Ammonia radio frequency plasma treatment of composite-filled poly

Nov 23, 1992 - 3077. Ammonia Radio Frequency Plasma Treatment of. Composite-Filled Poly(tetrafluoroethylene) and. Poly(tetrafluoroethylene) Laminated ...
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Langmuir 1993,9,3077-3084

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Ammonia Radio Frequency Plasma Treatment of Composite-Filled Poly(tetrafluoroethylene) and Poly(tetrafluoroethylene) Laminated to Copper Mebrahtu G. Fessehaie,? Skye McClain,+ Carlos L. Barton,: Gwo S. Swei,: and Steven L. Suib*pt*s Department of Chemistry and Institute of Materials Science, University of Connecticut, Storrs, Connecticut 06269-3060, Rogers Corporation, Rogers, Connecticut 06263, and Department of Chemical Engineering, University of Connecticut, Storrs, Connecticut 06269 Received November 23, 1992. I n Final Form: July 19, 1999 The surfaces of composite-fiied poly(tetrafluoroethy1ene)(PTFE) and PTFE laminated to copper were modified by ammonia radio frequency plasma treatment. The contact angle decreased significantly after treatment. Ammonia plasma treatments resulted in an enhanced peel strength of the composite-filled PTFE as compared to argon, air, and nitrogen plasma treatments. Ammonia plasma treatment generated nitrogen- and oxygen-containing functional groups on the surface.

Introduction Our research focused on the plasma modification of the surface of two different types of poly(tetrafluoroethy1ene) (PTFE), the first of which had added ceramic composite fillers, and the second of which had been laminated to copper metal. The control samples termed "skived" were thin sections (approximately 2 mm in thickness) mechanically sliced from an untreated block of PTFE. By treating these surfaces with a variety of plasma gases, we hoped to be able to modify the surfaces, making them more wettable (lowering their contact angles) and improving their mechanical properties (i.e., peel strength). We also wanted to look at what specific chemical changes occurred to these surfaces to see if there was any correlation between them and changes in the physical properties we observed. The mechanical properties of PTFE can be controlled to some extent by the addition of ceramic fillers. Specific properties showing improvement, as seen by researchers at Rogers Corp., include the creep resistance, dimensional stability, dielectric constant, and thermal coefficient of expansion. Controlling such properties can be especially useful in a variety of applications, including the development of flexible printed circuitry. Polymer surfaces subjected to plasma treatment undergo changes that may include the breaking of chemical bonds, cross-linking reactions of free radicals, and the incorporation of chemical functional groups originating from the plasma. This modification of the polymer surface may improve polymer-metal adhesion and the wettability of the polymer surface.13 A plasma is a soft radiation source. Hence, plasma treatment affects only a range of several hundred to several To whom correspondenceshould be addressed at the Department of Chemistry, University of Connecticut. + Department of Chemistry and Institute of Materials Science, University of Connecticut. t Rogers Corp. Department of Chemical Engineering,University of Connecticut. 8 Abstract published in Advance ACS Abstracts, September 15, 1993. (1) Wu, 5.Polymer Interface and Adhesion; Marcel Dekker: New York, 1982. (2) Yasuda, H.K.;Cho, D. L.; Yeh, Y. S . In Polymer Surfaces and Interfaces; Feast, W.J., Munro, H. S.,E&.; Wiley: New York, 1987.

(3) Yaeuda,H.; Sharma, H.K.;Yaeuda, T. J.Polym. Sci.,Polym. Phys. Ed. 1981,19,1285. (4) Hanaen, R. H.;Schonhom, H.J. Polym. Sci., B 1966,4, 203. (5) Mantell,R. M.; Ormand,W. Ind. E m . Chem.Prod. Res. Dev. 1964, 3,300.

g"- v*,= YLVCOS e Figure 1. Definition of the contact angle (8) and Young's equation. thousand angstroms deep.6 The bulk of the material, including its bulk properties, therefore remainsunchanged. Plasma surface treatment also offers the advantage of greater chemical flexibility. By choosing different reactant gases, or a mixture of gases, for generating reactive chemical species, different surfaces can be produced. Also in dry gas, plasma-surface interactions are generally free of secondary reactive species causing undesirable side reactions.' In this work the use of radio frequency (RF) plasma treatment for the modification of composite-filled PTFE surfaces and the polymer side of PTFE laminated to both shiny copper and electrodeposited rough copper is reported. The surfaces were analyzed by using X-ray photoelectron spectroscopy (XPS), Fourier transform infrared-attenuated total reflectance (FTIR-ATR) spectroscopy, scanning electron microscopy (SEMI,and contact angle measurements. The changes observed were correlated with physical properties in terms of peel strength. The principal method of measuring a polymer's surface tension is by measuring the angle it makes with a drop of liquid resting on its surface. The relationship between this contact angle, 6, and the three surface tensions at work on the surface is given by Young's equation: ysv y s =~ ~ L cos V 6, where y is the surface tension between each specific interface, solid-vapor (ysv), solid-liquid (ysv), and liquid-vapor ( y ~ v (Figure ) 1). A low contact angle is indicative of high surface tension as well as greater wettability. Improved wettability is desirable in increasing ~~

~~

~~

(6) Garbaeei,F.;Morra, M.; Occhiello,E.;Barino, L.; Scordamaglia,R.

