ng

unecul

Hard mask

oupC2Fweve grobeleersanithanalysis of the lms etched under a C2F6/Ar gas mixture revealed the existence ofF (i.e. TiF ) over the lm. C F compounds were not detected on the lm surface,

s a crucpropeepositus anduld impto a ra

conductor devices and

coupled plasma reac-ch rate and etch prole

Thin Solid Films 587 (2015) 2027

Contents lists available at ScienceDirect

Thin Soli

.etions in solar cells, light-emitting diodes, photo catalysts, photo-splitting of water and gas sensors [5,6]. In the semiconductor industry,TiO2 has been proposed as a replacement for SiO2 dielectrics of capaci-

of a variety of gases (Cl2, HBr, C2F6, Ar)were examined, and the effects ofetch parameters including coil rf power, dc-bias voltage, gas pressureand gas concentration on the etch prole and etchmechanismwere in-hard mask capabilities [2].The etching of hardmaskmaterials such as Ta, Ti, TiN and TiO2 under

several etch chemistries has recently been widely investigated [3,4].TiO2 has received a great deal of attention due to its numerous applica-

is necessary for their future applications in semias a hard mask material.

In this study, we investigated the inductivelytive ion etching (ICPRIE) of TiO2 thin lms. The etemployed in the next generation of electronic devices owing to theirlow selectivity and severe faceting during etching. Metallic and organichard mask materials have been proposed as possible replacements forpreviously conventional inorganic hard masks owing to their better

and surface analysis of TiO2 thin lms have previously been discussed[913], but physical evidence of redeposition-free etch proles with ahigh degree of anisotropy is scarce. Development of nanoscale aniso-tropic etching processes for TiO2 thin lms and their characterizationtors owing to its high dielectric constant and l

Corresponding author at: 244E, 2nd building, Inha UIncheon, 402-751, Republic of Korea. Tel: +82 32 860 74

E-mail address: cwchung@inha.ac.kr (C.W. Chung).

http://dx.doi.org/10.1016/j.tsf.2014.11.0550040-6090/ 2014 Elsevier B.V. All rights reserved.pid density growth anddevices [1].s SiO2 and SiC cannot be

pear to produce further improvement of the etch rate and etch prole[912].

The etch rates, etch selectivity, plasma modeling, etch mechanisms

physical limitations of semiconductor storage

Conventional inorganic hard masks such anew techniques and materials that coperformance of these devices owing1. Introduction

The etching of electronic materials iof semiconductor devices because imdefects such as faceting, trenching, redto the lm. There has been a continuo 2014 Elsevier B.V. All rights reserved.

ial step in the fabricationr conditions can lead toion, and plasma damagerapid implementation ofrove the scalability and

and as a hard mask material owing to its high selectivity and strongadhesion.

The etch properties of TiO2 thin lms under an inductively coupledplasma (ICP) were previously investigated under Cl2, HBr, BCl3, CF4and non-corrosive gases such as CH4/H2/Ar. The results of these studiesindicated that, as the concentration of halogen gas in the gas mixtureincreases, the etch rate increases and aggressive etch parameters ap-p

x x y

robably due to contamination with atmospheric carbon.photoelectron spectroscopyetch byproducts containingInductive couple plasma reactive ion etchithin lms

Adrian Adalberto Garay, Su Min Hwang, Chee Won ChDepartment of Chemistry and Chemical Engineering, Center for Design and Applications of Mol

a b s t r a c ta r t i c l e i n f o

Available online 22 November 2014

Keywords:Titanium dioxideThin lmsInductively coupled plasma reactive ion etchingHBr/Ar gasCl2/Ar gasC2F6/Ar gas

