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Pyramidal structural defects in erbium silicide thin films

A new pyramidal structural defect, 5 to 8 micron wide, has been discovered in thin films of epitaxial erbium disilicide formed by annealing thin Er films on Si(001) substrates at temperatures of 500 to 800C. Since these defects form even upon annealing in vacuum of TiN-capped films their formation is not due to oxidation. The pyramidal defects are absent when the erbium disilicide forms on amorphous substrates, which suggests that epitaxial strains play an important role in their formation. We propose that these defects form as a result of the separation of the silicide film from the substrate and its buckling in order to relieve the compressive, biaxial epitaxial stresses. Silicon can then diffuse through the silicide or along the interface to fully or partially fill the void between the buckled erbium disilicide film and the substrate.

Rare earth (RE) silicides are of considerable technological interest for potential use as Schottky sources/drains in metal-oxide-semiconductor field-effect transistors (MOSFET) due to their low Schottky barriers (∼0.3 eV) to n-type Si [1]. Schottky source/drain technology in semiconductor-on-insulator (SOI) has advantages in scaling and fabrication compared with traditional MOSFETs [2]. However, 'pits' or 'pinholes' are common in RE silicide films formed by solid phase reactions of rare earth metals and Si [3][4][5][6][7]. Such defects can degrade the performance of Schottky barrier devices [8]. The pits are of considerable lateral extent and penetrate deeply into the Si substrate relative to the silicide thickness. The shape of these defects is inherited from the substrate structure: triangular on Si(111) and square on Si(001).
In this article, we report the observation of a new type of structural defect in ErSi 2-x films formed by rapid thermal annealing (RTA) of Er films on Si(001) substrates. This defect has a pyramidal shape with the apex directed away from the substrate. We characterize this pyramidal structural defect and propose a mechanism for its formation. Unlike pits that form because of local depletion of Si atoms [3,4,7,9] the formation of these defects is associated with ErSi 2-x /Si epitaxial strains.
The ErSi 2-x films were grown by sputter depositing Er on p-doped Si wafers (5-10 Ω cm) followed by rapid thermal annealing. The wafers were cleaned using the standard RCA method, then dipped in dilute HF for 1 minute prior to the deposition of a 50 nm thick erbium film at room temperature in a chamber with a base pressure of 3×10 -7 torr. In some samples, a 15 nm thick TiN layer was deposited in-situ on top of the 50 nm Er (to prevent oxidation).
This was followed by a rapid thermal anneal for 60 seconds at between 400 and 800°C in a N 2 ambient or in vacuum (1×10 -6 torr). The 400°C anneals did not produce ErSi 2-x . Additional wafers were amorphized using Si ion implantation (30 keV, 1×10 15 ions/cm 2 ) prior to Er deposition in order to determine the role of epitaxy in the defect formation process. E.J. Tan et al.

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Phases were identified using x-ray diffraction (Bruker D8 GADDS x-ray diffractometer with 2D detector), electron diffraction and secondary ion mass spectroscopy (SIMS). Figure 1 implies that this is not simply a fiber texture, but that there is a strong in-plane texture as well.
The inset in Fig. 1  annealing in vacuum or in N 2 , indicates that the formation of these defects is not associated with oxidation. To our knowledge, the presence of such pyramidal defects has not previously been reported. On the other hand, earlier reports suggest that pits penetrating into the Si substrate form when ErSi 2-x films are grown on Si(001) at low temperature [3,4,9].  The hexagonal ErSi 2-x forms on Si(001) with its [0001] axis parallel to Si<110>. The mismatch parallel to the c-axis is -6.5% (compressive) and +1.1% (tensile) perpendicular to the c-axis. The diffraction data and previous reports [11][12][13] show that the ErSi 2-x film consists of alternating grains with the c-axis parallel to [110] and [110]. The grain size is typically 50-150 nm [11,12], which is much smaller than the lateral extent of the pyramidal E.J. Tan et al.

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defects. Therefore, the misfit strain, as measured on a scale larger than several grains is biaxial and compressive, -2.7%.
The proposed pyramidal defect formation mechanism is illustrated in Fig. 4. The film buckles away from the substrate to relieve the very large epitaxial strain energy (Fig. 4(b)).
We note that the pyramidal defect density is too low to significantly relieve the total strain energy stored in the film. Such buckling or blistering must originate at a pre-existing defect or region where the bonding between substrate and film is weak. These may either be processing defects or form as the result of plastic deformation. While it is possible for a compressively stressed film to buckle while remaining attached to the substrate, the observations of voids beneath the pyramidal defect ( Fig. 3(b)) and the highly defected Si where the void was filled (insets to Fig. 3(b)), both suggest that the film separates from the substrate upon buckling. Further, if the film remained attached to the substrate upon buckling, the density of the pyramidal defects would be much larger than that observed (to relax much of the strain energy stored in the ErSi 2-x film).
After the pyramidal defect forms by buckling, Fig. 4(b), the open region between the film and substrate begins to fill with Si, Figs. 4(c)-(e). This filling occurs in order to lessen the surface energies; filling the void below the buckled ErSi 2-x film covers part of both the Si free surface and the ErSi 2-x free surface. One indication that this filling occurs is the observation that the Si that fills in, under the buckled ErSi 2-x film, has a very different defect structure from that in the original Si substrate. That is, this Si contains a high density of {111} stacking faults (see Fig. 3(b)). The Si atoms located at the ErSi 2-x /Si interface close to the defect diffuse along or near the interface towards the defect.  4(c)). As shown in Fig. 4(c), silicon may grow simultaneously from the two surfaces. In the case of Si growing on the Si(001) surface, only one crystal orientation is possible. In contrast, there is considerable evidence that Si formed on free ErSi 2-x surfaces may grow with different orientations (see [15]). Eventually, the two re-crystallization fronts meet as shown schematically in Fig. 4(e), resulting in the filled pyramidal defect of Fig. 3(a). Figure 3(b) shows the case where a void remains (i.e., where the two fronts have not yet met) as in Fig.   4(d).
Examination of a large number of the pyramidal defects in the scanning electron microscope shows that many of the defects are cracked (these can also be seen in the optical micrograph in Fig. 2(a)). The cracking, represented in Fig. 4(f), is likely a result of the large tensile stresses formed near the four edges of the pyramid that meet at the apex and the contraction of the ErSi 2-x film upon cooling. The coefficient of thermal expansion of erbium silicide is larger than that of silicon hence large differential strains occur on cooling from the annealing temperature to room temperature.
The mechanistic model described above suggests that a balance between the elastic energy stored in the unbuckled film and the surface and interface energies determines the size of the pyramidal defects. Therefore, once the ErSi 2-x film forms and buckles on rapid thermal annealing, the size of the pyramidal defects should not increase on further annealing. As a check on the validity of the mechanistic model, we measured the pyramidal defect size in  showing the shape and height profile more clearly.
3. Cross-sectional TEM micrographs of pyramidal structural defects on an erbium silicide film annealed at 500°C for 60 s.  Fig. 3(b), (e) a completely filled defect as in Fig. 3(a), and (f) the formation of a crack in the ErSi 2-x film in the pyramidal defect upon cooling.