Anal. Chem. 1980, 52, 1467-1473 (12) (13) (14) (15) (16) (17) (18) (19) (20) (21) (22) (23)
J. A. Nelder and R. Mead, Comput. J., 7, 308 (1965). S. N. Deming and S. L. Morgan, Anal. Chem., 45, 278 (1973). S. L. Morgan and S. N. Deming. Anal. Chem.. 48, 1170 (1974). M. W. Routh, P. A. Swartz, and M. B. Denton, Anal. Chem., 49, 1422 (1977). M. J. S. Dewar, and P. J. Student, Quantum Chemistry Program Exchange. Bloomington. Ind. Program No. 228. See S. N. Deming, S. L. Morgan, and M. R. Willcott, Am. Lab., 8, 13 (1976), and references therein. R. R. Ernst, Rev. Sci. Instrum., 39, 998 (1968). G. L. RMer. S.R. Lowry, C. L. Wikins, and T. L. Isenhour, Anal. Chem., 47, 1951 (1975). T. F. Lam, C. L. Wilkins, T. R. Brenner, L. J. SoRzberg, and S. L. Kaberline. Anal. Chem., 48, 1768 (1976). See also: P. C. Jurs and T. L. Isenhour, “Chemical Applications of Pattern Recognition”, John Wiley and Sons, New York, 1975. C. L. Wilkins and S. L. Kaberiine, Department of Chemistry, University of Nebraska, Lincoln, Neb. 68588. private communication, 1979. P. R. Adby and M. A. H. Dempster, “Introduction to Optimization Methods”, Chapman and Hall, London, 1974.
1467
(24) J. Kowalik and M. R. Osborne, “Methods for Unconstrained Optimization Problems”, Elsevier, New York, 1968. (25) M. J. D. Poweli. ACM Trans. Math. Software, 1, 97 (1975). (26) W. T. Fishback, “Projective and Euclidean Geometry”, 2nd ed., John Wiley and Sons, New York, 1969. (27) M. Abramowitz and I.Stegun, “Handbook of Mathematical Functions”, Dover Publishing, New York, 1965. (28) H. Rosenbrock, Cotnput. J. 3, 175 (1980). (29) K. E. Hillstrom, ACM Trans. Math. Software, 3 , 305 (1977). (30) T. Umeda and A. Ichikawa, Id. Erg. Chem.. Process Des. &vel., 10, 229 (1971). (31) M. B. Denton. Department of Chemistry, University of Arizona, Tucson, Ariz. 85721, private communication, 1979.
RECEIVED for review June 8, 1978. Resubmitted March 24, 1980. Accepted April 10, 1980. One of us (R.L.B.) thanks Uniroyal, Inc., for a summer research grant.
Measurement of Sputtering Yields and Ion Beam Damage to Organic Thin Films with the Quartz Crystal Microbalance Dale M. Ullevig and John F. Evans* Department of Chemistry, University of Minnesota, Minneapolis, Minnesota 55455
An apparatus Is described for use In the measurement of Sputtering yields of organic and organometallic thln films. The experimental arrangement Involves the use of a dip-coated quartz crystal microbalance as the target for an Ion beam. The apparatus Is first tested uslng sliver fllms deposited on the quartz crystal, and then extended to measurements on organic overlayers In the 3000-7000 A thickness reghe. Inltlai results for Sputtering of thln fllms of polystyrene and polymethylmethacrylate using 1-KeV argon Ions Indicate that the sputter rate lnitlally decreases In an exponential fashion and uttlmatety becomes constant. These long time sputtering ylelds are distlnctly dlfferent for the two polymers examined, atthough the rates of approach to these values are very slmllar. The resuits are Interpreted to be lndlcatlve of a common damage cross section for the collision of argon Ions with these polymer films of vastly different lnitlal structures.
