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Adhesion and internal stress in thin films of aluminium
Thin Solid Films, 79 (1981) 15-19
15
METALLURGICAL AND PROTECTIVE COATINGS
ADHESION AND I N T E R N A L STRESS IN T H IN FILMS OF ALUMINIUM M. LAUGIER
Wimet Research and Development Laboratories*, Torrington Avenue, Coventry CV4 9AD (Gt. Britain) (Received October 24, 1980; accepted November 27, 1980)
Measurements of both adhesion and internal stress were made using thin aluminium films on glass substrates. The films were up to 1000/~ thick and were prepared by vacuum evaporation at pressures below 5 x 10 -5 Torr. Large compressive intrinsic stresses were found which could be explained in terms of the incorporation of oxygen during growth. The initial film adhesion as measured by the scratch test was poor but substantial increases in adhesion occurred with time, confirming earlier results. The observation of high compressive stresses explained in terms of oxygen incorporation provides evidence for the oxide layer model of adhesion in aluminium.
l. INTRODUCTION
Film stress and adhesion may be related through film stability. Internal stresses exceeding the film cohesive strength may lead to internal rupture although actual debonding is unlikely to be a direct result of internal stress except in the thickest coatings. The adhesion of vacuum-evaporated aluminium films on glass has been investigated and adhesion has been found to increase significantly after an aging period ~. These increases in adhesion have been attributed to oxygen migration to the Al-glass interface, leading to the formation of an oxide bonding layer ~. However, aluminium rapidly forms a protective oxide skin on exposure to air, which is an effective barrier to further oxygen diffusion from the atmosphere. Therefore oxygen must have been incorporated within the aluminium structure during film deposition if the increases in adhesion are to be attributed to the subsequent formation of an interfacial oxide bonding layer. Aluminium has a strong gettering effect for oxygen and the chamber pressure can be observed to fall during the vacuum evaporation of aluminium which suggests that oxygen may be incorporated in films prepared in this manner. For pure metals, intrinsic stress was found to be tensile and almost constant in vacuum-evaporated films beyond the early growth stages 2. However, 1 at.~o O * Now called Sandvik Research and Development Laboratories. 0040-6090/81/0000-0000/$02.50
© Elsevier Sequoia/Printed in The Netherlands
16
M. LAUGIER
incorporation in nickel films has been found to produce a significant compressive stress3. Measurements of intrinsic stress may therefore provide a sensitive technique for the investigation of impurity incorporation in thin films if the intrinsic stress is known in the pure structure. The intrinsic stress in aluminium films evaporated in ultrahigh vacuum has been investigated and the expected tensile intrinsic stresses associated with pure structures have been found4. The adhesion increases reported relate to aluminium films evaporated at a pressure in the region of 5 x 10m5 Torr when significant residual oxygen is present. Measurements of intrinsic stress under these conditions provide evidence of oxygen incorporation and lend support to the oxide layer model of adhesion in aluminium. 2.
