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Mechanical stresses in D.C. reactively sputtered Fe2O3 thin films
Thin Solid Films, 68 0980) 333-343 © Elsevier Sequoia S.A., Lausanne--Printed in the Netherlands
333
MECHANICAL STRESSES IN D.C. REACTIVELY SPUTTERED Fe20 a THIN FILMS V. ORLINOV AND G. SAROV
Instttute of Electronics, Bulgarian Academy of Sciences, Sofia 1113 and Institute of Semtconductor Technology, Botevgrad (Bulgaria) (Received October 17, 1979; accepted November 2, 1979)
We investigated the effect of substrate temperature during deposition, of deposition rate, of oxygen content in the Ar + 02 gas flow, of film thickness and of heat treatment on mechanical stresses in d.c. reactively sputtered thin films of Fe 2O3 on fused quartz substrates. On the basis of the results obtained some conclusions are drawn about the character and the origin of the mechanical stresses in such films. It is shown that it is possible to produce Fe203 films free of mechanical stresses on quartz substrates by optimizing the deposition process or by additional heat treatment.
1. INTRODUCTION The interest in thin films of Fe203 in the last decade has mainly arisen because of their use in microelectronics for the preparation of selectively transparent photomasks. These photomasks allow visual observation of the topology of the silicon wafer which facilitates the alignment of the images in the photolithographic process. In earlier work 1'2 on Fe203 thin films we have indicated the possibility of depositing such films by d.c. reactive sputtering, so that they are suitable for the preparation of selectively transparent photomasks. In our subsequent papers 3'4 we have reported final and systematic results of investigations of the deposition procedures, the optical, chemical and mechanical properties of these films and some structural features. Other papers 5-7 report methods of deposition and the investigation of these films with a view to their use in microelectronics for the purposes indicated. However, .comparatively little is known about the reasons for the decrease in the resolution of these photomasks during their preparation, i.e. of their chemical etching. One of the reasons may be the existence of large mechanical stresses which under suitable conditions can cause film cracking, rupture, bending and loss of adhesion between the film and the substrate. Such effects are particularly undesirable when precise geometrical configurations are to be made in the film. Mechanical stress is a force in the plane of the film acting per unit area of the film cross section. The mechanical stress may be compressive or tensile in character. The total mechanical stress o of a film consists of two terms: o = O,+OT
(I)
334
v . ORLINOV, G. SAROV
where crI is the so-called intrinsic stress, which is apparently a fundamental result of the conditions and method of film growth and is to a large degree a reflection of the film structure and the presence of impurities, and aT is the thermal stress in the film, which is given by the expression O"T = Ef(0Cf - - 0~s)( Td - -
TM)
(2)
where Er is the Young modulus for the film, ~f and ~sare average thermal coefficients of the film and the substrate and Td and TMare the film deposition temperature and the temperature during the stress measurements, respectively. Therefore, the thermal stress of the film is a result of the difference in the temperatures of the film during deposition and measurement. A number of phenomenological models of limited application have been proposed to explain the origin of the intrinsic stresses, according to which the stresses can arise for various reasons such as point defects and defect clusters in the film, growth processes at the grain boundaries, surface tension and the fact that the surface of the film during deposition is at a substantially higher temperature than the substrate. It is worth pointing out the model proposed in ref. 8 in which the magnitude of the intrinsic mechanical stresses is an indication of the amount of disorder initially present in the surface layer before it becomes covered by the condensation of the succeeding particles. Because of this ai will be determined by the relation between the annealing rate M of disordered material on the growing surface and the deposition rate I4/.At low substrate temperatures and high deposition rates, i.e. when M < W, large mechanical stresses a i will arise, whereas, for M > W, ai will be small. Results of experimental investigations of mechanical stresses in thin films of metals, semiconductors and dielectrics, deposited by different methods, including cathode sputtering, are also given in the literature. However, no such data have been obtained on thin Fe203 films until now. In the present work the results of the investigation of the deposition conditions and the subsequent heat treatment on the mechanical stresses in d.c. reactively sputtered thin FezO 3 films are presented. Some conclusions on the character and origin of the mechanical stresses and their influence on the quality of the photolithographic picture are made. 2. EXPERIMENTAL PROCEDURE
For the measurement of the mechanical stresses thin films of Fe203 were deposited onto optically flat circular quartz substrates with a diameter of 14 mm and a thickness of 150 0m. The deposition process was usually carried out at a working pressure p of Ar and 02 of 6 Pa and a cathode voltage U, of 3.5 kV. The cases in which deviations from these conditions were employed are pointed out in the text. The target was a 120 mm x 5 mm iron plate CArmco"). The target-to-substrate distance was in all cases 6 cm. The substrates were placed on a thin metal holder with a welded chromel-alumel thermocouple on its back side. The holder temperature could be varied by an additional electronically controlled heater. According to elasticity theory the mechanical stress of the deposited films can be determined as a function of the measured substrate deformation when the
MECHANICAL STRESSES IN REACTIVELY SPUTTERED F e 2 0 3
THIN HLMS
335
adhesion of the film to the substrate is strong enough to suppress slippage. Under this condition the average stress in the film is given by the expression E~ d~2 1 6(1-v~) de R~
(3)
where E, and v, are respectively the Young modulus and Poisson ratio of the substrate, d, and de are respectively the substrate and the film thicknesses and R, is the radius of curvature of the substrate. Rs can be determined by measuring the diameters of the Newton interference rings which appear when the film side of the substrate is placed on an optical reference plane and is exposed to monochromatic light of wavelength 2: (4)
Din2 - - Dn2
R~ = 42(m--n) where Dm and Dn are the diameters of the mth and the nth interference rings. Since the final substrate curvature is dependent on parameters such as the initial substrate curvature, R~ was measured twice: once with the deposited film in place and secondly after the film had been stripped by chemical etching. The following expression was used for the determination of the mechanical stress: a = 6 ( l _ v s ~ ) df
1 Rs2
where R,~ and R~2 are, respectively, the radii of curvature of the substrate with and without the film. In this expression R,~ and R,2 have a positive sign when the stress is tensile and a negative sign when it is compressive. It is to be noted that for tensile stresses the film tends to contract with respect to the substrate and the deformation shown in Fig. l(a) occurs. In the presence of compressive stresses the film tends to expand with respect to the substrate and the deformation shown in Fig. l(b) occurs. The same signs for tensile and compressive stresses follow from eqn. (1) for the thermal stress aT. sub.rate
J (a)
b
__
substrate
I
~reference plate
(b)
~reference plate
Fig. 1. View of the substrate deformation when (a) tensile or (b) compressive stresses exist in the thin film.
