Crystallization of nanoscale NiTi alloy thin films using rapid thermal annealing

Transcription

1 Crystallization of nanoscale NiTi alloy thin films using rapid thermal annealing Running title: Crystallization of nanoscale NiTi alloy thin films Running Authors: Hou, Hamilton, and Horn Huilong Hou a), Reginald F. Hamilton a), b), Mark W. Horn a) Department of Engineering Science and Mechanics, The Pennsylvania State University, University Park, Pennsylvania a) American Vacuum Society member. b) Electronic mail: rfhamilton@psu.edu This work utilizes short time heat treatments of submicrometer-thickness NiTi alloy films fabricated using biased target ion beam deposition (BTIBD) and investigates crystallization. Films were fabricated on Si substrates and thicknesses were about 150 nm, which were much less than conventional thicknesses on the order of micrometers. To understand the composition dependence Ni concentrations were varied such that alloys ranged from Ti-rich to near-equiatomic. Rapid thermal annealing (RTA) was used for the heat treatment and temperatures ranged from 465 C up to 540 C for 10 minutes. X-ray diffraction measurements for each of the NiTi alloy compositions revealed that the crystallization temperature was equivalent (~490 C) and the B2 austenitic atomic crystal structure existed. Evolutions of surface morphologies, measured using atomic force microscopy, as a function of heat treatment temperature confirmed the composition independence of the crystallization temperature. To investigate the structure using transmission electron microscopy, 150 nm-thickness films were also deposited on ultrathin SiN substrates and heat treated, which confirmed equiaxed grains existed. Crystallization and annealing heat treatments for nanoscale films can be carried out for time on the order of minutes, which should curtail detrimental diffusion effects known to compromise shape memory behavior. 1

2 27 28 I. INTRODUCTION Shape memory alloys (SMAs) thin films exhibit the largest work output per 29 volume compared to other smart materials. 1-3 Nickel titanium alloy (NiTi or Nitinol) is the flagship SMA composition. Conventional NiTi films are fabricated with thicknesses on the order of microns. Our previous work demonstrated that a novel biased target ion beam deposition (BTIBD) technique is suitable to produce nanoscale NiTi 33 alloy thin films. 4-6 Films of nanoscale thickness are an attractive class of material to integrate into small scale planar systems as advanced actuators. Understanding crystallization in the nanoscale NiTi films is essential, as crystallization is prerequisite for as-deposited SMA thin films for exhibiting shape memory actuations. As-deposited films are typically amorphous and require heat treatments for crystallization. The crystalline intermetallic phase with the B2 atomic structure undergoes the underlying crystallographically reversible solid-solid martensitic phase transformation (MT), which is essential for shape memory response. The heat treatment apparatus commonly used are vacuum furnace 7-12 or lamps 42 heating in deposition chambers For sputtered NiTi films, more common heat 43 treatments were C and the time was on the order of hours. 7,18-21 Surface 44 roughness values increased with increasing temperatures. 22 To exhibit the MT, the ratio of interdiffusion layer thickness to film thickness needs to be below a critical limit, ~ The interdiffusion layer tends to oppose the lattice distortion of the MT 15,23 and 47 could even suppress shape memory response. 15 One strategy to diminish interdiffusion layer is to limit both ramping and holding time. Rapid thermal annealing (RTA) uses rapid ramp rates (up to 200 C/sec) and short annealing time (typically less than 15 mins). 2

