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Gas adsorption on thin films of chromium studied by internal stress measurements
Thin Solid Films, II 1(1984) 303-3 11 METALLURGICAL AND PROTECTIVE COATINGS
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GAS ADSORPTION ON THIN FILMS OF CHROMIUM INTERNAL STRESS MEASUREMENTS
STUDIED
BY
R. ABERMANN AND H. P. MARTINZ Institute of Physical Chemistry,
University of Innsbruck, Innrain SZa, A-6020 Innsbruck (Austria)
(Received July 18,1983; accepted October 17,1983)
The effect of 02, H,O, CO, H,, N, and CH, adsorption on the internal stress of evaporated chromium films was investigated under high vacuum conditions. While the stress in chromium films is not altered during exposure to H,, CH, and Nz distinct stress changes are found when chromium films are exposed to H,O, CO and 0,. The adsorption of H,O and CO on the metal film gives rise to a compressive stress, because the surface tension of the metal film is lowered owing to saturation of free surface valences at the film and the grain boundary surfaces. A different behaviour is observed for the O,-Cr interaction. On exposure of the film to O2 a slight compressive stress is observed, which on further exposure changes to a tensile stress. While the compressive stress is again believed to be due to the formation of an adsorption layer, the subsequent tensile stress is interpreted as indicating the formation of a protective layer of bulk chromium oxide. The thickness of this protective layer depends on the O2 exposure pressure. The chromium oxide layer is impermeable to 0, and CO, but not to H,O.
1. INTRODUCTION
In recent studies’*’ we have investigated the mechanical properties of thin chromium films, using a stress-measuring apparatus based on the cantilever beam principle3-5. In these experiments the internal stress of the film was measured continuously during the deposition process. In particular we have investigated the influence of 02, CO, H,O, H, and N, present during the evaporation of chromium. In the experiments reported in this paper we first evaporated chromium films in a high vacuum (2 x 10m6 Torr) and then studied the effect of gas adsorption on the internal stress of these films. 2.
EXPERIMENTAL DETAILS
The cantilever beam principle was used to measure the internal stress. The experimental set-up has been described earlier3-‘. With a glass substrate of thickness 0.22 mm the sensitivity of the apparatus was approximately 10 dyn cm-‘. Positive values are assigned to tensile stress and negative values to compressive stress. 0040-6090/84/$3.00
0 Elsevier Sequoia/Printed in The Netherlands
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R. ABERMANN, H. P. MARTIN2
The base pressure of 2 x lo-’ Torr was obtained by using an oil diffusion pump with additional liquid-nitrogen-cooled traps. A fresh MgF, layer (mean thickness, about 30 A) was deposited before the chromium evaporation to eliminate the effect of contamination layers on the surface of the bending glass beam. Both the chromium and the MgF, were evaporated from resistance-heated tantalum crucibles at an evaporation rate of 2 +O.l 8, s-l. The pressure during the film evaporation was (l-3) x 10m6 Torr. The main reason for this increase in pressure was Hz produced from the MgF2. However, we have shown earlier’ that Hz had no effect on the internal stress of chromium films deposited at room temperature. At the end of the chromium deposition (d = 500 A) the evaporation sources were switched off and after a few seconds the pressure in the bell-jar was again about 2 x lo- ’ Torr. 30 s after terminating the metal evaporation the partial pressure was increased by leaking the gas under study into the bell-jar through a needle valve and the resulting stress changes were recorded. Simultaneously the gas uptake by the chromium film was measured with a quartz crystal oscillator with a sensitivity of 1OOng Hz-‘. While the evaporation rates of the two evaporation sources were being adjusted, chromium as well as MgF, was deposited onto the quartz crystal. To reduce the gas uptake of the chromium film evaporated during this period an MgFz film 100 A thick was deposited onto the quartz crystal before the actual experiment. 3.
