Sublimationsschneiden von Silizium mit ultrakurz gepulster Laserstrahlung
Aachen / Apprimus Verlag (2016) [Dissertation / PhD Thesis]
Page(s): 1 Online-Ressource (Seite i-xix, Seite 1-143) : Illustrationen, Diagramme
Sublimation cutting with ultra-short pulsed Laser radiation is due to the very short interaction time between laser beam and material a promising singulation technology for semiconductor parts. The short interaction time leads to a direct vaporization of the illuminatedarea on the part instead of melting and therefore increased precision and less heat affection.In contrast to mechanical singulation technologies no mechanical load on the sensitive material has to be considered. By decreasing the cutting kerf width and cutting street widthit is possible to maximize the wafer efficiency, the number of elements per wafer. Becausethe material thickness of the chips is limited to a certain minimal value, a reduction of the cutting kerf width leads to increased aspect ratios – the ratio between kerf depths to kerf width. The goal of this thesis is to localize the absorption of the incoming Laser radiation in thecutting kerf in order to increase the resulting aspect ratio in the sublimation cutting process.The local increase in temperature is realized by an additional CW Laser source. This Laser radiation is focused into the cutting kerf and leads to a temperature profile, which is depending on the process parameters like average power or feed rate. Furthermore thisthesis focus on the correlation between the aspect ratio and the following process parameters: Laser peak intensity, wavelength, pulse duration, beam profile, polarization and pulsedistance. By optimizing the peak intensity the resulting aspect ratio can be increased by up to 50 %.The influence of the investigated wavelength of 355 nm, 532 nm and 1064 nm on the aspectratio can be distinguished in two different ranges of intensity. In the range of low intensity, very close to the ablation threshold, a wavelength of 355 nm is best suitable in orderto obtain maximum aspect ratios. At intensities much higher than the ablation threshold the use of Laser radiation with a wavelength of 1064 nm is advantageous. P-polarized Laser radiation leads to higher aspect ratios than s-polarized Laser radiation, because of the higher amount of absorbed intensity. An increased temperature of the silicon substrateleads to a lower ablation threshold and to increased aspect ratios. A reduction of the spatialpulse distance to a few microns results in thermal accumulation effects and finally to anincreased substrate temperature. Investigations of the influence of the pulse duration andthe beam shape on the aspect ratio do not deliver a clear result.