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Platinum thin films are used in various microelectronic andmicro-sensor applications. The microstructural, chemical, and electricalstability of these films under high-temperature conditionsare of major concern. In addition, stability is also a concern forpotential extended use in specialized microelectronic applications,especially when the films are used as thin, two-dimensional interconnectsor electrodes connecting active components at elevatedtemperatures. Typical applications of these high-temperaturefilms are aligned with electrodes/interconnects for chemical sensors,micro-heaters and -hotplates within microelectromechanicalsystems (MEMS) [1–6]. Recently, more advanced MEM systemshave been applied within extreme environments, whichincludes high temperatures and harsh chemical reactants, such asmicro-chemical emission sensors, -structural monitoring sensors,-thermocouples, and -fuel cell systems that are utilized at temperatures>600–800 ◦C [7–13].High melting point noble metals are most suited for extremeenvironment applications. Platinum, with its relatively high meltingpoint (1773 ◦C) and excellent chemical inertness, has longbeen utilized for MEMs devices capable of operating at elevatedtemperatures. Pt and other noble metals have a great chemicalinertness; however, these metals show poor adhesion and high surfacetension toward oxide surfaces. Budhani et al. demonstrated aninterface modification between thermally grown aluminum oxide(Al2O3) and thin Pt films via reactive sputtering with low levelsof oxygen in order to obtain a 20–30 nm PtxO1−x layer prior topure platinum metal deposition. Adhesion tests showed a higherlevel of adhesion compared to the conventional Pt + Al2O3 couple.The authors indicated that strong PtxO1−x to Al2O3 bonding andinterdiffusion at the interface were responsible for the enhancedadhesion [14].Although the controlled oxidation of a sub-layer of Pt showedpromise for enhanced wetting and adhesion to oxide substrates,various researchers have focused on incorporating alternativemetal/metal oxide layer compositions. These thin coatingswere deposited to improve noble metal adhesion, as well as, toimprove the thermal stability over prolonged exposure to hightemperatures. High temperature operating conditions lead tothe development of many structural defects, such as hillocks,film delamination, surface cracking, voids and grain coarsening,which all eventually result in non-uniform film morphology andvariable electrical response [1–5,15–18]. At high temperatures(≥700 ◦C), grain coarsening and hillock formation are the majormechanisms that break the percolated granular network across thepolycrystalline film [3,19–22]. Since low-temperature sputteringand evaporation techniques typically produce films with highsurface area granular structures, these films possess an extremelyhigh driving force for sintering and grain growth processes. Hightemperature
operation permits the required diffusional kinetics
for accelerated grain growth, resulting in the coalescence of the
grains and the formation of a poorly percolated structure [21,23].
In other words, the total interfacial and surface energy of the thin
film can be minimized by reducing ceramic–metal contact area by
creating islands of Pt material. The destruction of the integrity of
the continuous film eventually results in complete loss of electrical
continuity, which diminishes the functionality, reliability and
sensitivity of the micromachined devices.
Metals such as Ti and Ta have been proposed and demonstrated
with variable success to decrease both Pt grain coarsening and
hillock formation. Lee et al. optimized the procedure first defined by
Budhani et al. for deposition of Pt over insulating oxide layers with
improved adhesion. According to this procedure, platinum deposition
under an oxidation atmosphere, followed by inert atmosphere
deposition of Pt and subsequent annealing of silicon substrate
at 400–1300 ◦C, removed the remaining O2 in the Pt film [24].
Recently, Tiggelaar et al. compared the use of the PtxO1−x adhesion
layer to the use of Ti or Ta adhesion layers. These layers were
deposited by sputtering onto silicon and Si3N4 substrates. After
annealing between 400 and 950 ◦C under inert and oxygen containing
atmospheres, their electrical and structural performances were
characterized [25]. The authors concluded that the operational reliability
of Pt films with Ti and Ta adhesion layers are limited to
temperatures below 650 ◦C and 850 ◦C, respectively. In the same
study, the fast diffusion behavior of Ti and the resultant changes
to the wetting characteristics of Pt on the Ti layer over different
ceramic layers (Al2O3, Ta2O5, SiO2 and Si3N4) were also described.
Firebaugh et al. used a similar Ta adhesion strategy on silicon rich
silicon nitride. This study states that the adhesion layer migration
and co
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