RESULTS
During the deposition of a chromium film (mean thickness, 500 A) under high vacuum conditions (total pressure during deposition, about 2 x 10V6 Torr), a positive, i.e. tensile, force per unit width of about 6 x lo4 dyn cm-’ is built up’*‘. This value has been chosen as the zero point of the figures presented in this paper. Fig. l(a) shows the recorded stress changes in this 500 8, chromium film when the film is exposed either to the residual gas atmosphere of the vacuum system (total pressure, about 2 x lo- ’ Torr) or to a particular gas at a partial pressure of 4.5 x 10e6 Torr. Because of the adsorption of residual gases the film stress built up during the deposition of the chromium film is reduced by about 4000 dyn cm- ’ within 5 min (curve l), while the frequency of the quartz crystal oscillator increases by lo-15 Hz during this period. This compressive stress then levels off and finally becomes constant after about 60min; a total change of about 8000 dyn cm-’ is recorded. The corresponding change in frequency of the quartz oscillator is about 50 Hz. When the chromium film is exposed to one of the three gases H,, Nz and CH, at 4.5 x lop6 Torr the same stress and frequency changes are observed. However, when the chromium film is exposed to CO (curve 2) or H,O vapour (curve 3) at a partial pressure of 4.5 x 10m6 Torr, compressive stress changes of about 6500 dyn respectively are measured after an exposure time of 5 min. cm-‘and8OOOdyncm-’ While this stress change is more or less complete after about 4 min for exposure to CO, it is not complete until after about 7 min for exposure to HZ0 vapour. This prolonged effect of the water vapour is also reflected in the frequency change of the quartz oscillator. The frequency’ levels off after about the same time as the stress does. The total frequency change is significantly greater for H,O exposure than for CO exposure. When the chromium film is exposed to O2 at a partial pressure of 4.5 x 10e6
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Fig. 1. (a) Stress changes in chromium films 500 8, thick during exposure to the iesidual gas, H,, N, and CH, (curve l), CO (curve 2), H,O (curve 3) and 0, (curve 4); (b) dependence on the thickness of the chromium film of the stress change after an exposure of 1500 langmuirs to O2 (A), CO (I) and H,O (0).
Torr, a small compressive stress develops during the initial stage only, as can be seen from Fig. l(a), curve 4. Thereafter a tensile stress change of about 1400 dyn cm- ’ is found. This stress change is completed in less than 30 s. Then the stress change is again compressive and similar to that found during exposure of the fresh chromium film to the residual atmosphere. The O2 uptake of the quartz oscillator is comparable in magnitude with the CO uptake but about twice as fast. Figure l(b) shows the dependence of the stress changes discussed above on the thickness of the chromium film which is exposed to the three gases. In each experiment the gas exposure was 1500 langmuirs. The tensile stress built up during the O2 exposure shows no significant further increase above a mean film thickness of about 100 A. The mass gain measured simultaneously with the quartz oscillator is already rather high (Af = 20 Hz) at the lowest film thickness (about 20 A) measured. As the chromium film thickness is increased to 500 8, the mass gain only increases to a value corresponding to about 30 Hz. In contrast with the O2 effect the stress change due to the adsorption of CO and H,O clearly increases with increasing chromium film thickness. The compressive stress changes are consistently somewhat higher for H,O-Cr than for CO-Cr interaction. The gas uptake increases to values corresponding to 20 Hz and 25 Hz during exposure to CO and H,O respectively at 1500 langmuirs, as the chromium film thickness reaches 250 A, and levels off at higher thickness. The mass gain due to gas adsorption on a film 1000 A thick corresponds to about 35 Hz for CO and to 45 Hz for H20. Figures 2-5 show the effect of successive exposures of the chromium film to any two of the three gases CO, H,O and OZ. The changes in the internal stress during exposure of a film 500 8, thick to H,O then O2 and vice versa are shown in Fig. 2. As the chromium film is exposed to O2 the stress changes mentioned above are measured. Lowering the total pressure to 2 x lo-’ Torr after 3 min of O2 exposure
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Fig. 2. Changes in the internal stress of a chromium film during successive exposures to 02 and H,O.
has no effect on the stress change. During subsequent exposure of this film to Hz0 vapour a compressive stress again develops. Although delayed in time this compressive stress change is only slightly less than the compressive stress change measured during exposure of the fresh chromium film to H,O vapour. Reducing the partial pressure of the H,O vapour after 3 min of exposure results in a small tensile stress change which coincides with a mass loss (about 10%) on the quartz crystal. During subsequent O2 exposure of this chromium film (pre-exposed to Hz0 vapour) a small compressive stress change and then a tensile stress change are again measured. These stress changes are considerably delayed in comparison with those during exposure of the virgin film to OZ. Figure 3 summarizes three experiments in which a fresh chromium film 500 A thick was first exposed to 0, and subsequently (after a pumpdown period of 1 min) to Hz0 vapour. Whereas the partial pressure during the O2 exposure was varied, the partial pressure of the Hz0 vapour was kept constant at 5 x 10Y6 Torr. Increasing the O2 pressure has only a rather small effect on the magnitude of the initial compressive stress change; however, it significantly affects the subsequent tensile stress change. This tensile stress change is significantly greater and its rate is enhanced when the chromium film is exposed to O2 at higher pressures. The frequency corresponding to the mass gain on the quartz oscillator increases from 30 Hz (po, = 4.5 x 10e6 Torr) through 45 Hz (po, = 5 x lo-’ Torr) to 90 Hz (po, = 5 x 10e4 Torr). After this tensile stress reaches its maximum, the small compressive stress change mentioned above is again observed. The latter effect is smaller the higher is the 0, pressure of the previous exposure. The compressive stress measured during exposure of a film to Hz0 vapour at a given pressure is significantly smaller in cases where the films were previously exposed to O2 at higher pressures. The same trend is seen for the tensile stress change measured during the pumpdown cycle after the 3 min exposure to HZ0 vapour. The effect of successive exposures of a fresh chromium film to CO and H,O vapour on the internal film stress is shown in Fig. 4. The mass loss on the microbalance again indicates that part of the previously adsorbed HZ0 vapour
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Fig. 3. Dependence of the internal stress change on the partial pressure during exposure of the chromium film to Oz. Each film was then exposed to H,O at 5 x 10m6Torr.
