Anal. Chem. 1999, 71, 3551-3557
Charged Particle Activation Analysis of Carbon on Silicon Plates and Its Use for the Monitoring of Organic Contamination of the Air Tadashi Nozaki,*,† Hirochika Yagi,‡ Hisashi Muraoka,† Akira Nagano,‡ and Masakazu Kohno‡
Purex Company, 735 Nippacho, Kohoku, Yokohama, 223-0057 Japan, S.H.I. Examination and Inspection, Ltd., Toyo, Ehime, 799-1393 Japan
Air pollution with carbonaceous substances has become a serious problem in modern human life, and numerous studies have been undertaken for determining carbonaceous contaminants in the air as a whole and in component compounds.1 Further efforts, however, are needed for realizing more reliable, sensitive, and rapid determination in order to understand in detail the effect of the pollution on nature and human health. In the semiconductor industry, also, surface contamination of substrate wafers with organic substances is attracting keen attention these days.2 Surface carbon on semiconductor silicon wafers treated in a high-grade clean room is usually of too low a level to be determined reliably with ease and is likely to increase rapidly on exposure to another ambient atmosphere. Most physical methods of surface analysis, such as X-ray photoelectron spectroscopy, Auger electron spectroscopy, and secondary ion mass spectroscopy, only give infor-
mation in a high vacuum under irradiation with photons, electrons, or ions.3 Mass spectrometry and often gas chromatography are sensitive enough and can give information on the chemical form; however, complete desorption of the contaminant should be guaranteed together with its quantitative introduction into the analyzer chamber without any external contamination. We formerly reported on the usefulness of charged-particle activation analysis for light elements on the surface, showing some results for oxygen, carbon, and boron.4,5 For carbon, we have used the activation with the 12C(d,n)13N reaction (Eth ) - 0.33 MeV), for which the ultra-compact cyclotrons now popular in nuclear medicine are well-suited. Our method is characterized by the sample arrangement in the charged particle bombardment which makes analysis of the surface under ordinary environmental conditions possible. As shown in Figure 1, two plates of the sample with identical surfaces are kept in intimate contact with each other and bombarded with a deuteron beam of energy sufficient to activate the carbon on the inside surfaces with the maximum cross section (about 200 mb at 2.3 MeV), and the 13N (9.965 min, β+, no γ) formed there is measured after a rapid chemical separation. Under this arrangement, neither any additional contamination of the inside surfaces with carbon or 13N nor any notable escape of them is possible during the bombardment under an ambient atmosphere. We intended to improve the accuracy, sensitivity, ease, and accessibility of this method and to establish a routine procedure for total carbon. Because of the Coulomb barrier, only light elements can be activated in bombardment with low-energy charged particles. Hence, the bombardment of high-purity silicon with a few mega electron volt deuterons give only short-lived 30P (2.498 min, β+, low γ intensity) from 30Si (3.10% abundance), low activities of 17,18F, 13N, and 11C from oxygen, carbon, and boron, respectively, existing as impurity and/or surface contaminant, and negligible activities of a few other radionuclides, unless the surface is enormously contaminated. This bombardment is thus markedly different from neutron activation of natural and low-purity substances; and we thought we would be able to use it profitably for the monitoring of surface contamination with carbon in the atmosphere. In the
* Corresponding author. Tel.: 045-541-9493. Fax: 045-541-7544. E-mail:
[email protected]. † Purex Co. ‡ S.H.I. Examination and Inspection, Ltd. (1) Fox, D. L. Anal. Chem. 1997, 69, 1R-13R. (2) National Technology Road Map for Semiconductors, 1997; Semiconductor Industry Association: San Jose, CA, 1997.
(3) McGuire, G. E.; Weiss, P. S.; Kushmerick, J. G.; Johnson, J. A.; Simko, S. J.; Nemanich, R. J.; Parikh, N. R.; Chopra, D. R. Anal. Chem. 1997, 69, 231R-250R. (4) Iwamoto, M.; Nozaki, T. J. Radioanal. Nucl. Chem. 1988, 125, 143-146. (5) Kataoka, S.; Taruni, Y.; Yagi, H.; Tomiyoshi, S.; Nozaki, T. J. Radioanal. Nucl. Chem. 1997, 216, 217-219.
