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Background: Miniaturized piezoresistive sensors, particularly strain gauges, pressure sensors, and accelerometers, have been used for measurements and control applications in various fields, such as automotive, aerospace, industrial, biomedical, sports, and many more. A variety of different materials have been investigated for the development of these sensors. Among them, diamond- like carbon (DLC) thin films have emerged as one of the most promising piezoresistive sensing materials due to their excellent mechanical properties, such as hardness and high Young’s modulus. At the same time, metal doping has been studied to enhance their electrical properties. Objective: This article explores the use of co-sputtered tungsten-doped diamond-like carbon (WDLC) thin films as microfabricated strain gauges or piezoresistors. Methods: Different serpentine thin-film resistors were microfabricated on co-sputtered W-DLC thin films using photolithography, metallization, lift-off and RIE (reactive ion etching) processes. In order to evaluate their piezoresistive sensing performance, Gauge Factor (GF) measurements were carried out at room temperature using the cantilever beam method. Results: GF values obtained in this study for co-sputtered W-DLC thin films are comparable to those reported for W-DLC films produced and characterized by other techniques, which indicates the feasibility of our approach to using them as sensing materials in piezoresistive sensors. Conclusion: W-DLC thin films produced by the co-magnetron sputtering technique can be considered as sensing materials for miniaturized piezoresistive sensors due to the following key advantages: (i) easy and well-controlled synthesis method, (ii) good piezoresistive properties exhibiting a GF higher than metals, and (iii) thin-film resistors formed by a simple microfabrication process.

The wearable sensors have attracted a growing interest in different markets, including health, fitness, gaming, and entertainment, due to their outstanding characteristics of convenience, simplicity, accuracy, speed, and competitive price. The development of different types of wearable sensors was only possible due to advances in smart nanostructured materials with properties to detect changes in temperature, touch, pressure, movement, and humidity. Among the various sensing nanomaterials used in wearable sensors, the piezoresistive type has been extensively investigated and their potential have been demonstrated for different applications. In this review article, the current status and challenges of nanomaterials and fabrication processes for wearable piezoresistive sensors are presented in three parts. The first part focuses on the different types of sensing nanomaterials, namely, zero-dimensional (0D), one-dimensional (1D), two-dimensional (2D), and three-dimensional (3D) piezoresistive nanomaterials. Then, in second part, their fabrication processes and integration are discussed. Finally, the last part presents examples of wearable piezoresistive sensors and their applications.

This work shows the design and structural analysis of a tensile testing machine for paper used as a substrate for the manufacture of piezoresistive sensors similar to strain gauges. This article also discusses the manufacturing process and the performance analysis of a low cost machine to characterize electromechanical structures made of graphite on polymeric substrate. Graphite in thin paper films have been investigated for a wide range of applications due to their excellent electrical and mechanical properties. Graphite sensors on flexible polymer substrates can be applied to portable wearable devices, as well as sensors for the Internet of things, bioengineering and applications in various areas of science and technology.

The chemical, structural, morphological, and optical properties of Al-doped TiO2 thin films, called TiO2/Al2O3 nanolaminates, grown by plasma-enhanced atomic layer deposition (PEALD) on p-type Si &lt, 100&gt, and commercial SLG glass were discussed. High-quality PEALD TiO2/Al2O3 nanolaminates were produced in the amorphous and crystalline phases. All crystalline nanolaminates have an overabundance of oxygen, while amorphous ones lack oxygen. The superabundance of oxygen on the crystalline film surface was illustrated by a schematic representation that described this phenomenon observed for PEALD TiO2/Al2O3 nanolaminates. The transition from crystalline to amorphous phase increased the surface hardness and the optical gap and decreased the refractive index. Therefore, the doping effect of TiO2 by the insertion of Al2O3 monolayers showed that it is possible to adjust different parameters of the thin-film material and to control, for example, the mobility of the hole-electron pair in the metal-insulator-devices semiconductors, corrosion protection, and optical properties, which are crucial for application in a wide range of technological areas, such as those used to manufacture fluorescence biosensors, photodetectors, and solar cells, among other devices.

Diamond-like carbon (DLC) thin films have been investigated for a wide range of applications due to their excellent electrical and mechanical properties. In the last decade, several researches and development activities have been conducted on the use of these thin films as piezoresistors in MEMS pressure sensors. This paper provides an overview on the design of a piezoresistive pressure sensor constituted of a silicon circular diaphragm with four DLC thin-film piezoresistors arranged in the Wheatstone bridge configuration. The sensor was designed from analytical formulas found in the literature.

