Electrochemical behaviors of silicon wafers in silica slurry

Journal of University of Science and Technology BejingMaterialsVolume 15, Number 4, August 2008, Page 495EL SEVIERElectrochemical behaviors of silicon wafers in silica slurryXiaolan Song", Haiping Yang", Xunda Shif), Xi He", and Guanzhou Qiu')1) Dep)epartment of Inorganic Materials, School of Minerals Processing and Bioengineering. Central South University, Changsha 410083, China2) Silicon Wafer Manufacture Department, Grinm Semiconductor Materials Co. Luid, Beijig 10088 China(Received 2007-09-03)Abstract: The electrochemical behaviors of n-type silicon wafers in silica-based slurry were investigated, and the infuences of thepH value and solid content of the slurry on the corrosion of silicon wafers were studied by using electrochemical DC polarization andAC impedance techniqucs. The results revealed that these factors afeeted the corrosion behaviors of silicon wafers to diferent de-grees and had their suitable parameters that made the maximum corrosion rate of the wafers. The corrosion potential of (100) surfacewas lower than that of(11), whercas the current density of (100) was much higher than that of(111).◎2008 University of Science and Technology Bijing. All rights reserved.Key words: silicon wafers; electrochemical behavior, impedance; corrosion; polarization curves[This study was financially supported by the National Natural Science Foundation of China (No.59925412) and the Natural ScienceFoundation of Hunan Province of "China (No.03JJY3015.]1. Introductionand flatness, it is commonly considered to be the onlyand best method of achieving global planarization.In the current semiconductor materials, silicon isOnly CMP satisfies the local and global planatity de-always in the predominant position (approximatelymands imposed by current lithography methods [7].98% of semiconductor products are made from siliconWhen polishing, the silicon wafer reacts with oxidantsmaterials). Because of their wide applications in thein the slurry adsorbed on the polishing pad and thelarge-scale integrated circuit, silicon was called“theproducts escape from the surface under mechanicalsill of information ages". With the rapid developmentfriction, so a very thin sheet of material can be re-of ULSI (ultra large scale integration), the device di-moved from the silicon surface and the goal of surfacemensions become smaller and smaller, and the diame-superfinishing can be achieved.ters of silicon wafers used as substrate materials be-The reactions between silicon wafers and polishingcome larger and larger (up to φ300 mm). The re-quirements for total and local thickness variations ofsolution are primarily electrochemical ones, so the re-searchon the electrochemical behaviors of siliconsilicon wafers are becoming stricter and stricter [1].wafers in the slurry, which can offer a theoreticalBefore 1960s, the substrate polishing of semicon-guidance of the electrochemistry for silicon polishing,ductors mostly adopted mechanical polishing methods.is of important scientific meaning and technical value.Although the "smooth surface" can be offered by tra-This article studied the factors that influence the cor-ditional planarization techniques, all of them are arosion rates of silicon wafers and how to find out thepartial planarization technique, which can not accom-best polishing parameters, which are very important toplish global planarization [2-5]. Because chemicalimprove the polishing quality.mechanical polishing (CMP), which was first put for-ward in 1967 by IBM [6] and replaced gradually the2. Experimentaltraditional methods such as single chemical polishing2.1. M:中国煤化工or single mechanical polishing, not only makes a bet-THCNMHGter surface but also provides a higher polishing rateIn the expeunlells, all u-iype silicon rod with aCorresponding author: Xiaolan Song, E-mai: xlsong@hnu.cnAlso available online at www.sclencedirect.com。