Konstantin Seibel

Sensitivity of CMOS based imagers and scaling perspectives

CMOS based imagers are beginning to compete against CCDs in many areas of the consumer market because of their system-on-a-chip capability. Sensitivity, however, is a main weakness of CMOS imagers and enhancements and deviations from standard CMOS processes are necessary to keep up sensitivity with downscaled process generations. In the introductory section several definitions for the sensitivity of image sensors are reviewed with regard to their potential to allow meaningful comparison of different detector structures. In the main section, the standard CMOS sensor architecture is compared to detector structures designed to improve the sensitivity, namely the photogate (PG), the pinned photodiode (PPD) and the thin film on ASIC (TFA) approach. The latter uses a vertical integration of the photodiode on top of the pixel transistors. A careful analysis of the relevant electrical, optical and technological parameters and many previously published experimental data for different imagers reveals that only the PPD and the TFA enhancements provide satisfactory sensitivity and withstand scaling down to 0.18 /spl mu/ processes. Due to the higher fill factor and the higher quantum efficiency TFA provides significantly better values than PPD. The radiometric sensitivity of a 5 /spl mu/m/spl times/5 /spl mu/m TFA pixel is found to amount to 11.9 V/(/spl mu//cm/sup 2/) for a 0.25 /spl mu/m process and 27.5 V/(/spl mu/J/cm/sup 2/) for a 0.18 /spl mu/m process.

A programmable planar electroosmotic micropump for lab-on-a-chip applications

A programmable planar micropump for lab-on-a-chip applications has been developed. The device consists of an electroosmotic micropump combined with a mass flow sensor in a closed control loop. The micropump design with a vertical arrangement of multiple narrow polymer pumping microchannels reduces the pump area to 1/10 compared to planar micropumps with widened shallow pumping channels. This design allows the fabrication of the channel system in only one process step and is compatible with post-CMOS processing. An analytical model is presented to estimate the flow rate in a field-free pressure-driven section of the channel. It is shown that the micropump with optimized dimensions of rib structures allows high pressure low voltage pumping. The electroosmotic micropump with a suggested design using microchannels of SU-8 and polyacrylamide gel electrodes has been fabricated and tested. The pumping rate is bidirectionally linear and reaches 10 nl min−1 in a 1 cm long pressure-driven channel at an applied voltage of 40 V, which corresponds to a zero-flow pressure of 65 Pa. The micropump has been operated successfully in a closed control loop together with an on-chip mass flow sensor and external control circuitry for flow rates between 0 and 30 nl min−1.

Monolithic integrated optical detection for microfluidic systems using thin film photodiodes based on amorphous silicon

For the fabrication of micrototal analysis systems (/spl mu/TAS) methods of micropatterning, sealing, and for the manufacturability, as well as techniques for connecting the microstructures to the macroworld are of particular significance. The idea is to create a monolithic instrument which consists of an assembly of three-dimensional integrated modules (e.g. fluidic networks, electrical and optical sensors, microelectronic circuits) to support the advantage of efficient interconnections. To demonstrate the feasibility of an application specific lab-on-microchip, published in Ref.1, results obtained with a fabricated micro flow cytometer and monolithically integrated optical detectors based on amorphous silicon are reported. In this work, we use an a-Si:H pin diode for the optoelectronic detection of organic fluorescence sensors, namely anthracene derivatives, which are driven through the fluidic channel network hydrodynamically.

UV Imager in TFA Technology

An image sensor with enhanced sensitivity for near ultraviolet radiation (UVA) has been fabricated in TFA (Thin Film on ASIC) technology. The device employs an amorphous silicon pin detector optimized for UV detection by carbonization and layer thickness variation. The front electrode consists of an Al grid or TCO. Measurements show a peak responsivity of 90 mAW -1 at 380 nm. The UV Imager prototype consists of 128 × 128 pixels with a size of 25 μm × 25 [tim each, fabricated in a 0.7 μm CMOS process. Global sensitivity control serves to achieve a dynamic range in excess of 80 dB. The sensor can be used in fields such as chemical, medical and astronomical applications. Furthermore, a UV monitor has been developed, suited to warn of excessive sunlight exposure, considering skin type and sun protection factor.