3 RTA has been employed successfully to crystallize micrometer-thick sputtered NiTi thin films, which exhibited shape memory response The BTIBD films exhibited ultra-smooth surfaces (roughness being less than half 53 of the sputtered films) 4 and minimal film/substrate interdiffusion. 5 We postulated that 54 BTIBD facilitates nucleation and can allow for tailoring grain size. 6 BTIBD promotes adatom mobility. The enhancement can be attributed to its unique configuration of an independent ion source (without the need to strike a plasma in the deposition chamber). Furthermore, BTIBD utilizes an ultra-low processing pressure (10-4 Torr in contrast to Torr in magnetron sputtering). 4-6 The capability of RTA to crystallize BTIBD NiTi alloy films of nanoscale thickness has been demonstrated previously by the current 60 authors in a comparative study of BTIBD and magnetron co-sputtering. 6 BTIBD films with a Ti-rich composition and Ni-rich compositions ( at.% Ni concentrations) were deposited onto SiN chips and heat treated in two-steps of 450 C for 1 min followed by 500 C for 5 min. The results suggested the crystallization temperature was near 500 C. Transmission electron microscopy (TEM) was used to confirm crystallization. The grain size was composition-dependent and larger grains were observed for the Ti-rich composition. For this report, a comprehensive approach was implemented to hone our determination of the crystallization temperature. BTIBD was used to fabricate 150-nmthick NiTi films by co-sputtering elemental Ni and Ti targets and films were deposited on Si substrates. We used an expanded heat treatment temperature range ( C) with respect to our previous work and fixed the time. Compositions of deposited thin films were Ti-rich and near-equiatomic as our previous work suggested the lower Ni 3

4 concentration NiTi alloys readily crystallize. Furthermore, for scrutiny of crystal structures and the dependence on heat treatment temperature, x-ray diffraction measurements were conducted. Atomic force microscopy (AFM) augmented the diffraction measurements in order to visualize the evolution of surface morphology for the increasing heat treatment temperatures. The SiN chip configuration was employed for TEM to characterize grain morphology. The systematic analysis presented here confirms the crystallization and annealing temperatures II. EXPERIMENTAL NiTi thin films were deposited using the biased target ion beam deposition system (BTIBD, 4Wave Inc.) in the Nanofabrication Laboratory of The Pennsylvania State University. Before actual depositions, a seasoning procedure was conducted for 600 s. The processing pressure during actual depositions was Torr. The Ar flow rate in the ion source was 60 sccm. A bias of 805 V was applied to the Ni and Ti targets. The ratio of pulse width to pulse period in the Ti target was 100/100. The ratio in the Ni target was varied to generate a range of compositions. The relation between power ratio 89 and film composition was established previously. 4 The substrate rotated at 20 rpm. The film thickness was measured using a stylus profilometer (P16+, KLA-Tencor Corp.) and the thicknesses were nm. Three films with the compositions Ti-50.8 at.% Ni and Ti-50.3 at.% Ni (near equiatomic) and Ti-49.7 at.% Ni (Ti-rich) were deposited on commercial four-inch (100) Si wafers. After deposition, each film/wafer was then cleaved into four symmetrical pieces. Therefore, twelve samples were produced. The atomic crystal structures in each 4

5 sample were examined using a PANalytical X Pert MPD diffractometer system in grazing incident x-ray diffraction (GIXRD) mode with a Cu K 1 x-ray source (λk =1.5418Å). The x-ray diffraction patterns were collected from 20 to 80 with a step size equal to The hold time at each step was 0.5 s. Surface morphologies were examined using an AFM (Dimension Icon, Bruker Corp.) in PeakForce tapping mode. Scan sizes were 1 m by 1 m with 1.0 nn PeakForce. The scan rate was 0.75 Hz and images were captured using 512 by 512 line scanning. Optical images were taken with an optical microscope (L200ND, Nikon Corporation) in bright-field mode. Heat treatments were carried out in a RTA furnace (RTA, Allwin21 Corp.). The heat treatment time was 10 minutes and the temperatures were 465 C, 490 C, 515 C, and 540 C. The procedure began with ramping to the target temperature at a rate of 30 C/s. The film was held at the temperature for the desired duration. Finally, the film cooled to room temperature in the furnace. Using the aforementioned deposition parameters, additional films (with nm thickness) were deposited on SiN chips for TEM analysis. The films were examined in a TEM (JEM-2010, JEOL Inc.) operated at 200 kv accelerating voltage. Details of the SiN chip configuration are provided 112 elsewhere. 5 The chip was circular with a 3 mm diameter. The support frame was etched so that a square window ~500 m 2 remained in the middle for electron-transparency. To crystallize the as-deposited thin films on SiN chips, a two-step heat treatment was used: 450 C, 1 min, and then 500 C for 5 min. The same two-step treatment in RTA was 116 employed in our previous work. 4,6 Here the heat treatment is used to crystallize a 117 different composition, Ti-50.8 at.% Ni

