Display Week 2015 Review

Display Materials and Processes

Display Materials and Processes

In addition to three major categories of new and evolving display materials – display glass, flexible transparent conductors, and quantum dots – a potentially disruptive material-and-process combination appeared at Display Week this year.

by Ken Werner

WITHOUT new materials and new manufacturing processes, progress in display technology is limited to evolutionary rather than revolutionary changes.  On the show floor at Display Week this year, we saw three major categories of new and evolving materials:  display glass, flexible transparent conductors, and quantum-dot products.  In addition, both on and off the show floor, Sharp was talking much more than previously about how it has implemented gate drivers on a display’s image area to create its attention-grabbing free-form displays.  Off the show floor, there was considerable discussion about micro-LEDs.  Candice Brown-Elliott, Nouvoyance CEO and creator of the Pentile matrix configuration widely used in Samsung OLED displays, said this was the only truly disruptive technology she saw at Display Week this year.  And there were additional interesting materials developments that did not fit into any of these categories.

Glass

The three leading manufacturers of display glass – Corning, Asahi Glass Company (AGC), and Nippon Electric Glass (NEG) – along with glass fabricator Cat-i Glass Manufacturing (Elgin, Illinois) were all on the show floor.  Also in the exhibit hall were LCD re-sizer Tannas Electronic Displays (Orange, California) and Litemax Technology (Fremont, California), which resizes LCDs  and fabricates custom-sized signs and monitors using its resized panels.  In an aisle, I also ran into Larry and Like Linden of glass-cutter TLC International (Phoenix, Arizona).  I have known TLC as a scriber of straight, curved, and circular lines in glass, but I did not know until this meeting that it also cuts complete LCDs.

Corning was showing several technologies, including its Iris Glass designed to replace polymer light-guide plates in television displays, its flexible Willow glass, two types of Gorilla Glass, and NXT Glass, its “next-generation” product.  Each of these varieties is designed to fit specific design needs, although in some cases their capabilities overlap.

According to Corning, an Iris light-guide plate “eliminates space and components, features excellent transmission, and enables thinner, brighter TVs with accurate colors.”  Also on display was the second generation of 100-mm-thick Willow glass on a carrier of conventional display glass.  This allows the glass to be processed on a conventional manufacturing line and then separated from the carrier.  The display is now on a very flexible sheet of glass that can be rolled to a rather tight radius, while the expensive carrier can be resurfaced and re-used.  If used as the substrate for a flexible OLED display, the Willow glass blocks moisture and oxygen, unlike polymer substrates.

Corning had an extensive display of Gorilla Glass for automotive demonstrations, including “cold form,” in which a flat piece of Gorilla Glass is bent to fit the application, and pieces that are hot-formed for applications requiring 3D surfaces or a localized bend – bends that vary in curvature across the sheet (Fig. 1).

Fig. 1:  Corning Gorilla Glass can be formed with a “local bend” for automotive applications.  Photo courtesy Ken Werner.

Gorilla Glass 4 was announced at CES.  Corning reps were happy to explain that it has been engineered with increased fracture resistance if a phone (for instance) is dropped on the display side, while Gorilla Glass 3 is engineered for maximum scratch resistance and scratch concealment.  Since the two versions optimize different characteristics, both will be produced.  Corning discovered that dropping a Gorilla 3 phone face down on a slightly rough surface such as concrete, asphalt, or sandpaper is more likely to produce fracture than a similar drop onto a smooth surface such as hardwood, granite, or steel.  The design of Gorilla 4 resolves that issue, says Corning.

Lotus NXT Glass, Corning’s next-generation display glass, is described by the company as “stable glass for high-performance displays.”  Under typical display processing, this glass exhibits a significantly lower “total pitch variation” – less variation in the pitch of the TFT array relative to the color-filter array.  A glass poster showed the improvement to be significant.  Lotus NXT is available in thicknesses as low as 0.4 mm.

