Capability of Sputtered Micro-patterned NiTi Thick Films
Christoph Bechtold1 • Rodrigo Lima de Miranda1,2 • Eckhard Quandt2
Published online: 4 August 2015
� ASM International 2015
Abstract Today, most NiTi devices are manufactured by
a combination of conventional metal fabrication steps, e.g.,
melting, extrusion, cold working, etc., and are subsequently
structured by high accuracy laser cutting. This combination
has been proven to be very successful; however, there are
several limitations to this fabrication route, e.g., in respect
to the fabrication of more complex device designs, device
miniaturization or the combination of different materials
for the integration of further functionality. These issues
have to be addressed in order to develop new devices and
applications. The fabrication of micro-patterned films using
magnetron sputtering, UV lithography, and wet etching has
great potential to overcome limitations of conventional
device manufacturing. Due to its fabrication characteris-
tics, this method allows the production of devices with
complex designs, high structural accuracy, smooth edge
profile, at layer thicknesses up to 75 lm. The aim of this
study is to present recent developments in the field of NiTi
thin film technology, its advantages and limitations, as well
as new possible applications in the medical and in non-
medical fields. These developments include among others
NiTi scaffold structures covered with NiTi membranes for
their potential use as filters, heart valve components or
aneurysm treatments, as well as micro-actuators for con-
sumable electronics or automotive applications.
Keywords NiTi � Shape memory � Magnetron
sputtering � Lithography � Medical applications �Integration of functionality
Introduction
NiTi is a well-established alloy in the medical industry. It
is used in orthodontic applications, for a variety of medical
instruments or for temporary or permanent vascular
implants [1–4]. Its superelastic properties have numerous
advantages, among them constant force delivery, kink
resistance, and different deployment methods [5]. The
latter allow for the use of smaller catheter diameters to
deliver implants and instruments, which is in particular
important for minimal invasive treatments. The increasing
popularity of vascular disease treatments utilizing stents in
the 1990s and the technological ability to fabricate and
structure high-quality NiTi tubes was a main trigger for the
rise of NiTi in medical applications [6]. Stent markets have
been growing since, and NiTi is still one of the main alloys
in this field.
The conventional fabrication route for NiTi implants
comprises a series of processing steps, including melting,
extrusion, hot or cold drawing, laser cutting, and surface
finishing [7–9]. This mass fabrication route has been pro-
ven to be very successful in providing millions of medical
products with high-quality standards. An ongoing trend in
medical device development is further miniaturization of
the medical device while enhancing its overall function-
ality allowing for less invasive treatments and shorter
healing times. This trend is a challenge for the established
This article is an invited paper selected from presentations at the
International Conference on Shape Memory and Superelastic
Technologies 2014, held May 12–16, 2014, in Pacific Grove,
California, and has been expanded from the original presentation.
& Christoph Bechtold
1 Acquandas GmbH, Kaiserstrasse 2, 24143 Kiel, Germany
2 Inorganic Functional Materials, Christian Albrechts-
Universitat zu Kiel, Kaiserstrasse 2, 24143 Kiel, Germany
123
Shap. Mem. Superelasticity (2015) 1:286–293
DOI 10.1007/s40830-015-0029-9
fabrication route. Some of these issues can be addressed
using an approach where NiTi structures are fabricated by a
sequence of microsystem technology processes, i.e., mag-
netron sputtering, UV lithography, and wet chemical
etching. This process is described in detail by Lima de
Miranda et al. [10]. This fabrication route has gained
increasing interest in recent years, especially since the
maximum thickness of thus prepared structures exceeds
75 lm and is therefore in a suitable range for many med-
ical applications. Magnetron sputtering is a vacuum
deposition technique. Inside a high vacuum chamber, Ar-
ions are accelerated towards a target, in our case a cast-
melted NiTi alloy target, target atoms are released due to
the ion bombardment and deposit on a cold substrate sur-
face. Oxide and carbide inclusions that are present in cast-
melted NiTi alloys with sizes of a few micrometers are not
transferred to the deposited film. Thus, a main source of
mechanical failure is eliminated, since oxide and carbide
inclusions, in particular in combination with voids, act as
crack initiation sites and decrease fatigue life [11]. Also,
the corrosion resistance of the material increases signifi-
cantly and exceeds 1000 mV [12]. The biocompatibility of
sputtered NiTi films has been subject of several studies,
which certify excellent properties [13, 14]. The lateral
homogeneity of the deposited films in terms of thickness
and chemical composition depends on the sputtering sys-
tem and on the target homogeneity. Many different sput-
tering systems exist. Small development systems with
static substrate and static magnetron can be used for pro-
totype fabrication, but the area of homogeneous thickness
and composition on the substrate may be small. Lager
systems with larger targets and magnetrons, moving sub-
strates or moving magnetrons yield better homogeneity. In
Fig. 1a, b, the lateral thickness and compositional homo-
geneity of a binary NiTi film on a 600 substrate fabricated
within a production system are shown. The thickness varies
by ±1 % over an area of 150 mm in diameter (measured
with an inductive digital comparator Mahr Extramess
2000), chemical composition was measured by EDX (EDX
error for each data point approx. 0.5–1.0 at.%). Since this
is a wafer-based technology, it is highly scalable and
reproducible.
