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Wear 263 (2007) 1477–1483

Oxidation and removal mechanisms duringchemical–mechanical planarization

D. Ng a, M. Kulkarni a, J. Johnson b, A. Zinovev b, D. Yang c, H. Liang a,∗a Mechanical Engineering, Texas A&M University, College Station, TX 77843-3123, USA

b Energy Technology Division, Argonne National Laboratory, 9700 South Cass Ave., Argonne, IL 60439, USAc Hysitron Inc., 10025 Valley View Rd., Minneapolis, MN 55344, USA

Received 15 August 2006; received in revised form 3 November 2006; accepted 30 November 2006Available online 23 May 2007

bstract

This paper studies surface properties of metals during chemical–mechanical polishing (CMP). In order to pinpoint the effects of chemistry andechanical impacts separately, during CMP, we polished Cu and Al surfaces using two distinct slurries of hydrogen peroxide (H2O2) and alumina

anoparticles. After polishing, detailed characterization of the surface quality and chemical composition were conducted using scanning electronicroscopy (SEM), X-ray photoelectron spectroscopy (XPS), and nanoindentation techniques. It was found that nanoparticles were effective

n removing surface materials while passivation provides a high quality layer. The application of H2O2 slurries in combination with friction

timulation produced an oxide layer. Depending on the nature of metals, it was found that Cu forms an active and non-equilibrium oxide layerhile Al has a stable one. The oxide layer resulted from two competing mechanisms, passivation and abrasion. It was deduced from kinetics thatxidation dominated the initial formation of the surface layer while mechanical sweeping determined the final film thickness. New material-removalechanisms are proposed.2007 Elsevier B.V. All rights reserved.

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eywords: Chemical–mechanical planarization (CMP); Passivation; Abrasion;

. Introduction

Mechanical polishing plays an important role in metal fab-ication [1]. The polishing mechanisms involve friction thatctivates surface reaction sites by bond stretching, bond break-ng, reformation, mass transport of ions, and nucleation [2–12],s well as increased diffusivity [13], improved boundary lubri-ation of steel [14–16], metals generating metallic bonds, plasticeformation, mechanically mixed layers during dry sliding, andriction-stirred welding [17].

CMP has been used in the microelectronic industry as aajor planarization process step in making chips since the 1980s

18–21]. It generates a super smooth surface with an averageoughness of less than 10 A across a 300 mm wafer. Metal

MP is possible when a passivation layer is formed and then

emoved through nanoabrasive particles. Oxide CMP involvesnteraction between abrasive silica particles and the oxide sur-

∗ Corresponding author.E-mail address: hliang@tamu.edu (H. Liang).

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043-1648/$ – see front matter © 2007 Elsevier B.V. All rights reserved.oi:10.1016/j.wear.2006.11.023

on; Wear

ace, where material is removed through a “snow-ball” action22].

It has been reported that oxide layers of silicon, tungsten, andopper are formed after CMP [23–35]. These metal-oxide layersre generally less than a few nanometers in thickness on super-mooth wafer surfaces (∼250 mm in diameter). Metals have highurface energies and oxides have lower ones. The surface energyetermines whether a material wets another material and formsuniform adherent layer. Such a uniformly adherent layer mightenefit from friction as it enhances diffusion of surface atomsnd reduces residual stress, chemical reactions, and any misfitetween film and bulk.

Our recent XPS work on copper CMP has shown that non-quilibrium CuO and Cu(OH)2 are formed during polishing36,37]. This means that mechanical stimulation during CMPlays a crucial role. In order to understand the synergy of thehemical–mechanical process here, we will study the chemical

nd mechanical effects separately. This allows us to simplify ourtudy and identify those reactions in two body wear. The focusill be on the properties of surfaces in non-equilibrium states.he approach used here is to conduct CMP experiments that

1478 D. Ng et al. / Wear 263 (

Table 1Physical and mechanical properties of metal materials

Crystalstructure

Density(g/cm3)

Tensile modulus(GPa)

Yield strength(MPa)

CA

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2

2

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2

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opper fcc 8.96 129.8 270luminum fcc 2.70 70.6 110–170

inpoint mechanical removal verses oxidation. This is furthernhanced through surface characterize using XPS, SEM, andanoindentation techniques.

