Anal. Chem. 1991, 63, 561-568
561
Role of Electronic Particle-Surface Interactions during the Sputter Degradation of Polymers Graham J. Leggett* and John C. Vickerman The Centre for Surface and Materials Analysis, Department of Chemistry, UMIST, P.O. Box 88, Sackville Street, Manchester, United Kingdom M60 1QD
Rates of damage for polymer surfaces subjected to sputtering may be evaluated by observing the time dependence of fragment Ion signal Intensltltes, provided due care Is taken to exclude effects due to sample charglng. The fragment Ion signal time dependence has been Investigated In detall for two polymers, poly(ethylene terephthalate) (PET) and poly(tetrafluoroethylene) (PTFE). Charged and neutral primary partlcles have been compared, and the effects of prlmary particle mass have also been estimated. I n general, for PET the decay rates were much faster for bombardment wlth charged partlcles, Indicating that electronic Interactions between the surface and the bombarding particle were an important contrlbullon to sample degradation. For the heavler xenon Ions, however, mass effects were relatively more lmportant. For PTFE, the difference between damage rates for charged and neutral species was greater still. I t was observed that at low doses, much of the time-dependent behavior could be eliminated by the use of neutral primary particles.
INTRODUCTION There has recently been considerable interest in the sputtering of organic materials, particularly in relation to the surface analytical technique of static secondary ion mass spectrometry (SSIMS) (1-4). SSIMS aims to derive structural information regarding a solid surface by observing the fragment ions ejected during the low flux bombardment of a surface. The bombarding particles are either ions or neutral species. In order for the derived information to be of value in the study of, for example, a polymer, the sputtered species must be characteristic of the unmodified or virgin polymer surface. This leads to the proposal of certain criteria, which, if they are met, allow us to relate the sample surface structure to the sputtered fragment ion structures (5). Most important of these criteria is the requirement that no point on the surface be struck more than once by primary particles. This necessitates very low primary particle fluences-at most of the order of nA cm-2 (6). A stronger criterion defining static conditions relates the degree of disruption of the surface structure to the amount of energy deposited into the sample. The precise mechanisms of polyatomic secondary ion formation and emission are not well understood and have been the subject of considerable debate (7-10). However, the sputtering theory of (in particular) Sigmund has provided us with a good general picture of the sputtering event. Primary collision events give rise to a number of processes (11). Single knock-on events involve the transfer of energy from the primary particle to a target atom which, following a few further collisions, is ejected from the surface. More important is the generation of collision cascades within the upper layers of the solid. In the linear cascade regime, the spatial density of moving atoms is small, and during a spike, the spatial density of moving atoms is high. These collision cascades deposit energy into a large area of
the surface (compared to the size of an individual atomic impact site), and so, in fact, the number of primary particle impacts permissible before fragment ions are ejected from nonvirgin regions of the surface is much smaller than we might originally have expected. The area of surface disturbed by each collision event has been estimated to be lo3A2, with the consequence that the maximum dose at which static conditions are expected to pertain is estimated to be 1013 ions cm-2. Besides the dynamic effects of the collision cascades they initiate, incident charged particles are involved in a variety of electronic interactions with the surface. Electrons may be transferred from the surface to an incoming positive ion in either a resonant process or an Auger transition (12). It has generally been thought that most incident ions are neutralized before they impact with the surface. The neutralization of incident particles will disrupt the electronic structure of atoms and molecules in the surface region of an insulator, besides giving rise to sample charging. The transfer of energy from the primary particle to the target atom(s) will also result in electronic excitation of the target. The electronic processes which may occur during ion sputtering are extremely complex (13-15). For insulators, excited-state lifetimes may be quite substantial and if the lifetime of an antibonded excited state is longer than the time required for the separation of two bonded atoms, then dissociation may occur. A number of complex processes are possible, leading to fragmentation and dissociation (13-15). These considerations are of crucial importance to the SIMS user. Static SIMS studies of polymers (our specific interest here) have advanced in a fairly ad hoc manner. In order to assess the usefulness of the data obtained, however, we need to have a good estimate of the reliability of the spectra. This particularly concerns the reproducibility of the spectra. Leaving aside the question about the ion formation mechanism, we need at the very least to know to what extent our spectra reflect the chemical structure of the virgin polymer surface. At very low primary particle doses, spectra will be highly reproducible as (assuming uniformity of surface structure) primary collision events will be equivalent (we make no specification here of what constitutes “low doses”). As structural damage accumulates, Le., as total dose increases, primary collision events will no longer necessarily be equivalent. This may be expected to manifest itself in the form of time-dependent changes in the spectrum, with the spectrum becoming increasingly characteristic of a damaged surface as time proceeds. Briggs has illustrated some of these effects for poly(viny1 chloride) (PVC) and poly(methy1methacrylate) (PMMA) (16). At low doses the observed spectra were readily interpreted in terms of simple free-radical steps similar to those which occur during the thermal and radiation degradation of polymers. At high doses, however, the observed SIMS spectra were markedly different, exhibiting aromatic ions formed by the decomposition of polyene structures formed as a consequence of primary beam damage. Our objective in the present study is to investigate spectral time dependence for two polymers, poly(tetrafluoroethy1ene)
0003-2700/9 1 /0363-0561$02.50/00 1991 American Chemical Society
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ANALYTICAL CHEMISTRY, VOL. 63, NO. 6, MARCH 15, 1991
(PTFE) and poly(ethy1ene terephthalate) (PET). In an earlier work, it has already been suggested that electronic particlesurface interactions are an important factor in the degradation of polymeric materials during ion bombardment, manifested in a decreased signal half-life for polystyrene (PS) (17). We seek here to make a comparative study for PTFE and PET, comparing argon and xenon particles, both charged and uncharged, and to evaluate the differences in the damage rates observed. It is hoped that this will provide an assessment of the extent of the advantage offered by the use of FAB as opposed to ion bombardment.
