PAALD 방법으로 증착된 Al₂O₃(BOX Layer) 박막을 이용한 SOI 구조 형성에 관한 연구

Abstract

In the past couple of decade, silicon on insulator (SOI) structures have been investigated for various device applications, such as three-dimensional integrated circuits, power devices, higher immunity to radiation, novel thin film transistor (TFT) devices and micro machining technologies. Because silicon on insulator (SOI) has solved for the problems of bulk silicon devices associated with junction area, leakage, isolation, and capacitance, SOI has become an attractive technology for future generations of mainstream Ultra Large Scaled Integrated (ULSI) device products. However, with increasing power dissipation it is becoming more and more problem to transfer the heat from the active region of the circuits to the heat sink. For SOI, the situation is even worse than in bulk technology due to the poor thermal conductivity of the buried insulator. These self heating effect degrades the SOI MOSFETs for ULSI devices in terms of threshold voltage, sub-threshold swing, mobility, etc. Nowadays, many research have been achieved with replacement of the buried oxide (BOX) with Al₂O₃ films which were grown atomic layer deposition (ALD) method. Because, the thermal conductivity of Al₂O₃ is several tens of times higher than that of silicon dioxide, and as well as it's high breakdown field voltage, and low leakage current characteristics. ALD Al₂O₃ has been considered as a possible candidate for gate oxides as well as Ta₂O_(5), HfO₂, and ZrO₂. In addition, Al₂O₃ has been recently regarded as a good passivation layer, which can prevent hydrogen penetration into the active region of semiconductor devices. In preparing high dielectric metal oxides with a thickness in the sub-hundred angstrom range, atomic layer deposition attracts considerable interest owing to its inherent merits in thin film depositions, because of its digital controllability of film thickness which derives from self limiting layer by layer deposition, owing to the surface limited reaction between reactant gases. Moreover, ALD recently became a serious alternative to the conventional chemical vapor deposition because of its excellent film step coverage on sub-micrometer features, which is one of the most important requirements for fabricating micro-electronic devices. In this work, we suggest a SOI buried with Al₂O₃ which was deposited by plasma assisted atomic layer deposition (PAALD). Al₂O₃ films which were grown with trimetylaluminum (TMA) and H₂O by using thermal ALD (TALD) method had poor physical characteristics in comparison with that of TMA and O₂ plasma assisted ALD method in high temperature annealing. In the case of PAALD, we supposed that oxygen radicals generated by plasma act as an active oxidant with TMA. Then, concentration rate of OH group and other unstable state elements inside of Al₂O₃ film is so small, and as a results, it decreases the thermal stress of the films efficiently. We could prove there are some different formation characteristics of Al₂O₃ films from between TALD and PAALD method by using HRXRD, spectroscopic ellipsometry, residual gas analysis, thermal stress analysis, C-V measurement and, XPS. After Al₂O₃ films were grown onto 4 inch Si(100) wafer, through standard RCA cleaning, high temperature silicon direct bonding and CMP process, we fabricated the Al₂O₃ buried SOI wafer. Scanning acoustic microscopy (SAM), infra-red transmission (IR), HRTEM results indicated the PAALD Al₂O₃ buried SOI structure was fabricated completely, by conventional silicon direct bonding (SDB) without micro defects in the interface. Also, we could measure the reasonable electric characteristics from the Pseudo-MOSFET devices which is widely accepted as a characterization method for the SOI wafer. It confirms that PAALD Al₂O₃ films have sufficient quality to be used as the buried oxide of SOI wafer. It is believed that our work is successful in improving the Al₂O₃ buried SOI characteristics in the view point of control of micro defects generation in the interface, during fabrication of the SDB SOI process.