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Effects of annealing on the temperature coefficient of resistance of nickel film deposited on polyimide substrate
Vacuum 160 (2019) 18–24
Contents lists available at ScienceDirect
Vacuum journal homepage: www.elsevier.com/locate/vacuum
Effects of annealing on the temperature coefficient of resistance of nickel film deposited on polyimide substrate
T
Baoyun Sun, Pengbin Wang, Binghe Ma∗, Jinjun Deng, Jian Luo Key Laboratory of Micro, Nano Systems for Aerospace, Ministry of Education, Northwestern Polytechnical University, Xi'an, Shaanxi, 710072, China
A R T I C LE I N FO
A B S T R A C T
Keywords: Annealing Temperature coefficient of resistance Nickel film Magnetron sputtering Polyimide substrate
This study investigates the effects of annealing temperature as well as time on the temperature coefficient of resistance (TCR) of nickel (Ni) films deposited on flexible polyimide substrate by direct current magnetron sputtering. The morphological and microstructural characteristics of as-deposited and annealed Ni films were analyzed to understand the mechanisms of TCR variation. The crystalline phase, grain size change tendency and residual stress of films were evaluated by X-ray Diffractometer (XRD) technique. (111) is the preferred orientation of Ni and the grains have a growth after annealing. The residual stress in as-deposited films is about −446.3 MPa and reduced to −208.8 MPa after annealing at 400 °C for 6 h. Focused Ion Beam Electron Microscope (FIBEM) and Energy Dispersive X-ray Spectroscopy (EDS) investigations showed the microstructure of annealed films has a relative improvement and the oxygen content was little increased with increasing annealing temperature and time. Mean surface roughness determined by Atomic Force Microscope (AFM) is decreased from 4.5 nm to 1.25 nm with increasing annealing temperature. The TCR value increases with the increasing annealing temperature and time. When the annealing time exceeds 6 h, the TCR almost has no change or even begin to decline.
1. Introduction Wall shear stress is one of the most important fluid mechanical parameters, which is a tangential force in the direction of motion due to the viscosity of the fluid [1]. Its measurement is essential for drag reduction, active flow control, boundary layer separation and transition detection in fluid mechanics applications [2]. Over the past few decades, numerous shear stress measurement techniques have been developed [3–6]. Among all these approaches, the flexible thermal shear stress sensors fabricated on polyimide substrate were most often used since they have merits that well fit on curved surfaces, capable of multipoint measurement, have less interference to the flow and high temporal/spatial resolution. The typical flexible thermal shear stress sensors structure mainly consists of substrate, thermistor and leading wire. Polyimide has stable mechanical, electrical, chemical and excellent temperature resistance properties [7], which is a good choose as a flexible sensor substrate. Because of its highest TCR of all metals [8,9], Ni was often being selected as the thermistor material [10–14]. TCR defines the resistance change with the temperature, which can be expressed by the following equation:
∗
TCR =
R − R0 × 106ppm / °C (T − T0) R 0
(1)
where R is the resistance at temperature T, R0 is the resistance at room temperature T0 (20 °C). Regardless of their differences in form, working principle of all thermal shear stress sensors is same [6,8]. The sensing thermistor is heated by current and forced-convective heat transfer with the passing fluid flow (Fig. 1). As the temperature of the thermistor varies with changes of the velocity gradient, so does the resistance. Thus, the sensitivity of the sensor is determined by the magnitude of the TCR. Hence, a high TCR value is desired for the thermistor of thermal shear stress sensor. Vacuum thermal annealing has been reported as one effective method to improve the electrical properties of thin film [15–17]. Therefore, for the sake of getting Ni film with highest TCR value, it is essential to explore the correlation between the TCR of Ni thin film and annealing treatment. In the present experimental studies, a polyimide based flexible thermal shear stress sensor has been developed. The effects of annealing temperature and time on the TCR value of Ni film deposited by direct current magnetron sputtering have been investigated. The crystalline
Corresponding author. E-mail address: [email protected] (B. Ma).
https://doi.org/10.1016/j.vacuum.2018.11.016 Received 23 July 2018; Received in revised form 7 November 2018; Accepted 8 November 2018 Available online 09 November 2018 0042-207X/ © 2018 Published by Elsevier Ltd.
