International audienceIn this paper, we report on the use of tantalum and iridium as adhesion layers for platinum electrodes used in high temperature SAW devices based on langasite substrates (LGS). Unlike iridium, tantalum exhibits a great adhesive strength, and a very low mobility through the Pt film, ensuring a device lifetime of at least half an hour at 1000°C. The latter is limited by morphological modifications of platinum, starting by the apparition of crystallites on the surface, and followed by important terracing and breaking of the film continuity. SNMS and XRD measurements allowed us to show that these phenomena are likely intrinsic to platinum film, whatever be the nature of the adhesion layer. Finally, after having outlined a possible scenario leading to this deterioration, we consider some solutions that could replace platinum in order to increase the lifetime of LGS-based SAW devices in high temperatures conditions

Alnot, Patrick

Assouar, Badreddine

Aubert, Thierry

Bournebe, Zoumnone

Bouvot, Laurent

Elmazria, Omar

HEHN, Michel

Oudich, Mourad

Weber, Sylvain

English

Archive Ouverte en Sciences de l'Information et de la Communication

HAL Id: hal-02114812
https://hal.archives-ouvertes.fr/hal-02114812
Submitted on 29 Apr 2019
HAL is a multi-disciplinary open access
archive for the deposit and dissemination of sci-
entific research documents, whether they are pub-
lished or not. The documents may come from
teaching and research institutions in France or
abroad, or from public or private research centers.
L’archive ouverte pluridisciplinaire HAL, est
destinée au dépôt et à la diffusion de documents
scientifiques de niveau recherche, publiés ou non,
émanant des établissements d’enseignement et de
recherche français ou étrangers, des laboratoires
publics ou privés.
Study of tantalum and iridium as adhesion layers for
Pt/LGS high temperature SAW devices
Thierry Aubert, Omar Elmazria, Badreddine Assouar, Laurent Bouvot,
Zoumnone Bournebe, Michel Hehn, Sylvain Weber, Mourad Oudich, Patrick
Alnot
To cite this version:
Thierry Aubert, Omar Elmazria, Badreddine Assouar, Laurent Bouvot, Zoumnone Bournebe, et al..
Study of tantalum and iridium as adhesion layers for Pt/LGS high temperature SAW devices. IEEE
International Ultrasonics Symposium, Sep 2009, Rome, Italy. ￿hal-02114812￿
Study of tantalum and iridium as adhesion layers for 
Pt/LGS high temperature SAW devices 
 
Thierry AUBERT, Omar ELMAZRIA, Badreddine ASSOUAR, Laurent BOUVOT, Zoumnone BOURNEBE, Michel 
HEHN, Sylvain WEBER, Mourad OUDICH, Patrick ALNOT 
Institut Jean Lamour 
UMR 7198 CNRS – Nancy University 
54506 Vandoeuvre lès Nancy, France 
thierry.aubert@lpmi.uhp-nancy.fr 
 
 
 
Abstract—In this paper, we report on the use of tantalum and 
iridium as adhesion layers for platinum electrodes used in high 
temperature SAW devices based on langasite substrates (LGS). 
Unlike iridium, tantalum exhibits a great adhesive strength, and 
a very low mobility through the Pt film, ensuring a device lifetime 
of at least half an hour at 1000°C. The latter is limited by 
morphological modifications of platinum, starting by the 
apparition of crystallites on the surface, and followed by 
important terracing and breaking of the film continuity. SNMS 
and XRD measurements allowed us to show that these 
phenomena are likely intrinsic to platinum film, whatever be the 
nature of the adhesion layer. Finally, after having outlined a 
possible scenario leading to this deterioration, we consider some 
solutions that could replace platinum in order to increase the 
lifetime of LGS-based SAW devices in high temperatures 
conditions. 
 