EXPERIMENTAL SECTION The SIMS system used for these studies was based on a VG MM 12-12quadrupole mass analyzer. The primary beam source could be used alternatively in ion or in fast atom mode. The accelerating potential was 2 kV in both modes. A static beam was employed in both modes, incident upon the target at an angle of 4 7 O to the surface normal. Charge compensation in ion mode was achieved via a VSW EG5 electron flood gun operating with an accelerating potential of 700 V. Primary particle currents were measured by using a Keithley Model 610C electrometer. A blank metal disk is fitted to the front of the sample manipulator and may be positioned in the path of the primary beam. A bias potential of about 50 V may be applied to this disk to suppress secondary electron emission, the residual current being the primary ion current. The primary particle current may be measured to an accuracy of *I%, meaning that the total primary particle dose may be calculated to a high degree of accuracy. In atom mode, the primary beam fluence I, may be determined from the equation (provided no bias potential is applied to the target) lobs
= 71,
(1)
where Iobs is the measured target current (measured on the blank metal disk) and y is the secondary electron yield for the material. For a conducting (metal) target, y is assumed to be equal for primary ions and atoms, as the primary ions are neutralized prior to impact with the surface. Thus y may be determined from the relation Iobs =
(1 + r)Z,
(2)
where Ii is the ion beam fluence (measured with a bias potential applied) and Iobs is the measured current in ion mode with no applied bias potential. With these relationships, the primary particle flux densities were calculated. In this study, the values employed were in the range (5-15) X lo9 particles cm-* s-l in both ion and atom mode, giving a range of current equivalents (1.5-2.5) nA cm-2. An alternative method of beam calibration is to insert a metal sample and to adjust the gun emission such that the signal intensity was identical with some fixed value (say, the signal recorded by using a 1 nA cm-2 ion current) in both modes. As charging effects can be neglected for a conducting sample, equal flux densities in ion and atom mode should generate equal signal intensities. This method can also be used t o check the validity of the calibration of primary atom flux using secondary electron yields. Again, using the stainless steel stub used for current measurement, we measured signal intensities for ion and atom currents by using measurements of ion current (in ion mode) and secondary electron current (in atom mode). The secondary ion yield for 56Fe+for unit current was calculated in each case, and the agreement was found to be very good. The ratio of secondary ion yields was found to be 1.03:1.00,giving a discrepancy of less than 5% in current measurement using the two methods. The validity of the equation of secondary electron yields in the two modes is further demonstrated in ref 18.
METHODOLOGY Our approach is based upon the assumption that the composition of the static SIMS spectrum-the relative abundances of the polyatomic or fragment ions formed-will change as the surface structure is degraded. This kind of damage-related
-1 '
0
20 40 Target bias potentiaIN
60
80
Figure 1. Variation of measured current with target bias potential for a metal target (a) with an argon ion beam, (b) with an argon atom beam and the target in the vicinity of a source of low-energy electrons (in this case an ionization gauge), and (c) with an atom beam with no source of low-energy electrons.
