Analysis the physical essence of microscopic fluid-based wear process inthe chemical mechanical planarization processXuesong Han Citation: J. Appl. Phys. 110, 063525 (2011); doi: 10.1063/1.3626798 View online: http://dx.doi.org/10.1063/1.3626798 View Table of Contents: http://jap.aip.org/resource/1/JAPIAU/v110/i6 Published by the American Institute of Physics. Additional information on J. Appl. Phys.Journal Homepage: http://jap.aip.org/ Journal Information: http://jap.aip.org/about/about_the_journal Top downloads: http://jap.aip.org/features/most_downloaded Information for Authors: http://jap.aip.org/authors

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Analysis the physical essence of microscopic fluid-based wear processin the chemical mechanical planarization process

Xuesong Hana)

School of Mechanical Engineering, Tianjin University, Tianjin 300072, People’s Republic of China

(Received 31 May 2011; accepted 21 July 2011; published online 26 September 2011)

Chemical mechanical planarization (CMP) has become the process of choice for surface global

planarization for materials surfaces in the fabrication of advanced multilevel integrated circuits (ICs)

in microelectronic industry. The surface planarization in the CMP is mainly realized by the tribology

behavior of nanoparticles. The suspending abrasive particles impinge on the surface at some velocity

and angle thus imparting energy to the surface, resulting in strain, weakened bonds, and eventually

material removal. Large-scale classical molecular dynamic (MD) simulation of interaction among

nanoparticles and solid surface has been carried out to investigate the physical essence of fluid-based

surface planarization process. The investigation shows that the plastic deformation plays an important

role in this nanoscale wear process while the contribution of dislocations to the yield stress becomes

insignificant. The depth of wear is gradually decreased which makes the fluid-based wear cannot real-

ize the global surface planarization by itself. The abrasive wear process leads to characteristic surface

topography running in the same direction as the sliding motion while the adhesive wear leads to the

atoms of the substrate materials adhere to the opposing surface. The adhesion wear plays an impor-

tant role at lower moving speed while the abrasive wear dominates the wear process at higher moving

speed which means the moving speed is one of the key factors that influence the particle wear mecha-

nism at the nanometer scale. Different tribology behavior involved in the CMP indicates that the final

surface planarization is accomplished by the synergetic effect of different wear mechanism. VC 2011American Institute of Physics. [doi:10.1063/1.3626798]

I. INTRODUCTION

Wear is the basis of many manufacturing technology

such as grinding, cutting, and polishing which can fabricate

parts with highly controlled shapes and surface finishes. In

semiconductor fabrication, it is often necessary to remove

the surface irregularities from the previous processing step to

flatten the wafer surface before adding additional circuit ele-

ments. This accomplished through a process called chemical

mechanical planarization (CMP) or polishing, where a pol-

ishing pad rubs abrasive and corrosive chemical slurry

against the wafer to remove the uneven material. With

advanced CMP processes, heterogeneous surfaces can be

polished to a roughness of a few angstroms.

In spite of being a historically ancient technology, CMP

has never attracted so much attention as it has in the last ten

years. This is because of its applicability in planarizing the

dielectrics and metal films used in the silicon integrated cir-

cuit (Si IC) fabrication. Continued miniaturization of the de-

vice dimensions and the related need to interconnect an

increasing number of devices on a chip have led to building

multilevel interconnections on planarized levels. The differ-

ence between the historical uses of CMP and those in the Si

IC fabrication lies in the amount of material that can be

removed prior to achieving the desired planarity. Very thin

(usually less than 0.5 lm) materials have to be removed pre-

cisely, ending up on a different material and on a sea of

embedded metal and dielectric surfaces. Maintaining the pre-

cise control on the remaining thickness, which is also very

small (< 0.5 lm), to within 0.01–0.05 lm while maintaining

the integrity of underlying structures are added requirements.

Such requirement has approached the limit of manufacturing

technology. Under this circumstance, any change of the flow

field or conflict in physical-chemical factors could deterio-

rate the surface quality and any tiny hard particle may gener-

ate large pits or mark in the surface. Further study on

physical aspects of nanometer manufacturing technology is

crucial for obtaining planarization surface with nanometer

level roughness. Today, chemical mechanical planarization

technology has already become attractive research item.

Despite intense theoretical and experimental research on

CMP,1–9 there is still serious lack of fundamental under-

standing on this process. The application of CMP still rests

on the semi-empirical stage. Presently, the researchers can-

not give convinced illumination about the mechanism of

CMP. The reason for this is that the CMP process is a com-

plex system characterized by multiphase, multiscale, and

multilevel at the same time being a micro/nano-tribology

behavior based chemical-physical process. It is not the geo-

metrical downsizing but should exist some new discipline

that dominates the material remove and surface generation

process in the CMP technique.

