Francis Pressecq

The advent of MEMS in space

Abstract An overview of the MEMS activity at the French Space Agency (CNES) is presented. At present, MEMS are in the introduction phase and failure analysis and quality assurance (FA/QA) activities are underway for a large variety of MEMS technologies. This paper presents the CNES roadmap and methodology developed to assess the reliability of MEMS for space applications. The presented approach focuses, in particular, on the use of advanced analytical models, test vehicles, process control monitoring test structures and simulation tools to control and validate the quality of a technology. It also discusses the FA know-how and associated techniques acquired by CNES to observe, characterize, prepare and test in controlled environment MEMS processes. Finally, case studies illustrating the CNES MEMS FA/QA strategy are presented.

A new method for the hermeticity testing of wafer-level packaging

Until now, the determination of microelectronic packages hermeticity has been related to the MIL-STD-883 method 1014 which is based on the helium leak detection method. But this method is no longer suited for small packages due to the resolution limit of the apparatus. Indeed, leaks induced by nonhermetic MEMS packages are often one order of magnitude smaller than the resolution of the helium leak tester. Consequently, characterization of MEMS packages requires new methodologies to measure hermeticity accurately. Two methods will be investigated in the context of this study: the membrane deflection measurement, when exposed to different pressures, using optical profilometry, and the measurement of the variation of gas concentration in a sealed silicon cavity by Fourier transform infrared spectroscopy (FTIR). The calculated leak rates are compared for samples where the standard fine leak test gave no results. The values obtained for the leak rates with the optical test and FTIR test for the same sample are identical, showing the relevance of these two methods. FTIR spectroscopy is a promising method which enhances standard detection limits. It can be used as a reliable process quality control tool.

Environmental test bench for reliability studies: influence of the temperature on rf switches with metallic membranes

RF switches are believed to replace PIN diodes and MESFETs in numerous future RF applications. But most of the actual applications require high reliability and long lifetimes for their devices. As MEMS is a new technology, aging tests and qualification procedures have yet to be demonstrated. MEMSCAP and CNES are developing an environmental test bench for the study of RF switch failure modes. This paper focuses, in particular, on the influence of the temperature in metallic RF switches/ Actually, architectures, such as the metallic air bridge, the membrane switch and the dielectric switch, display good RF performances. We will show in this paper that most of todays switches are sensitive to buckling. In particular, a few tens of degrees Celsius are enough to create a deformation that drives the air bridge switch (in ON or OFF position) to fatal failure. The influence of tensile pre-stress is also studied since it increases the buckling critical temperature. However, the required pre-stress range will degrade significantly the actuation voltage. Finally, stress relaxation structures are believed to decrease the sensitivity to high temperature while keeping a reasonable actuation voltage.

Physical and reliability issues in MEMS microrelays with gold contacts

This paper presents the work we have done on micro-relays with gold micro-contacts in MUMPs. Firstly, the theoretical physical principles of MEMS micro-relay are described. This study is divided in two parts: the micro-contact and the micro-actuator. The micro-contact part deals with resistance of constriction, contact area, adhesion, arcing and wear. Whereas the micro-actuator part describes general principles, contact force, restoring force and actuator reliability. Then, in a second part, an innovative electrostatic relay design in MUMPs is presented. The concept, the implementation and the final realization are discussed. Then, in the third part, characterization results are reported. This part particularly focuses on the micro-contact study. Conduction mode, contact area, mechanical and thermal deformation, and adhesion energies are presented.

Reliability of microsystems based on a failure mechanism approach

Compactness, complexity of the interconnections and specific packaging, which are characteristics of Microsystems (MEMS), rule out the use of statistical procedure to assess reliability in space applications. Predictable reliability is the method recommended in this paper that uses a similar approach as CALCE already did for hybrid and microelectronic circuits. This method based on a failure mechanism approach is recalled at first and an example to illustrate this procedure based on the evolution of material crystal properties under radiation is presented.