. Experimental procedure

.1. Materials

Two metals, copper and aluminum, were investigated in thistudy. Samples were cut from copper and aluminum wafers.he physical and mechanical properties are shown in Table 1.he polished area of these samples measured 10 mm × 10 mm.olyurethane pads (Rohm & Haas IC1000) were used as counter-arts to rub against the metal surfaces. Three types of polishinglurries were prepared; they are pure deionized water, waterontaining �-alumina (Buehler), and a slurry containing H2O2.heir composition and pH are listed in Table 2. As mentionedarlier, these three slurries were prepared in order to precipitatehe desired interaction.

.2. Polishing experiments

Polishing experiments were conducted with a disk-on-disketup and reciprocating motion (CETR’s UMT). Frictionalotion is produced by a scotch yoke mechanism driven by a vari-

ble speed motor. The upper sample was loaded against the lowerample by a 2D force sensor, which enabled the measurement ofhe sliding friction force as well as controlling the loading force.he testing conditions are as follows: normal load at 5 N, recip-

ocating speed at 200, 400, and 800 rpm, sliding time at 30 min,nd operating under ambient conditions. Prior to each test, theetal samples were cleaned using acetone. During experiments,

he slurry was applied to cover the pad surface completely. Theriction coefficients were recorded and tests were repeated ateast three times under each condition.

Before polishing, average surface roughness and wettingngle were measured. The surface roughness was measured

sing a profilometer (Talysurf 3+) with a stylus tip of radius�m. Data shown in the following sections are the average of

our roughness values. The contact angle measurement was car-ied out using a goniometer, the water droplet being delivered

b

me

able 2he composition and pH of slurries

DI water 3 wt.% �-alu

pH 5 pH 7.5 pH 10 pH 5

opper√ √

luminum√ √ √

2007) 1477–1483

nto the sample surface using a syringe. The measurement wasade after waiting for 20 s when the droplet is stabilized. The

ontact angle data was also the average of four measurements,ade at different locations on the metal surface.After polishing, sample surfaces were characterized using

llipsometry, scanning electron microscopy (SEM), X-ray pho-oemission spectroscopy (XPS), and nanoindentation. XPS wassed to study surface chemical compositions of the films. XPSata was collected using Mg K� (1253.6 eV) radiation with HEAn FAT mode (in vacuum of 3 × 10−10 Torr). Spectra calibrationas carried out using a Gold XPS line Au4f7/2 (BE 84 eV).ll data were corrected for inelastic scattering by subtracting ahirley background from the raw spectra. Peak fits were donesing pseudo-Voigt shape peaks with different relative contentf Gaussian and Lorentzian components. The Ar+ ion beam wassed for depth profiling of the metal surface. The ion beamurrent was 30 �A/cm2 and a voltage of 3 kV. Average sput-ering rate, which was calculated based on Sigmung’s theoryas 4 and 3.7 nm/min for copper and aluminum, respectively

38].Nanoindentation was performed on four polished samples

sing a Hysitron’s TriboIndenter®. The samples were labeled asopper wafer No. 1 polished by 3 wt%. H2O2 (Cu 1); Copperafer no. 2 polished by 3 wt.% Al2O3 (Cu 2); Aluminum wafero. 1 polished by 3 wt.% H2O2 (Al 1); and Aluminum wafero. 2 polished by 3 wt.% Al2O3 (Al 2). Eight or more indenta-ions were performed on each sample using a 142.3◦ Berkovichiamond indenter tip with a triangular load function consistingf a 5 s loading segment and a 5 s unloading segment. A min-mum normal load of 1000 �N and maximum normal load of1,000 �N were applied on all four samples for the tests. A highormal load was applied to get reproducible data. In addition,n situ surface probe microscope (SPM) images were obtainedrom all surfaces to estimate sample roughness.

. Results and discussions

.1. Friction and nanomechanical properties

The average friction coefficients were obtained from fourests and results are shown in Fig. 1. The shaded bars are frictionoefficient and the top portion shows the standard deviation. Its seen that from the slurries tested, both copper and aluminumave friction coefficients between 0.4 and 0.6. The exceptions that of Al polished with a slurry containing alumina. This

ehavior is explained later.

Nanoindentation tests were performed to obtain the reducedodulus (Er) and the hardness (H). Four samples were

valuated—Cu 1, Cu 2, Al 1 and Al 2. Prior to the nanoin-

mina 3 wt.% H2O2

pH 7.5 pH 10 pH 5 pH 7.5 pH 10√ √ √ √√ √ √

D. Ng et al. / Wear 263 (2007) 1477–1483 1479

du1uRo(RTfitapsTpis

F(r

Fir

dfmdf

3

TosfCstrong effect of H2O2 on surface passivation.

