006) 2230–2234www.elsevier.com/locate/tsf

Thin Solid Films 515 (2

Quantitative mapping of elastic properties ofplasma-treated silica-based low-k films

Le Jiang ⁎, Holm Geisler, Ehrenfried Zschech

AMD Saxony LLC & Co. KG, Materials Analysis Department, Wilschdorfer Landstrasse 101, D-01109 Dresden, Germany

Received 21 December 2005; received in revised form 26 April 2006; accepted 16 May 2006Available online 27 June 2006

Abstract

A modulus mapping technique is described which provides quantitative information about the elastic properties of the near-surface region ofboth unpatterned and patterned silica-based low dielectric constant (low-k) thin films. The extent of oxygen plasma-induced damage has beenstudied using this method. Compared with quasi-static nanoindentation, the modulus mapping technique is more surface sensitive. This is firstdemonstrated for thin Ta layers on top of low-k films, where the effective modulus systematically increases with the Ta film thickness.Correspondingly, the effective modulus of the near-surface region of a plasma-treated low-k film is significantly higher than that measured on thesame type of low-k film without treatment. Modulus mapping performed on integrated Cu/low-k structures showed that a sub-micrometer spatialresolution is clearly achievable. Furthermore, the effective modulus of patterned low-k material between inlaid copper interconnects was found tobe considerably higher than the modulus measured on a blanked low-k film of the same type.© 2006 Elsevier B.V. All rights reserved.

PACS: 81.70.Bt; 81.65.CF; 68.35.Gy; 68.37.Ps; 68.60.Bs; 52.77.Bn; 85.40.-eKeywords: Modulus mapping; Nanoindentation; Low-k; Plasma damage

1. Introduction

Low-k dielectric materials for insulating thin films areneeded to minimize the signal delay between on-chip metalinterconnects in leading-edge microelectronic products [1–3].Damage of organosilicate glasses (OSG) caused by plasma etchand clean process steps is one of the major concerns for low-kmaterial integration since this damage increases the permittivity(k value) of the insulating material at least locally. It is knownthat oxygen plasma processes used to pattern OSG thin filmsinduce so-called trench or via sidewall damage, i.e., a portion ofthe Si–CH3 bonds is broken and the carbon is replaced with a Sidangling bond, resulting in the formation of silanol (Si–OH)groups through a variety of intermediate reactions [4]. On theother hand, the parameters for plasma-enhanced chemicalvapour deposition can be adjusted to modify OSG materialslocally. As a consequence, adhesion can be improved and thestiffness of the backend-of-line (BEoL) layer stack can be

⁎ Corresponding author.E-mail address: le.jiang@amd.com (L. Jiang).

0040-6090/$ - see front matter © 2006 Elsevier B.V. All rights reserved.doi:10.1016/j.tsf.2006.05.026

increased significantly. Densification and carbon depletion innear-surface regions of plasma-treated OSG materials lead to anincreased permittivity, an improved stiffness, a loss of filmthickness and an adsorption of moisture [5–7]. Therefore, it isan extremely challenging task for process engineering to changethe local electronic polarizability and the chemical bonding ofthe interlayer dielectric (ILD) material in such a way that thestiffness of the material is increased significantly, whereas theeffective k value is increased only slightly or even stays con-stant. In addition, the adhesion between the etch stop layer andlow-k material should be improved.

As the interconnect line spacings of ultra-large-scaleintegrated circuits continue to shrink, optimization of theelectrical performance of Cu/low-k structures and the mechan-ical properties of the BEoL layer stack becomes increasinglyimportant for low-k integration since the effect of thin, plasma-modified regions on the effective material properties, e.g., theeffective permittivity keff, increases. For an optimization of ILDmaterials, including both the integral thin-film material and inparticular the near-surface regions with a typical extensionbetween 10 nm and 100 nm, it is necessary to understand the

Fig. 1. Schematic diagram of Hysitron TriboIndenter combined with nanoDMAand SPM techniques. The dynamic stiffness measurement (DSM) moduleprovides alternation of the signal configurations necessary for quasi-static ordynamic testing.

2231L. Jiang et al. / Thin Solid Films 515 (2006) 2230–2234

relationship between changed electronic polarizability andchemical bonding, between permittivity and elastic modulus[8]. Particularly, the extent of damage has to be studied quan-titatively for optimizing the patterning process. The goal of thispaper is to propose a methodology for evaluation and quan-tification of the extent of plasma-induced modification of OSGthin films which is based on changed nanomechanical pro-perties. The near-surface mechanical properties are examinedquantitatively by a modulus mapping technique. This techniquecan be applied for mapping of elastic properties of patternedwafers, and it allows to distinguish between ultra-thin layers ofdifferent thickness quantitatively.

