Micromachined transmission lines for microwave applications

This thesis reports on the design, fabrication, characterization, and modeling of micromachined strip lines. The challenge of structuring this three-dimensional transmission line is solved by using the photosensitive resin SU-8 as the micromachinable material and as the dielectric in the strip line. Additionally, electrodeposition is applied to reinforce conductive structures to a thickness of 3µm and to fill out 15 µm deep vias. The fabrication of the strip line, which is independent of the substrate, is described in detail. The characterization of the strip lines is performed directly on-wafer with microwave coplanar probes. The S-parameters of the strip lines are measured with frequencies of up to the millimeter-wave range (48 GHz). Based on these measurements the transmission line properties of the strip line are analyzed and the corresponding equivalent circuit model is also extracted. To the author’s best knowledge, this is the first time that the dielectric constant and the loss tangent of SU-8 are measured in the millimeter-wave frequency region. <br>Micromachining techniques applied to microwave transmission lines were limited to planar ones, like microstrip or coplanar lines. In this work the property of micromachining techniques to fabricate three-dimensional lines, such as the strip line, is demonstrated. The value of the dielectric thickness in the strip line is 30 µm. The conductors in the strip line consist of a sputtered TiW-Ni seed layer followed by a electrodeposited 3 µm thick Cu-layer. The structuring of SU-8 is performed by using the simplest method of IC-technology: spin-on and photolithography. In particular, with SU-8 excellent structures with large aspect ratios can be fabricated. Moreover, SU-8 itself is photosensitive making an additional process step superfluous. The feasibility of SU-8 as the micromachinable material and as dielectric in strip lines is investigated. In spite of the simple processing of SU–8, there were some problems, which are reported: poor adhesion and cracking. These problems are systematically solved leading in the end to an improved SU–8 process. In this process there is almost no cracking and adhesion of SU-8 to copper and nickel layers is improved. The total dielectric thickness was achieved by deposition and structuring of two 15 µm thick SU-8 layers. <br>Strip lines with strip lengths ranging between 0.5 mm and 3 mm and with strip width values between 4 µm and 30 µm are designed in order to completely characterize the strip lines. The transition from the strip line to the contact pads, also placed on the wafer, is discussed. These contact pads, which have coplanar configuration and 50 * characteristic impedance, are indispensable for on-wafer characterization of the strip lines. <br>The Finite Element (FE) code was programmed in order to calculate the characteristic impedance of the strip line by solving the Laplace equation of the electrical potential along the line’s cross section, which is a two-dimensional domain. This program can be applied to sensitivity calculations of the characteristic impedance on deviations related to the process and the geometrical dimensions. <br>The S-parameters of the fabricated strip lines are measured on-wafer with frequencies of up to 48 GHz. From the S-parameters the characteristic impedances and the propagation constants for the strip lines are calculated and the elements per unit length of the corresponding equivalent circuit model are extracted. For the first time the relative dielectric constant and loss tangent of SU-8 are measured with millimeter-wave frequencies. At 48 GHz, the relative dielectric constant of SU-8 is 3.2 and the loss tangent is 0.043. The strip line with 48.2 * characteristic impedance shows an attenuation constant of 0.58 dB/mm at 48 GHz. From full-wave simulations, an amount of 0.2 dB/mm is assigned to the skin-effect in the copper conductive layers, whereas 0.38 dB/mm corresponds to the loss tangent of the SU-8 dielectric.

Author dc:creator

• Osorio, Ricardo

Contributors dc:contributor

• Korvink, Jan G.

• SU-8
• Mikrowellenleitungen
• MEMS
• RF-MEMS
• strip line
• photoresist

Repository record source_url

https://freidok.uni-freiburg.de/data/1067

OAI identifier oai:identifier

oai:freidok.uni-freiburg.de:1067