The contact angle and average surface roughness versus speed(rpm) are shown in Table 4. Contact angle changed when H2O2was used as a polishing slurry but not when other slurries were

Fig. 1. Average friction coefficient of metals CMP. (a) Cu and (b) Al.

entation tests, in situ atomic force microscope (AFM) wassed to estimate the surface roughness of these samples. Cuand 2 showed the root-mean-square (RMS) roughness val-

es of ∼20 and ∼2 nm, respectively. Al 1 and 2 showed theMS roughness values of ∼28 and ∼15 nm, respectively. Inrder to obtain reproducable Er and H data, high normal loadswith large indentation depths) are used for nanoindentation.esults are shown in Figs. 2 (Cu samples) and 3 (Al samples).he figures show that the two copper surfaces are visibly dif-

erent in depth under the same load, while the depth for Al 2s greater than for Al 1. Table 3 summarizes the nanoinden-ation results. The reduced modulus and hardness data show

slight difference in material properties between the samplesolished by H2O2 and by Al2O3; the polished copper surfaceeems to have significantly higher values than that of aluminum.

he Vickers hardness of pure copper is between 49 and 87 andure aluminum is between 21 and 48 [39]. The high hardnesss shown in the nanoindentation data for Cu (Table 3). At thistage, we do not know the actual hardness of the metal oxides

ig. 2. Multiple load–displacement plots of Cu 1 (light blue) and Cu 2 (pink).For interpretation of the references to colour in this figure legend, the reader iseferred to the web version of the article.)

TS

L

C

1

A

C

1

A

ig. 3. Multiple load–displacement plots of Al 1 (pink) and Al 2 (blue). (Fornterpretation of the references to colour in this figure legend, the reader iseferred to the web version of the article.)

ue to the very thin layer. However, we should note that theriction value for copper is lower than that of aluminum. Thisay be due to the fact that the hard copper surface is less

eformed than the soft aluminum so that sliding is relativelyavorable.

.2. Surface properties

Ellipsometry data for Cu and Al are shown in Fig. 4(a and b).he delta gives a measure of the change in oxide film thicknessf the metal. The figures show that when the polishing surfacepeed changes (rpm value as the x-axis), delta changes slightlyor water and Al2O3 slurries. However, delta changes more whenu and Al were polished in H2O2 slurries. This indicates the

able 3ummary of results from nanoindentation

oad (�N) Reduced modulus(GPa)

Hardness(GPa)

Contact depth(nm)

opper 11200 169 ± 55 3.0 ± 1.0 111.6 ± 29.34000 159 ± 3 3.0 ± 0.2 227.7 ± 11.16000 160 ± 17 2.9 ± 0.5 280.9 ± 25.20000 163 ± 5 3.2 ± 0.3 355.1 ± 16.5

luminum 11200 76 ± 6 0.9 ± 0.3 210.8 ± 37.63000 75 ± 3 0.8 ± 0.1 379.3 ± 35.76000 63 ± 3 0.7 ± 0.1 597.2 ± 33.1

opper 21200 163 ± 18 3.0 ± 0.1 105.2 ± 3.54000 150 ± 8 2.9 ± 0.0 223.3 ± 1.06000 160 ± 2 2.9 ± 0.2 283.1 ± 5.10000 167 ± 2 3.2 ± 0.0 356.4 ± 2.4

luminum 21200 61 ± 5 0.8 ± 0.1 231.8 ± 21.03000 64 ± 1 0.7 ± 0.0 422.5 ± 10.06000 73 ± 8 0.7 ± 0.1 672.1 ± 24.0

1480 D. Ng et al. / Wear 263 (

Fig. 4. Surface properties of metals measured with an ellipsometry.

Table 4Contact angle and surface roughness as a function of polishing speed

Speed (rpm) DI water H2O2 Al2O3

pH 7.5 pH 10 pH 7.5 pH 10 pH 7.5 pH 10

(a) CuContact angle

0 82.63 82.63 82.63 82.63 82.63 82.63200 75.8 76.53 34.57 48.73 72.53 64.5400 71.37 70.07 51.07 26.63 81.3 67.83800 73 74.6 15.13 23.13 82.2 70.63

Surface roughness (�m)0 0.13 0.13 0.13 0.13 0.13 0.13200 0.083 0.07 0.37 0.22 0.153 0.07400 0.09 0.063 0.073 0.107 0.073 0.08800 0.103 0.057 0.137 0.083 0.1 0.053

