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Surface Evolution of NiTi and NiTiHf Thin Films

Chen Zhang*, Ralph H. Zee* and Paul E. Thoma**

*Materials Research and Education Center, Auburn University, Auburn, AL 36849;

**Corporate Technology, Johnson Controls, Inc., 1701 West Civic Dr., Milwaukee, WI 53209

ABSTRACT

The microstructure evolution of Ti-rich NiTi thin films and (TiHf)-rich NiTiHf thin films

containing 9at% Hf was investigated. These films were deposited from single NiTi and NiTiHf

targets using a DC magnetron sputtering system. Free-standing films were obtained by usingsingle crystal silicon substrates. The thickness of these films was controlled between 10-12 m.

In this investigation, the effects of deposition temperature on the surface and cross-sectional

microstructures of these films were studied. Substrate temperature during deposition was variedbetween 300C to 700C at 100C intervals. The influence of post deposition heat treatment

(HT) temperature on the microstructure of these films was also studied. The post deposition HT

temperature was varied between 300C and 800C at 100C intervals. Both surface and cross-sectional microstructures were examined using a scanning electron microscope (SEM). The

crystallinity and the phases present were determined using x-ray diffractometry. All the as-

deposited films were found to be crystalline, even when the substrate temperature was as low as300C. Results from the microstructure analysis show that all the films have a relatively fine

grain size ranging from 0.2 m to 2.5 m, and the grain size increases with increasing substrate

deposition temperature. The effect of post deposition HT on grain size was found to be minimal.

INTRODUCTION

Many investigations have been devoted to better understand the behavior of bulk shape

memory alloys (SMAs) since their discovery in the 1960s. The achievements in development ofSMAs have been significant. As a result, their commercial value has increased rapidly in the past

few years. Many applications have been developed including actuators, electronic connectors,

and medical implants such as stents and guidewires.Recently their applications have been extended to microactuators due to the increasing

demands for miniature devices. Since shape memory alloys are used as an actuator element, by

making it into a thin film, a new type of microactuator with high output stress and output strainbecomes possible. Meanwhile, a drawback of bulk SMAs, such as slow response, can be

overcome by increasing the actuator elements surface to volume ratio.

NiTi thin films have been successfully fabricated and studied in the past ten years. TernaryNiTiHf SMA with high Hf concentration becomes more and more attractive due to its high

transformation temperatures. The possibility of fabricating NiTiHf thin films, containing 10at%

Hf, was first reported by Johnson et al. [1]. However, a reliable method for manufacturingmicron-thick shape memory thin films, especially for high temperature shape memory thin films,

is still not available. Further studies are needed to better understand the effect of processing.

In this investigation, two specific shape memory thin films, a Ti-rich binary NiTi alloy and aternary NiTiHf alloy containing around 9at% Hf, were successfully fabricated. Processing

techniques for manufacturing these thin films were explored. The effects of processing

Mat. Res. Soc. Symp. Proc. Vol. 648 2001 Materials Research Society

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conditions on the microstructure of these thin films and on their transformation properties are

discussed.

EXPERIMENTAL SETUP

Ti-rich binary NiTi and ternary NiTiHf thin films were deposited from single targets of NiTiand NiTiHf respectively. The compositions of the targets are Ni47Ti53 and Ni42Ti48Hf10 (atomic

ratio). A DC magnetron sputtering system equipped with a cryogenic pump was used to depositthe films. Base pressure of 5x10

-7torr was achieved prior to sputtering to prevent contamination.

Both NiTi and NiTiHf ingots were arc melted and vacuum annealed at 900C for 100 hours by

Johnson Controls, Inc. The ingots were then electrical-discharge machined into 2 inch (5.08cm)diameter targets. The thickness of the targets varied between 0.125 inch and 0.250 inch.

The deposition process was described in authors previous papers [2,3]. Thickness of the

films was controlled between 10 m 12 m. After deposition, the films were peeled from thesilicon surface and cut into small pieces for heat treatment. The HT was conducted in an electron

beam furnace and all films were heat treated for one hour. During heat treatment, the vacuum

was maintained at around 5x10

-7

torr to minimize oxidation. A K-type thermocouple was used tomeasure the temperature. The deposition temperatures and post deposition HT temperatures

investigated in this study are listed in Table I.