Total carbon on the surface of silicon plates was determined by charged-particle activation with the 12C(d,n)13N reaction. A procedure was established for routine determination, including the methods for the transportation and bombardment of the sample without disturbing the surface carbon and for the separation of the 13N. Silicon plates, after undergoing different surface treatments, were exposed to the air in clean rooms and ordinary living areas, and the surface carbon was analyzed to evaluate the usefulness of this method and to get information required in the semiconductor industry. Surface carbon of (0.2-3) × 1015 atoms/cm2 was found on silicon plates left in usual nonsmoking rooms after any treatment; it was reduced to 5 × 1012 atoms/cm2 by heating to 900 °C for 30 min in purified air and was then kept under 1 × 1013 atoms/cm2 for a few days in high-grade clean rooms. Etching with NH4OH/H2O2 or a fluoride-containing reagent often gave higher and varying surface-carbon quantities. This method of analysis is sensitive to 1 × 1012 atoms/cm2 of total carbon and is hoped to offer meaningful information concerning clean rooms and to be utilized generally for monitoring carbonaceous air pollution.
10.1021/ac981402g CCC: $18.00 Published on Web 07/17/1999
© 1999 American Chemical Society
Analytical Chemistry, Vol. 71, No. 16, August 15, 1999 3551
Figure 1. Target arrangement.
activation, care should be taken for the fluctuation of deuteron energy, which is usually inevitable in the use of a cyclotron. A deuteron energy change of 0.1 MeV, for example, at the acceleration to 9 MeV results in the change of about 0.25 MeV after the energy is degraded to 2.3 MeV by a silicon or aluminum absorber, because of the decrease in the energy loss (MeV/(mg/cm2)) with the energy.8,9 Noticeable disagreements exist in reported excitation functions for the 12C(d,n)13N reaction.6,7 We examined these reports and also measured the excitation function by ourselves. As shown later in this paper, all of the 13N atoms formed on the inside surfaces are pushed into the forward plate by nuclear recoil. Hence, we need to measure the 13N in this plate only. Chemical separation of the 13N is usually indispensable, because of the simultaneous formation of a strong 30P radioactivity by the 29Si(d,n)30P reaction and some other radioactivities. We developed a wet separation method, used it together with the dry RF fusion method,10 and checked the reliability of the two methods using specially prepared silicon plates containing high 13N activities. A small cyclotron, though becoming popular in nuclear medicine, is usually of rather limited accessibility. For wider utilization of this method, we devised a simple technique of encapsulating the sample set for transportation and bombardment, without any notable disturbance of the surface carbon, and proved its usefulness. We exposed silicon plates which had undergone different surface treatments to a variety of atmospheres and analyzed their surface carbon, expecting that some results will provide useful information to semiconductor industries. We intend to survey carbonaceous air pollution in the public, using silicon plates of minimized surface carbon as a passive monitor. (6) Landoldt-Bo ¨rnstein. Numerical Data and Functional Relationships in Science and Technology, New Series, Group I, Vol. 13, Subvol. F, Production of Radionuclides at Intermediate Energies, Shopper, H., Ed.; Springer: Berlin, 1995; p 266. (7) Michelmann, R. W.; Krauskopf, J.; Meyer, J. D.; Bethge, K. Nucl. Instrum. Methods 1990, B51, 1-4. (8) Littmark, U.; Ziegler, J. F. Handbook of Range Distributions for Energetic Ions in All Elements; Pergamon: New York, 1980. (9) Ziegler, J. F. Handbook of Stopping Cross-sections for Energetic Ions in All Elements; Pergamon: New York, 1980; p 147. (10) Fukushima, H.; Kimura, T.; Hamaguchi, H.; Nozaki, T.; Itoh, Y.; Ohkubo, Y. J. Radioanal. Nucl. Chem. 1987, 112, 415-423.
3552 Analytical Chemistry, Vol. 71, No. 16, August 15, 1999
Figure 2. Device for sample encapsulation. D: Encapsulation device; Rh: ring holder of push-in type; P: central pin; Sf: forward sample plate; Sm: mask sample plate; F: aluminum foil; R: aluminum ring; T: target holder; Tr: backside of T; A: target assembly; Sp: spacer.
