Substrate effect on texturized microstructures in NiCr thin films

Thin Sohd Fdm6, 209 (1992) 67-72

67

Substrate effect on texturized microstructures in N i - C r thin films P.

Mengucci

Dtparttmento dt Sctenze det Materah e della Terra, Faeolta" dt lngegnerta, Umverstta' dl Ancona, Vta Breeze Blanche, 1-60131 Ancona (Italy)

M.

Costato

Dtparttmento dt Ftstca, Untverstta" dl Modena, Vta Campt 231/a, Modena (Italy)

G.

Majni

DaRartlrnento dt Scwnze det Materlah e della Terra, Facolta" dt Ingegnerta, Untverstta" dt Ancona, Vta Brecce Btamhe, 1-60131 Ancona (ltah')

(Recewed May 30, 1991, accepted October 7, 1991)

Abstract N1 Cr thin films with a composmon of 40at'¼,Nl and 60at %Cr have been sputter deposited on stainless steel and glass substrates The mlcrostructure and composition have been analysed by scanning electron microscopy, Rutherford-backscattermg spectroscopy, X-ray &ffractlon and transmission electron microscopy Ewdence is found of the metastable ~r phase and of the two termmal sohd solutions ~-Cr and y-Nl The mlcrostructure is characterized by a cubic close-packed lattice alternating with a hexagonal one within the same grams, resultmg m a stackmg fault presence Ewdence is found that the films have a fibre texture with the axis tdted 20 with respect to the surface normal and that &fferent substrates reduce &fferent fibre axis orientations (111) for the stamless steel substrate and (110) for the glass one This characteristic could be interpreted m terms of &fferent relatwe concentrations of the two terminal sohd solutions due to the &fferent substrate temperature during thm film deposmon

1. Introduction H i g h density N i - C r thin films are a t t r a c t i n g a g r o w i n g interest because o f their t r i b o l o g i c a l wear a n d c o r r o s i o n resistance, their use in m i c r o e l e c t r o n i c h y b r i d circutts m terms o f p o t e n t i o m e t e r s a n d fusible links in r e a d - o n l y m e m o r i e s , their use as resistors w~th high s t a b i h t y a n d low t e m p e r a t u r e coefficient o f electrical resistivity ( T R C ) a n d their g o o d l o n g - t e r m s t a b d i t y [1, 2]. The electrical resistiwty a n d structure o f N i - C r films have been studied for v a r i a b l e c h r o m m m c o n t e n t ( 0 - 5 0 at.%) a n d to evtdence the effects o f the presence o f oxygen [ 2 - 5 ] . N i - C r films have been investigated to evidence l a s e r - r e d u c e d t r a n s f o r m a t i o n s [6] a n d to obt a m with traces o f sdicon in solution a l m o s t zero T R C [7]. Studies have been d o n e to d e t e r m i n e the effects o f alloy c o m p o s m o n on pulsed i o n - b e a m - i n d u c e d reactions in N ~ , C r t _ , f i l m s d e p o s i t e d on silicon ( w h e r e x = 0.25 a n d 0 75) [8]. F u r t h e r m o r e , X - r a y m i c r o s t r u c ture analyses have been r e p o r t e d for samples o f a p p r o x imate c o m p o s i t i o n 7 8 a t . % N 1 - 2 2 a t % C r in thin films r a n g i n g f r o m 20 to 200 n m thick s h o w i n g on the a t o m i c scale N I - C r structures p r e d o m i n a n t l y with a cubic c l o s e d - p a c k e d lattice [9]. N I - C r thin films, used as low T R C reststors, with a fixed c o m p o s i t i o n 4 0 a t . % N i - 6 0 a t . % C r (40:60) have

0040-6090/92/$5 00

been a n a l y s e d in this s t u d y using scanning electron m i c r o s c o p y ( S E M ) , R u t h e r f o r d b a c k s c a t t e r m g spect r o s c o p y ( R B S ) , X - r a y diffraction ( X R D ) a n d transmission electron m i c r o s c o p y ( T E M ) . T h e aim o f this w o r k ts to present p r o o f o f the g r o w t h o f u n i f o r m texturtzed layers o f highly dense solid soluttons o f c h r o m t u m a n d ntckel wtth an o r i e n t a t i o n which is different a c c o r d i n g to the type o f s u b s t r a t e on which the film is deposited: on stainless steel it is parallel to (111) a n d on glass it is parallel to (110).

