Grigoris Kaltsas

Characterization of a silicon thermal gas-flow sensor with porous silicon thermal isolation

In this paper, the results of full characterization of a micromachined-silicon thermal gas flow sensor will be presented. The sensor is composed of two series of thermocouples on the right and left side of a polysilicon resistor, used as heater. The resistor and the hot contacts of the thermocouples lie on a thick porous silicon layer, which assures local thermal isolation, while the thermopile cold contacts lie on bulk silicon. Gas flow is parallel to the surface of the sensor and perpendicular to the resistor, which is heated at constant temperature. The power of the heater is stabilized by an external circuit, which provides a feedback current to compensate changes in the resistance of the heater under flow. Characterization of the sensor both under static conditions and under flow of different gases will be presented. The sensor shows high sensitivity [of the order of 175 /spl times/ 10/sup -3/ mV/(m/s)/sup 1/2/ per thermocouple] and very rapid response, below 1 ms, which makes it appropriate for use both under laminar and under turbulent flows.

Planar CMOS compatible process for the fabrication of buried microchannels in silicon, using porous-silicon technology

This work presents a new method for the fabrication of buried microchannels, covered with porous silicon (PS). The specific method is a two-step electrochemical process, which combines PS formation and electropolishing. In a first step a PS layer with a specific depth is created at a predefined area and in the following step a cavity underneath is formed, by electropolishing of silicon. The shape of the microchannel is semi-cylindrical due to isotropic formation. The method allows accurate control of the dimensions of both PS and the cavity. The formation conditions of the PS layer and the cavity were optimized so as to obtain smooth microchannel walls. In order to obtain stable structures the area underneath the PS masking layer was transformed into n-type by implantation, taking advantage of the selectivity of PS formation between n- and p-type silicon. With this technique, a monocrystalline support for the PS layer is formed on top of the cavity. Various microchannel diameters with different thickness of capping PS layer were obtained. The process is CMOS compatible and it uses only one lithographic step and leaves the surface of the wafer unaffected for further processing. A microfluidic thermal flow sensor was fabricated using this technology, the experimental evaluation of which is in progress.

A Silicon Thermal Accelerometer Without Solid Proof Mass Using Porous Silicon Thermal Isolation

A Si thermal accelerometer without solid proof mass, based on porous silicon (PS) technology has been developed and characterized. The device is compatible with silicon technology and it consists of a polysilicon heater and two thermopiles, situated symmetrically on each side of a heater. A thick PS layer provides thermal isolation from the Si substrate. The operation principle is based on the movement induced thermal convection variations between the heater and the hot thermopile contacts, which are caused by the movement of the hot fluid medium on top of the heater with respect to the sensor die. A detailed simulation by the FEA package Ansys was carried out in order to find the optimum geometrical parameters and the theoretical device behavior. The porous silicon thermal accelerometer (PSTA) was tested in a specially designed vibration system for various frequencies and accelerations. Different packaging configurations were also evaluated for both air and oil surrounding environments. The dependence of the PSTA response on applied power was studied for each surrounding environment. In each case, the response was compared with a commercial reference accelerometer and the corresponding sensitivities were extracted.

High crystalline quality erbium silicide films on (100) silicon, grown in high vacuum

Abstract Erbium silicide films were grown in high vacuum (10 −8 Torr) on (100) silicon substrates by erbium evaporation on a heated substrate and subsequent annealing. The substrate temperature was between 400–450°C and a second annealing step was given at 800–870°C for 30 min. Films with a thickness in the range of 40–50 nm were prepared. X-ray and electron diffraction were used for the characterisation of the grown films. Transmission electron microscopy characterisation revealed the very high crystalline quality of the films. They were almost single crystalline, of very large ‘grains’, up to a few hundreds of nm, slightly misoriented with respect to each other. The erbium silicide was found to possess a modulated structure, derived from a basic one of the ThSi 2 type.

Application of Porous Silicon to Bulk Silicon Micromachining

A fully C-MOS compatible process for bulk silicon micromachining using porous silicon technology and front-side lithography is developed. The process is based on the use of porous silicon as a sacrificial layer for the fabrication of deep cavities into monocrystalline silicon, so as to avoid back side lithography. Cavities as deep as several hundreds of micrometers are produced with very smooth surface and sidewalls. The process is used to produce : a) suspended monocrystalline silicon membranes, b) free standing polysilicon membranes in the form of bridges or cantilevers with lateral dimensions from a few μms to several hundreds of μms. Important applications to silicon integrated devices as sensors, actuators, detectors etc., are foreseen.

Study and Evaluation of a PCB-MEMS Liquid Microflow Sensor

This paper presents the evaluation of a miniature liquid microflow sensor, directly integrated on a PCB. The sensor operation is based on the convective heat transfer principle. The heating and sensing elements are thin Pt resistors which are in direct electrical contact with the external copper tracks of the printed circuit board. Due to the low thermal conductivity of the substrate material, a high degree of thermal isolation is obtained which improves the operating characteristics of the device. The sensor is able to operate under both the hot-wire and the calorimetric principle. In order to fully exploit the temperature distribution in the flowing liquid, multiple sensing elements are positioned in various distances from the heater. A special housing was developed which allowed implementation of the sensor into tubes of various cross sectional areas. The sensor sensitivity and measurement range as a function of the sensing element distance were quantified. A minimum resolution of 3 μL/min and a measurement flow range up to 500 μL/min were achieved.

A CMOS compatible thermal accelerometer without solid proof mass, based on porous silicon thermal isolation

A Si thermal accelerometer without solid proof mass has been developed and characterized. The device is CMOS compatible and consists of a polysilicon heater and two thermopiles, situated symmetrically on each side of a heater. A thick porous silicon (PS) layer assures thermal isolation from the Si substrate. The operating principle is based on the movement induced thermal convection variations between the heater and the hot thermopile contacts, which is caused by the movement of the hot air on top of the heater relative to the sensor die. The porous silicon thermal accelerometer (PSTA) was tested in a specially designed vibration system for different frequencies and accelerations. In each case, the response was compared to a commercial accelerometer. Appropriate read out electronics were fabricated in order to reduce the noise of the thermopiles and to amplify the signal. The dependence of the response on applied power and the surrounding environment was also studied.

A disposable flexible humidity sensor directly printed on paper for medical applications

The present study demonstrates an inkjet – printed interdigitated electrode array on paper substrate and its evaluation as humidity sensor. Inkjet droplet formation analysis has been performed in order to achieve repeatable results regarding generated droplets, based on the driving pulses applied on the inkjet piezoelectric element. Droplet formation has been monitored using stroboscopic effect. Three different paper substrates, namely high glossy inkjet photo paper, glossy inkjet photo and matte inkjet photo paper have been evaluated to investigate compatibility with the ink. Relative humidity measurements have been carried out in a controlled environment. Material degradation, long term response and memory effect are some of the aspects which were studied within the frame of the present work. The proposed sensor provides the opportunity for novel biomedical applications given the flexible substrate nature and the low – cost, single – step fabrication approach.