In its booth, Asahi Glass Company (AGC) featured Dragontrail, its competitor for Gorilla Glass.  New was a flexible version called Dragontrail X (Fig. 2).  AGC also showed soda-lime glass as thin as 0.23 mm and “Spool,” an ultra-thin developmental glass that is 0.05 mm thick.  Two or three years ago at Display Week, AGC showed its own version of thin glass on a carrier, along the lines of Corning’s Willow, but I did not see it this year.

Fig. 2:  AGC’s Dragontrail X is a flexible version of its Dragontrail product, which competes with Corning’s Gorilla Glass.  Photo courtesy Ken Werner.

AGC also showed its new “Glass Plus” glass-resin composite component.  Glass Plus is a display cover glass (which may contain a touch-panel sensor) bonded to a surrounding polymer frame that can be flush to the glass on one or both sides.  The component can therefore do away with the separate frame or bezel that often surrounds the cover glass, decreasing product thickness and removing both a component and an assembly step.

Nippon Electric Glass (NEG), which won an award for best medium-sized exhibit on the show floor this year, featured its own 0.2-mm glass called G-Leaf.  NEG’s Ted Shimizu highlighted G-Leaf’s roll-to-roll processing and possible use as a flexible OLED substrate with inherent barrier qualities.  He also mentioned heat shields for laboratory and industrial workers as a possible application that would leverage G-Leaf’s impressive transparency.

When it comes to ultra-thin glass, glass-makers are ahead of their display-making customers.  Rollable display glass is available now or will soon be available from the three leading fabricators, but display-makers have not yet developed the processes needed to make use of it.  LCD manufacturers may not feel justified in spending a lot of money to make major changes to plants and processing to incorporate roll-to-roll, especially since there are difficult problems to solve.  One of these is maintaining cell thickness when a flexible LCD is rolled to even moderate radiuses.  (Merck KGaA thinks it has a solution for this problem and is looking for development partners.  You will find more details later in this article.)

A nearer-term application of ultra-thin glass is OLED displays, even though this application requires a transition to printed OLED front planes.  That has been a subject of serious R&D for years.  At the beginning of Display Week, DuPont Displays and Kateeva announced they would collaborate to optimize ink-jet printing for the mass production of OLED TVs.  “With Kateeva and DuPont combining their considerable expertise in ink-jet printing and OLED materials, the industry is poised to take a significant step forward in achieving low-cost mass production of OLED TV,” said Steven Van Slyke, CTO at Kateeva.  From another source that might sound like standard commercial puffery.  From Van Slyke (co-inventor of the practical OLED display), it deserves to be taken seriously.

Quantum Dots

A lot of the conversation about quantum dots at Display Week this year revolved around the European Commission’s rejection of its own technical committee’s recommendation that cadmium-based quantum dots continue to be exempt fromprohibition because cadmium is on the European list of dangerous substances.

Initially, this generated some angst in the cadmium contingent and some jubilation in the non-cadmium (mostly indium phosphide) crowd.  But a consensus soon emerged that the EC’s rejection was based on one minor technical and one procedural matter and that the technical committee would certainly correct the minor issues, after which the exemption would be continued.  EC exemptions are often based on there being no alternative solution available, so the issue revolved around the current availability of indium phosphide.  However, indium has also been added to the EU’s list of hazardous substances.  It was generally regarded as irrelevant to the regulators that neither cadmium nor indium is biologically available when encased in a quantum-dot shell.

