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Surface mechanical property assessment of ultra-thin HfO2 films
Thin Solid Films 544 (2013) 212–217
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Thin Solid Films journal homepage: www.elsevier.com/locate/tsf
Surface mechanical property assessment of ultra-thin HfO2 films Wei-En Fu ⁎, Bo-Ching He, Yong-Qing Chang Centers for Measurement Standards, Industrial Technology Research Institute, No.321, Sec.2, Kuangfu Rd., Hsinchu 30011, Taiwan
a r t i c l e
i n f o
Available online 10 April 2013 Keywords: Thin film X-ray diffraction Wear resistance HfO2
a b s t r a c t As the devices scale down, HfO2 is an excellent gate dielectric material and can replace SiO2 in complementary metal-oxide semiconductor technology. However, the mechanical property-based reliability such as wear resistance and deformation mechanism are rarely understood. This paper describes the effect of annealing treatment on 20-nm-thick HfO2 films under varying applied normal forces (31.3–104.2 μN). According to grazing incident X-ray diffraction analysis, the HfO2 thin films changed from amorphous to polycrystalline structure after annealing treatment. The scratch depth relative to initial surface was proportional to normal force. In addition, plowing behavior dominated the deformation mechanism in the form of lumps along the edge of groove by atomic force microscopy images. The annealing-induced crystallization resulted in reduced penetration depth, coefficient of friction, and wear rate at all applied normal forces, indicating that the surface hardness and wear resistance of HfO2 thin films can be enhanced through appropriate annealing treatment. Furthermore, substrate effect caused negative correlation between wear resistance and normal force was not obvious to annealed samples. It could be attributed to the broadening of HfSixOy interfacial layer which enhanced the structure strength. © 2013 Elsevier B.V. All rights reserved.
1. Introduction HfO2 is a material that possesses a wide range of technological applications. Due to unique chemical and physical properties such as high chemical stability, hardness [1], and thermal stability, HfO2 is a good protective coating. In semiconductor industry, HfO2 is one of several materials that may replace SiO2 as gate dielectric in complementary metal-oxide semiconductor due to better functionality and performance at lower cost [2]. Although many studies have been conducted on the microstructures and dielectric properties of HfO2 under different growth and thermal treatment conditions, there are few discussions on mechanical behaviors. The thickness of gate dielectric in transistors has shrunk from micrometer to nanometer range in order to meet the demand of miniaturization of electronic devices. The shrinkage of the film thickness to nanoscale leads to varied nano-mechanical properties like wear resistance and fatigue [3,4]. Furthermore, annealing treatment usually accompanies with changes of thin film structure. Consequently, cyclic thermal stress and wire bonding-induced stress during the semiconductor processing could possibly damage the films, and thus lower the structure reliability, if nano-mechanical properties of HfO2 thin films are not fully understood. Nanoindentation technique is frequently used to measure the nanomechanical properties of thin films such as hardness and Young's modulus [5,6], it is difficult to remove substrate effect while film thickness shrinks to nanoscale [7,8]. Nanoscratch technique can avoid this difficulty encountered in nanoindentation and is well suited for characterizing ⁎ Corresponding author. Tel.: +886 3 5732220; fax: +886 3 5726445. E-mail address: [email protected] (W.-E. Fu). 0040-6090/$ – see front matter © 2013 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.tsf.2013.03.117
practical adhesion failure of thin films and coatings. The wear resistance of thin film can also be accessed through elastic deformation and pileup [9]. Moreover, the residual penetration depth, coefficient of friction (COF), and wear rate can be measured and assessed for the surface hardness and wear characterization under varied normal force and annealing temperatures. In the previous study [10], the authors suggested that the formation of the HfSixOy interfacial layer caused the increase of the surface hardness for the annealed HfO2 films. In order to understand the hardness increase due to the HfSixOy interfacial layer, the coefficient of friction (COF) and wear rate were evaluated with respect to varied normal
Fig. 1. Schematic diagram of AFM nanoscratch experiment.
