sputtering materials for vlsi and thin film devices || troubleshooting in sputter deposition

CHAPTER

8Troubleshooting in SputterDeposition

8.1 IntroductionIn Chapter 3 we discussed those aspects of magnetron sputtering that would influence the perfor-

mance of sputtering targets and as a result the process yield. These can primarily be divided into

two categories i.e., the characteristics of sputtering targets and the process variables specific to

sputtering tools and applications. In the sputtering target industry, the term control parameter is

used for identifying the characteristics of a sputtering target, while in the semiconductor industry

the term recipe is used for defining process variables for sputtering. The major control parameters

for sputtering targets and process variables are listed in Table 8.1. The deposition tool hardware is

also known to have considerable influence on the occurrence of troubleshooting in sputter deposi-

tion. Because of the interdisciplinary nature of the subject, any examination of the attributes of

deposition tool hardware is out of the scope of this discussion. Well-developed quality control sys-

tems in the sputtering target industry and specifications developed in the semiconductor industry

are intended to meet the requirements for trouble-free sputtering of films. However, it is not

uncommon to see occasional failure in meeting requirements and encountering troubleshooting in

semiconductor manufacturing facilities.

Here we briefly review most commonly seen problems as applied to DC magnetron sputtering

and then we expand them in the following sections. One of the major examples of troubleshooting

in the early life of a target is the long burn-in requirement before attaining desired film properties

such as sheet resistance (Rs), thickness uniformity and resistivity. Out of control inhomogeneity of

the sputtering material, inadequate surface finish and contaminations at the sputter surface can lead

to longer target burn-in. As sputtering progresses, sputtering targets may also start to produce

nodules in certain areas of the sputter face because of the re-deposition of the sputtered material.

These nodules can flake under the influence of thermal stresses and can produce particles on the

deposited films. Similarly, re-deposited material on the process kit can produce particles on depos-

ited films. This is a very common phenomenon seen especially in the reactive sputtering of

materials.

Pasting is a high power sputtering step in metal modes that cleans up the sputter surface and

also seals the re-deposited nodules with thin metal layers. If pasting is not done at recommended

intervals of time, flaking of re-deposited materials from the sputtering target and from the process

kit is likely to cause particle formation in the deposited films. Another significant source of in-film

particles is the occurrence of arcing inside the deposition tool. Arc generated at the embedded

dielectric inclusions in the sputtering target can lead to the formation of metal droplets (splats) and

particle formation in the deposited films.

567J. Sarkar: Sputtering Materials for VLSI and Thin Film Devices. DOI: http://dx.doi.org/10.1016/B978-0-8155-1593-7.00008-4

© 2014 Elsevier Inc. All rights reserved.

http://dx.doi.org/10.1016/B978-0-8155-1593-7.00008-4

These examples show that a wide variety of troubleshooting can take place from early-life to

the end-of-life of a target. The consequence of troubleshooting in sputtering is a possible reduction

in the process yields of users’ targets, while failure of delivering satisfactory targets to users is a

quality issue for target suppliers. In the case of troubleshooting that has originated from a defective

target, both the user and the target supplier work closely to resolve the issue with an emphasis on

the root-cause-analysis. Once the reason for failure is known, the necessary changes are brought

into the target manufacturing system to prevent it. Because of the proprietary nature of such trou-

bleshooting information, specific to the deposition tool, process recipe and the type of sputtering

target, we only discuss selective examples here that have already been discussed in the literature.

8.2 Long burn-in of sputtering targetBurn-in of a sputtering target is an essential but unproductive step in the sputter deposition of thin

films. Hence the semiconductor industry places a great deal of emphasis on the consistency of

burn-in performances of targets to keep the down-time to a minimum. There are examples in which

extension of burn-in time has been noticed due to the changes made by the target supplier of the

sputtering materials. Reduced burn-in sputter targets are also in demand for expensive sputtering

materials and advanced applications [1�5]. In the following sections, we discuss the role of indi-

vidual factors in the burn-in performance of a target in greater detail. It is important to recall that

the burn-in performance of a sputtering target is usually evaluated on the basis of time required to

achieve the controlled (within specification) deposition rate, film sheet resistance, film thickness

uniformity, film resistivity and in-film particle count. It should be noted that because of the proprie-

tary nature of these results, normally owned by the original equipment manufacturer (OEM), we

are not in a position to present enough results as a function of target life (in kWh) that would other-

wise illustrate the nature of problems seen at troubleshooting. As a result, primarily we have

addressed those issues that cause troubleshooting and reduce the process yield.

