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Materials Science in Semiconductor Processing

Materials Science in Semiconductor Processing 27 (2014) 390–396

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Galvanic corrosion characteristics of poly silicon–tantalumnitride couple immersed in dilute HF solutions

R. Govindarajan, M. Keswani n, S. RaghavanDepartment of Materials Science and Engineering University of Arizona, Tucson, AZ 85721, USA

a r t i c l e i n f o

Keywords:Galvanic corrosionPoly-SiTantalum nitridedissolved O2

Dopant concentrationHF solution

x.doi.org/10.1016/j.mssp.2014.07.01001/& 2014 Elsevier Ltd. All rights reserved.

esponding author at: Department of Maring, 1235 E James E Rogers Way, Tucson4361; fax: 5206218059.ail address: manishk@email.arizona.edu (M.

a b s t r a c t

Galvanic corrosion characteristics between poly-Si and tantalum nitride (TaN) in dilute HFsolutions (0.1 to 1%) have been investigated. Variables such as poly-Si to TaN area ratio,dissolved oxygen concentration in HF solutions and doping level of poly-Si were investigated.Extent of galvanic corrosion was directly measured as well as estimated using Tafelpolarization. Morphological changes on poly-Si due to galvanic corrosion were characterizedusing Scanning Electron Microscopy. Increase in exposed cathode (TaN) area as well asaeration results in higher corrosion of poly-Si. In de-oxygenated HF solutions (less than 4 ppmof O2), irrespective of the area ratio, there appears to be no significant silicon loss.

& 2014 Elsevier Ltd. All rights reserved.

1. Introduction

In the formation of high-k/metal gate stack structuresusing the ‘gate first’ integration scheme, HF based solutionsare used for removing oxide hard mask and for etchinghigh-k with high selectivity with respect to field oxide andsilicon [1]. These solutions are also used to remove post-etch residues after gate electrode patterning in memorydevices. Metal gate stacks (MG) typically consist of a layer ofmetallic material such as tantalum nitride (TaN), titaniumnitride (TiN), molybdenum (Mo) or ruthenium (Ru) on topof poly-Si [2]. Due to contact between the metal and thepoly-Si, there is an increased risk of corrosion of the lessnoble material during wet chemical etching/cleaning/rin-sing. Galvanic corrosion occurs when dissimilar metals withdifferent electrode potentials come into contact in presenceof an electrolyte. Electropotential difference drives thecorrosion of less noble metal preferentially [3].

terials Science andAZ 85721, USA. Tel.:

Keswani).

When silicon coupled to a more noble conductive material(M) is immersed in aerated HF solutions, it may suffer galvaniccorrosion; dissolution of silicon is the anodic reaction andoxygen reduction on the nobler surface is the cathodicreaction, as represented by the following equations:

Siþ6 HF - SiF2�6 þ6 Hþ þ4e E0anode ¼ 1:24 V ð1Þ

O2þ4 Hþ þ4e-M2 H2O E0cathode ¼ 1:23 V ð2Þ

If the exchange current density for oxygen reduction onM is high, the corrosion rate of silicon under coupledconditions would be higher than that under uncoupledconditions.

Garaud et al., investigated the propensity for galvaniccorrosion between different gate materials (TiN, Mo andRu) and poly-Si (doped and undoped) in HF solutions (0.5to 2%) containing different levels of NH4F (0 to 10%) usingelectrochemical methods [4]. They found the galvaniccorrosion current density to increase with the total fluor-ide content of the solution. In addition, poly-Si conductiv-ity and dissolved oxygen content were shown to have asignificant impact on corrosion behavior. Uniform corro-sion of poly-Si was observed in the case of doped poly

R. Govindarajan et al. / Materials Science in Semiconductor Processing 27 (2014) 390–396 391

whereas localized corrosion at poly-Si/metal interface wasobserved for undoped poly. In another study, Garaud et al.varied metal (Ru) to poly-Si area ratio from 1:1 to 7:1 andfound that increasing the metal to poly-Si area ratioincreased the galvanic current many fold [5].