Nanosys, which won an award from SID for best small exhibit at the show, had three side-by-side TVs that clearly showed why indium-phosphide quantum dots (QDs) are a poor substitute for cadmium.  The typical conventional LCD TV with white-LED backlighting in the Nanosys booth had a measured color gamut of less than 60% of Rec.2020, a luminance of 500 nits, and a power consumption of 130 W.  The same model of TV modified with blue LEDs and a cadmium QD sheet in the backlight measured greater than 90% of Rec.2020, 400 nits, and a power consumption of 130 W (with the original color-filter array) (Fig. 3).  And another example of the same model TV with an indium-phosphide QD sheet measured about 75% of Rec.2020, 350 nits, and 130 W.  Clearly, if the goal is to get close to Rec.2020, indium phosphide is not the way to go.  Subjectively, the difference between the cadmium QD-enhanced TV and the standard model was dramatic.  The difference between the indium-phosphide-enhanced set and the standard one was visible, but sufficiently subtle that consumers might not be strongly motivated to pay a premium for it.

Fig. 3:  Shown is the cadmium quantum-dot-enhanced example from the comparison shown in Nanosys’s prize-winning booth.  The demo made it very clear that cadmium quantum dots deliver a much greater color gamut than indium-phosphide dots.  Photo courtesy Ken Werner.

Nanosys Corporate Communications Manager Jeff Yurek wanted me to know that Nanosys has now reached a level of manufacturing volume such that the EPA required it to submit a pre-manufacturing notice, which was accepted.  He also announced a follow-on investment from Samsung Venture Investment Corporation.  The new funds will be used to expand production capacity as demand increases.

Also at Display Week, Nanosys partner 3M Display Materials and Systems Division showcased LCDs in several sizes with color gamuts of up to 93.7% of the Rec.2020 color gamut.  Among the demos was a 4K monitor with 93.7% Rec.2020, which demonstrated, as the booth signage read, “one of the largest known color gamuts in an otherwise commercially available 4K LCD monitor.”

QD Vision was exhibiting available commercial products using its IQ Color linear QD element.  Among these were a Philips 29-in. monitor, a TCL 65-in. TV, and a Hisense 65-in. curved TV.  This is the first curved TV, said CMO John Volkmann, and it uses one edge light and one IQ Color element on each of the left and right edges.

I asked Volkmann if he was concerned that an increasing percentage of TV sets are using direct backlighting for local-area dimming and therefore cannot use QD Vision’s linear array.  His answer:  “There will be a lot of edge-lit TVs made for the foreseeable future.”  He also said the company was looking at other form factors.  As previously stated, the company is working on a QD-on-chip approach and is closer than its competitors.  There was a 94% Rec.2020 demo in the booth.  To get higher than that, Volkmann said, wide-gamut color-filter arrays as well as high-quality QDs (such as QD Vision’s) are required.  Volkmann was confident that cadmium would remain legal in the EU and did not mention any fall-back materials for QD Vision.

If Nanosys, 3M, and QD Vision are among the leading QD companies, Quantum Materials Corp. (San Marcos, Texas) is one of the hopefuls.  Although not exhibiting at Display Week, QMC announced in a June 1 press release that it had “launched their new QDX class of high-stability cadmium-free quantum dots….”  The release continued, “QDX quantum dots have been tested to withstand heat resistance to 150°C for 4 hours with no oxidation performance degradation in an open-air environment.”  When I asked him, QMC PR person Art Lamstein told me the company is in a “pre-revenue” stage.  In addition to the company’s original cadmium-based quantum tetrapods based on a Rice University patent, QMC is now also making indium-phosphide dots based on a Bayer patent the company purchased in 2014.

Nanoco (Manchester, UK) was not on the show floor, but I spoke briefly with COO Keith Wiggins and Business Development VP Steve Reinhard.  Since Nanoco has for some time emphasized that its QDs are free of not only cadmium, but also of other heavy metals, the company has been almost gleeful in welcoming the European Parliament’s decision to turn down the RoHS exemption for cadmium despite its approval by the technical committee.  However, as mentioned above, the majority opinion is that this potential gift to Nanoco is likely to be short-lived.  As is well known, Nanoco has licensed its technology to Dow Chemical for volume manufacturing.

Transparent Flexible Conductors

Transparent flexible conductors (TFCs) provide added value based on their thinness, light weight, and ruggedness even when they are being applied to displays with rigid substrates.  Now that flexible displays are entering the marketplace in significant numbers, that value becomes even more pronounced.