Table 8.1 A Summary of Components that are Known to Influence Sputtering Performance and

Process Yield

Sputteringtarget

Target metallurgy Material purity, microstructure and its homogeneity, crystallographictexture and the distribution of texture components, sputter facecondition (uniformity and depth of deformed layer)

Target design Thickness of target and backing plate materials, roughness of particle-traps (e.g., arc-sprayed region) around sputter face, proper bevel atthe sputter face edge

Target ferromagneticproperties

Pass-through flux and permeability

Processvariables

Recipe Programs for bake-out, burn-in, deposition cycles and pasting

Handling andmaintenance

Sputtering target, process kit

568 CHAPTER 8 Troubleshooting in Sputter Deposition

8.2.1 Inhomogeneous materialIn a strict sense, most sputtering materials do show some degree of microstructural and textural

inhomogeneity with respect to the shape and dimensions of the sputtering targets. These apply to

both high purity metals and compound sputtering materials. Inhomogeneity of the microstructure

and texture seen in sputtering materials primarily depend on the processing history, i.e., material

fabrication methods used and forming methods involved in the manufacturing process.

Inhomogeneities in the three dimensions can be manifested in the plane of the sputter face as well

as in the through-thickness direction.

It is important to remember that while a certain degree of material inhomogeneity can be toler-

ated for less advanced processes, the same degree of material inhomogeneity can be a risk factor

for advanced processes. Hence, based on the process requirements, acceptable control limits for

material inhomogeneity are always established by the target suppliers in conjunction with the users.

Here a specific example is presented in which we distinguish within specification and out-of speci-

fication Rs-uniformity values that resulted from magnetron sputtering of a relatively homogeneous

target and a rather inhomogeneous target.

Rs-uniformity values for deposited films are determined from sheet resistance maps developed

using commercial four-point-probe equipment (see the book’s companion website for details).

Figure 8.1 shows sheet resistance maps of tantalum bi-layers resulting from magnetron sputtering

of these two targets [6]. Identical recipes were used to run these tests up to the same sputtering life.

Note that the contours in Figure 8.1(a) show relatively small variation in sheet resistance from the

center to the edge, while the contours in Figure 8.1(b) show relatively large variation in sheet resis-

tance data. As a result, for more homogeneous sputtering target, the Rs-uniformity value is low

enough to meet the specification. In contrast, for a relatively inhomogeneous sputtering target, the

Rs-uniformity value exceeds the specification limit. The following types of inhomogeneities have

been identified for sputtering materials.

FIGURE 8.1

Sheet resistance maps of tantalum by-layers that show (a) variation in values from the center to the edge of

the wafer leading to “within specification” Rs-uniformity resulted from a relatively homogeneous sputtering

target and (b) large variation in values, reflected in more number of contours, from the center to the edge of

the wafer leading to “out of specification” Rs-uniformity resulted from inhomogeneous sputtering target [6].

5698.2 Long burn-in of sputtering target

8.2.1.1 Inhomogeneous grain structuresInhomogeneity in the grain structure can result from inherent properties of the material and also

because of the deficiencies in the fabrication and processing methods. In general, dilute ductile alloys

in the form of flat products show better homogeneity in grain structure than curved or hollow products.

For example, dilute aluminum alloys (e.g., Al-0.5 wt% Cu, Al-0.2 wt% Si-0.5 wt% Cu) show nearly

homogeneous grain structure after conventional processing using casting and thermo-mechanical pro-

cessing steps, while high purity titanium and tantalum may show inhomogeneous grain structures.

Electron-beam melting of titanium and tantalum are quite common and subsequently these are

thermomechanically processed to achieve the desired microstructure and texture. Figure 8.2 shows

an example of banding of grains in cold-rolled and recrystallized titanium [7]. With careful selec-

tion of thermo-mechanical processing steps and heat treatment schedules, it is possible to produce

titanium without significant inhomogeneity in grain structure.

Apart from material, complex-shaped products may pose challenges in attaining homogeneous

grain structure. Curved and hollow sputtering targets are good examples of this problem. For exam-

ple, hollow cathode magnetron targets are known to have significantly different grain structure in

the round corners of the dome-shaped configuration than the side walls. This is primarily because

the material in the vicinity of the corners is stretched more than the walls during metal-forming

operations and the subsequent recrystallization step often fails to produce the equiaxed microstruc-

ture at the corners. Though inhomogeneity in grain structure is seen in various sputtering materials,

acceptable control limits have been established by sputtering target suppliers in conjunction with

users. Unfortunately, no quantitative relationship between grain structure inhomogeneity and longer

burn-in is available at this point of time.

8.2.1.2 Inhomogeneous texture in through-thickness directionIn addition to the inhomogeneous grain structure, often through-thickness texture variation is also

seen in sputtering materials [8]. In fact, inhomogeneity in grain structure and through-thickness

FIGURE 8.2

Microstructure of titanium deformed to 70% and annealed at 700�C for 15 min showing grain size

inhomogeneity [7].


texture gradient can co-exist and the origin of the through-thickness texture variation lies in mate-

rial properties such as slip systems, stacking fault energy, deformation history and recrystallization

heat treatments. In a rolled and recrystallized product, it is common to see variation of volume frac-

tions of different crystallographic texture components as a function of depth from the surface to the

plane at half-thickness. A large volume of literature is available on the through-thickness texture

variation in aluminum alloys, tantalum and many other materials [8,9]. For example, Figure 8.3

shows the texture gradient from surface to the mid-plane in a high purity tantalum plate [8]. This

subject has been discussed in greater detail in section 5.6.1 of Chapter 5. Readers are suggested to

refer to crystallographic texture basics given on the book’s companion website for the interpretation

of orientation distribution functions (ODFs).

