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Pulsed selective laser removal of nano- and micro-particles Shishir Shukla,*1,2 Sergey I. Kudryashov,2 Kevin Lyon2 and Susan D. Allen1,2 1

2

Department of Mechanical Engineering, University of Memphis, Memphis, TN 38152 Department of Chemistry and Physics, Arkansas State University, Jonesboro, AR 72467-0419

ABSTRACT Selective laser removal of micro-particles of one chemical composition from their mixture with micro-particles of another chemical type pre-deposited on hydrophobic or hydrophilic surfaces have been demonstrated by means of steam laser cleaning method realized with nanosecond IR laser and various liquid energy transfer media (ETM). Microscopic imaging of particle mixture deposition, ETM dosing and final particle removal has been performed with the help of timeresolved optical microscopy. Optimal ETM/particle combinations for selective targeting and removal of specific particles from their mixture on the surfaces have been revealed. Keywords: Nanosecond CO2 laser, isopropyl alcohol, de-ionized water, sub-micron and micron sized particles, selective laser cleaning

1. INTRODUCTION In the recent time a lot of interest is being shown in the field of nano-scaled structures. One of the applications in this area is nano-clustered devices1. These nano-clustered devices have found vast application, such as nano-wires, nanomagnets, nano-electronics and many others. Proper spatial arrangement of nano-particles is important for operation of the resulting nano-device (especially in a case of composing nano-particles of different chemical types, e.g., inorganic and organic particles or bio-species), while nano-particle manipulation capabilities are important for repairing such devices, or improving their functions. Potentially, selective motion or removal (erasing) of separate nano-particles or their ensembles of specific chemical type (composition) can be realized by means of well-recognized advanced laser cleaning methods2-4 developed for total removal of nano-and micro-particles of different chemical compositions from various critical substrates in a non-contact fashion without damage2-4. These methods exist in basically two variants, dry laser cleaning (DLC) and steam laser cleaning (SLC). In the 1990s DLC was predominantly used to clean Si surfaces2,6. This process efficiently removes particles by “trampoline” and “hopping” effects occurring during thermal expansion of the Si wafer or the particles5, respectively. In the case of SLC cleaning, an energy transfer medium (ETM) is employed to clean particulate contaminants8-10. In this mechanism, condensed vapor acts as an energy transfer medium between the laser energy and the Si substrate and/or particles. The ETM vapor, upon condensation, forms a uniform thin layer on the substrate depending on the ETM-substrate interaction. The substrate is irradiated by the laser at a specified time after the ETM dosing pulse, causing the surface to undergo a rapid temperature increase. When the temperature exceeds the 0.9Tcr (where Tcr is the critical temperature of the ETM) of the ETM, the ETM layer in the vicinity of the Si substrate undergoes interfacial explosive boiling and lifts off from the substrate10. The combination of the “gas piston” force from ETM explosive boiling under the particles and the viscous drag force on the particles exerted by the moving ETM layer helps to remove the particles from the surface9. If the ETM layer thickness is less than or equal to thickness of the lasersuperheated ETM layer, the “gas piston” mechanism prevails. In this work we used nanosecond IR CO2 laser to move or to remove (erase) nano- and micron-sized particles as the model structures of interest from Si substrates in DLC and SLC variant. We show that particles of different sizes and chemical types can be selectively cleaned or moved from the Si surface via the manipulation of the ETM type and thickness as well as the energy fluence.

Laser-based Micropackaging, edited by Friedrich G. Bachmann, Willem Hoving, Yongfeng Lu, Kunihiko Washio, Proc. of SPIE Vol. 6107, 61070S, (2006) · 0277-786X/06/$15 · doi: 10.1117/12.647056

Proc. of SPIE Vol. 6107 61070S-1

2. EXPERIMENT Monodispersed polystyrene (PS, Kiesker Bio) and alumina particles of radii 0.3-10 µm were deposited on a 0.25-mm thick commercial silicon (Si-100, Motorola) wafers with the help of spin-coater. The output of a 10.6-µm, 100-ns TEA CO2 laser (TEM00) beam was apertured in the center by a 5-mm wide iris and focused using a spherical calcium fluoride lens (focal length f = 11 cm) onto a surface of the Si substrate which was mounted on 2-D manual stage of the microscope (Fig.1). The temporal profile of the beam is shown in Fig. 2. The sample was placed under the microscope for real time monitoring of laser-driven manipulations with particles using a TV monitor. The laser beam fluence was attenuated using polyethylene sheets. The beam fluence was measured by splitting off a part of the beam to a pyroelectric detector (Gentec ED-500).

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Fig.1 (left). Experimental setup (PD − photo diode, BS − beam splitter, L − lens). Fig.2 (right). Temporal profile of the TEA CO2 laser beam.

