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272 DIFFUSION PROCESSES IN ADVANCED TECHNOLOGICAL MATERIALS

each at the electrical fields for both the control sample and the sample

with a SAM between Cu and SiO

. The SAM-coated sample had a very

small shift in capacitance-voltage plots, and a shift of only about 4 to 5 V

was seen after 650 minutes, at which time the capacitor failed. On the

other hand, capacitance-voltage plots shifted considerably with time for

the control sample, and after 150 minutes, when it failed, the capaci-

tance-voltage shift was greater than 18 V. In the control sample, the leak-

age current increases rapidly and continuously with time. On the other

hand, in the SAM-coated sample, a relatively constant current leakage of

about 10 to 30 nA/cm

persists until failure (at 650 minutes), when cur-

rent shoots up to 100 mA/cm

. The shifts in capacitance-voltage and the

increased leakage currents are indicative of’ the diffusion of ionic Cu

under electrical bias. These results seem to establish the effectiveness of

SAMs as APDB materials.

The barrier properties of SAMs were explained in terms of the size

and configuration of the terminal group and the molecular chain length.

[9]

The larger volume occupied by the aromatic rings (compared with, for

example, aliphatic groups) sterically hinder Cu diffusion between the

molecules through the SAM layer. Self-assembled monolayers with long

chain lengths apparently screen Cu atoms from the influence of the under-

lying SiO

, thereby preventing ionization and consequent acceleration by

the externally applied electric field, as happens in the absence of an APDB

layer between Cu and SiO

Both approaches, the use of Mg-Cu or Al-Cu alloy and the use of

SAMs, are very promising but will need validation in actual devices or

circuits fabricated, aged, and tested in actual manufacturing conditions.

Both offer APDB layers in thickness ranges of 1 to 2 nm or less. The Cu

alloys offer near-Cu resistivities and will not act as an insulator between

two vertical interconnections. The SAM layer may interfere with electri-

cal conduction, and thus may require selective removal from the via bot-

tom or an externally induced change in conduction behavior.

5.5.3 Zero-Flux Diffusion Zones (Multicomponent

Diffusion Effects)

My co-workers and I have taken a completely new approach in our

research to address diffusion-related instabilities in metal intercon-

nects.

[69, 70]

The concept of multicomponent diffusion effects is new to the

area of metal interconnections. The diffusion behavior in multicomponent

alloys (three or more elements) could be significantly different from what

is observed in single-component and binary diffusion. In multicomponent

alloys, the flux of each element depends on the concentration gradient of

Ch_05.qxd 11/29/04 6:20 PM Page 272

DIFFUSION BARRIERS IN SEMICONDUCTOR DEVICES, MURARKA 273

all the elements in the alloy. Hence, certain special effects such as uphill

diffusion and zero flux planes are likely to be observed. Such effects could

be used to limit and even inhibit the diffusion of a certain species within

the alloy.

One example of a multicomponent effect would be the occurrence of

a zero-flux plane (ZFP). AZFP is a region in the reaction zone of the dif-

fusion couple where the flux of a component goes to zero.

[71]

The occur-

rence of ZFPs for Al or Cu in an Al or Cu alloy would imply that diffusion

of Al or Cu is held in check by virtue of interactions with the interdiffus-

ing alloying elements. We could thus limit the diffusive spreading of Al or

Cu, thereby improving the resistance to electromigration and diffusion-

induced interactions.

A careful approach has been taken to select alloying elements to alu-

minum to achieve the desired characteristics.

[69, 70]

The research method-

ology is based on choosing homovalent alloying elements that will have

negligible solid solubility in Al (a) to minimize the undesirable increases

in resistivity associated with the scattering behavior of the solute atoms in

Al, and (b) to cause “grain boundary stuffing”.

[2]

The search for these ter-

nary (or higher component) alloys of homovalent metals (such as Al, In,

and La) that exhibit so-called ZFPs is also, therefore, undertaken to

explore the use of multicomponent diffusion effects in Al alloys. Note that

the multicomponent concepts are not unique to Al alloys and can easily be

extended to and thus explored in the Cu alloys.

To study the existence of ZFPs and other multicomponent effects, a

PC-based software program called Profiler is used. This program calcu-

lates the variation, with time and distance, of the concentrations of n

species diffusing across the interface of the single-phase diffusion cou-

ple.

[72]

It requires the investigator to input the diffusivity [D] matrix. In the

case of the ternary system, [D] is a 2  2 matrix. The elements of this

matrix are interaction terms that describe the pair-wise interactions of ele-

ments in the presence of a third element. In general, the flux of compo-

nent i is linked to the concentration gradients of all the elements in the

alloy via the pair-wise interaction coefficients, D

D

(∂c

∂x). (19)

If the off-diagonal terms of the [D] matrix are comparable in magnitude

to the diagonal terms, we can expect strong interactions between the pair

of elements for which the [D] matrix is considered. Such strong interac-

tions among the elements lead to the occurrence of multicomponent

effects such as the ZFP.

