Selective Oxidation and Carbonization by Laser Writing into Porous Silicon

Abstract

The selective formation of either oxidized or carbonized features into 2.5 µm thick porous silicon (PS) films using laser writing at a wavelength of 405 nm is demonstrated. Oxidized features are formed in air while carbonized features are achieved during the flow of propane at 600 sccm. Voids which have been previously associated with the use of propane are not observed, largely due to the rapid heating and high flow rates achieved in the experiment. Carbonized regions with feature widths down to 1.8 µm are achieved and chemical resistance to both hydrofluoric acid and potassium hydroxide is demonstrated. Once carbonized regions are formed, the surrounding areas can be overwritten in air to convert the surrounding regions into oxidized PS allowing films to be created with as‐fabricated, oxidized and carbonized regions. Energy dispersive X‐ray and Raman analysis confirms the presence of carbon within the written structures. At high optical powers, cracking around the carbonized features is observed which is attributed to a contraction of the film. Such cracking is not observed during selective oxidation of features. This work significantly enhances the ability to engineer and pattern the composition of PS films enabling selective control of the material's properties and functionality.

Morphological and Optical Transformation of Gas Assisted Direct Laser Written Porous Silicon Films

“…As seen in Figure 4 there is no significant increase in profile width with increasing laser fluence, as the width of the profile is entirely controlled by the width (focus) of the laser. This is contrary to results obtained in oxidised PS, [ 26 ] for which increasing the laser power created wider oxidised features in the film and further suggests carbonization results in increased thermal conductivity, preventing a localised in‐plane temperature rise. Overall, this component of the work demonstrates the ability to create highly selective carbonized regions of varying geometry, as the conditions for writing deeper or wider features are independent of each other.…”

“…The underlayer cracking indicates that damage occurs at power levels below those which cause previously reported visible damage on the surface. [ 26 ] For all PS films, including that seen in Figure 2, the onset of damage initiated at the Si/PS boundary, most likely due to the sudden differential stresses induced in the PS film from rapid contraction of the film due to carbonization. The stress is induced because the substrate acts as a zero displacement and constant temperature boundary while the porous film does not, preventing the contraction of the film, and increasing the stress in the film along this boundary.…”

“…Erfantalab et al showed that, for PS films of similar porosities to this study, nitrogen passivation at 600 °C resulted in a thermal conductivity of 0.7 W m −1 K −1 . [38] The thermal conductivity is similarly expected to be much lower for DLW nitridized PS samples than DLW carbonized PS samples, [26] resulting in the higher temperatures during DLW in nitrogen, causing the observed melting of the PS film in Figure 6. The sintered or melted PS in the laser affected region is not present in either carbonized or oxidised samples and does not represent ablated material.…”

“…Further, carbonization requires heating in acetylene, and at the temperatures required, safety issues complicate these processes. These limitations drive the requirement for methods to selectively, spatially passivate PS to enable the formation of complex PS-based devices that are compatible with standard mass production microfabrication processes.Studies have shown e-beam lithography, [17,18] imprinting using dies, [19][20][21] and direct laser writing [22][23][24][25][26][27] to be successful in patterning PS, however e-beam lithography is slow and costly, reducing capacity for mass-production, while imprinting does not allow mixed layers of patterned and unpatterned porosity, and can potentially damage the PS film due to its fragile structure. The method of direct laser writing (DLW) for creating PS structures, first demonstrated by Vlad et al for holographic gratings and later by Rossi et al for buried PS waveguides, [22,28] can overcome the limitations of both e-beam lithography and imprinting.…”

“…Studies have shown e-beam lithography, [17,18] imprinting using dies, [19][20][21] and direct laser writing [22][23][24][25][26][27] to be successful in patterning PS, however e-beam lithography is slow and costly, reducing capacity for mass-production, while imprinting does not allow mixed layers of patterned and unpatterned porosity, and can potentially damage the PS film due to its fragile structure. The method of direct laser writing (DLW) for creating PS structures, first demonstrated by Vlad et al for holographic gratings and later by Rossi et al for buried PS waveguides, [22,28] can overcome the limitations of both e-beam lithography and imprinting.…”