TFCs compete on a combination of cost (low is good), sheet resistance (measured in ohms per square; low is good), transmittance or transparency (high is good), lack of color-ation (none is good), haze (the lower the better for most but not all applications), degree of flexibility (measured by the diameter of a mandrel around which the film can be bent), and maintenance of sheet resistance with repeated flexing.

The first technologies in the market were fabricated metal matrices and silver nanowire (AgNW) inks.  AgNW inks have taken the lead because metal matrices have a regular pitch that produces moiré interference with the pitch of the pixels unless the matrix is especially designed for each display, and their relatively large feature size means they cannot be used with displays having very fine pixel pitches.  AgNW patterns are random and can be used with virtually any pixel pitch, with the silver wires from some makers now so fine that they produce very little haze in bright sunlight.  Cambrios is the current AgNW leader.

However, as we saw at Display Week, other ways to play the game have already escaped from university labs and corporate skunkworks.  Here, in no particular order, are the entries that appeared on the show floor.

Richard Jansen, VP of Sales and Marketing at SouthWest NanoTechnologies (SWeNT; Norman, Oklahoma), said his company uses both AgNW and carbon nanotubes (CNTs) in two layers.  The CNTs are screen-printed on top of the AgNWs, where they serve as a patterning mask.

The unprotected AgNWs are washed away with water and then reclaimed.  When AgNWs are used by themselves and adhered to the underlying film, Jansen said, they require laser patterning or photolithography.  Thus, SWeNT offers an easier and quicker patterning process.  The company is several months away from customer sampling.

Canatu Oy (Helsinki, Finland) uses carbon nanobuds for its conducting medium.  These budlike structures appear on the exterior walls of CNTs when they are grown, said Canatu marketing and sales VP Erkki Soininen, but Canatu forms its nanobuds directly through the reaction of gasses.  The nanobuds, said Soininen, literally fall out of the reacting gasses onto a film in a roll-to-roll process.  There is enough adhesion between the buds and the film so the product can be shipped in this form.  The customer patterns the film and adds an overcoat and any other films needed for his application.  The Canatu process produces TFCs with sheet resistances as low as 100 Ω/□ at 95% transparency.  Canatu has just announced its first design win, a flexible consumer product.  What product?  Soininen can’t say.  Not yet.

Kelly Ingham, COO of Cima NanoTech (St. Paul, Minnesota and Singapore), told me the company is currently making a major transition to manufacturing and is ramping up high-volume film production in China.  Cima NanTech spent 10 years in R&D mode, so this is a very significant change.  Ingham and two other members of the strong senior management team are former 3M employees and presumably familiar with high-volume films.

The company’s SANTE technology applies proprietary nanoparticles on PET or other polymer film in a wet roll-to-roll process.  The nanoparticles then self-assemble into a random metal mesh with 3–6-µm conductors.   The process can produce films with a 25-Ω/□ sheet resistance at 87% transmissivity (including the PET).  The SANTE’s “shading” – the transmissivity loss caused by the metal mesh alone – is only part of the total loss.  NanoTech’s first app is a game table with 10-finger touch from a U.S. company, in which the large display size and requirement for 6-msec response time demands very low sheet resistance.  The technology can go as low as 10 Ω/□ for large sizes.

At Display Week, Stanford spin-out C3nano (Hayward, California) culminated a string of major announcements this year by introducing its highly flexible ActiveGard hardcoat for its AgNW TFC product.  C3nano deposits an ink containing silver nanowires that overlap each other in a loose web.  The wire web is open enough so light can pass through but dense enough to provide good conductivity.  C3nano’s wrinkle is “Nanoglue” technology, a catalyst-mediated process that causes the AgNWs to fuse where they cross.  This results in greater conductivity for a given wire diameter, which can be used to deliver lower sheet resistance, less haze, or a combination of the two, said CEO Cliff Morris.

