Lead Inventor: James S. Im, Ph.D.

Tech Ventures Reference: M10-054, M10-044, M10-038

Problem or Unmet Need:
Active-matrix organic light-emitting diode (AMOLED) displays offer superior image quality over active-matrix liquid-crystal displays (AMLCDs): wider viewing angle, better color saturation, larger contrast ratio, and faster response time; and all that with lower power consumption. Furthermore, mass manufacturing may be cheaper and is more easily performed on flexible substrates. Recently, mass manufacturing of small AMOLED displays has taken off and manufacturers are seriously looking into the TV market.

AMOLED displays require a higher-performance active matrix to drive the pixels, and amorphous silicon, so popular with LCD TVs, is lacking both performance and stability. Low-temperature polycrystalline Si (LTPS) offers higher performance and greater stability, and for many years the process has successfully been used in manufacturing of small LCDs for high-end applications such as smart phones. Pulsed-laser based crystallization processes in particular have been successful and at present most AMOLED displays are manufactured using one such process: line-beam excimer-laser annealing (ELA).

However, ELA is suffering from limited throughput, insufficient uniformity, and a fundamentally narrow process window. Sequential lateral solidification (SLS), first proposed at Columbia University, has been developed in response to these issues and the process is now successfully used in manufacturing of small LCDs. Even so, both ELA and SLS are requiring further developments before they can successfully and economically be scaled to TV manufacturing, for example using gen-8 mother glass (~2.20x2.50m2). For example, multiple laser tubes are needed to increase power and thus throughput; however, this is challenging: due to inaccuracies in laser triggering circuitry ("jitter") it is difficult to achieve simultaneous firing of multiple tubes with high precision and pulse-to-pulse uniformity is degraded.


Details of the Invention:
Here, we circumvent the issues around jitter by sequentially firing the laser tubes. The delay is long enough to result in separate melting and solidification cycles, while it is short enough to ensure significant overlap between the pulses (a few to tens of microseconds). Using sample alignment techniques, the overlapped pulses can be placed exactly where pixel circuits are to be fabricated, while the area between circuits may be left unirradiated. Such a selective-area crystallization technique offers benefits in throughput enhancement as well as better control of the crystallization process for example in avoiding edge areas of the beam (eliminating stitching requirements).

According to the invention, a non-periodic laser pulse sequence is thus created with high efficiency, that is, without interruption of laser pulses. Such a pulse sequence may in particular benefit the "2-shot SLS" process using 2D projection optics; the scheme used in SLS-based small LCD manufacturing. Here, a rectangular beam is patterned into an array of narrow beamlets each resulting in complete melting followed by lateral crystal growth. In the second irradiation, each beamlet overlaps with two neighboring areas previously laterally crystallized and thereby concludes complete crystallization of the film via lateral crystal growth.



Applications:
• The non-periodic-pulse based selective-area crystallization process may be used for AMOLED TV manufacturing or ultra-high definition LCD (UDLCD) manufacturing.

Advantages:
• Stage requirements may be significantly relaxed as a result of the short time interval between overlapped pulses resulting in less sensitivity to stage error as resulting from wobble.
• Close overlapping of patterned beams results in improved robustness against image distortion from aberrations in the projection optics otherwise degrading microstructure uniformity.
• The crystallization details of the non-periodic pulse SLS process are identical to the convention SLS process and are thus tested and proven in manufacturing.
• Crystallization systems according to this method are presently being developed.
• A non-periodic pulse sequence in combination with selective-area crystallization may also be applied to an ELA process, offering the advantage of higher throughput and more accurate control of the pulse energy sequence that leads to the desired microstructure in this cumulative crystallization process.

Patent Status: Patents Pending

Licensing Status: Available for Licensing



Further Information
Paul van der Wilt
Email: TechTransfer@columbia.edu

File Number: M10-054