The effect of water content in HF-Isopropyl Alcohol(IPA) mixture [IPA (97%), HF (1.5%) and H2O (1.5%)] ongalvanic corrosion between Mo and doped poly-Si hasbeen reported by Watanabe et al. [6]. The galvanic currentgradually decreased with decrease in water content of HF–IPA mixture. It was hypothesized that the anodic reactionof silicon dissolution was inhibited in IPA containingsolutions.

Galvanic corrosion studies reported to date havefocused on HF concentrations (0.5 to 2%) that are typicallyused for the removal of oxide hard masks. For selectiveremoval of high-k materials with respect to SiO2, moredilute HF (0.05 to 0.1%) solutions are used [7,8]. Theobjective of this work was to investigate the galvaniccorrosion characteristics between poly-Si and TaN in verydilute HF solutions containing controlled levels of dis-solved oxygen. The effect of cathode to anode area ongalvanic current has also been elucidated. Morphologicalchanges on poly-Si due to galvanic corrosion have beencharacterized using Scanning Electron Microscopy.

2. Materials and methods

Boron doped and undoped poly-Si films of thickness�100 nm, stacked on 100 nm silicon dioxide on siliconprovided by Micron Technology Inc., were used in thisstudy. The doped samples were prepared by PECVD at550 1C using silane (SiH4) and diborane (B2H6) at apressure of �8 Torr. The doping level was controlled byadjusting the flowrate of B2H6. As can be seen from Fig. 1,where sheet resistance (Rs) and dopant concentration areplotted as a function of B2H6 flowrate, the value of Rsinitially decreases from 1300 to 1000 Ω/sq and thenfurther increases to 1500 Ω/sq as the flowrate of B2H6

increases from 75 to 400 SCCM. A minimum in Rs isobserved at about 1000 Ω/sq, which corresponds to aboron concentration of 2.5–4.0E21 #/cm3. Two types ofdoped samples were used in this study: �4E21 #/cm3 and�3E20 #/cm3 boron. Tantalum nitride (TaN) films of

Fig. 1. Sheet resistance (Rs) and dopant concentration (B atoms/cm3) as afunction of B2H6 flowrate (SCCM).

thickness �100 nm deposited on carbon doped silicondioxide (on silicon) with a sheet resistance of 41 Ω/sq wereused in the experiments. Semiconductor grade hydrofluo-ric acid (49%) purchased from Sigma-Aldrich was dilutedappropriately for experiments.

Pre-cleaning of poly-Si and TaN samples was done usingreagent grade isopropyl alcohol (IPA) to remove organiccontaminants followed by immersion in 0.1% hydrofluoricacid for 60 s for removing native oxides. Each pre-cleaningstep was followed by thorough de-ionized (DI) waterrinsing and N2 drying. Aerated and de-oxygenated solutionswere prepared by bubbling air and nitrogen respectivelythrough the solutions for 45 min. In some experiments, anoxygen scavenger (sodium dithionite) was used to controlthe level of dissolved oxygen in HF solutions. It was difficultto measure the dissolved oxygen (DO) content in HFsolutions using a DO sensor, due to material compatibilityissues. Since very dilute HF was used for experiments, DOcontent was measured for DI water under similar condi-tions and assumed to be the same for HF solutions. Airsaturated DI water at 25 1C was used for preparation of HFsolutions. The saturation of DI water with air was achievedby leaving the water exposed to ambient air for 2 h. Themeasured oxygen level using a suitable sensor (RosemontAnalytical model 499 A) was between 8.3 and 8.5 ppm. TheDO content in air saturated DI water and water de-oxygenated by bubbling nitrogen or using oxygen scavengerwas measured to be �8.5 ppm, �4 ppm and �0.2 ppmrespectively.

