Evolution in LTPS AMLCD Manufacturing via Advances in Laser Crystallization Techniques and Systems

Substantial R&D activities are presently under way toward the development of advanced pulsed-laser-based production tools and processes that can enable sophisticated yet effective crystallization of Si films for manufacturing advanced active-matrix liquid-crystal displays (AMLCDs) that utilize polycrystalline-Si films. Implementation of such techniques in volume production, which has actually already begun, is poised to potentially permit low-temperature polycrystalline-Si (LTPS) AMLCD technology to pursue a more aggressive product and manufacturing roadmap.

James S. Im, A. B. Limanov, P. C. van der Wilt, U. J. Chung, and A. M. Chitu

IN LESS THAN 20 YEARS, flat-panel displays (FPDs) have evolved from being a niche product category into a ubiquitous electronic product that continues to expand and advance. One of the reasons for such dynamic growth can no doubt be traced to the fierce battles that have been waging within the FPD industry involving various technical approaches on virtually all facets and levels of the technology. For instance, in addition to the often-mentioned competition between various light-generation/manipulation schemes (e.g., LCDs, OLEDs, PDPs, FEDs, etc.), a less-visible area within which much competition is taking place concerns the formation of backplanes and additional electronics on glass, plastic, and other FPD-enabling substrates.

Currently, hydrogenated amorphous-Si (a-Si) TFT-based active-matrix backplane technology can be recognized as being the prevailing technology, as it is being utilized in the manufacture of AMLCDs of all sizes that currently dominate the FPD market. This particular combination has thrived because the synergistic match between what is needed for AMLCD pixel TFTs and the device characteristics of a-Si TFTs [which are actually substantially inferior to metal-oxide-semiconductor field-effect transistors (MOSFETs) fabricated on a single-crystal Si wafer] is at least quite reasonable.

However, that is not the end of the story. Low-temperature polycrystalline-Si (LTPS) TFT technology can be recognized as being technologically more enabling than its a-Si counterpart. As the name suggests, LTPS TFT technology uses Si films with a poly-crystalline (poly-Si) microstructure as the semiconductor material on which to make devices. By doing so, this alternative-microstructure-based technology permits TFTs to attain performance characteristics that can approach those associated with conventional microelectronics. The resulting high-mobility/high-stability TFTs can apparently confer various small and not-so-small technical advantages (e.g., high-aperture ratio, integration of driver circuits, etc.) for making AMLCDs, as well as being instrumental in realizing AMOLED displays. Without a doubt, LTPS TFTs can be recognized as being technologically more enabling than their amorphous counterparts. As can be typically expected from a better-performance-oriented technology, however, the fabrication of these devices requires some extra procedures that are not for free; minimizing such costs while maximizing the benefits associated with the technology is clearly what must be done in order to increase the competitiveness. In any event, this particular capability/value-adding evolutionary path of TFTs is of interest to researchers in the field of large-area electronics (LAE) in that the path can potentially lead to various LAE products, some of which may not, in fact, even involve displays.

As far as LCDs are concerned, LTPS-based AMLCDs have been firmly established in the past few years as the preferred "high-end" displays for mobile applications due to a combination of factors that make them better suited for high-resolution, low-power-consumption, and small-form-factor LCDs than can be realized using a-Si TFTs. In principle, there are many ways to obtain poly-Si films for making LTPS TFTs (which can involve, for example, direct deposition of the poly-Si films or solid-phase crystallization of as-deposited amorphous films). In practice, however, rapid melt-mediated crystallization of amorphous films on glass substrates via pulsed-laser irradiation corresponds to the approach that has been established in actual volume production of LTPS TFT-based AMLCDs (and AMOLED displays).

In general, a number of R&D-level laser-crystallization schemes and approaches exist (for some examples, see Refs. 1–7). The selection of a particular crystallization method for manufacturing can be identified as being a tactically and strategically important process in that the choice will effectively dictate how enabling and functional the resulting LTPS TFT backplanes can ultimately be, the extent to which such backplanes can be cost effectively manufactured at high throughputs and yields, and the future processing and product-related options that can be realized. Specifically, the following manufacturing- and business-related factors can be identified as being dependent on the exact method through which the films are crystallized:

•  The microstructural quality of the resulting materials, and therewith the ultimate TFT performance characteristics that are attainable.

•  The all-too-significant manufacturing characteristics such as processing window, production yield, substrate throughput, and maintenance and running costs.