W.-E. Fu et al. / Thin Solid Films 544 (2013) 212–217
Fig. 2. GIXRD patterns of as-deposited and annealed HfO2 thin films.
force and contact stress, respectively. Nanoscratch experiments were conducted by atomic force microscope (AFM) with a diamond coated tip. The deformation mechanism was assessed by measured groove morphologies and cross-section scratch profiles. The reliability of HfO2 thin films deposited on Si wafer was assessed for semiconductor applications based on the evaluation of surface hardness and wear resistance. Additionally, the crystalline structure of as-deposited and annealed HfO2 thin films were characterized by grazing incident X-ray diffraction (GIXRD). 2. Experiments A 20-nm-thick HfO2 layer was grown on 8-inch (1 0 0) p-type Si wafer by using atomic layer deposition (ALD, Polygon 8200, ASM,
213
USA) method that makes atomic scale deposition control possible due to the characteristics of self-limiting and surface reactions. Detailed illustration of ALD process can be found elsewhere [11]. During the process, alternating surface saturating reactions between hafnium tetrachloride and water precursors at 300 °C were performed. Since no cleaning process before the deposition was applied to the bare Si wafer, a native SiO2 thin layer existed between HfO2 and Si wafer. The SiO2 interface was not significantly affected by the ALD process. The as-deposited HfO2 thin film was then cut into 20 mm × 20 mm samples for the subsequent annealing process. Due to a major structural change from amorphous to crystalline phases at ~ 450–550 °C [11,12] and current source/drain dopant activation requirements at ~900–1050 °C [13], the annealing processes were performed at 500 °C for 1 min and 900 °C for 2 min by rapid temperature annealing method with a heating rate of 50 °C/s under an argon environment. The structures of HfO2 thin film were characterized by GIXRD (X'Pert PRO MRD, PANalytical, Netherlands) technique. A metalceramic Cu Long Fine Focus X-ray tube operated at 45 kV and 40 mA was used for the generation of X-rays with wavelength of 0.154 nm. In order to increase diffraction volumes for thin films, an incident angle of 0.5° from the outermost film structures was selected. For the nanoscratch tests, AFM (Dimension Icon, Bruker, USA) was used to perform the experiments, as illustrated in Fig. 1. A nanoscale scratch on the HfO2 thin film surface was obtained using a diamondcoated Si tip (DT-NCHR, Nanosensors, USA). This coating on tip features with extremely high wear resistance due to the unsurpassed hardness of diamond. The tip radius of curvature is ~100 nm on a 10-μm-tall triangular pyramid probe. As far as mechanical specification is concerned, the probe includes a spring constant of ~42 N/m and a resonance frequency of ~330 kHz. The scratches were carried out with constant normal loads of 31.3, 52.1, 72.9, and 104.2 μN. The scratch speed was
Fig. 3. AFM-2D images of scratches on as-deposited HfO2 under constant normal forces of (a) 31.3, (b) 52.1, (c) 72.9, and (d) 104.2 μN.
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1 μm/s, and the length of each groove was 2 μm. The scratching direction along x-direction, as shown in Fig. 1, was perpendicular to the direction of cantilever length. At the same time, the lateral force was measured in situ through lateral force mode [14]. The profiles of the grooves were probed using a SiN tip (SCANASYSTAIR, Bruker, USA) in peak force tapping mode to prevent additional damage to the groove. The pyramid shape SiN tip had a typical tip radius of 2–12 nm with a spring constant of ~0.4 N/m and a resonance frequency of 70 kHz. The penetration depths of the as-deposited and annealed HfO2 thin films were obtained from the average values of scratch depths along the total scratch length. The surface hardness of the as-deposited and annealed HfO2 thin films was characterized by the analysis of the scratch depths.
