Brush Scrubbing for Post-CMP Cleaning

Chapter 4

Brush Scrubbing for Post-CMP Cleaning Ting Sun1, Zhenxing Han2 and Manish Keswani3 1

Sichuan Normal University, Chengdu, Sichuan, China, 2Micron Technology Inc., Boise, ID, United States, 3University of Arizona, Tucson, AZ, United States

Chapter Outline 1 Introduction 2 Particle Removal Mechanism 3 Process and Tool Kinematics 4 Consumables 4.1 Brush 4.2 Chemical Formulations

109 110 112 115 115 120

5 Related Issues 122 6 Summary 124 Appendix: Application of Tribology to Post-CMP Brush Scrubbing 125 References 131

1 INTRODUCTION In integrated circuit (IC) manufacturing, planarization techniques such as thermal-flow, sacrificial resist-etch back and spin-on glass are inadequate to achieve planarity for an interconnect system with more than three metal layers [1]. These processes provide only a limited degree of local planarization (i.e., on the micrometer scale) and are not capable of achieving global planarization (i.e., on the centimeter scale). Driven with the planarization challenge in IC manufacturing for denser transistors and more metal layers, IBM developed chemical mechanical planarization (CMP) in the mid-1980s based on the conventional polishing technique. Both chemical and mechanical actions are simultaneously involved during CMP to selectively remove the exposed material from elevated features, resulting in a wafer surface with improved planarization of interlevel dielectrics and metal layers [2,3]. Since its inception, CMP has become an enabling technology for manufacturing ICs, including microprocessor chips, memory circuitry, data storage devices, communication chips, graphic chips, and R. Kohli & K.L. Mittal (Eds): Developments in Surface Contamination and Cleaning, Vol 9. DOI: http://dx.doi.org/10.1016/B978-0-323-43157-6.00004-5 © 2017 Elsevier Inc. All rights reserved.

109

110  Developments in Surface Contamination and Cleaning

various specialized application chips. Moreover, to remove the slurry residuals and other contaminants, a cleaning process is required after CMP to achieve a defect-free wafer surface. Brush scrubbing is widely accepted in post-CMP applications as a viable cleaning option for next-generation technology due to process flexibility, single-wafer processing configuration, and reduced cost of ownership (COO) [4]. In brush scrubbing, cleaning is based on direct contact between a soft polyvinyl alcohol (PVA) brush and the wafer surface in which the brush asperities engulf the wafer surface contamination, while the rotating motion of the brush, as well as the cleaning fluid at the surface, dislodge and carry the particles away from the wafer. As such, the cleaning performance of brush scrubbing not only depends on the choice of chemistry, tool kinematics, and type of process equipment [5–9] but also on the mechanical properties of the brush itself [10,11]. In this chapter, a review of the brush scrubbing process with a focus on the important aspects of cleaning mechanism, tool kinematics and consumables is provided.

2  PARTICLE REMOVAL MECHANISM Nanoparticles are widely used in CMP slurry for ensuring precise material removal and local or global uniformities. Removing the residual particles becomes the main concern during post-CMP cleaning [1–3]. To understand how the particles (from slurry residue or other sources) are removed from the wafer surface during brush scrubbing, one first needs to understand the adhesion of the particle to the wafer surface. The main adhesion forces are believed to be van der Waals’ force, capillary force, and double-layer interactions [12–19]. Detailed discussion regarding particle forces can be found elsewhere [17–19]. These adhesion forces keep the particle adhered to the wafer surface. During the brush scrubbing process, the combination of brush compression and rotation in the environment controlled by certain cleaning solution chemistry generates so-called removal forces. When the removal forces are adequate to overcome the adhesion forces, the particle will be dislodged from the wafer surface. As shown in Fig. 4.1, a typical brush roller with cylindrical nodules is continuously compressed against the wafer surface during scrubbing. Direct contact between the PVA brush and the adhered particle ensures mechanical scrubbing, which is one possible removal force. A cleaning solution is applied throughout the scrubbing process. Therefore, the hydrodynamic forces also need to be considered. Significant experimental and modeling efforts have been focused in this direction to understand the removal forces. During scrubbing, the surfaces of the wafer and the brush slide on each other, affecting the tribological characteristics of the process. Fundamentals of applications of tribology to scrubbing can be found in the appendix. The lubrication mechanism during scrubbing is believed to be boundary or elastohydrodynamic, which means that the brush is always in contact with the wafer surface [20]. The induced shear force from this direct contact is essential to the

Brush Scrubbing for Post-CMP Cleaning  Chapter | 4  111

FIGURE 4.1  Schematic of brush–particle–wafer contact.

removal of particles. The magnitude of the friction force may be large enough to move relatively large particles (>0.1 μm) away from the wafer surface [21]. The hydrodynamic forces have been calculated and considered inadequate in overcoming the adhesion forces. Use of chemical formulations with specific additives for manipulating the electrostatic forces is generally required for particle removal [22]. A more thorough model has been developed by employing both contact model and lubrication hydrodynamic theories to analyze the fluid film between the soft, porous brush nodule surface and the flat, hard wafer surface. The findings support early theories that the combined shear force due to direct contact between the brush and the wafer and the hydrodynamic dragging force provides the removal mechanism that works against the adhesion forces dominated by the van der Waals’ force. Chemicals in the cleaning solution can weaken the adhesion through electrostatic interactions [23–25]. Experimental study, employing a fluorescence technique and particle-tracking velocimetry for in situ characterization during scrubbing clearly detects both frictional and hydrodynamic removal of the particles and leads to a three-step removal process—i.e., moving start (particles detach from the wafer surface when the removal forces are large enough to overcome the adhesion forces), moving along the substrate (particles are still in close neighborhood of the wafer surface but are rolling or sliding), and breaking away from cleaning surface (particles are finally moving away from the wafer surface into the bulk of the cleaning fluid). Process parameters (such as down force and brush-rotation speed) and the addition of surfactant strongly impact particle removal during scrubbing [26,27]. Keeping these findings in mind, the improvement of cleaning performance requires consideration of brush design, cleaning chemistry, tool design, and process tuning, which will be discussed in the following sections.

