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In Situ ATR-FTIR Analysis of Surfactant Adsorption onto Silicon from Buffered Hydrofluoric Acid Solutions A. Marcia Almanza-Workman,† Srini Raghavan,*,† and Roger P. Sperline‡ Department of Materials Science and Engineering and Department of Chemistry, University of Arizona, Tucson, Arizona 85721 Received September 10, 1999. In Final Form: January 28, 2000 Buffered hydrofluoric acid (BHF) solutions containing HF and NH4F are widely used in the manufacturing of silicon-based integrated circuits. The adsorption/desorption characteristics of a commercially available, high purity, polyglycidol type surfactant (OHS) onto/from silicon from buffered hydrofluoric acid (BHF) solutions was studied by in situ attenuated total reflection-Fourier transform infrared spectroscopy (ATRFTIR). The challenge in these measurements was to resolve the C-H peaks, in the 2800-3000 cm-1 region of the surfactant spectrum, that were masked by the strong absorbance due to N-H caused by a large amount of NH4+ ions in the solution. A technique has been developed to overcome this limitation. The principle of this technique is to carry out the surfactant adsorption in BHF solutions followed by the replacement of NH4+ ions by alkali-metal cations, such as K+ and Cs+, to allow better resolution of the C-H peaks from the baseline. Extrapolation of the adsorption density to time zero yields the adsorption density in the presence of NH4+. Using this technique, the adsorption density of OHS surfactant in a buffered HF solution containing 7 parts of NH4F (40%) and 1 part of HF (49%) was found to be approximately 20% higher than that in dilute HF solutions.
Introduction In the fabrication of silicon-based integrated circuits, many process steps involve the use of aqueous-based chemicals. These chemicals include etchants for SiO2 such as dilute hydrofluoric acid (HF) and buffered hydrofluoric acid (BHF). The addition of surface active agents to HFbased chemicals is gaining popularity and is known to offer many benefits during the processing of silicon, such as enhanced wetting,1-3 reduced particulate contamination,1-5 and lowering of surface microroughness,6-8 etc. Similar benefits have also been observed when surfactants are added to alkaline cleaning solutions.9,10 Nonionic surfactants based on alkylphenol poly(ethylene oxide) alcohol or alkylphenol polyglycidol are used in HF-based solutions to achieve effective chemical penetration into narrow hydrophobic trenches. Ideally, the surfactant should adsorb quickly onto the silicon surface during cleaning or etching but completely desorb from the surface when rinsed with DI water. The removal of adsorbed surfactant is becoming an increasingly important issue because any residual surfactant molecules remaining on † Department of Materials Science and Engineering, University of Arizona. ‡ Formerly with the Department of Chemistry, University of Arizona.
(1) Kikuyama, H.; Miki N. 9th ICCCS Proc. 1988, 378. (2) Kikuyama, H.; Miki, N.; Ohmi, T. Proc. IES 1990, 332. (3) Kikuyama, H.; Miki, N.; Saka, K.; Takano, J.; Kawanabe, I.; Miyashita, M.; Ohmi, T. IEEE Trans. Semicon. Manufact. 1990, SM-3, 99. (4) Kikuyama, H.; Takano, J.; Miki, N.; Ohmi, T. Proc. 35th Annu. Meet. IES 1989, 369. (5) Kezuka, T.; Ishii, M.; Unemoto, T.; Itano, M.; Kubo, M.; Ohmi, T. Proc. 40th Annu. Meet. IES 1994, 283. (6) Miyamoto, M.; Kita, N.; Ishida, S.; Tatsuno, T. Electrochem. Soc. Ext. Abstr. 1993, 93 (2), 561, Abstr. 342 (7) Miyamoto, M.; Kita, N.; Ishida, S.; Tatsuno, T. J. Electrochem. Soc. 1994, 141, 2899. (8) Jeon, J. S.; Raghavan, S.; Parks, H. G.; Lowell, J. K.; Ali, I. J. Electrochem. Soc. 1996, 143, 2870. (9) Jeon, J. S.; Raghavan, S.; Sperline, R. P. J. Electrochem. Soc. 1995, 142, 621. (10) Jeon, J. S.; Ragahavan, S.; Carrejo, J. P. J. Electrochem. Soc. 1996, 143, 277.