2008 University of Science and Technology Beijing. All rights reserved.496J. Univ. Sei Technol Bejing, VoLIS, No.4, Aug 2008diameter of 1.30 cm was sliced into 0.6-cm thick disks.about -100 mV. The corrosion current densities andEach disk mounted in a cylinder of Teflon with onethe potentials of Fig. 1 determined by the corrosionexposed surface was used as the samples for electro-software of EG&G Model 352 are listed in Table 1. Itchemical measurements, including electrochemicalis found that the current density increases at first withimpedance spectroscopy (EIS) and DC polarizationthe pH and reaches the maximum (9.912 μA/cm) atcurves. The resistivity was 0.1 S.cm . The chemicalspH 10.0, then with the further increase of pH, the cur-including H2SO4 (98vol%)， H2O2 (30vol%),rent density decreases gradually and falls to 8.000CH3COOH (99.5vol%), silica colloid (40vol%), andμA/cm' at pH 11.5. This implies that the dissolutionC2HsNH2 (70vol%) were used for the experiments. Allrate is the fastest in the slurry at pH 10.0. The work ofthe chemicals used were of analytically pure grade. .Palik et al. [8-9] showed that the release of H2 was at-The slurry, which was composed of deionised watertributed to the reaction among H20, OH and Si. Soand silica colloid, was deoxygenated by purging ni-the gross reaction could be expressed astrogen throughout the experiments. The pH of the testSi+2H2O+20H~→2H2+SiO2(OH)令(1)solutions was adjusted with CH3COOH or C2HsNH2.6002.2. Methods400... pH=10.0To block the crevice between the Teflon holder and200the electrode, epoxy was used. The exposed surface--- p1Hi-i1.50}was ground with 800 grit grinding paper and thencleaned with a sulfuric acid/hydrogen peroxide mix-ture (4:1, by volume) for 10 min. Further the samples-400were rinsed with deionised water.-600The electrochemical test cell was composed of the-800silicon working electrode, the platinum counter elec-. -0000 -8.0-7.0→6.0-5.0-4.0trode, and the Ag/AgCl reference electrode with aLuggin probe. The static electrochemical DC polariza-lg[/(A.cm2)]tion measurements were conducted using a Potentio-Fig.1. Potentiodynamic polarization curves of the n (111)stat/Galvanostat of EG&G Model 273A, whereas thesilicon wafer in the slurries with 2wt% colloidal silica atcorrosion software of EG&G Model 352 was adoptedvarious pH values.for electrochemical calculations. During the potentio-Table 1. Corresponding electrochemical values ofFig.1dynamic scans, the working electrode potential waspH_Econ/ mVlom/ (uA-cm-3)_varied with a sweep rate of 5 mV/s from -0.8 to0.4 V8.5- 124.26.784to measure the corrosion current density and potential.9.5一235.48.625The EIS measurements were performed by superim-10.0-245.69.912posing an AC signal with the ampltude of+5 mV in10.5- 295.59.376the frequency range of 100 KHz to 1 mHz. The EIS11.0-353.59.608results were analyzed by using PARC M398 software.11.5-414.58.000All the experiments were conducted at the laboratorytemperature (25+1°C). .According to the study of Seidel [9], the relation-3. Results and discussionship between the dissolution rate and concentrationcould be ftted asFig. 1 shows the potentiodynamic polarization cur-ves for the n(111) silicon wafer tested in the slurriesR=k[H2O][OH]'4(2)with 2wt% silica at various pH values. As can be seenwhere R is the dissolution rate, and k is a constant.from Fig. 1, the corrosion potential decreases gradu-Namely, the dissolution rate is related to water and theally with the increase of pH. The shape at pH 8.5,concentration of bydroxide ions. Glembocki et al. [10]which shows the active dissolution behavior with noproposed a model to explain the existence of the dis-obvious evidence of passivation, is much differentsolu中国煤化工- ssumption that bothfrom those at other pH values. However, the dissolu-freeetching species.tion rate is poor due to the lower OH concentration.TheYHCNMHGewrittenasThe curves shift towards the positive direction at otherR=CTH2Ore"[OH ],pH values, indicating that the current density increasesobviously, and the passivation behavior is found