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Fig. 4. Changes in the internal stress of a chromiqm film during successive exposures to CO and H,O.
desorbs during the pumpdown cycle which results in a small tensile stress. Reducing the CO pressure has only a very small effect on the film stress and on the quartz oscillator reading. During exposure to CO and H,O of a film pre-exposed to Hz0 and CO respectively, a compressive stress change is again measured. The frequency change on the microbalance again indicates that significantly more Hz0 vapour than CO is adsorbed. Finally Fig. 5 shows the effect of successive exposures of the chromium film to CO and 0, and vice versa. During the first gas exposure of the fresh films, the stress changes discussed above are found. Reducing the O2 pressure and subsequently
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Fig. 5. Changes in the internal stress of a chromium film during successive exposures to 0, and CO.
exposing this film to CO has no effect on the internal film stress and the quartz oscillator reading. However, when the chromium film pre-exposed to CO is exposed to 02, a small compressive stress change and then a tensile stress change are measured. Although the magnitudes of the compressive as well as the tensile stresses are comparable with those found during exposure of the virgin film to O2 the interaction is somewhat delayed in time. 4. DISCUSSION In earlier papers’*2 we have shown that the high tensile stress in chromium films (deposited onto MgFz substrates at room temperature) is reduced when the films are evaporated at higher partial pressures of O,, H,O and CO. When CO and H20 are present during the chromium deposition, even at the highest gas pressures (10m4 Torr) only weak and diffuse oxide and carbide diffraction rings were detectable by transmission electron diffraction (TED), and the principal component of the deposited film was the chromium metal phase. From these results and from the changes in resistivity of these films it was concluded that the observed compressive stress contributions were due to the formation of chromium oxide or carbide on the surface of the film and in the grain boundaries. The oxide and carbide on the surface of the grains thus reduced the recrystallization in the chromium film as was concluded from the elimination of changes in the incremental film stress. In contrast with these results a different behaviour was found when chromium was evaporated in an O2 atmosphere. Increasing the 0, pressure during the chromium deposition changed the chromium film stress and resistivity significantly more than the use of higher partial pressures of CO and Hz0 did. In the case of O2 at a partial pressure of 1 x 10m6Torr the stress of the chromium film was reduced by about 60”/ while the stress reduction due to CO or Hz0 uptake by the chromium film was only about 20%. When the 0, partial pressure was above 3 x 10m6 Torr, insulating transparent chromium oxide films were deposited as confirmed by TED2. The internal stress in these oxide films is higher than that of the pure chromium film. From these experiments we concluded that the interaction between chromium and
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CO or H,O is confined to the surface and the grain boundary region of the chromium film, while in the case of 0, at pressures above 3 x 10d6 Torr chromium oxide is formed. No significant effect on the internal film stress and on the film resistivity was found when the chromium film was evaporated in an H,, N2 or CH4 atmosphere. As was to be expected from the experiments mentioned above, exposing a chromium film, which had previously been deposited under best vacuum conditions, to H,, N, and CH4 has no effect on the film stress (Fig. 1, curve 1). The gradually increasing compressive stress found when the chromium film is exposed to the residual gas (curve 1) is believed to be due to the adsorption of Hz0 vapour, the principal component of the residual gas after the evaporators were switched off. This compressive stress change is clearly greater when the chromium film is exposed to a higher H,O dose (Fig. 1, curve 3). A similar, but somewhat smaller effect is measured during the adsorption of CO on the chromium film. In each case the resulting compressive stress is believed to arise because the surface tension of the metal film is lowered as a result of the saturation of free surface valences due to the formation of small amounts of chromium oxide and/or carbide6,’ on the film surface and in the grain boundaries. As in the experiments published earlier2 a special behaviour is observed for the interaction between 0, and a chromium film (Fig. 1, curve 4). On exposure of the film to 02, at first a small compressive stress is again found, which on further exposure changes to a tensile stress. On the basis of the argument above the compressive stress is again believed to be due to the formation of an adsorption layer. The more or less constant value of this compressive stress may indicate that during this process the gas is only adsorbed on the surface of the chromium film, and that during this time no significant adsorption in the grain boundaries occurs. The subsequent tensile stress is then believed to indicate the formation of a bulk chromium oxide layer6**, on the basis of the stress measurements published earlier for chromium oxide film$. The special behaviour in the case of O2 compared with CO and H20 is also demonstrated by the fact that the stress changes measured during CO and H20 exposure are dependent on the thickness of the underlying chromium film while the tensile stress change due to O,-Cr interaction stays more or less constant as soon as the chromium film thickness exceeds 100 A. We therefore believe that H,O and CO not only adsorb on the film surface but also penetrate the polycrystalline chromium film along the large number of grain boundaries, thus eventually covering the “effective inner film surface”, which increases with increasing film thickness. For O2 it is assumed that only the top layers of the chromium film are involved in the interaction. On exposure to O2 at a pressure of 4.5 x lO-‘j Torr we estimate the thickness of the protective layer to be lo-30 A. This is concluded from the stress change (Fig. l(a), curve 4; Fig. l(b)) and the mass gain indicated by the quartz microbalance. At higher O2 exposure pressures this thickness increases, as concluded from the higher tensile stress (Fig. 3) as well as from the greater gas uptake as measured by the quartz oscillator. In this argument the possible influence of the residual Hz0 vapour from the ambient atmosphere has been neglected. Whether its influence and that of other gases present in small amounts is indeed significant can only be decided from similar measurements under well-defined ultrahigh vacuum conditions, which are planned in this laboratory for the near future.