Recently there has been a growing interest in thin and ultrathin organic and organometallic films in applications ranging from the modification of electrode materials (1-11) to the preparation of chromatographic materials (12-1 7). In virtually all of these systems, regardless of whether the modifier layer is covalently bonded to the substrate material or simply adsorbed onto it, a knowledge of the thickness of the overlayer is useful in understanding the performance characteristics and/or limitations of the composite thin fibs. Conventional thin film analysis using an ion beam for the controlled removal of surface material with concurrent or subsequent analysis using surface sensitive probes, such as X-ray photoelectron spectroscopy (XPS), Auger electron spectroscopy (AES),and/or secondary ion mass spectrometry (SIMS), would be a convenient method to ascertain organic film thicknesses. However, before such an approach can be successfully applied, it is essential that the sputtering behavior 0003-2700/80/0352-1467$01 .OO/O
of organic materials be understood on a quantitative basis. It is known ( 1 4 1 9 )that ion bombardment of organic materials can lead to structural changes on the molecular level (new bonding arrangements brought about by bombardment-induced chemistry in the solid state), and such processes could in turn lead to changes in the rate a t which surface material is removed. Recent studies involving thin film analyses (using SIMS and AES detection) of organosilane polymer layers covalently attached to conductive carbon and metal oxide substrates (20) have indicated that there are significant nonidealities in the sputtering behavior of 200-500 A thick siloxane layers. The interpretation of nonideal behavior in the case of SIMS detection is complicated not only by the possibility of variations in the rate of removal of the organosilane surface layers but also by the fact that ion yields may vary as rearrangement in the film or damage to the film occurs. The initial results reported here deal with the former problem, that of ascertaining the sputter yields and variations in these for two distinctly different polymer overlayers. The measurements are made using a quartz crystal microbalance (QCM), coated with the organic material of interest, as the target for sputtering studies. The use of QCM to determine sputtering yields is not novel (21-24). However, the technique has not been previously applied to the study of the sputtering process associated with ion bombardment of organic materials. We wish to emphasize that our use of the term “sputtering” in the case of organic polymers is not meant to imply that we view the removal of organic material by ion bombardment to be mechanistically analogous to the sputtering of metals, metal halides, or metal oxides (25-28).Rather, we use the term to include all processes which lead to loss of material from the organic surface as either direct or indirect consequences of ions colliding with these surfaces. In the case of organic materials, these processes include what may be viewed as conventional sputtering (direct momentum transfer from the primary ion), volatilization due 0 1980 American Chemical Society
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ANALYTICAL CHEMISTRY, VOL. 52, NO. 9, AUGUST 1980
to local heating, and depolymerization due to secondary photons and/or electrons which result from the collision process. The QCM used here is part of a multitechnique surface analysis system which includes XPS, AES, and SIMS capabilities. This combination allows for the evaluation of mass loss phenomena induced by either ion, electron, or X-ray photon bombardment as well as in-situ analysis of the effects of each on surface composition. While structural changes which lead to mass loss induced by X-ray irradiation or electron bombardment (29, 30) will not be dealt with here in an extensive fashion, we wish to point out the general utility of the QCM in surface analytical work.
EXPERIMENTAL Vacuum System. The ultrahigh vacuum system used in this work is an ion pumped commercial XPS/AES/SIMS spectrometer (Physical Electronics model 548) fitted with a rapid introduction system (Physical Electronics model 2100) with auxiliary turbomolecular pumping. For the sputtering studies reported here, a special rod/sample mount system (see below), designed to accommodate the mounting of the QCM crystals, was used in place of the standard ambient temperature insertion rod. The Pa. system base pressure (analytical chamber) was 5 5 x The ion gun employed is a Physical Electronics model 04-161 which has been modified to reduce the spot size of the ion beam at the sample. This is achieved by changing the exit aperture of the gun to one which has a 0.32-cm diameter orifice. Sputtering experiments have been carried out using this gun in a backfilled mode with research grade argon (Air Products) at pressures Pa. In all cases, an acceleration ranging from 1.3 to 6.7 X potential of 1 kV was used. The corresponding current densities for these conditions were in the range 2 to 8 X lo* A-cm-*. All sputtering