Anal. Chem. 1900, 6 0 , 1401-1404
1401
Application of a CF,' Primary Ion Source for Depth Profiling in Secondary Ion Mass Spectrometry Wilhad Reuter* and Gerald J. Scilla
IBM T. J. Watson Research Center, Yorktown Heights, New York 10598
We have used a CF3+ prlmary Ion source for some typical depth proflllng appilcations In secondary ion mass spectrometry and compared Its performance with the conventlonally used 02+source. At the same accelerating potentlal, steady-state secondary ion emlsslon condltlons are reached much faster under CF,' than under 02+bombardment. Shallow Implant profiles can be measured with better definltion. This improvement Is directly related to the shorter converted layer range produced at 5 keV CF,' versus 5 keV 02+.The better depth solutlon found under CF,' bombardment may also be ratlonaked by the same reason of a shorter intermlxed range at the Interface. However, this Is shown at least In part to be due to a smoother surface generated compared to the surface after 02+bombardment.
Recently we reported (1)results of a study on the secondary bomion emission from metal targets under CF3+and 02+ bardment. We established that the ionization probabilities p+ for metal ion (M+)emission are higher under CF3+ bombardment relative to the ionization probabilities under 02+ bombardment for those elements which form weak metalbombardment. No increase in p + oxygen bonds under 02+ was found with CF3+ for those elements which form strong bombardment. In such cases, metal-oxide bonds under 02+ oxygen bombardment gives M+yields per sputtered atom or M+ yields per primary ion larger than 0.1 (2, 3) and the use of CF3+as primary ion will not result in a significant increase in the ionization probability. The observed trends in p+ were rationalized by an X-ray photoemission spectroscopy study of metal targets after saturation bombardment with 02+ and CF3+, respectively. Generally metal carbides and fluorides are formed. The fluorine uptake ranges from 9 to 24 atom %. At such low fluorine uptakes the matrix effects, i.e. the influence of A in a binary, AxB1-x,on the ionization probability of B+, are small. In this paper, we report our results for the application of a CF3+primary ion beam to the depth profiling of copper and boron implanted silicon, of a chromium-nickel sandwich structure, and of an A1,,,Gao,6As layer imbedded in GaAs.
EXPERIMENTAL SECTION Most of the work reported here has been performed with a quadrupole-based secondary ion mass spectrometer described bombardment a in detail in an earlier publication ( 4 ) . For 02+ cold cathode ion source developed by Wittmaack (5)and marketed by Atomika GmbH was used. The same ion source was used for CF,+ bombardment operating the source with a gas mixture of 3 Pa O2 and 3 Pa CF,. A Wien filter was employed for the mass separation of the primary beam. The primary ion currents measured on the target (+90 V bias) were 50 and 200 nA for CF3+ respectively. The primary beam was raster scanned over and 02+, an area of 600 X 600 pm with the secondary ion signal gated to accept the signal only over the central 5% fraction. The beam's size was about 150 pm as measured optically on shallow craters produced by a stationary focused beam. Matching of the beam size under CF3+and 02+operation was accomplished by slightly defocusing the CF3+beam to match the beam spot size of 02+
as judged by the luminescence produced on a manganese-doped film of zinc sulfide. Finally the success of this beam size matching can best be judged by comparing the Talystep tracings of the sample. craters shown in Figure 1produced on a G~AS/G%,&,~A~ All sample studies had sufficient electrical conductivity to allow measurements of the secondary ion intensities in the peak of the secondary ion energy distribution.
