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Sensors and Materials, Vol. 36, No. 8 (2024) 3479–3490
MYU Tokyo
S & M 3746

Crack Nucleation and Propagation Mechanismsof Sputtered AlSi Alloy Thin Filmunder Quasistatic and Dynamic Tension
Takahiro Namazu,1* Ginnosuke Ina,2 Hiroki Tsuma,3 and Tsuyoshi Nishiwaki3
1Kyoto University of Advanced Science, 18 Yamanouchigotanda-cho, Ukyo, Kyoto 615-8577, Japan
2University of Hyogo, 2167 Shosha, Himeji, Hyogo 671-2280, Japan
3MIRISE Technologies Corporation,

500-1 Minamiyama, Komenoki-cho, Nisshin, Aichi 470-0111, Japan(Received July 16, 2024; accepted August 6, 2024)

Keywords: AlSi alloy thin film, sputtering, tensile test, fatigue test, crack nucleation, crack propagation,
Si segregation, degradation

In this study, we examined the fatigue crack nucleation and propagation mechanisms of an
AlSi sputtered thin film subjected to the quasistatic tensile test and cyclic loading test by
scanning electron microscopy. An AlSi thin film with 1.0 wt% Si was prepared by dc magnetron
sputtering at a substrate temperature of 260 ℃, followed by annealing at 490 ℃. Two types ofuniaxial tensile test equipment were prepared, namely, manual tensile loading equipment andautomatic tensile loading equipment. The combination of the use of manual-type equipment andelectron backscattered diffraction analysis enabled us to visualize crystal grain subdivision
mechanisms. The automatic-type equipment enabled us to understand fatigue crack propagation
mechanisms in the AlSi thin film after crack nucleation originating from grain subdivision. A
fatigue crack propagated in a zigzag manner under the low stress amplitude condition during thefatigue test, whereas it propagated straightly under the high stress amplitude condition. Thisphenomenon is discussed from the viewpoints of crack detouring around Si segregates and Si
brittleness.

1.

Introduction

Owing to their low electric resistance, high strength-to-weight ratio, nonbrittle characteristic,
and ease of deposition by sputtering and vacuum evaporation, pure Al and Al alloy films arecommonly used as wiring and structural materials in various semiconductor and MEMS
devices.(1–5) In the dry processes to deposit Al-related films, it is easy to include additiveelements into the Al film. A certain amount of an additive element can be contained in thetarget, so film composition can be precisely controlled. In the case of 3D MEMS structures
made of an Al alloy film, the mechanical characteristics of the film should be examined
experimentally to improve its performance and reliability. A uniaxial tensile test directly*Corresponding author: e-mail: namazu.takahiro@kuas.ac.jp

https://doi.org/10.18494/SAM5240

ISSN 0914-4935 © MYU K.K.
https://myukk.org/

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measures the mechanical properties of a thin film if a sophisticated evaluation technique isprepared.(6–13) A dynamic uniaxial tensile test is important for investigating fatigue
characteristics against cycling external loading. As examples of studies of the fatigue test of Al
alloy films, Barrios et al. developed a system for the high-throughput fatigue test to investigate
fatigue cracking in sputtered Al films.(14) They showed that fatigue cracks exhibited significant
tortuosity and tended to follow hillocks on the Al film surface. Koike et al. discussed in detailfatigue cracking and deformation mechanisms in sputtered AlSi films during a thermal cycling
test.(15) On the basis of the microstructural observation after the test, they demonstrated that theinitial increase in the amount of residual stress was caused by the increase in the amount oflattice dislocations and their tangling, and the following decrease in the amount of residual stress
was caused by crack formation and delamination. Shindo et al. demonstrated the effectiveness ofa Si additive into the Al film for the extension of lifetime in the AlSi film.(16) Understanding of
fatigue mechanisms including fatigue crack nucleation and propagation during cyclic loading
enables us to design semiconductor and MEMS devices with high performance and reliability.
(17)

The objective of this research is to experimentally characterize the degradation of sputtered
AlSi thin film under cyclic loading. In particular, we focus on clarifying crack initiation andpropagation under quasistatic uniaxial tension loading and dynamic tension loading whileobserving crystal grain degradation and cracking around Si segregates by electron backscattereddiffraction (EBSD) and energy dispersive X-ray (EDX) analyses, respectively. Lastly, theinfluence of stress amplitude during the fatigue test on fatigue cracking is discussed.