We would have to conduct several interdiffusion experiments to

determine the [D] matrix. A procedure has been developed to evaluate

Ch_05.qxd 11/29/04 6:20 PM Page 273

all four elements of the [D] matrix by conducting a single interdiffu-

sion experiment. Figure 5.7 schematically shows the terminal compo-

sitions in composition space. Terminal alloy compositions are represented

by points P and Q in the unrotated orthogonal composition space.

Schut and Cooper

[73]

point out that as long as the line PQ, joining Pand

Q, is not parallel to the eigenvector of the [D] matrix, an S-shaped dif-

fusion path will result. Profiler is used to generate the penetration

profile and the diffusion path for a given diffusion couple. The proce-

dure developed for calculating [D] involves rotation of the axes to

align one of the two axes parallel to PQ. Using the diffusion path from

the actual experiment, the penetration profiles for alloys in the rotated

composition space are determined. Appearance of extrema in the pen-

etration curves for components in the rotated space enables the deter-

mination of all elements of the [D] matrix from three equations. These

equations are:



(l)

()

2ldc

D



 D



, (20)



(l)

()

2ldc

D



 D



, (21)

]  [B][D][B]

1

, (22)

where

[B] 



[B] is the rotation matrix, C

1,2

is the composition in the rotated space, and

[D*] is the diffusivity matrix in the rotated space. At the extremum for a

Cosa Sina

Sina Cosa

274 DIFFUSION PROCESSES IN ADVANCED TECHNOLOGICAL MATERIALS

Figure 5.7 Schematic showing the terminal compositions in composition space.

Ch_05.qxd 11/29/04 6:20 PM Page 274

DIFFUSION BARRIERS IN SEMICONDUCTOR DEVICES, MURARKA 275

given component in the concentration profile, the derivative of concentration

at that point will become zero. Thus two out of the four D

will be deter-

mined. The other two diffusivity elements can be determined at the

Matano interface. Clearly, the number of interdiffusion experiments

required to generate concentration profiles that would be needed to deter-

mine the [D] matrix is minimized. Not only does this method save time

and cost, it also avoids time-consuming calculations.

The Profiler program can be used to simulate the concentration pro-

files that would develop during diffusion in several diffusion couples,

eschewing the need to perform “real” experiments. Subsequently, the con-

centration profiles from each of the simulations are analyzed for ZFPs. If

the ZFP for Al (or Cu) is predicted for a certain diffusion couple, the cou-

ple could be deposited and tested for interdiffusion of Al (or Cu) to verify

the occurrence of the ZFP.

5.6 Brief Discussion of an APDB for

Low-k ILD Materials

We are concerned with the metal diffusion into and metal adhesion

to low-k dielectrics and vice versa. Low-k materials can be inorganic,

polymers, polymer-inorganic blends, or porous materials. Thus there is

a serious concern related to finding an APDB material that may be

unique for a given low-k ILD or universal for all low-k materials. This

discussion focuses on polymer and polymer-type materials only.

Although there are differences in the adhesion behavior of a metal on a

polymer and a polymer on a metal, the adhesion is generally weak. As

mentioned in Sec. 5.1, we expect chemical bonding across the interface

to provide a good adhesion. This, however, may be challenged by stress

in the films and substrate and by the process parameters, such as shear

stress during chemical-mechanical polishing. Interface stability also

plays an important role in keeping adhesion properties constant for a

reasonable time. Near-interface changes of polymer properties due to

diffusion and/or precipitation processes can affect adhesion. Small

atoms, like Cu, can form different precipitates and clusters in polymers.

The metal/polymer near-interface regions can contain metal-related

defects after thermal treatment, and metal can diffuse into the bulk of

polymers (see the discussion below). Short-range diffusion (tens of

nanometers) is, however, not expected to affect polymer properties and

adhesion unless strong clusters form. There are many examples of

process-affected and chemistry-dependent metal-polymer adhesion,

such as interfacial oxide formation, metal-oxygen complex formation,

carbide formation, and polymer cross-linking.

Ch_05.qxd 11/29/04 6:20 PM Page 275

Polymer and metal interactions are strong and lead to metal diffu-

sion in most polymers.