EXPERIMENTAL SECTION Preparation and Encapsulation of Samples. Silicon plates of 2 × 2 cm size and 210 ( 3 µm and about 600 µm thicknesses, with mirror-flat surfaces, were prepared from semiconductor silicon crystals of low carbon content (5-20 wt ppb), the thinner being the backside (mask) plate and the thicker the forward plate in Figure 1. The following surface treatments were undertaken usually in a clean room equipped with a filter for organic vapors: (1) washing with organic solvents; (2) NH4OH/H2O2 etching, namely SC-1 etching (conc. NH3(aq)/30%/H2O2/water ) 1 vol:1 vol:5 vol; 70 °C, 10 min); (3) exposure to O3 in air under UV illumination for 20 min; (4) heating to 900 °C for 30 min in a flow of purified air; (5) some combinations of above treatments; and (6) other treatments commonly used in semiconductor industry, such as that with organic alkali solutions, NH4F/HF solution, and dilute HF. Silicon plates were left for assigned times in places of organic pollution measurement or kept in a closed vessel with an organic substance, usually after treatment 2 or treatment 4. Reagents used were mostly of EL-grade. The forward plate and mask plate were then encapsulated in close contact with each other to make the target assembly, by the following procedure, using the device shown in Figure 2: (1) the two plates were placed in the shallow cavity of the target holder and covered with the aluminum foil; (2) the target holder was fixed with its backside hole on the central pin of the device,
and the aluminum ring was held around the ring holder; and (3) the upper part of the device was pressed down to encapsulate the target plates between the holder and foil with the aid of the ring. Before the encapsulation, the aluminum sample holder and cover foil were washed with acetone and heated to 500-530 °C for 30 min in a flow of purified air. To take out the bombarded target, the target assembly was placed on the spacer which was fixed on the central pin, and then the upper part of the device was pressed down to push off the ring. The following substances were tested as envelops and covers for protecting cleaned silicon plates from carbonaceous contamination in transportation and storage: heat-treated aluminum foil, quartz wool heated to 1000 °C in purified air, and a commercial flexible filter containing activated charcoal for clean room use. Silicon plates, heat-treated to minimize the surface carbon, were carefully covered with them and left several days in ordinary rooms, and the change of the surface carbon was measured. Bombardment. The sample assembly was bombarded with deuterons from the compact cyclotron of SHI Examination and Inspection, Ltd., Toyo, Ehime, Japan, called Cypris, which gives 18 MeV protons and 9.0 MeV deuterons. The target assembly was set on a water-cooled target-setting plate by air suction through holes in the plate, as shown in Figure 1. This plate was then pushed into the bombardment chamber, through which helium was passed for cooling. The deuteron beam was extracted from the cyclotron vacuum into the bombardment chamber through titanium foils. Daily minute change in deuteron energy was compensated for by changing the thickness of the energy-adjusting foil in Figure 1. Every day a stack of 5 sheets (each 1.70 mg/cm2 thick) of Mylar (CO2C6H4CO2C2H4)n was bombarded (0.1 µA, 30 s) behind a degrador plate slightly thinner than the mask plate, and the 13N radioactivity in each sheet was measured; from the result, the thickness of the energy-adjusting foil was obtained by the use of Ziegler’s tables.8,9 The silicon plate samples were bombarded with a 2 µA deuteron beam usually for 10 min. As the activation standard for the surface carbon, the data for the Mylar sheet, giving the maximum count rate, was used. Radioactivity Measurement. The annihilation radiation from 13N was measured by a pair of BGO detectors (2" × 2", 6-mm aperture) operated in coincidence. In experiments to ascertain the radiochemical purity of the separated 13N, the decay was followed usually for 50 min or more; in routine analysis of the silicon sample, the counting was continued usually