8.2.1.3 Inhomogeneous texture in the form of bandsTexture inhomogeneity can also be manifested in the form of banding in which similarly oriented

grains align in a particular direction, and these bands are usually separated by grains that have dif-

ferent crystallographic orientations. The alignment of grains originates from differential stretching

of grains and also selective growth of grains in specific orientations during recrystallization heat

treatment. Inherently, grains with certain crystallographic textures have greater resistance to

FIGURE 8.3

Two-dimensional orientation distribution function sections measured at different depths in a 4 mm thick

annealed tantalum plate. Texture gradient in through-thickness direction ranges from (001)[110]. texture at

the surface to (111), uvw. in the mid-plane of the tantalum plate [8].


deformation than neighboring grains. On subsequent recrystallization heat treatment, grains with

specific orientations may align in particular directions as seen in metallographic samples. This leads

to banding of grains and, in recent years, the electron backscattered method of crystallographic tex-

ture measurements has revealed these bands very clearly [8].

Electron-beam melted and thermomechanically processed high purity tantalum is known to have

significant texture banding, which is shown in Figure 8.4 [10]. The effects of microstructure and

texture inhomogeneity can be large enough to cause delay or complete failure in attaining desirable

film properties. This is the case for texturally inhomogeneous tantalum sputtering targets for

advanced chip making (below 65 nm node) and in particular for 300 mm wafer applications in

which tight control on film thickness uniformity is essential.

8.2.2 Surface characteristics8.2.2.1 Surface roughness and sub-surface deformed layerStandard sputtering targets are usually finished with precision machining or grinding followed by light

polishing depending on the nature of the metal or alloy targets. An average roughness (Ra) in the range

of 4�40 µinch is desired for the majority of production sputtering targets, and measurements are usu-

ally made at specific locations at the finished sputtering surface. It should be noted that in various

materials, it is difficult to achieve identical roughness at all parts of the target surface. In such cases

greater roughness is witnessed, with a relatively large Ra number, in the vicinity of the center in a

round target. This is primarily because of the wearing of the cutting tool (flank wear) with elapsed

time required for machining. Typically 5�30 minutes time is required for the final machining step and

the cutting tool starts to wear as it moves from the target periphery towards the center.

In the process of machining and grinding, a relatively thick deformed layer (e.g., as large as

40 µm in tantalum) is generated underneath the machine-finished surface [11]. A direct

FIGURE 8.4

Scanning electron micrograph showing the occurrence of banding of grains in longitudinal section of a

tantalum sample swaged to a true strain of 1:28 and annealed at 900�C for 30 min [10].


proportionality between the average roughness Ra and the depth of the deformed layer has been

established in materials such as tantalum, titanium, copper and aluminum [2]. Thickness of the

deformed layer was measured from peak broadening using x-rays.

Figure 3.10, in Chapter 3, shows the variation of the depth of the deformed layer as a function

of average roughness in tantalum sputtering targets [2]. The deformed layer retains large cold work

because of the high dislocation density and also residual stresses because of the severe plastic

deformation caused by the machining operations. The crystallographic texture in the deformed layer

is often significantly different than the bulk sputtering material. Hence, the deformed layer pos-

sesses different metallurgical characteristics as compared to the bulk sputtering material. It is

believed that, for certain metals, the deformed layer found at the sputter surface has a negative

influence on the burn-in performance and is an important issue for expensive sputtering material

and processes for advanced chip making. It has been shown that until the entire deformed layer

gets removed in the burn-in process or artificially removed in a specially prepared target, the

desired film properties are not achieved [3].

It is worth noting that the early material removal from the sputtering face of a flat and round

target usually starts from narrow concentric rings, which gradually spread to much wider erosion

grooves (also known as racetracks). This indicates that, for certain processes, the severely

deformed and greater roughness concentric areas found in the vicinity of the center (with

thicker deformed layer) of the sputter surface is first exposed to plasma during burn-in operations,

and this in fact may lead to a situation where burn-in requirement would be even more severe.

This may not be the case for those targets that have deepest racetracks towards the periphery.

For certain types of sputtering targets, it has been shown that if the deformed layer is removed

completely and the roughness is maintained small, it would be possible to achieve reduced

burn-in [3].

A systematic study has been conducted taking into account the surface roughness, the thickness

of the deformed layer, contamination level and their relationships to the burn-in performance of

titanium sputtering targets [2]. Various types of surface finishes were achieved by using a combina-

tion of two different machining methods (ordinary and precision), wet polishing and chemical etch-

ing techniques [3]. Reduction in Rs uniformity and an early saturation of deposition rate were

recorded because of the removal of the deformed layer [3]. Figure 8.5 shows the variation of depo-

sition rate with sputtering life for sputtering targets with different finishes [3]. It is obvious that tar-

get 4, prepared using precision machining, wet polishing and a chemical etch, shows a large

reduction in burn-in, while others do not.

