Effects of varying laser trimming geometries on thin film resistors

Purpose - This paper studies the effects of varying laser trim patterns on several performance parameters of thin film resistors such as the temperature coefficient of resistance (TCR) and target resistance value. Design/methodology/approach - The benefits and limitations of basic trim patterns are taken into consideration and the plunge cut, double plunge cut and the curved L-cut were selected to be modelled and tested experimentally. A computer simulation of the laser trim patterns has been developed for the modelling process of the resistors. The influence of the trim length and resistor dimensions on the TCR performance and resistance value of the resistors is investigated. Findings - It is found that variation in trim length, within the range of 5 to 15 mm, can give significant increases in the TCR of the thin films. Thus, for the plunge TCR cut can reach up to 11.51 ppm/oC, for the double plunge cut up to 14.34 ppm/oC and for the curved L-cut up to 5.11 ppm/oC. Originality/value – Research on the effects of various laser trimming geometries on the TCR and target resistance accuracy is limited, especially for patterns such as the curved L-cut,which is investigated in this paper

Resistor trimming geometry; past, present and future

This paper explores the key developments in thin film resistive trimming geometry for use in the fabrication of discrete precision resistors. Firstly an introduction to the laser trimming process is given with respect to well established trim geometries such as the plunge, 'L' and serpentine cuts. The effect of these trim patterns on key electrical properties of resistance tolerance and temperature co-efficient of resistance (TCR) of the thin films is then discussed before the performance of more recent geometries such as the three-contact and random trim approaches are reviewed. In addition to the properties of the standard trim patterns, the concept of the heat affected zone (HAZ) and ablation energy and the effect of introducing a 'fine' trim in areas of low current density to improve device performance are also studied. It is shown how trimming geometry and laser parameters can be systematically controlled to produce thin film resistors of the required properties for varying applications such as high precision, long term stability and high power pulse performance

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

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

An investigation into the effects of pattern geometries on laser trimmed film resistors

This study investigates the influence of different trim patterns on the performance of laser trimmed film resistors. A variety of popular trim pattern geometries including the plunge and L-cut were modelled and tested and their effect on resistance value, temperature coefficient of resistance (TCR) and heat-affected-zone (HAZ) sensitivity was investigated. It is shown that variation in resistor dimensions and trim length in the trimming algorithm can increase the TCR of the resistors with results of 100–140 ppm/°C for the plunge cut and 100–130 ppm/°C for the L-cut. It is also found that the L-cut has lower sensitivity in the HAZ with a value of 11% in comparison with the plunge cut with a value of 12% for the same resistor dimensions