6 III. RESULTS AND DISCUSSION Fig. 1 shows the XRD spectra of NiTi thin films for each of the four heat treatment temperatures. The curves for 465 C in Fig. 1(a) (c) have broad peaks, indicating the thin films are amorphous. The curves for 490 C, 515 C and 540 C show sharp peaks and the sharpness increases with temperature. The B2 crystalline phase is indexed. The B2 atomic crystal structure is the typical austenite phase in NiTi, which is known to undergo the martensitic transformation. The Ti 2 Ni and Ni 4 Ti 3 microconstituent phases are indexed. In Fig. 1(a), the precipitate Ti 2 Ni has a major peak (551), and is 127 expected for Ti-rich composition in the NiTi phase diagram. 31 For the Ti-50.3 at.% Ni and Ti-50.8 at.% Ni, the Ni 4 Ti 3 precipitates are identified by the major peaks (140) and (212) in Fig. 1(b) and (c). The precipitation of the Ni-rich Ni 4 Ti 3 precipitates is expected 130 for Ni concentration greater than 50 at.%. 31 The co-existence of the Ti 2 Ni for the Ni-rich NiTi alloy film suggests compositional non-uniformity may arise for the increased Ni concentration films. The Ni 4 Ti 3 precipitates are beneficial for facilitating complete 133 transformation strain recovery in Ni-rich SMAs. 31 The relative intensity of (551) versus B2 (110) is higher at 540 C than at 490 C and thus the fraction of Ti 2 Ni increases. Similar trends on relative intensity of Ti 2 Ni and Ni 4 Ti 3 precipitates versus B2 are discernable in Fig. 1(b) and (c). The results illustrate that when heated at 490 C and above, thin films become crystallized, and the crystallization temperature is between 465 and 490 C. 6

7 FIG. 1. (Color online) XRD spectra of NiTi thin films deposited on Si substrates after heat treatment: (a) Ti-49.7 at.% Ni films, (b) Ti-50.3 at.% Ni films, and (c) Ti-50.8 at.% Ni For the NiTi alloy films, Fig. 2 shows the evolution of surface roughness with increasing temperature to further interpret the spectra in Fig. 1. At the highest temperatures (515 and 540 C), the surface roughness parameters, root mean square 7

8 (RMS), are on the order of nm. For each composition heat treated at 465 C, RMS measurements are on the order of 2 3 nm. The 490 C, 10 min heat treatment produces surface features that are long strips with relatively large roughness. At 515 and 540 C, the roughness measurements are on the order of nm. After heat treatment at 490 C, the thin films become crystallized (see Fig. 1), and hence the significant roughness can be attributed to crystallization and the formation of grains with the B2 crystal structure

9 FIG. 2. (Color online) AFM three-dimensional images of NiTi thin films deposited on Si substrates after conducting heat treatment at four conditions. Color contour scale at the bottom applies for all images TEM micrographs for the Ti-50.8 at.% Ni are shown in Fig. 3. As-deposited thin films are amorphous, which is indicated by the diffuse rings in the diffraction pattern in Fig. 3(a). The amorphous films result in featureless contrast in the bright-filed image in Fig. 3(b) and (c). After heat treatment, diffraction rings with finite width and diffraction spots are present in Fig. 3(d). The bright spots in Fig. 3(d) collect from diffraction attributed to large grains. Fig. 3(e) shows the grain morphology after polymorphous crystallization, during which grains grow circularly and remain the same composition in 166 amorphous and crystalline status. 21 The representative grains highlighted in Fig. 3(f) have a size ~120 nm. The observed circular grains remain nearly the same in all directions, i.e. equiaxed, which is consistent with structure zone model 32 and through 169 thickness analysis. 33 The grains remain circular during growth until the external perimeter reaches the film/substrate interface or impinges with other grains. 33 impinging grain boundary tends to flatten as in Fig.3(f). The 9

