Ultrasound is widely used for imaging, measurement and diagnostics in the MHz region and is perhaps most familiar as a medical or non-destructive imaging or measurement tool. In the MHz frequency range the wavelength is typically measured in microns and is many times longer than the wavelength of visible light, limiting its resolution to objects much larger than the nano-scale.
It is possible to perform ultrasonic imaging and measurement at much higher frequencies, in the GHz region. Here the acoustic wavelength is typically less than that of light permitting the higher resolutions than optical microscopy and the ability to probe micro and nano-scale objects.
At these high frequencies ultrasonics has much to offer the nano-world as a powerful diagnostic tool: it could be used in circumstances where optical microscopy, electron microscopy and probe microscopy cannot, such as inside living objects.
Despite the potential that ultrasonics offers for imaging and measurement at the micro and nano-scale,
performing ultrasonics at the nano-scale is hampered by many problems that render the techniques typically used in the MHz region impractical.
In this paper we discuss some of the practical problems standing in the way of nano-ultrasonics and some of the solutions, especially the use of pico-second laser ultrasonics and the development of nano-ultrasonic transducers and their application to ultrasonic imaging inside living cells.
In pump-probe type experiments the signal of interest is often a very small fraction of the overall light intensity reaching
the detector. This is beyond the capabilities of conventional cameras due to the necessarily high light intensity at the
detector and its limited dynamic range. To overcome these problems, phase-sensitive or lock-in detection with a single
photodiode is generally used. In phase-sensitive detection, the pump beam is modulated and the probe beam is captured
with a photodiode connected to a lock-in amplifier running from the same reference. This provides very narrowband
detection and moves the signal away from low frequency noise. We have developed a linear array detector that can
perform shot-noise limited lock-in detection in 256 parallel channels. Each pixel has four independent wells to allow
phase-sensitive detection. The depth of each well is massively increased and can be controlled on a per-pixel basis
allowing the gain of the sensor to be matched to the incident light intensity, improving noise performance. The array
reduces the number of dimensions that need to be sequentially scanned and so greatly speeds up acquisition. Results
demonstrating spectral parallelism in pump-probe experiments are presented where the a.c. amplitude to background
ratio approaches 1 part in one million.
Many optical measurements that are subject to high levels of background illumination rely on phase sensitive lock-in
detection to extract the useful signal. If modulation is applied to the portion of the signal that contains information, lockin
detection can perform very narrowband (and hence low noise) detection at frequencies well away from noise sources
such as 1/f and instrumental drift. Lock-in detection is therefore used in many optical imaging and measurement
techniques, including optical coherence tomography, heterodyne interferometry, optoacoustic tomography and a range of
pump-probe techniques. Phase sensitive imaging is generally performed sequentially with a single photodetector and a
lock-in amplifier. However, this approach severely limits the rate of multi-dimensional image acquisition. We present a
novel linear array chip that can perform phase sensitive, shot-noise limited optical detection in up to 256 parallel
channels. This has been achieved by employing four independent wells in each pixel, and massively enhancing the
intrinsic well depth to suppress the effect of optical shot noise. Thus the array can reduce the number of dimensions that
need to be sequentially scanned and greatly speed up acquisition. Results demonstrating spatial and spectral parallelism
in pump-probe experiments are presented where the a.c. amplitude to background ratio approaches 1 part in one million.
Proc. SPIE. 6518, Metrology, Inspection, and Process Control for Microlithography XXI
KEYWORDS: Signal to noise ratio, Point spread functions, Principal component analysis, Interferometers, Photons, Optical testing, Artificial neural networks, Optical resolution, Critical dimension metrology, Classification systems
We have recently described a technique for optical line-width measurements. The system currently is capable of
measuring line-width down to 60 nm with a precision of 2 nm, and potentially should be able to measure down to 10nm.
The system consists of an ultra-stable interferometer and artificial neural networks (ANNs). The former is used to
generate optical profiles which are input to the ANNs. The outputs of the ANNs are the desired sample parameters.
Different types of samples have been tested with equally impressive results. In this paper we will discuss the factors that
are essential to extend the application of the technique. Two of the factors are signal conditioning and sample
classification. Methods, including principal component analysis, that are capable of performing these tasks will be
In this paper, we will describe a technique that combines a common path scanning optical interferometer with artificial
neural networks (ANN), to perform track width measurements that are significantly beyond the capability of
conventional optical systems.
Artificial neural networks have been used for many different applications. In the present case, ANNs are trained using
profiles of known samples obtained from the scanning interferometer. They are then applied to tracks that have not
previously been exposed to the networks. This paper will discuss the impacts of various ANN configurations, and the
processing of the input signal on the training of the network.
The profiles of the samples, which are used as the inputs to the ANNs, are obtained with a common path scanning
optical interferometer. It provides extremely repeatable measurements, with very high signal to noise ratio, both are
essential for the working of the ANNs. The characteristics of the system will be described.
A number of samples with line widths ranging from 60nm-3μm have been measured to test the system. The system can
measure line widths down to 60nm with a standard deviation of 3nm using optical wavelength of 633nm and a system
numerical aperture of 0.3. These results will be presented in detail along with a discussion of the potential of this