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<<< Motorola Inc. (NYSE: MOT - message board) has made a materials discovery that could really shake up the economics of making high-speed electronic chips.
The multinational firm announced this week that its scientists have come up with a way -- albeit by accident -- that makes it possible to "grow" gallium arsenide (GaAs) on top of a silicon wafer (see Motorola Makes Materials Breakthrough ). Working together with epitaxial wafer vendor IQE PLC, Motorola created a 12-inch (300mm) GaAs-on-silicon wafer, which is twice the diameter of the largest GaAs wafers available today.
The breakthrough could significantly reduce the costs of substrates and processing for gallium arsenide, the company claims.
Simply put, Motorola has found a way to combine the robustness and low cost of silicon processing with the superior optical and electronic properties of so-called "compound semiconductors" such as gallium arsenide and indium phosphide.
The details of how this was achieved are complicated. To give an idea of how complicated, try this: Motorola says it has filed no fewer than 270 patents on the new technology!
However, the basic principle is easier to understand. It boils down to the fact that semiconductors are essentially single crystals. When depositing a thin layer of one material on top of another, the integrity of the crystal is maintained at the boundary, even though the lattice constant (the distance between atoms in the crystal) may be different. This creates stress.
As the thickness of the layer grows, at some point the stress in the crystal is relieved by generating atomic-level cracks. These are bad news because they degrade the electrical and optical properties of the material. This is what normally happens when people try to grow gallium arsenide on silicon.
The key to this problem cropped up unexpectedly. Motorola scientists were using an oxide, strontium titanate (STO), to develop ultra-thin transistors. They found that oxygen diffused through the STO layer, forming an "amorphous," or random, interface between the STO and silicon.
At first, the scientists thought this was a nuisance. Then one of them, Dr. Jamal Ramdani, realized that the amorphous layer removed the strain from the STO, allowing it to relax back to its natural crystal lattice constant. What's more, the lattice constant of the STO was close to that of GaAs, allowing the two materials to bond without cracks.>>>
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- jk |
