Why Back End of Line (BEOL) Photoresist “Track” Tools are and must be Different from Front End of Line (FEOL) Photoresist Processors

In earlier writings we have discussed cost of ownership issues (COO) related to photoresist processing tools. In this entry, we will discuss the important functional differences between tools generally intended for the FEOL, and those more adaptable to the needs of BEOL wafer fabrication.

As the semiconductor industry has packed more and more functionality and speed into ever smaller chips (here one almost always must refer to Moore’s Law well know to those in the industry), the size and cost of the packaging of the chip has made necessary the creation of new types of wafer processing, which is needed to do wafer-level chip scale packaging. In this specific technology, photolithographic processing is used so chip outputs and inputs can be repositioned, so as to adapt to interconnect circuitry directly without the intervention of a typical pin out type package. In this case the chip is “rewired” and “bumped” so that it can be “flipped and bonded” directly to the chip interconnect circuitry. Such rewiring and wafer bumping is accomplished on the finished device wafer prior to singulation of the wafer into chips.

While the patterning concept is identical to that required to make the chip itself in practice, the geometries are far larger and the need for organic insulating film more prevalent than in the creation of the chip. This means that the spin cast films are much thicker and there is far more effluent of material leaving the wafer during the required “bakes” done in the thermal modules of the requisite tools. These thicker films require the following considerations in the design and implementation of the tool.

  • The cost of the process must be kept low, thus requiring tools with a low COO.
  • The spin cup must be designed to handle thicker films and capable of modulating spin exhaust conditions on a step-by-step basis throughout the process recipe.
  • The higher viscosity resists that are used to spin the thicker films means pumps must be geared to accomodate those viscosities.
  • The higher viscosity resist requires that the dispense height be preferably programmable rather than adjustable so that it can be varied during the dispense.
  • The much thicker films evolve far more solvent and other effluent requiring that the effluent be directed away from cold surfaces where it can condense and require frequent bake module cleaning, or create defective films owing to effluent reflux.

While FEOL tools may be utilized in BEOL, their high cost can be prohibitive, and their performance while tuned to the FEOL is not appropriate to the BEOL provider.