Keywords-high temperature; platinum; SAW; langasite; 
tantalum 
I.  INTRODUCTION 
It is now well established that SAW devices can be used as 
pressure, gas or temperature sensors. Those devices are 
particularly interesting for applications in harsh environments 
where no electronics can survive. Indeed, because they can be 
remotely powered and interrogated, embedded electronic is not 
required for wireless applications. Many industrial sectors, 
such as aeronautics, metallurgy, automotive, are looking 
forward those small, robust and wireless devices [1,2]. Efforts 
are now focused on selecting materials able to withstand harsh 
conditions, such as high temperatures up to 1000°C. 
Regarding to the substrate, several studies have shown that 
langasite (La3Ga5SiO14) is well suited for this kind of 
applications [3]. It is only limited by its relative important 
propagation losses, especially at high frequencies, making it 
unsuitable for applications at high temperatures above 1 GHz 
[4], which could be an embarrassing limitation for wireless 
applications. The challenge now is to find a reliable 
composition of materials to realize IDT structure with high 
stability and good performances. 
Up to now, most of the studies have been conducted on 
platinum electrodes [1,2]. Indeed, this noble metal has a 
relatively high melting temperature (1773°C), and an important 
chemical inertness appreciable at high temperature. 
Nevertheless, this interesting property has also a drawback : the 
consequence of the platinum weak chemical affinity for others 
elements is a very poor adhesion of platinum films on oxide 
substrates such as quartz or langasite. The solution consists in 
using a very thin (10 to 20 nm) intermediate layer between the 
substrate and the platinum film, so-called “adhesion layer”. It 
must be constituted by a refractive metal, more reactive than 
platinum, such as titanium, zirconium, tantalum … Bonding 
between the adhesion layer and the platinum film is ensured by 
alloying [5].  
Several studies have shown the limitations of titanium as 
adhesion layer at temperatures above 600°C [6]. The diffusion 
of titanium in the platinum film is actually large, leading to the 
formation of oxide precipitates (TiO2) in the Pt grain 
boundaries. The consequences are the progressive vanishing of 
the adhesion layer leading to delamination, the modification of 
the electrical properties of the IDTs and even their destruction. 
Pt/Zr combination has shown far better performances until 
700°C. In particular, AES (Auger electron spectroscopy) 
measurements have highlighted that Zr has not diffused at all 
through Pt after 8h of annealing at 700°C [7]. Nonetheless, it 
has been shown that this combination is inadequate for 
continuous operation beyond about 700°C, because of de-
wetting phenomena [3]. 
More recently, Da Cunha et al. have studied alternative 
solutions, replacing platinum by a Pt/10%Rh alloy, while 
maintaining the adhesion layer of zirconium [3]. This 
combination exhibited a very good stability until 950°C, which 
is a considerable advance. This stability was even improved by 
co-deposing the alloy with ZrO2, and by using a SiAlON 
protective over-layer [8]. 
Our purpose in this work is to study new adhesion layers 
for platinum, not studied yet in high temperature SAW 
applications, to reach better results than the ones with 
zirconium, and then to ensure a device lifetime of at least 30 
minutes at 1000°C. We will focus our efforts on two potential 
candidates : tantalum and iridium. 
Tantalum as adhesion layer for platinum has already been 
studied in other fields than SAW applications. Maeder et al., in 
particular, have noted that its performance is very close to that 
of zirconium after an annealing of 10 minutes in a low pressure 
O2 atmosphere at 620°C [9]. On the other hand, iridium is a 
noble metal, just like platinum. Nevertheless, a thin oxide film 
forms on its surface, even at room temperature, foreshadowing 
a better chemical reactivity than that of platinum, which could 
allow bonding with the substrate [10]. 
II. EXPERIMENTAL  
Platinum and tantalum thin films were deposited on YX 
langasite substrates by sputtering method, while iridium was 
evaporated in electron beam system. The thickness of the 
adhesion layers was systematically 10 nm, while that of 
platinum film was 100 nm. SAW delay lines operating at 167 
MHz were then processed combining photolithography and ion 
beam etching (IBE). All devices were then electrically 
characterized with a probe station before and after annealing. 
These were performed in a tube furnace under air atmosphere 
at temperatures from 900°C to 1000°C and for a duration 
varying between 30 minutes and 2 hours. The samples were 
quickly inserted in the pre-heated furnace, and pulled out of the 
hot zone as fast as possible. Langasite substrates showed great 
resistance to this thermal shock treatment. 
The morphology of the thin metal films was studied by X-
ray diffraction (XRD) and scanning electron microscopy 
(SEM). Second neutral mass spectroscopy (SNMS) was used to 
obtain their compositional depth profiles. 
III. RESULTS 
A. Pt/Ir Combination 
If the adhesion strength of the as-deposited Pt/Ir films 
seemed good, the lithography process revealed that it was 
actually poor. In fact, after this phase, approximately 70 % of 
the achieved devices were unusable, because of the presence of 
numerous scratches on the IDTs. We can suppose that their 
origin is related to a poor adhesion of the Pt/Ir film on the 
substrate, making the devices particularly delicate. This 
fragility also appeared when the prober tips come into contact 
with the IDT pads during electrical characterization.  
Significant scratches were then systematically observed. This 
behavior was confirmed after the high temperature annealing. 
SEM images revealed the formation of numerous blisters as 
well as delamination, leading to a strong thinning of the IDTs 
fingers, threatening their continuity (see Fig. 1). However, the 
IDTs have not been completely broken, as the S21 response 
attests (see Fig. 2). One can guess that the result would have 
been probably different if the experiment had lasted over half 
an hour. This strong lack of adhesion, partially expected due to 
the noble character of iridium, leads us to focus our efforts on 
the Pt/Ta combination.  
 
