Can MicroLEDs and Quantum Dots Revitalize Inorganic Displays?
For the past several years, the flat-panel display technology battle has been between LCD and OLED. But innovations in inorganic semiconductor materials and devices have opened the door for microLEDs and quantum dots to move from supporting to leading roles – as dominant FPD technologies in their own right.
by Paul Semenza
Light-emitting diodes (LEDs) have been used in displays for decades, as the emitters in low-resolution videowalls and monochrome displays, and as backlights for LCDs, particularly after they took off in large TV panels in 2008.1 However, these applications use relatively large LED chips in bulky packages, placed in relatively sparse arrangements. Most backlight designs achieve high luminance with a small number of LEDs and use lightguide plates to distribute the light across the display. Large LED walls use RGB emitters in each pixel, but the chips are large and the pixel density is very low, due to the viewing distances.
Two developments have created new opportunities for the use of LEDs in displays. First, the ability to create LEDs smaller than 50 microns on a side – so-called “microLEDs” – makes them viable as emitters in displays and backlights with very high pixel density. Second, the development of massively parallel transfer techniques opens up the possibility that millions of devices could be transferred from LED wafers to display or backlight substrates.2
As analyzed by Yole Développement and Knowmade, over 120 companies/research organizations have filed ~1,500 patents in more than 500 families relating to microLEDs.3 In addition, companies including Apple, Facebook, and Sharp have made investments in or acquired startups in the space, while Sony and Samsung, among others, have developed prototype products using the technology. MicroLEDs are transferred from wafers and attached to substrates as bare dice, with no chip packaging. MiniLEDs are a transitional form between micro and traditional LEDs; they can be in the range of 50 to 200 microns on a side and implemented as bare dice or packaged devices.
Micro-MicroLED Displays
MicroLEDs have been used to construct two types of displays – small “microdisplays” used in near-to-eye or projection mode, and large-area, direct-view flat-panel displays. Generally, microdisplays are constructed by integrating microLEDs onto silicon wafers using flip-chip or wafer-scale integration, or by growing the LED devices on the silicon wafer; an alternative approach involves depositing TFTs onto the LED wafer.4 These highly integrated devices have very high pixel densities, and the spacing can be similar to that of the devices as grown; combined with the fact that they are often constructed on wafers, this means that they can use direct-transfer techniques. Near-to-eye microdisplays often use monochrome LED arrays with a color-conversion layer, while projection-mode devices require very high luminance. In all of these cases, the ability to accurately and reliably transfer or otherwise integrate millions of microLEDs and electrically connect to the drive circuitry (TFT or silicon devices) is critical for commercial success.
MicroLEDs on the Big Screen
Direct-view displays, used in wearable devices, automotive displays, smartphones, tablets, mobile PCs, monitors, and TVs, among other applications, represent the vast majority of revenues in the display market. Thus, the potential for microLEDs to create a new type of flat-panel display is intriguing. In particular, existing LED manufacturing techniques can be used to produce more than 100 million microLEDs on a single wafer. If such devices could be efficiently transferred to large substrates and connected to an active matrix, microLED displays could be cost competitive with existing LCD and OLED technologies.
The key challenge is the need to distribute the microLEDs over a much larger area than the wafer on which they were grown, and to do so with the minimum number of transfer steps (fewer transfer steps mean that more displays can be produced with a given number of transfer machines). For example, a typical wafer might be 6 inches in diameter, whereas a desirable target for a high-end TV might be a 55-in. diagonal panel. If the devices are transferred directly from the wafer to the substrate, many hundreds of transfer operations would be required. This has led to the idea of an intermediate transfer step, perhaps using an interposer or larger wafer, which can reduce the number of transfer operations by an order of magnitude or more.5
Cok et al. have simulated the number of transfer operations required to populate a full-HD display from 150-mm LED wafers. For the 6,220,800 devices required to construct 1,920 × 1,080 RGB pixels, populating the display using a roughly 1-in. square “stamp” would require 1,260 transfers; using a larger stamp and/or an intermediate substrate could cut the required transfers to less than 100 (Fig. 1). (For background on a monolithic process for microLEDs, see this issue’s Business of Displays interview with Plessey.)
Fig. 1: Representative schemes for populating a 1,920 × 1,080 RGB micro LED display include using (A) a 22.4 × 27-mm stamp, (B) a 112 × 94.5-mm stamp, (C) the small stamp with a 300-mm intermediate substrate, and (D) the large stamp and the intermediate substrate.