Changes in the inductively cthe addition of HBr, Cl2 andthe etch rate increased; hoconcentration increased, thanisotropic etch prole wasCl2. Field emission scanningprole; hence, etch parametgas pressure on the etch ratethe etch prole increased w

j ourna l homepage: wwwow leakage current [7,8],

niversity, 100 Inharo, Nam-gu,73; fax: +82 32 872 0959.characteristics of TiO2

g ar Catalysts, Inha University, 100 Inharo, Nam-gu, Incheon 402-751, Republic of Korea

led plasma reactive ion etching characteristics of TiO2 thin lms in response to6 to Ar gas were investigated. As the HBr, Cl2 and C2F6 concentration increased,er, the etch prole degree of anisotropy followed a different trend. As HBreatest anisotropic etch prole was obtained at 100% HBr, while the greatesttained at concentrations of 25% when etching was conducted under C2F6 andctron microscopy revealed that 25% C2F6 generated the greatest vertical etchwere varied at this concentration. The effects of rf power, dc-bias voltage andd etch prole were also investigated. The etch rate and degree of anisotropy inincreasing rf power and dc-bias voltage and decreasing gas pressure. X-ray

d Films

l sev ie r .com/ locate / ts fvestigated. The etch rates were obtained using a surface prolometerand etch proles were observed by eld emission scanning electronmicroscopy (FESEM). Additionally, the surface chemistry of TiO2 lmsunder the proper etch gas was analyzed by X-ray photoelectronspectroscopy (XPS).

at 100% C2F6, respectively. The etch selectivity gradually decreasedfrom a maximum of 1.08 at 100% Ar to a minimum of 0.477 at 100%C2F6. The etch selectivity of TiO2 thinlms over PRmask showed the ten-dency of slightly decreasing with increasing C2F6 concentration, whilethe etch rate increased. Although an unsaturated uorocarbon protec-tive layer is formed when the lms are etched in a C2F6/Ar plasma, theetch rate of PR is so fast that the sidewall of TiO2 cannot be properlyprotected. It is believed that the etch selectivity of TiO2 lm over PRmask can somehow inuence the overall etch prole. Fig. 6 shows theFESEM micrographs of the TiO2 samples etched under a variety of C2F6concentrations. The TiO2 samples etched in pure Ar showed heavy side-wall redeposition (Fig. 6(a)), while the addition of 25% C2F6 to Ar gas(Fig. 6(b)) revealed an etch slope with a high degree of anisotropy. Asthe C2F6 concentration increased from 50% to 100% (Fig. 6(c) to (e)),the etch slope becamemore slanted and the surface roughness increasedconsiderably. At 100% C2F6, the formation of an area near the sidewallthat differed in height from the rest of the lm was clearly evident.These ndings suggest the existence of a polymer layer, presumablycontaining CxFy compounds, over the surface.

All halogen gas chemistries explored above showed a chemical en-

21A.A. Garay et al. / Thin Solid Films 587 (2015) 20272. Experimental details

ICPRIE of TiO2 thin lms was conducted using HBr/Ar, Cl2/Ar andC2F6/Ar gas mixtures. Additionally, 100 nm TiO2 thin lms were pre-pared on a SiO2/Si substrate by rf magnetron reactive sputtering usinga Ti target in a Ar (35 sccm)/O2 (5 sccm) atmosphere.

Photolithography was carried out on TiO2 thin lms using a 1.2 mthick photoresist (PR: AZ1512) to pattern the lm. The lms were pat-terned as parallel lines of different widths. The TiO2 thin lms werethen etched using a conventional ICPRIE system (A-Tech System,Korea) equipped with a main chamber and a load lock chamber. To dis-sipate the heat generated during the etching process while avoidingdamage to the sample, a cold uid around 12 C to 15 C was circulatedthrough the susceptor, while circular void channels between the sub-strate and the susceptor were lled with He to improve sample cooling.

High density plasmawas generated by a coil located at the top of themain chamber, that was connected to a 13.56 MHz rf power supply. Tocontrol the kinetic energy of the ions generated on the plasma, a self-induced dc bias voltage was generated by an rf power generator at13.56 MHz that was capacitively coupled to the susceptor. The vacuumcondition of the main chamber was generated by a turbo molecularpump backed up by a mechanical pump, making it possible to obtainbase pressures lower than (2.74) 104 Pa.

