Nanoparticle removal from substrates with pulsed-laser induced plasma and shock waves

Removing particles with nanometer scale diameters from substrates is a challenging task with numerous critical applications. A novel removal method for nanoparticles is developed and tested. The technique, which is dry and non-contact, takes advantages of shock wavefronts initiated by plasma formation under a focused laser beam pulse and its interaction with the substrate. Experimental results indicate that silica particles down to 500 nm on silicon wafers can be removed without substrate damage. In the reported experiments, a Q-switched Nd : YAG pulsed laser with a 5-ns pulse width and 360-mJ pulse energy at 1064 nm wavelength is employed as a plasma generation source. It is reported that the traditional dry laser cleaning method based on the rapid thermal expansion under direct laser irradiation often results in surface damage in the nanometer scale due to light diffraction around nanoparticles and/or stress localization in the thermal skin. This occurs when the characteristic dimensions of the particles are comparable to the wavelength of incident beam. In the laser plasma method, various mechanisms are responsible for the removal effect. The strong shock wave in air generates complex pressure wavefields resulting in both drag/lift on the particle and acceleration of the substrate. However, shock waves are not transmitted to the solid substrate due to a large difference between the relevant wave phase speeds in the two media. The effects of the number of shots and the distance between the surface and the plasma boundary on the removal efficiency are reported.     

Efficiency studies of particle removal with pulsed-laser induced plasma

The detachment and removal of micro- and nanoparticles from surfaces is of importance in many industries. The cleaning of silicon wafers is of great interest in the semiconductor, microelectronics, and optics industries. The development and adoption of dry, rapid, non-contact, and non-damaging particle removal technologies is a critical process. The proposed laser-induced plasma (LIP) removal technique is a novel method for detaching and removing fine particles from substrates. The current technique is a dry, non-contact method that takes advantage of the strong shock wavefront from expanding plasma, created by focusing a laser pulse in air above the substrate. The transient pressure field acts on the target particles to produce a rotational moment and a rolling mode of detachment from the substrate. In the current study, the LIP removal technique is employed repeatedly to remove particles over an area of a silicon wafer, and a systematic efficiency study for the removal effectiveness is performed. A Q-switched Nd:YAG pulsed laser operating at 1064 nm with a 370 mJ pulse energy and 5 ns pulse length is used in experiments. 0.99 μm diameter silica spheres and 3.063 μm diameter polystyrene spheres were successfully detached and removed with no substrate damage. The removal efficiency at various gap distances between plasma core and substrate is determined and reported. This work is the first laboratory demonstration of the LIP technique over an extended area. The reported results substantiate the LIP removal technique as a serious option for particle detachment and removal from extended areas.     

Potential thermo-mechanical substrate damage in nanoparticle removal with pulsed lasers

Difficulties associated with the removal of nanoparticles with traditional cleaning methods have been widely discussed and reported in the literature. It has been projected that the minimum feature sizes of semiconductor devices by the year 2008 will be reduced to 70 nm. Laser cleaning as an emerging technique has been gaining momentum in recent years. The two main features of the laser methods: its dry and non-contact nature, make it particular attractive. However, as damage in pulsed laser processing has been an issue in micro-manufacturing applications, so concerns about damage in laser cleaning applications must also be adequately addressed. In laser cleaning, detachment often is achieved by creating a normal inertial force exerted on the particle due to the normal surface acceleration under a rapid laser thermal loading. The value of the required detachment acceleration is proportional to 1/D ² (D: particle diameter). In this study, the onset of thermal and mechanical substrate damage in nanoparticle removal has been computationally studied for silicon and copper substrates. The maximum and minimum values of the components of the acceleration vector and axisymmetric stress tensor as well as the temperature are computed at various points on the substrate surface and in the interior of the substrate. The minimum and maximum values of the thermoelastic quantities along the radial and axial directions have been reported. It is computationally determined and demonstrated that the type of first-damage depends on the substrate and particle material properties as well as particle diameter.     

