Mask-less Laser Direct Imaging & Adaptive Patterning for Fan-Out Heterogeneous Integration
Authors: Sandstrom Clifford , Olson Tim 
Published: February 2024

Rapid growth of advanced FOWLP (fan-out wafer level packaging) or simply fan-out, is upon us as the industry moves from historical monolithic SoC (system on chip) scaling to multi-die architectures using chiplets, tiles, dielets or whatever they’re called. The emerging multi-die devices can be either homogeneous or heterogeneous or combinations of both and are being driven by today’s most advanced applications including 5G, AI, autonomous driving as well as the Internet of Things. In this paper, we will explore the use of LDI (laser direct imaging) in combination with Adaptive Patterning® as a platform for photo lithography in support of this ever more complicated packaging integration. LDI is not new to the wafer level packaging industry since Deca first introduced the concept in 2010 and went to production in 2011 in support of WLCSP (wafer level chip scale packaging). Deca collaborated with an LDI technology leader in the PCB industry, Hitachi Via Mechanics (now ADTEC) to drive the technology to the precision required for wafer level packaging, a novel concept at the time.

With the long-term vision of Adaptive Patterning in mind for embedded die structures such as M-Series™ fan-out, Deca utilized the advantages of rapid design-to-manufacturing in a mask-less environment and elimination of photomask costs as initial value drivers for WLCSP. The mask-less environment further allowed designers the opportunity to change layout attributes such as thieving structures for plating, RDL routing for modeling and more in real-time. Using digital masks allowed time and expense savings with the avoidance of initial tooling cost and storage. The key challenge is using LDI photolithography was the 405nm wavelength of the approach. Conventional wafer level packaging photoresists and photo imageable dielectrics were generally designed for either broadband or i-line exposure. Some of the broadband materials performed well with LDI, while Deca worked with several leading manufacturers to develop new photo-activation packages tailored for 405nm sensitivity. A combination of classic spin-on materials as well as novel dry film photoresists were successfully used in product qualifications and high-volume production.

The process flow for dry film photoresist is different than liquid spin on. The dry film process consists of lamination, exposure, coversheet removal followed by develop. Typical throughput on a lamination tool is between 40 to 60 wafers per hour and not dependent on resist thickness. The conversion time from a standard 7 to 15µm RDL film to a 50 to 150µm Cu Pillar film can be accomplished in less than 15minutes. The advantages of a dry film include high throughput of greater than 100 wafer per hour for RDL steps with imaging down to 8µm, elimination of harmful solvents, waste handling of liquid photoresist, edge bead removal and associated film thickness variation due to resist swelling. Lower cost per wafer was another key factor in utilizing dry films with LDI over liquid resist. Current dry film engineering data suggests strong imaging performance down to 4µm at the 405nm wavelength with additional work underway on 2µm and finer features.

The resolution of an optical system is dependent on exposure wavelength and NA (numerical aperture). Conventional photo steppers improve their imaging performance with higher and higher levels of NA rather than migrating to shorter wavelengths due to the use of cost-effective Hg lamps. The disadvantage of high NA steppers is the reduction in DOF (depth of focus). LDI offers a high DOF due to its inherent low NA, often 2x or greater DOF as compared to steppers. A higher DOF becomes critical as linewidths are scaled ever smaller upon the inherent imperfect planarity of organic FOWLP structures.

A new generation of LDI tools are coming to market with enhanced capability to achieve 2µm lines and spaces and below utilizing 405nm or mixed wavelengths including 375nm. The shorter wavelength option opens the door for using more broadly available i-line sensitive materials. A breakthrough capability of LDI is the elimination of reticle size limits since there are no glass photomasks used in the process. Industry leading steppers are forced to use reticle stitching as package body size grows above the typical range of 16mm to 24mm. Several recent customer-driven designs for heterogeneous integration exceed 35mm on a side requiring multiple levels of reticle stitching which significantly reduces factory throughput driving up product cost. Using Adaptive Patterning design tools including AP Studio, product designers are limited only by the physical wafer or panel size where packages devices could grow to 300mm x 300mm if desired using a 600mm x 600mm manufacturing format.

Another key LDI advantage is the ability to utilize a unique optimized design per wafer to precisely overcome embedded die shift related to natural variation in die placement and compression molding. Adaptive Patterning design tools are utilized by package designers to establish rules for Adaptive Alignment, Adaptive Routing or Adaptive Metal Fill for each package design. Within the presentation, the complete Adaptive Patterning process will be explained for a straightforward understanding of how complex heterogeneous multi-chiplet systems can be constructed in a cost-effective manner with optimum design rules and assurance of high yields in manufacturing. A survey of currently available LDI tools will be presented and a few examples of advanced M-Series fan-out technologies based heterogeneous chiplet-based products designed for LDI and Adaptive Patterning will be shared.