Anal. Chem. 1983. 55, 1434-1437
1434
signal can be seen in tracing b. Here, the ICP conditions and the sensitivity of the detection electronics remained unchanged. The noise of the output signal is reduced by a factor of 3 when this filter unit is used. A further reduction in the electronic noise of the output signal can be obtained if the 220-V power line filter unit is added to the rf generator. Tracing c is the output signal obtained when the 110-V power line filter unit is used for the detection electronics and the 220-V power line filter is used in the rf generator. For this tracing, the same ICP conditions and detection electronic sensitivity were used. The noise of the output signal has been reduced by a factor of 20 in this tracing as compared to tracing a. Clearly, the use of these filtering units allows a significant reduction in the rf electronic noise associated with this ICP system. Since the torch “box” used in these measurements has been modified for diagnostic studies and for ICP excited ICP fluorescence measurements, the rf leakage may be greater than normally expected for an ICP system. Hence, other systems may not exhibit as great of a reduction in the rf
electronic noise with the insertion of these filter units. If, however, the rf electronic noise makes up a smaller percentage of the noise associated with ICP measurements, the use of these filter units can reduce the amount of background noise. For the system described in this report, the use of rf filters significantly reduces the electronic noise and allows the attainment of lower LOD values for ICP emission and ICP excited ICP fluorescence measurements.
LITERATURE CITED (1) (2)
(3) (4)
Nomenclature, Symbols, Units and Their Usage In Spectrochemical Analysis-11. Data Interpretation. Pure Appl. Chern. 1976, 45, 99. Winefordner, J. D. “Trace Analysis: Spectroscopic Methods for Elements”: Why: New York, 1976: Vol. 46. Kosinski, M. A,: Uchida, H.; Winefordner, J. D. Anal. Chem. 1983, 55, 668. Ott, H. W. “Noise Reduction Techniques in Electronic Systems”: Wiley: New York, 1976.
RECEIVED for review January 24,1983. Accepted March 21, 1983. Work supported solely by Contract AF-AFOSRF49620-80-C-0005.
Characterization of Primary Beams in Fast Atom Bombardment and Liquid Secondary Ion Mass Spectrometry Sources Bryan L. Bentz* and P. Jane Gale RCA Laboratories, David Sarnoff Research Center, Princeton, New Jersey 08540
The successful application of secondary ion mass spectrometry to the study of large, thermally labile organic molecules dissolved in liquid matrices has been widely demonstrated (1-3). The novel means of sample preparation has resulted in long-lasting currents of molecular ions produced with high intensity. The increasing reliance of analytical chemists on the technique has been accompanied by interest in refining it through more careful specification of experimental conditions. With such information, findings among different laboratories engaged in similar research can more easily be compared ( 4 ) . More exact specification of experimental conditions can also make clearer the influence of each experimental parameter upon the production of secondary ions. These observations permit inferences about the mechanism of sputtering in the organic systems to be made, Chief among the parameters which must be specified are the characteristics of the primary atom or ion beam. These include the chemical identity and charge state of the particles, their energy, the size and shape of the beam as it impinges on the target (the beam spot), and the number of particles which reach the target per unit time. The latter two items taken together constitute the current density of the primary beam and strongly influence the absolute intensity of secondary ions produced, as well as the extent of beam damage induced in the uppermost surface layers. The characterization of primary beams has received much attention in the fields of sputtering and ion implantation (5). At least two general methods exist. If optical access t o the target is available, the target can be coated with a fluorescing material such as ZnS or KBr. The impact of the primary particles on the coated target causes it to fluoresce, allowing in situ visual inspection of the sue and shape of the beam spot. Very rough estimates of beam intensity can also be made from the brightness of the fluorescence. An electrical method for obtaining quantitative estimates of charged particle beam intensity employs a Faraday cup in place of the target (6). If
secondary electrons are properly collected, the ion current entering the cup can be measured. Perhaps the simplest, yet heretofore not widely adopted, method for obtaining estimates of beam size and shape takes advantage of an optical effect characteristic of certain refractory metal films (7) or oxides (8). As a result of multiple light-beam interferences, these materials appear colored when illuminated by visible light (9). The color perceived is dependent on the thickness of the oxide film; differences in color indicate differences in thickness. Bombarding such a f i i with a beam of either energetic ions or neutrals erodes the surface, resulting in a thinner f i i at the impact site. Thus, sputtering records in the film a permanent image of the position, size, and shape of the beam. If the duration of the exposure of the metal oxide film to the beam is known, the effective sputter rate (material removed per unit time) can be determined under analytical operating conditions. We report here the use of tantalum oxide, Ta205,to study the beam produced by a PHI Model 04-303 ion gun attached to an H P 5985B GC/MS modified for molecular secondary ion mass spectrometry (SIMS) (10). The method can easily be used to characterize the primary beams employed by any of the instruments currently being used for fast atom bombardment (FAB) or liquid SIMS.
