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Anal. Chem. 1992, 64, 2123-2129
Determination of Boron and Phosphorus in Borophosphosilicate Thin Films on Silicon Substrates by Capillary Electrophoresis Ronald A. Carpio' and &ne Mariscal SEMATECH, 2706 Montopolis Drive, Austin, Texas 78741
John Welch WATERS, 2626 South Loop West, Houston, Texas 77054
The application of the new technique of eledrophoretlc capillary ion analysis (CIA) to the analysis of boroohosphodilcale thln flknr (BPSG) k described. CIA and ion chromatography (IC) were shownto produceequivalent analytkal results for boron, but the inductively coupled plasma qmctrorcOplc ~W~eddure urlngOptlc~rlddectkn( ICP-AES) ykld.d results whlch were 4.5 wt % higher. The +5 phosphorus contentsfor BPSQ films measured by CIA, IC, and ICP-AES are In agreement. The use of vapor-phase decomposition (VPD) for this analyrk k introduced.
INTRODUCTION The importance of accurately monitoring and controlling both the boron and phosphorus concentration in borophosphosilicate thin films, which are now routinely utilized as interlevel dielectrics in semiconductor very large scale integrated fabrication, is well known.'-3 These dopants govern the reflow characteristics, etch rate, film stress, moistureabsorption characteristics, and dielectric properties of these films. The analytical methods developed over the last 10 years for analysis of this type of film generally fall into two categories: namely destructive techniques which involve dissolution of the film from the silicon wafer surface and those which are nondestructive in nature. In general, most modern semiconductor fabrication facilities prefer to utilize a faster nondestructive technique for this monitoring, generally FTIRQand less commonly X-ray fluorescence (XRF) spectrometry6 or an electron microprobe.6 These spectroscopictechniques are calibrated using a wet chemicalanalysis procedure. Calibration checks, and often entire recalibrations, must be performed if processing alterations are made. Thus, the destructive, wet chemical techniques, such as ion chromatography (IC), play a vital role in processing borophosphosilicate films. One of us has utilized the ion chromatographic technique for a number of years as the primary destructive method for performing the boron and phosphorus analyses for these films,7,8 and IC has also been adopted by others in the (1)Kern, W.;Schnable, G . L. RCA Reo. 1982,43,423. (2)Kern, W.;Smeltzer, R. K. Solid State Technol. 1985,28,171. (3)Kern, W. Semicond. Znt. 1985,8 (7),122. (4)Adhihetty, I. S.;McGuire, J. A.; Wangmaneerat, B.; Niemczyk, T. M.; Haaland, D. M. Anal. Chem. 1991,63,2329-2338.
(5) Parekh, N.; Nieuwenhulizen, C.; Borstrok, J.; Elgersma, O.J.Electrochem. SOC. 1991,138,1460. (6)Hughes, M. C.;Wonsidler, D. R. J. Electrochem. SOC. 1987,134,
1488. (7)Tong, J. E.; Schertenleib, K.; Carpio, R. A. Solid State Technol. 1984,27 (lo),161-170. (8)Carpio, R. A. Semicond. Int. 1989,12(9), 100-104.