These are the TCFs that were on the show floor.  Still in laboratories are carbon nano-tubes, graphene, and who knows what else.  For a category often thought of as simply “ITO replacements,” TCFs have become very interesting indeed.

Off the Show Floor

It took five contributors to produce the reporting for Information Display’s coverage of Display Week, and we did not come close to seeing and hearing everything.  There were many, many technical and business presentations at Display Week, some of quite general interest, some by specialists for a handful of their fellow specialists.  Here are short summaries of a very few materials-and-process-oriented presentations I was smart enough to seek out or lucky enough to stumble upon.

For some time, Sharp has been showing examples of its “free-form” displays, which do both the “row” and “column” driving through one edge of the display, leaving the rest of the display to be cut in curves or other unusual shapes (Fig. 4).  But until this Display Week, Sharp had not been willing to describe in detail how it distributed the gate drivers throughout the display so that conventional row drivers mounted on a vertical display edge are not necessary.

Fig. 4:  This recent demo of Sharp’s “free-form” LCD technology has a curved top and almost no bezel on the top three sides.  The display incorporated touch on the outside edges of the display, not the surface.  Photo courtesy Ken Werner.

In the Sharp booth, Automotive Marketing Director for Display Products Thomas Spears did his best to explain the situation but it was hard for him to do so in any detail amidst the cut and thrust on the show floor.  More detail was available from the invited paper by Hidefumi Yoshida and 13 colleagues from Sharp in Nara, Japan.  The paper, “Flexible Flat-Panel-Display Designs with Gate Driver Circuits Integrated within the Pixel Area,” described Sharp’s truly clever approach.

Yoshida and colleagues began with a well-known method, gate-driver monolithic circuitry (GDM).  With GDM, the shift registers and output transistors of the gate drivers are deposited on the vertical edge of the display at the same time as the switching transistors are fabricated.  This is an alternative to the more conventional approach of using ICs for the gate-driver circuitry.  Since GDM circuitry can occupy significant real estate at the vertical edge of the display, especially when implemented in amorphous silicon, it requires a wide bezel, which is not compatible with current display preferences or with gracefully curved display contours.

Here is where Sharp’s cleverness comes into play.  First, instead of putting the GDM circuitry on the vertical edge(s) of the display, Sharp locates it in one or more vertical “bands” within the display area (Fig. 5).  I’ve put “bands” in quotes because Sharp has done far more than simply shifting the left-edge circuitry into the image area.  Sharp disperses the transistors of the GDM circuitry so individual transistors are located at individual pixel locations and interconnected via additional surface connections and a large number of through holes in the display. Thus, the gate-driver control signals enter through the bottom edge of the display, which is also where the source drive ICs are located.  The gate signals travel from the dispersed GDM circuits horizontally to the pixels, but entirely within the image area.  This allows the left, right, and top edges of the display to have very thin bezels, which can be shaped with great freedom.  Sharp has widely shown a triple curve that is appropriate for the tachometer, speedometer, and combined temperature/gas gauge in a primary automotive instrument display.  This is a significant innovation in display architecture that is, as Yoshida et al. carefully note, just as applicable to OLED displays as to LCDs.

Fig. 5:  At right, a conventional display has gate-driver circuits located in the bezel area.  At left, the Sharp display has gate drivers integrated within the pixel area.  (Graphics: Yoshida et al.).

Dow Corning’s EA-4600 HM RTV hot-melt adhesive was initially developed as an alternative to double-sided tape in the assembly of cell phones and other electronic devices.  In this role, it can be 20% of the cost of DS tape in large-volume applications.  But because the material requires dispensing equipment that costs in the vicinity of $100,000, it takes high volumes for the much lower material cost to deliver maximum savings.

