the slope of the curve in Figure 6, a hydrogen solubility coefficient a t 850 OC of (1.20 f 0.05) X g atom H/ (g [1.03 X g atom H/(g atom U X atom U X was calculated compared with 1.3 X g atom H/ (g atom U X 6) [1.14 X g atom H/(g atom U X measured by Mallett and Trzeciak (6). A similar treatment of the data in Figure 7 yielded a hydrogen s o h bility coefficient a t 850 "C of (1.43 f 0.06) X g atom H/(g atom Ni X G) [1.24 X g atom H/ (g atom Ti X for nickel compared with 1.60 X g atom H/ (g atom Ni X &) [1.38 X g atom H/ (g atom Ni X measured by Sieverts (7). Note that the hydrogen solubility coeffients for nickel and uranium are three orders of magnitude smaller than that for titanium. The hydrogen analysis chamber H2 base pressure for nickel or uranium analyses can be six orders of magnitude greater than that for hydrogen analysis of titanium before comparable near-equilibrium interactions occur between the evolving hydrogen and the sample. Furthermore, the hydrogen solubility coefficients for uranium and nickel decrease with de-
m)
a)]
m]
a)]
a)]
creasing temperature (6, 7); thus, sublimed metal at temperatures below the furnace temperature has less affinity for hydrogen than that at furnace temperature and, therefore, does not pose a significant hydrogen absorption problem. LITERATURE CITED (1)J. 6.Condon, R. A. Strehlow, and G. L. Powell, Anal. Chem., 43, 1448 (1971). (2)J. T. Sterling, F. J. Palumbo, and L. L. Wyman, J. Res. Nat. Bur. Stand., Sect. A, 66, 483 (1962). (3)G. L. Powell and J. B. Condon, Anal. Chem., 45, 2349 (1973). (4) G. L. Powell, Anal. Chem., 44, 2357 (1972). (5)A. D. McQuillan, Proc. RoyalSoc. Ser. A, 204, 309 (1950). (6) M. W. Malled and M. J. Trzeciak, Amer. SOC.Metals, Trans. Quart., 50, 981 (1958). (7)A. Sieverts, 2. Metallk., 21, 37 (1929).
RECEIVED for review August 28,1974. Accepted December 4, 1974. Work performed at the Oak Ridge Y-12 Plant under Contract W-7405-eng-26 with the U.S. Atomic Energy Commission.
Surface Sputter Effects in a Hollow Cathode Discharge E. H. Daughtrey, Jr., D. L. Donohue, P. J. Slevin, and W. W. Harrison Department of Chemistry, University of Virginia, Charlottesville, Va. 22903
A scanning electron microscope is used to study sputter effects in a hollow cathode discharge. Copper, stainless steel, and graphite were sputtered as cathode materials. The effects of fill gas, net sputter time, and acid cleaning were noted. Both mlcro- and macro-changes were observed. Conical hillocks formed rapidly after sputter initiation. An approximation toward a spherical cavity emerges with long term sputtering. These phenomena appear to be significant to analytical hollow cathode emission studies.