atwhere C is a constant. According to the model, theXL Song et al, Electrochemical behaviors of silicon wafers in silica slurry197water is composed of free water and bound water, andpermittivity of the oxide film, and Eo the vacuum per-free water and hydroxide ions are the major pariclesmittivity. According to the formula, the thickness ofthat participate in the chemical reaction. As the pH in-the passive films can be calculated. The calculatedcreases, the OH concentration increases and thevalues (in the range of 0.4-0.5 nm) are very near to theH2Otrce concentration decreases. So these two com-results (0.7-0.8 nm) measured by Bertagna [14] withpeting efects produce a peak in the etch rate.Ellipsometer, indicating that the reliability of EIS isEIS is a very effective technique, which can help tovery good. It is also found that the film thickness isanalyze various steps involved in an electrochemicalinversely proportional to the capacitance, and thereaction by measuring the impedance system responsethickness is the thinnest at pH 10.0 corresponding toto a small AC potential signal in a wide frequencythe value of 4.134.range [11-12]. Fig. 2 presents the Nyquist plots forsilicon in the slurries with 2wt% silica at various pHvalues. As shown in Fig. 2, the EIS spectra for siliconconsist of only one irregular capacitive semicircle in3211.0the measured frequency range from 100 kHz to 1 mHz.b)-CapacitanceThe capacitive semicircle at high frequency is due tothe thin oxide films formed on the silicon surface.2810.5Because of only one semicircle, the EIS spectra can be26fitted by using a simple parallel RC circuit as shown2410.0当in Fig. 3(a). Fig. 3(b) ilustrates the simulated values心of resistance and capacitance as a function of pH. As22can be seen from Fig. 3(b), the films' resistance de-9.+ -Resistancecreases gradually with the increase of pH, indicating18that the dissolution rate gets faster which accounts for8.59.09.510.010.511.011.53the decrease in corrosion potential and the enhance-pHment of current density in Fig. 1. After reaching theminimum 19.506 kS2 at pH 10.0, the films' resistanceFig.3. Equivalent circuit and simulated values: (a)equivalent circuit; (b) resistance and capacitance as a func-increases gradually as the pH increases. However, thetion of pH. R,- the solution resistance between the workingchanging tendency of the films' capacitance is justelectrode and the reference electrode; R- the resistance ofopposite to the resistance, and the capacitance rangesthe oxide flms formed on the sllicon surface; Cr- the ca-from 9 to 11 μF/cm2.pacitance of the oxide flms.Fig.4. presents potentiodynamic polarization cur-20 t一- pH=-8.5ves of different crystal planes at pH 8.5 and 10.0. It is8+--←pH-10.0evident that the corrosion potential is lower for Si65(100) compared with Si (11), whereas the current4F--- pH-ii.0density is higher than that of the ltter (the right shift2t-- pH-11l.5of the polarization curve), both at pH 8.5 and at pH10.0. It is well known that the corrosion current den-sity is a parameter, which represents the corrosion rateof a material in the presence of an electrolyte. So thecorrosion current density can be converted to the cor-10152025303540responding corrosion rate according to Faraday's law[15]. Based on the DC potentiodynamic curves ob-ReZ/ (kQ2cm2)tained, all the electrochemical parameters and the cor-Fig. 2. Nyquist plots of the n(111) silion wafer in therosion rates calculated using Faraday's law are listedslurries with 2wt% colloidal silica at various pH values.in Table 2. As shown in Table 2, the current density isIn a rough approximation, the oxide layer couldmuch higher for (100) compared with (111), and thepossibly be considered, as was assumed by Schmukicorres中国煤化工ne tendency in thesame:ate reaches 0.186et al. [13], as a compact insulator of area A and thick-ness d, thus constituting a capacitor:nm/mTMHCNMHGhlowerthan0.669nm/min of (100), and the corrosion rate ratio of aboutC=feoA/d,3.5:1 for (100)(111) is