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From the experiments in which the chromium film was successively exposed to any two of the gases mentioned above, additional information concerning the interaction between the chromium film and these gases was obtained. It can be seen from Figs. 2 and 4 that a small part of the Hz0 adsorbing on the chromium film desorbs again when the Hz0 pressure in the gas phase is lowered. Figures 4 and 5 suggest a similar, though much smaller, effect for CO. A second exposure to O2 or H,O of the chromium film pre-exposed to O2 or Hz0 respectively shows that both stress effects are delayed in time but the mechanism of the interaction in both cases seems to be the same as that during exposure of the fresh film to the respective gases. The fact that the stress change due to the adsorption of H,O on films pre-exposed to oxygen is only slightly less than that when Hz0 is adsorbed on the fresh film indicates that the oxide protective layer is not able to prevent diffusion of the H,O into the underlying chromium film (Fig. 2). However, with increasing thickness the protective layer is more and more impermeable to H,O. This delays the effect of the Hz0 vapour during exposure to the residual atmosphere after the formation of the protective oxide layer as well as during the exposure to H,O vapour at a high pressure (Fig. 3). The permeability of the protective layer to O,, however, seems not to be affected by the interaction with Hz0 since subsequent 0, exposures have no effect on the film stress. The stress effect due to the interaction between CO and the chromium film is very similar to that between Hz0 and chromium, the only difference being that the total amount of Hz0 adsorbed is greater than that of CO. The fact that during an exposure of 1500 langmuirs less CO is adsorbed than H,O may be the reason that the stress change during the subsequent 0, exposure (Fig. 5) is only slightly affected by the preceding CO interaction, while pre-adsorption of H,O clearly delays the stress changes during the subsequent O2 interaction. Finally it is seen from Fig. 5 that CO is not able to penetrate through the oxide protective layer into the chromium film. 5.
CONCLUSIONS
The bending beam apparatus has previously been used to study the dependence of the internal film stress on gas adsorption during film deposition. We hope to have demonstrated in this paper that the technique can also be used to study the effect of gas adsorption on the internal stress of films after deposition. The adsorption of gases on a metal film generally gives rise to a compressive stress depending on the strength of the interaction between the metal and the particular gas. A different behaviour is observed in the case of 0, interaction with chromium films. On exposure of the film to O2 a low compressive stress is observed which, on further exposure, changes to a tensile stress. While the initial compressive stress is due to the formation of an adsorption layer the subsequent tensile stress is indicative of the formation of a protective layer of bulk chromium oxide. ACKNOWLEDGMENT
We gratefully acknowledge support for this work from the Fonds zur FGrderung der wissenschaftlichen Forschung of Austria (Project 4647).
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REFERENCES
1 H. P. Martinz and R. Abermann, Proc. 8th Int. Vacuum Congr., Cannes, 1980, in Vide, Couches Minces,Suppl.,201(1981)311. 2 H. P. Martinz and R. Abermann, Thin Solid Films, 89 (1982) 133. 3 R. Abermann, R. Kramer and J. Miser, Thin Solid Films, 52 (1978) 127. 4 R. Abermann, H. P. Martinz and R. Kramer, Thin Solid Films, 70 (1980) 127. 5 H. P. Martinz, Thesis, Innsbruck, 1982. 6 J. C. Fuggle, L. M. Watson, D. J. Fabian and S. Affrossman, Surf. Sci., 49 (1975) 61. 7 R. Kieffer and F. Benesovsky, Hartstoffe, Springer, Vienna, 1963. 8 G. Gewinner, J. C. Peruchetti and A. JaCglt, Surf. Sci., I22 (1982) 383.