A diagram of the experimental set-up for the measurement of the radius of curvature of the substrate is shown in Fig. 2. A helium lamp was used as a monochromatic light source with a wavelength 2 of 5880 A. The diameters of the interference rings were measured using a microscope with an eyepiece micrometer. A typical case of an interference picture observed during the measurement is shown in Fig. 3. The average radius R~ of curvature of the substrate was determined9 from the
336
v . ORLINOV, G. SAROV
radii of the 4th and 8th rings along the major and the minor axes of the elliptical interference rings.
/subst~te ~ L _ _ ~ t h i n film I - ~ ] ~ n c e
plate
\ t
semffransparent
he b,mp
miffor
to
microscope
Fig. 2. Schematic diagram of the experimental set-up for the determination of mechanical stress in thin films. Fig. 3. A typical interference picture obtained while determining the mechamcal stress in the samples.
3" 2 1
0
f f J
r,"
-1 -2 -3
-4 -5 I
~00
l
2O0
I
3O0
I
400
I
~O'C
Fig. 4. Dependences of the thermal stress aT, the intrinsic stress a~ and the total mechanical stress a on the substrate temperature T, during film deposition.
3.
EXPERIMENTAL RESULTS AND DISCUSSION
The experimental dependences of the total mechanical stress a and its
MECHANICAL STRESSES IN REACTIVELY SPUTTERED F e 2 0 3 THIN FILMS
337
components (the thermal stress a T and the intrinsic stress at) on the substrate temperature T~are shown in Fig. 4. T~was varied during the deposition of F e 2 0 3 by varying the discharge current. The deposition process was carried out under the conditions given previously in Section 2 and at an oxygen content Qo, in the gas flow of 3%, a deposition rate W of 60 A m i n - 1 and a film thickness df of 1600 A. The separation of the two components of ~ was made by measuring a at room temperature and at a temperature equal to that of the substrate during the deposition. Thus, from the measured data at T~and according to eqn. (2), at A T = 0, a T = 0 and a = tri and the thermal component O"T at room temperature will be a - ai, where a is the value of the total mechanical stress measured at room temperature. The behaviour of the experimentally determined dependence tTT(T~)is in good agreement with its theoretical behaviour given by eqn. (2). Furthermore, since the thermal stress t7T has a positive sign, i.e. it is a tensile stress, it follows from eqn. (2) that ~f > ~,. This is in agreement with reported data 1°'11 for these thermal expansion coefficients (~Fe203 = 2.66 x 10 - 6 K - 1 and ~sio2 = 0.5 x 10 - 6 K - l ) . Taking these data into account we obtained from the dependence ¢rT(T~)in Fig. 4 a value of 5.33 x 1011 Pa for the Young modulus of Fe203 thin films. As can be seen from Fig. 4 the intrinsic mechanical stresses a, in thin films of Fe203, deposited by d.c. reactive sputtering at temperatures T~ < 450 °C have a negative sign, i.e. ~7i is compressive. This result confirms the conclusion drawn in ref. 12 that in dielectric and semiconducting thin films compressive intrinsic stresses usually predominate. If we accept the assumption expressed in ref. 13 that the intrinsic mechanical stress is related to point defects and defect clusters in the film, the behaviour of cri(T~)in Fig. 4 is in good agreement with the model proposed in ref. 8. According to this model, annealing of these defects and ordering of the film will increase as the substrate temperature increases and as a result a decrease in the intrinsic stresses will be observed. In order to verify the contribution of the defects in our case to the origin of the compressive stress in Fe203 films, the following experiment was carried out. Thin F e 2 0 3 films were deposited onto quartz substrates under the same conditions as those used for the films in Fig. 4 where tensile stresses arose and were measured. After that the films were implanted with a dose ¢~i of Ar + ions with a definite energy E1 and the stresses were measured again. Finally the substrates were subjected to annealing at temperatures equal to the deposition temperatures for a time t and the residual mechanical stresses were again measured. The results of this experiment are given in Table I.
TABLE I Deposition and treatment condltzons
a(Pa)
After deposition of FezO 3 at T, --- 360 °C, Qo2 = 3%, W = 60,4 m i n - 1 and df = 1600,~
After Ar + Implantation at E. = 80 k e V and ~l = 5 x 101 a ions c m - 2
After annealing of the film at T. = 360 °C for t = 60 min
+lxl0a
- 0 . 3 × 10a
+ l . 2 x 10a
338
v . ORLINOV, O. SAROV
The results obtained show that the defects introduced in the films during ion implantation actually lead to the appearance of compressive mechanical stresses; this confirms the assumption about the nature of the intrinsic stresses and the explanation for the behaviour of oi(T,) given earlier. In addition, the X-ray data described in ref. 4 show that these films are a mixture of amorphous and polycrystalline phases and that the relative portion of the latter phase increases with increasing deposition temperature or after annealing of the films. Therefore we can conclude that in the ease of d.c. reactively sputtered films of Fe20 a at substrate temperatures T~ ~<450 °C the annealing rate M of disordered material is not sufficient to compensate for the formation of defects during the growth process and as a result compressive intrinsic stresses will be developed. With an increase in the temperature T~the rate M also increases, i.e. the number of defects decreases and the percentage of the polycrystaUine phase increases. This results in a decrease in the compressive intrinsic stresses in the films. An important conclusion from a practical point of view can be drawn from the relationship o(T~) shown in Fig. 4: using d.c. reactive sputtering of thin Fe203 films onto quartz substrates we can achieve films with negligible total mechanical stresses by choosing the right substrate temperature T, during the deposition (in our case a value of 330 °C).
x
3 2 1 I
0 -I
-2 -3 -4 -5 -6
I
tOO
I
200
I
300
i
400
I
500°C
Fig. 5. Dependence of the total mechanical stresses o on the substrate temperature T,. 1, heated by the discharge power alone; 2, heated by an external heat source.