semiconductorfield."" One of the chief attributes of the IC method is ita ability to quantitate phosphorus in both the +3 and +5 oxidation state. This is especiallyimportant for BPSG films which have been deposited by plasma processes or if it is desired to use unannealed films for process monitoring. In contrast, the inductively coupled plasma spectroscopic by optical emission (ICP-AES) method is used to establish the total phosphorus.12 Colorimetric procedures are sensitive to phosphate, so the etchant solution is oxidized to convert all phosphorus to the +5 oxidation state. In order to quantitate boron by IC, it is converted to the BF4- anion, enabling it to be detected by use of a conventional conductivity detector. Boron can also be determined in the tetrafluoroborate form by colorimetry.13 The BF4-peak, obtained by using the most popular ion chromatographic columns, generally tails badly unless organicadditivesare employedin the eluent.7.8JlThese additives limit the lifetime of the column and the eluent. Moreover, the eluents are basic, and this complicates the analysis of the etchant solutions which are acidic. The goal that is generally established for BPSG process control is to be able to measure boron to AO.1 wt !% For a 200-mm wafer, this translates into establishing boron levels which lie in the range of 2-5 wt !% in films and which weigh approximately40mg f 40pg. The phosphorusconcentrations lie within a similar w t !% range, but the phosphorus controls are less stringent at f0.2 w t 5%. It was decided to explore whether the new advances in electrophoretic capillary ion analysis,l4-16 now referred to as CIA, could be utilized for borophosphosilicateanalyses, and if so, whether there are any benefits to be gained by using this technique rather than ion chromatography or inductively coupled plasma spectroscopy. The mobility of ions in an electric field is the basis of capillary electrophoresis separations, which is different than for the ion chromatographic analysis. The potential speed and resolution advantage, as well the hope that the techniquewould be matrix independent, provided incentives for investigating capillary electrophoresis.
.
EXPERIMENTAL SECTION A Waters Quanta 4000 capillary electrophoresis system was utilized for this work. Water Maxima software was employed for data acquisition and processing. Data was acquired at a rate of 20 point&. Indirect UV detection, using a mercury lamp at (9) Lai, S. T.; Nishina, M. M.; Sangermano, L. J. High Resol. Chromatogr. Chromatogr. Commun. 1984, 7,336. (10)Becker, F. S.; Pawlik, D.; Schafer, H.; Staudigl, G. J. Vac. Sci. Technol. B 1986,4 (3), 732. (11)Merrill, R. M. Liq. Gas Chromatogr. 1988,6 (5), 3. (12)Levy, R.A.; Kometani, T. Y.J.Electrochem. SOC. 1987,134,1565. (13)Graveaon, D.;Grondm, G. Semicond. Znt. 1988,I1 (7),140. (14)Jandik, P.;Jones, W. R.; Weston, A.; Brown, P. R. LC-GC 1991,
6 (9), 634. (15)Jones, W. R.; Jankik, P. Am. Lab. 1990,June, 57. (16)Jones, W. R.;Jandik, P.; Preifer, R. Am. Lab. 1991,May, 40.
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Table I. Compositions, Conductivities, and pH of Electrolytes El E2 E3 0.4% HF chromate (mM) 10 10 10 OFM BT (mM) 0.05 0.1 0.5 1.06 0.94 3.5 conductivity (i2-m-l X lo3 0.98 7.93 8.02 7.89 PH 254 nm, was employed. This method is based on signal perturbation of a continuously monitored background substance, sodium chromate, in the carrier electrolyte by the separated analytes. A variety of different voltages were evaluated with the negative power supply. Both 75-pm4.d. x 60-cm-long and 75wm X 100-cm fused silica capillary columns were employed. Sample loading, generally for 10 or 20 s, was accomplished hydrostatically from plastic microcentrifugetubes, approximately 600 pL in volume. The electrolyte on the detector side was the same as that on the injection side. The current flowing through the capillary was less than 30 pA, and a fan was always in operation during the run, so Joul'e heating was not a problem. Fresh electrolyte was drawn through the capillary with a 2-min vacuum purge on the detector side before sample injection to restore the surface conditions between runs. The electrolytes that were employed for the CIA work were prepared by mixing a measured volume of a concentrate which was 200 mM in NazCrO4 and 1.36 mN in &Soh, with a measured volume of the Waters OFM Anion-BT concentrate which is 20 mM and then by diluting with DI water. The OFM, electroosmotic flow modifier, is a cationic surfactant that reverses the direction of electrosmotic flow inside the fused silica capillary