Anal. Chem. 1987, 59, 440-443
440
Spectrometry of X-ray Induced Emission in Sputtering Deposition: A New Technique for in Situ Thin-Film Chemical Analysis Michel Hecq* and Jacques Leleux
Laboratoire de Chimie Inorganique, Universitt? de Mons, 23, avenue Maistriau, 7000 Mons, Belgium
A new technlque Is presented that allows the study In 8th of the chemlcal compodtlon of sputtered films durlng thelr growth. A spulferlng chamber Is coupled with a vacuum X-ray spectrometer allowhg the analysis of X-ray emlseion Induced by the fast electrons of the sputterlng discharge (rf or dc mode). Most elements can be detected. Some appllcatkns of this technlqud are ghrene detection of the end polnl of the presputtering time, process control of Indlum-tln oxlde reactive sputterlng, and dmuskn of subatrate atom in thln flhns.
The sputtering process is a very widespread technique for thin film deposition (1). Most inorganic compounds can be synthesized by sputtering deposition, including compounds that are thermodynamically unstable in the bulk state (2). Basically, one can describe the technique as follows: The compound or a mixture of elements in the solid state is placed on the cathode in a vacuum chamber and polarized to a few thousand volts. If at a given pressure the cathode-anode distance is large enough, a plasma (glow discharge) is produced. The target is bombarded by positive plasma ions with the result that the chamber walls are covered by sputtered particles. Ar is generally used as the discharge gas but it is also possible to mix Ar with some reactive gase. In thus case chemical reactions between sputtered and gas particles are possible and compounds chemically different from the target are deposited on the walls. A thin film can thus grow on a substrate placed on the anode (Figure 1). During this growth, the film is bombarded by ions, electrons, and photons (3). Film composition and microstructure are not well controlled. This is why several methods have been developed to study correlations between plasma variable and film composition. The electrical parameters of the plasma are measured by Langmuir probes ( 4 ) ,whereas chemical composition can be studied by optical emission spectroscopy (5) or mass spectroscopy (6). However, the chemical composition and microstructure of the films are only determined after deposition. There are several methods for analyzing thin films (7) but most of them require high vacuum and an external excitation source (secondary ion mass spectrometry, electron spectroscopy for chemical analysis, Auger electron spectroscopy, ...). Techniques that can be applied during the sputtering deposition probe the plasma (optical emission spectroscopy or mass spectrometry) and not the film itself. The technique that will be presented in this paper allows chemical analysis in situ during f i i growth provided that the discharge is in the capacitive mode (dc or rf). The principle of the technique is as follows: Under ion impact there is emission of neutrals, ions, photons, and electrons from the target surface. The mean energy of these secondary electrons is about a few electron volts but they are accelerated across the cathode sheath and enter the negative glow with an energy equivalent to the discharge voltage. These fast electrons hit the gas particles and produce atomic or molecular ionizations and excitations, which attenuate their energy somewhat.
Table 1. Variation of the 0 X-ray Emission as a Function of the Presputtering Time time, min
R (0 Ka)
time, min
1.0
40.6 16.9 7.9 3.4
15.5 20.0
4.0 8.0 11.5
24.5
R (0 K a ) 1.0
0.2 0.0
However, most arrive on the walls with a large part of their initial energy (3). It is well-known that these fast electrons are mainly responsible for the substrate heating. Of the energy dissipated by electrons in the thin film or in the substrate, a fraction is used to produce X-ray emission. We have discovered that the X-ray characteristic emission is large enough to enable a chemical analysis.
