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Determination of the Implantation Dose in Silicon Wafers by X-ray Fluorescence Analysis Reinhold Klockenkamper,* Maria Becker, Henning Bubert, and Peter Burba
Institut fur Spektrochemie und angewandte Spektroskopie, Postfach 10 13 52, 0-4600 Dortmund, Federal Republic of Germany Leopold Palmetshofer
Institut fur Experimentalphysik, Johannes-Kepler- Universitat, A-4040 Linz, Austria
The ion dose implanted in silicon wafers was determined by X-ray fluorescence analysis after the implantation process. As only near-surface layers below 1-bm thickness were considered, the callbration could be carried out with external standards consisting of thin films of doped gelatine spread on pure wafers. Dose values for Cr and Co were determined between 4 X 1015 and 2 X 10’’ atoms/cm2, the detection limits being about 3 X loq4atoms/cm2. The resuits are precise and accurate apart from a residual scatter of less than 7 YO. This was confirmed by flame atomic absorption spectrometry after volatilization of the silicon matrix as SIF,. It was found that ion-current measurements carried out during the implantation process can have considerable systematic errors.
Ion implantation into solids is gaining increasing importance in modern material technology. By application of this method, the physical and chemical properties of near-surface regions can be modified in a well-controlled manner. Above all, ion implantation is used for the production of novel semiconductor devices, while this method is still in the stage of development in the field of metallurgy and ceramic or glass industry (e.g., ref 1 and 2). Besides, ion implantations are performed to produce calibration samples for surface analysis and depth profiling. In the ion implantation process, accelerators are used when a definite kinetic energy of the ions is desired above some tens of kiloelectronvolts and up to some megaelectronvolts. The implantation dose, i.e., the number of the implanted ions/cm2, is usually determined by ion-current measurements (ICM). The determination of the ion dose is thought to be generally reliable. However, various effects-such as the incomplete suppression of the emission of secondary electrons generated by the ion bombardment or the reduction of the ion-current density caused by the broadening of the ion beam during scanning-can lead t o considerable systematic errors. They would feign too high values of the dose ( 3 ) . Therefore, it is necessary to check the given values after the implantation process, especially when the ion dose is the basis for the quantification of depth profiles, which is the case for the so-called integration methods (4, 5 ) . This investigation was made for implantations of Cr and Co ions in silicon wafers. Naturally, the real value of the implanted ions can be determined afterward by digesting the target (the wafer) chemically and analyzing the solution by inductively coupled plasma optical emission spectrometry (ICP-OES), flame atomic absorption spectrometry (FAAS), or total-reflection X-ray fluorescence (6). However, these methods are time- and material-consuming. Other instrumental methods as neutron activation analysis (NAA), Rutherford back-scattering (RBS), or synchrotron-induced
Table I. Calibration Samples (C1-425)for the Determination of Cr and Co sample pipetted element mass m, no.
Cr-C1
Cr-C2 Cr-C3 Cr-C4
Cr-C5
pg
0.1 0.5 2
10 40
no. of atoms n
1.16 X 5.80 X 2.32 X 1.16 X 4.63 X
1015 1015 loL6
sample pipetted element mass m, no. pg Co-C1 Co-C2
0.1
0.4
loi7
co-c3 co-c4
2 10
1017
CO-c5
50
no. of atoms n 1.02 X 4.09 X 2.04 X 1.02 X 5.11 X
1015 10l6 10I6 loi7 lo”
X-ray fluorescence (7) are nonconsumptive but highly sophisticated and cannot be applied generally. For instance, 52Cr cannot be detected by NAA in principle. Therefore, we used a classical method that is direct, simple, and even nonconsumptive: the X-ray fluorescence analysis (XRFA).