The use of wide bandgap semiconductor thin films as sensing materials for micro-electrical---mechanical systems (MEMS) sensors has been the subject of much discussion in the academic and industrial communities. The motivation is that such materials are recognized as being suitable for extreme environment applications, namely: high temperatures, intense radiation and corrosive atmospheres. Among the wide bandgap semiconductor materials, aluminum nitride (AlN), zinc oxide (ZnO), diamond-like carbon (DLC) and silicon carbide (SiC) are highlighted due to their inherent sensing properties and compatibility with MEMS fabrication processes. Here we show an overview on the development technologies and applications of AlN, ZnO, DLC and SiC thin films in piezoelectric and piezoresistive MEMS sensors. Emphasis is placed on the influence of the temperature on the piezoelectric and piezoresistive properties of these films.

Silicon carbide (SiC), in bulk or film form, has been shown as a promising material to replace the silicon as sensing element in devices for harsh environments. This has motivated several studies on growth and characterization of SiC thin films on different substrates such as silicon, silicon dioxide (SiO2), aluminum nitride (AlN) and silicon nitride (Si3N4), among others. However, less attention has been given to the investigation on how substrates affect the piezoresistive properties of SiC thin films. In this work, we have investigated the effect of substrates on the piezoresistive properties of SiC thin films produced by magnetron sputtering. Three types of substrates were utilized: (100) monocrystalline silicon, AlN on silicon and thermally oxidized silicon. Test structures were fabricated using photolithography, metallization lift-off and RIE (reactive ion etching) processes in order to perform piezoresistive characterization of the SiC samples produced.

The use of thin films as sensing elements for microsensor applications has been shown very attractive due to their low-cost fabrication, potential for integration with standard CMOS technologies and possibility of deposition on different substrate types. In particular, piezoresistive sensors based on thin films have been commonly developed because can be easily implemented using microfabrication processes and present the best relation between sensitivity and system complexity, which showing great advantages in term of device integration. In our previous works (Fraga et al. 2010, 2011a), we studied undoped and nitrogen-doped PECVD a-SiC thin films as alternative materials to replace the silicon piezoresistors in strain and pressure sensors for harsh environments. Here, we focused our attention on the piezoresistive properties of sputtered silicon carbide (SiC), diamond-like carbon (DLC) and titanium dioxide (TiO2) thin films. These materials were evaluated in terms of sensitivity or gauge factor and of the influence of the temperature on this sensitivity, allowing a preliminary analysis of the applicability of these thin films in high temperature piezoresistive sensors.

In order to evaluate the potential of amorphous silicon carbide (a-SiC) films for piezoresistive sensors applications, a pressure sensor has been developed based on this material. The deposition conditions and properties of a-SiC films deposited on thermally oxidized (100) Si substrates by two techniques enhanced by plasma, PECVD (plasma enhanced chemical vapor deposition) and RF magnetron sputtering, are briefly described and compared. Among the SiC films produced, we choose the nitrogen-doped PECVD SiC film to fabricate the piezoresistors of the sensor. The structure of the sensor consists of six a-SiC piezoresistors, configured in Wheatstone bridge, on a SiO2/Si square diaphragm. The sensor was tested for applied pressure ranging from 0 to 12 psi and supply voltage of 12 V. A preliminary study of the influence of the temperature on the performance of the sensor was performed by experimental measurements and theoretical investigations.

In this study, the effect of nitrogen doping on piezoresistive properties of non-stoichiometric amorphous silicon carbide (a-Si x C y ) thin-film strain gauges was investigated. The films were prepared by plasma-enhanced chemical vapor deposition (PECVD) and the strain gauges were patterned by fabrication processes like conventional photolithography, metallization and reactive ion etching (RIE). The structure of the strain gauges consists of a a-Si x C y thin-film resistor with one Ti/Au electrical contact at each extreme of the resistor. In order to determine the piezoresistive properties, each strain gauge type was bonded near the clamped edge of a stainless steel cantilever beam and on the free edge calibrated weights were applied. The influence of the temperature on piezoresistive properties also was evaluated through the temperature coefficient of resistance (TCR) measurements from room temperature up to 250oC. It was observed that nitrogen doping increased the piezoresistive coefficient and TCR of a-Si x C y thin film strain gauges.

A discussion on the so-called kinetic compensation effect, which has been observed in several heterogeneous catalytic processes and in particular in the direct synthesis of dimethyldichlorosilane, is presented. A possible explanation about the origin of the phenomena in relation to the anisotropic character of the reaction is proposed. A complementary explanation about the mechanism of promoters' action on the direct synthesis reaction is also proposed. These findings were obtained by comparison with analogous characteristics of another silicon reaction, namely, the silicon etching by alkaline solutions, for which similar phenomena are observed.