CMP as a process for achieving globally planar surfaces

originated with the advent of slicing single-crystal wafers

from grown single-crystal silicon. The planarization process

was done primarily to remove surface damaged layers cre-

ated by the previous process and to achieve a specified wafer

a)Author to whom correspondence should be addressed. Electronic mail:

hanxuesongphd@yahoo.com.cn.

0021-8979/2011/110(6)/063525/9/$30.00 VC 2011 American Institute of Physics110, 063525-1

JOURNAL OF APPLIED PHYSICS 110, 063525 (2011)

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thickness and surface planarization. The damage may extend

for several microns into the silicon wafer, and thus, the pla-

narization process had to be rapid, yet maintain a planar sur-

face. The planarization process is viewed as being a

chemical softening of the materials surface and the mechani-

cal scraping of this softened layer by the abrasive particle.

CMP is a technology that accurately produces geometrically

dimensional shapes in the nanometer order. Planarization is

carried out without letting fine abrasive particles that gener-

ate brittle fractures on the work surfaces, while removing

these materials little by little only by means of plastic defor-

mation, to finally produce a smooth mirror surface. Fine ab-

rasive particles are retained on the pad surface resiliently and

plastically, and the work surfaces are scratched microscopi-

cally. Planarization actions are by far smaller if compared

with lapping, contributing to the successful applications to

the brittle materials such as single crystal silicon. The final

surface integrity acquired using CMP technique is mainly

depended on the micro-tribology behavior of nanoparticles.

The complex multibody interaction among nanoparticles and

materials surface is different from interaction in the macro-

scopic scale which makes the traditional classical materials

machining theory cannot accurately uncover the mystery of

the surface generation in the CMP.

FIG. 1. (Color online) MD computation model.

FIG. 2. (Color online) Schematic of three-body abrasion.

FIG. 3. (Color online) MD simulation

results of surface wear process at the

nanoscale purple: particle 1, cyan: parti-

cle 2, pink: particle 3.

063525-2 Xuesong Han J. Appl. Phys. 110, 063525 (2011)

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Planarization may occur by either solid-based or by

fluid-based wear. During solid-based wear, the abrasive par-

ticles are dragged across the surface and act as cutting tools.

Removal volumes are determined by abrasive particle load-

ing and film properties. Removal rates are governed by ve-

locity and particle loading. During fluid-based wear,

abrasive particles are not dragged across the surface, but

rather impinge on the surface at some velocity and angle. As

particles collide with the surface, they impart energy to the

surface, resulting in strain, weakened bonds, and eventually

material removal. Whether CMP occurs as solid-based or

fluid-based wear is not clear and has been the subject of

some debate. The difference between the two wear modes is

in the slurry fluid layer between the pad and wafer. If the

fluid layer is not continuous, then pad-wafer contact occurs.

Note, however, that the pad does not contact the wafer sur-

face directly, but rather the pad presses abrasive particles

against the surface. In such instances, the pad will drag the

abrasives across the surface, resulting in solid-based wear. If

the fluid layer is continuous, then the pad does not contact

the wafer surface, and solid-based wear will not occur.

Instead, the collisions between abrasive particles and the pad

accelerate the abrasive particles. The particles then impinge

on the wafer surface, resulting in fluid-based wear. The ve-

locity and angle of approach of the abrasive particles will

determine the kinetic energy that the particles transfer to the

surface, and hence will affect removal rates. The kinetic

energy of the particles is a function of pad velocity and

hydrodynamic pressure of the fluid layer. In addition,

because the particles lose energy (slow down) as they move

through the fluid layer, local fluid layer thickness and slurry

viscosity will also affect the particle velocity. Planarization

mode and the role of the fluid layer are poorly understood at

this point. It is clear, however, that planarization mode and

fluid layer thickness and continuity have important implica-

tions for polish rates and planarity. This area of CMP is still

poorly understood, yet has important implications as to the

removal mechanisms of CMP. The traditional model for slid-

ing wear such as the Archard wear equation cannot give a

convinced explanation about this discrete particle tribology

behavior. New powerful theoretical tool is needed to investi-

gate this microscopic process and uncover the essence of

CMP.

