the Nanoscale Device Fabrication Challenge
Integrated Design for Manufacturability Must be Implemented
Moslehi, PhD, is chief technology officer and senior vice president,
semiconductor technology research, for The Noblemen Group, a boutique
investment banking, strategic advisory, and business development firm.
Moslehi has 20 years' experience working in the semiconductor and semiconductor
equipment industries. He can be reached at email@example.com.
the past year, many chip manufacturers have introduced their 90-nm process
technologies and ramp plans, and several have begun volume production.
Crossing the 100-nm line marks the entry into nanometer-scale devices,
a world where various second-order effects become major technology issues.
These new challenges have started to dominate device behavior and have
a negative impact on fabrication. A few of these problems have emerged
as serious integration limiters, threatening to diminish the performance
benefits that device scaling has provided up to this point.
top technical challenge facing the development of sub-100-nm device and
process technologies is power. Off-state device leakage, overall power
consumption, and thermal heating have progressively and alarmingly increased
with each new nanoscale CMOS technology node, a trend that will only continue
to worsen. Poor device behaviors caused by heating and the subsequent
temperature rise during chip operation accelerate circuit performance
degradation. Other possible undesirable effects include shortened battery
life and extra packaging and cooling costs.
attempts to address these problems result in fixes that significantly
compromise performance. Recent high-profile and widely publicized cancellations
or postponements of some high-performance products underscore the seriousness
of these problems. It is clear that major innovations and new methodologies
are urgently needed to overcome the leakage and power issues.
the manufacturing front, the fabrication of nanoscale devices faces several
important challenges, including process variabilities, narrowing process
windows, and diminishing manufacturing tolerances that do not scale in
step with the technology nodes. This combination of factors can increase
the variability of device characteristics and circuit performance, resulting
in lower yields. Such yield inhibitors include interconnect variabilities
(caused by CMP dishing and erosion), critical dimension (CD) variations,
and dopant fluctuations. As always, process engineers and equipment suppliers
continue to work on major process and product innovations and offerings
designed to attack these problems. For example, the recently introduced
ECMP tools, based on electrochemical mechanical planarization technology,
should alleviate many manufacturing problems associated with the copper
noted, the process community has traditionally addressed virtually all
manufacturing issues. Consequently, design and process engineers have
worked in isolated environments with minimal interaction; in most cases,
designs have been "thrown over the wall" to manufacturing. In the sub-100-nm
world, those days have to come to an end. All indications point to an
inescapable fact: the process and fab teams are no longer able to address
and solve certain critical variability issues on their own.
variabilities are often atomic-scale and statistical in nature, including
process fluctuations that affect the threshold voltages of the transistors
and line-edge roughness or CD variations that weaken device speed and
performance. These effects require solid statistical physical models as
well as integrated simulation and design software tools. Although advanced
process control (APC) and other innovations have helped tighten the process
windows and improve tolerances, the accepted manufacturing methods appear
to be insufficient for addressing certain process variabilities and atomic-scale
device fluctuations. New methodologies are required. The conventional
approach of relying only on design rules (defined based on the capability
of the manufacturing processes) and process specifications (established
based on design requirements) needs to be overhauled to factor in these
main goals, though simple, are quite challenging. Nanoscale designs must
be manufacturable with high yields in conventional wafer-fab environments
and with minimum design respins. Furthermore, circuits must function properly
and as intended during their typical operation under various realistic
environmental conditions within a system. Ultimately, the shortest possible
time-to-yield, time-to-market, and time-to-profits must be achieved with
the lowest possible costs and targeted net-profit margins.
response to these mounting challenges, the concept of design for manufacturability
(DFM) has emerged as an absolute necessity. DFM goes beyond the earlier
critical moves toward design-for-testability (DFT), a trend that has been
evolving over the past few decades. For DFM to succeed, there must be
a broad-based, integrated, and interdisciplinary approach, where process
and design groups closely work together and fully understand the issues,
constraints, and challenges of each other's domains. DFM requires cooperation
and seamless interaction among design, process, and mask-shop engineers,
which also implies that the long-established views on product and layout
design, as well as design flow, must be significantly modified.
the nanoscale regime, designers (and designs/circuits) must become process
aware, while process engineers (and processes/manufacturing methods) have
to be increasingly design and product aware. Robust layout and circuit
designs must factor in those manufacturing variabilities that cannot be
addressed by process, never losing sight of sound product design for yield
(DFY) methodologies and goals. This approach links directly to overall
costs and ultimate profits. DFM must provide optimized product designs,
layouts, and design rules that are fully characterized on suitable representative
structures for critical processes with optimized lithographic resolution
enhancement techniques (RET). It must also factor in high yields and the
desired margins in manufacturing, with minimum possible sensitivity to
such critical parameters as CD, overlay, and process defects.
semiconductor industry's experience with subwavelength lithography, where
the use of various RET methods has enabled nanoscale technologies, serves
as an interesting and relevant example. Several years ago when I worked
at VLSI Technology, we started using optical proximity correction (OPC)
techniques beyond the 180-nm node. Fortunately, we soon realized that
the only way to succeed was to get the engineers from the design, lithography,
mask, device, and module-integration groups to directly work together
as a cohesive, functional team. We also collaborated very closely with
the mask and OPC/design tool suppliers, effectively leveraging their knowledge,
experience, and support. We also changed the design flow to reflect the
paradigm shift in this iterative process. This exercise, a practical,
hands-on introduction to DFM methods, proved to be quite successful.
methodologies require the integration of design, process, and mask-making
functions, something that may give many integrated device manufacturers
(IDMs) an advantage. To effectively integrate these functions, the fabless
and outsourced/contract manufacturing models need to be modified to strengthen
partnerships and expand information sharing (including process, yield,
and fab data) with customers throughout the design flow and across the
supply chain. Whether in an IDM, fabless, or foundry setting, circuit
and layout designers, process engineers and other fab groups, mask shops,
electronic design automation tool providers, process equipment manufacturers,
process control vendors, and factory automation suppliers all play critical
roles and must collaborate with each other and with their customers.
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