NOVEL ELECTROSTATIC DISCHARGE PROTECTION STRUCTURE FOR A MONOLITHIC GAS SENSOR SYSTEMS-ON-A-CHIP ** J. A. Salcedo, J. J. Liou, M. Y. Afridi*, A. R. Hefner* Department of Electrical and Computer Engineering University of Central Florida Orlando, FL 32816, USA *Semiconductor Electronics Division National Institute of Standards and Technology Gaithersburg, MD 20899, USA ** Contribution of the National Institute of Standards and Technology; not subject to copyright. ABSTRACT A new on-chip Electrostatic Discharge (ESD) protection scheme is demonstrated for MicroElectroMechanical Systems (MEMS)- based Embedded Sensor (ES) System-on-a-Chip (SoC). The ESD protection scheme includes ground-referenced protection cells implemented with novel multifinger thyristor-type devices for 1) the Input/Output (I/O) protection, 2) the power supply clamp, and 3) the protection at the internal sensors’ electrodes. The I-V characteristics of the thyristor-type protection cells are adjusted for providing an optimum ESD protection per unit area. Transmission Line Pulsing (TLP) measurements and ESD testing show superb high conductance on-state I-V characteristics with no latch-up problem when thyristor-type devices are subjected to an ESD event, while very low leakage current is obtained at the SoC operating voltage. 1. INTRODUCTION The MEMS (MicroElectroMechanical Systems) microhotplate-based chemical gas sensor is an emerging CMOS (Complementary Metal Oxide Semiconductor)-based technology that has cost and performance advantages over the existing commercial gas sensing technologies [1]. The microhotplate gas sensor platform, heater power amplifier, signal conditioning, and control circuitry have been formulated as a Virtual Component (VC) conforming to the SoC block-based design methodology [2]. The SoC design methodology is necessary due to the complexity of large digital systems and facilitates functional block design reuse. Formulating the gas sensor as a VC enables incorporation into CAD (Computer Aided Design) libraries and facilitates the development of single chip gas sensing and classification solutions. The implementation of ES-VCs requires the use of a standard digital interface and standard DFT (Design for Test) functionality. A major concern in fabrication, packaging and assembly and, even during normal handling and testing of CMOS ICs is the Electrostatic Discharge (ESD) induced damages. The ESD protection usually consists of protection devices for each Input/Output (I/O) pad and a power supply clamp. Because of the chip area constraints, sensor micromachining process, and presence of the ES, the design of ESD protection for the gas sensor SoC is more stringent than that for typical VLSI circuits. The gas sensor SoC requires ESD protection at the peripheral I/O pads and at the internal electrodes of the ES. Fig. 1 illustrates such an ESD protection scheme. Bidirectional ground-referenced ESD protection elements are shown connected to the I/O pads and sensor electrodes. Fig. 2 depicts a Scanning Electron Microscope (SEM) micrograph of a microhotplate-based gas-sensor showing in more detail the internal sensor electrodes with ESD protection included. Sensor / Heater Selector Gas Sensing Film Heater Current Driver MOS Switch - + Digital Core Circuit Ri I/O Heater & Temp. Sensor Sensor Electrodes Protection I/O ESD Protection I/O ESD Protection Ri I/O I/O ESD Protection Supply Clamp Vdd Sensor 1 Sensor n Fig. 1. ESD protection scheme for the gas sensor SoC. Electrode Pad 2 Pit Electrode Pad 1 Heater (+) Heater (-) ESD Cell ESD Cell Sensing Electrodes Suspended Membrane Fig. 2. An SEM micrograph of microhotplate showing sensor electrodes and ESD protection points. In this paper, a novel ESD protection scheme is integrated in the gas sensor SoC. The microhotplate gas sensors are produced using a standard 1.5 µm CMOS foundry process as 416 0-7803-8834-8/05/$20.00 ©2005 IEEE.