EXPERIMENTAL SETUP The method (8)requires the use of two different techniques, sputtering and X-ray spectrometry. The apparatus is shown in Figure 1. It includes a sputtering chamber and an X-ray vacuum spectrometer. They are connected by means of a tube containing Soller slits. The tube can be closed by a gate valve. X-rays are diffracted by a flat crystal (ammonium dihydrogen phosphate, thallium hydragen phthalate, lead stearate) according to the Bragg equation and detected by a gas flow counter (10% CHI in Ar). The counter operates at 200 torr and has a 1-pm polypropylene window. The sputtering system is a standard diode unit with watercooled cathode shutter and substrate holder anode. The massdeposition rate is monitored by a water-cooled quartz microbalance. The cathode can be powered by dc or rf current. In the rf mode, an impedance matching box is connected between the target and the rf generator. Vacuum is maintained in both chambers by means of turbomolecular pumps. The residual pressure is checked by a quadrupole mass spectrometer. Gases are admitted through mass-flow controllers. The film thickness is measured by X-ray fluorescence. RESULTS Determination of the Presputtering End Point. A sputtering deposition must always be preceded by a presputtering of targets. During this time, a discharge is established between the target and a shutter located close to substrate. Target surface contamination is removed and the system is outgassed. The amount of time needed for this is usually established empirically. However, with our technique it is possible to determine the end point by monitoring the characteristic X-ray emission of a contaminant atom. In this study we choose to monitor the oxygen K a emission. Oxygen production can result from wall outgassing or target oxidation. In Table I, the X-ray signal to background ratio ( R ) and presputtering time are given. This ratio (R = IJI,.,) is preferred to the counting rate due to the difficulty in determining the incident beam current. I p is the maximum intensity of the peak and Ib the background under the peak (found by linear extrapolation). If it is assumed that most of the
0003-2700/87/0359-0440$01.50/00 1987 American Chemical Society
ANALYTICAL CHEMISTRY, VOL. 59, NO. 3, FEBRUARY 1, 1987
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Figure 1. Experimental setup: 1, turbomolecular pump; 2, tltanlum sublimation pump; 3, gate valve; 4, quadrupole mass spectrometer; 5, quartz microbalance; 6, cathode; 7, shutter; 8, substrate holder; 9, window; 10, X-ray crystal analyzer; 11, gas flow counter; 12, Soller slit.
0
75
0
7
L */
2
4
%02
8
Flgure 3. Variation of the thin-film resistlvity as a function of the gas mixture.
25CR
5 0:/ 25
0
u 2
4
6
%02
Flgure 2. Optical transmission at 550 nm as a function of the gas mixture.
background radiation is due to “bremsstrahlung”, Ip/Ibserves as a measure of composition which is less affected by absorption and backscatter effects than the characteristic intensity alone (9). The ratio does not depend on the discharge current. From Table I, the presputtering end point is obtained after 24.5 min. The experiment was carried out by using a silicon target and pure Ar as the discharge gas. In order to estimate the detection limit for 0 in Si, we sputtered a Si target in a mixture of Ar and O2 (20%) to deposit a thin film of SiOz. The minimum detectable amount is calculated by (10) CDL
= (3/m)(lb/Tgb)”2
where m is the peak intensity in counts per second per 1% for the element in question and Tb is the time to count the background (100 s). Assuming a composition of SiOz for the film, m was found to be 214 and I b 51 C I S (at 28 124’ using the TlAP crystal). This yields a CDLof 0.01 or 100 ppm. Control of Reactive Sputtering Deposition. Reactive sputtering is a convenient way to produce indium-tin oxide films (IT0or In203:Sn). These films are of great technological interest because they are transparent and conductors (11).
0
2
4
6
8 %02
Flgure 4. Peak to background ratio for the 0 Kcu line as a function of the gas mixture.
