[MUSIC] Hello everyone, in this module let's learn about printed electronics. Particularly we will focus on the electro hydrodynamic printing method for electronic devices. Nowadays, the structure of electronic devices can be deformable. And that kind of freeform electronics can be categorized into two different approaches. One is, to make stretchable geometry of electronic devices. And these stretchable electronic devices can be attached onto the non planar surfaces. And in that case these electronic devices can be located near the surface area of the object. And the other method is to use printing techniques. If we print the electronic devices, then we can print the three dimensional object and also we can print the electronic devices. So these electronic devices can be embedded inside of the three dimensional objects, later than the surface areas. So we will focus on that kind of printed electronics. So currently the silicon-based electronic devices, or our displays, use the photolithography process. And that kind of photolithography process are based on the two-dimensional, the planar structure of the substrate. For example, in this slide in the top several images you will see the sequence of the photolithography process to make the basic transistor structures for the source and drain patternings. For example, we use silicon wafers. In that case, we can oxidize the top surface of the silicon layers. And then we can deposit and then we can expose the light through the photo mask. And then after developing, we can make the patterns. Then we can wet edge the oxide layers. And then we can do the ion implantation method to talk to the selective area of the silicon wafers. And then after depositing the metals using the vacuum deposit method, and then by lifting off the structures we can make the source and drain pattern of the 40 transistors. And actually that kind of a sequence should be repeated multiple times to stack the three-dimensional transistor structures. So and that kind of multiple processes are really time consuming and it's cost is also very expensive. So people try to find other alternative method and that can be the printing techniques. So, although the current, the photolithography process is very, very powerful. It's really complicate because it is based on two-dimensional structurea. So multiple processing to stack the individual device layers can be very expensive. And another difficulty is that kind of photolithography process is limited for the non-planar substrate. So we need another alternative method to print the electronic materials directly on to the kind of non-planar substrate. So the kind of printing electronic devices can be combined with the flexible electronics as well. There are many various printing techniques, but today we will focus on the Inkjet Manufacturing System. The inkjet printing techniques has multiple advantage. First, the functional ink material can be printed directly, through the nozzle to the substrate. So we don't need very complicated processes. So it's very simple process. So it can reduce the multiple processing steps compared to the photolithography process. And also in the case of the inkjet, just one of the droplet can print large area of the substrate. So we can use all the functional ink materials very efficiently so we can reduce the ink waste in that case. And also the inkjet printing system, it is suitable for the very largest substrate. For example, we can use two meter by two meter scale of the class substrates for this place. So the kind of huge substrate size can be suitable for the inkjet printing. So it's definitely good approach for the electronic devices. And also a very chemical sensitive materials such as DNA, protein, and cell. These materials can be printed through the inkjet as well. So the inkjet method can be applied for electronic devices and also for the biotechnologies as well. So because of that advantages many interesting devices has been demonstrated. For example, organic thin-film transistors and even the inorganic thin-film transistors based on the indium gallium oxide semiconductor materials. And also, we can print the actual metrics of the for this place as well. And also people can print the color printers for the large area LCD displays. And also we can print the circuit geometries directly on to the non-planar plus different frames as well. So these will be very interesting approaches. So let's learn about the inkjet method more in detail. The commercial inkjet use the Thermal or Piezolectric print head, typically. So these two printers are represented here in the case of the commercial inkjet system. In the case of the thermal inkjet system, there is a heater inside the ink chamber. By heating the ink material then it generate the primitive pressures. So through the capillary pressure. So ink droplet can be ejected from that kind of heater region. So that's the some of the inkjet system. And the other inkjet system is plezoelectric type inkjet printer. So in the chamber there is a piece of material. Very thin membrane so it fire right, so it generate the pressure directly. So the ink droplet can be ejected through the nozzle in that case. So for those two approaches these print thermal inkjet print head or piezoelectric type print head generate pressure. And if the pressure is higher than the capillary pressure then ink droplet can be ejected through the nozzle. And that capillary pressure is dependent on the nozzle diameter as well as indicated in this relationship. So as the nodule size becomes smaller, the capillary pressure increases. So the print head should generate much higher pressures compared to the capillary pressures. That's the limitation why we are very difficult to reduce the nozzle size. So as the nozzle size becomes a smaller, then capillary pressure becomes very large for example. So typically the commercial print heads are used the minimum nozzle size of just 5 micrometer. So if the nozzle size becomes smaller than 5 micrometers then capillary pressure becomes too large. So the print head cannot generate the threshold, the pressures to eject the droplet. So droplet ejection becomes very difficult. In that case, if we use very tiny nodules, then ink droplet can be sprayed better than the ejected injections. So typically, droplet diameters are bigger than the nozzle size in that case. So the reduction of the droplet diameter is limited because of the limitation in the nozzle size. And another problem is droplet placement error. So statistically, droplet placements should be deviated from the jet trajectory, so it increases the printed area. So that's the reason why in the case of the inkjet printing system, its printing resolution is relatively coarse. So typically, the commercial inkjet printing system generate droplet volume of water under 1. And if the droplet is printed on to the glass substrate, the typical diameter is around 10 micrometer, or bigger than the 10 micrometer. So that's the minimum feature size that the commercial inkjet system can print directly. So the resolution is not enough to make very high definition of the electronic devices. So we need another method to increase the printing resolutions further. So these two image shows the typical inkjet printed electrode patterns, for example, gold electrode patterns, or the Inkjet printed DNA microarrays. So it's shown here, the minimum feature size is much bigger than the 10 micrometer scales and also the line area is relative rough. So we need another better method to improve the printing resolutions. So that's the summary of the current, the inkjet printing systems. So current inkjet printing system use the thermal or piezoelectric type inkjet print head and that print head should generate pressure bigger than capillary pressures in order to eject the droplet. But the capillary pressure is dependent on the nozzle diameter. So as the nozzle diameter becomes decreases, the required capillary pressure becomes increases. So print head cannot generate that kind of very high pressures above that huge capillaries. So droplet ejection becomes very difficult. So that's the reason why the minimum nozzle size is limited. So the minimum nozzle size is a few micrometers scale. For that kind of micro scale, the minimum droplet diameter is around 3 micrometer. Its volume is just about 1 or a little smaller than 1. And once that droplet is printed on to the glass micro substrate, the typical printed resolution is about 10 micrometers. So that's the summary and the limitation of the current inkjet system.