EXPERIMENTAL
DETAILS
The glass substrates were thoroughly cleaned with Teepol just prior to use and before film deposition they were subjected to a final plasma-cleaning process. Aluminium was evaporated from tungsten spirals at rates between 4 and 6 A s- ‘. The pressure at the beginning of deposition was 5 x 10m5 Torr but it fell below 1 x 10e5 Torr during deposition. Adhesion measurements were performed using the scratch test5 in which a loaded smoothly rounded stylus is drawn across the film surface. The load at which film removal occurs, leaving a clear channel, is taken as a measure of adhesion. The styli used were made of chrome steel of tip radius 45 pm. The intrinsic stress was determined in two ways, both of which made use of a sensitive cantilevered plate technique with capacitative detection6. In the first method, intrinsic stress was determined from the deformation of the cantilever after thermal equilibrium had occurred. This simple method suffers from several disadvantages which include possible stress relief, “frozen-in” thermal stresses and the need for a separate stress determination for each thickness value7. The second method is dynamic and relates the instantaneous substrate deformation occurring during film deposition to film stresses as a function of film thickness, thereby overcoming the disadvantages inherent in the sequential method7. This technique is more complex and relies both on theory and on careful measurements of the temperature of the substrate and the temperature difference between substrate faces, for which thin film thermocouples may be employed’. Substrate deformation is most directly related to the bending force per unit width. The observed substrate deformation may be related to a total bending force STo, which may be written7 STot = S,, + S,,(f)
+ S,,,
+ SE,, + SInt
where S,, is the thermal bending force resulting from a temperature differential AT bending force resulting from between substrate faces’, S *=(f) is a thermal constrained expansion of the film relative to the substrate during growth (Aa is the difference in coefficients of thermal expansion of the film and the substrate and f is the film thickness), S,,, is the bending force transmitted by the momentum of the vapour stream and S,,,, is simply the force that results from expansion of a 200 A layer of aluminium deposited onto the rear face of the substrate required for measurement purposes. The intrinsic film stress a,,, is given by glnt = dS,,,/dJ
ADHESION AND INTERNAL STRESS IN THIN FILMS OF
A1
17
3. RESULTS The adhesion, the changes in adhesion with time and the internal film stress were investigated in aluminium films evaporated at 5 x 10-s Torr onto glass for films deposited layer by layer and for continuously deposited films. Films 1000 A thick were deposited in layers of less than 200 A at a rate of 5 ]k s- 1 and the intrinsic stress in these films was determined using the sequential technique. Films 900 thick were deposited at rates between 4 and 6 A s- 1 and the intrinsic stress was determined by the dynamic technique. Films 400 A thick were deposited at a rate of 10 A s- 1 and the intrinsic stress was not determined in these films. 3.1. Adhesion
Figure 1 shows the measured adhesion for the 400 and 900 A aluminium films deposited continuously and for the 1000 A sequentially prepared films. In each case, aging proceeded in the same manner: adhesion was initially relatively poor (10 gf) and remained at this level for about 9 days at which stage a steady increase began, leading to a significantly improved final adhesion after a total aging period of about 21 days. The final adhesion value found for both the 1000 A sequentially prepared films and the 900 A continuously prepared films was close to 110 gf. The final adhesion value found in the 400/~ films was 90 gf. 3.2. Internal stress
Figure 2 shows the bending force Sin t which was observed in the 1000/~ films using the sequential technique. The bending force was found to be compressive and linear and the corresponding intrinsic stress O'lnt was about 7.4 × 108 dyn c m - 2.
o?o
/. .'/.. 6 110
-/ /
~'~ °~.°
/
100 9O
8O
.
[3.
70
~60
3O ~
•/
20 10 0
/ 2
4
6
8 10 12 !4 16 18 20 22 24 26 28 Time,lay.j3
0
1
2 3 4 5 6 Thickness L-IO2 ~3
7
8
9
10
11
Fig. 1. Changes in adhesion after aging of aluminium films on glass. The critical normal load is plotted against time for films of 400 A (e), 900/~ (.) and 1000/~ (O). Fig. 2. Compressive intrinsic force in aluminium deposited at 5 A s - 1 by the sequential method. The symbols each refer to a separate film.