•  The possibility of realizing future roadmap scenarios involving various manufacturing options and products (e.g., efficient utilization of Gen 5 or larger substrates; production of small-to-large AMLCDs and/or active-matrix organic-light-emitting-diode (AMOLED) displays; manufacturing highly integrated system-on-glass (SOG) products; use of flexible substrate materials; use of solution processed/ink-jet-printed Si films, especially for large displays; etc.).

•  The risks associated with the LTPS-TFT backplane manufacturing investment (which can depend, for instance, on whether or not the crystallization method has been validated in volume production and on the investment-cycle matching availability of suitable manufacturing-tested and refined production systems).

We now transition to manufacturing-related advances that have recently been made in the field. In particular, we report on the dynamic status associated with the sequential lateral solidification (SLS) technology. This advanced laser-crystallization method can now be qualified as the only volume-production-proven "alternative" to the conventional excimer-laser annealing (ELA) method, which for the past 10 years has been essentially the only LTPS manufacturing crystallization method (see, for example, Refs. 8–10). In the process, SLS is enabling production engineers to manufacture advanced AMLCD products while providing for more-beneficial manufacturing attributes and better strategical options than can be realized using ELA.

ELA, C-SLG, and SLS Methods

SLS corresponds first and foremost to a method that builds on the concept of controlled super-lateral growth (C-SLG) (Ref. 2 and references therein) that was conceived and demonstrated during the early-to-mid 90s. The development of the C-SLG approach itself resulted from the fundamental findings regarding the melt-mediated transformation scenarios that are manifested in pulsed-laser-induced melting and solidification of thin Si films.^11,12 Technologically, it was conceived in order to overcome the material-, process-, and device-related issues and limitations that were identified and concluded as being apparently innate to the flood-irradiation-based ELA method.

As regards to ELA, despite the valiant and exhaustive efforts by many production and equipment engineers over the years in improving the technique, it can still be recognized as being:

•  Ineffective in terms of providing microstructurally optimal materials (i.e., it essentially produces small-grained poly-Si films that can furthermore be non-uniform over various length scales).

•  Incapable of conferring a well-defined and forgiving process window, as the method is found to be extremely sensitive to minute variations in processing and sample preparation details.

 (a)  (b)  (c)

Fig. 1: SEM images of some microstructures obtained via various SLS schemes: (a) A uniform large-grained polycrystalline material via two-shot SLS, (b) a directionally solidified material via line-scan directional SLS, and (c) location-controlled single-crystal regions via dot SLS. Note that the samples were treated with a wet etchant to delineate the grain boundaries.

•  Inefficient under real manufacturing conditions (i.e., the method requires a significant number of pulses to "crystallize" an area, which does mean high investment and running costs).

These limitations are essentially impossible to avoid, as they can be identified as stemming fundamentally from the stochastic and critical nature of the partial-melting/near-complete-melting regimes within which the ELA technique operates.^9-12

The aforementioned factors translate to the harsh real-world penalties of high-maintenance, high-cost, and low-throughput backplane manufacturing using limited-size substrates with less-than-optimal TFTs. Since these limitations in turn make the LTPS technology itself that much less compelling and competitive, it can be advocated that there exists a need for a better manufacturing-crystallization solution to facilitate its progress and growth.

The SLS method avoids encountering much of the difficulties and challenges associated with ELA in that it is designed to follow a fundamentally different melt-mediated crystal-growth path. As a result, when properly implemented, SLS can be characterized as being substantially more deterministic and parametrically less sensitive. Technically, SLS builds on iteratively executing the following material-processing requirements: (1) inducing spatially localized and well-defined complete melting in irradiated region(s) of the film that subsequently leads to lateral solidification to transpire within the region(s); and (2) repositioning and re-irradiating the film in order to induce seeded lateral growth to proceed epitaxially from the large grains that resulted via lateral solidification from the previous irradiation.

Over the years, SLS has been demonstrated to be an unusually flexible approach. It has been shown numerous times that a number of distinct technical schemes and procedures can be employed – provided that the above fundamental SLS requirements are properly fulfilled – to form a variety of low-defect-density microstructures. Some examples of SLS-processed Si materials (Fig. 1) include uniform large-grained poly-Si films (e.g., two-shot micro-structure),^13,14 polycrystalline-Si films with a directionally solidified micro-structure,^1,4 and location-controlled single-crystal regions^15,16 (which can furthermore be surface-orientation controlled and free of intra-grain defects¹⁷).

