Simulating flow in silicon Y-bifurcated microchannels

Microfluidic devices are excessively used for various biomedical, chemical, and engineering applications. The most common microfluidic platforms are obtained from polydimethylsiloxane (PDMS). Platforms based on etched silicon wafers anodically bonded to Pyrex glass are more mechanically rigid, have better sealing and there is no gas permeability compared to those obtained from PDMS [1,2]. The aim of our work is to numerically analyze fluid flow in anisotropically etched silicon microchannels sealed with Pyrex glass. We present simulations of fluid flow in Y-bifurcated microchannels fabricated from the etched {100} silicon in 25 wt% tetramethylammonium hydroxide (TMAH) water solution at the temperature of 80°C [3]. We have explored two symmetrical Y-bifurcations that are defined with acute angles of 36.8° and 19° with the sides that are along the and crystallographic directions in the masking layer [3], respectively. The angles between obtained sidewalls and {100} silicon of two ingoing microchannels for the first and second Y-bifurcation are 72.5° and 80.7°, respectively. The sidewalls of outgoing microchannel in both cases are defined with crystallographic directions and they are orthogonal to the surface of {100} silicon wafer. The appropriate widths of ingoing and outgoing microchannels are 300 and 400 μm, respectively. The depth of microchannels is 55 μm. All simulated flows are three-dimensional (3D), steady and laminar [4], while the investigated fluid is water. Velocities and pressure values are defined at the inlet and outlet boundaries, respectively. The resulting flows are illustrated by velocity contours. The obtained conclusions from fluid flow simulations of presented simple Y-bifurcations provide guidance for future fabrication of complex microfluidic platforms by a cost-effective process with good control over microchannel dimension

Two Color Photodiodes Mounted on the Micromachined Carrier

In this paper, two color detector based on silicon photodiodes is studied and fabricated. Standard IHTM photodiode’s design is modified to allow mounting one photodiode above another using special micromachined carrier. The carrier is fabricated using wet silicon etching in 25% TMAH water solution and anodic bonding of etched silicon and Pyrex glass. The fabricated carrier also allows easy wire thermocompression bonding from the photodiode’s pads to TO-5 housing. Output currents of the photodiodes were measured by applying light of 900 nm and 1060 nm. Obtained results verify applicability of the new packaging for two color detector

Etching of Uncompensated Convex Corners with Sides along <n10> and <100> in 25 wt% TMAH at 80 °C

This paper presents etching of convex corners with sides along and crystallographic directions in a 25 wt% tetramethylammonium hydroxide (TMAH) water solution at 80 °C. We analyzed parallelograms as the mask patterns for anisotropic wet etching of Si (100). The sides of the parallelograms were designed along and crystallographic directions (1 and crystallographic directions were smaller than 45°. All the crystallographic planes that appeared during etching in the experiment were determined. We found that the obtained types of 3D silicon shape sustain when n > 2. The convex corners were not distorted during etching. Therefore, no convex corner compensation is necessary. We fabricated three matrices of parallelograms with sides along crystallographic directions and as examples for possible applications. Additionally, the etching of matrices was simulated by the level set method. We obtained a good agreement between experiments and simulations

Etch rates of Si crystallographic planes in a 25 wt % TMAH water solution

Nagrizane su silicijumske strukture koje su na početku nagrizanja definisane kvadratnim ostrvima od termičkog silicijum dioksida. Stranice kvadrata su projektovane u različitim kristalografskim pravcima. Određene su kristalografske ravni koje se pojavljuju tokom nagrizanja ovih struktura u vodenom rastvoru TMAH koncentracije 25 tež. % na temperaturi od 80 0 C. Merenjem odgovarajućih parametara nagrizanih kristalografskih ravni sa vremenom odredili smo brzine nagrizanja uočenih kristalografskih ravni.We etched Si structures that had been defined at the beginning of etching by square islands of thermal SiO2. The sides of the squares were designed with orientations along various crystallographic directions. All the planes that appeared during etching of these Si structures in a 25 wt % TMAH water solution at a temperature of 80 deg C were determined. By measuring the time dependence of the appropriate parameters of the etched Si crystallographic planes we indirectly calculated their etch rates

