Shock testing of MEMS devices.

Micro-Electro-Mechanical Systems (MEMS) are being considered for use in a wide range of devices that are subjected to mechanical shock environments. MEMS devices are of interest because they are expected to survive and function following severe shock environments due to their small relative size and mass. Tests have been conducted on surface micromachined MEMS devices, using a Hopkinson pressure bar, to quantify shock survivability and the results will be presented. MEMS die, consisting of both simple structures and complex actuators, were fabricated in the Sandia SUMMiTTM and Cronos MUMPS processes and subjected to compression, tension, and shear shocks up to 200,000 g’s. During a field test, surface micromachined and bulk micromachined MEMS devices were subjected to the shock environment of a penetrator shot into a hard target. The unpackaged MEMS die were inspected before and after testing to quantify damage as a function of shock loading. Sensitivities to the direction and magnitude of the shock input, as well as associated failure mechanisms have been identified and will be presented. INTRODUCTION Micro-Electro-Mechanical Systems (MEMS) technology is producing devices that are expected to survive high-g shock environments. This work quantified shock levels at which MEMS structures and actuators fail, either by coming apart in a catastrophic manner or by failing to operate after a shock event. Field tests have exposed MEMS devices to the shock environment of a penetrator shot into a hard target. The field tests had shock levels of a few thousand g’s with durations of milliseconds. Laboratory tests at high-g shock levels up to two hundred thousand g’s with durations of tens of microseconds have been performed. The laboratory tests were focused on testing Surface micromachined (SMM) MEMS devices. These devices are produced by the Sandia SUMMiTTM and Cronos MUMPS processes which deposit alternating layers of polysilicon and sacrificial oxide from 1 to 3 microns thick. Average devices have lateral dimensions of 50 to 1000 microns. Because of the small size of SMM MEMS, inertial forces on these devices are small. The field tests were performed on both SMM MEMS and Deep Reactive Ion Etched (DRIE) MEMS. DRIE is done by etching through a silicon wafer to create features with thickness of 500 microns and lateral dimensions from tens of microns to a few millimeters. The inertial forces on DRIE MEMS are an order of magnitude higher than SMM MEMS. By using silicon on insulator wafers the DRIE devices can have different features etched on the frontside and backside, allowing for more complexity in design. The DRIE devices tested were produced by the Sandia Compound Semiconductor Research Laboratory. SURFACE MICROMACHINED TEST STRUCTURES Several modules were designed for MEMS shock testing. Die #1 was a single module, fabricated using the SUMMiTTM process, and populated with actuators, cantilever beams, and fixed-fixed beams. The beams were 20 microns wide, with the length varying from 100 to 1000 microns. The square anchor cut attaching the beam to the Poly0 layer varied from 4 to 18 microns. The module also contained standard component library designs for a microengine, torsional ratcheting actuators (TRA), and thermal actuators. Die #2 was designed and fabricated using the MEMSCAP PolyMUMPS (formerly Cronos MUMPS) process. This module contained cantilever beams, thermal actuators, resonators, and other standard components found in the MEMSCAP Consolidated Micromechanical Element Library (CaMEL). The beams were 20 microns wide, and the length varied from 100 to 1000 microns. The square anchor cut attaching the beam to the Poly0 layer varied from 2 to 8 microns. Die #1 and #2 were used for a series of laboratory tests which were previously summarized in [1] and these die were included in the field tests described herein. Die #3 was designed for further investigation of shock survivability. This module was a double module and fabricated using the SUMMiTTM process. A picture of Die #3 is shown in Figure 2. Cantilever beams with a width of 30 microns had length variations of 200, 400, 600, 800 and 1000 microns. The square anchor cut had side length variations of 4, 6, 8, 10, and 12 microns. The cantilever beams were fabricated with both the Poly1Poly2 and the Poly3 layer. The Poly1Poly2 beams have a 2.5 micron thickness and a 2.0 micron gap of between the beam and Poly0 base layer. The Poly3 layer beams have a 2.25 micron thickness and 6.8 micron gap. This set of 50 beams was placed on the module twice, with one set oriented 90 degrees from the other. The module also contained 2 microengines, 1 TRA, 2 spiral springs, 4 resonators, 2 bent beam thermal actuators, 2 differential leg thermal actuators, and a rotary table. The laboratory shock test results for Die #3 will be presented herein. DRIE TEST STRUCTURES Other MEMS devices produced for various projects were included in the field test. The g-sensor