Cathode surface sputtering in the hollow cathode discharge provides the basis for two trace element analysis techniques. Hollow cathode emission (HCE) has been used in analysis for many years ( I , 2), providing excellent elemental sensitivity for solids (3, 4 ) as well as for certain elements deposited from solution as a film in the cathode cavity (5-7). The recently developed (8) hollow cathode ion source (HCIS) has been shown to be a sensitive, stable ion source for analytical use in solids mass spectrometry. It can also provide valuable information about the species produced in the sputtering and ionization processes of the hollow cathode discharge. However, large changes in sensitivity and reproducibility, especially with the emission source, were observed periodically and appeared to correlate with changes in appearance of the cathode surface, some visible to the unaided eye, some requiring microscopic examination. A number of factors affected these surface phenomena. Cathode material and its recent surface history, including preparation, cleaning, and net sputter time, were of particular significance. Basic studies of sputtering in the hollow cathode discharge have received little attention, generally because of the lack of control over sputtering parameters, making in-
terpretation of results difficult. A study of sputtering rates in the planar glow discharge cites the increased role of backsputtering or redeposition as a factor in material transport as the discharge pressure range increases (9).The incident ion energy, and angle of incidence cannot be as carefully controlled in the hollow cathode configuration as in ion bombardment experiments (IO, I l ) , which generally operate a t pressure low enough to allow escape of the sputtered atoms. The energy and angle of escape of the sputtered atoms will also be influenced by the hollow cathode configuration. The cylindrical cup hollow cathode geometry, as opposed to the simple flat disk target used in ion bombardment experiments, further complicates sputter considerations. Because of these factors, rigorous agreement with sputtering theories and ion bombardment experiments cannot be expected. Because of interest in this laboratory in both hollow cathode emission and ionization, a study of hollow cathode sputter effects was conducted, using a scanning elecron microscope as the major examination tool. The main objective was to determine the micro- and macro-effects which the hollow cathode discharge produced on a cathode surface. We hoped to see some relationship of these surface phenomena with observed analytical sensitivities and experimental difficulties. EXPERIMENTAL The cathodes were sputtered using a Glomax demountable hollow cathode tube (Barnes Engineering Company), modified for high current operation, coupled to an existing vacuum system (7). The tube was powered by a Kepco BHK power supply which was capable of supplying 200 mA controlled current at up to 1000 V. The Cambridge Stereoscan scanning electron microscope was used to provide high resolution examination of each cathode surface. The samples were prepared in two different ways. In the first ANALYTICAL CHEMISTRY, VOL. 4 7 , NO. 4, APRIL 1975
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a Figure 1. Effect of fill gas on cam
b hode
(a)Helium: (b) neon: (0argon m c t h w l , a disk of t h e selecred material. slightly smaller i n diameICI than the cathode cavity. was placrd in [lie b o i w m of rhc cavity. T h e carhodr was inrrnrd ~n the cathode hlork. rhr system BVRCIIated and iilled w i t h t h e desired fill gas at t h e p r o p r r prrssurr. and rhc rathwlr arid disk sputrrred for a p r e - d e l e m i n e d perivd of timr. T h e disk was then remwrd from the v n t h d e nnd m i m n t ~ d w i t h r i l crmenr ~ ~ ~ to thr sample holder for S K M rxaminarion. Thr E C C O ~method ~ of sample prQparat:On W ~ scromplished S by the sacrifice of B previously sputtered cathode. T h e sputtered cathode
was cut in half lengthwise and then again crosswise to ubservr .iputtrring changes alm: the Icnknh uf t h e cnthode. 'l'he crow.cut was nrcessary 1 0 i i t fhc t a t h o d e o n the snmplestsgr.
RESULTS AND DlSCUSSION Copper has served as a favorable cathode Inaterial fiM' much of our hollow cathode work. If SPUtterS very well, i s available in reasonable purity, and contributes little spec-
a Figure 3. Long term SpUner effectson the bottom of
C
b copper hollow cathodes, 500X
(a)5 mi". initial hillock formation; (b)7.5 hr, hillock crowding; (0change in (b)after acid cleaning