In addition to the potential for efficient creation of light-emitting pixels, microLED displays offer the intriguing potential of using the same transfer techniques to create the active-matrix drive circuits. Instead of TFT devices, micro-integrated circuits can be created using existing CMOS techniques, and transferred to the display substrate using the same processes and equipment as were used to transfer the microLEDs. Pixel controllers with more than 200 transistors have been fabricated in chips smaller than 40 microns on a side, transferred from silicon wafer to substrates, and used to drive RGB microLED pixels.6 Such miniaturization opens up the possibility for transparent displays, as the microLEDs and controller might only fill 1/10th of the pixel area, depending upon the resolution. In an early demonstration, Semprius and Kodak used microcontrollers instead of TFTs in AMOLED displays.7
MicroLEDs thus have the potential to be used to produce large displays with the luminance, color, and contrast to provide high-dynamic-range (HDR) performance, but without the need for the thin-film transistors critical to large LCD and OLED panels. Manufacturing of such displays can utilize the existing LED supply chain for the microLEDs, and would thus only need to combine the transfer operation with relatively simple metallization to connect the LEDs and controllers. No vacuum processes would be needed, and the assembly process would not require multiple displays to be produced on each substrate, opening up the potential for modular display production lines. Thus, the capital expenditure, in the billions of dollars for state-of-the-art LCD and OLED fabs, could be dramatically reduced.
Micro- and MiniLEDs Take the Stage
At Display Week 2018 in Los Angeles, a high level of activity was evident around these new LED form factors, both in the backlight and for the display itself. For example, AUO won the Best in Show Award in part for its prototype 8-in. microLED display, boasting 1,280 × 480 pixels (169 pixels per inch or ppi). The prototype used an LTPS-TFT backplane and color conversion from blue LEDs to full color. In the I-Zone at Display Week, Jade Bird Display and PlayNitride were named honorees for their microLED demonstration displays. Jade Bird showed a 5,000-ppi, 3-million-nit projection microdisplay, while PlayNitride demonstrated a transparent 0.89-in.display with 64 × 64 pixels (105 ppi), as well as a 3.12-in. 256 × 256 pixel display. In addition, the Display Week Technical Symposium featured 15 technical papers relating to microLEDs.
LCD panel makers AUO, BOE, Tianma, and others also showed panels with miniLED backlights, utilizing large numbers of chips to implement local dimming and high dynamic range. This could represent a step toward microLED displays, as the techniques used to fabricate miniLED backlights may be helpful in maturing the assembly processes. In the meantime, such backlights can bring significant performance improvement to existing LCDs, thus continuing the competition in front-of-screen performance between LCD and OLED displays.
Quantum Dots: Nanotechnology Meets Displays
Quantum dots (QDs) are semiconductor particles at the nanometer scale that emit light at specific and tunable wavelengths if electricity or light is applied to them; the tuning is accomplished by the choice of dot size, shape, and material. Similar to the use of LEDs in LCD backlights, quantum-dot materials were initially adopted to improve LCD color gamut and power efficiency, particularly as LCDs faced competition from OLEDs. The narrow emission spectra available from QD materials enable purer primary colors, and thus a broader palette of total available colors to display. Quantum dots have become critical to meet the wide color-gamut performance required by standards such as Rec. 2020, which are becoming prevalent in high-end TVs and other displays. The use of quantum dots also improves system efficiency, as blue LEDs can be used, which are less expensive and more efficient than white LEDs. Since the light emitted from the QD material has narrow spectra, less light is lost at the color-filter layer.
To date, most quantum-dot materials used in displays have been embedded in a film that is part of the backlight (Fig. 2), though there have been cases of implementing the quantum dots via a “rail” along with edge-lit displays as well as in the LED itself. The QD material converts broadband light to narrow spectra. These applications are examples of photo-enhancement, to improve the performance of LCD backlights. As described by QD supplier Nanosys, there are two additional modes available for the use of QDs in displays.8
Fig. 2: Current quantum-dot implementation works by embedding the materials in a film (or coated on glass) in the backlight.8 Source: Nanosys.
The next step is the use of quantum-dot material in a photoemissive mode. In this approach, similar to the backlight method, the light source is monochromatic (typically blue), and the QD material converts some of the blue light to red and green, while still transmitting the rest of the blue light. In LCDs, this would allow for the elimination of the color filter, which could provide significant gains in efficiency as well as color gamut. Another envisioned architecture is to combine the photoemissive QD with a backplane of blue OLED, which would be a simpler implementation than both existing LC and OLED displays. Finally, some developers of microLED microdisplays (near-to-eye) have used blue microLEDs with QD color-conversion materials, which means that full color can be achieved with only one type of LED (Fig. 3).
Fig. 3: In photoemissive mode, a patterned quantum-dot layer would convert blue light to red and green. This would allow for elimination of the color filter in an LCD (a) or of the green and blue emitters in an OLED (b).8 Source: Nanosys.
Perhaps the ultimate performance of a QD display would be achieved in the electro-emissive mode, in which QD material is used to create a direct-emitting display, based on the principle of stimulating the QD material electrically, rather than using a light source. In such a display (Fig. 4), QD materials emitting in red, green, and blue would be deposited on top of an active-matrix backplane. The emission spectrum (full width at half maximum) of QD materials is in the range of 20–30 nanometers (nm), compared to 40–60 nm for OLED, enabling a wider color gamut.
