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Growth, structure and stress of sputtered TiB2 thin films
Materials Science and Engineering
A191 (1995) 233-238
Growth, structure and stress of sputtered TiB, thin films J. Chen, J.A. Barnard The University ofAlabama,
Department of Metallurgical and Materials Engineering, USA
Tuscaloosa, AL 35487-0202,
Received 24 May 1994; in revised form 2 1 June 1994
Abstract Thin hexagonal TiB, films were deposited using d.c. magnetron sputtering. The structure of films with different thicknesses was evaluated by X-ray diffractometry and transmission electron microscopy. It was found that the microstructure of TiB, films depends on thickness. Films below approximately 1000 A are made up of randomly oriented fine grains. A (001) texture starts to develop above this thickness. More detailed study of the texture reveals that the textured crystallites align more ideally as the thickness of films increases. The residual stresses in all films are compressive and increase slightly with increasing film thickness. After 400 “C annealing. the film stress systematically shifts toward the tensile direction with the thinner films exhibiting larger changes. This is attributed to the elimination of a larger density of defects and more pronounced grain growth in thinner films. Keywords: Stress; Sputtering;
Titanium; Boron; Thin films
1. Introduction Titanium diboride is a hard intermetallic compound which has a high resistance to erosion and corrosion and shows good electrical conductivity. The structure is hexagonal with a = 3.028 A and c = 3.228 A. It has been under considerable study in recent years because of its potential applications as a protective coating. Several methods have been reported for depositing TiB?, e.g. spray coating [ 11, chemical vapor deposition [2-51, electrodeposition from a molten alkali metal borate bath [6], and sputtering [7,8]. Among all the techniques, sputtering offers advantages over other methods, such as low temperature during deposition and the absence of toxic or explosive gases. It has been recognized for some time that internal stresses in thin films, built up either during or after their deposition, may significantly affect their properties. Films can fail by buckling or cracking depending on the state of stress. The general problem of stress in films deposited by sputtering was described by Thornton and Hoffman [9]. They observed a tensile to compressive stress transition as a function of operating pressure. The compressive stress was attributed to atomic peening, i.e. lattice distortion produced by energetic particles striking the condensing films. More 0921-5093/95/$9.50 0 1995 - Elsevier SSZIlO921-5093(94)09632-7
Science
S.A. All rights reserved
recently, stress has been related to the sputtering voltage [lo]. These experiments show that for a given target-substrate distance the same stress results at a given sputtering voltage. Hence, the stress in a sputtered film appears to be related to the energetics of substrate bombardment. Various models have been developed to describe the thermalization process, the process by which collisions in the sputtering gas reduce the energy of the reflected neutrals and the sputtered atoms themselves [ 1l- 131, which aim to give a measure of the degree of “atomic peening” occurring at the growing surface. In the present study, deposition conditions such as power and pressure are fixed. Stresses, as deposited and after heat treatment at 400 “C, are studied as a function of film thickness. Results are correlated to the microstructure of the films as a function of film thickness and temperature.
2. Experimental details 2.1. Sample preparation A Vat-Tee model 250 batch side sputtering system was used to deposit the TiBz thin films using d.c.
J. Chen, J.A. Barnard /
234
MaterialsScience and Engineering A191 (1995) 233-238
magnetron sputtering. Films with different thicknesses (200-5000 a) were deposited on 2 in diameter oxidized Si( 111) wafers, 7059 Corning glass, and carbon-coated copper grids at ambient temperature. A 99.5% TiB, disk (4 in diameter) was used as target material. Base pressure in the chamber before sputtering was 3 x 10 -h Torr. The sputtering condition was fixed at 400 W power control and 5 mTorr Ar. The resulting deposition rate was 6 A s-l. Film thickness was measured using a Dektak IIa surface prolifometer. 2.2. Structure investigation The structure of deposited films was studied using a Rigaku D/Max-2BX X-ray diffractometer (Cu Ka radiation) with a thin film goniometer. Scans were made in different modes, namely 8-28 (BraggBrentano mode), grazing angle (Seeman-Bohlin mode) and texture analysis (TA mode). The principles of these different modes have been summarized by Flinn and Waychunas [ 121. In the present study, 5” was used as the incident beam angle in the SB mode. In the TA mode, the measurements were carried out with a fixed Bragg angle of 68.5”, corresponding to diffraction by the (102) plane of TiB,, and the value of a, the angle between the incident X-ray beam and the sample, was varied. A peak near 8 - B ( 19,the Bragg angle of the ( 102) plane, and /3, the angle between the (00 1) plane which aligns parallel to the surface and the ( 102) plane) is expected to be seen if the material is textured. If the textured grains are ideally aligned, the a peak position should be at 2.7”. Transmission electron microscopy (TEM) specimens of thickness less than 1000 A were directly deposited on carbon-coated Cu grids, while those of thickness more than 1000 A were prepared by back thinning from the substrate side using ion milling. Specimens were examined in a Hitachi H-8000 transmission electron microscope operating at 200 keV.