A previously described, dosing system was used to deposit a uniform layer of ETM on Si substrates7,8. Pulsed vapor dosing was achieved by using pressurized nitrogen gas with a triggered valve connected to a bubbler immersed in a glass flask filled with heated isopropyl alcohol (IPA) as ETM. The vapor from the flask was directed on the sample surface using a heated nozzle attached to the flask. The optimum distance between the doser nozzle and the substrate for uniform deposition of a thin IPA layer was 5 cm. The dosing conditions for these experiments were gas pressure of 0.7 bar and ETM temperature of 44oC for IPA. The thickness of the IPA layer was measured using a HeNe laser at an angle of 30o from the normal to the substrate. The reflected beam was captured by a photodiode (ThorLabs DET 210). The reflected beam produced optical interference fringes used to calculate the IPA layer thickness (Fig.3(a,b)). The linear relationship between layer thickness L and dosing time Tdose can be seen in Fig.3b. The same dosing system (gas pressure of 0.7 bar, flask water and nozzle temperatures of 40 oC, dosing times Tdose=0.10.9 s) was used to deposit onto the Si wafers 3-6 cm wide (depending on Tdose) layers of separate water droplets generated by condensing a steam vapor a from de-ionized water. Deposition, laser removal and natural drying of the water layers were monitored in real time both by time-resolved optical microscopy (Fig.4) and by observing the optical reflectance/scattering of a HeNe probe laser focused on the center of the irradiated area (Fig.5). Time-resolved optical microscopy visualization of the transient water droplets on the Si wafer surfaces was performed at magnification of 400× using a Mitutoyo WH microscope equipped with a digital camera (Olympus 3030). Subsequent frames of water-dosed Si surfaces were taken at a rate 30 frames/s and then these movies were analyzed using Windows Movie Maker graphics software to track spatiotemporal evolution of separate water droplets frame by frame. It was observed that at the instant tlas when the KrF laser fires the deposited water layer consists of separate micron-sized droplets (Fig.4b) as the several nanometer-thick native silicon oxide surface film on the Si wafers has de-wetting properties relative to water with an expected contact angle of 20-45o.11 Coalescence and Ostvald ripening of water droplets were found to occur on a multi-


ms timescale during and after the dosing pulses (Fig.4c,d), thus, being negligibly slow in the microsecond of PA measurements performed in this work. Importantly, at tlas surface coverage, S, of the Si wafers by water droplets, i.e., the total surface of the water/Si boundary, was nearly linearly proportional to Tdose within the range of 0.1-1.0 s (Fig.6), indicating about constant thickness of large water

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Fig.3 (left). (a) Transients of the HeNe reflectance on the IPA dosed Si surface for different dosing lengths Tdose; (b) Calculated IPA layer thickness L as a function of Tdose.

droplets, H≈7-10 µm, at different Tdose. This fact is consistent with results of absolute measurements of deposited water mass, M, performed with CaSO4 absorbent (drierite), exhibiting linear increase of M with increasing Tdose at the overall deposition rate of 0.007 g/s. A similar linear trend has been obtained for natural drying times, tdry, vs. Tdose (Fig.6) measured by tracking the HeNe scattering from the water-dosed Si surfaces (Fig.5), demonstrating local natural drying of the deposited water at the almost constant vaporization rate. The linear change of tdry, vs. Tdose can be interpreted in terms of the constant droplet thickness H, while the maximum total surface of the water/air boundary, being approximately equal to S for multi-micron sized droplets with their lateral widths W»H, linearly increases with Tdose. Most probably, during the coalescence/Ostvald ripening and reverse natural drying process the water droplets are adjusting their shapes (mainly, widths W of their top and bottom bases) to keep the droplets thickness constant for the reason of contact angle constraints for this combination of air/water/Si surfaces. In cleaning experiments the heating CO2 laser was fired at the instant tlas=0.06 s after the end of each liquid deposition step, accounting for the nearly 0.04 s delay for the dosing jet to propagate between the nozzle and the Si substrate surface. The gas valve and CO2 laser were triggered manually in a single-shot mode with the corresponding delay times using a pulse generator (Stanford Research Systems DG 535). The experiments were performed under the Mitutoyo microscope under ambient conditions. For measurements of cleaning thresholds, examination of the laser-irradiated spots on contaminated samples was done measuring the area of the cleaned spot. The experiments were monitored by using the digital camera mounted on the top objective of the microscope. The camera is kept in movie mode (Olympus Camedia) to capture the surface processes. The movie has been recorded with the help Hi8 recorder (Sony Hi8). Another


visual output is given to a black and white TV monitor to watch the experiments in real time. These movies are then digitized by using Microsoft Movie Maker Software. The separate frames of these movies are then extracted by using movie editing software (VirtualDub, http://www.virtualdub.org).

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Fig.4. Snapshot of the wet Si substrate at dosing time Tdose = 0.3 s: 33 ms before tlas (a), at t = tlas (b), 33 ms after tlas (c), 100 ms after tlas (d) (frame size 0.37×0.37 mm). Note that the heating laser has not been fired in this visualization series.


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Fig.5. Transients of dose HeNe reflectance/scattering from the Si substrate dose dosed with water with (1) and without (2) excimer laser irradiation and the corresponding electrical pulse for the dosing valve (3) of duration Tdose=0.3 s. Fig.6. Deposited water mass M (curve 1), drying time tdry (curve 2) and Si surface coverage S (the product of square and surface density of water droplets, curve 3) as a function of Tdose.

