Direct Analysis of Thin Layers by Spark Source Mass Spectrography J. B. Clegg, E. J. Millett, and J. A. Roberts Mullard Research Laboratories, Redhill, Surrey, England Standard techniques of spark source mass spectrography are unsuitable for the analysis of evaporated films, and thin diffused and epitaxial layers in semiconducting single crystals. A precision scanning device i s described which can be mounted in the existing source unit and allows the uniform erosion of the surface of a plane parallel slice between 2 and 3 cm in diameter. Ion yield is independent of all instrumental parameters and analytical sensitivity is proportional to the mass eroded. The random penetration of the spark discharge i s the limiting factor in the analysis of layers below 1 pm in thickness where dilution with the substrate occurs. The attainment of detection limits of 0.05 ppm atomic on 1 pm thick layers is demonstrated together with the identification of surface impurities at 10” atoms/cm2, or less than one thousandth of a monolayer.
MODERNSEMlCONDUCTOR DEVICES and integrated circuits consist of layers within a single crystal containing deliberately (atom ratio) and added impurities at concentrations up to with thicknesses in the range of 0.1 to several hundred microns. Extraneous impurities may be significant at concentrations as low as 10-8 and the dimensions of the active areas of individual devices may be measured in microns. Ultimately, it would be desirable to determine impurities at this concentration in a volume of one cubic micron, but as this would represent a gram, this is unlikely to detection limit of 100 atoms or be attained directly by any technique currently available. In practice, the devices are formed in successive layers, in patterns delineated by oxide masking, on slices of single crystal silicon about 3 cm in diameter. The impurities introduced during processing could therefore be explored by analyzing complete layers deposited over the whole area of an experimental slice. Spark source mass spectrography is the only instrumental technique capable of simultaneously determining a large range of impurities in bulk semiconductor materials at concentrations down to During the impurity analysis of a rod sample of silicon, approximately 10 mg of the sample (2 x lozoatoms) are consumed to provide a 1-pC exposure on the photographic plate giving a limit of detection of 10-9 or 1011 atoms for most elements. A layer 3 cm2 x 7 pm in depth contains sufficient material for an analysis at maximum sensitivity and a layer I-pm thick would permit analysis with a limit of detection of 10-8. A contaminant on the surface would be seen at a concentration as low as of a monolayer irrespective of the sampling depth. The analysis of surface films by manual scanning of the counter electrode over a small area of the sample has been reported but the local penetration cannot be controlled because of the superimposition of successive spark discharges and ranges from 3-5 pm to 100-500 pm (1-4). (1) W. M. Hickam and G. G. Sweeney, “Mass Spectrometric Analysis of Solids,” A. J. Ahearn, Ed., Elsevier, 1966,pp 138-164. (2) W. M. Hickam and G. G. Sweeney, Reo. Sci. Instrum., 34, 783 (1963). (3) R. A. Wallace and J. Roboz, A.S.T.M., Mass Spectrometry Conference, San Francisco, Calif., 1963. (4) A. J. Ahearn, J . Appl. Phys., 32, 1197 (1961).