RESULTS AND DISCUSSION In Figure 2 the depth profiles are shown for a polycrystalline oscillating layer structure of 150 8,Cr/150 8,Ni with a primary (5 keV), N2+(5 keV), CF3+ (5 keV), and Ar+ (12 beam of 02+ keV), respectively, all under normal incidence conditions. The bombardment is the apmost interesting feature under 02+ pearance of the maxima in the Ni+ signals at the Ni/Cr interfaces. This can best be understood from the results obtained by us (6) in a secondary ion mass spectrometry (S1MS)-X-ray photoemission spectroscopy (XPS) study of saturation bombardment. We have Ni/Cr alloys after 02+ shown in that study that the ionization probability of Ni+, normalized to the sputter yield and the nickel concentration, increases with the increasing chromium concentration by as much as a factor of 7 for the binary Cr0,31N&,66 with the highest chromium concentration. This increase in the Ni+ ionization probability can be rationalized by the increase in the Ni bonding energy found by us after saturation bombardment of Cr-Ni alloys versus pure nickel. Of course the question remains open whether the interface mixing is due to ion beam induced mixing or some interdiffusion prior to the analysis. When the target is tilted to 45' incidence or analyzed in a Cameca 4F (-40' off normal incidence), the double peak structure in the Ni+ signal is not observed. We have shown (7) by Auger spectroscopy that the oxygen uptake of nickel is reduced by a factor of 3 if the angle of incidence of 02+is changed from the normal direction to 45' and that nickel is predominantly in the metallic state (XPS). Artifacts in SIMS profiles can be reduced or eliminated by the reduction of bombardment-induced chemistry using oblique incidence of bombardment. Employing oxygen back-flooding in the Cameca 4F sample chamber recovers the double peak structure of the Ni+ signal. Under N2+ and Ar+ bombardment, an inverse result is obtained; i.e. now the Cr+ signal exhibits two maxima a t the film structure interfaces. A depth profile of 0- (not shown here) obtained in a Cameca 3F with a Cs+ beam shows an oxygen intensity higher by an order of magnitude in the Cr versus the Ni layer with weak maxima near the nickel interfaces. The Cr- depth profile follows precisely the 0- profile. This suggests that during the film deposition chromium is slightly oxidized particularly a t the interfaces while switching the in situ evaporation sources. The onset of the Cr+ signal is not shown for the top Cr layer for N2+and Ar+ bombardment because the Cr+ signal was far higher in the top layer due to the presence of a few monolayers of a chromium oxide. The depth profiles of Cr+ and Ni+ under CF3+5-keV bombardment has several features distinctly different from those observed for the other primary projectiles. The Cr+ signal in the top Cr layer reaches steady state after the removal of
0003-2700/88/0360-1401$01.50/00 1988 American Chemical Society
ANALYTICAL CHEMISTRY, VOL. BO, NO. 14, JULY 15, 1988
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I t
a W
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a W
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Figure 1. Talystep tracing of craters produced by CF,’ and 02+ bombardment of GaAs after beam size matching by luminescence on a manganesedoped ZnS film.
TIME (arb units)
Flgure 2. Cr and Ni depth profiles obtained from a 150 A 01150 A Ni sandwich target obtained with the respective prlmary ion beams
under normal incidence bombardment condition. only about 40 A in spite of the presence of a few monolayers of chromium oxide. At least in part, this is due to the partitioning of the primary ion energy between each atom of the CF3+ion upon impact on the target. Assuming equal partitioning of the energy, the effective primary ion energy is reduced to 1.25 keV. If we assume that steady-state signals are observed after the removal of 2R,! where R, is the projected range, the observed steady-state signal after the removal of 40 is in reasonable agreement with an estimated R, value (8) of 15 A for 1-keV Ne in iron. I t remains, however, noteworthy that in spite of the presence of a few monolayers of chromium oxide, steady state is reached quickly. This implies that either oxygen is very effectively removed or that the fluoride formation overrides the residual oxide in its effect on the ionization probability of Cr. Artifacts in the depth profile introduced either by the presence of oxygen in the chromium layer (Ne+ and Ar+) or by changes in the ionization probability of Ni+ in the intermixed interface region (02+) are absent in the depth profile under CF3+ bombardment. Since the implanted fluorine concentration in Cr and Ni is 19 and 11 atom %, respectively
a
TIME ( a r b u n i t s )
Flgure 3. Depth profile of the respective secondary ion signals for the indicated film structure obtained with a 5-keV primary beam of CF,’