and tf the thickness of the film. In this formula R is the net curvature, obtained after subtracting the curvature of the substrate before sputtering. Eq. (1) is valid for deposited film thicknesses much less than the substrate thickness (a condition met in this study). In this thin film approximation the curvature change is independent of the mechanical properties of the film. The variation in stress with temperature was also investigated. The chamber was backfilled with Ar gas during the measurements. The highest temperature investigated was 400 “C with a heating ramp of 5 “C min-‘.
3. Results and discussion 3.1. Structure A representative X-ray diffraction (XRD) scan of a sample of 5000 A thickness in 8-28 mode is shown in Fig. 1. An Si (111) peak from the substate which is close to the (001) peak of TiB, is present in the scan. The ratio of the (001) peak to the (010) and (101) peaks of TiB, is high which indicates a (001) texture in the as-deposited films since in the powder pattern (001) is only 60% of ( 101). However, the presence of (0 10) and ( 10 1) peaks also indicates that a significant portion of the film is incompletely textured or untextured. In the grazing angle scans (Fig. 2(a)), obtained from different film thicknesses, the (001) peak is still intense relative to the other two peaks while the Si substrate peak is eliminated. The approximately 9” difference between 8 and a (8 = 14” and a = 5”) for this reflection is consistent with well-defined texture with a spread in a of less than 18”. In Fig. 2(a), it can be clearly noted that the intensity ratio of (001) and ( 101) peaks is almost constant in films with thickness more than 1300 A. At a thickness 4000
2.3. Stress measurement A thin film stress tester (Flexus Stress Measurement System, model FLX 2320) with in situ annealing facilities was used to evaluate stress at room temperature of as-deposited and annealed films. A laser beam is reflected from the surface of the water, and the displacement of the reflected beam is determined as the wafer is scanned. The curvature of the substrate is measured before and after film deposition, and stress can be calculated using a=(l/6)[E,l(I
- Qt,‘lt,)lR
(1)
where E, and v are Young’s modulus and Poisson’s ratio of the substrate, [, is the thickness of the substrate
$
t”““““““,““,““,““,““j
2000
';' 1500 .-
25
30
35
40
45
50
55
2 theta Fig. 1. 8-28 X-ray scan of TiB, film 500 A thick.
60
J. Chen, J.A. Barnard
1
235
Materials Science and Engineering Al 91 (1995) 2.T.GZ.i,y
50 0 20
30
40
50
60
70
0
5
IO
x c v) k
250
;
200
r
150
-
100
-
50
r
Fig. 3. Texture TiBz film.
20
25
30
35
40
scan of the (102) retlcction
of a 2800
A thick
Table 1 Peak positions of the (102) reflection of TiBz films obtained X-ray diffraction texture analysis Thickness (A) Peak (deg) Shift (deg)
C
0-
20
theta
2 theta
(a)
15
30
40
50
60
70
2 theta (b) Fig. 2. Grazing angle (5”) X-ray scans of TiBz film: (a) with different thicknesses; (b) an 860 A thick film.