112  Developments in Surface Contamination and Cleaning

3  PROCESS AND TOOL KINEMATICS The tools used in brush scrubbing (scrubbers) are designed to accommodate different types of polishers. Currently available tools or scrubbers are either single-sided (brush in contact and scrubs the front side of the wafer only) or double-sided (brush scrubs both sides, front, and back of the wafer simultaneously as illustrated in Fig. 4.2). The latter type is more commonly used these days. The introduction of the cleaning solution can be either through a spray-on process (solution comes into contact with the wafer and the brush simultaneously) or through brush core delivery (absorbed by the brush and then applied to the wafer) [29]. To improve the tunability of a scrubber, two cleaning-solution delivery mechanisms are often both available in one tool. Since the role of friction is one of the key factors impacting cleaning performance, the applied down force and pressure of a brush need to be adjustable automatically, and a torque monitor is usually utilized in a scrubber [29]. Moreover, the brush is normally set at the center of the wafer substrate (Fig. 4.3) due to the design convenience and the mechanical stability of the tools. A study on the aspect of the contact trajectories between the brush nodule and the wafer surface shows a center-heavy and localized contact feature of this commonly used configuration (Fig. 4.4). Combined with fluid dynamics analysis, a new configuration—eccentric scrubbing—has been proposed and compared for its effectiveness in removing particles [28]. As a matter of fact, novel scrubbers provide an adjustable brush-roller holder to allow certain amount of tuning in terms of brush–wafer contact concentricity, which makes the concentricity a process parameter. Brush oscillation during scrubbing is also often employed in manufacturing, which is

FIGURE 4.2  Schematic of double-sided scrubbing. (Reprinted from Ref. 28).

Brush Scrubbing for Post-CMP Cleaning  Chapter | 4  113

FIGURE 4.3  Illustration of concentric (left) and eccentric (right) scrubbing. (Reprinted from Ref. 28).

FIGURE 4.4  Contact trajectories of brush nodules and wafer surface for concentric (left) and eccentric (right) scrubbing. (Reprinted from Ref. 28).

another way to ensure uniform scrubbing. Other details need to be considered, especially those that might impact the electrical characteristics of the product when designing a scrubber. A product yield drop has been reported to be related to the charging mechanism on the scrubber and recovered by improving the scrubber alone [30]. Tool kinematics involving parameters such as down force (pressure), rotation speed (wafer and brush), and cleaning-solution flow rate are believed to be of immense importance for achieving superior scrubbing performance. By employing fluorescence spectroscopy, movement of particles during scrubbing can be visualized and analyzed [27]. As shown in Fig. 4.5, the particle removal rate is clearly a strong function of down force, brush speed, and composition of the cleaning formulation (to be discussed in Section 4.2). Since the main removal mechanism comes from the friction force (through direct contact) and hydrodynamic

114  Developments in Surface Contamination and Cleaning

FIGURE 4.5  Effect of down force, brush-rotation speed, and surfactant on particle removal. (Reprinted from Ref. 27).

dragging force (through fluid dynamics), shear-force analysis can shed light on the effect of tool kinematics on scrubbing performance. The work of Gu and coworkers [31,32] showed strong correlation between product defects (in terms of scratches and device electrical characteristics) and scrubbing tribological behavior (refer to the appendix for fundamentals), suggesting that high brush-rotation speed and low down force are desired. Investigation of tribological attributes of scrubbing yields strong dependence of the coefficient of friction (COF) on down force and no significant dependence on cleaning-solution flow rate. Also, the contact area of the brush and the wafer has been found to be determined by down force and brush-rotation speed [31–33]. In summary, the general parameters that can be used for scrubber tuning are shown in the following. 1. Down force (applied pressure): higher down force leads to higher contact area and COF. 2. Brush-rotation speed: faster rotation increases not only hydrodynamic dragging but also brush–wafer contact area and COF. 3. Flow rate of the cleaning solution: this is believed to exhibit a threshold, ensures effective scrubbing performance, and has no significant impact on COF or on performance once the threshold is met, but it is of importance in terms of COO. 4. Wafer rotation speed: often set to be different from brush rotation to ensure uniform coverage. 5. Brush oscillation: often employed to ensure uniform coverage.

Brush Scrubbing for Post-CMP Cleaning  Chapter | 4  115

The scrubbing process is another important regime in cleaning design for post-CMP application. Two-stage scrubbing, with the first stage being wet treatment using ozonized water and hydrofluoric acid and the second one being brush scrubbing, results in higher particle removal in comparison to a singlestep scrubbing in post-Si CMP cleaning [34]. A cleaning process combining buffing with dilute HNO3–benzotriazole (BTA) aqueous solution and the brush scrubbing process is shown to be effective for colloidal silica removal in post-Cu CMP application [35]. Another so-called hybrid cleaning process that combines acidic and basic cleaning in sequence has been developed and demonstrates the advantages of both acidic and basic cleaning and achieves superior performance in reduction of defects, including polish residues, foreign materials, slurry abrasives, and scratches, compared to an all-basic brush cleaning process [36,37]. In manufacturing, various brush scrubbing processes have been designed to accommodate a certain post-CMP cleaning demand. In general, the scrubbing process in use these days can be separated into four steps: 1. In prerinse, a certain cleaning chemistry is used to treat the wafer surface before mechanical scrubbing with a PVA brush. 2. In scrubbing, a brush comes into contact with the wafer surface under the same or a different chemical environment as that of the prerinse step (mechanical scrubbing may have more than one step with various consumable sets to remove different types of defects). 3. Postrinse involves rinsing the wafer surface with deionized water after mechanical scrubbing. 4. Spin-drying employs high-speed rotation to dry the wafer.