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Fig. 1. Schematic view of the thermal shear stress sensor sensing principle.
phase, grain size, residual stress, composition, microstructure and surface roughness of as-deposited and annealed films were evaluated by XRD, EDS, FIBEM and AFM techniques, respectively. The resistance values of Ni thermistors at different temperatures were measured by four-wires method in a constant temperature water bath. 2. Experimental 2.1. Sensor manufacture The fabrication processes of the flexible thermal shear stress sensor are shown in Fig. 2 [18]. A ready-made 50 μm thick UBE UpilexS polyimide foil was used as the sensor substrate. The polydimethylsiloxane (PDMS) was spun-on a glass wafer, then the polyimide foil was bonded onto the glass wafer by the PDMS adhesive layer and cured in a vacuum drying oven (a). The Ni thermistor film of 1 μm thickness was then magnetron sputtered (b) onto the polyimide foil. After that, a 200 nm thick copper (Cu) lead film was sputtered onto the Ni film as a seed layer (c). To reduce the lead resistance, the thickness of the Cu film was increased to 3 μm according electroplating process (d). Finally, the Cu and Ni films were patterned with photolithography and wet-etched (e-f), respectively. To improve the TCR and make the sensor's electrical resistance stable, annealing (g) was carried out to eliminate lattice defects in Ni films. Parylene C was vapor deposition on the sensor as a protective layer (h). Three-dimensional schematic view and finished sensors are shown in Fig. 3. 2.2. Ni film preparation The Ni films were deposited on polyimide substrate by DC magnetron sputtering system from a pure Ni (with a purity of 99.99%) target with diameter of 5 inch. The films were deposited without intentional heating of the substrates. Base pressure of the deposition chamber was approximately 8.0 × 10−4 Pa. Pure argon (with a purity of 99.999%) Fig. 3. (a) Three-dimensional schematic view of the sensor structure. (b) Flexible hot-film sensors.
was used as sputtering gas and flow rate was 60sccm. Target was presputtered for 10min to remove any surface impurities. All deposition was performed using an input power of 150 W, which gave a deposition rate of about 25 nm/min. Films with a thickness of about 1 μm were deposited. After the deposition, some films were annealed at 200 °C, 250 °C, 300 °C, 350 °C and 400 °C for 2, 4, 6 and 8 h (h) at a base pressure of 1.0 × 10−3Pa, respectively. The decomposition temperature of this type polyimide is about 500 °C. If the annealing temperature is over 400 °C, the polyimide will transition to fragile and the deposited film will start peeling off from the substrate surface. Therefore, the highest annealing temperature is set at 400 °C. Fig. 2. The flexible hot-film shear stress sensor fabrication processes. 19
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2.3. Film characterization The crystalline phase and average grain size change tendency of the Ni films were analyzed from XRD data obtained in standard BraggBrentano geometry. XRD studies were carried out with a SHIMADZU XRD-7000 X-RAY DIFFRACTOMETER using a Cu Kα (λ = 0.154 nm) radiation source operated at 40 kV and 30 mA. For phase identification, XRD patterns were collected in the 2θ range of 30–80°. The residual stress in Ni films were measured using the sin2ψ technique [19–24] with the help of another XRD in Bragg Brentano configuration (Bruker D8 ADVANCE, Germany). During measurement, the positions of Ni (311) (2θ∼92.947°) peaks were chosen to determine the stress. The tilt angles ψ were changed from 0° to 45° with steps of 9° at two different rotation angles ϕ of 0° and 180°, respectively. The Poisson's ratio and Young's modulus of Ni material used for stress calculation were ν = 0.31 and E = 210Gpa, respectively [25]. The surface and cross-sectional microstructures were observed by a FIBEM (FEI Helios G4 CX) and the composition of the films were determined by a Thermo NS7 EDS attachment of FIBEM. The EDS was calibrated by using the cross-section of silicon. The film surface morphology was investigated by an AFM (BRUKER Dimension Fast Scan™). All the measurements were conducted without Parylene C layer deposition. The resistance of the patterned thermistor was measured by fourwires method with an Agilent 34410A digital multimeter. The resistance values at various temperatures were obtained in a thermostatic water bath. The temperature was monitored by a standard platinum resistance thermometer. The TCR values can be calculated by Eq. (1).
Fig. 5. XRD patterns of the Ni films (a) as-deposited and annealed at 300 °C for (b) 2 h, (c) 4 h, (d) 6 h.
inversely proportional to the FWHM value. As a result, the grains growing bigger with increasing annealing temperature below 400 °C. Fig. 5 shows the XRD patterns of the as-deposited Ni films and annealed at 300 °C for different times. The XRD patterns show Ni (111), Ni (200) and Ni (220) diffraction peaks as well as Fig. 4. The FWHM value of Ni (111) decreases from 0.520 of as-deposited film to 0.456 of film annealed at 300 °C for 6 h, which means the grains growing bigger with increasing annealing time below 6 h, too. The residual stress in Ni films determined by the sin2ψ technique is shown in Table 1. The results indicate that the residual stress in both asdeposited and annealed Ni films tends to be compressive in character. The residual stress of as-deposited films is about −446.3 MPa and has a remarkable relaxation after annealing. Moreover, the magnitude of this stress is progressively decreased with increasing annealing temperature and time. The smallest value of stress can be reduced to about −208.8 MPa after annealing at 400 °C for 6 h. The reduction in residual stress may be attributed to the point-defect annihilation take place during the annealing process [27,28].