303
GAS ADSORPTION ON THIN FILMS OF CHROMIUM INTERNAL STRESS MEASUREMENTS
STUDIED
BY
R. ABERMANN AND H. P. MARTINZ Institute of Physical Chemistry,
University of Innsbruck, Innrain SZa, A-6020 Innsbruck (Austria)
(Received July 18,1983; accepted October 17,1983)
The effect of 02, H,O, CO, H,, N, and CH, adsorption on the internal stress of evaporated chromium films was investigated under high vacuum conditions. While the stress in chromium films is not altered during exposure to H,, CH, and Nz distinct stress changes are found when chromium films are exposed to H,O, CO and 0,. The adsorption of H,O and CO on the metal film gives rise to a compressive stress, because the surface tension of the metal film is lowered owing to saturation of free surface valences at the film and the grain boundary surfaces. A different behaviour is observed for the O,-Cr interaction. On exposure of the film to O2 a slight compressive stress is observed, which on further exposure changes to a tensile stress. While the compressive stress is again believed to be due to the formation of an adsorption layer, the subsequent tensile stress is interpreted as indicating the formation of a protective layer of bulk chromium oxide. The thickness of this protective layer depends on the O2 exposure pressure. The chromium oxide layer is impermeable to 0, and CO, but not to H,O.
1. INTRODUCTION
In recent studies’*’ we have investigated the mechanical properties of thin chromium films, using a stress-measuring apparatus based on the cantilever beam principle3-5. In these experiments the internal stress of the film was measured continuously during the deposition process. In particular we have investigated the influence of 02, CO, H,O, H, and N, present during the evaporation of chromium. In the experiments reported in this paper we first evaporated chromium films in a high vacuum (2 x 10m6 Torr) and then studied the effect of gas adsorption on the internal stress of these films. 2.
EXPERIMENTAL DETAILS
The cantilever beam principle was used to measure the internal stress. The experimental set-up has been described earlier3-‘. With a glass substrate of thickness 0.22 mm the sensitivity of the apparatus was approximately 10 dyn cm-‘. Positive values are assigned to tensile stress and negative values to compressive stress. 0040-6090/84/$3.00
0 Elsevier Sequoia/Printed in The Netherlands
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R. ABERMANN, H. P. MARTIN2
The base pressure of 2 x lo-’ Torr was obtained by using an oil diffusion pump with additional liquid-nitrogen-cooled traps. A fresh MgF, layer (mean thickness, about 30 A) was deposited before the chromium evaporation to eliminate the effect of contamination layers on the surface of the bending glass beam. Both the chromium and the MgF, were evaporated from resistance-heated tantalum crucibles at an evaporation rate of 2 +O.l 8, s-l. The pressure during the film evaporation was (l-3) x 10m6 Torr. The main reason for this increase in pressure was Hz produced from the MgF2. However, we have shown earlier’ that Hz had no effect on the internal stress of chromium films deposited at room temperature. At the end of the chromium deposition (d = 500 A) the evaporation sources were switched off and after a few seconds the pressure in the bell-jar was again about 2 x lo- ’ Torr. 30 s after terminating the metal evaporation the partial pressure was increased by leaking the gas under study into the bell-jar through a needle valve and the resulting stress changes were recorded. Simultaneously the gas uptake by the chromium film was measured with a quartz crystal oscillator with a sensitivity of 1OOng Hz-‘. While the evaporation rates of the two evaporation sources were being adjusted, chromium as well as MgF, was deposited onto the quartz crystal. To reduce the gas uptake of the chromium film evaporated during this period an MgFz film 100 A thick was deposited onto the quartz crystal before the actual experiment. 3.
RESULTS
During the deposition of a chromium film (mean thickness, 500 A) under high vacuum conditions (total pressure during deposition, about 2 x 10V6 Torr), a positive, i.e. tensile, force per unit width of about 6 x lo4 dyn cm-’ is built up’*‘. This value has been chosen as the zero point of the figures presented in this paper. Fig. l(a) shows the recorded stress changes in this 500 8, chromium film when the film is exposed either to the residual gas atmosphere of the vacuum system (total pressure, about 2 x lo- ’ Torr) or to a particular gas at a partial pressure of 4.5 x 10e6 Torr. Because of the adsorption of residual gases the film stress built up during the deposition of the chromium film is reduced by about 4000 dyn cm- ’ within 5 min (curve l), while the frequency of the quartz crystal oscillator increases by lo-15 Hz during this period. This compressive stress then levels off and finally becomes constant after about 60min; a total change of about 8000 dyn cm-’ is recorded. The corresponding change in frequency of the quartz oscillator is about 50 Hz. When the chromium film is exposed to one of the three gases H,, Nz and CH, at 4.5 x lop6 Torr the same stress and frequency changes are observed. However, when the chromium film is exposed to CO (curve 2) or H,O vapour (curve 3) at a partial pressure of 4.5 x 10m6 Torr, compressive stress changes of about 6500 dyn respectively are measured after an exposure time of 5 min. cm-‘and8OOOdyncm-’ While this stress change is more or less complete after about 4 min for exposure to CO, it is not complete until after about 7 min for exposure to HZ0 vapour. This prolonged effect of the water vapour is also reflected in the frequency change of the quartz oscillator. The frequency’ levels off after about the same time as the stress does. The total frequency change is significantly greater for H,O exposure than for CO exposure. When the chromium film is exposed to O2 at a partial pressure of 4.5 x 10e6
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Fig. 1. (a) Stress changes in chromium films 500 8, thick during exposure to the iesidual gas, H,, N, and CH, (curve l), CO (curve 2), H,O (curve 3) and 0, (curve 4); (b) dependence on the thickness of the chromium film of the stress change after an exposure of 1500 langmuirs to O2 (A), CO (I) and H,O (0).
