Mechanical and tribological properties of 100-nm thick alumina films prepared by atomic layer deposition on Si(100) substrates

The study investigates mechanical and tribological properties of alumina (Al2O3) films prepared on Si(100) substrates. The 100-nm thick films were deposited by atomic layer deposition. Nanoindentation and nano-scratch tests were performed with Berkovich and sphero-conical diamond indenters, respectively. Energy-dispersive X-ray spectroscopy and optical and scanning electron microscopy were used to analyse the surface morphology and chemical composition of the thin films. X-ray diffraction was used to characterize their crystal structure. Crystallization was found to start at 1100 °C after 3 hours of annealing. The hardness and tribological properties of the alumina films were influenced by the substrate in nanoindentation and nano-scratch tests. Within the relatively low load range (5–50 mN), the coefficient of friction of Si and alumina against diamond depended on the load, most likely due to a change in the elastic/plastic deformation behaviour within the Si substrate.


INTRODUCTION
* Alumina films are an important technological material used for chemical and mechanical protection of engineering components [1] and in the manufacturing of optical devices and micro-electromechanical systems.Preparation of patterned alumina films is an important aspect of modern technology to study the influence of surface textures on friction and wear [2].
Atomic layer deposition (ALD) is widely used for the preparation of alumina films [3].Nanoindentation and nano-scratching are well-known methods to determine the mechanical properties of thin films.The nanohardness of alumina films can vary between 6 and 12 GPa [4,5].However, there is a lack of investigations of mechanical properties of thin alumina films by the nano-scratch method.
This study focuses on mechanical and tribological properties (within 0.2-400 mN of load) of 100-nm thick alumina films deposited on Si(100).

EXPERIMENTAL METHODS
The alumina thin films of 100-nm thickness were prepared on Si(100) substrates by the ALD method with subsequent annealing.The deposition temperature was 300 °C.The temperature and duration of annealing for different samples are shown in Table 1.
The nanoindentation tests were carried out by means of a nano-mechanical testing system (NanoTest, Micro Materials Ltd.) using a diamond Berkovich indenter.
The applied loads were varied between 0.2 and 0.45 mN with the increment of 0.05 mN.The nanoindentation was performed with a series of 10 indentations at each load on each sample.The mechanical properties of the Si substrate and alumina films, such as the hardness and elastic modulus, were determined by the Oliver-Pharr method [6].The nano-scratch tests were performed using a sphero-conical diamond indenter with a radius of 5 µm (the tests details are listed in Table 2).The friction force (F x ) and friction coefficient (COF) were the parameters of interest for nano-scratch tests.The surfaces of samples after the scratch tests were examined by optical microscopy.