The Effect of Time and Humidity on Particle Adhesion and Removal

Time and humidity greatly influence particle adhesion and removal in many particlesubstrate systems. The effect of time (aging) and humidity on the adhesion and removal of 22 μm PSL (Polystyrene Latex) particles on polished silicon wafers is investigated. The results show that the effect of time on the adhesion and removal of the 22 μm PSL particles on silicon substrates in high humidity environment is very significant. The removal efficiency of PSL particles significantly decreased after the samples were aged for more than one day in high humidity environment. The combined effect of the van der Waals force and the capillary force tend to accelerate the adhesion-induced deformation process. When capillary force occurs at the particle substrate interface, the removal efficiency decreases quickly by more than 50% within 24 hours. Without the capillary force, the adhesion-induced deformation is negligible within the first 24 hours.     

The Effect of Time and Humidity on Particle Adhesion and Removal

Time and humidity greatly influence particle adhesion and removal in many particlesubstrate systems. The effect of time (aging) and humidity on the adhesion and removal of 22 μm PSL (Polystyrene Latex) particles on polished silicon wafers is investigated. The results show that the effect of time on the adhesion and removal of the 22 μm PSL particles on silicon substrates in high humidity environment is very significant. The removal efficiency of PSL particles significantly decreased after the samples were aged for more than one day in high humidity environment. The combined effect of the van der Waals force and the capillary force tend to accelerate the adhesion-induced deformation process. When capillary force occurs at the particle substrate interface, the removal efficiency decreases quickly by more than 50% within 24 hours. Without the capillary force, the adhesion-induced deformation is negligible within the first 24 hours.     

Hydrophobic and superhydrophobic surfaces fabricated by plasma polymerization of perfluorohexane,perfluoro(2-methylpent-2-ene), and perfluoro(4-methylpent-2-ene)

Fluoropolymer films were deposited on silicon (1 0 0) wafers, glass, epoxy, and hierarchical dual-sized filler epoxy composite surfaces by plasma polymerization of perfluorohexane, perfluoro(2-methylpent-2-ene), and perfluoro(4-methylpent-2-ene). The procedure involved continuous wave plasma-enhanced deposition, followed by a discharge-off period, with the monomer gas feed maintained. Silanization of silicon wafers and glass surfaces with triethoxyvinylsilane was employed to improve plasma fluoropolymer bonding to these substrates. The presence of double bonds in perfluoro(2-methylpent-2-ene) and perfluoro(4-methylpent-2-ene) was found to influence fluoropolymer coating topography, thereby increasing surface roughness in modified glass and epoxy substrates. All fluorocarbons provided a similar level of hydrophobization of flat substrates, exhibited by water contact angles (WCA) of about 110°. Hydrophobization of nanocomposite hierarchical surfaces by plasma polymerization provided superhydrophobic surfaces, with WCA of 160° and contact angle hysteresis below 8°.     

Plasma-deposited fluoropolymer film mask for local porous silicon formation

ABSTRACT: The study of an innovative fluoropolymer masking layer for silicon anodization is proposed. Due to its high chemical resistance to hydrofluoric acid even under anodic bias, this thin film deposited by plasma has allowed the formation of deep porous silicon regions patterned on the silicon wafer. Unlike most of other masks, fluoropolymer removal after electrochemical etching is rapid and does not alter the porous layer. Local porous regions were thus fabricated both in p+-type and low-doped n-type silicon substrates.     

A New Method for Lift-off of III-Nitride Semiconductors for Heterogeneous Integration

The release and transfer of GaN epilayers to other substrates is of interest for a variety of applications, including heterogeneous integration of silicon logic devices, III–V power devices and optical devices. We have developed a simple wet chemical etching method to release high-quality epitaxial III-nitride films from their substrates. This method builds on a nanoepitaxial lateral overgrowth (NELO) process that provides III-Nitride films with low dislocation densities. NELO is accomplished using a nanoporous mask layer patterned on GaN substrates. Chemical removal of the SiO₂ layer after growth of III-Nitride overlayers causes fracture at the interface between the GaN film and the original GaN substrate, resulting in free-standing GaN films with nanostructured surfaces on one side. These layers can be transferred to other substrates, and the nano-structured surface can be used in photonic devices, or planarized for power devices.     