2.

Experimental Procedure

Figure 1 shows a photograph of the tensile test specimen produced using conventional
semiconductor fabrication technologies. First, a 4-inch n-type Si (001) wafer of 300 μm thickness
was oxidized under wet thermal conditions at 1100 ℃ for 2 h to grow a 1-μm-thick amorphous
Si oxide (SiOx) film on the entire wafer. On the top surface, an AlSi alloy film of 5 μm thickness
was deposited by dc magnetron sputtering using an AlSi alloy target with a Si content of 1.0
wt%. The substrate temperature during sputtering was kept constant at 260 ℃. After thedeposition, annealing was conducted at 490 ℃ in a hydrogen + nitrogen atmosphere for 1 h.
After annealing, a dog-bone-shaped self-standing monolayer film specimen for the tensile
mode fatigue test was produced using conventional micromachining processes. The specimen
shape was patterned onto the deposited film by a photolithography technique, followed by wet
etching with a mixed solution of phosphoric acid, nitric acid, and acetic acid. As shown in theenlarged photograph in Fig. 1, a free-standing monolayer film specimen consists of a parallel
section and a curvature section. At the boundary between the two sections, four isosceles
triangles each measuring approximately 7 μm in height and 10 μm in bottom width were made,which are used as gauge marks to measure specimen elongation during tensile tests.(16) The
length and width of the parallel section were set to be 500 and 200 μm, respectively. After the
specimen section fabrication, through holes were made by photolithography and a dry etching
method, called deep reactive-ion etching (DRIE) from the back side of a wafer. Through DRIE

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Fig. 1.

3481(Color online) Photograph of AlSi film specimen for quasistatic and dynamic tension loading.

with the BOSCH process recipe using sulfur hexafluoride (SF6) and octafluorocyclobutane(C4F8) gases, two square holes, four Si springs, and a space beneath a film specimen wereformed. After the etching, one end of the specimen section, which is the portion supported byfour Si springs, became movable. The spring width was only 100 μm, and a film specimen was
easily deformed along the tensile direction only, so no misalignment between the specimen’s
longitudinal direction and the tensile direction happened during the tensile test. The square holes
were used for hooking vertical rectangular solid pins installed in the tensile test apparatus toapply a tensile force. The space beneath a film specimen makes the film in the tension section
free-standing. A film in the dog-bone section exists as a self-standing monolayer, whereas thefilm in other portions exists as the double layers of the film and substrate. Lastly, after the SiOx
layer on the back side of a film specimen was completely removed by wet etching with bufferedhydrofluoric acid, the preparation of the tensile test specimen was completed.
For crystal grain degradation evaluation under quasistatic tension, double notches at thecenter of the specimen parallel section were fabricated using a focused ion beam (FIB). The
length and width of each notch were 25 and 2 μm, respectively, and the distance between the two
was 20 μm. For crack propagation evaluation under dynamic tension, no notches were introduced
into the films.
Figure 2 shows photographs of the uniaxial tensile test equipment for thin film evaluation.
Two types of uniaxial tensile test equipment were prepared, namely, manual tensile loading
equipment and automatic tensile loading equipment. The manual-type equipment, which was
specially designed for crystal grain degradation observation at the tip of a notch using EBSD,

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Fig. 2.

