There are several different printing techniques in common use amongst developers of printed electronics

Each process has particular characteristics in relation to image resolution, dry ink film thickness, substrate characteristics, ink characteristics, printing pressure, speed and controllability. These characteristics become particularly significant in

consideration of the suitability of a process for use in printing electronics. As it happens, all of the processes have been tried to some extent although at present there is no doubt that screen printing and inkjet printing are the most popular. However, some sizeable experiments or prototype runs have also been made with flexo and litho.

Screen printing

Screen printing is a stencil printing method. It operates at fairly low speed, is very tolerant of ink and substrate properties, exerts almost no pressure on the substrate, and deposits a relatively thick ink film over what can be a large area. Because of this it is being

developed particularly for printing some of the circuitry on the back of displays, and for the lay down of the electro-

luminescent material itself.

The elements of screen printing are the screen, stencil, squeegee, ink, press bed and substrate. It is basically a

simple technique but in fact has many variables that can affect the quality of the printed image. Such variables are:

•    Printing speed;

•    Angle and geometry of the squeegee;

•    Distance between the screen and

the substrate;

•    Mesh material;

•    Mesh count;

•    Viscosity of the ‘ink’.

The thickness of the ink film increases with increasing viscosity (although not necessarily linearly). Higher mesh counts lead to thinner films.

Early screen printed produced results which were not very different from those achieved with spin coating methods. However, the printed layers had

undesirable features due to mesh effects that were a few hundred nanometres high, 100 microns long and 20 microns wide, resulting in non-uniform light

emission. Subsequently these features have been eliminated.

Screen printing can now deposit ultra thin layers of organic materials for use in OLEDs fabrication. Films having a thickness lower than 20 nanometres and an rms roughness of less than 1.5nm can be obtained using screen printing.

Inkjet printing

Inkjet printing involves creating fine ink droplets and positioning these as required on the substrate. It has

particular advantages of no physical contact with the item being printed, and being a computer controlled process making it suited to small

volumes and/or complex designs. There are two basic methods: continuous and drop-on-demand.

Figure 2 shows the continuous process. An ink chamber which is

supplied with ink under pressure is excited by a vibrating transducer. Providing the pressure, viscosity (and other rheological factors) and frequency are appropriate, a regular stream of droplets is ejected from the nozzle. These are passed through a charge

electrode which induces a charge on each droplet. As a consequence the droplets can be deflected by an electric field. This is controlled in synchronism with the droplets so that the flight path of each droplet can be controlled. Droplets which are not required are deflected into a catcher. Another applied electric field at right angles deflects the droplets sideways by controlled amounts. This sideways deflection

coupled with the motion of the substrate enables a raster pattern of droplets to be deposited, thus producing an image.

This process has been widely applied to batch and date coding and high speed direct mail applications. Its resolution at best (at present) tends to be limited to about 200–300 drops per inch which is

somewhat low for printing electronics.

However the second inkjet method shown in Figure 3 is more controllable and works at higher resolutions. In this system, the ink chamber is fed with ink at ambient pressure. Ink droplets can be made to eject from the nozzle in one of two ways:

•    In the heat system, a small heater causes the ink to vapourise locally resulting in the formation of a bubble in the ink chamber. This happens very rapidly and hence the resulting pulse causes a droplet to be ejected.

•    In the piezo system, a voltage pulse is applied to the piezo element which deflects, thus creating a pulse in the ink and the ejection of a droplet.

The nozzle itself is held fairly close to the substrate and moved, so no other control of droplet flight is required.

For printing electronics applications, the piezo approach is clearly preferred since there is no requirement that the ink should vapourise. This method is commonly used in desktop office

printers, and a number of experiments related to printing electronics have been done on this class of equipment. High speed and high quality DoD systems are used for printing labels, some packaging materials and for proofing purposes. The method is also being adapted as a means of making litho printing plates.

DoD inkjet has the advantage of being able to produce very small drop sizes (<10 pico litres) and the ability to place these precisely. Piezo systems are suited to use with many different types of ‘ink’. These are characteristics that make the process well suited to printing electronics, especially in relatively low volumes. Many companies and university-based organisations are now experimenting with inkjet systems.

To create RGB colour displays, each pixel must be filled with a precise amount of OLEP material. Conceptually, inkjets are ideal precision metering devices for dispensing a variety of

materials without contacting the substrate. Thus, the individual layers and pixels of a flat panel display can be printed with an inkjet system jetting solutions of OLEP material. But to make this work in practice requires the

integration of precision hardware,

‘electronic’ inks and specially designed inkjet printheads: a new inkjet head from Spectra Inc. has been designed specifically for printing displays.

Other companies such as Xaar are also developing new heads, one of which will enable independent control over drop volume and drop velocity. Xaar are shortly (expected May 2004) introducing the Omnidot inkjet head, a greyscale device with 760 nozzles arranged in two rows producing three pl drops. Looking ahead to 2005, they have also issued a draft print head specification as shown in the table above.

Despite this improved control of the inkjet printing process, the method does still have its resolution limited by droplet flight variations and the spreading and splashing of the ink material on the substrate. This gives rise to shorting, making the maximum channel resolution in the tens of micrometres.