Figure 1. Single-beam spectra of HF and BHF solutions.
the silicon surface after the rinsing process are considered to be organic contaminants. An in situ ATR-FTIR technique has been used to measure the adsorption of surfactants onto silicon from HF solutions11 and trace level organics in hydrofluoric acid.12 Although surfactants are more commonly used in buffered HF solutions, there have been no published reports on the characterization of the adsorption/desorption behavior of high purity surfactants onto silicon from BHF solutions. The main reason for this lack of this information is the difficulty presented by the IR absorption of NH4+ in the frequency range 3300-3500 cm-1. To illustrate the challenge in measuring adsorption from BHF solutions, single beam or open beam spectra of 50:1 HF and 7:1 BHF solutions are shown in Figure 1. It may be seen from this figure that the characteristic O-H peak for water is shifted and the envelope is broadened in BHF solutions due to strong absorption of -N-H bonds. The baseline absorption becomes so large in the 2700-3200 cm-1 region that the resolution of C-H peaks riding this baseline is extremely difficult. The N-H, O-H, and C-H (11) Haworth, P. D.; Kovach, M. J.; Sperline, R. P.; Raghavan, S. J. Electrochem. Soc. 1999, 146, 2284-2288. (12) Chyan, O. M. R.; Chen, J. J.; Xu, F.; Wu, J. J. Anal. Chem. 1997, 69, 2435.
10.1021/la991200v CCC: $19.00 © 2000 American Chemical Society Published on Web 03/18/2000
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bonds all have their characteristic stretching frequencies in the range of 2700-3600 cm-1. To allow the quantitative analysis of surfactant adsorption onto silicon from BHF solutions, a new technique, referred to as the “ion replacement technique” in this paper, was developed. Using this technique, the adsorption of a nonionic surfactant, OHS, at the silicon/BHF solution interface has been estimated. Experimental Procedures Materials. An electronic grade alkylphenol polyglycidol nonionic surfactant, OHS, was supplied by Olin Microelectronic Materials as a 20% (v/v) aqueous solution. According to the manufacturer13 the surfactant has the following generic structure:
Figure 2. Replacement of NH4+ ions by K+ or Cs+.
( )
As 2deff ) sCsdeff + s (Cit) (1) N dp with a molecular weight of approximately 1000. Semiconductorgrade 7:1 BHF (7 parts of 40% NH4F solution and one part of 49% HF solution), 50:1 HF (50 parts of water and 1 part of 49% HF solution), and 49% HF solutions were also supplied by Olin Microelectronic Materials. The deionized water used to characterize the desorption of the surfactant from silicon was produced in a Millipore Milli-Q four-bowl system and had a resistivity of 18 MΩ cm. Cesium fluoride (CsF) and potassium fluoride (KF) were purchased from Aldrich Chemical Co. (99.9%) and used as received. An undoped single-crystal silicon parallelogram plate (50 × 20 × 1 mm), polished on both faces and beveled at 45°, was used as an internal reflection element (IRE). Up to 40 solution sensing internal reflections of an IR beam incident normal to the beveled faces were achieved. Methods. Adsorption and desorption measurements were carried out using a Perkin-Elmer Spectrum 2000 FTIR spectrometer equipped with a mid-infrared DTGS (deuterated triglycerine sulfate) detector. The resultant unpolarized IR spectra were collected at 4 cm-1 resolution and averaged over 5-10 individual spectra scans. A demountable liquid transmission cell with CaF2 windows was utilized for molar absorptivity measurements of OHS solutions. Interference from atmospheric H2O and CO2 were minimized by flushing the spectrometer optics and sample compartment with ultrapure nitrogen gas. The adsorption density of surfactants was measured using an ATR-FTIR liquid flow cell made from PTFE (Harrick Scientific model MEC-1T0). The cell was installed in a twin parallel mirror reflection attachment (Harrick Scientific model TMP-F-PO5 45°). Prior to exposure to surfactant solutions, the silicon IRE was cleaned in 4:1 H2SO4 (98%):H2O2 (30%) solution for 15 min at ∼90 °C to remove organic contaminants, thoroughly rinsed in DI water, and then etched in 50:1 HF to remove the oxide film. Finally, the sample was rinsed with DI water and dried with filtered N2 gas and then mounted immediately in the spectrometer sample compartment. For the calculation of adsorption density, the IR spectrum of aliphatic region (2800-3000 cm-1) was used and the integrated spectral absorbance (As) was determined by subtracting a background spectrum of the alkali-metal fluoride/HF solution from the area of aliphatic stretching region. The extent of adsorption/desorption of surfactant (Γ) in mol/cm2 on silicon was calculated from the following equation for the absorbance per reflection (As/N) based on a step-type concentration profile:14,15 (13) Scardera, M.; Roche, T. S. (Olin Corporation) US Patent 4,761,245, 1988. (14) Sperline, R. P.; Muralidharan, S.; Freiser, H. Langmuir 1987, 3, 198. (15) Tompkins, H. G. Appl. Spectrosc. 1974, 28, 335.
