Effect of radical species density and ion bombardment during ashing of extremeultralow- κ interlevel dielectric materialsM. A. Worsley, S. F. Bent, N. C. M. Fuller, T. L. Tai, J. Doyle, M. Rothwell, and T. Dalton Citation: Journal of Applied Physics 101, 013305 (2007); doi: 10.1063/1.2405123 View online: http://dx.doi.org/10.1063/1.2405123 View Table of Contents: http://scitation.aip.org/content/aip/journal/jap/101/1?ver=pdfcov Published by the AIP Publishing Articles you may be interested in Effect of low-frequency radio frequency on plasma-enhanced chemical vapor deposited ultra low-κ dielectric filmsfor very large-scale integrated interconnects J. Appl. Phys. 115, 144107 (2014); 10.1063/1.4870453 H2/N2 plasma damage on porous dielectric SiOCH film evaluated by in situ film characterization and plasmadiagnostics J. Appl. Phys. 109, 084112 (2011); 10.1063/1.3562161 Effect of energetic ions on plasma damage of porous SiCOH low- k materials J. Vac. Sci. Technol. B 28, 450 (2010); 10.1116/1.3372838 Modifications of dielectric films induced by plasma ashing processes: Hybrid versus porous SiOCH materials J. Vac. Sci. Technol. B 26, 1964 (2008); 10.1116/1.3006021 Time of flight secondary ion mass spectroscopy investigation of ultralow- k dielectric modifications in hydrogenand deuterium plasmas J. Vac. Sci. Technol. B 24, 2695 (2006); 10.1116/1.2382949

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Effect of radical species density and ion bombardment during ashingof extreme ultralow-� interlevel dielectric materials

M. A. Worsley and S. F. BentDepartment of Chemical Engineering, Stanford University, Stanford, California 94305

N. C. M. Fullera�

Semiconductor Research and Development Center, IBM Research Division,Yorktown Heights, New York 10598

T. L. TaiIBM Systems and Technology Group, Hopewell Junction, New York 12533

J. Doyle, M. Rothwell, and T. DaltonSemiconductor Research and Development Center, IBM Research Division,Yorktown Heights, New York 10598

�Received 3 July 2006; accepted 26 October 2006; published online 9 January 2007�

The significance of ion impact and radical species density on ash-induced modification of anextreme ultralow-� interlevel dielectric �ILD� material ���2.0� in a patterned single damascenestructure exposed to Ar/O2 and Ar/N2 dual frequency capacitive discharges is determined bycombining plasma diagnostics, modeling of the ion angular distribution function, and materialcharacterization such as angle resolved x-ray photoelectron spectroscopy. Radical species densitywas determined by optical emission actinometry under the same conditions and in the same reactorin a previous study by the present authors. ILD modification is observed and correlated with changesin the plasma for a range of pressures �5–60 mTorr�, bias powers �0–350 W�, and percent Ar in thesource gas �0%, 85%�. For the Ar/O2 discharge, extensive modification of the ILD sidewall wasobserved for significant ion scattering conditions, whereas minimal modification of the ILD sidewallwas observed under conditions of minimal or no ion scattering. Further, for an identical increase inthe O-radical density �� an order of magnitude�, a different degree of modification was induced atthe ILD trench bottom surface depending on whether pressure or percent Ar was used to increase theradical density. The different degrees of modification seemingly correlated with the relative changesin the ion current for increasing pressure or percent Ar. For the Ar/N2 discharge, reduced damageof the ILD sidewall and trench bottom surfaces was observed for increasing pressure �increasingN-radical density� and decreasing ion current to both surfaces. It is, thus, proposed that themechanism for modification of the porous ILD is dominated by the creation of reactive sites by ionimpact under the present conditions. A detailed discussion of the results which support this proposalis presented. © 2007 American Institute of Physics. �DOI: 10.1063/1.2405123�

I. INTRODUCTION

The challenge of integrating ultralow-� �ULK� and ex-treme ultralow-� �eULK� interlevel dielectric �ILD� materi-als in dual damascene integration schemes is a key issue inthe microelectronics industry.1 One of the critical obstaclesto successful integration of low-� ILD materials is the pho-toresist ash process. A first-order approximation of the re-sponse of an ILD material exposed to a photoresist removalprocess can be obtained by exposing the blanket ILD film tothe said plasma process and investigating any changes in thefilm properties postexposure by a variety of analytical tech-niques. Due to the accessibility of blanket films and the rela-tively simple experimental setup, various blanket film studiesof this nature have been performed to address the modifyingeffect that photoresist ash plasmas have on a variety of denseand porous organosilicate glass �OSG� materials.2–10 Theseblanket film studies have allowed for some fundamental un-

derstanding of how the modification of the ILD due to expo-sure to ash plasmas can change as a function of plasmachemistry, substrate temperature, and porosity. However, theusefulness of blanket film studies is ultimately limited whentrying to draw conclusions that would apply directly to inte-gration because results observed on a blanket film may ormay not accurately correspond to what would be observed ina patterned structure. One of the main reasons for this dis-crepancy is the presence of multidimensional interconnectstructures comprised of vias and trenches. These multidimen-sional structures have surfaces that are simply not present inblanket films �i.e., sidewalls� and as such may exhibit differ-ences in the degree of modification depending on the ashconditions. Specifically, the introduction of these featuresmay cause local perturbations in the plasma, namely, in thereactive species density, which may vary as a function ofaspect ratio or pattern density. Further, the ion current inci-dent on the sidewalls is likely to be much less than that onhorizontal surfaces. Therefore, the results of a study involv-ing a patterned ILD structure would be more suitable fora�Electronic mail: nfuller@us.ibm.com

JOURNAL OF APPLIED PHYSICS 101, 013305 �2007�

0021-8979/2007/101�1�/013305/11/$23.00 © 2007 American Institute of Physics101, 013305-1

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addressing detailed questions concerning the modification ofeULK or ULK ILD films in patterned structures by photore-sist ash plasmas.