B. Pt/Ta Combination 
Unlike Pt/Ir combination, Pt/Ta films showed very good 
adhesion to the substrate at room temperature, but also after 
annealing. The frequency response shows no deterioration after 
annealing at 1000°C for 30 minutes (see Fig. 3). SEM images 
confirmed this result, showing only a starting modification, 
with the apparition of crystallites at the surface of the platinum 
film (see Fig. 4). 
To study the limitations of this kind of device at high 
temperature, we made it undergo a new annealing process at 
900°C for 2 hours. The result was no appeal : the SAW signal 
vanished while SEM images revealed that the crystallites have 
disappeared, leaving a substantial terracing and breaking of the 
IDTs fingers, which explains the loss of the SAW signal (see 
Fig. 5). 
 
 
Figure 1. SEM image of a Pt/Ir IDT after annealing for 30 min. at 1000°C 
 
  
Figure 2. Frequency response of a Pt/Ir/LGS SAW device before and after 
annealing for 30 min. at 1000°C 
  
Figure 3. Frequency response of a Pt/Ta/ LGS SAW device before and after 
annealing for 30 min. at 1000°C 
 
Figure 4. Grazing SEM image of a Pt/Ta IDT before and after annealing for 30 
min. at 1000°C 
 
Figure 5. Grazing SEM image of a Pt/Ta IDT finger tip after 2 hours. of 
annealing at 900°C 
In order to increase the lifetime of this kind of devices at 
very high temperatures, it is essential to understand the origin 
of the crystallites appearing at the surface of the platinum film 
during annealing. Indeed, it seems clear that they constitute a 
prerequisite for the deterioration of the IDTs. That is why some 
unprocessed Pt/Ta/LGS samples were then investigated by 
SNMS (Fig. 6) and XRD (Fig. 7). 
IV. DISCUSSION  
On SNMS depth profile curve, the atomic concentration of 
several elements is plotted versus the time of abrasion, which is 
related to the depth inside the sample. The two vertical lines 
symbolize the approximate interfaces Pt/Ta and Ta/LGS. One 
can see that there has not been massive diffusion of tantalum 
through the Pt film during annealing, and a slight one into the 
LGS substrate. This result, coupled with the excellent adhesive 
strength of the Pt/Ta films, shows how tantalum is a good 
adhesion layer for platinum thin films and for high temperature 
applications. Moreover, tantalum will probably play no role in 
the development of the crystallites on the surface of the Pt 
layer, implying that this phenomenon is intrinsic to platinum. 
XRD measurements allow confirming this hypothesis. θ-2θ 
scan pointed out a strong [111] orientation of the platinum 
films, which is a conventional result for FCC metals, like Pt 
[11]. Another interesting information is given by the shift of 
this [111] Pt peak if one compares its position before and after 
annealing. The peak of as-deposited films is at a slightly lower 
angle of diffraction (39,538°) than its theoretical value 
(39,763°), which means that the interplanar value d111 of these 
films is larger than the bulk one. This indicates that the Pt layer 
is slightly in compression in the plane of the film. A likely 
reason for this compressive stress is peening, by which the 
subsurface of a sputtered film is compressed due to 
bombardment by reflected neutral atoms [11]. To the opposite, 
the peak after annealing has shifted to a much higher value 
(39,850°). This can be explained by the difference of the 
thermal expansion coefficient α between Pt (8,8.10-6 K-1) and 
LGS (5,6.10-6 K-1), which induces, after cooling down at a high 
rate a strain of the film and an associated “thermal” stress. 
Here, as αLGS is lower than αPt, we can deduce that the Pt film 
is stretched in the plane, implying a lower d111 value than for 
the bulk, and thus a higher diffraction angle than the theoretical 
value. More important, it can be clearly seen on this spectrum 
that the full width at half maximum (FWHM) of this [111] Pt 
peak decreases significantly while its intensity increases after 
annealing (Fig. 7). It is well known that the width of diffraction 
peaks decreases while increasing crystallinity, so we can 
conclude that the Pt layer strongly recrystallized during 
annealing, and that the apparition of the crystallites is a 
harbinger of this phenomenon. This scenario can also explain 
the mechanism leading to the loss of continuity of the Pt film 
after long-time annealing. Indeed, if the temperature or the time 
of annealing increases, the phenomenon of recrystallization 
goes on, and crystals grow up out of plane more and more, 
which implies that some areas of the film will be depleted of Pt 
in order to “feed” these baby-crystals. 
 
 
Figure 6. SNMS depth profile of Pt/Ta/LGS sample after 30 min. of annealing 
at 1000°C 
 
Figure 7. Pt [111] XR diffraction peak before and after annealing for 30 min. at 
1000°C 
 