(1)
In the above equation, N is the number of solution sensing internal reflections, s is the integrated molar absorptivity (L/ mol-cm2) in the aliphatic region determined from transmission IR spectra, dp (cm) is the depth of penetration of the evanescent wave, deff (cm) is the effective depth, t (cm) is the thickness of adsorbed layer, Ci is the surfactant concentration at the interface (mol/L) and Cs is the bulk solution concentration of the surfactant. The quantity (Cit)/1000 is the same as the adsorption density (Γ) in mol/cm2. The values of dp and deff were determined from the optical constants of the IRE system using well-known equations derived from Harrick.16 It should be noted that this equation is only valid for cases in which dp is much greater than the thickness of the adsorbed layer as it is here. A constant residual organic background absorption inherited with the spectrometer’s optical system12 can be seen as two small but distinct IR absorptions at approximately 2928 and 2856 cm-1 (indicated by arrows in Figure 1). In this work, any increase of IR absorption intensities over the preexistent spectrometer’s organic absorption background was considered to be indicative of adsorbed organic on the silicon IRE crystal surface. This increase became measurable upon subtraction of the background spectra. The extent of adsorption of OHS from BHF solutions onto silicon was determined by the “ion replacement technique” (Figure 2). The experimental technique involved several steps: (i) the BHF-surfactant solution was injected into the system and kept inside the cell until equilibrium was reached (∼30 min), (ii) two cell volumes (4 mL) of KF/HF or CsF/HF solution, with the same surfactant concentration as the BHF solution, were introduced into the cell rapidly (in less than 10 s), and (iii) more KF/HF or CsF/HF solution containing surfactant was pumped into the system at 0.2 mL/min for 1 h. Spectra were recorded every minute during the first 15 min and then less frequently for a total time of 60 min. The adsorption density of the surfactant in BHF solutions was estimated by extrapolating the calculated adsorption density at different times in step iii to zero time. Finally, DI water was pumped at 4 mL/min through the cell to investigate the desorption behavior of the adsorbed surfactant.
Results and Discussion As described in the Methods section, the extent of adsorption of OHS from BHF solutions was measured after replacement of NH4+ ions with alkali-metal cations. It is important that during the replacement of NH4+ ions by alkali-metal cations, the ionic strength and pH of the system remained the same as that of the BHF system. Since 7:1 BHF solution has a very high ionic strength (between 11 and 12 M), solubility in HF was a very (16) Harrick, N. J. J. Opt. Soc. Am. 1965, 55, 851.
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Figure 4. Single-beam spectra obtained by slow replacement of NH4+ ions by K+ ions at a flow rate of 0.2 mL/min. Spectra were taken every minute.
Figure 3. Single-beam spectra of NaF, KF, CsF, HF and BHF solutions. Table 1. Properties of Different Fluoride Salts
salt NaF KF NH4F CsF
cation metal hydrated alkali atomic radius (Å) metal radius (Å) 0.95 1.33 1.48 1.69
3.6 3.3 3.3
maximum solubility in water at 25 °C g/100 mL
M
4.22 92.3 100 366.5
1 6.3 27 24.1
important factor to be considered when selecting an alkalimetal fluoride salt. Table 1 lists the solubility of NaF, KF, CsF and NH4F in water. An ionic strength almost equal to that of the 7:1 BHF solution can be achieved only in the CsF/HF system. Figure 3 shows the single-beam spectra of HF solutions containing different alkali-metal fluoride salts. The pH values of the different solutions were adjusted to 4, and the ionic strengths were adjusted to the values allowed by the maximum dissolution of the respective salts in HF. It can be seen that the 2800-3000 cm-1 region of the spectra of KF and CsF solutions containing HF approached the desired baseline (HF single-beam spectrum). It is also worth noting from this figure that the background absorption in alkali-metal metal fluoride/HF solutions is significantly lower than that in BHF solutions at 28003000 cm-1. The first series of tests was conducted to find out the total volume of KF/HF solution required to totally replace the NH4+ ions contained in the cell. In these tests the silicon IRE was exposed to 7:1 BHF solution containing 100 ppm of OHS for half an hour. At the end of the 30 min period, KF/HF solution containing 100 ppm of OHS was pumped through the cell at a flow rate of 0.2 mL/min and the spectra was recorded every minute until the desired baseline was approached. The single-beam absorption spectra obtained are shown in Figure 4. The arrow in this figure shows how the BHF spectrum progressively changed to the KF/HF spectrum. It may clearly be seen from this figure that approximately 4 mL of the KF/HF solution was required to replace almost all the NH4+ ions in the cell. Since the replacement of BHF solution by KF/HF solution was done slowly, it was thought that some reorganization of the adsorbed surfactant could occur at the interface. To minimize this, it was decided to replace BHF solution contained in the cell with 4 mL of KF/HF solution containing the surfactant at a very rapid rate
Figure 5. (a) Variation in OHS adsorption density after rapid replacement of BHF/OHS in the cell by (alkali-metal fluoride)/ HF/OHS. (b) Short-term profile for data presented in part a.