One such question involves the mechanism by whichmodification of the ILD occurs. It is widely held that theprocess that results in modification depends on the flux ofvarious plasma species �ions and neutrals� to the surface.Previous work by the present authors on blanket films andpreliminary work �also by the present authors� on patternedstructures suggest that reactive neutral species density andion bombardment may play a key role in modifying theILD.8,11,12 Blanket film studies showed that increasing thefraction of hydrogen in the source gas resulted in a signifi-cant decrease in the carbon content of the ILD film.8 Pat-terned structure studies showed that more oxidizing andmore reducing plasma conditions caused more damage �lesscarbon content in the ILD� than less oxidizing or less reduc-ing conditions respectively.11,12 These results would suggestan effect due to reactive species density. The patterned struc-ture studies also showed a strong dependence of dielectricdamage �as measured by intralevel capacitance on nestedcomb structures� on downstream ash process �negligible ioncurrent� conditions, suggesting an effect due to ionbombardment.11,12 Though these studies suggest the role ofspecies density and ion bombardment on the modificationprocess, the results are not conclusive. Due to the variety ofplasma conditions used and the absence of plasma diagnos-tics, it would be difficult to draw strong conclusions on theeffect of species density or ion bombardment independent ofother possible influences. Therefore, a controlled study, fo-cused solely on determining how ion bombardment and re-active neutral species densities affect the modification pro-cess of patterned ULK/eULK ILD structures, would be animportant step toward understanding how to tune the photo-resist ash process such that modification of the ILD is mini-mized.

This study reports measurements of the modification of asingle damascene patterned eULK ILD structure due to pho-toresist removal as well as modeling of the ion angular dis-tribution function �IADF� in Ar/O2 and Ar/N2 discharges. Aprevious work13 reports the neutral species densities of therelevant plasmas in the same reactor used in the presentstudy. Modification of the patterned ILD was determined us-ing angle resolved x-ray photoelectron spectroscopy�ARXPS�. The ARXPS technique was used to determine thecomposition of the ILD surface on the sidewall and trenchbottom. The composition of the sidewall surface and trenchbottom surface were differentiated by collecting data at twodifferent sample orientations. Modeling of the IADF in theplasma sheath was done by comparing the ion mean freepath ��i� with the plasma sheath thickness �sm�. It is assumedfor this study that for �i�sm scattering events �collisionalregime� occur, and for �i�sm there are little to no scatteringevents that occur �collisionless regime�. It is then concludedthat in a collisional regime ion bombardment will occur atboth the sidewall surface and the trench bottom surface,while in a collisionless regime, ion bombardment will belimited to the trench bottom surface.

II. EXPERIMENT

A. Test structure

Figure 1 shows a schematic of the test structure used inthis study before and after photoresist removal. The teststructure is similar to that described in more detail in previ-ous work.11,12 Briefly, the single damascene structure con-sists of a 460 nm layer of a porous SiCOH �OSG� materialsandwiched between two 30 nm layers of TaN with a300 nm line/space array patterned in 450 nm of photoresist.The porous SiCOH �OSG� film used in this study was Den-driglass, a spin-on porous methyl silsesquioxane that hasbeen widely characterized.8,14–20 In this study, a porogenloading of 40% was used to achieve the target � value of lessthan 2.0. The preparation and curing of the OSG is identicalto that found in a previous study.8

B. Equipment

The ash tool used in this study is a commercial dualfrequency capacitively coupled system identical to that usedin a previous study.13 The power is delivered at frequenciesof �13.56 and �27 MHz for the bias and source, respec-tively. Discharge parameters for the different plasma chem-istries are shown in Table I. The total flow rate for all caseswas set at 332.5 SCCM �SCCM denotes cubic centimeter perminute at STP�. The concentration of radical species in theoxygen discharge was varied by changing the percent of car-rier gas, Ar, in the source flow from 0% to 85%. Previouswork has shown that this method of varying the radical den-sity is not as efficient in the nitrogen discharge, so it is notincluded.13 The concentration of active species in the sourcegas was also modulated using a pressure range of5–60 mTorr. For all cases source power was set at 300 W.Bias powers are in the range of 0–200 W for the oxygenchemistries and of 100–350 W for the nitrogen chemistry.The higher bias power settings were necessary to achieveappreciable ash rates in the Ar/N2 discharge. A 25%“overash” condition was used for all experiments to elimi-

FIG. 1. �Color online� Schematic of test structure and ARXPS strategy todetect ILD trench bottom and sidewall modification independently.

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nate any residual photoresist on the TaN which might con-tribute to the C and/or O signal from the ILD.

The modification of the ILD composition at sidewall andtrench bottom surfaces was determined by ARXPS on aThermo VG Theta 300 MK1 tool for which the angular ac-curacy was less than 2°. A charge neutralizer was used dur-ing all scans to minimize the x-ray induced charging effect.ARXPS spectra of the Si2p, O1s, C1s, and N1s peaks wereobtained using Al �K�� radiation after the photoresist re-moval step. Example spectra are shown in Fig. 2. Modifica-tion is defined as any deviation in ILD composition from thatof the pristine material as determined by XPS. From previ-ous work,8 the composition of the pristine porous SiCOHfilm by XPS is SiO1.3C. Standard deviations represent theuncertainties in the integrated peak area, determined with theGaussian peak-fitting method, for multiple measurements.

