Langmuir 1989,5, 1331-1334
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the ratio of the draft length and the build-up width was one to one.
and Prof. T. Takenaka of Kyoto University Chemical Research Laboratory for their helpful comments.
Acknowledgment. We thank Managing Director Dr. H. Mizuno of Matsushita Electric Industrial Co., Ltd.,
Registry No. AD, 84882-92-8; w-TSA, 65119-95-1;Si, 744021-3; SiO,, 7631-86-9.
Depth Dependence of Alkali Etching of Poly(tetrafluoroethylene): Effect of X-ray Radiation R. R. Rye* and G . W. Arnold Sandia National Laboratories, Albuquerque, New Mexico 87185 Received March 31, 1989 Using Rutherford backscattering spectroscopy (RBS),we have shown that defluorination due to alkali etching of poly(tetrafluoroethy1ene) (PTFE or Teflon) extends to a depth of at least 3000 A. Equivalent alkali etching after irradiation with Mg Ka X-rays gives no indication of defluorination within the depth resolution of RBS (150 A); the RBS spectrum is identical with that of a reference PTFE sample. In contrast, X-ray photoelectron spectra (XPS) reveal comparable defluorination for both irradiated and nonirradiated samples. Given the sampling depth of XPS and the depth resolution of RBS, this limits the defluorination depth of the irradiated surfaces of PTFE to between 30 and 150 A. An equivalent radiation effect is seen in the depth profile of Na, the active etching agent, deliberately left on and/or in the surface by rinsing with n-hexane after etching. The fact that this radiation-modified etching behavior is known to correlate with adhesion strength suggests that the important factor for adhesion to PTFE is mechanical interlocking due to the three-dimensionalnature of the etched surface. Introduction
Chemical and physical inertness is the major characteristic of poly(tetrafluoroethy1ene) (PTFE or Teflon). As a result, adhesion to PTFE requires relatively drastic methods. The most widely used is chemical etching with sodium: either Na in liquid ammonia or as a commercially available 1:l complex of sodium and naphthalene (Tetra-Etch, W. L. Gore and Associates, Newark, DE). Such alkali etching leads to a highly porous, extensively defluorinated surface’ with the alkali attack reported to extend to depths of the order of 10 000 As2 The basis of strong adhesion to PTFE is suggested to be due to a mechanical interlocking with this highly porous surfacea3 We have recently shown that irradiation of the surface with electrons or photons has a dramatic effect on the alkali etching of PTFE and leads to a photolithographic technique for adhesion in which preirradiation of selected regions of the surface is used to tailor both the spatial extent of adhesion and the adhesion strength.'^^ As an example, Figure 1 contains a photograph of two PTFE samples which were alkali etched after irradiation of one-half of the surface with Mg Ka X-rays (1253.6 eV);l the near white region with the appearance of unetched PTFE is the irradiated half. Similar results have been obtained over a wide energy range with both electrons (500 eV < E < 3 keV)4 and soft X-rays (25 eV < hu < 1000 eV).S With electrons, the resolution (1) Rye, R. R. J.Polym. Sic.,Polym. Lett. Ed. 1988,26, 2133. (2) Benderly, A. A. J.Appl. Polym. Sci. 1962, 20, 221. (3) Rye, R. R.; Martinez, R. J . J. Appl. Polym. Sci. 1989, 37, 2529. (4) Rye, R. R. Langmuw, in press. (5) Rye, R. R.; Shinn, N. D. Langmuir, in press.