10 FIG. 3. (Color online) Upper panels, (a) (c) for Ti-50.8 at.% Ni thin film in asdeposited condition, and lower panels, (d) (f) for the same film after heat treatment at 450 C, 1 min C for 5 min. (a) and (d) are TEM diffraction patterns. (b), (c), (e), and (f) are bright-field micrographs. In (e) and (f), while lines are inserted for clarity of representative grain morphology The images in Fig. 4 show surfaces of the NiTi thin films before and after heat treatment. Note that the at.% Ni concentration is Ni-rich. Images for differential NiTi alloy film compositions deposited on SiN having thicknesses between nm, unequivocally confirm that the trends in Fig. 4 are composition independent. Thus, the behavior in Fig. 4 is expected for the Ti-rich and near-equiatomic NiTi alloy compositions as well. Before deposition of the thin films, a square window of the bare SiN membrane is viewed from top (Fig. 4(a)) and bottom (Fig. 4(b)), respectively. The surface of the as-deposited films is wrinkled/buckled in the square area in Fig. 4(c). The bottom view in Fig. 4(d) shows both the membrane and films are wrinkled. After heat 10

11 treatment, the surface in the square area is flat and the color is uniform in Fig. 4(e). The area is also flat when viewed from the bottom-side in Fig. 4(f). Additional stress curvature measurements were carried out on 800 nm thick NiTi alloy thin films fabricated using BTIBD and deposited on Si wafers that had 49.7 at.%, 50.3 at.%, and 51.8 at.% Ni concentrations. The analysis revealed that the as-deposited NiTi films were under compressive stresses that ranged 200 MPa to 600 MPa. After heat treatments, the stress states became tensile and magnitudes ranged from 300 to 900 MPa. For the current 150 nm thick films deposited on SiN, the flat surface after heat treatment is concomitant with the switch from compressive to tensile stress states

12 FIG. 4. (Color online) Images from top and bottom view of Ti-62.5 at.% Ni thin films on SiN chips. (a), (b) Bare SiN substrate with a square window in the center. For clarity, dashed yellow lines trace the window. The Si frame beneath the window was etched away, leaving a large opening at the bottom. (c), (d) As-deposited film with a wrinkled/buckled surface. (e), (f) Heat treated film with a flat surface Short time heat treatment using RTA is herein effective for crystallized phases and grain morphology in NiTi alloy thin films prepared by BTIBD. Primarily in literature, in-situ TEM heating was reported to crystallize NiTi thin films from amorphous in short time (5 10 mins), to observe grain growth morphology and to 209 measure kinetic parameters such as nucleation rate and growth rate. 21,34-36 The grain 12

13 morphology of thin films after heat treatment using RTA in this work is consistent with the in-situ TEM heating in short time showing the formation of eqipaxed grains. Moreover, the crystallization temperature found herein for BTIBD films agrees with the 213 reported effective range for sputtered films. 24,28 The equiaxed grain morphology after RTA heat treatment is consistent with Zone structure 3 in the classic Thornton structural zone model. 37, IV. SUMMARY AND CONCLUSIONS RTA is able to crystallize nanoscale NiTi thin films under high ramp rate (30 C/s) and short annealing time (on the order of 10 mins). The crystallization temperature is about 490 C and the B2 phase, typical of NiTi shape memory alloys, exists. X-ray diffraction analysis shows that the crystallization temperature is independent of Niconcentration. Diffraction measurements also show 515 and 540 C heat treatment temperatures promote precipitation and annealing. AFM measurements reveal that the surfaces of films heat treated at higher temperature tend to be smoother. The surfaces of films heated to temperatures close to the crystallization temperature are the most rough 226 surface. Reported RMS values for the B2 austenite are on the order of 3 nm, 39,40 and the current values (< 5 nm) are close. The well-known Thornton structure zone model developed for sputtered films 32,41 provides phenomenological underpinnings: at the higher temperatures corresponding to Zone 3, equiaxed grains are expected to result in 230 smooth surfaces. 32 The smoothest surfaces developing for higher heat treatment temperatures in this work may be due to grain growth, which suggest annealing occurs C above the crystallization temperature. It is interesting to note that annealing can 13