Ahearn showed that surface contamination equivalent to 0.01 of a monolayer could be detected ( 4 ) and this would, of course, interfere in the quantitative analysis of the bulk concentration of impurities in finite layers. Axelrod et at. have reported the analysis of molybdenum tantalum ratios in 0.4pm films using successive linear traces 3 pm deep across a rectangular sample and estimated a limit of detection by extrapolation of 10 ppm atomic (5). Malm (@ and Brown (7) have recently described the use of a rotating disk with manual scanning to increase the area sampled in the analysis of discrete layers between 0.5 and 5 pm in thickness, but give no indication of the sensitivity to be expected. Random penetration of such discrete layers only reduces the overall sensitivity in proportion to the volume of the substrate sampled and gives rise to interferences from the matrix elements of substrate and the impurities in it. As the substrate is chemically different in this case, the dilution can be directly estimated and the interferences corrected for quantitatively. In the analysis of doped semiconductor layers, the matrix element is constant and quantitative analysis of the distribution of an impurity in depth requires controlled penetration with a discrimination of a micron or better. Malm using manually scanned rotation could only establish such a concentration profile by measurements on a series of samples with successive layers removed by controlled etching, deducing the profile by difference (6). Hickam and Sweeney using a fixed counter electrode and a rotary scan report the generation of individual spark discharge craters 0.2 pm in depth or less, but with such small volumes of sample the concentration sensitivity is naturally very poor ( I ) . Whatever the geometry of the individual spark craters may be, it is evident that the uniform erosion of a finite volume of sample can only be achieved by superimposing successive discharges in a controlled manner using a mechanical scan in two dimensions. The generation of such a mechanical scan presents practical difficulties but was solved using a device originally developed independently in this laboratory (8). This paper describes the device, the experimental determination of the factors affecting spark crater geometry and erosion depth, and the attainment of practical measurements of impurity concentrations at the 10-8 level in semiconductor materials at controlled depths down to 1 pm. EXPERIMENTAL
Device Construction. The simplest mechanism that springs to mind for a two-dimensional mechanical scan is a spiral track generated by a linear traverse across a rotating disk. ( 5 ) N. N. Axelrod, B. H. Vromen, H. G. Guberman, D. J. Harring-
ton, and N. Schwartz, “Physical Measurement and Analysis of Thin Films,” E. M. Murt and W. G. Guldner, Eds., Plenum Press, New York, N. Y.,1969, pp 159-165. (6) D. L. Malm, “Physical Measurement and Analysis of Thin Films,’’ E. M. Murt and W. G. Guldner, Eds., Plenum Press, New York, N. Y., 1969, pp 158-159. (7) R. Brown, 16th Annual Conference on Mass Spectrometry (A.S.T.M. E-14), Pittsburgh, Pa., 1968. (8) E. J. Millett, British Patent 45912, (1965) and 6th Annual M.S.7. Users Conference, Manchester, 1966. ANALYTICAL CHEMISTRY, VOL. 42, NO. 7, JUNE 1970
a
713
A Tantalum 0 Silicon
Figure 3. weignt mses ot silicon ais&ana tantalum counter electrode as a function of counter electrode diameter
Sample platen
.
Wormwheel worm
\
T 0
R.F SUPPIY
P
-
i = wI
-7 I
Motor driven spindle
Fixed bolt drive
Figure 2. Mechanical design of scanning system
nowever at a consranr ~ravcrscsperu anu angurar r v r a u u ~ ~ , the dwell time increases as the counter electrode approaches the center, so that with a 10-mm radius sample the erosion depth would be expected to be five times as great 2 mm from the center as at the edge. This was overcome by a mechanism incorporating a fixed ball drive to both rotate and traverse the specimen so that a spiral path is traced out by the R.F. spark maintained between the disk surface and the counter electrode at constant speed (Figure 1). The optimum values of spiral pitch and surface scanning speed to give complete erosion were calculated from experimental results with rod electrodes of silicon. As the normal silicon consumption rate is 10 mgjhr with a spark pulse repetition rate of 300jsec and a pulse length of 100 fisec, it was estimated that lo-* gram of material were consumed per pulse. (All pulse parameters are the nominal instrumental values.) If each pulse produced a hemispherical crater, the radius would be 10 microns. To form a spiral 714
-
ANALYTICAL CHEMISTRY, VOL. 42, NO. 7, JUNE 1970
