Focal Extension – A Novel Lithography Technique in Direct-Write Laser Lithography to Enable Fine-Pitch Patterning over Large Topography

Extending fan-out wafer-level packaging (FOWLP) to finer I/O pitches requires the ability to scale down wiring. This enables the ability to interconnect a greater number of dielets (in the same package). This in turn requires the ability to print finer and smaller features over greater topography and large areas. Direct-write laser lithography offers the versatility necessary to pattern fine-pitch wiring in FOWLP and forms the crux of TrueAdapt™ [1]. In addition to the ability to make changes to the patterning on the fly to account for die-shift, there is a need to pattern fine-features over topography, which can range from tens to hundreds of microns depending on the amount of warpage. We introduce a novel exposure technique called "focal extension," which increases the depth-of-focus (DOF) of laser direct-write systems which utilize a Gaussian laser. A description of the working principle of focal extension is presented along with some real-world applications. Using multiple exposures at different focal lengths, we can produce an image that is in focus in a wider range of focus heights. We identify that the contrast of the resist is correlated with the amount of extension possible, with higher contrast resists offering a greater focal extension. Using a specific example, we have extended the DOF of a 405 nm laser direct-write system from 30 µm to almost 100 µm for 2 µm features using a standard MUV resist (AZ 5214 E). We then apply the process on the FlexTrate™ platform to enable fine-pitch patterning in a fully flexible, biocompatible substrate.