3. Results and discussions 3.1. XRD analysis Fig. 4 shows the XRD patterns of the as-deposited Ni films and annealed at different temperatures for 4 h. The reflections at about 44.4°, 51.8° and 76.3° represent the (111), (200), (220) planes of face centered cubic structure. The results indicate that (111) is the preferred orientation of Ni films. When the annealing temperature increased to 400 °C, the full width at half maximum (FWHM) value of Ni (111) decreases from 0.528 of as-deposited film to 0.442 of film annealed at 400 °C for 4 h. According to the Scherrer formula [26], the grain size
3.2. FIBEM analysis The composition of Ni films record by the EDS is shown in Table 1. The oxygen content of as-deposited films is less than the annealed films. The oxygen composition of annealed Ni films is progressively increased with increasing annealing temperature and time, but the variation is very little. These results indicate that the annealing process was accompanied by the formation of oxide. Fig. 6 shows the surface and cross-sectional microphotographs of Table 1 The residual stress of Ni films measured by XRD and the composition of Ni films record by EDS. Annealing temperature (°C)
200 300 300 300 400 No annealing
Fig. 4. XRD patterns of the Ni films (a) as-deposited and annealed at (b) 200 °C, (c) 300 °C, (d) 400 °C for 4 h. 20
Annealing time (h)
6 4 6 8 6
Residual stress (MPa)
−400.9 −399.7 −325.5 −256.7 −208.8 −446.3
± ± ± ± ± ±
18.3 18.2 17.6 15.9 12.0 18.5
Composition (at.%) Ni
O
96.04 96.35 96.04 95.90 95.85 96.39
3.96 3.65 3.96 4.10 4.15 3.61
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the Ni film is impossible to get high TCR. When the films were annealed at 200 °C (Fig. 6(b)), 300 °C (Fig. 6(c)) and 400 °C (Fig. 6(d)) for 4 h, the surface morphology has a relative improvement and the columnar crystals were denser with larger grain size. The possible reason for these structural changes is annealing provides thermal energy for the adatoms growing and moving to the suitable site [29]. When the films were annealed at 300 °C for 2 h (Fig. 6(e)), 4 h (Fig. 6(c)) and 6h (Fig. 6(f)), there is a grain growth and the crosssections show a more compact morphology with increasing annealing time. The possible reason for improvement in the structural properties is the ad-atoms have enough time to move and relocate on the substrate surface. 3.3. AFM analysis The three-dimensional AFM images of the Ni films as-deposited and annealed at different temperatures and times are shown in Fig. 7. The surface roughness was determined by the AFM data. The values of the mean roughness (Ra) and the root mean square roughness (Rq) of asdeposited film and films annealed at different temperatures are present in Fig. 8. The results show that the surface roughness of the Ni films decreases with increase in annealing temperature up to 400 °C. This is attributed to more uniform growth take place during the annealing process. Commonly, smoothing and roughening are two basic stages accompanied with the film growth [30]. It is obviously that our annealing of Ni films below 400 °C is in the smoothing stage. It is well known that higher annealing temperature provides more thermal energy for adatoms moving. The rearrangement of adatoms increases the surface density by sintering of intergranular pores and coalescence of grains. Furthermore, we can see that the grains have a growth after annealing from the XRD and FIBEM results. The growth of grains at this stage is two-dimensional (except vertical direction) and along their preferred directions, until they impinge on each other. The above reasons are expected to cause more uniform film growth and smooth surface [27,31,32]. Due to the limitation of the substrate decomposition temperature, the Ni films cannot be annealed at higher temperature over 400 °C. However, if the films be annealed at higher temperatures, the surface roughness is supposed to increase above a critical temperature. It is attributed to non-uniform three-dimensional grain growth and partial recrystallization during annealing at higher temperatures. As known, the resistivity of metal is proportional to the surface roughness, which is because of the rougher surface leads to more electron scattering at the boundary and shorter mean free path length [33–35]. According to Matthiessen's rule, the relationship between TCR α and resistivity ρ can be written as: αρ = constant
(2)
so, the TCR has a negative correlation with the surface roughness. 3.4. Electrical properties The resistance change with respect to the ambient temperature variation of films annealed at different temperatures are shown in Fig. 9. As we can see, the resistance of Ni films is linear increase with increasing ambient temperature from 10 °C to 90 °C, and the linearly dependent coefficient is over 0.999. The TCR of the as-deposited film and films annealed at different temperatures and times are present in Fig. 10 and Fig. 11, respectively. The TCR value of as-deposited film is only about 2900 ppm/°C, which is much lower than the bulk material (∼6800 ppm/°C). In Fig. 10, the TCR is shown to be affected dramatically by annealing and exhibits a nonlinear increase with the increase of annealing temperature. When the annealing temperature is over 300 °C, the TCR variation is greater
Fig. 6. FIB Electron Microscope microphotographs of the Ni films, (a) as-deposited, (b) 200 °C, 4 h, (c) 300 °C, 4 h, (d) 400 °C, 4 h, (e) 300 °C, 2 h, (f) 300 °C, 6 h.