EXPERIMENTAL SECTION The XRF-analyses were carried out sequentially by a wavelength-dispersive spectrometer. Standard samples were used for calibration that were made by solutions of gelatine pipetted on original silicon wafers and dried. Calibration Standards. Different standard solutions based on an aqueous solution of gelatine were mixed and doped with a Cr and Co standard (Merck-Titrisol). The appropriate viscosity was found for a mixture of 10 mg of Merck gelatine and 1 mL of distilled HzO. These solutions (20 gL) were pipetted on small parts of a silicon disk (Wacker Chemitronic) with a size of 15 X 15 mm2 and dried under IR light within 5 min. The residual homogeneous films had a diameter of about 8 mm, a thickness of 3 Fm, and a mass of nearly 200 fig. They can be regarded as “infinitely”thin films for X-rays of Cr or Co Ka (8). Their number n of atoms can be calculated by the pipetted mass rn of the Cr or Co dosage: n = (rn/M)N
(1)
where N = Loschmidt’s number and M = mass per mole. Table I presents the corresponding values for the calibration samples. Analyte Samples. The implantation was carried out with a 350-kV low-current implanter. The vacuum in the target station was about 2 X lo4 Pa during the implantation. Wafers of n-type silicon, (100) orientated, with a size of 15 X 15 mm2, were implanted with 52Crand 59C0ions, respectively. To avoid channelling, an azimuthal angle of 40’ or 14’ and a polar angle of 4.3’ or 7.5”, related to the crystal orientation, were chosen for Cr or Co, respectively. The kinetic energy of the ions was 300 keV, resulting in a Gaussian-like implantation profile with an average projected range (depth) between 200 and 260 nm and a range straggling (width) of about 80 nm depending on the implantation dose and the kind of ions. Therefore, the implantation zone can also be regarded as a thin film quite near the surface for X-rays of Cr or Co KO (8). The ion current measurements (ICM) were performed with conventional Faraday cups (9). Three independent cups were arranged at the corners of the scanned area outside the main target area. This allowed monitoring of the dose uniformity (IO). The
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Table 11. Analyte Samples (Al-A4) Represented by Silicon Wafers Implanted with Cr and Co sample element no.
implanted ions/cm2
Cr-A1 &-A2 Cr-A3 Cr-A4
5 X 10l6 1.6 X 10l6 5 X 10l6 1.6 X 1017
sample element
implanted
no.
ions/cm2
Co-A1 CO-A~ Co-A3 Co-A4
5 X 1015 1.6 X 10l6 5X 1.6 X loi7
implanted doses for the different samples are controlled by ICM with a repeatibility of 2%; their values are listed in Table 11. XRF Analysis. The flat-crystal Philips PW 1400 X-ray spectrometer was used. A Mo tube was chosen for excitation, the tube potential was set at 80 kV, and the tube current at 30 mA. A LiF (200) crystal was used for dispersion, a tandem of a scintillation and a gas-flow detector for registration of X-rays. A circular aluminum mask with a diameter of 10 mm (Fo= 0.785 cm2) was placed in front of all specimens, enclosing any of the gelatin films entirely. The K a lines of Cr and Co were adjusted sequentially, and the gross-intensities measured within 200 s. The net intensities were determined by subtraction of the blank intensities measured for the pure silicon wafer. By means of the external calibration samples, the number n of atoms was determined for the homogeneous analyte samples (Al, ...,A4). By relating this value to the area Foof the circular mask, the atomic density D, defined by
D = n/Fo
(2)
was calculated. This value presents the desired value of the ion-implantation dose. The complete procedure was carried out in triplicate. Digestion of Wafers. Under heating, small wafer samples (120-160 mg) were digested in an open PTFE vessel by a mixture of 3 mL of 40% HF (suprapur, Merck AG) and 0.6 mL of 65% “OB (prepurifiedby subboiling) according to ref 6. After matrix volatilization as SiF4,the dry residue was redissolved in 1 mL of the above-mentioned acid mixture and heated to dryness. Finally, the residue was dissolved in 1 mL of 10% “OB for the determination by FAAS. FAAS Determination. After digestion, the Co and Cr content in wafer samples were determined by a microtechnique of FAAS (injection technique, 50-pL samples, Pye-Unicam SP 9 spectrometer, resonance lines 240.7 nm Co and 357.9 nm Cr, deuterium background correction) (11). Sample solutions containing a higher Co or Cr concentration had to be diluted (1 + 4) by 10% HN03 before FAAS determination. For calibration, four synthetic standard solutions being adequate to the dissolved samples were used. The digested wafer solutions were determined 3-fold, and the standard solutions were measured before and after these measurements.
RESULTS AND DISCUSSION Calibration. The net intensity of the X-ray measurements was plotted against the number of atoms for the different calibration samples (Cl, ...,C5). The precision, described by the relative standard deviation of repeated measurements, is chiefly determined by the sample preparation and is nearly constant. Therefore, a logarithmic plot and regression are demanded and carried out (12). Figure 1 presents the two regression lines for Cr and Co, which are both straight lines. The slope is about 1 (0.98 for Cr and 1.01 for Co). This was to be expected as analyte line intensity is directly proportional to analyte mass for “infinitely” thin films, even within 3 orders of magnitude. The residual scatter of measured values above the calibration line is 5% for Cr and 7% for Co. The detection limits amount to about 3 x 1014atoms/cm2. Analysis. By means of this external calibration, the implanted wafers (Al, ..., A4) were analyzed by XRFA and their ion doses determined by eq 2. Since all these values deviate systematically from the ICM values, a further determination was performed by FAAS after matrix volatilization. This
/
103
11
11
I
I 1 1 1 1 1 1 1
id5
I
I
I I 1 l 1 1 1
10l6 number n of atoms
I
1
1
1
1
10”
Figure 1. Calibration by a logarithmic regression of Ka line intensity versus number of Cr and Co atoms. Doped gelatine films were used as calibration samples; 0 Co, A Cr values.
