2. experimental procedure for laser-induced plasma …

J. Adhesion Sci. Technol., Vol. 18, No. 7, pp. 779–794 (2004) VSP 2004.Also available online - www.vsppub.com

Post-chemical mechanical polishing cleaning of siliconwafers with laser-induced plasma

VAMSI KRISHNA DEVARAPALLI 1, YING LI 2 and CETIN CETINKAYA 3,∗1 Department of Mechanical and Aeronautical Engineering, Wallace H. Coulter School of

Engineering, Clarkson University, Potsdam, NY 13699-5725, USA2 IBM Microelectronics Division, Hopewell Junction, NY 12533, USA3 Department of Mechanical and Aeronautical Engineering, Center for Advanced Materials

Processing, Wallace H. Coulter School of Engineering, Clarkson University, Potsdam,NY 13699-5725, USA

Received in final form 15 March 2004

Abstract—During chemical-mechanical polishing (CMP), wafers are subjected to various particlesources such as slurry, polishing pads and polishing machines. Consequently, wafer post-CMPcleaning is crucial in micro- and nano-manufacturing to improve yield. In conventional cleaningprocess, the wafer is subjected to forces whose magnitudes are large enough to potentially causesubstrate damage. This damage concern is becoming more severe as the characteristic feature sizeshrinks to sub-100-nm region. The development of dry, rapid, non-contact and non-destructive particleremoval methods has been emerging as a critical requirement for post-CMP cleaning. In recent years anovel technique for particle removal using the pressure field generated by pulsed laser-induced plasma(LIP) has been introduced for cleaning surfaces and trenches. In the current study, it is demonstratedthat the LIP removal technique is capable of removing ceria CMP particles with size of 100 nm andabove without any substrate damage. The results reported in this study prove that LIP can also beapplied over extended areas for post-CMP cleaning. It is possible to remove smaller particles at lowergap distances but the minimum particle size that can be removed is limited by the risk of substratedamage. Results reported in this study indicate that the LIP particle removal technique has significantpotential for post-CMP cleaning in the near future.

Keywords: Nanoparticles; ceria; particle removal; laser cleaning; chemical-mechanical polishing(CMP); micro-contamination; nano-contamination.

1. INTRODUCTION

Chemical-mechanical polishing (CMP) is an established technology for the pla-narization of dielectric and metal layers in micro- and nano-manufacturing. During

∗To whom correspondence should be addressed. Tel.: (1-315) 268-6514. Fax: (1-315) 268-6438.E-mail: [email protected]

780 V. K. Devarapalli et al.

CMP, wafers are subjected to various particle sources such as slurry, polishing padsand polishing machines. Wafer cleaning after CMP, therefore, is a critical step inmicro- and nano-manufacturing in order to minimize yield loss. As the complexityof nano-fabrication increases, surface cleaning for post-CMP and during many otherprocesses, such as reactive ion etching and thin-film deposition, becomes extremelycrucial in many industries, including semiconductors, optics, photonics and micro-electromechanical systems (MEMS). A dry, rapid, non-contact and non-destructiveparticle removal method is desirable to reduce the risk of substrate damage andchemical usage while removing smaller particles. Minimizing the use of chemicalagents in LIP cleaning is also a major advantage for workplace safety and environ-mental conservation.

A brief review of the laser removal techniques based on substrate acceleration ispresented. Pulsed-laser particle removal methods based on substrate accelerationand shock-wave generation from superheated thin liquid films have been employedin the past decade. It is known that a direct-pulsed laser irradiation generateshigh surface accelerations and strong shear waves in the near field [1] due to rapidthermal expansion in a thin top layer. Particle removal using pulsed lasers has beenreported in numerous contexts since it was first reported in 1991 [2, 3]. However,possible substrate damage in direct-pulsed irradiation has been a concern for sub-micrometer particles [4]. Laser steam cleaning, in which particle removal is dueto superheating of a liquid film deposited on the substrate to nucleate cavitationbubbles under pulsed laser heating, appears more effective [5] than the direct-irradiation method. However, it is inadequate when dry cleaning of a surface isrequired and/or particle-liquid/surface-liquid interactions at high temperatures arenot desirable, for instance in the removal of dry pharmaceutical powders. Directlaser cleaning has been applied to a large number of cleaning applications such aswafers, disk drive heads, stencils and sputtering targets [6, 7]. In direct surfaceirradiation methods, recent interest has been in thermo-elastic stresses generatedon the substrate [4]. Also, the interaction between the laser beam and the particle(near-field diffraction), and consequent substrate damage has been a concern [8, 9].It is feared that this interaction presents a serious challenge for the direct method innanoparticle removal applications since lasers with short wavelengths are preferredin direct laser cleaning due to their higher absorption coefficients.

