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October 16, 2013

Improved Compensation for a Reduction Stepper to Meet the Challenges for Advanced Packaging Applications

Presented at IMAPS Microelectronics, October 2013


Advanced Packaging manufacturers require steppers that will provide solutions for the challenges encountered with new advances in wafer-level packaging technologies such as TSV, eWLB, silicon and glass interposers being utilized in leading edge mobile devices. Step and repeat photolithography systems capable of finer imaging with tighter overlay are being introduced to meet the challenging manufacturing requirements associated with the mix and match needed for volume production on larger wafers.

A 2X reduction stepper with unique features that extend the range of compensation is necessary to achieve the tighter specifications needed for many Advanced Packaging applications printed on 300 to 450mm wafers. A high throughput projection optical system is used to expose circuit patterns from a reticle mask onto a substrate to image features with the optimal fidelity required for Advanced Packaging technologies. The camera incorporates 350-450nm light from a mercury arc lamp that is transmitted through the mask containing circuit patterns. The imaging field prints a large 52mm x 66mm area in a single exposure. These features enable a system to process wafers in fewer shots which result in higher throughput using lower power.

Substrates are positioned with a precise x, y, Θ stage by locating marks using an off-axis, bright field alignment system with fully trainable mark feature capability. The approach results in precisely placed features within a layer and from layer to layer without directly referencing the reticle. The integrated metrology and precision positioning subsystem technologies are combined with a low distortion projection lens and a wide range of adjustments, allowing the stepper to be integrated into a production line in a mix and match setup with other lithography systems. This equipment can be used to image critical layers on substrates while ensuring grid registration and alignment with other lithography systems that are also printing images in the same process line.

Several important global and intra-field image placement relationships for devices requiring multiple layer patterning have been combined in the stepper matching correction software. Further adjustment to the tool can be made to improve overlay when incorporated with fab-wide yield management software for automated, real-time process control. The types of adjustments needed and techniques that can be applied to compensate for image placement errors over large areas are discussed.

To download the full white paper