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Date: Wed, 22 Mar 95 14:38:01 -0500
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The reason for this effect is not known. However, several theories exist. The theory that is probably most correct is that the magnesium or calcium ions actually bind to the carboxylic acids attached to the photoresist binders.
Aqueous photoresist use polymer binders with carboxylic acid groups attached as side chains. These acids allow the resist to develop and strip in high pH solutions (10-11 for developing 12-13 for stripping). The magnesium and calcium ions, which are the main hardness ions in most tap water, will bind to these acid groups. In fact, they probably bond strong enough to replace most of sodium or potassium ions (from the carbonate developer solution). One would expect the magnesium and/or calcium to change the properties of the binder and therefore the photoresist. For example, hard water can make resists more resistance to alkaline etching solutions. Chemists familiar with magnesium and calcium, would probably expect them to cause this type of effect.
Of course, some resist may be more sensitive to the hardness of the rinse than others, and I certainly can't speak for the suppliers of all photoresists. But, I doubt that any resist is damaged by hard water rinses. Since most tap water is usually hard enough to give good resist properties, and since it is the cheapest water available, we always recommend using tap water for developer rinsing (even for the final developer rinse).
The hardness effect appears to occur in both the developer sump and the rinses. However, the effect is strongest in the rinse water, because much of the calcium is precipitated out by carbonate in the developer sump. (Magnesium carbonate has higher solubility than calcium carbonate but calcium is usually the predominant ion in tap water.) Therefore, using soft or softened water for the developer sump make-up (to reduce scale build-up from CaCO3), and tap water for the rinses works well. However, others use tap water for the developer sump make up with no problems.
All of this assumes that the tap water is not harder than 300-320 ppm as CaCO3 equivalent. Water harder than this can cause problems with scale build up not only in the developer sump but in the first rinse as well (from carbonate carried over from the developer). Some have mixed soft water with tap water to achieve a lower hardness.
Sidney Cox,
From: "G. SIDNEY COX"
Message-Id: <75106122305991/6021592@JLCL01>
To: "G. Sidney Cox"
Subject: Hard vs. Soft Water for Photoresist Developing
Resist vendors suggest using hard water for photoresist developer rinsing because of empirical results. For example, some resist will have a straighter sidewall and a smaller resist foot when rinsed in hard water after developing. In general, the hard water rinse makes the resists slightly more resistant to attack by other process chemistries and less sensitive to over development.
DuPont Electronic Materials
Chairman of the Imaging/Developing Subcommittee, IPC