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Hello! I'm TR of R&D department.
In this article, I would like to talk about our attempt for the pretreatment, posttreatment, which are extremely important in the plating process.
Five points of pretreatment and posttreatment in the plating process
In the plating process of wafers and substrate products, it is necessary to keep the substrate surface clean by pretreatment and ensure sufficient wettability so as not to interfere with plating. If the surface of the base film is oxidized or the wettability is poor, problems such as voids (pores), uneven appearance, and poor adhesion may occur.
1．Removal of resist residue
When forming a wiring pattern with a photoresist or dry film, it is necessary to remove the resist residue after development. Generally, plasma ashing or wet treatment with chemicals is performed. If there is a resist residue, a gap will be occurred at the bottom of the wiring when the resist is peeled off after plating, and the etching of the seed layer will further proceed undercutting, it causes the wiring to fall or detach.
2．Improved wettability of the substrate surface
When dipping the substrate in the processing tank, if the wettability is poor, the phenomenon that air bubbles adhering to the substrate surface cannot be easily removed occurs. If it is processed as it is, it will not be possible to plate the part where air bubbles are adhering. The compatibility between the substrate material and water and the roughness of the surface affect the quality of wettability. If the wettability is poor, the substrate surface is irradiated with oxygen plasma or UV light to perform modifications such as introducing hydroxyl groups (-OH). Degassed water is also effective for improving the wettability of the resist surface.
3．Ensuring wettability and surface activation with pretreatment liquid
Since the surface tension is reduced by the surfactant in pretreatment liquid, air bubbles are easily removed from the substrate surface. In addition, acid activation treatment that removes the thin natural oxide film of the underlying metal film on the substrate surface is also important. Chemical manufacturers supply pretreatment liquids suitable for various applications, and we also obtain the advice of chemical manufacturers as necessary to select the optimum pretreatment liquid.
4．Countermeasures to unevenness of the substrate surface
Vias are formed by laser processing on a multi-layer build-up printed circuit board. After drilling, unevenness is occurred on the side wall of the via by exposing the glass fiber and filler in Glass epoxy resin. If the pre-plating treatment is inadequate, air bubbles may remain on the unevenness and may cause voids to be generated by plating. The more miniaturized the wiring, the more severe management is required.
Even with patterned plating for through silicon vias (TSVs), wiring with deep trench structures, and thick dry films such as copper pillars, As the aspect ratio increases, it becomes a factor to remain bubbles. Methods such as applying ultrasonic waves, reducing the pressure, and absorbing air bubbles with degassed water from which dissolved oxygen in the pretreatment liquid and pure water has been removed are used with the substrate immersed in the treatment tank.
5．Handling of corrosive metals
In magnetic alloy plating containing a large amount of iron, the quality deteriorates by oxidation on the plating film as soon as the substrate surface is dried after plating. In order to avoid this, it is necessary to take measures such as promptly washing away the plating solution on the substrate surface, transporting while constantly pouring pure water so that the surface does not dry, and quickly drying with a spin rinse dryer (SRD).
In addition, in continuous laminated plating of different kind of metals, galvanic corrosion (local battery action) may occur due to the potential difference of the metal. It is necessary to think out not only the final washing but also the intermediate washing. It is necessary to consider a treatment method that suppresses the progress of corrosion as much as possible.
Lastly
What do you think?
In the plating process, optimizing the pretreatment and posttreatment conditions is an important consideration. We will compare and examine the process conditions, and based on the results, we will support you consistently from specification determination, design, and manufacturing of production equipment.
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