It should be noted that both target 4 and target 5 have no deformed layers and target 5 has high-

er roughness than target 4. Interestingly, target 5 does not show a large reduction in burn-in though

it is free from the deformed layer. This result implies that after removing the entire deformed layer

from the sputter surface if the roughness remains high, a significant reduction in burn-in may not

be achieved. It should also be noted that these types of surface preparations are relatively complex

and in each case the entire process is expected to take several hours. Disposal of acid waste is com-

plicated and expensive. Hence, there have been efforts to prepare reduced burn-in sputtering targets

using dry methods such as ion cleaning, and this led to the development of techniques that are inex-

pensive and environment friendly [5,12,13].

One of the studies on titanium shows that it is not necessary to remove the entire deformed

layer, but the removal of the contaminants from the sputter surface and transforming the deformed


layer into a soft layer can cause an equivalent effect, i.e., a significant reduction in the burn-in with

improved particle performance [12,13]. During preparation of this manuscript it was noted that

only few target manufacturers could supply reduced burn-in titanium and tantalum targets to chip

manufacturers.

8.2.2.2 ContaminationsIn addition to the surface roughness and deformed layer at the sub-surface, sputter surface contami-

nation may play a role in burn-in performance of a sputtering target. In various sputtering targets,

final shiny finish is achieved by applying polishing steps after completing final machining step.

The polishing media can be emery papers as well as abrasive material reinforced polymeric media.

In both cases, abrasive materials are fine ceramic particles either bonded to the paper or to the

polymeric media. During final polishing sputtering targets may retain these fine ceramic particles

and also organic compounds from the polishing media. These ceramic particles and organic com-

pounds are known to cause severe arcing and out-gassing, respectively.

The other source of organic contamination is the plastic bag used in the packaging of the sput-

tering target. The thickness of organic compound layer can be several mono-layers thick and may

lead to degassing problems if a suitable packaging material is not used. In some cases, moisture

pick up by the plastic bags can cause stain generation on the sputter surface as well as on the back-

ing plates. Though some of these issues may not be as critical as the particle contamination, sput-

tering targets with even minor cosmetic differences are rejected by the users. This helps users to

keep the production schedule and quality in control and meet the process yield.

FIGURE 8.5

Variation of deposition rate with sputtering life (power consumed) for sputtering targets with different surface

finishes [3].


8.3 In-film defect generationDefects such as particles and splats (submicron to tens of micron in size) are known to influence

the process yield in chip, flat panel display and other thin film device manufacturing. Defects are

generated at different steps of a manufacturing process, but we focus on the in-film defect genera-

tion during sputtering steps. Primarily defects are caused by the flaking of re-deposited nodules

from the target itself, flaking of deposited films from the process kit (chamber shield and clamping

ring) and also by arcing activities [14�30].

A variety of sputtering materials such as tungsten�titanium, titanium, indium�tin�oxide and

aluminum alloys show particle problems [14�30]. Figure 8.6 shows a W-Ti particle capable of

causing bridging of metal lines leading to short circuit in an integrated circuit [15]. The severity of

the defect formation depends on the quality of the target material, design of the target and also on

process variables such as deposition rate, maintenance/replacement schedule of the process kit, etc.

In order to keep the interpretations simple, we draw some examples from the studies carried out on

the tungsten�titanium alloys (W-Ti), indium tin oxide (ITO) and aluminum alloys used in the

VLSI and flat panel display applications, respectively [14�30].

8.3.1 Flaking of nodules from the sputtering targetIn-film defect formation due to the flaking of nodules from the sputter surface has the following

steps: (i) nucleation and growth of nodules on the target surface because of the re-deposition of

sputtered materials, (ii) disintegration of nodules because of arcing and (iii) the flaking of nodules

from the sputter surface [25]. Detailed studies on the W-Ti alloy systems have disclosed that nucle-

ation and growth of the nodules preferentially take place in the vicinity of the voids in a target that

is less dense [25].

Figure 8.7 shows less than 1 µm size nodules around a void in a W-10 wt% Ti alloy target that

is 95% dense [26]. In addition, surface imperfections such as crevices and cracks also favor nodule

formation, and shape of the nodules can be remarkably different. These imperfections result from

machining and grinding operations. Evidence of nodule formation have been found on those parts

FIGURE 8.6

Ti-W particle causing bridging of metal lines [15].

5758.3 In-film defect generation

of the sputter surface where magnetic fields are weak. This means that the areas other than erosion

grooves are preferential sites for nodule formation. In round targets, these areas are usually found

close to the periphery and the center of the sputter face. This implies that there would be a differ-

ence in the nodule density and the size distribution along the radial direction from the center to the

periphery.