In recent years a novel method for particle removal using the pressure fieldgenerated by pulsed laser-induced plasma (LIP) has been introduced for cleaningsurfaces [10–12] and trenches [13]. The schematic in Fig. 1 depicts a simple set-upfor the technique. In the experiments reported in the current study, an incident laserbeam is focused through a convex lens. Near the focal point of the lens, the energydensity is large enough to cause the breakdown of air into plasma [14]. The pressurefield generated by the expanding plasma core acts over the surface of the particle,creating an equivalent moment about the contact zone [15]. If the moment due to thepressure field is strong enough, an interfacial crack is initiated between the particleand the substrate. The crack propagates at the leading edge and closes at the trailing

Post-CMP cleaning of silicon wafers with LIP 781

Figure 1. Schematic of the laser-induced plasma removal set-up. d is the distance from substrate tothe center of the beam, fl the focal length of the lens and r0 the incident beam radius.

edge as the particle rolls. The removal of the particle begins when the particleloses contact with the surface due to various reasons, such as interactions withother particles and surface features. It is observed that the removal effectivenessdepends heavily on the distance between the center of the plasma and the substrate,since it dictates the magnitude of the applied pressure field on the surface of theparticle [11]. Moving the substrate closer to the blast center increases the magnitudeof the moment experienced by the particle, which increases the effectiveness of theremoval process. However, the risk of substrate damage increases as the substrate ismoved closer to the plasma core. These two issues must be successfully mitigatedfor damage-free particle removal. The LIP experiments performed to date indicatethat the main reason for surface damage is the physical contact between the plasmaand the substrate. The required pressure levels predicted in Ref. [15] and measuredin Ref. [16] are in the range of 20–50 kPa, which are low compared to the yieldstrengths of typical substrate materials (e.g. silicon). Therefore, it is reasonable toconclude that surface damage risk due to the magnitude of shockwave pressure islow and the main cause of damage is chemical reactions between the plasma andthe surface.

2. EXPERIMENTAL PROCEDURE FOR LASER-INDUCED PLASMA REMOVAL

The substrates used in the experiments were reclaimed 6-inch, [111] n-type dopedsilicon wafers with approximately 1-µm-thick thermal oxide layer. The wafer wascut into small square pieces of 1.5 cm×1.5 cm area using a diamond scriber to avoidthe geometrical constraints, while the sample was imaged and analyzed under ascanning electron microscope (SEM). These samples were individually washed withde-ionized water and ethyl alcohol to get rid of the initial contamination. In orderto locate the same area before and after pulsed-laser irradiation, a diamond-shapedreference pattern was engraved on the square wafer piece with the same diamond


Figure 2. Particle size distribution bar chart (Vp) from the ultrafine particle size analyzer(MICROTRAC-UPA150) indicates that the ceria slurry used in experiments has particles with diame-ters (D) in the range of 30–400 nm. The left y-axis corresponds to the cumulative volume percentage(Cv) of the slurry passing through the analyzer channels versus the particle diameter (D).

scriber. The shape of the pattern was observed to change with the application ofLIP, which hindered the process of locating the exact cleaned area. To eliminatethis problem, prior to slurry deposition the bare sample piece was cleaned bylaser shots at 1.4 mm firing distance with 5 pulses over the diamond-shapedarea.