the Ni films as-deposited and annealed at different temperatures and times. As we can see from the cross-sectional morphologies in Fig. 6, all the Ni films exhibit a columnar grain structure. Most of the grains in asdeposited film (Fig. 6(a)) are agglomerated with each other but they are not arranged densely. This is attributed to there is not enough kinetic energy for ad-atoms moving to the proper location. With this structure, 21
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Fig. 7. 3D AFM images of the Ni films, (a) as-deposited, (b) 200 °C, 4 h, (c) 300 °C, 4 h, (d) 400 °C, 4 h, (e) 300 °C, 2 h, (f) 300 °C, 6 h.
roughness resulted in higher conductivity and TCR. Even though the oxygen composition has a small increase in the annealed films, the TCR was still increased with the annealing temperature below 400 °C and time below 6 h. The possible reason for the saturated TCR after 6 h is the ad-atoms reach stable state within 6 h and grain cannot grow anymore with the annealing time increasing. Another reason maybe the additional increase oxidation of Ni films with increasing annealing time. In the previous study by other researchers [36], the results show that TCR is inversely proportional to the annealing time at a given temperature, which is quite different from our experimental results. We believe that the reason for this phenomenon is the formation of too much nickel oxide on the film surface, which is due to the vacuum
than the lower annealing temperatures. Fig. 11 shows that the TCR has a positive correlation with the annealing time below 6 h. The highest TCR value can reach approximately 5000 ppm/°C after annealing. The evolution of TCR can be explained by the improvement of the films structural and morphological properties. It has been known from XRD patterns, FIBEM microphotographs and AFM images that the asdeposited Ni films have smaller grain size which not arranged densely and often contained many structural defects, higher residual stress and rougher surfaces. All these above reasons lead to a relatively small TCR value. With increasing annealing temperature and time, the ad-atoms got more thermal energy and time to migrate and grow. Larger grain size decreased the grain boundaries, defects annihilation and smaller 22
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Fig. 10. TCR of the films annealed at different temperatures. Fig. 8. Mean surface roughness (Ra) and root mean square roughness (Rq) of the as-deposited Ni film and films annealed at different temperatures for 4 h.
Fig. 11. TCR of the films annealed at different times.
2. Annealing treatment provided enough thermal energy and time for the ad-atoms to grow and rearrangement at the suitable site on the substrate surface. Resulting in the appearance of morphological and microstructural improvements, including grain growth and residual stress relaxation. Meanwhile, the two-dimensional growth below critical temperature lead to a smaller surface roughness. 3. Whereas the oxygen composition has an increase after annealing, the microstructure improvements still make the TCR values show a significant nonlinear increase with the increase of annealing temperature and time. The highest TCR value can reach approximately 5000 ppm/°C after annealing at 400 °C for 6 h. 4. When the annealing time exceeds 6 h, the TCR almost has no increase or even begin to decline. This is may be attribute to 6 h is enough for the ad-atoms to reach stable state below 400 °C annealing and additional increase oxidation of Ni films.
Fig. 9. Resistance change with respect to the ambient temperature variation of films annealed at different temperatures. R is the resistance at temperature T, R0 is the resistance at room temperature T0 (20 °C). A, B and C means three films annealed at same temperature for same time.
degree of the annealing furnace is not enough. Hence, if the base pressure of the annealing furnace is equal or less than 1.0 × 10−3Pa, the TCR value will increases with increasing annealing time within 6 h. 4. Conclusions The Ni films were deposited by direct current magnetron sputtering system on flexible polyimide substrates. The films were annealed at temperatures range from 200 °C to 400 °C, and duration times rang from 2 h to 8 h. The crystalline phase, grain size, residual stress, composition, microstructure and surface roughness of as-deposited and annealed films were investigated to understand the effects of annealing on the TCR.
Acknowledgments This work is supported by the National instrumentation program of China (Grant No. 2013YQ040911), National Natural Science Foundation of China (Grant No.51775446), National Key Basic Research Program of China (2015CB057400) and Advance Research
1. The as-deposited Ni film has high residual stress and surface roughness, the grain size is small and not arranged densely in nature, resulting the much lower TCR value about 2900 ppm/°C. 23
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Program (41406020201).