yield measurements were made under conditions where the positioning of the QCM relative to the ion beam was such that the maximum in the ion current distribution function (position of highest ion flux) was coincident with the center of the QCM (position of highest mass sensitivity). Both the ion beam current intensity distribution function (ion beam cross section) and the QCM mass sensitivity function were assumed to be cylindrically symmetric about the point of coincidence defined above in all planes parallel to and including the face of the QCM. The calculation of mean ion current density was made by first determining the ion beam current distribution function using a Faraday cup mounted on an X-Y-Z manipulator and a picoammeter, followed by numerical integration of this function and normalization to the act've area of the QCM. Sputtering yield correction factors were similarly calculated using these radial ion distribution functions. However, in these cases the nonuniformity of the ion current distribution function and the radial sensitivity function (see below) were convolved numerically. The actual size and shape of the ion beam cross section could be varied by adjustment of the potential applied to the focusing element of the ion gun. A focusing potential of 750 V (defocused ion beam) was found to give an ion beam FWHM of 0.74 to 1.04 cm depending on the argon pressure. At a focusing potential of 840 V (focused ion beam), the ion beam cross section was found to be much more Gaussian in shape and had a FWHM of 0.22 cm. The ion beam distribution function was found to be affected by argon pressure variation to a negligible extent in the case of a focused ion beam. For sputtering yield measurements, a defocused ion beam was used since this more closely approximates the ideal situation of a constant ion beam flux across the QCM surface. This minimizes errors introduced by the above mentioned correction. For determination of the form of the QCM mass sensitivity function a focused ion beam was used. In both cases, variation of either the argon pressure, the ion gun filament emission current, or both allowed for variation in the intensities of ions impinging on the QCM targets. Charge neutralization was not used in the sputtering experiments carried out on polymer films. It was found that slight (ca. 1.0 eV) charge-induced shifting was evident in XPS spectra of the polymer films and, consequently, the possibility of charging during ion bombardment cannot be entirely ruled out. Future experiments will be carried out using a flood gun type charge
E TCP
V'C
h
Figure 1. Diagam of the QCM insertion rod assemb : (A) quartz crystal ( - 0.5 mm thick), (8) silver electrodes (- 2000 thick) over which polymer layer is applied, (C) rapid insertion rod, (D)stainless steel outer sleeve (2.2-cm diameter), (E) mounting rods for attachment of D to C, (F) M a m ceramic insert to electrically isolate the rear face of the crystal, held in D by set screws (not shown), (G) spring clips for positioning of the crystal and electrical contact to the front face, (H) gokl leaf contact to the rear face of the crystal, (I) ultrahigh vacuum electrical feedthrough connected to H and to a BNC connector on the outboard end of the insertion rod
f
OSCILLATOR CIRCUI-
-
e -
1 FREQUENCY METER
-
3lGll
STRIP C H A R T R ECOROER
Flgue 2. Block diagam of the measurement system. The front (lower) face of the QCM, that pesented to the ion beam, is connected to gourd for sputter rate measurements or to a picoammeter for measurement of the ion beam current. Each line between the frequency meter, the digit selection circuit, and the DAC represents 4 bits (one digit) of BCD code
neutralizer to ascertain the extent to which charging may influence the sputtering of thin polymer films. The geometry of the various sources and detectors in the vacuum system was defined by the port arrangement on the Physical Electronics model U-44UE bell jar. The critical consideration here is the angle of incidence of the ion beam on the face of the QCM. This angle is 30.3O relative to the surface normal. The QCM crystals which serve as targets for ion bombardment are mounted on a special holder attached to a standard size insertion rod as shown in Figure 1. Contact to the outer face of the QCM (which is exposed to the ion beam) is made through two spring clips, so that the outer face of the QCM is at the same potential as the insertion rod body which is held at ground potential during sputtering. As such, target current measurement can be made by connection of a picoammeter (Keithley model 415) to the rod since it is electrically isolated from the rest of the bell jar by the Teflon seals present in the insertion system. Electrical contact to the inner face of the QCM ia made through a small piece of gold leaf via an ultrahigh vacuum feedthrough welded in the center of the end of the insertion rod. QCM Electronics. The oscillator circuit used to drive the QCM (31)is mounted directly at the end of the insertion rod on the atmospheric side. The frequency is measured using a Fluke model 1953A counter-timer equipped with BCD outputa aa shown in the block diagram of Figure 2. The eight digits of BCD data are output to a digital circuit allowing three digits of the desired significance to be fed into a digital to analog converter (Burr Brown model DAC80),the output of which is presented to a strip chart recorder (Heath-Schlumbergermodel SR-204). The overall
