Failure Analysis of Thermally Shocked NiCr Films on Mn-Ni-Co Spinel Oxide Substrates
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Failure Analysis of Thermally Shocked NiCr Films on Mn-Ni-Co Spinel Oxide Substrates Min-Seok Jeon, Jun-Kwang Song, Eui-Jong Lee, Yong-Nam Kim, Hyun-Gyu Shin and Hee-Soo Lee Material Testing Team, Korea Testing Laboratory, 222-13 Guro-dong, Guro-gu, Seoul 152-848, KOREA ABSTRACT NiCr films were thermally evaporated on the Mn-Ni-Co-O thick-film substrates. The NiCr/Mn-Ni-Co-O bi-layer systems were tested in a thermal shock chamber with three temperature differences of 150, 175 and 200°C. The systems were considered to have failed when the sheet resistance of NiCr films changed by 30% relative to an initial value. As the cyclic repetition of thermal shock increased, the sheet resistance of NiCr coatings increased. The Coffin-Manson equation was applied to the failure mechanism of cracking of NiCr coatings and the SEM observation of cracks and delamination in NiCr coatings due to thermal cycling agreed well with the failure mechanism. INTRODUCTION NiCr films are applied for humidity sensor, thin film resistor or infrared sensor [1-3]. In our case, the NiCr films represent an infrared absorbing coatings or electrodes because they have a high emissivity of 0.97 comparable to gold black and low cost in fabrication. Tailored Mn-Ni-Co-O thick film thermistor is one of promising candidates for commercial infrared sensing element. The output of thermistor is large in comparison with other infrared sensing element and an infrared sensor using it is able to operate at temperature of up 150°C. NiCr films absorb an infrared and transport its heat to the NTC (negative temperature coefficient) thermistor. The resistance of Mn-Ni-Co oxide decreases with its temperature and this change of the resistance is converted in the form of output voltage for sensing a infrared from objects. NiCr/Mn-Ni-Co-O bi-layer sensing element experiences periodical temperature changes during long term operation. In addressing the issue of ceramic sensor reliability, an understanding of the failure mechanisms is required. Accelerated degradation testing is being used frequently to predict the failure and reliability of various materials [4]. In addition the temperature cycling test is extensively used in microelectronic industry to qualify new products [5]. Therefore the assessment of the durability and characteristics of the sensing element are needed by an accelerated degradation testing (ADT) of thermal cyclic method. One of important aspect of maintaining the stability of NiCr/Mn-Ni-Co oxide sensing element is the control of mechanical stresses [6,7]. These stresses are able to be generated and accumulated in the NiCr films by an operation condition of periodical temperature changes. Therefore the thermo-mechanical stability of NiCr films is required to be evaluated through the testing in an accelerated condition. Thermal shock was given to NiCr/Mn-Ni-Co oxide thick films and their characteristics were compared before and after the accelerated testing. This paper is aimed at analyzing failure mechanisms of NiCr/Mn-Ni-Co oxide sensing element
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