Commercially available ceria slurry supplied by NYACOL Nano Technologies(Ashland, MA, USA), with a particle size in the range of 30–400 nm was used(Fig. 2). The slurry was diluted with methanol in order to reach a concentrationwhere particles could be deposited with minimal agglomeration. The diluted CMPslurry was deposited onto the silicon wafer using the drop-agitation technique. Theobjective of deposition was to achieve a uniform distribution of slurry particles withminimal agglomeration and proper density to facilitate analysis in the target areasof the wafer. The wafer pieces were attached to a rigid post positioned in a jewelry-cleaning machine. The surface vibrations in the jewelry-cleaning machine wereused in order to reduce particle aggregation while the suspension was drying onthe surface. The diluted slurry was applied to the central area of the wafer. Thewafer was allowed to completely dry following deposition. To demonstrate that LIPcould be applied over extended areas the complete 6-inch wafer was taken, cleanedand slurry was deposited according to the procedure explained above and then thepulsed-laser irradiation was carried out.

Both initial cleaning and deposition were conducted in a class-10 cleanroom. Thewafers were then taken to the laser setup outside of the cleanroom for applicationof the LIP. The pulsed laser utilized in the experiments was a Q-switched Nd:YAGoperating at a fundamental wavelength of 1064 nm (Quantel Brilliant series Q44)


with pulse energy of 370 mJ. It had a pulse length of 5 ns, a repetition rate of 10 Hz,and a beam diameter of 5 mm. A 25 mm diameter, 100 mm focal length lens with a1064-nm-thick specific antireflective coating was used to converge the beam.

The wafer was placed onto a translation stage and held in place with a lightadhesive tape on an aluminum stud. Horizontal translation was achieved in twodimensions using sliding posts with millimeter markings. More exact translationwas required in the vertical dimension to control the critical parameter d (Fig. 1).A linear translation stage with a 20 ± 10 µm resolution was used for this purpose.In this particular experimental setup, d was set at 1.4 mm. A He-Ne laser wasemployed for positioning of the lens and vertical alignment of the sample withthe Nd:YAG beam. A diode laser was used to mark the horizontal position of theplasma and to align the sample. Two sets of experiments were carried out, one on a6-inch wafer and the other on a small wafer piece; this is because the 6-inch wafercould not be analyzed under the SEM, because of the geometrical limitations in theSEM chamber. The pulsed-laser irradiation procedures for the two samples were asdescribed below.

2.1. Irradiation procedure for the wafer piece sample

The wafer was positioned on the translation stage and the required test area whereLIP was to be applied was identified with the diode laser. Previous experimentalresults showed that a single pulse affected a circular area with a diameter of about2–3 mm [11]. As the test area was approximately 2 mm2, a single attempt wassufficient to cover the whole test area. In this particular experiment 10 pulses wereimparted.

2.2. Irradiation procedure for the 6-inch wafer

The wafer was placed on the translation stage. LIP was performed at the center ofthe wafer in an 8 mm × 8 mm grid (Fig. 3a). The size of the grid compared to thewafer is depicted in Fig. 3b by a square with white lines. The wafer was positionedso that the lower left grid corner was the first to be cleaned; the laser irradiationthen proceeded sequentially over the entire grid for cleaning the entire zone withan area of 8 mm × 8 mm. After each alignment, a single pulse was applied to eachpoint on the grid. Optical microscope (Olympus model BH2) images were capturedon the entire 6-inch wafer. Particle counting was achieved using a surface analysissystem (SAS) (PMS SAS 3600 XP by Particle Measuring Systems, Boulder, CO,USA). SEM analysis was then conducted to characterize the particles and to obtainthe before and after images of smaller square shaped wafer pieces. A JEOL® model6300 microscope was employed for capturing SEM images.


(a)

(b)

Figure 3. SAS analyses of the silicon substrate before LIP exposure (a) with the 8 mm × 8 mmcleaning grid in the inset and after LIP exposure (b). The white square in (b) indicates the cleaningarea grid.


3. PARTICLE REMOVAL RESULTS

The ceria slurry was analyzed in an ultrafine particle analyzer (MICROTRAC-UPA150, Honeywell, Phoenix, AZ, USA). Figure 2 shows the particle size dis-tribution (Vp) chart of the CMP slurry used in the experiments. It was observed thatthe slurry had particles with diameter D in the range of 30–400 nm. The findings ofthe experiments for the 6-inch wafer and the wafer piece samples are summarizedbelow.

