Pioneered in Finland, Atomic Layer Deposition (ALD) continues to be strongly associated with the country, thanks both to the companies active in the field and to the cutting-edge research that continues to be done in the field by researchers at the University of Helsinki and elsewhere.
Atomic layer deposition makes use of a sequential gas phase chemical process to build up thin films atom by atom, layer by layer, ending up with a total thickness in the order of just a single nanometre, a full 79,000 nanometres thinner than an average strand of human hair.
Typically linked with thin-film electroluminescent displays in the early days of its commercial application, ald has since spread its wings into a much wider range of areas, particularly microelectronics – where its ability to produce highly conformal films with atomic-level control over thickness and composition gives it excellent and growing potential in a world where miniaturisation is very much the name of the game.
The unique self-limiting film growth mechanism intrinsic to ALD is proving a real winner here, as an ALD-produced film will always be conformal, whatever the target shape, 2D or 3D, to be coated, with the right chemistry.
“ALD is a perfect example of a technology that was in many senses ahead of its time when it was first developed,” says Professor Markku Leskelä, Head of the Laboratory of Inorganic Chemistry at the University of Helsinki’s Department of Chemistry, and someone with decades of ALD research to his credit.
“As a step-wise technique, its was also seen by some early on as slow in terms of the thickness of layers deposited in an hour. Its critics paid less attention, however, to the volume that can be produced in a batch and to the quality of the film it produces. These benefits have come to be better appreciated over the years, particularly over the last 10 to 15 years when older technologies have come to the end of their potential when called on to produce thinner, more conformal surfaces.
“ALD, in contrast, has really got into its stride, thanks to things like miniaturisation in the electronics industry and nanotechnology applications.”
|Professor Markku Leskelä (on the left) and Professor Mikko Ritala are two of the most well-known names in the world of ALD research. Photo: Peter Herring
Room for everyone
Finnish researchers – at the University of Helsinki, VTT Technical Research Centre of Finland, and Aalto University’s school of Science and Technology – have been at the cutting edge of work on ALD since the 1970s. The team at the University of Helsinki was the largest working on ALD at a university until very recently and remains a highly respected powerhouse. Leskelä and his colleague, professor Mikko Ritala, are among the most-cited scientists in the material sciences worldwide; there are only 20 researchers with such ISI highly cited scientist status across all fields in Finland.
Leskelä, Ritala, and their fellow researchers have always worked closely with industry, and particularly closely with ASM Microchemistry, which signed a cooperation agreement with the University back in 2003 that has now been extended to 2013 and which has seen ASM set up their own laboratory on campus.
“Although ASM and companies like Picosun and Beneq compete, naturally, there’s plenty of room in the ALD field for everyone as new application areas continue to be opened up,” says Ritala.
“As academic researchers, what we’re most interested in is developing new ALD chemistries. The drive for these typically comes from industry, when someone wants to use a new material for which a chemistry doesn’t yet exist or, if it does, it doesn’t perform as well as needed.
“One of the materials that we were recently challenged with was germanium antimony telluride, which lacked precursors that were safe and possessed sufficiently high reactivity. Using a novel precursor, we were able to achieve crystallisation properties similar to those of sputter-deposited films, without the limitation that this technology involves.”
Given the wide application of ALD technology and the large number of partners the University team works with, it is only natural that its research extends over a broad field as well, from energy applications, such as solar cell and Li-Ion batteries, to optics, particularly X-ray optics, micromechanical systems (MEMS), sensors, and other areas of electronics.
|The University of Helsinki has four campuses in the city, including the City Centre Campus for Human Sciences can be seen here. Photo: Peter Herring