4 CONSUMABLES 4.1 Brush PVA brush is a soft, elastic, and porous material used to remove submicrometersized particles, nanoparticles, and metallic contaminants in post-CMP scrubbing. The cleaning performance of brush scrubbing not only depends on the chemistries used (refer to Section 4.2) and the tool design (refer to Section 4.3) but also on the physical properties of the brush material(s) [10]. The formation of a typical porous sponge is accomplished by dissolving semicrystalline, hydrolyzed PVA material in deionized water (DIW) followed by exposure to high temperature. Surfactant and cross-linking agents are added to the viscous solution to produce the desired cell structure. Current technology for the introduction of cleaning solutions onto the PVA in post-CMP cleaning involves the use of a flow-through brush core or external manifold (spray or drip) techniques or both. Different cross-linking agents are used to modify the physical properties of the brush such as hydrophilicity, flexibility, and softness. Fig. 4.6 shows a PVA brush sponge with the cylindrical design. Porosity in PVA affects the flow characteristics and other material properties such as the ability

116  Developments in Surface Contamination and Cleaning

FIGURE 4.6  PVA brush with cylindrical nodule design. (Source: ITW Rippey Corp., http://www. rippey.com/300_mm.htm).

FIGURE 4.7  Scanning electron microscope (SEM) image of pore distribution in a PVA brush. (Source: ITW Rippey Corp., http://www.rippey.com/newF2.htm).

of the brush to engulf particles during scrubbing. Pore-forming agents are usually employed to generate an evenly distributed and stabilized pore networks as the insoluble sponge is formed [29]. Fig. 4.7 shows the consistent open structure of a PVA brush. Mechanical properties such as stiffness and damping, reported

Brush Scrubbing for Post-CMP Cleaning  Chapter | 4  117

FIGURE 4.8  Mechanical properties of a PVA brush: dry (left) and damp (right). (Reprinted from Ref. 10).

as storage modulus and tan delta, have been measured using a dynamic mechanical analyzer for dry and damp PVA brush and are summarized and compared in Fig. 4.8 [10]. The mechanical responses of a dry PVA brush are obviously quite different when wetted by DIW (i.e., for a damp brush). Different types of brushes, however, do not show significant variations in these classical mechanical properties. The relationships between applied load and contact area for different types of brushes are clearly different (Fig. 4.9), which indicates correlation to tribological attributes during scrubbing [10]. Although it is widely accepted that the mechanical properties of a PVA brush are closely related to its scrubbing performance, the actual correlation is not yet clear. Brush nodule design and electrostatic interaction of the brush are other properties of immense importance. When the brush nodule is compressed against the wafer surface, the nodule will experience a localized increase in pressure and density and a corresponding decrease in porosity. The localized pressure around the brush nodule induces pumping action of expelling the cleaning solution from the pores. Significant effort has been made in the past decade to determine the optimum nodule design for the brushes. Compared to knobby design (cylindrical nodule), a ridged brush geometry (as shown in Fig. 4.10) is believed to be

118  Developments in Surface Contamination and Cleaning

FIGURE 4.9  Relationship between applied load and contact area for comparison between two types of brushes. (Reprinted from Ref. 10).

FIGURE 4.10  Examples of nodule design: (A) knobby and (B) ridged. (Reprinted from Ref. 38).

more effective in removing particles given the continuous nature of the contact line between the roller and the wafer surface [38]. Another study compares the frictional attributes of two types of brush rollers (with and without nodule as shown in Fig. 4.11) in a post-interlevel dielectric CMP scrubbing process. The existence of nodules not only increases COF (refer to the appendix) dramatically for both blanket and patterned wafer substrates regardless of the solution pH but also alters the lubrication mechanism of the scrubbing process [39].

Brush Scrubbing for Post-CMP Cleaning  Chapter | 4  119

FIGURE 4.11  Two types of brush rollers with nodule (left) and no nodule (right). (Reprinted from Ref. 39).

FIGURE 4.12  New nodule designs. (Reprinted from Ref. 41).

During brush scrubbing, not only are the particles adhering to the wafer surface removed but also a tiny amount of wafer material may be etched away. A study on etching of copper during scrubbing showed significant variance in material removal between the wafer center and the edge. It is believed that the brushing effect concentrates in the wafer center area and results in a faster material removal than at the edge [40]. This identifies the unevenness of scrubbing with the most commonly used uniform cylindrical nodule design. To address this issue, new designs have been proposed, as shown in Fig. 4.12. The elongated nodule at the edge increases the scrubbing strength in the wafer edge region significantly. The twisted elongated nodule design tunes down the strength at the wafer edge but still promotes edge scrubbing enough to improve cleaning performance and uniformity [41]. Prevention of particle reattachment to the wafer surface in the scrubbing process is achieved by controlling the electrostatic interaction (represented by the zeta potential) among the polished wafer surface, the brush, and the particles

120  Developments in Surface Contamination and Cleaning

TABLE 4.1 IEP of Certain Materials of Interest [28] Material

IEP

Thermally oxidized wafer

3.0–4.0

Neutral PVA brush

2.0–2.5

SiO2 particles

2.0–3.0

WO2 and WO3 particles

2.3–2.5

CeO2 particles

6.5–7.0

Al2O3 particles

8.0–9.9

to be removed. Zeta potential is the potential difference measured in the liquid between the shear plane around a particle and the bulk of the liquid. Thus, zeta potential and isoelectric point (IEP) of the brush, wafer surface, and particles are of great interest in designing a scrubbing process. Keeping the zeta potential of all the surfaces of interest at the same polarity enhances the mutual repulsion required for effective scrubbing. Table 4.1 gives the IEP for selected materials used in CMP and post-CMP brush scrubbing processes. The charges on the surface are usually controlled by the pH of the cleaning solution. Newer developments in PVA brushes have enabled charge modification of brush surfaces, which further aids in improving the scrubbing effectiveness [41]. Traditional PVA brushes can be treated to possess more negative zeta potential (see Fig. 4.13), which expands the tunable range of a scrubbing process to accommodate more and more demanding post-CMP cleaning applications.