[74]

Besides externally induced forces (thermal,

electrical, chemical, and occasionally mechanical), the structure, crys-

tallinity, porosity, and surface functional groups play very important

roles in these interactions. The effect of an applied electric field on the

polarization and charge generation within the dielectric is expected to

influence the diffusion and interactions. With respect to diffusion in so-

called crystalline polymers, the microcrystallites in polymers can be

regarded as impermeable islands embedded in a continuum of perme-

able, amorphous media, and a nonuniform distribution of diffusant can

be expected during thermal processes. In amorphous phases, diffusion

and viscoelastic properties are expected to be strongly influenced by the

amount of free volume. However, not enough experimental data are

available on void distribution in most polymers as a function of prepa-

ration conditions. It is not surprising, therefore, that many theories have

been developed describing diffusion in polymers. These theories can be

classified as either molecular or free-volume models.

[75, 76]

Fractal mod-

els for describing diffusion in polymers are particular promising. While

there are other theories of diffusion in polymers,

[77]

metallic diffusion

cannot be described in terms of these models. Discussion of the rela-

tionship between polymer structure and diffusivity of species is beyond

the scope of this chapter. The entire Chapter 7 in this book is devoted to

this topic.

Generally, metal atoms diffuse at the metal-polymer interface.

However, there are cases where reactive groups of ions present on the

polymer chain-react with the metal and produce species that diffuse in the

metal. The reactivity is highest for oxygen containing moieties (for example,

CO, COOH, CHO, COC), followed by those containing

nitrogen (such as CN, NH

, and NH). Alkyl groups have the least

reactivity, although phenyl-type groups have comparatively higher reac-

tivity.

[78]

Fluorine and other halogens also have considerable reactivity; a

classic example is the interactions between Al films and fluorinated poly-

mers.

[79]

The interactions sometimes lead to interfacial reaction products

that form barriers to further interactions and diffusion. The literature is

full of metal-polymer interactions, organometallic formations, and

decompositions, and metal’s chemical reactivity has been reported to play

a role in the diffusion (or in the drift under electrical bias) in polymers.

[80–82]

For example, Mallikarjunan et al.

[80]

studied a hybrid organosiloxane (OS)

polymer with k  2.5 in conjunction with different gate metals such as

aluminum, tantalum, and platinum, using C-V and BTS measurements on

the metal/polymer/SiO

/Si capacitors. The results obtained with Cu and Al

metals could not be explained satisfactorily with existing understanding;

therefore, gate metals Ta and Pt were also investigated. Tantalum, pure or

276 DIFFUSION PROCESSES IN ADVANCED TECHNOLOGICAL MATERIALS

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DIFFUSION BARRIERS IN SEMICONDUCTOR DEVICES, MURARKA 277

as a nitride, is useful as an adhesion promoter/diffusion barrier layer and

is similar to aluminum. Copper is, comparatively, much less chemically

active. Platinum, on the other hand, is a noble metal and is not expected

to diffuse or drift into, or interact readily with, the dielectrics. The total

number of charges obtained from measured C-V shifts under BTS

increases in the order Pt, Cu, Ta, and Al, with no charges detected for Pt

and maximum charge detected with Al as the gate metal. This is because

both aluminum and tantalum react strongly with SiO

and could be

trapped by dangling bonds at the SiO

/OS interface. From the plot of the

number of charges as a function of the heat of formation for the oxides of

these metals per oxygen atom, it was concluded that the number of

charges (or metal ions)/cm

that reach the OS/SiO

interface is directly

related to the metal’s oxidation tendency as measured by the oxide’s heat

of formation. Also note that the first ionization energies of Pt, Cu, Ta, and

Al are 9.0, 7.726, 7.89, and 5.986 eV, respectively. These numbers are rea-

sonably in line with the oxidation tendency.

For metals such as Al, Ti and Ta that show strong reactions with SiO

the formation of a stable metal-oxide diffusion barrier limits diffusion or

drift into SiO

. However, in polymers with significant organic content,

such a reaction may not occur. Instead, oxygen-containing groups may

increase the ionization tendency of the metal and aid the drift process.

Loke et al.

[83]

have used such a model to qualitatively explain Cu drift

kinetics in different polymers.

Low-k dielectric/metal technologies need a low-resistivity metal or a

thin, low-k insulator as the APDB material suitable for the chosen ILD mate-

rial. This is a real challenge, in view of the complexity of such ILD materials.

5.7 Summary

This chapter begins with a brief review of the early history of the

diffusion barrier thin films used in the vintage semiconductor devices

and circuits of the last century. It provides a short overview of the dif-

fusion processes and the various influencing material factors that must

be considered in the development of diffusion barriers. Special aspects

of diffusion barrier materials recently developed to contact interlayer

dielectric films with the main conductor are covered for modern circuits,

notably the Cu metallization introduced in the industry during the past

few years. In the context of Cu, an entirely new concept of using alloys

with zero flux planes is introduced. Finally, recent innovations and

novel concepts used in the adhesion-promoting diffusion barrier films

for applications in Al and Cu metallization and low-k dielectrics tech-

nologies are discussed.

Ch_05.qxd 11/29/04 6:20 PM Page 277

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