In our experiments the target is an alloy of 90% In and 10% Sn. The ar and O2gas discharge mixture is adjusted by means of mass-flow controllers. The discharge is run in the dc mode at operating voltage of 3800 V and total pressure 4 X torr. The substrate holder is maintained at 200 “C. Films are deposited on glass. Several thin films have been synthesized in different gas mixtures. The transmission has been measured at 500 nm (Figure 2). The film is fairly transparent as long as the oxygen concentration is above 2%. Under this value, the transmission drops sharply. The resistivity (Figure 3) decreases with decreasing oxygen content of the discharge gas down to a concentration of 1% , where the curve shows a discontinuity. Under this oxygen concentration, the film is highly absorbing and it has a metallic reflectivity. Nevertheless, the presence of oxygen is detected
442
ANALYTICAL CHEMISTRY, VOL. 59, NO. 3, FEBRUARY 1, 1987
in the film by X-ray emission (Figure 4). It is then assumed that the film is composed of a metallic matrix with oxygen inclusion. One observes that the resistivity is higher than that of the oxide film. The mechanisms of electrical conduction in semiconducting oxide films are completely different from those of thin metal films (11). The conductivity of I T 0 films is mainly obtained by anion vacancy and tin doping (11). In reactive sputtering, a variation of the oxygen content in the gas mixture does not change the impurity level (tin doping) but could change the anion vacancy. A decrease in the film resistivity (Figure 3) should be thus correlated to an increase of oxygen vacancy (12). In Figure 4 we have plotted the ratio R of peak intensity to background (Zp/Zb) for the 0 Ka peak, R depends on the 0 concentration in the film. A plateau is observed above 2% in oxygen and a sharp decrease between 0 and 2%. There is no straightforward relation between the variation of the resistivity of the oxide film (Figure 3) and the ratio R (Figure 4). However, if we compare Figure 2 and Figure 3, the most interesting values (9070transmission and 3.6 X lo4 Q cm resistivity) are found for an oxygen concentration of 1.8% in the gas discharge. For this concentration, R is 95% of the plateau value. Good electrical and optical characteristics need a relatively large number of oxygen vacancies (12). We point out that any charge in the experimental procedure (for example, an increase in the sputtering rate) will change the oxygen concentration in the gas phase for the optimum physical characteristics and another set of thin-fiim-deposition parameters should be necessary. Experimental time can be reduced if a chemical analysis can be carried out during the deposition. Atomic Diffusion from the Substrate. With the capability of making a chemical analysis of the film during its growth, it is possible to observe the diffusion of substrate atoms into the film. This is achieved by monitoring the extinction of the characteristic radiation of certain elements of the substrate. As the film thickness increases, the energy of the electrons arriving through the film to the substrate decreases and will eventually fall below the critical energy (E,) needed to excite the given line. The critical energy (E,) needed to produce Na Kcu emission is 1.041 keV and using the empirical range formula (13),we can calculate at what thickness the line should disappear. This critical depth (R,) will depend on the difference between the initial energy (E,) and the critical energy for a given material
R, = 2 5 0 A / d " / 2 ( E o "- E,") with n = 1.2/(1 - 0.29 log 2) where A is the atomic mass, a the mass density, and 2 the atomic number. Assuming that Eo is given by the sputtering voltage, in our case E, = 3.8 keV, we find for an In metallic matrix, R, = 910 A and for an In203matrix, R, = 930 A. For this thickness, only 50% of Na K a intensity is absorbed. Thus the critical depth is mainly determined by the excitation energy of the electrons. Thus, after a thin-film thickness of 1000 A, the Na K a line must disappear. The evolution of the Na K a line has been monitored during the deposition of In,03:Sn or 1n:Sn on a glass substrate and at different substrate temperatures. In Figure 5, we have plotted Zp/Zb as a function of the film thickness calculated from the mass deposition as measured by the microbalance (assuming a bulk mass density). When the film is made of In203 and for the different substrate temperatures, the Na K a line disappears after a thickness of 2100 A which is higher than the critical thickness. This shows that Na diffuses into the film but with a diffusion rate lower than the deposition rate. Sputtering of InSn alloys has also been carried out in pure Ar and at different substrate temperatures (Figure 6). In
1I
10
4: Figure 5. Peak to background ratio of the Na Ka line as a function of the I T 0 thin-film thickness for the following conditions: T (substrate temperature) = 100 OC, gas composition = 1.1% O2(0);T = 300 OC, 1.1% 0,(m); r = 300 OC, 1.7% 0,(A).