UK company Plastic Logic has

overcome this problem by employing a process of substrate surface energy

patterning which directs the flow of the water-based conducting polymer inkjet droplets. Using these techniques, water-based polymer ink droplets are confined to a hydrophilic (water-attracting)

substrate with a pattern of narrow (water-repelling) hydrophobic surface regions. This surface free energy pattern is

generated on the substrate prior to any of the polymer deposition steps, and can be accomplished by a broad range of techniques, including laser printing or deposition of teflon-like, self-assembled monolayers patterned using a soft rubber stamp or ultraviolet light exposure.

Once the substrate has been patterned, the source and drain conducting polymer is inkjet printed. Then semiconductor is printed over the transistor channel and dielectric added by spin coating. In this way the high resolution definition of channel lengths down to five microns and below is

possible and the construction of transistors and other components can be done as described later.

For displays, Plastic Logic buys in glass which is pre-patterned with indium oxide doped with tin oxide for the data lines and pixels. The plastic electronic thin film transistor source, drain and channel are then defined by surface energy patterning. The substrate is hydrophobic, but the applied energy

patterning is hydrophilic. The transistor source and drain are then inkjet printed on to the energy-patterned substrate. The water based conductive polymer is attracted to the hydrophilic surface but repelled by the hydrophobic areas. This stops the conductive polymer spreading or splashing on the substrate and gives the very high resolution required. The transistor semiconductor is then inkjetted into the gap before the transistor gate dielectric layer is spin coated from

solution across the entire area. Metal is then deposited to form TFT gates and gate interconnects. The backplane is then complete and ready for integration with the display itself.

Gravure printing

Gravure printing uses an engraved cylinder (with a copper surface) with the image to be printed in the form of square cells which are typically up to 25–30 microns deep. There may be 150–250 cells per inch or even more in specialist applications. If etched, each cell may be of more or less constant area, but vary in depth, or alternatively, both area and depth may alter to create tonal effects. In recent years, mechanical engraving has been much more common which produces an inverted pyramid shape cell. In the last few years, laser

engraving has been introduced which

is more flexible.

The cells are flooded by a liquid ink, the excess removed by a doctor blade, and the ink then transferred out of the cells by contact with the substrate under high pressure. Because of the high pressure and direct contact with the metal engraved cylinder, it is not possible to print on rigid substrates or glass, but an offset gravure process using an intermediate cylinder would be possible to overcome this.

The cell pattern extends across all types of subject matter including

pictures, text and line work. The aim when printing is to arrange a set of

conditions where the ink just floods the gap between the cells and so prints as a true solid or as a joined line. This is achieved by finding the precise balance between ink viscosity, depth of

engraving and printing speed. However, even when this is done, the edge of lines will still have a characteristic zig-zag pattern arising from the cell formation. Modern engraving techniques overcome this to some extent (e.g. by engraving half cells).

The process has not been widely investigated in relation to printing

electronics. However, it does have good potential in that a wide range of ink systems could be used (water and solvent based systems are commonly used and UV curing systems have been tried),

and a reasonably thick ink film can be

produced especially at low speeds using higher viscosity inks.

Gravure printing is currently being used by VTT Technical Research Centre of Finland who are having some success in printing components and light emitting elements using conductive inks and OLEPs, although they have been experiencing difficulty with the efficiency of the latter. ANITRA Medienprojekte GmbH in Germany are also planning on using a flatbed gravure-based method within a project called Contact. One aim of this project is to develop a reference method for the production of transistors using organic semiconductors.

Flexo printing

Flexo printing uses a printing plate made of rubber or more commonly

photopolymer, where the image areas

are raised relative to the surrounding

surface by a small distance (fractions of a millimetre up to a few millimetres). Ink is applied to the printing surface by what is termed an anilox roller. The anilox roller is engraved all over with a fine cell pattern. Cells may be of many different shapes and dimensions but 200–500 cells per inch would be typical. The anilox roller often sits in an ink bath (very liquid ink similar to that used for gravure printing) and has the excess ink removed by a reverse angle doctor blade. Alternatively the ink may be picked up by a fountain roller and applied to the anilox roller. A different surface speed or counter-rotation may be used to give a wiping action.

The printing plate is slightly resilient in nature and hence transfers ink to the substrate which is supported by a metal impression cylinder, using very light pressure. Inks may be water or solvent based, of UV curing. The process requires careful control of all variables to achieve high quality consistent results. Speed, ink viscosity, and pressures between all rollers in the system must be carefully set.

The process is well-suited to printing electronics since it is capable of printing quite fine line work (nearly as good as litho) if plates are made with the latest techniques, and thin plates are used. However it is a letterpress-type process which produces a typical ink-squash pattern, especially around the edges of solid areas and lines. This produces a particularly thick ink film just at the edge (and slightly outside it), of an area, and an ink depleted area just inside. Careful control of pressures and ink viscosity together with

appropriate choice of anilox roller cell specification can minimise the effect but it is generally present to some extent. However, by careful design of the shape of conductors this should not present a great problem in printing electronics. At the other extreme the process has been found (under experimental conditions) to be able to print large areas with or without a pattern on to glass with a remarkable degree

of evenness.