Molecular dynamics simulation,10 by virtue of its high

temporal and spatial resolution, can offer an ideal approach

to gain insights into atomic scale process and understand

their mechanisms.11–16 A remarkable enhancement in com-

putational capability (computer hardware) and high perform-

ance computation techniques (parallel computation) has

enabled us employing large scale classical MD method to

investigate the nanometric tribology process and gain

insights into this atomistic behavior.

II. MOLECULAR DYNAMICS SIMULATIONMETHODOLOGY

The atomic configuration of the system studied is illus-

trated in Fig. 1. Silicon atoms of particle and substrate are

initially arranged in diamond cubic structure with a constant

lattice parameter of 5.43 A. The dimension of silicon

FIG. 4. (Color online) The stress distri-

bution after planarization.

063525-3 Xuesong Han J. Appl. Phys. 110, 063525 (2011)

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workpiece is 180� 55� 50A3 along x, y, and z directions.

The moving speed is 20 m/s, the environment temperature is

293 K, the depth of cut is 8 A, the particle diameter is 16 A,

and the timestep is 2:5� 10�15s.

For covalent systems, the Tersoff potential17 is used to

depict the interaction between the silicon atoms and atoms of

the abrasive particle as follows:

/ij ¼ fcðrijÞ½fRðrijÞ þ bijfAðrijÞ� (1)

With Eq. (1), the interaction force between silicon atoms can

be obtained by calculating the negative gradient of /.

The computation of atom trajectory requires numerical

integration of the differential equations from initial state

which the abrasive particle is approaching the wafer but has

not touched yet to the final state which a layer of material

has been removed from the wafer. There variety of methods

available for performing this numerical integration such as

fourth-order Runge-Kutta method, Leap-Frog method, Verlet

method, Velocity-Verlet method, and so on. The Velocity-

Verlet method is a symplectic algorithm which can prevent

the energy dissipation and have high computation efficiency,

this paper adopted this method as follows:

rnþ1i ¼ rn

i þ hvn þ h2

2mFn

i (2)

vnþ1i ¼ vn

i þ hðFni þ Fnþ1

i Þ=2m (3)

here rnþ1i , vnþ1

i , and Fnþ1i are position, velocity, and force at

nþ 1 step of the ith atom while rni , vn

i , and Fni are position,

velocity, and force at n step of the ith atom, h is timestep,

and m is mass of atom.

Fluid-based wear in the CMP is caused by hard particles

that are free to roll or slide between two sliding surfaces

(polishing pad and silicon wafer) which is also termed as

three-body abrasion (Fig. 2). Despite the technological im-

portance of wear, no simple and universal model has been

developed to describe it. As with many other tribological

phenomena, the multitude of physical mechanisms contribut-

ing to wear makes it difficult to develop a general compre-

hension of how wear occurs at the nanoscale. Understanding

the micro-mechanisms, contributing to three-body abrasion

is crucial for exploring fundamentals of CMP.

III. PARTICLE TRIBOLOGY BASED SURFACEGENERATION AT NANOSCALE

A. Plastic deformation and materials removal atnanoscale

Figures 3–6 shows the MD simulation results of fluid-

based wear process in the CMP. It is different from the mac-

roscopic scale process, as the plastic deformation plays an

important role at this nanoscale wear process. The wear pro-

cess showed in the MD simulation results involve plastic de-

formation of material at small localized regions where

asperities of the opposing surface and particles make contact.

This process belongs to tangential contact problem in which

complete sticking exists in the contact area where partial

sliding takes place. The onset of plasticity for such small

FIG. 5. (Color online) Variation of friction force between particle and substrate.

063525-4 Xuesong Han J. Appl. Phys. 110, 063525 (2011)

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regions can be dramatically different than the plastic yield

stress determined by macroscopic measurements (Fig. 7).

When the size of the region undergoing plastic flow becomes

much smaller than the typical distance between dislocations,

the contribution of dislocations to the yield stress becomes

insignificant (shown in Fig. 8). It is also shown that there are

not any obvious dislocations in the substrate materials and

the plastic deformation (yield stress) is governed by the force

needed to slide one plane of atoms over another. Neither brit-

tle crack at the root of the surface peak nor the crack extend-

ing into the bulk is observed. Most of the materials removed

from the substrate stick to the particle surface which means

this plastic deformation based wear is generated by adhesive

force which exceeds the yield stress of the sliding materials.

The notion that wear should be related to adhesion seems

fairly natural: the same bonding mechanism that makes it

difficult to pull surfaces apart should also make it difficult to

slide them over each other. The traditional model for sliding

wear, namely, the Archard wear equation cannot fit for this

microscopic adhesive wear process. The depth of wear grad-

ually decreased as the particle moving ahead because there

are no fixed rigid constraints which minimized the effect of

planarization (shown in Fig. 7).

