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`frontline technology
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`Micro-LED Technologies and Applications
`
`Light-emitting diodes (LEDs) offer extraordinary luminance, efficiency, and color quality,
`but to date are largely used in displays as backlights or packaged pixel elements in large-area
`LED billboard displays. Building high-performance emissive displays in a smaller form
`factor requires a new micro-LED technology separate from what is used for large LED
`billboards. Several approaches have been proposed to isolate micro-LED elements and
`integrate these micro-LEDs into active-matrix arrays. Technologies that use micro-LEDs
`offer the potential for significantly increased luminance and efficiency, unlocking new
`possibilities in high dynamic range, augmented/mixed reality, projection, and non-display
`light-engine applications.
`
`by Vincent W. Lee, Nancy Twu, and Ioannis Kymissis
`
`THE luminance and power efficiency of a
`
`light source are the key factors for determining
`suitable applications. Light-emitting diodes
`(LEDs) offer very high luminance levels,
`greater than 50,000,000 cd/m2, giving them a
`proven ability to perform in high-ambient
`display applications. LEDs also offer some of
`the highest efficiencies for converting electrical
`power to optical power. Depending on the
`material system, an energy conversion of over
`60% can be achieved. Due to these benefits
`and the small solid-state form factor, emissive
`LEDs can become a solution for display
`applications of all sizes.
`In today’s display applications, LEDs are
`most commonly used as the illumination
`source for liquid-crystal displays (LCDs) of
`practically all sizes, including 100-in. TVs to
`
`Vincent Lee is the founder and CEO of
`Lumiode, a startup company commercializing
`micro-LED microdisplay technology. He can
`be reached at vincent@lumiode.com. Nancy
`Twu is a Research and Development Engineer
`at Lumiode. Ioannis (John) Kymissis is a
`faculty member in the electrical engineering
`department at Columbia University and a
`co-founder and scientific advisor to Lumiode.
`
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`0.5-in. microdisplays. Individually packaged
`LEDs are also used as the direct pixel element
`in large-area billboard displays, which are
`currently the only format of directly emissive
`LED displays. What remains underexploited
`is the use of LEDs as individual pixel elements
`in all other smaller display formats.
`In large-area displays, discrete packaged
`LED pixels, each containing a red, green, and
`blue LED chip in the package, form the active
`elements in emissive video walls. Emissive
`video walls are attractive for stadium and
`advertising applications given the high
`luminance (excellent viewability under
`bright ambient light conditions) and energy
`efficiency of LEDs. Although the size of
`each packaged LED pixel is relatively large,
`full-resolution displays are easily achieved
`in these large-area applications. Building
`smaller displays with packaged LEDs is
`more difficult. When using the smallest
`available packaged LED pixels (approxi-
`mately 0.75 mm), 70 in. is the current mini-
`mum achievable size for a FHD-resolution
`(1920 × 1080) full-color display. When using
`packaged LED pixels, displays smaller than
`70 in. can be produced, albeit with lower
`resolution.
`
`0362-0972/6/2016-016$1.00 + .00 © SID 2016
`
`New applications such as high-dynamic-
`range (HDR) television and augmented reality
`are demanding the same high-performance
`specifications as that of large-area displays,
`but at dimensions that are difficult to scale
`for fully packaged LEDs. For example,
`TV displays require a peak luminance of
`10,000 cd/m2 for future HDR content, and
`microdisplays need to reach 100,000 cd/m2
`to support the luminance needs of augmented-
`reality and mixed-reality (AR/MR) glasses.
