This EArly-concept Grant for Exploratory Research (EAGER) grant provides funding for testing the feasibility of pulsed laser processing that would enable high rate and cost effective exfoliation of thin single crystalline silicon carbide (SiC) layers suitable for fabrication of micro-devices. The attractiveness of single crystalline SiC in a variety of device applications is counteracted by the very high cost of substrates. The main goal of this project is to exfoliate multiple thin layers from one standard thickness SiC wafer using hydrogen ion implantation and laser processing, and transferring such layers to silicon or polycrystalline SiC substrates in order to enable a broader use of this material. Hydrogen ion implantation into SiC can form a zone of voids and microcracks at a depth approximating the implantation range, and lead to exfoliation at subsequent very high temperatures. The proposed approach of laser-assisted exfoliation would utilize a lower implantation dose and lower annealing temperatures, thus reducing damage and allowing bonding of SiC to temperature-sensitive substrates. Interactions between ion implantation conditions, laser irradiation, and heating will be explored to gain preliminary understanding of the path leading to exfoliation of continuous single crystalline layers suitable for electronic devices. Feasibility studies of the proposed process will be conducted and structural and electrical properties of processed samples will be investigated.
This research will concentrate on a 4H polytype of SiC, as it is of most relevance for power and high voltage applications, but laser-assisted exfoliation should also be extendable to other semiconductors and wide bandgap materials, such as gallium nitride (GaN). If successful, the results of this exploratory research will lead to development of a rapid and cost-effective method of exfoliating single crystalline layers of SiC and bonding them to lower cost substrates that are compatible with high performance electronic, photonic, sensor, or MEMS device operation.
The objective of this EAGER-funded program was to test whether short laser pulses could selectively peel off (exfoliate) a thin layer of single crystalline silicon carbide (SiC) from a larger single crystalline wafer. Such a thin layer, of the order of 1 micrometer in thickness could then be transferred to a lower cost substrate and utilized for fabrication of micro-devices. Although laser ablation and laser induced evaporation from a solid surface are well known and extensively studied, at the onset of our program there was no evidence whatsoever that a crystalline layer of controlled thickness can be removed from the surface of a crystalline substrate and transferred to a new substrate. Our strategy was to produce by ion implantation a very narrow highly opaque layer parallel to the surface and positioned about 1 um under it. Since a 4H polytype of SiC is a wide bandgap semiconductor, in the absence of implantation-induced defects it is transparent over a broad range of light wavelengths. Hydrogen ion implantation of 6E16 ions/cm2 at 180 keV produced a highly absorbing layer, only 150 nm thick and centered at 1.07 um (1070 nm) under the surface. Laser irradiation from the back side of the crystal, opposite to the implanted side, was utilized. Each 5 ns pulse from a frequency doubled Nd:YAG laser (532 nm wavelength) traversed the entire thickness of a 360 um thick wafer and was largely absorbed in the hydrogen-rich thin subsurface layer. Initially we were counting on the unique properties of hydrogen implanted into a semiconductor crystal, i.e. the fact that under furnace annealing hydrogen can form nano-voids inside the crystal that grow and lead to controlled exfoliation. Using a laser that locally produces temperatures much higher than the 800-1000C used in a furnace, might potentially lead to a similar outcome on a much shorter time scale. However, we discovered that hydrogen nanovoids are not required for laser induced material exfoliation. A series of experiments with different H implantation conditions, and eventually samples implanted with boron ions, where nanovoids never form during thermal annealing in a furnace, demonstrated that very high thermal stresses produced by strong absorption of light in a narrow layer are sufficient to produce delamination of the near-surface layer. Thus the mechanism of laser-induced exfoliation is entirely different than the one that is responsible for exfoliation in a furnace. Exfoliation under isothermal conditions present is a furnace is only possible for hydrogen implanted layers and is uniquely tied to the agglomeration of hydrogen into voids of nanoscale dimensions that lead to microcracks and eventually to layer separation from the main substrate. Laser exfoliation occurs under approximately adiabatic conditions, with the fracture in a narrow laser heated zone occurring faster than conductive heat transfer out of this very small and momentarily very hot region. Our data demonstrate that laser exfoliation is possible whenever highly absorbing subsurface zone is produced through crystalline lattice modification, regardless of the implanted species. The very high laser-induced thermal stresses are responsible for micro-cracking of the crystal along the highly absorbing plane and exfoliation of a near-surface layer. Our observations mean that exfoliation of thin surface layers can be achieved in a broader range of materials than those that are suitable for hydrogen induced furnace exfoliation. The data show that an order of magnitude lower implant doses can be used with ions heavier than hydrogen, although this advantage is somewhat countered by a need for higher accelerating voltage to achieve a given depth of implantation, which translates into the film thickness.
