AIST Stories No2

Leading the way AIST!5at a glanceTerminologyIn 2007, TOTO commercialized a coating technology based on the AD method for semiconductor fabrication equipment. This coating technology, which employs ceramic yttria with hardness on par with sapphire, has become indispensible for next-generation semiconductor fabrication equipment.A semiconductor chip is fabricated by forming a thin film on a silicon wafer and etching it to create fine patterns that form circuits. Currently, patterns as fine as the order of 20 nm (nanometer; n: 10–9) in width have been attained. Fabrication with conventional equipment is fast reaching its limits because the etching process entails generating a plasma inside the equipment to abrade the wafer; in doing so, the plasma also abrades the inner walls of the equipment. This in turn leads to the formation of abrasion residue. If this abrasion residue falls onto the chip, unintended patterns can end up forming: the finer the patterning, the more apparent the effects of the abrasion residue become, leading to a fall-off in yield.“Fabs want to reduce their production costs by raising chip yields and one way of doing so is to prevent formation of this abrasion residue by coating the inner walls of the equipment. Yttria coatings applied using the AD method are an answer to such a need. These coatings are extremely reproducible, we can’t proceed with R&D directed at commercialization unless we understand the guidelines for discovering the optimum parameters.”“After all, I found that AIST was being asked to clarify the mechanism. Thereupon, based on the previous theory of film formation through particle fragmentation, I believed that the key to understanding was the mechanical fracture properties due to particle collisions more than the velocity of particles sprayed onto the substrate; I thus focused my efforts on determining the relationship between film formation properties and the optimum size, mechanical properties, and pretreatment conditions required for the raw material particles.We found that to obtain a dense film, raw material particles of a certain diameter range were required. Although dependent on the material and its internal structure, films are formed only from particles with diameters of around 0.1–1 µm (micrometer, μ: 10–6). If the particles are smaller, they do not bond but end up forming a compacted powder, whereas if they are larger, deformation and bonding do not occur even after collisions and the fragments scatter and end up abrading the substrate in a manner akin to sand blasting. This became an important guideline in transforming this film forming method into a practical technology.”If the mechanism can be understood, the research route will then become clear. From around this time, significant progress was made in research in the form of a large-scale national project called ”Nanostructure Forming for Advanced Ceramic Integration Technology’’ (2002, NEDO) that was completed in 2006. Subsequently, joint research was carried out with a number of companies starting with TOTO towards development of numerous applications, their practical application, and commercial-scale production. “Through this research, one major result achieved jointly with these private sector-companies was the firming up of strategic patents at an international level.”Application in semiconductor fabrication equipment; exploiting a hardness that cannot be abraded even by plasmaA phenomenon whereby impacting a substrate with high velocity fine ceramic particles results in the crystalline particles fragmenting on impact, then once again bonding together. Through this action, a robust film can be formed at room temperature. The discovery of this phenomenon resulted in the birth of the AD method.Substrate (metal)Substrate (metal)Raw material particles at high speed, fragments and undergoes deposition, while the finely fragmented active surfaces form strong inter-particle bonds.High-speed collisionsParticlefragmentationCrystal size5~500nm AD methodDiffusion reaction between particles in the furnace sees them adhere together and crystals grow.Crystal size1,000~10,000nmABCCConventional methodReaction in furnace >900°CFusing and bondingCrystal growthFilm-forming mechanisms: AD method versus conventional methodRoom temperature impact consolidation phenomenon▲Plasma erosion-resistant components for TOTO’s semiconductor fabrication equipment (yttria coating)Photograph: TOTO