`These requirements are easily satisfied by
`LEDs, which can have luminances up to
`50,000,000 cd/m2. Like other emissive
`displays, an emissive LED display offers the
`luminance and efficiency of the pixel source
`without the typical loss associated with light
`selection and modulation elements (polarizers,
`color filters, etc). Emissive LED arrays there-
`fore have a huge luminance advantage
`High-luminance emissive FHD-resolution
`LED displays smaller than 70 in. cannot be
`made from packaged LED pixels, and thus
`require the development of new manufacturing
`techniques and technologies. Specifically,
`these smaller display formats require fabrication
`and use smaller LED elements or “micro-
`LEDs.” Loosely defined, micro-LEDs are
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`devices in which the LED emission area per
`pixel is below 50 × 50 µm, or 0.0025 mm2. An
`array of micro-LEDs makes up a micro-LED
`display, which ranges in size from fractions
`of an inch up to 70 in.
`Micro-LED displays take advantage of the
`exceptional luminance of LEDs by spreading
`the generated photons over a larger area than
`the area occupied by the micro-LEDs them-
`selves, either by distributing the LED
`elements spatially or by dispersing light
`optically. This is illustrated in Fig. 1. There
`are wide differences between these two tech-
`nologies, despite confusing nomenclature that
`refers to both as “micro-LED.” The former
`technology, shown in Fig. 1(a), distributes
`LED elements spatially and can be used to
`build displays ranging from 3 to 70 in. In this
`article, these are referred to as “direct-view
`micro-LED displays.” The latter technology,
`shown in Fig. 1(b), disperses light optically
`and is used to build displays < 2 in., which are
`referred to as “micro-LED microdisplays.”
`In direct-view micro-LED displays, micro-
`LEDs are fabricated with a small pixel pitch,
`separated into individual dice, and transferred
`to an active-matrix backplane using pick-and-
`place methods. This allows for the develop-
`ment of an LED display in which the active
`LED area occupies only a small fraction of the
`total area. The area expansion allows for
`direct viewability of high-luminance micro-
`LEDs (up to a full 50,000,000 cd/m2 per LED)
`because the micro-LEDs are spatially separated,
`resulting in a lower apparent luminance per
`pixel. The area unoccupied by micro-LEDs is
`available for a black matrix and integration of
`interconnection electronics. With larger
`current-distribution buses available, this
`approach allows for passive-matrix display
`development and integration and also lends
`itself to active-matrix approaches using large-
`area electronics.
`Micro-LED microdisplays use semiconductor
`integration techniques to combine an array of
`small pixel-pitch micro-LEDs with a transistor
`back plate, which are then integrated with an
`optical system such as projection lenses or
`see-through glasses. Because the < 20-µm
`pixel pitch of micro-LEDs for microdisplays
`is even smaller than that of direct-view
`displays, the scaling of micro-LEDs for
`microdisplays requires full integration at the
`wafer-fabrication level. There are several
`strategies to perform the semiconductor inte-
`gration between micro-LEDs and transistors,
`
`Fig. 1: Shown are two approaches used to build micro-LED displays. Both methods start with
`a micro-LED array but use either (a) a pick-and-place technology for direct-view displays or
`(b) semiconductor integration for microdisplays.
`including pixel-to-transistor bonding, LED
`epitaxial transfer to silicon CMOS, and inte-
`gration with thin-film-transistors (TFTs).
`Because of fundamental differences in
`technology approaches and display sizes,
`micro-LEDs for direct-view displays and
`micro-LEDs for microdisplays target different
`markets. Together they offer the promise of
`
`Direct-View Micro-LED Displays
`Using Pick-and-Place Technologies
`Today, stadium and large street displays use
`fully packaged surface-mounted LEDs in a
`
`replacing all displays now and in the future
`with the most efficient and highest-luminance
`systems possible.
`
`Fig. 2: In (a) and (c), selected individual chips are picked from a wafer using an elastomer
`stamp. In (b) and (d), the stamp is then moved to a non-native “target” substrate where the
`devices are placed, typically in a sparse array. Multiple devices are picked and placed in each
`transfer, and multiple transfers are used to complete a final display.1
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`tiled format. With reductions in packaged
`LED sizes, large-area tiled displays have
`scaled to smaller sizes and higher resolution
`displays, as small as 70 in. with FHD resolu-
`tion. Direct-view micro-LED technologies
`are the natural extension of efforts to further
`shrink stadium-sized LED displays for new
`applications. Instead of using packaged
`LEDs, direct-view micro-LED technologies
`use smaller unpackaged LED dies and pick-
`and-place techniques to build emissive LED
`displays in the 3–70-in. size range. The result-
`ing direct-view micro-LED displays show
`increased luminance and improved color
`gamut for HDR displays, provide different
`form factors for wearable and flexible displays,
`and address the push for ever-increasing
`power efficiency in these applications.