change has been illustrated for a number of polymers by Briggs (16). For a static regime in which successive collision cascades give rise to the ejection of fragments from previously undisturbed regions of the surface, the spectrum should exhibit no time dependence. During the sputtering of polymers, two processes occur which may simultaneously affect the relative intensities of sputtered fragment ions. One is the accumulation of structural damage (of principal concern here); the other is sample charging. For our present purposes, we need to eliminate sample charging as a source of time-dependent behavior. Seah et al. have demonstrated clearly the effect upon relative polyatomic ion signal intensities of varying target bias potential (19). For a nickel film, they showed that substantial variations occurred, with the Nizf intensity being much greater than that for NiOH2+ with a target bias potential of -5.0 V, whereas the reverse was true at a target bias potential of +8.5 V. These differences in the spectrum are due to differences in the secondary ion energy distributions for the various ions. In particular, polyatomic ions exhibit much sharper energy distributions than elemental ions and thus are more sensitive to variations in the target bias potential. Their intensities decrease rapidly as the bias potential increases, whereas the intensities of the elemental ions decrease more slowly. Furthermore, there are also differences in the energy distributions for different polyatomic ions, which may peak a t different energies. For a neutral beam, charging arises due to the ejection of secondary electrons by primary particles. Surface potential accumulates until the surface potential is sufficiently large to inhibit the ejection of further electrons. The generated surface potential may be compensated for by adjustment of the target bias. Supposing some ion X has a secondary ion energy distribution /(E), the spectrometer may be tuned such that the maximum of f(E)lies within its transmission window (defined by the ion optics system, in our case, a Wittmaack box arrangement (20, 21)). Accumulation of a sample charge with a potential of A V with respect to the earth shifts the energies of the secondary ions by an amount eAV, and to compensate for this, the accelerating voltage applied to the secondary ions may be decreased by an amount AV. Thus, by judicious reoptimization of the target bias potential, the maximum of / ( E ) for X may be maintained within the quadrupole transmission window. Figure 1 illustrates results of measurements taken for the blank metal stub (stainless steel) used for current measurement in three instances. Measured current is plotted against applied target bias potential. As the (positive) target bias potential is increased, an increasingly large potential barrier is applied, preventing an increasing proportion of the secondary electrons from escaping from the surface. When d-
(Iob,)/dV = 0, the potential barrier is sufficiently large to inhibit all secondary electron emission. For the case of the ion beam only (curve a) this occurs at a target bias potential of 50 V, but at 20 V the value of Zob is already very much less than the initial value, suggesting that the energies of most of the secondary electrons are less than 20 eV. For the case of an atom beam with the target in the vicinity of a source of low-energy electrons (such as an ionization gauge, curve b), the observed current does not reach a steady value but, having passed through zero, becomes increasingly negative as the target bias potential is increased. This is due to the increased attraction of low-energy electrons to the sample that accompanies the increasing target bias potential. For an atom beam only (curve c) the behavior is quite similar to that observed for the ion beam only, except that the observed current is still negative a t large target bias potentials. This is probably a consequence of the generation of secondary electrons by primary atoms and recoiling primary atoms that strike surrounding metalwork and, in particular, the screening plate of the secondary ion energy analyzer. This helps establish the extraction potential and is negative in positive SIMS mode. From these observations we can make some predictions about the rate of charging during atom beam bombardment. With no source of additional electrons present, charging will be quite rapid during an early period until a surface potential of 10-15 V is attained, after which the rate of charging is expected to be quite slow. For P E T samples, for example, the rate of increase of the surface potential was very slow at primary atom doses of greater than 5 X 10l2atoms cm-2 and the value of the target bias potential required for optimum signal intensity varied only slightly. Consequently, the surface charge could easily by compensated for by judicious adjustment of the target bias, and a t higher doses than 5 X 10l2 particles cm-2 charging effects could be eliminated from the data. For bombardment with a positive primary ion beam charge compensated by the application of a high-energy electron flood, the situation is less straightforward. Whilst a detailed discussion is not appropriate here, it is important to note that in this instance the variation of the target bias will do more than simply compensate for sample charging. It will affect the rate of conduction of electrons through the sample and the rate of transfer of electrons from the electron beam impact site to the ion beam impact site (for charge compensation is thought principally to occur via low-energy secondary electrons generated by the impact of the high-energy primary electrons), besides affecting secondary electron yields and secondary ion energies. Consequently, we cannot simply compensate for sample charging by reoptimizing the target bias potential. Instead, we must seek an alternative solution. Experiments performed in our laboratory have shown that a certain time interval is required before the surface potential is stable during charge-compensated ion bombardment. This interval is dependent upon the number of incident positive at which a steady state charges and is defined by a dose, d, has been attained. Prior to attainment of the steady state, suitable alteration of the target bias can improve signal intensity. After attainment of the steady state, alteration of the target bias cannot improve the signal intensity any further. The surface potential is assumed to be stable. The value of d,, depends on a number of factors, including the electrical properties of the polymer under consideration, the method of sample preparation, etc. We examined two alternative polymers for surface potential stability. For PET (in the form of free-standing film samples, 100 pm thick), d, was typically (5 f 1) X 10l2ions cm-2. Consequently, at doses greater than this, changes in the spectrum may be attributed to damage-
, -m/z
207
m/z 91 n
L
? h c c W + .-c
2
1
0
dose/1013 particles cm-2 Figure 2. Variation of cluster ion signal intensitiies with primary argon ion dose for a polystyrene film sample.
n m L
k
c
c e
.-c
70
80
90
t a r g e t bias p o t e n t i a l / V
Figure 3. Secondary ion energy distributions for some polystyrene cluster ions.