In a poster paper entitled “Silicon Hot-Melt Adhesive Providing Protection, Waterproofing and Reworkability for Precision Assembly of Electronic Devices,” Ryan Schneider, Glenn Gordon, and colleagues from Dow Corning explained that one advantage of the silicon hot melt is that it can be used to make to make beads of 0.5 mm or less when, for example, making a peripheral seal on cell-phone window glass, where maximum screen area is crucial.  It is, said Gordon, impossible to cut DS tape that finely.

Although the original conception was to use the hot melt as an adhesive for assembly, if you deposit a peripheral bead on only one surface and allow it to cure, it forms a gasket that can be used to provide water- and dust-proofing for a snap-on cover – and the cover can be removed and re-snapped indefinitely while still retaining its water-proofing characteristics.  This approach was used to waterproof the back cover of a recent, popular smartphone model.  Although Schneider and Gordon would not identify the model in question, reliable industry sources tell me it was the Samsung Galaxy S5 (Fig. 6).  Dow Corning is talking to other manufacturers about adopting the technique.

Fig. 6:  Dow Corning’s EA-4600 HM RTV hot-melt adhesive (presumably) forms this waterproof gasket in the author’s Samsung Galaxy S5 phone.  Photo courtesy Ken Werner.

Merck KGaA (Darmstadt, Germany) offered a substantial number of technical presentations.  Two were particularly interesting.  In an invited paper, Merck’s Martin Engel and colleagues discussed the company’s ultra-bright fringe-field-switching (UB-FFS) formulation, which provides 15% more transmittance than standard FFS.  The product is currently available but various parameters – including switching speed and reliability – still need to be improved, said Engel.

Engel noted that in both UB-FFS and FFS, transmittance depends on the polarity of the applied voltage, and this produces flicker.  The reason is not fully understood, but some formulations can reduce the flicker/switching-speed trade-off.

In the Q&A, Facebook/Oculus VR executive Mary Lou Jepsen asked if the diffraction seen at the edge of the fringe field is any less than in UB-FFS.  Engel speculated that there would be less diffraction because there is less tilt at the edge of the fringe with UB-FFS.

In “Opening the Door to New LCD Applications via Polymer Walls,” another invited paper from Merck KGaA, Nils Greinert and his colleagues revealed a practical way of making LCDs with internal polymer walls.  Currently, most of the interest in flexible displays is focused on OLED displays, which are amenable to being bent if they are fabricated on a flexible substrate.  It’s harder with LCDs, which depend on a precisely maintained cell gap for proper operation.  Bending a conventional LCD decreases the cell gap.  (Current curved LCD TVs side-step this problem by bending the LCD so slightly that cell-gap reduction and substrate misalignment remain insignificant.)

The problem could be solved by fabricating walls between the flexible substrates (and in between the pixels) to stabilize the cell gap when the display is bent.  This is not a new idea.  NHK showed a simple ferroelectric LCD with walls in the early 2000s, and the Merck authors cite other early efforts.  But there was not a process for fabricating the walls that was efficient and compatible with standard LCD fabrication techniques.  That is the problem Merck KGaA has solved.

Greinert and his colleagues mix polymer precursors together with the LC host and homogenize the mixture by heating it above the liquid-crystal clearing point.  The authors call the resulting mixture a “polymer wall LC mixture.”  The mixture is enclosed between the two substrates and UV-irradiated through a photomask.  The walls form and the LC host settles down to its expected orientation and tilt angle.  Remarkably, if the proportion of precursor to LC material is chosen properly, all of the monomers are incorporated into the polymer walls and the LC characteristics are very, very close to what they are in a conventional process (Fig. 7).

Fig. 7:  (a) The “wall LC” mixture consists of the LC host (blue rods) and polymer precursors (red dots).  (b) The mixture is deposited in the display and exposed to UV radiation through a photomask, which results in polymerization-induced phase separation.  (c) Polymer walls form in the irradiated regions.  The liquid-crystalline phase is restored and, equally important, has aligned itself with the polyimide layer.  (Graphic:  Greinert et al.)