NiTi films that are deposited at room temperature are
amorphous and require a subsequent heat treatment for
crystallization [15]. Hence, the microstructure of sputter-
deposited NiTi differs to that of bulk materials which
crystallize from the melt and often experience a high
degree of cold work to reduce the dimension of the semi-
finished product. Microstructural investigations of sput-
tered films have been carried out for a large variety of
binary to quaternary compositions and heat treatments,
e.g., [16–22], which demonstrates the large influence of the
two parameters on microstructural features (grain size,
distribution, type, and size of precipitates), transformation
temperatures and consequently their functional properties.
Using sputtering techniques, alloy optimization by varying
the chemical composition can be achieved in numerous
ways, ranging from different target compositions, changing
sputtering power or target-substrate distances to complex
combinatorial methods where a large area of the compo-
sition spread can be investigated simultaneously by high-
throughput methods [22–24].
Standard UV lithography allows for structuring photo-
sensitive resist with feature sizes in the submicron range
[25]. The pattern of the structured resist is then transferred
to the NiTi film, e.g., using wet chemical etching. NiTi
films with thicknesses of 75 lm have been deposited fol-
lowing the process described in [10]. Since sputtered edges
are not perfectly vertical, the minimum feature size
increases with increasing film thickness, i.e., *1 lm for
Fig. 1 a Thickness homogeneity over a 600 wafer area, nominal thickness 70 lm ± 1 %. b Chemical homogeneity over a 600 wafer area
Shap. Mem. Superelasticity (2015) 1:286–293 287
123
very thin NiTi films, and *10 lm for film with 75 lmthickness. Thus, aspect ratios of up to 7–8 can be achieved.
This is in particular interesting for the fabrication of thin
mesh structures where maximum pore or strut size can be
in the range of a few tens of micrometers only.
The NiTi layer can be released from the substrate using
a sacrificial layer technique. When flat substrates are used,
the released, amorphous film is obviously also flat. The
cylindrical shape of vascular implants can be achieved by a
constraint heat treatment, during which the amorphous film
crystallizes within a quartz tube of the desired diameter.
For many applications, an open design (beginning and end
of circumference are not joint) may be feasible, for others
the joint circumference may be required. Pulsatile fatigue
investigations on stent structures with laser-welded joints
of the circumference showed that the fatigue resistance of
the micro-welded joints is sufficiently high, and fractures
were rather observed at stent struts than the welding tags
[26]. Also, the overall functional behavior of NiTi sheet
metal was found to be mainly unaffected by welding, since
no significant change in the stress–strain transformation
plateau was detected [27]. The necessity of jointing can be
avoided when depositing NiTi directly on cylindrical sub-
strates, e.g., using a hollow cathode setup [28] or a rotating
substrate approach [29]. The latter is economically less
interesting due to the significant decrease in the effective
sputtering rate.
Except for laser-welded joints, structured, sputtered
NiTi films exhibit no heat-affected zones (HAZ), in con-
trast to laser-cut tubes. The microstructure is hence less
affected which in combination with the lack of oxide and
carbide inclusions promises higher fatigue life and
improved mechanical properties. Investigations on the
fatigue life of sputtered diamond-shaped samples reveal
indeed a high fatigue endurance limit. Up to mean strains
of 6 %, maximum alternating strains of ±1.5 % did not
lead to a fracture of the specimen during 3 9 10E6 cycles,
a significantly higher value compared to ±0.5 % maximum
alternating strain for NiTi standard material [26].