MEMS physical analysis in order to complete experimental results return

The emerging MicroElectroMechanical Systems (MEMS) technologies are entering in an active phase of high volume production and successful commercial applications. The expertise and the qualification for space application of such devices have already begun. But these technologies are still recent and important efforts on the reliability issue have to be done. This paper defines the role of technological analysis in the actual MEMS design process. Afterwards, it presents MEMS technological analysis techniques developed at CNES applied to an open MEMS technology. In particular, it is shown how these technological analyses respond to designer needs and that the designer and the founder still need a strong interaction. We also present the MEMS reliability issue at CNES and replace it in the current worlds one.

Thermal and electrostatic reliability characterization in RF MEMS switches

The reliability of RF MEMS switches is closely linked to their operational and environmental conditions. This paper examines the reliability of five different capacitive switch designs by a combined use of modeling and experimental tools. Three-dimensional multiphysics finite element analysis was performed to estimate the actuation voltage and deflection vs. temperature variations of the micro-switches. The effect of temperature and temperature cycles on switch dilatation and pull-in voltage are studied, as well as the influence of different operational signals on switch reliability.

Mechanical qualification of RF-filters for space applications

Micromachined RF-filters for Millimeter wave communications are expected to replace the actual bulky off-chip filters since they offer high performances at low cost. Nevertheless, their large aspect ratio between area (several mm/sup 2/) and their thickness (a few microns) may make them sensitive to mechanical stress such as vibrations or shocks. In this paper, we focus on their mechanical characteristics. The studied filters are built on a thin SiO/sub 2//Si/sub x/N/sub y/ dielectric membrane and the signal lines are in gold. This composition of material makes the analytical approach rapidly complex. So, in counterparts, we ran FEM simulations to study the influence of process parameters such as stress on flexion modes. Then, we measured by vibrometry the natural resonant frequency of several filter sizes (from 1 mm/sup 2/ up to 70 mm/sup 2/) and compared results with previous simulations. Finally, we applied at die level, standard procedures of shock and vibration used for space qualification. All the membranes withstood the tests without structural damage.

A new method for hermeticity testing of wafer-level packaging

Until now, the determination of the hermeticity of microelectronic packages is related to the MIL-STD-883 method 1014 which is based on the He leak detection method. But this method is no more suited for small packages due to the resolution limit of the apparatus used conventionally. Indeed the minimum detectable leak rate is of the order of 5.10-11 atm.cm3.s-1. Leaks induced by non hermetic MEMS packages are often one order of magnitude smaller. So, the sensitivity of the He leak detector method is too low and this method can not be applied anymore. The MEMS packages produced with wafer level encapsulation techniques, require new methodologies to measure hermeticity appropriately and accurately. The purpose of this paper is to present the development of alternative methods for testing the hermeticity of MEMS micro-cavities. Two methods will be investigated in the context of this study: The membrane deflection measurement exposed to different pressures, using optical profilometry, and the measurement of the variation of gas concentration in a sealed silicon cavity by Fourier-transform infrared spectroscopy (FT-IR). The calculated leak rates are compared for samples where standard fine leak test gave no results. The values obtained for the leak rates within optical test and FT-IR test for the same sample are identical, showing the relevance of these two methods.

Reliability issues of an accelerometer under environmental stresses

COTS (Commercial-off-the-shelf) MEMS components are very interesting for space applications because they are lightweight, small, economic in energy, cheap and available in short delays. The reliability of MEMS COTS that are used out of their intended domain of operation (such as a space application) might be assured by a reliability methodology derived from the Physics of Failure approach. In order to use this approach it is necessary to create models of MEMS components that take into consideration environmental stresses and thus can be used for lifetime prediction. Unfortunately, today MEMS failure mechanisms are not well understood today and therefore a preliminary work is necessary to determine influent factors and physical phenomena. The model development is based on a good knowledge of the process parameters (Young’s modulus, stress...), environmental tests and appropriated modeling approaches, such a finite element analysis (FEA) and behavioural modeling. In order to do the environmental tests and to analyse MEMS behaviour, we have developed the Environmental MEMS Analyzer EMA 3D. The described methodology has been applied to a Commercial-off-the-shelf (COTS) accelerometer, the ADXL150. A first level behavioral model was created and then refined in the following steps by the enrichment with experimental results and finite element simulations.