and 02+under normal incidence bombardment. ( I ) , the residual oxygen concentration in the chromium layer introduced during the film deposition will not affect the Cr+ signal as it does under Ar+ and N2+bombardment. Using the same binary Cr/Ni alloys used in our study in ref 6, we found the normalized ionization probabilities of Ni+ and Cr+ invariant with the alloy composition with CF3+bombardment. We argued previously (1) that strong matrix effects are expected only if a nearly complete stoichiometric conversion occurs in the target under reactive ion bombardment. Since this condition does not prevail for nickel or chromium under CF3+bombardment (1)no double peak structures are observed in the profile. Noteworthy but not understood are the large differences in the sputter yields of Ni and Cr seen here and also reported by us in ref 1. Neither nickel nor chromium forms volatile fluorides. Chemical etching rather than physical sputtering has so far been observed only at very low (less than 1 keV) bombardment energies (9) and for this reason can be excluded as the responsible mechanism. We have shown in ref 1that the fluorine uptake in chromium is larger than in nickel. Since one could expect the surface binding energy of fluorides to be higher than of metals, one could on this basis rationalize the large difference in sputter yields. In Figure 3 the depth profiles are shown for the respective secondary ion signals of a 400-AAb cG% ,As molecular beam epitaxial layer embedded under a 3000-A top layer of GaAs. The sputtering time was normalized to the center of the embedded layer. Molecular ions were monitored for the Ga and A1 profiles because of excessive signal intensities of the M+ ions. The appearance of an AlO’ signal in the top layer of GaAs is probably due to the presence of a pin hole. Again we observe that steady-state conditions are reached faster bombardment, attributed to the under CF3+than under 02+ shorter projected range for CF3+. This sample was selected for a study of the depth resolution. For this reason, particular attention was given to the equivalence in the beam size of 02+ and CF3+using the procedure outlined in the Experimental Section. The Talystep tracings of the two craters shown in Figure 1 prove that we reasonably satisfy this condition. It is apparent from the AlO+ and the A”+ profile that the depth resolution is significantly better under CF3+ bombardment. Electron micrographs taken from the bottom of the craters at 50000X magnification show a perfectly smooth surface for CF3+ whereas some structure becomes detectable under 02+ bombardment. This suggests that at least in part the better depth resolution for CF3+bombardment is topography related, although the reduced depth of the bombardment-induced intermixed layer at 1.25-keV F+ versus 2.5-keV O+ will contribute. Although the atomic concentration of As is constant in this film structure, the As+ signal increases by about a factor of 20 in the Alo4Ga, &s layer when probed with the CF3+beam. Also noteworthy but not understood is the absence of mo-
ANALYTICAL CHEMISTRY, VOL. 60, NO. 14,JULY 15, 1988
'O" 10 16:
t
\
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2000
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4000 DEPTH
MASS/CHARGE Figure 4. Mass scan obtained from GaAs with normal incidence bombardment CF,+ and 02+.
I
1
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~
~
~
I
I
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400
600
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DEPTH
(a)
Figure 5. Depth profiles of a 5-keV boron Implant at a dose of 3 X 1015atoms/cm2in silicon taken with 5-keV CF3+ and 02+ incident at 40' off the normal direction.
lecular ions formed with the primary projectile if GaAs is bombarded with CF3+. In Figure 4 mass scans are shown projectiles. taken in the 3000-A GaAs layer with CF3+and 02+ The molecular ions GaF+ and AsF+ are not detected whereas peaks are seen for GaO+ and AsO'. The 19F+peak appears only at low intensity whereas in silicon under CF3+ bombardment (I) and other metallic targets this component in the mass spectra belongs to the most intense peaks. Molecular interferences are less likely to occur in GaAs if the 02+ beam is substituted by CF3+. This is an exception since mass spectra taken by us of numerous metal targets and of silicon indicate more complex molecular ion structures for CF3+ bombardment. Another application is shown in Figure 5 for the depth profiling of a shallow 5-keV boron implant in silicon. Such shallow implants became increasingly more important in the semiconductor industry as dimensions of device structures decrease. Depth profiles were taken with 5-keV CF3+and 02+ beams under 45' incidence, a condition closely matching that of the Cameca instrument normally used for our boron depth profiling work. With CF3+bombardment, the boron implant concentration can be determined very close to the surface, whereas oxygen bombardment introduces artifacts in this region due to oxygen loading extending nearly 200 A into the target. To some extent this problem can be reduced if oxygen bleeding is employed during 02+bombardment. The better performance under CF3+bombardment is clearly related to the reduced projected range. Finally, we used the CF3+ ion source to determine the 200-keV copper implant profile in silicon. The result is shown in Figure 6 for normal incidence 5-keV CF3+ along with the depth profiles obtained under 10-keV 02+ bombardment at various angles of incidence. The large profile distortion introduced by normal versus 45' incidence 02+ bombardment
I
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6000
0000
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(a
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O$,4Oo AND 50°
Figure 6. Depth proflles of a 200-keV copper implant at a dose of 1 X loi5atoms/cm2 in silicon taken with normal incidence bombardment 5-keV CF3+ and 10-keV 0 , ' at the respective angles of incidence.