3. Results and discussions 3.1. Structure analysis Typical XRD patterns of the HfO2 thin films between 2 θ from 20° to 48° are shown in Fig. 2. There is no clear diffraction peaks observed at collected angles for the as-deposited sample. It indicates that amorphous structure dominated for the as-deposited films. For the HfO2 film annealed at 500 °C, multiple peaks at various 2 θ diffraction angles appeared. According to the International Center for Diffraction Data, these peaks are monoclinic phases with different crystal directions except the peak 30.68° identified as (1 1 1) orthorhombic phase [11,15,16]. Therefore, a translation from amorphous to polycrystalline structure can be generated with annealing treatment at 500 °C. While annealing temperature was increased to 900 °C, the orthorhombic phase disappeared and only monoclinic phases existed. It demonstrates
that monoclinic phases dominated the structure at higher annealing temperature in HfO2 thin films [17]. 3.2. Surface property analysis The surface hardness of the HfO2 thin films can be evaluated through the volume of the removed material using scratch technique. Fig. 3 shows the AFM image of scratch topographies for as-deposited HfO2 thin films under different normal forces. It was found that lumps appeared along the two sides of scratch groove and became significant as normal force increased. The formation of lumps was a plastic deformation mechanism that the surface material underwent a plowing behavior by AFM tip as a result of pileups on the groove edges. Furthermore, the degree of plastic deformation is proportional to the volume of lump [18]. Hence, the plastic deformation became significant accompanying with lager normal force from increased lump. It is worth noting that there was no agglomerated continuous ribbon-like debris and only a small amount of wear plate-like debris around the grooves. It means that HfO2 thin films exhibited good scratch resistance and high fracture strength under normal force range from 31.3 to 104.2 μN. It also indicates that the material removal mechanism is primarily based on plowing behavior, but not cutting behavior and brittle fracture [19]. Figs. 4 and 5 show AFM images of scratch topographies for HfO2 thin films after annealing treatment at 500 and 900 °C, respectively. It is found that the plowing behavior still dominated on annealed samples and less debris was observed comparing with as-deposited one, indicating a promoted anti-cutting capability through annealing-induced crystallization [17]. It is worth noting that there are right angle tails at two ends of each scratch. The contact forces increased, when the tip approached to the beginning of scratch. The contact forces decreased, when the tip withdrew at the end of scratch.
Fig. 4. AFM-2D images of scratches on HfO2 thin films annealed at 500 °C under constant normal forces of (a) 31.3, (b) 52.1, (c) 72.9, and (d) 104.2 μN.
W.-E. Fu et al. / Thin Solid Films 544 (2013) 212–217
215
Fig. 5. AFM-2D images of scratches on HfO2 thin films annealed at 900 °C under constant normal forces of (a) 31.3, (b) 52.1, (c) 72.9, and (d) 104.2 μN.
The changes of the contact forces during the scratches would change the tilt angle of cantilever and lead to a lateral movement of contact position between the tip and the film surface. The tilt angle and the lateral movement induce a lateral tail-like scratch. Based on the AFM images, more islands can be found on the surface of annealed HfO2 compared with as-deposited one. This is due to the crystallization of HfO2 occurring through the annealing treatment [20], which is in consistent with XRD results. 3.3. Cross-section analysis of scratches Fig. 6 shows the cross-section profiles of scratch depth. The values of roughness average (Ra) are 0.28, 0.33, and 0.38 nm for as-deposited, 500 °C, and 900 °C samples, respectively. Compared with minimum scratch depth (~2 nm) in this experiment, the Ra is small enough to be ignored. The groove width and depth increased as normal force increased in all samples. It indicated that HfO2 thin films partly absorbed the loading energy given from AFM tip in the form of plastic deformation. In addition, larger normal force contributed to relative significant pileups due to plowing behavior. The normal force with respect to scratch depth in this experiment was exhibited in Fig. 7. A positive correlation between normal force and scratch depth can be observed in all samples. Obviously, the scratch depths after annealing treatment were smaller than as-deposited one under same normal forces. The surface hardness can be assessed by scratch depth [19]. Therefore, the results exhibit that annealed samples possess higher surface hardness than as-deposited one. From GIXRD analysis (see Fig. 2), crystallization was formed after annealing treatment. Hence, the polycrystalline structure had larger resistance to tip penetration compared with amorphous one. All the scratch depths
were less than 16 nm, which is below the thickness of HfO2 thin films. It implies that the 20-nm-thick HfO2 thin films possessed an anti-scratch effect while normal force ranged from 31.3 to 104.2 μN in this experiment [18]. 3.4. Coefficient of friction and wear rate The COF is the relation between normal and lateral force [19,21]. In Fig. 8, the COF for annealed samples is smaller than as-deposited one at the same normal forces, indicating that crystallization by annealing treatment can enhance the wear resistance [22]. The best resistance can be achieved with 900 °C-annealed sample under the maximum applied normal force (104.2 μN). Fig. 9 shows the wear rate with respect to contact stress. The contact stress σ was evaluated based on the normal force F divided by contact area A: σ¼
F : A
ð1Þ
The contact area can be obtained by an approximated model which mixes the Hertzian model of elastic contact of a sphere (tip) and a flat surface (HfO2) with other models [23,24]: A¼π
3Rt F 2=3 4E
ð2Þ
where Rt is the tip radius of curvature and E* is the reduced Young's modulus of tip and HfO2. The tip radius was determined by a tip calibration method (tip qualification model, nanoscope analysis 1.4, Bruker). These parameters were reported in Table 1. The wear rate
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W.-E. Fu et al. / Thin Solid Films 544 (2013) 212–217
Fig. 8. The normal force with respect to COF.
