By Dr. Phil Garrou, Contributing Editor

It is true, as Shakespeare one said, that “A Rose by any other name would smell as sweet” but … in our times, it is important to have an unambiguous name that clearly indicates what you are talking about. Unfortunately, this is not the case for micro LED displays (µLED Displays). There are those for which this term brings to mind a millennial trying to read the Wall Street Journal on his smart watch with a magnifying glass or possibly a display worn by the Astonishing Antman.

The problem obviously is the grammatical issue of what the µ modifies…i.e. a micro (LED display) or a (micro LED) display. While it may be fun to consider how to create a display for the Antman, we will be talking about the latter. IFTLE thought a primer on the subject was in order since this technology has become more and more dependent on interconnect and assembly technologies being supplied by the packaging community.

IFTLE certainly considers this a “futures” technology, meaning still a few years out, but always remember “…the leading edge is where the money is made!”

Comparing the Display Technologies

TFT LCD (thin film transistor liquid crystal displays) were being developed in the 1970s, but by the late 1980s it was publically thought to be way too difficult to yield (“.. you cannot have bad pixels on a TV set”) and certainly a much too expensive a technology to ever displace the cathode ray tubes used for TV and desktop computer displays. Well we all know what happened next and clearly without that technology transformation there would be no laptops or smartphones or smart watches today.

Next on the time line came OLED developed initially by Eastman Kodak in the late 1980s. An organic light-emitting diode (OLED) is a LED in which the emissive layer is a film of organic compound that emits light in response to an electric current. This layer is sandwiched between two electrodes (typically the upper electrode is transparent such as ITO). An OLED display works without a backlight, and is thinner and lighter than a LCD. OLEDs have traditionally been expensive to manufacture and only LG and Samsung made them. Samsung has been working in OLED devices for a decade and is currently using the technology in their smart phones and televisions. Today, the cost has come down dramatically, and now OLED TVs are very affordable. Apple began using OLED displays in its watches in 2015 and in its laptops in 2016.

The term “Micro-LED” was first used by Cree in its US patent “Micro- led arrays with enhanced light extraction” in 2001. The patent describes arrays of interconnected LEDs with individual sizes of less than 30μm.

While there currently are no µLED displays in production, the companies developing the technology believe that it has the potential to challenge OLED and LCDs in the future. Like OLED, it does not require a backlight, producing light in each individual pixel. µLED have the advantage of lower power consumption, higher brightness, ultra-high definition, high color saturation, faster response rate, longer lifetimes and higher efficiencies compared to LCDs and OLEDs.

The three technologies are compared in cross section in the figure below [link]

µLEDs were placed onto the industry technology roadmap 2 years ago following Apple’s acquisition of LuxVue, which claimed that its technology was 9x brighter than OLED and LCD. Then Oculus (Facebook) acquired InfiniLED another µLED company which claimed “… a 20 – 40X reduction in power consumption” [link].

In most cases, the µLED chips are manufactured separately then positioned and connected to the transistor matrix via a pick and place process show in the figure below.

Singulation of μLED display chips is typically achieved by bonding the epi wafer to a carrier and plasma etching in the die streets. According to Yole Developpement LEDs as small as 5μm have been demonstrated, but applications requiring >1500 PPI (pixels per inch) might require even smaller sizes.

Traditional pick and place equipment cannot pick up such small dies. Such tools typically have throughput around 25,000 places per hour. If large displays are to incorporate millions of tiny LEDs they cannot be assembled by such a method. Thus a requirement for µLED displays is a massively parallel pick and place technology. Players who have developed such technology include Luxvue and X-Celeprint who we have discussed on IFTLE before [see IFTLE 203, “Apple Acquires LuxVue µ-assembly Technology”]

X-Celeprint has developed MEMS-like sacrificial release processes for LED chips. Luxvue uses an electrostatic technology for their massively parallel pick-up, while Xceleprint uses an elastomeric stamp.

The µLED display concept was first validated by Sony in 2012 [link]. Their 55 inch “Crystal LED TV“ which utilized 6MM tiny LEDs (2 million each for red, green, blue subpixels) to reproduce a picture in Full HD resolution. Sony claimed that it had 3.5 times the contrast ratio, 1.4X the color range, and 10X faster response time compared to a traditional LCD.

HVM at costs acceptable to the proposed applications still faces significant engineering and manufacturing challenges. Most expect to see smart watches, being worked on now, as the first application to reach commercialization in the next few years with Apple in the lead. The drivers for this application include battery life and display brightness.

µLED performance and supply chain players are compared below [link].

While it will likely take considerable time, effort and investment to establish an HVM infrastructure, µLED could emerge as an alternative to OLED in the future as LCD fades away.

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