FEATURE
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The big screen

Stephen Mounsey evaluates the photonics contribution to flat-panel display manufacture

A feature of modern society is that consumers have a continual appetite for new technologies; consumers have opted for DVD players, for example, at a rate which rendered the VHS standard technology obsolete within the space of a few years. Similarly, flat-panel displays (FPDs) have enjoyed a steady rise in popularity over the last decade, to the point that few retailers still stock anachronistic cathode ray tube TVs and monitors.

The pace of technological change in the home and office is creating consequential demands for technological change in the manufacture of high-value consumer electronic goods, and the manufacture of FPDs is a case in point. High definition FPDs can require features of micrometre dimensions to be created millions of times on a single screen. Although lasers may not always have been a cost-effective technique for display manufacture, laser-based techniques have become the most effective way of mass-producing affordable FPDs. The most important factors in FPD manufacture are precision and high-throughput – lasers are able to deliver on both.

Cutting

Glass used as the substrate for FPDs can range in thickness from 30μm to 1mm. The conventional mechanical method of cutting glass is to scribe the surface of the pane with a hard blade, in order to create a partial-thickness crack. A mechanical stress is then applied in order to propagate the crack through the thickness of the pane. Mechanical cutting creates debris, a cut which may not be perpendicular to the pane’s surface, and an edge with residual stress and micro-cracks. Because stress is concentrated at the tip of any crack, the presence of these micro-cracks weakens the glass. They are polished out of the edge after splitting to improve fracture toughness, and debris from the cutting process is often removed through cleaning.

Laser cutting greatly reduces both debris and micro-cracks, and also leaves far fewer residual stresses in the edge of the glass, meaning that laser-cut glass has two to three times the fracture toughness of mechanically cut glass, and edges that do not need to be cleaned or polished.

Laser cutting can be accomplished in two ways: laser scribing and through-thickness laser cutting. Within the FPD industry, scribing is used for thicknesses of 0.3-0.7mm (the effective thickness range of the technique is 0.2-10mm), whereas through-thickness cutting is used for glass with thickness of 0.2mm or less (effective range 30μm-1mm).

The lasers used in both techniques are most commonly CO2 CW lasers, although UV DPSS lasers are sometimes used to achieve greater precision where needed in laser scribing. The 10.6μm wavelength emitted by CO2 lasers is absorbed by the glass, meaning that the surface of the pane heats up quickly in both cases. When laser scribing, the laser is scanned along the surface, at speeds up to 1.5m/s, before being rapidly cooled by a nozzle delivering water, ethanol, or CO2 (see Figure 1). The shock of being heated and cooled so rapidly creates a crack in the glass that is approximately 100μm deep. The pane is then stressed using a roller, which propagates the crack through-thickness to complete the process. Again, the cleavage is produced perpendicular to the surface, relatively free of micro-fractures and debris.

In through-thickness cutting, the initial heating and cooling is still carried out, creating a scribed crack as before, but this is followed up with a second laser beam in order to reheat and expand the surface, driving the crack through the glass without mechanical deformation. This process can be carried out at up to 20mm/s.

Patterning

Cut glass alone would not make much of a display without patterning. Each pixel of a display can be independently addressed by way of circuit elements that are built onto the glass substrate. These structures may be applied using conventional lithography, although the process is expensive, requiring highly specialised equipment and expertise. Laser direct patterning (LDP) is a cost-effective alternative, wherein laser radiation is used to ablate the material of the thin film selectively and precisely, without damaging the film’s substrate or the component’s structures.

Transparent electrodes are present in some display technologies, consisting of a thin film of indium-tin oxide (ITO) on the glass. As the name suggests, finding ways of depositing laser energy into the thin film making up the ‘transparent conductor’ is a challenge, and one that can necessitate the use of UV wavelengths at high powers. Visible and infrared light can be used, but these wavelengths transmitted through such a film without absorption, means less precision.

Fig 1: Laser scribing creates a continuous partial-thickness crack in the glass, which is separated later by mechanical rollers. Image courtesy of Coherent.

Rainer Paetzel, director of marketing for Coherent (Deutschland), explains: ‘The more precisely you want to do the machining, the more precisely you have to match the wavelength, so that you can have deposition of the energy in a small area. Too long a wavelength would allow penetration of light into the substrate and heat into the surroundings. If the feature size is large, this can be tolerated, but high precision, thin films of ITO and other transparent conductive oxides need UV lasers.’

When structuring metallic electrodes with 100μm features, UV is not used, as reasonable absorption is achieved at the near-IR wavelengths, which are cheaper to produce, and the feature sizes can be created with these wavelengths. In contrast, features on an FPD can range from 100μm (at the ITO transparent contacts), to 5μm in the colour-filter level.

