An Integrated ISFETs Instrumentation System in Standard CMOS Technology

Published versio

Heterogeneously integrated impedance measuring system with disposable thin-film electrodes

We propose a novel integrated impedance measurement system with disposable thin-film electrodes. Most modern CMOS-based biosensors use on-chip electrodes to interface between the electronics and biosamples, which forces disposal of the CMOS chip after a few measurements, since most biological reactions are non-reversible. The sensor performance is also limited by the design of on-chip electrodes due to the physical dimensions and the CMOS design rules restrictions. In this work, we extract the electrodes from the silicon chip for relocation onto a low-cost, disposable substrate. This enables reusability of the high-performance CMOS chip, at the same time providing a low-cost route for manufacture of the active thin-film electrodes using large-area processing. The use of disposable thin-film chip also enables customised designed electrodes for different applications, such as extra high sensitivity concentration sensors. In this work, DNA concentration measurements are performed, and it shows a doubling of sensitivity over the previously reported system.This work is partially supported by Isaac Newton Trust.This is the accepted manuscript of a paper published in Sensors and Actuators B (Ma H, Li J, Cheng X, Nathan A, Sensors and Actuators B 2015, 21, 77–82, doi:10.1016/j.snb.2015.01.044)

Robust low power CMOS methodologies for ISFETs instrumentation

I have developed a robust design methodology in a 0.18 [Mu]m commercial CMOS process to circumvent the performance issues of the integrated Ions Sensitive Field Effect Transistor (ISFET) for pH detection. In circuit design, I have developed frequency domain signal processing, which transforms pH information into a frequency modulated signal. The frequency modulated signal is subsequently digitized and encoded into a bit-stream of data. The architecture of the instrumentation system consists of a) A novel front-end averaging amplifier to interface an array of ISFETs for converting pH into a voltage signal, b) A high linear voltage controlled oscillator for converting the voltage signal into a frequency modulated signal, and c) Digital gates for digitizing and differentiating the frequency modulated signal into an output bit-stream. The output bit stream is indistinguishable to a 1st order sigma delta modulation, whose noise floor is shaped by +20dB/decade. The fabricated instrumentation system has a dimension of 1565 [Mu] m 1565 [Mu] m. The chip responds linearly to the pH in a chemical solution and produces a digital output, with up to an 8-bit accuracy. Most importantly, the fabricated chips do not need any post-CMOS processing for neutralizing any trapped-charged effect, which can modulate on-chip ISFETs’ threshold voltages into atypical values. As compared to other ISFET-related works in the literature, the instrumentation system proposed in this thesis can cope with the mismatched ISFETs on chip for analogue-to-digital conversions. The design methodology is thus very accurate and robust for chemical sensing

Circuito condicionador de ultrabaixo consumo para sensor ISFET

Dissertação (mestrado) - Universidade Federal de Santa Catarina, Centro Tecnológico, Programa de Pós-Graduação em Engenharia Elétrica, Florianópolis, 2015Este trabalho apresentou o projeto, simulação e medição de dois circuitos condicionadores, empregando componentes discretos, para ISFET (Transistor de Efeito de Campo Sensível a Íons). O primeiro circuito condicionador desenvolvido foi o seguidor de fonte-e-dreno, em que a operação do ISFET em modo CVCC (Tensão Constante e Corrente Constante) permitiu a leitura do pH no terminal de fonte do ISFET com uma linearidade igual a1, obtida pelo coeficiente de determinação R2. Os resultados experimentais indicaram uma faixa de variação de 1,404 V aplicando um potencial de referência de -0,290 V a 0,136 V e uma responsividade igual a 3,3 V/V. O segundo circuito condicionador apresentado, denominado neste trabalho por pHCO, foi desenvolvido como alternativa ao condicionamento do sinal do elemento sensor via medição da frequência de pulsos na saída do circuito. Neste caso, a informação do pH está representada digitalmente e codificada no domínio do tempo. Isto eliminou o uso de ADCs no projeto,reduzindo o consumo elétrico geral e minimizando a instrumentação do circuito condicionador. Ademais, a representação digital efetuada diretamente pelo circuito condicionador proporciona uma maior integridade da informação, melhor interfaceamento com blocos digitais além de permitir que a informação seja processada remotamente por um dispositivo microcontrolador. Os resultados experimentais obtidos com este circuito indicaram uma faixa de variação igual a 9 kHz, um coeficiente de determinação R2 igual a 0,993 e uma responsividade igual a 9 kHz/pH. Complementarmente, uma versão integrada do circuito condicionador pHCO foi projetada em tecnologia IBM 0,18 mm, utilizando sete níveis de metal e modelo de transistor BSIM3v3. Os resultados de simulação mostraram que o circuito condicionador integrado consome apenas 114 mW de potência elétrica e é competitivo com demais trabalhos de relevância da área. A motivação deste trabalho e do projeto do circuito condicionador integrado está relacionada na medição da glicemia para diagnóstico e tratamento da diabetes melito via redes corporais sem fio (WBANs).Abstract: This work presented the design, the simulation and the measurementof two conditioning circuits for the sensor ISFET (Ion Sensitive Field EffectTransistor). The first conditioning circuit designed was based in the so-calledsource-drain follower configuration. The experimental results indicated a coefficientof determination R2 equal to 1 measured in a span equal to 1.404 V.The voltage range applied to the reference potential was varied from -0.290V to 0.136 V and the sensitivity obtained was equal to 3.3 V/V.The second conditioning circuit, called in this work by pHCO, convertsthe pH input in a digital output representation whose information isencoded in the frequency domain. This approach has enabled the minimizationof the analog front-end instrumentation providing reduction of the overallpower consumption. Furthermore, the digital output representation providesbetter information integrity rather than interfacing with digital blocks. Theexperimental results obtained with this circuit showed a span equal to 9 kHz,a coefficient of determination R2 equal to 0.993 and a sensitivity equal to 9kHz/pH.In addition, an integrated version of the pHCO conditioning circuitwas designed in the IBM 0.18 mm technology, using seven metal layers andthe BSIM3v3 transistor model. The simulation results showed that the circuituses only 114 mW of electric power and is competitive with other relevantworks in the area. The motivation of this work and the design of the integratedconditioning circuit is related to the treatment of diabetes mellitus via wirelessbody area networks (WBANs)