Viewing Structures at the Atomic Scale
|An atomic landscape: A scanning tunneling microscope image
of an infrared detector consisting of alternating layers of indium
arsenide and gallium indium antimonide. Individual atomic orbitals
separated by 4-6 Angstroms are clearly visible in the image,
obtained by 'cleaving'the semiconductor wafer under ultrahigh
vacuum conditions to expose the interior of the device.
Electrical and Computer Engineering Professor Edward Yu has developed new imaging technologies that are allowing scientists to literally look inside of transistors and other devices, where they can view properties and substructures at previously invisible scales.
These technologies are playing a critical role in the ongoing drive to miniaturize, both to make way for nanotechnology and to keep pace with Moore's Law, which says transistor counts on microchips shall double every 18-24 months. Yu points out that success at building smaller devices is greatly enhanced when engineers can see the even smaller building blocks.
Using a scanning tunneling microscope Yu has captured images of the individual atoms of compound semiconductor "heterostructures" — electronic devices composed of two or more semiconductor materials. Such devices, though less common than conventional silicon transistors, are coveted for their ability to operate at extreme power and temperature ranges and at high frequencies. And, because of enhanced light sensitivity compared to silicon, heterostructures are widespread in photonic and LED applications. Images generated in Yu's Nanoscale Characterization and Device Laboratory show layers of gallium arsenide, indium phosphide, and other materials with resolution sufficient to Viewing Structures at reveal individual atomic orbitals, rendered to fractions of a nanometer.
Using atomic force microscopy (AFM) and related techniques, substructures of devices have been rendered including gates, sources, and drains of conventional silicon transistors, and individual defects such as dislocation lines in more exotic semiconductor materials such as gallium nitride. These tools allow the electronic, magnetic, optical, and other characteristics of new materials and devices to be measured with spatial resolution in the 10 nanometer range. That's about a tenth the size of today's smallest commercially available transistor gate (0.13 micron) in Intel's Pentium 4.
Yu and his teams start out with commercially available technology and then extend and enhance. They have done pioneering work with proximal probes, which interact directly at the nanometer scale with the materials they are rendering. Techniques developed in Yu's laboratory allow specific regions of actual device structures to be imaged. STM probes have the sharpest of tips, consisting of two or three atoms at the apex, which accounts for their extremely high resolving capabilities. In comparison, instruments such as transmission electron microscopes, while also providing extremely powerful imaging capability, obtain output that represents averages through a column of atoms, which can blur or wash out atomic-scale details in an imaged structure. For more details on Yu's work, visit his Nanoscale Characterization and Devices Laboratory at http://kesey.ucsd.edu/group/