18
M. LAUGIER
Figures 3, 4 and 5 relate to the dynamic measurements on the 900 A films and show respectively the observed total bending force Sxot, the thermal bending forces SAt, Sa, and S.... and the intrinsic bending force S~nt,found by subtraction. A simple calculation shows that the contribution of momentum transfer Smo m is negligible under these experimental conditions % ~o (Smom ~ 13 dyn c m - ~ assuming a sticking coefficient of unity and a rate of deposition of 10 A s- x). The maximum substrate temperature during deposition was close to 15 °C and differential temperatures AT as high as 0.35 °C were recorded. For thicknesses greater than 200 A, intrinsic bending force S~nt was again found to be compressive and linear and the corresponding intrinsic stress O'lnt was again close to 7.4 × 108 dyn c m - 2. 12T i l
l'q
'~
° Q ~
~ 1~ 0
0
1
2 3 4 5 Thickness E102~]
6
7
8
9
10
t~ -1
.."~" °
o~--o ~ / ° j ~ ° ~o'
4
~'
~'
5
6
7
8 -9
Thickness ElO2~q
Fig. 3. Compressive total bending force STot v s . thickness: II, 4 A s 1; O, 5 A S 1; @, 6 A S 1. Fig. 4. Compressive thermal bending force SAt (B, 4 A S- 1 ; O, 5 A S- 1; O, 6 A S- 1) and thermal bending forces SA, (compressive) and Sr~ar (tensile) (D, S t , , ; ©, SA~)v s . film thickness.
8F
6!
5~ } 4i
.~'."/"
"
.~
I ~ i .... 0
1
2
3
4
5
6
7
8
9
Thickness EIO2 ~']
Fig. 5. Compressive intrinsic force
S~, t vs.
thickness: II, 4 A s 1 ; O, 5/~ s 1 ; e , 6 • s - 1.
4. DISCUSSION The adhesion results were in good agreement with those of ref. 1 and support the suggestion of Benjamin and Weaver 1 that, if adhesion increases are due to the formation of an oxide layer at the interface, then the oxygen must have been trapped in the film during growth, because the rapid formation of a protective surface oxide layer would have prevented the diffusion of oxygen from the atmosphere. If this were
ADHESION AND INTERNAL STRESS IN THIN FILMS OF AI
19
not so, the onset of aging should have had a thickness dependence and this was not found. The slightly lower final adhesion found in the 400/~ films would simply be due to incomplete formation of the bonding oxide layer. On this basis, adhesion increases are observed once a critical thickness of oxide has formed at the interface; below this value, no change in adhesion should occur. More oxygen might be expected to be incorporated in the 1000 /~ films produced sequentially, but the adhesion-aging behaviour of the sequentially produced films was indistinguishable from that of the 900 A films produced without interruption of deposition. The compressive intrinsic stresses found in both sequentially deposited and continuously deposited films were also closely similar and if compressive intrinsic stress in vacuum-evaporated aluminium films is directly related to oxygen content these results suggest that most oxygen incorporation took place during the deposition process. Compressive stresses of the order of 4 x 109 dyn cm- 2 were attributed to the incorporation of 1 at.~o O in vacuum-evaporated nickel films3 and on this basis the compressive stresses observed in aluminium correspond to only 0.18 at.~o O incorporation. No evidence for the presence of oxygen could be obtained by electron diffraction, emphasizing the sensitivity to small amounts of impurity of this technique. 5. CONCLUSION
Measurements of the adhesion of vacuum-evaporated aluminium films to glass were made by means of the scratch test and good agreement with previous results was obtained. Anomalous compressive intrinsic stresses found in these same films were interpreted in terms of oxygen incorporation during film growth and provide evidence for the oxide layer model of adhesion. REFERENCES 1 2 3 4 5 6 7 8 9 10
P. Benjamin and C. Weaver, Proc. R. Soc. London, Ser. A, 254 (1960) 177. E. Klokholm and B. S. Berry, J. Electrochem. Soc., 115 (1968) 823. P.M. Alexander and R. W. Hoffman, J. Vac. Sci. Technol., 13 (1976) 96. E. Klokholm, J. Vac. Sci. Technol., 6 (1969) 138. P. Benjamin and C. Weaver, Proc. R. Soc. London, Ser. A, 254 (1960) 163. M. Laugier, J. Mater. Sci., 15 (1980) 1147. M. Laugier, Thin Solid Films, 66 (1980) L7. M. Laugier, Thin Solid Films, 67 (1980) 163. M. Laugier, Thin Solid Films, 66 (1980) L11. M. Laugier, Thin Solid Films, 75 (1981) 213.