of 860 A, the peaks are low and broad which is caused by smaller diffraction volume and smaller grain size. An enlarged scan plot of the 860 A thick film is shown in Fig. 2(b). The ratio of the (001) peak to the (101) peak has decreased compared with that of films thicker than 1300 A. This decrease could be due to the grains that are more randomly oriented or to textured grains which are aligned more closely to the surface resulting in a decrease in the diffraction volume at this particular angle (9” tilt from the surface). XRJI scans were also made in a TA mode to investigate the distribution of textured grains. A typical scan of a 2800 A thick film is shown in Fig. 3. The peak is broad indicating that some of the grains are randomly oriented which is in agreement with the results from grazing angle scans. As discussed in the previous section, the TA peak should be centered at 2.7” if most of the textured grains are oriented ideally. However, a shift of 5.3” in the peak position is noticed in 2800 A thick film which indicates that most of the textured
350 No -
860 No -
1300 10.3 7.6
1800 8.7 6.0
2800 8.0 5.3
5000 3.5, 1.2
by
Ideal 2.7 0
grains are oriented about 5.3” away from the ideal position. Table 1 lists the peak position and shift of peak position from the ideal obtained by texture analysis as a function of film thicknesses. A peak was not observed in films with thicknesses less than 860 A which indicates that grains are essentially randomly oriented in those films. The conclusion drawn from these scans as well as grazing angle scans is that films thinner than 860 A contain mostly randomly oriented fine grains. The (001) texture starts to develop as the films grow thicker. A systematic shift of the peak toward the ideal position is clearly observed in the table as film thickness increases. Bright field TEM images of films at different thicknesses are shown in Fig. 4. The films deposited are all polycrystalline with a grain size which increases with thickness. At 350 A thickness, small spherical grains are observed with a size of about 5 nm. With increasing thickness, the grain size increases from about 10 nm for 860 A films to about 50 nm for the 1300 A thick film. Grain size does not change after further increase of thickness. Studies of the microstructure of TiB2 using different diffraction geometries and TEM indicate a clear dependence of film structure on the thickness of the films. The first approximately 1000 A consist of randomly oriented fine grains with the grain size increasing with thickness. (001) texture starts to develop on further increase in film thickness but the
236
J. Chen, J.A. Barnard
Fig. 4. TEM bright field micrographs
/
Materials Science and En&eeringAl91
of as-deposited
(1995) 233-238
TiBz film (a) 350 A. (b) 860 A, (c) 1300
A and (d) 1800 A thick.
grain size remains nearly constant. However, a large portion of the grains are still untextured. The same growth pattern was also reported in other material systems [ 14,151. Interestingly, the textured grains are oriented closer to the ideal position with increasing film thickness. 3.2. Stress measurement The results of room temperature stress measurements of films as deposited and after 400 “C annealing are presented in Fig. 5. In the investigated thickness range, the stresses as deposited are all compressive and become more compressive with increasing film thickness. Beyond 2000 A, the film stress is essentially constant. According to the “atomic peening” mechanism suggested by Thornton and Hoffman [9], neutral working gas atoms rebounding from the target can bombard the growing film and modify the film structure and create a compressive stress. We consider that the above mechanism can be applied to explain the existence of compressive stresses in sputtered TiB, thin films. From the stress-thickness plot, it can be concluded that compressive stress accumulates during sputter deposition. Under similar deposition conditions, compressive stress was not found in pure Ti films [ 161. It is believed that the large magnitude of compressive stress observed in as-deposited TiB, thin films is associated with its high melting point. Relaxation of stress during growth by elimination of defects is not effective if T/T, is low and adatom mobility is low. High compressive stresses can therefore accumulate during the deposition process. The plateau in the stress-thickness plot above 2000 A may be associated with more effective defect elimination as the temperature of the film increases during prolonged growth. A plot of film stresses after 400 “C annealing in Fig. 5 reveals that, in all the films, the compressive stress is
400 “C annealed.
2
as deposited
Z
5000
6000
-2000 I,~,rl~~~lllll,l,r~~I~~~~-
-3000 0
1000
2000
3000
Thickness
4000
(A)
Fig. 5. Stress as a function of TiBz film thickness.