Torr, a small compressive stress develops during the initial stage only, as can be seen from Fig. l(a), curve 4. Thereafter a tensile stress change of about 1400 dyn cm- ’ is found. This stress change is completed in less than 30 s. Then the stress change is again compressive and similar to that found during exposure of the fresh chromium film to the residual atmosphere. The O2 uptake of the quartz oscillator is comparable in magnitude with the CO uptake but about twice as fast. Figure l(b) shows the dependence of the stress changes discussed above on the thickness of the chromium film which is exposed to the three gases. In each experiment the gas exposure was 1500 langmuirs. The tensile stress built up during the O2 exposure shows no significant further increase above a mean film thickness of about 100 A. The mass gain measured simultaneously with the quartz oscillator is already rather high (Af = 20 Hz) at the lowest film thickness (about 20 A) measured. As the chromium film thickness is increased to 500 8, the mass gain only increases to a value corresponding to about 30 Hz. In contrast with the O2 effect the stress change due to the adsorption of CO and H,O clearly increases with increasing chromium film thickness. The compressive stress changes are consistently somewhat higher for H,O-Cr than for CO-Cr interaction. The gas uptake increases to values corresponding to 20 Hz and 25 Hz during exposure to CO and H,O respectively at 1500 langmuirs, as the chromium film thickness reaches 250 A, and levels off at higher thickness. The mass gain due to gas adsorption on a film 1000 A thick corresponds to about 35 Hz for CO and to 45 Hz for H20. Figures 2-5 show the effect of successive exposures of the chromium film to any two of the three gases CO, H,O and OZ. The changes in the internal stress during exposure of a film 500 8, thick to H,O then O2 and vice versa are shown in Fig. 2. As the chromium film is exposed to O2 the stress changes mentioned above are measured. Lowering the total pressure to 2 x lo-’ Torr after 3 min of O2 exposure
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Fig. 2. Changes in the internal stress of a chromium film during successive exposures to 02 and H,O.
has no effect on the stress change. During subsequent exposure of this film to Hz0 vapour a compressive stress again develops. Although delayed in time this compressive stress change is only slightly less than the compressive stress change measured during exposure of the fresh chromium film to H,O vapour. Reducing the partial pressure of the H,O vapour after 3 min of exposure results in a small tensile stress change which coincides with a mass loss (about 10%) on the quartz crystal. During subsequent O2 exposure of this chromium film (pre-exposed to Hz0 vapour) a small compressive stress change and then a tensile stress change are again measured. These stress changes are considerably delayed in comparison with those during exposure of the virgin film to OZ. Figure 3 summarizes three experiments in which a fresh chromium film 500 A thick was first exposed to 0, and subsequently (after a pumpdown period of 1 min) to Hz0 vapour. Whereas the partial pressure during the O2 exposure was varied, the partial pressure of the Hz0 vapour was kept constant at 5 x 10Y6 Torr. Increasing the O2 pressure has only a rather small effect on the magnitude of the initial compressive stress change; however, it significantly affects the subsequent tensile stress change. This tensile stress change is significantly greater and its rate is enhanced when the chromium film is exposed to O2 at higher pressures. The frequency corresponding to the mass gain on the quartz oscillator increases from 30 Hz (po, = 4.5 x 10e6 Torr) through 45 Hz (po, = 5 x lo-’ Torr) to 90 Hz (po, = 5 x 10e4 Torr). After this tensile stress reaches its maximum, the small compressive stress change mentioned above is again observed. The latter effect is smaller the higher is the 0, pressure of the previous exposure. The compressive stress measured during exposure of a film to Hz0 vapour at a given pressure is significantly smaller in cases where the films were previously exposed to O2 at higher pressures. The same trend is seen for the tensile stress change measured during the pumpdown cycle after the 3 min exposure to HZ0 vapour. The effect of successive exposures of a fresh chromium film to CO and H,O vapour on the internal film stress is shown in Fig. 4. The mass loss on the microbalance again indicates that part of the previously adsorbed HZ0 vapour
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Fig. 3. Dependence of the internal stress change on the partial pressure during exposure of the chromium film to Oz. Each film was then exposed to H,O at 5 x 10m6Torr.