Surf. Interface Anal. 1989,14,585. (7) Hallahan, J. R.; Stafford,B.B.J. Appl. Polym. Sci. 1969,13,807.

0743-7463/93/2409-3077$04.00/00 1993 American Chemical Society

3078 Langmuir, Vol. 9,No.11,1993

INLET PORTS FOR PLASMA GAS

Fessehaie et al. DRIVEN ELECTRODE

I

I

PTFE SAMPLE

GLASS SLIDE

I

2.5" I

-

M ~ A CLIP L

III I I I I 1 1 qpox15-n

GROUNDED ELECTRODE

7 13"

Figure 2. Plasma treatment apparatus.

the adhesive properties of a polymer surface. This is due to the fact that improved wettability will ensure a more intimate contact between the polymer and the surface of the substance to which it is adhering. PTFE has an extremely low surface tension as compared to most other polymers (on the order of 18dyn=cm-1)8and as a result has extremely poor adhesion characteristics. The reader is recommended to see the work of Zisman for more information on the measurement of contact angles and their relationship to adhesion.8

Experimental Section A. Composite-Filled PTFE. 1. Plasma Treatments. Ceramic-filled PTFE composites RO-2800 and DUROID-6002 manufactured by Rogers Corp. were wet sanded with 600 grit and cleaned before plasma treatment. While the composition of these samples is proprietary, the data reported are still useful since they compare treated samples to control samples, and all conclusions are drawn from the changes we observed. S b p l e s were placed on the grounded electrode of a planar, capacitively coupled plasma reactor (Figure 2). The plasma conditions used were 250 W, 0.5 Torr, and 200 std ~m~0min-l (SCCM) using a radio frequency of 13.56 MHz. Argon, nitrogen, oxygen, and ammonia were used as plasma-supporting gases, and the samples were exposed to plasma for 5 and 20 min. 2. Contact Angle Measurements. A standard goniometer was used for angle measurements, with distilled deionized water as the contacting liquid. The reported contact angle measurements were averages of four measurements a t four different locations across the sample's surface. Drops were applied by hand, and both advancing (adding a small amount of liquid from the existing drop) contact angles, as well as the initial contact angles, were measured. 3. Peel Strength Measurements. Peel strength measurements were made by following the testing procedure as prescribed by the Institute of Printed Circuitry (IPC) for the measurement of the "peel strength of flexible printed wiring materials". In this procedure, peel strips 1/4 in. in width were tested with a peel angle of 90" maintained throughout the measurement. A copper coatingwas first deposited on the composite-filledPTFE samples and was then built up by electroplating. The electrodelesscopper deposition process was carried out for 30 min using a MacDeranld METEX 9650, and electroplating was carried out for 60 min at 0.17 A/in.2 using a Slenex CUBATHM. 4. Aging. For aging studies, RO-2800 and skived PTFE films were ammonia plasma treated for 1,5,10,15, and 30 min. The plasma treatment conditions were 500 W, 0.5 Torr, and 200 std cm3*min-l(sccm)using a radio frequencyof 13.56 MHz at ambient temperature. The two storage conditions used were (A) open air of 50% relative humidity at 22 "C and (B) in a desiccator at 22 "C. The aging process was monitored by measuring the initial contact angle 1,24,48,96,240, and 480 h after plasma treatment. B. PTFE Laminated to Copper. 1. Plasma Treatments. The PTFE laminated to copper samples were treated with ammonia plasma on the PTFE side. Samples were placed on the grounded electrode of a planar, capacitively coupled plasma reactor (Figure 2). The plasma conditions used were 250 W, 0.5 ~

(8) Zisman, W. A. Advances in Chemistry Series; ACS Applied Publications: Washington, DC, 1964;No. 43,p 1.