(Color online) Tensile test equipment for quasistatic and dynamic tension loading.

consists of a sample stage with two hooking pins, a spring structure, a micrometer, and a SEM
stage. The equipment is set on the stage of a commercial SEM system (ZEISS, Ultla 55 SEM)
and can apply quasistatic tensile loading to a film specimen by rotating a micrometer because
the two pins are manually moved away from each other. Another micrometer is set outside theSEM system, which is connected to the micrometer equipped with the manual-type jig via a
flexible tube. The automatic-type equipment was designed and developed to be of compact size,
which enables us to conduct pulsating-tension cyclic loading tests in a SEM system (Hitachi
High-Tech, S4800 Type II). The equipment is composed of a piezoelectric actuator for automatic
quasistatic/dynamic tensile force application, a load cell, a course moving stage, a SEM holder, apair of sample holders with a square vertical pin for hooking a specimen, and an actuator casewith a lever structure to amplify a PZT actuator’s elongation by a factor of 4.1 along the tensile
direction. By setting the equipment into the SEM system, it can be operated using a personal
computer outside of the SEM system. The system enabled us to do automatic pulsating-tension
fatigue tests inside the SEM system.
By using a specimen and automatic-type equipment, the pulsating tension mode fatigue test
was carried out in a high vacuum in the SEM system. The frequency was set to be 10 Hz. A
sinusoidal waveform was applied to the specimen and the amplitude was set on the basis of areference stress value derived from quasistatic tensile tests.(16)3.

Results and Discussion

Figure 3 shows representative SEM images around notch tips under quasistatic uniaxial
tension along with EBSD maps and its experimental setup and color definition. EBSD is known
as an effective analysis tool to deeply understand the crystallinity and degradation ofgrains.(13,16,18) In the EBSD analysis, the inverse pole figure (IPF) distribution in the specimen’s
axial direction (AD), width direction (WD), and normal direction (ND) were obtained. Here, tomainly investigate the influence of axial tensile loading on crystal grain deformation anddegradation around notches, we used the AD IPF data only. Although the data were omitted inthis paper, all the AlSi films evaluated had a highly (111) preferred orientation in ND and aSensors and Materials, Vol. 36, No. 8 (2024)

Fig. 3.

3483(Color online) SEM and EBSD images of area around notch tip under quasistatic tension.

random orientation in AD and WD. The strain shown beneath each figure was calculated on thebasis of the elongation of the parallel section measured using the two-gauge marks in the tensile
direction. The tips of the notches on the left and right sides were artificially made inside a crystal
grain and on the crystal grain boundary, respectively. Before applying tension, it is found fromthe ESBD images that the crystal grains are randomly arranged in-plane. Even when a strain of
0.8% was applied, no significant change can be seen in the SEM image, although the width ofeach notch was slightly enlarged. In the EBSD map, no significant change can be seen as well,
except at the tip of the left-side notch. It is found that the color of the left side of the notch ischanged to slightly darker than that of the right side. This indicates that the crystal grain aroundthe notch degraded slightly. When applying tension up to 1.2%, although the notches appear
clearly widened, it is difficult to find changes in the SEM image. In the EBSD map, however,
two apparent changes can be found. The first change is on the tip of the left notch. One crystal
grain looks divided into three different grains. The second change is on the tip of the right notch,
which was made on the boundary of the two grains. Each grain consists of multiple colors. The
left-side grain looks divided into three colors at the inclined angles of around 30° and 70° fromthe vertical line (grain boundary). The right-side grain looks divided into two colors at the angleof around 45°. This implies that shear deformation at around the tip of the notches happened ineach grain and the grain subdivision started. At a tensile strain of 2.7%, it is found that the grainaround the left-side notch was completely separated into at least two grains. In the grains aroundthe right-side notch, grain subdivision was proceeding because the difference in color in eachgrain, that is, the subgrain boundary, became clearer than at 0.8% tensile strain. At 3.9% tensile
strain, it is found that the left-side grain around the right-side notch was subdivided into three orfour grains and the right-side grain was subdivided into two grains. By summarizing the EBSD
analysis results obtained here, we found that the crystal in a grain around a notch tip rotatedpartially owing to shear deformation, and then a subgrain boundary was made along the shear3484