EXPERIMENTAL SECTION Anodic tantalum oxide films were formed at room temperature following a simple procedure described by Pawel et al. (21). A polycrystalline tantalum strip, initially cleaned with methanol, was immersed in an electrolytic cell containing aqueous Na2S04 (0.5%) and a Ta cathode. The driving voltage for the reaction was provided by a Hewlett-Packard constant dc current power supply (Model 6186B). During anodization, current density was limited to several mA/cm2 to assure slow growth rates necessary for uniform film thickness. The oxidized metal strip was attached with silver paste to the sample probe tip used for sputtering (the FAB probe) (12). The
0003-2700/83/0355-1434$01.50/0 0 1983 American Chemical Society
ANALYTICAL CHEMISTRY, VOL. 55. NO. 8. JULY 1983
Flgure 1. Erosion contours formed on a 1970 A thick Ta,O, film by impact of a focused, 5 keV Ar' beam coupled to an HP 59858 GCIMS working distance of ion gun (PHI Model 04-303). -5 cm: diameter of spot, 1 mm. From the c o b changes obsewed and a 2 min "beam on" time, a sputter rate of -480 A/min is deduced.
1435
-
probe was then inserted into its position in the path of the beam and the gun turned on. After a given period (usually minutes) the gun was turned off, the probe was withdrawn, and the strip was examined. At the point of impact of the primary beam, a spot defmed by changes in the mlor of the fhcould he discerned. Figure 1is a black-and-white enlargement of a submillimeter-sized spot showing in gray tones the erosion contours. It was not necessary to scan the mass spectrometer during this procedure. On those occasions on which secondary ion mass spectra were acquired during sputtering, spectra similar to those shown in Figure 2 were obtained. Correlation of color with oxide thickness requires preparation of a standard 'calibration test strip". For this, a strip of tantalum was subjected to repeated cycles of (a) anodization a t a given voltage, followed by (b) slight retraction of the strip from the cell and further anodization of the section of strip which remained in the electrolyte at a higher voltage. This method of multiple anodizations produced on the strip a series of colored oxide films ranging in thickness from 300 to 3600 A, shown in the photograph of Figure 3 as shades of gray. (A color photograph of this 'calibration strip" is available upon reprint request.) The various colors correlate with accurately known thicknesses from the anodization parameters (12). The f h were then washed in distilled water and air-dried. Once formed, they are inert and will not he stripped from the base metal by handling.
RESULTS AND DISCUSSION The three-dimensional image recorded in a sputtered TeO, strip contains information from which the physical dimensions of the primary beam and its intensity can be determined. The position of the spot on the strip shows where on the probe the primary beam impinges and thus from where secondary particles are produced. Knowledge of the area of origin of sputtered particles can suggest means of improving the efficiency with which secondary ions are collected. From the area and depth of the spot the sputtering rate can be calculated. Since the rate of sputtering is related to the intensity of the primary beam, the sputter rate and implied current density can be useful indicators of the experimental conditions under which sputtering experiments are conducted. The sputter rate, i,is defined as the thickness of material removed, Aq per unit time, t, usually given as Angstroms of oxide thickness per minute. The color of the spot produced
I.$;
i
Flgure 2. (a) Positive-ion SIMS spectrum of tantalum oxide, Ta,O,: static, 8 heV XeCprimary beam used on a Finnigan TSO organic mass spectrometer modified for liquid SIMS: area bombarded on sample, -3 mm2:ions at mle 23 (Na') and m/e 39 (K') arise from impurities. (b) Negative-ion SIMS spectrum of tantalum oxide, Ta,O,: same conditions as in Figure 2a; signal due to oxygen contained in the oxide sample.