The HBr/Ar, Cl2/Ar and C2F6/Ar gasmixtures used as etch gases werefed into the main chamber at a rate of 40 sccm. The etch rates, etch se-lectivity and etch proles of the TiO2 thin lm and photoresist were ex-amined under varying concentrations of HBr/Ar, Cl2/Ar and C2F6/Ar.Additionally, the effects of etch parameters such as ICP rf power, dc-bias voltage to the substrate, and gas pressure on the etch rate and pro-le were investigated. Etch by products and the surface chemistry ofTiO2 lms prepared under the proper etch gas were analyzed by XPS.

The etch rates were obtained using a surface prolometer (Tencor P-1) and etch proleswere observed by FESEM(FESEM-Hitachi 4300SE) atan operating voltage of 15 kV. Etch byproducts, the surface chemistryand etch mechanism of TiO2 lms under the proper etch gas were ana-lyzed by using an ex-situ K-alpha source XPS analyzer (ThermoScienticK-Alpha). All XPS analysis sampleswere TiO2 thinlmswithout photore-sist. To remove any contamination from the lm surface, all sampleswere Argon pre-sputtered for 30 s at beam acceleration energy andbeam current of 1 keV and 2 A, respectively. The binding energieswere calibrated using Au 4f7/2 = 84.00 eV as a reference.

3. Results and discussion

Etching of TiO2 thin lms was carried out under various HBr/Ar, Cl2/Ar and C2F6/Ar concentrations at an ICP power of 800W, dc-bias voltageto substrate of 300 V and gas pressure of 0.67 Pa.

Fig. 1 shows the etch rate of TiO2 thin lms and the etch selectivity ofTiO2 over PR etched under the HBr/Ar gas mixture. The etch rate in-creased fromaminimumof 23.81 nm/min at 100%Ar gas to amaximumof 64.62 nm/min at 75% HBr gas. At 100% HBr, the etch rate decreased to63.12 nm/min. The etch rate of PR increased from a minimum of 22.02nm/min under pure Ar to a maximum of 160.3 nm/min at 25% HBr,then decreased to 116.2 nm/min at 100%HBr.We consider this decreasein etch rate of TiO2 thin lms at 100% HBr to be related to plasma insta-bilities and/or the considerable reduction of ion bombardment onto thesample due to the absence of Ar gas. The etch selectivity underwent arapid decrease from 1.08 nm/min under pure argon to a minimum of0.197 at 25% HBr, after which it increased to a maximum value of0.543 at 100% HBr. FESEM micrographs that describe the etch proleof TiO2 thin lms etched under various HBr concentrations are shownin Fig. 2. PR patterned TiO2 thin lms before etching are shown inFig. 2(a). Fig. 2(b) shows the etch prole of TiO2 thin lms etched inpure Argon. Heavy sidewall redeposition and a slanted slope couldclearly be seen, probably due to the absence of any chemical reactions

and the dominance of physical sputtering. As HBr concentrationsincreased from 25% to 75% (Fig. 2(c) to (e)), there was a slight improve-ment in the etch slope and no redeposition could be seen near the side-wall. At 100% HBr (Fig. 2(f)), the etch slope clearly improved relative toother samples, but the angle was still less than 60. In general, the intro-duction of 25% HBr was sufcient to obtain a slanted, but redepositionfree etch prole that improved as concentration increased.

Fig. 3 depicts the etch rate of TiO2 thin lms and the etch selectivityof TiO2 over PR etched under a Cl2/Ar gas mixture. The etch rate slowlyincreased from a minimum of 23.81 nm/min at 100% Ar gas to amaximum of 134.76 nm/min at 100% Cl2. The etch rate of PR underwenta steeper increase from a minimum of 22.02 nm/min to a maximum of447.6 nm/min. The etch selectivity showed a maximum value of 1.08 at100% Ar that gradually decreased to its minimum value of 0.3 at 100%Cl2.

Fig. 4 depicts the FESEM micrographs of the TiO2 thin lms etchedunder various Cl2/Ar concentrations. Fig. 4(a) and (b) shows thetransition of the etch prole from 100% Ar and 25% Cl2/Ar, respectively.The sample etched in pure Ar showed heavy sidewall redeposition,while the prole of the sample etched at 25%Cl2 improved considerably.A further increase in Cl2 concentration led to negative effects on the etchprole degree of anisotropy (Fig. 4(c) to (d)).

