Spa Water Used for the Fabrication of Visible-Light-Emitting Porous Silicon
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conventional infrared (IR) imaging of a bonded wafer, it is difficult to determine whether a lack of visible voids on the interface corresponds to strong bonding. Using dynamic surface energy measurement, researchers Xuanxiong Zhang of Shanghai Research Center for Wireless Communication in China and Jean-Pierre Raskin of Université Catholique de Louvain in Belgium have studied bonding strength and bonding uniformity of bonded wafer pairs before and after post-bonding annealing. Low-temperature O2 plasma exposure was used to bond Si samples (3 in. diameter, -oriented, p-type, 15–25 Ω cm) that had been pre-cleaned using a standard procedure and then exposed to O2 plasma for 5 s. The researchers discovered that, overall, the bonding strength increased significantly after 10 h of annealing at 120°C. As the researchers reported in the August issue of Electrochemical and Solid-State Letters (p. G268; DOI: 10.1149/ l.2012288), some areas seemingly free of voids in IR images have much lower surface energy values than the mean value, indicating that these areas may be de-bonding or have voids. After 20 h of annealing, the overall bonding strength is improved further, but the non-uniformity observed earlier still exists. With longer annealing times of up to 120 h, the bonding strength rose further by a small amount, but the uniformity of the bonding strength is remarkably enhanced. In particular, in almost all tested samples, the bonding strength at the rim of a bonded pair is higher than in the center of the bonded pair. The researchers believe that the increase of bonding strength and the enhancement of bonding strength uniformity over the annealing time may be due to out-diffusion of bonding by-products (i.e., hydrogen, oxygen, and water vapor). Therefore, they conclude that extended post-bonding annealing is necessary to obtain uniform bonding strength on the whole wafer level and improve the percentage of successful bonding yield in practical applications. SHIMING WU
Spa Water Used for the Fabrication of Visible-Light-Emitting Porous Silicon Visible light emission from porous silicon has potential application in silicon-based optoelectronic devices. Most porous Si (p-Si) layers are prepared by anodic etching on p-type Si wafers or photoetching p-type Si wafers in an HF solution. Both methods have disadvantages involving either complicated fabrication steps or special handling of toxic chemicals. A group of researchers in Japan have now used potable spa water as the solution in photoetching to produce visible-light-emitting p-Si. As reported in the August issue of Electrochemical and Solid-State Letters (p. G251; DOI: 10.1149/1.2001791), S. Adachi and K. Tomioka from the Department of Electronic Engineering of Gunma University in Japan have fabricated visible-light-emitting porous Si by photoetching Si in spa water. The spa water they used is a colorless alkaline solution from one of the Japanese hot springs, the Tenkeisen Spa. The researchers photoetched an n-type Si(111) wafer in the Tenkeisen Spa water (TeSW) under 5
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