where L was 2 μm for all scratches and the wear volume was calculated by integrating the surface area of each 2D profile (extracted from different locations on 3D profile) over distance. Due to the crystallizationenhanced hardness, annealed samples exhibited smaller wear rate compared with as-deposited one. It was found that increased normal force would cause an increase in COF and wear rate due to substrate effect (hard film on soft substrate). However, annealing treated samples did not follow the tendency. According to previous study [10], HfSixOy interfacial layer generates and increases with increasing annealing temperatures. Since the hardness of HfSixOy was larger than that of HfO2, the interfacial layer would compensate the substrate effect, which induced unchanged and even decreases in COF and wear rate of the annealed sample at higher normal forces.
4. Conclusion
Fig. 6. Cross-section profiles of AFM scratches at various normal forces on the (a) as-deposited, (b) 500 °C-annealed, and (c) 900 °C-annealed HfO2 thin films.
w relates the wear volume V to the normal force F with product of the sliding distance L:
w¼
V FL
ð3Þ
Fig. 7. The normal force with respect to scratch depth.
The mechanical behaviors of HfO2 thin film were investigated through nanoscratch tests with AFM system. The groove morphology, depth cross-section, COF and wear rate were considered for evaluating the material removal mechanism, surface hardness, and wear resistance. From AFM images, lump along the scratch groove without the ribbon-like debris denoted that plowing behavior dominated the plastic deformation. It was found that the scratch depth decreased after annealing treatment, which means that the structure transformation from amorphous to polycrystalline phases can enhance the surface hardness of the HfO2 thin films. Compared with as-deposited sample, the annealed ones exhibited not only lower COF and wear rate under the same normal forces condition attributed to the crystallizationinduced increased wear resistance, but also less influence from substrate effect, which decreases the wear resistance at larger normal force, due to the broadening of harder HfSixOy interfacial layer.
Fig. 9. The contact stress with respect to wear rate.
W.-E. Fu et al. / Thin Solid Films 544 (2013) 212–217 Table 1 The parameters used to calculate contact area [25,26].
HfO2 Diamond-coated tip
Young's modulus (GPa)
Poisson's ratio
Radius (nm)
380 1104
0.2 0.2
60
[7] [8] [9] [10] [11] [12] [13]
Acknowledgments
[14] [15]
The financial support provided by the Bureau of Standards, Metrology and Inspection (BSMI) Nanometer-Scale Metrology Project is gratefully acknowledged.
[16] [17] [18]
References [1] J. Wang, H.P. Li, R. Stevens, J. Mater. Sci. 27 (1992) 5397. [2] S.A. Campbell, T.Z. Ma, R. Smith, W.L. Gladfelter, F. Chen, Microelectron. Eng. 59 (2001) 361. [3] Materials Research Society Proceedings, Thin films: stresses and mechanical properties, I–VIII, Materials Research Society, Pittsburgh (PA), 1988–1998. [4] R.W. Hoffman, in: G. Hass, R.E. Thun (Eds.), Physics of Thin Films, vol. 3, Academic Press, New York, 1966, p. 211. [5] B. Bhushan, Handbook of Micro/nano Tribology, CRC Press, 1995. [6] N.R. Moody, W.W. Gerberich, N. Burnham, S.P. Baker, Fundamentals of Nanoindentation and Nanotribology, Materials Research Society, 1998.
