Novel Technique for Backside Alignment Using Direct Laser Writing

Backside alignment is a key microfabrication process step, especially in micro-electromechanical systems (MEMS). The double-side mask aligners used for this purpose are unaffordable for many research centres. We propose a new method that aligns the backside mask to the features on the topside using a direct laser writer, which is available in many cleanrooms. In this method, the corner co-ordinates of the sample are used as alignment features, and a transformation matrix is developed to map the design co-ordinates to the stage co-ordinates. This method has been validated on copper features as small as 100 μm on silicon substrates. Test samples are cut from a 2 inch Si wafer, and copper features are sputtered and developed onto the topside. Backside patterns that are aligned to these copper features are created using photolithography through the application of this alignment method. This method exhibited challenges for samples without sharp right-angled corners, where the estimation of the corner co-ordinates resulted in misalignment. Sixteen areas over nine samples were analysed. An average alignment resolution of 23 ± 1 μm was established in the x and 8 ± 4 μm in the y direction, and a rotation misalignment of less than 1° was achieved. Differences in alignment were due to the individual quality of each sample's corners and to the clarity of the corner co-ordinates. This new approach provides a route towards low-cost microfabrication process development.