每日科技报告 第13期Watching Crystals Provides Clues to mak-ing Smoother Films

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Watching Crystals Grow Provides Clues to mak-ing Smoother, Defect-Free Thin Films

To make thin films for semiconductors in electronic devices, layers of atoms must be grown in neat, crystalline sheets. But while some materials grow smooth crystals, others tend to develop bumps and defects -- a serious problem for thin-film manufacturing.



In the online edition of the journal Science (Jan. 22, 2010), Cornell researchers shed new light on how atoms arrange themselves into thin films. Led by assistant professor of physics Itai Cohen, they recreated conditions of layer-by-layer crystalline growth using particles much bigger than atoms, but still small enough that they behave like atoms.
"These particles are big and slow enough that you can see what's going on in real time," explained graduate student Mark Buckley. Using an optical microscope, the scientists could watch exactly what their "atoms" -- actually, micron-sized silica particles suspended in fluid -- did as they crystallized. What's more, they were able to manipulate single particles one at a time and test conditions that lead to smooth crystal growth. In doing so, they discovered that the random darting motion of the particles is a key factor that affects how the crystals grow.
A major challenge to growing thin films with atoms is that the atoms often form mounds, rather than crystallizing into thin sheets. This happens because as atoms are deposited onto a substrate, they initially form small crystals, called islands. When more atoms are dumped on top of these crystals, the atoms tend to stay atop the islands, rather than hopping off the edges -- as though there were a barrier on the crystals' edges. This creates the pesky rough spots, "and it's game over" for a perfect thin film, Cohen said.
Conventional theory says when films are being formed at the atomic scale, atoms land on top of each other and form mounds or "islands" and feel an energetic "pull" from other atoms that prevents them from hopping off the island's edges and crystallizing into smooth sheets. The result is rough spots on the thin films used to produce semiconductors. Cornell University-led researchers eliminated this pull by shortening the bonds between their particles. But they still saw particles hesitate at the island's edges. In this image, green particles are the ones that encounter a step edge or corner barrier. The orange particle encounters smaller barriers as it moves from site to site. The #1 indicates the bond being broken. The #2 indicates the bond that is forming. Near a step edge or corner the atoms do not have a new neighbor to form a bond with (so no #2 particle). This is what sets up the barrier.

sea_star666

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wuhao080910

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明心见性

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liuxn_2005

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a850338480

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a850338480

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sea_star666

回复 6# a850338480


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Mr.BM

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