2. Sample preparation and experimental techniques T h e N i - C r alloy thm film d e p o s i t i o n was carried o u t in an r.f. s p u t t e r i n g system at 2 kV a n d 800 W. The target consisted o f a c t r c u l a r - s h a p e d , 2 0 . 4 c m radius, b i n a r y 4 0 a t . % N i - 6 O a t . % C r alloy. It was p r e - s p u t t e r e d clean for 1 h at 5 × 10 3 T o r r in an a r g o n a t m o s p h e r e before d e p o s i t i o n . A f t e r outgassing, the s p u t t e r i n g c h a m b e r was first e v a c u a t e d d o w n to a b o u t 10 - 7 T o r r a n d then filled with high p u r i t y a r g o n gas. The deposition was m a d e at 1 0 - 4 T o r r . T h e s u b s t r a t e - t o - t a r g e t distance was a b o u t 5 cm. D u r i n g d e p o s i t i o n the substrate lay on a steel plate w i t h o u t coohng. T h e sputtering system was e q u i p p e d with a liquid n l t n g e n tray. T h e

'.( 1992 - - Elsevier Sequoia All rights reserved

P Mengu(ct et al

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/

Te~turtzed mwrostru(tures m Nt

sputtered film thickness was between 200 and 280 nm with a typical deposition rate of 5 nm mln ~ Two types of samples have been analysed: the first obtained by deposition on AISI 303 stainless steel, the second on Cornlng 0211 glass. Both substrates consisted of square pieces with sides of 25 mm and a thickness of 0.5 mm Different experimental techniques have been here used to analyse the structure of the samples. (1) By SEM we have verified the flat morphology of the sample surface. (2) 1.8 MeV 4He+ RBS was used to obtain data on the atomic concentration and on the depth distribution of the elements. (3) X R D was used to identify the various compounds, the crystalline structure of the N I - C r layers and the texture axis orientation. A scintillation counter dlffractometer with Cu K s radiation was used in the common B r a g g - B r e n t a n o reflection-focusing geometry. (4) T E M was performed with a Philips CM 12 equipped with an LaB6 filament at 120 kV to evidence the microstructure and the average grain size and to confirm the texture axis orientation determined by the X R D investigations.

3. Results

Figure 1 shows the Rutherford backscatterlng spectrum of an N i - C r sample 200 nm thick deposited on a stainless steel substrate. The spectrum is representative of all other investigated samples and similar results are also obtained for N t - C r layers deposited on glass

[

I

I

Cr thm [thn,s

substrates. The arrows indicate the energy of the 4He+ particles scattered from surface atoms of the correspondlng elements. The nickel and chromium shoulders extend to lower energies, indicating a mixture of chromium and nickel From the average height of each shoulder the composition ratio of nickel and chromium in the alloy was determined and corresponds to that of the cathode, t . e 40 at.% NI and 60 at."/,, Cr (40:60). F r o m the energy width of the shoulder the film thickness can be obtained. From 525 to 975 keV the scale of the backscattering yield has been magnified 10-fold to put into evidence the peak associated with oxygen present at the surface of the sample. The area of the peak is proportional to the atomic concentration of the surface oxygen, less than l at.% Other peaks present in this energy range are mainly due to noise effects and in any case are comparable with the sensitivity limit of the RBS technique. No other impurities are detected on the surface or in the depth of the deposited film. Figure 2 shows the X R D spectra obtained from two samples with different substrates. Figure 2(a) refers to an N I - C r film 240 nm thick deposited on stainless steel, whereas Fig. 2(b) refers to a film 280 nm thick deposited on glass. Each figure shows a pattern where the peaks are relatively broad owing to small crystalhte sizes and possible non-uniform strains. Moreover, the relative intensities of the peaks differ significantly from those of the corresponding powders, indicating the presence of a fibre texture in the microstructure of both films. The orientation of the fibre axis is different for

I

20

~

A ,,i

16

4He+

~

1 8 MeV

,

C

._1 UJ

> (.9

12

z

LU

8

I---

< (j

09 (J < m

I,

4

X 10

"t

I

300

500

700

900

1100

\

1300

ENERGY (KEY) F~g

l

Rutherford backscattermg spectrum of an N~-Cr (40 60) film 200 nm thick deposited on a stainless steel substrate

P Mengu~(1 et a l / Texturtzed mtcro~tructure~ m Nt

!