4.2  Chemical Formulations During brush scrubbing, the cleaning solution chemistry provides the electrostatic repulsion required to dislodge the particles from the polished wafer surface, and it also chemically etches the wafer surface to remove microscratches that may have resulted from previous processing steps. Table 4.2 summarizes the commonly used cleaning solutions in IC manufacturing. For a particular application, the cleaning chemical formulations require careful tuning and design. Studies on modulating and optimizing the composition of cleaning solutions are intended to provide general guidelines for the scrubbing process. In post-Cu CMP cleaning, the effect of additives in citric acid–based cleaning solutions on silica particles adhesion force has been studied and reported [42]. A slight increase in the zeta potential of silica and Cu is observed when citric acid is added due to the adsorption of citrate. When

Brush Scrubbing for Post-CMP Cleaning  Chapter | 4  121 Zeta potential (ZP) variation with pH for the untreated and negative ZP modified PVA 1.0

Untreated (control) Neg ZP modified

Zeta potential (normalized)

0.5 0.0 –0.5

0

1

2

3

4

5

6

7

8

9

10 11 12

–1.0 –1.5 –2.0 –2.5

pH

FIGURE 4.13  Zeta potential variation with pH. (Reprinted from Ref. 41).

TABLE 4.2 Commonly Used Cleaning Solutions [28] Solution

Common Name(s)

Effective in Removing

NH4OH/H2O2/H2O

RCA-1 or SC-1 or APM

Light organics, particles, metals; dissolves and reforms a fresh hydrous chemical oxide film

HCl/H2O2/H2O

RCA-2 or SC-2 or HPM

Heavy metals, alkali metals and metal hydroxides; leaves a protective chemical oxide film

H2SO4/H2O2

Piranha or SPM

Heavy organics and nonimplanted photoresist (PR) and PR residues

HF/H2O

DHF

All types of silicon dioxide (including chemical oxide) and, to some extent, silicon nitride

HF/NH4F/H2O

BOE or BHF

Silicon dioxide

concentration of the acid increases, the adhesion between silica and Cu weakens due to more repulsive electrostatic interactions. The addition of benzotriazole, tetramethylammonium hydroxide, or NH4OH with citric acid has been explored, and the use of NH4OH shows the lowest adhesion force. The best silica removal efficiency, as expected, is observed when using the chemical formulation that yields the weakest adhesion [42]. Organic residue from CMP slurry is another contaminant that needs to be removed during post-CMP cleaning. In post-Cu CMP cleaning, the removal of BTA has been studied from the perspective of

122  Developments in Surface Contamination and Cleaning

cleaning chemistry [43]. Metal corrosion also needs to be controlled to prevent defects. In post-Cu CMP cleaning, additives, inhibitors and chelating agents are added to the chemical formulation to minimize defects and enhance performance [44]. A study focusing on pad debris removal in post-Cu CMP showed that higher additive concentration in a basic solution is more effective. The residue from a polyurethane pad (most commonly used pad material in CMP) is rather stable when subject to different kinds of cleaning solutions. A strong repulsive force in the chemical environment is essential to dislodge or remove this debris from the Cu surface [45]. To improve the cleaning solution performance, the addition of surfactant has become a regular practice. An in situ study on particle removal employing fluorescence provided direct evidence that the addition of the surfactant speeds up the particle removal, especially at low brush-rotation speed [26]. The surfactant not only modifies the chemical environment of cleaning but also impacts the frictional attributes during scrubbing. A higher shear force has been observed for one type of cleaning solution containing a surfactant at moderate and high brush-rotation speeds [46]. This indicates that the effect on the tribological aspect needs to be considered when designing a cleaning chemistry. For a single cleaning step consideration, it is assumed that the highly complicated chemical composition of cleaning solutions may meet all the requirements—i.e., high removal efficiency of particulate and organic contaminants, minimum microscratches, and so on. However, another approach is to use lesscomplicated chemistry but employ more than one cleaning step [47]. Different contaminants and defects can be the focus of separate cleaning steps, which is a common practice nowadays. This again emphasizes the concept of a consumable set for a particular process and the corresponding process design coupled with it. With the increasingly demanding requirements for cleaning, not only scrubbing consumables are set to provide superior performance, but also the process itself has to be accordingly modified smartly.

5  RELATED ISSUES The chemistry of the cleaning solution needs to be tuned to prevent particle reattachment during the scrubbing process. Besides the slurry residue particles, it has been suspected that PVA brush itself might generate particles especially after extensive usage [48]. Furthermore, it has been shown that the particle counts after scrubbing increase with brush compression distance (due to increase in the friction force) and scrubbing time when tested on an H-terminated Si surface. Raman spectroscopy is used to detect spectral features of PVA in these particles. A mechanism has been proposed that the opposite zeta potential between PVA particles generated by a large friction and the H-terminated Si wafer surface in DIW makes the particle adsorb strongly [48]. This underlines the importance of giving serious consideration to the brush materials and their properties when tuning the cleaning solution chemistry. As a matter of fact, all the consumables

Brush Scrubbing for Post-CMP Cleaning  Chapter | 4  123 1

4

1

2

3 Good (i.e., perfectly concentric) brush

4

2

3 Eccentric brush

FIGURE 4.14  Illustration of concentric (left) and eccentric (right) brushes. (Reprinted from Ref. 49).