15
METALLURGICAL AND PROTECTIVE COATINGS
ADHESION AND I N T E R N A L STRESS IN T H IN FILMS OF ALUMINIUM M. LAUGIER
Wimet Research and Development Laboratories*, Torrington Avenue, Coventry CV4 9AD (Gt. Britain) (Received October 24, 1980; accepted November 27, 1980)
Measurements of both adhesion and internal stress were made using thin aluminium films on glass substrates. The films were up to 1000/~ thick and were prepared by vacuum evaporation at pressures below 5 x 10 -5 Torr. Large compressive intrinsic stresses were found which could be explained in terms of the incorporation of oxygen during growth. The initial film adhesion as measured by the scratch test was poor but substantial increases in adhesion occurred with time, confirming earlier results. The observation of high compressive stresses explained in terms of oxygen incorporation provides evidence for the oxide layer model of adhesion in aluminium.
l. INTRODUCTION
Film stress and adhesion may be related through film stability. Internal stresses exceeding the film cohesive strength may lead to internal rupture although actual debonding is unlikely to be a direct result of internal stress except in the thickest coatings. The adhesion of vacuum-evaporated aluminium films on glass has been investigated and adhesion has been found to increase significantly after an aging period ~. These increases in adhesion have been attributed to oxygen migration to the Al-glass interface, leading to the formation of an oxide bonding layer ~. However, aluminium rapidly forms a protective oxide skin on exposure to air, which is an effective barrier to further oxygen diffusion from the atmosphere. Therefore oxygen must have been incorporated within the aluminium structure during film deposition if the increases in adhesion are to be attributed to the subsequent formation of an interfacial oxide bonding layer. Aluminium has a strong gettering effect for oxygen and the chamber pressure can be observed to fall during the vacuum evaporation of aluminium which suggests that oxygen may be incorporated in films prepared in this manner. For pure metals, intrinsic stress was found to be tensile and almost constant in vacuum-evaporated films beyond the early growth stages 2. However, 1 at.~o O * Now called Sandvik Research and Development Laboratories. 0040-6090/81/0000-0000/$02.50
© Elsevier Sequoia/Printed in The Netherlands
16
M. LAUGIER
incorporation in nickel films has been found to produce a significant compressive stress3. Measurements of intrinsic stress may therefore provide a sensitive technique for the investigation of impurity incorporation in thin films if the intrinsic stress is known in the pure structure. The intrinsic stress in aluminium films evaporated in ultrahigh vacuum has been investigated and the expected tensile intrinsic stresses associated with pure structures have been found4. The adhesion increases reported relate to aluminium films evaporated at a pressure in the region of 5 x 10m5 Torr when significant residual oxygen is present. Measurements of intrinsic stress under these conditions provide evidence of oxygen incorporation and lend support to the oxide layer model of adhesion in aluminium. 2.
