Echoes of the Past in Silicon
By Heather Rock Woods
Thermal oxide is the real on-off switch
for your computer. The nanometers-thick film on the surface of silicon
transistors helps turn on and off the flow of electricity through the
transistor, providing the 0 and 1 binary signals modern electronics run
on. There are several million transistors on each computer chip.
of the silicon atom positions near the interface (the horizontal
line) of crystalline silicon and amorphous thermal oxide (SiO2)
for a crystal structure. The small dots in the thermal oxide
(above the horizontal line) represent where the silicon atoms
would be if the crystal structure had expanded without
disordering. The outline of the four silicon unit cells is
shown below the interface. The outline of four expanded lattice
cells in the oxide is shown above the interface. (Graphic
courtesy of SSRL)
As technology produces smaller chips that
require thinner oxides, the ability of thermal oxide to act as the basis
for integrated circuits is starting to break down.
“We’re pushing the fundamental limits,”
said materials researcher Sean Brennan (ESRD). “Anything you can do to
learn more about the thermal oxide is a huge plus.”
Thermal oxide is ‘grown’ on the surface of
silicon wafers by diffusing oxygen atoms into the silicon’s crystal
lattice. The oxygen atoms break silicon-silicon bonds and form
silicon-oxygen bonds, in the process disrupting the perfectly repeated
and regular crystal structure. This layer of oxidized silicon (SiO2) is
thermal oxide, and was long believed to be completely amorphous, in
other words, an unpredictable structure without long-range order.
New evidence from Brennan and former SSRL
graduate student Anneli Munkholm, now at Lumileds Lighting, is
overturning that assumption. Their research, recently published in
Physical Review Letters, shows that thermal oxide holds “weak
crystalline ‘echoes’ of the silicon’s former self buried within the
non-crystalline oxide,” said Munkholm.
X-ray scattering at SSRL revealed that
thermal oxide has faint memories of the former position of the silicon
atoms. Each silicon chip is a single crystal, meaning it follows a
regular three-dimensional pattern. Think of a three-dimensional chess
board—all the black squares are in predictable locations (forward 1,
over 1, up 1). In contrast, in thermal oxide the oxygen atoms randomly
attaches to silicon in any direction, so the structure is not a crystal.
So Munkholm and Brennan were surprised to
find their scattering patterns show the silicon atoms are relatively
close to the positions they held before being disrupted by oxidation.
The new model based on the data also shows that the crystal memory is
stronger closer to the pure silicon (at the interface between the
silicon and thermal oxide, where oxygen atoms are less dense), and fades
closer to the surface of the thermal oxide. The silicon lattice also
expands as oxygen diffuses in, but expands less at the interface.
“It is only through the use of an intense
synchrotron x-ray beam from SPEAR that we were able to observe the
residual order,” Munkholm said.
They found residual order in a wide range
of oxidation recipes with oxide thicknesses from 6 nanometers (nm) up to
100 nm and on silicon with different surface orientations (the crystal
sliced at different angles).
“We have seen evidence that different
recipes result in different amounts of disorder,” Brennan said.
This suggests researchers may be able to
relate certain amounts of order with specific electrical properties in
thermal oxide that could be better for running new integrated circuits.
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