Post-chemical mechanical polishing cleaning of silicon wafers with laser-induced plasma

During chemical-mechanical polishing (CMP), wafers are subjected to various particle sources such as slurry, polishing pads and polishing machines. Consequently, wafer post-CMP cleaning is crucial in micro- and nano-manufacturing to improve yield. In conventional cleaning process, the wafer is subjected to forces whose magnitudes are large enough to potentially cause substrate damage. This damage concern is becoming more severe as the characteristic feature size shrinks to sub-100-nm region. The development of dry, rapid, non-contact and non-destructive particle removal methods has been emerging as a critical requirement for post-CMP cleaning. In recent years a novel 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 demonstrated that the LIP removal technique is capable of removing ceria CMP particles with size of 100 nm and above without any substrate damage. The results reported in this study prove that LIP can also be applied over extended areas for post-CMP cleaning. It is possible to remove smaller particles at lower gap distances but the minimum particle size that can be removed is limited by the risk of substrate damage. Results reported in this study indicate that the LIP particle removal technique has significant potential for post-CMP cleaning in the near future.     

Investigation of diamond deposition uniformity and quality for freestanding film and substrate applications

In this paper, we report on microwave CVD deposition of high quality polycrystalline diamond and on related post-processing steps to produce smooth, flat and uniformly thick films or diamond substrates. The deposition reactor is a 2.45 GHz microwave cavity applicator with the plasma confined inside a 12 cm diameter fused silica bell jar. The deposition substrates utilized are up to 75 mm diameter silicon wafers. The substrate holder is actively cooled with a water-cooled substrate holder to achieve a substrate surface temperature of 600–1150 C. The pressure utilized is 60–180 Torr and the microwave incident power is 2–4.5 kW. Important parameters for the deposition of thick films with uniform quality and thickness include substrate temperature uniformity as well as plasma discharge size and shape. As deposited thickness uniformities of ± 5% across 75 mm diameters are achieved with simultaneous growth rates of 1.9 μm/h. The addition of argon to the deposition gases improves film deposition uniformity without decreasing growth rate or film quality, over the range of parameters investigated. Post-processing includes laser cutting of the diamond to a desired shape, etching, lapping and polishing steps.     

Non-contact removal of 60-nm latex particles from silicon wafers with laser-induced plasma

In micro- and nano-manufacturing particles are created and circulated as by-products of the processes utilized. The removal of micro- and nano-particles during manufacturing processes is of great importance in many industries, including semiconductors, optics, photonics and microelectromechanical systems (MEMS). One specific application is in the cleaning of silicon wafers. According to the 2002 update of the International Technology Roadmap for Semiconductors, by the year 2006, techniques for the removal of polystyrene latex (PSL)-equivalent particles with size less than 35 nm will be required and currently, according to the roadmap update, there is no known method to remove PSL-equivalent particles smaller than diameter D = 40 nm. The current study reports and demonstrates a non-contact laser-induced plasma method for removal of particles with D = 60 nm. Non-contact techniques are desired to reduce the risk of substrate damage. Chemical usage in traditional cleaning methods is also a serious concern for process cost, workplace safety and environmental conservation. The development of dry, rapid, non-contact and non-damaging nanoparticle removal methods is, therefore, a critical need for various applications. Using a scanning mechanism, the process discussed can be automated for rapid cleaning of large silicon wafers.     

A Method of Determining the Contact Area Between a Particle and Substrate Using Scanning Electron Microscopy

A new technique for determining the contact radius between a micrometer size particle and a contacting substrate using scanning electron microscopy has been developed. The Contact Area Measurement (CAM) technique, which is especially suited for small surface-force-induced contact radii, involves evaporating a thin, uniform coating of a conductive material, such as aluminum, over a sample comprised of particles on a substrate while the sample is rotated slowly. The sample is examined before and after particle removal to determine both the radii of the particle and its respective contact. Where the particle contacted the substrate, no metal deposition occurred. The resulting differences in the secondary electron emissions provide a contrast mechanism that the SEM can image. The CAM technique is shown to be useful in examining rigid particles on rigid substrates, where the inherent contacts are small, making measurements difficult, and for examining irregularly-shaped particles and contact areas.     

