UC Berkeley

Based on the second law of thermodynamics, energy has dispersive nature. When dispersive energy is confined, useful functions can be accomplished. Lasers offer a facile manner to confine energy in spatial and time domain. A laser light can be directly delivered in 2D/3D location of a target. Due to the monochromatic feature of laser light, the delivered laser light can be selectively absorbed in a specific material of the target. Furthermore, by confining light energy in varied time scales such as femtosecond, nanosecond, and continuous, the light energy absorbed in the specific material can initiate various reactions such as melting, deposition, dewetting, crystallization, and ablation, which are demonstrated throughout this study.

We demonstrated the production of graphene folds induced by femtosecond laser ablation. A single laser pulse irradiation on supported graphene produced an ablated spot featuring circumferentially periodic graphene folds in its proximity. The graphene fold structure was constructed through folding of a single layer graphene segment. We investigated the laser fluence effect on the graphene fold structure. Interestingly, the graphene fold structure was not further produced by subsequent pulses. Based on these findings, we achieved submicron graphene patterns without folding structures by multi-pulse scanning. We also performed ablation on suspended graphene and verified that interaction with the underlying substrate is required for the formation of graphene folds. We expect this one-step folding method may provide a controlled process to explore unique properties of graphene folds.

Due to the multi-photon absorption, various thin films can be effectively ablated using femtosecond lasers, even if the photon energy corresponding the laser wavelength is smaller than the bandgap. We invented the laser-induced pattern transfer (LIPT) method for the transferring and subsequent patterning of graphene in a single processing step. Via the direct graphene patterning and simultaneous transferring, the LIPT method greatly reduces the complexity of graphene fabrication while augmenting the flexibility in graphene device design. Femtosecond laser ablation under the ambient conditions is employed to transfer graphene/PMMA micron-scale patterns on arbitrary substrates, including a flexible film. Suspended cantilever structures are also demonstrated over a pre-fabricated trench structure via the single step method. The feasibility of this method toward the fabrication of functional graphene devices is confirmed by measuring the electrical response of a graphene/PMMA device under laser illumination. In addition, we invented the laser ablation and wet etching (LAWET) method for producing suspended structures. By combining femtosecond laser ablation of multilayered sample such as a SiO2 layer on a silicon substrate and subsequent anisotropic wet etching, various suspended structure with simultaneous removal of ablation debris were demonstrated.

Irradiation of an ultrathin film with a beam-shaped nanosecond laser was performed to achieve site-selective controlled dewetting of the film into nanoscale structures. Nanosecond pulse is longer than the electron-lattice relaxation time, therefore, the dewetting can be initiated, and oxidation can be minimized. As a proof of concept, the laser-directed dewetting of an amorphous silicon thin film on a glass substrate was demonstrated using a donut-shaped laser beam. Upon irradiation of a single laser pulse, the silicon film immediately melted and dewetted on the substrate surface. The irradiation with the donut beam induced an unconventional lateral temperature profile in the film, leading to thermocapillary-induced transport of the molten silicon to the center of the beam spot area. As the transported molten silicon was solidified, the process was completed, resulting in phase transformation from amorphous to crystalline structure. As a result, the ultrathin amorphous silicon film was dramatically transformed to a crystalline silicon nanodome of increased height. This morphological change enabled further dimensional reduction of the nanodome as well as removal of the untreated film material by isotropic silicon etching.

We demonstrated glass melting and semiconductor material deposition on a glass substrate using continuous CO2 laser. We note that CO2 laser is absorbed in glass materials that are transparent to visible lasers; however, the useful property cannot be utilized in nano/micro manufacturing, due to the relatively large beam size. Therefore, we proposed the adiabatic thermal energy confinement (ATC) strategy, whereby heat dissipation through a substrate was removed. As a proof of concept, site-specific deposition and melting mediated deformation were demonstrated on ATC platforms. The ATC strategy can be applied to visible lasers with material sequence modification.