3. RESULTS AND DISCUSSION Our CO2-laser cleaning results presented in Fig.7 and Fig.8 show that 3-µm and 10-µm PS particles – larger than 3 µm − are completely cleaned from the Si substrates with pre-deposited micron-thick IPA layers. Such CO2-laser cleaning of these particles can be achieved at very thin ETM thickness, although similar cleaning under dry conditions does not remove any of these particles from Si surfaces. This may mean SLC rather DLC cleaning mechanism for the PS particle/IPA ETM combination under CO2-laser irradiation, when particle/substrate interaction may be strongly reduced in presence of IPA ETM because of the lyophylic character of bulk polystyrene and polystyrene particles with respect to IPA (note that in presence of ETM particle/substrate interactions should be described by triple Hamaker constants, which can be positive or negative, indicating attraction or repulsion of a particle to the substrate in presence of the particular ETM). The SLC cleaning threshold fluence for these PS particles is lower than the damage threshold of the Si substrate. In contrast, both CO2-laser SLC and DLC are completely inefficient for alumina particles (potentially, because of their sub-micron size).

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A Fig.7. Microscope images of the same spot on the Si substrate with pre-deposited 10-µm PS particles before (left) and after (right) SLC with IPA ETM (maximum fluence =10.1J/cm2). Frame size is 2.5×2.5 mm.


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Fig.8. Microscope images of the same spot on the Si substrate with pre-deposited 3-µm PS particles before (left) and after (right) SLC with IPA ETM (maximum fluence =10.1 J/cm2). Frame size is 2×2 mm.

Similarly, Fig.9 and Fig.10 show CO2-laser SLC results for 3-µm and 10-µm PS particles cleaned with water as ETM. In this case the cleaning threshold for particles was also smaller than the Si damage threshold. Best cleaning results are achieved at longer dosing pulses (in Fig.9 and Fig.10 Tdose = 0.7 s) indicating the SLC character of the cleaning process, while for shorter dosing pulses and DLC the cleaning is very incomplete or absent, respectively .Also sub-micron alumina particles can be cleaned well with the water as ETM.

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Fig.9. Microscope images of the same spot on the Si substrate with pre-deposited 10-µm PS particles before (left) and after (right) SLC with water ETM (maximum fluence =9.5 J/cm2). Frame size is 2.5×2.5 mm.


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Fig.10. Microscope images of the same spot on the Si substrate with pre-deposited 3-µm PS particles before (left) and after (right) SLC with water ETM (maximum fluence =9.5 J/cm2). Frame size is 2×2 mm.

Summarizing, comparative analysis of our cleaning results shows (Fig.11) that there is some selectivity in cleaning of PS and alumina particles in presence of IPA or water ETMs, respectively. In particular, in the presence of IPA as an ETM we can clean PS particles for size ≥3 µm but cannot clean alumina particles at all. When water is used as an ETM then it is possible to clean all type of particles. In principle, one can ascribe the ETM effects to a quasi-static reduction (“screening”) of corresponding particle-substrate interactions by ETM. However, one should also take into account that CO2-laser fluences of 9-12 J/cm2 used in this work are well above the corresponding explosive boiling thresholds of IPA (0.6 J/cm2) and water (1.5 J/cm2) at the 10.6-µm wavelength, so there is a dynamic near-critical bulk superheating of liquid ETMs − IPA layers or water droplets − by the intense laser radiation. Under such superheating conditions lyophobic and lyophylic effects in particle/ETM and ETM/substrate interactions are strongly enhanced by local density augmentation phenomena,12 determining dynamic viscous, tension and other important properties at solid/liquid interfaces in these experiments. In general, bulk superheating of various liquid ETMs by CO2-laser radiation provides good opportunities to study these fundamental near-critical phenomena and their various applications.

Fig.11. Comparative figures of PS (left) and alumina (right) particles cleaning using water (top), IPA (middle) and IPA-water mixture (bottom). Frame size is 2.5×2.5 mm.


4. CONCLUSIONS In this work we studied pulsed nanosecond CO2-laser cleaning of sub- and micron-sized polystyrene and alumina particles from Si substrate in dry and steam laser cleaning regimes. Our results show no dry laser removal for any these particles, while their 100% steam laser cleaning was observed for specific particle/ETM combinations (for particles not smaller than 3 microns in diameter). Particularly, we have demonstrated selective cleaning of polystyrene particles with IPA ETM, while no cleaning was observed for alumina particles in presence of IPA. For water as ETM, both types of particles can be cleaned at appropriate ETM dosing. We ascribed this selectivity to dynamic effects in highlysuperheated ETM such as local density augmentation effect strongly affecting dynamic characteristics – viscosity, tension and others − of liquid ETMs.

ACKNOWLEDGEMENTS Authors gratefully acknowledge current support by NSF CTS Grant # 0218024 under the technical direction of Dr. Triantafillos J. Mountziaris.

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