related effects rather than to charging effects. For PS (cast as a film on a piece of Al), the rate of variation in the surface potential was very slow at a similar dose but in fact the surface potential continued to drift slightly thereafter, suggesting incomplete attainment of the steady state. Tuning of the spectrometer involves adjustment of the target bias potential such that the largest observed fragment ion signal intensity is maximized. In the case of PS, this ion was the m / z 207 ion. An increase in the surface potential causes an increase in the energy of the sputtered ions, necessitating a reduction in the target bias potential in order to maintain maximum intensity for the largest fragment ion. Although for PS the drift necessitated the adjustment of the target bias by only about 5 V over the course of the timedependence graph in Figure 2, this was possibly sufficient to explain the observed variations in the signal intensities (because of the sensitivity of fragment ion signal intensities to surface potential variations). Figure 3 shows the variation of fragment ion intensity with target bias potential for the PS cluster ions, showing effects equivalent to those observed as the surface potential varies. Consequently, it was decided not to proceed further with these studies for PS films: in this case we could not be sure of having eliminated effects due to surface potential fluctuations from the data. This clearly emphasizes the need for care in the interpretation of data. Suggested explanations of time-dependent behavior must be viewed with suspicion unless due attention is given to the effects of sample charging. It is also possible to circumvent the charging problem by monitoring the appearance of new peaks (characteristic of the damaged polymer) during the course of sputtering. P T F E exhibits a number of peaks separated by m / z 2 (i.e. the difference between two fluorine atoms and three carbon atoms). The lower mlz peak of each pair has a lower fluorine content and often increases dramatically with primary particle dose.
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ANALYTICAL CHEMISTRY, VOL. 63, NO. 6, MARCH 15, 1991 81
I
a
.
.
I
Ill
190
I
,
I1
I
II
m/z
290 b
-
+
2791 281
c 3
-
=I1 c
I
I
C
I
2171219
- 5 1 1
d
I
-
2791281 190
m/z
290
Figure 4. Variation of the PTFE SIMS spectrum with an argon atom atoms cm-2, (c) 6 X atoms cm-2, (b) 4 X dose of (a) 2 X atoms cm-2. atoms cm-2, and (d) 8 X
Figure 4 shows the variation in a region of the PTFE spectrum over a range of doses, from 2 x 1013particles cm-2 up to 8 x 10l3particles cm-2 during bombardment by an argon atom beam of current equivalent 1 nA cm-2. Two pairs of peaks are highlighted. At a dose of 2 X 1013 particles cm-2, the C7F7+peak ( m / z 217) is slightly smaller than the C4Fg+peak (m/z 219). Likewise, the CgFg+peak (m/z 279) is considerably smaller than the C6Fll+ peak ( m / z 281). After a dose of 4 x 1013atoms cm-2, the C7F7+peak is already bigger than the C4F9+peak. The C9Fg+peak is increased in intensity and continues to rise relative to the C6Fll+peak, equalling it at 6 X 1013particles cm-2 and being much larger a t 8 x 1013particles cm-2. The rise in intensity of the lower fluorine content peaks possibly reflects preferential sputtering of fluorine and the development of structural damage in the polymer (which could take the form of intramolecular processes, such as the formation of double bonds within the polymer chain, or alternatively could arise in the form of intermolecular processes, such as polymer chain cross-linking). The general shape of the spectrum also changes. At low doses, a smaller number of distinctive peaks are evident. As damage accumulates, the intensities of these peaks decrease and the intensities of the peaks corresponding to the defluorinated polymer increase, giving a generally less distinct spectrum. However, P T F E is known to be extremely susceptible to electron-stimulated desorption (ESD) (besides also being highly susceptible to damage by irradiation with X-rays). At the primary electron energies employed in the present study, the effects of ESD in the site of impact of the primary electron beam would be expected to be great. This has been illustrated graphically by Briggs (6) who, whilst attempting to record SIMS spectra of P T F E by using a primary ion beam with a coincident 700-eV electron beam, was able to record ESD spectra that were quite intense even in the high-mass regions. The problem was solved by moving the site of impact of the electron beam so that it was not coincident with the site of impact of the primary ion beam ( 4 ) . Under these conditions, satisfactory charge compensation may be achieved by virtue of the generation of secondary electrons of low energies in the
electron beam impact site. These secondary electrons are then attracted to the ion beam impact site to neutralize positive charge. Consequently, no ESD contribution to the SIMS spectrum is observed. ESD undoubtedly still occurs, but it does not represent a contribution to the SIMS spectrum. Neither can it contribute to the damage rate observed, for the ions observed in the SIMS spectrum are ejected from the ion beam impact site rather than from the electron beam impact site. We confirmed this by performing time-dependence studies similar to those described below. It has been suggested by Wittmaack (22) that little surface charging occurs for a polymer bombarded by electrons with energies of this magnitude. He found that conduction of the electrons through the sample was virtually unhindered. This means that an electron beam may be incident upon a polymer sample (such as a piece of PTFE) for some time without an incident positive ion beam, and little charging will occur. The effects of the electron beam upon the damage rate observed in the time dependence study may be studied by recording secondary ion signal intensities a t intervals over a long period of time. The procedure employed was to set up an ion beam with a typical current density (e.g. 1 nA cm-2)and to set the electron beam up in the same manner as would be employed in order to record time-dependence data. This involved establishing stable secondary ion signals and ensuring that there was no ESD contribution to the SIMS spectrum. A fresh sample was inserted, and then secondary ion signal intensities were recorded a t reasonably long intervals (time intervals which would, under ion bombardment, be equivalent to doses of ca. ions cm-2,up to a maximum equivalent to about 5x ions cm-2). In between, the ion beam would be blanked off. Under such conditions, there was no evidence of charging (signal intensity was instantaneously restored upon application of the ion beam) and there was no evidence whatsoever of a decline in signal intensity either. It was thus confirmed that ESD did not contribute to the observed damage rate under ion bombardment. By monitoring peak ratios for these neighboring peaks we can obtain an estimate of the relative rate of damage of the polymer under ion and atom bombardment, again free from any effects due to sample charging. So it was concluded that by observing the variations in signal intensity for PET at doses greater than 5 X 10l2particles cm-2 and for certain selected P T F E fragment ions we can monitor the rates of damage of these polymers under bombardment by primary beams of ions and atoms free from the effects of sample charge accumulation.