Two-Shot SLS of Si Films

Among the various microstructures that can be generated via SLS, the consideration and optimization of the aforementioned manufacturing-centric factors leads one at present to identify the two-shot SLS material as being best suited for enabling high-throughput and high-yield manufacturing of advanced active-matrix displays.^13,14 Still, it should be appreciated how the other low-defect-density SLS microstructures, most of which actually provide higher-mobility TFTs than can be typically realized with the two-shot material, are available for a potential future usage. Some of these microstructures can also be recognized as being well matched to address other electronic applications (such as poly-Si-based thin-film solar cells and three-dimensional integrated circuits).

The two-shot SLS-processed Si films are noteworthy because they can be very efficiently produced, they can lead to uniform TFTs with overall performance characteristics that are superior to (and thus more enabling than) those that can be obtained from the ELA-processed films, they have now been demonstrated to be fully compatible with volume manufacturing of advanced AMLCDs on Gen 4 glass substrates, and they can be expected to enable realization of effectively all future manufacturing and product-related options and scenarios that may be reasonably ascertained at this point. Successful utilization of the material in volume manufacturing demonstates that those issues often mentioned about the material (such as its surface morphology and the directionality of the grains) do not stand in the way of production.

 Japan Steel Works, Ltd.

Fig. 2: Shown is the excimer-laser-based flexible lateral crystallization (FLX) system from Japan Steel Works, Ltd., which utilizes a multiple-beamlet-based SLS approach and is presently used in a two-shot SLS volume production, but which is the most flexible approach in creating a wide range of microstructures.

As far as directional SLS-processed films are concerned, it has been previously shown to be the case that the high-mobility TFTs that are fabricated on the resulting films can actually be not as uniform as those that can be obtained on the otherwise identically configured and treated two-shot SLS-processed films.¹³ Recent microstructural investigations of the directionally solidified Si films reveal spatial variations in the density of intragrain defects and crystallographic orientations in the material that appear to fundamentally underlie the device characteristics.^18,19 For this microstructural and additional process-related reasons (e.g., substantially lower throughputs, smaller energy-density windows, etc.), we presently consider the directional SLS of as-deposited Si films, by itself, as being a somewhat-less-competitive LTPS manufacturing option. As for the uniformity of devices fabricated on two-shot SLS-processed Si films, it is important to recognize how the periodic and predictable nature of the grains can be beneficially leveraged in order to further improve the uniformity of the devices by commensurately and optimally engineering the size, shape, and orientation of the devices.²⁰

Advanced Manufacturing Systems

The two-shot SLS-processed films can be technically realized either by (1) a multiple-beamlet-based SLS approach^14,21 (e.g., using the manufacturing-implemented "FLX" systems from Japan Steel Works (JSW), Ltd., see Fig. 2), or (2) a line-scan SLS technique^13,22using a single line beam (e.g., using the highfrequency excimer-laser-based "TDX" systems presently being developed at Team Cymer Zeiss (TCZ) GmbH,²³ see Fig. 3, or solid-state laser-based systems from Innovavent GmbH, see Fig. 4). These manufacturing systems are presently configured to handle Gen 4 substrates, and, building on the success of the Gen 4 manufacturing equipment, an FLX model capable of processing Gen 5 (or 5.5) substrates is presently being developed by JSW and Coherent engineers.

Conclusion

In summary, we conclude that consideration of various manufacturing-relevant and product-related factors identify the two-shot SLS material as presently corresponding to an optimal manufacturing choice for producing LTPS TFT-based AMLCDs. As such, recent availability and utilization of two-shot-SLS-capable manufacturing systems can be viewed as contributing and catalyzing in terms of increasing the overall competitiveness of the LTPS technology. For instance, SLS systems are already playing a leading role in terms of accelerating efficient and effective utilization of the Gen 5 plus substrates for future LTPS manufacturing. We presently also anticipate, based on various technical considerations and observations, the two-shot SLS approach to provide high-yield production-enabling solutions for high-throughput manufacturing of small-to-very-large AMOLED displays.

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 Team Cymer Zeiss GmbH

Fig. 3: The excimer-laser-based "TDX" system presently being developed at Team Cymer Zeiss GmbH that utilizes a line-scan SLS technique with a single line beam which is optimized for generating directional and two-shot SLS materials.

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 Innovavent GmbH

Fig. 4: The solid-state-laser-based "Lava"system available from Innovavent GmbH utilizes a single line beam and is capable of generating directional and two-shot SLS materials.