Both surface and cross-section microstructures of the thin films were studied using a HitachiScanning Electron Microscope (SEM) at room temperature. For cross-section microstructure

analysis, thin film samples were first sandwiched between two stainless steel plates. The samples

were cold mounted and then polished on 600 and 1000 grit abrasive paper consecutively. The

rough polishing was followed by polishing on red cloth using 6 m and 1 m diamond paste.

The final polishing was done using a 0.05 m SiO2 suspension on imperial cloth. Samples were

then etched in a fresh etchant for 2 minutes prior to SEM examination. The etchant used was 50ml glycerol, 18 ml HNO3 and 2 ml HF. Samples for surface structure analysis were directly

etched without polishing. The same etchant was used on these samples.

Table I. Target Composition, Substrate Temperature and Post Heat Treatment Temperature.

Target Composition Substrate Temperature (Ts) Heat Treatment (HT)

Ni47Ti53

300C

400C

500C

Ni42Ti48Hf10

300C

400C

500C600C

700C

300C

400C

500C

600C

700C800C

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RESULTS

As discussed in two previous papers by the authors [2,3], the martensite and austenite

transformation temperatures of the NiTi and NiTiHf films are affected by the depositiontemperature as well as post deposition HT temperature. This is also true for the microstructure of

these films. The evolution of the surface and cross-sectional microstructure of these films with

changes in substrate temperature and post deposition HT temperature are reviewed in thefollowing sections. Films heat treated at 800C are not discussed in this paper due to the partial

surface oxidization which made these films very difficult to examine.

NiTi Thin Films

All NiTi thin films with the various deposition temperatures and post HT temperatures show afine grain structure. The grain size increases with increasing deposition temperature as shown in

figures 1a and 1b. The film deposited on a 300C substrate has a grain size less than 0.2 m

(figure 1a), while the film deposited on a 500C substrate shows a grain size several times larger.After HT, the grain size of these films remains the same despite the higher HT temperature used

(700C) as shown in figure 1c. Figure 1c shows the surface microstructure of a film deposited ona 500C substrate and followed by a 700C HT. Compared with figure 1b, figure 1c shows moresubstructures inside the grains. No significant grain growth is observed in the microstructure

shown in figure 1c.

Figures 1d, 1e and 1f show the cross-sectional microstructure of NiTi thin films. All threefilms show a tapered structure except for the as-coated film at 300C. Figure 1d shows large R-

phase grains with small tapered martensite grains starting to develop inside the R-phase grains.

The presence of these phases is confirmed based on results from x-ray diffraction.These substructures (fine needles) can also be found in the cross-sectional views of the films

as shown in figure 1f. Comparing figures 1e (as-coated) and 1f (heat treated), both films show

tapered structures except that more substructures are revealed in the film. The substructures are

observed in all the films heat treated at higher HT temperature. The substructures grow withincreasing HT temperature and become most significant in the films heat treated at 700 C.

NiTiHf Thin Films

The effect of deposition temperature and post deposition HT temperature on NiTiHf thin

films is similar to that of NiTi films. In general, grain size increases with increasing deposition

temperature while it remains the same for different post deposition HT temperature.Figures 2a to 2d show the surface microstructure of NiTiHf films deposited on substrates at

temperatures from 300C to 700C. With elevated deposition temperature, the development of

microstructure as a function of deposition temperature is obvious. In figure 2a, a martensite grain

structure is just beginning to form, but is still surrounded by the retained austenite structure. Thiscorresponds well with the x-ray and DSC results in the authors earlier publications [3,4]. During

cooling, austenite transforms to martensite. With further cooling, austenite continues totransform to martensite. Detailed information can be found in authors previous papers [2-4].

With increasing deposition temperature, the martensite grain structure continues to develop.

At 600C (figure 2c), the microstructure consists mainly of martensite as confirmed by XRDresults. Figure 2d is the as-coated NiTiHf film deposited on a 700C substrate. It not only shows

a larger grain size when compared with figures 2a - 2c, but also shows clear twin structures.

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(a) (b) (c)

(d) (e) (f)

Figure 1. Surface microstructure of NiTi films (a). as-coated at 300C, (b). as-coated at 500C,(c). as-coated at 500C followed by heat treatment at 700C for one hour. The reference line is

2 um. Cross-sectional microstructure of NiTi films (d). as-coated at 300C, (e). as-coated at

400C, (f) as-coated at 400C followed by heat treatment at 700C for one hour. The referenceline is 5 um.