tral interference to either the optical or mass spectra. Thus, the SEM studies here were done in large part with copper cathodes. Oxygen-free copper was used in this study, althcugh this grade is not required. Stainless steel and graphite are also of interest for comparison purposes, particularly where low sputtering is desired. Studies with Copper. Effect of Fill Gas. Because of the large differences observed in the emission intensity of both gold (12) and boron (7)with various fill gases, copper disks sputtered hy helium, neon; and argon were exmined with the SEM. Disk inserts, spotted with a sample, have been used successfully with HCE analysis (13).It is known that sputtering generally increases with the weight of the homharding ion (111,(although some discrepancies exist a t low energies (IO)), hut of interest was the extent of net sputtering difference which occurs in the hollow cathode configuration and in the short time involved with a HCE experiment. Each disk was sputtered in the hollow cathode discharge for fifteen minutes a t 200 mA and under conditions known to yield optimum emission intensity from copper for the particular gas. Figure 1shows the surfaces ohserved a t 500 magnification. With helium (Figure la), some slight pocking of the surface is observed, hut otherwise there is little evidence of surface changes due to sputtering. With neon (Figure lb), an area showing the disk cut marks was selected. Some rounding of the striations is ohserved, but still little sputtering has occurred. With argon (Figure le), the change in the surface is pronounced with the formation of regular conical structures, referred to elsewhere as hillocks UI), 20 to 30 microns high. Argon clearly sputters the copper surface more heavily than do the other two gases. However, the sensitivity for boron and gold emission with argon as the hornbarding gas is dependent not only on the high sputter efficiency, hut also on the formation of the conical surface structure. Effect of Net Sputter Time. The dependence of the emission intensity on the surface history of the cathode suggested a need to investigate surfaces generated after various lengths of sputter time. Figure 2 shows the effects on copper disks of increasing sputtering time a t 200 mA in argon. A freshly cut surface (Figure 2a) shows prominent striations due to the cutting of the disk. At three minutes (Figure 2b), sputtering has greatly reduced the striations, and at five minutes (Figure 2c), etching to the crystal structure of the copper is ohserved under higher magnification. At ten minutes (Figure 2d), the crystal structure is destroyed and rough hillocks are beginning to form. At fifteen
Figure 4. Cross-section of hollow cathode sputtered for 15 hr Leners refer io SEM photomicrographs in Figure 5
minutes (Figure 2e), the previously described conical hillocks are observed. This formation seems to he a favored configuration as further sputtering shows the same general type surface, with some rounding. Correlation of this hillock formation in the hollow cathode discharge to the type of sputter etching obtained by ion bombardment of flat surfaces shows certain similarities. The surfaces shown in Figure 2c and d, resemble $0some degree those produced (11)by hornbarding polycrystalline copper with 400 eV mercury ions. At low ion beam density, etching to the crystal structure was ohserved, while a t higher ion beam bombardment, the crystal structure was destroyed. Cones (or hillocks) have also heen produced in a normal glow discharge (14) and observed with a SEM. These were attributed to the presence of inclusions which were more resistant than the matrix to sputtering. Hillock formation by ion bombardment has been observed ( 1 I ) , with the orientation dependent on the angle of incidence of the ion beam and the crystal structure of the target material, among other things. The formation of these regular conical hillocks appears to he more extensive than in glow discharge or ion bombardment experiments. I t is known that sputtering is favored a t angles away from the normal (15). No ohservahle damage pattern is reported (16) for normal ANALYTiCAL CHEMISTRY, VOL. 47, NO. 4, APRIL 1975
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a
b
d
e
C
f
Figure 5. Surface changes for various regions of the cathode shown in Flgure 4
incidence in a glow discharge. In this same study, hillock formation was shown to he parallel to the ion beam direction. The geometry Considerations in the hollow cathode cavity make extrapolations from ion bombardment studies hazardous. because neither the anale of ion incidence nor the ion energy is well known in thelhollow cathode ( I 7 , 1 8 ) . The formation of microscopic hillocks in a glow discharge has been attributed to redeposition; their geometry was dependent on the metal matrix (19).The role of redepositon, which must be a factor at hollow cathode pressures, is nnclear but is probably a large consideration in the formation of the conical hillocks, which are less frequently observed with ion bomhardment where the pressure is low enough that escape