High-precision alignment system for DMD-based maskless lithography in multilayer exposure applications

Maskless lithography based on Digital Micromirror Devices (DMDs) has advanced rapidly in recent decades and offers a flexible and efficient alternative to conventional lithography methods that require physical photomasks. In many applications, multilayer exposure and patterning of photoresist (PR) layers are common, essential, and required for most lithography systems. Consequently, effective methods are needed to ensure accurate layer-by-layer alignment of the successive exposure patterns. However, despite the importance of alignment, current DMD-based maskless lithography systems lack methods to achieve high-precision alignment across multiple layers. Accordingly, this study proposes a simple yet effective alignment system for DMD-based maskless lithography incorporating a downward image sensor and an upward image sensor. The downward image sensor captures the alignment targets on the substrate or previous layer, and the coordinates of these targets are determined using digital image analysis techniques. The upward-facing sensor simultaneously measures the offset distance between the optical engine and the downward-facing sensor. Given the information acquired from the two sensors, the current posture of the substrate relative to the DMD can be accurately determined and the planned exposure coordinates for the subsequent layer corrected as needed. The experimental results show that the proposed method achieves an average alignment accuracy of $1.76\mu m$ in the $x$-direction and $0.63\mu m$ in the $y$-direction, with a maximum error of $3\mu m$. This level of precision meets the alignment requirements for circuit board exposure. Alignment exposure tests conducted on commercial substrates confirm the applicability of the proposed alignment system for real-world lithography.