15t
'Ot
I* l
0
9
. 2
4
6
Figure 6. Peak to background ratio for the Na K a line as a function of the InSn thin-film thickness at the following temperatures: T = 100 OC, 0 ; T = 200 OC, m; = 300 OC, A.
r
these cases, the Na K a line is always detectable; even at thicknesses beyond 6000 A, there is a steady state between the deposition rate and the diffusion rate. It can be shown that the Na diffusion rate relative to the deposition rate goes up with an increase in the substrate temperature.
CONCLUDING REMARKS In this paper we have shown that the spectrometry of X-ray induced emission in a sputtering discharge may be used to monitor the deposition process. We are working on improving the technique by automation of the spectrometer and data acquisition via a microcomputer. The sensitivity could be increased by the use of a more highly reflective crystal and a thinner counter window. Soft X-ray emission results from electron transition in the outer shells of the atoms so that chemical bonds can produce observable line shifts. A study of the line profile by means of a high-resolution spectrometer will give information on the chemical state of the film. Most elements can be detected by using K, L, M, or N X-ray lines, provided the critical energy is lower than the discharge voltage. Thus, our technique has advantages over emission spectroscopy or glow discharge mass spectrometry in that it can give direct information on the chemical composition of the sputtered film and that it may be used for process control (14). ACKNOWLEDGMENT The authors are indebted to M. Bettignies for technical assistance and to Alexander Martin for helpful discussions. Registry No. ITO, 50926-11-9.
Anal. Chem. 1907, 5 9 , 443-448
(1) (2)
LITERATURE CITED Vossen, J. L.; Cuomo, J. J. In Thin Fllm Processes; Vossen, J. L., Kern, W., Eds. Academic: New York, 1978; pp 12-73. Hecq, M.; Hecq, A.; Delrue, J. p.; Robert, T. J . Less-Common Met.
1079, 84, 25. (3) Chapman, B. olow Discharge Processes; Wiley: New York, 1980. (4) Bail, D. J. J . Appl. Phys. 1072. 42, 3047. (5) Greene, J. E.; Sequeda-Orsorio,F. J . Vac. Scl. Technoi. 1073, 10, 1144. (6) Harrison, W. W.; Hess, K. R.; Marcuss, R. K.; King, F. L. Anal. Chem. W86. 58. 341A. (7) Werner, H. W.; Garten, R. P. H. Rep. Prog. PhYs. 1964, 4 7 , 221. (8) Belglan Patent 01216896, 1986. (9) Statham, P. J. In Microbeam Analysis; Nemerg, E., Ed.; Sari Francisco Press: San Francisco, CA, 1979; p 247.
----. - - .
443
(IO) Berth. E. P. Princlpies and Practice of X-ray Spectrometric Analysis;
(12)
Plenum: New York, 1975. Vossen, J. L. In Physics of Thin Films Hass, G., Francombe, M. H., Hoffman, R. W., Eds. Academic: New York, 1977. Manifacier, J. C.; Spessy, L.; Bresse, J. L.; Perotln, M.; Stuck, R. Ma-
(13)
Roche, A.; Charbonnier,M.; Gaillard, F.; Romand,
(11)
fer.Res. Bull. 1070, 74, 163.
Surf. Sci. 1081, 9 , 227. (14) Lowry, R. K. Anal. Chem. 1086, 58, 23A.
M.; Bador, R. Appl.
RECEIWDfor review June 23, 1986. Accepted September 17, 1986. The authors thank Le Ministere de 1'Energie de la RBgion Wallonne for the support of this research.
Serially Interfaced Gas Chromatography/Fourier Transform Infrared Spectrometer/Ion Trap Mass Spectrometer System Edwin S. Olson* and John W. Diehl
University of North Dakota, Energy Research Center, Box 8213, University Station, Grand Forks, North Dakota 58202
A serial gas chromatography/Fourier transform infrared spectrometer/mass spectrometer (GC/FTIR/MS) system has been developed with an ion trap detector or mass analyzer that Is interfaced to the light pipe In the FTIR spectrometer. A modlficatlon of the manufacturer-supplied open-spilt interface to the ion trap was required to obtain chromatographic r e d s free of dlscrimlnation and activation effects. The flow rate of the helium make-up gas in the light pipe was used to control the amount of material that enters the ion trap. Hydrogen carrier gas was used for the chromatographic separatkns with no adverse effects on the mass spectra obtained. The analyses of three samples are described to demonstrate the capabilities of the system. These include the Grob test mixture, a steam c#sHlate from a gadfkation water treatment plant, and a sample of methyl esters of coal oxidation products.
