Bottom-up Au electroplating of high-aspect-ratio Si-based X-ray microgratings

High-aspect-ratio Au microgratings are critical optical components for grating based X-ray interferometry, which we recently demonstrated to be more dose efficient than conventional computer tomography in imaging of human breast under close-to-clinical condition. High quality dense gratings (50% duty cycle) on large area (4-inch wafers at least) are needed for this medical application. Silicon-based manufacturing allows high precision pattern definition by lithographic techniques and high-aspect-ratio silicon template fabrication by pattern transfer methods such as deep reactive ion etching . Uniform and voids-free gold filling of the high aspect ratio silicon trenches is the final step of our grating microfabrication. 

The electrolytes [1-3] used for Au bottom-up-fill (BUF) are simple and nontoxic, containing Na₃Au(SO₃)₂, Na₂SO₃, and micromolar concentrations of Bi³⁺ additive. A typical BUF goes through an incubation period, Bi-activated local deposition at the bottom, sustained bottom-up filling, and self-passivation [1-2]. High-aspect-ratio void-free Au BUF can be realized by controlling the solution concentration and pH, as well as the plating potential. Au BUF can also be realized in bent gratings attached to curved holders, reducing the elastic strains with respect to planar gratings bent to the same radius [3]. 

We systematically investigated the process reproducibility and the filling quality. Void-free BUF of high-aspect-ratio gratings is challenging because deposition on the sidewalls can happen during active filling, causing truncated BUF. Electroplating parameters need to be adjusted in accord with the feature dimensions. We successfully filled Au microgratings with various feature sizes with pitch ranging from 1 [4] to 5 µm [3], and we are challenging ourselves with uniform filling in more extreme structures (submicron pitch and aspect ratio higher than 100) and larger wafer sizes (up to 8-inch wafer scale).