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deformation direction. With increasing tensile strain, the grain subdivision proceeded
continuously. Crack nucleation is expected to form around a notch, i.e., a stress-concentrated
portion, after the grain subdivision happened, and the crack will propagate along the shear
direction.
Figure 4 shows representative SEM images of the degraded portion taken during cyclic
tensile loading. The fatigue test was carried out at a stress amplitude of 35 MPa. The SEM
images until 150400 cycles were taken around the edge of the AlSi film where degradation
happened, and the last two images were taken such that the entire film was covered. At 16000
cycles, multiple slip lines can be seen on the AlSi film surface. The angle of the slip lines wasaround 60° from the axial direction of the film specimen. At 50000 cycles, crack nucleation wasformed at the edge of the film. Although it is difficult to directly see in the SEM image, it can beconsidered that grain subdivision happened just before the crack initiation occurred. At 105000
cycles, a crack was found to start propagating along the slip lines. The crack propagated acrossmultiple slip lines, but it can be seen that up to 145000 cycles, the direction of propagation does
not differ significantly from the direction of the slip lines. However, at 150000 cycles, the crackchanged its propagation direction from top right to top left at the portion indicated by a dashedcircle. At 150400 cycles, the crack became large and opened, and then it rapidly propagated into
the specimen width direction up to 150800 cycles. The degradation of area A is discussed later indetail. After that, the crack slightly changed its propagation position to the left side, thenpropagated to the top side rapidly, leading to catastrophic fatigue failure.
In the fatigue cracking phenomena shown in Fig. 4, several interesting findings wereobtained. The first one is that fatigue cracking was initiated where a dislocation glide occurred.
After tensile load application, crystal grains at the stress concentration area degraded, and grain
subdivision took place. Slip lines were produced owing to the degradation inside the grains. As aresult, crack nucleation probably newly formed in the dense slip line portion, leading to fatigue
cracking. The second one is that at the beginning of the cyclic loading test, a crack propagatedFig. 4.

SEM images of fatigue cracks in AlSi thin film under dynamic tension.

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along the slip lines that appeared in advance. This indicates that the dislocation glide would haveinfluenced the crack propagation. That is, it can be said that the dislocation glide formed thecrack propagation path. The third one is that a crack changed its propagation direction suddenly.
A crack propagated in a slightly zigzag manner at the beginning of the test. Then, it changed its
direction considerably in the middle of the test. Figure 5(a) is the SEM image showing multiple
slip lines, which are the evidence that a dislocation glide occurred. Most of the slip lines faced
the same direction (in this case, from the bottom left to the top right). Some of the slip lines wereformed in a different direction, which provided cross slips in the crystal grain. Note that there
are several areas where no slip lines could be detected in the SEM image. According to the EDX
map of the same region, the Si element shown in red was detected in the areas without slip lines.
That is, it can be said that Si segregates prevented dislocation glides during the cyclic loading
test. Figure 5(b) shows that there were some relationships between cracking and Si segregation.
Namely, when the crack propagated in front of the Si segregates, it probably stopped for a shorttime and then detoured to prevent hitting the segregates.(16) Si segregates worked well to prevent
cracking inside the AlSi film, which contributed to the extension of the lifetime of the Al alloy
film including the Si additive. The fourth one is that the degradation of the film including
dislocation glides and local cracks occurred ahead of a crack. As seen in the SEM images in Fig.
6, which are the enlarged images of area A in Fig. 4, a mottle pattern in front of the crack tip wasFig. 5. (Color online) SEM and EBSD images of fatigue crack stopping and the detouring around Si segregates for
zigzag cracking under dynamic tension.

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Fig. 6.