Considerable research efforts have been devoted to the development of MEMS sensor packaging based on different material types (ceramic, plastic and metal). In this work, we describe the design, fabrication and test of a metal packaging with high reliability and low cost. The novelty of our research is on the packaging assembly by mechanical interference replacing the expensive plasma welding process that is used commonly in the development of traditional metal packaging. Keywords— packaging; MEMS; pressure sensors; low-cost

Amorphous silicon carbide (a-SiC) thin films have been grown on aluminum nitride (AlN) intermediate layers on (100) Si substrates by RF magnetron sputtering technique. Profilometry, four-point probe method, Rutherford backscattering spectroscopy (RBS) and Fourier transform infrared (FTIR) were employed to characterize the as-deposited SiC thin films. Test structures have been developed to investigate the piezoresistive properties. These structures consist of SiC thin-film resistors on AlN/Si substrates defined by reactive ion etching (RIE) with Ti/Au pads formed by lift-off process. Gauge factor (GF) and temperature coefficient of resistance (TCR) measurements have been performed and demonstrated the potential of these resistors to be used as sensing elements in devices for high temperature application.

In recent years, the piezoresistive properties of different semiconductor thin films, with chemical and mechanical stability, have been studied in order to use them as base material in the fabrication of strain sensors for high temperature applications. In this context, this work compares the performance of strain sensors based on sputter-deposited semiconductor thin films such as titanium dioxide (TiO2), silicon carbide (SiC) and diamond-like carbon (DLC) operated at temperatures up to 250°C. The structure of each sensor consists of four thin-film resistors, configured in Wheatstone bridge, with Ti/Au electrical contacts. These strain sensors reported here differ from our previous works in the types of materials used and in the quantity/configuration of the thin-film resistors. The strain sensors were fabricated by photolithography techiques in conjunction with lift-off processes. The beam-bending experiments were performed to characterize the sensors.

In this work, we studied the fabrication and characterization of strain sensors based on semiconductor materials for high temperature applications: non-stoichometric amorphous silicon carbide (a-Si x C y ) thin film and SOI (Silicon-On-Insulator) substrates. a-Si x C y were deposited onto thermally oxidized (100) Si wafers by plasma enhanced chemical vapor deposition (PECVD) technique using silane (SiH 4 ) and methane (CH 4 ) as precursor gases. The SOI wafer used had a sandwich structure of a 0.2 µm thick top p-type Si layer, 0.5 µm thick buried oxide (BOX) layer and 250µm Si substrate. The piezoresistive strain sensitivity element in the sensors is a a-Si x C y thin-film resistor or a Si p-type resistor formed on SOI substrate. Gauge factor (GF) measurements were done using the beam-bending method. One resistor of each type was bonded near the clamped edge of a stainless steel cantilever beam and on the free edge were applied different forces. The electrical resistance of each resistor was measured without applied load on the beam and during subsequent tensile load. The temperature coefficient of resistance (TCR) also was investigated from room temperature up to 250°C. The results indicate that the a-Si x C y thin-film resistor has a GF of 48 and a TCR of 35 ppm/°C whereas p-type Si on SOI substrate has gauge factor of 22 and TCR of 140 ppm/°C.

O presente trabalho descreve uma metodologia para obtenção de membranas de nanofibras do material fluoreto de polivinideno (PVDF) com propriedade piezoelétrica. O processo utilizado para a obtenção das membranas foi a eletrofiação, foram exploradas duas variações do processo, uma utilizando coletor rotativo e outra eletrodos paralelos. Análises de espectroscopia Raman e difração de raio X confirmam a presença da fase ? nas membranas, tal fase é responsável pela piezoeletricidade do PVDF. Com as membranas, foram construídos dispositivos. Um arranjo experimental foi montado para constatar a presença da piezoeletricidade nos dispositivos contendo as membranas eletrofiadas de PVDF. A máxima resposta obtida foi um pico de tensão de 292 mV diante da deformação mecânica ocasionada por solenoide excitando a amostra com frequência de 1 Hz e tensão de 15 V. Respostas semelhantes em diferentes amostras sugerem reprodutibilidade. Estes resultados mostram aplicabilidade das membranas eletrofiadas de PVDF em sensores ou em colheita de energia. This work describes a methodology for obtaining nanofibre membranes of polyvinylidene fluoride (PVDF) material with piezoelectric properties. The process used to obtain the membranes was electrospinning, two process variations was used, the first using a rotary collector and another parallel electrodes. Analyzes of Raman spectroscopy and X-ray diffraction confirm the presence of the ? phase in the membranes, this phase is responsible for the piezoelectricity of PVDF. With the membranes, devices were built. An experimental arrangement was set up to verify the presence of piezoelectricity in devices containing PVDF electrospinning membranes. The maximum response obtained was a peak voltage of 292 mV, due to the mechanical deformation caused by solenoid excited the sample with a frequency of 1 Hz and a voltage of 15 V. Similar responses in different samples suggest reproducibility. These results demonstrate applicability of PVDF electrophilic membranes to sensors or energy storage.