of scrubbing process need to be studied and tuned collectively just like those in a CMP process. Another area of interest relates to the brush concentricity. A concentric cylinder is the most commonly used brush geometry (Fig. 4.14) in today’s IC industry. The inner and outer diameters are set to accommodate different types of cleaning tools. The concentricity is required to ensure uniform compression when the brush is rotating. An abnormal brush may possess eccentricity as shown in an exaggerated view in Fig. 4.14. It is rather difficult to identify eccentric brushes before using them on the cleaning tools due to their soft nature (the softness of the brush causes problems in measuring the dimensions accurately). This may result in extra tool downtime or even unexpected yield drop. A potential rapid method has been reported to detect these abnormal brushes through contact pressure and tribology screening: the contact pressure and contact area of the eccentric brush vary significantly under different brush orientations (Fig. 4.15); variance of shear force obviously increases for the eccentric brush especially at high rotation speed (Fig. 4.16) [49]. The lifetime of a PVA brush is an important parameter to evaluate the performance of a roller and the COO of the scrubbing process. The soft and elastic PVA foam may be damaged under long hours of continuous stress, which will result in unsatisfactory cleaning performance. The stable behavior of brush– wafer contact pressure, contact area, and friction can be used to indicate cleaning and mechanical consistency of PVA brushes over their lifetime. A 48-hour marathon test under accelerated stress has been developed and used to evaluate PVA brushes. The stability in the COF value over 48-hour scrubbing time is shown in Fig. 4.17. The average value of COF shows minimal variation throughout the stress test. The total range, however, increases dramatically after 8 hours of the accelerated stress test [50]. The tribological analysis of the roller

124  Developments in Surface Contamination and Cleaning

FIGURE 4.15  Pressure contour map and contact area for concentric (left) and eccentric (right) brushes measured under four different brush orientations. (Reprinted from Ref. 49).

FIGURE 4.16  Summary of shear force variance (lbf2) at various brush rotational velocities for both types of brushes. (Reprinted from Ref. 49).

may shed light at the instance when the brush fails to perform, which provides important insight into the actual lifetime of the PVA brush.

6 SUMMARY Brush scrubbing is a widely utilized cleaning process for post-CMP applications. With the emerging cleaning challenges and more stricter demands for the CMP tool, the scrubbing process requires comprehensive development and

Brush Scrubbing for Post-CMP Cleaning  Chapter | 4  125

FIGURE 4.17  Stability of COF value over brush lifetime. (Reprinted from Ref. 50).

tuning. To summarize the studies covered in this chapter, a generalized checklist for scrubbing process design is as follows: 1. What application—wafer surface material and defect challenges. 2. What slurry—contaminants to be removed. 3. What consumable set—type of PVA brush and cleaning chemistry to be chosen and tuned collectively. 4. What tool kinematics—tool parameters to be decided, including down force, brush and wafer rotation speed, brush oscillation, flow rate of cleaning solution, and so on. 5. What process—the process modules to be used such as prerinse, scrubbing (may be more than one steps), postrinse, and spin dry.

APPENDIX: APPLICATION OF TRIBOLOGY TO POST-CMP BRUSH SCRUBBING Tribology is the study of friction, wear, and lubrication between solid surfaces. Earlier studies in tribology used a journal bearing: a simple bearing in which a shaft ( journal) or crankshaft rotates in the bearing with a layer of oil or grease separating the two parts through fluid dynamic effects with a shaft inside as shown in Fig. 4.A1. The shear force between the shaft and the wall of the journal bearing is recorded, and the COF is calculated as the shear force divided by the normal

126  Developments in Surface Contamination and Cleaning

FIGURE 4.A1  (A) Journal bearing-shaft setup and (B) schematic (side view) of a journal bearing [51].

force applied to the shaft. The Hersey number in the journal bearing-shaft setup is defined as [51]:

Hersey number =

µ⋅u P

(4.A1)

where u represents the relative linear velocity of the shaft to the bearing, μ is the viscosity of the lubricant, and P denotes the applied pressure to the shaft. It should be noted that the Hersey number is in the unit of length. The journal bearing-shaft setup is a two-body contact system. Based on the plot of COF versus the Hersey number, the Stribeck curve characterizes three different lubrication regimes as shown in Fig. 4.A2. The first region is known as boundary lubrication and is in the leftmost portion of the curve. Boundary lubrication occurs where two solid surfaces are rubbing in intimate contact—in the extreme case, without any fluid between the surfaces. In boundary lubrication, there is almost no change in COF with an increasing Hersey number. The next region corresponds to the partial lubrication regime and lies on the portion of the curve where the onset of a steeply decreasing slope in COF occurs. Partial lubrication occurs as velocity increases and pressure decreases,

Brush Scrubbing for Post-CMP Cleaning  Chapter | 4  127

FIGURE 4.A2  Stribeck curve of journal bearing-shaft system [51].

causing a partial levitation of the shaft from the journal bearing. The final lubrication regime is hydrodynamic lubrication where the shaft has completely separated from the journal bearing due to the high velocity and low pressure. In this region, full fluid film thickness exists between the journal bearing and the shaft [52]. It must be noted that in post-CMP brush cleaning a three-body contact system is developed instead of the two-body system. The shear force is induced due to the intimate interaction among the brush, the wafer, and the particles. Even though the post-CMP brush cleaning system appears to be very different from the model that Stribeck developed, the same principles of tribology can be incorporated by redefining the Hersey number. The Sommerfeld number, So, is a dimensionless parameter with the addition of a characteristic length to the denominator of the Hersey number [52]:



µ ×U So = p × heff

(4.A2)

where μ is the viscosity of the cleaning liquid, U is the relative brush–wafer velocity, p is the applied brush pressure on the wafer, and heff is the effective liquid film thickness. The Stribeck curve shown in Fig. 4.A3 is a log–log plot

128  Developments in Surface Contamination and Cleaning

FIGURE 4.A3  Generic Stribeck curve for post-CMP brush scrubbing process (h is the fluid film thickness and Ra is the surface roughness) [52].

of COF versus the Sommerfeld number. Reporting friction data in this manner gives direct evidence of the level of contact at the brush–wafer interface [52]. Generally speaking, there exist three distinct operational lubrication regimes for two or more surfaces in contact as shown in Fig. 4.A3. The first mode of contact is known as boundary lubrication, where all solid bodies are in complete contact with one another. This regime generally occurs at lower values of the Sommerfeld number with the curve acquiring a flat shape. In this contact mode, larger values of COF are expected due to intimate contact between the wafer and the brush, whereby the presence of a thin film with minimum microroughness separates the interface. The second mode of contact, called partial lubrication, occurs at intermediate values of the Sommerfeld number. A liquid film layer (generally of the same thickness as the brush surface roughness) develops, partially separating the wafer and the brush. As Stribeck curve transitions from boundary lubrication to partial lubrication, the slope of the line measuring COF transfers from a flat line to a rapidly decreasing line. The hydrodynamic lubrication regime is observed at larger values of the Sommerfeld number when the film thickness separating the wafer and the brush is larger than the brush surface roughness. COF is small signifying little or no contact between the brush and the wafer.