I

Steel

Substrate

a)

i I

a (11o) 7 (111) hop(002) I

I 20

i 0"(041) 10

v >l-I

,

I

B

'

7(220)

a(211)

;

1

Glass

b)

Substrate

3o 7(220)

2 0 p-

0 30

a (110) hop 7 (111) 0"(002) (100) ~hcp(O02)

I 40

~

I 50

I ]

i

I 60 2 e

I

20 (deg)

~ Index

a Index

7 Index

39 6 41 4 44 3 45 5 53 7 75 1 83 1

--

002

-100hcp 111 002hcp --220 --

--

110 --211

041 150 --

I

'

Z Z

69

TABLE 1 XRD data forNi-Cr thm films

I

30 ~

Cr thmfilm~

i 70

k

I 80

L

I 90

(degrees)

Fig 2 X R D spectra (a) Ni Cr film 240nm thick deposited on stainless steel, (b) N1 Cr film 280 nm thick deposited on glass

the two types of films investigated and depends on the substrate nature as revealed by the different main peaks in Fig. 2(a) (20 = 44.3 °) and Fig. 2(b) (20 = 75.1°). The films deposited on stainless steel exhibit a fibre texture with the axis oriented along the (111) direction, whereas the films deposited on glass exhibit a fibre texture with the axis oriented in the (110) direction. From the compositional point of view the two films are constituted of the same phases, namely (1) a 7-Ni terminal solid solution with (f.c c.) structure and lattice parameter a = 0.3591 nm, (2) an ~-Cr terminal solid solution with (b.c.c.) structure and lattice parameter a = 0.2872 nm and (3) a metastable cy phase (approximately 30 at.% Cr) with tetragonal structure and lattice parameters a = 0.882 nm and c = 0.4567 nm. Furthermore, the presence of an h.c.p, lattice can be unambiguously inferred from the weak peak at 20 = 41.4" The (002) peak of the h.c.p lattice overlaps the f.c.c. (111) and the b.c.c. (110) peaks. The complete set of results obtained by X R D is reported in Table 1, where each peak is identified by the indices of the corresponding phases.

Figure 3 shows the results obtained by TEM observations on N 1 - C r deposited on a glass substrate. Figure 3(a) is a bright field image where the size of the crystallite forming the film is seen to be about 15 nm. The contrast variation within each crystallite could be attributed to the presence of extended defects such as stacking faults. Figure 3(b) represents the electron diffraction pattern of the same sample taken with the electron beam perpendicular to the film surface. The shape of the rings is typical of a texturized structure with a fibre axis tilted with respect to the surface normal. When the electron beam exactly coincides with the fibre axis, the intensity of the rings becomes continuous. This situation is shown in Fig. 3(c), taken with a specimen tilt of 20 °. Therefore the film investigated exhibits a fibre texture with the axis tilted 20 ° to the surface normal. A similar tilt of the fibre axis was found for samples deposited on stainless steel (not shown here). The results of the electron diffraction pattern analysis are summarized in Table 2, which shows that the reflection rings correspond to 7-NI, Qt-Cr and oxides of various structures. The experimental d spacings are reported in the second column and are compared with those reported in powder diffraction files from the literature [10] in the third to sixth columns The first three rings (N = 1, 2, 3) can be fairly unambiguously attributed to chromium oxides, C r 3 0 4 and Cr203 which are present on the surface of the samples as indicated by the oxygen peak in the Rutherford backscattering spectrum. The remaining rings (N = 4 to 9) are typical of nickel and chromium, where both sequences give the complete set of planes which are responsible for the diffraction. It is worth noticing that, because of the experimental uncertainty, rings 4 and 8 can arise from the overlapping of nickel and chromium. In this table the (hkl) Miller indices referring to nickel and chromium show a selection rule of the type h = k. This allows us to calculate the zone axis of the diffraction pattern, which, for the particular conditions of specimen orientation used, corresponds to the fibre axis. In particular, one can deduce that the fibre axis is parallel to the ( l l 0 ) direction. TEM observations of the films

