An improved micromechanical method for investigating the mechanical properties of poly-silicon membranes

Freestanding poly-silicon membranes are of increasing importance for designing MEMS devices such as pressure sensors, microphones and gyroscopes. It is crucial to accurately determine the mechanical properties of such membranes not only to access parameters for designing new devices but also for assuring proper performance and quality in service. Classically, microscopic tensile tests [1-3] or bulge tests [4] were conducted to obtain Young’s modulus and strength of the membrane material. These methods however are prone to artifacts due to crack initiation at edge defects (e.g. predefined notches in tensile specimens [3] or slits in bulge test samples [4]). In search of a method more sensitive to the membrane surface rather than specimen geometries, a novel approach has been introduced more recently. By loading the center region of a circumferentially clamped membrane with a spherical probe, the membrane is stretched all the way up to rupture while precisely recording the load-deflection data. Complementary FEA simulations allow for determining the failure stresses of individual membranes, based on the mechanical test data. In a subsequent step the tests are analyzed via a two-parameter Weibull approach to statistically evaluate the characteristic fracture strength. The membranes tested in the given project had a thickness of only 330 nm over a diameter of 1 mm. The necessity to apply minute forces while testing the compliant membranes at quite large deflections with high precision proves to be challenging. Additionally the need for statistical verification requires conducting multiple tests in a reasonable time frame. In the presented work a commercial nanoindenter has been used to match the aforementioned requirements. Lately some methodological improvements have been implemented to maximize throughput by automation and improve accuracy by refining the data analysis to capture the experimental conditions most realistically. Some of these approaches will be illustrated by recent data and explained in detail

Intrinsic stress measurement by FIB ion milling becomes an industrial-strength method

Intrinsic stresses in semiconductor and MEMS devices significantly affect functional behaviour and reliability. Trusted knowledge on stress amount and sign is a basic need developing new products. Electronics and MEMS devices often demand an extremely high spatial resolution of stress states. Only a few methods, like X-ray / electron diffraction [1, 2] and microRaman spectroscopy [3, 4] have been established as indirect stress measurement tools. Even finite element simulation reaches its limits to predict reliably mechanical stresses, if systems are rather complex and material laws are insufficiently known [5, 6]. Stress measurement by means of FIB based ion milling and subsequent quantification of stress relief pattern is a new approach, published first, 10 years ago [7]. In the meanwhile the method has been utilized and strengthened by several research labs [8-10]. Currently an extensive European program is realized to qualify this method for commercialization and to apply it under industrial conditions [11]. This contribution gives an overview on the measurement method, the current state-of-art on the method qualification, on measurement capabilities and limits. For example, typical research lab applications on thin layer stacks and 3D integration components like TSVs are demonstrated as well