Figure 8.8 shows the variations of nodule density as a function of radial distance from the center

to the periphery for three W-Ti alloy targets that have different densities (94% for target 3 and

86.4% for target 5), i.e., different void contents [25]. It is clear that the largest densities of nodules

are found close to the periphery of the sputter face followed by the center, while nodule densities

are at their minimum in the erosion grooves. On the contrary, nodule sizes are larger at the bound-

aries of the erosion groove and the smallest at the periphery (Figure 8.9) [25]. These nodules either

undergo fracture or flaking that form particles on the deposited films.

Figure 8.10 (p. 578) shows a nodule that has fractured [25]. Similar observations have been made in

indium�tin�oxide sputtering targets. Figure 8.11 (p. 578) shows two fully grown nodules in which

one of the nodules has the tip intact and the other one has lost the tip because of the fracture [27].

Unlike voids in W-Ti alloys, indium�tin�oxide sputtering targets are sensitive to the distribution of

the oxide phase (SnO2), which has been found to cause arcing during sputtering. With improved metal-

lurgical practices, the distribution of tin oxide can be made uniform and diffusion of Sn into In2O3 can

be completed. Figure 8.12 (p. 579) shows two indium�tin�oxide targets with varying amount of

nodules because of the difference in the target metallurgy. Target (a) has more uniform distribution of

tin oxide than in target (b) and shows 6 times less arcing than the target (b) [28].

8.3.2 Flaking of brittle films from the process kitSputtering materials get accumulated on process kit components such as the chamber shield and the

clamping ring during sputter deposition. In fact, the large surface area of the chamber shield

FIGURE 8.7

SEM image showing W-Ti re-deposited nodules in the vicinity of voids in a sputtered W-10 wt% Ti target [26].


initially helps capture dust and particles inside the chamber. With continued sputtering, in particular

in reactive sputtering, highly stressed layers of sputtered material are deposited on the chamber

shield. These stressed layers can flake easily unless pasting (applying pure metal coating) is done

at regular intervals. In spite of pasting, certain materials such as TiN, TaN, W-Ti layers may flake

FIGURE 8.9

Variation of nodule size along the radial direction in a W-10 wt% Ti target after 27 kWh sputtering [25].

FIGURE 8.8

Nodule density distributions along the radial direction in W-Ti targets after 27 kWh sputtering [25].


under the influence of chamber conditions and generate particles on the deposited films. In order to

reduce particle formation from flaking of brittle layers, the chamber shield needs to be replaced at

OEM recommended intervals of time. These clearly show that particle performance of a process is

controlled by both flaking of nodules from the sputtering target itself and the flaking of sputtered

layers from the process kit.

8.3.3 ArcingArcing is a common phenomenon in sputtering, and in this section we first discuss the role of mate-

rial imperfections on the arcing problem, and as a result, defect formation in deposited films.

FIGURE 8.10

SEM image of a broken nodule from W-Ti sputtering target [25].

FIGURE 8.11

Two fully grown nodules at the sputter face of an indium-tin-oxide sputtering target. The left nodule shows a

particulate at the tip (x 250) [27].


Figure 8.13 shows these imperfections (dielectric inclusions and films, voids, entrapped gases, con-

taminations, etc.) that are of metallurgical origin and can be reduced by improved manufacturing

practices [16]. The other major issue is the prevention and suppression of arcing using advanced

power supply units [17].

Arc is a general term used for representing a high power density short circuit. A possible cause

of arc generation is the rapid accumulation of charge on a small area (often dielectric films and

inclusions) at the sputter surface. An arc resembles a miniature explosion and can cause local

FIGURE 8.12

Indium-tin-oxide sputtering targets showing nodule contents in (a) conventionally processed target and (b)

improved target [28].

FIGURE 8.13

Sputtering target showing undesirable attributes that are of metallurgical origin and may cause arcing [16].


melting of the sputtering material inside a deposition chamber. The molten material ejected from the

target surface can form particles and splats on the deposited film (Figure 8.14) [23]. Pareto analysis

of chip manufacturers shows that arc-related defects reduce the overall yield of deposited wafers.

Arcs are broadly classified into three groups based on the mechanisms of formation;

namely, microarcs, unipolar arcs and bipolar arcs (Figure 8.15) [30]. Microarcs (also known as

FIGURE 8.14

A splat on the deposited film caused by the arcing at aluminum oxide particles embedded in an aluminum

sputtering target [23].

FIGURE 8.15

Types of arcs that are generated during sputtering: (a) Microarcs, (b) unipolar arc and (c) bipolar arc [30].


plasma-target/cathodic arcs) are ignited between the plasma and the dielectric parts of the target,

which are often native oxide films (Figure 8.15(a)). Arcs are caused by the electric breakdown of

the dielectric films on the target surface; the mechanism of breakdown is explained in Figure 8.16

[16]. The accumulated positive charge on the front face of dielectric film develops a large voltage

difference across the film because the back of the film is in contact with a negatively charged tar-

get. A breakdown occurs when the voltage across the film (the same as in a capacitor) exceeds the

dielectric strength of the film. As a result, plasma discharges to a point where charge builds up

(inevitably dielectric regions) and electric field is favorably distorted to allow shorter path for

grounding to occur. This is equivalent to the bridging of the cathode dark space. A majority of arc-

ing events, as large as 99%, are caused by the microarcs [30].