for 10-15 min. The counting was automated by a personal computer, which gave the record of counting, the decay curve, the most probable 13N activity at the end of bombardment, and the saturation activity for 1 µA bombardment. A Ge(Li) counter was also used for highly radioactive samples, i.e., Mylar, graphite, and 13N-implanted silicon, because the change of counting geometry is much easier than that in the coincidence counter. A 22Na standard was also measured by the Ge(Li) counter. Measurement of Excitation Function and Thick Target Yield. The excitation function for the 12C(d,n)13N reaction was measured by the bombardment of the Mylar stacks behind degradors of various thicknesses for deuteron energies to 7 MeV, but in particular detail for those under 3.5 MeV. The deuteron energy at each sheet was known from the Ziegler’s tables.8,9 The
Figure 3. Excitation function for the 12C(d,p)13N reaction. A: Curve obtained under our measurement condition. B: The most reliable reported curve.7
absolute cross section was obtained by the 22Na standard. The thick target yield was measured by bombarding graphite plates behind silicon degrador plates of various thicknesses. Separation of 13N. Various possible methods were examined and tried for the separation of 13N in a limited thickness of the sample surface; the wet procedure thus devised is described later in this paper. This method and the dry volatilization by RF fusion were used, with the 13N in the dry fusion being trapped on-line by the titanium sponge.10 To examine the precision for both methods, two kinds of silicon plates containing 13N in the surface layers were prepared by the following procedures: (1) carbon atoms were implanted into silicon wafers (1016 and 1015 atoms/ cm2 C, 700-nm mean implantation depth) in Sumitomo Eaton Nova Corp., Toyo, Japan, and the wafers were bombarded by the deuteron beam and (2) the mask plate in the target assembly was replaced by a sheet of carbon (0.1-mm-thick) with an aluminum backing for deuteron energy adjustment, and the assembly was bombarded with the deuteron beam (0.1 µA, 30 s) in order for the 13N formed in the carbon near the interface to be pushed into the silicon plate by the recoil energy at the nuclear reaction. The 13N in the C-implanted sample was measured either nondestructively or after chemical separation by one of the methods. The recoil-implanted silicon plate was lightly etched with NH4OH/ H2O2 (1 min) for particle-contamination removal, and the 13N was measured first nondestructively and then after either of the separations. RESULTS AND DISCUSSION Fundamental Data Concerning the Activation. The excitation function for the 12C(d,n)13N reaction is shown in Figure 3 in two forms: (A) the result of our observation shown by a mean over 8 measurements and (B) reproduction of the curve of Mickelmann et al. that we have judged to be the most reliable of all the reported curves.6,7 Some spreading in deuteron energy took place in our measurement, in which deuterons from a fixed-energy cyclotron (9 MeV) were passed through a silicon plate and titanium and aluminum foils before impinging on the Mylar foil stack.8 This and the finite thickness of the Mylar foil resulted in the smoothing of peaks to give Curve A, Figure 3, which can be regarded as in fairly good agreement with Curve B. This smoothing due to the deuteron energy straggling is profitable or even indispensable in our surface analysis, because it suppresses the change of the 13N formation cross section due to the fluctuation of the mask-plate thickness. The thick target yield obtained by direct measurement agreed well with that calculated from the excitation function. Analytical Chemistry, Vol. 71, No. 16, August 15, 1999
3553
Table 1. Recovery of sample 1. C-implanted 2. C-implanted 3. 13N-recoil a
13N
in the Two Chemical Separations procedure
repetition
mean ( σa
6 4 3 4 4 6 6
(1.035 ( 0.044) × (1.022 ( 0.105) × 1016 (1.244 ( 0.096) × 1016 (6.60 ( 0.40) × 1014 (6.56 ( 0.53) × 1014
wet dry nondstruc wet dry wet dry
ratio to nondstruc 1016
83.2 ( 7.6 82.1 ( 10.4 (100) 82.1 ( 3.9 82.8 ( 3.7
Surface carbon found (atoms/cm2).