14 change the intrinsic stress state and optical images provide evidence: the surface of the as-deposited thin film is wrinkled. After heat treatment, the surface becomes flat similar to the bare substrate. The abilities of RTA will be useful in tailoring of the microstructure and the associated shape memory properties at nanoscale ACKNOWLEDGMENTS The authors thank Nichole Wonderling of Materials Characterization Laboratory, The Pennsylvania State University for the assistance on x-ray diffraction analysis. Components of this work were conducted at The Pennsylvania State University node of the National Science Foundation-funded National Nanotechnology Infrastructure Network. This work was supported by the National Science Foundation under Grant No. CMMI P. Krulevitch, A. P. Lee, P. B. Ramsey, J. C. Trevino, J. Hamilton, and M. A. Northrup, J. Microelectromech. S. 5, 270 (1996). 2 S. Miyazaki and A. Ishida, Mater. Sci. Eng., A 273, 106 (1999). 3 Y. Q. Fu, H. J. Du, W. M. Huang, S. Zhang, and M. Hu, Sens. Actuators, A 112, 395 (2004). 4 H. Hou, R. F. Hamilton, M. W. Horn, and Y. Jin, Thin Solid Films 570, 1 (2014). 5 H. Hou, R. F. Hamilton, and M. W. Horn, J. Vac. Sci. Technol., A 33, (2015). 6 H. Hou, R. F. Hamilton, and M. W. Horn, J. Vac. Sci. Technol., B 34, (2016). 7 S. Sanjabi, S. K. Sadmezhaad, K. A. Yates, and Z. H. Barber, Thin Solid Films 491, 190 (2005). 8 S. Sanjabi and Z. H. Barber, Surf. Coat. Technol. 204, 1299 (2010). 9 H. J. Lee, X. Huang, K. P. Mohanchandra, G. Carman, and A. G. Ramirez, Scr. Mater. 60, 1133 (2009). 10 M. Kabla, H. Seiner, M. Musilova, M. Landa, and D. Shilo, Acta Mater. 70, 79 (2014). 11 A. Ishida and M. Sato, Acta Mater. 51, 5571 (2003). 12 E. Saebnoori, T. Shahrabi, S. Sanjabi, M. Ghaffari, and Z. H. Barber, Philos. Mag. 95, 1696 (2015). 13 J. Cui, Y. S. Chu, O. O. Famodu, Y. Furuya, J. Hattrick-Simpers, R. D. James, A. Ludwig, S. Thienhaus, M. Wuttig, Z. Zhang, and I. Takeuchi, Nat. Mater. 5, 286 (2006). 14 X. G. Ma and K. Komvopoulos, J. Mater. Res. 20, 1808 (2005). 15 Y. Q. Fu, S. Zhang, M. J. Wu, W. M. Huang, H. J. Du, J. K. Luo, A. J. Flewitt, and W. I. Milne, Thin Solid Films 515, 80 (2006). 16 Y. Q. Fu, J. K. Luo, A. J. Flewitt, S. E. Ong, S. Zhang, H. J. Du, and W. I. Milne, Smart Mater Struct 16, 2651 (2007). 14

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16 FIGURE CAPTIONS FIG. 1. (Color online) XRD spectra of NiTi thin films deposited on Si substrates after heat treatment: (a) Ti-49.7 at.% Ni films, (b) Ti-50.3 at.% Ni films, and (c) Ti-50.8 at.% Ni FIG. 2. (Color online) AFM three-dimensional images of NiTi thin films deposited on Si substrates after conducting heat treatment at four conditions. Color contour scale at the bottom applies for all images FIG. 3. (Color online) Upper panels, (a) (c) for Ti-50.8 at.% Ni thin film in asdeposited condition, and lower panels, (d) (f) for the same film after heat treatment at 450 C, 1 min C for 5 min. (a) and (d) are TEM diffraction patterns. (b), (c), (e), and (f) are bright-field micrographs. In (e) and (f), while lines are inserted for clarity of representative grain morphology FIG. 4. (Color online) Images from top and bottom view of Ti-62.5 at.% Ni thin films on SiN chips. (a), (b) Bare SiN substrate with a square window in the center. For clarity, dashed yellow lines trace the window. The Si frame beneath the window was etched away, leaving a large opening at the bottom. (c), (d) As-deposited film with a wrinkled/buckled surface. (e), (f) Heat treated film with a flat surface