SAN DIEGO The world's first direct observation of the "melting" of a semiconductor material could reveal the details of many atomic-level processes relevant to chip manufacturing, physics, chemistry and biology. Researchers at the University of California, San Diego, heading a multinational interdisciplinary group, used ultrafast laser pulses of light combined with X-ray pulses to instantly "slice" molecules apart without disrupting their crystalline structure, which is inevitable with thermal melting. The technique is able to record movies of the real-time reorganization of molecules as they undergo changes that until now were unobservable.
"We are the first group to directly observe the non-thermal rearrangement of atoms in a material with ultrafast lasers we saw the symmetric crystal lattice of germanium atoms fall apart not just on the surface, but inside the material, too," said Craig Siders, one of the UCSD researchers.
Another UCSD researcher, Andrea Cavalleri, added, "Usually when you melt a semiconductor, the atoms become hot and vibrate out of their positions in the crystal, but our ultrashort laser pulses removed the glue holding them in the crystal, thereby instantaneously freeing them without vibrating the whole structure."
"We could see not only the internal structure, but also how structural changes occurred, and that will have a strong impact in many fields of research, not only in solid-state physics," said professor Klaus Sokolowski-Tinten of the University of Essen (Germany), a collaborator on the project. Other researchers include Kent Wilson, Csaba Toth, Christopher Barty and Ting Guo (now at UC-Davis) at UCSD; Dietrich von der Linde of the University of Essen; and Martin Kammler and Michael Horn von Hoegen of the University of Hannover, Germany.
Non-thermal melting results when electrons in a material are pushed into an extremely excited state with a laser pulse, forcing the crystal's atom to go from a cold solid to a hot liquid without passing through thermal equilibrium. Other scientists have used ultrashort pulses of laser light to "pump and probe" many fundamental physical processes, but the technique developed by the UCSD team of physicists, chemists and engineers also included extremely short bursts of X-rays to allow them to directly observe the non-thermal melting of a germanium semiconductor.
The group's laser pulses, which were just a femtosecond long, pushed the germanium beyond the normal boundaries of liquid and solid, into an excited state unattainable in nature, except perhaps at temperatures and pressures found only at the center of the Earth and Sun. "We can simulate the condition of matter in the interior of the Earth or inside a star, only for the briefest instant, but long enough to study the structure of its constituent atoms as they rearrange themselves," Siders said.
The group's laboratory setup to create the ultrafast laser pulses fits on a desktop, unlike competing groups in France and at Lawrence Livermore National Labs at UC-Berkeley, which require cyclotron-sized accelerators to produce the ultrashort X-rays.
The technique captures the Fourier transform information from an ongoing process, then spools it to a computer data file in real-time. Later, off-line, the group's software reconstructs what the atomic structures must have looked like as the laser sliced them, in the same manner that real-time microscope images of biological cells are rendered into movies from captured Fourier data. In theory, the process could be built into a real-time computer program that could render a visual image of the atoms as they come apart, plus enable slow-motion replays of segments of interest.
"We can do a structural analysis as a function of time on the Fourier data we collect, just like people do to render movies of living cells. But since we are looking at the molecules themselves we see much more detail. In fact, applying our techniques to living cells should finally let us see how photosynthesis works," Cavalleri said.
In semiconductor processing, the new technique can perform real work besides just revealing the inner nature of semiconductor processes. For example, the ultrashort laser pulses could be used for a higher-precision laser annealing that is not only orders of magnitude faster than current laser-annealing techniques, but which also recovers the exact crystal rather than a replica, and puts it back into its original structure with zero errors. That solves a long-standing problem with conventional annealing processes: that is, a slight mixing of adjacent regions on the chip accumulates after annealing steps, resulting in semiconductor "soup."
For instance, when doping materials, a subsequent annealing step is required to reestablish the perfection of the crystal after it was disturbed by forcing the doping material into the lattice. Normal laser annealing reestablishes a crystalline structure around the doping materials, but ultrafast lasers, according to the UCSD group, recover exactly the original structure, right down to the individual atoms.
"We can't name individual atoms and see that the same ones are in the same places as before, but we can confirm that not just a similar, but exactly the same, crystalline structure is reestablished, something that is not possible with ordinary laser annealing," Cavalleri said.
The technique can be used to create extremely high temperatures and impossible-to-attain pressures in tiny regions in just femtoseconds. For instance, if the ultrafast laser pulses are scanned across a thin layer of material in real-time, then the instantaneous pressure sets up an ultrafast acoustic wave propagating across the material.
"Ultrashort laser pulses give you the possibility to dump energy into a material so fast that it has no time to react that's when strange things can hap-pen. If the temperature rises very quickly, say, several thousand degrees in just a few picoseconds, then that solid is well above its normal melting point while still a solid. Nobody knows how such strongly overheated solids will behave, but ultrafast X-ray diffraction gives us a means to investigate these and other exotic states of matter," Sokolowski-Tinten said.
The UCSD group's next goal is to use its ultrafast laser pulses to examine the structure of carbon. Because of the high temperature required to melt carbon, no one has ever observed it in the liquid state. Even with an apparatus that could achieve the high temperature required to melt carbon, it could not be contained in any solid material because the container would melt.
"Carbon atoms have the highest melting point of any element. Nobody has ever melted it because there is no container that it wouldn't melt through, but we can observe liquid carbon in such a brief amount of time that it doesn't matter if it subsequently melts through our workbench," Cavalleri said.
Besides assisting in the technological development of semiconductor processing, the ultrafast laser technique also promises to be a boon to the next generation of nanoscale mechanical devices. Nanotechnology attempts to build solid structures with atomic precision. The ultrafast laser can make movies of self-assembly processes as well as exert direct forces on individual molecules in a structure so that they can be sliced, diced and melted into precise atomic alignment.