Sensors and Materials, Vol. 36, No. 8 (2024)(Color online) SEM images of degrading AlSi film just before fatigue failure (Detail of A in Fig. 4).

observed in the final part of the fatigue test. The mottle pattern is possibly the degradation
caused by multiple short dislocation glides that happened owing to large stress concentrations
originating from a fatigue crack opening process produced in the area. Also, a local crack was
produced beside the mottle-pattern region. Because of Si segregation, the region might havebeen brittle as compared with other portions, which would have brought about a somewhat
brittle fracture in the local area because a large stress was definitely concentrated around the
local crack. With the combination of the two mechanisms above, consequently, in the last part of
the test, the AlSi film probably catastrophically fatigue fractured, which was likely a brittle
fracture, along the specimen width direction perpendicular to the loading direction.
From the crack propagation observation results, the fatigue crack in the AlSi film was foundto propagate in a zigzag manner as it detoured the Si segregates. To deeply understand the effectof the Si additive on fatigue cracking in the AlSi film, the stress amplitude during the fatigue test
was changed. Figure 7 shows photographs of the top surfaces of AlSi films after the fatigue test
at stress amplitudes of 35, 40, and 45 MPa. At 35 MPa, it is found that a crack initiated at theedge of the specimen propagated in a zigzag manner from the cracking start to around threefourths of the specimen width. After that, a straight crack was observed until fracture. Many slip
lines were visible near the propagating crack. These trends are very similar to that shown in Fig.
4. At a stress amplitude of 40 MPa, basically, a trend similar to zigzag cracking was seen.
However, the degree of the zigzag of the crack appears to be smaller than that at 35 MPa. Whenthe stress amplitude increased to 45 MPa, the crack propagation pattern changed to straighter
than the other two specimens, regardless of having multiple slip lines inclined at around 45°,
which was almost the same as the other two specimens. That is, the Si additive in the AlSi film
was found to be more effective for crack stopping and then detouring under the low stressamplitude condition than under the high stress amplitude one.
To understand the influence of stress amplitude on fatigue cracking in more detail, the crack
path was analyzed as illustrated in Fig. 8. The photographs shown in the figure were taken
before the fatigue test. The red thick lines on each photograph indicate the crack path under eachcondition during the fatigue test. We tried to roughly estimate the crack propagation rate duringSensors and Materials, Vol. 36, No. 8 (2024)

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Fig. 7. (Color online) Fatigue cracking paths at stress amplitudes of 35, 40, and 45 MPa during dynamic tension
loading.

Fig. 8.

(Color online) Crack path analysis in AlSi films under dynamic tension.

the tests by comparing the propagated crack length measurable from the snapshots with thenumber of loading cycles at the moment. All the figures in Fig. 8 include multiple information,
crack path, crystal grain, grain boundary, slip lines, and crack propagation rate at the moment.
At the stress amplitude of 35 MPa, the crack propagation rate was initially 0.174 nm/cycle. After
that, the rate gradually increased to 1.85 nm/cycle at one-third of the specimen width. At aroundthe middle of the specimen, the crack propagation direction considerably changed from thebottom right to the bottom left. At the moment, the propagation rate decreased slightly to 0.984
nm/cycle, around half of the last rate. In the latter half, the crack propagated to roughly thebottom-left side of the picture with increasing propagation rate. At the last moment, the rate was
3.76 nm/cycle. At 40 MPa, the initial crack propagation rate was 0.47 nm/cycle, which was 2.7