P Mengu~ct et al / Textunzed mtcrostructures m Nt Cr thm fihns

70

TA B LE 2 Electron &ffractlon data for N1 Cr thin films deposited on glass substrate N

1 2 3

4 5 6 7 8 9

dm (nm)

0 3706 0 2805 0 2352

0 0 0 0 0 0

2013 1432 1164 1049 1015 0873

~(nm) Cr~O4

hkl

Cr20~

hkl

202 113

0 3633 ---

012

0 2860 0 2340

N1

hkl

Cr

hkl

0 2035

Ill

0 2040 0 1443 0 1178

110 002 112

0 1063 0 1017 0 0881

113 222 004

0 1020

220

d e p o s i t e d on stainless steel substrates yield the same result o b t a i n e d by the X R D investigation, n a m e l y that the fibre axis is oriented a l o n g the (111) direction.

(a)

4. Discussion

iN

(b)

Ic) Fig 3 TEM results for an N1-Cr film 280 nm think deposited on glass (a) bright field ~mage,(b) selected area electron &ffracUon pattern taken with the beam normal to the surface, (c) selected area electron diffraction pattern of the same zone taken with 20 specimen ult

X R D p a t t e r n s a n d T E M d a t a u n a m b i g u o u s l y show that on the a t o m i c scale the N i - C r thin films exhibit a c.c.p, structure mixed with a small a m o u n t o f h.c.p. layers. In fact, we can identify the p e a k at 20 -- 44.3 n as the o v e r l a p p i n g o f the (111) reflection o f the nickel f.c.c, lattice, as the (002) reflection o f the nickel h.c.p. lattice a n d as the (110) reflection o f the c h r o m i u m b c.c. lattice. This n o n - u n i q u e a t t r i b u t i o n is p r i m a r i l y due to the b r o a d e n i n g o f the p e a k caused by the n e a r coincidence o f the Ni(111) a n d Cr(110) l n t e r p l a n a r distances. H o w e v e r , the p o s i t i o n o f the p e a k m a x i m u m is exactly coincident with w h a t one can expect f r o m the Ni(111) reflection. F r o m this o b s e r v a t i o n one can c o n c l u d e that the m a i n c o n t r i b u t i o n to the p e a k at 20 = 44.3 r' is due to the nickel lattice, a c c o r d i n g to the d a t a r e p o r t e d xn T a b l e 1 by H u a n g a n d H o w a r d for an N i - C r s a m p l e 200 n m thick [9]. The possibthty o f having a mixture, on the a t o m i c scale, o f cubic a n d h e x a g o n a l latuces can be a t t r i b u t e d to a structure m a d e o f individual grains with a cubic structure a l t e r n a t i n g with a h e x a g o n a l one. This is clearly confirmed in Fig. 3(a), where the c o n t r a s t variations within the d a r k grains are typical o f the presence o f lattice defects such as stacking faults which arise whenever the plane sequence typical o f the f.c.c, structure is c h a n g e d to that o f the h c.p. one. The p e a k at 20 = 75 1• is indexed as the (220) reflection o f the 7-Ni f.c.c, lattice. The p e a k at 2 0 - - 8 3 . 1 ~ is indexed as the (211) reflection o f the 7-Cr b.c c lattice The equilibrium