Unipolar arcs are ignited between conducting sputtering material and the dielectric parts of the

target (Figure 8.15(b)) [30]. Dielectric materials can be oxide inclusions as well as re-deposited

materials in the vicinity of the periphery and the center of the sputtering face. Formation of AlOx

type amorphous oxide and TiN has been reported in the reactive sputtering of aluminum oxide and

TiN films, respectively. The re-deposition is favored by the weak magnetic fields in these areas. It

should be noted that the discharge takes place between the dielectric and the conducting spots on

the target itself unlike between plasma and non-dielectric films in the microarc case. Unipolar arcs

can initiate microarcs, and in the literature these are often used interchangeably.

Bipolar arcs are generated between the sputtering target and the shield/substrate (Figure 8.15(c))

[30]. Strong electrical instability in bipolar arcs can cause complete shutdown of the system. The

frequency of occurrence of bipolar arcs is relatively low and initiated by the bridging flakes or

improperly centered hardware.

FIGURE 8.16

Electrical breakdown of a dielectric film found on the sputter surface and arc generation [16].


The probability ratio of arcing for bipolar, unipolar and microarcs is reported to be 1:10:100000

[30]. In the ranking of energy associated with each type of arcing, microarcs are placed on the

lower side, while bipolar arcs are on the higher side (Emicroarcs,Eunipolar,Ebipolar). The above

information provides a basic understanding of the various types of arcs that are encountered in sput-

tering systems. In order to reveal the role of target metallurgy on the formation of arcs, we provide

some examples.

First, we will discuss material imperfections (dielectric inclusions and films, voids, surface pro-

trusion, entrapped gases) that can affect in-film defect formation in the form of particles or arcs.

Subsequently, on a more fundamental basis, the impact of size and shape of dielectric inclusions on

the arc formation will be discussed.

It is a fact that the level of imperfections in the sputtering targets may vary based on the target

manufacturer and even from one lot to the other supplied by the same manufacturer. Fine dielectric

inclusions or films have their origin at skin oxides formed at the surface of the starting material, which

subsequently disintegrate and distribute during melting practices and also metal-forming operations.

Voids and entrapped gases result from the solubility difference of gases in the molten and solid states.

For example, higher solubility of hydrogen in molten aluminum than in solid aluminum may cause

entrapment of hydrogen gas in the form of small voids or porosity in the solid ingots.

Studies on aluminum sputtering targets with different levels of such undesirable metallurgical

attributes reveal that inclusions and voids are the key sources of splats and particles. Splats as large

as 500 µm in size and particles in the range of submicron to tens of micron were recorded. A signif-

icant reduction in in-film defects was achieved by improving the metallurgical practices in target

fabrication processes. Figure 8.17 shows the variation of in-film defects as a function of sputtering

life (in kWh) for a standard target and an improved aluminum target [16].

FIGURE 8.17

Defect-density versus life of sputtering showing reduced defects in improved target as compared to the

standard target [16].


While we recognize the relative contributions of more than one metallurgical properties to the

in-film defect formation, only a few systematic laboratory scale studies have been carried out to

record the impact of shape and size of the dielectric inclusions on the arc generation and defect for-

mation on the deposited films [22,23]. Attempts have been made to measure the velocity of the

ejected particles, preferred angles of ejection and temperature of the molten droplets. In these stud-

ies either pure aluminum targets or Al-0.5wt% Cu alloy targets have been used and aluminum

oxide inclusions of chosen size and shape were embedded on the racetracks.

Figure 8.18 shows such a target (7.6 cm diameter), which has an aluminum oxide inclusion

embedded in the racetrack [22]. The size of the aluminum oxide inclusion varied between 100 µmand 3000 µm and various power densities (8�56 W/cm2) were used to record the in-film particle

density caused by the arcing [23]. The arc rate was calculated by dividing the number of arcs with

the deposition time (1 min for all the experiments). Figure 8.19 reveals the linear relationship

between the aluminum oxide inclusion size and the arc rate at various power densities [23]. For the

given set of experimental conditions, larger oxide inclusions showed a steady increase in the arc

rate at higher power densities. As a result, in-film particle density rose steadily up to a value of

380 per cm2 for a very coarse oxide inclusion case . For a given particle size (2940 µm), increased

power density enhanced the number of particles in all size categories and more than 60% of the

particles were less than 1.1 µm size (Figure 8.20) [23].

A critical inclusion size for the initiation of arcing was estimated to be 4406 160 µm and also

argued that this size effect was only related to the condition for initiation of arcing and not related

FIGURE 8.18

Schematic of a laboratory scale aluminum alloy sputtering target that has aluminum oxide inclusion

embedded in the racetrack [22].

FIGURE 8.19

Relationship between oxide inclusion size and the arc rate at various power levels [23].


to the condition required for the ejection of molten metal from the sputter surface to form particles.