The substrate materials undergone uniformly distributed

shear stress in the whole CMP process (Fig. 4). This plastic

deformation induced at nanoscale is different from the plas-

tic deformation at the macroscopic scale, as there is neither

dislocation nor stress concentration being induced in the sub-

strate. The animate movies show that the adhesion occurs af-

ter the particles touch the surface peak, followed by plastic

shearing that plucks off part of the asperities; these bits then

adhere to the outer layer of the particles before eventually

becoming loose wear debris. Higher adhesion forces in the

contact zones are more likely to pull out a wear fragment.

Consequently, higher surface energy should result in higher

wear rates, since adhesive forces scale with surface energy.

Figure 5 shows the variation of friction force of wear

process in the CMP. There are different changing tendency

for these three particles which means different wear mecha-

nism induced. The average value of the friction force

induced by particle 1 and particle 2 is larger than that of fric-

tion force induced by particle 3 and the wear mechanism of

these two particles should belong to the abrasive wear. The

abrasive wear process leads to a characteristic surface topog-

raphy running in the same direction as the sliding motion as

shown in Fig. 3. While the wear mechanism induced by par-

ticle 3 should belongs to the adhesive wear. During sliding,

the atoms of the substrate materials adhere to the opposing

surface and become detached from the particle 3 (Fig. 3).

There are still exist interaction force between particles and

substrate materials after they detaching which justifies the

substrate materials sticking to the outer layer of the particle.

Figure 6 shows the changing tendency of substrate

potential energy in the surface planarization process. This

graph shows that high surface energy induced in the contact

zones thus more likely to pull out wear fragment and result

in higher wear rates. Consequently, it can be concluded that

the adhesive forces should scale with the surface energy and

dominate the plastic deformation based wear process. There

are basically four wave peak in the potential energy graph,

which denotes four intensive substrate interaction accident.

The animate movie shows that it should have something to

do with the four impacting accident between substrate and

the particle1.

B. The effect of velocity upon the atomic scale wearprocess

Figure 9 shows the fluid-based wear process with differ-

ent moving speed in the CMP. As the particles gradually

being lifted in the surface planarization process, the actual

fluid-based wear process only induced at the initial stage as

FIG. 6. (Color online) Variation of potential energy of substrate.

FIG. 7. (Color online) Dynamic track of particle tribology behavior. FIG. 8. Cross-Sectional HRTEM images of silicon.

063525-5 Xuesong Han J. Appl. Phys. 110, 063525 (2011)

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FIG. 9. (Color online) Atomic wear process at different speed.

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FIG. 9. (Color online) (Continued)

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shown in Fig. 9(a). When two materials (particle and sub-

strate) are pressed against each other, initial solid–solid con-

tact occurs at the high points. This area where the asperities

touch is usually an extremely small fraction (< 1%) of the

total area covered by the surfaces, but the forces generated

between the contacting atoms in this small area are responsi-

ble for most tribological phenomena—friction, wear, adhe-

sion, etc. Consequently, understanding how forces acting on

asperities distort the material around the points of contact

can provide an important basis for understanding tribology

behavior based process.

Larger attractive force is induced as the particle

approach substrate materials at the beginning of the planari-

zation. The attractive force is gradually increased with the

increasing of the moving speed. This means the materials tri-

bology behavior is closely related to how the individual

atoms are bound to their neighboring atoms in the material.

When two similar atoms are brought together, a bond will

form between them: attractive force, which gradually

decreases as the atoms move apart; repulsive force, which

increases rapidly as the atoms are squeezed together due to

the overlap of the electron clouds that surround each atom.

There are particle cluster generated no matter at low or

high moving speed. This aggregation behavior should be

considered as some elementary kinds of chemical reaction

among particles. The conformation of the cluster gradually

changed from compact to dendritic with the increasing of

moving speed. The diameter of nanoparticle used in the

CMP is in the range of several hundreds of nanometer or

smaller. Too large atomic clusters will induce surface dam-

age and thus degrade the quality of ultra-large scale inte-

grated circuit.

The wear rate (materials removal rate) is gradually

increased with the increasing of particle moving speed

(Fig. 10). This variation trend gradually approaches its satu-

rated point at larger moving speed which means there are

may be different wear mechanisms at different moving

speed. The adhesion wear plays an important role at lower

moving speed while the abrasion wear dominates the wear

process at higher moving speed. Most of the removal sub-

strate atoms stick to the out layer of particles in the adhesion

wear process. While majority of removal substrate atoms dis-

persed in the surrounding environment in the abrasion wear

process.