We also note that the relationships presented in Fig. 4 are similar to those obtained in ref. 14 for Si3N4 and SiO2 films. In Fig. 5 we compare two o(T,) dependences. In the first, which was taken from Fig. 4, T~was varied by varying the discharge current I, (i.e. by changing the pressure in the vacuum chamber) while in the second dependence I, was constant and equal to 40 mA and T~was varied with the aid of an external heat source. As can be seen from these figures the way in which the heating was performed influences the stress dependences. Probably in the case of curve 1 the growing film is subjected to a more
MECHANICAL STRE~ES IN REACTIVELY SPUTTERED F e 2 0 3 THIN HLMS
339
intense ion bombardment compared with that of curve 2. This increases the ability of the deposited atoms to migrate and so it is possible to grow a film having a more perfect structure at a given substrate temperature. o
X
Y 6 5 4 3
l
t
1~=4
2 7 0 -1 -2 -3
I
loo
I
200
I
300
I
4oo
I[ ' ~
5oo
Fig. 6. Dependenceofthe total mechamcalstressesa on the annealingtemperaturesT,at the two extreme substrate temperatures T~m Fig. 4 The relationship between the total mechanical stress and the annealing temperature T~ of the films which were annealed for 20 min after their deposition at the two extreme substrate temperatures T~in Fig. 4 (T~ = 200 °C and T~ = 400 °C) is given in Fig. 6. As expected at T, < T~ no change occurs in the values of a in both cases. A change is later observed in a, which is in agreement with the mechanism of structural changes that has already been considered (a decrease in the number of defects and an increase in the polycrystalline phase) as the substrate temperature rises. Furthermore, the comparison between the relationship a(T~) at T~= 200 °C and the relationship o(T~) given by curve 2 in Fig. 5 shows that the annealing of the already deposited film and its heating by an external heat source during the deposition process lead to approximately equal changes in a.
-I
-2
I
I
I
t
I
I
I
I
lo
20
30
4o
so
~o
70
~0
Fig. 7. Dependence of the mechanical stresses~ on the deposition rate W.
w_..
340
V. O R L I N O V , G . S A R O V
The relationship a(W) given in Fig. 7 and obtained by changing the glow discharge voltage Va in the range 2-4 kV is also in agreement with the physical model proposed in ref. 8. The substrate temperature T, is held constant at 300 °C using an external heat source. The other conditions of the experiment were Qo2 = 3% and df = 1600 A. As the deposition rate Wis increased, more and more of the particles falling onto the surface will not succeed in building a defect-free structure before being covered by the incident flow of sputtered particles. This will result in an increase in the compressive intrinsic stresses in the film. A further point concerning the relationship a(W) given in Fig. 7 is that under the conditions of this experiment the decrease in W is accompanied by a decrease in the discharge power and the substrate temperature caused by it. In agreement with the results shown in Fig. 5, the decrease in W will cause a change in a which is in opposition to the change caused by the variation of the sputtering rate, since the maintenance of T, at a constant value by the external heat source only partially compensates for this effect. Furthermore, keeping Qo, constant and varying W during measurement of the dependence a(W) creates an additional influence of the oxygen in the gas flow on a which is equivalent to the effect that occurs when Qo, is varied and W is held constant. From the results shown in Fig. 8 (which we shall discuss later) it can be seen that the increase in Qo~ for constant w (which is equivalent to a decrease in W for constant Qo) leads to an increase in the compressive stresses a. Therefore the aforementioned effect of 02 in the gas flow on ,r in the dependence a(W) is exactly opposite to that caused by the change in W.Thus, we can conclude that the influence of the deposition rate W on the mechanical stresses ~ of Fe20 3 films obtained by d.c. reactive sputtering is considerably greater than that shown in Fig. 7 owing to the additional effects which accompany the experiment.
-2
-3
-4
~o
2
I
i
3
4
I
I
i
I
I
5 e
I
8
10
20
30
I
I
I
40 50 eo
I
I ~Ut
8o t00
Fig. 8 T h e m e c h a n i c a l stresses o a s a f u n c t i o n o f 0 2 c o n t e n t m the A r + 0 2 g a s flow
Let us now consider in greater detail the dependence a(Qo) given in Fig. 8. This was measured at T~ = 320 °C by heating the substrate using only the discharge power which was also kept constant (Ia = 50 mA and V~= 3.5 kV). The other conditions of the experiment were W = 60/~ min- 1 and df = 1600/~. The behaviour of this dependence is in accordance with the commonly accepted opinion 15 that
MECHANICAL STRESSES IN REACTIVELY SPUTTERED F e 2 0 3 THIN FILMS
341
gases captured in the growing film increase the mechanical stresses in the film and with the experimental results in ref. 16 where it is shown that the increase in the partial pressure of H20 and 02 leads to an increase in the compressive stresses in thermally evaporated thin films of SiO2. The explanation of this is as follows. The oxygen may react with the grain walls or it may be absorbed by the grains thus increasing their volume. This will cause the grains to press against one another and compressive mechanical stresses will arise in the film. The fact that a is approximately constant, and even its slight decrease at Qo~ > 40%, can be explained by the previously established decreasea in W and the slight increase in T~ in this range of Qo~ which cannot be disregarded in this experiment. Figure 9 gives the experimentally determined dependence o(df). Initially samples of maximum thickness df were deposited. Then consecutive chemical etching of the samples by HC1 was carried out and some of them were used for the measurement of (7 and others for the measurement'of dr. The behaviour of o(df) is in agreement with the ideas that we have already considered about the origin of the mechanical stresses in thin films. As cr is the average stress value along the thickness of the film, it follows from the figure that the majority of it is concentrated near the interface at which the greatest number of imperfections are to be expected in the growing of the film structure, owing to the difference between the two structures. A similar behaviour of the dependence o(df) has been reported in ref. 17 for reactive d.c. sputtering of tungsten. 0 n
1
O
2 3
6 I
20
I
I
so
I
I
100
I
I
140
I
I
180
I
I
220
I
I
260
Fzg. 9. The mechanical stresses ~ as a functzon of the thzckness dr of the thin film.