Neste trabalho foram estudadas as etapas de processo envolvidas na tecnologia Pin-in-Paste (PIP) de soldagem por refusão de componentes convencionais (THCs - Through Hole Components ou Componentes de Furo Passante) em placas de circuito impresso (PCIs), utilizando pasta de solda sem chumbo (lead-free) com liga SAC (Sn-Ag-Cu) de forma a atender as novas exigências ambientais para a montagem eletrônica. Inicialmente foi feito o projeto da PCI de teste com três diferentes componentes THCs e três componentes SMD com encapsulamentos distintos, com o objetivo de reproduzir uma PCI comercial. Foram gerados dois diâmetros de furos diferentes para inserir os THCs, possibilitando o estudo da variação de preenchimento com solda no PTH. Foi proposta uma equação para o cálculo do volume de pasta de solda a ser impresso sobre os furos no processo de montagem. A partir desta equação foram calculadas as dimensões dos furos do estêncil para a PCI de teste. Os parâmetros de impressão foram otimizados em função da variação de pressão e da velocidade do rodo. Duas curvas de refusão foram utilizadas, uma convencional e outra otimizada para verificar a variação na geração de defeitos. A impressão de pasta de solda ficou superior ao projetado, o que resultou em todas as amostras terem solda acima do parâmetro mínimo de aceitabilidade de volume de 75% de preenchimento do PTH. Esta sobre impressão ocasionou defeitos em boa parte dos componentes, excesso de solda nos filetes e resíduos de fluxo na solda nos PTHs. Tais defeitos foram expressivos para todos os THCs, mostrando que o excesso de pasta impressa foi decisivo na geração de defeitos para todas as combinações das variáveis estudadas. Os SMDs tiveram solda aceitável, apresentando apenas alguns casos de excesso de fluxo ou pouca solda em alguns QFPs devido ao uso de ilhas com dimensões maiores que o exigido em norma. O processo de Pin In Paste se mostra viável como substituto da solda onda em linhas de montagem para placas com SMDs e THCs, mas estudos posteriores deverão ser realizados para a geração de um modelo confiável de projeto de PCIs e estêncil com solda lead-free para que tal processo seja utilizado em grande escala na indústria. This study describes the process steps involved in Pin-in-Paste (PIP) reflow soldering technology in printed circuit boards (PCBs) using lead-free solder paste with SAC alloy (Sn-Ag-Cu) in order to attend new environmental requirements for the electronics assembly. Initially it was designed a PCB test with three different THCs (Through Hole Component) and three different SMD (Surface Mount Device) packages in order to reproduce a commercial board. It was generated two different diameters of holes to insert the THCs, aiming to study the solder fill variation in PTH. An equation was proposed for calculating the volume of solder paste to be printed over the holes in the assembly process. From this equation it was calculated the dimensions of the holes of the stencil. The printing parameters were optimized according to the variation of pressure and speed of the squeegee. Two reflow curves were used in the process, a conventional one and an optimized one to determine the variations in the generation of soldering defects. The printed solder paste volume was higher than projected, which resulted in solder excess, causing defects in most of the components, such as excess solder in the fillet and solder flux residues in PTHs. Such defects were significant for all THCs, showing that the excess paste that was printed caused critical defects for all combinations of variables. Regarding that all samples were above the reflow minimum acceptable volume of 75% coverage of PTH. The SMDs solders were acceptable, with only few cases of solder flux excess. The Pin in Paste process was observed as a good option to replace the wave soldering thermal process for mixed PCBs. Further studies should be conducted to generate a reliable model of PCB and stencil design.