ANALYTICAL CHEMISTRY, VOL. 52, NO. 9, AUGUST 1980
sensitivity to mass change is limited by the 0.1-Hz sensitivity of the counter-timer (10-s gating) and corresponds to a 3 X 10.' g mass loss or gain. QCM Crystals. The crystals used in this work were 1.3-cm diameter A T cut planar quartz having a nominal oscillating frequency of 5 MHz (Sloan Technology Inc.). As received, these crystals have had a circular silver electrode vapor deposited on each face. Following use in sputtering experiments, the electrodes are stripped from the crystal using aqua regia and new electrodes (99.9% silver) are vapor deposited to a thickness of ca. 2000 A. These films are then vacuum annealed at 100 O C for 8 h after which they are slowly cooled to room temperature over a 12-h period. (Best results are obtained when the crystals remain in the vacuum during the cooling period.) This procedure has been found to yield crystals with much more stable frequencies than those obtained without annealing or annealing without vacuum (32). No difference in behavior was noted between "as received" crystals and those prepared as described above. Prior to use, the crystals were cleaned with spectroscopic grade acetone followed by spectroscopic grade methanol. In nearly all cases, including experiments where the QCM had been coated with a polymeric film, a very slight frequency increase was noted when the QCM was first placed in the vacuum system ( - 5 X lo-' Pa). This corresponds t o a mass loss of approximately 5-10 X lo4 g which is attributed t o the desorption of gas from the silver electrode of the QCM or to the evaporation of residual solvent in the case of polymer film samples. The oscillator frequency was allowed to stabilize at this pressure for at least 10 min before backfilling with argon and proceeding with a sputtering experiment. No frequency shifts were observed upon backfilling to the desired pressure. Chemicals and Coating Procedures. Polystyrene (PS) and polymethylmethacrylate (PMMA) were obtained from Monomer Polymer Labs (Philadelphia, Pa.) as linear polymers of nominal molecular weights 2.3 X lo5 and 1.0 X lo6 g-mol-', respectively. These polymers were dissolved in spectroscopic grade chloroform for dip coating onto the QCM crystals. Concentrations were 4 X g-mI-' and 2 X lo-' g-mL-' for PS and PMMA, respectively. The QCMs to be coated were dipped into the appropriate solution, withdrawn, and allowed to air dry, giving coatings of approximately 7000 A for PS and 3200 A for PMMA as calculated using the frequency difference between the loaded and unloaded QCM, and assuming bulk densities. THEORY The basic principles of the operation of the QCM were first investigated by Sauerbrey (33),and have been reviewed elsewhere (34). Consequently, only a brief presentation of the principles will be given here. The microbalance consists of a crystalline quartz wafer, the faces of which have been coated with a conductor. An oscillator circuit ( 3 1 , 3 5 )is connected across the wafer and the resonant frequency is monitored. The frequency responds to an added (or subtracted) mass by the following equation:
where Am is the mass change (g), A is the active area of the crystal (cm2)defined by the projected overlap of the exciting electrodes, p is the density of quartz (2.65 g - ~ m - ~A)f ,is the change in frequency associated with a change in mass Am, f is the resonant frequency of the quartz crystal and N is the frequency constant of quartz (1.67 X lo5 Hz-cm). Two subtleties of Equation 1 should be noted. First, it is assumed that the measured frequency change accompanying a mass change is small compared to the initial frequency of the system. Secondly, although it appears that C embodies all of the constants of the particular QCM being utilized, and therefore would seem t o be a constant sensitivity factor, such is not necessarily the case. Sauerbrey's derivation (33) of Equation 1 assumes that the mass change per unit area is constant over all active regions. Only under these conditions may Equation 1 be used. In cases where the mass change may vary as a
1469
function of radial position on the QCM surface, consideration must be given to the fact that C in Equation 1 is an integral sensitivity function. In terms of cylindrical coordinates, C may be defined by Equation 2:
C=
1'"1"S(r,e)r 0
0
drd8
where r, is the radius of the circular active area of the QCM and S(r,B) may be considered to be an areal sensitivity function (Hz-g-'-cm-*). Such considerations become important in the present work because of the radial inhomogeneity in the ion beam which in turn leads t o an inhomogeneity in the mass loss across the active area of the QCM. For a time-dependent mass loss measurement, Equation 1 becomes: Am =
_ .