toward the anode. In the work described here, the polarity of the injection end of the capillary is made negative, so the observed mobility of all anionic analytes is increased.I4 Three electrolytes whose compositions are given in Table I were used for this work. Electrolyte 1was used for the majority of the work. Electrolyte 3 is the standard Waters electrolyte. Also, listed in Table I are electrolyte pH values and ionic conductivities at 25 "C that were measured with a Rosemont Model RC-18A conductivity bridge. The conductivity for the sample blank is shown for comparison. BPSG films, approximately 6000 8, in thickness, which had been deposited a t atmospheric pressure on 150-or 200-mm silicon wafers using a Watkin-Johnson 999-4 CVD reactor and silane, phosphine, diborane, and oxygen reactant gases, were etched with aqueous HF solutions. The BPSG films were etched in cylindrical polypropylene containers that were covered with closefitting lids. The etchant solution, which was either an aliquot of 10% or 20% v/v aqueous HF, was applied to the horizontal wafer surface, and the dish was rotated until the solution completely covered the film. Etching of the film was evident when the wafer surface become hydrophobic and the etching solution formed beads on the wafer surface. The etchant solution was allowed to remain on the wafer for 1 h to ensure complete conversion of the boron oxide to BF4- as described previously.7 This etchant solution was then quantitatively transferred to a 100-mL volumetric flask and made up to volume with DI water. The resultant solution was analyzed directly by the CIA and also by IC and ICP-AES. The concentrations of phosphorus and boron in the diluted sample solutions ranged from approximately 5 to 20 ppm. Film weights were established to the nearest 0.1 mg by weighing the wafer both initially and after etching, DI water rinsing, and oven drying with a Mettler AJlOO balance which was equipped with an air filter chamber. Caution had to be exercised to weigh the films immediately after removal from the reactor since these films are hygroscopic. Ion chromatographic analyses were performed with a dualcolumn Dionex 4500i system, equipped with conductivity detectors. For comparison purposes, two different columns were employed. For the AS-5 column, a procedure which was described and used previously was adopted. For the AS-4A column, the procedure described by MerrilP was utilized. A 20-pL sample volume was employed with the AS-5 column and a 50-pL sample loop was used for the AS-4A column. The eluent flow rate was set at 1 mL/min. The eluents employed were as follows. AS-5 column: 3 mM sodium carbonate, 2 mM sodium hydroxide, 0.5
Table 11. ICP-AES Operating Parameters for Boron and Phosphorus setting parameter boron phosphorus wavelength, nm 249.773 213.618 incident power, kW 1.0 1.0 spectral band width, nm 0.5 0.5 plasma gas flow rate, L/min 12 12 auxillary gas flow rate, L/min 0.6 0.6 nebulizer gas flow rate, Limin 0.7 0.7 observation height, mm 15 15 integration time, ms 32 32 number of replicates 12 12 mM methylparaben, 0.5 % v/v acetonitrile. AS-4A column: 0.75 mM sodium bicarbonate, 2.0 mM sodium carbonate, 0.1 mM 4-cyanophenol. In the case of the AS-4A procedure, it was not found necessary to adjust the pH of the solution as was done by MerrilP to obtain reproducible results for the phosphorus concentrations when the HF concentration was 0.1% v/v. Some of the analyses were performed on the chromatographic files using the Galactic Industries LabCalc software package. A Perkin-Elmer Model 40 inductively coupled atomic emission spectrometer (ICP-AES) was utilized for performing comparisons with the chromatographic results. The experimental parameters are summarized in Table 11. Solutions were aspirated into the argon plasma using a cross-flow pneumatic nebulizer and injected with a peristaltic pump. An HF resistant ceramic torch was employed. The boron and phosphorus levels were generally computed by comparison of the measured intensities of the BPSG control against the sample. The boron and phosphorus levels of this control had been established by IC. This approach was adopted to provide for exact matrix matching. Also, results were computed using the results from standard solutions which closely matched the solutions obtained from the BPSG films with the exception that the silicon component was omitted. Standards were prepared from Baker Ultrex boric acid for boron, Baker Ultrex potassium dihydrogen phosphate for orthophosphate, and Baker phosphorus acid for phosphite. The exact concentration