`Several academic groups and companies
`have demonstrated pick-and-place approaches
`for transferring LED dies to a substrate board
`and connecting the elements to each other.
`All of these techniques begin with fabrication
`of densely packed small-pitch micro-LEDs.
`
`The micro-LEDs are then separated into
`individual dies, transferred to a secondary
`substrate, and physically spread out to a large
`pitch via proprietary pick-and-place processes.
`The choice of secondary substrate depends on
`the specific application and resolution. Appli-
`cations such as flat-panel displays use a secon-
`dary glass substrate with active-drive transistors,
`while wearables such as watches and wrist-
`bands can use a flexible secondary substrate.
`While several pick-and-place methods are
`being developed, few are publicly disclosed
`in detail. One paper by Bower et al. from
`X-Celeprint highlights some key aspects of
`pick-and-place processes.1 Figures 2(a) and
`2(c) show an array of densely packed devices
`(micro-LEDs) from which a subset of devices
`is sparsely picked up by an elastomer stamp.
`This stamp is moved to a secondary substrate,
`shown in Figs. 2(b) and 2(d), placing the
`devices in a dispersed array. The stamp can
`pick up many micro-LED devices at one time
`to lower the number of transfers needed to
`populate a full display.
`
`Figure 3 shows X-Celeprint’s process for
`transferring a small-pitch (~20 µm) micro-
`LED array to a larger pitch (~200 µm) on a
`glass substrate.2 The micro-LEDs have a
`sacrificial release layer that is engineered into
`the LED epitaxial growth and later undercut
`to release the micro-LEDs from the growth
`substrate [Fig. 3(b)]. The micro-LEDs are
`then picked up by the elastomer stamp and
`transferred to a glass substrate with some
`pre-defined metal lines [Fig. 3(c)]. A second
`metal layer is then deposited [Fig. 3(d)] to
`electrically connect the transferred micro-
`LEDs to the glass substrate. By using this
`transfer process, X-Celeprint demonstrates a
`100 × 100 color passive-matrix display
`[Fig. 3(e)]. Active-matrix formats can also be
`achieved by transferring the micro-LEDs to a
`secondary substrate with indium gallium zinc
`oxide (IGZO) or low-temperature polysilicon
`(LTPS) transistors.
`Sony’s micro-LED technology, initially
`demonstrated at CES in 2012, was recently
`released. Based on available technical data,
`
`Fig. 3: Image (a) shows the pick-and-place process of micro-LEDs to a secondary substrate. Image (b) is a micrograph of ready-to-transfer
`micro-LEDs on a source wafer with an undercut etch of the sacrificial layer. The LED size is approximately 10 x 10 µm. Images (c) and (d) are
`micrographs of transferred micro-LEDs and deposited metal layer for interconnection between micro-LEDs and a secondary substrate, respec-
`tively. Image (e) shows a full-color passive-matrix micro-LED display.2
`
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`micro-LEDs approximately 50 × 50 µm in
`size are placed on a 320 RGB × 360 tile.3
`Much like conventional stadium displays,
`these micro-LED based tiles are then further
`arrayed to form the FHD display. Not much is
`known about the pick-and-place method or the
`backplanes used in Sony’s demonstrations.
`Two other companies in the space are
`InfiniLED and LuxVue. InfiniLED’s micro-
`LED technology uses a unique parabolic
`micro-LED structure for light collimation and
`light extraction [Fig. 4(a)].4 This type of
`shaping allows for control of the micro-LED
`emission angle and potential improvements to
`the overall efficiency of the display. LuxVue,
`
`recently acquired by Apple, uses a MEMS-
`based pick-and-place process for micro-LEDs.