IV. CONCLUSION

Fluid-based wear is one of the important factors of the

surface planarization which can fabricate parts with highly

FIG. 9. (Color online) (Continued).

FIG. 10. Wear rates of different speed.

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controlled shapes and nanometer dimensional tolerances

with only a few angstroms of roughness. Nanoparticles have

properties which are fundamentally different from those of

discrete molecules or the relevant bulk solid. The complex

multibody interaction among suspending nanoparticles and

crystal wafer is crucial for uncover the mystery of the mate-

rials removal and surface defect generation in the CMP. The

authors investigate CMP process of single crystal silicon

using MD simulation method, after that drew some

conclusions:

(1) There are no brittle crack observed at the root of the

surface peak or the under the materials surface. It is different

from the macroscopic scale process that the plastic deforma-

tion plays an important role at this nanoscale wear process.

When the size of the region undergoing plastic flow becomes

much smaller than the typical distance between dislocations,

the contribution of dislocations to the yield stress becomes

insignificant.

(2) The depth of wear gradually decreased as the particle

moving ahead because there is no fixed rigid constraint

which minimized the effect of planarization. This phenom-

enon justifies that the fluid-based wear cannot realize the

global surface planarization in the CMP by itself.

(3) The different tribology behavior for the different par-

ticles means different wear mechanism induced. The wear

mechanism of particle 1 and particle 2 should belong to the

abrasive wear. The abrasive wear process leads to a charac-

teristic surface topography running in the same direction as

the sliding motion as shown in Fig. 3. While the wear mecha-

nism of the particle 3 should belongs to the adhesive wear.

During sliding, the substrate atoms adhere to the opposing

surface and become detached from the particle 3 (Fig. 3).

(4) Larger attractive force is induced as the particle

approaching substrate materials at the beginning of the pla-

narization. The attractive force is gradually increased with

the increasing of the moving speed. This means the materials

tribology behavior is closely related to how the individual

atoms are bound to their neighboring atoms in the material.

(5) The roughness of the local surface gradually deterio-

rated with the increasing of particle moving speed. The plas-

tic flow within crystalline materials gradually transform from

sliding between the atom layer to delamination, namely, the

moving speed changes the particle wear mechanism.

(6) The wear rate (materials removal rate) is gradually

increased with the increasing of particle moving speed

(Fig. 9). This variation trend approaches the saturated point

at larger moving speed which means there are may be differ-

ent wear mechanisms at different moving speed. The adhe-

sion wear plays an important role at lower moving speed

while the abrasion wear dominates the wear process at higher

moving speed.

Single crystal silicon has already become general sub-

strate materials in semiconductor industry for its excellent

dielectric property. It is difficult for single crystal silicon to

acquire nanometer level machined surface, as it is a hard and

brittle material. In the case of CMP technique, the depth of

cut is in the range of nanometer or sub-nanometer, the mate-

rials removal and surface generation process are different

from pure fracture mode exists in brittle materials machining

process or plastic shear mode exists in metals machining pro-

cess. The materials removal processes involve plastic defor-

mation of material in small localized regions where

asperities of the opposing surfaces or hard particles make

contact. The onset of plasticity for such small regions is dra-

matically different than the plastic yield stress determined by

macroscopic measurements.

Theoretically, planarization may occur by either solid-

based or by fluid-based wear. During solid-based wear (like

nano-cutting process), the abrasive particles are dragged

across the surface and act as cutting tools. During fluid-

based wear, abrasive particles are not dragged across the sur-

face, but rather impinge on the surface at some velocity and

angle. As particles collide with the surface, they impart

energy to the surface, resulting in strain, weakened bonds,

and eventually material removal. Whether CMP occurs as

solid-based or fluid-based wear is not clear and has been the

subject of some debate. The interaction strength between

slurry particle and the substrate in the solid-based wear is far

beyond the corresponding interaction strength in the fluid-

based wear process which results in different surface integ-

rity and subsurface structure. It is clear that planarization

mode and fluid layer thickness and continuity have important

implications for polish rates and planarity. This area of CMP

is still poorly understood, yet has important implications as

to the removal mechanisms of CMP.

ACKNOWLEDGMENT

This research was supported by Specialized Research

Fund for the Doctoral Program of Higher Education of the

Ministry of Education of China (No. 200800561097).

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