The ARXPS used in this study is a simplified version ofthat performed by Oehrlein.21 The concept of “shading”21

was utilized, but the XPS detector remains at a fixed anglewith respect to the sample surface normal. The composition

of the sidewall surface and trench bottom surface were dif-ferentiated by positioning the XPS detector in two differentorientations with respect to the sample, as shown in Fig. 1:�1� parallel or 0° with respect to the trenches to probe thetrench bottom and �2� perpendicular or 90° with respect tothe trenches to probe the sidewall. In both orientations, thedetector was positioned at an angle of 59° from the surfacenormal. This angle was determined based on the geometry ofthe test structure �Fig. 1� after etching �250 nm into SiCOH.When the detector is oriented perpendicular to the trenches,at angles 43° from the surface normal, the only electrons toreach the detector from the ILD surface originate from thesidewall since the line of sight from the trench bottom to thedetector is nonexistent. The angle of 59° from the surfacenormal was chosen to ensure that only the sidewall issampled even if slight variations in the etch depth of the ILDoccur. When the XPS detector is positioned parallel to thetrenches, the only electrons to reach the detector from theILD surface originate from the trench bottom since the lineof sight from the sidewall to the detector is nonexistent.

TABLE I. Discharge parameters.

Parameter Oxygen discharge Nitrogen discharge

Upper electrode �rf� 300 W ��27 MHz� 300 W ��27 MHz�Lower electrode �rf� 0–200 W ��13.56 MHz� 100–350 W ��13.56 MHz�Total gas flow 332.5 SCCM 332.5 SCCMChamber pressure 10–60 mTorr 5–60 mTorr% Ar 0 85 85% A2 where A2 is O2 or N2 100 15 15

FIG. 2. Sample XPS spectra of theC1s, O1s, N1s, and Si2p regions withthe XPS detector oriented parallel �0°�and perpendicular �90°� to the trenchesin the test structure. Ash conditions forthis sample were Ar/N2, 10 mTorr,and 200 W bias power.

013305-3 Worsley et al. J. Appl. Phys. 101, 013305 �2007�

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In both orientations, the TaN from the top surface issampled. However, the top surface contribution to the C, O,and N XPS spectra is eliminated or accounted for. As inprevious work by Fuller et al.,11 an overash condition wasused to eliminate the contribution of C or O from the TaNsurface. The N signal coming from the TaN layer creates abase line N/Si ratio �0.1. Thus, observing a N/Si ratio�0.1 would indicate no N in the ILD. No Si signal would beexpected from the TaN layer. In this manner, the contributionof C, O, and N from the TaN surface are accounted for oreliminated. Therefore, using the ARXPS technique, the com-position of the ILD was determined, and changing the orien-tation of the sample to the XPS detector allowed the modi-fication to the ILD sidewall and trench bottom to be probedindependently.

C. Modeling

Ion scattering in the sheath was estimated to be eitherscattering �collisional regime� or nonscattering �collisionlessregime� using modeling of the Ar+ mean free path and thesheath thickness based on a 100% Ar plasma at 30 mTorrand 300 W source power, using experimental values of Te

�2.5 eV� and the plasma density �1010 cm−3�. This method-ology facilitated reasonable approximations of the ion scat-tering regimes for 85% Ar/A2 �A2=O2 or N2� plasmas. Theion mean free path �i was modeled by the following equa-tion:

�i = 1/�ngi� , �1�

where ng is the gas density and i is the ion-atom scatteringcross section. Ion-atom scattering experimental data fromMcDaniel et al.22 were then used to estimate the scatteringcross section for the Ar+ resulting in an expression relatingthe mean free path of the Ar+ �in centimeters� to pressure p�in Torr�.

�i = 1/�330p� . �2�

Thus, the ion mean free path as a function of pressure can beplotted �Fig. 3�. The sheath thickness was estimated usingthe collisional sheath model given by Lieberman andLichtenberg.23 A collisional analysis is required because fortypical processing discharges, the ions will likely be in-volved in one or more collisions as they cross the sheath.Only under certain circumstances, as revealed by this analy-sis, will the ions find themselves in truly collisionless re-gimes. �A collisionless sheath analysis was also conducted. Itpredicted a slightly higher bias power for the boundary be-tween collisionless and collisional regimes. For the exampleat 20 mTorr, the collisionless sheath analysis predicted that�i and sm were equal at �25 W compared to �10 W ob-tained with the collisional sheath analysis. Thus, the colli-sional analysis gives a lower bound for the boundary be-tween the two regimes. This is sufficient because the focus ofthis study is on determining regimes where �i�sm and �i

�sm, so only an approximate determination of the boundaryis necessary.� Following the derivation by Lieberman andLichtenberg23 and through simple manipulation it is foundthat the sheath thickness can be approximated by

sm5/2 � 1.68�0�2e

M

V3/2�i1/2

ensus, �3�

where e is the elementary charge, ns is the plasma density atthe sheath edge, us is the modified Bohm velocity, �0 is thepermittivity of free space, M is the ion mass, V is the ionsheath voltage, and sm is the sheath thickness. It can be seenfrom this expression that the sheath thickness is dependenton bias power �by way of the sheath voltage� and the pres-sure �by way of the ion mean free path and plasma density�.This dependence of sheath thickness on bias power and pres-sure is shown in Fig. 3 for the conditions of our system.

For the purposes of this study, ion scattering falls intotwo different regimes as defined by the difference betweenthe sheath thickness and the ion mean free path: �1� for �i

�sm, a collisional regime exists with significant ion scatter-ing. �2� for �i�sm, a collisionless regime exists with no ionscattering.

The boundary for the two regimes at a particular opera-tion pressure can be determined from Fig. 3. Then, depend-ing on the bias power applied, the specific regime is deter-mined and the significance of ion scattering can beestablished. For example, if operating a discharge at20 mTorr, with an electron temperature of 2.5 eV and elec-tron density of �1010 cm−3, the sheath thickness and ionmean free path are equal at an �10 W bias power �Fig. 3�.That means that in a 20 mTorr discharge, the boundary forthe two regimes lies at an �10 W bias power, so operatingwell below a 10 W bias power �e.g., 0 W� puts the system ina collionless regime, and operating well above 10 W �e.g.,100 or 200 W� puts the system in a collisional regime. In thisfashion, the specific regime is identified and the degree ofion scattering is determined.