3.1. Results from the 6-inch wafer

Single pulses were imparted on the 6-inch wafer. In Fig. 4, the before andafter optical microscope pictures of the affected areas of the silicon wafer at100 × magnification factors are presented. These images verify that larger particlesand agglomerates in these areas have been removed. It is evident that the LIPtechnique at a gap distance of d = 1.4 mm has been quite successful in removingthe smaller particles also, including the clumped particles. Since smaller particlesare not clearly visible under the optical microscope and as the 6-inch wafer has ageometrical constraint to be analyzed under SEM, so experiments with small waferpieces were carried out with a higher number of pulses to check the removal ofparticles.

3.2. Results from the wafer piece sample

In Fig. 5, a set of SEM images of the deposited ceria slurry particles on the waferis presented. These images were captured for the characterization and verificationof the particle size distribution in the deposited ceria slurry. The characterizationprocess is also important for a more complete understanding of removal from theentire area and nature of the deposited particles. The SEM images confirm a particlediameter (D) range of approximately 30–450 nm. This estimation seems to agreewith the range of particle diameters found using the ultrafine particle analyzer inFig. 2. Figure 6 presents a set of SEM images of the substrate before and after LIPremoval. It is evident that a vast majority of the particles deposited on the markedareas on the wafer has been removed. While no damage is detected by the SEManalysis in the cleaning areas, there is some substrate damage observed near thewalls of the inscribed diamond reference, which can be attributed to the change inthe surface roughness imparted by the diamond scriber while marking the reference.

4. CONCLUSIONS AND REMARKS

It is demonstrated that the laser-induced plasma (LIP) removal technique is capableof removing particles as small as 100 nm present in the ceria CMP slurry froma silicon wafer. The objective of the study was to evaluate the potential of LIPas a post-CMP cleaning technique. The wafer used as a substrate was n-type


(a)

(b)

Figure 4. Optical microscope images (100×) of the silicon substrate (a) before and (b) after LIPexposure.


(a)

(b)

Figure 5. SEM images of deposited ceria slurry on the silicon wafer showing the particle sizedistribution with (a) and (c) at 4000×, (b) at 5000× and (d) at 10 000× magnifications. The ap-proximate size of some of the individual particles measured by SEM is shown in panels (a), (b)and (c).


(c)

(d)

Figure 5. (Continued).


(a)

(b)

Figure 6. SEM images of ceria CMP slurry particles deposited on a silicon wafer. (a–d) at 1000×,(e–h) at 3000× and (i, j) at 6000× magnifications are the pairs of images before and after LIPexposure. Approximate size of some of the individual particles are identified on all the before images.The dot-dash lines indicate the boundaries of the markers and the cleaning area.


(c)

(d)



(e)

(f)



(g)

(h)



(i)

(j)



doped [111] silicon with an approximately 1-µm-thick thermal oxide layer. Thedata from the ultrafine particle analyzer confirmed that the particles in the ceriaslurry ranged from 30 nm to 400 nm, and the SEM analysis conducted for removalverification confirmed the particle sizes to range from roughly 35 nm to 450 nm.The particles were deposited on the substrate with the drop-agitation technique. Inthe after cleaning SEM images, no substrate damage was detected at the 1.4-mmgap distance. Successful experiments were performed to prove that LIP could alsobe applied over extended areas for post-CMP cleaning. It is possible to remove ceriaparticles smaller than 100 nm at lower gap distances, but the lower limit for a gapdistance is constrained by the risk of substrate damage. Further trials are neededto verify the ability of the LIP technique to remove particles smaller than 100 nm.The current results provide a good indication that the LIP removal technique hassignificant potential in post-CMP cleaning as a near-future cleaning technique.

Acknowledgements

The authors acknowledge the National Science Foundation (Nanoscale ExploratoryResearch Program, Award ID 0210242), the New York State Science and Technol-ogy Foundation, and the Center for Advanced Materials Processing (CAMP) fortheir partial financial supports for this research project.

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