_____)

Figure 3. Contact angle measurement apparatus. Torr, and 200 sccm using a radio frequency of 13.56 MHz. Samples were exposed for 3,10, and 30 min. The contact angle, X-ray photoelectron spectroscopy (XPS),and Fourier transform infrared (FTIR) analyses were done within one week of plasma treatment. 2. Contact Angle Measurements. A Rame-Hart NRL contact angle goniometer was used for the measurement of the laminated PTFE samples. A section approximately 3 X 6 cm2 was cut from each sample sheet for analysis, taking care to keep the same sample orientation. The shorter edges of each section were held in place on the goniometer using clean glass microscope slides and large metal clips (Figure 3). Distilled deionized water was used as the contacting liquid. Drops were applied by hand using a small (1mL) syringe. The drop size averaged a volume of approximately 4 pL and covered an area of approximately 0.15 mm.2 Fifteen drops were evenly distributed across each sample's surface. Contact angles were measured for both the right and left sides of each drop immediately after application. The reported contact angles are an average of 13 drop measurements, after discarding the high and low drop values. 3. Scanning Electron Microscopy (SEM). Sections approximately 1X 1mm were cut from each sample and mounted on standard aluminum SEM pegs using carbon paint. These mounted samples were then sputter coated with gold to a thickness of approximately 5 nm. Samples were imaged using an Amray 1810D SEM. The electron beam current was kept a t 10 kV for most of the images in an attempt to prevent sample charging. Two samples were imaged with higher beam currents (15and 30 kV) to try to obtain greater detail of the fine structure. Little sample charging occurred for either of these samples. Working distances were all either 7 or 8mm. The condenser lens was maintained at a setting of 5.0 for all images. 4. X-ray Photoelectron Spectroscopy (XPS). The XPS data were collected using a Leybold Heraeus LHS-10 spectrometer equipped with an EA-10 hemispherical analyzer. A Mg Ka X-ray source was used for all analyses. The X-ray beam current and voltage were 10 mA and 13 kV, respectively. The pressure was 1X 10-8 mbar in the preparation chamber and 4 X mbar in the analysis chamber. Survey scans were collected in a constant relative resolution mode with a retardation ratio of 3.0. The narrow scans were collected at a constant resolution mode with a transmission energy of 50 meV. The instrument was calibrated by setting the Cu 2p 3/2 peak to 932.7 f 0.1 eV and the Au 4f 7/2 peak to 75.1 f 0.1 eV. Charge correction was carried out by setting the adventitious C 1s peak to 284.6 eV. Unresolved peaks were resolved by using the Proctor LOGAFIT program. Quantitative analysis was carried out using sensitivity factors provided by the manufacturer.9 5. FTIR-ATR. FTIR-ATR surface analysis was performed using a Nicolet SX-60 FTIR spectrometer. A mercury cadmium telluride detector was used. A germanium prism was used as an internal reflectance element with a 45" incidence angle. The system was purged with dry air to prevent interference of atmospheric moisture.

Results A. Composite-Filled PTFE. 1. RO-2800. The contact angle and the peel strength values of RO-2800are (9)Wagner, C. D.; Riggs, W. M.; Davis, L. E.; Moulder, L. F.; Muilenberg, G. E. Handbook of X-ray Photoelectron Spectroscopy, Perkin-Elmer Corp., Physical Electronics Division: Eden Prairie, MN, 1979;55344.

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Table I. Contact Angle and Peel Strength Values of Plasma-Treated RO-2800 Composite-Filled PTFE contact angle (deg) plaema gaa control argon nitrogen oxygen ammonia

time (min)

initial

advancing

receding

peel strength (psi)

0 5 20 5 20 5 20 5 20

115 89

96 92 74 82 94 92 77 36 29

84 65 55 68 72 74 61 25 13

1.5 1.4 1.9 2.0 1.3 1.3 1.2 4.1 2.6

86 101 103 111 99 51 31

Table 11. Contact Angle and Peel Strength Values of Plasma-Treated DUROID-6002 Composite-Filled PTFE contact angle (deg) plaama gaa control argon nitrogen oxygen ammonia

time (min)

initial

advancing

receding

peel strength (psi)

0 5 20 5 20 5 20 5 20

111 79 75 98 103 115 102 41 31

99 93 74 66

91 60 55 69 70 77 56 25 20

0.6 1.8 1.5 1.5 1.9 1.2 1.4 2.5 1.2

84

90 53 36 31

Table 111. Contact Angles of Ammonia Plasma Treated RO-2800 Composite-Filled PTFE at Different Storage Times contact angle (deg) storage control 1 min 5 min 10 min 15 min 30 min time (h) oDen air desiccator o w n air desiccator o w n air desiccator oDen air desiccator oDen air desiccator own air desiccator ~

1 24

~

~

240

50 73 57 74 78

480

66

48 96

65 79 78 79 74 54

59 83 96 87 127 122

63 69 59 69 76 64

43 63 65 52 63 64

shown in Table I. The peel strength for the control was 1.5 (psi). There was no significant change after 5 min of argon plasma treatment. There was, however, an increase of 0.4 psi after 20 min of treatment. Similarly, nitrogen plasma treatment resulted in 2.0 and 1.3 psi peel strengths after 5 and 20 min of treatment, respectively. Oxygen plasma also did not show any enhanced strength. On the other hand, ammonia plasma treatment resulted in both a 73 % and a 173% increase in peel strength after 20 and 5 min of treatment, respectively. The initial contact angle of the untreated control sample was 115'. After argon plasma treatment it was 89' and 86' after 5 and 20 min, respectively. With nitrogen plasma treatment, the initial contact angles were only 101' and 103'. Similarly, oxygen plasma treatment results only in initial contact angles of 111' and 99', after 5 and 20 min of treatment, respectively. However, ammonia plasma treatment resulted in a dramatic decrease of the initial contact angles. After 5 min of treatment the initial contact angle was 51', and after 20 min it was 31'. The advancing and receding angles of the treated samplesremained lower than those of the untreated control samples. In all cases, the advancing angles were higher than the corresponding receding angles. 2. DUROID-6002.Table I1 shows the contact angles and peel strengths for DUROID-6002. The control untreated sample had contact angles of 11l0,99', and 91' as the initial, advancing, and receding angles,respectively. After 5 min of argon plasma treatment the values had decreased to 79', 93', and 80' as initial, advancing, and receding angles, respectively. After 20 min these values were further reduced to 75', 74', and 55'. The nitrogen and oxygen plasma treatments resulted in similarreduction