Enhancement of the performance of multipurpose thermopile-based sensor using proprietary mems technology for soi piezoresistive pressure sensors fabrication

MEMS senzori na bazi Zebekovog efekta su tema dugogodišnjeg istraživanja u IHTM-CMTM-u. Oni su posebno interesantni zbog činjenice da imaju raznovrsnu primenu (senzori protoka, senzori vakuuma, termalni konvertori, IC detektori, akcelerometri, inklinometri, biološki i hemijski senzori, senzori vrste gasa, senzori sastava binarne smeše gasova, ...). U IHTM-u je do sada realizovano nekoliko varijanti ovog tipa senzora. Prilikom razvoja ovih senzora teži se da se iskoriste postojeći tehnološki procesi razvijeni za IHTM piezorezistivne senzore pritiska. Tako je poslednja generacija senzora sa temoparovima tehnološki kompatibilna sa IHTM Si piezorezistivnim senzorima pritiska. U medjuvremenu je osvojena tehnologija izrade SOI piezorezistivnih senzora pritiska koja će biti primenjena za realizaciju sledeće generacije termalnih senzora. U ovom radu je dat osvrt na dizajn, tehnologiju izrade i poboljšanje performansi koje se očekuje kod SOI višenamenskih senzora sa termoparovima.MEMS sensors based on Seebeck effect have been a part of the long-term research at IHTM-CMTM. They are of special interest because of the fact that they have broad range of applications (flow sensors, vacuum sensors, thermal converters, IR detectors, accelerometers, inclinometers, biological and chemical sensors, gas type sensors, binary gas mixture composition sensors, ...). Till now several types of this kind of sensors have been developed at IHTM. During the development process we aim at using aleady existing technological processes developed for IHTM piezoresistive pressure sensors. Thus, the last generation of thermopile-based sensors is technologicaly compatible with IHTM Si piezoresisitve pressure sensors. In the meantime, a technology of SOI piezoresistive pressure sensors have been conquered and it will be applied for realization of the next generation of thermal sensors. This paper gives overview of the design, fabrication technology and performance improvement expected to be reached with SOI multipurpose thermopile-based sensors

Study of possibilities of application of a thermopile-based gas sensor

The goal of this work is exploring the possibilities of application of a thermopile-based gas sensor. The main task was to study for which kind of gases this type of sensor would be suitable. For this purpose self-developed 1D analytical model was used. Modelling was done for multipurpose sensors developed at ICTM, but also for the same structure that would be fabricated on SOI substrate. Output signal of thermopile-based sensor depends on thermal conductivity of the surrounding gas. When this type of sensor is applied as a gas sensor, prerequisite is that the gases have different thermal conductivities so that the sensor can distinguish between them. According to simulation results, thermopile-based sensors could be applied for a number of gases which are important in industrial safety, homeland security, healthcare, domestic safety, etc. The results obtained for hydrogen detection were already presented, so in this work simulation data for other gases of interest will be given. This includes methane, ammonia, hydrogen sulfide, chlorine. Important conclusion is that thermopile-based sensor is capable to detect wide variety of gases

SOI piezoresistive low pressure sensor for high temperature environments

Silicon-On-Insulator (SOI) wafers utilizing a novel maskless wet etching technique. The prototype has a structured square diaphragm with a concentric boss. The SOI pressure sensors have piezoresistors dielectrically isolated from each other and from the substrate by silicon dioxide. A high temperature transducer prototype has been made utilizing the fabricated sensor. A high temperature method for pressure measurements has been formed. The transducer prototype performance was measured at temperatures up to 300 0С. These SOI pressure sensors are intended for extreme environmental conditions and high operating temperatures that are often needed in military-grade applications and where it is necessary to perform sensitive low pressure measurements. Some examples include aerospace applications like aircraft engines, wind tunnels, various missiles, etc

Reinforcement of the pressure sensor diaphragm by etching in 25%tmah water solution

