MILC Used to Convert Silicon in TFTs
Peter Singer, Editor-in-Chief -- Semiconductor International, 8/1/2000
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Several methods have been studied for converting amorphous silicon to polysilicon; excimer laser annealing is currently thought to be the most preferable. However, researchers at Korea's Seoul National University think they have found a better alternative, using a phenomenon known as metal-induced lateral crystallization (MILC). They are now working to fine-tune the MILC effect demonstrated in earlier research to produce TFTs with lower leakage current and better thermal stability.
In work first reported in 1996, the researchers -Seok-Woon Lee, Tae Hyung Ihn and Seung-Ki Joo- fabricated high-mobility p-channel polysilicon TFTs using a self-aligned MILC technique. At temperatures below 500°C, a one-step anneal resulted in mobilities two to three times higher than that of polysilicon TFTs fabricated by conventional solid-phase crystallization at around 600°C.
Researchers have known since the early '90s that the crystallization temperature of amorphous silicon can be lowered by adding different metals, such as Al, Pd or Ni. This process, called metal-induced crystallization (MIC), was interesting but not very useful for fabricating TFTs due to the metal contamination. MILC is a twist on metal-induced cyrstallization. Large grains of polysilicon - several tens of microns - are formed laterally, away from the metal-doped area. This results in a polysilicon "channel" free enough from contamination and defects to form fairly high-quality TFTs.
Joo and company initially used a self-aligned approach where the gate was centered over the centerline that marked the starting point of the lateral diffusion. In more recent work, reported in the July 2000 IEEE Electron Device Letters, they have used an asymmetric MILC approach to further improve leakage current and thermal stability.
The devices were fabricated on a Corning 1737 glass, covered with 1000Å of SiO2. 1000Å of amorphous silicon was then deposited by PECVD at 250°C. After patterning this layer to form active islands, a 1000Å-thick SiO2 film was deposited for the gate dielectric, followed by a 3000Å-thick Mo gate electrode. The Ni-offset mask pattern was formed on top of the TFTs using photoresist. A 20Å-thick Ni film was deposited and patterned using the lift-off method. Ion doping was then performed, followed by a one-step anneal at 500°C that activated the dopants and crystallized the channel area.
Silicon Focus of New MRS Volume
Applications requiring large-area semiconductor coverage rely increasingly on amorphous and heterogeneous silicon materials because they can be deposited at low cost on a variety of substrates. A new volume from the Materials Research Society, Amorphous and Heterogeneous Silicon Thin Films: Fundamentals to Devices -1999, covers the range from fundamental research to the device applications of these materials. A special session on medium-range order is featured and confirms the belief that ordering correlates with the electronic quality of a-Si:H films. Topics include: growth and properties; high-rate deposition; recrystallization, amorphization and porous silicon; ordering and hydrogen; metastability; defects, band tails and transport; heterogeneous materials and devices; thin-film transistors and displays; solar cells; and detectors, imagers and other devices. Available for $65.00 to MRS members, $77.00 list. See www.mrs.org/publications for more details.
New Solar Cells Are 26% Efficient
Emcore Corp. (Somerset, N.J.) has developed triple-junction solar cells with 26% efficiency. The cells are now in production and are being shipped to customers for space qualifications. During qualification testing, the cells demonstrated a power loss of only 8% under a typical 15-year Geostationary orbit characteristic of communication satellites. "This triple-junction solar cell complements the dual-junction solar cells also in production at a delivered average efficiency of 23.3%," explained Emcore president and CEO Reuben Richards.