36 43 44 54 55 55

11 36 53 68 51 52

9 35 45 50

59

60

9 41 67 56 49 49

12 35 39 63 52 53

33 55 52 64 54 61

55 65 78 68 70 67

of the contact angles. Ammonia plasma treatment, on the other hand, resulted in a sharp decrease of the contact angles. After 5 min of treatment the angles became 51°, 36', and 25' as initial, advancing, and receding angles, respectively. Upon 20 min of treatment the corresponding values were reduced further to 31°, 31', and 20'. As can be seen from Table 11,the advancing angles were greater than the corresponding receding angles. The peel strengths of the plasma-treated samples were at least twice that of the untreated sample. The 5-min ammonia plasma treatment resulted in about a &fold increase. At a longer ammonia plasma treatment time (20 min), the peel strength showed only a 2-fold improvement. However, the contact angle at 5 min of treatment was higher than that at 20 min. This may indicate that the peel strength was not solely determined by the wettability of the surface. 3. Aging. In order to determine the shelf life of the ammonia-treatedsamples,the initial contact angles of RO2800 were measured (Table 111). The effect of aging of the ammonia plasma treated skived PTFE was also determined for comparison (Table IV). The contact angle of the controlwas above 100' after 1h of storage. However, it decreased to 67' before it started to increase after prolonged storage. After 480 h the contact angles were 95' and 115' for the open-air- and desiccator-stored samples, respectively. The contact angles of the ammonia plasma treated skived PTFE were much lower after 1h of storage for all treatment times as compared to the correspondingcontrol. The lowest contact angle was observed for the 10-min treated sample that was stored in the open air. However, the 10-min sample that was stored in the desiccator did not show the lowest contact angle as compared to the other

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Table IV. Contact Angles of Ammonia Plasma Treated Skived PTFE at Different Storage Times contact angle (deg) storage control 1 min 5 min 10 min 15 min 30 min time (h) open air desiccator open air desiccator open air desiccator open air desiccator open air desiccator open air desiccator 1 24 48 96 240 480

105 74 67 69 97 95

116 67 94 89 95 115

32 36 42 47 43 43

33 29 44 37 42 31

14 27 38 37 43 37

28 25 29 40 44 43

10 28 30 27 37 34

26 45 26 31 40 30

15 28 27 38 38 38

17 28 29 36 31 37

14 36 28 31 39 43

37 61 47 41 44 40

Table V. Contact Angles of Ammonia Plasma Treated PTFE Laminated to Shiny and to Electrodeposited Copper sample exposure contact angle (deg) lamination time (min) average std dev shiny 3 32 3.8 10 19 4.1 shiny shiny 30 42 4.4 electrodeposited 3 46 2.6 electrodeposited 10 46 2.0 electrodeposited 30 59 5.3

surfaces stored there. On the whole the contact angles for the open-air-stored samples were higher than those of the correspondingsamples stored in a desiccator. An exception was the 30-min treated sample where the reverse was true. In the RO-2800 sample the contact angle of the control is higher than that of the 1-min treated sample after 1h of aging. The minimum contact angles were measured for the 1-h stored samples that had been exposed to the 10and 15-min plasma treatments. The 15-min treated sample keeps the lowest contact angle of 49' after 4800 h of aging, compared to the other treatment times. For the most part, the contact angles of the air-stored samples were higher than those stored in the desiccator. However, just as in the case of the skived PTFE, the 30-min treatment in the desiccator resulted in more hydrophobic surfaces as compared to open air storage. B. PTFE Laminated to Copper. 1. Contact Angle. The results of contact angle measurments for PTFE laminated to copper are shown in Table V. In the shiny samples the 10-min treatment resulted in the lowest contact angle while that at 30 min resulted in the highest contact angle. In the electrodeposited samples the 30min treatment resulted in the highest contact angle while those at 3 and 10 min resulted in similar contact angle values. The contact angles of the electrodeposited samples are higher (av 50') than those of the shiny samples (av 31'). 2. SEM. The results of the SEM imaging are shown in Figures 4-8. Figure 4 shows the skived PTFE control sample at a magnification of 2400X. Representative images of the electrodeposited samples are shown in Figures 5 and 6. These surfaces are extremely rough when compared to the shiny copper laminates and the control, with many pits and ridges. Figure 5 shows the electrodeposited surface at 2780X after being exposed to the plasma for 3 min. Figure 6 is of a higher magnification (5500X) to more completely show the changes caused by 30 min of plasma exposure. Figures 7 and 8 are representative images of the PTFE samples laminated to shiny copper. The shiny laminates had considerably fewer surface features as they are a much smoother surface. Figure 7 shows the PTFE laminated to shiny copper at a magnification of 2460X after being exposed to the plasma for 10min and is typical of exposure times of 3 and 10min. Figure 8 is of similar magnification

Figure 4. Low-magnification (2400X) SEM image of untreated skived PTFE.