EXPERIMENTAL
DETAILS
The glass substrates were thoroughly cleaned with Teepol just prior to use and before film deposition they were subjected to a final plasma-cleaning process. Aluminium was evaporated from tungsten spirals at rates between 4 and 6 A s- ‘. The pressure at the beginning of deposition was 5 x 10m5 Torr but it fell below 1 x 10e5 Torr during deposition. Adhesion measurements were performed using the scratch test5 in which a loaded smoothly rounded stylus is drawn across the film surface. The load at which film removal occurs, leaving a clear channel, is taken as a measure of adhesion. The styli used were made of chrome steel of tip radius 45 pm. The intrinsic stress was determined in two ways, both of which made use of a sensitive cantilevered plate technique with capacitative detection6. In the first method, intrinsic stress was determined from the deformation of the cantilever after thermal equilibrium had occurred. This simple method suffers from several disadvantages which include possible stress relief, “frozen-in” thermal stresses and the need for a separate stress determination for each thickness value7. The second method is dynamic and relates the instantaneous substrate deformation occurring during film deposition to film stresses as a function of film thickness, thereby overcoming the disadvantages inherent in the sequential method7. This technique is more complex and relies both on theory and on careful measurements of the temperature of the substrate and the temperature difference between substrate faces, for which thin film thermocouples may be employed’. Substrate deformation is most directly related to the bending force per unit width. The observed substrate deformation may be related to a total bending force STo, which may be written7 STot = S,, + S,,(f)
+ S,,,
+ SE,, + SInt
where S,, is the thermal bending force resulting from a temperature differential AT bending force resulting from between substrate faces’, S *=(f) is a thermal constrained expansion of the film relative to the substrate during growth (Aa is the difference in coefficients of thermal expansion of the film and the substrate and f is the film thickness), S,,, is the bending force transmitted by the momentum of the vapour stream and S,,,, is simply the force that results from expansion of a 200 A layer of aluminium deposited onto the rear face of the substrate required for measurement purposes. The intrinsic film stress a,,, is given by glnt = dS,,,/dJ
ADHESION AND INTERNAL STRESS IN THIN FILMS OF
A1
17
3. RESULTS The adhesion, the changes in adhesion with time and the internal film stress were investigated in aluminium films evaporated at 5 x 10-s Torr onto glass for films deposited layer by layer and for continuously deposited films. Films 1000 A thick were deposited in layers of less than 200 A at a rate of 5 ]k s- 1 and the intrinsic stress in these films was determined using the sequential technique. Films 900 thick were deposited at rates between 4 and 6 A s- 1 and the intrinsic stress was determined by the dynamic technique. Films 400 A thick were deposited at a rate of 10 A s- 1 and the intrinsic stress was not determined in these films. 3.1. Adhesion
Figure 1 shows the measured adhesion for the 400 and 900 A aluminium films deposited continuously and for the 1000 A sequentially prepared films. In each case, aging proceeded in the same manner: adhesion was initially relatively poor (10 gf) and remained at this level for about 9 days at which stage a steady increase began, leading to a significantly improved final adhesion after a total aging period of about 21 days. The final adhesion value found for both the 1000 A sequentially prepared films and the 900 A continuously prepared films was close to 110 gf. The final adhesion value found in the 400/~ films was 90 gf. 3.2. Internal stress
Figure 2 shows the bending force Sin t which was observed in the 1000/~ films using the sequential technique. The bending force was found to be compressive and linear and the corresponding intrinsic stress O'lnt was about 7.4 × 108 dyn c m - 2.
o?o
/. .'/.. 6 110
-/ /
~'~ °~.°
/
100 9O
8O
.
[3.
70
~60
3O ~
•/
20 10 0
/ 2
4
6
8 10 12 !4 16 18 20 22 24 26 28 Time,lay.j3
0
1
2 3 4 5 6 Thickness L-IO2 ~3
7
8
9
10
11
Fig. 1. Changes in adhesion after aging of aluminium films on glass. The critical normal load is plotted against time for films of 400 A (e), 900/~ (.) and 1000/~ (O). Fig. 2. Compressive intrinsic force in aluminium deposited at 5 A s - 1 by the sequential method. The symbols each refer to a separate film.
