Platinum silicide

Platinum silicide, also known as platinum monosilicide, is the inorganic compound with the formula PtSi and forms an orthorhombic crystalline structure when synthesized.

The crystal structure of PtSi is orthorhombic, with each silicon atom having six neighboring platinum atoms. The distances between the silicon and the platinum neighbors are as follow: one at a distance of 2.41 angstroms, two at a distance of 2.43 angstroms, one at a distance of 2.52 angstroms, and the final two at a distance of 2.64 angstroms. Each platinum atom has six silicon neighbors at the same distances, as well as two platinum neighbors, at a distance of 2.87 and 2.90 angstroms. All of the distances over 2.50 angstroms are considered too far to really be involved in bonding interactions of the compound. As a result, it has been shown that two sets of covalent bonds compose the bonds forming the compound. One set is the three center Pt-Si-Pt bond, and the other set the two center Pt-Si bonds. Each silicon atom in the compound has one three center bond and two two center bonds. The thinnest film of PtSi would consist of two alternating planes of atoms, a single sheet of orthorhombic structures. Thicker layers are formed by stacking pairs of the alternating sheets. The mechanism of bonding between PtSi is more similar to that of pure silicon than pure platinum or Pt2Si, though experimentation has revealed metallic bonding character in PtSi that pure silicon lacks.

PtSi can be synthesized in several ways. The standard method involves depositing a thin film of pure platinum onto silicon wafers and heating in a conventional furnace at 450-600 °C for a half an hour in inert ambients. The process cannot be carried out in an oxygenated environment, as this results in the formation of an oxide layer on the silicon, preventing PtSi from forming. A secondary technique for synthesis requires a sputtered platinum film deposited on a silicon substrate. Due to the ease with which PtSi can become contaminated by oxygen, several variations of the methods have been reported. Rapid thermal processing has been shown to increase the purity of PtSi layers formed. Lower temperatures (200-450 °C) were also found to be successful , higher temperatures produce thicker PtSi layers, though temperatures in excess of 950 °C formed PtSi with increased resistivity due to clusters of large PtSi grains.

Despite the synthesis method employed, PtSi forms in the same way. When pure platinum is first heated with silicon, Pt2Si is formed. Once all the available Pt and Si are used and the only available surfaces are Pt2Si, the silicide will begin the slower reaction of converting into PtSi. The activation energy for the Pt2Si reaction is around 1.38 eV, while it is 1.67 eV for PtSi.

Oxygen is extremely detrimental to the reaction, as it will bind preferably to Pt, limiting the sites available for Pt-Si bonding and preventing the silicide formation. A partial pressure of O₂ as low at 10⁻⁷ has been found to be sufficient to slow the formation of the silicide. To avoid this issue inert ambients are used, as well as small annealing chambers to minimize amount of potential contamination. The cleanliness of the metal film is also extremely important, and unclean conditions result in poor PtSi synthesis.

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