A three electrode setup was used to carry out galvaniccorrosion experiments as shown in Fig. 2. The workingelectrode (WE) was poly-Si (1 cm2 area). Tantalum nitride(1 or 4 cm2 area) was used as the ground electrode (GE)and a platinum foil (99.9% purity, 1 cm2 area and 0.5 mmthick) was used as the pseudo reference electrode (RE).Poly-Si and TaN were used as separate samples butcoupled to each other in HF solution for galvanic corrosionstudies. Galvanic corrosion measurements were conductedin specially designed laboratory scale electrochemicalcells. The cell body was made of polytetrafluoroethylene(PTFE) with a liquid holding volume of 100 ml or 500 ml

Fig. 2. Schematic of three electrode experimental set-up used forgalvanic corrosion studies.

Fig. 4. Galvanic current density as a function of time for poly-Si coupledto TaN (area ratio 1:1) in 1:1000 HF aerated (�8.5 ppm O2) solutions.

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and an opening at the bottom or side to expose 1 cm2 or4 cm2 of the TaN sample to solution, respectively. Thepoly-Si sample was held in a PTFE holder placed verticallyin the electrochemical cell at a fixed distance from TaN,exposing 1 cm2 of surface to the solution. The platinum foilreference electrode was placed vertically close to the poly-Si sample. Galvanic corrosion and Tafel polarizationexperiments (scan rate: 1 mV/s) were carried out usingPARSTAT 2273 potentiostat (Princeton Applied Research).PARSTAT 2273 has an in-built zero resistance ammeterthat counters the effect of the inserted resistance byapplying a voltage across the resistor from within theinstrument that is exactly equal to the voltage drop acrossthe resistor. This applied voltage is divided by the resis-tance and reported as the current in the circuit.

The corrosion current density of poly-Si was calculatedfrom the integration of galvanic current versus time dataup to 1200 s. Morphological changes in the poly-Si werecharacterized using a field emission scanning electronmicroscope (Hitachi S4800 FESEM).

3. Results and discussion

3.1. Effect of HF concentration

Galvanic corrosion between poly-Si and TaN was inves-tigated by exposing electrically coupled poly-Si (4E21 B,1 cm2) and TaN (1 cm2) surfaces to dilute HF solutionsprepared by diluting 49% HF by 100, 1000 and 5000 timeswith de-ionized water. Fig. 3 shows a plot of galvaniccurrent density measured as a function of time in aeratedHF solutions (8.5 ppm dissolved O2). The current densitystarts off at 2.7 mA/cm2 and 0.2 mA/cm2 for 1:100 and1:1000 HF solutions respectively, and then levels out at1.6 mA/cm2 and 0.15 mA/cm2 after �500 s. In the case of1:5000 HF solutions, the measured current density wasvery low, on the order of nA/cm2. With the electrodeconfiguration employed, a positive current was recorded,

Fig. 3. Galvanic current density as a function of time for poly-Si (1 cm2)and TaN (1 cm2) coupled in 1:100, 1:1000 and 1:5000 aerated (�8.5 ppmO2) HF solutions.

which indicates that poly-Si was the anode and TaN wasthe cathode.

3.2. Effect of doping level

The effect of doping level on galvanic corrosion wasinvestigated in 1:1000 HF solutions under aerated condi-tion (8.5 ppm dissolved O2). Galvanic current densitybetween coupled poly-Si (1 cm2) and TaN (1 cm2) samplesis shown in Fig. 4 as a function of time. Undoped poly-Sishowed a very low galvanic current density (almost belowmeasurable limit). Poly-Si with a doping level of 3E20B/cm3, showed a galvanic current density of 3.3 nA/cm2

and the measured value increased to 0.15 mA/cm2 for4E21 B/cm3 doped poly-Si. The average grain size was�8 nm in 4E21/cm3 doped sample while it was roughly�14 nm in the 3E20/cm3 sample indicating that the grainsize affects galvanic corrosion.

