Is there an explanation of why a negative photoresist take-off after contact with ferric chloride (like in the pictures) ?

The issues observed in the photos of the negative photoresist after contact with ferric chloride can be attributed to several factors. Here's an analysis and explanation of the possible causes:

Factors Leading to Negative Photoresist Lift-Off

Inadequate Crosslinking of Photoresist:Cause: If the exposure time during UV curing is insufficient or the intensity of UV light is inadequate, the acrylate polymer may not fully crosslink. Effect: This results in a weaker resist that the etchant may be able to undermine, which causes the peeling or lift-off seen.

Improper Development:: Incomplete development of the negative photoresist can leave unpolymerized regions, which can be attacked by the ferric chloride. Effect: These regions will lift off during the etching process, as seen in the images.

Adhesion Issues:Cause: Poor adhesion of the photoresist to the substrate can occur due to surface contamination, insufficient surface preparation, or inappropriate baking conditions. Effect: This causes the resist to lift off when exposed to the etchant.

Over-Etching:Cause: Extended exposure to ferric chloride can attack the photoresist, especially if it is not fully crosslinked or developed. Effect: The resist layer will start to degrade and peel off.

Chemical Compatibility: Cause: The chemical resistance of the acrylate polymer may not be sufficient to withstand prolonged exposure to ferric chloride. Effect: The etchant penetrates and undermines the resist, causing it to lift off.

Mechanical Stress:Cause: During the etching process, mechanical stresses can build up due to differential thermal expansion or contraction between the resist and the substrate. Effect: This stress can cause the resist to crack and lift off.

Recommendations for Improvement

Optimize UV Exposure:Ensure that the UV exposure time and intensity are sufficient to fully crosslink the photoresist. Conduct a dose-response experiment to determine the optimal exposure conditions.

Improve Surface Preparation:Thoroughly clean and prepare the substrate surface to enhance adhesion. Use a suitable adhesion promoter if necessary.

Control Development Process:Optimize the development time and conditions to ensure complete removal of unexposed resist. Use a fresh developer solution and maintain consistent agitation.

Monitor Etching Time:Carefully monitor the etching process to avoid over-etching. Shorten the etching time if necessary or use a less aggressive etchant if possible.

Enhance Chemical Resistance:Consider using a different photoresist formulation with higher chemical resistance to ferric chloride or add a post-bake step to harden the resist.

The photoresist lift-off that was seen after contact with ferric chloride was probably caused by a mix of poor crosslinking, development, adhesion, over-etching, and maybe even the resist's natural chemical resistance. By optimising the exposure, development, and etching processes, and ensuring good adhesion and chemical resistance, the integrity of the photoresist during etching can be improved.

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