitride, both widely used in MEMS technology, were an essential contribution to the acceptance of a standard value for the modulus of elasticity of these materials.