This is a follow-up blog to our metal lift processes post. When coating a liquid photoresist onto a wafer, the resist is often dispensed at the wafer center. The resist then moves outward to the wafer’s edge via centrifugal force as the wafer spins. Ideally, the coated resist’s thickness is uniform from the center to the edge. In reality, there is typically a build-up of resist at the wafer edge that can be as much as 3x the thickness at the wafer center. Additionally, the resist can, and usually does, wrap around the edge of the wafer to coat the bottom of the wafer.
Both of these deviations from a uniformly thick resist are problematic for almost all wafer fabrication operations. The extra thickness on the wafer top will translate at exposure to undesired and uncontrolled critical dimension variations after resist develop. However, the resist on the wafer bottom can be even more problematic, causing poor leveling in the exposure tool, resist build-up in the coat cup and down-stream wafer track, and in non-litho tools that will subsequently handle the affected wafers.
The solution to this problem is edge bead removal (EBR). EBR comes in two forms: chemical top-side removal and wafer edge exposure (WEE) with bottom-side chemical dispense. Topside chemical EBR processes have the advantage of removing resist, any underlying organic anti-reflective coatings (BARCs), and any resist coating the bottom of the wafer in a single step. The disadvantage to top-side EBR is that in poorly controlled processes there can be considerable splashing and misting of the solvent leading to resist coating defects. There is also a lower limit to the size of the edge bead removal zone; generally, this means the edge bead removal zone must be greater than 2 mm (4-5 mm is often chosen).
In the case of WEE there is no splashing of solvent, and edge bead removal zones can be reduced below 2 mm. This latter point thus allows a manufacturer to reduce his Yield Zone Exclusion to <2 mm. This can increase as a result the total number of useful die per wafer, thus reducing overall manufacturing costs. The down-side to WEE is two-fold: there is no provision for removing an organic BARC, and the manufacturer must still utilize a bottom-side chemical dispense to remove resist from bottom of the wafer. Without careful process and tool set-up, many of the same chemically-originating problems from topside chemical EBR will occur, chiefly misting and splashing.
In order to minimize the defects that can come as a result of EBR splashing or misting, certain tool and process parameters must be optimized: wafer centering, exhaust air flow, EBR flow rates, height and angle of the EBR dispense nozzle and dispense cup integrity. The use of a high speed camera can be used to verify wafer centering, EBR dispense nozzle height, and angle optimization. Air Exhaust Flow and EBR flow rates must be measured and maintained via an active preventative maintenance (PM) program. Similar procedures are required to optimize bottom-side EBR dispense for the WEE process.
The last set of considerations are the rotation speed and time during EBR dispense, as well as the actual choice of chemistry used in the EBR process. Generally speaking, the appropriate chemistry for any resist is easily determined from the casting solvents used in formulating the resist in question. Common casting solvents include propylene glycol methyl ether acetate (PGMEA), ethyl lactate, or combinations of various organic solvents. The key requirement of any EBR chemistry is that it not cause swelling in the resist film at the interface between the edge bead and resist film, that the resist is quickly dissolved by the EBR solvent, and that the chemistry displays wide latitude for the various tool and process conditions chosen for the EBR process; that is to say, slight variations in exhaust flow rates or EBR flow rates cannot cause process variability.
Various high quality/low metal containing EBR solvents can improve your yield by reducing your defect rate. Ask an expert on how you can optimize your EBR processes via tool, process or chemical optimization.