Fig. 5 shows the etch rate and selectivity of the TiO2 thin lms etchedunder C2F6/Ar gas mixture at varying concentrations. The etch rate ofTiO2 and PR increased from a minimum of 23.81 nm/min and 22.02nm/min at 100% Ar to a maximum of 105.6 nm/min and 221.2 nm/min

0 25 50 75 1000

50

100

150

200

Etc

h R

ate

(nm

/min

)

% HBr in HBr/Ar

TiO2 PR TiO2/PR

0

1

2

Sel

ectiv

ity

Fig. 1.Etch rate of TiO2 thinlms and PR, and etch selectivity of TiO2/PR under varying TiO2concentrations in HBr/Ar mixtures and the following etch conditions: ICP rf power of800 W, dc bias voltage of 300 V and gas pressure of 0.667 Pa.hancement of the TiO2 etch rate over pure Ar sputtering, but the etch

(c)

(a)

300nmPhotoresistTiO2

22 A.A. Garay et al. / Thin Solid Films 587 (2015) 2027rate of TiO2 thin lms in Cl2/Ar was the fastest, followed by C2F6/Ar andHBr/Ar. The high etch rates in the Cl2/Ar gas mixture may be associatedwith the higher total ux of neutral reactive species, as well as thehigher volatility of TiClx etch byproducts compared to TiBrx etch byproducts [11]. Both Cl2/Ar and HBr/Ar appear to follow an ion enhancedetching mechanism commonly known as reactive ion etching [14]. The

(e)

Fig. 2. FESEM micrographs of TiO2 thin lms before etching (a) and etched in

0 25 50 75 1000

100

200

300

400

500

Etc

h R

ate

(nm

/min

)

% Cl2 in Cl2/Ar

TiO2 PR TiO2/PR

0

1

2

Sel

ectiv

ity

Fig. 3.Etch rate of TiO2 thinlms and PR, and etch selectivity of TiO2/PR under varying TiO2concentrations in Cl2/Ar mixtures and the following etch conditions: ICP rf power of800 W, dc bias voltage of 300 V and gas pressure of 0.667 Pa.(b)

(d)

TiO2

SiO2etch rates of TiO2 thinlmsunder SF6/Ar gasmixtures have been report-ed to bemuch faster than those under Cl2/Ar gasmixtures, which bringsinto question the reasons why the etch rates of TiO2 thin lms etchedunder a C2F6/Ar gasmixturewere considerably lower than those obtain-ed under a Cl2/Ar gas mixture [9]. We believe that etching of TiO2 thinlms in a C2F6/Ar gas mixture is hindered by the formation of an inhibi-tion layer over the lm, similar to those generated in response to freonfeed gases (e.g., C2F6 and CHF4) that yield unsaturated polymer-formingspecies in plasmas [14].

The SEM micrographs of the TiO2 thin lms etched under HBr/Ar,Cl2/Ar and C2F6/Ar revealed that the best etch prole was obtained at25% C2F6/Ar. We believe that there is a balance between the formationof an inhibition layer and its removal by the effect of Ar sputtering atthis concentration, which helps maintain the high degree of anisotropyin the etch slope. Based on the results obtained upon etching of TiO2 thinlms in varying gasmixtures and concentrations, 25% C2F6 was selectedas the standard gas concentration for evaluation of the etching parame-ters due to its degree of anisotropy and plasma stability. The standardconditions were 25% C2F6 in a C2F6/Ar gas mixture at an ICP rf powerof 800 W, dc-bias voltage to substrate of 300 V and gas pressure of0.67 Pa.

Fig. 7(a) shows the etch rates of TiO2 thin lms and PR togetherwiththe selectivity of TiO2 thin lms to PR for a variation of ICP power whilethe other conditions were xed. As the ICP rf power increased from700 W to 900 W, the etch rates of TiO2 thin lms and PR increased

(f)

(b) pure argon, (c) 25% HBr, (d) 50% HBr, (e) 75% HBr, and (f) 100% HBr.