Sabemos que em placas de circuito impresso montadas com tecnologia SMD Surface Mount Device podem ocorrer problemas com a migração eletroquímica. O fenômeno aparece principalmente porque os novos encapsulamentos possuem terminais com espaçamentos muito próximos. A migração eletroquímica pode tornar-se um potencial problema no processo de soldagem eletrônica quando é utilizado a tecnologia Lead Free na montagem das placas. O processo de migração eletroquímica ocorre quando temos metal, isolante e metal, em ambiente de alta umidade e sobre polarização elétrica, o metal deixa a posição inicial em forma de íon e se redeposita sobre o isolante. Em uma placa de circuito impresso, dois terminais adjacentes podem tornar-se eletrodos, dessa forma as dendritas crescem do cátodo para o ânodo. Podem aparecer diferentes morfologias com diferentes elementos envolvidos no processo de migração, dependendo da composição da pasta de solda ou acabamento da placa de circuito impresso. Uma estrutura do tipo pente comb feita sobre laminado FR4 foi utilizada nos experimentos. A distância entre as trilhas foram de 102 e 254 mícrons para simular uma distancia real dos terminais dos dispositivos. Os fatores considerados durante os experimentos foram: A distancia entre os terminais na estrutura (102 ou 254 mícrons), tensão aplicada (2 ou 3 V). Foi observado que a pasta de solda e o acabamento final não influenciam no processo de migração eletroquímica. O Estanho foi o principal metal que migrou. Todos os resultados obtidos nesse estudo concordam com a literatura. It is well known that in printed circuits boards assembled by SMT technology may occur Electrochemical migration (ECM). This phenomenon appears mainly because the new packaging has the terminals very close. Also the Electrochemical migration may become a potential reliability problem in electronic soldering when lead free technology is used in soldering electronic devices. Electrochemical migration is an electrochemical process where metal on an insulating material, in a humid environment and under an applied electric field, leaves its initial location in ionic form and redeposit. In a PCB two adjacents terminal may behave as electrodes so the dendrites grow from cathode to anode. It can show different morphologies with the different migration elements involved depending on the solder paste composition or PCB surface finishing. A structure with a comb shape printed on FR4 substrate was used in the experiments. The distance between the fingers in the structure was 102 or 254 microns, in order to simulate a real distance between dispositive terminals. The factors considered during the experiments were surface finishing (ENIG or HASL), solder paste composition, distance between terminals (102 or 254 microns) and applied voltage (2 or 3 V). It was observed that the solder paste and the surface finishing dont influence the ECM process. Tin was the main metal that migrates. All the results obtained in these study agrees with the literature.

Nos últimos anos tem se dado uma evolução muito grande na área industrial de MEMS, e esta se caracteriza por desenvolvimento dos processos de microeletrônica para diminuição de custos e a própria integração de diferentes sistemas que passam a ser muito confiáveis em diferentes aplicações. Motivado pelas tendências de elevar os conhecimentos de processos para obtenção de MEMS, foi proposto um trabalho de corrosão anisotrópica de silício utilizando solução de hidróxido de potássio (KOH) junto com a adição de sais metálicos. O objetivo é comprovar o efeito de compensação cinética da reação e a influência destes metais no caráter anisotrópico e nas taxas de corrosão. Foi analisado o efeito de zinco, alumínio e cobre, e puderam ser observadas alterações nas taxas de corrosão e, conseqüentemente, nas geometrias resultantes no corpo de silício. Com a caracterização dessas mudanças comprovou-se suposições anteriores de que o efeito de compensação cinética encontrado na síntese direta de dimetilclorosilana (DMDCS) e silício, também ocorreria para a corrosão de silício em KOH, pois esses processos obedecem a lei de Arrhenius e tem um comportamento anisotrópico em silício. Os resultados obtidos, graças a um grande detalhamento de todos as etapas de processo e caracterização voltada à tecnologia do silício, comprovaram as suposições teóricas e mostraram como pode ser modificada a taxa de corrosão na anisotropia com a adição de metais em solução. In the past few years it has been a huge evolution in industrial area of MEMS, and this one is characterized by the development of the processes of microelectronic to reduce cost and to integrate different systems that become very trustful in different applications. It is being done a work in silicon anisotropic etching in Potassium hydroxide (KOH) added with metallic impurities aiming the study of the kinetic compensation effect and the influence of these metals in the anisotropy and in etch rates. It was analyzed the effects of Zinc, Aluminum and Cupper and it could be observed changes in etch rates, and, consequently, in the resulting geometries. After characterization of theses changes it was proved that previous suppositions that the kinetic compensation effects found in direct synthesis of dimethyldichorosilane (DMDCS) and silicon would also occur for silicon anisotropic etching in KOH, due to both reactions follows Arrhenius law and are anisotropic reactions. The obtained results proved the theory suppositions that the etch rate and anisotropy can be modified when adding metals to the solution.