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24
Contents lists available at ScienceDirect
Vacuum journal homepage: www.elsevier.com/locate/vacuum
Effects of annealing on the temperature coefficient of resistance of nickel film deposited on polyimide substrate
T
Baoyun Sun, Pengbin Wang, Binghe Ma∗, Jinjun Deng, Jian Luo Key Laboratory of Micro, Nano Systems for Aerospace, Ministry of Education, Northwestern Polytechnical University, Xi'an, Shaanxi, 710072, China
A R T I C LE I N FO
A B S T R A C T
Keywords: Annealing Temperature coefficient of resistance Nickel film Magnetron sputtering Polyimide substrate
This study investigates the effects of annealing temperature as well as time on the temperature coefficient of resistance (TCR) of nickel (Ni) films deposited on flexible polyimide substrate by direct current magnetron sputtering. The morphological and microstructural characteristics of as-deposited and annealed Ni films were analyzed to understand the mechanisms of TCR variation. The crystalline phase, grain size change tendency and residual stress of films were evaluated by X-ray Diffractometer (XRD) technique. (111) is the preferred orientation of Ni and the grains have a growth after annealing. The residual stress in as-deposited films is about −446.3 MPa and reduced to −208.8 MPa after annealing at 400 °C for 6 h. Focused Ion Beam Electron Microscope (FIBEM) and Energy Dispersive X-ray Spectroscopy (EDS) investigations showed the microstructure of annealed films has a relative improvement and the oxygen content was little increased with increasing annealing temperature and time. Mean surface roughness determined by Atomic Force Microscope (AFM) is decreased from 4.5 nm to 1.25 nm with increasing annealing temperature. The TCR value increases with the increasing annealing temperature and time. When the annealing time exceeds 6 h, the TCR almost has no change or even begin to decline.
1. Introduction Wall shear stress is one of the most important fluid mechanical parameters, which is a tangential force in the direction of motion due to the viscosity of the fluid [1]. Its measurement is essential for drag reduction, active flow control, boundary layer separation and transition detection in fluid mechanics applications [2]. Over the past few decades, numerous shear stress measurement techniques have been developed [3–6]. Among all these approaches, the flexible thermal shear stress sensors fabricated on polyimide substrate were most often used since they have merits that well fit on curved surfaces, capable of multipoint measurement, have less interference to the flow and high temporal/spatial resolution. The typical flexible thermal shear stress sensors structure mainly consists of substrate, thermistor and leading wire. Polyimide has stable mechanical, electrical, chemical and excellent temperature resistance properties [7], which is a good choose as a flexible sensor substrate. Because of its highest TCR of all metals [8,9], Ni was often being selected as the thermistor material [10–14]. TCR defines the resistance change with the temperature, which can be expressed by the following equation:
∗
TCR =
R − R0 × 106ppm / °C (T − T0) R 0
(1)
where R is the resistance at temperature T, R0 is the resistance at room temperature T0 (20 °C). Regardless of their differences in form, working principle of all thermal shear stress sensors is same [6,8]. The sensing thermistor is heated by current and forced-convective heat transfer with the passing fluid flow (Fig. 1). As the temperature of the thermistor varies with changes of the velocity gradient, so does the resistance. Thus, the sensitivity of the sensor is determined by the magnitude of the TCR. Hence, a high TCR value is desired for the thermistor of thermal shear stress sensor. Vacuum thermal annealing has been reported as one effective method to improve the electrical properties of thin film [15–17]. Therefore, for the sake of getting Ni film with highest TCR value, it is essential to explore the correlation between the TCR of Ni thin film and annealing treatment. In the present experimental studies, a polyimide based flexible thermal shear stress sensor has been developed. The effects of annealing temperature and time on the TCR value of Ni film deposited by direct current magnetron sputtering have been investigated. The crystalline
Corresponding author. E-mail address: [email protected] (B. Ma).
https://doi.org/10.1016/j.vacuum.2018.11.016 Received 23 July 2018; Received in revised form 7 November 2018; Accepted 8 November 2018 Available online 09 November 2018 0042-207X/ © 2018 Published by Elsevier Ltd.
