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Nature of group Ib metal films deposited on polymer surfaces photooxidized with far-ultraviolet (185 nm) radiation
IB44
Surface Science I30 (1983) L344-L348 North-Holland Publishing Company
S U R F A C E SCIENCE LETTERS NATURE OF G R O U P lb METAL FILMS DEPOSITED ON POLYMER S U R F A C E S P H O T O O X I D I Z E D WITH FAR-ULTRAVIOLET (185 nm) RADIATION R. SRINIVASAN, V.B. JIPSON and M.J. POIRIER * I B M Thomas J. Watson Research Center, Yorktown Heights, New York 10598, USA Received 12 April 1983
Solid samples of three polymers (polymethyl methacrylate, polycarbonate, and polyimide), whose surfaces were photooxidized with far-ultraviolet (185 nm) radiation in air, were used as substrates for the deposition of gold, silver, and copper films in vacuo. The electrical conductivities of the films were studied in situ as a function of the film thickness (0-15 nm). At a thickness of 10 nm, silver films showed 104 greater conductivities when deposited on the treated surfaces of all three polymers. Gold films showed no detectable differences between treated and untreated surfaces. Copper behaved similar to gold on PMMA but showed somewhat greater sensitivity ( < 20 at 5.4 nm thickness) on photooxidized polyimide. Transmission electron microphotographs of silver films on treated and untreated surfaces showed that photooxidation of the polymer caused the metal to be deposited with better coverage of the surface, i.e., with fewer voids. A partial explanation for the observations would be that photooxidation increases the number of nucleating sites for the metal on the polymer surface. In some way, this must be specific to silver because the effect does not extend to either a more reactive element such as copper or a less reactive element such as gold.
In the past decade, several elegant methods [1,2] have been developed to probe the nature of organic films deposited on the surfaces of metals. The reverse situation in which a metal film lies on an organic surface lends itself less readily to similar investigation. For this reason, there is very little known on the structure and dynamics of the deposition of a metal on an organic solid. In this communication it is shown that organic polymer surfaces which are photooxidized in a controlled manner with far-UV radiation (185 nm) in air show great selectivity in their behavior towards the deposition of group l b metals. The oxidation of polymer surfaces by oxygen plasmas [3] or mid-UV radiation and air [4] is well-documented. Photoelectron spectra of such surfaces
* Present address: Department of Electrical Engineering, University of Rhode Island, Rhode Island, USA.
0 0 3 9 - 6 0 2 8 / 8 3 / 0 0 0 0 - 0 0 0 0 / $ 0 3 . 0 0 © 1983 North-Holland
R. Srini~)asan et al. / Nature of group lb metal films
L345
show the progressive formation of oxygen containing groups such as -O
-C-O-,
-COO-
and
C=O -O
with increasing exposure [5]. ATR infrared spectra support these deductions [6]. The hydrophilic nature of the surface following photooxidation has been established by contact angle measurements [7]. In the present work it was found convenient to oxidize the polymer surfaces by exposure in air to far-UV radiation at 185 nm [8]. After 5 rain of exposure the contact angle for water did not show any further decrease, indicating a surface saturation effect. A film of metal was deposited by e-beam evaporation [9] on a polymer whose surface had been prepared by photooxidation. The relative conductivity of the deposited layer was monitored in situ as a function of its thickness [10]. The conductivity was measured relative to a 1 M~? resistor in series with the sample (fig. 1) by measuring the voltage across the latter. A second sample of the same polymer which had not been photooxidized was also placed in the vacuum chamber at the same distance from the source, and used as a reference. A typical result for silver on PMMA is shown in fig. 1. The preoxidized surface shows a striking preference for forming a strongly conducting film at a thickness as little as 5 nm. The critical thicknesses [11] T~ and T~x of various films (with and without photooxidation respectively) at which the film resistance is equal to the 1 M~2 reference resistance are given in table 1. The smaller the ratio T c x / T ~ is in comparison to unity for a given polymer and metal, the greater the effect that photooxidation has on the nucleation of the metal film.
I.O
iO0 0 0 " 0 - 1
/
o Ag on OXIDIZED PMMA
o
W
_>
~o-e-e
o
/
o.8
[] Ag on UNTREATED PMMA
o
/
..J bJ
~o.6 :P
O
~-- SAMPLE
[a.I
0.4 I-_J o>
Vs
iM,Q
ol
/f
0.2
O
o
_
! !Tcx I [].--r.,~B~=B .~" t 4-
8
12
Tc I
I
I
I
16
20
?_4
28
,.
THICKNESS (nm) Fig, I, Plot of thickness versus relative conductivity of silver films on polymethyl methacrylate (PMMA). (The nature of the circuit shown makes relative voltages given as ordinate equal to relative conductivities.) Open circles, photooxidized polymer; crossed squares, untreated polymer.