of the surface atoms is possible. The conical hillock formation is a complex process, governed by fadors which include ion incidence angle, the ejection angle of the sputtered species, discharge current and pressure, relative sputtering and redeposition rates, and possibly inclusions and grain boundaries. The sputtering process is not clearly defined even for the controlled ion bombardment experiments. Geometry and pressure considerations in the hollow cathode discharge further complicate the resolution of this problem. For long term sputtering studies, a series of copper cathodes were sputtered for u p to 7.5 hours and then sacrificed by a lengthwise cut of the cathode, followed hy SEM analysis. Figure 3 shows two examples from this study, near the hottom of the cathode cavity. The fill gas was again argon, the current 200 mA. At five minutes of sputtering (Figure 686
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3a), hillocks are already observed arising out of a rather rough background. After one hour, the hillocks begin to crowd together in the cavity and a t 7.5 hours, macrochanges in the cathode surface are observed with the formation of a bulb shape in the bottom of the cavity. The bottom of this cavity (Figure 3b) shows hillocks which are no longer arising out of a planar surface hut a concave one, running the hillocks together. This also causes a difference in perspective which makes the hillocks less recognizable, as any angle of observation is then more normal t o the concave surface. Overall, comparing the inserted disk us. intact cathode surfaces, there appears to be more vigorous sputtering of the intact cathode for a given time interval and also more evidence of redepositon of the sputtered material as a factor in the surface changes. This difference in sputtering could be attributed to imperfect electrical contact of the disk with the cathode, yielding a slightly smaller acceleration of the argon ions into the surface. This could affect sensitivity for hollow cathode emission applications where a sample is spotted on a disk for subsequent spntteringexcitation. Effect of Acid Cleaning. We had previously noted that acid cleaning of a copper cathode greatly reduced the emission response from boron. This could be, due to surface changes in the cavity surface as a result of interaction with the acid t o alter the effective surface area. To investigate this effect, the 7.5-hour sputtered cathode was washed with 5%nitric acid for anuroximatelv four minutes. followed hv rinsing with distilled, deionized water. The bright surface I.
a
b
C
d Flgure 6. Cathodic spuitering of (a) stainless steel. 15 min. IOOX, (b) stainless steel. 15 min. 5OOOX. (c) graphite, unsputtered. 200X. and graphite, 15 min, 2OOX. Argon HCD, 1 Torr, 200 mA
darkened on reaction with the acid. As seen in Figure 3c, SEM analysis of the surface revealed general pocking of the surface and partial erosion of the hillock formation. This change in the surface structure reduces the boron emission response by at least one order of magnitude. Sputter renewal of the cathode surface restores the sensitivity to its former level. These differences may be due to net effective surface area changes, occlusion of the sample hy the acid etched surface, or formation of less favored sputter angles hy hillock erosion. Cathode Geometry Changes. When long-term sputtered cathodes were sacrificed for examination, gross surface changes in the cavity were observed, visible to the unaided eye. This is shown in Figure 4, a cross-section of a cathode sputtered with argon for 15 hours at 200 mA. Several of the changes generated by the sputtering process are worth noting. The mouth of the cavity is heavily sputtered; the previously sharp edge has been severely eroded. The neck of the cavity, however, is fairly smooth and relatively unchanged. An hourglass-like shape is observed in the bottom two-thirds of cavity, giving evidence of heavy sputtering and redeposition in this area. The wall thickness a t the middle of the lower bulb is about 0.025 inch, and a t the neck between the bulbs, 0.060 inch, compared to an original wall thickness of 0.047 inch. At the bottom of the cathode, the thickness is 0.185 inch, compared to an original thickness of 0.146 inch, indicating significant redeposition in rhe bottom of the cathode cavity. Observation of lhe bulb furmation with time shows that the lower hulh is tbrmed first,
(4