This interpretation means that, whatever be the nature of 
the adhesion layer, the deterioration of the platinum film at 
high temperature cannot be avoided, because of this 
recrystallization phenomena. This is consistent with the fact 
that similar observations of Pt film recrystallization during 
annealing have been made in the case of a zirconium adhesion 
layer [3]. 
As our metal thin films are polycrystalline, grain 
boundaries are very numerous and the total surface of grains is 
large, which implies that interface diffusion phenomena are 
probably predominant under bulk ones, and govern the 
recrystallization process. Moreover, the onset of surface 
diffusion is about the Hüttig temperature (which is defined as 
0,3Tm with Tm the melting temperature in K, i.e. for Pt about 
340°C), whereas lattice mobility becomes significant over the 
Tammann temperature (defined as 0,5Tm, i.e. for Pt about 
750°C) [11]. However, it is clear that the higher is the melting 
temperature, the latter the recrystallisation phenomena will 
start. A trivial solution to increase the lifetime of the device 
would be to replace platinum by a noble metal having a 
significantly higher melting temperature, i.e. ruthenium or 
iridium. Nonetheless, they start oxidizing at temperatures about 
700°C, and their oxides are highly volatile [12]. Another 
interesting way is that followed by Da Cunha et al. who use a 
Pt/10%Rh alloy. Indeed, in theses kind of alloys, precipitates 
form in the grain boundaries, which prevents grain boundaries 
diffusion [13]. 
V. CONCLUSION  
To summarize, we have shown that tantalum has the 
required properties to be used as an adhesion layer for 
platinum IDTs. The limitation of 30 min at 1000 °C is 
attributed to platinum and not to the layer of tantalum. The 
deterioration of Pt/Ta electrode is explained by a strong 
recrystallization of platinum as evidenced by the results of X-
ray diffraction.  
REFERENCES 
[1] J. A. Thiele and M. P. da Cunha, “Platinum and palladium high- 
temperature transducers on langasite,” IEEE Trans. Ultrason. Ferroelec. 
Freq. Contr., Vol. 52, No. 04, April 2005, pp. 545-549. 
[2] T. Aubert, F. Sarry, O. Elmazria, L. Bouvot, B. Assouar, P. Nicolay and 
M. Hehn, “Extraction of COM parameters on Pt/LGS for high 
temperature SAW sensor,” Proc. IEEE Ultrasonics Symp., 2008, pp. 
820-823. 
[3] M. P. da Cunha, T. Moonlight, R. J. Lad, G. Bernhardt and D. J. 
Frankel, “Enabling very high température acoustic wave devices for 