Monolithic Integrated a-Si:H based pin-Diodes with Orthogonal Liquid Light Guidance Structures for Lab-on-Microchip Applications

We report the fabrication of an amorphous silicon based fluorescence sensor for miniaturized total analysis systems along with experimental results on optical excitation and detection elements. The pin-photodiode exhibits a dynamic range of 110dB and a room temperature dark current of less than 3000 charge carriers per ms according to a detector area of 0.1256mm 2 . The spectral response is ranging from 320nm to 780nm with a maximum at 600nm @ 80% quantum efficiency. To provide high sensitivity, the excitation light irradiates the fluid orthogonally to the active sensor detection direction by means of specifically designed microfluidic capillaries filled with e.g. methylene iodide or 1,2-o-dibrombenzene. The liquid core, which is enclosed by solid cladding materials, has been calculated to dimensions of a width of 16.75µm or 59.67µm with a height from 15µm to 50µm according to a number of propagating modes inside of 16 or 57, respectively.

Characterization of a Micro Capillary Zone Electrophoresis System With Integrated Amorphous Silicon Based Optical Detectors

A pplication specific L ab-on- M icrochips (ALMs) making use of the combination of complex microfluidic networks with microelectronic circuits and micro optical components allow the realization of miniaturized application specific biological and chemical processing and analysis devices. Fluorescence sensing is one of the most widely used detection technologies, e.g. for DNA fluorescence labelling in M icro C apillary E lectrophoresis (µCE) due to its superior sensitivity and specificity. Unfortunately, commercially available fluorescence sensing systems are physically very large, non portable, expensive and constrain the analysis in portable diagnostic and medical care. Integrated semiconductor optoelectronic devices can provide a portable, parallel and inexpensive solution for on chip fluorescence sensing. Most µCE applications working in the spectral range of visible light. For the integration of optical detection components a photon energy range of 1.6 eV - 3.1 eV is of interest. The a-Si:H technology accomplished due to the low dark current and high absorption coefficient against to crystalline silicon the requirements in that spectral range. In this paper we combine a:Si-H photo sensors with a fluidic micro system to detect the fluorescence of a rhodamine analyte mixture. The analyte mixture was excited by light with a wavelength in the range of λ Ex = 450 - 490 nm. The a-Si:H detector reveals a low dark current density on the order of 10 -10 A/cm 2 and a sufficient dynamic range of ∼100 dB under illumination of ∼1000 lx as a function of bias voltage. The measurement shows that the movement of the rhodamine plug in the microchannel causes a significant rise in the pin-diode photo current, which correlates to the evaluated signal of a microscope image detector. The photo current difference for excitation and additional fluorescence amounts to 2.4 µA.

A Planar Electroosmotic Micropump for Lab-on-Microchip Applications

The scope of the paper is to provide a theoretical and experimental treatment allowing to optimize critical design parameters for planar electroosmotic micropumps. The suggested design with a vertical arrangement of multiple narrow polymer pumping microchannels reduces the pump area to 1/10 compared to planar micropumps with widened shallow pumping channels. This design allows the fabrication of the channel system in only one process step, compatible with post-CMOS processing and suitable for monolithical integration on labchips. A simple analytical model has been developed to characterize the flow rate in a field free pressure-driven section of the channel. It is shown that the micropump with optimized dimensions of rib structures makes possible high pressure low voltage pumping. For high pressure capacity the distance between the ribs must be on the order of 0.5-1 µm with an aspect ratio of 10-20. The electroosmotic micropump with microchannels of SU-8 and polyacrylamide gel electrodes has been fabricated and tested. The pumping rate is bidirectionally linear and reached 10 nl/min at applied voltage of 40 V in 1 cm long pressure-driven channel, which corresponds to zero-flow pressure of 65 Pa.