At
Af _-1 --
c
At
(3)
where A t is the gating period (s) for the frequency measurement. Rigorously speaking, the usual definition of sputtering yields for elemental targets (atoms sputtered per incident ion) cannot be applied to the sputtering of a molecular material such as a polymer, except in cases where the identities of all sputtered particles as well as their distributions are known. Since, in the present case, such information is unavailable, we must define sputtering yields for polymers in terms of mass loss per incident ion, Y,, and we distinguish it from the conventional sputtering yield of atoms per incident ion, Y,. T o determine these quantities using the QCM, it is essential t o have a knowledge of the ion fluence or the arrival rate of bombarding ions, R, during the sputtering experiment. This quantity can be calculated from the current density of the ion beam, j , according to:
(4) where j has units of A-cm-2, q is the charge on the incident ion, No is Avogadro's number, and F is the Faraday. Under the assumption that the current density is relatively uniform over the active area of the QCM crystal ( 2 4 ) , the integral sensitivity factor of Equations 1 and 3 may be used to calculate Y , according to:
(5) The validity of this assumption will be addressed in the Results and Discussion section below. For the sputtering yields measured for silver films, the conventional atomic sputtering yield, Y,, may be used since this is a pure elemental film. In terms of Y,, Y , is given by:
YmN0 Y, = W where W is the gram atomic weight of the element comprising the film. R E S U L T S AND DISCUSSION The sputtering yield of silver films was determined using the present experimental configuration both to give a relative calibration and to evaluate whether or not anticipated artifacts such as heating of the QCM by the ion beam or the differential radial sensitivity of the QCM would compromise the measurements to be made on organic films. Each of these considerations will be addressed in turn. T h e r m a l Effects. It is well known that the resonant frequency of a quartz crystal oscillator is dependent upon its
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ANALYTICAL CHEMISTRY, VOL. 52, NO. 9, AUGUST 1980 *O
I
0
b
ION PUMP STARTED
IO
5
IS
20
25
30
35
40
45
T I M E (MIN.)
Flaure 3. Test for thermal freauency shifts induced by ion bombardment. The ion beam current density was 7.2 X lo-' A-cm-'; 61 s correspond tosix gating periods of the Fluke frequency meter
temperature (34). These thermal effects are minimized through the use of AT cut crystals which have little frequency dependence on temperature over an approximately 30 O C temperature range about ambient (20 "C) temperature. The possibility that ion bombardment a t the current densities employed here could induce a sufficient degree of heating of the crystal to cause a thermal frequency shift convolved with that due to mass loss was still of fundamental concern. T o determine whether or not this was the case, a silver coated crystal was sputtered for 10 min a t which point the argon was pumped from the vacuum chamber and the ion beam turned off. These results are shown in Figure 3. During the 40-min period following the evacuation of the chamber, the frequency of the QCM was continually monitored, and showed no measurable frequency shift. Therefore, we conclude that there are no thermal artifacts present in the system. At these current densities, the thermal conductivity of the attachment clips is apparently sufficient to sink any heat generated by the ion beam into the insertion rod. Radial Sensitivity of t h e QCM. Several authors have reported data which indicates that the mass sensitivity of a QCM is a function of radial distance from the center of a quartz crystal of round cross section (24,33). We have used a focused condition of the ion beam to reinvestigate this phenomenon in an attempt to ascertain its degree of significance in our system. A silver coated QCM target was moved relative to the stationary ion beam such that the ion beam traversed the diameter of the QCM. During this operation, the frequency shift per unit time was recorded as a function of distance from the center of the crystal. The results, presented in Figure 4, are found to be in good agreement with the masking studies of both Sauerbrey (33) and Blank and Wittmaack (24). From this plot, it is obvious that the sensitivity is a very strong function of position. This emphasizes the advantages of using a defocused ion beam of well-defined geometry. The most intense part of the ion beam should be placed at the center of the QCM face (the most sensitive point on the QCM) if errors in yield measurements due to differential radial sensitivity are to be minimized. From Figure 4, it should also be noted that the active area extends to the edge of the crystals. This is not surprising since the circular silver electrode covers nearly the entire face of the crystals. It is important to realize that the location of the mounting clips, required for attachment and electrical contact to the front face of the crystal, must be at the outside edges of the device where sensitivity is lowest. This ensures that
ION 0 E A M FWHM
0 22 c m
-
I
2 RADIAL
3 POSITION
4
5
6
(cm)
Flgure 4. Plot of
the radial sensitivity across the face of the QCM. An ion beam current density of 8.7 X lo-' A-cm-' was used. The position given is that of the beam center with respect to the crystal center. (cf., Sauerbrey (33)and Blank and Wittmaack ( 2 4 ) )
mechanical loading and stress are placed at the least sensitive points on the QCM face. Sputtering Yield for Silver. For silver coated QCMs the relationship between moles of silver sputtered per second and the ion beam target current, jA was found to be linear over the range of 0.3 to 0.9 X lo4 A. Three crystals were used for 28 separate determinations. At 95% confidence a value for Y , of 6.44 (&0.21) silver atoms sputtered per incident argon ion was found. When corrected for the angular dependence of the sputtering yield (36)to give a value for normal incidence of the ion beam, and also corrected for the inhomogeneity in the ion beam cross section this value becomes 4.03 (*0.13) silver atoms sputtered per incident argon ion. This value agrees remarkably well with the value of 3.8 reported by Wehner (37) for analogous sputtering conditions (1keV argon a t normal incidence) which was obtained by conventional
ANALYTICAL CHEMISTRY, VOL. 52, NO. 9, AUGUST 1980
6'48
1471
1 SPUTTERING OF DIP-COATED PS
\\
4.32
-
T
E
0
-\
rn
N
-
0
-
\
B
.(
*E 2.16
/
n
/ v
SF-----+ I
0
#
8
0
24
16
32
ION
DOSE
40
48
56
C
,
187
195
m3
( r1010eq.)