in the oxide hy exposure to the beam indicates the thickness of the oxide film remaining. The difference between the thickness of the initial oxide layer and the thickness after exposure to the beam gives Az, the depth of the sputtered crater or the thickness of the layer removed. Knowing the length of time, t, the oxide was exposed to the beam, the sputter rate, i, can easily he calculated i = Az/t
The speed of erosion of the target is an indirect measure of the primary beam intensity, since it is the impact of primary particles that causes sputtering. To convert the sputter rate to the current density, the absolute sputtering yield (i.e., the number of secondary particles produced by the impact of a primary particle of given mass and energy) must he known. Because yields for many material/beam combinations are not known, we recommend the use of sputter rates for comparisons of experimental conditions (see Appendix). By use of the color changes in sputtered Ta,O, as a measure of primary beam flux, a number of simple experiments can be designed to understand more fully the characteristics and capabilities of a given primary particle source. In systems employing ion guns, parameters such as beam energy, emission current, and gas pressure can he varied, with central interest in observing how their variations influence the sputter rate. The ratio of ions to neutrals in the flux from sources reputed to produce neutral beams, such as ion guns fitted with charge exchange cells (13) or high-pressure nozzle-type sources (24),
1438 * ANALYTICAL CHEMISTRY, VOL. 55, NO. 8. JULY 1983
atom (ion) bombardment for the routine analysis of heretofore difficulbto-analyze organic materials, the need to specify experimental Conditions becomes increasingly impartant, both t o enhance the usefulness of the technique and to eliminate ambiguity as to the conditions under which particular results were obtained. ACKNOWLEDGMENT The authors thank Charles W. Magee for many helpful suggestions and discussions. APPENDIX The number of particles per unit time, t, multiplied by the electronic charge factor, herein designated Q, is the current,
i,
i,
= npQ/t
where the subscript p indicates reference to primary particles. The average current density J, is equal to the primary current impacting area A J , = ip - = - npQ
A
At
As indicated in the text, the number of secondary particles produced, %, per primary particle impact is given by the yield,
Y
Flgure 3. "Calibrationstrip" of tantalum oxide. used for side-by-side comparisons with the COIM pattern(s) roducad by sputtering on a test strip. ranging in thickness from 300 i(top of photograph) to 3600 A (bonom of photograph). Increments of oxide thickness are approx-
imately 300 A.
can also be investigated. By use of electrostatic deflection plates to remove ions from the composite beam before it impacts the Ta205target, the sputter rate due to the neutral component of the beam can be measured. This then can be compared with the sputter rate obtained with no ion deflection, giving the fraction of primary particles which is neutral. The recent observations of Aberth e t al., reported in this journal (E),underscore the need for a means of chacterizing primary beams. In pointing out the apparent increase in sensitivity using beams of Cs+ as opposed to Xeo, the authors suggest a "tighter focus" available when ion beams were used for sputtering. Such an effect from focusing implies an increase in current density. Comparison of the spots produced in Ta205with the two different primary beams provides the means for probing exactly such differences. It should be noted that the use of the Ta20, strip is not limited to characterization of the primary beam at the target. It can be physically inserted at any point along the ion flight path in a mass spectrometer. Beam images recorded with the strip in such positions can afford insight into focusing of intermediate images, checks on the quality of beam collimation, and observation of ion flight paths before or after a collision cell or charge exchange cell. The generally low intensity of a beam in the secondary ion flight path, however, may require long term exposure of the Ta205to the beam to accumulate sufficient ion dose to reveal surface damage. CONCLUSION The method described here for the use of a tantalum oxide strip a~ a sputter target provides a simple method for obtaining information about the site and rate at which sputtering occurs. With the growth in reliance on the use of liquid SIMS or fast
Substituting this expression for np in the expression for J , gives J
= -n.Q
YAt
n.can be calculated from the dimensions of the sputter crater PXV n,=-=M
PN&A M
where p is the target density in g/cm3, M is the molecular weight of the target material in grams/mole, No is Avagadro's number, 6.023 X loz3particles per mole, and Vis the volume of the sputtered crater, &A, in em3. Thus
Collecting t e r n and simplifying results in the expression give
where K is a constant incorporating Registry No. Ta,O,, 1314-61-0.
p,
No,M, and Q.
