Inkjet printing equipment for organic LED mass production

The organic LED (OLED) display market has grown rapidly in recent years, largely due to the adoption of OLED for small-size mobile products. New advances in the development of ultra-thin and flexible OLED displays have further expanded the opportunity for OLEDs in mobile applications.

However, OLEDs have yet to achieve widespread commercial success in the large-size display market because volume-manufacturing costs remain prohibitive. Today's large-size OLED panels primarily use vacuum-based coating to deposit the active organic films that make up each emissive OLED pixel. Ideally, OLED displays are made using repeating arrays of individually patterned red, green, and blue (RGB) subpixels. This is the simplest device structure with the best overall display performance.

It is possible to fabricate such a display using a so-called fine metal mask (FMM) to achieve patterned RGB coatings of the OLED active materials,^{1, 2} but material efficiency, scalability, and yield are not acceptable for mass production of large-size displays. As a result, existing large-size products use an alternative technique where an unpatterned white-emitting OLED structure is combined with conventional RGB color filters.^3,4 However, this approach requires a more complex architecture (adding to cost), and the resulting displays have lower overall performance than true RGB-patterned OLED displays. Even in a high-volume, high-yield mass-production line, this approach will not deliver a product that is cost-competitive with LCDs.

Since the late 1990s, inkjet printing has been explored by many groups to address the OLED cost-competitiveness problem.^{5, 6} The goal is to develop a low-cost method of manufacturing true RGB OLED displays without the difficulties of vacuum-based FMM processing. Inkjet was a logical candidate because this technology can deliver highly scalable, pattern-wise precision deposition of organic thin films with high material efficiency. Making inkjet successful, however, requires good inkjet equipment and good solution-processable OLED materials.

Historically, two challenges have impeded the adoption of inkjet printing for OLED mass production. First, device performance (color, efficiency, and lifetime) failed to reach the standard required for display commercialization due to intrinsic material limitations and the use of wet processing. Second, inkjet OLED panels suffered from visual nonuniformities, in particular, a type of pattern nonuniformity referred to as ‘mura.’ Mura is caused by imperfect inkjet droplet deposition onto the panel substrate and nonuniform ink drying over the substrate surface.

Lately, the intrinsic performance of wet-processed inkjet printable materials has improved dramatically^7–13 and has reached the level necessary to transition inkjet into mass production. In addition, progress in inkjet technology has resulted in the demonstration of large-size, uniform, mura-free panels.^14,15 This progress has made inkjet a realistic near-term solution to enable low-cost mass production of large-size OLED panels. However, the transition from research and development to mass production is difficult, especially for inkjet equipment.

Between 2008 and 2013, we at Kateeva developed the first commercial inkjet technology engineered from the ground up for OLED mass production—the YIELDjet™ (see Figures 1 and 2)—and our mass-production tools are now operating in the field.

Figure 1. Kateeva YIELDjet™ inkjet technology.

Figure 2. Close-up of inkjet head array showing three packs of three printheads on an existing YIELDjet mass-production system.

In addition to the baseline requirements of a precision inkjet-printing system, the technology has three key characteristics. First, it is engineered to enable very low substrate particle contamination. This is achieved through the systematic application of semiconductor equipment design principles to each component of the system, and also through the careful design of gas flow within the system to minimize turbulence.

Second, the technology uses a production-worthy integrated ultrapure nitrogen processing environment. Given the sensitivity of OLEDs to ambient contaminants such as oxygen and moisture, this critical feature maximizes OLED performance, especially device lifetime. The challenge was to provide a high-purity N₂ environment while maintaining production worthiness. We accomplished this through a ground-up design of the inkjet machine architecture to ensure that the system can be routinely serviced without exposure to air, and that in rare cases of air exposure, clean nitrogen is directed and purged through all system components to sweep away residual air, enabling a remarkably short recovery time.

Third, the system uses advanced inkjet printing algorithms and process monitoring to provide consistent, uniform, mura-free film deposition with a wide process window. This means that in production, the panels consistently meet specification. This contrasts with the typical research and development environment, where it is sufficient to demonstrate a small number of high-quality samples and near-perfect repeatability is not required.

To achieve such reliability, we have developed several key innovations. Specifically, we developed a novel in-flight inkjet droplet monitoring technology that is 50 times faster than previous techniques. This technology uses laser reflection and interference to perform highly accurate in situ measurements for each nozzle of droplet volume, velocity, and trajectory—key parameters needed to control and calibrate the print algorithm—at rates that for the first time are compatible with high-volume manufacturing. We have also developed a unique print engine, advanced printing uniformity algorithms, and novel drive electronics. Combined, these technologies enable sustained, on-the-fly adaptation of the printing process based on continuous measurements of nozzle status with feedback to the print engine, resulting in high uniformity and repeatability, and requiring minimum down time for tool recalibration and tuning.¹⁶

Today, inkjet printing is already enabling OLED thin-film encapsulation (TFE), key technology required for manufacturing ultra-thin and flexible OLED screens. Companies using inkjet for TFE OLED mass production are attracted by the high coating quality, as well as the cost-of-ownership advantages over competing technologies. With recent advancements in material quality and equipment technology, as described here, inkjet printing for RGB pixel patterning is now poised to follow the same path as TFE into OLED mass production to enable manufacturing of cost-effective large-size OLED displays. As our next step, we are scaling up and incorporating these technologies into our forthcoming G8 glass (i.e., glass plates large enough to process six 55" TV panels on one plate) RGB mass production systems for high-volume, low-cost manufacturing of large-size OLED displays.