A Study on Interface Circuits for CMOS-Integrable Bio-Chemical Sensors

Abstract

본 연구에서는 CMOS와 집적 가능한 생화학센서의 신호검출에 적합하도록 여러가지 인터페이스회로를 고안하고 CMOS 상에서 IC로 검증하였다. 많은 경우에 있어서, 집적 가능한 용량성 센서의 경우에는 MEMS 공정을 통해서 제작이 되고, 집적 가능한 저항성 센서의 경우에는 나노-와이어를 활용해서 제작이 된다. 이처럼, 집적을 통한 센서의 고밀도화 가능성을 십분 활용하기 위해서는 어레이 구조가 매력적이라 할 수 있다. 용량성 센서의 신호를 검출하기 위해서는 전하증폭기를 활용한 일련의 방법들을 제안하였고, 저항성 센서의 신호를 검출하기 위해서는 전류-모드 검출을 활용한 일련의 방법들을 제안하였다. 또한 두 가지 모두의 경우에 있어서, 어레이 구조에서 발생할 수 있는 센서 초기값의 넓은 산포를 보상할 수 있도록 동적 영역을 확장시키는 방법을 구현하고자 하였다.

용량성 센서의 검출을 위한 첫 번째 연구에서는, 함께 집적될 차동 용량 센서에 적합하도록 반복적인 전하 적분을 활용함으로써, 복잡도는 낮추면서도 정확도는 높인 CMOS 검출 회로를 제안하였다. 두 번째 연구에서는, 적분 캐패시터를 방전시킬 때 잔여 전하를 보존함으로써, 정확성이 더욱 향상된 검출 회로를 제안하였다. 세 번째 연구에서는, 낮은 복잡도는 유지하면서도 해상도를 향상시키기 위해 델타-시그마 변조 기법을 활용하면서, 캐패시터의 초기값을 보상함으로써 검출 가능한 동적 영역의 손실을 줄이는 회로를 제안하였다.

저항성 센서의 검출을 위한 첫 번째 연구에서는, 배열 구조의 CNT 센서에 적합하도록 동적 영역 확장 기능 및 전류-모드 델타 시그마 변조 기법을 활용한 검출 회로를 제안하였고, 두 번째 연구에서는 TDC를 활용한 회로를 제안함과 동시에 CNT-SnO2 센서망을 구현된 CMOS 회로상에 함께 집적하여, 통합된 시스템으로 NH3 가스 검출을 성공적으로 시행하였다.

In this study, several methods were invented and verified by the silicon for the readout of CMOS-integrable bio-chemical sensors. These sensors are generally capacitive or resistive. In many cases, integrable capacitor-type sensors are in the form of micro-electro-mechanical system (MEMS) sensors and integrable resistor-type sensors are in the form of various nanowire sensors. To get benefit from high-density CMOS-integration and to characterize the sensing environment sufficiently, these sensors are made into array. For the detecting of signals from capacitor-type sensors, the scheme using charge amplifier has been invented and developed. For the detecting of signals from resistor-type sensors, schemes using current-mode detection have been invented and developed. In any type of sensors, schemes to extend dynamic range or methods to compensate for the variation of sensors initial values in array structure are considered and developed.

In the first study for the readout of capacitive sensors, a low-complexity CMOS circuit for reading out monolithically integrated differential capacitive sensors has been proposed. It directly converts the differential capacitance of a MEMS sensing device to a frequency by accumulating the charges produced by repeated charge integration and charge conservation. A prototype chip was designed and fabricated in 0.35μm CMOS technology. Experimental results show that differential capacitance is linearly converted to output frequency. In the second study for the readout of capacitive sensors, a more accurate capacitance-to-frequency converter has been presented, which produces a single pulse stream in a wide range of frequencies. This circuit saves residual charges and accumulates them when discharging an integrator capacitor. Implemented in 0.35μm CMOS technology, the proposed circuit improves the accuracy from about 6% to 0.13%. In the third study for the readout of capacitive sensors, a low-complexity interface circuit for capacitive sensors has been presented which are integrated into sensor microsystems. To reduce hardware cost while keeping high resolution, a first-order delta-sigma modulator (DSM), which balances the charge from the capacitive difference between the sense and reference capacitors with the charge from a fixed-quantity capacitor, is employed. A charge-mode digital-to-analog converter and a successive approximation register are utilized to automatically calibrate the zero point of the interface circuit, which may shift further than a dynamic range. A prototype circuit fabricated in a 0.35μm CMOS process. Its DSM operates at a sampling frequency of 1MS/s with an oversampling ratio of 128. This circuit can read a capacitive difference from -0.5pF to +0.5pF with a 0.49fF resolution. Capacitive offset that causes the zero point to shift can be cancelled in the range from -2pF to +2pF with a 31.25fF resolution.

In the first study for the readout of resistive sensors, a sensor readout integrated circuit for the carbon nanotube (CNT) bio-sensor array has been presented. The heart of the proposed circuit is the low-power current-input continuous-time ΔΣ modulator that is capable of dynamic range extension. Experimental results show that the prototype chip, designed and fabricated in 0.18μm CMOS process, achieves a dynamic range of 87.746dB and has a readout rate of 160kHz, which guarantees 1k sample/s per each sensor. It consumes 8.94μW/cell considering the 16x10 sensors and its core area is 0.085mm2. In the second study for the readout of resistive sensors, a tin-oxide-decorated CNT network gas sensor system has been implemented on a single die. The deposition of metallic tin on the CNT networks, its subsequent oxidation in air, and the improvement of the lifetime of the sensors have also been shown. The fabricated array of CNT sensors contains 128 sensor cells for added redundancy and increased accuracy. The read-out integrated circuit (ROIC) combines coarse and fine time-to-digital converters to extend its resolution in a power-efficient way. The ROIC is fabricated using a 0.35µm CMOS process, and the whole sensor system consumes 30mA at 5V. The sensor system was successfully tested in the detection of ammonia gas at elevated temperatures.