Finally, we shall consider the mutual relation between mechanical stresses in the film and the quality of the photolithographic process in using the films for the preparation of selectively transparent photomasks. For this purpose Fe203 films were deposited on polished quartz substrates at three different values of T~: 200, 330 and 400 °C, corresponding to maximum compressive stresses (a = - 4 x l0 s Pa), to zero stresses (a = 0) and to maximum tensile stresses (or = 3 x l0 s Pa) under the conditions in Fig. 5 (curve 1). These three different films were therefore coated with photoresist and were exposed through the same photomask. Then photomasks with a given geometrical configuration were prepared from them. Scanning electron micrographs of the Fe2Oa photomasks obtained are given in Fig. 10. As can be seen, the photomask with the highest quality is that in Fig. 10(b), corresponding to a = 0, whereas in Figs. 10(a) and 10(c) a clear trend of toothed edges is observed. We can conclude from these observations that the presence of both compressive and tensile stresses in Fe203 thin films causes local cracking, loss of adhesion or bending during
342
v. ORLINOV,o. SAROV
their chemical etching, which lead to a deterioration in the edges of the image and therefore the quality of the photomask.
m
(a)
(b)
(c) Fig. 10. Views of Fe203 photomasks prepared with films hawng different mechamcal stresses: (a) = - 4 x l0 s Pa;(b) ~r = 0;(c)o = 3 x 10s Pa.
4. CONCLUSION
We have undertaken an investigation which for the first time aims at understanding the influence of the deposition conditions and subsequent heat treatment on mechanical stresses in Fe203 thin films prepared by d.c. reactive sputtering on fused quartz substrates. It has been established that in the range of variation of the substrate temperature that we studied, the substrate being heated by the discharge power only, the intrinsic film stresses ai are compressive and decrease with T~, the thermal stresses aT are tensile and increase with T~and the total mechanical stresses t7 of the film change their character from compressive at low T~to tensile at high T~. Similar variations in ~rare observed both when a film is annealed after deposition or when an external heat source is used during the deposition of a film. The compressive mechanical stresses a in the film increase as either the film deposition rate W or the oxygen content Qo2 in the gas flow are increased. ~ initially falls abruptly with increasing film thickness dr. This is an important observation which shows that the
MECHANICAL STRESSES IN REACTIVELY SPUTTERED F e 2 0 3 THIN FILMS
343
fundamental part of the mechanical stresses is concentrated near the interface between the film and the substrate. Some conclusions have been drawn about the character and the origin of the mechanical stresses in the films investigated, on the basis of interpretations of the relations established. It has been shown that the difference between the deposition temperature and the temperature at which the film is investigated as well as the higher thermal expansion coefficient of the film compared with that of the quartz substrate cause the appearance of tensile stress. The presence of disorder (defects or an amorphous phase) in the film structure and of gases trapped during the film growth process leads to the appearance of compressive stress. In this study of the preparation of thin Fe203 films on quartz substrates using d.c. reactive sputtering we have shown that it is possible to obtain films which have minimum (approximately zero) mechanical stress by optimizing the deposition conditions or the subsequent heat treatment. This permits the fabrication of high quality selectively transparent photomasks. REFERENCES G A. Sarov, Electropromishlenost i Priborostroene, 4 (1973) 131. V. Orlinov and G. Sarov, Bulg. J. Phys., 2 (1975) 156. V. Oriinov, B. Goranchev, G. Sarov and V. Tsaneva, Thin SohdFilms, 67 (1979) 125. -V. Orlinov, B. Goranchev, G. Sarov and A. Mlsmk, Thin Solid Fdms, 67 (1979) 135. W.R. Sinclair, M V Smith and R. A. Fastnach, J. Electrochem. Soc., 118 (1971) 341. I B. MacChesney, P . B . O ' C o n n o r a n d M V Sullivan, J. Electrochem. Soc.,l18(1971)776. A.R. Ianus, SolidState Technol., 6 (1973) 33. E KlakholmandB. S. Berry, J. Electrochem. Soc.,115(1968)823. P.B. G h a t e a n d L H Hali, J. Electrochem. Soc.,l19(1972)491. K.P. Yakovlev, Kratky Fiztko-Tekhiucheski Spravochnik, Vol 3, FML, Moscow, 1962, p. 232. K.P. Yakovlev, Kratky Spravochnikpo Ftztke, FML, Moscow, 1963, p. 310. G Hass and R. E. Thun, Phystcs of Thin Films, Vol. 3, Academic Press, New York, 1966, Chap. 5. H S. StoryandR. W Hoffman, Proc R Soc. London, Ser. B, 70(1957)950. A.K. Smha, H. S. Lerinstein and T. E SmRh, J. Appl. Phys., 49 (1978) 2423. J . R . Priest and H. L. Caswell, Trans. 8th Natl. Symp. on Vacuum Technology, Pergamon, New York, 1961 16 J R. Priest, H. L. Caswell and J Budu, Vacuum, 12 (1962) 301. 17 R.C. Sun, T.C. TlsoneandP. D Ruzend, J. Appl. Phys., 46 (1975) l12.