(B) AlContact angle

0 84.17 84.17 84.17 84.17 84.17 84.17200 70 78.73 37.93 32.93 56 23.6400 55.83 65.6 37.63 35.97 46.17 30.63800 59.67 71.4 27.7 24.33 56.13 19.9

Surface roughness (�m)0 0.37 0.37 0.37 0.37 0.37 0.37200 0.66 0.41 0.49 0.67 0.81 0.76400 0.38 0.34 0.36 0.42 0.54 0.65800 0.42 0.37 0.35 0.53 0.68 0.52

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2007) 1477–1483

sed. Most roughness values were low and did not vary muchith polishing speed with the exception of the low polishing

peed of 200 rpm, where H2O2 for neutral pH is shown to giverough surface.

SEM micrographs are shown in Fig. 5, where (a and b) are forl surfaces and (c and d) for Cu surfaces. Fig. 5(a) shows pitsue to corrosion on the surface while (b) shows more scratchesue to mechanical abrasion. The most interesting one is Fig. 5(c)hat shows a passivation layer of H2O2 on the copper surface.he other micrographs exhibit regularly polished or scratchedurfaces as already stated. To study the chemical nature of theselms, we decided to conduct XPS experiments.

.3. XPS analysis

A “survey” spectrum was taken over a binding energy rangingrom 0 to about 1000 eV for the aforementioned samples. Afterhe elements contained in the film were identified, a detailederies of spectra were taken over the specific range of the ele-ents of interest.The XPS analysis results are shown in Fig. 6. Cu 1 and

u 2 spectra are shown in Fig. 6(a), whereas Al 1 and Alare in (b). The Cu 1, which was polished in H2O2, had aosaic surface according to SEM. Before any treatment, Cuwas found to contain Cu, K, C, and O. This confirms the

DAX analysis for this sample. The sample was then sput-ered and analyzed continuously until pure Cu was attained.he total thickness of the surface layer was around 20 nm.u 2 was polished in Al2O3 and showed no film formed inn SEM image. Potassium was not detected in this sample asoncurs with the EDAX results. Most peaks shown are simi-ar, although there were some changes between 230 and 300 eVhere C and K were found on the Cu 2. The C was from theolishing pad as reported in our previous study [25,26] and Kas used as a dispersion addition in the polishing slurry. The

ounts obtained depend on the depth of the photoelectrons sohat the exact concentration of the surface elements is approxi-

ate.The XPS results showed no difference between surfaces of

l 1 and Al 2, as shown in Fig. 6(b). This indicates that noatter what slurry is used, the oxidation and removal states of

oth surfaces are the same. In other words, the tribochemistryuring Al-CMP is not affected by the addition of H2O2.

Comparing Cu and Al, it is interesting to see their oxidesave a different reaction to mechanical removal. The Cu 2oes not show any remaining oxide layer after mechanical pol-shing while Al 1 and 2 show no difference. Apparently, thisepends on the bonding strength of the oxide. The Al2O3 isnown for its high hardness and thus it is difficult to removehe oxide through pad sweeping. On the contrary, the CuO andu(OH)2 are much softer than Al2O3 and we have found they

re in non-equilibrium state [40,41]. A higher friction coeffi-ient for Al than for Cu in Al2O3 is seen in Fig. 1. Resultsndicate that these oxides, in equilibrium or not, dominate theolishing performance. The oxidation is due to tribochemicaleactions.

D. Ng et al. / Wear 263 (2007) 1477–1483 1481

Fig. 5. SEM micrographs o

Fig. 6. XPS spectra for Cu (a) and Al (b).

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h

wi

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f metals after CMP.

.4. Kinetics of film formation

Our results showed evidence of a 20 nm thick layer formedn the Cu surface when H2O2 was present. The formationf such a film was due to passivation as well as mechani-al abrasion. Apparently, there is a balance between these tworocesses. For passivation, the thickness of oxide growth, hox,epends on the time of oxidation, t, through the parabolic relation42]

ox = Kt0.5 (1)

here K is the parabolic oxidation rate constant.For our analysis, an empirical formula was chosen in the form

f the power-law dependence,

ox(t) = Ktn + h0 (2)

here hox(t) is the thickness of the newly formed oxide film, ns the polynomial constant (n < 1), and h0 is the initial thickness.