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Contents lists available at ScienceDirect
Thin Solid Films journal homepage: www.elsevier.com/locate/tsf
Surface mechanical property assessment of ultra-thin HfO2 films Wei-En Fu ⁎, Bo-Ching He, Yong-Qing Chang Centers for Measurement Standards, Industrial Technology Research Institute, No.321, Sec.2, Kuangfu Rd., Hsinchu 30011, Taiwan
a r t i c l e
i n f o
Available online 10 April 2013 Keywords: Thin film X-ray diffraction Wear resistance HfO2
a b s t r a c t As the devices scale down, HfO2 is an excellent gate dielectric material and can replace SiO2 in complementary metal-oxide semiconductor technology. However, the mechanical property-based reliability such as wear resistance and deformation mechanism are rarely understood. This paper describes the effect of annealing treatment on 20-nm-thick HfO2 films under varying applied normal forces (31.3–104.2 μN). According to grazing incident X-ray diffraction analysis, the HfO2 thin films changed from amorphous to polycrystalline structure after annealing treatment. The scratch depth relative to initial surface was proportional to normal force. In addition, plowing behavior dominated the deformation mechanism in the form of lumps along the edge of groove by atomic force microscopy images. The annealing-induced crystallization resulted in reduced penetration depth, coefficient of friction, and wear rate at all applied normal forces, indicating that the surface hardness and wear resistance of HfO2 thin films can be enhanced through appropriate annealing treatment. Furthermore, substrate effect caused negative correlation between wear resistance and normal force was not obvious to annealed samples. It could be attributed to the broadening of HfSixOy interfacial layer which enhanced the structure strength. © 2013 Elsevier B.V. All rights reserved.
1. Introduction HfO2 is a material that possesses a wide range of technological applications. Due to unique chemical and physical properties such as high chemical stability, hardness [1], and thermal stability, HfO2 is a good protective coating. In semiconductor industry, HfO2 is one of several materials that may replace SiO2 as gate dielectric in complementary metal-oxide semiconductor due to better functionality and performance at lower cost [2]. Although many studies have been conducted on the microstructures and dielectric properties of HfO2 under different growth and thermal treatment conditions, there are few discussions on mechanical behaviors. The thickness of gate dielectric in transistors has shrunk from micrometer to nanometer range in order to meet the demand of miniaturization of electronic devices. The shrinkage of the film thickness to nanoscale leads to varied nano-mechanical properties like wear resistance and fatigue [3,4]. Furthermore, annealing treatment usually accompanies with changes of thin film structure. Consequently, cyclic thermal stress and wire bonding-induced stress during the semiconductor processing could possibly damage the films, and thus lower the structure reliability, if nano-mechanical properties of HfO2 thin films are not fully understood. Nanoindentation technique is frequently used to measure the nanomechanical properties of thin films such as hardness and Young's modulus [5,6], it is difficult to remove substrate effect while film thickness shrinks to nanoscale [7,8]. Nanoscratch technique can avoid this difficulty encountered in nanoindentation and is well suited for characterizing ⁎ Corresponding author. Tel.: +886 3 5732220; fax: +886 3 5726445. E-mail address: [email protected] (W.-E. Fu). 0040-6090/$ – see front matter © 2013 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.tsf.2013.03.117
practical adhesion failure of thin films and coatings. The wear resistance of thin film can also be accessed through elastic deformation and pileup [9]. Moreover, the residual penetration depth, coefficient of friction (COF), and wear rate can be measured and assessed for the surface hardness and wear characterization under varied normal force and annealing temperatures. In the previous study [10], the authors suggested that the formation of the HfSixOy interfacial layer caused the increase of the surface hardness for the annealed HfO2 films. In order to understand the hardness increase due to the HfSixOy interfacial layer, the coefficient of friction (COF) and wear rate were evaluated with respect to varied normal
Fig. 1. Schematic diagram of AFM nanoscratch experiment.