1~346
R. Srinivasan et al. ,/ Nature of group Ib metal fitms
Table 1 Critical thicknesses of metal films deposited on untreated and photooxidized surfaces Substrate
P M M A b) PM M A PMMA Polycarbonate c ~ Polyimide a ~ Polyimide
Metal
Gold Silver Copper Silver Silver Copper
Critical thickness ~'~(nm) (untreated}
(photooxidized)
6.7 l 3.0 5.0 12.6 13.2 5.g,
6.6 7,2 4.7 6.8 7.0 4. I
Ratio T . ~ / / ~
0.98 0.55 0.94 0.54 0.53 0.76
~'~ This thickness was measured at the point where the conductivity of the film was equal to the conductivity of the 1 M,f2 resistance. ~'~ Commercial polymethyl methacrylate .g/-- 200,000. ~ Commercial material from General Electric Co. ~ Commercial film-condensate of pyromellitic dianhydride and p,pLdiamino diphenyl ether.
The condensation of a metal vapor on an organic surface is usually pictured to begin with the formation of islands of solid metal at a few nucleating sites. The islands then expand by the condensation of more metal atoms on them until eventually the islands meet to form first a networked film and eventually a continuous film [11]. The activation of a polymer surface by photooxidation can be due to the formation of more nucleating sites per unit area for the condensing metal atoms. This would lead to the formation of a continuous film at a lower average thickness of the film. There must be a specific chemical interaction between a metal atom and an "'active site" on the polymer surface since gold, which is a less reactive element than silver, does not respond to the p r e t r e a t m e n t of the surface in the same way as silver. Fig. 2 shows transmission electron microphotographs of 15 nm films of silver which were grown to the same thickness on both far-UV treated and untreated surfaces of a p o l y c a r b o n a t e [12]. At this thickness both films are well above the critical thickness for film networking, The film on the treated surface is clearly free from the voids that are quite evident in the film on the untreated surface. These data lend support to the view proposed above. However, this picture cannot be complete since copper which is a much more reactive element than silver towards the formation of organic c o m p o u n d s did not exhibit the same behavior as silver towards photooxidized P M M A . At best, it showed only a modest preference towards treated polyimide surfaces. It is possible that in addition to the chemical reactivity of the metal atom, its packing on the surface is also a factor in the growth of a continuous film, The chemical reactivity of silver films deposited on photooxidized polymer surfaces is under investigation.
R. Srinivasan et al. / Nature of group lb metal fihns
L347
tOOnm Fig. 2. Transmission electron microphotographs of 15 nm silver films deposited on polycarbonate: (a) photooxidized polymer; (b) untreated polymer.
The authors thank Mr. Harold Lynt for preparation of the samples, Ms. S. Herd for the TEM photographs and Dr. Praveen Chaudhari for helpful discussions. References [1] P.F. Kane and G.B. Larrabee, Eds., Characterization of Solid Surfaces (Plenum, New York, 1974), [2] G,A. Somorjai, Chemistry in Two Dimensions (Cornell University Press, Ithaca, NY, 1981).
L348
R. Srinivasan et al. / Nature of group lb metal films
[3] For a review, see D.T. Clark, A. Dilks and D. Shuttleworth, in: Polymer Surfaces, Eds. D.T. Clark and W.J. Feast (Wiley, New York, 1974) p. 206. [4] D.J. Carlsson and D.M. Wiles, in: Ultraviolet Light Induced Reactions in Polymers, Ed. S,S. Labana,'Am. Chem, Soc. Symp. Set. (1976) p. 321. [5] D.T. Clark and A. Dilks, J. Polymer Sci, Chem. Ed. 15 (1977) 232l. [6] P. Blais, M. Day and D.M. Wiles, J. Appi. Polymer Sci. 17 (1973) 1895. [7] M.M. Millard, K.S. Lee and A.E. Pavlath, Textile Res. J. 42 (t972) 307. [8] An Osram H N S 1 0 - W / U oz mercury resonance lamp at a distance of 5 mm was the source. For details of the photoetching process see: R. Srinivasan, Polymer 23 (1982) 1863. [9] The sample and reference were mounted (at 60 ° to normal incidence) equidistant from the source of an e-beam evaporator. The chamber was evacuated to 10 - 7 Torr. Metal deposition occurred at 0.1 n m / s The thickness was measured by a quartz crystal detector. [10] Electrical contact was established by gold contact pads which were 1 cm apart on all polymer samples. [11] K.L. Chopra, Thin Film Phenomena (McGraw-Hill, 1969). The choice of 1 M/2 as the point where ~ is measured is arbitrary, but we are mainly concerned with the relation between T~ and Tc~. [12] Similar TEM photographs were obtained from silver films deposited on photooxidized and untreated PMMA.