LITERATURE CITED Barber. M.; Berdoll. R. 5.;EllioH. 0. J.; SBdgwICk. R. 0.; Tyler. A. N. Anal. Chem. 1982, 5 4 . 645A-657A. Hunt. 0.F.; %one. W. M.; Shabanowitr. J.; Rhodes. J.; Belkd. J. M. Anal. Chem. 1981. 53, 1704-1706. Williams. 0.H.; Bradley. C.; Bolesen, 0.;Sontlkam. S.; T a m . L. C. E. J . Am. Chem. Soc. 1981, 103, 5700-5704. Murphy. R. 6.:Chy, K. L. Presented at lhe 3Olh Annual Conterencs on Mass Specbometry and Allied Topics; HOI~OIUIU, HI, June 6-11, 1982. Andersen. H. H.; Bay. H. L. In "To~Icsin Applied Physics"; Behrlsch, R., Ed.; Spingar-Vedag: Berlin. 1961; VoI. 47. pp 145-218. While. F. A. "Mass Spectrometry in Science and Techndagy"; Wiley: New Ywk, 1968: pp 84-119. Guthrle. J. W.; Blewer, R. S.Rev. Scl. I n s b m . 1972. 43. 654-655. Delphncke, J. L. Suri. Technol. 1982. 16. 153-162. Born. M.: Wolf, E. "Principles of Optlcs", 5th ed.: Pergamn R e s : Oxford. 1975 pp 323-367. Gale. P. J.; Bentz. 8. L.; Harrlngton, W. L.; Magee. C. W. Presented at the 30th Annual Conference on Mass Spctrometry and A l l ! Topics; Honolulu. HI, June 6-11. 1982.
Anal. Chern. 1983, 5 5 , 1437-1440 (11)
Pawel, R. E.; Pemsler, J. P.; Evans, C. A,, Jr. J . .€/ectrot:hem. SOC.
1972, 119, 24-29. (12) Hodgkin, N. M. In "Metallographic Specimen Preparation"; McCall, J. L.. Mueller. W. M..Eds.: Plenum Press: New York. 1974 DO 297-306. rr (13) Taylor, L. C. E. IniiTkes./Dev. 1961, 23, 124-i28. (14) Maiioney, J. F.; Perel, J.; Forrester, A. T. Appl. fhys. Lett. 1981, 38,
-. -._.
I
1437
320-322. (15) Aberth, W.; Straub, K. M.; 2029-2034.
Burlingame, A. L. Anal. Cbem. l982!, 5 4 ,
~
RECEIVED for review January 21, 1983. Accepted April 4,1983.
Lineair Mass Reflectron with a Laser Photoionization Source for Time-of-Flight Mass Spectrometry D. M. L,ubman,*lW. E. Bell,* and M. N. Kronlck' Quanta-Ray, Inc., 1250 Charleston Road, Mountaln View, California 94043
Time-of-flight devices have many features which make them the mass spectrometer of choice in photoionization experiments. Most significantly, the time-of-flight mass spectrometer (TOFMS) can display the full mass spectrum of ions created by a single laser pulse. For laser photoionization sources with low repetition rates this is especially important because of the low duty cycle. The TOFMS in theory can provide unlimited mass range in contrast to the quadrupole mass spectrometer in which the transmission falls off rapidly as the mass increases. The T O F device is also mechanically and electrically simple compared to other types of mass spectrometers and is thus less susceptible to failure since there are no movable parts or scanning fields. Perhaps the most significant drawback of present linear TOF devices based on the design of Wiley and McLaren ( I ) is the limited resolution achievable. Typically the resolution of such devices built in-house has been on the order of 250 for a 1.5-m device using a thermal beam. The main limitation on the resolution is the initial energy spread of the ions created, which causes a spread in time of the ions arriving at the detector. The initial energy spread must be minimized or compensated for in the field free drift region if the width of the ion packets is to be minimized and the resolution increased. Recently, Mamyrin and Shmikk have reported a new TOFMS with substantially increased resolution (2,3) which has found use in several laser-based applications ( 4 4 ) . This device, named the mass reflectron, can achieve a resolution of as high as 3000 in the width of the peak at half-height with a drift length of 1.6 m. The device compensates for the difference in the time-of-flight of the ions of different energies by means of a system of electrostatic fields and an ion reflector which results in focusing of the ion packets in space and time at the detector. The ion reflector which is the essential feature of the reflectron, allows ions with greater velocities to penetrate a greater distance into the reflector region than ions with slower velocities. Thus, ions with greater velocities will travel a greater distance and spend more time in the reflector region than ions with slower velocities. Of course, in the field free drift region the faster ions will spend less time. The reflector scheme therefore compensates for the initial spread in ion velocities. If the total flight time in the drift and reflector region can be made almost the same for every ion of the same m l e ratio, then the ion packet widths will be short, as is necessary for the attainment of high resolution. The original devices built by Mamyrin and collaborators used a V-shaped trajectory. The problems with this design were the difficulty of focusing the ion beam in the angles of emission from the source and the increased diameter of the Present address: Department of Chemistry, University of Michigan, Ann Arbor, MI 48109. Present address: Jerome Instruments Corm. Jerome. .AZ. Present address: Applied Biosystems, 85O'Lincoln Centre Dr., Foster City, CA 94404.
