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TECHNOLOGY TRENDS

Adhesive bonding for flexible microLED

display assembly

By Goutham Ezhilarasu, Subramanian Iyer [UCLA] Ajit Paranjpe [Veeco Instruments, Inc.]
Jay Lee [DISCO Corporation] Frank Wei [DISCO Hi-Tec America, Inc.]
I n recent years, there has been thickness), sequential pick and place for action of a voltage-driven stamp to attract

growing interest in fabricating high-
assembly is impractical. Therefore, a mass
and pickup microLEDs for transfer. This
resolution information displays using
a large block of devices from the growth
micro-scale inorganic light emitting transfer process is generally used wherein approach is very sensitive to substrate
planarity and surface contamination, and
diodes (i.e., microLEDs) as an alternative substrate are transferred and assembled onto involves a relatively complex process with
to organic LED (OLED) or liquid crystal the target in a massively parallel fashion [2] specialized equipment such as an active
display (LCD) displays. The main reasons (Figure 1). Before releasing from the growth stamp. Approaches based on electromagnetic
for this interest is the remarkable quality of
displays made using microLEDs that possess
several benefits [1]: 1) microLEDs are based
on compound semiconductor material
systems like GaN, GaAs or InP that have
far superior emission properties like sharper
line width, higher quantum efficiencies,
and strong luminance exceeding 10 cd/m ;
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2
2) Resistant to environmental conditions
such as temperature and humidity due to
their chemical stability leading to longer
operational lifetimes (>100,000 hours); and
3) Ultra-fast response times typically in
the nanosecond range because of the high
mobility of carriers. Such high-quality
microLED displays can have a wide variety
of applications ranging from the commercial/ Figure 1: Schematic of a typical mass transfer process (left); Summary of various mass transfer approaches
defense sectors for augmented reality (AR) in literature (right)
and virtual reality (VR) device displays,
automobile and heads-up displays (HUDs), substrate, the prefabricated microLEDs are forces also suffer from similar drawbacks
to the medical sector for visible light therapy attached to a temporary carrier by bonding of the electrostatic case in addition to
and optogenetics. (metallic or adhesive) or stiction (Van der limited scalability to handle very small
In spite of the enormous market potential Waals [VdW] in elastomers) for accurate microLEDs (<10µm pitch). Mass transfer
for microLED displays, the technology is, registration. The release process of the using viscoelastic stamps is the most popular
however, mostly in the research phase [1]. devices from the growth substrate is done technique reported in literature, and is
The main reason for this is the difficulty in either chemically (for GaAs, InP) or optically being explored for commercialization by the
mass manufacturing microLED displays at (for III-N), depending on the material startup XDC.
competitive costs because of the poor yields system. After release, a transfer printing The viscoelastic mass transfer technique
in assembly. Unlike OLED materials that can process is utilized wherein a “stamp” is used relies on the relatively weak Van der
be directly deposited on a target substrate to pick up selected patterns of microLEDs Waals stiction between the stamp and
to fabricate the LED device, microLEDs from the temporary carrier to the target the microLEDs for pickup. They are,
contain a complex stack of compound substrate for final assembly. For a full color therefore, not very reliable in holding the
semiconductor material layers that are grown display assembly, this mass transfer has to devices in place during the transfer printing
either epitaxially, or using metal organic be repeated three different times to assemble process as a small percentage of devices
chemical vapor deposition (MOCVD) on the three colors for each pixel. could shift or even fall off during the
lattice-matched growth substrates at high Different methods of transfer printing transfer. The viscoelastic property of the
temperatures exceeding 900ºC. In order to using different types of stamps have been elastomeric stamp also causes it to exhibit
assemble such devices on a target such as a explored and are summarized in the table peel-rate dependent interfacial adhesion, a
Si CMOS backplane or a flexible substrate, in Figure 1. Although all the transfer property that is exploited to allow the same
the prefabricated devices on the growth approaches do work in principle, their polydimethylsiloxane (PDMS) stamp to
substrate are to be released from it and application to product manufacturing have both pick up devices from the source using
then transferred onto the target. Because of mostly been stifled [2]. Transfer approaches a higher pull rate and release the devices
the extremely small size and thickness of based on electrostatic forces as developed to the target using a lower pull rate. The
the individual LED (<100µm side, <7µm by LuxVue utilize the electrostatic gripper complex transfer physics involved makes

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