of the phosphite stock solution was established by ICP measurements which in turn was calibrated against phosphate standards. For comparison, the following Spex solution standards were utilized for phosphorus, 1 mg/mL phosphorus prepared by dissolving NH4H2PO4 in water; and for boron, 1 mg/mL boron which was prepared by dissolving (NH&B407'4H20 dissolved in water. A Spex silicon standard that had a concentration of 1mg/mL which consisted of (NH1)2SiFe dissolved in water with a trace of HF was also used to study the effect of silicon on the ICP calibration curve. Aldrich fluorophosphoric acid was used as an aid in CIA and IC peak identification. All solutions containing HF were prepared and stored in plastic containers. A GeMeTec vapor-phase decomposition unit was used for a portion of the work. This technique has become popular for wafer cleaning and for the determination of trace metal impurities on thermally oxidized or bare silicon surface^.'^ In this procedure the film is etched in an HF ambient for 1h a t room temperature. The small droplets which were formed on the wafer surface were quantitately recovered and evaporated to dryness in a Teflon beaker. The residue was then dissolved in DI water. Note: Hydrofluoric acid causes severe burns and requires that extreme caution be exercised in its use. This includes working with the concentrated acid in a well ventilated hood and wearing proper safety equipment at all times.
RESULTS AND DISCUSSION A typical electropherogram for a BPSG film is shown in Figure 1. The migration time could be dramatically altered by variations in the applied voltage, the OFM concentration, (17) Wong, M.; Moslehi, M. M.; Reed, D. W. J. Electrochem. SOC. 1991, 138, 1799-1802.
ANALYTICAL CHEMISTRY, VOL. 64, NO. 18, SEPTEMBER 15, 1992
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f 10
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Flgurr 1. Typical electropherogram for a 0.1 % (v/v) HF solution contalning a dissolved BPSG film. Capillary: 75-pm X 100-cm fused silica. Electrolyte: electrolyte 1 descrlbed In text. Voltage: 20 kV (negatlve). Injectlon: 20 s, hydrostatlc. Detection: indirect UV at 254 nm.
or the column length. With a 60-cm column, the entire electropherogram could be obtained in less than 4 min without loss of resolution. The main peaks of analytical interest are the tetrafluoroborate peak which precedes the main fluoride peak and the phosphate peak which lies in a dip following the main fluoride peak. The two peaks which precede the BFd- peak and the dip following the main fluoride peak are of interest from the perspective of CIA in general. These peaks and the dip after the main fluoride peak are present in the electropherogram of the HF blank. By spiking the sample with various anions, it was found that the migration time of the first peak coincides with that for bromide, while that of the second peak coincides with that of sulfate. Neither anion is a significant impurity in the grade of HF which was used for these studies. Sulfate is, however, present in the carrier electrolyte. It was established by secondary mass spectroscopicanalysis of the residue that remained after evaporation of a small quantity of the OFM solution on a wafer surface that bromide was present in the OFM. A stacking phenomenon is responsible for the appearance of carrier electrolyte anion peaks in the electropherogram. Stacking is a consequence of differences in the electric field strength in different regions of the capillary where there is a difference in matrix or difference in electricalconductivity.18 In normal electrostacking, which is employed for the concentration of sample ions, the conductivity of the carrier electrolyteis higher than that of the sample which is prepared in water or the carrier electrolyte itaelf.18 In this procedure, the conductivity of the sample solution is greater than that of the carrier electrolyte, as shown by the conductivity data in Table I. The carrier electrolyte peaks were present in the electropherogram of the HF blank, regardless of HF concentration, both when DI water and the carrier electrolyte itaelf were used as the diluent. Both the bromide and the sulfate peak areas were linearly related to the HF concentration in aqueous HF solutions over the range of 0.1 to 0.6% HF (v/v). The bromide and sulfate peak areas were also linearly related to the boron concentration for boron calibration standards which were made from boric acid and (18)Chien, R.-L.; Burgi, D.S.Anal. Chem. 1992,64 (S),489A-496A.
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