W.-E. Fu et al. / Thin Solid Films 544 (2013) 212–217
Fig. 2. GIXRD patterns of as-deposited and annealed HfO2 thin films.
force and contact stress, respectively. Nanoscratch experiments were conducted by atomic force microscope (AFM) with a diamond coated tip. The deformation mechanism was assessed by measured groove morphologies and cross-section scratch profiles. The reliability of HfO2 thin films deposited on Si wafer was assessed for semiconductor applications based on the evaluation of surface hardness and wear resistance. Additionally, the crystalline structure of as-deposited and annealed HfO2 thin films were characterized by grazing incident X-ray diffraction (GIXRD). 2. Experiments A 20-nm-thick HfO2 layer was grown on 8-inch (1 0 0) p-type Si wafer by using atomic layer deposition (ALD, Polygon 8200, ASM,
213
USA) method that makes atomic scale deposition control possible due to the characteristics of self-limiting and surface reactions. Detailed illustration of ALD process can be found elsewhere [11]. During the process, alternating surface saturating reactions between hafnium tetrachloride and water precursors at 300 °C were performed. Since no cleaning process before the deposition was applied to the bare Si wafer, a native SiO2 thin layer existed between HfO2 and Si wafer. The SiO2 interface was not significantly affected by the ALD process. The as-deposited HfO2 thin film was then cut into 20 mm × 20 mm samples for the subsequent annealing process. Due to a major structural change from amorphous to crystalline phases at ~ 450–550 °C [11,12] and current source/drain dopant activation requirements at ~900–1050 °C [13], the annealing processes were performed at 500 °C for 1 min and 900 °C for 2 min by rapid temperature annealing method with a heating rate of 50 °C/s under an argon environment. The structures of HfO2 thin film were characterized by GIXRD (X'Pert PRO MRD, PANalytical, Netherlands) technique. A metalceramic Cu Long Fine Focus X-ray tube operated at 45 kV and 40 mA was used for the generation of X-rays with wavelength of 0.154 nm. In order to increase diffraction volumes for thin films, an incident angle of 0.5° from the outermost film structures was selected. For the nanoscratch tests, AFM (Dimension Icon, Bruker, USA) was used to perform the experiments, as illustrated in Fig. 1. A nanoscale scratch on the HfO2 thin film surface was obtained using a diamondcoated Si tip (DT-NCHR, Nanosensors, USA). This coating on tip features with extremely high wear resistance due to the unsurpassed hardness of diamond. The tip radius of curvature is ~100 nm on a 10-μm-tall triangular pyramid probe. As far as mechanical specification is concerned, the probe includes a spring constant of ~42 N/m and a resonance frequency of ~330 kHz. The scratches were carried out with constant normal loads of 31.3, 52.1, 72.9, and 104.2 μN. The scratch speed was
Fig. 3. AFM-2D images of scratches on as-deposited HfO2 under constant normal forces of (a) 31.3, (b) 52.1, (c) 72.9, and (d) 104.2 μN.
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W.-E. Fu et al. / Thin Solid Films 544 (2013) 212–217
1 μm/s, and the length of each groove was 2 μm. The scratching direction along x-direction, as shown in Fig. 1, was perpendicular to the direction of cantilever length. At the same time, the lateral force was measured in situ through lateral force mode [14]. The profiles of the grooves were probed using a SiN tip (SCANASYSTAIR, Bruker, USA) in peak force tapping mode to prevent additional damage to the groove. The pyramid shape SiN tip had a typical tip radius of 2–12 nm with a spring constant of ~0.4 N/m and a resonance frequency of 70 kHz. The penetration depths of the as-deposited and annealed HfO2 thin films were obtained from the average values of scratch depths along the total scratch length. The surface hardness of the as-deposited and annealed HfO2 thin films was characterized by the analysis of the scratch depths.
