An Investigation of Varying Laser Trim Pattern Characteristics on the Performance and Reliability of Discrete Resistive Components

Alafogianni, Maria (2018) An Investigation of Varying Laser Trim Pattern Characteristics on the Performance and Reliability of Discrete Resistive Components. Doctoral thesis, Northumbria University.

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Abstract

This project investigates the effects of varying laser trimming geometries on key performance parameters of bar shape thin film resistors. Previous studies related to the effect of the trim geometry and in-service performance are quite limited. As a result, industries are still relying on basic patterns and this research aims to focus on the development of new trim pattern models in order to study the effect of the geometries on the accuracy and long-term stability of the material. In addition to this, the application of the model to thin and thick film materials improves the performance of the resistors. Laser trimming is an essential process in the manufacture of precision thin film resistors due to the fact that it is impossible to deposit batches of the product with resistance tolerances better than ~10%. However, it is quite an expensive process which significantly increases the production costs of integrated circuits. Several popular trim patterns including the plunge cut, double plunge cut, L-cut and scan cut as well as novel patterns; the curved L-cut, the angled L-cut, semi-circle and elliptical cut were modelled and tested experimentally. The effect of these geometries on target resistance value, heat affected zone (HAZ) sensitivity and temperature coefficient of resistance (TCR) were analysed. The HAZ sensitivity is an important parameter that characterises the aging performance of the film caused by the high energy required to vaporize the material in the cut, while the TCR indicates how the resistors behave under cold and high operating temperatures. The modelling process of the different laser trim patterns included numerical simulations to predict the performance of laser–trimmed resistors taking into account the HAZ around the trim and computer-aided models. Conductive paper and thin films were used for the experiments in order to emulate the resistors and verify the theoretical results from the simulations. It is found that variation in trimming shape and length has a direct effect on the performance of resistors giving significant increases in the TCR of the resistor with values of 17.22 ppm/°C for the curved L-cut and 8.52 ppm/°C for the semi-circle. The resistance gain, the power coefficient of resistance (PCR) and the trimming time needed for the cuts were also investigated for clarification of the characteristics of each pattern and it is shown that the semi-circle pattern offers the larger resistance increase in relation to the other patterns.

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