For abrasion, the wear volume can be written as [43],

= kwLS

H(3)

here V is the volume removed, kw the wear coefficient, S the

liding distance, L the load on the sample, and H is the hardnessf the material.

As mentioned earlier, this research focuses on a simplifiedolishing process so that we will focus on the two-body abrasive

1 263 (2007) 1477–1483

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482 D. Ng et al. / Wear

ear only. The two-body-volume wear rate can be expressed as,

dV

dt= kwL

Hv (4)

here v is the velocity of two surface in contact (=dS/dt).If it is assumed that the contact area does not change during

liding contact, the above equation can be written as,

dh

dt= kw

Hpv (5)

ntegrating the above equation, we get an expression for thenitial thickness, h0

0 = kw

Hpvt (6)

here p is the pressure applied during polishing (=L/A) and t ishe polishing time.

Thus the Arrhenius equation for the thickness due to theechanical (pad) sweeping should be equal to [44]:

wear = h0 e−t/τ, where τ = LP

Er(7)

here τ is the exponential decay time constant, L the pad thick-ess, E the pad modulus, and r is the removal rate.

The balance of thickness between oxidation and pad sweep-ng is,

= 1

2

(Ktn + h0 + kw

Hpvt e−t/τ

)(8)

As shown in Eq. (1), the oxidation is a function of time expo-entially. During CMP, the oxidation takes place in the H2O2lurry. Different metals have different constant, K. The subse-uent removal of such an oxide layer depends on the bondingtrength and stability. Our results showed that the Cu forms anxide layer that is more active than that of Al. The evidence iseen in the remaining thickness of oxide on Cu 2. Furthermore,he copper oxide is in a non-equilibrium state.

How does the resulting oxide layer affect removal? Theear through the pad abrasion is expressed in Eq. (4). This

s for a two body abrasive wear model so that we focus onhe mechanical abrasion through pad sweeping. Through aew simple steps in Eqs. (5)–(7), we derive an equation thatummarizes the resulting thickness of the oxide layer, shownn Eq. (8). It indicates that the resulting oxide thickness inMP, h, depends on several factors: oxidation constant, oxi-ation time, initial thickness, wear coefficient, hardness ofaterial, polishing pressure, polishing speed, polishing time,

ad thickness, pad modulus and removal rate. Since this workocuses on identifying separate mechanical and chemical reac-ions in two body wear, a detailed study will be carried in theuture.

Fig. 7 illustrates competing mechanisms between oxidationnd abrasion. The initial thickness of the surface layer increases

ue to the faster oxidizing rate. After that, the thickness stabi-izes, reaching equilibrium. According to Eq. (8), the shape ofhe balancing thickness curve depends largely on the mechani-al abrasion factor, this is only a qualitive illustration. In future,

aE3g

Fig. 7. Illustration of surface film formation under manipulation.

etailed quantitative comparisons will be made using an indus-rial polisher.

. Conclusions

In this research, we used a non-conventional approach totudy CMP mechanisms. Our experiments were able to separatehe effects of oxidation and mechanical removal; we focusedn the surface properties and their changes that take placeuring CMP instead of removal rate. The results showed thathere was a surface layer formed on copper with H2O2. Thexide layer thickness for Cu was 20 nm after CMP, whichas not been seen before. Without H2O2, however, the pol-shed surface showed either scratches or corroded pits (forluminum). In addition, sliding friction introduces a non-quilibrium oxide for Cu. This tribo-oxidation was not foundn Al.

Kinetic analysis of film formation was conducted to bal-nce two competing mechanisms. An equation expressing oxidehickness showed that there are several factors affecting the finalhickness: oxidation constant, oxidation time, initial thickness,ear coefficient, hardness of material, polishing pressure, pol-

shing speed, polishing time, pad thickness, pad modulus, andemoval rate.

The surface residuals and their resulting properties not onlyeflect the CMP quality, but also reveal fundamental mechanismsf polishing. The mechanical impact and chemical interaction,.e., manipulation, apparently plays an important role in CMP.his research was able to quantify these effects through kineticalculations predicting the resulting film thickness. This opensreas of investigation for future research.

cknowledgements

This research was in part sponsored by the National Scienceoundation (CMS-00239136), Texas Engineering and Experi-ent Station (TEES), and Texas A&M University. J. Johnson

nd A. Zinovev were supported by the U.S. Department ofnergy, Office of Science, under Contract no. W-31-109-ENG-8. Data collection assisted by Helen Xu and Melloy Baker wasreatly appreciated.

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D. Ng et al. / Wear

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