Eliminating False
Positives in Semiconductor
Wafer Inspection
The critical need for low noise and power stability

Overview
Semiconductor wafer fabrication processes often produce unnecessary waste of materials
and time due to inefficiencies in inspection.Laser noise, when used as a measurement method
for wafer inspection, can be an incredibly accurate metrology process in such environments.
However, variation in certain laser properties may falsely identify errors on the wafer, leading to
the creation of false-positives, and therefore, an avoidable increase in rejections, and ultimately,wasted material and processing time.
This issue can largely be eliminated by utilising an ultra-low noise laser with long-term powerstability and one that is designed specifically for such demanding 24/7 inspection applications.

BackgroundIn high-end metrology applications, measuring feature size is a key metric in semiconductor
process monitoring. An efficient method for performing this measurement is by rapidly
scanning a laser line across a wafer substrate, using the laser as the probe. The reflectedsignal is subsequently measured, and laser noise, on interaction with the sample, indicates thefeature size. The lower the laser noise, the higher the precision in accurately characterising themeasurements of these features.

How is a low noise laser created?
Low noise specifications are achieved largely by designing a specific low noise cavity at theheart of the laser, working in tandem with an onboard electronic control subsystem designed
specifically to support low noise. Novanta’s Laser Quantum systems feature a patented, low
noise cavity based on a SANOFF filtering technique. SANOFF is a selective mode-stripping
function designed to reduce laser mode competition, leading to ultra-low noise specifications.
The electronic subsystem incorporates a seriesof further noise reduction features that operate
in conjunction with the optimised cavity design
LASER
DETECTOR
in order to deliver high levels of signal purity
and consistency. One such feature is a realtime
feedback function, where the laser beam signal
is monitored in realtime by an ultrafast photo
diode. Any residual noise present in that signal
SEMICONDUCTOR
is then compensated in real time at the pump
WAFER
diode, effectively cancelling out any residual noisepeaks before they reach the sample. This function
works together with the optical design to create
an ultra-low noise laser that is able to actively,
and intelligently maintain ultra-low noise levels
Figure 1: Showing the set up of semiconductor wafer
irrespective of surroundings.
inspection©2023 Novanta Corporation. All rights reserved.

Eliminating False Positives in Semiconductor Wafer Inspection							2Power Stability
High-level and long-term power stability in a laser is also a critical parameter in this application,with integrated realtime power feedback being a prerequisite of a reliable, consistent system. All
Novanta Laser Quantum lasers are designed with an advanced power management architecture
designed to feedback micro fluctuations in power to the controller, enabling power stability toremain constant. The ability to customise the automation of this feedback loop affords OEMs theability to perform micro adjustments from PID settings in order to precisely manage dead bandissues in the most demanding applications.

Figure 2: Showing the set up of semiconductor wafer inspection

Reliability
As a lead function within a mission critical application, the laser system must be capableof delivering reliable consistency. Novanta is a manufacturer of embedded OEM systems,and every system is designed to deliver this mission critical performance as standard. Everylaser manufactured is subjected to a rigorous testing regime that simulates a wide variety ofdemanding performance environments including temperature cycling and vibration testing
amongst many more. It is one of the many design philosophies within Novanta that its laser
systems are tested under higher power cycling conditions in order to ensure that additional
power can be made available when necessary, and to ensure a long lifetime of consistentperformance.
The axiom laser system features all the technology describedin this application paper making it ideally suited to themost demanding applications where power stability, low
noise and consistent performance are required.

Interested in speaking to one of ourknowledgeable representatives?
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