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Fig. 1. Schematic view of the thermal shear stress sensor sensing principle.
phase, grain size, residual stress, composition, microstructure and surface roughness of as-deposited and annealed films were evaluated by XRD, EDS, FIBEM and AFM techniques, respectively. The resistance values of Ni thermistors at different temperatures were measured by four-wires method in a constant temperature water bath. 2. Experimental 2.1. Sensor manufacture The fabrication processes of the flexible thermal shear stress sensor are shown in Fig. 2 [18]. A ready-made 50 μm thick UBE UpilexS polyimide foil was used as the sensor substrate. The polydimethylsiloxane (PDMS) was spun-on a glass wafer, then the polyimide foil was bonded onto the glass wafer by the PDMS adhesive layer and cured in a vacuum drying oven (a). The Ni thermistor film of 1 μm thickness was then magnetron sputtered (b) onto the polyimide foil. After that, a 200 nm thick copper (Cu) lead film was sputtered onto the Ni film as a seed layer (c). To reduce the lead resistance, the thickness of the Cu film was increased to 3 μm according electroplating process (d). Finally, the Cu and Ni films were patterned with photolithography and wet-etched (e-f), respectively. To improve the TCR and make the sensor's electrical resistance stable, annealing (g) was carried out to eliminate lattice defects in Ni films. Parylene C was vapor deposition on the sensor as a protective layer (h). Three-dimensional schematic view and finished sensors are shown in Fig. 3. 2.2. Ni film preparation The Ni films were deposited on polyimide substrate by DC magnetron sputtering system from a pure Ni (with a purity of 99.99%) target with diameter of 5 inch. The films were deposited without intentional heating of the substrates. Base pressure of the deposition chamber was approximately 8.0 × 10−4 Pa. Pure argon (with a purity of 99.999%) Fig. 3. (a) Three-dimensional schematic view of the sensor structure. (b) Flexible hot-film sensors.
was used as sputtering gas and flow rate was 60sccm. Target was presputtered for 10min to remove any surface impurities. All deposition was performed using an input power of 150 W, which gave a deposition rate of about 25 nm/min. Films with a thickness of about 1 μm were deposited. After the deposition, some films were annealed at 200 °C, 250 °C, 300 °C, 350 °C and 400 °C for 2, 4, 6 and 8 h (h) at a base pressure of 1.0 × 10−3Pa, respectively. The decomposition temperature of this type polyimide is about 500 °C. If the annealing temperature is over 400 °C, the polyimide will transition to fragile and the deposited film will start peeling off from the substrate surface. Therefore, the highest annealing temperature is set at 400 °C. Fig. 2. The flexible hot-film shear stress sensor fabrication processes. 19
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2.3. Film characterization The crystalline phase and average grain size change tendency of the Ni films were analyzed from XRD data obtained in standard BraggBrentano geometry. XRD studies were carried out with a SHIMADZU XRD-7000 X-RAY DIFFRACTOMETER using a Cu Kα (λ = 0.154 nm) radiation source operated at 40 kV and 30 mA. For phase identification, XRD patterns were collected in the 2θ range of 30–80°. The residual stress in Ni films were measured using the sin2ψ technique [19–24] with the help of another XRD in Bragg Brentano configuration (Bruker D8 ADVANCE, Germany). During measurement, the positions of Ni (311) (2θ∼92.947°) peaks were chosen to determine the stress. The tilt angles ψ were changed from 0° to 45° with steps of 9° at two different rotation angles ϕ of 0° and 180°, respectively. The Poisson's ratio and Young's modulus of Ni material used for stress calculation were ν = 0.31 and E = 210Gpa, respectively [25]. The surface and cross-sectional microstructures were observed by a FIBEM (FEI Helios G4 CX) and the composition of the films were determined by a Thermo NS7 EDS attachment of FIBEM. The EDS was calibrated by using the cross-section of silicon. The film surface morphology was investigated by an AFM (BRUKER Dimension Fast Scan™). All the measurements were conducted without Parylene C layer deposition. The resistance of the patterned thermistor was measured by fourwires method with an Agilent 34410A digital multimeter. The resistance values at various temperatures were obtained in a thermostatic water bath. The temperature was monitored by a standard platinum resistance thermometer. The TCR values can be calculated by Eq. (1).
Fig. 5. XRD patterns of the Ni films (a) as-deposited and annealed at 300 °C for (b) 2 h, (c) 4 h, (d) 6 h.
inversely proportional to the FWHM value. As a result, the grains growing bigger with increasing annealing temperature below 400 °C. Fig. 5 shows the XRD patterns of the as-deposited Ni films and annealed at 300 °C for different times. The XRD patterns show Ni (111), Ni (200) and Ni (220) diffraction peaks as well as Fig. 4. The FWHM value of Ni (111) decreases from 0.520 of as-deposited film to 0.456 of film annealed at 300 °C for 6 h, which means the grains growing bigger with increasing annealing time below 6 h, too. The residual stress in Ni films determined by the sin2ψ technique is shown in Table 1. The results indicate that the residual stress in both asdeposited and annealed Ni films tends to be compressive in character. The residual stress of as-deposited films is about −446.3 MPa and has a remarkable relaxation after annealing. Moreover, the magnitude of this stress is progressively decreased with increasing annealing temperature and time. The smallest value of stress can be reduced to about −208.8 MPa after annealing at 400 °C for 6 h. The reduction in residual stress may be attributed to the point-defect annihilation take place during the annealing process [27,28].