2. Experimental details

In this study, carbon-doped oxide films comprised of Si, C, Oand H (OSG, also called SiCOH) were deposited on bothunpatterned and patterned wafers using plasma-enhanced chem-ical vapor deposition. For comparison of the modulus mappingtechnique with quasi-static nanoindentation, OSG thin films witha thickness of 500 nmwere capped with ultra-thin physical-vapordeposited Ta films of various thicknesses between 10 nm and30 nm. The elastic properties of these ultra-thin films were firstlymeasured quantitatively using both the modulus mapping tech-nique and quasi-static nanoindentation with small normal forces.An unpatterned 300 nm thick low-k OSG film with an original kvalue of k=2.7 was modified applying an oxygen plasma treat-ment. The as-deposited and plasma-damaged films were sub-sequently investigated using the modulus mapping technique.Furthermore, quasi-static nanoindentation was carried out on thesame samples with a normal forceFn ranging from5μN to 80μN.Additionally, using the modulus mapping technique a patternedCu/low-k structure with 1.8 μm wide copper lines was inves-tigated. Areas with a size of 5 μm×5 μm were scanned with aBerkovich tip mounted on the nanoindentation tool. The mea-suring parameters (e.g., oscillation frequency=200 Hz, DC-force=2 μN and AC-force=1.5 μN) were identical for all thesamples.

The modulus mapping technique used here was implementedin a TriboIndenter nanomechanical testing tool with in situ scan-ning probe microscopy (SPM) imaging capability (Hysitron, Inc.,Minneapolis, MN) [9]. A scheme of the Hysitron TriboIndentermeasuring system is shown in Fig. 1. A similar experimental setupwas reported by Syed Asif et al. [10], and earlier a tool forcontinuous stiffness measurement without scanning capabilitywas introduced by Oliver and Pharr in [11]. During the mappingprocess, the dynamic test is performed by oscillating the indentertip with small forces while monitoring the resultant oscillationamplitude and phase lag due to the material response. Simul-taneously, SPM imaging allows the indenter tip to scan across thesample surface. The system continuouslymonitors the stiffness ofthe sample, and it provides a plot of the contact stiffness as afunction of the position on the sample. The contact stiffness isgiven at each pixel of the image, and the elastic modulus can becalculated if the geometry of the probe tip is known. Thismodulusmapping result is equivalent to a dynamic indentation test per-formed at each pixel in a 256×256 pixel image. The complex

elastic modulus information obtained by this modulus mappingtechnique includes real and imaginary parts, E=E′+ iE″ andprovides the storage (E′) and loss (E″) characteristics of a mate-rial. The E′ and E″ values are given by [10]

E V¼ ksffiffiffi

pp

2ffiffiffiffiffi

Acp ð1aÞ

EW ¼ xCsffiffiffi

pp

2ffiffiffiffiffi

Acp ð1bÞ

respectively, where ks is the tip-sample contact stiffness, Cs is thedamping coefficient of the contact, ω is the frequency in radians,andAc represents the contact areawhich is calculated based on theradius of curvature of the probe tip. The contact stiffness ks andthe contact damping coefficient Cs can be calculated from thedisplacement amplitude X, the phase shift φ of the displacementsignal and the frequency ω. This has been described in detailelsewhere [10,12].

3. Results and discussion

All samples showed small or negligible viscoelastic properties,leading to a loss modulus of nearly zero. Therefore, the storagemodulus was used for comparison with the quasi-static nano-indentation results. As a short-form, the term modulus is used inthe following instead of reduced elastic modulus, which is usuallymeasured by nanoindentation [11]. For OSG/Ta samples, verysmall forces were applied during the nanoindentation tests whichcaused very shallow indents compared to the total thickness of thefilm stack. As expected, the modulus values obtained with quasi-static indentation andmodulusmapping increased for the OSG/Tasamples compared to the bare OSG film. Furthermore, both theindentation experiment and the modulus mapping show an in-crease in the modulus of the samples with increasing Ta filmthickness (Fig. 2). However, there is no significant difference inmodulus for the samples with 10 nm and 20 nm Ta film thicknessbased on the indentation technique, whereas cross-sectional pro-files of the quantitative modulus maps show a clear difference in

Fig. 2. (a) Elastic modulus vs. contact depth, obtained from quasi-static depthsensing indentation on a pure OSG film and on OSG films with ultra-thin Talayers of different thicknesses on top; error bars are indicated for one sample; (b)cross-sectional modulus profiles taken from modulus maps of the pure OSG filmand of OSG films with ultra-thin Ta layers of different thicknesses on top.