lower and in thinner films it has been changed to a tensile stress. Thermal and intrinsic stress changes can be clearly distinguished if the stress is plotted as a function of temperature. Some representative plots of the first thermal cycle are shown in Fig. 6. Specimens were first heated to 400 “C with 5 “C min-’ ramp and were then held for 1 h before cooling down. On heating, the elastic strain due to the difference in thermal expansion between TiB, film and the silicon resulted in a linear change in stress with temperature of - 2.7 mPa “C-l. On further heating beyond 150 “C, densification, defect annihilation, and grain growth occurred in all the films causing a drop of stress. During the isothermal 1 h anneal at 400 “C the stresses further decreased owing to the same effects. Comparing a thinner film (860 A) with a thicker film (2800 A), a much more rapid drop of stress during heating above 150 “C is observed in the thinner film. Note also that the stress change during the isothermal anneal increases as film thickness decreases. These changes are attributed to the removal of a larger density of defects and more pronounced grain
J. Chen, J.A. Barnard
/
Materials Science and Engineering A 191 (199.5) X-.38
growth in thinner films. Structural defects can include self-interstitials, possibly generated by the Ar “atomic peening” process, and entrapped Ar atoms. In the initial stage of deposition, the substrate temperature rise is very low and the structural defects cannot be activated effectively resulting in a larger density of defects in thinner films. Films are grown at an effectively higher temperature as the substrate warms during prolonged growth. On annealing at higher temperatures, the elimination of defects, causing lateral shrinkage of the films, produces a relative tensile stress in the thinnest films. Grain boundary elimination is another cause of the drop in compressive stress. According to the grain growth model [17], the grain boundary has a porous structure, and the coalescence of grains eliminates the grain boundaries, and thus the lateral shrinkage of the film again induces a relative tensile stress. From the point of view of energy, this process transfers energy from grain boundary energy to strain energy. It is also an activated process. TEM micrographs of micro-
*Ooo t’l
237
structures after 1 h of 400 “C annealing are shown in Fig. 7. As can be seen from the micrograph, the grain size indeed increased for thinner films (compared with the grain size observed as deposited in Fig. 4). The grain size changed from about 5 nm to a maximum of 20 nm in the film 3.50 A thick. While grain size change can still be observed in 860 A thick film, no obvious size change can be found in films thicker than 1300 A. It is therefore concluded that the very fine spherical grains grown in the first 1000 A are susceptible to grain growth at modest temperatures while larger more textured grains are much more stable. In TiB, films, any stress relaxation associated with grain growth is thus more important in the thinner films. The volume fraction of randomly oriented grains (those most susceptible to growth during annealing) decreases as film thickness increases. XRD scans of samples made after the stress-temperature cycle (not shown here) exhibit no significant change in film texture or lattice parameter. A slight increase in peak intensity and sharpness is noted for the thinnest films. During the second heating and cooling cycle, only elastic behavior was observed for all the films, indicating that the microstructure was stabilized by the first annealing cycle.
4. Conclusions
100
200
300
Temperature Fig. 6. Stress-temperature thicknesses.
400
500
(C)
plots for TiBz films with different
Fig. 7. TEM bright field micrographs
The microstructure of sputter-deposited TiB, films depends on thickness. Films below approximately 1000 A contain randomly oriented fine grains whose size increases with increasing film thickness. (001) texture starts to develop above film thicknesses of about 1000 A and textured grains align more ideally when the thickness of the films increases. The residual stresses of TiB, films are all in compression as deposited and increase slightly with increasing film thickness. After 400 “C annealing, the
of 400 “C annealed TiB, film (a) 350 a, (b) 860 A, (c) 1300 A and (d) 1800 A thick.
238
J. Chen, J.A. Barnard
1
Materials Science and Engineering A191 (1995) 233-238
stresses of all films shift toward the tensile direction with the thinner films exhibiting larger changes. This is attributed to the elimination of a larger density of defects and more pronounced grain growth in thinner films. Acknowledgment
This research is funded by the National Science Foundation Grant EHR-9 10876 (EPSCoR-Alabama). References
[ll
A.W. Mullendore, D.W. Mattox, J.B. Whitley and D.J. Sharp, Thin Solid Films, 63 (1979) 243. [21 A.J. Becker and J.H. Blanks, Thin Solid Films, 119 (1984) 241. [31 A.W. Mullendore, J.B. Whitley, H.O. Pierson and D.M. Mattox, J. Vat. Sci. Technol., 18 (1981) 1049.
