August
01, 2005 --- In a study published in the August 1
issue of Applied
Physics Letters , John
Booske , a UW-Madison professor of electrical
and computer engineering , and Keith Thompson,
David Larson and Tom Kelly of the Madison-based
company Imago
Scientific Instruments , used Imago's local
electrode atom probe (LEAP) microscope to pinpoint
individual atoms of boron — a common additive,
or dopant, in semiconductors — within a sea of
silicon atoms.
The precise placement of dopants has long concerned
engineers because these elements control the electrical
properties of silicon transistors — the tiny,
voltage-controlled switches found by the millions on semiconductor chips. But
as manufacturers have relentlessly reduced the size of transistors in order
to squeeze more of them on chips, locating dopants has become progressively
difficult.
"
As we start studying and manufacturing electronic
devices whose sizes are approximately 100 nanometers
or less in all three dimensions, and we want to
know where the dopants and the defects are, we're
getting into the realm of asking, 'Just where are
all the individual atoms and what type of atoms
are they?' " says
Booske.
With its ability to analyze roughly 50 million atoms an hour, LEAP can now routinely
map the properties of even the tiniest semiconductor devices, producing findings
that manufacturers can use to diagnose and correct the defects underlying electrical
failures. Until now, a lack of suitable tools has forced industry to pursue a
laborious and largely blind, trial and error approach to new device development,
says Thompson, a former Texas Instruments employee who is now senior applications
engineer at Imago.
LEAP is an advanced atom probe microscope invented by Kelly. A former UW-Madison
professor of materials science and engineering, he left the university to start
Imago with Larson, then a graduate student. To create images, Kelly's instrument
applies voltage to a specially prepared, needle-shaped sample, ripping atoms
from the sample's tip one by one. An electric field then pulls the charged atoms
to a detector, which identifies them and records their location in the original
specimen.
The paper's results represent the first practical application of LEAP to semiconductors,
says Thompson. Atom probing has traditionally been restricted to metallic specimens
because of the need for them to readily conduct electricity.
The project originally began when Thompson was a postdoctoral researcher in
Booske's laboratory and the pair was studying silicon transistors. Both to
achieve faster switching and fit more transistors onto a single silicon wafer,
manufacturers have been decreasing the size of the "gate" — the part of the transistor that
controls switching — by 30 percent every two years.
"As the gate gets smaller, every other feature in the device has to get smaller — you
can't just shrink one part," says Thompson. "And as this happens, you really
start worrying where every atom is."
For atomic-scale transistors to function properly, boron must be implanted at
very high concentrations within the first 200 to 300 atomic layers of the silicon
surface, he explains. But a critical heating step for making transistors often
causes boron atoms to diffuse more deeply, ending up in regions where another
dopant, usually arsenic, resides.
"Before, when semiconductors had feature sizes of one to two microns (1000 to
2000 nanometers), a 20- to 30-nanometer region of overlap between the dopants
resulted in an error of only two percent," says Thompson. "Now, with feature
sizes of 30 and 40 nanometers, even 10 nanometers of inter-diffusion represents
a 25 to 30 percent error."
Booske and Thompson devised a microwave rapid-heating technique that they believed
would limit diffusion. But the pair soon realized the system's minute scale left
them no way to confirm the boron atoms were staying put.
"After searching around, we came to the conclusion that the only technology with
the potential to make these kinds of measurements would be Imago's LEAP instrument," says
Booske. "So, Imago provided the instrument facility and Keith and I worked on
the materials and specimen preparation end of things." Eventually, the collaboration
proved so successful Imago hired Thompson as a full-time employee.
LEAP has now revealed in precise, three-dimensional detail that rapid-heating
techniques do indeed cause boron to spread from its original location. For example,
images of boron-laden regions atop the transistor gates show dopant atoms amassed
like pebbles in the crevices between seemingly boulder-sized crystals of silicon.
Movement and clustering of boron "is something that had never been experimentally
verified," says Thompson. "It had only been assumed. Physics says that it should
happen, but there was never a technique to actually show it happens." In one
boron pile-up at the junction of three silicon grains, the scientists counted
exactly 264 atoms, a feat of enumeration not possible with any other method.
Because clustering can lead to electrical failure, the results demonstrate the
need to further investigate rapid-heating techniques. With the new information
LEAP provides, semiconductor manufacturers can now do just that as they continue
their push to miniaturize chip components.
"The timing is right for an instrument like LEAP to come along," says Booske. "The
need is there."
Copyright © University of Wisconsin
|
