Nano Pastes, New Printing Process Enable Micro Structures

OLEDs, organic FETs, organic batteries, plasma displays or fuel cells: all these products require multilayer high precision printing processes for their manufacturing. Until now these applications have been limited due to the printing processes used that only allowed thin layers. This is now changing. With nano pastes and a new printing process, micro structures also can be produced using screen printing and can be built into multiple layers up to very high layer thickness.

“Screen printing now enters completely new applications,” said Joachim Biegel, Product Manager, Essemtec. During the last few years Biegel worked with the PV Lab of the EPFL Neuchâtel to develop a process and machine. PV Lab (Photovoltaic and thin film electronics laboratory) researches and develops Heterojunction Solar Cells. Using the new printing process and printing machine, these cells are now more efficient than others by several percentage points. However, the innovation from the solar industry can also bring new wind into the thick film technologies.

50µm wide structures with 40µm thickness
Conductor lines with widths from 100µm to 150µm and a height of about 12µm have been state of the art for a long time when using screen and stencil printers. However, Essemtec’s SP900-S can print structures that are only 50µm in width and 40µm or more in height. The PV Lab in Neuchâtel uses this machine to produce its Heterojunction Solar Cells.

The efficiency of many products depends on the optimal design of the conductors, as do solar cells. The traces on these cells are called bus bars and fingers. They should be as thin as possible to maximize the active area. The cross section also should be as big as possible to minimize the inner resistance of the cell.

Using the SP900-S, solar cell manufacturers can maximize the height/width ratio of the conducting traces. No other printing machine allow such a high layer thickness to be produced and such high cross sections to be achieved. This is advantageous not only in the solar industry but also in other products that require thin lines with high electrical current capacity.

Multilayer screen printing
Narrow but high traces can be produced by printing multiple layers on top of each other until the desired height is reached. The idea is not new, but until now almost no one could print such thin lines so precisely on top of each other. The SP900-S has been optimized for this task. Design engineers have implemented a new type of printing control and have developed new vision, drive and damping systems to fulfil the demanding requirements of such a process.

“It is similar to newspaper printing,” said Joachim Biegel. The paper printing process also requires an exact positioning of the four colors relative to each other, otherwise the cover story only gives headaches. There, printing machines make use of so-called registration marks to line up the colors. Therein lies the difference: On solar cells there are no such marks as this would reduce the active area.

Therefore, the SP900-S must use structures on the wafer itself for alignment. For the first printing cycle, are edges and corners of the wafer. When printing a further layer, the printing screen must be lined up with the pre-printed fingers or bus bars. The vision system on the SP900-S can use both alignment methods. Screen and substrate are positioned with an accuracy of ±8µm to one another.

From force control to position control
High profile conducting lines are printed in multiple layers as previously described. In the past, a reduced aperture technology was used for such processes. For the top layer the screen apertures were slightly smaller than for the first layer and were positioned on top of the pre-printed structure. However, local bleed-out often occurred that floated down on the bottom layer, resulting in improper printing. Additionally, the production of two different screens was expensive.

The new process uses the same screen for the first and the second layers. For the second print, the screen is lifted up several micrometers or the substrate is lowered, depending on the printing machine. The printing head no longer regulates to a predefined printing pressure but to an exactly defined location in the vertical axis.

In this way, the distance between the screen and the substrate is adjustable within microns. No more bleed out occurred and the print thickness is precisely controllable. Of course, this process requires a perfect parallelism of squeegee, screen and substrate and also places special demands on the vertical drive of the print head. Normal printers are not suitable; however, the SP900-S is optimized for this.

Squeegees need support
The vertical axis of the SP900-S print head is equipped with a powerful, high-resolution stepping motor drive that positions the squeegee within a few microns. The same accuracy also is possible with the printing table vertical drive on which the substrate is mounted. This leaves the squeegee as the only inaccurate part of the system, but engineers have found a solution for this problem too.

Normally, printing squeegees have a large tolerance when measuring the deformation under pressure. These tolerances can even destroy all efforts for accurate drive systems. To overcome this problem, so-called backbone squeegees have been introduced. They work like this: A massive back plate restricts the squeegee's deformation so that only the front edge of the blade retains the flexibility that is needed for the printing process.

Precision Is Everything
The SP900-S is designed especially robust and features a special damping system that decouples the printing table from the outer frame and avoids the transmission of vibration. The printing accuracy of a few microns also for this has required exceptional solutions from the machine designers.

During process development, different materials were tested for their process capability, such as the screens. Ultra fine screen printing requires a very fine mesh that does not deform under pressure. Presently, the best results are achieved using Vecry fibre mesh, which combines the advantages of stainless steel and polymer fibers.

Outlook
The uniquely thin and high traces on solar cells produced with the SP900-S printer have brought the researchers of the PV Lab a big step further. Furthermore, the industry now possesses a machine that has been developed using practical experience. It features a very compact design with small footprint, easy-to-use software, a high flexibility and low cost of ownership. It is very well suited for use in pilot production lines, laboratories and small productions.

Development is not at an end yet. The next step is already targeted, which is another reduction of the printable structures down to 25 µm. This will bring screen printing into an area of applications that until now have required expensive offset or flex print technologies. Screen printing, however, will reduce the cost for production, will be much more flexible, and will enable higher electrical currents to be transmitted through thinner traces.

Essemtec