Table 111. Implanted Ion Dose D and Standard Deviations [ 1015 atoms/cm2], Determined by Different Methods sample element no.
ICM D
D
Sa
D
Sb
Cr-A1 Cr-A2 Cr-A3 Cr-A4 &-A1 CO-A~ &-A3 &-A4
5 16 50 160 5 16 50 160
4.03 11.8 41.6 137 4.55 14.2 45.8 138
0.3 0.5 1.6 11 0.1 0.6 1.9 8
4.7 11.1 44.1 141 4.82 16.3 46.0 140
0.3 0.3 0.9 2 0.1 0.2 0.5 1
XRFA
FAAS
Determined by three independent determinations. Determined by 3-fold repeated measurements. method is material consumptive and completely independent of XRFA. All values are listed in Table 111. The XRFA and the FAAS values correspond very well. Their individual values D have a relative standard deviation of 5% for XRFA and 2.5% for FAAS. Within these limits, their results are not significantly different. However, the ICM values amount to significant deviations of +24% for the Cr and of +12% for the Co doses with respect to the mean values of XRFA and FAAS. These deviations could be put down to systematic errors mainly caused by a broadening of the ion beam during scanning and by a maladjustment of the current integrator. The emission of secondary electrons could be excluded as an error source since it was largely suppressed.
CONCLUSION The systematic errors, found for the ion-current measurement during the implantation process, can be avoided in future or-at least-be corrected for. However, it is necessary to check this method and to verify the ion dose by an independent method afterward. By XRF analysis, a simple and nonconsumptive method was established to determine the ion-implantation dose in silicon wafers after the implantation process. The method can easily be calibrated with external standards prepared as thin gelatine films. The results are accurate since calibration and analyte samples present “infinitely” thin films of less than micrometer thickness. Registry No. Cr, 7440-47-3; Co, 7440-48-4; silicon, 7440-21-3. LITERATURE CITED (1) Dearnaley, G . N u d . Instrum. Methods Phys. Res. 1987, 824125, 506-5 11.
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(2) Sioshansi, P. Thh SolM Fiims 1984, 178, 61-71, (3) Sal’man, V. M.; Kiseleva, K. V.; Krasnopevtsev, V. V. Kratk. Soobshch. Fiz. 1982, 7 , 14-20. (4) Werner, H. W. Acta Electron. 1976, 19, 53. (5) Morrison, G. H. Ouantlflcatlon of SIMS: in SIMS I I I ; Benninghoven, et al., Eds.; Springer-Veriag: Berlin, FRG, 1982. (6) Reus, U. Spectrochim. Acta 1989, 446. 533-541. (7) Bowen, D. K.: Davies, S. T.; Ambridge, T. J . Appl. Phys. 1985, 58, 260-263. (8) Klockenkamper, R.; von Bohlen, A. Spectrochim. Acta 1989, 4 4 8 , 461-469. (9) Dearnaley, G.; Freeman, J. H.; Nelson, R. S.; Stephen, J. Ion Implantation; North Holland: Amsterdam, 1973; pp 416f. (10) McKenna, C. M. In Ion Implantation Techniques; Ryssel, H., Giaw-
ischnig, H., Eds; Springer Series in Electrophysics IO; Springer: Berlin 1982; pp 73f. (11) Berndt, H.; Jackwerth, E. At. Abs. Newsl. 1976, 15, 109-113. (12) Bubert, H.; Klockenkamper, R. Fresenlus’ 2.Anal. Chem. 1983, 316, 186-193.
RECEIVED for review February 1,1990. Accepted April 9,1990. This work has been supported financially by the “Ministerium fur Wissenschaft und Forschung des Landes NordrheinWestfalen” and the “Bundesministerium fur Forschung und Technologie”.