III. RESULTS

The damage to the ILD after exposure to the photoresistremoval plasma process is characterized by the deviation of

FIG. 3. Sheath thickness �increasing with bias power� and ion mean freepath of Ar+ �flat with bias power� as a function of bias power�Te=2.5 eV, ns=1010 cm−3�. The values for the 20 mTorr plasma are boldedto illustrate ion scattering regimes.

013305-4 Worsley et al. J. Appl. Phys. 101, 013305 �2007�

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the composition of the SiCOH film from that of the pristinematerial as determined �in this case� by ARXPS analysis. Itis well known that in an oxygen discharge, a fraction of theporous SiCOH volume is converted to a more silicon diox-idelike composition.5–9 Therefore, the deviation from thepristine porous SiCOH due to exposure to an oxygen dis-charge can be followed by observing the increase in O con-tent and the decrease in C content at the surface. In the caseof a nitrogen discharge, it has been observed that a siliconnitride layer is formed at the surface of the material.4,6,10

Thus, in the case of the nitrogen discharge, the deviationfrom the pristine porous SiCOH can be tracked by observingthe increase in N content, as well as the decrease in O and Ccontents at the surface. In this study, the composition is pre-sented in terms of atomic percentages and as ratios. Theratios are determined by taking the O, C, and N atomic per-centages and ratioing them to that of a reference element �Siin this study�. These ratios are helpful in highlighting therelative changes in concentration of a given elemental spe-cies to that of an element that is not altered during exposureto the plasma ash process �Si, for example�. Using thesemetrics, absolute damage to the surface of the ILD can bedetermined using XPS analysis.

It is also of interest to quantify damage occurring at thesidewall relative to that occurring at the trench bottom. Asmentioned earlier, data are collected at different orientationsallowing the composition of the sidewall surface and trenchbottom surface to be probed independently. Thus, absolutedamage due to exposure to the oxygen discharge, as deter-mined by oxygen content at the surface postexposure, forexample, on both surfaces can be determined independently.Then, obtaining the ratio of the oxygen content at the side-wall surface �defined as the oxygen-to-silicon ratio,�O:Si�SW� to the oxygen content at the trench bottom sur-face, �O:Si�TB, leads to the following statements.

�1� For �O:Si�SW/ �O:Si�TB�1, the damage to the sidewallis less than the damage to the trench bottom.

�2� For �O:Si�SW/ �O:Si�TB=1, the damage to the sidewallis equal to the damage to the trench bottom.

�3� For �O:Si�SW/ �O:Si�TB1, the damage to the sidewallis greater than the damage to the trench bottom.

A similar series of statements would be valid for the N con-tent in the case of the nitrogen discharge.

�1� For �N:Si�SW/ �N:Si�TB�1, the damage to the sidewallis less than the damage to the trench bottom.

�2� For �N:Si�SW/ �N:Si�TB=1, the damage to the sidewallis equal to the damage to the trench bottom.

�3� For �N:Si�SW/ �N:Si�TB1, the damage to the sidewallis greater than the damage to the trench bottom.

For the C content, which decreases in atomic percentagewith increasing damage, the following statements would bevalid.

�1� For �C:Si�SW/ �C:Si�TB�1, the damage to the sidewallis greater than the damage to the trench bottom.

�2� For �C:Si�SW/ �C:Si�TB=1, the damage to the sidewallis equal to the damage to the trench bottom.

�3� For �C:Si�SW/ �C:Si�TB1, the damage to the sidewallis less than the damage to the trench bottom.

These statements regarding the C content are applicable toboth the oxygen and nitrogen discharges. Using these ratiosof elemental content at the sidewall surface relative to thetrench bottom surface, damage to the sidewall relative to thetrench bottom is determined.

Figure 4 shows the Si, O, N, and C percentages at thesidewall surface after the photoresist removal for processesoperating at 30 mTorr, 300 W source power, and 85% Ar.The bias power was 200 W for the oxygen chemistry and350 W for the nitrogen chemistry. As expected, the damagedue to the oxygen chemistry is characterized by a significantremoval of C leaving an oxidelike layer at the surface. Thenitrogen chemistry, also as expected, caused decreases in theC and O percentages, and the appearance of N at the surface.These results qualitatively show the effects these two plasmachemistries have on ILD materials within the parameterspace of this study and are consistent with previous work.6,8

We now examine the different plasma chemistries inturn, beginning with the oxygen plasma. Figures 5 and 6illustrate the effect of bias power on the modification to theILD for the oxygen discharge with 85% Ar at 30 mTorr and

FIG. 4. Composition of ILD sidewall surface �ARXPS angle at 59° fromsurface normal� as a function of ash chemistry. Plasma conditions include300 W source power, 85% Ar, and a pressure of 30 mTorr. 200 W biaspower was used for the O2 plasma and 350 W bias power for the N2 plasma.

FIG. 5. O:Si ratio at the ILD surface after ashing in the O2 discharge with85% Ar �300 W source power and 30 mTorr pressure� as a function of biaspower.

013305-5 Worsley et al. J. Appl. Phys. 101, 013305 �2007�

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300 W source power. The error bars in all figures denote onestandard deviation. Figure 5 shows the O content after expo-sure to the oxygen discharge as a function of bias powervaried from 0–200 W for both the trench bottom and side-wall. At the sidewall, the O content increases with biaspower, while it remains constant at the trench bottom withinthe uncertainty of the data. The damage at the sidewall rela-tive to the trench bottom, �O:Si�SW/ �O:Si�TB, as a functionof bias power is shown in Fig. 6. Here it is evident thatdamage to the sidewall relative to the trench bottom is sig-nificantly impacted by the increase in bias power.