RESULTS P E T samples were cut from sheets of 100 I.tm thick IC1 Melinex “0” and were cleaned, prior to analysis, by treatment in ether in an ultrasonic bath for 1 h. They were attached to stainless steel sample stubs by using double-sided adhesive tape. The PTFE samples were cut from a reel of PTFE tape and were likewise adhered to the stubs by using double-sided tape. The PTFE tape, doubled over and with clean faces in contact, was pressed into place in order to ensure uniform contact between the sample and the stub. This ensured there were no air pockets trapped between the sample and the stub which might give rise to differential charging across the sample. (a) PET. The time dependence of PET was monitored with both argon and xenon primary particles in both ion and atom modes. The objective was to evaluate the contribution to damage rates of electronic particle-solid interactions; comparison was made for particles of differing masses to examine whether electronic or mass effects dominate as particle mass is increased.
ANALYTICAL CHEMISTRY, VOL. 63, NO. 6, MARCH 15, 1991
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Table I. PET Cluster Ion Identities ion,
mlt
structure +
77 91
104
0
149 1
0
2
3
193
dose / 1013 ions cmq2
O ' -CH,-CH;
lo4
decay rates. For argon at least, mass effects are clearly less important with regard to the damage rate than electronic effects. Figure 6 shows that the order of damage rates is ArO C XeO < Ar+ < Xet indicating that, provided the lower secondary ion yields may be tolerated, Aro is the bombarding particle of choice for the SIMS analyst. If we assume a simple exponential decay process where the sputter yield a t time T is given by S, = So exp(-kT)
-
'In +
c
sc 0
v) c 01 + .-c
0 0
2
1
d ~ s e / l O ' ~ a t o mcm-2 s Figure 5. Variation of signal intensity for some PET cluster ions with (a, top) primary ion dose and (b, bottom) primary atom dose.
I
.-2 1.21 -r. C
2
1.0
-
0.8
xenonatoms xenonions argon atoms
.-C C
.? 0.6
-
0.4
llj
.-
z8
0.2 0.0
0
1
2
3
~ ~ ~ ~ /particles 1 0 l cm-* 3 Figure 6. Variation of normalized signal intensitites for the m l z 104 ion observed for PET with primary particle dose.
Figure 5 shows the general form of the time dependence data obtained (a) for an argon ion beam and (b) for an argon atom beam. The data were obtained by measuring the intensities of specified ions a t time intervals of 40 s and with primary particle current equivalents between 2 and 3 nA cm-* (in both ion and atom mode). It is immediately obvious that the signal intensity decay rate is considerably greater for the argon ion beam than for the argon atom beam. For the case of the atom beam, there is an early period of rising signal intensity due to surface potential stabilization, followed by a decline in intensity as damage accumulates. For the ion beam, there is a steady decline in intensity as damage accumulates. Figure 6 shows the values normalized (to assist with comparison) to the maximum intensity observed for each primary particle together with data for xenon beams. Although secondary ion intensities for xenon particles tended to be 2-3 times higher than for the corresponding argon particles (in terms of yield per unit primary current equivalent), Figure 6 reveals that this is at the cost of an increase in the signal
then a graph of In S, against Twill have a gradient proportional to the signal decay rate k . The following data are obtained for the four particles above. Aro Xeo Art Xe' particle halI-life/1012 particles cm-2 50 26 9 6 The magnitude of the difference between Aro and Ar+ (ca. 6:l) as opposed to Xeo and Xe+ (ca. 4:l) suggests that electronic effects are (relatively) more important for argon than for the heavier xenon particles, for which mass effects are a more significant contribution to the damage process. This explains the observation by Briggs (23) that a substantial increase in polyatomic ion signal intensity could be obtained for Xet as opposed to Ar+ without a correspondingly large increase in the damage rate (although the damage rate was still increased). This consideration suggests that the charge-induced degradation which occurs during ion bombardment simply disrupts the surface chemical structure without contributing appreciably to the sputtered ion yield. These data clearly strengthen the argument in favor of the use of neutral particles and in particular show that the use of argon atoms reduces the rate of sample degradation appreciably. Thus far we have only concerned ourselves with (normalized) absolute intensities. These give us an estimate of the intensity of a spectrum (which clearly decreases as damage accumulates) but not of the reproducibility of the spectra (the degree of variability of the relative intensities of the structurally important cluster ions). One way of monitoring spectral reproducibility is to monitor the change in relative ion intensity ratios with primary particle dose. Figure 7 shows the variation of the ratios m / z 193/149 and m / z 149/91 for an argon ion beam and for an argon atom beam (secondary ion identites are shown in Table I). The m / z 193 and 149 ions are highly characteristic of PET; the m / z 91 ion (tropyllium ion) is characteristic of aromatic polymers in general. It is evident that for both sets of data, there is little difference in the decay rate of the larger fragment relative to the smaller fragment for bombardment with ions and with atoms. A slight decline is observed but the magnitude of this decline is not great considering the long time period of the experiment. Clearly, for both primary ions and atoms, fragment ion spectra will be highly reproducible for
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ANALYTICAL CHEMISTRY, VOL. 63, NO. 6, MARCH 15, 1991
1
31 argon ions argon atoms -
I
t' 0
I
I
1
2
iJ
3
dose/1013 atoms ~ m - ~
0.0 0
'
I
1
2
3
I
o
01
0.2
0.3
0.4
0.5
0.6
0.7
0.8
~ o s e / 1 0 1 3particles cm-* Figure 7. Variation of normalized intensity ratios for PET with primary particle dose (a, top) for the ratio of m l z 1931149 and (b, bottom) for the ratio of m l z 149191.