Figure 5. Low-magnification (2780X) SEM image of 3-min ammonia plasma treated PTFE laminated to electrodeposited copper. (2400X) and shows the surface after 30 min of plasma

exposure. Higher magnification images were obtained for the shiny laminates, but they showed little detail due to the lack of surface features. 3. XPS. The elemental compositions of the treated (polymer) side of the PTFE laminated to copper samples are shown in Table VI. The samples contained some Zn, Na, C1, and S as impurities. In the shiny samples the percentage of Cu, F, and C increased with treatment time. The F/C ratio also increased with treatment time. After 30 min of treatment the ratio was 0.50 which is consistent with PTFE. The N/C ratio was 0.27 for the 3-min treatment, decreasing to 0.13 for thd 10-min treatment, and then increasing to 0.19 for the 30-min treatment. In the electrodeposited sample, the F/C ratio increased after 10 min of plasma treatment and decreased after 30

Composite-Filled PTFE and PTFE Laminated to Copper

Langmuir, Vol. 9,No. 11,1993 3081

300.0

295.0

290.0

285.0

Binding Energy / e v

Figure 6. High-magnification (5500X) SEM image of 30-min ammonia plasma treated PTFE laminated to electrodeposited copper.

p”

Figure 7. Low-magnification (2460X) SEM image of 10-min ammonia plasma treated PTFE laminated to shiny copper.

Figure 8. Low-magnification (2400X) SEM image of 30-min ammonia plasma treated PTFE laminated to shiny copper.

min of treatment. The N/C ratio, on the other hand, did not change with treatment time. The relative percentage of C decreased with treatment time. We recognize that curve fitting XPS spectra can be quite arbitrary and that no specific curve fitting is unique. We feel, however, that using peak positions reported by other

Figure 9. C 1s core level spectrum of 3-min ammonia plasma treated PTFE laminated to shiny copper.

researchers for specific elemental environments (see references in each appropriate section), and choosing appropriate peak widths for the conditions under which the data were collected, our method of curve fitting has a strong basis in truth. The C 1s region of the XPS spectra indicated the presence of six peaks (Figure 9). These peaks are consistent with9 C-C and C-H (284.6 eV); C-N and C-CF2 (286.1 eV); C-N, C=O, and CF-C (287.4 eV); CF-CF2 and 0-C-0 (288.9 eV); CF2-CF2 (291.2 eV); and CFB (292.9 eV). Table VI1 shows the relative percentage of the deconvoluted peaks of the C 1s region. The C1 percentage decreased significantlyin both the shiny and electrodeposited samples as compared to its percentage in the untreated control sample. In the shiny sample the C1 percentage decreased significantly (average 41 % decrease) from the 3-min treatment to the 10- and 30-min treatments. No such change was noted in the electrodeposited samples however. The C2 percentage increased with treatment time for the shiny sample (+22% from 3 to 30 min) while for the electrodeposited sample it was lowest after 10 min of treatment (average 27% less than the 3- and 30-min treatments). The percentages of the C3 and C4 components varied only slightly during all treatment times for both the shiny and electrodeposited samples. There was a difference though when comparing the two sample types. The C3 component in the shiny samples was approximately 15% higher than that in the electrodeposited samples. The opposite was true for the C4 component, with the electrodeposited samples being an average of 25 % higher than those of the shiny samples. The C5 percentage was the highest in both the shiny and the electrodeposited samplefor the 10-mintreatment. This may indicate the sample was also being etched during plasma treatment. In the 3-min treatment the surface modification may have been a more important factor than any etching that had taken place up to that point. For samples treated for 30 min the etched surface may have been modified again, with the modification becoming a more important factor as compared to the etching. The trend of the C6 percentage follows that of C5, probably for the same reason. The N 1s region is fitted with three peaks (Figure 10). The peak positions are 397.3, 399.1, and 401.1 eV. The 397.3-eV peak is consistent with nitride-type nitrogen.lO (10)Taylor, J. A.; Rabalis, J. W.J . Chem. Phys. 1981, 75, 1735.