The maximum and minimum forward recoil energies of 13N in the 12C(d,n)13N reaction can be calculated from the conservation of energy and momentum. It is clear that the 13N travelling on the same straight line as the incident deuteron is provided with the maximum or minimum forward recoil energies, which are given as the solutions of the following pair of simultaneous equations:
2Vd ) 13VN + Vn
(1)
(2/2)Vd2 ) Ed ) (13/2) VN2 + Vn2 + Q
(2)
Here, Vd, VN, and Vn are the velocities of the deuteron, 13N, and neutron, respectively; Ed is the incident deuteron energy; and Q is the Q-value of this reaction (Q ) - 0.28 MeV). Hence, the recoil energy of 13N for Ed ) 2.3 MeV, denoted by EN, is given to be
EN ) 0.62 MeV and 0.035 MeV for which the recoil ranges are 0.4 µm and 0.02 µm, respectively, in silicon.9) In the wet chemical separation, hence, it is sufficient to dissolve 2 µm thick of surface layer of only the forward plate, even though some irregularity in etching depth is inevitable. From Figure 3 and the observed thick target yield, it is known that 1.0 × 1016 atoms/cm3 carbon in the bulk of the forward plate gives nearly the same 13N activity as 2.0 × 1013 atoms/cm2 carbon on the inside surfaces, for the deuteron energy of 2.3 MeV. With the wet separation, the interference of bulk carbon is suppressed to about 1/10 of this value. The bulk carbon can be measured reliably by activation with the same reaction and by IR spectrophotometry.11 Modern semiconductor silicon crystallized by the Czochralski process in a vacuum has proven to contain only 1.0 × 1015 atoms/cm3 carbon or still less, though older products usually contain 1.0 × 1016 atoms/cm3. Only on special occasions, hence, does the interference of bulk carbon need to be corrected for by the use of the above values. Chemical Separation. The following procedure for the wet separation has proven to be useful. (1) Prepare a wide-mouth 50mL conical flask fitted with a glass tube (about 15-mm diameter, 5-cm length). Partly fill the tube with silica wool, and make it wet with an aqueous NH4Cl solution (0.3 mol/L, 1 mL). (2) Put the bombarded sample plate in the flask, with the surface to be (11) ASTM F1.06 Committee.Standard Test Method for Substitutional Atomic Carbon Content of Silicon by Infrared Absorption (ASTM Standards F 139193); ASTM: West Coshocken, PA, 1994.
3554 Analytical Chemistry, Vol. 71, No. 16, August 15, 1999
analyzed upward, and cover it with solutions of TiCl3 (4 wt %, 0.05 mL) and NaOH (4 mol/L, 0.15 mL), which are sustained by surface tension. Set the tube on the flask, and place the flask on a hot plate of 180-200 °C, until the solution is brought to near dryness. Nearly 3 mg (0.7 mg/cm2, 3 µm) of silicon surface is thus dissolved. (3) Cool the flask briefly, and add water (5 mL) through the column into the flask. Then shake the flask to catch completely gaseous 13NH3 by water, and wash the column with a new portion of water (20 mL total) for complete elution of the 13NH . (4) Transfer the solution into a beaker containing NaBPh 3 4 (0.3 mol/L, 1.5 mL), chloroacetic acid, and sodium chloroacetate (both 2 mol/L, 2 mL), and collect the NH4BPh4 precipitate thus formed, after a rapid warming for coagulation, on a small filter paper under suction. This separation took about 15 min, which was about 3 min more than the dry separation took. Both separations gave 13N in satisfactory radiochemical purity. Table 1 shows the precision in the two chemical separations. In the 1016 atoms/cm2 C-implanted sample, the three methods can be compared, but the S/N ratio in the 1015 atoms/cm2 C-implanted sample was not high enough for reliable nondestructive measurement. The recoil implantation gave silicon plates of much higher 13N activity and S/N ratio than the other method; thus, its results are regarded as the most reliable, though the uncertainty in the beam-current measurement is not included. The two separations are seen to indicate almost the same 13N activities, which, however, are lower by nearly 20% than the result of nondestructive measurement. In the dry fusion, a part of the 13N is known to be lost principally by being gettered in the sublimate of matrix silicon.12 The recovery of NH4Cl carrier as NH4BPh4 in the wet process was found to be 94-95%, indicating that the 13N was partly converted into forms other than NH4+. The pronounced dependence of efficiency on the source shape and size in the coincidence counting of annihilation radiation and the presence of 18F (110 mim, β+) formed from 17O in the bombarded sample can be partial causes of uncertainty, usually a positive bias in the nondestructive measurement. It is certain, however, that the loss of 13N in both separations is within the range of 1.20 ( 0.10; we now use the correction factor of 1.20 for the separation loss. More details of our former study concerning the correction factor are described in ref 12. Further study on the 13N recovery is underway. Sensitivity, Precision, and Accuracy. In our analysis, 1 × 1012 atoms/cm2 surface carbon gives about 5 counts/min of 13N after 20 min from the end of bombardment with 2 µA deuterons for 10 min. In the bombardment of two sample plates of identical (12) Kataoka, S.; Higaki, Y.; Tarumi, Y.; Tazawa, S.; Imai, S.; Nozaki, T. J. Radioanal. Nucl. Chem. 1993, 168, 377-384.