Flgure 5. Sputtering behavior of PS as a function of ion dose. Curve A shows the mass yield Y,. Curve B shows the logarithmic approach of Y, to a steady-state value Ym,ss,The slope of the linear regression line is 3.2 X 10' equlv-'. An Ion beam current density of 5.5 X lod A-cm-' was used C ( I s 1 X P S OF PS
n' SHAKE UP
INITIAL
'lo
,
,
'
I
I
295 BINDING
289 ENERGY, aV
,
I
2 83
Flgure 6. Carbon (1s)XPS spectra of PS before sputtering (upper) and following attainment of a steady state value of Y , (lower) weighing techniques applied before and after prolonged ion bombardment. We suspect that our value may be higher due
to the fact that prolonged sputtering of silver can lead to severe cone formation (38)which would reduce the effective sputtering yield. Such an artifact would have a more pronounced effect on the experimentally determined sputtering yield when larger ion doses (ions-cm-*)are required to remove a measurable mass of material. With the QCM technique employed here, the ion doses required are orders of magnitude lower than those required using conventional mass measurement techniques (e.g., mechanical or electrobalance measurements). Sputtering Yield for Linear Polystyrene (PS). Exemplary of the nonideality in polymer sputtering behavior is the data of Figure 5, curve A for PS where the mass sputtering yield, Y,, is plotted as a function of ion dose. Although there is a rapid decrease in Y,, it should be noted that ultimately a steady-state value is observed. In all samples examined, this behavior was found. It is emphasized that no silver SIMS or XPS signal could be detected either during or after this type of experiment. Therefore, the observed trend is definitely not due to a transition from measurement of polymer sputtering yield to that for silver. Rather it would appear that in the PS film a significant structural and chemical transformation takes place initially and ultimately the damage process is a t steady state with the removal of damaged polymer by a sputtering process. Although neither the exact mechanism by which this ion beam induced reaction proceeds nor the exact chemical and physical nature of the product(s) is (are) known, it is suspected that a t steady state the outer layer is a highly cross-linked (and possibly even graphitic) form of carbon. Visual examination of the PS films after ion beam bombardment to steady-state yield reveals the films are dark greyish in appearance although a certain degree of translu-
ANALYTICAL CHEMISTRY, VOL. 52, NO. 9, AUGUST 1980
1472
0
I
64
\
40
-
-
A
SPUTTERING OF DIP-COATED P M M A
32
I
2 0
N N
0
X
>! 16
'
v v
O 0
v r
24
48
/
ION DOSE ( x 1O1'eq.) Flgure 7. Sputtering behavior of PMMA as a function of ion dose. Curve A shows the mass yield Y,. Curve B shows the quasi-lo arithmic approach of Y, to Y,,,-. The slope of the linear regression line is 3.3 X 10' equiv-'. The ion beam current density used was 7.0 X 10- A-cm-'
q
cency is retained. The bombarded films are also found to be insoluble in chloroform, the solvent used to apply the PS in the dip coating procedure. Comparison of the C(1s) high resolution spectra obtained before sputtering and after reaching the steady-state situation (Figure 6) clearly indicates the destruction of the aromatic ring system of the monomer as evidenced by loss of the shake-up satellite (at higher binding energy) after ion bombardment (cf. Ref. 39). Curve B of Figure 5 shows the damage reaction to be first-order process, depending only on the cumulative ion dose. Although implanted argon is detected by X P S after the steady-state yield is reached, quantitation reveals that this element is present a t a concentration of 1-2 at.