SPRAY COATING
Spray coating of substrates can be a sensible coating technique for photoresists in cases where the substrate size or its texturing do not permit spraying with the required homogeneity of the coating pattern. This section describes the technology of spray coating, the physical processes between droplet formation at the spray head and the finished resist layer and provides explanations and answers to common problems associated with spray coating.
The basics of spray painting
Basic principle of spray painting
In spray coating, the paint layer is deposited from the spray mist formed from atomised photoresist with typical droplet sizes in the μm range. The droplets are formed, for example, via a nitrogen - pressurised nozzle or via ultrasonic atomisation and land, carried by an air or nitrogen stream, on the substrate surface where they combine to form a closed resist layer .
Possible advantages
In addition to an at least theoretically very high coating yield, spray coating offers the potential to coat substrates of almost any shape and texture, for which spinning is not technically feasible or does not deliver the required results in terms of homogeneity of the coating layer thickness over the entire substrate or its textures (edge coverage). With a spray coating suitable for the respective process and optimised spray parameters, spray coating can be used to provide the trenches, flanks and edges of textured substrates with a sufficiently uniform coating thickness for most applications.
Limits of spray coating
In practice, most of the paint droplets end up in the extraction system and only a small proportion on the substrate. The achievable lacquer yield typically reaches values of around 5 - 15 %, which is still significantly greater than the values of typ. < 5 % is achieved. The paint droplets do not "rain" onto the substrate in a laminar flow, but are deposited locally, sometimes very inhomogeneously, by micro-turbulence over the surface, especially of textured surfaces. As a result, the coating layer on textured substrates is not uniformly thick over the textures, but is usually thinner at the upper edges and thicker in the trenches near the flanks. The formation of thin (approx. < 1 μm) closed paint layers tends to be difficult, as the landing sites of the droplets on the substrate and the droplet diameters are statistically distributed. Therefore, a certain droplet density on the substrate surface is necessary to enable coalescence into a continuous layer. However, this process of flowing on a μm scale, which is desirable for a smooth coating layer, also leads to a thinning of the coating layer over the edges and corners of textured substrates (low edge coverage).
Spray mist formation
Suitable equipment
The technically simplest way to produce a spray mist is to atomise the paint using a nozzle, as is done in commercially available airbrush guns. To prevent contamination of the paint with humidity or particles condensing on the low-pressure side, we recommend using ultra-pure nitrogen. If airbrush guns are to be used, attention must be paid to the stability of the seals used in them against the solvents used in the paint, such as PGMEA, MEK or acetone . Another option for forming the spray mist is ultrasonic atomisation, in which the paint is atomised by a high-frequency mechanical oscillation applied to the paint and directed onto the substrate by a carrier gas.
Spray paints suitable for atomisation
Coatings optimised for spraying are usually too viscous to form a sufficiently fine spray mist. Each technology requires a relatively low viscosity of the spray paint of usually a few cSt to produce a droplet size distribution suitable for spray painting. When selecting and concentrating the solvents, incompatibilities between different solvents and the photoresist must be taken into account, as well as the fact that photoresists that are diluted too much age more quickly and form particles.
Processes in the spray mist
Evaporation of solvent
Between the spray head and the substrate, enough solvent should evaporate from the droplets of the spray mist so that no macroscopic flow of the coating takes place on the substrate, which would impair the homogeneity of the coating pattern and, if relevant, the edge coverage . However, the viscosity of the coating layer should also be low enough to enable it to be smoothed on a μm scale and thus to create a closed coating layer. If too much solvent already evaporates in the spray mist, the droplets cannot flow together to form a closed paint layer. In extreme cases, especially the smaller paint droplets with their larger surface/volume ratio already dry in flight to form resin beads, which do not adhere to the substrate but are released into the cleanroom air as particles in the event of inadequate extraction. The parameters temperature, flow velocity and saturation of the environment with solvent as well as the solvent composition determine the evaporation rate for each droplet size as a function of the solvent saturation of the droplet surface. This concentration in turn depends on the temperature- and solvent concentration-dependent diffusion of the different solvents from the inside of the droplets to their surface. In practice, two or more solvents with very different vapour pressures (e.g. PGMEA with MEK or acetone) are combined in order to predetermine a certain viscosity of the droplets when they land on the substrate.