`For pick-and-place technologies, the manu-
`facturing challenges are similar across all of
`the techniques. The primary challenge is
`pixel transfer yield, as modern displays
`require nearly zero dead pixels across a FHD
`screen. To reach the needed pixel yields,
`some groups have proposed transferring
`redundant micro-LEDs,4 as shown in Fig. 4(b),
`or performing individual pixel repair transfers
`for dead pixels.2 These workarounds will add
`to either the base material cost or manufacturing
`time to build a display, reducing scalability.
`In addition, each pick-and-place process
`
`requires careful engineering of the LED
`materials, sometimes even custom LED
`epitaxy, to ensure that the electrical and
`optical performances are not affected through-
`out the fabrication process (LED ohmic
`contacts, undercut etch of the micro-LEDs,
`transfer of pixels, etc).
`
`Micro-LED Microdisplays Using
`Semiconductor Integration Technologies
`For microdisplays with a panel diagonal
`< 2 in., pick-and-place technologies cannot
`scale to the smaller pixel pitch required for
`FHD displays. Microdisplays < 2 in. using
`passive-matrix schemes also cannot achieve
`sufficient resolution or luminance, even
`though small pixel pitches have been demon-
`strated.5,6 Building bright high-resolution < 2 in.
`microdisplays requires direct integration of
`micro-LED arrays with arrays of transistors
`that provide active-matrix switching. There
`are several transistor technologies and
`approaches to building the integrated active-
`transistor matrix. At a high level, these
`technologies can be sorted into the three
`categories as shown in Fig. 5: (a) chip-level
`
`Fig. 4: The top image (a) shows a micro-LED schematic with reflective sidewalls and a curved
`shape for light collimation and extraction. The lower image (b) demonstrates a top-view design
`of a twin micro-LED emitter for redundancy and improvement of micro-LED yield.4
`
`Fig. 5: Shown are three approaches to the
`integration of silicon transistors and micro-
`LED arrays for microdisplays: (a) chip-level
`hybridization of foundry CMOS chip with
`micro-LED arrays; (b) wafer-level transfer of
`LED epitaxial layers to CMOS wafer; and (c)
`wafer-level fabrication of TFTs directly on
`micro-LED arrays.7
`
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`micro-LED pixel–to–CMOS-transistor bond-
`ing, (b) LED epitaxial transfer to silicon
`CMOS, and (c) micro-LED array integration
`with TFTs.
`Under first consideration is chip-level
`micro-LED pixel–to–CMOS-transistor bond-
`ing, a process also known as flip-chip bonding.
`Because of the ubiquity of foundry CMOS
`processes, many technologies in this category
`start with a completed silicon chip and work
`within back-end processes to add functionality,
`namely, chip-level assembly of micro-LED
`arrays as shown in Fig. 5(a). The LED arrays
`are fabricated by lithographic patterning of
`contacts and mesa etches. Bump bonds are
`then fabricated either on the silicon CMOS
`chip or on the micro-LED array. Next, the
`micro-LED arrays are die separated and bump
`bonded to the silicon CMOS chip. This fabri-
`cation flow offers researchers the advantage
`of using the highest-performing transistors,
`demonstrated by researchers at the University
`of Strathclyde,8 the Hong Kong University of
`Science and Technology,9 the Industrial Tech-
`nology Research Institute,10 and mLED.11
`Devices fabricated by mLED with conven-
`tional flip-chip bonding have been hybridized
`and demonstrated up to nHD (640 × 360)
`resolution with a pixel pitch as low as 20 µm.