Figure 7 shows the effects of pressure for two differentbias powers �0 and 200 W� on the modification to the ILDfor the oxygen discharge with 85% Ar at 300 W sourcepower. Figure 7�a� shows the O content at the sidewall sur-face as a function of pressure. This plot shows that the Ocontent is the same at 10 mTorr for both bias powers and isalso the same at 30 mTorr with 0 W bias power. At30 mTorr, the O content increases with the increase to200 W bias power �as observed above in Fig. 5�, and thesame O content is observed at 60 mTorr for both bias pow-ers. Figure 7�b� shows the O content at the trench bottomsurface as a function of pressure. This plot shows that the Ocontent is already enriched at 10 mTorr and only increasesslightly with increasing pressure, suggesting that pressurehas only a small effect on modification to the trench bottomover the range observed.

Figures 8 and 9 show the effect of percent Ar on themodification to the ILD for the oxygen discharge at 300 Wsource power and 200 W bias power. Figure 8 is a plot of theO:Si ratio as a function of percent Ar. The O:Si ratio isobserved to increase with percent Ar for both the sidewalland trench bottom. Figure 9 shows the composition of theILD as a function of percent Ar in the oxygen discharge. Thecomposition of the pristine material is also included for ref-erence. This figure shows that the surface exposed to the 0%Ar discharge is close to that of the pristine surface, while thesurface exposed to the 85% Ar discharge has a compositionthat shows a greater deviation from the pristine surface.These figures suggest that the 85% Ar discharge is signifi-cantly more damaging than the 0% Ar discharge.

We now turn to the nitrogen plasma. Figure 10 illustratesthe effect of bias power on the modification to the ILD forthe nitrogen discharge with 85% Ar at 20 mTorr and 300 Wsource power. Specifically, Fig. 10 shows the N:Si ratio andC:Si ratio after exposure to the nitrogen discharge as a func-tion of bias power varied from 100–350 W. In Fig. 10�a�, itis observed that the N content is unchanged from100 to 200 W bias power, but increases at 350 W biaspower, for both the sidewall and the trench bottom. In Fig.10�b�, a corresponding decrease in C content is observedwith increasing bias power. It can be inferred from Fig. 10that �N:Si�SW/ �N:Si�TB and �C:Si�SW/ �C:Si�TB are �1.

FIG. 6. Sidewall damage relative to trench bottom after ashing in the O2

discharge with 85% Ar �300 W source power and 30 mTorr pressure� as afunction of bias power.

FIG. 7. O:Si ratio at the ILD surface after ashing in theO2 discharge with 85% Ar �300 W source power� as afunction of pressure at �a� sidewall and �b� trench bot-tom. Results for 0 and 200 W bias power are shown.

FIG. 8. O:Si ratio at the ILD surface after ashing in the O2 discharge �300 W source power, 200 W bias power, 30 mTorr pressure� as a functionof percent Ar.

013305-6 Worsley et al. J. Appl. Phys. 101, 013305 �2007�

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Therefore, the damage at the sidewall is relatively the same,within error, to that of the trench bottom for the nitrogenplasma at different bias powers.

Figure 11 shows the effects of pressure on modificationof the ILD due to exposure to the nitrogen discharge with85% Ar at 300 W source power. In Fig. 11�a�, the N:Si ratiois plotted as a function of pressure with 200 W bias power.Figure 11�a� shows that the N content decreases upon reach-ing 20 mTorr. A small decrease in N content at higher pres-sures is also observed in Fig. 11�b� where the N:Si ratio isplotted against pressure with a 350 W bias power. Figures11�c� and 11�d� show that the C:Si ratio increases with pres-sure, which is consistent with less modification of the ILD.Figure 11 therefore suggests that less damage is induced athigher pressures for the nitrogen discharge. Again, no signifi-cant difference is observed between the postash compositionat the sidewall and that for the trench bottom for the nitrogenplasma.

IV. DISCUSSION

A. Modeling of ion density

To gain more insight into the role of ion bombardment inthe modification process, modeling of the relative ion densi-ties as a function of pressure was performed using calcula-tions similar to those outlined by Fuller et al.24 Given thatthe discharge is minimally dissociated ��5% �,13 it is as-sumed that dominant positive ionic species besides Ar+ areO2

+ and N2+ for the Ar/O2 and Ar/N2 discharges, respectively.

The relative total ion density nion, assuming similar loss pro-cesses �diffusion to the sheath edge� for A2

+ �where A2+=O2

+ orN2

+� and Ar+, is given by24

nion �nA2

+ + nAr+

nArref+

�kiz�A2�nA2

+ kiz�Ar�nAr

kiz�Arref�nArref

, �4�

where nA2+ is the density of species A2

+, nAr+ is the density ofspecies Ar+, nArref

+ is the reference density of species Ar+ at10 mTorr, kiz�A2� is the rate constant for ionization of A2 toA2

+, kiz�Arref�is the rate constant for ionization of Ar to Ar+ at

10 mTorr, nA2is the density of A2, and nArref

is the density ofAr at 10 mTorr. Cross sections for electron-impact ionizationthat were used to determine kiz�A2� and kiz�Ar� were taken fromreports by Straub et al.25,26 The electron energy distributionfunctions �EEDFs� used are identical to those determined forthis tool in a previous study for the relevant conditions.13