18
M. LAUGIER
Figures 3, 4 and 5 relate to the dynamic measurements on the 900 A films and show respectively the observed total bending force Sxot, the thermal bending forces SAt, Sa, and S.... and the intrinsic bending force S~nt,found by subtraction. A simple calculation shows that the contribution of momentum transfer Smo m is negligible under these experimental conditions % ~o (Smom ~ 13 dyn c m - ~ assuming a sticking coefficient of unity and a rate of deposition of 10 A s- x). The maximum substrate temperature during deposition was close to 15 °C and differential temperatures AT as high as 0.35 °C were recorded. For thicknesses greater than 200 A, intrinsic bending force S~nt was again found to be compressive and linear and the corresponding intrinsic stress O'lnt was again close to 7.4 × 108 dyn c m - 2. 12T i l
l'q
'~
° Q ~
~ 1~ 0
0
1
2 3 4 5 Thickness E102~]
6
7
8
9
10
t~ -1
.."~" °
o~--o ~ / ° j ~ ° ~o'
4
~'
~'
5
6
7
8 -9
Thickness ElO2~q
Fig. 3. Compressive total bending force STot v s . thickness: II, 4 A s 1; O, 5 A S 1; @, 6 A S 1. Fig. 4. Compressive thermal bending force SAt (B, 4 A S- 1 ; O, 5 A S- 1; O, 6 A S- 1) and thermal bending forces SA, (compressive) and Sr~ar (tensile) (D, S t , , ; ©, SA~)v s . film thickness.
8F
6!
5~ } 4i
.~'."/"
"
.~
I ~ i .... 0
1
2
3
4
5
6
7
8
9
Thickness EIO2 ~']
Fig. 5. Compressive intrinsic force
S~, t vs.
thickness: II, 4 A s 1 ; O, 5/~ s 1 ; e , 6 • s - 1.
4. DISCUSSION The adhesion results were in good agreement with those of ref. 1 and support the suggestion of Benjamin and Weaver 1 that, if adhesion increases are due to the formation of an oxide layer at the interface, then the oxygen must have been trapped in the film during growth, because the rapid formation of a protective surface oxide layer would have prevented the diffusion of oxygen from the atmosphere. If this were
ADHESION AND INTERNAL STRESS IN THIN FILMS OF AI
19
not so, the onset of aging should have had a thickness dependence and this was not found. The slightly lower final adhesion found in the 400/~ films would simply be due to incomplete formation of the bonding oxide layer. On this basis, adhesion increases are observed once a critical thickness of oxide has formed at the interface; below this value, no change in adhesion should occur. More oxygen might be expected to be incorporated in the 1000 /~ films produced sequentially, but the adhesion-aging behaviour of the sequentially produced films was indistinguishable from that of the 900 A films produced without interruption of deposition. The compressive intrinsic stresses found in both sequentially deposited and continuously deposited films were also closely similar and if compressive intrinsic stress in vacuum-evaporated aluminium films is directly related to oxygen content these results suggest that most oxygen incorporation took place during the deposition process. Compressive stresses of the order of 4 x 109 dyn cm- 2 were attributed to the incorporation of 1 at.~o O in vacuum-evaporated nickel films3 and on this basis the compressive stresses observed in aluminium correspond to only 0.18 at.~o O incorporation. No evidence for the presence of oxygen could be obtained by electron diffraction, emphasizing the sensitivity to small amounts of impurity of this technique. 5. CONCLUSION
Measurements of the adhesion of vacuum-evaporated aluminium films to glass were made by means of the scratch test and good agreement with previous results was obtained. Anomalous compressive intrinsic stresses found in these same films were interpreted in terms of oxygen incorporation during film growth and provide evidence for the oxide layer model of adhesion. REFERENCES 1 2 3 4 5 6 7 8 9 10
P. Benjamin and C. Weaver, Proc. R. Soc. London, Ser. A, 254 (1960) 177. E. Klokholm and B. S. Berry, J. Electrochem. Soc., 115 (1968) 823. P.M. Alexander and R. W. Hoffman, J. Vac. Sci. Technol., 13 (1976) 96. E. Klokholm, J. Vac. Sci. Technol., 6 (1969) 138. P. Benjamin and C. Weaver, Proc. R. Soc. London, Ser. A, 254 (1960) 163. M. Laugier, J. Mater. Sci., 15 (1980) 1147. M. Laugier, Thin Solid Films, 66 (1980) L7. M. Laugier, Thin Solid Films, 67 (1980) 163. M. Laugier, Thin Solid Films, 66 (1980) L11. M. Laugier, Thin Solid Films, 75 (1981) 213.