RESULTS AND DISCUSSION
Scanning Electron Microscopy (SEM) images taken of the Al 2 O 3 films are shown in Fig. 1.The as-deposited film (sample 1 in Table 1) has a uniform smooth layer (Fig. 1a).A similar surface morphology was observed after annealing at 500, 700, and 900 °C.The holes within the oxide layer (sample 6) can be seen in the SEM image after annealing at 1100 °C for 3 h (Fig. 1b).This demonstrates how the patterned alumina films can be prepared.
SEM images taken in the backscattered-electron (BSE) mode are shown in Fig. 2a and Fig. 2b along with the energy-dispersive X-ray spectroscopy (EDS) patterns.The spectrum taken within the hole shows a relatively strong Si signal, indicating the peeling of the film after annealing at 1100 °C for 3 h in comparison with the spectrum corresponding to the pristine alumina film surface, which possesses a strong Al peak.The X-ray diffraction (XRD) patterns taken on samples 5 and 6 indicate the formation of a crystalline structure after 3 h of annealing (Fig. 2c).The study of the literature suggests that most likely multiphase alumina was formed [7].The structure of samples 1-5 was amorphous.
The nanohardness and Young's modulus of the Si(100) substrate were in the range of 10.6-13.2GPa and 142-169 GPa, respectively, which is in good agreement with the literature [8].The hardness and Young's modulus of as-deposited and annealed at 500, 700, 900, and 1100 °C alumina films were in the range of 10-13 GPa and 150-190 GPa; thus the Si(100) substrate influenced the properties of the alumina films (Table 3).
The scratch test on the Si(100) substrate, illustrated in Fig. 3, was performed at the peak load of 200 mN.The results of the scratch test are in good agreement with the literature [9].The main transitions on the Si substrate are as follows: L y -onset of non-elastic deformation, Lc 1 -pop-in observed by an accompanied change in the colour of the scratch track, and Lc 2 -prominent chipping observed at the edges of the track.
The behaviour of alumina film (sample 1) under the peak load of 200 mN is shown in Fig. 4a.Chipping starts at Lc 2 = 78 mN.The response of Al 2 O 3 film (sample 6) at the peak load of 200 mN is shown in Fig. 4b; no clear Lc 1 transition was observed.Chipping starts at Lc 2 = 89 mN.There is a good correlation with the value of 63 mN (Lc 1 ) observed on Si(100), which shows the influence of substrate on the properties of alumina films, i.e. when severe deformation starts within the Si substrate.
Figure 5a shows that there is a correlation between the normal load and friction force.According to Amontons' law of friction, the friction force is directly proportional to the normal load F x = µN, where F x denotes frictional (tangential) force, N is normal load, and µ is the coefficient of friction.The COF is the coefficient of proportionality and it is independent of load [10,11].Figure 5b shows COF versus normal load.It should be noted that the COF value increased for alumina and Si within stage 1, which is a disagreement with Amontons' law of friction.According to data shown in Figs 3 and 4, the elastic deformation as well as plastic deformation dominated in this stage.Most likely, the COF value changed due to the mainly continuous change of the elastic-plastic behaviour of Si.Therefore, contact conditions between the indenter and sample surface changed continuously as well.However, for stage 2, the COF values were nearly constant; such behaviour shows agreement with the above-mentioned law of friction.
The plastic deformation and wear (chipping) were dominant processes for stage 2 (Figs 3 and 4).The COF  for Si(100) substrate within the load range of 200-400 mN was 0.1, which is in good agreement with the literature [12].The dependence of the COF values on the load between alumina and diamond in sliding tests was observed in previous studies as well [13].There is a clear difference between COF values for alumina and Si for low loads (5-20 mN), where the elastic-plastic deformation dominated (Figs 3 and 4).In other words, in spite of the strong influence of the substrate observed in nanoindentation tests even for the lower loads (up to 0.45 mN), see discussion above, the tribological properties of alumina differed from those for the Si substrate in nano-scratch tests.The difference in COF values for higher loads (200-400 mN), where severe wear was the dominant process, probably means that the alumina films were not fully delaminated from Si substrates.

CONCLUSIONS
Nanoindentation and nano-scratch tests were performed to evaluate the mechanical and tribological properties of thin alumina films deposited on Si(100) substrates.The measurements of hardness and Young's modulus of Al 2 O 3 thin films were influenced by the Si substrate.After annealing at 1100 °C, formation of holes within Al 2 O 3 films was observed.Disagreement with Amontons' law of friction was found for the loads in the range of 5-20 mN, which probably corresponds to a continuous change from elastic to plastic deformations within the Si substrate.However, for the higher loads (200-400 mN), where the wear is the dominant process, a good agreement was found with Amontons' law.

Fig. 3 .
Fig. 3. Nano-scratch test on the Si substrate at the peak load of 200 mN.

Fig. 2 .
Fig. 2. BSE images and EDS spectra taken on sample 6: (a) within the hole, (b) on the film surface, and (c) XRD pattern after 2 and 3 hours of annealing at 1100 °C.

Table 3 .Fig. 4 .
Fig. 4. Nano-scratch tests on alumina along with traces taken by means of optical microscopy: (a) as-deposited sample 1 and (b) sample 6.

Table 1 .
Post-deposition annealing parameters of films