is a mass on a folded spring suspension and is used as an inertial sensor. The g-sensor is created by using Deep Reactive Ion Etching to pattern silicon on insulator wafers. The dual environment sensor is also a mass on folded spring suspension, but it is created by using advanced DRIE to create features on two levels. A thin clamp must be moved out of engagement by an inertial force before a perpendicular inertial force can move the mass. LAB TEST SETUP The die were shocked in three directions, and the conventions of compression, tension and shear have been assigned. Compression referred to the direction of shock in which a MEMS structure moved closer to the substrate due to its inertia and anchors were put into compression. Tension referred to the direction of shock in which the MEMS structures moved away from the substrate and anchors were put into tension. The direction of both tension and compression shocks were normal to the plane of the MEMS die. Shear referred to a shock direction parallel to the plane of the MEMS die. All shock tests were performed on a Hopkinson pressure bar. For the tests done in compression and tension, each die was bonded into a cavity of an aluminum fixture. The die was protected from dust and handling by covering the cavity with a machined cover. The fixture was held by vacuum to the end of a 3⁄4 in. aluminum Hopkinson pressure bar. For the compression tests the fixture cavity faced away from the bar, as shown in Figure 1. For tension tests the cavity faced towards the bar. Previous work by Bateman [2] determined there was no significant impedance mismatch between the aluminum and silicon substrate; so the pulse was not attenuated at the silicon and aluminum interface. The reference measurement for all tests was a strain gage bridge located in the center of the aluminum bar. The uncertainty was +6% for these strain gage measurements [3]. Pulse durations were measured at 10% of the positive peak acceleration magnitudes. For tests conducted at -65F or 165F the end of the Hopkinson bar was placed in an environmental chamber. Each die was visually and electrically inspected before and after shock testing. Actuators have been observed to fail in two ways, by a noticeable physical failure or by not functioning even though the actuator is intact. A beam was considered to have failed if any part of it was broken off. The method of visual inspection used before and after shock tests did not determine if the beams were free-standing or stuck down to the substrate. Figure 1. Shock fixture on end of Al Hopkinson bar Figure 2. Die #3 layout LAB TEST RESULTS In Figure 3 the results of the Lab shock testing for Die #3 are shown as the percentage of structures that survived. Each device or structure is represented by a column. The data are ordered in rows first by orientation as this is the most significant factor for survivability and secondly by shock level, and finally by the temperature at which the test was run. The percentage is calculated by dividing the number of working actuators or intact beams after the test by the number that were working or intact before the test. This accounts for devices and structures that were damaged during release, bonding or handling. The color coding divides the survivability into three categories. Red is shown when the survivability is between 0% and 33%, gray for 34% to 66%, and green for 67% to 100%. The columns start on the left with the most complicated actuator, with decreasing complexity toward the right. Then the beams are listed by increasing length. One would expect the complicated actuators and longest beams to fail at lower shock levels and the results confirm this is true. A microengine consists of 2 combdrives that are linked at 90 degree angles to a drive gear. When the microengine fails due to the connection to the drive gear, the combdrives may still be functional. Thus, both the survival percentage of the microengine as a system and the combdrives individually are shown. The beams of a given length are grouped together without regard to the variable size anchor cuts. Orientation Shock Level (1000 g's) Temp (deg F) Micro engine TRA Comb drive Resonator Thermal Actuator Beam length = 200 Beam length = 400 Beam length = 600 Beam length = 800 Beam length = 1000 165 100% 100% 100% 100% 100% 100% 100% 100% 100% 100% 72.5 50% 0% 63% 100% 100% 100% 100% 100% 100% 100% -65 50% 50% 50% 100% 100% 100% 100% 100% 100% 100% 165 75% 100% 100% 88% 100% 95% 98% 95% 83% 73% 72.5 50% 0% 50% 100% 100% 100% 100% 100% 100% 100% -65 25% 50% 75% 75% 25% 88% 90% 93% 75% 75% 165 100% 100% 100% 100% 100% 100% 98% 98% 100% 100% 72.5 25% 100% 50% 100% 100% 100% 98% 93% 90% 90% -65 25% 50% 75% 100% 100% 97% 98% 100% 100% 100% 165 75% 50% 75% 75% 100% 100% 90% 83% 98% 98% 72.5 0% 50% 75% 100% 100% 88% 95% 95% 89% 90% -65 0% 0% 0% 100% 100% 100% 98% 98% 100% 98% 165 50% 100% 50% 100% 100% 95% 80% 75% 68% 80% 72.5 25% 0% 50% 100% 100% 48% 93% 58% 65% 70% -65 0% 50% 50% 63% 100% 93%