Fig. 3. 5 μm×5 μm modulus maps of plasma-damaged (a) and as-deposited (b)OSG films; the grey scale indicates the modulus value. (c) cross-sectionalmodulus profiles of plasma-damaged and as-deposited OSG films at the posi-tions indicated by lines in (a) and (b).

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modulus between these two samples. Extracted fromFig. 2(b), themean modulus values are 9.5, 11, 14, 18 GPa for the bare OSGfilm and OSG films capped with 10 nm, 20 nm, and 30 nm ultra-thin Ta films, respectively. The modulus mapping result of the500 nm thick OSG film is in good quantitative agreement with theaverage quasi-static indentation modulus for the same specimen,but for the films with Ta on top, the quantitative agreementbetween the two methods is not satisfying. This difference isascribed to the different information depths probed by both tech-niques. The modulus mapping shows more systematic increase inmodulus with increasing Ta film thickness as compared to quasi-static indentation. It should also be noted that in the case of quasi-static indentation, the Ta coated samples show first a decrease inmodulus with increasing indentation depth, and then an increase.The initial decrease is certainly due to the fact that Ta is muchstiffer thanOSG, and the shallower the contact depth, themore themodulus is influenced by the thin Ta layer on top. The modulus isexpected to show a steep increase with further decreasing contactdepths, i.e., between 0 and 5 nm, but this range was not accessibleby quasi-static indentation. In this region the scattering of the dataincreases significantly with decreasing contact depth. For deeperindents, the quasi-static indentation data are more and moreinfluenced by the compliant OSG film, leading to the minimum

Fig. 4. Modulus vs. contact depth of as-deposited and plasma-damaged OSGfilms, obtained from quasi-static depth sensing indentation.

Fig. 5. (a) 5 μm×5 μm modulus map of a Cu/low-k structure on a patternedwafer; (b) cross-sectional modulus profile of the Cu/low-k structure at theposition indicated by a line in the map (a).

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observed at contact depths of about 15 nm. With further in-creasing contact depth, the modulus tends to increase and thismight already be due to substrate effects.

For about 300 nm thick unpatterned low-kOSG films with andwithout plasma treatment, different image contrasts were ob-served in the modulus maps, as shown in Fig. 3(a) and (b).Extracting cross-sectional profile data from themaps, i.e., plottingthe values of each point from one horizontal line in the maps, ameanmodulus value of 9.2 GPawas obtained for the as-depositedfilm, compared to a modulus equal to 15.5 GPa for the plasma-damaged OSG film, as shown in Fig. 3(c). The plasma-treatedfilm showed an increased scatter in modulus compared to theuntreated film. Since the average surface roughness for bothsamples are very similar based on the results from atomic forcemicroscopy (AFM), the increase in scatter for plasma-damagedfilm could relate with the higher local roughness and the positionwhere to extract the cross-sectional profile data from the quan-titative modulus map. The corresponding results from small-loadquasi-static indentation are shown in Fig. 4. The modulus de-creases initiallywith increasing contact depth, and after reaching aminimum value it increases again. The modulus values are higherfor the plasma-damaged sample than for the as-deposited sample.

The elastic modulus obtained from indents pressed into thesample surface depends on the stiffness of the material, which isdetermined by the slope of the unloading segment of the force-displacement curve using a power-law fit to the initial unloadingdata, and the area function of the indentation probe based on theOliver–Pharr theory [11]. At very shallow penetration depths, theaccuracy of the tip area function determined on a calibrationspecimen becomes more critical for the calculation of the elasticmodulus. Nevertheless, using the area function which had beencarefully determined before at very small contact depths (less than10 nm), the modulus of the OSG film decreases with increasingcontact depth. This decrease cannot easily be ascribed to in-dentation size and strain gradient plasticity effects [13,14]. Thesenear-surface effects are usually used to explain increasing hardnessin near surface regions of metallic samples. The root cause for theobserved behavior close to the surface of amorphous OSG films isstill not well understood. After reaching a minimum value, themodulus starts to increase with further increasing contact depths.

For these 300 nm thin films, this effect is probably due to theconstraint of the stiffer substrate. Considering that the maximumpenetration depth is about 3.5 nm for the modulus mappingmethod, and that the contact depth is even less in this case, it isreasonable that the mapping technique results in higher modulusvalues, approaching the values fromvery small-depth indentations.