Limitations of the NiTi sputtering technology are the
limited film thickness and lateral dimension (the latter
determined by the wafer size used, e.g., 600, 800, etc.,), thenecessity to utilize the usable area effectively in order to be
cost efficient (good if many parts fit into the wafer area),
and the lack of cold work as parameter to influence
microstructure and thus functional properties.
The unique features of NiTi sputtering, however, have
high potential to realize some of the next generation medical
implants or tools, where the current technology cannot pro-
vide, e.g., the freedom of design or the ability to combine and
structure a series of additional materials. The capability of
NiTi sputtering is discussed in the following chapter.
Results and Discussion
We have divided this chapter according to what we have
named NiTi sputter technology platforms, namely 2D, 2.5-
dimensional (2.5D), and multi-layered structures. For each
platform, the fabrication process is adjusted in order to
realize different features of a NiTi-based sample.
The 2D platform is the base platform; it describes the
arbitrary, two-dimensional structuring of a NiTi film with
homogeneous thickness up to 75 lm. The characteristics of
the produced samples have been mentioned before: feature
resolution dependent on film thickness, but below 10 lmfor 75 thick films, aspect ratios up to 7–8, thickness and
compositional homogeneity of ±1 % and ±0.4 at.%,
respectively (depending on the sputtering system and target
homogeneity), highly pure material, excellent fatigue
properties, possibility of shape setting, and lateral dimen-
sion limited to wafer size. Examples are shown in Fig. 2.
Figure 2a shows an 8-lm-thick mesh structure with
8 lm strut width and 75 9 20 lm2 diamond-shaped pore
size. Strut width and pore size can be optimized according
to application requirements, e.g., when used as embolic
filters or as surfaces to engineer tissue growth. Figure 2c, d
shows a multitude of parallel, fine lamellae connected to a
supporting beam; Fig. 2e, f a connector structure; and
Fig. 2g, h generic, shape set stent structures of different
diameter.
2.5D structures offer a limited freedom of design per-
pendicular to the film plane. This can be achieved in two
ways. Either the film thickness is homogeneous, but lens-
shaped cavities are patterned into the structured films, see
Fig. 3.
A different approach is shown in Fig. 4 where a second
NiTi layer was deposited and patterned on top of a struc-
tured NiTi base layer. In this case, a 40-lm-thick ring was
structured on top of a 5-lm-thick layer. Within the prior
given limitations, the dimension of each layer can be varied
freely. Hence, reservoirs can be created, e.g., for drug
eluting systems or for micropump systems. Volume and
shape of the reservoirs can be varied in wide range to adjust
for the specific requirements of the application. Using the
second approach novel components with unique properties
can be created.
Figure 5a–c shows a supporting structure (50 lm thick)
with a fine mesh (5 lm thick, 75 9 20 lm2 diamond-
shaped pore size) between the supporting beams. Despite
its stiffness, these kinds of superelastic structures can be
crimped into a catheter of 3 mm without breaking. Fig-
ure 5e, f shows a micro-patterned strut surface, and
Fig. 5g, h stent-like structures with flaps attached.
The application potential of the 2.5D platform in the
medical field is manifold: micro-patterned surfaces for
288 Shap. Mem. Superelasticity (2015) 1:286–293
123
drug eluting systems, tissue engineering or friction control,
robust filters with scaffolds, artificial heart valves, or flow
diverters for aneurysm treatments can be realized.
The third platform involves the deposition and struc-
turing of other materials on a NiTi structure. An example is
the deposition of a radiopaque material, preferentially a
heavy metal such as Ta, Au, or Pt–Ir [4] on a NiTi struc-
ture. In Fig. 6a, a SEM image of a 50-lm-thick stent-like
structure with 35 lm strut with coated with 10 lm Ta is
shown, the corresponding radiopacity results are shown in
Fig. 6b: areas 1–6 contain samples covered with Ta, areas
7–9 uncovered samples of identical NiTi thickness. Areas
2, 3, 5, 6, 7, and 8 contain stent structures, and areas 1, 4,
and 9 diamond-shaped control specimen.