dose/i013 ions cm-2 Figure 8. Variation of some key PTFE cluster ion intensitites with primary particle dose (a, top) for argon atoms and (b, bottom) for argon ions.
P E T at doses less than 1013 particles cm-2. Experiments performed in this laboratory utilizing a tandem SIMS instrument (24, 25) have led us to believe that the processes which lead to the formation and ejection of fragment ions from P E T are quite straightforward, occurring in simple steps originating from a repeat unit of the polymer. As bombardment proceeds, the polymer chains lying in the surface become increasingly segmented. As the degree of segmentation increases, the probability of large fragments of the intact polymer chain (e.g. whole repeat units) being desorbed decreases. "Unzipping" processes are facilitated once a small degree of segmentation has occurred, and these may lead to the formation of smaller fragments at the surface. As damage increases, we expect the intensities of the larger fragment ions to decrease both relative to each other (e.g. m / z 193 relative to m / z 149) and relative to the smaller fragment The fact that the intensities of the larger ions (C4H3+,C5H5+). fragment ions relative to each other change slowly suggests that the primary reason for the decline in the signal intensities monitored in Figure 6 is the increase in the random segmentation of the polymer chains. This has the statistical effect that, for any given collision cascade, it is more likely that the polymer chain at the end of the cascade will already have been segmented and less likely that the polymer chain at the end of the cascade will be undamaged. The acceleration of the decline in the absolute intensities of the fragment ions which occurs with the ion beam reflects the increase in the rate of this random segmentation of the polymer chain which occurs when charged particles are employed. As the relative intensities of the large fragments change only slowly relative to each other (although decreasing relative to the elemental and small fragment species), we conclude that the random segmentation of the polymer chain proceeds without significant modification of the chemical structure of the remaining segments. We term this kind of degradation physical degradation in contradistinction to the chemical degradation which occurs for PTFE. (b) PTFE. We have already observed qualitatively the defluorination of P T F E in the changes which occur in the
SIMS spectrum as damage accumulates. We can investigate these effects more quantitatively, in the same fashion as for PET, by observing changes in absolute and relative cluster ion yields. Figure 8 shows the general form of PTFE time dependence for some key pairs of PTFE fragment ions for bombardment with (a) argon atoms and (b) argon ions. There is again, initially, a short period of rising signal intensity which is due to surface potential stabilization. After the maximum signal had been attained, further increases could not be achieved by the adjustment of the target bias potential, matching the criteria for a stable surface potential described above for PET. Note the dramatic differences in relative polyatomic ion intensitites for PTFE for the ion beam compared to the atom beam, despite the much lower range of primary ion dose. For ease of comparison, some normalized signal intensitites (normalization again being to the maximum intensity observered for each species with the repective primary particle) are plotted in Figure 9 for the fragment ions C3F7+( m / z 169) and C4Fg+(mlz 219). As was the case for PET, there is a substantial difference in the decay rates-greater, in fact, than was the case for PET. The slightly higher fragment ion yields obtained by using the ion beam during the early period of sputtering is illustrated for C3F,+ in Figure 10, which shows absolute intensities for ion and atom beams with an equal flux density of 1O1O particles cm-2 s-l. The important feature of these data is the maximum signal intensity. The early period of rising intensity in the graph is due to initial fluctuations in the surface potential. Relative intensity variations were monitored for three pairs of ions, each separated by mlz 2: C4F5+and C7F3+(mlz 143 and 141); C3F7+and C6F6+( m / z 169 and 167); and C4Fg+and C7F7+(mlz 219 and 217). The data obtained are plotted in Figure lla-c. These data again show the remarkable difference in the rate of change for the ratios of these ion intensitites with argon atom and argon ion beams. Even at very high doses, each ratio is within ca. 30% of its original value for atom bombardment whereas, for ion bombardment, the
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ANALYTICAL CHEMISTRY, VOL. 63, NO. 6, MARCH 15, 1991
1.2
I
1.0 E
'Mz
2r
? I
I
.-
argon atoms argon ions
i
0.8
1
0.6
-a
-$ ;5 0.4
.e
'-
e E o
z0 2
0.2
.r
o.a
n
2
1
4
3
"0
4
Dose/1013 particles cm-2
~ ~ ~ ~ /particles 1 0 l 3cm-2
-
3
2
1
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argon atoms
1
0.6
'
argon atoms argon ions
0.4
0.0
0
3
2
1
4
I
0.2
1
0
2
3
4
Dose/1013 particles cm-*
Dose/1013 particles cm-2 Flgure 9 . Variation of normalized secondary Ion yields (a, top) for C3F7+( m l z 169) and (b, bottom) for C,F9+ (mlz 219) for PTFE with argon ion and atom beams.