Those conditions include 85% Ar Ar/A2 �A2=O2 or N2� dis-charges operating at a fixed source power of 300 W withpressures from 10 to 60 mTorr and pure O2 discharge with apressure of 30 mTorr also operating at the same fixed sourcepower of 300 W. Figure 12 summarizes the results of mod-eling with the relative Ar+ density also shown as a functionof pressure for reference. For the Ar/O2 discharge, there isan overall increase in ion density �approximately two times�for an increase in pressure from 10 to 60 mTorr. Changingthe percent Ar from 85% to 0% �pure O2 at 30 mTorr� re-sulted in an �75% drop in ion density. In the Ar/N2 case,the ion density appears to fall by �10% with increasingpressure. The observed ion density trends follow fromchanges in the EEDF as a function of pressure and percentAr discussed in a previous report.13

The relative ion density coupled with changes in theelectron temperature and ion energy provide insight into howthe ion current may change with pressure and percent Ar. Forthe Ar/O2 discharge and the case of rising pressure, thoughsome marginal decrease in electron temperature and ion en-ergy is expected,23 there is a two times increase in ion den-sity �Fig. 12�. Thus, some increase in ion current might beexpected with an increase in pressure for Ar/O2, where ioncurrent is proportional to ion density times ion energy. Forthe case of Ar/N2 and rising pressure, the decrease in elec-tron temperature and ion energy is accompanied by a simul-taneous decrease in ion density, resulting in a lower ion cur-rent. So a decrease in ion current would be expected at highpressures for an Ar/N2 discharge. For the case of increasingpercent Ar in an Ar/O2 discharge, since the ion density �Fig.12� increases by a factor of �3.8 times and electron tempera-ture is likely to increase �the ionization potential of O2

FIG. 9. Composition of the ILD surface after ashing in the O2 discharge �300 W source power, 200 W bias power, 30 mTorr pressure� as a functionof percent Ar. Pristine ILD composition is included for reference.

FIG. 10. �a� N:Si and �b� C:Si ratios at the ILD surfaceafter ashing in the N2 discharge with 85% Ar �300 Wsource power, 20 mTorr pressure� as a function of biaspower.

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�12.1 eV whereas that of Ar is 15.76 eV�, it is highly likelythat the ion current increases by at least a factor of �3.8times under these conditions.

The following discussions will now examine the resultshighlighted in Figs. 4–11 in relation to changes in the plasmawith respect to radical species density, ion scattering, iondensity, and ion energy.

B. Oxygen chemistry

1. Bias power

Figures 5 and 6 illustrate the O:Si content of the sidewalland trench bottom of the porous SiCOH film as a function ofbias power at a fixed pressure of 30 mTorr for an 85%Ar/O2 discharge. Under these conditions, previous work bythe authors13 has shown that the O-radical density is essen-tially unaffected by bias power. However, based on the mod-eling of ion scattering with input experimental data �Te

=2.5 eV, ns=1010 cm−3� �Fig. 3�, it was determined that forthe 30 mTorr discharge varying the bias power from0 to 200 W allowed sampling of both the collisionless andcollisional regimes in terms of ion scattering. At 0 W biaspower, the discharge operates in a collisionless regime,meaning the ions can cross the plasma sheath region withoutundergoing any collisions. With no collisions to redirect ions,they are simply accelerated across the sheath by the plasmapotential resulting in ion bombardment solely on surfacesparallel to the bottom electrode �i.e., trench bottom�. There-fore, the ILD sidewalls would not be subject to significantion bombardment. Assuming that ion bombardment facili-tates the modification process, one would expect to see moremodification at the surfaces subject to ion bombardment. So

at 0 W, one would expect to observe a larger O content at thetrench bottom relative to that at the sidewalls. This is indeedwhat is observed in Figs. 5 and 6. At 100 and 200 W, basedon the modeling of ion scattering, the discharge operates in acollisional regime. This means that ions collide and are redi-rected within the sheath region. This increased ion scatteringresults in increased ion bombardment of the sidewall sur-faces of the test structure. Again, assuming that ion bombard-ment plays a role in the modification process, one wouldexpect to see increased O content of the sidewalls underthese conditions. This is consistent with the observation inFigs. 5 and 6. As the bias power increases to 100 W and thento 200 W, absolute damage to the sidewall is increased �Fig.5� and so is damage to the sidewall relative to the trenchbottom �Fig. 6�. O content at the trench bottom is already

FIG. 11. �a� N:Si ratio at 200 W biaspower, �b� N:Si ratio at 350 W biaspower, �c� C:Si ratio at 200 W biaspower, and �d� C:Si ratio at 350 Wbias power at ILD surface after ashingin the N2 discharge with 85% Ar �300 W source power� as a function ofpressure.

FIG. 12. Relative ion densities for O2 plasma �0% Ar and 85% Ar� and N2

plasma �85% Ar� modeled as a function of pressure. Relative Ar+ densityalso shown to illustrate the contribution of Ar+ ions to total ion density. Arref

+

at 10 mTorr was used to normalize the densities.

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near saturation �O:Si�2� at 0 W bias power, so no signifi-cant increases are observed as bias power is increased.

2. Pressure

Figure 7 highlights the O:Si content of the sidewall andtrench bottom of an 85% Ar/O2 discharge for fixed biaspowers of 0 and 200 W as a function of pressure�10–60 mTorr�. Figure 7�a� highlights these changes in theO:Si ratio as a function of pressure for the sidewall only.Examination of this figure reveals that the O:Si ratio at thesidewall surface ��1.4� is close to that found in the as-deposited porous SiCOH ��1.3� for samples exposed to the10 mTorr discharge at 0 and 200 W bias power and the30 mTorr discharge at 0 W bias power. The commonalityamongst these three conditions is that they all fall into theregime of no �sm���� or minimal ion scattering �sm����,implying that ion bombardment to the sidewall surface isnegligible. The increase in pressure from 10 to 30 mTorr re-sults in a four times increase in O-radical density; however, itis significant to note that no change in the O:Si ratio is ob-served for these three above conditions. Figure 7�a� also hasthree conditions in which the O:Si ratio at the sidewall sur-face approaches that of oxide ��2.0�. This occurs forsamples exposed to discharges at 30 mTorr with 200 W biaspower and 60 mTorr with 0 and 200 W bias powers—conditions coinciding with the collisional regime—indicatingthat ion bombardment to the sidewall surfaces occurs. Thus,the overall increase in the O:Si ratio at the sidewall from�1.4 for “collisionless” to �2.0 for “collisional” conditionssuggests that ion bombardment plays a dominant role. Thenonchanging O:Si ratio ��1.4� for the increase in O-radicaldensity by a factor of four times for the increase in dischargepressure from 10 to 30 mTorr �0 W bias power� suggests adiminished role in the “damaging” process by O radicals.