The NiTi structure can be completely or partly coated.
In the former case, the thickness of the radiopaque material
has to be sufficiently high to increase radiopacity but small
enough not to hinder the stress-induced martensitic trans-
formation of the underlying NiTi. Also, delamination of the
radiopaque layer must be avoided during mechanical
loading. In the second case, the radiopaque layer can be
patterned so that heavily loaded areas are uncoated and
thus free to transform, whereas less loaded areas are coated
(Fig. 6c).
Fig. 2 Examples of 2D structures: a, b fine mesh structures, c, d thin lamellae connected to a supporting beam, e, f connector structures, g,h shape set generic stent structures
Fig. 3 Sputtered NiTi film with
patterned lens-shaped cavities
Shap. Mem. Superelasticity (2015) 1:286–293 289
123
A novel field is the combination of flexible NiTi sub-
strates with structured isolating and conducting layers.
Circuit paths can be deposited on NiTi with a top and
bottom isolating layer. The top isolation might be absent in
certain areas so that electrodes for stimulation, sensing, or
mapping can be realized.
So far we have considered mostly superelastic materials
for medical applications. On the other hand, Ti-rich films
exhibiting the shape memory effect can be used as minia-
turized actuator components (Fig. 7). Due to the high work
output of NiTi small, flat, lightweight actuators can be
realized for applications within the automotive or con-
sumable electronics industry. Since the 2D design of the
actuators is free to choose, optimized force-stroke rela-
tionships and additional design elements to facilitate sys-
tem integration of the actuator component (instead of wire
crimping) can be realized.
The lack of oxide and carbide inclusions in sputtered
films promises good fatigue life also for actuator
applications.
Often Joule heating is used as a simple heatingmechanism
to increase the sample temperature above the austenite finish
temperature. Using the combination with isolating and
conducting layers, local heating elements can be created on
the NiTi surface and the NiTi actuator structure can be
activated in selected areas only, if required.
Fig. 4 Patterned NiTi rings of
40 lm thickness on top of a
patterned NiTi film
Fig. 5 Examples for 2.5D structures: a–d fine, thin mesh structure in between thick supporting beams to enhance crimpability, e, f stent-likestructure with micro-patterned dots on the strut surface, g, h stent-like structure with flaps attached for flow diversion in aneurism treatment
290 Shap. Mem. Superelasticity (2015) 1:286–293
123
Summary and Conclusion
The present paper investigates the capability of magnetron
sputtering, UV lithography, and wet chemical etching to
fabricate different NiTi-based components.
The characteristic features of sputtered micro-patterned
NiTi films (feature resolution, maximum thickness, thick-
ness and compositional homogeneity, certain microstruc-
tural aspects, fatigue performance) have been discussed
briefly. The main focus of this paper, however, is not on the
functional properties or the microstructure of sputtered
films, but the freedom of design that can be realized with
the combination of the above-named microsystem tech-
nology processes. In particular, 2.5D structures offer in our
view novel design opportunities in the medical field for
drug eluting systems, filters, valves, tissue engineering, or
flow diverters. Prototypes of 2D and 2.5D structures can be
fabricated with only few adjustments (different lithography
masks) required. The integration of additional functionality
using radiopaque, electrically conducting or insulating, or
magnetic layers is a promising approach to design and
fabricate next generation medical devices.
Experimental
The underlying process to fabricate NiTi films presented in
this paper is a combination of three microsystem technol-
ogy processes: UV lithography, physical vapor deposition,
and wet etching as described in [10]. In order to obtain
Fig. 6 a Stent strut coated with
Ta, b radiopacity results for
coated and uncoated samples,
c partial strut coating with Ta
Fig. 7 a Plain tensile actuator
design with features to facilitate
system integration, b disk-
shaped actuator
Shap. Mem. Superelasticity (2015) 1:286–293 291
123
more complex geometries such as 2.5D structures or
structures that are combined with other materials, some of
the above process steps are repeated or slightly altered. The
NiTi films are deposited using magnetron sputtering devi-
ces at base pressures below 1 9 10-7 mbar, Ar gas flow,
and deposition rates between 3.8 and 5.6 nm/s, depending
on sputtering system.
The processing sequence (Fig. 8) starts with the depo-
sition of a Cu sacrificial layer and a NiTi seed layer.
