Wafer bonding of highly oriented diamond to silicon
Introduction
Integration of diamond with other electronic materials is of considerable technological interest. Silicon-on-diamond (SOD) technology would significantly enhance power characteristics of devices fabricated on silicon due to diamond's thermal characteristics. Wafer bonding is a promising way to create SOD. The interface obtained during bonding will determine the properties of SOD substrates. Yushin et al. [1], [2] were the first to conduct transmission electron microscopy (TEM) studies of the bonded silicon/diamond interface obtained by fusion of polished polycrystalline diamond to silicon in the temperature range between 950 and 1100 °C. In that work, the variation in the polishing rate for crystallites of various orientations limited the RMS roughness of mechanically polished polycrystalline diamond samples to 15 nm. Highly oriented diamond (HOD) has several advantages over polycrystalline diamond including higher thermal conductivity and more uniform mechanical properties, which enable fabrication of smooth surfaces with low RMS roughness [3], [4], [5], [6]. Minimizing the surface roughness is crucial for high quality wafer bonding. In this work we present the TEM analysis of HOD/Si interface fabricated by high temperature UHV fusion.
Section snippets
Experimental details
In all bonding experiments, commercially available (100) Si wafers and mechanically polished Plasma Assisted Chemical Vapor Deposited (100) HOD films grown on Si substrates were used. Microscopic roughness measurements of the sample surfaces were ascertained by atomic force microscopy (AFM) using a Digital Instruments D-3000 microscope. Studies of the macroscopic surface roughness (waviness and bow) of the samples were performed on a Zygo GPI XP Laser Interferometer.
Prior to fusion, Si samples
Atomic force microscopy and optical interferometry
Stresses generated during the HOD growth caused bending of silicon substrates and resulted in concave surfaces on the unpolished diamond samples. As-grown HOD samples exhibited 3–6 μm bow across a 5×5 mm2 area, as ascertained by interferometry measurements. Polishing decreased the difference in the surface height to ∼2–3 μm. Higher material removal rate at the edges of the specimen during polishing was responsible for the convex shape of the polished diamond surface . The inevitable polishing
Summary
HOD was found to be a promising material for silicon-on-diamond technology. Successful fusion was achieved at temperatures above 850 °C. Fusion resulted in the formation of an abrupt Si/diamond interface in the areas away from diamond grain boundaries, where diamond surface was sufficiently smooth. Regions of the interface close to HOD grain boundaries showed voids, formation of amorphous material and generation of planar defects in Si. It is anticipated that further reduction in the diamond
Acknowledgements
This work was sponsored by the Army Research Office under grant no. DAAG5598D0003.
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