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Dry Film Stripping
The removal of photoresists (PR stripping) on the surface of the substrate is essential after bump electroplating finishes. Photoresist layer on microelectronic devices is usually thicker than that of the frontend process of IC manufacturing. The thickness of the photoresist ranges between of: 25 ~ 250μm. Since the negative photoresist is known to cross-link, thus difficult for the removal process, NMP (N-methyl-pyrrolidinone) is usually used as the PR stripper. In general, wet bench batch processing is well-used to remove dry film photoresist. Figure 2 shows the 300mm-wafer automatic wet bench processor by GPTC. PR stripper is added in the first two process tanks: the first tank is used to remove bulky photoresists to age the stripper. The remaining PR would then be cleaned off in the second tank, where the stripper is "more fresh," and capable of removing PR in whole. The third tank, QDR (quick-dump-rinse), serves to rinse off the residual chemicals and photoresist residues. The four tank is where the wafers are spun-dried. Also, re-deposition must be avoided during PR stripping.
A before-and-after PR stripping is inspected under an optical microscope (OM) shot in Figure 2. Over a decade of successful manufacturing development partnership with the clients, has enabled GTPC to claim 99% of Taiwan's bumping fabrication market share with its 300mm automatic wet bench systems.
A before-and-after PR stripping is inspected under an optical microscope (OM) shot in Figure 2. Over a decade of successful manufacturing development partnership with the clients, has enabled GTPC to claim 99% of Taiwan's bumping fabrication market share with its 300mm automatic wet bench systems.
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Figure 1、A high-performance wet bench system is used for GTPC's 300mm wafer automatic PR stripping (Source: GPTC WBD) -
Wet Bench equipment
Figure 2、A before-and-after schematic of photoresist removal (Source: GPTC WBD))
Five criteria must be taken into consideration for evaluating the effectiveness of dry film PR strip solvent ：
| item | Critical items to be considered for quality dry film PR strippe |
|---|---|
| 1 | Photoresist removal rate |
| 2 | Lifetime of PR stripper |
| 3 | The appearance of the substrate after PR stripping. Bump and UBM substrates should not be damaged (attacked) or oxidized. |
| 4 | The particles must be filtered out of PR stripper, and not re-deposit after PR removal |
| 5 | Waste chemicals should be easy to process, without hurting the environment |
PR strip agents contain four chemicals; their properties are listed below：
| item | Chemical | Uses |
|---|---|---|
| 1 | Strong base solutions: KOH (potassium hydroxide) or NaOH (sodium hydroxide) | These chemicals disrupt photoresist cross-linking. The alkaline hydrolysis of esters, a process known as saponification, produces carbonyl acid (R-CO2-1). Carbonyl acid is also called fatty acid soaps, which foams during photoresist removal. Photoresists removed by strong alkali are shaped like sheets. |
| 2 | Weak base solutions: Mono-ethanol-amine, also known as the MEA | This weak base solution breaks up the internal link of the photoresists, while disrupting the link between the resists and substrates. The two are compatible in their removal rate. The photoresists stripped by the solutions are grainy. |
| 3 | Water-soluble solvents: 2-butoxy ethanol; aka-ethylene-glycol mono-butylether | This stripper contains 2-butoxy ethanol, which can penetrate PR to make it sponge-like, enabling easy removal. |
| 4 | Anti-tarnish agents: Organo-amines | This solvent protects the bottom of substrate to avoid copper oxidation and the loss of copper luster. |