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Table VI. Relative Atomic Composition of Ammonia Plasma Treated PTFE Laminated to Shiny and to Electrodeposited Copper lamination control shiny shiny shiny electrodeposited electrodeposited electrodeposited

plasma exposure time (min) 0 3 10 30 3 10 30

Zn 3.52 0.00 0.00 0.00 0.00 0.00 0.00

Na 1.63 38.30 10.54 5.40 4.80 6.60 2.80

Cu 3.26 1.10 2.62 2.74 12.00 12.70 16.50

elemental percentage F 0 N 12.82 40.54 2.11 11.14 5.14 9.17 22.40 9.49 6.30 25.62 4.95 9.43 9.60 21.20 9.50 16.20 19.50 7.40 13.00 21.80 8.80

C 34.33 34.00 47.49 50.65 39.80 34.60 34.80

C1 0.86 0.84 0.91 1.03 2.40 2.70 1.90

S 0.93 0.30 0.24 0.18 0.60 0.40 0.30

F/Cratio 0.37 0.33 0.47 0.51 0.24 0.47 0.37

N/Cratio 0.06 0.27 0.13 0.19 0.24 0.21 0.25

Table VII. Relative Percentage of the Component Peaks of the C 1s Core Level XPS Spectra of Ammonia Plasma Treated PTFE Laminated to Shiny and to ElectrodeDosited Comer relative percentage of C Is components plasma C4 (289.7 eV) C5 (291.2 eV) C6 (293.1 eV) exposure Cl(284.6 eV) C2 (286.1 eV) C3 (287.7 eV) C=N, (C-N, C-CF2) C=O, CF-C) (CF-CF2,0=C-O) (CF2-CFz) (CFs) lamination time (min) (C-C, C-H) ~

shiny shiny shiny electrodeposited electrodeposited electrodeposited

29.6 17.1 17.9 27.2 30.4 26.6

3 10 30 3 10 30

20.8 23.2 25.3 28.8 21.1 29.4

17.6 15.2 17.1 14.2 14.5 13.6

10.6 10.2 10.4 14.3 12.3 14.8

14.9 23.0 20.4 11.0 15.4 10.3

6.6 11.4 8.9 4.6 6.3 5.3

Table VIII. Relative Percentage of the Component Peaks of the N 1s Core Level XPS Spectra of Ammonia Plasma Treated PTFE Laminated to Shiny and to Electrodeposited Copper relative percentage of N 1s components plasma exposure N2 (399.1 eV) N3 (401.4 eV) (C=NR, RsN, CmM V",44) lamination time (min) N1 (397.3 eV) (-N) shiny shiny shiny electrodeposited electrodeposited electrodeposited

14.4 17.7 5.3 11.3 22.8 25.8

3 10 30 3 10 30 1

I

404 0

400 0

396 0

B i n d i n g E n e r g y / eV

Figure 10. N 1s core level spectrum of 3-min ammonia plasma treated PTFE laminated to shiny copper.

The 399.1-eV peak is consistent with imine," amine,12 and nitrilel3 types of nitrogen. The 401.4-eV peak is consistent with ammonium14 and nitros015 functional groups. Table VI11 shows the relative percentages of the component peaks of the N 1s regions. The percentage of the nitride peak, N1, in the shiny sample goes from 17.7% after 10min of treatment of 5.3% after 30 min of treatment (11) Nefedov, V. I.; Salin, J. V.; Walther, D.; Uhlig, E.;Dinjus, E.2. Chem. 1977,17, 191. (12) Lee, T. H.; Rabalais, J. W. J . Electron Spectrosc. Relat. Phenom. 1977, 11, 123. (13) Lindberg, B. J.; Hedman, J. Chem. Scr. 1975, 7, 155. (14) Escard, J.; Mavel, G.:Guerchais, J. E.:Keraoat. - R. J.Znora. Chem. 1974, 13, 695. (15) Batich, C. D.; Donald, D. S.J . Am. Chem. SOC.1984,106,2758.

72.2 72.8 79.0 73.1 69.2 66.4

13.4 9.5 15.7 15.7 7.9 7.8

(a 70% decrease). In the electrodeposited sample N1 increased steadily with treatment time from 11.3% after 3 min of treatment to 25.8% after 30 min of treatment (a 125% increase). The N2 (-C=N) peak comprises at least two-thirds of the total area. This component increased with treatment time in the shiny samples and decreased with treatment time in the electrodeposited samples. The N3 component is 13.4% for the shiny sample after 3 min of treatment, decreasing to 9.5 % after 10min of treatment, and then increasing to 15.7% after 30 min of plasma treatment. In the electrodeposited samples, however, N3 was 15.7% after 3 min, decreasing by about half to 7.9% after 10 min and remaining the same after 30 min. The F 1s region was fitted with two peaks (Figure 11). Table IX shows the relative percentage of the component peaks. The higher binding energy peak is assigned to C-F, and the lower binding energy peak is assigned to F-.9J6J7 There is a slight difference in the binding energies of both components in the two seta of samples. In the shiny samples the higher binding energy peak is dominant. The opposite is true in the 10- and 30-min treated electrodeposited samples where the lower binding energy is the dominant peak. After 3 min of treatment of the electrodeposited sample the two component peaks were of about the same intensity. Moreover, in the electrodeposited samples, the percentage of the higher binding energy peak (F2) decreased with treatment time (by an average of 30 5% ) and that of the lower binding energy peak (Fl)increased (again by an average of 30%1. In the shiny samples, however, the F1 component nearly doubles on (16) Van Der Laan, G.; Westra, C.; Haas, C.; Sawatzky, G. A. Phys. Reo. B 1981,23,4369. (17)Nefedov,V. I.; Salyn, Y. V.; Leonhardt,G.;Scheibe,R.J.Electron Spectrosc. Relat. Phenom. 1977,10, 121.