[41 D.M. Mattox, Thin Solid Films, 63 (1979) 213. [51 H.O. Pierson and A.W. Mullendore, Thin Solid Films, 95 (1982)99.
bl D.M. Flinn, F.X. McCawley, G.R. Smith and P.B. Needham, Rep. Invest. 8332, US Department of the Interior, Bureau of Mines, 1979. W.A. Zadaniewski and J. Wu, J. Mater. Res., 6 (1991) 1066. ti; G. Hilz and H. Holleck, Mater. Sci. Erg. A, 139 (1991) 268. [91 J. Thornton and D.W. Hoffman, Thin Solid Films, 171 (1989) 5. [lOI T.J. Vink and J.B.D. van Zon, J. Vat. Sci. Technol. A, 9 (1991) 124. [I11 F.J. Cadieu and N. Chencinski, IEEE Trans. Mag. I I ( 1975) 227. W.D. Westwood, J. Vat. Sci. Technol., 1.5 (1978) 1. t::i R.E. Somekh, Vacuum, 34 (1984) 987. [I41 U. Hwang, Y. Uchiyama, K. Ishibashi and T. Suzuki, Thin Solid Films, 147( 1987) 231. J.K. Howard, J. Vat. Sci. Technol. A, 4 (1986) 2975. 1::; M. Chinmulgund, MS. Thesis, University of AlabamaTuscaloosa, 1994. [I71 E. Klokholm and B.S. Berry, J. Electrochem. Sot., 115 (1968) 823.
A191 (1995) 233-238
Growth, structure and stress of sputtered TiB, thin films J. Chen, J.A. Barnard The University ofAlabama,
Department of Metallurgical and Materials Engineering, USA
Tuscaloosa, AL 35487-0202,
Received 24 May 1994; in revised form 2 1 June 1994
Abstract Thin hexagonal TiB, films were deposited using d.c. magnetron sputtering. The structure of films with different thicknesses was evaluated by X-ray diffractometry and transmission electron microscopy. It was found that the microstructure of TiB, films depends on thickness. Films below approximately 1000 A are made up of randomly oriented fine grains. A (001) texture starts to develop above this thickness. More detailed study of the texture reveals that the textured crystallites align more ideally as the thickness of films increases. The residual stresses in all films are compressive and increase slightly with increasing film thickness. After 400 “C annealing. the film stress systematically shifts toward the tensile direction with the thinner films exhibiting larger changes. This is attributed to the elimination of a larger density of defects and more pronounced grain growth in thinner films. Keywords: Stress; Sputtering;
Titanium; Boron; Thin films
1. Introduction Titanium diboride is a hard intermetallic compound which has a high resistance to erosion and corrosion and shows good electrical conductivity. The structure is hexagonal with a = 3.028 A and c = 3.228 A. It has been under considerable study in recent years because of its potential applications as a protective coating. Several methods have been reported for depositing TiB?, e.g. spray coating [ 11, chemical vapor deposition [2-51, electrodeposition from a molten alkali metal borate bath [6], and sputtering [7,8]. Among all the techniques, sputtering offers advantages over other methods, such as low temperature during deposition and the absence of toxic or explosive gases. It has been recognized for some time that internal stresses in thin films, built up either during or after their deposition, may significantly affect their properties. Films can fail by buckling or cracking depending on the state of stress. The general problem of stress in films deposited by sputtering was described by Thornton and Hoffman [9]. They observed a tensile to compressive stress transition as a function of operating pressure. The compressive stress was attributed to atomic peening, i.e. lattice distortion produced by energetic particles striking the condensing films. More 0921-5093/95/$9.50 0 1995 - Elsevier SSZIlO921-5093(94)09632-7
Science
S.A. All rights reserved
recently, stress has been related to the sputtering voltage [lo]. These experiments show that for a given target-substrate distance the same stress results at a given sputtering voltage. Hence, the stress in a sputtered film appears to be related to the energetics of substrate bombardment. Various models have been developed to describe the thermalization process, the process by which collisions in the sputtering gas reduce the energy of the reflected neutrals and the sputtered atoms themselves [ 1l- 131, which aim to give a measure of the degree of “atomic peening” occurring at the growing surface. In the present study, deposition conditions such as power and pressure are fixed. Stresses, as deposited and after heat treatment at 400 “C, are studied as a function of film thickness. Results are correlated to the microstructure of the films as a function of film thickness and temperature.
2. Experimental details 2.1. Sample preparation A Vat-Tee model 250 batch side sputtering system was used to deposit the TiBz thin films using d.c.