3. Results and discussions 3.1. XRD analysis Fig. 4 shows the XRD patterns of the as-deposited Ni films and annealed at different temperatures for 4 h. The reflections at about 44.4°, 51.8° and 76.3° represent the (111), (200), (220) planes of face centered cubic structure. The results indicate that (111) is the preferred orientation of Ni films. When the annealing temperature increased to 400 °C, the full width at half maximum (FWHM) value of Ni (111) decreases from 0.528 of as-deposited film to 0.442 of film annealed at 400 °C for 4 h. According to the Scherrer formula [26], the grain size
3.2. FIBEM analysis The composition of Ni films record by the EDS is shown in Table 1. The oxygen content of as-deposited films is less than the annealed films. The oxygen composition of annealed Ni films is progressively increased with increasing annealing temperature and time, but the variation is very little. These results indicate that the annealing process was accompanied by the formation of oxide. Fig. 6 shows the surface and cross-sectional microphotographs of Table 1 The residual stress of Ni films measured by XRD and the composition of Ni films record by EDS. Annealing temperature (°C)
200 300 300 300 400 No annealing
Fig. 4. XRD patterns of the Ni films (a) as-deposited and annealed at (b) 200 °C, (c) 300 °C, (d) 400 °C for 4 h. 20
Annealing time (h)
6 4 6 8 6
Residual stress (MPa)
−400.9 −399.7 −325.5 −256.7 −208.8 −446.3
± ± ± ± ± ±
18.3 18.2 17.6 15.9 12.0 18.5
Composition (at.%) Ni
O
96.04 96.35 96.04 95.90 95.85 96.39
3.96 3.65 3.96 4.10 4.15 3.61
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the Ni film is impossible to get high TCR. When the films were annealed at 200 °C (Fig. 6(b)), 300 °C (Fig. 6(c)) and 400 °C (Fig. 6(d)) for 4 h, the surface morphology has a relative improvement and the columnar crystals were denser with larger grain size. The possible reason for these structural changes is annealing provides thermal energy for the adatoms growing and moving to the suitable site [29]. When the films were annealed at 300 °C for 2 h (Fig. 6(e)), 4 h (Fig. 6(c)) and 6h (Fig. 6(f)), there is a grain growth and the crosssections show a more compact morphology with increasing annealing time. The possible reason for improvement in the structural properties is the ad-atoms have enough time to move and relocate on the substrate surface. 3.3. AFM analysis The three-dimensional AFM images of the Ni films as-deposited and annealed at different temperatures and times are shown in Fig. 7. The surface roughness was determined by the AFM data. The values of the mean roughness (Ra) and the root mean square roughness (Rq) of asdeposited film and films annealed at different temperatures are present in Fig. 8. The results show that the surface roughness of the Ni films decreases with increase in annealing temperature up to 400 °C. This is attributed to more uniform growth take place during the annealing process. Commonly, smoothing and roughening are two basic stages accompanied with the film growth [30]. It is obviously that our annealing of Ni films below 400 °C is in the smoothing stage. It is well known that higher annealing temperature provides more thermal energy for adatoms moving. The rearrangement of adatoms increases the surface density by sintering of intergranular pores and coalescence of grains. Furthermore, we can see that the grains have a growth after annealing from the XRD and FIBEM results. The growth of grains at this stage is two-dimensional (except vertical direction) and along their preferred directions, until they impinge on each other. The above reasons are expected to cause more uniform film growth and smooth surface [27,31,32]. Due to the limitation of the substrate decomposition temperature, the Ni films cannot be annealed at higher temperature over 400 °C. However, if the films be annealed at higher temperatures, the surface roughness is supposed to increase above a critical temperature. It is attributed to non-uniform three-dimensional grain growth and partial recrystallization during annealing at higher temperatures. As known, the resistivity of metal is proportional to the surface roughness, which is because of the rougher surface leads to more electron scattering at the boundary and shorter mean free path length [33–35]. According to Matthiessen's rule, the relationship between TCR α and resistivity ρ can be written as: αρ = constant
(2)
so, the TCR has a negative correlation with the surface roughness. 3.4. Electrical properties The resistance change with respect to the ambient temperature variation of films annealed at different temperatures are shown in Fig. 9. As we can see, the resistance of Ni films is linear increase with increasing ambient temperature from 10 °C to 90 °C, and the linearly dependent coefficient is over 0.999. The TCR of the as-deposited film and films annealed at different temperatures and times are present in Fig. 10 and Fig. 11, respectively. The TCR value of as-deposited film is only about 2900 ppm/°C, which is much lower than the bulk material (∼6800 ppm/°C). In Fig. 10, the TCR is shown to be affected dramatically by annealing and exhibits a nonlinear increase with the increase of annealing temperature. When the annealing temperature is over 300 °C, the TCR variation is greater
Fig. 6. FIB Electron Microscope microphotographs of the Ni films, (a) as-deposited, (b) 200 °C, 4 h, (c) 300 °C, 4 h, (d) 400 °C, 4 h, (e) 300 °C, 2 h, (f) 300 °C, 6 h.
