Gerhard Lammel

Tunable optical filter of porous silicon as key component for a MEMS spectrometer

We present a microspectrometer based on a tunable interference filter for infrared or visible light that scans the desired part of the spectrum within milliseconds. A single pixel detector measures serially the intensity at selected wavelengths. This concept avoids expensive linear detectors as used for grating spectrometers. The tunable filter is fabricated by a new porous silicon technology using only two photolithography steps. A Bragg mirror or a Fabry-Perot bandpass filter for transmission wavelengths between 400 nm and 8 /spl mu/m at normal incidence is created by modulations of the refractive index in the filter plate. Two thermal bimorph micro-actuators tilt the plate by up to 90/spl deg/, changing the incidence angle of the beam to be analyzed. This tunes the wavelength transmitted to the detector by a factor of 1.16. The filter area can be chosen between 0.27 /spl times/ 0.70 mm/sup 2/ and 2.50 /spl times/ 3.00 mm/sup 2/, the filter thickness is typically 30 /spl mu/m. The spectral resolution of /spl Delta//spl lambda///spl lambda/ = 1/25 is sufficient for most sensor applications, e.g., measurement of CO/sub 2/ and CO in combustion processes by their IR absorption bands as will be presented.

A novel micromachining process for the fabrication of monocrystalline Si-membranes using porous silicon

We report a new surface micromachining technology to fabricate monocrystalline silicon membranes covering a vacuum cavity for applications like piezoresistive pressure sensors. The main process steps are: (i) local anodic etching of layered porous silicon with different porosities, (ii) thermal rearrangement of the porous silicon, and (iii) epitaxial growth of the silicon membrane layer. In contrast to conventional bulk micromachining the new technology has the benefit of a considerable freedom in the design of mono-crystalline silicon membranes. The membrane geometry is only determined by the porous region. Further, the new fabrication method is fully CMOS compatible. In fact, except for anodic etching, all process steps are part of a standard mixed signal IC production line. Various aspects of the used key process steps are discussed, particularly with regard to the oxygen and fluorine desorption during the porous silicon annealing. A piezoresistive pressure sensor with integrated ASIC based on the new fabrication method is demonstrated.

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.

The future of MEMS sensors in our connected world

The MEMS market is year after year growing faster than the average semiconductor industry. Over that time the largest technology driver for MEMS changed from automotive applications to consumer electronics dominated by smartphones. Beyond that, MEMS sensors become the heart of whole classes of new devices like fitness trackers, smart watches, virtual reality glasses and smart sensor nodes for the Internet of Things. Silicon chips are only one part of the MEMS story, you need as well special mixed signal circuitry, low power data processing, smart algorithms and connectivity to transform raw signals into meaningful information. Multi-sensor applications & modules are playing an increasingly important role.

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.