Robust PECVD SiC membrane made for stencil lithography
Stencil lithography (SL) is a shadow mask based technique which allows parallel, resistless, micro- and nano-patterning of material through apertures in a membrane (stencil) onto a substrate . The stencils are usually made of LPCVD low-stress SiN due to its outstanding physical and chemical stability. However, limitations are found in some special designs, where membranes could be distorted because of the deformations induced by stress from the deposited materials. To solve this problem, the fabrication process has to become more complex by introducing corrugations on the membranes for reinforcement . Here we report on a recently developed PECVD SiC based shadow mask for applications in SL. The SiC stencil demonstrated a better performance than the shadow mask made of SiN in terms of robustness to deformation and resistance to etching. The SiC was grown by PECVD at temperatures below 400 oC. The intrinsic stress of SiC can be tuned from compressive to tensile by varying the annealing time to fulfill the low stress requirement for the stencil membrane. During this process, the amorphous SiC is starting to crystallize and thus enhance its robustness, which translates into a higher Young’s modulus . The fabrication of the SiC stencil (Fig 1a-d) started from a Si wafer with 540 nm thick PECVD SiC on the front side and 200 nm thick PECVD SiN on the backside. SiC was patterned by dry etching in AMS 200 DSE, following by a DRIE process on the backside to open the membrane windows. The residual 50 m thick Si left after the DRIE process was etched away by KOH. Fig 1e shows the SEM image of a released 1 mm x 1 mm area SiC membrane. The SiC stencil was then placed on a Si substrate for metal deposition. Fig 2 shows the SiC stencil and the accurately duplicated corresponding patterns on the substrate after 5 nm Ti and 50 nm Au deposition in an ebeam evaporator. The results are comparable to those from SiN stencil deposition, where the main challenge is the enlargement of the pattern, known as blurring . The robustness was compared between the low-stress (110 MPa) SiC and the low-stress (200 MPa) SiN by depositing Cr film, which has a high tensile stress. Fig 3 shows the different bending behaviors of cantilever structures in the membranes when coated with 25 nm Cr. The SiC cantilevers bend 75% less than the SiN ones by measuring the maximum deformation, providing a wider possibility for the choice of the deposited material. The higher robustness to deformation of the SiC membrane indicates the possibility of having in general a more precise pattern duplication from the stencil to the substrate. Another advantage of SiC is its high chemical inertness, enabling it to be a good mask for both dry and wet etching. The SiC stencil was used as a mask to etch 500 nm SiO2 by RIE without any metal protection layer on the membrane due to its high etching selectivity (Fig 4a). By comparison, a metal protection layer is essential for SiN stencil under similar conditions for dry etching . The transferred patterns (Fig 4b) show a sub 100 nm loss in diameter for a 3 m diameter aperture, as shown in Fig 4c and d. There is no evident damage to the SiC stencil after the dry etching.