Morphological changes of poly-Si (1 cm2) samples withdifferent doping levels coupled to TaN (1 cm2) after expo-sure to 1:1000 HF solution under aerated conditions(8.5 ppm O2) were characterized using FESEM and theresults are shown in Fig. 5. Poly-Si samples with differentdoping levels showed distinct grain boundaries and grainsizes before exposure to dilute HF solution. Exposure ofdoped poly-Si/TaN couple to 1:1000 HF solution for 20 minresulted in dissolution of poly Si grains, as may be evidentfrom the ‘haziness’ of the grains in the SEM micrographs.The extent of attack on the grains increased with thedoping level, which is consistent with the results ofgalvanic current measurements.

3.3. Effect of dissolved oxygen

The effect of dissolved oxygen in dilute HF solutions ongalvanic corrosion was investigated by exposing poly-Si(area: 1 cm2) doped with 4E21 B/cm3 and TaN (area:4 cm2) couple with an area ratio of 1:4 in HF solutionscontaining different levels of dissolved O2. Dissolved oxy-gen in 1:1000 HF solutions was removed by purging withnitrogen or by using an oxygen scavenger (sodium

Fig. 5. FESEM images of as-received poly-Si (1 cm2) with different doping levels (a) Undoped, (b) 3E20 B/cm3 (c) 4E21 B/cm3 and poly-Si (1 cm2) samplescoupled with TaN after 1:1000 HF exposure for 20 min (d) Undoped, (e) 3E20 B/cm3 and (f) 4E21 B/cm3.

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dithionite). Aqueous solutions of dithionite are acidic innature. Highly acidic conditions induce decomposition ofdithionite to sodium bisulfite (and perhaps SO2). However,both bisulfite and SO2 exhibit oxygen scavenging ability [9].In addition, hydrogen terminated silicon surface is morecommon in HF solution and passivation of silicon by sulfurhas never been reported. Small amount of dithionite (�1.5times the dissolved O2 level) was used for adjusting thedissolved O2 levels in HF solution for the experiments.Fluorine concentration was high compared to the dithio-nite levels in HF solutions and hence passivation of siliconby sulfur was highly unlikely. A blanket of nitrogen wasmaintained on top of the solution in the cell to avoidoxygen diffusion from atmosphere back into solutionduring the measurements.

From the data shown in Fig. 6(a), an average galvaniccurrent density of 34.7 mA/cm2 was measured in aeratedHF solution (8.5 ppm O2), which corresponds to a poly-Sicorrosion rate of 6.5 Å/min. When HF solutions were de-oxygenated to 4 ppm and 0.2 ppm dissolved O2, theaverage galvanic current density dropped to 0.58 mA/cm2

and 0.03 mA/cm2, respectively. Comparing with data inFig. 4 for poly-Si (4E21 B/cm3) to TaN area ratio of 1:1,it may be noticed that increasing the cathode area by 4times results in an increase in galvanic corrosion densityby at least an order of magnitude. FESEM images in Fig. 6(b)indeed confirm that galvanic corrosion of poly-Si is morepronounced in HF solution containing higher dissolvedoxygen concentration. The importance of low oxygen con-tent in HF solution in reducing galvanic corrosion has alsobeen pointed out by Vos et al. [1].

3.4. Estimation of galvanic current density using Tafelpolarization

Galvanic corrosion current densities in different aera-ted HF solutions were estimated from Tafel measurementsmade on 4E21 B/cm3 doped poly silicon (1 cm2 area) andTaN (1 cm2 area). The Tafel plots are shown in Fig. 7. Theanodic Tafel slope for poly-Si immersed in 1:100 HF issmaller compared to that in 1:1000 HF solution. Cathodicpolarization curve for oxygen reduction on TaN immersedin 1:100 HF solution intersects poly-Si anodic curve at ahigher current density compared to the oxygen reductioncurve in 1:1000 HF solution. The estimated magnitude ofcorrosion current density (icorr) decreased with increasingdilution of HF solution (0.98 mA/cm2 in 1:100 vs 22 nA/cm2

in 1:5000 solution).Increasing the HF concentration from 0.1% (1:1000 HF)

to 1% (1:100HF) increases galvanic corrosion rate. Since therelative concentration of different fluorine based species(HF, F� , HF2

