J. Bausells

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.

TMAH/IPA anisotropic etching characteristics

Abstract The main advantages of tetramethyl ammonium hydroxide (TMAH)-based solutions is their full compatibility with IC technologies. In this work a new etching system of TMAH/IPA (isopropyl alcohol) is suggested. The influence of the addition of IPA to TMAH solutions on their etching characteristics is presented. The etch rates of (100) oriented silicon crystal planes decreases linearly with decreasing the IPA concentration for all experimental conditions. Etch rates for TMAH/IPA solutions (10–45 μm/h) are lower than those for KOH/IPA solutions (30–100 μm/h) but they are still applicable for micromachining purposes. The etch rates of most commonly used masking layers in IC technologies has been investigated. Low-pressure chemical vapour deposited (LPCVD) Si 3 N 4 and thermally-grown SiO 2 have excellent stability in TMAH/IPA solutions. Low-temperature deposited silicon oxide (LTO) etch rates are low enough to be used as masking layers in anisotropic etching processes. Quality of etched surfaces is mainly dependent on TMAH wt.% concentration. For pure TMAH solutions the observed undercutting ratio (5–7) is much larger than in KOH case. The addition of IPA to TMAH solutions reduce the undercutting by a factor of more than 2 and leads to smoother surfaces of sidewalls etched planes. We have studied briefly the p ++ etch-stop characteristics by means of heavily boron-implanted layers. The etching selectivity with respect to high boron-doped silicon is improved in TMAH/IPA solutions. Implant doses used in our experiments (2 × 10 16 ion/cm 2 ) stands the etching during more than 90 min.

A simple REFET for pH detection in differential mode

Abstract A simple reference field-effect transistor (REFET) for pH detection in differential mode measurements is described. The device is based on a pH-insensitive polymeric PVC membrane cast on the gate insulator of an ion-sensitive field-effect transistor (ISFET) device, that has been previously silylated. The REFET shows low pH sensitivity (1.8 mV/pH) and is only slightly affected by the concentration of Na + and K + . The pH response of a combined solid-state device consisting on a pH ISFET (of 45 mV/pH sensitivity), the REFET and a quasi-reference electrode (QRE) (Pt) is 43.7 mV/pH from pH 2 to 9.

Etching front control of strips for corner compensation

Abstract In this paper a scheme of convex corner compensation for etching in aqueous KOH using only strips is presented. Turns and branches are used to control the etching front on the strip surface for better compensation quality. The design rules as well as the effects of turning and branching on the effective compensation length are presented. The etching fronts on the bottom are investigated by experiment. Applications of the new structures are shown.

New technology for multi-sensor silicon needles for biomedical applications

Abstract A multi-sensor silicon needle including two ion-sensitive field effect transistor (ISFET) sensors, a platinum pseudo-reference electrode (Pt) and a temperature sensor has been fabricated by using a CMOS-compatible technology and silicon micromachining. This paper presents a summary of the fabrication process and results of the device characterisation. The feasibility of the fabrication technology has been demonstrated and all devices have operated satisfactorily, with a response showing good sensitivity and linearity. The multi-sensor has been developed for the detection of myocardial ischemia during cardiac surgery.

Impedimetric immunosensor for human serum albumin detection on a direct aldehyde-functionalized silicon nitride surface.

In this work we report the fabrication and characterization of a label-free impedimetric immunosensor based on a silicon nitride (Si(3)N(4)) surface for the specific detection of human serum albumin (HSA) proteins. Silicon nitride provides several advantages compared with other materials commonly used, such as gold, and in particular in solid-state physics for electronic-based biosensors. However, few Si(3)N(4)-based biosensors have been developed; the lack of an efficient and direct protocol for the integration of biological elements with silicon-based substrates is still one of its the main drawbacks. Here, we use a direct functionalization method for the direct covalent binding of monoclonal anti-HSA antibodies on an aldehyde-functionalized Si-p/SiO(2)/Si(3)N(4) structure. This methodology, in contrast with most of the protocols reported in literature, requires less chemical reagents, it is less time-consuming and it does not need any chemical activation. The detection capability of the immunosensor was tested by performing non-faradaic electrochemical impedance spectroscopy (EIS) measurements for the specific detection of HSA proteins. Protein concentrations within the linear range of 10(-13)-10(-7) M were detected, showing a sensitivity of 0.128 Ω μM(-1) and a limit of detection of 10(-14) M. The specificity of the sensor was also addressed by studying the interferences with a similar protein, bovine serum albumin. The results obtained show that the antibodies were efficiently immobilized and the proteins detected specifically, thus, establishing the basis and the potential applicability of the developed silicon nitride-based immunosensor for the detection of proteins in real and more complex samples.

Hydrogen-selective microelectrodes based on silicon needles

The fabrication of hydrogen-selective microelectrodes on silicon needle-shaped substrates is described. The microelectrodes are based on an ion-selective poly(vinyl chloride) (PVC) membrane with an intrinsically conducting polymer (polypyrrole (PPy)) solid contact layer. The polypyrrole is prepared with the dopant anion cobaltbis(dicarbollide) [3,3 0 -Co(1,2-C2B9H11)2] � , which gives a high stability to the polymer layer. The performance of the resulting solid-contact ion-selective microelectrodes (SCISME) is investigated by using potentiometric measurement and electrochemical impedance spectrometry. The feasibility of the fabrication technology is demonstrated and the devices operate satisfactorily, with a response showing good sensitivity and selectivity against common interfering cations in background solutions. The SCISME has been developed for organ monitoring during cardiac surgery or during transportation for transplants. # 2003 Elsevier Science B.V. All rights reserved.

Application of a new phosphadithiamacrocycle to ClO−4-selective CHEMFET and ion-selective electrode devices

Abstract A new dithiamacrocycle incorporating a phosphine donor group (I) has been synthesized and tested as neutral carrier in PVC membranes. ClO−4-selective CHEMFET and ISE devices based on these plasticized membranes have been developed. Both devices have shown Nernstian response and a wide working pH range. Most common anions, except BF−4, do not cause interferences. The response properties and selectivity found for perchlorate ions compare favourably with the commercial and conventional ClO−4 electrodes based on quaternary ammonium ion-exchange sites. This is probably the first example of perchlorate selective CHEMFETs.

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.

Analysis of the effects of constant‐current Fowler–Nordheim‐tunneling injection with charge trapping inside the potential barrier

Charge trapping and the generation of interface traps in thermally grown SiO2 and its interface with silicon, produced by Fowler–Nordheim tunneling injection at low temperatures from highly doped Si substrates, have been investigated. The results that can be obtained with the constant‐current‐injection method, when a moderate amount of charge is trapped inside the potential barrier, have been analyzed. This has afforded information about the position of the charge trapped in the oxide. No increase in the interface‐trap density has been produced immediately after injection at 77 K, but, as the temperature is raised after injection, the growing of a peak of interface states has been observed. This phenomenon had been reported to be produced as a consequence of a previous hole trapping but, in this case, this intermediate stage of positive‐charge building has not been observed. This effect is discussed, taking into account published models.