Laser4nduced Capillary Vibration for Ultramicroanalysis Jiaqi Wu, Takehiko Kitamori, and Tsuguo Sawada”
Department of Industrial Chemistry, Faculty of Engineering, University of Tokyo, 7-3-1 Hongo, Bunkyo, Tokyo 113, Japan
A highly sensltive, novel method for ultramicroanalysis of flowlng llquid sample in a Capillary tube is reported. An intenslty-modulated argon laser beam of 70-mW power was irradiated on the caplllary tube in which liquld sample was flowing, and the beam deflection signal of a He-Ne probe laser beam passlng just over the tube could be detected. The signal generation mechanism was clarified as not due to ordinary photothermal effects but to vlbration of the Capillary tube Itself based on measured tenslon dependences and resonant peaks In frequency characterlstics of the signals. The signal amplltude was found to be proportional to the absorbance of the sample. As the detection volume of the method was 1.0 X I O 2 pL for a 50 pm 1.d. capillary tube, the detection limit of the absorbance was 1.5 X IOJ, which corresponded to an absolute amount of 6.0 fg (13 amol) for sunset yellow dye.
INTRODUCTION Ultramicroanalysis is required for dealing with small sample amounts in many fields of analytical chemistry, such as capillary liquid chromatography (LC)and capillary zone elecwhich are powerful separation tools ( I ) . trophoresis (CZE), Capillary LC and CZE require a detector with a sampling volume of less than 1 nL, while maintaining a superior detection limit, to ensure their separation efficiency and ultramicroanalysis ability. However, a conventional spectrophotometric detector is not sensitive enough for small sampling volumes (2), and a fluorometric method requires derivatization for most analytes ( 3 ) . On the other hand, the laser-induced photothermal beam deflection (PBD) method has been proved to be one of the most promising methods for determination of liquid samples in a capillary tube ( 4 , 5 ) ;a detecting volume of about lo2pL has been achieved and an ability to measure weak optical absorbance in the lo* range has been shown ( 4 ) . When it was used as a detector for CZE, the detection limit absorbance reached to lo-’ order according to the evaluation criterion of the CZE system in short-term stability (5). However, as the probe beam passes through the capillary tube in the PBD method, the deflection of the probe beam is affected by the temperature and composition of the *To whom all correspondence should be addressed.
mobile phase, and the curvature of the interface between the sample and capillary tube ( 4 , 5 ) . Hence the method stability is insufficient and the fluctuating base-line drift makes quantitative analysis difficult ( 4 , 5 ) . In addition, this method is difficult to use in gradient mobile phase liquid chromatography due to the composition change of the mobile phase, and the optical alignment has to be changed as mobile phases have different refractive indexes. Furthermore, since the interaction of the probe beam and the liquid-filled capillary tube produces a set of diffraction fringes, the detector should be located near the fringe center ( 4 , 5 ) ,making a troublesome microadjustment of the optical system necessary to focus the probe beam a t the capillary tube center. To avoid these difficulties of the PBD method, a novel method for ultramicroanalysis of flowing liquid sample in a capillary tube is proposed in the present study. An intensity modulated laser beam irradiates the capillary tube and the beam deflection signal of the probe beam passing just above the irradiated point of the capillary tube is measured. The signal generation mechanism is experimentally confirmed as dominated by vibration of the capillary tube rather than the photothermal deflection phenomena. Results are shown from applying this method to ultramicrochemical determination of flowing dye solutions in a capillary tube.
EXPERIMENTAL SECTION Apparatus. A block diagram of the experimental arrangement is shown in Figure 1. A 50 Fm i.d. fused quartz capillary tube was used. It was coated by polyimide, and a portion of the coating was removed for irradiation by the focused excitation beam. The tube was mounted on a two-axis stage. It was subjected to constant stress by hanging a weight at one end, and both ends of the tube were fiied. The liquid sample was pumped into the capillary tube and kept flowing during the measurement. The excitation beam was an argon ion laser beam of 488-nm wavelength and 70-mW output power. The beam intensity was modulated by an acoustooptic modulator. The excitation beam was focused on the capillary tube by a 50 mm focal length lens which was mounted on a one-axis stage. As shown in Figure 1,a He-Ne laser was used as the probe beam, and it was focused by a 50 mm focal length lens just above the tube at the point where the excitation beam was focused. The probe beam deflection was measured by using a knife edge and a photodiode detection system as in the conventional optical beam deflection method (6). The photodiode output was amplified by a 20-dB preamplifier and fed into an autophase lock-in amplifier. Reagent. The liquid samples were aqueous solutions of sunset yellow dye. They were prepared by stepwise dilutions of the stock
0003-2700/90/0362-1676$02.50/0 0 1990 American Chemical Society