argon atoms ions
argon
I 1 u-
0
1
2
3
4
D ~ S ~ /particles I O ~ cm-2 ~
3
2
4
~ ~ ~ ~ particles / 1 0 l cm-2 3 Flgure 11. Variation in normalized Intensity ratios with argon ion and atom beams for (a, top) m l r 143l141 (b, middle) m l z 169/167, and (c, bottom) m l z 219l217 for PTFE.
Flgure 10. Comparison of absolute signal Intensities observed for C3F7+( m l z 169) for PTFE with argon ion and atom beams.
ratio is markedly different at an early stage. Examination of the variation in the intensity ratio over a restricted dose range (less than l O I 3 particles cm-2)-see Figure 12 for m / z 143/ 141-reveals that, whilst, for the ion beam, the value of the ratio falls markedly (from 1 to ca. 0.3), for the atom beam, it remains nearly constant. Chemical degradation is (as may be seen from Figure 4) a key contribution to spectral time dependence for PTFE. It is not easy to speculate as to the exact mechanism of this degradation, but it clearly involves the preferential sputtering of fluorine. The massive yield of F- observed for PTFE in the negative ion mode, combined with the very high yield of CF3+in positive ion mode, might at first sight seem a possible factor, but the similarity of the relative intensitites in ion and atom mode at low doses suggests that ion stabilities are similar in both modes. A more likely explanation would be in terms of surface degradation steps which do not contribute to the SIMS spectrum (there is no increase in secondary ion signal in proportion to the increase in damage rate on switching from atom to ion mode). The neutral species sputtered will be predominantly the stoichiometric CFzospecies (25),but other species are formed. One other common loss is the fluorine atom. We have observed that the formation of ions during
' argon atoms argon ions
1
"0 ~ 0 ~ 1 0 particles 1 3 cm-2
Flgure 12. Variation in m l z 1431141 signal intensity ratio at low particle doses.
collisionally activated dissociation (CAD) in a tandem SIMS instrument via the loss of Fois more favored (relative to the loss of CFzO)when the collision target is xenon rather than argon (26). Thus we conclude that the activation energy for the loss of Fo is high relative to that for the loss of CFzO. If the surface-particle electron transfer left the polymer in an excited electronic state, i t is possible that the elimination of fluorine atoms might become relatively more favored during sputtering (compared to the situation pertaining to the impact of a particle with the polymer in its electronic ground state)
Anal. Chem. 1991, 63,568-574
560
and consequently that this process (which may involve the formation of carbon-carbon double bonds or of cross-linking between the polymer chains) or a similar one might be more common in the case of ion bombardment than in the case of atom bombardment. The exaggerated difference in time dependence behavior for ion beams, as compared to atom beams, observed for PTFE, as compared to PET, certainly suggests that particle-surface electronic interactions are a major contributor to this preferential sputtering of fluorine.
CONCLUSIONS Comparison of spectral time-dependence behavior for P E T and PTFE has provided data concerning the nature of polymer degradation during particle bombardment. In particular, electronic particle-surface interactions have been shown to be a key contributing factor to degradation. In PET, this degradation is by straightforward physical degradation (random segmentation of polymer chains), which is accelerated by the use of charged primary particles. For PTFE, in addition to this process, an additional chemical degradation mechanism operates, which is, to a large extent, the consequence of electronic particle-surface interactions. This involves the preferential sputtering of fluorine and is manifested by the observation of a rise in peaks with a low fluorine to carbon ratio. At doses less than 1013 particles cmV2,this degradation mechanism may be nearly eliminated by the use of primary neutral particles rather than charged particles. Registry No. PTFE, 9002-84-0; PET, 25038-59-9;Ar, 744037-1; polystyrene, 9003-53-6; xenon, 7440-63-3.
LITERATURE CITED (1) Benninghoven, Alfred. I n Ion Formation from Organic SolMs: Benninghoven, Alfred, ed.; Springer-Veriag: Berlin, 1983: pp 64-89. (2) Fenseiau, Catherine. Reference 1, pp 90-100.