Substrates are then coated with positive photoresist
(AZ1518) with 2.3 lm thickness using spin coating fol-
lowed by UV exposure on a Karl Suss mask aligner in soft
contact mode for 2 s, a soft bake on a 105 �C hotplate for
2 min and rehydration for another 2 min. The resist is then
developed using AZ716 MIF solution for 1 min, forming
the desired geometry. Hard bake is performed at 120 �C for
20 min. The third step consists of wet etching the NiTi
seed layer using a HF solution with an etching rate of
10 nm/s. In the fourth step, the sacrificial layer is wet
etched which results in mushroom-like structures due to
undercutting during isotropic etching of the sacrificial
layer. As selective etchant, a standard BASF Selectipur
Chromium Etch (etching rate 13.5 nm/s) is used. After
etching, the photoresist is removed using acetone. To
achieve the required Nitinol film thickness, a thick NiTi
layer is now deposited on top of the seed layer. The
mushroom-like structures allow for the growth of the
structured NiTi film while avoiding any coalescence with
adjacent areas in between the mushroom-like structures. As
a final step, the sacrificial layer is removed, again using
BASF Selectipur Chromium Etch, which results in a free-
standing micro-patterned NiTi structure.
Subsequently, the amorphous samples were crystallized
in a high vacuum chamber in order to avoid oxidation
during the annealing process. Heat treatment was carried
out ex situ by means of a rapid thermal annealing system.
The halogen-lamp driven heating chamber enables typical
heating rates of 50 K/s in a vacuum environment of about
10-6–10-7 mbar. The annealing temperature was held
constant for 10 min at the maximum temperature of
650 �C, which turned out to be sufficient for crystallization
of amorphous NiTi. In a second step, films were annealed
for further 10 min at 450 �C, which is a common procedure
for Ni-rich Nitinol samples to induce the formation of
Ni4Ti3 precipitates in order to adjust phase transformation
temperatures.
Acknowledgments We thank Dr. C. Zamponi for EDX measure-
ments and Dr. Jens Trentmann at the department of Radiology and
Neuroradiology at the UKSH Kiel for radiopacity investigations.
References
1. Thompson SA (2000) An overview of nickel–titanium alloys
used in dentistry. Int Endod J 33:297
2. Stockel D (2000) Nitinol medical devices and implants. Minim
Invasive Ther 9(2):81–88
3. Pelton A, Schroeder V, Mitchell MR, Gong X-Y, Barney M,
Robertson SW (2008) Fatigue and durability of Nitinol stents.
J Mech Behav Biomed Mater I 1(2):153–164
4. Stockel D, Pelton A, Duerig T (2009) Self-expanding Nitinol
stents for the treatment of vascular disease. In: Yoneyama T,
Miyazaki S (eds) Shape memory alloys for biomedical applica-
tions. CRC Woodhead Publishing, Boca Raton, pp 237–256
5. Duerig T, Pelton A, Stockel D (1999) An overview of Nitinol
medical applications. Mater Sci Eng A 273:149–160
Fig. 8 Process steps for the production of freestanding micro-patterned NiTi 2D structures
292 Shap. Mem. Superelasticity (2015) 1:286–293
123
6. Pelton A, Duerig T, Berg B, Hodgson D, Mertmann M, Mitchell
M, Proft J, Wu M, Yang J (2005) Nitinol medical devices. Adv
Mater Proc 163(10):63
7. Hodgson D, Russell S (2000) Nitinol melting, manufacture and
fabrication. Minim Invasive Ther Allied Technol 9(2):61
8. Favier D, Orgeas L, Ferrier D, Poncin P, Liu Y (2001) Influence
of manufacturing methods on the homogeneity and properties of
nitinol tubular stents. J Phys IV 11(PR8):541
9. Poncet P, Adler PH, Carpenter S, Wu MH (2003) Manufacture of
Nitinol tubing. In: ASM Materials & Processes for Medical
Devices Conference, Anaheim
10. Lima de Miranda R, Zamponi C, Quandt E (2013) Micropat-
terned freestanding superelastic TiNi films. Adv Eng Mater
15(1–2):66
11. Rahim M, Frenzel J, Frotscher M, Pfetzing-Micklich J, Steeg-
muller R, Wohlschlogel M, Mughrabi H, Eggeler G (2013)
Impurity levels and fatigue lives of pseudoelastic NiTi shape
memory alloys. Acta Mater 61:3667
12. Wohlschlogel M, Steegmuller R, Schussler A (2014) Nitinol:
tubing versus sputtered foil—microcleanliness and corrosion
behavior. In: SMST Conference 2014, Pacific Grove
13. Kealey CP, Whelan SA, Chun YJ, Soojung CH, Tulloch AW,
Mohanchandra KP, DiCarlo D, Levi DS, Carman GP, Rigberg
DA (2010) In vitro hemocompatibility of thin film nitinol in
stenotic flow conditions. Biomaterials 31(34):8864
14. Habijan T, Lima De Miranda R, Zamponi C, Quandt E, Greulich
C, Schildhauer TA, Koller M (2012) The biocompatibility and
mechanical properties of cylindrical NiTi thin films produced by
magnetron sputtering. Mater Sci Eng C 32:2532
15. Ishida A, Sato M, Takei A, Miyazaki S (1995) Effect of heat
treatment on shape memory behavior of Ti-rich Ti–Ni thin films.
