MiniLEDs
Using diamond films to enhance thermal performance in electronics packaging
IEEE Electronics Packaging Chapter, Santa Clara Valley Chapter, USA, recently organized a seminar on using diamond films to enhance thermal performance in electronics packaging.
Artificial diamond films are deposited from a mixture of methane and hydrogen — and the deposition of the material is not an expensive process. However, the integration of diamond films and electronic devices requires the development and optimization of new processing lines, which is a costly procedure. LEDs, for instance, have become low-cost components.
Dr. Joana Catarina Mendes, Researcher at the Instituto de Telecomunicações in Portugal said a diamond is metastable allotrope of carbon, where carbon atoms are arranged in a variation of the face‐centered cubic crystal structure known as diamond lattice. It has small atomic radius, extremely strong covalent bonding between sp3 hybrid orbitals, and set of extreme properties. Some properties include high hardness, high demand inertness, high young modules, high thermal conductivity, high bandgap/breakdown field, high electron mobility, and low dielectric constant.
Natural diamond forms 150-200 km inside the earth’s mantle under extreme conditions. Despite their high commercial value in jewelry, natural diamond crystals have too many defects, and cannot be used for electronic applications. Their use is typically limited to tri-bological apps.
Diamond synthesis
Artificial diamond can be formed under high pressure and high temperature (HPHT). The HPHT method reproduces diamond formation conditions inside the earth’s mantle. Diamond seeds are placed at the bottom of a press at 5 GPa. The internal part of the press is heated above 1400°C, and melts the solvent metal. The molten metal dissolves and drags atoms from high purity carbon source, which precipitate on the diamond seed.
Another method is chemical vapor deposition (CVD). CH4 and H2 are typical input gases. The input gases are dissociated and activated. The activated radicals flow and react with C atoms on a substrate. Atomic H etches away non‐sp3 C bonds. Due to their short wavelength (12 cm at 2.45 GHz) the MW power can be supplied as TEM or TM waves. Conductive plasma replaces the outer conductor of coaxial line in plasma discharge region.
Single crystal diamond (SCD) substrate leads to homoepitaxial diamond films. They have the highest thermal conductivity. They are ideal for electronic devices and thermal management apps. Non‐diamond substrate leads to heteroepitaxial/polycrystalline diamond films (PCD). Here, different substrates are possible, such as Si, SiC, GaN, etc.
Diamond films enhance thermal performance in electronics packaging
We can use diamond films to enhance thermal performance in electronics packaging. We can start by integrating diamond and GaN high-electron-mobility transistors (HEMTs). In some cases, the amount of heat generated per unit volume is comparable in magnitude to that encountered at nuclear reactors and at the surface of the sun! We need to cool down the hotspot. We can also grow diamond on the back of GaN wafer.
Next, we have capping diamond, where, films are deposited at 700°C. Metal heat spreaders transfer the heat to the underlying HEMT holder. Thermal resistance is reduced by ≈ 40 percent, and junction temperature is lowered by 100°C @ 25 W/mm. 4’’ GaN‐on‐diamond wafer volume manufacturing was achieved in 2021. Radios and power amplifier modules are available for satellite apps.
Diamond substrate needs GaN/diamond wafer bonding. We can do thermocompression using adhesive layer, as well. We can also do surface-activated bonding (SAB). Another method is Van der Waals (VdW) bonding. The process was initially employed for GaAs thin films.
Other uses
Diamond can be used as chip‐carrier of power LEDs. We can also have diamond carriers for high power LED dice. Diamond can also be used as power board. Depending on the activation energy of the aging processes, LEDs mounted on diamond board will age 60-90 percent slower @350 mA and 90-99 percent @700 mA.
Conclusion
Diamond has been successfully used to improve the thermal management of different devices.
For GaN HEMTs, we have diamond‐capping of passivated HEMTs, direct growth of diamond on back of GaN wafers, bonding of GaN wafers/HEMTs and diamond substrates, commercial GaN‐on‐diamond‐based RF power amplifiers are available for satellite communications. Companies such as Mitsubishi Electric Corp. and Fujitsu are involved in research.
Diamond as chip‐carrier has similar impact of PCD and SCD carriers on LED characteristics. It improves stability of the wavelength with the current, and increases LED lifetime significantly. Diamond as power board increases LED lifetime considerably, when compared to standard MCPCBs. The results can be extrapolated to other devices.
MicroLED and miniLED technology and market outlook
Leo Liu, Director of Taiwan Operations, DSCC, presented the microLED and miniLED technology and market outlook at the ongoing Display Week Conference 2022.