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times higher than that at 35 MPa. Owing to the straighter cracking at 40 MPa than at 35 MPa,
the propagation rate increased monotonically throughout the fatigue test. Immediately before
fatigue fracture, the propagation rate at 40 MPa was 11.23 nm/cycle, around three times higher
than that at 35 MPa. At 45 MPa, the crack propagation was faster than that under the other two
conditions. At the initial and final moments, the crack propagation rates were 2.70 and 59.4 nm/
cycle, respectively, which were 5.3–5.7 times higher than those at 40 MPa.
Figure 9 shows the relationship between the crack propagation rate and the propagated crack
length. The circle, triangle, and squared plots indicate the stress amplitudes of 35, 40, and 45
MPa, respectively. The crack propagation rates are based on the estimation results in Fig. 8. The
propagated crack length indicates the traveled distance of the crack tip from one edge to theother edge of the film specimen. At 35 MPa, in the first stage ranging from 0 to 60 μm in crack
length, the crack propagation rate of the AlSi film was found to increase from around 0.2 to 1
nm/cycle. After that, in the middle stage from 60 to 180 μm in crack length, the crack
propagation rate of the AlSi film was maintained at around 1 nm/cycle. In the final stage, thepropagation rate rapidly increased until fatigue failure. In fact, the shape of the propagation
rate–crack length relationship looks like a creep S-shaped curve. It is well known that a creep
curve can be divided into three regions, primary (transient) creep, secondary (steady state) creep,
and tertiary creep. On the basis of these regions, the change in crack propagation rate (fatigue
degradation rate) can be explained as follows. In the first stage, a crack initiated at the specimenedge started to propagate rapidly toward the inside. The reason for the fast propagation can be as
follows. The energy that accumulated around the specimen edge owing to stress concentration
was released in a short period, which accelerated crack propagation. In the second stage, theFig. 9.

Relationship between fatigue crack propagation rate and crack length.

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crack propagation rate stayed almost constant because the driving forces for cracking andmechanical resistance were balanced. Actually, the crack propagated in a zigzag manner as it
passed the middle section of the specimen. In the third stage, the cross-sectional area of thespecimen rapidly became small, so the propagation rate increased considerably. At the stressamplitudes of 40 and 45 MPa, a trend almost similar to that observed at 35 MPa can be seen
except for two things: the slope in the second stage and the crack propagation rate in every stage.
At 40 MPa, the crack propagation rate in the second stage did not remain constant, but gradually
increased linearly in the semi-logarithmic graph as the crack length increased. The propagation
rate at 60 nm in crack length was roughly 4 nm/cycle, which increased to around 10 nm/cycle at
160 nm in crack length. At 45 MPa, the propagation rates at 60 and 160 nm in crack length werearound 15 and 50 nm/cycle, respectively, which were roughly 5 times higher than that at 40 MPa
at every crack length. The reason why the crack propagation rate and crack length relationshipdid not remain constant can be that the driving force for cracking was always greater than themechanical resistance originating from Si segregates, as seen in the comparison between the
AlSi alloy film and the pure Al film.(16) Moreover, with increasing stress amplitude, the slope ofthe crack propagation rate in relation to crack length increased. The AlSi alloy films used in thisstudy contained 1 wt% Si. Si was probably distributed almost uniformly, but Si segregatedportions were definitely Si rich. Si segregated portions are definitely hard mechanically because
Si is harder than Al, but possibly brittle as compared with the portion with less Si. The higher
stress amplitude during the fatigue test gave rise to a faster pulsating-tension loading, leading toa higher kinetic energy for crack propagation. This provided the ease of brittle-like fracture at
the Si segregated portions, which would have made the Si segregates less effective in the
stopping and detouring of fatigue crack under large stress amplitude conditions.

4.

Conclusions

In this paper, the fatigue crack nucleation and propagation mechanisms of a sputtered AlSi
alloy film were examined by the combination of quasistatic/dynamic tensile loading tests, SEM
crack observations, and EBSD and EDX analyses. Quasistatic tension loading demonstrated thatcrystal grain subdivision happened first at the part with stress concentration. Then, dynamic
tension loading demonstrated that slip lines originating from the dislocation glides formed, thencrack nucleation was initiated, and then the crack started propagating along the slip lines. A
crack propagated in a zigzag manner under slow loading conditions so that it stopped and thendetoured Si segregates, whereas a crack propagated straightly under fast loading conditions. It
was found that the balance between the driving force for cracking and the mechanical resistancearound the Si segregates determined the cracking manner during the fatigue test. Si added into
the AlSi film was confirmed to work well in reducing the crack propagation rate, contributing to
the long-term reliability of the AlSi film.

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