(a)

(c)

300nm

23A.A. Garay et al. / Thin Solid Films 587 (2015) 2027and etch selectivity increased slightly. The increase in the etch rate ofTiO2 thin lms as rf power increased can be attributed to an increaseof both neutral species and ions. The increase in rf power causes in-creases in the densities and uxes of positive ions and F atoms throughan increase in both dissociation and ionization rates, which leads to an

(e)

Fig. 4. FESEM micrographs of TiO2 thin lms etched in (a) pure a

0 25 50 75 1000

100

200

% C2F6 in C2F6/Ar

Etc

h R

ate

(nm

/min

)

TiO2 PR TiO2/PR

0

1

2

Sel

ectiv

ity

Fig. 5.Etch rate of TiO2 thinlms and PR, and etch selectivity of TiO2/PR under varying TiO2concentrations in C2F6/Ar mixtures and the following etch conditions: ICP rf power of800 W, dc bias voltage of 300 V and gas pressure of 0.667 Pa.(b)

(d)increase in the etch rate of TiO2 thin lms [12]. FESEM micrographs ofTiO2 thin lms etched at various ICP rf powers are shown inFig. 7(b) to (d). The etch prole of the sample etched at 700 W showsa slanted etch slope with no redeposition near the sidewall (Fig. 7(b)).Fig. 7(c) depicts the sample etched at 800 W (standard conditions).The etch prole seemed to be similar to those observed in response tovarying C2F6/Ar concentrations, suggesting good reproducibility. Asthe rf power increased to 900 W (Fig. 7(d)), the etch slope appearedto improve slightly compared to TiO2 thin lms etched at 800 W.

The effects of dc-bias voltage on the etch rates and selectivity of theTiO2 thin lms and PR are presented in Fig. 8(a). As the DC bias in-creased from 200 V to 400 V, the etch rate of both TiO2 thin lms andPR rapidly increased relative to the rf power variation. The etch selectiv-ity of TiO2 thin lms to PR steadily increased from 200 V to 300 V, but afurther increase in the dc bias to 400 V caused a decrease in the etch se-lectivity. FESEM micrographs revealed that the etch slope becameslanted and the etch depth near the sidewall differed greatly from theaverage etch depth at a dc-bias voltage of 200 V (Fig. 8(b)). The etchslope at a dc-bias voltage of 400 V (Fig. 8(c)) was steeper than that at200 V. The increase in dc-bias voltage caused an increase in the overallion energy, increasing the ion bombardment onto the lm surface.This caused an expedited increase in the sputtering effect of the lmthat enhanced the bond breaking and desorption of etch productsfrom the TiO2 surface, resulting in an increased etch rate and improvedetch prole.

rgon, (b) 25% Cl2, (c) 50% Cl2, (d) 75% Cl2, and (e) 100% Cl2.

24 A.A. Garay et al. / Thin Solid Films 587 (2015) 2027(a)

(c)

300nmFig. 9(a) shows the etch rate and selectivity of TiO2 thin lms etchedat different gas pressures. As pressure increased from 0.133 Pa to1.33 Pa, the etch rate of the TiO2 thin lms and PR decreased, whilethe etch selectivity of TiO2 over PR decreased slightly. The SEM micro-graph of the sample etched at 0.133 Pa (Fig. 9(b)) revealed a steepetch slope with no visible redeposition along the sidewall, while theetch prole of the sample etched at 1.33 Pa (Fig. 9(c)) exhibited aslanted etch slope. In general, lowering the pressure leads to a propor-tional decrease in the concentration of neutral etchant, while the rela-tive rate of energetic ion-enhanced etching increases [15]. At lowpressures (0.133 Pa), ion bombardment onto the sample is enhancedbecause high energy and low incident angle ions are abundant due tothe large mean free path. This results in an increased etch rate and de-gree of anisotropy of the etch prole. Conversely, at high pressures(1.33 Pa), many ions with relatively low energy and high-incident an-gles exist. The lack of high energy ions and diminished ion bombard-ment consequently reduce the etch rate due to inefcient TiO bondbreaking and/or the formation of a greater amount of etch byproducts.We suspect that the increase in neutral active species (chemical etch-ing) at high pressures results in the formation of a polymer layer overthe lm, which reduces the etch rate and degree of anisotropy of theetch prole [14,16].

