Evaluation of quality of mechanical components of MEMS pressure sensors

Abstract

The pressure MEMS sensors are becoming popular in many various practical applications. They are e.g. applied to measure relatively high pressure in engine machines. The microfabrication process is not enough controlled to obtain high repeatability of manufactures structural mechanical components. This relates in particular to very stiff membranes used in high pressure sensors. This relates in particular to the membranes being sensing , important mechanical component of the sensor. There is a need for methods of the identification of the mechanical properties of the membranes as well such methods should enable to estimate the general quality of the structures .In particular for designer of such structures it is interesting to compare properties of modeled and real microcomponents after fabrication . The crucial characteristics describing the quality of the micro membranes of pressure sensors concern in particular stiffness of the structures i.e. force-displacement behavior of fabricated structures. The statistics of the results of the stiffness measurements enables to correct the fabrication process. Another important information related to the quality of the mechanical components of a sensor is the position of the membrane against the substrate i.e. during testing it is crucial identification of the distance between the membrane and the substrate as well as checking how many membranes are just laying on the substrate, what is evident failure of the product. The estimation of the stiction force to the substrate is also additional important task. Such structures are relatively stiff so the serious problem appears with the selection of the right method of studies and equipment which can be used. We elaborated the method of testing with the use of Atomic Force Microscope (AFM) which can be used not only to identify load-displacement (stiffness) characteristics but also to estimate quality of membranes and repeatability of the fabrication process. A set of fabricated membranes shown in Fig. 1 was studied. The method enabled to recognized the membranes which were touching the substrate . their distance from the substrate, stiction to the substrate and of course the load-displacement (stiffness) characteristics. Many different AFM cantilevers were tested to find right cantilever to perform the experiments. Finally it was selected very stiff steel cantilever with spring constant 426 N/m and diamond tip (with radius 125 nm) which usually serves to perform nano-wear tests on hard films. The force-distance curves were then found for each membranes. The moment of the contact of the tip with the membrane as well as the contact of the membrane with the substrate were precisely identified. The membranes laying on the substrate were easily found. A wide set of membranes with various diameter was tested and the results are presented in Figs. 2 and 3. The elaborated method is enough sensitive to identify most important characteristics of MEMS very stiff micro membranes used as sensing structures in MEMS pressure sensors and can be proposed to evaluate quality of the fabricated components as well repeatability of the manufacturing of membranes in the production process.