% in the analysis volume. We presume that its presence does not substantially affect the magnitude of the steady-state mass yield for PS determined in this work. The average raw steady state mass yield of 4.5 x g-ion-' when corrected for nonuniform radial sensitivity and ion dose and adjusted to normal incidence (see above) gives a value of 2.8 X lo-= g-ion-'. No studies of the effect of variation in ion beam energy on the sputtering behavior of PS have as yet been undertaken. However, there appears to be no significant effect on the observed kinetics and steady-state mass yield caused by changing the current density over the range of 2-8 X lo4 A-cm-2. Sputtering Yield for Linear Polymethylmethacrylate (PMMA). Whereas PS is known to cross-link when subjected to electron beam bombardment (30),PMMA undergoes depolymerization under electron or X-ray flux (29, 30). Consequently, for comparison to PS, the sputtering behavior of
PMMA was investigated. It was anticipated that more ideal behavior might be observed for PMMA, but, as described below, such was not the case. It was noted that during XPS analysis of PMMA coated QCMs prior to ion bombardment, significant weight loss was induced by the Mg K a excitation source employed (400-W applied power). Indeed, this mass loss was linear with photon dose, having a value of 3.6 X g-s-'. (Unfortunately, the volatile depolymerization products could not be speciated since the vacuum system employed in these studies was not equipped with a residual gas analyzer.) Under ion bombardment, a similar decay in the mass loss per incident ion was found for PMMA as compared to PS as shown in Figure 7 , curve A. Again, a steady-state sputtering yield could be observed after an initial exponential decrease in this value. However, the initial yield is an order of magnitude higher for PMMA and the steady-state yield of 3.0 X g-ion-' (1.9 x g-ion-', corrected) is likewise much larger than that found for PS. Again, neither SIMS nor X P S spectra revealed the presence of observable silver during or following the sputtering experiments. The surface was also visibly darkened after ion bombardment for PMMA samples, and the C(1s) XPS spectrum changed from the expected three features before bombardment (Figure 8) to one indicating predominately one type of carbon bonding environment after ion beam damage had proceeded to steady state. The O/C ratio was found by XPS to decrease from 0.43 initially to 0.03 indicating significant expulsion of oxygen-containing moieties during the ion beam induced surface reaction (cf. Ref. 39). During the acquisition of post-ion bombardment X P S
ANALYTICAL CHEMISTRY, VOL. 52, NO. 9, AUGUST 1980
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ACKNOWLEDGMENT The authors acknowledge Grant Albrecht for the suggestion of using the QCM for these measurements and the assistance of Michael Ross in the measurement of some of the ion beam current distributions.
C ( l s ) X P S OF P M M A
LITERATURE CITED Landrum, H. L.; Salmon, R. T.; Hawkridge, F. M. J. Am. Chem. SOC. 1977, 99,3154. Diaz, A. J . Am. Chem. SOC.1977, 99,5838. Doblhofer, K.; Noke, D.; Ulstrup, J. Ber. Bunsenges Phys. Chem. 1878, 82, 403. Merz, A,; Bard, A. J. J. Am. Chem. SOC.1978, 100, 3222. Nowak, R.; Schultz, E. A,; UmaRa, M.; Abrufia, H.; Murray, R. W. J. Nectroanal. Chem. 1978, 94,219. Phom, M. C.; Dubois, J. E.; Lacaze. P. C. J ' . Electroanal. Chem. 1979, 99,331. Kerr, J. 8.; Miller, L. L. J. Electroanal. Chem. 1979, 101, 263. Kaufman, F. B.; Engler, E. M. J. Am. Chem. SOC. 1979, 101, 547. Bolts, J. M.; Bocarsly, A. B.; Pahzzotto. M. C.; WaRon, E. G.; Lewis, N. S.;Wrighton, M. S . J. Am. Chem. SOC. 1979, 101, 1378,and references fherein. Oyama, N.; Yap, K. B.; Anson, F. C. J. Nectroanal. Chem. 1979, 100,
233.