Brush Scrubbing for Post-CMP Cleaning  Chapter | 4  129

FIGURE 4.A4  Examples of Stribeck curves for post-CMP brush scrubbing process corresponding to different applied pressures: (□) 4 kPa (0.58 psi), (◇) 3.1 kPa (0.45 psi), (○) 2.4 kPa (0.35 psi), and (△) 1.7 kPa (0.25 psi). (Reprinted from Ref. 11).

Due to the design of the nodules and the inherent softness of the brush (and hence, the significant deformation and nodule collapse during scrubbing), the effective brush–wafer distance is approximated by the following expression:



heff = δn × (1 − α) ×

AP −ref AP

(4.A3)

where δn is the height of each nodule and α is the ratio of the total area of the up features of the nodules (i.e., parallel to the direction of shear) to the total area of the outer core of the brush. The term AP−ref in Eq. (4.A3) denotes the brush– wafer contact area at a reference pressure, while AP represents the contact area at the actual operating pressure. Given the presence of relatively tall nodules on the surface of the brush, even at the highest pressure of 4 kPa (0.58 psi), the distance between the surface of the brush and the surface of the wafer can never be small enough to cause boundary lubrication between the two surfaces, thus limiting the contact modes to partial lubrication or hydrodynamic lubrication. Fig. 4.A4 shows examples of Stribeck curves at various applied pressures for post-CMP brush scrubbing process. Comparison with the generic Stribeck curve (Fig. 4.A3) indicates that at high to moderate pressures the tribological

COF

130  Developments in Surface Contamination and Cleaning 1

1

0.1

0.1

0.01 1.00E–06 1.00E–05 1.00E–04

0.01 1.00E–06 1.00E–05 1.00E–04

Sommerfeld number

Sommerfeld number

FIGURE 4.A5  Examples when Stribeck curves fail to yield useful lubrication information. (Reprinted from Ref. 35).

mechanism is that of partial lubrication, where increasing brush velocity reduces the COF from its original high value to more moderate values. The Stribeck curves, unfortunately, are most useful when applied to rigid sliding bodies rather than an elastomeric substance sliding on a rigid body, which is the case for brush scrubbing. Stribeck curves frequently fail to generate any useful information regarding the lubrication mechanism during scrubbing as shown in Fig. 4.A5 (two examples of nonuseful Stribeck curves). Therefore, a new method has been proposed to determine the lubrication mechanism during scrubbing—i.e., σ 2 criterion. The friction force variance σ 2 represents the total amount of mechanical energy caused by stick–slip phenomena at the brush– wafer interface, either as a result of sliding at the surface or brush deformation in the bulk. For the case of hydrodynamic lubrication, there is no direct contact between the brush and the wafer, resulting in a very low shear-force variance. On the other hand, intimate contact at the brush–wafer interface exists for the case of boundary lubrication, which leads to a much higher force variance. In the intermediate region, there is partial lubrication. Studies have shown that this so-called σ 2 criterion can represent the lubrication regimes as follows [53]: 1. σ 2>0.01 indicates “boundary lubrication”; 2. 0.01> σ 2>0.0001 indicates “partial lubrication”; 3. σ 2<0.0001 indicates “hydrodynamic lubrication.” By applying the σ 2 criterion, the lubrication mechanism can be determined without performing multiple tool kinematics tests. Moreover, brush scrubbing falls in the situation of elastomer sliding on rigid body, where the Stribeck curves often fail to provide any useful information on tribological characteristics.

Brush Scrubbing for Post-CMP Cleaning  Chapter | 4  131

This σ 2 criterion provides a valid way to determine the lubrication mechanism during scrubbing.