Figure 7�b� highlights the changes in the O:Si ratio at thetrench bottom for the 85% Ar/O2 discharge as a function ofpressure. In this figure an increase in the O:Si ratio from�1.7 �1.6� to �2.0 �1.9� for 200 W �0 W� bias power isobserved for the respective increase in pressure from10 to 60 mTorr. Compared to the O:Si ratio of �1.3 in thepristine ILD, the O:Si ratio at the trench bottom after photo-resist removal increased by �0.4 �0.3� at 10 mTorr and �0.7�0.6� at 60 mTorr for the 200 W �0 W� bias. This representsa factor approximately two times increase in the amount ofoxygen incorporated into the ILD due to plasma exposure forthe respective increase in pressure from 10 to 60 mTorr. Ithas been shown13 that the O species density nO increases by� an order of magnitude from 3 1012 cm−3 at 10 mTorr to2 1013 cm−3 at 60 mTorr. Further, modeling data shown inFig. 12 reveal that the ion density increases by a factor ofapproximately two times for the corresponding change inpressure. No precise data for the electron temperature areavailable for the same range of conditions; however, at30 mTorr and 300 W source power for 100% Ar plasma,Te=2.5 eV. Given that an increase in pressure would in-crease the electron-neutral collision frequency, it is expectedthat some drop in the Te will occur for this six times increasein pressure. Given that the ion current varies directly with theion density and the square root of the Te, there would have to

be an unlikely 4 eV drop in Te for the ion current to remainconstant over this range. Hence, it is more likely that somemoderate increase in ion current �less than two times� occursat this same pressure range. Thus, given that both theO-radical density and ion current increase �by differing fac-tors� for the six times increase in pressure, it is reasonable toask which variable�s� is/are contributing to the observed in-crease in the O:Si ratio at the trench bottom over these con-ditions. Examination of data presented in Fig. 8 will aid inanswering this question.

3. Percent argon

Figures 8 and 9 show the O:Si ratio at the sidewall andtrench bottom for an Ar/O2 discharge at 200 W bias power�collisional regime� and 30 mTorr pressure as a function ofAr fraction �0%, 85% Ar�. Figure 8 shows that the amount ofoxygen incorporated into the ILD due to plasma exposureincreases significantly, from 1.4 �1.5� to 2.0 �2.0�, for thetrench bottom �sidewall� as the Ar fraction increases from0% to 85%. This represents a factor of approximately seventimes �four times� increase in the amount of oxygen incorpo-rated into the ILD relative to the pristine ILD as the Arfraction is increased. It has been determined13 that theO-radical density nO also increases by an order of magnitude�as observed for the six times increase in pressure� from 1 1012 to 1 1013 cm−3 as the Ar fraction increases from 0%to 85%. Figure 12 also reveals that the relative ion densityincreases by a factor of �3.8 times for the correspondingincrease in Ar fraction. Given the higher ionization potentialof Ar �15.76 eV� versus that of O2 �12.1 eV�, it is expectedthat the increase in Ar fraction would also increase Te andion energy. Thus, the overall ion current to the wafer wouldincrease by a factor of at least 3.8 times. Given that theO-radical density increase associated with the increased pres-sure is the same as that caused by the change in Ar fraction,whereas the ion current differs significantly for these sets oftwo experiments, the observation that the oxygen incorpora-tions at the trench bottom are very different �an increase ofapproximately two times and seven times respectively� be-tween the percentage Ar and pressure experiments indicatesthat the ion current dominates the process. In other words,the relative difference in the increased ion current ��2 timesand �3.8 times, respectively� associated with these respec-tive discharge changes is responsible for the observed differ-ences in the damage content of the porous SiCOH ILD filmunder these conditions.

C. Nitrogen chemistry

1. Bias power

Figure 10 shows the N:Si and C:Si ratios at the sidewalland trench bottom for an 85% Ar/N2 discharge at 20 mTorras a function of bias power. The N:Si ratio increases from�0.2 �for both sidewall and trench bottom� to �0.45 as thebias power is increased from 100 W/200 W to 350 W.There is a corresponding decrease in the C:Si ratio for bothsurfaces from 0.5�100 W� to 0.2�350 W�. On the other hand,these ratios are the same for both the sidewall and trenchbottoms at each bias power studied. At 20 mTorr, it was

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determined �Fig. 3� that above �10 W bias power, the sys-tem operated in the collisional regime. Therefore, from100 to 350 W, significant ion scattering occurs and ionbombardment is present at all surfaces. Also, over this rangeof conditions, it was found12 that the bias power had noeffect on the N-radical density. It is, thus, quite likely that theobserved increase in damage �as measured by the increasedN:Si ratio and decreased C:Si ratio� over these range of con-ditions can be attributed to the associated increased ion en-ergy as the bias power is increased by a factor 3.5 times.Further, the equivalent damage to the sidewall and trenchbottom, as shown in Fig. 10, may be indicative of similar ioncurrents being incident on both the sidewall and trench bot-tom surfaces, given that the discharge is operating in a col-lisional regime.