Surface Science I30 (1983) L344-L348 North-Holland Publishing Company
S U R F A C E SCIENCE LETTERS NATURE OF G R O U P lb METAL FILMS DEPOSITED ON POLYMER S U R F A C E S P H O T O O X I D I Z E D WITH FAR-ULTRAVIOLET (185 nm) RADIATION R. SRINIVASAN, V.B. JIPSON and M.J. POIRIER * I B M Thomas J. Watson Research Center, Yorktown Heights, New York 10598, USA Received 12 April 1983
Solid samples of three polymers (polymethyl methacrylate, polycarbonate, and polyimide), whose surfaces were photooxidized with far-ultraviolet (185 nm) radiation in air, were used as substrates for the deposition of gold, silver, and copper films in vacuo. The electrical conductivities of the films were studied in situ as a function of the film thickness (0-15 nm). At a thickness of 10 nm, silver films showed 104 greater conductivities when deposited on the treated surfaces of all three polymers. Gold films showed no detectable differences between treated and untreated surfaces. Copper behaved similar to gold on PMMA but showed somewhat greater sensitivity ( < 20 at 5.4 nm thickness) on photooxidized polyimide. Transmission electron microphotographs of silver films on treated and untreated surfaces showed that photooxidation of the polymer caused the metal to be deposited with better coverage of the surface, i.e., with fewer voids. A partial explanation for the observations would be that photooxidation increases the number of nucleating sites for the metal on the polymer surface. In some way, this must be specific to silver because the effect does not extend to either a more reactive element such as copper or a less reactive element such as gold.
In the past decade, several elegant methods [1,2] have been developed to probe the nature of organic films deposited on the surfaces of metals. The reverse situation in which a metal film lies on an organic surface lends itself less readily to similar investigation. For this reason, there is very little known on the structure and dynamics of the deposition of a metal on an organic solid. In this communication it is shown that organic polymer surfaces which are photooxidized in a controlled manner with far-UV radiation (185 nm) in air show great selectivity in their behavior towards the deposition of group l b metals. The oxidation of polymer surfaces by oxygen plasmas [3] or mid-UV radiation and air [4] is well-documented. Photoelectron spectra of such surfaces
* Present address: Department of Electrical Engineering, University of Rhode Island, Rhode Island, USA.
0 0 3 9 - 6 0 2 8 / 8 3 / 0 0 0 0 - 0 0 0 0 / $ 0 3 . 0 0 © 1983 North-Holland
R. Srini~)asan et al. / Nature of group lb metal films
L345
show the progressive formation of oxygen containing groups such as -O
-C-O-,
-COO-
and
C=O -O
with increasing exposure [5]. ATR infrared spectra support these deductions [6]. The hydrophilic nature of the surface following photooxidation has been established by contact angle measurements [7]. In the present work it was found convenient to oxidize the polymer surfaces by exposure in air to far-UV radiation at 185 nm [8]. After 5 rain of exposure the contact angle for water did not show any further decrease, indicating a surface saturation effect. A film of metal was deposited by e-beam evaporation [9] on a polymer whose surface had been prepared by photooxidation. The relative conductivity of the deposited layer was monitored in situ as a function of its thickness [10]. The conductivity was measured relative to a 1 M~? resistor in series with the sample (fig. 1) by measuring the voltage across the latter. A second sample of the same polymer which had not been photooxidized was also placed in the vacuum chamber at the same distance from the source, and used as a reference. A typical result for silver on PMMA is shown in fig. 1. The preoxidized surface shows a striking preference for forming a strongly conducting film at a thickness as little as 5 nm. The critical thicknesses [11] T~ and T~x of various films (with and without photooxidation respectively) at which the film resistance is equal to the 1 M~2 reference resistance are given in table 1. The smaller the ratio T c x / T ~ is in comparison to unity for a given polymer and metal, the greater the effect that photooxidation has on the nucleation of the metal film.
I.O
iO0 0 0 " 0 - 1
/
o Ag on OXIDIZED PMMA
o
W
_>
~o-e-e
o
/
o.8
[] Ag on UNTREATED PMMA
o
/
..J bJ
~o.6 :P
O
~-- SAMPLE
[a.I
0.4 I-_J o>
Vs
iM,Q
ol
/f
0.2
O
o
_
! !Tcx I [].--r.,~B~=B .~" t 4-
8
12
Tc I
I
I
I
16
20
?_4
28
,.
THICKNESS (nm) Fig, I, Plot of thickness versus relative conductivity of silver films on polymethyl methacrylate (PMMA). (The nature of the circuit shown makes relative voltages given as ordinate equal to relative conductivities.) Open circles, photooxidized polymer; crossed squares, untreated polymer.