Because QD materials are available in solution, they can be deposited through low-cost processes such as spincoating and ink jet or screen printing. Such displays have been referred to variously as QD-LED, AMQLED, and QD-EL. BOE has reported producing 5-in. and 14-in. displays by printing the QD materials, using LTPS and IGZO backplanes, respectively.9 It is not yet known what type of quantum efficiency might be achievable with such a display architecture.
Fig. 4: An electro-emissive quantum-dot display would be an alternative to existing LCD and OLED display architectures.8
Back to the Future
The emerging forms of LED and QD display architectures are interesting, given that some of the earliest display approaches were based on inorganic materials. The original display technology – the cathode-ray tube (CRT) – utilized inorganic phosphors to emit light when excited by a scanning electron beam (more recently, attempts were made to make thin versions of the CRT through field emission, also to excite phosphors). In the 1960s, plasma displays, using phosphors excited by a gas plasma, and thin-film electroluminescent displays, using electrical stimulation of inorganic materials, were both demonstrated. Decades of development of all of these technologies followed.
Ultimately, all of these inorganic-based displays were eclipsed by the LCD, which uses organic liquid-crystal materials to shutter externally generated light, generally through control of polarization. The fact that LCDs need a high luminance light-source input, which is then modulated with very low total transmission efficiency, means that they are generally more complex – and expensive – than the inorganic display types. However, they offer excellent image quality and have for some time had the benefit of the largest number of manufacturers and supply chain participants.
There has long been a sense that there should be a less-complex approach to flat-panel displays than LCDs. In some ways, this desire has been answered by the OLED, which is self-emissive, like the inorganic approaches. OLEDs have been very successful, particularly in smartphones and in the emerging flexible display formats, but they have their own challenges, one of which relates to the fact that the semiconducting materials are organic, which means that they are susceptible to degradation when exposed to oxygen or water vapor. This is a particular challenge when using plastic substrates, which generally have unacceptably high water-vapor transmission rates, requiring encapsulation to protect the organic materials. Thin-film encapsulation using dyads of metal oxides and organic materials, which combine to create a tortuous path for the water molecules, have been developed, but these impose significant manufacturing costs.
Inorganic displays based on microLEDs and quantum dots offer the promise of simple, rugged, self-emissive displays. Of course, much development is still needed to achieve the potential, including material performance and manufacturing processes. Whether such displays will be able to be competitive with the incumbent LCD and OLED panels will depend on addressing manufacturing cost challenges. In the case of microLED displays, one of the key issues is the assembly cost – the equipment and processes for transferring millions of devices from wafers to substrates. For QD displays, material development, including lifetime, and optimization of the display design are challenges. Meanwhile, the majority of innovation in FPDs is in the form of incremental steps to improve the dominant LCD and OLED architectures, to which both LED and QD materials also contribute.
References
1J. Kim and P. Semenza, “How LEDs Have Changed the LCD Industry,” Information Display 4(12), 20–22, 2012.
2P. Semenza, “Can Advanced Assembly Techniques Alter the Dynamic of Display Manufacturing?” Information Display 3(15), 32–34, 2015.
3E. Virey, “Are MicroLEDs Really the Next Display Revolution?” Information Display 3(18), 22–27, 2018.
4V. Lee, N. Twu, and I. Kymissis, “Micro-LED Technologies and Applications,” Information Display 6(16), 16–23, 2016.
5A. Paranjpe, J. Montgomery, S. Lee, and C. Morath, “Micro-LED Displays: Key Manufacturing Challenges and Solutions,” SID Symposium Digest of Technical Papers, 597–600, 2018.
6R. Cok et al., “Inorganic Light-Emitting Diode Displays Using Micro-Transfer Printing,” Journal of the SID 25(10), 589–608, 2017.
7C.A. Bower, E. Menard, S. Bonafede, J.W. Hamer, and R.S. Cok, “Active-Matrix OLED Backplanes using Transfer-Printed Microscale Integrated Circuits,” Proc. Elec. Comp. Tech. Conf. (60th ECTC), Las Vegas, NV (2010); C.A. Bower, D. Gomez, K. Lucht, B. Cox, and D. Kneeburg, “Transfer-Printed Integrated Circuits for Display Backplanes,” IDW ’10, 1203–1206, 2010.
8C. Hotz, “Quantum Dot Displays: Advances and Outlook,” SID Seminar Lecture Notes SE-7, Society for Information Display Conference, Los Angeles, 2018.
9Y. Li et al., “Developing AMQLED Technology for Display Applications,” SID Symp. Digest of Tech. Papers, 1076–1079, 2018. •
Paul Semenza spent two decades managing market research for The NPD Group, where he was responsible for the DisplaySearch and Solarbuzz businesses; iSuppli, where he was vice president of display research; and Stanford Resources. He was also founder and director of commercialization of NextFlex, a public-private institute focused on flexible hybrid electronics manufacturing. Semenza is currently an independent industry consultant and adjunct professor in engineering management. He has degrees in electrical engineering from Tufts University and in public policy from the Harvard Kennedy School. He can be reached at psemenza@gmail.com.