Platinum thin films are used in var

Platinum thin films are used in various microelectronic and
micro-sensor applications. The microstructural, chemical, and electrical
stability of these films under high-temperature conditions
are of major concern. In addition, stability is also a concern for
potential extended use in specialized microelectronic applications,
especially when the films are used as thin, two-dimensional interconnects
or electrodes connecting active components at elevated
temperatures. Typical applications of these high-temperature
films are aligned with electrodes/interconnects for chemical sensors,
micro-heaters and -hotplates within microelectromechanical
systems (MEMS) [1–6]. Recently, more advanced MEM systems
have been applied within extreme environments, which
includes high temperatures and harsh chemical reactants, such as
micro-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 extreme
environment applications. Platinum, with its relatively high melting
point (1773 ◦C) and excellent chemical inertness, has long
been utilized for MEMs devices capable of operating at elevated
temperatures. Pt and other noble metals have a great chemical
inertness; however, these metals show poor adhesion and high surface
tension toward oxide surfaces. Budhani et al. demonstrated an
interface modification between thermally grown aluminum oxide
(Al2O3) and thin Pt films via reactive sputtering with low levels
of oxygen in order to obtain a 20–30 nm PtxO1−x layer prior to
pure platinum metal deposition. Adhesion tests showed a higher
level of adhesion compared to the conventional Pt + Al2O3 couple.
The authors indicated that strong PtxO1−x to Al2O3 bonding and
interdiffusion at the interface were responsible for the enhanced
adhesion [14].
Although the controlled oxidation of a sub-layer of Pt showed
promise for enhanced wetting and adhesion to oxide substrates,
various researchers have focused on incorporating alternative
metal/metal oxide layer compositions. These thin coatings
were deposited to improve noble metal adhesion, as well as, to
improve the thermal stability over prolonged exposure to high
temperatures. High temperature operating conditions lead to
the 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 and
variable electrical response [1–5,15–18]. At high temperatures
(≥700 ◦C), grain coarsening and hillock formation are the major
mechanisms that break the percolated granular network across the
polycrystalline film [3,19–22]. Since low-temperature sputtering
and evaporation techniques typically produce films with high
surface area granular structures, these films possess an extremely
high 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