Presenting the LED industry outlook, Haitz Law is Moore’s Law for the LED industry. Given, lumen/package can increase 20x and cost/lumen can decrease 10x for every decade, Haitz Law has held on from the 1960s through the 2020s, and appears to be on track for the 2030s. This trend will help miniLED and microLED penetrate display apps.
LED usage (by area) on miniLED BLU is ~100x higher, compared to conventional BLU on the iPad. The ratio of LED usage in microLED will be much higher than in miniLED. 2021 is the first year of miniLED, and it will keep growing in next five years. MicroLED will be the successor.
There is no clear definition about miniLED and microLED. Both miniLED and microLED are adopted in very tiny LEDs (either chips or packages). MiniLED is mainly used as backlight in display with local dimming control. MicroLED is the direct-view display – RGB chips or blue chips with color conversion.
There are examples for microLEDs from 2022 Touch Taiwan, such as AUO with transparent display, spherical full flight simulator, watch display, TV, etc. Innolux, with color conversion AM microLED display, TV, watch, AR/VR, etc. Panel Semi, with flexible display, rollable display, watch, etc. And, PlayNitride, with automotive, watch, TV, etc.
There are examples for miniLEDs from 2022 Touch Taiwan, such as AUO with notebook, monitor, automotive, etc., and Innolux with VR display, automotive, notebook, watch display, etc.
MiniLED will help LCD to have a significant improvement in performance. There is affordable cost. Except upgrading performance, panel makers (AUO, Innolux, Sharp, etc.) are also proposing entry-level MiniLED solution. By adopting fewer number of LEDs and reasonable dimming zones, entry-level miniLED can offer good-enough performance under competitive cost.
Trends
There are trends for miniLED cost reduction. For LED, there should be reduced number of LEDs on each panel. Thanks to efficacy enhancements and optical improvement, the number of LEDs on each panel will decrease annually. Cost of LEDs will continue decreasing annually. The decreasing number of LEDs multiplies the LED cost reduction, and will accelerate the LED cost drop.
Regarding transfer, new transfer equipment and technology with faster speeds and higher yields are under development. Fewer LEDs can lead to better transfer yields. Quality improvement on backplanes can contribute to transfer yield.
As for backplane, PCB-based units will still be the mainstream. Combined with a reduced number of LEDs and discrete LED driver ICs, circuit on PCB will become simpler with fewer layers of PCB. Accordingly, the cost of the PCB backplane will drop. BOE can become aggressive on promoting glass-based as backplane in miniLED backlights.
For LED driver IC, discrete IC (QFN package) can move to discrete “bare IC” (bonding together with LED dies). Central IC can have more and more dimming zones/IC (scanning). For vertical integration, another way for cost reduction is to shorten (or integrate) the supply chain. Integrating transfer and backlight assembly seems to be the most cost-effective.
There are some challenges with microLED display manufacturing. There is lack of reliable, cost-effective manufacturing equipment. Materials are also not ready for high volume production. We need to overcome challenges at low costs for high-volume manufacturing.
Forecast
First, let us look at the miniLED LCD forecast. 12.9” iPad Pro and 14”/16” Macbook Pros will maintain large contributions to miniLED tablet and notebook shipments before 2025. In notebook, miniLED will keep increase before 2025. But, the penetration rate will start to drop after 2026 due to the competition from OLED.
For large-size panels (monitor and TV), penetration of miniLED looks more promising than tablet and notebook. Samsung, LG and main TV brands in China are pushing miniLED TV to the market. This will drive TV to become the largest app for miniLED shipments after 2026.
Looking at the microLED display forecast, in 2021, there are small volumes of microLED displays for TV and AR/VR. DSCC forecasts microLED revenues to grow from $0.8 million in 2021 to $0.9 billion in 2026.
In 2021, Samsung and LG released their first microLED LCD TVs. MicroLED will get a niche in the very large TV market or signage, where LCD panels and OLED become less cost efficient. Except TV, Taiwan panel makers (AUO, Innolux, Panel Semi) are very aggressive in adopting microLED into transparent and curve large-size display (rollable, foldable).
MicroLED and miniLED are among the emerging display technologies, and have high potential to compete with other existing or new display technologies. Although the cost of miniLED and microLED is high and the yield is still low now, we believe people will find the way to overcome hurdles to allow microLED and miniLED costs become affordable. MiniLED will start to dominate LED industry from 2023, and MicroLED will have a significant jump after 2025.
Apple has outstanding shipments in miniLED IT products, and the shipment of Samsung and TCL miniLED TV is also good. Such news encourages more people to enter the miniLED market. MicroLED will take time to penetrate into market. Niche apps, such as high-end TV, signage, transparent display, AR, wearable, etc, are the main apps for microLED before 2025.
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