Primenom maskless tehnike, koja se bazira na vlažnom hemijskom nagrizanju u vodenom rastvoru TMAH koncentracije 25 tež. %. na temperaturi od 80 0 C, na Si pločice uspešno su napravljene dijafragme sa ojačanjem. Ojačanje je projektovano za ravne kvadratne dijafragme debljine 30 μm i površine 2040 μm x 2040 μm, čiji je nominalni opseg rada 1 bar. Vrednost pritiska na kojoj neojačana dijafragma puca je 12 bar. Eksperimentalno je pokazano da je ojačanje povećalo pritisak na kojem puca dijafragma 1.8 puta za dijafragmu sa ojačanjem širine 90 μm, odnosno 2.5 puta za dijafragmu sa ojačanjem širine 40 μm. Ovo poboljšanje pokazuje da je u okviru samog senzora moguće MEMS tehnologijama povećati vrednost pritiska na kojem dolazi do pucanja dijafragme i njenog nepopravljivog oštećenja.Reinforcements of a pressure sensor diaphragm have been designed and fabricated on the Si wafers by maskless wet etching technique. Maskless wet etching technique has been performed in the 25% TMAH water solution at the temperature of 800. Reinforcements are designed for the 30 μm thick and flat square diaphragm. Area of the diaphragm is 2040 μm x 2040 μm. Operation pressure range of the flat diagrapham is 1 bar. Measured burst pressure of the flat diaphragm is 12 bar. For the samples of the diaphragm with the 90 μm wide reinforcement measured burst pressures are 1.8 times higher than for the flat one. For the samples of the diaphragm with the 40 μm wide reinforcement measured burst pressures are 2.5 times higher than for the flat one. Higher measured burst pressures of the diaphragms with reinforcements show that the improvement is possible on the sensor level by using maskless wet etching technique

Influence of geometrical parameters on performance of MEMS thermopile based flow sensor

The aim of this work is to study how performance of thermal flow sensors depends on variation of specific geometrical parameters. Self-developed 1D analytical model was applied at a MEMS sensor based on Seebeck effect. The main elements of the analysed structure are: p+ Si/Al thermocouples, p+ Si heater, thermally and electrically isolating membrane and residual n-Si layer in membrane area. Two thermopiles consisting of N thermocouples are placed symmetrically at both sides of the heater. In this type of flow sensor output signal is obtained as a difference between the Seebeck voltages generated at the downstream and upstream thermopile. It was assumed that sensor is placed in the constant air flow. Several parameters of interest were calculated including flow induced temperature difference established between the downstream and upstream thermopile, output voltage and sensitivity. Simulations were performed in order to analyse dependence of these parameters on residual n-Si layer thickness (dnSi), distance between the hot thermopile junctions and the heater (Δl) and thermocouple width (wTP) and length (lTP). Simulation results show that sensitivity of the thermal flow sensor is improving with increasing Δl and lTP. On the other hand, performance of the sensor will also increase if dnSi or wTP are decreasing

Aggregation problem of dye monolayer in dye sensitized solar cells

Dye sensitized solar cells are photovoltaic devices which simulate photosynthesis [1,2]. Comparing to other solar cells, they cost less and are easy to manufacture. One of themost important parts of this system is adye. A monolayer of dye, adsorbed onto semiconductor surface collects sunlight, which is further being transformed into electrical energy. Many types of sensitizers are known today: organic, inorganic, synthetic, natural [3].Cells with synthetic ruthenium dyes have the highest efficiency of conversion sun energy into electrical. The greatdisadvantage of ruthenium dyes is their price. Natural pigments, on the other hand, extracted from plants, fruits andflowers are available and affordable, though they gain efficiency of conversion of only a fewpercent. The basic problem with using natural dyes as sensitizers is their aggregation on the semisonductor surface. A major factor responsable for the low phtoconversion efficiencyof an organic dye sensitized solar cell is the formation of dye aggregate on the semiconducuctor surface. Such an aggregation effect can bring about significant changes in the absorption and photosensitizing properties of the sensitizing dye molecule. One can minimize aggregation effect on the semiconductor surface using dyes with particularanchoring groups [4] or antiaggregation coadsorbents [5]. Computational calculations (density functional theory(DFT), time-dependent DFT (TDDFT)) are also a convenient approach for investigation of aggregationof dye molecules [6]