J. Chen, J.A. Barnard /
234
MaterialsScience and Engineering A191 (1995) 233-238
magnetron sputtering. Films with different thicknesses (200-5000 a) were deposited on 2 in diameter oxidized Si( 111) wafers, 7059 Corning glass, and carbon-coated copper grids at ambient temperature. A 99.5% TiB, disk (4 in diameter) was used as target material. Base pressure in the chamber before sputtering was 3 x 10 -h Torr. The sputtering condition was fixed at 400 W power control and 5 mTorr Ar. The resulting deposition rate was 6 A s-l. Film thickness was measured using a Dektak IIa surface prolifometer. 2.2. Structure investigation The structure of deposited films was studied using a Rigaku D/Max-2BX X-ray diffractometer (Cu Ka radiation) with a thin film goniometer. Scans were made in different modes, namely 8-28 (BraggBrentano mode), grazing angle (Seeman-Bohlin mode) and texture analysis (TA mode). The principles of these different modes have been summarized by Flinn and Waychunas [ 121. In the present study, 5” was used as the incident beam angle in the SB mode. In the TA mode, the measurements were carried out with a fixed Bragg angle of 68.5”, corresponding to diffraction by the (102) plane of TiB,, and the value of a, the angle between the incident X-ray beam and the sample, was varied. A peak near 8 - B ( 19,the Bragg angle of the ( 102) plane, and /3, the angle between the (00 1) plane which aligns parallel to the surface and the ( 102) plane) is expected to be seen if the material is textured. If the textured grains are ideally aligned, the a peak position should be at 2.7”. Transmission electron microscopy (TEM) specimens of thickness less than 1000 A were directly deposited on carbon-coated Cu grids, while those of thickness more than 1000 A were prepared by back thinning from the substrate side using ion milling. Specimens were examined in a Hitachi H-8000 transmission electron microscope operating at 200 keV.
and tf the thickness of the film. In this formula R is the net curvature, obtained after subtracting the curvature of the substrate before sputtering. Eq. (1) is valid for deposited film thicknesses much less than the substrate thickness (a condition met in this study). In this thin film approximation the curvature change is independent of the mechanical properties of the film. The variation in stress with temperature was also investigated. The chamber was backfilled with Ar gas during the measurements. The highest temperature investigated was 400 “C with a heating ramp of 5 “C min-‘.
3. Results and discussion 3.1. Structure A representative X-ray diffraction (XRD) scan of a sample of 5000 A thickness in 8-28 mode is shown in Fig. 1. An Si (111) peak from the substate which is close to the (001) peak of TiB, is present in the scan. The ratio of the (001) peak to the (010) and (101) peaks of TiB, is high which indicates a (001) texture in the as-deposited films since in the powder pattern (001) is only 60% of ( 101). However, the presence of (0 10) and ( 10 1) peaks also indicates that a significant portion of the film is incompletely textured or untextured. In the grazing angle scans (Fig. 2(a)), obtained from different film thicknesses, the (001) peak is still intense relative to the other two peaks while the Si substrate peak is eliminated. The approximately 9” difference between 8 and a (8 = 14” and a = 5”) for this reflection is consistent with well-defined texture with a spread in a of less than 18”. In Fig. 2(a), it can be clearly noted that the intensity ratio of (001) and ( 101) peaks is almost constant in films with thickness more than 1300 A. At a thickness 4000
2.3. Stress measurement A thin film stress tester (Flexus Stress Measurement System, model FLX 2320) with in situ annealing facilities was used to evaluate stress at room temperature of as-deposited and annealed films. A laser beam is reflected from the surface of the water, and the displacement of the reflected beam is determined as the wafer is scanned. The curvature of the substrate is measured before and after film deposition, and stress can be calculated using a=(l/6)[E,l(I
- Qt,‘lt,)lR
(1)
where E, and v are Young’s modulus and Poisson’s ratio of the substrate, [, is the thickness of the substrate
$
t”““““““,““,““,““,““j
2000
';' 1500 .-
25
30
35
40
45
50
55
2 theta Fig. 1. 8-28 X-ray scan of TiB, film 500 A thick.
60
J. Chen, J.A. Barnard
1
235
Materials Science and Engineering Al 91 (1995) 2.T.GZ.i,y
50 0 20
30
40
50
60
70
0
5
IO
x c v) k
250
;
200
r
150
-
100
-
50
r
Fig. 3. Texture TiBz film.
