Tino Fuchs

Mechanical Characterization of Polycrystalline and Amorphous Silicon Carbide Thin Films Using Bulge Test

This paper aims at determining the mechanical parameters such as Youngs modulus, Poissons ratio, and intrinsic stress of polycrystalline and amorphous silicon carbide thin films using the bulge test. A suitable method for the bulge test of highly compressive amorphous SiC films was devised by developing tensile composite layers involving a highly tensile substrate layer. A setup suitable for use in high temperatures was developed to hold membrane chips mechanically for applying pressure. The center deflection was measured with optical interferometry. The bulge test was done on square and rectangular membranes in order to ascertain the Poissons ratio and the exact Youngs modulus of the material. The tests in this paper were conducted at room temperature. The tensile polycrystalline SiC films were obtained by low-pressure chemical vapor deposition (LPCVD) at 825°C, using monomethylsilane as the precursor. The amorphous SiC (a-SiC) films were deposited by plasma enhanced chemical vapor deposition (PECVD) process at a temperature of 870°C with silane and acetylene as precursors. Test chips were pressurized from outside the membrane cavity, and membrane deflection was measured for each pressure step. The poly-SiC layers indicated a Youngs modulus of 280 GPa with a Poissons ratio of 0.237. The Youngs modulus of a-SiC films was found to be 232 GPa, but the Poissons ratio could not be determined as we used a composite layer. A comparison with the existing literature values of Youngs modulus was also done.

Mechanical characterization between room temperature and 1000 °C of SiC free-standing thin films by a novel high-temperature micro-tensile setup

A novel high-temperature micro-tensile setup allows the characterization of the elastic and plastic as well as creep behavior of free-standing thin films at temperatures of up to 1000 °C. Correspondingly, a new layout for free-standing thin film tensile test structures has been developed, enabling accurate self-alignment upon loading. Furthermore, a differential optical strain measurement technique as well as optimizations of the optical path has been implemented, providing a strain resolution of well below 1 × 10(-4) at 1000 °C. Two different polycrystalline SiC free-standing thin films have been investigated in tension to acquire stress-strain data and corresponding Youngs modulus at up to 1000 °C. The high sensitivity of the strain measurement technique makes it also possible to identify creep strains in the high-temperature regime.

A 10 μm thick poly-SiGe gyroscope processed above 0.35 μm CMOS

This paper describes a monolithically integrated omegaz-gyroscope fabricated in a surface-micromaching technology. As functional structure, a 10 mum thick Silicon-Germanium layer is processed above a standard high voltage 0.35 mum CMOS-ASIC. Drive and Sense of the in plane double wing gyroscope is fully capacitively. Measurement of movement is also done fully capacitively in continuous-time baseband sensing. For characterization, the gyroscope chip is mounted on a breadboard with auxiliary circuits. A noise floor of 0.01 degs/sqrt(Hz) for operation at 3 mBar is achieved.

A new highly selective sacrificial layer technology for SiC MEMS

A new sacrificial layer technology for the fabrication of silicon carbide (SiC) based micro electro mechanical systems (MEMS) is described using silicon germanium (SiGe) as sacrificial material. An extremely high etch selectivity between SiC and SiGe of up to 1∶1000000 was achieved when using chlorine trifluoride (ClF3) as etchant in a dry plasmaless etch process. This enormous selectivity enables the fabrication of free standing SiC microstructures like combs or membranes with a strongly reduced need for perforation patterns, which are typical for silicon dioxide sacrificial layers etched selectively to silicon by hydrofluoric acid (HF) in liquid or vapour phase [4]. Only few and small openings are necessary to fully release large SiC structures with the sacrificial layer technology presented, therefore allowing higher degrees of design freedom for MEMS compared to traditional release technologies.

Reliability of platinum electrodes and heating elements on SiO 2 insulation layers and membranes

In this work, failure mechanisms of Pt electrodes including adhesion problems, material migration due to thermally induced compressive stress and electromigration that could occur in the platinum electrodes and heater structures at temperatures above 600 °C have been systematically studied, after the deposition. Lifetime determination, scanning electron microscopy and XRD analysis have been applied for samples which have experienced different loading conditions in order to qualitatively and quantitatively understand the phenomena. Electromigration testing is performed with the aim to enable time-to-failure prediction for sensor elements and compare different platinum layers in terms of their stability. Dedicated, application-related test structures are used so that the results are applicable to sensor lifetime estimations. Furthermore, a method for the determination of thermal conductivity of thin insulating films has been adapted for the characterization of plasma-enhanced chemical vapor deposition (PECVD) silicon oxide and successfully applied on two materials with different deposition recipes. These two materials are used for the fabrication of platinum-based heating elements with PECVD SiO2 as insulation or membrane layer. The results for the two recipes are similar but with a significant difference. A slight increase of the conductivities has been observed due to a thermal anneal of the test structures at temperatures above 700 °C.

Reliability characterization of a soot particle sensor: Analysis of stress- and electromigration in thin-film platinum

In this work we present a systematic investigation of failure mechanisms for thin-film platinum heater structures and interdigitated electrodes as components of a resistive type soot particle sensor. We study stress-migration and electromigration and effects of their interaction. Lifetime determination and SEM imaging are applied for samples which have experienced different load conditions to quantitatively and qualitatively understand the phenomena. We use dedicated, application-related test structures to ensure that the results are transferable to sensor lifetime estimations.

Piezoresistivity and Electrical Conductivity of SiC Thin Films Deposited by High Temperature PECVD

We introduce a novel high temperature PECVD process and use it for the deposition of silicon carbide thin films on oxidized silicon wafers at 900°C substrate temperature. A variation of the atomic composition over a wide range is achieved by altering the flow ratio of the precursors silane (SiH4) and acetylene (C2H2). XPS analysis is performed to verify the silicon to carbon ratio in the deposited layers. The resistivity of the obtained thin films shows a strong dependence on the Si/C-ratio. Four point measurements show the resistivity ranging between 5•10-3Ωcm for C-rich layers and >107Ωcm for near stoichiometric layers. We investigate the piezoresistivity of the SiC layers at room temperature under compressive and tensile strain using the four point bending method. The same method is used to analyze selected layers at elevated temperatures up to 600°C. Based on the results we evaluate the applicability of the obtained thin films for strain transducing in harsh environment MEMS sensors.

Characterization of LPCVD SiC thin films at elevated temperatures for robust MEMS sensor applications

In this work we present a systematic characterization of the mechanical properties of thin-film silicon carbide electrodes and released structures as components of a μ-contact type combustion pressure sensor. We developed, fabricated and successfully applied designated MEMS test structures for the determination of the Youngs modulus, residual stress, stress gradient and coefficient of thermal expansion of thin SiC films. In particular, a novel model, describing the capacitance-voltage characteristic of the test structures, has been developed and used for the purely electrical determination of the Youngs modulus and stress gradient of the SiC layers.

Further miniaturization of O 3 -sensors for smart integration - heterointegration

This paper focuses on packaging and miniaturization of ozone sensors and review aspects of their heterointegration together with other physical sensors for consumer electronics (CE) and internet of things (IoT) applications. We present a concept and simulations of an optically regenerated gas sensor for ozone detection [1]. A demonstrator of innovative stand-alone miniaturized gas sensor according to a previously evaluated concept [2] is shown and measured performance is compared to an unpackaged sensor. In this work we also evaluate the impact of the integration of such sensor together with other environmental sensors i.e. pressure sensor. Measured is the performance of the pressure sensors with and without ozone sensor elements.