It was believed that the force exerted by any arc was strong enough to overcome the surface tension

of the molten aluminum at sputter surface to generate particles. This study also established relation-

ships between arc rates and the in-film particle densities (Figure 8.21) [23].

These results also imply a saturation of particle density with increased arc rate. The role of alu-

minum oxide inclusion area, shape and orientation on the arcing and particle generation was also

reported [23]. The existence of a relationship between the plasma dark-space distance and the diam-

eter of a circle having the same surface area as that of an oxide inclusion was also proposed. It was

reported that below a critical inclusion area, no arcing activity was recorded. When the inclusion

length exceeded twice the dark space distance, the orientation of the inclusion with respect to the

racetrack tangent played an important role in arc generation. Elongated inclusion placed perpendic-

ular to the racetrack tangent showed greater tendency of particle formation than the inclusion

placed parallel to the racetrack tangents.

Though the above experimental results provided a great deal of information about arcing char-

acteristics, the quoted oxide inclusion diameter of 520 µm6 110 µm corresponding to the critical

surface area requirement for the initiation of arc appears unrealistically large and it is believed that

FIGURE 8.21

Relationship between the arc rate and particle density in the deposited films [23].

0.5 0.75 1.1 1.6 2.4 3.6 5.4 8.1 12Particle size (µm)

020406080

100120140160180

Par

ticle

den

sity

(cm

–2) AI2O3 Inclusion:

2940 um SputteringPower density

(W/cm2)8

16 3224

FIGURE 8.20

Particle size distributions in the deposited film after sputtering at various power densities. Particles are

generated by an aluminum oxide inclusion (2940 µm in size) embedded in the sputtering target [23].


even much smaller inclusions could initiate arcing. For example, in aluminum alloy sputtering tar-

gets for flat panel display applications, 10 µm diameter inclusions were found to be large enough to

cause arcing and particle generation [29,31].

Studies involving high-speed video analysis of arcing revealed some characteristics of ejected

molten droplets such as velocity, trajectory and temperature [22]. Velocities from 5 to over 500 m/

sec were recorded for aluminum droplets, and it was not possible to verify size dependence of

velocity as found in the case of copper [32]. The ejection angles of molten aluminum droplets con-

centrated around 30� from the target surface (Figure 8.22) [22]. The temperature range of the dro-

plets was estimated to be between 1750 K and 3000 K6 1000 K. Other works on copper reported

temperatures between 2000 K and 3200 K [33,34].

It has long been realized that though arcing can be reduced by improving target design, metal-

lurgical practices and also by optimizing the process variables, modifications are required in the

design of power supply. Over the last few decades emphasis has been placed on the detection and

suppression of the arcs and yet improvements have been required with the evolution of the

advanced deposition tools. The subject of arc detection and suppression is out of the scope of this

discussion and details can be found elsewhere [35�38].

8.3.4 Trace elements of sputtering targetHigher trace element content of a sputtering material can cause excessive particle formation during

sputtering and one such example is high purity titanium with higher oxygen content. In a produc-

tion environment, the number of in-film particles continues to increase with sputtering life (kWh).

In order to keep the in-film particle content under control, pasting of chamber shield and process

kit change are required at recommended intervals of time.

It has been shown that by lowering the oxygen content of the titanium, it is possible to maintain an

in-film defect level significantly low up to a large sputtering life [39]. This is primarily because of the

reduced oxygen available to the titanium to form titanium oxide. In various other sputtering materials,

trace elements can be of different chemical nature (e.g., gases, alkali, transition and noble metals) that

may have a tendency to cause arcing in more direct ways or form undesirable inclusions that may lead

to arcing and generation of particles.

0 15 30 45 60 75 90Angle (Degree)

0

5

10

15

20

25

30

No.

of m

acro

-pat

icle

s

90°

0°Target

FIGURE 8.22

Molten metal ejection angle distribution for aluminum alloy sputtering target [22].


8.3.5 Re-deposited material in hollow cathode magnetron (HCM) targetsThe same as flat sputtering targets, hollow cathode magnetron targets also suffer from particle pro-

blems. The primary source of particles is the re-deposited material. If re-deposited material fails to

stick to the target material properly, re-deposited material will starts to flake under the influence of

cyclic thermal stresses. Figure 8.23 shows an example of a re-deposited area and the phenomenon

of flaking in a titanium HCM target [6]. The shiny and relatively smooth regions are the

re-deposited areas, which in this case extend to the side wall up to a height of one-third of the verti-

cal height of the HCM. The boundaries between the sputtered and re-deposited regions often show

flaking of re-deposited material. Figure 8.24 shows an example of nodule formation because of

re-deposition in a copper HCM target [6]. The structure of re-deposited material in a HCM target

can be drastically different than what we have seen in flat sputtering targets.

FIGURE 8.23

Concentric shiny areas inside a sputtered titanium HCM target showing re-deposited material. Flaking of

re-deposited material is also visible in the inset [6].

FIGURE 8.24

A sputtered copper HCM target showing re-deposited nodules at the edge of the target [6].