Table 2. Carbon on Silicon Surface after Various Surface Treatmentsa treatment
no. of samples
surface carbon
(1013 atoms/cm2)
acetone wash SC-1b O3c O3 + SC-1 SC-1 + O3 heatingd SC-1+ heating O3 + heating
2 4 3 6 3 6 3 2
range 22-40 8.0-30 18.5-34 1.52-14.7 2.7-6.6 0.59-1.65 0.71-1.74 1.69-2.4
mean ( σ 31 15.5 ( 9.4 26 8.5 ( 4.0 3.8 1.40 ( 0.39 1.31 2.0
a In a clean room under a fairly good control. b Etching with NH4OH/H2O2 (conc NH3(aq)/30% H2O2/water ) 1:1:5 vol, 70 °C, 10 min). c Exposure to ozone under UV irradiation. d Heating to 900-950 °C for 30 min in purified air.
surfaces which are in contact, the observed radioactivity should be divided by 2 in the calculation of the carbon on each surface. Hence, we can determine 1 × 1012 atoms/cm2 surface carbon with an uncertainty of less than 50%. For all samples ever analyzed, about 400 samples in all, we have always detected more than 3 × 1012 atoms/cm2 surface carbon. In the monitoring of a highly cleaned atmosphere, it is essential to prepare passive monitor plates sufficiently free from surface carbon. Well-reproducible results have mostly been obtained for samples heat-treated and left in a well-cleaned atmosphere. Four silicon samples, after being etched with NH4OH-H2O2 and left in a carefully controlled clean room for a week, gave the results of (1.590 ( 0.044) × 1013 atoms/cm2 carbon, where ( indicates the standard deviation, though much larger variations have been observed for the samples left in less clean atmospheres after this etching. The method of analysis, hence, can be regarded as fairly good in reproducibility, but the quantity of surface carbon itself is suspected to sometimes vary noticeably from sample to sample after a quite similar treatment. As for the absolute result, chargedparticle activation analysis for light elements in semiconductor silicon is thought to be so reliable that it is being used in the standardization of the infrared spectrophotometry for oxygen and carbon.11,13 Although Mylar standard in this analysis is less prominent than graphite standard in the bulk carbon analysis, because of its lower thermal and radiation stability, this analysis is expected to give more reliable absolute values than other methods. The behavior of volatile compounds, which usually exist in more quantities than less volatile compounds in both normal and polluted air, should be taken into account in the treatment of the result. About 1.5 ppm of methane, for example, exists in the normal atmosphere and is thought to be absorbed on surfaces covered with less volatile organic contaminants but to be desorbed on mild heating or in a flow of methane-free air. However, our target assembly shown in Figures 1 and 2 is regarded as highly effective for suppressing the escape of volatile compounds and we intend to study the behavior of them in more detail. Minimization of Surface Carbon. Table 2 shows the surfacecarbon amount after various treatments in a clean room controlled relatively well. The samples after the treatments were immediately (13) Iizuka, T.; Takasu, S.; Tajima, M.; Arai, T.; Nozaki, T.; Inoue, N.; Watanabe, M. J. Electrochem. Soc. 1985, 132, 1707-1713.
Table 3. Change of Surface Carbon by Exposures after the Two Treatments (1013 atoms/cm2) exposure
sample treatment
clean rooma (10 min)
clean rooma (2 d)
BHTb vaporc (2 d)
acetone vaporc (2 d)
heat treatment NH4OH/H2O2
0.54 1.07
1.40 8.4
2.1 11.5
2.0 1.6
a Clean room of a medium grade. b Butylated hydroxytoluene ) 2,6di-tert-butyl-4-methyl-phenol. c Air saturated with these vapors at room temperature.