Additively patterned ferroelectric thin films with vertical sidewalls

The functional properties of electroceramic thin films can be degraded by subtractive patterning techniques used for microelectromechanical (MEMS) applications. This work explores an alternative deposition technique, where lead zirconate titanate (PZT) liquid precursors are printed onto substrates in a desired geometry from stamp wells (rather than stamp protrusions). Printing from wells significantly increased sidewall angles (from ~1 to >35 degrees) relative to printing solutions from stamp protrusions. Arrays of PZT features were printed, characterized, and compared to continuous PZT thin films of similar thickness. Three‐hundred‐nanometer‐thick printed PZT features exhibit a permittivity of 730 and a loss tangent of 0.022. The features showed remanent polarizations of 26 μC/cm², and coercive fields of 95 kV/cm. The piezoelectric response of the features produced an e_31,f of ?5.2 C/m². This technique was also used to print directly atop prepatterned substrates. Optimization of printing parameters yielded patterned films with 90° sidewalls. Lateral feature sizes ranged from hundreds of micrometers down to one micrometer. In addition, several device designs were prepatterned onto silicon on insulator (SOI) wafers (Si/SiO₂/Si with thicknesses of 0.35/1/500 μm). The top patterned silicon was released from the underlying material, and PZT was directly printed and crystallized on the free‐standing structures.     

The effect of laser irradiation on peel strength of temporary adhesives for wafer bonding

Thin silicon wafers are usually used for many devices and electronic fabrication development. But handling thin wafers are not easy since thin wafers may lose the supporting strength and crack. Using adhesives is the one of the possible solutions for thin wafer handling, and how to synthesize adhesives materials and investigate debonding behaviors for temporary bonding and debonding are an important research. In this work, laser irradiation is considered for debonding temporary adhesives in a 3D multi-chip package process because of its very fast debonding time about few seconds and irradiation stability than thermal or chemical debonding. The thermal curable adhesives were fast cured on high temperature by the laser irradiation. The emphasis is the choice of the specific laser process parameters such as the out-focusing length, the line spacing, and the scan speed. The surface morphology with various sets of these parameters is examined by optical microscopy. Also, peel strength before/after the laser irradiation is investigated. Based on this study, suitable process parameters and conditions are proposed for clean surface of silicon wafers and lower peel strength for easy debonding.     

The adhesion-induced deformation and the removal of submicrometer particles

Adhesion-induced deformations of submicrometer polystyrene particles on silicon substrates were observed as a function of time using scanning electron microscopy. The contact area between the particle and the substrate was found to increase with time for a period of approximately 72 hours before reaching a constant value. The ratio of the final contact radius to the particle radius was ~ 0.4. The time dependence of this deformation appears similar to the creep phenomenon in bulk polymers. These results are related to the studies of particle removal conducted for different time periods, using hydrodynamic and centrifugal removal forces. The removal efficiency was found to decrease with time. This correlates well with the increase in the adhesion force on the particles with time as observed from the SEM measurements. The effect of the particle diameter on the removal efficiency and the correlation between the time dependent adhesion-induced deformation and particle removal efficiency is discussed.     

Laser-transfer processing of functional ceramic films

Pulsed excimer laser irradiation through a UV-transparent fabrication substrate has been successfully employed to separate PZT thick films from their sapphire host substrates. Films of 20 μm in thickness were prepared by a hybrid particle sol–gel synthesis route. The microstructure, morphology and ferroelectric properties of the thick films after laser-transfer have been examined. Films were irradiated with a 248 nm, 15 ns pulse, and transferred to a platinised silicon substrate (Pt/Ti/SiO₂/Si). A laser fluence of 250 mJ/cm² was sufficient to delaminate the original PZT/sapphire interface. The pulsed energy density used here is lower than reported by other groups utilising a laser-transfer process for PZT. This is believed to be due to higher levels of porosity at the film/substrate interface in this study.     

