Identification and Formation Mechanism of a New Interstitial Defect Found in Ion Implanted Silicon

For 40 years, extended defects are found in silicon after ion implantation and annealing. Widely studied for shallow implantation, thermal evolution of these defects follow the same path in the particular case of a Si layer transferred by Smart Cut™ technology. Being extrinsic or intrinsic precipitates, these defects are formed under supersaturated condition and grow by interchanging interstitials, or vacancies, during annealing. All along their growth, the precipitates rearrange them-selves to transform into a more stable structure. A transferred Si layer needs a combination of very high-temperature annealing in neutral and oxidizing atmospheres to thin and smooth the fractured surface. Despite these treatments, irregular topography has been detected on the extremely smooth surface. Studying them in detail, we have found a new type of interstitial-type crystalline defects, extremely stable, that we have named “tent-like” defects in reference to their shape. The presence of such structures was a surprise as it is generally believed that the high-temperature treatment following fracture was sufficient to dissolve all the precipitates initially present in the transferred Si layer. It is the goal of this work to report the characteristics, formation mechanism and origin of these structures. From that, we infer, and then experimentally confirm the process conditions promoting or hampering their formation. For this work, 300 nm-thick Si layers were transferred by using H+ and other species, wafer bonding and thermal initiation of the fracture. Afterwards, the transferred layers were subjected to different annealing steps, oxidizing or not, then annealed at temperatures up to 1200°C under Ar atmosphere. Figure 1a is a typical Scanning Electron Microscopy (SEM) image of one of these structures. Such images were obtained using a low voltage SEM with the ability to image mono-atomic steps separating flat (001) terraces all over the surface of a 300 mm wafer. Step bunching imaged on the right of the defect, as contour lines, reveals some “erosion” around the rectangular shape, at the origin of a shallow hole which was initially detected. In general, these structures are elongated along one of the two in-plane <110> directions (Fig. 1a). Their length increases during oxidizing annealing whereas their width stays almost constant. Since silicon oxidation results in the injection of Si interstitials into the underlying crystal, this growth suggests they are extrinsic defects i.e., contain excess Si atoms. Figure 1b is a HREM image of such a defect, cut in its body part, as shown in Fig. 1a. Two intersecting {111} stacking faults (SF) define a V-shape defect (Fig. 1b). Figure 1c shows the geometric phase distributions around the structure obtained by Dark Field Electron Holography (DFEH) for g= 004, g=111 and g=-1-11. From such images (no phase jump for g=004 and mirror symmetry between g=111 and g=-1-11), the Burgers vector of the dislocation, seen edge-on at the SFs intersection, is found to be 1/6[-1-10], i.e. that of a stair-rod Lomer-Cottrell dislocation. This finding suggests that the tent-like defect probably results from the dissociation of straight dislocation segments, each following a path such as 1/3<-1-11>=1/6<-1-10>+1/6<-1-12>. Based on these experimental data, we propose a scenario describing the formation of such structures. At the beginning an interstitial loop (Frank or perfect) was buried in the matrix. Such loops are known to form in ion implanted silicon annealed at high enough temperature. During oxidation, such a loop can grow up to, or may be cut by, the SiO2/Si interface. It then forms a half-loop. Based on energetic criteria, we show that such a half-loop tends to elongate and becomes segmented. During high-temperature annealing, the dislocation lines dissociate: a 2D defect transforms into a 3D “obtuse wedge elongated defect” bound by four {111} SFs and five segments of stair-rod Lomer-Cottrell dislocations. This dissociation is similar to the one describing the formation of tetrahedral defects from triangular Frank dislocation loops. In summary, we have found a new type of extended defects in Si, of interstitial character, which may appear as a result of the precipitation of silicon interstitials. This defect has not been identified before as the conditions for its formation are quite stringent. Beyond the process conditions under which they have been observed at first, the mechanisms which lead to their formation requires that a highly I-supersaturated region is initially located very close to a free surface (possibly an interface) where a half-dislocation loop can form and then dissociate, a condition rarely found in real cases with the notable exception of layers transferred by the Smart Cut™ process. Figure 1