sensor and frequency control applications,” Proc. IEEE Ultrasonics 
Symp., 2007,  pp. 2107-2110. 
[4] R. Fachberger, G. Bruckner, R. Hauser, C. Ruppel, J. Biniasch and L. 
Reindl, “Properties of radio frequency Rayleigh waves on langasite at 
elevated temperature,” Proc. IEEE Ultrasonics Symp., 2004, pp 1223-
1226. 
[5] D. S. Ballantine, R. M. White, S. J. Martin, A. J. Ricco, E. T. Zellers, 
G.C. Frye and H. Wohltjen, Acoustic Wave Sensors, Academic Press, 
1997. 
[6] M. N. Hamidon, V. Skarda, N. M. White, F. Krispel, P. Krempl, M. 
Binhack and W. Buff,  “Fabrication of high temperature surface acoustic 
wave devices for sensor applications,” Sens. and Actuators A, Vol. 123-
124, pp 403-407, 2005. 
[7] G. Bernhardt, C. Silvestre, N. LeCursi, S.C. Moulzolf, D. J. Frankel and 
R. J. Lad, “Performance of Zr and Ti adhesion layers for bonding of 
platinum metallization to sapphire substrates,” Sens. and Actuators 
B:Chem., Vol. 77, pp. 368-374, 2001. 
[8] M. P. da Cunha, R. J. Lad, T. Moonlight, G. Bernhardt and D. J. 
Frankel, “High température stability of langasite surface acoustic wave 
devices,” Proc. IEEE Ultrasonics Sympo., 2008 ; pp. 205-208. 
[9] T. Maeder, L. Sagalowicz and P. Muralt, “Stabilized platinum electrodes 
for ferroelectric film deposition using Ti, Ta and Zr adhesion layers,” 
Jpn. J. Appl. Phys., Vol. 37, 1998, pp. 2007-2012. 
[10] C. A. Krier and R. I. Jaffee, “Oxidation of the platinum-group metals,” J. 
Less-Common Metals, Vol.5, 1963, pp. 411-431. 
[11] R. M. Tiggelaar, R. G. P. Sanders, A. W. Groenland, J. G. E. Gardeniers, 
“Stability of thin platinum films implemented in high-temperature 
microdevices,” Sens. Actuators A: Phys. (2009), doi : 
10.1016/j.sna.2009.03.017. 
[12] M. Lisker, T. Hur’yeva, Y. Ritterhaus and E. P. Burte, “Effect of 
annealing in oxygen atmosphere on morphological and electrical 
properties of iridium and ruthenium thin films prepared by liquid 
delivery MOCVD,”  Surf. and Coat. Tech., Vol. 201, 2007, pp. 9294-
9298. 
[13] C. Alépée, “Technologies for high-temperature silicon microreactors” 
Ph.D. dissertation, École polytechnique fédérale de Lausanne (EPFL), 
Lausanne, Switzerland, 2000. 


Study of tantalum and iridium as adhesion layers for Pt/LGS high temperature SAW devices

https://hal.archives-ouvertes.fr/hal-02114812/file/Proc%20Roma.pdf

Study of tantalum and iridium as adhesion layers for Pt/LGS high temperature SAW devices

Abstract

Similar works

Full text

Available Versions

Archive Ouverte en Sciences de l'Information et de la Communication