Behind the Scenes - Slot Die Coater Development

Posted on 07 Oct 14:39

Following on from our post about the Innovate UK grant and what we achieved during the project, we thought we would give a behind the scenes look at one of the most exciting products to come out of it. The Ossila Slot Die Coater, launching in early 2016, will be an invaluable tool for up scaling cutting edge OLEDs, OPVs, OFETS, sensors and inorganic solar cells.

Spin coating is a commonly used tool in coating application and device manufacture, and our Spin Coater is one of our best selling products. Spin coating offers a large amount of control when creating organic electronic devices due to its well understood mechanics, its small scale batch production, and tight control of variables in repeatability through programs. However, spin coating has its limitations. It is difficult to coat substrates larger than 50 mm2 with the same control as smaller ones, it's a batch process, you are only able to coat one device at once. It is also not a compatible process with roll-2-roll manufacturing, the simplest way to scale up flexible electronics research.

These inherent qualities of spin coating prevent it from being a suitable method for commercialisation and scale up. Investigating alternative deposition techniques can introduce additional problems when fabricating devices, whether from the different deposition process or a different device size. This requires a significant investment in time and can be costly due to the required equipment and the amount of materials consumed per experimental run.

During the Innovate UK grant, we intended to make an instrument that would change this and enable investigations into high throughput coating techniques in the lab. Three criteria were identified for the unit to be lab ready; it had to create production quality coatings, be affordable, and be compact.

We initially looked at making a blade coater, with the aim of using 3D printed micro-patterned blade heads. Having seen research by Diao et al.1 into micro-patterned blade heads manufactured using nano lithography, we investigated the prospect of creating the blade heads using high resolution 3D printing. The patterning controls the flow of the coating material and creates uniform crystallinity. Unfortunately current 3D printing techniques are not able to create patterns of the required size and quality to allow for production of the blade heads.

3D printed micropatterend designs on blade heads. Not enough precision was possible for our intended designs.

 

After this set back, our attention turned to slot die coating. Slot die coating is a deposition method used in roll-2-roll manufacture, where the head deposits the ink material without contacting the substrate. The meniscus formed between the head and the substrate acts as the paintbrush, with coating thickness controlled by the flow rate of the material and the speed of the relative linear motion of the head.

The slot die thickness equation where d is the coating thickness [cm], f is the flow rate [cm3/min], ν the coating speed [cm/min], ω is the coating width [cm], c is ink concentration [g/cm3], and ρ is the density of dried film [g/cm3].

 

Development

To give a sneak peek behind the design process at Ossila, and give you an overview of the challenges we faced in the project, we have gathered a selection of images from the development of the prototype Slot Die Coater.

 

Here is a very early proof of concept design, used to demonstrate the dispense and linear travel control systems. The dispense system is not compact at this stage.

 

Our first attempt at a 3D printed slot die head. There are small holes that can be seen in the picture. These were individual channels, that fluids more viscous than water had difficulty getting through.

 

By this stage, the substrate holder has been designed and put in place. A new metal, 3D printed slot head sits on a static gantry above the platform. The syringe dispensing unit is now more compact. The electronics, now fully functioning, remain on a bread board until the next iteration.

 

Here are the first steps of consolidating the components into a small compact design, and the first time the components left the optical breadboard. Most of the machinery is able to be tucked below the coating surface. The user interface components are currently sitting in a remote control unit. Also shown is version 1 of the new gantry, the block holding the head above the substrate surface.

 

Revisions of the gantry are shown here. The gantries show the progression from a simple support structure, to including casters for substrate height tracking, to a streamlined all in one unit, to a modular structure for interchangeable slot die head designs.

 

The consolidated electronics and control circuitry is now fleshed out onto printed circuit boards (PCB). The device runs through an open source arduino board, located underneath the PCB.

 

Throughout the development, computer aided design (CAD) and renders have been an integral part. These renders were used to visualise potential problems with a design, get components 3D printed quickly and accurately, and get costings for casings and graphics during the prototyping process. Shown here is the look of the finished unit with the syringe dispense under the coating platform.

 

Here is another prototype, designed with the syringe dispense unit now vertical, creating a lower profile for the substrate coating area. The electronics are housed in a plastic box, but would be enclosed in the left hand side unit in the final design.

 

This CAD render of the latest prototype shows how all systems apart from the coating surface are now inside a single structural unit.

It is important to note that the development process has not yet finished. We are currently working on optimisation and user friendliness of the unit, and the complete feature list has not yet been finalised. If you have any suggestions, or specific requirements for what you would like the Slot Die Coater to do, please contact us.