A. Merlos

Ion-sensitive field-effect transistors fabricated in a commercial CMOS technology

Abstract The fabrication of pH-sensitive ISFET devices in an unmodified two-metal commercial CMOS technology (1.0 m from Atmel-ES2) is reported. The ISFET devices have a gate structure compatible with the CMOS process, with an electrically floating electrode consisting on polysilicon plus the two metals. The passivation oxynitride layer acts as the pH-sensitive material in contact with the liquid solution. The devices have shown good operating characteristics, with a 47 mV/pH response. The use of a commercial CMOS process allows the straightforward integration of signal-processing circuitry. An ISFET amplifier circuit has been integrated with the ISFET sensors.

Microtechnologies for PH ISFET chemical sensors

A review of different microtechnologies for the fabrication of pH ion sensitive field effect transistor (ISFET) sensors is presented. Integrated ISFETs are of interest due to the advantages of low price, fast response and small dimensions that they present compared to ISE electrodes. ISFETs can be also applied to the detection of different ions, using the proper sensitive membranes. A lot of work has been done during the last few decades to obtain commercial devices, and many technologies and structures can be found in the literature. In this paper, both front-side and back-side contacted devices are studied, in order to determine the compatibility of different processes, devices and materials with standard CMOS technologies, which seems to be a goal for present and future applications.

Three-dimensional interdigitated electrode array as a transducer for label-free biosensors.

A new transducer for biosensor applications has been developed based on a three-dimensional interdigitated electrode array (IDEA) with electrode digits separated by an insulating barrier. Binding of molecules to a chemically modified surface of the transducer induces important changes in conductivity between the electrodes. Three-dimensional sensor shows considerable improvement compared with a standard planar IDEA design. The potential of the developed device as a sensor transducer to detect immunochemical and enzymatic reactions, as well as DNA hybridization events is demonstrated. The immunosensor allows direct detection of the antibiotic sulfapyridine and shows the IC(50) parameter value of 5.6 microgL(-1) in a buffer. Immunochemical determination occurs under competitive configurations and without the use of any label. Each modified sensor is of a single use. Nevertheless, biochemical reagents can be easily cleaned off the sensor surface for its reuse. Layer-by-layer method of used to deposit polyethyleneimine and glucose oxidase showed that the sensor is also highly effective for detecting single and multilayered molecular assemblies.

A study of the undercutting characteristics in the TMAH-IPA system

A new etching system of tetramethyl ammonium hydroxide-2-propanol (TMAH-IPA) is suggested. The undercutting ratio for TMAH at 80 degrees C is about 7, much higher than for KOH etchants. The addition of IPA in the TMAH system maintains the main features of TMAH and reduces the undercutting ratio by a factor of 2-3.

Optimized technology for the fabrication of piezoresistive pressure sensors

In this work a technology for the fabrication of piezoresistive pressure sensors is presented, based on the use of silicon BESOI (bonded and etch-back silicon on insulator) wafers. The main purpose of the proposed technology is the optimization of the thin silicon diaphragm definition process that is one of the most critical steps in the fabrication of silicon pressure sensors. The buried silicon oxide layer of the BESOI wafers is used as an automatic etch stop of the silicon anisotropic etching, making it possible to obtain very precise control of the sensor diaphragm thickness. In addition, the type and thickness of the layers acting as masking materials on the backside of the wafers have been optimized in order to get a high-yield process. The experimental results obtained when using the proposed technology are presented and discussed.

Multilayer ISFET membranes for microsystems applications

The fabrication of microsystems in which chemical measurements play a role make the on-chip combination of chemical sensors and signal processing circuitry highly desirable. Chemical sensors use sensitive layers which are not a part of MOS devices and in this sense, the determination of the best sensors and technological processes to achieve good integrated smart sensors is very important. In this paper a study of compatibility of ISFET chemical sensors and CMOS circuitry is presented for application to biomedical microsystems.

Study of integrated RF passive components performed using CMOS and Si micromachining technologies

Silicon integrated RF passive components performed using standard CMOS processes and silicon micromachining post-processing are studied. The results indicate that the removal of the Si substrate under the components using micromachining technology is a very suitable procedure to improve their electrical performance. The self-resonant frequency and maximum quality factor of the inductors are increased by a factor about three. Similar results are obtained for interdigitized capacitors.

High-Quality Factor Electrolyte Insulator Silicon Capacitor for Wireless Chemical Sensing

This letter reports on the design, fabrication, and characterization of a high Q electrolyte insulator silicon (EIS) capacitive structure and its application to LC wireless chemical sensors. LC sensors are based on the variation of inductance or capacitance in response to changes in the magnitude of interest. In this new type of LC sensor, the EIS structure is used as a pH-to-capacitance transductor. A double electrode scheme, one for stable dc potential and another for low ac series resistance, has been adopted in order to obtain sufficient Q at the resonator. An almost linear response and a sensitivity of 1% frequency change per pH unit have been achieved

Electrochemical etching of porous silicon sacrificial layers for micromachining applications

Using porous silicon (PS) as a thick sacrificial layer, free-standing structures at a large distance from the bulk can be obtained. The very high specific surface of PS makes its removal using dilute alkaline solutions possible. In this work different technological conditions, such as adding ethanol or ultrasonic stirring, are studied in order to optimize the removal of PS using a one-step process with 0.1% KOH solution. Finally, cathodic polarization of the sample during the removal process is proposed as a method to enhance and improve the etching of PS whilst avoiding possible oxide formation. This electrochemical etching of a PS sacrificial layer allows good quality free-standing polysilicon structures featuring a smooth substrate to be obtained.

Mechanical sensors integrated in a commercial CMOS technology

Abstract Pressure sensors and aceelerometer structures have been fabricated in a commercial CMOS foundry technology (1.0 µm from Atmel-ES2) using a post-processing for back-side wafer micromachining. The overall technology (CMOS plus post-processing) can be used for integrated sensor system design through a specific design kit in the standard foundry design environment. Fabrication is then performed so that postprocessing is transparent to the user.