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Langmuir, Vol. 9, No. 11, 1993 3003

Table IX. Relative Percentage of the Component Peaks of the F 1s Core Level XPS Spectra of Ammonia Plasma Treated PTFE Laminated to Shiny and to Electrodeposited Copper ~~

lamination shiny shiny shiny electrodeposited electrodeposited electrodeposited

plasma exposure time (min)

relative percentage of F 1s components F1 (shiny,685.8 eV; ED,685.2 eV) (-F) F2 (shiny,688.4 eV; ED,688.9 eV) ((2-8')

3 10 30 3 10 30

10.3 20.2 14.8 49.9 60.5 69.3 I

I

i

1

695 0

690 0 Binding E n e r g y

685 0

680 0

/ ev

Figure 11. F 1s core level spectrum of 3-min ammonia plasma treated PTFE laminated to shiny copper.

going from 3- to 10-min treatment times, and then drops by about 25 7% after 30 min of treatment. Correspondingly, the higher binding energy peak (F2) drops by about 10% on going from 3- to 10-min treatment times, and then returns to near the 3-min level after 30 min of treatment. 4. FTIR-ATR. The FTIR-ATR spectra of treated PTFE samples did not indicate any differences from untreated PTFE. This is most likely due to the modified surface layer not being thick enough to produce a detectable change in the FTIR-ATR spectra.

Discussion A. Composite-FilledPTFE. Plasma treatments using different gases resulted in a decrease in the contact angles for both of the composite-filled PTFE samples. This decrease was more dramatic after ammonia plasma treatment. The corresponding changesin the peel strength were very small for argon-, oxygen-, and nitrogen-treated samples. For RO-2800, lower contact angles resulted in higher peel strengths with the nitrogen and argon plasma treatments. However, with oxygen and ammonia plasma treatment the reverse was true. In addition, in the oxygen and ammonia treatments the peel strength was greater for 5-min exposure as compared to the 20-min exposure. Compared to the control, the ammonia plasma treatment resulted in a 73-173% increase in peel strength. Hence, the increase in performance must be associated with ammonia plasma treatment. In the case of DUROID-6002,plasma exposure resulted in a lower contact angle with all gases used. However, even though the ammonia plasma treatment for 5-min exposure had a higher contact angle, it also resulted in a greater peel strength. This may indicate that the surface wettability is not the only factor that determines peel strength. Argon plasmas primarily generate Ar* and Ar+ species that may sputter the surface, resulting in morphological

89.7 79.8 85.2 50.1 39.5 30.7

changes. Oxygen plasmas may contain 0 2 + , 0+,and 0 2 species,18and electrons. Oxygen plasmas may sputter and etch the surface. Also, oxygen plasma treatment can result in oxygenated surfaces. However, in this experiment oxygen treatment a t long periods indicated that the etching process may have been a more significant factor than the surface modification process through oxygenation. In any case, in the described process there was no improvement in peel strength in either of the composite-filled PTFE samples. Nitrogen plasmas may contain N2+, N4+, N3+, and N species.19 It has been shown that atomic nitrogen breaks up organic compounds. This process generates some cyanides and ammonia. In the nitrogen plasma treatment of PTFE surfaces, nitrogen functional groups have been generated that are consistent with amines, amides, and -C=N functional groups. Similarly, in the ammonia plasma treatment, specieslike NH3*, NH2+,and NH2 may have been generated. The dramatic increase in peel strength in both composite-filled samples may be associated with surface modification that resulted when the surface was subjected to bombardment by these species. The surface may have incorporated -NH2, -C=NH, -C=N, and amide functionalgroups on the surface. These functional groups make the surface more wettable. It has been observed that wettability increases in the following order: CF3 < CF2 < CF.19 The surface of the polymer is probably altered by a net result of substitution of F with NH2 and NH. The aging study indicated that in both skived PTFE and RO-2800 the contact angles increase with aging. However, even after 4800 h of aging the contact angles are significantly lower for the aged samples than for the untreated samples. This increase of the contact angle, indicating a hydrophobic surface, may be due to macromolecular motion that moves polar groups generated on the surface during plasma treatment away from the surface into the bulk. Lavielle et al.19 studied the interaction between acrylic acid-grafted polyethylene and water and showed that the polar groups were attracted to the surface by contact with water. Occhello et a1.20also showed that air plasma treated polypropylene, when aged in water, didnot show adecrease in contact angle. However, when the treated sample was equilibrated in air, the contact angle increased, and when it was immersed in water, the contact angle decreased to the same value as that of the equilibrated and aged underwater samples. The same kinds of macromolecular movements are probably responsible for similar behavior in our sample, resulting in subsequent changes in contact angle. The fact that open air samples are more hydrophobic may simply indicate more hydrocarbon contamination during storage. On the other hand, for the 30-min (18)Boenig, H. V. Fundamentals of Plasma Chemistry and Technology; Technomic Publishing Co., Inc.: Lancaster, 1988. (19) Laviel, L.; Schultz, J. J. Colloid Interface Sei. 1985, 106,438. (20) Occhiello, E.; Morra, M.;Morini, G.; Garbassi, F.; Humphrey, P. J. Appl. Polym. Sci. 1991, 42, 551.