the Ni films as-deposited and annealed at different temperatures and times. As we can see from the cross-sectional morphologies in Fig. 6, all the Ni films exhibit a columnar grain structure. Most of the grains in asdeposited film (Fig. 6(a)) are agglomerated with each other but they are not arranged densely. This is attributed to there is not enough kinetic energy for ad-atoms moving to the proper location. With this structure, 21
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Fig. 7. 3D AFM images of the Ni films, (a) as-deposited, (b) 200 °C, 4 h, (c) 300 °C, 4 h, (d) 400 °C, 4 h, (e) 300 °C, 2 h, (f) 300 °C, 6 h.
roughness resulted in higher conductivity and TCR. Even though the oxygen composition has a small increase in the annealed films, the TCR was still increased with the annealing temperature below 400 °C and time below 6 h. The possible reason for the saturated TCR after 6 h is the ad-atoms reach stable state within 6 h and grain cannot grow anymore with the annealing time increasing. Another reason maybe the additional increase oxidation of Ni films with increasing annealing time. In the previous study by other researchers [36], the results show that TCR is inversely proportional to the annealing time at a given temperature, which is quite different from our experimental results. We believe that the reason for this phenomenon is the formation of too much nickel oxide on the film surface, which is due to the vacuum
than the lower annealing temperatures. Fig. 11 shows that the TCR has a positive correlation with the annealing time below 6 h. The highest TCR value can reach approximately 5000 ppm/°C after annealing. The evolution of TCR can be explained by the improvement of the films structural and morphological properties. It has been known from XRD patterns, FIBEM microphotographs and AFM images that the asdeposited Ni films have smaller grain size which not arranged densely and often contained many structural defects, higher residual stress and rougher surfaces. All these above reasons lead to a relatively small TCR value. With increasing annealing temperature and time, the ad-atoms got more thermal energy and time to migrate and grow. Larger grain size decreased the grain boundaries, defects annihilation and smaller 22
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Fig. 10. TCR of the films annealed at different temperatures. Fig. 8. Mean surface roughness (Ra) and root mean square roughness (Rq) of the as-deposited Ni film and films annealed at different temperatures for 4 h.
Fig. 11. TCR of the films annealed at different times.
2. Annealing treatment provided enough thermal energy and time for the ad-atoms to grow and rearrangement at the suitable site on the substrate surface. Resulting in the appearance of morphological and microstructural improvements, including grain growth and residual stress relaxation. Meanwhile, the two-dimensional growth below critical temperature lead to a smaller surface roughness. 3. Whereas the oxygen composition has an increase after annealing, the microstructure improvements still make the TCR values show a significant nonlinear increase with the increase of annealing temperature and time. The highest TCR value can reach approximately 5000 ppm/°C after annealing at 400 °C for 6 h. 4. When the annealing time exceeds 6 h, the TCR almost has no increase or even begin to decline. This is may be attribute to 6 h is enough for the ad-atoms to reach stable state below 400 °C annealing and additional increase oxidation of Ni films.
Fig. 9. Resistance change with respect to the ambient temperature variation of films annealed at different temperatures. R is the resistance at temperature T, R0 is the resistance at room temperature T0 (20 °C). A, B and C means three films annealed at same temperature for same time.
degree of the annealing furnace is not enough. Hence, if the base pressure of the annealing furnace is equal or less than 1.0 × 10−3Pa, the TCR value will increases with increasing annealing time within 6 h. 4. Conclusions The Ni films were deposited by direct current magnetron sputtering system on flexible polyimide substrates. The films were annealed at temperatures range from 200 °C to 400 °C, and duration times rang from 2 h to 8 h. The crystalline phase, grain size, residual stress, composition, microstructure and surface roughness of as-deposited and annealed films were investigated to understand the effects of annealing on the TCR.
Acknowledgments This work is supported by the National instrumentation program of China (Grant No. 2013YQ040911), National Natural Science Foundation of China (Grant No.51775446), National Key Basic Research Program of China (2015CB057400) and Advance Research
1. The as-deposited Ni film has high residual stress and surface roughness, the grain size is small and not arranged densely in nature, resulting the much lower TCR value about 2900 ppm/°C. 23
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Program (41406020201).
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