at 5.5 keV has been reported previously (IO). Depth profiling of 0- and Cu- done by us with a Cs beam after the removal 2 ' beam shows that the copper is driven of 5000 A with an 0 to the depth of the converted oxide layer (-200 A). The profile distortion decreases as one employs more oblique angles steady-state of incidence. XPS studies (11) done after 02+ bombardment of silicon clearly show that at angles of incidence larger than 15' off the normal direction lower oxidation states of silicon are found. Rapid diffusion of the copper to the converted layer depth apparently occurs only if this layer is fully oxidized. This "snow plow" effect is not observed under CF3+bombardment. Since the converted layer is primarily silicon carbide (I),we conclude that Cu will not diffuse rapidly in this matrix.
CONCLUSIONS The use of a CF3+ primary ion source offers some clear advantages over the conventionally used 02+ source, both operated at the same accelerating potential. Steady-state secondary ion emission conditions are reached much faster under CF3+ bombardment, allowing the determination of shallow implant profiles in the surface region with better primary source. This definition than is possible with an 02+ becomes more important as device structure in the semiconductor industry decreases in dimension requiring shallower implantation depth of dopants. In the few cases studied here it also appears that the depth reduction is better than CF3+ bombardment. These advantages at least in part can be attributed to the shorter projectile range of CF3+versus O,+ at the same accelerating potential. Further reduction of the bombardment to match the accelerating potential for 02+ projected range of CF3+is impractical due to large losses of extractable ion currents and difficulties in producing a well-focused ion beam under such conditions. In view of these results, it would appear to be an exciting prospect if ion sources can be developed which produce reasonable extractable ion currents of C3F7+or, better yet, of cluster ions of the halogens. Shallow concentration profiles can then be resolved with nearly monolayer resolution at high detection sensitivities for most elements. ACKNOWLEDGMENT We are indebted to D. S. Yee for the preparation of the nickel-chromium film structures. Registry No. CF3+,18851-76-8. LITERATURE CITED (1) (2) (3) (4)
Reuter, W. Aha/. Chem. 1987, 59, 2081-2087. Husinsky, W. J. Vac. Sci. Techno/. 1985, 6 3 , 1546-1559. Benninghoven, A. Surf. Scl. 1975, 53, 596-625. Frlsch, M. A.; Reuter, W.; Wttmaack, K. Rev. Sci. Instrum. 1980, 5 1 , 695-704.
Anal. Chem. isaa, 60, 1404-1408
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(5) Wittmaack, K. Nucl. Insbum. Methods 1977, 143, 1. (6) Yu, M. L.;Reuter, W. J . Appl. Phys. 1981, 53, 1478-1488. (7) Reuter. W.; Yu, M. L. J . Appl. Phys. 1982. 53, 3764-3786. (8) Benninghoven, A.; Ruedenauer. F. G.; Werner, H. W. Secondary Ion Mass Spectrometry; Wiley: New York, 1987. (9) Coburn, J. W.; Winters, H. F.; Chuang, T. J. J . ~ p p l Phys. . 1977. 4 8 , 3532.
(10) Boudewijn. P. R.; Akerboom, H. W. P.; Kempeners, M. N. C. Spectrochim. Acta, Part 8 1984, 398, 1567-1571. (11) Reuter, W. Nucl. Instrum. Methods 1986. 815,3-175.
RECEIVED
for review July 2% 1987. Accepted February 28,
1988.
Secondary Ion Emission and Sputter Yields from Metal Targets under F,+ Bombardment Wilhad Reuter* and J. G . Clabes IBM T. J. Watson Research Center, Yorktown Heights, New York 10598
Sputter yields and relative Ionization probabllltles have been determlned after Saturation bombardment of 13 elements using a massgeparated 10-keV F,' primary beam generated in a cold cathode ion gun operated at a pressure of 5.3 Pa of pure fluorlne gas. Sputter yleids are larger than those obtained under 0,' bombardment and are In good agreement wlth those obtalned from an available formallsm developed to give the best flt to experlmentai data on sputter ylelds of the element under Inert ion bombardment. IonIration probabllltles are hlgher by up to a factor of 50 for those elements glvlng poor ion yields under 0,' bombardment. Partlally fluorlnated metal surfaces are formed after bombardment to steady-state Ion emlsslon condltlons wlth a fluorlne uptake of about 10-20 atom %.
