Lasers play an enabling role in microelectronics and particularly in display manufacturing. Owing to its ability to interact selectively and flexibly, laser processing is successfully employed particularly in the fabrication of thin film microelectronics.
Substrate separation via laser lift-off
The Laser Lift-Off method facilitates the gentle separation of delicate films from rigid carriers. Releasing functional films from rigid substrates is desired to achieve lightweight, thin and high performance architectures. Separating the layers is achieved by layer-selective laser ablation of a thin polymer film. It is paramount to ensure that the adjacent microelectronic functional layer is not affected by the laser energy. Laser Lift-Off processing has thus become the domain of the short-wavelength excimer laser. Industrial scale excimer laser processing strategies comprise scanning a long (150 to 750 mm) and thin (0.3 to 0.4 mm) linebeam across the display panel as well as step & scan processing with rectangular beams as is preferred in the case of wafers.
Application example: Thin wafer
Advanced chip packaging strategies are aimed at realizing shorter signal path lengths and higher data transfer in conjunction with reducing energy consumption in mobile devices. In the same vein, Laser Lift-Off enables to reduce the device height of the mobile device by releasing carrier plates from ultra-thin wafers. Prior to thinning the Silicon wafers are temporarily bonded with their frontside to a glass carrier wafer. The bond has to be a temperature stable polymer. After thinning and backside processing, the thinned Silicon wafer is released via Laser Lift-Off from its carrier wafer.
Technically, this occurs by selective laser evaporation of the polymer film at a fluence of some 200 mJ/cm2. This process applies only to molecules most adjacent to the glass carrier and is hence unnoticed by the frontplane. Using a rectangular beam, a 300 mm size wafer is fully released by just some 1,000 laser pulses. Thin wafer technologies are pivotal to fabricating highly integrated, multilayer chips as employed in smartphones.
Application example: Flexible displays
Flexible displays, whether they form part of a smartphone, a tablet, a TV or an E-Reader share a distinct feature: the backplane is located on a bendable polymer foil instead of a rigid glass plate.
In manufacturing, a glass panel serving as temporary carrier for handling purposes is coated with a polymer film. Above the the polymer the thin film transistors are deposited followed by several display frontplane layers. Finally, Laser Lift-Off accomplishes the transition to full flexibility by passing an excimer laser beam through the glass carrier and evaporated the adjacent polymer film just within its first atomic layers. This separating effect is induced by applying a single laser pulse per area. By moving the substrate below the linebeam a generation 4 size glass carrier (730 mm x 920 mm) containing some 60 five inch displays is separated applying only a few thousand laser pulses.
En route to mobile devices with increasing functional density and decreasing height and weight, wafers of every diameter as well as displays of every diagonal are fast and gently released from their glass carriers by virtue of the excimer laser’s outstanding UV capabilities.