The powerful analytical capabilities of a gas chromatography/Fourier transform infrared spectrometer/ mass spectrometer (GC/FTIR/MS) system have been reported in previous publications (1,2). These systems have employed either splitting of the GC column effluent between the FTIR and MS in a parallel configuration (1)or serial interfacing where the column effluent passes through the light pipe and then directly into a jet separator on the MS (2). A GC/ F"lI'R/MS system that differs from those reported previously has been assembled in our laboratory. An ion trap detector (ITD) with a modified open-split interface was employed for the MS, and this was serially coupled to the light pipe exit in such a way that the make-up gas to the light pipe controls the amount of material actually entering the ITD transfer line. EXPERIMENTAL SECTION Gas Chromatography. A Hewlett-Packard Model 5890A gas chromatograph with a J&W on-column capillary injector was connected t o the Nicolet GC/FTIR interface as shown in the system configuration diagram (Figure 1). A J&W 30 m X 0.32 mm i.d. 1.0 pm film DB1701 column was used for analyses reported in this paper. The carrier gas was ultra-high-purity hydrogen further purified by a drying tube and oxygen trap in line 0003-2700/87/0359-0443$0 1.50/0
and the carrier flow was set at a linear velocity of 42 cm/s (2 mL/min) at 300 "C. The GC oven temperature programming used an initial temperature of 40 "C and then a rapid increase (30 "C/min) to 50 "C, followed immediately by a rate of 5 OC/min from 50 to 300 "C.
Fourier Transform Infrared Spectrometry. A Nicolet POSXB FTIR spectrometer with the Nicolet GC/FTIR light pipe interface was used. The gold-coated light pipe dimensions were 15 cm X 1.5 mm i.d. and the light pipe temperature was 250 "C. Helium (ultra-high-puritygrade, scrubbed with a General Electric GO-GETTER oxygen and water removal system) make-up gas for the light pipe was adjusted to a flow rate of 0-10 mL/min, depending on the analysis. The data system was the Nicolet 1280 computer equipped with a fast Fourier transform coprocessor and an 86-Mbyte hard disk. The number of scans per chemigram data point was varied between 4 and 16 to produce a data point every 1-3 s, respectively. Heated, 1/16 in. glass-lined tubing (GLT) connected both the light pipe entrance and exit to the inside of the GC oven (Figure 1). The GC column end was inserted through the GLT to the light pipe entrance. A short section of the same column which was used for the chromatography was inserted into the light pipe exit tube as far as bends in the GLT would allow, and the other end of this section was connected to the open-split interface inside the GC oven. Mass Spectrometry. The Finnigan Model 700 ion trap detector was used to obtain mass spectra of the GC eluents. This instrument has a mass range of 10-650 amu with unit resolution. A range of 50450 amu was used for the experiments in this paper. The interfacing of the mass spectrometer utilized a modification of the original Finnigan-supplied all metal open-split interface. The open-split interface was connected to the MS via a 4 ft section of heated metal tubing through which a flow restrictor was installed (Figure 2). This restrictor was a section of SGE 0.1 mm i.d., 0.21 mm o.d., 0.1 pm film BP-10 (OV 1701) column, with the restrictor extending 5 cm beyond the open split interface into the GC oven. The section of column coming from the light pipe exit was then inserted over this narrow piece of fused silica tubing and far enough into the open split interface to allow the column nut and ferrule to be tightened. A helium sweep of 2 mL/min in the open-split interface and a transfer line temperature of 250 "C were maintained. The data system for the ITD was an IBM PC/AT with a 30-Mbyte hard disk and standard GC/MS software for tuning, total ion chromatogram generation, selective ion monitoring, and library search routines. 0 1987 American Chemical Society