track with craters just touching, the surface scanning speed should be 20 microns of linear movement between pulses with a spiral spacing of 20 microns. For a pulse repetition rate of 300/sec, the required speed is 6 mmjsec. The mechanical design of the device is shown in Figure 2. The drive from an Escap dc micromotor powered by DEAC perma-seal nickel cadmium accumulators is transmitted to the sample platen via two rotating plates separated by a fixed ball mounted 2 mm from the axis of rotation. The surface speed measured directly above the ball is 7 mmjsec. To produce a constant track spacing, the carriage is also driven from the ball drive by means of a wormwheel and lead screw, and gives a pitch of 16 microns. Since the carriage can traverse 8 mm, the maximum scanning region of the disk is the annular area between 2 and 10 mm radius and represents 9 6 x of the available area for a 20-mm diameter disk. The device which operates in the source unit of the spectrograph (type MS7 made by GECjAEI Ltd) under a vacuum of torr and at R.F. potentials of up to 40 kV is partly enclosed by a molybdenum box to produce a field-free region near the spark. All moving parts are lightly lubricated with diffusion pump oil and the mechanism has given trouble free performance for over one hundred hours. Sample Presentation and Counter Electrode Geometry. The samples used for performance assessment were silicon disks 20 mm in diameter, 0.28 mm thick, plane parallel, and with resistivities of 0.02 and 5 n cm. The disks were mounted on the sample platen with a film of vacuum wax sufficiently thin to maintain a reasonably low electrical resistance (approximately 1000 n) between the two. The amount of wax used has to be carefully controlled to maintain the sample surface sufficiently parallel to the platen. Small variations in the spark gap have marked effects on the stability of the spark during scanning and the total displacement of the platen at right angles to the surface, measured at the edge, must be less than 15 fim per revolution. The device support was designed so that the sample was mounted at an angle to the axis of the spectrograph, in order to minimize self-shielding effects. Four disks were scanned with the same spark conditions with the device at 15' and 30", the latter representing the maximum angle obtainable due to size limitations within the source. The total ion charge was not significantly affected, so the 15" angle was used throughout the work reported here. Tantalum was initially chosen as a counter electrode as the purity is acceptable, the atomjmass ratio is favorable and it is easily machined or sharpened to a point. For electrode diameters less than 300 pm, the electrode tip geometry, Le., whether pointed or blunt, was found to have no significant effect on the diameter and distribution of craters formed at
Figure 4. Single track scanned a t 10 pps X 25 ps with 17.5 kV applied, at 7 mmjsec. Counter electrode diameter 0.45 mm Magnification XlOO (reduced X% in reproduction)
Figure 5. Single track scanned at 30 p.p.s. X 25 ps with 30 kV applied Magnification XlOO (reduced
Figure 6. R.F. breakdown oscillogram of a nominal 100-ps pulse. Time scale 20 ps per main division
32.5 kV applied
x%in reproduction)
low spark pulse repetition rates so all electrodes were deliberately blunted by putting a flat at right angles to the tip. Figure 3 shows the relationship between the weight losses of tantalum and silicon and the counter electrode diameter. With small diameters, the high erosion rate of the counter electrode required frequent adjustments of the spark gap. Electrodes larger than 0.5 mm in diameter gave reduced ion currents, presumably because of shielding. A diameter of 0.45 to 0.5 mm was selected as a convenient compromise, which gives a relatively large silicon weight loss and silicon/ tantalum ratio with an acceptable rate of erosion of the counter electrode. Crater Geometry and Erosion Profile. To investigate the crater geometry produced by this device, a series of single circular tracks were laid down starting with the lowest pulse repetition rate and pulse length. Figure 4 shows that at 10 p,p.s. (pulses per second) X 25 ps two clusters of craters are formed separated by a distance corresponding to the surface displacement between pulses (approximately 980 pm). The craters are spread over an area of roughly the same diameter as the counter electrode. Figure 5 shows a single track of 30 p.p.s. X 25 ps, At this repetition frequency an R.F. pulse is applied once each time the disk has moved through 300 pm, so that the area containing the crater patterns from one pulse just overlaps in the direction of the scan. Angled sections through tracks formed at 30 p.p.s. show that the craters are just less than one micron deep and are surrounded by a rim of material deposited above the original
Maanification XlOO (reduced X% in reoroduction) level of the surface. The results suggest that crater dimenrinno
fainrtinnr nf t h e slnnlinrl
..
..nl+nnn.
with
17
