Stefan Finkbeiner

Next generation pressure sensors in surface micromachining technology

One of the first MEMS products - the pressure sensor - has still room for innovation. We report a completely new pressure sensor generation based on a novel surface micromachining technology. Using porous silicon the membrane fabrication can be monolithically integrated with high synergy in an analog/digital semiconductor process suited for high volume production in an IC-fabrication facility. Only two mask layers and one electrochemical etching step are inserted at the beginning of a standard IC-process to transform the epitaxial silicon layer from the electronic process into a monocrystalline membrane with a vacuum cavity under it.

Monocrystalline Si membranes for pressure sensors fabricated by a novel surface micromachining process using porous silicon

We developed a novel surface micromachining process to fabricate monocrystalline silicon membranes covering a vacuum cavity without any additional sealing steps. Heart of the process is anodic etching of porous silicon, annealing and epitaxial growth. The porous silicon layer consists of two parts, a starting mesoporous silicon layer with low surface porosity and a nanoporous silicon layer with a high porosity. The following annealing step removes native oxide within the later cavity, and the surface is sealed for the subsequent epitaxial layer deposition. The observed stacking fault density in the epitaxial layer about 1E5 cm-2. The temperature budget of the following ASIC-process leads to a complete transformation of the nanoporous silicon layer into a large cavity. The whole structure can be used as a pressure sensor. The estimated pressure in the cavity is smaller than 1 mbar. First integrated pressure sensors have been fabricated using this process. The sensors show a good linearity over the whole pressure range of 200 mbar to 1000 mbar. This novel process has several advantages compared to already published processes. It is a “MEMS first” process, which means that after the epitaxial growth the surface of the wafer is close to a standard wafer surface. Due to full IC compatibility, standard ASIC processes are possible after the fabrication of the membrane. The use of porous silicon enables a high degree of geometrical freedom in the design of membranes compared to standard bulk micromachining (KOH, TMAH). The monocrystalline membranes can be fabricated with surface micromachining without any additional sealing or backside processing steps.

The effects of thermal treatment on the anisotropic etching behavior of Cz- and Fz-silicon

Abstract We investigated the effects of stepwise thermal treatment of [100]-CZ- and FZ-silicon on the crystal defects and the etching behavior in KOH solutions. The anisotropy (the quotient of the vertical etch rate and the underetching of the mask), formed crystal defects as well as the surface roughness of the exposed {100}- and {111}-planes are examined. Thermal treatment of the silicon substrate can result in a precipitation of the interstitial oxygen. The precipitated oxygen causes elastic stress in the crystal which can be relieved by a generation of defects [1] , [2] . We found out that the evolution of these defects accelerates the lateral etch rate considerably. For the reference, the samples without thermal treatment the anisotropy has a value of 120 for FZ-silicon and 70 for CZ-silicon, respectively. However, with rising temperature and process time the anisotropy decreases to 30 for FZ-silicon and 15 for CZ-silicon. At the same time, the surface quality of the {100}- and {111}-planes degrades with increasing temperature. The roughness value Ra rises from 5 nm to approximately 30 nm. Along with the variation of the etching behavior during thermal treatment, we further present a suitable model for the oxygen-dependence of the etch rate.

MEMS for automotive and consumer electronics

Micro-Electro-Mechanical Systems (MEMS) are sensing the environmental conditions and give input to electronic control systems. MEMS are miniature systems which usually combine tiny mechanical structures with electronic circuits. Typical MEMS structures have a size of a few micrometers. MEMS sensors make system reactions to human needs more intelligent, precise, and at much faster reaction rates than humanly possible. Today MEMS sensors can be found in nearly every motor vehicle, smart phone or laptop. Due to continuous product innovations, the sensors find their way into more and more applications in automotive and consumer electronics. According to IHS iSuppli an amount of 4.3 billion micromechanical sensors were sold in 2011 with an impressive increase to 9.8 billion sensors in 2015 - a growth rate of 23% per year! These growth rates are only possible with continuous efforts to improve the performance and to decrease the size, power consumption and costs of the sensors.

Influence of process variation on the functionality of a high-pressure sensor

A sensor designer has to keep in mind a couple of things developing a new product. These are performance, reliability and last but not least cost. Cost is mainly a function of yield. To be successful on the market, it is therefore necessary to predict yield for a new design, based on its process steps. This can be done only by modeling a sensor based on process parameters as input to the models. How to realize this, will be shown for the design of a high pressure sensor.

Smart Sensor Technology as the Foundation of the IoT: Optical Microsystems Enable Interactive Laser Projection

Consumer electronics such as smartphones, tablets, and wearables are part of our everyday life—visible everywhere and taken for granted. Less visible however are the small MEMS (micro-electromechanical systems) sensors that are an integral part of these devices. Smart sensor technology enables things to be sensed and connected—in all parts of our daily life, in homes, vehicles, cities. With the emergence of the Internet of Things (IoT), more and more devices become connected which results in demanding challenges for MEMS sensor technology providers—in addition to the trends of low cost, small size, low power consumption as well as overall system performance. The exciting developments in the IoT are advancing at an amazing pace. It is not just about how devices communicate or sense their surrounding environments, but increasingly about how technology interacts with human beings. Laser-projected virtual interfaces based on optical MEMS are a new fascinating solution in a world of previously unimaginable opportunities. They give any kind of device a unique personality of its own, enabling technology to interact with people, to make life simpler and more exciting. It is a ground-breaking solution for embedded projectors and augmented reality applications such as games, infotainment as well as in-car head-up displays or intelligent head lamps for automated driving.

Sensor measuring principles

There is a great number of sensors at work in motor vehicles. They act as the sensory organs of the vehicle and convert input variables into electrical signals. These signals are used in control and regulation functions by the control units in the engine-management, safety and comfort and convenience systems. Various measuring concepts are applied, depending on the task.

Material-related effects on wet chemical micromachining of smart MEMS devices

Smart MEMS devices such as pressure, mass flow and yaw rate sensors are presented in detail. One common point of this range of devices is their fabrication technology regarding anisotropic etching. This paper is first meant to give a short review upon the applications processed by using wet chemical etching in KOH-solutions. Furthermore, we will discuss about the impact of material and process related defects in the silicon crystal and on the anisotropic etching behavior.