Transport of the droplets to the substrate
The free fall speed in air of spherical paint droplets with a diameter of 1 μm is approx. 10 cm/hour and therefore far too low to coat substrates solely via the force of gravity, i.e. "in free fall". Therefore, a droplet transport through a carrier gas is required, which in the airbrush process is achieved by the nitrogen used for atomisation at the nozzle, and in ultrasonic atomisation by a corresponding carrier gas flow. Directly above the substrate surface to be coated, there is partly laminar flow parallel to the surface and partly turbulent flow defined by a texture which can make it difficult to coat edges and trenches evenly . The illustration shows how a closed, smooth coating layer forms on a smooth substrate (a non- textured silicon wafer) during spray coating.
Wetting, edge coverage and roughness of the paint layer
Wetting
Good wetting of the substrate promotes the formation of a closed coating layer from the droplets and is therefore a prerequisite for good coating results. Good wetting and adhesion can be achieved through optimised substrate pre-treatment.
Smooth paint surface and optimised edge coverage: always a compromise
If the solvent concentration of the paint layer formed is very high over a long period of time, the paint flows together to form a very smooth film due to its surface tension, but at the same time also withdraws from the edges of textures, which may only be covered very thinly or not at all by paint as a result. If, on the other hand, the spray paint and spray parameters are set so that the formed paint layer contains little residual solvent or reaches this state very quickly due to rapid evaporation, the edges of textures remain better covered. However, due to the suppressed flow of the varnish, the varnish layer is comparatively rough on a μm scale - in the extreme case, the droplets remain in their original form on the substrate where they landed.
Parameters for influencing the edge coverage and smoothness of the coating layer
In order to find the optimum compromise for a particular application between a smooth coating layer and - if necessary - optimum edge coverage, the viscosity of the coating droplets that have just landed and the viscosity over time (evaporation of the solvent) of the coating layer that has just been formed must be adjusted . A large number of parameters are available for this purpose . Better edge coverage with the compromise of a generally less smooth coating layer can be achieved if the coating droplets landing on the substrate are sufficiently highly viscous due to accelerated evaporation in flight between the spray head and the substrate, which can be achieved via a low proportion of high-boiling solvents
- A low proportion of high-boiling solvents such as PGMEA and a higher proportion of low-boiling solvents such as acetone or MEK in the spray paint
- Smaller droplet diameters, e.g. via a high solvent concentration in the spray paint or settings on the spray head
- A greater distance between the spray head and the substrate for a lower solvent concentration of the droplets landing on the substrate due to evaporation during the flight of the droplets can be implemented.
Alternatively or additionally, accelerated evaporation from the coating layer that has just formed can be achieved via
- A substrate heated to approx. 40 - 60°C
- A reduced spray rate
improve the edge coverage. The opposite measures can help to improve the smoothness of the lacquer layer - but at the expense of a tendency towards poorer edge coverage:
- A high proportion of high-boiling solvents such as PGMEA and a low proportion of low-boiling solvents such as acetone or MEK in the spray coating
- Larger droplet diameters, e.g. via a lower solvent concentration in the spray paint or
Settings on the spray head:
- A smaller distance between the spray head and substrate for a higher solvent concentration of the droplets landing on the substrate
- A higher spray rate
Suitable spray paints
With a suitable solvent composition, a spray lacquer can basically be prepared from any lacquer, which is why the choice of lacquer should primarily be based on which lacquer thickness of a positive, reverse or negative lacquer with which resolution is required for which application of the developed lacquer mask. The optimum composition with high- and low-boiling solvents depends on the substrate and the criterion of whether a smooth paint layer or maximum edge coverage is required, and also on equipment-related parameters such as the technology used for spray mist generation, the spray rate and the distance between the spray nozzle and the substrate. With paints such as AZ® 4999 or TI Spray , there are ready-to-use spray paints suitable for most applications. Adjustments to the solvent composition to optimise the coating result may nevertheless be advisable under certain circumstances.
