Surface acceleration during dry laser cleaning of silicon

We report on measurements of the surface acceleration for the application of dry laser cleaning. For that purpose, industrial silicon samples were irradiated by a frequency-doubled Q-switched Nd:YAG laser. The surface displacement was measured by a heterodyne interferometer and recorded by a digital storage oscilloscope. Several hundreds of shots were averaged to give smooth displacement curves which could be derived numerically. The experiments show that the highest accelerations, which are thought to be responsible for the cleaning, occur on the time scale of the laser pulse. Simple theoretical models are in good agreement with the experimental data. The maximal displacement depends only on the deposited energy, while the maximal acceleration shows also a strong dependence from the temporal pulse shape. This knowledge allows one to optimize the pulse shape for the cleaning process. PACS: 81.65.C; 79.60.Bm In order to remove particles with diameters below 1 μm from substrates, a cleaning force exceeding the strong van der Waals adhesion must be applied to the particle. Because the van der Waals force grows rapidly with decreasing particle diameter as compared to the forces used in conventional cleaning techniques (e.g., ultrasonic cleaning), these methods are not capable of removing particles below several hundreds of nanometers. A technique called dry laser cleaning has been proposed to remove such small particles from substrates [1– 4]: A short laser pulse irradiates the substrate, which expands during the pulse and accelerates the particles. The particles are ejected because of their inertia at the high negative acceleration (the rapid stop of the thermal expansion) at the end of the laser pulse. Dry laser cleaning has been proposed for cleaning surfaces like hard disk heads and silicon wafers in microelectronics, in which such small particles can lead to defects on disk heads and yield loss in chip production [5]. COLA’99 – 5th International Conference on Laser Ablation, July 19–23, 1999 in Göttingen, Germany We report on the surface acceleration of commercial silicon wafers in dry laser cleaning conditions with a frequencydoubled Nd:YAG laser. 1 Experimental setup The experimental setup is depicted in Fig. 1: The beam of a Q-switched Nd:YAG laser is frequency-doubled (λ= 532 nm) and guided to the silicon substrate by several prisms. The laser energy can be attenuated in a controlled way by 2n glass plates (n = 1 . . . 5); if necessary, a lens can be used to increase the energy density at the surface. We used commercial silicon wafers (Wacker siltronic, thickness 0.75 mm) as substrates. The surface displacement of the silicon substrate was measured by a heterodyne interferometer (B.M. industries, SH-130, bandwidth 200 kHz–45 MHz) and recorded on a digital storage oscilloscope (LeCroy 9450A, 400 MHz). A fast photodiode (FND100, rise time 1 ns) was used to register the temporal shape of the incident laser pulse. The pulse duration could be changed by variation of the flash lamp energy of the Nd:YAG laser between approximately 10 ns and 20 ns DM Nd:YAG BS L Si PD