Design and fabrication of embedded resistors in LTCC for high frequency applications
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This work was aimed at the RF performance of embedded resistors in LTCC, with tight tolerance, and low variation in resistance value, as basic required properties. In order to conduct this performance evaluation, two test vehicles were designed; one to evaluate the effect of physical design on embedded resistors, the other for characterization at high frequencies. DuPont 951 Green Tapes and CF021 resistor paste (100Ω/sq.) were employed to fabricate the substrates that were cofired at 875°C for 15 minutes. A four-probe Kelvin contact method and the HP-4263 LCR meter were used for the direct current (DC) resistance measurement, and an HP network analyzer was used for impedance measurements. It was found that although the majority of resistors had a 3a tolerance of 20∼30%, resistors less than lOmil wide or one square in aspect ratio had much higher tolerance and variation than the others. For the 2 to 5 square resistors in a certain width, the tolerance varied little. The apparent sheet resistivity demonstrated negative variation for resistors under 2 square and positive for over 2 square in aspect ratio. In a substrate with two resistor layers, the tolerance of most resistors on the upper layer was less than 15%, half of that on the lower layer, while their variation of apparent sheet resistivity were similar. Unlike the resistor width and aspect ratio, the overlap and extension had little influence on the tolerance and variation due to the short diffusion distance between termination and resistor. It is recommended that for low tolerance and low variation design, the preferred options are 20mil to 35mil width for one (1) to five (5) square aspect ratio, 10mil overlap and 5mil extension, and most importantly, no other embedded passives on the layers just over or under the resistors. With this design and the regular LTCC fabrication process, a tolerance less than 15% and variation no more than ± 5% have been achieved repeatedly in this work.