Tag Archives: nitride

Nakamura on blue light history and future

Nobel Laureate Shuji Nakamura provided the keynote address to the attendees at the 57th annual Electronic Materials Conference held this week in Columbus, Ohio. His talk on “The History and Developments of InGaN-based LEDs and Laser Diodes” informed and entertained the audience of materials researchers, particularly since he followed first-principles of materials science and his natural inspiration to create the world’s first commercially viable blue LEDs over 20 years ago.
Nakamura-sensei is now legendary for showing excellent GaN-based blue LED functionality in an era when ZnSe was the main material explored by almost all scientists in the world due to six orders of magnitude superior defectivity level for the latter material (due to near zero lattice mismatch between ZnSe and GaAs, instead of the extreme mismatch between GaN and sapphire). In the 57th EMC keynote, he confessed that the only reason he began work on GaN was that almost everyone else was ignoring it so he could easily get papers published on the way to earning a Ph.D., and he initially had no plans to try to create a blue LED with the material.
However, when he bought a new MOCVD reactor to grow GaN on sapphire substrates he found the capabilities of the tool to be lacking so he began daily hardware modifications and test runs, and after some months began to get surprisingly strong data. Soon his group at Nichia was reporting world record GaN optoelectronic properties, and had developed both n- and p-type GaN. However, from first principles it was known that a double-heterojunction (DH) structure would allow for band-gap and hence wavelength tuning, so he then developed the world’s first useful InGaN MOCVD process and by 1993 was able to issue a press release claiming 1000 mcd LED output. “Indium gallium nitride is the most important material, but the Nobel committee didn’t say anything about Indium gallium nitride,” reminded Nakamura.
Most of the rest of the story is well known by now, including his precedent-setting lawsuit with Nichia, move to UCSB, and founding of Soraa.
Nakamura’s vision for the the future of blue (and through integration with phosphors “white”) light can be summed up as LEDs are good but lasers are better. Relatively speaking, with lasers the current density can by many times higher, and BMW and Audi have prototype laser headlamps that can reach 2-3x farther down the road compared to the best lamps today. The challenges today are to improve efficiency and cost. Efficiency for blue LEDs are now 50-60% while lasers are only ~30%. Also, blue laser production cost is now ~10x higher than that for blue LEDs.

CMP Slurry Trade-offs in R&D

As covered at SemiMD.com, the CMP Users Group (of the Northern California Chapter of The American Vacuum Society) recently held a meeting in Albany, New York in collaboration with CNSE, SUNY Polytechnic Institute, and SEMATECH. Among the presentations were deep dives into the inherent challenges of CMP slurry R&D.
Daniel Dickmann of Ferro Corporation discussed trade-offs in designing CMP slurries in his presentation, “Advances in Ceria Slurries to Address Challenges in Fabricating Next Generation Devices.” Adding H2O2 to ceria slurry dramatically alters the zeta-potential of the particles and thereby alters the removal rates and selectivities. For CMP of Shallow Trench Isolation (STI) structures, adding H2O2 to the slurry allows for lowering of the particle concentration from 4% to <2% while maintaining the same removal rate. Reducing the average ceria particle size from 130nm to 70nm results in a reduction in scratch defects while maintaining the same removal rate by tuning the chemistry, but the company has not yet found chemistries that allow for reasonable removal rates with 40nm diameter particles. The ceria morphology is another variable that must be controlled according to Dickmann, “It can seem counter-intuitive, but we’ve seen that non-spherical particles can demonstrate superior removal-rates and defectivities compared to more perfect spheres.”
Selectivity is one of the most critical and difficult aspects of the CMP process, and arguably the key distinction between CMP and mere polishing. The more similarity between the two or more exposed materials, the more difficult to design high selectivity in a slurry. Generally, dielectric:dielectric selectivity is difficult, and how to develop a slurry that is highly selective to nitride (Si3N4) instead of TEOS-oxide (PECVD SiO2 using tetra-ethyl-ortho-silicate precursor) was discussed by Takeda-san of Fujimi Corporation. In general, dielectric CMP is dominated by mechanical forces, so the slurry chemistry must be tuned to achieve selectivity. Choosing <5 pH for the slurry allows for reducing the oxide removal rate while maintaining the rate of nitride removal. Legacy nitride slurries have acceptable selectivities but unacceptable edge-over-erosion (EOE) – the localized over-planarization often seen near pattern edges. Reducing the particle size reduces the mechanical force across the surface such that chemical forces dominate the removal even more, while EOE can be reduced because negatively charged particles are attracted to the positively charged nitride surface resulting in local accumulation.