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15
333
MECHANICAL STRESSES IN D.C. REACTIVELY SPUTTERED Fe20 a THIN FILMS V. ORLINOV AND G. SAROV
Instttute of Electronics, Bulgarian Academy of Sciences, Sofia 1113 and Institute of Semtconductor Technology, Botevgrad (Bulgaria) (Received October 17, 1979; accepted November 2, 1979)
We investigated the effect of substrate temperature during deposition, of deposition rate, of oxygen content in the Ar + 02 gas flow, of film thickness and of heat treatment on mechanical stresses in d.c. reactively sputtered thin films of Fe 2O3 on fused quartz substrates. On the basis of the results obtained some conclusions are drawn about the character and the origin of the mechanical stresses in such films. It is shown that it is possible to produce Fe203 films free of mechanical stresses on quartz substrates by optimizing the deposition process or by additional heat treatment.
1. INTRODUCTION The interest in thin films of Fe203 in the last decade has mainly arisen because of their use in microelectronics for the preparation of selectively transparent photomasks. These photomasks allow visual observation of the topology of the silicon wafer which facilitates the alignment of the images in the photolithographic process. In earlier work 1'2 on Fe203 thin films we have indicated the possibility of depositing such films by d.c. reactive sputtering, so that they are suitable for the preparation of selectively transparent photomasks. In our subsequent papers 3'4 we have reported final and systematic results of investigations of the deposition procedures, the optical, chemical and mechanical properties of these films and some structural features. Other papers 5-7 report methods of deposition and the investigation of these films with a view to their use in microelectronics for the purposes indicated. However, .comparatively little is known about the reasons for the decrease in the resolution of these photomasks during their preparation, i.e. of their chemical etching. One of the reasons may be the existence of large mechanical stresses which under suitable conditions can cause film cracking, rupture, bending and loss of adhesion between the film and the substrate. Such effects are particularly undesirable when precise geometrical configurations are to be made in the film. Mechanical stress is a force in the plane of the film acting per unit area of the film cross section. The mechanical stress may be compressive or tensile in character. The total mechanical stress o of a film consists of two terms: o = O,+OT
(I)
334
v . ORLINOV, G. SAROV
where crI is the so-called intrinsic stress, which is apparently a fundamental result of the conditions and method of film growth and is to a large degree a reflection of the film structure and the presence of impurities, and aT is the thermal stress in the film, which is given by the expression O"T = Ef(0Cf - - 0~s)( Td - -
TM)
(2)
where Er is the Young modulus for the film, ~f and ~sare average thermal coefficients of the film and the substrate and Td and TMare the film deposition temperature and the temperature during the stress measurements, respectively. Therefore, the thermal stress of the film is a result of the difference in the temperatures of the film during deposition and measurement. A number of phenomenological models of limited application have been proposed to explain the origin of the intrinsic stresses, according to which the stresses can arise for various reasons such as point defects and defect clusters in the film, growth processes at the grain boundaries, surface tension and the fact that the surface of the film during deposition is at a substantially higher temperature than the substrate. It is worth pointing out the model proposed in ref. 8 in which the magnitude of the intrinsic mechanical stresses is an indication of the amount of disorder initially present in the surface layer before it becomes covered by the condensation of the succeeding particles. Because of this ai will be determined by the relation between the annealing rate M of disordered material on the growing surface and the deposition rate I4/.At low substrate temperatures and high deposition rates, i.e. when M < W, large mechanical stresses a i will arise, whereas, for M > W, ai will be small. Results of experimental investigations of mechanical stresses in thin films of metals, semiconductors and dielectrics, deposited by different methods, including cathode sputtering, are also given in the literature. However, no such data have been obtained on thin Fe203 films until now. In the present work the results of the investigation of the deposition conditions and the subsequent heat treatment on the mechanical stresses in d.c. reactively sputtered thin FezO 3 films are presented. Some conclusions on the character and origin of the mechanical stresses and their influence on the quality of the photolithographic picture are made. 2. EXPERIMENTAL PROCEDURE
For the measurement of the mechanical stresses thin films of Fe203 were deposited onto optically flat circular quartz substrates with a diameter of 14 mm and a thickness of 150 0m. The deposition process was usually carried out at a working pressure p of Ar and 02 of 6 Pa and a cathode voltage U, of 3.5 kV. The cases in which deviations from these conditions were employed are pointed out in the text. The target was a 120 mm x 5 mm iron plate CArmco"). The target-to-substrate distance was in all cases 6 cm. The substrates were placed on a thin metal holder with a welded chromel-alumel thermocouple on its back side. The holder temperature could be varied by an additional electronically controlled heater. According to elasticity theory the mechanical stress of the deposited films can be determined as a function of the measured substrate deformation when the
MECHANICAL STRESSES IN REACTIVELY SPUTTERED F e 2 0 3
THIN HLMS
335
adhesion of the film to the substrate is strong enough to suppress slippage. Under this condition the average stress in the film is given by the expression E~ d~2 1 6(1-v~) de R~
(3)
where E, and v, are respectively the Young modulus and Poisson ratio of the substrate, d, and de are respectively the substrate and the film thicknesses and R, is the radius of curvature of the substrate. Rs can be determined by measuring the diameters of the Newton interference rings which appear when the film side of the substrate is placed on an optical reference plane and is exposed to monochromatic light of wavelength 2: (4)
Din2 - - Dn2
R~ = 42(m--n) where Dm and Dn are the diameters of the mth and the nth interference rings. Since the final substrate curvature is dependent on parameters such as the initial substrate curvature, R~ was measured twice: once with the deposited film in place and secondly after the film had been stripped by chemical etching. The following expression was used for the determination of the mechanical stress: a = 6 ( l _ v s ~ ) df
1 Rs2
where R,~ and R~2 are, respectively, the radii of curvature of the substrate with and without the film. In this expression R,~ and R,2 have a positive sign when the stress is tensile and a negative sign when it is compressive. It is to be noted that for tensile stresses the film tends to contract with respect to the substrate and the deformation shown in Fig. l(a) occurs. In the presence of compressive stresses the film tends to expand with respect to the substrate and the deformation shown in Fig. l(b) occurs. The same signs for tensile and compressive stresses follow from eqn. (1) for the thermal stress aT. sub.rate
J (a)
b
__
substrate
I
~reference plate
(b)
~reference plate
Fig. 1. View of the substrate deformation when (a) tensile or (b) compressive stresses exist in the thin film.