3. Results and discussions 3.1. Structure analysis Typical XRD patterns of the HfO2 thin films between 2 θ from 20° to 48° are shown in Fig. 2. There is no clear diffraction peaks observed at collected angles for the as-deposited sample. It indicates that amorphous structure dominated for the as-deposited films. For the HfO2 film annealed at 500 °C, multiple peaks at various 2 θ diffraction angles appeared. According to the International Center for Diffraction Data, these peaks are monoclinic phases with different crystal directions except the peak 30.68° identified as (1 1 1) orthorhombic phase [11,15,16]. Therefore, a translation from amorphous to polycrystalline structure can be generated with annealing treatment at 500 °C. While annealing temperature was increased to 900 °C, the orthorhombic phase disappeared and only monoclinic phases existed. It demonstrates
that monoclinic phases dominated the structure at higher annealing temperature in HfO2 thin films [17]. 3.2. Surface property analysis The surface hardness of the HfO2 thin films can be evaluated through the volume of the removed material using scratch technique. Fig. 3 shows the AFM image of scratch topographies for as-deposited HfO2 thin films under different normal forces. It was found that lumps appeared along the two sides of scratch groove and became significant as normal force increased. The formation of lumps was a plastic deformation mechanism that the surface material underwent a plowing behavior by AFM tip as a result of pileups on the groove edges. Furthermore, the degree of plastic deformation is proportional to the volume of lump [18]. Hence, the plastic deformation became significant accompanying with lager normal force from increased lump. It is worth noting that there was no agglomerated continuous ribbon-like debris and only a small amount of wear plate-like debris around the grooves. It means that HfO2 thin films exhibited good scratch resistance and high fracture strength under normal force range from 31.3 to 104.2 μN. It also indicates that the material removal mechanism is primarily based on plowing behavior, but not cutting behavior and brittle fracture [19]. Figs. 4 and 5 show AFM images of scratch topographies for HfO2 thin films after annealing treatment at 500 and 900 °C, respectively. It is found that the plowing behavior still dominated on annealed samples and less debris was observed comparing with as-deposited one, indicating a promoted anti-cutting capability through annealing-induced crystallization [17]. It is worth noting that there are right angle tails at two ends of each scratch. The contact forces increased, when the tip approached to the beginning of scratch. The contact forces decreased, when the tip withdrew at the end of scratch.
Fig. 4. AFM-2D images of scratches on HfO2 thin films annealed at 500 °C under constant normal forces of (a) 31.3, (b) 52.1, (c) 72.9, and (d) 104.2 μN.
W.-E. Fu et al. / Thin Solid Films 544 (2013) 212–217
215
Fig. 5. AFM-2D images of scratches on HfO2 thin films annealed at 900 °C under constant normal forces of (a) 31.3, (b) 52.1, (c) 72.9, and (d) 104.2 μN.
The changes of the contact forces during the scratches would change the tilt angle of cantilever and lead to a lateral movement of contact position between the tip and the film surface. The tilt angle and the lateral movement induce a lateral tail-like scratch. Based on the AFM images, more islands can be found on the surface of annealed HfO2 compared with as-deposited one. This is due to the crystallization of HfO2 occurring through the annealing treatment [20], which is in consistent with XRD results. 3.3. Cross-section analysis of scratches Fig. 6 shows the cross-section profiles of scratch depth. The values of roughness average (Ra) are 0.28, 0.33, and 0.38 nm for as-deposited, 500 °C, and 900 °C samples, respectively. Compared with minimum scratch depth (~2 nm) in this experiment, the Ra is small enough to be ignored. The groove width and depth increased as normal force increased in all samples. It indicated that HfO2 thin films partly absorbed the loading energy given from AFM tip in the form of plastic deformation. In addition, larger normal force contributed to relative significant pileups due to plowing behavior. The normal force with respect to scratch depth in this experiment was exhibited in Fig. 7. A positive correlation between normal force and scratch depth can be observed in all samples. Obviously, the scratch depths after annealing treatment were smaller than as-deposited one under same normal forces. The surface hardness can be assessed by scratch depth [19]. Therefore, the results exhibit that annealed samples possess higher surface hardness than as-deposited one. From GIXRD analysis (see Fig. 2), crystallization was formed after annealing treatment. Hence, the polycrystalline structure had larger resistance to tip penetration compared with amorphous one. All the scratch depths