analyzer chamber ( 2 ) . As a result, this group then designed a linear mass reflectron which circumvented these problems (3). For an ion drifft length of 0.6 m they were able to obtain a resolution of 1200. This design allows the construction of a mass spectrometer of small size with high resolution. The literature on the practical details of building reflectrons is rather sketchy. In this paper we present the design o f a linear mass reflectron based on the article of Mamyrin and Shmikk and present results obtained by using UV laser radiation as the ionizing source for gas-phase molecules. In addition, we discuss the utility of using UV laser radiation as the ionizing source in TOF mass spectrometry.
EXPERIMENTAL SECTION The reflectron consisted of three mechanical parts: ( I ) the reflectron focusing plate structure and reflector, (2) the vacuum chamber, and (3) the drift tube with the detector housing. The reflectron focusing plate structure was constructed as an iadependent unit on a 6 in. stainless steel conflat flange which could be mounted on the vacuum chamber (Figure 1). The structure was supported by four 1/4 in. diameter, 6'/4 in. long rods screwed into the flange. The threads on the rods were slotted in order to allow the threads to be pumped out. The plate spacings (see Figure 2) were the same as those used by Mamyrin and Shmikk (3) where dl = 0.25 cm, d2 = d3 = d5 = 0.5 cm, d4 = 10 cm, L1 = 2 cm, and L, = 56 cm. The voltages on the plates were originally intended to be the same as provided in this reference. The plates were 13/4in. stainless steel disks slotted for the four support rods with a 3/s in. aperture in the center. The focusing plates used a f i e Ni mesh in order to keep the ions traveling in straight paths. The mesh used was1 Ni (Buckbee-Mears, St. Paul, MN) LOO line@. with a translmission of 82%. Etched stainless steel mesh can also be used since it is less likely to warp, especially if heated, as compared to copper or nickel. However, a comparable stainless steel mesh will have significantly lower transmission. The fine mesh is clamped between two plates to keep it as flat as possible. It can also be spot-welded to one plate, but it is rather difficult to keep the grid perfectly flat by using this method. A warped grid will affect the trajectory of the ions and eventually ithe resolution. The spacers between the plates are made of Maror, a machinable ceramic (Corning Glass Works, Corning, NY). Each spacer has an extended lip to space the plates from the support rod to prevent shorting. The whole assembly is fastened to the rods by four nuts. In order to prevent complete loss of ions between the reflector and plate P3,guard rings were used to maintain a homogeneous field. Plates were spaced -3/ls in. apart so that 19 plates were used. Obviously a greater number of guard rings produces a more homogeneous field, but creates greater difficulty in assembly. A chain of 10-M Q resistors was used to create the appropriate potential on each guard ring as the voltage drops from -1000 V on the reflector to the -500 V on PB. The guard rings were also spaced by Macor spacers which included a lip to prevent shorting t o the support rods. The voltage for the reflector and focusing plates was brought through vacuum feedthroughs and the electrical leads were soldered to the plate with low-temperature silver solder. The leads were insulated with glass sleeving and stray fields were shielded with stainless steel tubing. The reflectfor
0003-2700/83/0355-1497$01.~0/0 0 1983 American Chemical Society