A diagram of the experimental set-up for the measurement of the radius of curvature of the substrate is shown in Fig. 2. A helium lamp was used as a monochromatic light source with a wavelength 2 of 5880 A. The diameters of the interference rings were measured using a microscope with an eyepiece micrometer. A typical case of an interference picture observed during the measurement is shown in Fig. 3. The average radius R~ of curvature of the substrate was determined9 from the
336
v . ORLINOV, G. SAROV
radii of the 4th and 8th rings along the major and the minor axes of the elliptical interference rings.
/subst~te ~ L _ _ ~ t h i n film I - ~ ] ~ n c e
plate
\ t
semffransparent
he b,mp
miffor
to
microscope
Fig. 2. Schematic diagram of the experimental set-up for the determination of mechanical stress in thin films. Fig. 3. A typical interference picture obtained while determining the mechamcal stress in the samples.
3" 2 1
0
f f J
r,"
-1 -2 -3
-4 -5 I
~00
l
2O0
I
3O0
I
400
I
~O'C
Fig. 4. Dependences of the thermal stress aT, the intrinsic stress a~ and the total mechanical stress a on the substrate temperature T, during film deposition.
3.
EXPERIMENTAL RESULTS AND DISCUSSION
The experimental dependences of the total mechanical stress a and its
MECHANICAL STRESSES IN REACTIVELY SPUTTERED F e 2 0 3 THIN FILMS
337
components (the thermal stress a T and the intrinsic stress at) on the substrate temperature T~are shown in Fig. 4. T~was varied during the deposition of F e 2 0 3 by varying the discharge current. The deposition process was carried out under the conditions given previously in Section 2 and at an oxygen content Qo, in the gas flow of 3%, a deposition rate W of 60 A m i n - 1 and a film thickness df of 1600 A. The separation of the two components of ~ was made by measuring a at room temperature and at a temperature equal to that of the substrate during the deposition. Thus, from the measured data at T~and according to eqn. (2), at A T = 0, a T = 0 and a = tri and the thermal component O"T at room temperature will be a - ai, where a is the value of the total mechanical stress measured at room temperature. The behaviour of the experimentally determined dependence tTT(T~)is in good agreement with its theoretical behaviour given by eqn. (2). Furthermore, since the thermal stress t7T has a positive sign, i.e. it is a tensile stress, it follows from eqn. (2) that ~f > ~,. This is in agreement with reported data 1°'11 for these thermal expansion coefficients (~Fe203 = 2.66 x 10 - 6 K - 1 and ~sio2 = 0.5 x 10 - 6 K - l ) . Taking these data into account we obtained from the dependence ¢rT(T~)in Fig. 4 a value of 5.33 x 1011 Pa for the Young modulus of Fe203 thin films. As can be seen from Fig. 4 the intrinsic mechanical stresses a, in thin films of Fe203, deposited by d.c. reactive sputtering at temperatures T~ < 450 °C have a negative sign, i.e. ~7i is compressive. This result confirms the conclusion drawn in ref. 12 that in dielectric and semiconducting thin films compressive intrinsic stresses usually predominate. If we accept the assumption expressed in ref. 13 that the intrinsic mechanical stress is related to point defects and defect clusters in the film, the behaviour of cri(T~)in Fig. 4 is in good agreement with the model proposed in ref. 8. According to this model, annealing of these defects and ordering of the film will increase as the substrate temperature increases and as a result a decrease in the intrinsic stresses will be observed. In order to verify the contribution of the defects in our case to the origin of the compressive stress in Fe203 films, the following experiment was carried out. Thin F e 2 0 3 films were deposited onto quartz substrates under the same conditions as those used for the films in Fig. 4 where tensile stresses arose and were measured. After that the films were implanted with a dose ¢~i of Ar + ions with a definite energy E1 and the stresses were measured again. Finally the substrates were subjected to annealing at temperatures equal to the deposition temperatures for a time t and the residual mechanical stresses were again measured. The results of this experiment are given in Table I.
TABLE I Deposition and treatment condltzons
a(Pa)
After deposition of FezO 3 at T, --- 360 °C, Qo2 = 3%, W = 60,4 m i n - 1 and df = 1600,~
After Ar + Implantation at E. = 80 k e V and ~l = 5 x 101 a ions c m - 2
After annealing of the film at T. = 360 °C for t = 60 min
+lxl0a
- 0 . 3 × 10a
+ l . 2 x 10a
338
v . ORLINOV, O. SAROV
The results obtained show that the defects introduced in the films during ion implantation actually lead to the appearance of compressive mechanical stresses; this confirms the assumption about the nature of the intrinsic stresses and the explanation for the behaviour of oi(T,) given earlier. In addition, the X-ray data described in ref. 4 show that these films are a mixture of amorphous and polycrystalline phases and that the relative portion of the latter phase increases with increasing deposition temperature or after annealing of the films. Therefore we can conclude that in the ease of d.c. reactively sputtered films of Fe20 a at substrate temperatures T~ ~<450 °C the annealing rate M of disordered material is not sufficient to compensate for the formation of defects during the growth process and as a result compressive intrinsic stresses will be developed. With an increase in the temperature T~the rate M also increases, i.e. the number of defects decreases and the percentage of the polycrystaUine phase increases. This results in a decrease in the compressive intrinsic stresses in the films. An important conclusion from a practical point of view can be drawn from the relationship o(T~) shown in Fig. 4: using d.c. reactive sputtering of thin Fe203 films onto quartz substrates we can achieve films with negligible total mechanical stresses by choosing the right substrate temperature T, during the deposition (in our case a value of 330 °C).
x
3 2 1 I
0 -I
-2 -3 -4 -5 -6
I
tOO
I
200
I
300
i
400
I
500°C
Fig. 5. Dependence of the total mechanical stresses o on the substrate temperature T,. 1, heated by the discharge power alone; 2, heated by an external heat source.