20
25
30
35
40
scan of the (102) retlcction
of a 2800
A thick
Table 1 Peak positions of the (102) reflection of TiBz films obtained X-ray diffraction texture analysis Thickness (A) Peak (deg) Shift (deg)
C
0-
20
theta
2 theta
(a)
15
30
40
50
60
70
2 theta (b) Fig. 2. Grazing angle (5”) X-ray scans of TiBz film: (a) with different thicknesses; (b) an 860 A thick film.
of 860 A, the peaks are low and broad which is caused by smaller diffraction volume and smaller grain size. An enlarged scan plot of the 860 A thick film is shown in Fig. 2(b). The ratio of the (001) peak to the (101) peak has decreased compared with that of films thicker than 1300 A. This decrease could be due to the grains that are more randomly oriented or to textured grains which are aligned more closely to the surface resulting in a decrease in the diffraction volume at this particular angle (9” tilt from the surface). XRJI scans were also made in a TA mode to investigate the distribution of textured grains. A typical scan of a 2800 A thick film is shown in Fig. 3. The peak is broad indicating that some of the grains are randomly oriented which is in agreement with the results from grazing angle scans. As discussed in the previous section, the TA peak should be centered at 2.7” if most of the textured grains are oriented ideally. However, a shift of 5.3” in the peak position is noticed in 2800 A thick film which indicates that most of the textured
350 No -
860 No -
1300 10.3 7.6
1800 8.7 6.0
2800 8.0 5.3
5000 3.5, 1.2
by
Ideal 2.7 0
grains are oriented about 5.3” away from the ideal position. Table 1 lists the peak position and shift of peak position from the ideal obtained by texture analysis as a function of film thicknesses. A peak was not observed in films with thicknesses less than 860 A which indicates that grains are essentially randomly oriented in those films. The conclusion drawn from these scans as well as grazing angle scans is that films thinner than 860 A contain mostly randomly oriented fine grains. The (001) texture starts to develop as the films grow thicker. A systematic shift of the peak toward the ideal position is clearly observed in the table as film thickness increases. Bright field TEM images of films at different thicknesses are shown in Fig. 4. The films deposited are all polycrystalline with a grain size which increases with thickness. At 350 A thickness, small spherical grains are observed with a size of about 5 nm. With increasing thickness, the grain size increases from about 10 nm for 860 A films to about 50 nm for the 1300 A thick film. Grain size does not change after further increase of thickness. Studies of the microstructure of TiB2 using different diffraction geometries and TEM indicate a clear dependence of film structure on the thickness of the films. The first approximately 1000 A consist of randomly oriented fine grains with the grain size increasing with thickness. (001) texture starts to develop on further increase in film thickness but the
236
J. Chen, J.A. Barnard
Fig. 4. TEM bright field micrographs
/
Materials Science and En&eeringAl91
of as-deposited
(1995) 233-238
TiBz film (a) 350 A. (b) 860 A, (c) 1300
A and (d) 1800 A thick.
grain size remains nearly constant. However, a large portion of the grains are still untextured. The same growth pattern was also reported in other material systems [ 14,151. Interestingly, the textured grains are oriented closer to the ideal position with increasing film thickness. 3.2. Stress measurement The results of room temperature stress measurements of films as deposited and after 400 “C annealing are presented in Fig. 5. In the investigated thickness range, the stresses as deposited are all compressive and become more compressive with increasing film thickness. Beyond 2000 A, the film stress is essentially constant. According to the “atomic peening” mechanism suggested by Thornton and Hoffman [9], neutral working gas atoms rebounding from the target can bombard the growing film and modify the film structure and create a compressive stress. We consider that the above mechanism can be applied to explain the existence of compressive stresses in sputtered TiB, thin films. From the stress-thickness plot, it can be concluded that compressive stress accumulates during sputter deposition. Under similar deposition conditions, compressive stress was not found in pure Ti films [ 161. It is believed that the large magnitude of compressive stress observed in as-deposited TiB, thin films is associated with its high melting point. Relaxation of stress during growth by elimination of defects is not effective if T/T, is low and adatom mobility is low. High compressive stresses can therefore accumulate during the deposition process. The plateau in the stress-thickness plot above 2000 A may be associated with more effective defect elimination as the temperature of the film increases during prolonged growth. A plot of film stresses after 400 “C annealing in Fig. 5 reveals that, in all the films, the compressive stress is
400 “C annealed.