found in the experiment,where f is the surface roughness factor, ε the dielectricshowing a certain anisotropic corrosion behavior.498J. Unir, Sci Technol Bejing, VoL15, No.4, Aug 2008Many scholars have investigated the anisotropic be-40wt% silica, indicating the content of silica has a lit-haviors of silicon wafers deeply and proposed varioustle influence on the films' thickness. The corrosionreaction models. Facisth and Palik [16] believed thatcurrent density increases gradually from 16.442the lower reaction rate for (111) was attributed to theμA/cm2 for 5wt% silica to the maximum 34.568existence of a passivation layer, which had existed onμA/cm2 for 20wt% silica, then with the further in-the (111) surface below the passivation potential. Sei-crease in silica content, the current density decreasesdel et al. [9] assumed that the anisotropy was due togradually and falls to 12.237 μA/cm2 for 40wt% silica.the different energy levels of the surface states for dif-As you know, the micelle structure of silica in theferent crystal orientations resulted from the binding ofalkali can be written ashydroxide ions to the surface atoms, and the energydffererce berwen the backbond surface states of {5i0OJmSiO}r :2(0 x)H'}* :2xHt.(100) and (111) surfaces corresponded roughly to thedifference between the activation energies of their400一pH-8.5---- pi-10.0etch rates. Because of the lower energy levels for the200surface states and the higher activation energies on theot(111) surface, the corrosion rate was found to be lower(111)than that of (100).E -200-1(1!!!n(100)Fig. 5 shows the potentiodynamic polarization cur-n(100。ves of the n(111) silicon wafer in the slurries with dif-ferent silica contents at pH 10.50. It shows that theshape of the polarization curves is very similar. The-80passivation occurs at around -100 mV except the cur--10-9-8-7-6--5-4 -3ves for 30wt% and 40wt% silica, where passivationlg([I/(A.cm 2)]occurs at around 0 mV. It is also found that the COrTO-Fig.4. Potentiodynamic polarization curves of differentsion potential changes a lttle, ranging from the mini-crystal planes.mum - -341.9 mV for 20wt% silica to - -268.9 mV forTable 2. Corresponding electrochemical values of Fig.4 and corrosion ratesSamplespHEcom/ mV1m/(A.cm-)_ CR /(nm.min-)__n(11)8.5-124.26.7840.127n(111)10.0-245.9.9120.186-343.423.7490.445-505.035.6830.669Note: CR- corrosion rate.After immerging the silicon wafer into the slurry,between the silicon surface and bulk solution, result-electrons would move from the silicon surface into theing in the decrease of corrosion rate.solution because of the higher original Fermi level of300the semiconductor as compared with the solution, so a200 t5wt% siO;---- 10wt% SiO2positive excess charge on the silicon surface built up,100 |一- 20wt% SiO2resulting in the formation of an eletric double layer0--..30wt% SiO2[17]. The colloidal particle with a negative charge-100--- 40wt% SiO;would be absorbed around the electric double layer百-200because of the absorption of some SiO groups. The勾-300silica colloid of different contents was likely to affect- 400the distribution of silica in the electric double layer,-500t-600 twhich further influenced the charge distribution of the-700electric double layer, and finally impacted on then _7n -6.0 -5.0 4.0Fermi levels, leading to the alteration of the electro-中国煤化工明chemical reaction's activation energies, therefore, in-creasing the current density gradually. When the con-Fig..MYHcNMHGcurvesofthen(1)tent reached 20wt%, the colloidal particle could holdsilicon wafer in the slurries with different silica contents atback the mutual diffusion of reactants and productspH 10.50.XL. Song et al, Electrochemical behaviors of silicon wafers in silica slurry4994. Conclusions[5] J.F. Luo and D. Dornfeld, A material removal mechanismin chemical mechanical polishing: theory and modeling,The electrochemical characteristics of n-type sili-IEEE Trans. Semicond. 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