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Fig. 4. Changes in the internal stress of a chromiqm film during successive exposures to CO and H,O.
desorbs during the pumpdown cycle which results in a small tensile stress. Reducing the CO pressure has only a very small effect on the film stress and on the quartz oscillator reading. During exposure to CO and H,O of a film pre-exposed to Hz0 and CO respectively, a compressive stress change is again measured. The frequency change on the microbalance again indicates that significantly more Hz0 vapour than CO is adsorbed. Finally Fig. 5 shows the effect of successive exposures of the chromium film to CO and 0, and vice versa. During the first gas exposure of the fresh films, the stress changes discussed above are found. Reducing the O2 pressure and subsequently
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0
1
2
3
4 TIME
5
6
7
(mid
Fig. 5. Changes in the internal stress of a chromium film during successive exposures to 0, and CO.
exposing this film to CO has no effect on the internal film stress and the quartz oscillator reading. However, when the chromium film pre-exposed to CO is exposed to 02, a small compressive stress change and then a tensile stress change are measured. Although the magnitudes of the compressive as well as the tensile stresses are comparable with those found during exposure of the virgin film to O2 the interaction is somewhat delayed in time. 4. DISCUSSION In earlier papers’*2 we have shown that the high tensile stress in chromium films (deposited onto MgFz substrates at room temperature) is reduced when the films are evaporated at higher partial pressures of O,, H,O and CO. When CO and H20 are present during the chromium deposition, even at the highest gas pressures (10m4 Torr) only weak and diffuse oxide and carbide diffraction rings were detectable by transmission electron diffraction (TED), and the principal component of the deposited film was the chromium metal phase. From these results and from the changes in resistivity of these films it was concluded that the observed compressive stress contributions were due to the formation of chromium oxide or carbide on the surface of the film and in the grain boundaries. The oxide and carbide on the surface of the grains thus reduced the recrystallization in the chromium film as was concluded from the elimination of changes in the incremental film stress. In contrast with these results a different behaviour was found when chromium was evaporated in an O2 atmosphere. Increasing the 0, pressure during the chromium deposition changed the chromium film stress and resistivity significantly more than the use of higher partial pressures of CO and Hz0 did. In the case of O2 at a partial pressure of 1 x 10m6Torr the stress of the chromium film was reduced by about 60”/ while the stress reduction due to CO or Hz0 uptake by the chromium film was only about 20%. When the 0, partial pressure was above 3 x 10m6 Torr, insulating transparent chromium oxide films were deposited as confirmed by TED2. The internal stress in these oxide films is higher than that of the pure chromium film. From these experiments we concluded that the interaction between chromium and
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CO or H,O is confined to the surface and the grain boundary region of the chromium film, while in the case of 0, at pressures above 3 x 10d6 Torr chromium oxide is formed. No significant effect on the internal film stress and on the film resistivity was found when the chromium film was evaporated in an H,, N2 or CH4 atmosphere. As was to be expected from the experiments mentioned above, exposing a chromium film, which had previously been deposited under best vacuum conditions, to H,, N, and CH4 has no effect on the film stress (Fig. 1, curve 1). The gradually increasing compressive stress found when the chromium film is exposed to the residual gas (curve 1) is believed to be due to the adsorption of Hz0 vapour, the principal component of the residual gas after the evaporators were switched off. This compressive stress change is clearly greater when the chromium film is exposed to a higher H,O dose (Fig. 1, curve 3). A similar, but somewhat smaller effect is measured during the adsorption of CO on the chromium film. In each case the resulting compressive stress is believed to arise because the surface tension of the metal film is lowered as a result of the saturation of free surface valences due to the formation of small amounts of chromium oxide and/or carbide6,’ on the film surface and in the grain boundaries. As in the experiments published earlier2 a special behaviour is observed for the interaction between 0, and a chromium film (Fig. 1, curve 4). On exposure of the film to 02, at first a small compressive stress is again found, which on further exposure changes to a tensile stress. On the basis of the argument above the compressive stress is again believed to be due to the formation of an adsorption layer. The more or less constant value of this compressive stress may indicate that during this process the gas is only adsorbed on the surface of the chromium film, and that during this time no significant adsorption in the grain boundaries occurs. The subsequent tensile stress is then believed to indicate the formation of a bulk chromium oxide layer6**, on the basis of the stress measurements published earlier for chromium oxide film$. The special behaviour in the case of O2 compared with CO and H20 is also demonstrated by the fact that the stress changes measured during CO and H20 exposure are dependent on the thickness of the underlying chromium film while the tensile stress change due to O,-Cr interaction stays more or less constant as soon as the chromium film thickness exceeds 100 A. We therefore believe that H,O and CO not only adsorb on the film surface but also penetrate the polycrystalline chromium film along the large number of grain boundaries, thus eventually covering the “effective inner film surface”, which increases with increasing film thickness. For O2 it is assumed that only the top layers of the chromium film are involved in the interaction. On exposure to O2 at a pressure of 4.5 x lO-‘j Torr we estimate the thickness of the protective layer to be lo-30 A. This is concluded from the stress change (Fig. l(a), curve 4; Fig. l(b)) and the mass gain indicated by the quartz microbalance. At higher O2 exposure pressures this thickness increases, as concluded from the higher tensile stress (Fig. 3) as well as from the greater gas uptake as measured by the quartz oscillator. In this argument the possible influence of the residual Hz0 vapour from the ambient atmosphere has been neglected. Whether its influence and that of other gases present in small amounts is indeed significant can only be decided from similar measurements under well-defined ultrahigh vacuum conditions, which are planned in this laboratory for the near future.
