Petra Kurzawski

Towards a versatile DRIE: silicon pit structures combined with electrochemical etch stop

A novel approach for fabricating low-pitch arrays of silicon membranes on standard CMOS wafers by combining deep-reactive ion etching (DRIE) and electrochemical etching (ECE) techniques is presented. These techniques have been used to fabricate membrane-based sensors and sensor arrays featuring different membrane sizes on a single wafer with a well defined etch stop. The described procedure is particularly useful in cases when the usage of SOI wafers is not an option. The combination of a grid-like mask pattern featuring uniform-size etch openings for the DRIE process with a reliable ECE technique allowed to fabricate silicon membranes with sizes ranging from 0.01 mm2 to 2.2 mm2 . The development of this new method has been motivated by the need to design a compact n-well-based calorimetric sensor array, where the use of a standard ECE technique would have significantly increased the overall size of the device

Direct Determination of the Enantiomeric Purity or Enantiomeric Composition of Methylpropionates Using a Single Capacitive Microsensor

Capacitive enantioselective sensors have been demonstrated to provide antipodal signals upon dosage of, e.g., the enantiomers of methyl lactate or methyl-2-chloropropionate. In a next step, these sensors have been used to not only qualitatively determine the nature of the respective enantiomer or to quantitatively measure its concentration upon dosage in the pure form but to also assess the enantiomeric composition of mixtures by using only a single capacitive-type sensor. The enantioselective coating material consisted of a modified gamma-cyclodextrin. It was shown that the absorption and desorption kinetics of the two enantiomers of, e.g., the methyl-2-chloropropionate, are sufficiently different and produce sensor signal features that enable an accurate determination of the enantiomeric purity and composition of the chiral analyte or mixture under investigation. The method even allows for detecting small impurities in commercially available samples labeled as 99% enantiomerically pure. Moreover, the results disclosed here show that sensor techniques can be used to reveal details of enantioselective analyte-receptor and analyte-matrix interactions.

Gas and liquid phase sensing of volatile organics with disk microresonator

The sensing of volatile organic compounds (VOCs) using a MEMS resonator with an in-plane vibrational mode is reported. VOCs are detected in both the gas and liquid phases by a polymer-coated disk microresonator, which is operated as the frequency determining element in an amplifying feedback loop. The functionalized disk microresonators exhibit a short term frequency stability of 1.1 times 10-7 in air and 3.4 times 10-6 in water. Using polymer membranes as chemically sensitive layers, different concentrations of o-xylene, benzene, octane, trichloroethane, and toluene have been detected in the gas phase, with the limit of detection for o-xylene being 2.2 ppm. M-xylene has been detected in the liquid phase with a limit of detection of 1.9 ppm.

Multi-transducer recordings from a single-chip gas sensor system coated with different polymers

The performance of a single-chip three-transducer CMOS gas sensor microsystems is demonstrated, which has been coated with different polymers. The three different transducers (mass-sensitive, calorimetric, capacitive) on the same chip inherently provide complementary information, such as analyte volatility (mass-sensitive), polarity (capacitive), and ab/desorption heat (calorimetric). By combining several of those single chip systems, each of which has a different polymeric coating, in an array, even more information on the detected analyte is accessible that can be used to classify and quantify organic volatiles as will be shown below.

The Use of Different Transduction Principles for Qualitative and Quantitative Chiral Gas Analysis

University of Tubingen, Institute of Organic Chemistry, auf der Morgenstelle 18, D-72076 Tubingen, Germany Trends in chemical gas sensor research over the last years include amongst others: (a) the search for highly selective bio/chemical layer materials, (b) the use of arrays of different partially selective sensors with subsequent pattern recognition and multi-component analysis methods

A Comparison of Multi-Transducer Arrays and Single-Transducer Arrays for the Determination of Multi-Vapor Mixtures

This study provides a comprehensive analysis of multi-transducer (MT) array performance, illustrating quantitatively the advantages of such arrays over their single-transducer (ST) counterparts. Calibrations were performed for 11 vapors with five cantilevers, five capacitive, and five calorimetric sensors coated with five different polymers. Using these data in Monte Carlo simulations coupled with a disjoint principal component regression pattern recognition algorithm we examine the predicted rates of recognition achievable for analyses of individual vapors and their binary and ternary mixtures. Optimal MT arrays consistently outperform optimal ST arrays of similar dimension, and a number of ternary mixtures could be reliably analyzed with MT arrays that could not be analyzed with any ST arrays. Recognition rates did not increase significantly by including > 5 or 6 sensors in the array.

Chiral Discrimination Performance of a Monolithic CMOS Gas Sensor Microsystem

The performance of a single-chip, three-transducer CMOS gas sensor microsystem for the discrimination of chiral molecules is demonstrated. For the discrimination of enantiomers, such as methyl lactate and methyl 2-chloropropionate, an enantioselective coating based on a modified gamma-cyclodextrin dissolved in a polysiloxane matrix was used. The entire system comprising micromechanical components, i.e., a mass-sensitive, calorimetric, and capacitive transducer, and all the required circuitry components as well as interface and communication units are integrated on a single chip. The three different transducers inherently provide complementary information on the target analyte such as its volatility, polarity, and sorption heat.