`
`Fig. 6: Image (a) shows a cross-section schematic of a silicon CMOS IC flip-chip bonded with
`a micro-LED array using indium bump bonds. An image of a monochrome microdisplay
`appears in (b) and a micrograph of a micro-LED array with indium bonds prior to flip-chip
`bonding is shown in (c).12
`
`Fig. 7: Image (a) shows a schematic cross section of the micro-tube fabrication process. Image (b) shows a scanning electron micrograph of
`micro-tubes fabricated on a CMOS wafer. In (c) and (d), micrographs and a photograph of flip-chip bonded micro-LED arrays with a 10-µm
`pixel pitch are shown.13,14
`
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`Within flip-chip bonding, as pixel-pitch
`shrinking becomes more aggressive, non-
`traditional bonding processes are being
`developed. Devices by Day et al. use indium
`bumps to bond a 160 × 120 array at a 15-µm
`pitch [Fig. 6(b)].12 Fabrication of this device,
`shown in Fig. 6(a), starts with foundry-silicon
`CMOS and a micro-LED array. The micro-
`LED array is then coated with indium through
`thermal evaporation and lithographically
`patterned to form indium bumps [Fig. 6(c)].
`The indium-patterned micro-LED array is
`then flipped and mated with the corresponding
`CMOS silicon contact and heated to reflow
`each of the LED-transistor contacts.
`More recently, Templier et al. at CEA-LETI
`demonstrated an innovative micro-tube
`technology to perform the bonding process.
`As in other approaches, a silicon CMOS chip
`and a micro-LED array are first fabricated
`separately. The silicon CMOS chip then
`
`continues through a series of back-end
`processes to form micro-tubes, as shown in
`Figs. 7(a) and 7(b).13 The micro-tubes are
`created through the use of a sacrificial layer
`and deposition of a hard metal layer. This is
`followed by a chemical mechanical polish and
`removal of the sacrificial layer to form the
`micro-tube core. On the micro-LED array, a
`soft metal is formed to be compressed into the
`micro-tubes during the die assembly process.
`The resulting hybridized chip, shown in Figs.
`7(c) and 7(d),14 contains 6-µm micro-LEDs
`with a 10-µm pitch, a resolution of 873 × 500,
`demonstrating luminances up to 10,000,000
`cd/m2.
`An alternative to flip-chip bonding is been
`being developed by Ostendo. Here, the
`process starts with silicon CMOS wafers, but
`instead of flip-chip bonding a completed array
`of micro-LEDs, LED epitaxial layers are
`transferred to the CMOS wafer. The LED
`
`material is then patterned and fabricated into
`the structure shown in Figs. 8(a) and 8(b).15
`In addition, vertical waveguide hole structures
`are fabricated into the surface, which can
`define the light-output cone angle. By changing
`the size of the hole structures, cone angles
`between ±17° to ±45° can be achieved.
`Ostendo has shown transfer of three colors
`(RGB) and a stacked RGB structure yielding
`a full-color microdisplay. In particular, nHD
`(640 × 360) and 720p (1280 × 720) resolution
`displays at 20,000 cd/m2 have been demon-
`strated, as shown in Fig. 8(d).
`Lumiode, the authors’ startup, integrates
`micro-LEDs with a thin film of silicon transis-
`tors in a fully monolithic process, meaning all
`fabrication work is done on a single wafer. As
`illustrated in Fig. 5(c), the process starts with
`the LED substrate, upon which pixels are
`patterned into a micro-LED array using photo-
`lithography.7 The pixel pitch can ultimately
`
`Fig. 8: A cross-section schematic of a single stacked micro-LED pixel with vertical waveguides appears in (a). Image (b) shows a cross-section
`schematic of each light-emitting layer within the stacked micro-LED pixel. An image of a micro-LED array after front-side processing appears in
`(c), and in (d), a photograph of a test image shown on a 720p micro-LED microdisplay is shown.15
`
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`scale to the limits of wafer-level photolithog-
`raphy tools. Next, instead of using a foundry
`CMOS process and bonding to the micro-
`LEDs, layers of silicon dioxide and silicon
`are deposited to form the active TFT films.
