13-12-2012, 04:14 PM
Wafer Inspection Technology Challenges for ULSI Manufacturing
[attachment=43674]
INTRODUCTION
Wafer inspection systems help semiconductor
manufacturers increase and maintain integrated circuit (IC)
chip yields. The manufacturers buy these systems at a rate
of about $700 million per year. This capital investment
attests to the value of these systems in manufacturing IC
chips.
The IC industry employs inspection systems to detect
defects that occur during the manufacturing process. Their
main purpose is to monitor whether the process is under
control. If it isn’t, the system should indicate the source of
the problem, which the manager of the IC fabrication
process (fab manager) can fix. The important inspection
system characteristics here are defect detection sensitivity
and wafer throughput. As we discuss later, sensitivity and
throughput are coupled such that greater sensitivity usually
means lower throughput. There are both physical and
economic reasons for this relationship.
The relative value of sensitivity and throughput depends
on the function of the inspection system. There are three
general functional requirements for these systems: first,
detecting and classifying defects in process development,
second, in monitoring a process line, and third, in
monitoring a station. In process development one is willing
to have low throughput in order to capture smaller defects
and a greater range of defect types. However, in monitoring
a production line or a station, cost-of-ownership (COO),
thus throughput, becomes relatively more important. In this
case, of course, the sensitivity must be adequate to capture
the yield-limiting defects.
HISTORICAL PERSPECTIVE
In the 1970’s and early 1980’s manual inspection was the
norm. Critical dimensions were micrometers and yield
limiting defects were easily seen visually. However,
production yields were low and their rate of increase slow.
Furthermore, the variance in the inspection process was
considerable, thus, the desire for automated inspection.
Two types of inspection tools appeared in the early
1980’s. One type was an automated microscope that
captured bright-field images of patterned wafers and looked
for defects by comparing die images (die-to-die
comparison). The inspection rate was about 0.1 100-mm
diameter wafers per hour (wph). About the same time a tool
that detected particles on bare silicon wafers became
available. This tool found particles by detecting laser light
scattered from the particle, commonly designated in
microscopy as dark-field. Its inspection rate was quite rapid
at the time, equivalent to about 30 100-mm wph. Its
minimum detectable dielectric particle diameter was about 3
μm.
System considerations
An inspection system obtains an image (electron or
photon), then processes it to determine if a defect is present,
classifies it according to some criteria, and finally passes
the information on to a yield management system. Each of
these steps may have certain limitations and we describe
briefly some of the system considerations.
Inspecting contacts and vias
Both optical and SEM inspectors are effective in helping
to develop and control IC manufacturing processes.
However, there is one major gap in the performance of
current systems. It is seeing small defects or residue at the
bottom of high aspect ratio structures.
Optically one can detect partially filled or missing
contacts in high resolution systems. However, if a residue
of 5 nm is at the bottom of a 250 nm diameter by 1000 nm
deep via, we are asking the optical system to detect a
volume difference equivalent to a 75 nm diameter sphere at
the bottom of the hole, a very difficult task (8). Thus, if
contact/vias must be checked individually, we are not going
to do it optically on real wafers. However, if all the
contact/vias within a local area are incompletely etched,
then optical means can detect it.
[attachment=43674]
INTRODUCTION
Wafer inspection systems help semiconductor
manufacturers increase and maintain integrated circuit (IC)
chip yields. The manufacturers buy these systems at a rate
of about $700 million per year. This capital investment
attests to the value of these systems in manufacturing IC
chips.
The IC industry employs inspection systems to detect
defects that occur during the manufacturing process. Their
main purpose is to monitor whether the process is under
control. If it isn’t, the system should indicate the source of
the problem, which the manager of the IC fabrication
process (fab manager) can fix. The important inspection
system characteristics here are defect detection sensitivity
and wafer throughput. As we discuss later, sensitivity and
throughput are coupled such that greater sensitivity usually
means lower throughput. There are both physical and
economic reasons for this relationship.
The relative value of sensitivity and throughput depends
on the function of the inspection system. There are three
general functional requirements for these systems: first,
detecting and classifying defects in process development,
second, in monitoring a process line, and third, in
monitoring a station. In process development one is willing
to have low throughput in order to capture smaller defects
and a greater range of defect types. However, in monitoring
a production line or a station, cost-of-ownership (COO),
thus throughput, becomes relatively more important. In this
case, of course, the sensitivity must be adequate to capture
the yield-limiting defects.
HISTORICAL PERSPECTIVE
In the 1970’s and early 1980’s manual inspection was the
norm. Critical dimensions were micrometers and yield
limiting defects were easily seen visually. However,
production yields were low and their rate of increase slow.
Furthermore, the variance in the inspection process was
considerable, thus, the desire for automated inspection.
Two types of inspection tools appeared in the early
1980’s. One type was an automated microscope that
captured bright-field images of patterned wafers and looked
for defects by comparing die images (die-to-die
comparison). The inspection rate was about 0.1 100-mm
diameter wafers per hour (wph). About the same time a tool
that detected particles on bare silicon wafers became
available. This tool found particles by detecting laser light
scattered from the particle, commonly designated in
microscopy as dark-field. Its inspection rate was quite rapid
at the time, equivalent to about 30 100-mm wph. Its
minimum detectable dielectric particle diameter was about 3
μm.
System considerations
An inspection system obtains an image (electron or
photon), then processes it to determine if a defect is present,
classifies it according to some criteria, and finally passes
the information on to a yield management system. Each of
these steps may have certain limitations and we describe
briefly some of the system considerations.
Inspecting contacts and vias
Both optical and SEM inspectors are effective in helping
to develop and control IC manufacturing processes.
However, there is one major gap in the performance of
current systems. It is seeing small defects or residue at the
bottom of high aspect ratio structures.
Optically one can detect partially filled or missing
contacts in high resolution systems. However, if a residue
of 5 nm is at the bottom of a 250 nm diameter by 1000 nm
deep via, we are asking the optical system to detect a
volume difference equivalent to a 75 nm diameter sphere at
the bottom of the hole, a very difficult task (8). Thus, if
contact/vias must be checked individually, we are not going
to do it optically on real wafers. However, if all the
contact/vias within a local area are incompletely etched,
then optical means can detect it.