encapsulated and kept there and analyzed on the same day. For as-supplied commercial silicon wafers, from 2 × 1014 to 2 × 1015 atoms/cm2 surface carbon was found and this did not decrease notably and sometimes even increased upon acetone washing or NH4OH/H2O2 etching in a usual room. Table 2 shows that heating to 900-950 °C in purified air for 30 min is highly effective for minimizing the surface carbon, which seems to be removed as oxide gas with simultaneous formation of an oxide film to cause a noticeable change of surface reactivity and adsorptivity. In a high-grade clean room, surface-carbon levels of about 5 × 1012 atoms/cm2 have often been maintained after the heating. This heat treatment, sometimes after washing with acetone, is simple and has been used for the preparation of our passive monitor. The heating at 500 °C was insufficient, leaving (1.0-1.5) × 1014 atoms/cm2 surface carbon. Table 2 also shows that the NH4OH/H2O2 etching has given surface carbon amounts higher and more variable than the heat treatment has. The O3 treatment seems to show no notable advantages for the minimization of surface carbon. When the sample was subjected successively to different treatments, the surface-carbon quantity was determined almost exclusively by the last treatment. Change of Surface Carbon on Exposure to Clean Room Air or Organic Vapor. No significant change in the surface carbon was observed when the heat-treated silicon plates were left in a high-grade clean room for 2 days or kept encapsulated as shown in Figure 2 for a week. Even for the NH4OH/H2O2 etched samples, the surface carbon remained under 8 × 1012 atoms/cm2 for more than 3 days in the same clean room. In a medium- or low-grade clean room, the surface carbon increased rapidly in the first 30 min, with the rate higher for the NH4OH/H2O2 etched silicon than for the heat-treated silicon. After exposure of the heat-treated silicon plates for 3-5 days in various clean rooms, the following increases in the surface carbon (in 1013 atoms/cm2) were observed: (1) from 0.7 to 1.12, (2) from 1.4 to 5.4, (3) from 1.0 to 51, and (4) from 1.0 to 125. The last clean room was made free only from particulates. Considerable aging of the filter was observed in one of the clean rooms. After the NH4OH/H2O2 etching, the surface carbon (in atoms/cm2) increased usually from 1 × 1013 to 1 × 1014 or slightly over in 10 min, as is exemplified by the following data: 0 min, 1.20 × 1013; 9 min, 1.55 × 1014; 75 min, 7.3 × 1014; 120 min, 1.00 × 1015. Table 3 shows a comparison of the heat-treated and NH4OH/H2O2 etched silicon plates in the exposure to a clean room atmosphere and organic vapors, by way of the mean of duplicate analyses which were in fair agreement. Analytical Chemistry, Vol. 71, No. 16, August 15, 1999
3555
Figure 4. Change in surface carbon with time in acetone and hexane vapor. Pretreatment of the sample: NH4OH/H2O2 etching (conc. NH3(aq)/30% H2O2/water ) 1:1:5 vol, 70 °C, 10 min).
Low-volatility compounds, such as di-tert-butyl-methyl-phenol (butylated hydroxytoluene, BHT), dioctyl phthalate, and some siloxanes are serious contaminants of semiconductor silicon wafers in the industry. The surface carbon adsorbed from their saturation vapors was measured easily; a few results are given in Table 3. In the exposure to the saturation vapors of acetone and hexane, the surface carbon on the heat-treated silicon was found to vary highly capriciously between 1 × 1013 and 1 × 1015 atoms/cm2. For the NH4OH/H2O2 etched samples, fairly large quantities of surface carbon were often found at the initial stage of the exposure, and the quantity then decreased with time, as exemplified by Figure 4. We are going to study these phenomena in more detail. The heat-treated silicon plate was shown to be transported without any notable change in the surface carbon when enveloped in the heat-treated aluminum foils and covered with heat-treated quartz wool or a commercial air-filter sheet impregnated with charcoal. It has, hence, proved to be possible to send the passive monitor with the encapsulating set to a remote place and, after the exposure to the environment, to send it back to the cyclotron laboratory without any notable effect of transportation. From all the data concerning the NH4OH/H2O2 treatment, this etching is suspected to give the nascent surface adsorptivity and reactivity sufficiently high to efficiently catch organic vapors from the ambient atmosphere. A potentiality thus exists for the elevation of sensitivity to organic air pollution by the use of the passive monitor prepared by this etching. Surface Carbon after Some Treatments Used in Industry. Surface treatment of silicon wafers with organic alkaline solutions and fluoride-containing reagents is used in the industrial production of integrated circuits. Treatment with a dilute choline solution in high-grade and low-grade clean rooms gave 2.5 × 1013 and 9.2 × 1013 atoms/cm2 surface carbon, respectively. The same solution, when added with a chelating agent, gave the corresponding values of 1.25 × 1013 and 3.4 × 1013 atoms/cm2. After etching with 0.1% tetramethylammonium hydroxide solution containing from 0.12 to 1.2% H2O2 at 70 °C for 10 min and sometimes after leaving the resultant plates in a high-grade clean room for 3 days, we found surface carbon of (1.01 ( 0.26) × 1013 atoms/cm2 to be the avarage of 11 samples, where the sign ( denotes the standard deviation, without any systematic dependence on the treatment conditions. Some other alkaline etchings of industrial use gave (0.6-1.9) × 1013 atoms/cm2 surface carbon in the same high-grade clean room. Treatment of silicon with dilute hydrofluoric acid, mixtures of hydrofluoric acid and ammonia water, or some other fluoride3556 Analytical Chemistry, Vol. 71, No. 16, August 15, 1999
Figure 5. Change of surface carbon with time in nonchemical rooms. Room of the exposure: A and B, physical laboratories; C, smoking-permitted office. D: curve for proportional increase of surface carbon with time.