and the upper bulb only after more than 7.5 hours of sputtering at 200 mA in argon. Figure 5 shows SEM photomicrographs of various regions of the 15-hour sputtered cathode. The mouth of the cathode (Figure 5a) shows evidence of heavy sputtering. At about 0.25 inch from the mouth (Figure 5b), lengthwise channels appear, suggesting a sputtering angle aligned with the axis of the cathode. Such channels or grooves have been shown in ion bombardment studies (11). The hulk of the neck has the appearance shown in the bottom portion of Figure 5c with evidence of only light sputtering. A transition line between the two bulbs, seen in Figure 4 and in the middle of Figure 5c shows little sputter etching b u t much redeposition. The surface of the two bulbs was very similar (Figure 5d) with rough irregular hillock formation seen. About 1 mm from the bottom of the lower bulb, a strange transition zone is observed (Figure 5e) which is vaguely molten in appearance. However, higher magnification examination (Figure 5f) shows the same crowded rough hillocks seen in the 7.5-hour sputtered cathode. Widely varying surface changes are thus observed along the length of the cathode cavity after sputtering in the hollow cathode discharge, indicating that a solution sample residue deposited on a cathode cavity surface would be subjected to varying degrees of sputter release, depending upon its location. Other observations of cavity formation have been reported. White (171 has suggested thar a spherical hollow cathode (formed by sputtering! is the only ultimately stable tiirm for a hollow cathode discharge. Knerr e/ a / . 1201 ANA-YT.CA- ChEM
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observed cavity formation with a copper cathode. Cavity formation was also observed by Townsend, noting that the size was dependent on pressure and current (21). The geometry changes observed on sputtering in the hollow cathode discharge are complex and poorly understood. I t is reported (20,22) that the current density is greatest a t the bottom of the cathode cavity a t the pressure range in which the hollow cathode is normally operated. This is reflected in this study by initial hillock formation occurring in the bottom center of the cavity, which would involve a combination of the sputtering and redeposition processes. The filling in of the cathode bottom with redeposited material (Figure 4) gradually occurs as the cylindrical cathode cavity assumes a spherical configuration. The formation of a neck a t the top of the bulb arises from redeposition of the sputtered atoms by effusion out of the negative glow onto the wall. The constriction of the neck has been suggested to act as a focusing lens (20), directing more ions to the bottom of the cavity. This would be supported by the heavily sputtered appearance of the bottom (Figure 5 f ) , which is surrounded by the transition zone (Figure 5e). As the neck of the bulb becomes more constricted, the ridge would start to be seen as a new bottom, and a second bulb formation could begin to form. This would accelerate the ridge formation from the upper side and promote further bulb formation. The bulb formation could be thought of as an approximation toward a spherical surface, where the eventual rates of sputtering and redeposition reach a steady state, with no net change in material transport. This change in macro-geometry would certainly have an effect on the current density in the cathode and could affect further sputtering. Studies with Other Cathode Materials. Because the emission response of trace elements deposited as a surface film in the hollow cathode cavity is dependent on the cathode material used, the SEM study of cathodic sputtering was extended to other cathode materials which have been useful in HCE. Studies of relative sputtering rates by ion bombardment by Wehner (23) indicated that the sputtering yield may be related to the filling of the electronic shells, especially the d shells. Boumans (9) has pointed out the difficulties in determining the sputter rates using the planar glow discharge. Although he was able to obtain reasonable agreement with the values of Wehner, the redeposition of sputtered material at the pressure range in which the discharge operates seriously complicates the considerations. Hollow cathode discharges have similar "high" pressure (-0.1-1 0 Torr) redeposition problems, further compounded by the closed cylinder geometry. However, qualitative surface changes can be used as indicators of sputtering action to compare one cathode material us. another. A disk of the selected material was sputtered in an argon hollow cathode discharge at 200 mA for 15 minutes, and removed for SEM examination. Figure 6 shows the surfaces, sputtered under these conditions, of two materials which are of particular interest to our HCE studies. The sputtered stainless steel surface is shown in Figure 6a at 100 magnification and in Figure 6b a t a t 5000 magnification. Only scattered hillock formation is observed with this material as compared to a similarly sputtered copper surface where the hillocks are both more densely packed and larger. The distribution of the hillock clusters is rather sporadic across the surface disk.
