Ion Beam Induced Exfoliation of Silicon Structures J.W. Mayer, T.L. Alford, Arizona State Univ.; S.S. Lau, UC-San Diego DMR-0308127

Fig.1 A sketch of the approach to achieve Si layer transfer using plasma hydrogenation.

Fig. 2 TEM shows a high-quality, exfoliated, defect-free Si layer of less than 20 nm thickness, which is not achievable by conventional techniques.

The International Technology Semiconductor Roadmap requires ultra thin silicon layers less than 20 nm thick by 2012. The conventional ion-cut process, utilizing hydrogen ion implantation, can be used to initiate layer delamination at desired depths, but the process causes significant residual ion damage in the transferred layer. We have demonstrated for the first time a novel approach with potential advantages for transfer of such ultra thin Si layers onto handle wafers. We first create a thin hydrogen trapping layer at a specific depth, e.g., by growth of a defective layer. This is then followed by plasma hydrogenation (Fig. 1). We found that a thin Sb-doped layer grown by MBE served as an effective trapping site for hydrogen. Our results shown in Fig. 2 provide direct evidence of the feasibility of using the plasma hydrogenation and defect trapping process to exfoliate a 15 nm relatively defect free silicon layer. We have achieved this with the hydrogen-plasma process whereas the thinnest layer (75 nm) transferred by the hydrogen ion implantation process is five times thicker. The next critical step is the evaluation of the device quality of such ultra thin transferred layers, and this work is under way.

Fig.1 Si wafer after hydrogenation Fig.2 Top Si(190nm)/Si0.8Ge0.2 (5nm)/Si substrate after hydrogenation

We show here that a buried nanometer thick layer can serve as H trapping centers. A thin SiGe layer is used as an example here. In conjunction with H plasma hydrogenation, lift-off of the top Si layer can be realized with cleavage occurring at the depth of the strained SiGe layer. This technique avoids irradiation damage within the top Si layer, caused by heavy ion implantation used to create H trapping layers in the traditional ion-cut method. We explain the strain-facilitated layer-transfer due to preferential vacancy agglomeration within the strained layer and subsequent trapping of hydrogen, which lead to cracking in a well controlled manner.

Ion Beam Induced Exfoliation of Silicon Structures J.W. Mayer, T.L. Alford, Arizona State Univ.; S.S. Lau, UC-San Diego DMR-0308127

Si layer transfer without any ion implantation

The major impact of our project is that it demonstrates that we can achieve transfer of Si layers without use of high energy hydrogen ions. This will enable a much wider range of applications because of the lower defect density in the silicon achieved by the new process. Other aspects are i) a collaborative program, based on the complementary nature of the facilities at UCSD and ASU, as well as the success of the past programs between the investigators. The research program is carried out by a culturally diversified team and with active involvement with a national laboratory, Los Alamo, ii) establishment of active undergraduate and graduate research opportunities, iii) continuation to integrate our research program into our class presentations, and iv) new research areas of Si layers on flexible substrates.

Ion Beam Induced Exfoliation of Silicon Structures J.W. Mayer, T.L. Alford, Arizona State Univ.; S.S. Lau, UC-San Diego DMR-0308127

Broader Impact

Graduates students, Prasad Shetty and Ramya Govindasamy, work with undergraduate student Edward Williams to demonstrate the temperature dependence of the exfoliation process.