Using themodulusmapping technique, the patternedCu/low-kstructure with 1.8 μmwide copper lines was investigated directlyand quantitatively on a scale of a few micrometers with a highsurface sensitivity. The copper interconnect line appearing asbright image contrast in the modulus map is much stiffer than theOSG material as shown in Fig. 5(a). Reasonable modulus valuesfor Cu (∼110 GPa) and OSG (∼23 GPa) were obtained from across-sectional profile through the map (Fig. 5(b)). Most inter-estingly, the modulus value of the patterned OSG measured hereis higher than themodulusmeasured on a blankedOSG film of thesame type. This implies that patterning, cleaning or polishingprocesses, or the combination of them, have changed the mechan-ical properties of the low-k material. Cu/low-k structures withdecreasing copper line pitch were studied [15] to demonstrate that

2234 L. Jiang et al. / Thin Solid Films 515 (2006) 2230–2234

this effect is not caused by the mechanical constraint from thecomposite structure, since these investigated structures did notshow a systematic increase of the modulus for the low-k materialwhen the distance between neighbored copper lines decreased.Asseen in Fig. 5(b), the modulus signal is smooth on top of the OSGregions, whereas variations are visible on top of the copper line.This variation on top of copper ismost probably caused by surfaceroughness which influences the contact area between indenter tipand sample surface, and therefore, the calculated modulus values.Most consistent modulus mapping results are expected forperfectly smooth surfaces as can be seen for the low-k material,which has a lower surface roughness than the copper. This resultwas confirmed using both AFM and topography scans recordedwith the SPM imaging mode of the nanoindentation tool. On theother hand, the variation of the modulus signal on copper could inprinciple also be due to local variations of the copper micro-structure, keeping in mind that copper is elastically highly aniso-tropic. The measured modulus for copper between 90 GPa and135 GPa observed in Fig. 5(b) are in the range of modulus valuesfor different copper crystallite orientations. However, in the mod-ulus map the image contrast on top of the copper line does notcoincide with the typical grain morphology of inlaid copper lines,so the explanation of the variation by surface roughness is morereasonable in this case.

Since the modulus mapping technique is more surface sen-sitive than the quasi-static indentation, it is possible to detectslight differences in the elastic properties of ultra-thin layers ontop of the OSG film. It should also be noted that with the quasi-static indentation method the data from very shallow indentscould be significantly affected by signal noise and drift effects,which can potentially cause errors. Therefore, the modulus map-ping technique should preferably be applied for the determinationof elastic properties of thin films and near-surface regions. Asurface acoustic wave (SAW) spectroscopy technique has beenused for quantitative mapping of elastic properties on thin films,too [16]. However, it is difficult to apply this technique topatterned wafers.

4. Conclusions

In summary, we have shown that modulus mapping is a validmethod for assessing process-induced modifications of dielectricmaterials at the near-surface regions, like OSG damage caused byoxygen-plasma treatment. The modulus mapping technique im-plemented in a nanoindentation tool with in situ SPM imagingcapability allows to map the elastic modulus quantitatively withsub-micrometer resolution on low-k filmswith a very high surfacesensitivity. Consequently, the extent of damage caused by a spe-cific process step can be quantified with high spatial resolution. Incontrast to the SAW technique, it can be applied directly topatternedwafers. Themodulus values extracted from themaps arein the same order of magnitude as low-load quasi-static nano-indentation data, and rather good quantitative agreement is ob-served for untreated OSG thin films. Small differences of themodulus can be resolved, as shown for OSG films of the samecomposition but with different surface treatments, as well as for

patternedCu/low-k structures, where a highermodulus of the low-k material was detected compared to a blanked low-k film of thesame type. As an implication, modulus mapping on patternedstructures provides important information regarding process-induced damage and local changes of mechanical properties oflow-k interlayer dielectrics which might not be detectable onblanked films that were not exposed to patterning and resiststripping. Our results show that modulus mapping is more surfacesensitive than quasi-static indentation. Modified near-surface re-gions of low-k films can be characterized, and ultra-thin layers canbe distinguished quantitatively.

Acknowledgements

The authors thank Frank Koschinsky, Christof Streck and PetraHofmann, all with AMD Saxony, Dresden, Germany, for prepar-ing the samples. Valuable discussions with Dmytro Chumakov,AMD Saxony, Dresden, Germany, are gratefully acknowledged.

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