(3) Benninghoven, Alfred. J. Vac. Sci. Techno/. 1985. A3, 451-460. (4) Briggs, David. B r . PolymerJ. 1889, 27,3-15. (5) Hearn, Martin J.; Briggs, David. Surf. Interface Anal. 1988, 7 7 , 198-2 13. (6) Briggs, David. Surf. Interface Anal. 1882, 4, 109-115. (7) Kidweil, David, A.; Ross, Mark, M.;Coiton, Richard, J. Int. J . Mass Spectrom. Ion Processes 1987, 78, 315-328. (8) Pachuta, Steven, J.; Cooks, R. Graham. Chem. Rev. 1887, 647-669. (9) Yu, Ming, L. Nucl. Instrum. Methods 1887, 8 7 8 , 542-548. (IO) King, B. V.; Tsong, I . S.T.: Lin, S. H. Int. J . Mass Spectrom. Ion Processes 1987, 78, 341-356. (11) Sigmund, Peter. I n Sputtering by Particle Bombardment I ; Behrisch, Rainer, Ed.; Springer-Verlag: Berlin, 1981; Chapter 2. (12) Murray, P. Terrence; Rabaiais, J. Wayne. J. Am. Chem. Soc. 1981, 703, 1007-1013. (13) Toik, N. H., Traum. M. M., Tully, J. C., Madey, T. E., Eds. Desorption Induced by Electronic Interactions DIET I ; Srpinger-Veriag: Berlin, 1983. (14) Brenig, W., Menzel, D., Eds. Desorption Induced by Electronic Transitions DIET I I ; Springer-Veriag: Berlin, 1985. (15) Avouris, Phaedon; Walkup, Robert E. Annu. Rev. Phys. Chem. 1988, 4 0 , 173-206. (16) Briggs, David; Hearn, Martin, J. Vacuum 1986, 36, 1005-1010. (17) Brown, Alan: van den Berg, Jaap A.: Vickerman, John C. Spectrochim. Acta 1985, 408, 871-877. (18) Eccles, A. John; van den Berg, Jaap A,; Brown, Alan: Vickerman, John C. Appl. Phys. Lett. 1886, 49, 188-190. (19) Hunt, C. P.; Stoddart. C. T. H.: Seah, M. P. Surf. Interface Anal. 1882, 703, 157-160. (20) Wittmaack, K.; Maul, J.; Schultz, F. Int. J . Mass Spectrom. Ion Phys. 1973, 7 7 , 23-35. (21) Wittmaack, K. Vacuum 1982, 32, 65-89. (22) Wittmaack, K. Surf. Interface Anal. 1987, 70,311. (23) Briggs, David; Hearn, Martin J. Int. J . Mass Spectrum. Ion Processes 1985, 6 7 , 47-56. (24) Leggett, Graham J.; Vickerman, John C.; Briggs, David. Unpublished work (UMIST, 1990). (25) Leggett, Graham J.: Briggs, David: Vickerman. John C. J. Chem. Soc., Faraday Trans. 1980, 8 6 , 1863-1872. (26) Leggett, Graham J. PhD. Thesis, UMIST, 1990.
RECEIVED for review August 27, 1990. Accepted December 7,1990. The financial support of the Science and Engineering Research Council is gratefully acknowledged.
Thermal Gradient Microbore Liquid Chromatography with Dual-Wavelength Absorbance Detection Curtiss N. Renn and Robert E. Synovec* Department of Chemistry, BG-IO, Center for Process Analytical Chemistry, University of Washington, Seattle, Washington 98195
Theoretical relationships are derived relating changes in the refractive index of the mobile phase in liquid chromatography to aperture limited absorbance measurements as applled to a single fiber-optic two-wavelength detector. The detection system was designed for remote sensing in thermal gradient microbore liquid chromatography (TG-pLC). TG-pLC was demonstrated with a reversed-phase separation of an unleaded gasoline sample for a temperature gradient of 25-150 OC over 30 min. The unique two-wavelength difference detection method, along with the single fiber-optic construction, virtually eliminated baseline drift associated with thermal Induced refractlve index ( R I ) aberrations. The detector provides a solution to R I aberrations not only for TEMLC but also for mobile-phase gradient liquid chromatography (MPG-LC) and other flow methods such as flow injection analyds (FIA). The advantages of TG-pLC are presented, including gradient separation capability for MLC, effective control of retention tlme comparable to MPG-LC, and separation efflclency over 72 000 theoretical plates/m using 5-pm packing material. 0003-2700/9 110363-0568$02.50/0
INTRODUCTION From the inception of modern liquid chromatography, UV-vis absorbance detection has played a fundamental role as a diagnostic tool to provide qualitative and quantitative information of complex mixtures ( I ) . The development of new separation techniques, however, has placed additional demands on absorbance detection that have not been previously solved. Mobile-phase gradient liquid chromatography (MPG-LC) ( 2 ) ,supercritical fluid chromatography (SFC) (3), and the more recent technique of thermal gradient LC (TGLC) have placed an additional burden on absorbance detection as a result of large refractive index changes of the mobile phase, interfering with aperture-limited absorbance measurements. To further hamper absorbance detector design, the trend toward microbore LC (pLC) has necessitated small volume flow cells, in the sub to low pL range, in order to preserve the integrity of the chromatographic separation ( 4 ) . Attempts have been made to reduce thc refractive index dependence of absorbance detectors by focusing the light past 0 1991 American Chemical Society