Diamond-like carbon films with End-Hall ion source enhanced chemical vapour deposition

A custom-designed End-Hall ion source was used to deposit diamond-like carbon (DLC) films in a plasma enhanced chemical vapour deposition (PECVD) mode. The deposition system was characterised and optimised for infrared transmission enhancement applications and large area deposition onto silicon or germanium substrates. Ion bombardment energy (in eV) on substrate was found to scale about 60% of the discharge voltage. Uniformity was about 2.5% and 5% for substrate diameters of 20 cm and 40 cm respectively. For the infrared enhancement applications the optimised ion bombardment energy was about 54 eV with a high deposition rate approximate 30 nm/min. Coating the DLC onto a single side of double-sided polished silicon wafers resulted in a transmission of 69.5% in the wavelength of about 4 μm, very close to the ideal value. Mechanical and reliability properties of the DLC films on silicon wafers were analysed at different environmental conditions. It was found that the DLC films produced in the ion source PECVD deposition system were satisfied with the requirements for the infrared transmission enhancement applications.     

Study of anodic layers and their effects on electropolishing of bulk and electroplated films of copper

Copper anodic layers in solutions of hydroxyethylidenediphosphonic acid (HEDP), phosphoric acid, and phosphoric acid with organic and inorganic additives were analysed in situ using electrochemical impedance spectroscopy (EIS). Salt films formed in HEDP solutions were detected. No salt film was found in solutions of phosphoric acid or phosphoric acid with copper oxide, ethylene glycol and sodium tripolyphosphate as additives. Analysis of EIS data suggests that H₂O molecules are the mass transport controlling species in solutions of phosphoric acid and of phosphoric acid with copper oxide, whereas Cu²⁺ ions are the mass transport controlling species in solutions of HEDP and of phosphoric acid with additives ethylene glycol and sodium tripolyphosphate. Very good results for electropolishing of bulk copper discs were obtained in all the above solutions, but the electropolishing of electroplated copper films on patterned silicon wafers is more challenging. In this case, good planarization was obtained only from HEDP solutions. Experimental results and theoretical analysis indicate that surface profile of anodic layers is an important factor influencing planarization of electroplated copper films on patterned silicon wafers. An electrically resistive anode film with macro surface profile can lead to planarization of the gentle surface undulations of electroplated copper films on trenched silicon wafers due to migration smoothing, diffusion smoothing, and ohmic levelling effects.     

Nanoparticle removal with a pulsed laser: thermoelastic transient surface acceleration simulations for analyzing edge effects

Damage-free removal of nanoparticles and thin films from substrates is a critical requirement in micro- and nano-manufacturing. In laser cleaning, particles are removed by application of inertial forces at the particles attached to a substrate by means of surface acceleration. While the technique is promising, experimental results indicate that damage can occur in the process due to high levels of fluence. Potential damage mechanisms include surface breakage, interferometric interactions due to bumps/surface features, diffraction related focusing, micro-cracks, and peeling of top layers. To reduce and/or eliminate damage risk by avoiding excessive heat deposition, the absolute acceleration requirement must be well understood and accurately modeled. In the current work, a set of transient simulations for the thermoelastic response has been conducted to determine the surface acceleration vector and temperature elevation under a nanosecond pulsed-laser. It is known that in nanoparticle removal rolling motion-based removal requires the least amount of acceleration. The distribution of the acceleration field on the surface of a half-space is obtained. The magnitude of a jump in the radial acceleration component at the boundary of the irradiated area and the dark zone is determined, and its use in nanoparticle removal has been discussed and demonstrated. Preliminary experimental data for a novel removal method based on this acceleration localization is presented and discussed.     

Diamond deposition on Ni3Ge single- and polycrystalline substrates

In order to find new materials for heteroepitaxial diamond growth Ni₃Ge single- and polycrystalline wafers were produced and used as substrates for diamond deposition in a microwave plasma system.The cubic phase Ni₃Ge substrate revealed to be an interesting and potential material for heteroepitaxial diamond chemical vapour deposition due to its: (1) lattice parameter matching within <1% the lattice parameter of diamond; and (2) coexistence with carbon up to its (congruent) melting point. Thus centimetre-size crystal boules were pulled from the melt using the Czochralski crystal growth method. These boules were sectioned into wafers and polished.Low-pressure diamond was grown on the Ni₃Ge wafers under various deposition conditions. The orientation of isolated diamond single crystals grown on the Ni₃Ge substrate surface show that heteroepitaxial nucleation occurred. Diamond nucleation was low, as seeding methods to enhance nucleation were not used.