REFERENCES 1. M. Oliver, Chemical Mechanical Planarization of Semiconductor Materials, Springer-Verlag, Berlin and Heidelberg, Germany (2004). 2. M. A. Fury, “Emerging Developments in CMP for Semiconductor Planarization”, Solid State Technol. 38, 81 (1995). 3. J. M. Steigerwald, S. P. Murarka, and R. J. Gutmann, Chemical Mechanical Planarization of Microelectronic Materials, John Wiley and Sons, New York, NY (1997). 4. D. Hymes, I. Malik, J. Zhang, and R. Emami, “Brush Scrubbing Emerges as Future WaferCleaning Technology”, Solid State Technol. 40, 209 (1997). 5. T. Myers, M. Fury, and W. Krusell, “Post-Tungsten CMP Cleaning: Issues and Solutions”, Solid State Technol. 38, 59 (1995). 6. S. R. Roy, I. Ali, G. Shinn, N. Furushawa, R. Shah, S. Peterman, K. Witt, S. Eastman and S. Kumar, “PostchemicaI-Mechanical Planarization Cleanup Process for Interlayer Dielectric Films”, J. Electrochem. Soc. 142, 216 (1995). 7. N. Mounmen and A. A. Busnaina, “Removal of Submicrometre Alumina Particles from Silicon Oxide Substrates”, Surf. Eng. 17, 422 (2001). 8. W. Krusell, J. M. de Larios, and J. Zhang, “Mechanical Brush Scrubbing for Post-CMP Cleaning”, Solid State Technol. 38, 109 (1995). 9. K. Bahten and D. McMullen, “Physical Property Effects as a Result of Compression and Rotational Velocity in Post-CMP PVA Brush Scrubbing Applications”, Proceedings of Chemical Mechanical Polishing ’99 (1999). http://www.rippey.com/papers/physprop.html. 10. A. Philipossian and L. Mustapha, “Effect of Mechanical Properties of PVA Brush Rollers on Frictional Forces During Post-CMP Scrubbing”, J. Electrochem. Soc. 151, G632 (2004). 11. A. Philipossian and L. Mustapha, “Tribological Attributes of Post-CMP Brush Scrubbing”, J. Electrochem. Soc. 151, G456 (2004). 12. R. Chein and W. Liao, “Modeling of Particle Removal using Non-Contact Brush Scrubbing in Post-CMP Cleaning Processes”, J. Adhesion 82, 555 (2006). 13. G. Zhang, G. Burdick, F. Dai, T. Bibby, and S. Beaudoin, “Assessment of Post-CMP Cleaning Mechanisms by Statistically Designed Experiments”, Thin Solid Films 332, 379 (1998). 14. K. Xu, R. Vos, G. Vereecke, G. Doumen, W. Fyen, P. W. Mertens, M. M. Heyns, C. Vinckier and J. Fransaer, “Particle Adhesion and Removal Mechanisms during Brush Scrubber Cleaning”, J. Vac. Sci. Technol. B 22, 2844 (2004). 15. R. Bowling, “An Analysis of Particle Adhesion on Semiconductor Surfaces”, J. Electrochem. Soc. 132, 2208 (1985). 16. F. Tardif, “Chapter 7 Post-CMP Clean”, Semicond. Semimetals. 63, 183 (1999). 17. M. Keswani and Z. Han, “Post-CMP Cleaning”, in: Developments in Surface Contamination and Cleaning: Cleaning Techniques, Volume 8, R. Kohli and K. L. Mittal (Eds.), Chapter 4, pp. 145–183, Elsevier, Oxford, UK (2015). 18. K. Reinhardt, R. Reidy and J. Daviot, Handbook of Cleaning for Semiconductor ManufacturingFundamentals and Applications, John Wiley & Sons, New York, NY (2011). 19. S. Beaudoin, P. Jaiswal, A. Harrison, J. Laster, K. Smith, M. Sweat and M. Thomas, “Fundamental Forces in Particle Adhesion”, in: Particle Adhesion and Removal, K. L. Mittal and R. Jaiswal (Eds.), Chapter 1, pp. 3–80, Wiley-Scrivener Publishing, Beverly, MA (2015).

132  Developments in Surface Contamination and Cleaning 20.

H. Liang, E. Estragnat, and J. Lee, “Mechanisms of Post-CMP Cleaning”, MRS Symp. Proc. 671, M741,(2001). 21. A. A. Busnaina, H. Lin, N. Moumen, J. Feng, and J. Taylor, “Particle Adhesion and Removal Mechanisms in Post-CMP Cleaning Processes”, IEEE Trans. Semicond. Manuf. 15, 374 (2002). 22. D. Ng, P. Y. Huang, Y. R. Jeng, and H. Liang, “Nanoparticle Removal Mechanisms during Post-CMP Cleaning”, Electrochem. Solid-State Lett. 10, H227 (2007). 23. Y. Huang, D. Guo, X. Lu, and J. Luo, “A Lubrication Model between the Soft Porous Brush and Rigid Flat Substrate for Post-CMP Cleaning”, Microelectronic Eng. 88, 2862 (2011). 24. Y. Huang, D. Guo, X. Lu, and J. Luo, “Mechanisms for Nano Particle Removal in Brush Scrubber Cleaning”, Appl. Surf. Sci. 257, 3055 (2011). 25. Y. Huang, D. Guo, X. Lu, and J. Luo, “Modeling of Particle Removal Processes in Brush Scrubber Cleaning”, Wear 273, 105 (2011). 26. Y. Huang, X. Lu, G. Pan, B. Lee, and J. Luo, “Particles Detection and Analysis of Hard Disk Substrate after Post-CMP Cleaning”, in: Advanced Tribology Proc. CIST2008 & ITSIFToMM2008, J. Luo, Y. Meng, T. Shao, and Q. Zhao (Eds.), pp. 772–773, Springer Verlag, Berlin and Heidelberg, Germany (2008). 27. Y. Huang, Y. Li, D. Guo, and C. Ming, “Probing Particle Removal in Brush Scrubber Cleaning with Fluorescence Technique”, Sci. China Technol. Sci. 56, 2994 (2013). 28. Z. Qi, W. Lu, and W. Lee, “A Novel Design of Brush Scrubbing in Post-CMP Cleaning”, Int. J. Machine Tools Manuf. 85, 30 (2014). 29. T. Sun, Pad-Wafer and Brush-Wafer Contact Characterization in Planarization and PostPlanarization Processes, Ph.D. Dissertation, University of Arizona, Tucson, AZ (2009). 30 S. Lariviere, F. Picore, P. L. Saez, J.-L. Baltzinger, B. Delahaye, J. Matha, X. Gilhard, S. D’Oliveira, J. M. Lagarde, J. L. Prebot, F. Merlot, F. Nogueira, D. Petit, S. Williams, and E. Besade, “Electro-Static Induced Metal Breakdown at Interlayer Dielectric Post CMP Brush Clean Process”, Proc. IEEE ASMC 2009, pp. 17–21 (2009). 3 1. X. Gu, T. Nemoto, A. Teramoto, T. Ito, S. Sugawa, and T. Ohmi, “Reduction of Scratch on Brush Scrubbing in Post CMP Cleaning by Analyzing Contact Kinetics on Ultra Low-k Dielectric”, ECS Trans. 19, 103 (2009). 3 2. X. Gu, T. Nemoto, Y. Tomita, A. Teramoto, S. Sugawa, and T. Ohmi, “Tribological Effects of Brush Scrubbing in Post Chemical Mechanical Planarization Cleaning on Electrical Characteristics in Novel Non-porous Low-k Dielectric Fluorocarbon on Cu Interconnects”, Jpn. J. Appl. Phys. 50, 05EC07 (2011). 3 3. A. Philipossian and L. Mustapha, “Effect of Tool Kinematics, Brush Pressure and Cleaning Fluid pH on Coefficient of Friction and Tribology of Post-CMP PVA Brush Scrubbing Processes”, MRS Symp. Proc. 767, F5.8 (2003). 3 4. Y. Kurokawa, H. Hirose, T. Moriya, and C. Kimura, “Cleaning by Brush-Scrubbing of Chemical Mechanical Polished Silicon Surfaces Using Ozonized Water and Diluted HF”, Jpn. J. Appl. Phys. 38, 5040 (1999). 3 5. P. Chen, J. Chen, M. Tsai, B. Dai, and C. Yeh, “Post-Cu CMP Cleaning for Colloidal Silica Abrasive Removal”, Microelectronic Eng. 75, 352 (2004). 36. W. Tseng, V. Devarapalli, J. Steffes, A. Ticknor, M. Khojasteh, P. Poloju, C. Goyette, D. Steber, L. Tai, S. Molis, M. Zaitz, E. Rill, S. Mittal, M. Kennett, L. Economikos, G. Ouimet, C. Bunke, C. Truong, S. Grunow, and M. Chudzik, “Hybrid Clean Approach for Post-Copper CMP Defect Reduction”, Proc. IEEE ASMC (Advanced Semiconductor Manufacturing Conference, 2013, pp. 346–351 (2013).