were less than 16 nm, which is below the thickness of HfO2 thin films. It implies that the 20-nm-thick HfO2 thin films possessed an anti-scratch effect while normal force ranged from 31.3 to 104.2 μN in this experiment [18]. 3.4. Coefficient of friction and wear rate The COF is the relation between normal and lateral force [19,21]. In Fig. 8, the COF for annealed samples is smaller than as-deposited one at the same normal forces, indicating that crystallization by annealing treatment can enhance the wear resistance [22]. The best resistance can be achieved with 900 °C-annealed sample under the maximum applied normal force (104.2 μN). Fig. 9 shows the wear rate with respect to contact stress. The contact stress σ was evaluated based on the normal force F divided by contact area A: σ¼
F : A
ð1Þ
The contact area can be obtained by an approximated model which mixes the Hertzian model of elastic contact of a sphere (tip) and a flat surface (HfO2) with other models [23,24]: A¼π
3Rt F 2=3 4E
ð2Þ
where Rt is the tip radius of curvature and E* is the reduced Young's modulus of tip and HfO2. The tip radius was determined by a tip calibration method (tip qualification model, nanoscope analysis 1.4, Bruker). These parameters were reported in Table 1. The wear rate
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W.-E. Fu et al. / Thin Solid Films 544 (2013) 212–217
Fig. 8. The normal force with respect to COF.
where L was 2 μm for all scratches and the wear volume was calculated by integrating the surface area of each 2D profile (extracted from different locations on 3D profile) over distance. Due to the crystallizationenhanced hardness, annealed samples exhibited smaller wear rate compared with as-deposited one. It was found that increased normal force would cause an increase in COF and wear rate due to substrate effect (hard film on soft substrate). However, annealing treated samples did not follow the tendency. According to previous study [10], HfSixOy interfacial layer generates and increases with increasing annealing temperatures. Since the hardness of HfSixOy was larger than that of HfO2, the interfacial layer would compensate the substrate effect, which induced unchanged and even decreases in COF and wear rate of the annealed sample at higher normal forces.
4. Conclusion
Fig. 6. Cross-section profiles of AFM scratches at various normal forces on the (a) as-deposited, (b) 500 °C-annealed, and (c) 900 °C-annealed HfO2 thin films.
w relates the wear volume V to the normal force F with product of the sliding distance L:
w¼
V FL
ð3Þ
Fig. 7. The normal force with respect to scratch depth.
The mechanical behaviors of HfO2 thin film were investigated through nanoscratch tests with AFM system. The groove morphology, depth cross-section, COF and wear rate were considered for evaluating the material removal mechanism, surface hardness, and wear resistance. From AFM images, lump along the scratch groove without the ribbon-like debris denoted that plowing behavior dominated the plastic deformation. It was found that the scratch depth decreased after annealing treatment, which means that the structure transformation from amorphous to polycrystalline phases can enhance the surface hardness of the HfO2 thin films. Compared with as-deposited sample, the annealed ones exhibited not only lower COF and wear rate under the same normal forces condition attributed to the crystallizationinduced increased wear resistance, but also less influence from substrate effect, which decreases the wear resistance at larger normal force, due to the broadening of harder HfSixOy interfacial layer.
Fig. 9. The contact stress with respect to wear rate.
W.-E. Fu et al. / Thin Solid Films 544 (2013) 212–217 Table 1 The parameters used to calculate contact area [25,26].
HfO2 Diamond-coated tip
Young's modulus (GPa)
Poisson's ratio
Radius (nm)
380 1104
0.2 0.2
60
[7] [8] [9] [10] [11] [12] [13]
Acknowledgments
[14] [15]
The financial support provided by the Bureau of Standards, Metrology and Inspection (BSMI) Nanometer-Scale Metrology Project is gratefully acknowledged.
[16] [17] [18]
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