Universal threshold for the steam laser cleaning of submicron spherical particles from silicon

The efficiency of the “steam laser cleaning” process is examined. For the investigation of the physics of particle removal from the particularly interesting surface of silicon we have deposited well-characterized spherical polymer and silica particles of different diameters ranging from several tens to hundreds of nanometers on commercial wafers. As a result of our systematic study we observe a sharp threshold of the steam cleaning process at 110 mJ/cm2 (λ= 532 nm, FWHM= 7 ns) which is independent of the size (for particles with diameters as small as 60 nm) and material of the particles. An efficiency above 90% after 20 cleaning steps is reached at a laser fluence of 170 mJ/cm2. Experiments with irregularly shaped alumina particles exhibit the same threshold as for spherical particles. PACS: 81.65.C; 79.60.Bm The removal of submicron particles from highly damageable surfaces poses an increasing challenge to engineers worldwide. Unless removed, these particles are responsible for production losses or malfunctions, e.g. in electronic circuits [1]. One way to overcome the strong adhesion forces acting on the particles is to make use of inertia effects. High accelerations on the order of 106 g are needed for particle removal [2] which cannot be provided by conventional methods such as ultrasonics [3]. It has recently been demonstrated that particles (Al2O3, SiO2, MgO, SiC, CeO2, BC) as small as 100 nm in diameter can efficiently be removed by laser cleaning [4–10]. In this “steam laser cleaning” process, a thin liquid film is condensed onto the substrate and then evaporated momentarily by irradiating the surface with a short laser pulse. The energy absorption in the substrate leads to a fast temperature increase both in the substrate and, due to heat transport, in the liquid film. Bubble nucleation at the solid/liquid interface and the subsequent explosive vaporization of the liquid cause the removal of contaminants. ∗Corresponding author. (E-mail: [email protected]) Previous investigations focused on the demonstration of the cleaning effect itself [4–7] and on the study of the bubble nucleation and growth process [12–14]. To the best of our knowledge only a few quantitative measurements on the influence of the process parameters of the steam laser cleaning (laser fluence, particle size and material) on the cleaning efficiency have been reported. However, these experiments were performed either with irregularly shaped particles that tend to form coagulates and thus inhibit a quantitative analysis of single particle removal [8–11] or the cleaning efficiency was investigated only for a few different kinds of particles [9, 10]. For a systematic investigation of the underlying physical processes on the cleaning behaviour we changed [15] from the irregularly shaped alumina particles commonly used as test particles to small monodisperse polystyrene (PS) or silica (SiO2) spheres [16] with diameters in a range from 60 nm up to 800 nm. This allows studies on the size and material dependence of the cleaning efficiency. Preliminary results for PS particles with diameters of 800 nm have already been presented in [17]. 1 Sample preparation–colloidal particles as model contaminants After rarefaction with isopropyl alcohol (IPA) the particle suspension was deposited onto the wafer surface by spin coating. Since we were interested in the preparation of samples with isolated particles, high evaporation rates and low concentrations of colloids were chosen for drying in order to keep coagulation to a minimum. We were able to prepare samples with essentially isolated spheres (> 95% for particles with diameter> 300 nm) at high particle densities. The percentage of isolated particles increases with particle diameter. An example of PS particles with diameters of 480 nm is shown in Fig. 1. For a comparison with previous studies [4–6, 8–11] we have in addition chosen commercially available micropolish powder [18] with a mean diameter of 300 nm as a source of alumina (Al2O3) particles that were applied to the sample in