P Menguccl et a l / Te~cturlzed mwrostructutes m N t - C r thm fibns

phase diagram of nickel and chromium shows that these structures are the expected terminal solid solutions for the bulk materials [11]. The peak at 20 = 39.5 ~ is a novel one for thin Ni Cr films. We have identified it as the metastable a phase which is present in bulk materials [ 12] but can also be seen in vacuum-deposited N I - C r films [13]. This phase is found in bulk materials for an alloy composition ranging from about 50 to 70 at.% Cr in the range of temperatures between 1100 and 1400 "C. At lower temperatures it decomposes to the normal ~-Cr and 7-NI terminal solid solutions. The existence of the cy phase in N 1 - C r thin films has been Inferred as being due to the presence of impurities [ 14] which are typically present in the type of films investigated here in concentrations lower than the sensitivity of the employed techniques. On the other hand it is important to notice that the composition of our films, t.e. 40 at.% NI and 60 at.% Cr (40:60), is fully contained In the above range valid for bulk materials. The presence of the ~ phase also permits us to identify the remaining peaks. The presence of fibre textures in the structure of N 1 - C r thin films is well known in the literature [3, 9]. In particular, Belu-Marian et al reported experimental evidence of an orientation dependence of the fibre axis on the N I - C r concentration ratio. They showed that the axis of orientation shifts from (111) towards (100) on changing the concentration from 80.20 to 50:50 [3]. On the other hand our results refer to thin films with a fixed N I - C r concentration (40:60) and show an axis orientation dependence on the substrate nature. This orientation shifts from (111) for the stainless steel substrate to (110) for the glass one. This effect could be attributed to several concomitant factors influencing the film growth, such as the substrate temperature during film deposition, the atomic surface moblhty and the binding energy of the elements to the substrate. However, we believe that the main contribution could be attributed to the different substrate temperatures, since m a n y of the above factors influencing the film growth are closely connected to this parameter. We expect that the temperature of the stainless steel substrate should be lower than that of glass because of the heat sink effect of steel. Although the temperatures of the two substrates during the film depositions was not measured because of several technical difficulties, we believe that the temperature differences are of the order of few tens of degrees because of the following observations" (1) the dimensions of the substrates are small in comparison with the underlying metal plate, which implies that the heat capacity of the metal plate is higher than that of each substrate type; (2) the small thickness of the substrates allows good heat dissipation even in the glass case. These temperature differences could be sufficient to shift the concentration ratio C

71

of the two terminal solid solutions, because thin film growth is a typical situation of non-thermodynamic equilibrium. By using the lever law in the phase diagram in the temperature range 1100-1400 °C where the ~ phase exists, we find that with increasing temperature the b.c.c. ~-Cr concentration increases with respect to the f c.c. y-Ni one, namely Ccr = 0.44CN, at 1100 °C, C~r = 0.79CN, at 1200 °C and Ccr = 1.18CN, at 1300 °C. The above considerations, coupled with the wellknown fact that the fibre texture in thin films occurs along the close-packed directions, which are (111) for the f.c.c, structure and (110) for the b.c.c, one [15], permit us to summarize as follows (1) The films deposited on stainless steel show a fibre axis in the (111) direction typical of f.c.c, lattices. This is due to the fact that the film growth is determined by the f.c c structure of the T-Ni solid solution, which is predominant with respect to the ~-Cr b c.c. one. (2) The films deposited on glass show a fibre axis in the (110) direction typical of b.c c. lattices This is due to the fact that, as a consequence of the higher substrate temperature during deposition, the concentration of ~-Cr b.c.c, solid solution becomes predominant with respect to the 7-Ni f.c.c, one Therefore a different substrate temperature during the deposition of N i - C r thin films can give rise to the same effects of variation in the texture axis orientation as a different elemental concentration of the target alloy, as already reported by Belu-Marlan et al. [3].

5. Conclusions Films of N 1 - C r with a fixed composition have been deposited by sputtering on two types of substrates. stainless steel and glass. F r o m the compositional point of view the thin films are constituted of the same phases: (1), two terminal solid solutions, namely f.c.c. T-Ni and b.c.c. ~-Cr, (2) a metastable ~ phase (approximately 30 at.% Cr) with tetragonal structure and lattice parameters a = 0.882 nm and c = 0.4567 nm. On the atomic scale the films show a cubic close-packed lattice alternating with a hexagonal one, resulting in a stacking fault presence within each grain. Furthermore, the two films exhibit a fibre texture with the axis tilted 20 ° from the surface normal and are oriented in a different direction according to the type of substrate. The film deposited on stainless steel shows the fibre axis oriented along the (111) direction, while the film deposited on glass shows the fibre axis oriented along the (110) direction. This latter characteristic could be interpreted in terms of different relative concentrations of the two terminal solutions due to the different substrate temperatures during film deposition.

72

P Menguc~l et al / Texturlzed mwro;tructures m Nt Cr thm film,~

Acknowledgments The authors wish to thank Gefran Sensori srl., Provagho d'Iseo, Brescta, Italy for supplymg the samples. The work is supported by CNR and MURST

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