We also note that the relationships presented in Fig. 4 are similar to those obtained in ref. 14 for Si3N4 and SiO2 films. In Fig. 5 we compare two o(T,) dependences. In the first, which was taken from Fig. 4, T~was varied by varying the discharge current I, (i.e. by changing the pressure in the vacuum chamber) while in the second dependence I, was constant and equal to 40 mA and T~was varied with the aid of an external heat source. As can be seen from these figures the way in which the heating was performed influences the stress dependences. Probably in the case of curve 1 the growing film is subjected to a more
MECHANICAL STRE~ES IN REACTIVELY SPUTTERED F e 2 0 3 THIN HLMS
339
intense ion bombardment compared with that of curve 2. This increases the ability of the deposited atoms to migrate and so it is possible to grow a film having a more perfect structure at a given substrate temperature. o
X
Y 6 5 4 3
l
t
1~=4
2 7 0 -1 -2 -3
I
loo
I
200
I
300
I
4oo
I[ ' ~
5oo
Fig. 6. Dependenceofthe total mechamcalstressesa on the annealingtemperaturesT,at the two extreme substrate temperatures T~m Fig. 4 The relationship between the total mechanical stress and the annealing temperature T~ of the films which were annealed for 20 min after their deposition at the two extreme substrate temperatures T~in Fig. 4 (T~ = 200 °C and T~ = 400 °C) is given in Fig. 6. As expected at T, < T~ no change occurs in the values of a in both cases. A change is later observed in a, which is in agreement with the mechanism of structural changes that has already been considered (a decrease in the number of defects and an increase in the polycrystalline phase) as the substrate temperature rises. Furthermore, the comparison between the relationship a(T~) at T~= 200 °C and the relationship o(T~) given by curve 2 in Fig. 5 shows that the annealing of the already deposited film and its heating by an external heat source during the deposition process lead to approximately equal changes in a.
-I
-2
I
I
I
t
I
I
I
I
lo
20
30
4o
so
~o
70
~0
Fig. 7. Dependence of the mechanical stresses~ on the deposition rate W.
w_..
340
V. O R L I N O V , G . S A R O V
The relationship a(W) given in Fig. 7 and obtained by changing the glow discharge voltage Va in the range 2-4 kV is also in agreement with the physical model proposed in ref. 8. The substrate temperature T, is held constant at 300 °C using an external heat source. The other conditions of the experiment were Qo2 = 3% and df = 1600 A. As the deposition rate Wis increased, more and more of the particles falling onto the surface will not succeed in building a defect-free structure before being covered by the incident flow of sputtered particles. This will result in an increase in the compressive intrinsic stresses in the film. A further point concerning the relationship a(W) given in Fig. 7 is that under the conditions of this experiment the decrease in W is accompanied by a decrease in the discharge power and the substrate temperature caused by it. In agreement with the results shown in Fig. 5, the decrease in W will cause a change in a which is in opposition to the change caused by the variation of the sputtering rate, since the maintenance of T, at a constant value by the external heat source only partially compensates for this effect. Furthermore, keeping Qo, constant and varying W during measurement of the dependence a(W) creates an additional influence of the oxygen in the gas flow on a which is equivalent to the effect that occurs when Qo, is varied and W is held constant. From the results shown in Fig. 8 (which we shall discuss later) it can be seen that the increase in Qo~ for constant w (which is equivalent to a decrease in W for constant Qo) leads to an increase in the compressive stresses a. Therefore the aforementioned effect of 02 in the gas flow on ,r in the dependence a(W) is exactly opposite to that caused by the change in W.Thus, we can conclude that the influence of the deposition rate W on the mechanical stresses ~ of Fe20 3 films obtained by d.c. reactive sputtering is considerably greater than that shown in Fig. 7 owing to the additional effects which accompany the experiment.
-2
-3
-4
~o
2
I
i
3
4
I
I
i
I
I
5 e
I
8
10
20
30
I
I
I
40 50 eo
I
I ~Ut
8o t00
Fig. 8 T h e m e c h a n i c a l stresses o a s a f u n c t i o n o f 0 2 c o n t e n t m the A r + 0 2 g a s flow
Let us now consider in greater detail the dependence a(Qo) given in Fig. 8. This was measured at T~ = 320 °C by heating the substrate using only the discharge power which was also kept constant (Ia = 50 mA and V~= 3.5 kV). The other conditions of the experiment were W = 60/~ min- 1 and df = 1600/~. The behaviour of this dependence is in accordance with the commonly accepted opinion 15 that
MECHANICAL STRESSES IN REACTIVELY SPUTTERED F e 2 0 3 THIN FILMS
341
gases captured in the growing film increase the mechanical stresses in the film and with the experimental results in ref. 16 where it is shown that the increase in the partial pressure of H20 and 02 leads to an increase in the compressive stresses in thermally evaporated thin films of SiO2. The explanation of this is as follows. The oxygen may react with the grain walls or it may be absorbed by the grains thus increasing their volume. This will cause the grains to press against one another and compressive mechanical stresses will arise in the film. The fact that a is approximately constant, and even its slight decrease at Qo~ > 40%, can be explained by the previously established decreasea in W and the slight increase in T~ in this range of Qo~ which cannot be disregarded in this experiment. Figure 9 gives the experimentally determined dependence o(df). Initially samples of maximum thickness df were deposited. Then consecutive chemical etching of the samples by HC1 was carried out and some of them were used for the measurement of (7 and others for the measurement'of dr. The behaviour of o(df) is in agreement with the ideas that we have already considered about the origin of the mechanical stresses in thin films. As cr is the average stress value along the thickness of the film, it follows from the figure that the majority of it is concentrated near the interface at which the greatest number of imperfections are to be expected in the growing of the film structure, owing to the difference between the two structures. A similar behaviour of the dependence o(df) has been reported in ref. 17 for reactive d.c. sputtering of tungsten. 0 n
1
O
2 3
6 I
20
I
I
so
I
I
100
I
I
140
I
I
180
I
I
220
I
I
260
Fzg. 9. The mechanical stresses ~ as a functzon of the thzckness dr of the thin film.