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Spectroscopic graphite showed little evidence of sputtering, as evidenced by the small change between an unsputtered (Figure 6c) and sputtered (Figure 6 4 graphite surface. Some smoothing of the graphite surface does occur. Pyrolytic graphite (Poco Graphite, Inc., North Haven, Conn.) was also studied and yielded the same type surfaces as the spectroscopic graphite. Higher magnification revealed no graphite hillocks. The surface changes observed seem to correlate with the observations of relative sputtering rates (23). Copper showed the greatest surface sputtering, stainless steel was only modestly sputtered, and graphite showed little change, in line with the sputering yield order of Cu > Fe > C observed by Wehner. For applied sample films which depend upon matrix sputter for transmission into the discharge plasma, copper serves as an excellent choice. CONCLUSIONS The results from these surface studies have certain practical implications for methods using the hollow cathode discharge as an emission or ionization source, particularly for applications where the sample is a solution, dried to a residual film. The high currents required for trace element analysis sensitivity will cause both micro- and macro-geometry changes in the cathode cavity. New cathodes of high sputter rate materials, such as copper, should be sputter conditioned to reach a stable surface microstructure for best emission results. Acid or abrasive cleaning of cathodes between analysis can alter this surface and affect the release rate of a sample residue. On a longer time scale, the formation of one or more bulb-like cavities, as a cathode is used repeatedly, will alter the overall discharge characteristics and contribute to current density gradients. The resultant nonuniform sputtering could produce emission differences between cathodes if the gross cavity geometries were not generally comparable. LITERATURE CITED (1) J. R. McNally, G. R. Harrjson, and E. Rowe, J. Op. SOC.Amer., 37, 93 (1947). (2) F. T. Birks, Spectfochim. Acta, 6, 1969 (1954). (3) G. A. Pevtsov and V.2 . Krasil'shckik, Zh. Anal. Khirn., 19, 1106 (1964). (4) V.2. Krasil'shchik, Zavod. Lab., 37, 181 (1971). (5) 2. A. Berezin, Zavod. Lab., 27, 859 (1961). (6) N. K. Rudnevskii, D. E. Maximov, T. M. Shabanova, and L. P. Lazareva, Zh. Prikl. Spektrosk., 16, 356 (1972). (7) E. H. Daughtrey and W. W. Harrison, Anal. Chim. Acta, 67, 253 (1973). (8) W. W. Harrison and C. W. Magee, Anal. Chem., 46, 461 (1974). (9) P. W. J. M. Boumans, Anal. Chem., 44, 1219 (1972). (10) M. Kaminsky. "Atomic and Ionic Impact Phenomena on Metal Surfaces," Academic Press, New York, N.Y., 1965. (1 1) G. Carter ana J. S. Colligon, "Ion Bombardment of Solids," American Elsevier, New York, N.Y., 1968. (12) W. W. Harrison and E. H. Daughtrey, Jr., Anal. Chim. Acta. 65, 35 (1973). (13) G. A. Pevtsov. V. 2 . Krasil'shchik, and A. F. Yakovleva, Zh. Anal. Khim., 23, 1785 (1968). (14) H. Jager and F. Blum, Spectrochim. Acta, Pad B, 29, 73 (1974). (15) G. K. Wehner, J. Appl. Phys., 30, 1762 (1959). (16) G. D. Magnuson, B. B. Meckel, and P. A. Harkins, J. Appi. Phys., 32, 369 (1961). (17) A.D. White, J. Appl. Phys., 30, 711 (1959). (18) G. F. Weston, "Cold Cathode Glow Discharge Tubes," lliffe Books, Ltd, London, 1968. (19) A. Guntherschulze and W. Tollmien, Z. Phys. 119, 685 (194.2). (20) G. Knerr, J. Maierhofer, and A. Reis, 2.Anal. Chem., 229, 241 (1967). (21) M. A. Townsend, BellSyst. Tech. J., 36, 755 (1957). (22) A. Lampe, R. Seeliger, and E. Wolter, Ann. Phys., 36. 9 (1939). (23) G. K. Wehner, Advan. Electron Phys., 7, 239 (1955).
RECEIVEDfor review August 9, 1974. Accepted December 9, 1974. This research supported in part by NIH Grant No. GM-14569.