The evolution of microstructure with increasing deposition temperature can also be observed

from a cross-sectional examination of these films. Figures 2e and 2f are the cross-sectionalmicrostructures of the as-coated NiTiHf films deposited at 300C and 600C. Figure 2e shows acoarse grain structure which is associated with the austenite structure. With increasing deposition

temperature, a greater amount of the tapered martensite grain structure is observed. The film

deposited at 600C is mainly composed of tapered martensite grains as shown in figure 2f.Similar to NiTi films, higher heat treatment temperature generates more substructure in the

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(a) (b) (c)

(d) (e) (f)

Figure 2. Surface microstructure of NiTiHf films (a). as-coated at 300C, (b). as-coated at

500C, (c). as-coated at 600C. (d). as-coated at 700C. Cross-sectional microstructure of NiTiHffilms (e). as-coated at 300C, (f). as-coated at 600C. The reference line is 10 um for (d) and

5 um for (a), (b), (c), (e), (f).

NiTiHf films. The size of substructures grows with increasing deposition temperature, similar to

what is observed in the martensite grains. The substructure is found in all the films heat treated at

higher HT temperatures. It becomes most distinct when the HT temperature is 700 C.

DISCUSSION

The microstructures of both NiTi and NiTiHf thin films are presented above. All the as-

deposited films are found to be crystalline, even when the substrate temperature is as low as300C. It is found that grain size increases with increasing deposition temperature, but it is not

affected by post deposition heat treatment temperature. This is related to the increased surface

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mobility of the atoms with increasing substrate temperature. As discussed in earlier papers [2-4],

in the thin film growth process during sputtering, the adsorbed particles collide with one anotherand form clusters. The clusters continuously collide with incoming particles and grow in size

until they reach a critical size at which they become thermodynamically stable. With increasing

deposition temperature, the mobility of the particles increases and the possibility for particles and

clusters interacting with one another also increases. As a result, large critical nuclei are formed.These critical nuclei continue to grow and eventually form grains. With increasing deposition

temperature, the grains increase in size.It is interesting that grain size is not affected by post deposition heat treatment temperature, as

observed in both the NiTi and NiTiHf thin films. This is because the as-coated films are already

in the crystalline state and the mobility of the deposited atoms decreases significantly after thefilm is formed. Thermal diffusion is too slow to result in grain growth even when the post

deposition HT temperature is higher than 700C. However, with increasing HT temperature, the

available thermal energy does permit dislocations to move and form substructures as shown infigure 2d.

It is found in both NiTi and NiTiHf thin films that martensite is primarily observed in the

films heat treated and deposited at higher temperatures. It is believed that large amounts ofdislocations formed at low deposition temperature and HT temperature might suppress the

martensite transformation. With increasing deposition temperature and HT temperature, more

and more dislocations are annealed out, therefore the martensite transformation temperatureincreases. However, the possibility of the existence of secondary phases such as Ti2Ni and

(TiHf)2Ni formed in the films may also suppress the martensite by the development of internal

stress, especially if these secondary phases are coherent. More research is needed to betterunderstand the above observations.

SUMMARY

The microstructure analysis shows that all the NiTi and NiTiHf thin films have a fine grainsize. The grain size increases with increasing deposition temperature, but no significant change

was observed with elevated post deposition HT temperature. However, at higher post deposition

HT temperature, more substructures were observed within the grains. This might be related todislocation reorganization or formation of precipitates during the heat treatment.

REFERENCES

1. A.D Johnson, V.V. Martynov and R.S, Minners,Journal de Physique IVColloque C8Supplement au Journal de Physique III vol. 5, C8-783-787 (1995).

2. C. Zhang, P.E. Thoma and R.H. Zee,Materials Processing Fundamentals, proceedings of

1999 EPD Congress, 139-145 (1999).

3. C. Zhang, P.E. Thoma and R.H. Zee,Materials for Smart Systems III, edited by Marilyn Wun-Fogle, K. Uchino, Y. Ito and R. Gotthardt, Materials Research Society Proceedings vol. 604,

129-134 (1999).

4. C. Zhang,NiTi and NiTiHf Shape Memory Thin Films, PhD dissertation, 2000.

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