Finally, we shall consider the mutual relation between mechanical stresses in the film and the quality of the photolithographic process in using the films for the preparation of selectively transparent photomasks. For this purpose Fe203 films were deposited on polished quartz substrates at three different values of T~: 200, 330 and 400 °C, corresponding to maximum compressive stresses (a = - 4 x l0 s Pa), to zero stresses (a = 0) and to maximum tensile stresses (or = 3 x l0 s Pa) under the conditions in Fig. 5 (curve 1). These three different films were therefore coated with photoresist and were exposed through the same photomask. Then photomasks with a given geometrical configuration were prepared from them. Scanning electron micrographs of the Fe2Oa photomasks obtained are given in Fig. 10. As can be seen, the photomask with the highest quality is that in Fig. 10(b), corresponding to a = 0, whereas in Figs. 10(a) and 10(c) a clear trend of toothed edges is observed. We can conclude from these observations that the presence of both compressive and tensile stresses in Fe203 thin films causes local cracking, loss of adhesion or bending during
342
v. ORLINOV,o. SAROV
their chemical etching, which lead to a deterioration in the edges of the image and therefore the quality of the photomask.
m
(a)
(b)
(c) Fig. 10. Views of Fe203 photomasks prepared with films hawng different mechamcal stresses: (a) = - 4 x l0 s Pa;(b) ~r = 0;(c)o = 3 x 10s Pa.
4. CONCLUSION
We have undertaken an investigation which for the first time aims at understanding the influence of the deposition conditions and subsequent heat treatment on mechanical stresses in Fe203 thin films prepared by d.c. reactive sputtering on fused quartz substrates. It has been established that in the range of variation of the substrate temperature that we studied, the substrate being heated by the discharge power only, the intrinsic film stresses ai are compressive and decrease with T~, the thermal stresses aT are tensile and increase with T~and the total mechanical stresses t7 of the film change their character from compressive at low T~to tensile at high T~. Similar variations in ~rare observed both when a film is annealed after deposition or when an external heat source is used during the deposition of a film. The compressive mechanical stresses a in the film increase as either the film deposition rate W or the oxygen content Qo2 in the gas flow are increased. ~ initially falls abruptly with increasing film thickness dr. This is an important observation which shows that the
MECHANICAL STRESSES IN REACTIVELY SPUTTERED F e 2 0 3 THIN FILMS
343
fundamental part of the mechanical stresses is concentrated near the interface between the film and the substrate. Some conclusions have been drawn about the character and the origin of the mechanical stresses in the films investigated, on the basis of interpretations of the relations established. It has been shown that the difference between the deposition temperature and the temperature at which the film is investigated as well as the higher thermal expansion coefficient of the film compared with that of the quartz substrate cause the appearance of tensile stress. The presence of disorder (defects or an amorphous phase) in the film structure and of gases trapped during the film growth process leads to the appearance of compressive stress. In this study of the preparation of thin Fe203 films on quartz substrates using d.c. reactive sputtering we have shown that it is possible to obtain films which have minimum (approximately zero) mechanical stress by optimizing the deposition conditions or the subsequent heat treatment. This permits the fabrication of high quality selectively transparent photomasks. REFERENCES G A. Sarov, Electropromishlenost i Priborostroene, 4 (1973) 131. V. Orlinov and G. Sarov, Bulg. J. Phys., 2 (1975) 156. V. Oriinov, B. Goranchev, G. Sarov and V. Tsaneva, Thin SohdFilms, 67 (1979) 125. -V. Orlinov, B. Goranchev, G. Sarov and A. Mlsmk, Thin Solid Fdms, 67 (1979) 135. W.R. Sinclair, M V Smith and R. A. Fastnach, J. Electrochem. Soc., 118 (1971) 341. I B. MacChesney, P . B . O ' C o n n o r a n d M V Sullivan, J. Electrochem. Soc.,l18(1971)776. A.R. Ianus, SolidState Technol., 6 (1973) 33. E KlakholmandB. S. Berry, J. Electrochem. Soc.,115(1968)823. P.B. G h a t e a n d L H Hali, J. Electrochem. Soc.,l19(1972)491. K.P. Yakovlev, Kratky Fiztko-Tekhiucheski Spravochnik, Vol 3, FML, Moscow, 1962, p. 232. K.P. Yakovlev, Kratky Spravochnikpo Ftztke, FML, Moscow, 1963, p. 310. G Hass and R. E. Thun, Phystcs of Thin Films, Vol. 3, Academic Press, New York, 1966, Chap. 5. H S. StoryandR. W Hoffman, Proc R Soc. London, Ser. B, 70(1957)950. A.K. Smha, H. S. Lerinstein and T. E SmRh, J. Appl. Phys., 49 (1978) 2423. J . R . Priest and H. L. Caswell, Trans. 8th Natl. Symp. on Vacuum Technology, Pergamon, New York, 1961 16 J R. Priest, H. L. Caswell and J Budu, Vacuum, 12 (1962) 301. 17 R.C. Sun, T.C. TlsoneandP. D Ruzend, J. Appl. Phys., 46 (1975) l12.
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