RIT in the US used flexo printing to print the antenna of an RFID badge for 5,000 conference delegates using Parmod inks, but little other published work has used this process.

Litho printing

Litho printing is a well established process that is capable of printing on a wide variety of materials. It is commonly referred to as an offset process because the plate transfers its image first to a rubber blanket, then to the substrate. The rubber blanket is compressible and hence deforms to small irregularities in the surface of the substrate, thereby making it quite a tolerant process.

Lithography relies on the action of two wetting functions on the surface of a smooth and unembossed printing plate. The plate chemistry repels water where the printed image is dark, allowing an oil-based ink to adhere. A water film repels the ink in light regions of the image. Contact with an ink and a moistening roller allows the printing plate to attract both water and ink as required, and to form the image to be printed. The image is not printed directly on to the substrate material (e.g. paper), but is instead transferred to an intermediate or ‘blanket cylinder’ that has a yielding surface. The blanket cylinder then presses the ink film onto the surface of the substrate, which is now supported

on a separate impression cylinder. The printed substrates rely on evaporation and/or oxidation of the ink film for the image to become fixed.

Litho printing can print high resolution images better than any of the other conventional print processes since there is no line or screen pattern interfering with line edges. It typically produces a flat even ink film, with little evidence of edge effects. In principle therefore the process is well suited to printing electronics. It does however print a rather thin ink film (2–3 microns) which may limit the range of applications or performance. This may be compared to the 50 microns thick-film conductors formed by screen printing processes, and 100 microns of copper typically laminated onto conventional circuit boards.

The idea of using litho for printing circuits is not new and has been

extensively reported by Ramsey, Evans and Harrison (“A Novel Circuit Fabrication Technique Using Offset Lithography”, B.J. Ramsey, P. S.A. Evans & D. Harrison, Journal of Electronics Manufacturing, Vol. No.7, No.1, March 1997).

The lithographic printing process offers excellent dimensional control and registration with substrate patterns, and circuits printed in this way exhibit sheet resistivities comparable with thick-film circuit produced by standard means. Critical elements of the process include the ink characteristics, the printing plate, the fountain solution and the printing machine.

Current resolution limits of the process lie between 0.1mm and 0.01mm (100–10 microns). Printed films show a track and gap width of 0.1mm (0.004in) that provides reliable conductivity. Track widths of 0.01mm result in a visible although non-conducting deposit.

The ability to construct circuits on both sides of a substrate is clearly advantageous. The difficulties encountered in double-sided printing of conductive lithographic films resemble those observed in two-sided printing of paper sheets in

standard commercial printing, and are overcome using similar methods.

Through-hole interconnection is an essential facet of any circuit board

fabrication process. There are two methods: firstly, a process utilising the printing operation to force ink under pressure through holes in the substrate, creating an electrical contact between films printed on alternate

substrate sides. The quantity of ink available to form the interconnect is limited by the small volume transferred by the litho printing process.

An alternative approach is to apply ‘blobs’ of conductive adhesive to the holes at the time that the

substrate is prepared for populating with components. This method is currently more reliable, as there is no restriction on the volume of

conductive adhesive that can be applied to each interconnect.

Thermal imaging

Thermal imaging is a method borrowed from litho plate production and is being adapted to the purposes of

printing electronics by Creo. Thermal imaging involves a chemical or physical change in a material induced by heating above a critical temperature.

It is therefore a binary process: if the critical temperature is not reached, no change takes place; above that

temperature and a complete change occurs. The process is not therefore very dependent on precise wavelengths. Writable and re-writable optical discs are examples of thermal imaging devices. Creo has a so-called SQUAREspot technology which is capable of remarkably fine resolution – it uses an 830nm laser diode and can produce square pixels of five, ten or 20 microns across (and others). For electronics applications, it is this imaging device which is of interest rather than a binary-change material.

The technology is currently being developed for:

•    Producing LCD colour filters;

•    ‘Printing’ inkjet barrier ribs;

•    Surface energy patterning.

Colour LCDs employ a matrix of red, green and blue pixels that filter the white back light. There is also a black matrix which blocks stray light and enhances contrast. This colour mixture is normally created by a moderately long process involving coating,

exposure to a mask, development and baking for each colour before adding final overall layers. Using thermal transfer processes this is reduced to a simple five-step process (resulting in a process similar to that used in proofing systems). This results in a 30 per cent reduction in overall costs.

The difficulty with control of inkjet systems for printing electronics have been previously described, and while

it is evident that much can be done to improve drop placement accuracy this does not overcome the tendency for drops to spread on impact over the

surface, thus constraining efforts to move to smaller features. Thermally transferred barrier ribs would be one way of preventing this ink drop spread – that is by creating a physical wall to contain the ink drop until it has dried.

The other approach as used by Plastic Logic is to use surface patterning. A variety of methods can be used for

this but thermal treatment is

particularly suitable enabling lines of 2–3 microns to subsequently be

produced by inkjet printing. Creo is developing partnerships with a number of other companies with a view to developing these concepts further

and have prototype machines under development.