1~346
R. Srinivasan et al. ,/ Nature of group Ib metal fitms
Table 1 Critical thicknesses of metal films deposited on untreated and photooxidized surfaces Substrate
P M M A b) PM M A PMMA Polycarbonate c ~ Polyimide a ~ Polyimide
Metal
Gold Silver Copper Silver Silver Copper
Critical thickness ~'~(nm) (untreated}
(photooxidized)
6.7 l 3.0 5.0 12.6 13.2 5.g,
6.6 7,2 4.7 6.8 7.0 4. I
Ratio T . ~ / / ~
0.98 0.55 0.94 0.54 0.53 0.76
~'~ This thickness was measured at the point where the conductivity of the film was equal to the conductivity of the 1 M,f2 resistance. ~'~ Commercial polymethyl methacrylate .g/-- 200,000. ~ Commercial material from General Electric Co. ~ Commercial film-condensate of pyromellitic dianhydride and p,pLdiamino diphenyl ether.
The condensation of a metal vapor on an organic surface is usually pictured to begin with the formation of islands of solid metal at a few nucleating sites. The islands then expand by the condensation of more metal atoms on them until eventually the islands meet to form first a networked film and eventually a continuous film [11]. The activation of a polymer surface by photooxidation can be due to the formation of more nucleating sites per unit area for the condensing metal atoms. This would lead to the formation of a continuous film at a lower average thickness of the film. There must be a specific chemical interaction between a metal atom and an "'active site" on the polymer surface since gold, which is a less reactive element than silver, does not respond to the p r e t r e a t m e n t of the surface in the same way as silver. Fig. 2 shows transmission electron microphotographs of 15 nm films of silver which were grown to the same thickness on both far-UV treated and untreated surfaces of a p o l y c a r b o n a t e [12]. At this thickness both films are well above the critical thickness for film networking, The film on the treated surface is clearly free from the voids that are quite evident in the film on the untreated surface. These data lend support to the view proposed above. However, this picture cannot be complete since copper which is a much more reactive element than silver towards the formation of organic c o m p o u n d s did not exhibit the same behavior as silver towards photooxidized P M M A . At best, it showed only a modest preference towards treated polyimide surfaces. It is possible that in addition to the chemical reactivity of the metal atom, its packing on the surface is also a factor in the growth of a continuous film, The chemical reactivity of silver films deposited on photooxidized polymer surfaces is under investigation.
R. Srinivasan et al. / Nature of group lb metal fihns
L347
tOOnm Fig. 2. Transmission electron microphotographs of 15 nm silver films deposited on polycarbonate: (a) photooxidized polymer; (b) untreated polymer.
The authors thank Mr. Harold Lynt for preparation of the samples, Ms. S. Herd for the TEM photographs and Dr. Praveen Chaudhari for helpful discussions. References [1] P.F. Kane and G.B. Larrabee, Eds., Characterization of Solid Surfaces (Plenum, New York, 1974), [2] G,A. Somorjai, Chemistry in Two Dimensions (Cornell University Press, Ithaca, NY, 1981).
L348
R. Srinivasan et al. / Nature of group lb metal films
[3] For a review, see D.T. Clark, A. Dilks and D. Shuttleworth, in: Polymer Surfaces, Eds. D.T. Clark and W.J. Feast (Wiley, New York, 1974) p. 206. [4] D.J. Carlsson and D.M. Wiles, in: Ultraviolet Light Induced Reactions in Polymers, Ed. S,S. Labana,'Am. Chem, Soc. Symp. Set. (1976) p. 321. [5] D.T. Clark and A. Dilks, J. Polymer Sci, Chem. Ed. 15 (1977) 232l. [6] P. Blais, M. Day and D.M. Wiles, J. Appi. Polymer Sci. 17 (1973) 1895. [7] M.M. Millard, K.S. Lee and A.E. Pavlath, Textile Res. J. 42 (t972) 307. [8] An Osram H N S 1 0 - W / U oz mercury resonance lamp at a distance of 5 mm was the source. For details of the photoetching process see: R. Srinivasan, Polymer 23 (1982) 1863. [9] The sample and reference were mounted (at 60 ° to normal incidence) equidistant from the source of an e-beam evaporator. The chamber was evacuated to 10 - 7 Torr. Metal deposition occurred at 0.1 n m / s The thickness was measured by a quartz crystal detector. [10] Electrical contact was established by gold contact pads which were 1 cm apart on all polymer samples. [11] K.L. Chopra, Thin Film Phenomena (McGraw-Hill, 1969). The choice of 1 M/2 as the point where ~ is measured is arbitrary, but we are mainly concerned with the relation between T~ and Tc~. [12] Similar TEM photographs were obtained from silver films deposited on photooxidized and untreated PMMA.