Brush Scrubbing for Post-CMP Cleaning  Chapter | 4  133 37.

W. Tseng, V. Devarapalli, J. Steffes, A. Ticknor, M. Khojasteh, P. Poloju, C. Goyette, D. Steber, L. Tai, S. Molis, M. Zaitz, E. Rill, S. Mittal, M. Kennett, L. Economikos, G. Ouimet, C. Bunke, C. Truong, S. Grunow, and M. Chudzik, “Post Copper CMP Hybrid Clean Process for Advanced BEOL Technology”, IEEE Trans. Semicond. Manuf. 26, 493 (2013). 38. D. W. Cooper, R. C. Linke, and M. T. Andreas, “Comparing the Effectiveness of Knobby and Ridged Post-CMP Cleaning Brushes”, MICRO Magazine, pp. 55–66 (1999). 39. A. Philipossian and T. Sun, “Frictional Analysis of Various Poly(vinyl alcohol) Brush Roller Designs for Post-Interlevel Dielectric CMP Scrubbing Applications”, Electrochem. SolidState Lett. 12, H84 (2009). 40. J. Noguchi, N. Konishi, and Y. Yamada, “Influence of Post-CMP Cleaning on Cu Interconnects and TDDB Reliability”, IEEE Trans. Electron Devices 52, 934 (2005). 41. R. K. Singh, C. Patel, D. Trio, E. McNamara, and C. R. Wargo, “PVA Brush Technology for Next Generation Post-CMP Cleaning Applications”, ECS Trans. 33, 167 (2010). 42. Y. Hong, D. Eom, S. Lee, T. Kim, J. Park, and A. A. Busnaina, “The Effect of Additives in Post-Cu CMP Cleaning on Particle Adhesion and Removal”, J. Electrochem. Soc. 151, G756 (2004). 43. C. Tran, P. Zhang, L. Sun, N. K. Penta, U. R. K. Lagudu, D. Shipp, and S. V. Babu, “Develop­ ment of Post-CMP Cleaners for Better Defect Performance”, ECS Trans. 44, 565 (2012). 4 4. S. Li, J. Liu, C. Tran, E. Tan, Q. Li, and R. Yan, “Cu Corrosion during Post-CMP Clean Cause and Prevention”, ECS Trans. 44, 573 (2012). 4 5. W. Tseng, E. Rill, B. Backes, M. Chace, Y. Yao, P. DeHaven, A. Tricknor, V. Devarapalli, M. Khojasteh, D. Steber, L. Economikos, C. Truong, and C. Majors, “Post Cu CMP Cleaning of Polyurethane Pad Debris”, ECS J. Solid State Sci. Technol. 3, N3023 (2014). 4 6. Y. Sampurno, Y. Zhuang, X. Gu, S. Theng, T. Nemoto, T. Sun, F. Sudargho, A. Teramoto, A. Philipossian, and T. Ohmi, “Effect of Various Cleaning Solutions and Brush Scrubber Kinematics on the Frictional Attributes of Post Copper CMP Cleaning Process”, Solid State Phenom. 145-146, 363 (2009). 4 7. Y. Ein-Eli and D. Starosvetsky, “Review on Copper Chemical–Mechanical Polishing (CMP) and Post-CMP Cleaning in Ultra Large System Integrated (ULSI)—An Electrochemical Perspective”, Electrochim. Acta 52, 1825 (2007). 4 8. N. Sato and Y. Shimogaki, “Particle Generation on Hydrogen-Terminated Si Surface by Brush Scrubbing of Polyvinyl Alcohol”, J. Electrochem. Soc. 158, D651 (2011). 4 9. T. Sun, Y. Zhuang, W. Li, and A. Philipossian, “Investigation of Eccentric PVA Brush Behaviors in Post-Cu CMP Cleaning”, Microelectronic Eng. 100, 20 (2012). 5 0. R. K. Singh, C. R. Wargo, and D. W. Stockbower, “Post-CMP Clean PVA Brush Design Advancements and Characterization in Cu/Low-k Applications”, Proc. International Conference on Planarization/CMP Technology (ICPT), VDE Verlag, Berlin and Offenbach, Germany, pp. 431–436, (2007). 5 1. M. D. Hersey, Theory and Research in Lubrication, John Wiley and Sons, New York, NY (1966). 5 2. K. Ludema, Friction, Wear and Lubrication: A Textbook in Tribology, CRC Press, Boca Raton, FL (1996). 5 3. T. Sun and A. Philipossian, “Method for Determining the Lubrication Mechanism of Post-ILD CMP Brush Scrubbing”, Electrochem. Solid-State Lett. 11, H214 (2008).