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3084 Langmuir, Vol. 9, No. 11, 1993 treated samples the higher contact angle for the samples stored in a desiccator may indicate relatively more etching and hence more hydrophobic surface exposure. B. PTFE Laminated to Copper. 1. Contact Angle Measurements. The PTFE laminated to shiny copper, as one would expect, had a consistently lower (by an average of 38%) contact angle than the PTFE laminated to shiny copper, the duration of the ammonia plasma treatment has the effect of first lowering the contact angle, and then raising it. For the PTFE laminated to electrodeposited copper, there is no change until one reaches 30-min exposure, when one observes an increase in the contact angle. The increase in the contact angle observed for both samples after prolonged exposure of the ammonia plasma treatment is most likely due to degradation of the modified surface by the plasma, exposing more unmodified “bulk” PTFE. The benefita from ammonia plasma in lowering the contact angle seem to be optimized at some point between 3- and 30-min exposure time. 2. SEM. The PTFE that has been laminated to electrodeposited copper shows an extremely rough, cratered surface (Figures 5 and 6). These craters range in size from approximately 2 to 8 pm. At the base of most craters there appear to be additional small holes that form a network structure, similar to a foam. There is little observable difference between the samples exposed to the 3- and 10-min plasma exposure times. It is possible to discern under higher magnification (5500X) (Figure 6) a “softening” of the ridges between the craters upon prolonged (30 min) plasma exposure. One can clearly see the impressions of milling marks (from the shiny copper) on the PTFE samples which had been laminated to the shiny copper (Figures 7 and 8). Again, there is little change between samples of differing plasma exposure times. One feature present in the 30min exposure sample (Figure 8) that does not appear to be in either the 3- or the 10-min exposure samples is the appearance of a “fibril”structure in the deeper depressions. These features were also observed in other samples that experienced long plasma exposure times (>15 min). These structures may be due to preferential removal of an amorphous region of material.21 3. XPS. To determine the surface modification that resulted in improved peel strength after ammonia plasma treatment, PTFE laminated to shiny copper and PTFE laminated to electrodeposited copper were plasma treated and samples were analyzed with XPS. (21) Sawyer, L.C.;Grubb, D. T. Polymer Microscopy; Chapman and Halk New York, 1987; p 112.

The XPS analyses indicated the presence of component peaks consistent with oxygenated and nitrogenated functional groups. The C 1s spectra indicate C-N, C-0, C=N, C=O, and CO-NH2 groups in all the samples. The N 1s region also showed peaks that are consistent with nitrides, imines, amines, and nitroso and ammonium types of functional groups. The presence of organic and inorganic fluorine complexes was also observed. Metal fluorides could also be formed during plasma treatment. Some of the fluoride could also have been formed during the cleaning process. Hence, the surface of the treated sample is more wettable than the treated samples. 4. FTIR-ATR. To further determine the presence of amine and amide functional groups, FTIR-ATR was used. However, no IR signals were observed at 3420,3260,1640, and 1410 cm-l consistent with the antisymmetric -NH2 stretch, symmetric -NH2 stretch, NH deformation, and C-N stretch.22 This indicates that the modified surfaces were between about 40 A (the depth of analysis of XPS) and about 1000 A (the limit of depth resolution of FTIRATR).

Conclusions The effects of argon, nitrogen, oxygen, and ammonia RF plasmas on the contact angle and peel strength of ceramic-filled PTFE composites were examined. The extent of aging of ammonia plasma treated skived PTFE and ceramic-filled composite PTFE was also studied. The chemical compositions of ammonia plasma treated copper laminated PTFE films were also determined. The principal conclusions that can be drawn are the following: (1)Ammonia plasma treatment improves wettability much more as compared to oxygen, nitrogen, or argon plasma treatments. (2) Wettability is not the only factor that controls peel strength. Sample morphology may also play a significant role. (3) In the ammonia plasma treated samples the modified surface thickness was less than 100 A. (4) Ammonia plasma treated surfaces maintain low contact angles for at least 480 h. Aging of the treated surface is also affected by storage conditions.

Acknowledgment. We thank the Department of Higher Education for support of this research through the DHE High Technology grant program. (22) Collins, G. C. S.; Lowe, A. C. Eur. Polym. J . 1973, 9, 1173.