containing reagents usually gave surface carbon of (0.1-5) × 1014 but fairly often as high as (0.5-3) × 1015 atoms/cm2. The larger values were observed almost always with poor reproducibility. The surface-carbon quantity was clearly dependent on the producer of the reagents and probably on the ambient atmosphere of the treatment. The surface carbon was reduced down to (0.6-8) × 1013 atoms/cm2 by: (1) passing the reagents through columns of silicon powder, (2) minimizing the contact of the reagents with plastics, and (3) undertaking the treatment in a high-grade clean room. Further studies are under way to clarify the reason for the high surface carbon and to establish a method for its suppression. Monitoring Environmental Organic Contamination. The increase of the surface carbon with time is shown in Figure 5 for the heat-treated silicon plates left in two nonchemical laboratories and a smoking-permitted office room. All three curves show the accumulation of total surface carbon, with its rate increasing with time in the exposure time between 1/3 and 3 days. For still longer exposure, saturations were observed in ill-reproducible and unsystematic forms. Outside, near a parking place, 1 day of exposure gave (7-11) × 1015 atoms/cm2 of surface carbon. Considerable scatters and occasional anomalous values sometimes appeared (not shown in Figure 5), possibly due to the change in organic vapor concentration within the monitoring time and area and to unexpected falling of carbonacious dusts on the monitor plate. We intend to utilize this air monitoring widely indoors and outdoors, while protecting the monitor plate from contamination with dusts, after further fundamental studies. Semiconductor silicon is a highly suitable substance for the passive monitor because (1) it is of well-controlled and extremely high purity, (2) its chemical and surface properties are well-understood, and (3) the deuteron bombardment gives no long-lived radioactivities. The bombardment can be shortened to 5 min in the monitoring of ordinary environments. Usefulness of This Method and Some Other Methods. This method possesses the following useful characteristics: (1) ease of preparation, exposure, and transportation of the monitor plates; (2) high sensitivity and fair accuracy and rapidity in the carbon determination; and (3) capability for monitoring organic contamination in a narrow aperture or from small sources and for measuring the inequality of carbon deposition within a given area and time. Actually, we measured the deposition of organic vapor originating from small metal chips which had been painted with various pigments a few to several months before.
Of the methods of analysis for adsorbed organic substances,3 desorption mass spectrometry, sometimes in conjunction with gas chromatography, is now used most effectively and is undertaken commercially by collecting gases desorbed from large silicon wafers. We measured surface carbon remaining on the wafer after the thermal desorption and found less than 1 × 1013 atoms/cm2 of carbon, indicating a satisfactory desorption. Reflection infrared spectroscopy, in which the sample need not be placed in a high vacuum, is usually deficient in sensitivity. CONCLUSION To realize highly reliable measurement of carbonaceous contamination in air and on silicon wafers, for individual purposes, at present, we need to first survey various potential methods and then to undertake two of them, making use of the complementary characteristics of each method. Our activation analysis, we believe, can play a part in these methods. We can now analyze about 25 samples in 8 working hours. Partial automation of the procedure
is under consideration to be able to analyze more samples each day and to suppress the radiation exposure of the workers. We intend to undertake this monitoring on a commercial basis, sending the passive monitor plates together with the encapsulating devices to those who order them. ACKNOWLEDGMENT We would like to express our thanks to the Cyclotron Operating Groups of SHI Examination and Inspection, Ltd., for the bombardment services and to Miss Asami Masuda, Miss Yoshie Takahashi, and Mrs Naomi Shibamoto for their aid in the sample preparation.
Received for review December 17, 1998. Accepted May 18, 1999. AC981402G
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