�and H2F2) can be a function of HF concentra-

tion, speciation versus pH diagrams were constructed for0.05 M (�0.1%) and 0.5 M (�1%) HF solutions. Thesediagrams were constructed using STABCAL software andthermodynamic data available in the literature [10,11] andshown in Fig. 8(a) and (b). As can be seen from thesefigures, at pH values 2.5 and below, the dominant speciesis H2F2; additionally higher the HF concentration higherthe proportion of H2F2. If it is postulated that the anodicreaction involves Si reaction with H2F2, then the highergalvanic corrosion rate measured at the higher HF con-centration may be expected. Table 1 compares estimated

Fig. 6. Effect of dissolved oxygen concentration in 1:1000 HF solutions on Poly-Si and TaN couple (area ratio 1:4): (a) Measured galvanic current density asa function of time; (b) FESEM images.

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Fig. 7. Tafel polarization plots for Poly-Si and TaN electrodes immersed in1:100, 1:1000 and 1:5000 HF solutions.

Fig. 8. Speciation vs pH diagrams for HF solutions (a) 0.1% and (b) 1%.

Table 1Comparison of estimated and measured corrosion current density valuesfor HF solutions of different dilutions.

HFdilution

Estimatedgalvanic currentdensity (A/cm2)

Measuredcurrentdensity(A/cm2)

Poly Si corrosionrate (Å/min) basedon measuredcurrent density

1: 100 0.98E-06 1.6E-06 0.31:1000 0.072E-06 0.15E-06 0.0271:5000 22E-09 1.0E-09 0.00019

Fig. 9. Tafel polarization of Poly-Si (area 1 cm2) and TaN electrodes (area1 & 4 cm2) immersed in 1:100 HF solution under aerated conditions.

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and measured corrosion current density values. Currentdensities from galvanic corrosion experiments conductedin different solutions agree roughly with those estimatedfrom Tafel plots.

The effect of anode to cathode ratio on galvanic corrosionin aerated 1:100 HF solutions was investigated by couplingpoly Si to TaN samples of different areas. Increasing the arearatio from 1:1 to 1:4 increased the measured galvanic corro-sion rate from 0.3 Å/min (icorr¼1.6 mA/cm2) to 6.5 Å/min(icorr¼34.7 mA/cm2). The influence of cathode area on galvaniccorrosion of poly-Si was also investigated by Tafel polarizationof TaN sample of 4 cm2 area. From the data shown in Fig. 9,the corrosion current resulting from coupling of TaN that isfour times the area of polysilicon was estimated to be 1.45 mA(estimated corrosion rate: 0.27 Å/min). In the case of 1:1 arearatio, the corrosion current is roughly 0.84 mA (estimatedcorrosion rate: 0.16 Å/min).

4. Conclusion

Corrosion of poly-Si coupled to TaN in dilute HF solu-tions was investigated. The effect of variables such as HFconcentration, B doping level, dissolved oxygen content,and cathode area on poly-Si corrosion was examined.Increasing the HF concentration and poly-Si doping levelincreased the galvanic current density and the poly-Sidissolution. The presence of dissolved oxygen in HF solu-tion and an increase in coupled TaN (cathode) arearesulted in enhanced galvanic corrosion of poly-Si.

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By reducing the level of dissolved oxygen in dilute HF solu-tions, galvanic corrosion of poly-Si was significantlysuppressed.

Acknowledgments

The authors acknowledge financial support from SRC/Sematech Engineering Research Center for Environmen-tally Benign Semiconductor Manufacturing at The Univer-sity of Arizona through Contract # 425.028. The authorswould like to thank Dr. Niraj Rana and Dr. Prashant Raghuof Micron Technology Inc., for providing the test samples.

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