The authors note that “total monomer concentration, photomask, cell, and UV equipment have to be considered and optimized in order to produce the desired polymer-wall pattern.”  However, they also say, “We have found that commercially available monomers do not satisfy the simultaneous requirements of good mask reproduction and mechanical stability.”  Merck KGaA is currently developing tailor-made monomers to solve this problem.

Following the paper, Robert Miller (Senior Business Manager, LC and Advanced Technologies at Merck’s U.S. subsidiary EMD Performance Materials) told me, “We feel we have demonstrated the effectiveness of the basic materials and process, and we are now looking for development partners.”

Last but Certainly Not Least: Micro-LEDs

John Rogers (a professor at the University of Illinois and co-founder of and technology advisor to X-Celeprint) presented a Monday seminar entitled “Microscale LEDs for Multifunctional Display Systems.”  You may recall that this is the technology Candice Brown-Elliott called disruptive.  Microscale LEDs (or micro-LEDs or µ-ILEDs) were not well known outside the relatively small community of people who work on them before Apple acquired LuxVue last year, at which point a much wider community started scrambling to learn about them.

It would be very attractive to make phone, tablet, and TV displays from inorganic LEDs, but there has been no inexpensive way to assemble LED chips into dense RGB arrays.  If it were possible, such displays could be several times as efficient as OLEDs and have longer lifetimes.

What Rogers and colleagues, along with a handful of micro-LED companies, have learned to do is to initiate the epitaxial growth of AlInGaP LEDs on recyclable GaAs wafers.  Rogers described a process for making multiple layers of LEDs with sacrificial layers in between that allow the layers to be lifted off.  That’s impressive, but solves only half the problem.  If we went no farther, we could make no more than wafer-sized displays (Fig. 8).

Fig. 8:  This diagram shows a multilayered epitaxial lift-off. (Graphic: John Rogers).

The second part of the solution was covered by Chris Bower, CTO of X-Celeprint (Cork, Ireland), who described the company’s technology for performing transfer printing of the chips using elastomeric stamps utilizing peel-rate-dependent adhesion.  To oversimplify shamelessly, if you place the stamp on the layer of chips and peel it off quickly, the chips adhere to the stamps.  By impressing the stamp on the target substrate and peeling it off slowly, the chips adhere to the target.  This is also impressive, but it still does not create LED arrays any larger than the original lattice-matched array.

As it turns out, it is relatively simple to impose patterns on the stamps that result in picking up every 10th, 20th, or nth LED before depositing them on the substrate.  In this way, you can go from the dense array of the original wafer to a sparse array on the target substrate.  In principle, this allows you to make µ-ILED displays of virtually any diagonal.  Bower said that X-Celeprint has made 150-mm stamps.  Making larger ones is just a matter of engineering, he said, not science.

Now, obviously, if you can transfer-print u-ILEDs you can also transfer-print CMOS switching circuits and no longer worry about the instability issues of a-Si and IGZO TFTs or the scalability issues of LTPS.  In fact, you can transfer-print many types of “chiplets” and even assemble them in three-dimensional structures.  Displays are only one application of the technology.

Some experts have speculated that the first µ-ILED display we see in a commercial product may come from LuxVue and appear in an Apple iWatch as early as next year.  While that may be a touch on the early side, it will be interesting to see if and when the technology starts to make inroads.  Is it possible that µ-ILED, not OLED, will become the universal display that replaces the LCD?  That is a question that should be commanding the attention of all of us in the display community.

We led off this article by saying new materials and new manufacturing processes are basic to major display developments.  That becomes very clear in the context of µ-ILED and transfer-printing technology.  •


Ken Werner is Principal of Nutmeg Consultants, specializing in the display industry, manufacturing, technology, and applications, including mobile devices and television.  He consults for attorneys, investment analysts, and companies using displays in their products.  He can be reached at kwerner@nutmegconsultants.com.