`Standard materials from nearly all LED
`suppliers can be used as the starting LED
`substrate, with the caveat that the maximum
`process temperature is limited to what the
`LEDs can withstand. Thus, the challenge is
`to build a high-performance low-temperature
`transistor process. This is achieved by using
`laser crystallization to convert the low-electron-
`mobility amorphous silicon to high-mobility
`polycrystalline silicon. This laser crystalliza-
`tion is similar to low-temperature polysilicon
`(LTPS) processes employed in flat-panel
`displays. Transistors are fabricated in the
`polycrystalline silicon to form the necessary
`display driving circuits. Figure 9 shows a cross
`section and top view of an integrated device.
`While significant strides have been made
`with regard to micro-LED microdisplays,
`much work still remains to be done before
`commercial products reach the marketplace.
`The integration of color-generating materials
`is a challenge, although some work has been
`demonstrated in using an optical combination
`of three micro-LED microdisplays16; integra-
`tion of phosphor materials11; and stacking of
`red-, green-, and blue-LED epitaxial layers.15
`
`Another challenge for micro-LED micro-
`displays is the scalability of the integrated
`semiconductor processes. Flip-chip, LED
`epitaxial transfer to CMOS, and TFT methods
`all have unique factors that affect overall scal-
`ability and the ability to yield high-resolution
`displays. Overall, these factors will drive the
`associated costs and thus associated address-
`able markets.
`Micro-LED microdisplay technologies all
`aim to address markets where small panels
`and high luminance are the primary require-
`ment, the largest market being displays for
`augmented reality and mixed reality (AR/MR).
`Glasses-based devices for AR/MR require a
`microdisplay focused through a see-through
`optic that can be viewed in outdoor environ-
`ments. This will require luminances well
`above 100,000 cd/m2, luminance levels that
`only emissive micro-LED displays can
`provide. In addition to AR/MR, micro-LED
`microdisplays have been demonstrated in a
`wide range of applications, including projection
`formats10,17 and light-field displays,18 as well
`as non-display markets such as maskless
`photolithography,19 optogenetics,20 and
`visible-light communications.21
`
`Micro-LEDs Will Maximize LED Usage
`Today’s consumer markets rely on LEDs as
`the illumination source for general lighting
`
`and nearly all displays. By extending the
`application of LEDs beyond illumination to
`directly emissive pixels, the specifications
`needed for next-generation displays can be
`satisfied. In this review, we categorized the
`activity within the micro-LED space by the
`two display-size regimes: direct-view displays
`(3–70 in.) which use pick-and-place technologies
`and microdisplays (< 2 in.) which use semi-
`conductor integration technologies. While the
`science behind micro-LEDs is straightforward,
`the success of any particular technology will
`depend heavily on overcoming the unique
`engineering and manufacturing challenges
`associated with each technology. Once
`commercialized emissive LED displays have
`the ability to replace all displays and can span
`the entire size range from 0.5-in. microdisplays
`all the way up to a 1000-in. stadium displays.
`
`References
`1C. Bower, M. Meitl, S. Bonafede, and
`D. Gomez, “Heterogeneous Integration of
`Microscale Compound Semiconductor
`Devices by Micro-Transfer-Printing,” IEEE
`Electronic Components & Technology Confer-
`ence, 963–967 (2015).
`2M. Meitl, E. Radauscher, S. Bonafede, D.
`Gomez, T. Moore, C. Prevatte, B. Raymond,
`B. Fisher, K. Ghosal, A. Fecioru, A. Trindade,
`D. Kneeburg, and C. Bower, “Passive Matrix
`
`Fig. 9: (a) A schematic cross section of a micro-LED pixel is integrated with a thin-film silicon transistor. Image (b) shows a micrograph of a
`prototype active-matrix micro-LED microdisplay.7
`
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`Displays with Transfer-Printed Microscale
`Inorganic LEDs,” SID Digest of Technical
`Papers, 743–746 (2016).
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