(e)

Fig. 6. FESEM micrographs of TiO2 thin lms etched in (a) pure argo(b)

(d)XPS analysis was conducted to elucidate the etch mechanism anddetermine whether etch byproducts were present on lms etchedunder a C2F6/Ar gas mixture. Bare TiO2 thin lms without photoresistmasks were used for the analysis and all species were pre-sputteredprior to analysis to remove contaminants from the surface.

Fig. 10(a) shows the narrow scan of Ti 2p3/2 peaks for TiO2 thin lms.The narrow scan of the as-deposited lm reveals the chemical state ofpure TiO2 (~458.8 eV) and Ti2O3 (~456.9 eV) [17]. However, when theTiO2 thin lms were etched in 25% C2F6/Ar (low C2F6 concentration)and 75% C2F6 (high C2F6 concentration), the main Ti 2p3/2 peak shiftedto a higher energy of around 459 eV. The shift of the peak indicatesthat compounds containing Ti were formed on the surface of TiO2 thinlms [12]. Fig. 10(b) presents the narrow scans of the F 1s peaks. TheF 1s peaks of the as-deposited lmwere not observed; however, the ex-istence of main peaks of 684.5 eV to 685.1 eV after the sample wasetched in 25% and 75% C2F6/Ar gas mixtures suggests the existence ofTiFx compounds over the lm [18]. Additionally, the presence of second-ary peaks located at approximately 683.8 eV suggests the presenceof structures corresponding to oxyuoride (FTiO) functionalgroups over the TiO2 surface [19]. In addition to the existence of TiFxor TiO2 xFx compounds after etching of TiO2 thin lms in a C2F6/Argas mixture, we expected to obtain signals corresponding to CxFy

n, (b) 25% C2F6, (c) 50% C2F6, (d) 75% C2F6, and (e) 100% C2F6.

00w

25A.A. Garay et al. / Thin Solid Films 587 (2015) 2027700 80

50

100

150

200

250

Etc

h R

ate

(nm

/min

)rf po

TiO2 PR TiO2/PR

(b)compounds; however, the Ti 2p3/2 and C 1s narrow scan (gure notshown) revealed the absence of this etch byproduct. Hazra et al. report-ed that uorinated carbon residues over the lm were extremely dif-cult to detect unless in situ XPS measurements were carried out afteretching [20].

The Ti 2p3/2 and F 1s spectra conrmed that F related compoundswere formed over the TiO2 layer after etching under C2F6 plasma chem-istries. Even though the etch rate increased as the C2F6 concentration in-creased, Ti, O and/or F containing compounds formed over the lm,even at low C2F6 concentrations (Figs. 6(b) and 10). Furthermore, de-tailed inspection of Fig. 6 FESEM micrographs clearly showed that theTiO2 surface became irregular and rougher as the C2F6 concentration in-creased,whichwas likely due to the formation of amuchdenser layer ofetch byproducts (TiFx compounds, CxFy polymers, oxyuorides, etc.)over the lm at high C2F6 concentrations. The Ti 2p3/2 spectrum(Fig. 10(a)) Ti2O3 peak intensity increasedwith C2F6 addition comparedto the as depositedlm. The destruction of TiObonds by ion sputteringgenerated highly reactive sites that not only increased the chemicaletching rate of the C2F6 neutral species, but also induced bonding

(d)

300nm

Fig. 7. (a) Etch rate of TiO2 thinlms and PR, and etch selectivity of TiO2/PRunder varying coil rf300 V and gas pressure of 0.667 Pa. FESEMmicrographs of TiO2 thin lms etched at the follow900er

0

1

Sel

ectiv

ity

(c)

(a)between the Ti and O elements of the lm to form several types of Tioxides.