2. Pressure

Figure 11 documents the N:Si and C:Si ratios at both thetrench bottom and sidewall as a function of pressure for an85% Ar/N2 discharge at bias powers of 200 and 350 W.Figures 11�a� and 11�c� show the N:Si and C:Si ratios, re-spectively, at 200 W bias power. The N:Si ratio decreasesfrom �0.4 to �0.2 as the pressure is increased from10 to 20 mTorr. The C:Si ratio increases from �0.2 to �0.4as the pressure is increased from 10 to 20 mTorr. Furtherincreases in pressure from 20 to 60 mTorr at the higher biaspower �350 W� result in a similar decrease of the N:Si ratiofrom �0.5 to �0.3 and an increase in the C:Si ratio from�0.2 to �0.5. Plasma diagnostic work conducted previouslyhas revealed that the N-radical density Nn increases by afactor of approximately six times from 7 1011 to 4 1012 cm−3 as the pressure is increased from10 to 60 mTorr. Clearly, this increased N-radical density isnot responsible for the observed decreased damage of theporous SiCOH ILD at both trench bottom and sidewall sur-faces as a function of increased pressure. However, as pres-sure increases from 10 mTorr, as shown in Fig. 12, there is a10% decrease in the relative ion density, attributable to thesimilar ionization potentials of Ar �15.76 eV� and N2

�15.6 eV� and the decrease in Te that would occur for anincrease in pressure from 10 to 60 mTorr. The decrease in Te

and ion energy that would occur with the increase in pressurewould result in a net decrease in ion current that would begreater than 10% due to ion density changes. Therefore, it islogical to attribute at least a portion of the observed decreasein damage as measured by the N:Si and C:Si ratios to thedecrease in ion current. These observations would be consis-tent with the assertion that ion current rather than radicalspecies density is the dominant mechanism for inducingdamage to this porous SiCOH ILD film in a N2-containingdischarge under these conditions.

V. CONCLUSIONS

The effect of ion impact and radical species density inphotoresist removal plasmas on the modification of a pat-terned extreme ultralow-� ILD single damascene structurewas determined by combining discharge modeling based oninput experimental data and several experimental techniques.

The photoresist ash discharge generated in a capacitivelycoupled dual frequency commercial reactor was first charac-terized under a range of ash conditions by OE actinometry13

and the ion angular distribution function �IADF� in thesheath and the positive ion density estimated. The OE acti-nometry allowed the radical species densities to be deter-mined, and the estimation of the IADF led to the identifica-tion of operating regimes under which ion impact occurredon the sidewall and trench bottom of the test structure or onthe trench bottom alone. A range of conditions were charac-terized including two different chemistries �Ar/O2 andAr/N2�, a range of pressures �5–60 mTorr�, bias power�0–350 W�, and percent Ar �0% and 85%�. Followingplasma characterization, photoresist was removed from thetest structure using the various characterized ash plasma con-ditions, and analysis of the sidewall and trench bottom sur-face compositions was done by angle resolved x-ray photo-electron spectroscopy �ARXPS�. ARXPS provided a methodto not only determine the composition of the ash-modifiedsurface but also to allow for the differentiation of the com-position at the sidewall from the composition at the trenchbottom. The modification of the test structure surfaces wascorrelated with plasma characterization data to draw conclu-sions regarding the effect of radical species density and ionimpact �ion current�.

While it is possible that both ion impact and radical spe-cies density play important roles in the modification of thisporous SiCOH film under all of the investigated conditionsof this study, the data strongly suggest that the dischargelikely never operates in a reactant starved regime and that themodification of the ILD is primarily driven by the ion currentto exposed surfaces. It is proposed that the process modifi-cation �removal of carbon and incorporation of oxygen�Ar/O2� or nitrogen �Ar/N2�� at the ILD surface is highlydependent on the creation of reactive sites by ion impactunder the present conditions. Though the radical species areundoubtedly responsible for the subsequent surface reactionsthat result in modification, without reactive sites created byion impact, those reactions would probably proceed at amuch slower rate.

The dominant effect of ion impact is illustrated in all ofthe results but most potently in the experiments where theO:Si ratio was measured as a function of increasing pressureand Ar fraction �Ar/O2 discharge� and where the N:Si andC:Si ratios were measured as a function of increasing pres-sure �Ar/N2 discharge�. In the former case, while there wasan identical increase in the O-radical density �� an order ofmagnitude� for increasing pressure and Ar fraction, the in-creased damage content of the film as measured by the in-creased O:Si ratio was very different �approximately twotimes and approximately seven times, respectively� for thechanges in these two variables and coincided with the rela-tive changes in the ion current ��2 times and �3.8 times,respectively� over the same range of conditions. In the lattercase, while there was an increase in the N-radical density�six times� for increasing pressure, the damage content of thefilm as measured by the N:Si and C:Si ratios decreased 40%–50% and once more coincided with the decrease in ion cur-rent of greater than 10% to the exposed surfaces.

013305-10 Worsley et al. J. Appl. Phys. 101, 013305 �2007�

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In summary, the significance of the ion current and radi-cal density on ash-induced modification of a patterned ex-treme ultralow-� ILD single damascene structure was deter-mined. It was observed that under the present conditions—whereby the reactant concentration greatly exceeds theconcentration of ILD surface sites—the ion current to theexposed ILD film surfaces is the limiting factor in the ILDmodification process.

ACKNOWLEDGMENTS

The authors would like to thank the IBM Microelectron-ics Research Lab �MRL� for support in preparation ofsamples. One of the authors �M.A.W.� supported by an IBMPh.D. Fellowship, gratefully acknowledges the National Sci-ence Foundation and General Electric for funding, and is aFord Fellow.

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