2
as deposited
Z
5000
6000
-2000 I,~,rl~~~lllll,l,r~~I~~~~-
-3000 0
1000
2000
3000
Thickness
4000
(A)
Fig. 5. Stress as a function of TiBz film thickness.
lower and in thinner films it has been changed to a tensile stress. Thermal and intrinsic stress changes can be clearly distinguished if the stress is plotted as a function of temperature. Some representative plots of the first thermal cycle are shown in Fig. 6. Specimens were first heated to 400 “C with 5 “C min-’ ramp and were then held for 1 h before cooling down. On heating, the elastic strain due to the difference in thermal expansion between TiB, film and the silicon resulted in a linear change in stress with temperature of - 2.7 mPa “C-l. On further heating beyond 150 “C, densification, defect annihilation, and grain growth occurred in all the films causing a drop of stress. During the isothermal 1 h anneal at 400 “C the stresses further decreased owing to the same effects. Comparing a thinner film (860 A) with a thicker film (2800 A), a much more rapid drop of stress during heating above 150 “C is observed in the thinner film. Note also that the stress change during the isothermal anneal increases as film thickness decreases. These changes are attributed to the removal of a larger density of defects and more pronounced grain
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Materials Science and Engineering A 191 (199.5) X-.38
growth in thinner films. Structural defects can include self-interstitials, possibly generated by the Ar “atomic peening” process, and entrapped Ar atoms. In the initial stage of deposition, the substrate temperature rise is very low and the structural defects cannot be activated effectively resulting in a larger density of defects in thinner films. Films are grown at an effectively higher temperature as the substrate warms during prolonged growth. On annealing at higher temperatures, the elimination of defects, causing lateral shrinkage of the films, produces a relative tensile stress in the thinnest films. Grain boundary elimination is another cause of the drop in compressive stress. According to the grain growth model [17], the grain boundary has a porous structure, and the coalescence of grains eliminates the grain boundaries, and thus the lateral shrinkage of the film again induces a relative tensile stress. From the point of view of energy, this process transfers energy from grain boundary energy to strain energy. It is also an activated process. TEM micrographs of micro-
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structures after 1 h of 400 “C annealing are shown in Fig. 7. As can be seen from the micrograph, the grain size indeed increased for thinner films (compared with the grain size observed as deposited in Fig. 4). The grain size changed from about 5 nm to a maximum of 20 nm in the film 3.50 A thick. While grain size change can still be observed in 860 A thick film, no obvious size change can be found in films thicker than 1300 A. It is therefore concluded that the very fine spherical grains grown in the first 1000 A are susceptible to grain growth at modest temperatures while larger more textured grains are much more stable. In TiB, films, any stress relaxation associated with grain growth is thus more important in the thinner films. The volume fraction of randomly oriented grains (those most susceptible to growth during annealing) decreases as film thickness increases. XRD scans of samples made after the stress-temperature cycle (not shown here) exhibit no significant change in film texture or lattice parameter. A slight increase in peak intensity and sharpness is noted for the thinnest films. During the second heating and cooling cycle, only elastic behavior was observed for all the films, indicating that the microstructure was stabilized by the first annealing cycle.
4. Conclusions
100
200
300
Temperature Fig. 6. Stress-temperature thicknesses.
400
500
(C)
plots for TiBz films with different
Fig. 7. TEM bright field micrographs
The microstructure of sputter-deposited TiB, films depends on thickness. Films below approximately 1000 A contain randomly oriented fine grains whose size increases with increasing film thickness. (001) texture starts to develop above film thicknesses of about 1000 A and textured grains align more ideally when the thickness of the films increases. The residual stresses of TiB, films are all in compression as deposited and increase slightly with increasing film thickness. After 400 “C annealing, the
of 400 “C annealed TiB, film (a) 350 a, (b) 860 A, (c) 1300 A and (d) 1800 A thick.
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Materials Science and Engineering A191 (1995) 233-238
stresses of all films shift toward the tensile direction with the thinner films exhibiting larger changes. This is attributed to the elimination of a larger density of defects and more pronounced grain growth in thinner films. Acknowledgment
This research is funded by the National Science Foundation Grant EHR-9 10876 (EPSCoR-Alabama). References
[ll
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