Mater Trans JIM 36(11):1349–1355
16. Miyazaki S, Ishida A (1999) Martensitic transformation and
shape memory behavior in sputter-deposited TiNi-base thin films.
Mater Sci Eng A 273–275:106–133
17. Ishida A, Sato M, Kimura T, Miyazaki S (2000) Stress-strain
curves of sputter-deposited Ti–Ni thin films. Philos Mag A
80(4):967–980
18. Otsuka K, Ren X (2005) Physical metallurgy of Ti–Ni-based
shape memory alloys. Prog Mater Sci 50:511
19. Ishida A, Sato M (2006) Microstructure and shape memory
behaviour of annealed Ti51.5Ni(48.5-x)Cux(x=6.5-20.9) thin films.
Philos Mag Lett 86:13–20
20. Ishida A, Sato M, Gao ZY (2013) Properties and applications of Ti–
Ni–Cu shape-memory-alloy thin films. J Alloys Comp 577(S1):S184
21. Zarnetta R, Takahashi R, Young ML, Savan A, Furuya Y,
Thienhaus S, Maaß B, Rahim M, Frenzel J, Brunken H, Chu YS,
Srivastava V, James RD, Takeuchi I, Eggeler G, Ludwig A
(2010) Identification of quaternary shape memory alloys with
near-zero thermal hysteresis. Adv Funct Mater 20(12):1917
22. Zarnetta R, Buenconsejo PJ, Savan A, Thienhaus S, Ludwig A
(2012) High-throughput study of martensitic transformations in
the complete Ti–Ni–Cu system. Intermetallics 26:98
23. Cui J, Chu Y, Famodu O, Furuya Y, Hattrick-Simpers J, James D,
Ludwig A, Thienhaus S, Wuttig M, Zhang Z, Takeuchi I (2006)
Combinatorial search of thermoelastic shape-memory alloys with
extremely small hysteresis width. Nat Mater 5:287
24. Zarnetta R, Savan A, Thienhaus S, Ludwig A (2007) Combina-
torial study of phase transformation characteristics of a Ti–Ni–Pd
shape memory thin film composition spread in view of
microactuator applications. Appl Surf Sci 254(3):743–748
25. Madou MJ (2002) Fundamentals of microfabrication, 2nd edn.
CRC Press LLC, Boca Raton
26. Siekmeyer G, Schußler A, Lima de Miranda R, Quandt E (2014)
Comparison of the fatigue performance of commercially pro-
duced Nitinol samples versus sputter-deposited Nitinol. JMEPEG
23:2437
27. Maletta C, Falvo A, Furgiuele F, Barbieri G, Brandizzi M (2009)
Fracture behaviour of Nickel–Titanium laser welded joints.
J Mater Eng Perform 18:569
28. Marton D, Boyle CT, Wiseman RW, Banas CE (2010) High
strength vacuum deposited Nitinol alloy films and method of
making same. US Patent 8083908 B2
29. Lima de Miranda R, Zamponi C, Quandt E (2009) Fabrication of
TiNi thin film stents. Smart Mater Struct 18:104010
Shap. Mem. Superelasticity (2015) 1:286–293 293
123