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From the experiments in which the chromium film was successively exposed to any two of the gases mentioned above, additional information concerning the interaction between the chromium film and these gases was obtained. It can be seen from Figs. 2 and 4 that a small part of the Hz0 adsorbing on the chromium film desorbs again when the Hz0 pressure in the gas phase is lowered. Figures 4 and 5 suggest a similar, though much smaller, effect for CO. A second exposure to O2 or H,O of the chromium film pre-exposed to O2 or Hz0 respectively shows that both stress effects are delayed in time but the mechanism of the interaction in both cases seems to be the same as that during exposure of the fresh film to the respective gases. The fact that the stress change due to the adsorption of H,O on films pre-exposed to oxygen is only slightly less than that when Hz0 is adsorbed on the fresh film indicates that the oxide protective layer is not able to prevent diffusion of the H,O into the underlying chromium film (Fig. 2). However, with increasing thickness the protective layer is more and more impermeable to H,O. This delays the effect of the Hz0 vapour during exposure to the residual atmosphere after the formation of the protective oxide layer as well as during the exposure to H,O vapour at a high pressure (Fig. 3). The permeability of the protective layer to O,, however, seems not to be affected by the interaction with Hz0 since subsequent 0, exposures have no effect on the film stress. The stress effect due to the interaction between CO and the chromium film is very similar to that between Hz0 and chromium, the only difference being that the total amount of Hz0 adsorbed is greater than that of CO. The fact that during an exposure of 1500 langmuirs less CO is adsorbed than H,O may be the reason that the stress change during the subsequent 0, exposure (Fig. 5) is only slightly affected by the preceding CO interaction, while pre-adsorption of H,O clearly delays the stress changes during the subsequent O2 interaction. Finally it is seen from Fig. 5 that CO is not able to penetrate through the oxide protective layer into the chromium film. 5.
CONCLUSIONS
The bending beam apparatus has previously been used to study the dependence of the internal film stress on gas adsorption during film deposition. We hope to have demonstrated in this paper that the technique can also be used to study the effect of gas adsorption on the internal stress of films after deposition. The adsorption of gases on a metal film generally gives rise to a compressive stress depending on the strength of the interaction between the metal and the particular gas. A different behaviour is observed in the case of 0, interaction with chromium films. On exposure of the film to O2 a low compressive stress is observed which, on further exposure, changes to a tensile stress. While the initial compressive stress is due to the formation of an adsorption layer the subsequent tensile stress is indicative of the formation of a protective layer of bulk chromium oxide. ACKNOWLEDGMENT
We gratefully acknowledge support for this work from the Fonds zur FGrderung der wissenschaftlichen Forschung of Austria (Project 4647).
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REFERENCES
1 H. P. Martinz and R. Abermann, Proc. 8th Int. Vacuum Congr., Cannes, 1980, in Vide, Couches Minces,Suppl.,201(1981)311. 2 H. P. Martinz and R. Abermann, Thin Solid Films, 89 (1982) 133. 3 R. Abermann, R. Kramer and J. Miser, Thin Solid Films, 52 (1978) 127. 4 R. Abermann, H. P. Martinz and R. Kramer, Thin Solid Films, 70 (1980) 127. 5 H. P. Martinz, Thesis, Innsbruck, 1982. 6 J. C. Fuggle, L. M. Watson, D. J. Fabian and S. Affrossman, Surf. Sci., 49 (1975) 61. 7 R. Kieffer and F. Benesovsky, Hartstoffe, Springer, Vienna, 1963. 8 G. Gewinner, J. C. Peruchetti and A. JaCglt, Surf. Sci., I22 (1982) 383.