Patterning can either be achieved by scanning a beam over the surface, or by using a mask approach and a higher powered laser. Scanned (direct-write) patterning is suitable for simple lines and features, and the powers obtained from a DPSS laser are sufficient, most commonly at 1,064nm. If a more complex feature is required, the manipulation of the beam could become complex, and so a mask can be used, although the challenge of patterning a thin film in this way necessitates the high power UV radiation, which is only available by using an excimer laser. In both cases, the lasers are pulsed, thereby minimising damage to the substrate, by Q-switching in order to produce pulses of 25-50ns duration.

When using DPSS lasers, the beam profile can need some consideration; beams of this type are normally Gaussian and circular, meaning that material at the centre of the beam is likely to receive too much fluence before material at the edge of the beam has received sufficient, leading to spots of damage to the substrate. To counteract this, optics such as a fly’s eye homogeniser or systems of bi-prisms are used to give a smoother beam profile and more uniform intensity.

Polysilicon annealing

Silicon is applied to a glass substrate by means of conventional chemical vapour deposition (CVD) techniques, typically to a thickness of around 50nm, in order to create the thin-film transistors (TFTs) that make up the active component of many display technologies, including LCD. This silicon is initially deposited as an amorphous solid, with very limited crystal structure present, meaning that the mobility of electrons within the layer is limited. Increasing electron mobility allows TFTs to be made smaller, and in turn allows higher resolution and higher contrast displays to be created. The electron mobility of amorphous silicon is improved by way of conversion into polycrystalline silicon, which is carried out using a low-temperature annealing process. Ric Allott, of the UK Displays and Lighting Knowledge Transfer Network, believes that green DPSS lasers (1,064nm, frequency-doubled to 532nm) are sufficiently powerful for this process. Fibre lasers are likely to have an impact on this market.

Fig 2: A process by which display circuitry can be put onto a flexible carrier: A) Display circuitry is built on sacrificial Si layer on glass substrate. B) Circuit is bonded to temporary substrate. C) Laser selectively ablates sacrificial layer. D) Circuit separates from glass substrate. E) Circuit bonded to flexible substrate. F) Temporary substrate removed. G) Display circuitry is now on a flexible substrate. Image courtesy of Coherent.

Coherent, on the other hand, promotes excimer lasers for annealing applications. The reason behind this, according to Paetzel, is that the process takes place on such a thin film that contamination from the substrate back into the film is a possibility if absorption is not limited to the thin film. Additionally, the output of an excimer laser can be shaped into a line, in order to scan across a work-piece and cover the whole of it in one pass.

Throughput is, therefore, an important factor: a short line used to anneal a large work piece would require the glass to be scanned many times, leading to an extended takt time (the time taken for a single work-piece to undergo a particular process on a production line), along with imperfections at the junctions. A laser line shaped to cover the whole width of the workpiece need only be moved across it once in order to completely anneal it, and there therefore exists a strong motivation for a long line, albeit one with sufficient power to achieve the fluence necessary for annealing. Paetzel believes that the combination of these factors makes a strong case for excimer-based laser systems.

Future of display manufacture

Significant effort has gone into the development of electronic paper and flexible displays. Displays based on electronic paper, such as are used in many current eBook readers, look much like a standard monochromatic LCD, except that they remain readable without power. Currently, this kind of display is still built onto a glass substrate, but a flexible substrate would make the device more versatile. More complicated flexible displays, such as those capable of showing colour video, are also under development. Laser manufacturing techniques being developed for these technologies include laser lift-off (see Figure 2). The circuitry of the display is constructed, layer by layer, on a temporary glass carrier using the conventional techniques described above. The difference, in the case of a flexible substrate, is that a sacrificial layer of silicon is added in between the glass carrier and circuitry of the final product. Once the display has been created, it can be released from the glass carrier by using a laser to remove the sacrificial layer. The delicate circuitry is floated in liquid until it can be adhered to a flexible substrate, such as the polyethylene terephthalate (PET) used in drinks bottles. A laser beam shaped into a long line, as used in the annealing process mentioned above, would be well suited to high throughput in this application.

In another novel application under development, lasers are used to modify the surface of a substrate at a very fine scale, in such a way as to make it either hydrophobic or hydrophilic. This is relevant to the up-coming methods of printing electronic components, allowing the designer a greater degree of control over where the various inks bond to the substrate.

Putting aside the break-through technologies of the future, the trend within the industry of today is to go to ever-larger panels in the processing fab; that is not to say the TVs are getting bigger, but only that a fab now produces them 12 at a time, rather than six at a time. This up-scaling allows ever-lower retail prices, and thereby allows manufacturers to take the remaining market share from CRT displays. From there, plasma screens will give way to LCD, and LCD will ultimately give way to OLED displays. The first 10th generation fab is due to be commissioned by Panasonic later this year, processing glass panes of 3 x 2.5m, 1mm thick. The pane is worked on by all the machines in the factory, before being sliced into 12 sections for use in 12 displays. From the point of view of laser developers, the challenge is to ensure that making twice as many displays does not take twice as long.