8.3.6 Contaminated particle-traps of the sputtering targetsWe know that various sputtering targets contain particle-trap zones in the form of grit-blasted or

arc-sprayed areas next to the sputter surface [40]. Such rough zones have a large surface area that

helps capture particles from the chamber and improve particle performance of the process.

However, if such regions are contaminated during subsequent finishing steps and handling, these

can be a major source of arcing and particle formation on the deposited films. For example, during

initial cleaning operations arc-sprayed regions may pick up fine cotton threads and dust particles

because of improper handling. Therefore, extra caution is required for the preparation of particle-

trap regions around the sputter face and handling of sputtering targets.

8.4 Bonding-related problemsA large variety of sputtering targets are bonded to appropriate backing plates using resins or elas-

tomers, solders and diffusion bonding methods (see section 4.5 of Chapter 4 for details). While

bond coverage and bond strength values are expected to be satisfactory, in many instances targets

fail to meet the requirements because of incomplete bonding. In the case of solder bonding, if

both target and backing plates are not flat, bond coverage can be significantly low and conse-

quently bond strength would be low. Other reasons include inadequately prepared surfaces, insuffi-

cient pre-coating, applying incorrect bonding temperature and handling problems. Figure 8.25

shows a bond coverage map developed using an ultrasound technique [6]. The central patch

FIGURE 8.25

Ultrasound map showing partial debonding at the target�backing plate interface in a titanium target [6].

5878.4 Bonding-related problems

clearly shows the severity of incomplete bonding. In some cases, defective targets made using sol-

der bonding can be re-bonded with proper measures.

Similarly, bonding defects are also found in diffusion-bonded and resin/elastomer-bonded tar-

gets. In the case of diffusion bonding, meeting surfaces of the target and backing plate are often

prepared using one of the grit-blasting methods to impart adequate roughness. Failure to obtain

desired roughness and sufficiently clean surfaces may lead to incomplete bonding. Minor bowing

of the target and also the backing plate do not pose a severe bonding problem because bonding

takes place under high pressure.

Figure 8.26 shows debonding of a diffusion-bonded titanium target [6]. The sputtering target

appeared to have weakly-bonded regions in the vicinity of the periphery. Separation of the titanium

target and backing plate developed during use under operating power of several kWs. It is worth

noting that such separation in the form of voids stops heat transfer from target to the backing plate.

This leads to abnormal temperature rise at the sputter face. As a result severe grain growth can

occur in target material, which is the case in this example. The shiny regions in the diametrically

opposite positions showed larger grains. Other extreme examples of cone formation because of

selective debonding of target material from the backing plate and the development of cracking

at the sputter face because of creep-related issues are shown in Figure 8.27 and Figure 8.28,

respectively [6].

FIGURE 8.26

A sputtered titanium target showing partial debonding at the target�backing plate interface. Shiny regions

showed large grains because of grain growth driven by heat build-up [6].


FIGURE 8.27

A tantalum target showing partial debonding at the target�backing plate interface and cone formation at the

sputter surface [6].

FIGURE 8.28

A titanium target showing crack formation in the vicinity of the center during sputtering [6].

5898.4 Bonding-related problems

8.5 Long pump-down time and out-gassingIt is not uncommon to encounter long pump-down time to reach the base pressure in a deposition

tool. Pump-down in sputtering tools usually consists of a rough pumping stage, leak check using

helium, cycle purge using nitrogen, bake-out at high temperature for several hours, cooling and rate

of rise check (air leakage while pump is off) before burn-in of targets start. Evidence shows that

the pump-down time can be long and rate of rise can be large as a result of unclean and out of con-

trol rough o-ring grooves of the sputtering targets. With a smoother and cleaner o-ring groove,

pump-down time has been found to meet specification. It is also recognized that if the density

of the sputtering target is not high enough and the sputter face of a target is contaminated because

of the usage of inappropriate packaging material or mishandling, pump-down time can be longer

than usual.

Once the chamber is ready for burn-in of the sputtering target, power is applied to the sputtering

target in steps to reach the desired power level. At this stage, the sputtering target starts to eject

material from the sputter surface and plasma starts to see the various inhomogeneities such as

inclusions, voids and entrapped gases in the target. Occurrence of prolonged out-gassing has been

reported during burn-in of sputtering targets. In most cases, powder metallurgy processed targets

have shown out-gassing. In the fabrication of sputtering targets, during mixing of powders, con-

taminants can be incorporated into the raw material if the container and the grinding media are not

clean or previously used for a different product. On subsequent processing steps, these contami-

nants may produce gaseous products that would be entrapped into the sputtering target. Similarly,

incomplete removal of the binders in the fabrication of sputtering targets can lead to the generation

of gases that can be entrapped into the sputtering target. During out-gassing these gaseous products

are removed in a low-pressure chamber condition. Though the out-gassing problem is less common,

long pump-down time can be an issue if cleanliness is not maintained at all stages of handling and

installation of the sputtering targets into the deposition chamber [39,41,42].

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