0743-7463/89/2405-1331$01.50/0
of the resulting pattern is limited to the beam diameter down to a 50-pm beam.6 In addition to this visual effect, the most dramatic result of preirradiation/etching of PTFE is a reduction in adhesion strength. For epoxy-bonded A1 test plugs, the adhesion strength for the irradiated and etched portions of the surface falls to less than 3% of the adhesion strength of the nonirradiated etched ~ u r f a c e . ~ ?For ~ ” 2-keV electrons, this maximum decrease in adhesion strength occurs for a minimum dose of 4 pA-s/cm2,a dose which corresponds to roughly 1.6 X absorbed electrons per CF, unit.4 X-rays are much less effective due to their range in PTFE, >50 000 A for Mg Ka X-rays; only the fraction absorbed in the near surface region is effective in inhibiting alkali etching. While there are major changes with radiation in the visual appearance of etched PTFE and in the resulting adhesion strength, there is no corresponding major change in the surface composition as measured by X-ray photoelectron spectroscopy (XPS). Both irradiated and nonirradiated portions of the surface in Figure 1 show a major loss of fluorine.’ The F 1s signal from the etched surface in Figure 1 decreases by a factor of =40 relative to untreated PTFE while the signal from the irradiated and then etched surface decreases by a factor of 20. The problem with the XPS measurement is that adhesion to PTFE is apparently a three-dimensional effect3 with alkali etching reported to perturb the surface region to a depth of =lo 000 A,, while XPS has a sampling depth of only =30 A. The basis of the etch inhibition is suggested to be radiation-induced cross-linkingand/or branch(6) Nelson, G.C.,unpublished results. (7) Rye, R. R.; Martinez, R. J. Mater. Res. SOC.Symp. R o c . 1988, 119. 265.
0 1989 American Chemical Society
1332 Langmuir, Vol. 5, No. 6, 1989
Rye and Arnold
POlytetrafluoroethylBne No X - r a y s
I
60 m i n
10 min
X-ray
X-ray
I Exposure
I
Rutherford Back S c a t l e i SD(1Ct,B
2 M e V He
No X - r a y s
8ec C h e m i c a l E t c h Fiyre 1. Chemically etched PTFE samples. Each sample was first exposed to Mg K a X-rays with one half covered with 0.25mm Ta, after which the entire sample was chemically etched. After being etched, the sampleswere rinsed in water and methanol. The near white portion of each sample is the irradiated area. Both Samples: 40
ing, which produces a more rigid surface resistant to deep alkali attack. Consistent with this model, the amount of low molecular weight fluorocarbon byproducts of crosslinking thermally desorbed after irrdiation is shown to increase with increasing dose' of Mg Ka X-rays. That the important feature is cross-linking atlor near the surface is seen from synchrotron studies of the photon energy dependence? As the photon energy is increased through the F 1s binding energy, there is a large increase in the absorption coefficient resulting in a concentrated energy deposition closer to the surface. Along with this, there is an increse in the fluorocarbon release rate and a decrease in the dose required to inhibit etching, showing that the important factor is the near surface component of the cross-linking distribution. Thus, there is strong evidence that this photolithographic effect resulting from preirradiation of the surface stems from cross-linking. The second part of the model, however, is that the result of cross-linking is a reduction of the etching depth for which there is currently no direct experimental evidence. In this report, we make use of Rutherford backscattering (RBS) spectroscopy to probe the depth dependence of the alkali etching of PTFE. The RBS spectra show clearly that the effect of irradiation prior to etching is to stop deep alkali attack.
Experimental Section The PTFE samples (1.2 an X 1.2 em) were cut from the same 0.79-un-thick piece of commercial PTFE. No specific pretreatment was performed othere than an ultrasonic rinse in methanol. Irradiation of PTFE prior to alkali etching was performed with the Mg K a X-ray source used previously.18 All irradiated samples were exposed for 20 min because previous work3 has shown that this dose is twice that required to obtain a maximum and constant decrease in adhesion strength. After exposure, the samples were removed from the vacuum system and chemically etched. After rinsing in water and alcohol. the samples were transferred in air to the RBS system for depth profiling. In order to probe the Na depth distribution, selected samples were rinsed in n-hexane insted of water and methanol. The RBS measurements were made with a 2-MeV He beam generated by the Sandia tandem Van de Graaff accelerator using The scattered He standard RBS technology and techniq~es.~ was detected with a silicon surface barrier detector at an angle (8) Rye, R. R.: Kelber, J. A. Appl. Surf. Sei. 1981.29.397. (9) Chu. W.K+ Mayer. J. W.;Nicolet, M.A. Bockscottoring Spec-
trometry; Academic Press:New York. 1978.
0 0
-
Reference PTFE
0
2 0 mln M g ( k d 4 0 sec C h e m i c a l E t c h
0
9 0 S c a t t e r e d He E n e r g y
(kev)
Figure 2. Rutherford backstatter spectra obtained by using 2-MeV He ions for a reference PTFE sample (tilted squares) and a sample (+) irradiated for 20 min with Mg Ka X-rays and then etched. of 164' relative to the beam direction. All measurementswere made with the He beam at normal incidence. Beam currents were