E
Dautartas, M. F.; Evans, J. F. J. Electroanal. Chem.. in press. RBhBk, V.; Smolkbva, E. Chromatographia 1976, 9 ,219. Gordon, A. L.; Taylor, P. J.; Harris, F. W. J. Chromatogr. Sci. 1976. 14,
i
i
428. Sebastin, 1.; Halasz, I. "Advances in Chromatography", Voi. 14;Marcel Dekker, Inc.: New York, 1976;Chapter 3. Grushka, E.; Kikta, E. J. Anal. Chem. 1977, 49, 1004A Jennings, W. "Gas Chromatography with Glass Capillary Columns"; Academic Press: New York, 1978. Novotny, M. Anal. Chem. 1978, 5 0 , 16A. Storp, S.; Holm, R. J . Nectron Spectrosc. Relat. Phenom. 1979, 16.
DAMAGED
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292
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Flgure 8. Carbon (1s)XPS spectra of PMMA before sputtering (upper) and following sputtering to a steady state value of Y , (lower)
spectra, a linear loss of mass was again observed. The rate was found to have decreased by a factor of two compared to that of the undamaged film. This observation together with the data reported above suggest that a highly cross-linked surface structure forms during ion bombardment as in the case of PS. However, it further suggests that the range of the 1-KeV argon ions in the damaged matrix is relatively short (10-100 A, as estimated by Storp and Holm (18))compared to the film thickness since the X-ray induced depolymerization reaction is readily observed. Although the argument for a first-order damage process cannot be made as convincingly in the case of PMMA as it can in the case of PS (compare Figure 5, curve B, and Figure 7 , curve B), linear regressions of these data to a first-order fit gives values of 3.2 X lo8 equiv-l and 3.3 X lo8 equiv-', respectively. If the densities of the undamaged films are assumed to be comparable, one is tempted to conclude that the damage cross sections for 1-KeV argon ions in these two chemically dissimilar polymers are nearly equal. Clearly, this hypothesis is very speculative. Future work on the sputtering of other polymeric materials will attempt to ascertain the extent to which this concept is valid.
Taylor, J. A.; Lancaster, G. M.; Rabalais, J. W. Appl. Surf. Sci. 1978, 1 , 503. Ross, M. R.; Evans, J. F., 1979,unpublished data. McKeown, D. Rev. Sci. Instrum. 1961, 3 2 , 133. Andersen, H. H. Radlat. E f f . 1973, 19,257. Andersen. H. H.; Bay, H. L. J. Appl. Phys. 1975, 46, 1919. Blank, P.; Wlttmaack, K. J. Appl. Phys. 1979, 50, 1519. Smith, D. L. J. Nucl. Mater. 1978, 75, 20. Wittmaack, K. Phys. Lett A 1978, 69,322. Honda, F.; Fukuda. Y.; Rabalais, J. W. J. Chem. Phys. 1979, 70, 4834, and references therein. Winograd, N.; Garrison, B. J.; Fleisch, T.; Delgass, W. N.; Harrison, D. E., Jr. J. Vac. Sci. Techno/. 1979, 16, 629,and references therein. Todd, A. J. Polym. Sci. 1980, 42,223. Bowden, M. J. CRC Crit. Rev. Solid State Mat. Scl. 1978, 8 , 223,and references therein. Benndorf. C.; Keller, G.; Seidel, H.; Thieme, F. J. Vac. Sci. Techno/. 1977, 14,819. Rlegert, R. P. Res.lDev. 1988, 19,46. Sauerbrey, G. 2. Phys. 1959, 155,206. Warner, A. W. I n "Ultra Micro Weight Determination in Controlled Environments", Wolsky, S. P., Zdanuk, E. J., Eds.; John Wiiey: New York, 1969;Chapters 5 and 7. Ramadan, 6.; Piyakls, K.; Kos, J. F. Rev. Scl. Instrum. 1979, 50, 867. Oechsner, H. Appl. Phys. 1975, 8 , 185. Wehner, G. K. in "Methods of Surface Analysis", Czanderna, A. W., Ed.; Eisevler: Amsterdam, 1975;Chapter 1. Evans. J. F.; Albrecht, M. G.; Ullevig, D. M.; Hexter, R. M. J. Electfaanal. Chem. 1980, 106, 209. Williams, D. E.; Davis, L. E. I n "Characterization of Metal and Polymer Surfaces", Vol. 2,Lee, L. H., Ed.; Academic Press: New York, 1977, p 53.
RECEIVED for review February 11, 1980. Accepted April 24, 1980. The financial support of the Graduate School, University of Minnesota, is hereby acknowledged; and acknowledgement is made to the Donors of the Petroleum Research Fund, administered by the American Chemical Society, for partial support of this research (Grant f: 10733-G3).