4. Conclusions

The ICPRIE characteristics of PR patterned TiO2 thin lms were in-vestigated using HBr/Ar, Cl2/Ar and C2F6/Ar gas mixtures. The etchrates and etch proles of TiO2 thin lms were examined by individuallyvarying the HBr, Cl2 and C2F6 concentrations. Generally, as the HBr, Cl2and C2F6 concentration in the Ar gas mixture increased, the etch ratesof TiO2 thin lm and PR increased, except for the HBr/Ar gas mixture.The etching of TiO2 thin lms using HBr/Ar, Cl2/Ar and C2F6/Ar gas mix-tures was found to follow the conventional reactive ion etchingmecha-nism. FESEM images revealed heavy sidewall redeposition on the TiO2samples etched in pure Ar, but as the HBr concentration increased, thedegree of anisotropy increased while for Cl2 and C2F6 it decreased asconcentration increased. Among the three halogen gases employed,the most anisotropic etch proles were obtained in a C2F6/Ar gas mix-ture. At 25% C2F6, the etch slope appeared to be greater than 80, with

power and the following etch conditions: 25% C2F6 in C2F6/Ar gasmixture, dc bias voltage ofing coil rf power: (b) 700 W, (c) 800 W, and (d) 900 W.

200 300 4000

50

100

150

200

250

Etc

h R

ate

(nm

/min

)dc bias

TiO2 PR TiO2/PR

0

1

Sel

ectiv

ity

(b) (c)

300nm

(a)

Fig. 8. (a) Etch rate of TiO2 thinlms and PR, and etch selectivity of TiO2/PR under varying dc-bias voltage and etch conditions of: 25% C2F6 in C2F6/Ar gasmixture, ICP rf power of 900Wandgas pressure of 0.667 Pa. FESEM micrographs of TiO2 thin lms etched at the following coil rf power: (b) 200 V and (c) 400 V.

0 5 100

50

100

150

200

250

Etc

h R

ate

(nm

/min

)

Pressure

TiO2 PR TiO2/PR

0

1

Sel

ectiv

ity

300nm

(b) (c)

(a)

Fig. 9. (a) Etch rate of TiO2 thinlms and PR, and etch selectivity of TiO2/PR under varying gas pressures and etch conditions of: 25% C2F6 in C2F6/Ar gasmixture, ICP rf power of 900Wanddc bias voltage of 300 V. FESEM micrographs of TiO2 thin lms etched at varying gas pressures of: (b) 0.133 Pa and (c) 1.33 Pa.

26 A.A. Garay et al. / Thin Solid Films 587 (2015) 2027

Ti 2p3/2

Inte

nsity

(arb

.uni

t)

25% C2F6 /Ar

75% C2F6/Ar

F 1s

Inte

nsity

(arb

.uni

t)

25% C2F6 /Ar

75% C2F6/Ar

(b)(a)

nder

27A.A. Garay et al. / Thin Solid Films 587 (2015) 2027no visible redeposition near the sidewall; however, as the concentrationincreased to pure C2F6, the lm surface became irregular and the etchdepth varied greatly with position, with the depth near the sidewallbeing deeper than between patterns. These changes may have beendue to the reduction of physical sputtering at this concentration, aswell as the formation of layers of CxFy and etch byproducts over thelm.

The etch parameters were explored using 25% C2F6 as the standardgas concentration. The etch rate, selectivity and degree of anisotropyof TiO2 thin lms increased with increasing rf power, dc-bias voltageand decreasing gas pressure.

XPS analysis of partially etched TiO2 blanket lms conrmed that thechemical reaction between TiO2 and C2F6 left etch byproducts, mainlyTiFx compounds, on the etched TiO2 surface. The existence of a CxFy poly-mer layer over the TiO2 thin lm could not be conrmed due to contam-ination with atmospheric carbon. These ndings suggest that etchingTiO2 thin lms in a 25% C2F6/Ar gas mixture is suitable for obtaining pat-terns with a high degree of anisotropy, even though post treatment ofthe lms might be necessary to reduce or eliminate uorine etchbyproducts from the lm.

Acknowledgments

This research was supported by a grant from the R&D Program forIndustrial Core Technology funded by the Ministry of Trade, Industryand Energy (MOTIE), Republic of Korea (grant no. 10044723). Thiswork was supported by an Inha University research grant (grant no.50047).

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