and then continue the flow of KF/HF solution at a slow flow rate to maintain an equilibrium with the adsorbed OHS. Using this fast injection followed by slow injection technique allowed the study of OHS adsorption from BHF solutions. In Figure 5a, the measured adsorption density of OHS surfactant is plotted as a function of time after the rapid injection of the alkali-metal fluoride/HF solution, which took approximately 10 s. The same type of experiments was also carried out with high ionic strength solutions containing CsF/HF. It can be noticed from this figure that the replacement of NH4+ by Cs+ resulted in the desorption of approximately 25% of the surfactant adsorbed onto silicon and replacement of NH4+ by K+ increased the surfactant adsorption by approximately 20%. The short time adsorption data is expanded and plotted in Figure 5b. The extrapolation of the short time adsorption data to zero time was taken to be the equilibrium adsorption density of OHS onto silicon from BHF solutions. Using the same technique, the adsorption of OHS in BHF solutions containing different levels of surfactant was also investigated. Table 2 summarizes and compares the estimated equilibrium adsorption density of OHS from 7:1 BHF solutions containing OHS with the one obtained in HF solutions. It may be seen from this table that the
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Table 2. HF and BHF Equilibrium Adsorption Density onto Silicon Surfaces surfactant concentration (ppm)
a
OHS equilibrium adsorption onto silicon ΓOHS (mol/cm2) 50:1 HF
(Ia
∼ 1.4 M)
7:1 BHF (I ∼ 12 M)
5
9.13 × 10-11 ( 0.5 × 10-11
100
1.59 × 10-10 ( 0.1 × 10-10
500
2.93 × 10-10 ( 0.2 × 10-10
1.15 × 10-10 (using KF/HF, I ∼ 8 M) 1.17 × 10-10 (using CsF/HF I ∼ 12 M) 1.93 × 10-10 (using KF/HF, I ∼ 8 M) 1.91 × 10-10 (using CsF/HF, I ∼ 12 M) 3.66 × 10-10 (using KF/HF, I ∼ 8 M) 3.58 × 10-10 (using CsF/HF I ∼ 12 M)
I indicates ionic strength.
Figure 6. Desorption behavior of OHS from silicon, when adsorbed for 60 min from alkali-metal fluoride/HF solutions containing 100 ppm of OHS.
adsorption density of OHS in 7:1 BHF solutions is roughly 20% higher than that in HF solutions. It is also worth noting that a significant increase in adsorption density begins to occur at concentrations exceeding the cmc (∼20 ppm) of OHS in 7:1 BHF. The removal or desorption of OHS adsorbed from KF/ HF and CsF/HF solutions was also investigated. If rinsing is carried out right after the establishment of equilibrium with the BHF solution, the peaks due to adsorbed OHS layer were not resolvable during the initial stages (∼20 min of DI water rinsing). This is due to the presence of NH4+ still in the system. However, if NH4+ ions are replaced by K+ or Cs+ (ion replacement technique), the removal of adsorbed surfactant can be followed very easily as a function of time of rinsing with DI water. The results of the rinsing experiments carried out after performing the “ion replacement” are shown in Figure 6. For comparison, the rinsing of OHS adsorbed from 50:1 HF is also shown. It is evident from the figure that much of the adsorbed OHS is rinsed away from the surface in approximately 10 min. Unlike the adsorption results, the desorption of OHS was independent of the alkali-metal fluoride used in the ion replacement step. One question that may be raised is whether the oxidation of silicon surface that typically occurs during DI rinsing would affect the desorption of surfactant. It is important to point out that the thickness of the native oxide that can form on hydrogen-passivated silicon during DI water rinsing is extremely small (