It has long been recognized in secondary ion mass spectrometry (SIMS) that the sensitive detection of electropositive elements in a metallic target requires bombardment with primary ions that form strong ionic bonds with the target atoms. Traditionally, ion sources are operated with oxygen gas to produce abundant amounts of 02+ions. Excellent secondary ion yields are obtained for those elements that can be completely oxidized and that form strong ionic bonds with oxygen. It has been shown in a static SIMS study of positive metal ion (M+) emission that for such elements as Mg, Al, Cr, Si, and Fe, more than 10% of all sputtered particles are emitted in the M+ state ( I ) . Secondary ion yields, however, may be smaller by up to several orders of magnitude for those elements (e.g., Cu, Ni, Zn, Pd, Cd, Ag) that form only weak bonds with oxygen or that are only partially oxidized under 02+ bombardment (2). Since the ionicity of metal-fluoride bonds is considerably larger than that in metal oxides, one would expect higher secondary ion yields for these elements under F2+bombardment for comparable projectile uptakes in the target. Anticipated problems associated with the high reactivity of fluorine gas with ion source materials led initially to the use of carbon tetrafluoride as the ion source gas. With the addition of nitrogen or oxygen, we produced a stable discharge and extracted several hundred nanoamperes of a mass-filtered CF3+beam ( 3 ) . However, neither F2+nor F+ is formed with sufficient abundance for analytical applications. Ionization probabilities relative to those obtained under 02+ bombardment increase by an order of magnitude for Ni, Cu, and Ag (3),i.e., for those elements giving relatively poor secondary source. The fluorine loading ion yields with the standard 02+
of the sample after saturation bombardment measured with X-ray photoemission spectroscopy (XPS) is smaller (10-20 bombardment, atom %) than the oxygen uptake under 02+ yet ionization probabilities can be larger under CF3+bombardment. The carbon uptake under CF3+bombardment is about 30 atom 70 with the carbon present mostly as metal carbides. In addition to Si,F,+, molecular ion species of the Si,C,+ type are formed; Le., mass interference problems are more likely to occur with this ion source as compared to 02+ operation. In view of these results, it appears to be of much interest to study whether under F2+bombardment the fluorine uptake and the secondary ion yields can be increased further with the additional advantage of reduced molecular interference probabilities.
EXPERIMENTAL SECTION The analyticalsystem combines XPS-SIMS capabilitiesunder ultrahigh vacuum (UHV) conditions and has been described earlier ( 4 ) . The cold cathode ion gun (5) was operated under an F2 pressure of 5.3 Pa. From the known conductance of the ion source aperture we calculated a daily consumption of 56 cm3 of F2 gas at 1-atm foreline pressure. With the addition of a 1-L balast tank in the gas foreline we could maintain constant gas flow within 10% during 8 h of operation without resupplying F2gas from the cylinder. The gas foreline and balast tank were constructed of Monel. We did not use a pressure regulator valve to avoid potential leakage in such devices. Instead we used a double valving system generating a pressure of 1/2 atm in the foreline, monitored by a Datametric Barocell gauge constructed of Monel. The gas cylinder and foreline were encased by a shroud connected to the house exhaust system and equipped with a photohelic alarm system. On the basis of the experience of Matheson Gas Products, we passivated the inner Monel surfaces of the gas foreline by exposure to atm of a 20180 F2/N2gas supply. The foreline pressure rapidly increased to about 3/10 atm, probably due to the reaction of fluorine with the absorbed water layer. This pressure rise is only observed in this start-up procedure. The ion source pressure of 5.3 Pa was maintained through a variable-leak Granville Phillips valve without any leakage upon closing over a 6-month operating period. With these precautions, no problems were encountered in the safe operation of the ion source. With the gun in operation, the sample chamber pressure into 7 X Pa. Additional peaks creased from about 1 X appear in the residual gas spectrum at masses 69, 50,31, and 19 with a cracking pattern very similar to that of CF4. This means that most if not all of the fluorine gas reacts with components in the primary beam section before entering the sample chamber. Probably for this reason we have not observed any unusual deterioration of the electron multiplier with time. After ignition of the plasma the discharge current stabilizes in about 1 h at about 3 mA and drops finally to near zero after 2-6 days of operation. The anode cylinder exhibits an insulating
0003-2700/88/0360-1404$01.50/0 ?Z 1988 American Chemical Society
