To the high-speed optoelectronic devices for high-speed (up to 40 Gb/s) optical fiber communication systems, one of the key issues is feeding high-frequency microwave signals onto the chips. It is a common practice to mount the device chip onto a submount and connect the signal electrodes through wire bonding. The microwave circuit usually consists of a low-loss microwave transmission line and an impedance-matching thin-film resistor. To ensure low microwave power reflection, resistance of such a resistor should be close to 50 Ω in a wideband microwave frequency up to 40 GHz. Ta2N thin-film are often adopted as resistors for its high thermal stableness and radio-frequency (RF) impedance stableness.
In our lab, the Ta2N thin films were deposited on the substrate with reactive magnetron sputtering technique and processed into small squares as microwave impedance-matching resistors. However, the X-Ray diffraction analysis of such Ta2N thin films have shown relatively poor crystal quality, that may has a serious influence on the microwave characteristics of the submounts. Therefore, it is very necessary to improve crystal quality of the Ta2N thin films and optimize the microwave characteristics.
Our recent progress on the experimental optimization of Ta2N thin-films based on thermal annealing technique will be introduced. Two kinds of thermal annealing processes are adopted in this study: in vacuum environment and N2 environment. After vacuum high-temperature annealing, the X-ray rocking-curve response of Ta2N characteristic peak was dramatically improved from 102 to 105 count/s levels, and the contact resistivity between Ta2N film and metal electrode was reduced from 10-4 to 10-5 Ω•cm levels. On contrast, in N2 environment, the characteristics of Ta2N thin-films have no significant change after thermal annealing process. The measurement and further experiment optimization of microwave characteristics of such Ta2N thin films will be also discussed.
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