Claudio I. Zanelli

A Robust Hydrophone for HIFU Metrology

The high acoustic intensities generated by HIFU systems cause conventional hydrophones to fail before measurements can be reliably made. To address this challenge, we present a new piezoelectric needle hydrophone, which is resistant to cavitation while possessing a flat frequency response (+/− 3 dB from 1 to 10 MHz) and a small effective aperture (400 micron effective diameter). This hydrophone has been used in a high intensity field (1.5 MHz tone burst of 30 microseconds and 3% duty cycle, with rarefactional pressures exceeding 4 MPa and positive pressures exceeding 15 MPa) without degradation in the hydrophone’s performance, as indicated in before‐and after calibration checks of the device.

Ultrasonic based detection of interventional medical device contact and alignment

A method and apparatus for remotely monitoring the location of an interventional medical device (IMD) using ultrasonic signals. Both the proximity and alignment of the IMD are calculated from ultrasound signals reflected off the tissue surface. The inclusion of an offset between the distal end of the IMD and the ultrasound transducer enables accurate position and alignment monitoring of when the IMD is in contact with, or very close to, the tissue surface. The timing of the reflected signal is used to measure proximity or contact. A comparison of the strength between multiple reflected signals is used to measure the alignment of the IMD in 3D space or perpendicularity to a given surface. The present invention may be used as a location indicator within a wide variety of IMDs, and in a wide variety of medical procedures.

HIFU Transducer Characterization Using a Robust Needle Hydrophone

A robust needle hydrophone has been developed for HIFU transducer characterization and reported on earlier. After a brief review of the hydrophone design and performance, we demonstrate its use to characterize a 1.5 MHz, 10 cm diameter, F‐number 1.5 spherically focused source driven to exceed an intensity of 1400 W/cm2at its focus. Quantitative characterization of this source at high powers is assisted by deconvolving the hydrophone’s calibrated frequency response in order to accurately reflect the contribution of harmonics generated by nonlinear propagation in the water testing environment. Results are compared to measurements with a membrane hydrophone at 0.3% duty cycle and to theoretical calculations, using measurements of the field at the source’s radiating surface as input to a numerical solution of the KZK equation.

Characterization of Cavitation in Ultrasonic or Megasonic Irradiated Gas Saturated Solutions Using a Hydrophone

Emerging ultrasonic and megasonic cleaning demands in various applications (solar cell, storage devices, wafer and mask cleaning, etc.) dictate the need to understand the acoustic cavitation under different operating conditions to optimize efficiency of cleaning and reduce damage. Major parameters that affect cavitation include frequency of the sound field, operating power of the transducer and the cleaning chemistry. Previous studies have reported the use of common techniques such as multi-bubble sonoluminescence [1] and sono-electrochemistry [2] to understand acoustic cavitation. The disadvantage with sonoluminescence technique is that it characterizes cavitation mainly in the bulk of the solution, which may not be pertinent to wafer cleaning applications where the interest is in understanding cavitation phenomena close to the wafer surface. Although, sono-electrochemical techniques employing microelectrode are capable of measuring cavitation in the vicinity of a solid surface, they are limited to measurements on an extremely small area due to the miniscule size (5-25 μm) of the electrode. In this context, hydrophone measurements offer significant benefit as they can be taken near a solid surface as well as on a relative large area (1-2 mm diameter) of the pressure sensitive tip.

Comparison of invasive and non-invasive methods of measuring high intensity acoustic fields

We present two methods of characterizing high intensity acoustic fields, namely, a non-invasive Schlieren method, and an invasive fiber-optic based one. The instant display makes the Schlieren method very attractive, although because it is a projection method it does not convey all the information available by the more localized sampling provided by the optical fiber. Numerical comparisons as well as the limitations of each method are described in the context of therapeutic ultrasound applications.

Schlieren imaging of sound at the turn of the century

The mystery of sound fields lies largely in that, despite the complex presence of diffraction effects, at a human scale, they cannot be seen. We sample an acoustic field two points at a time, and mapping it completely, even in two dimensions, requires a constant field and a lot of patience. Vision allows capturing two‐dimensional images, and even mental three‐dimensional images. Yet light can only be seen when it is interfered with, and scattered into, our retinas. Like sound, we must destroy its field in order to sense it. Quite possibly, the lack of sensory perception of acoustic field maps is responsible for our slowness in understanding the nature of waves. The limitations of point‐to‐point mapping of fields leads many to take a few samples and then work directly with mathematical models. Schlieren is an ancient yet rediscovered, fascinating technique to generate instantaneous maps of acoustic fields based on the interaction of light and sound. Its value as a guide to intuitively connect with the unde...

Characterization of acoustic cavitation from a megasonic nozzle transducer for photomask cleaning

Megasonic agitation continues to be used in advanced 193i and EUV photomask cleaning processes. The trend to adopt higher frequencies is driven by the need to control a tighter process window with shrinking feature sizes and a zerotolerance defect requirement. Despite continued use of megasonics, the effect of the acoustic field applied to the mask substrate remains unclear. Photomask cleaning is a dynamic process with many parameters that contribute to the particle removal efficiency and pattern damage. These include transducer type, transducer position, drive frequency, power setting, flow rate, chemistry, gas concentration, etc. To add to the complexity, when the acoustic waves from the transducer interact with a quartz mask the energy may reflect from, transmit through, or couple into the substrate. An in-situ measurement of the acoustic field, as present at the feature location, is required to correlate acoustic parameters with cleaning performance. This work introduces a photomask-shaped cavitation sensor capable of measuring the absolute pressure from the direct field, stable cavitation, and transient cavitation present at the surface. In contrast to previous work characterizing skirt-type transducers, this measurement instrument is sensitive to higher drive frequencies while withstanding concentrated pressure levels from nozzle transducers. Here, a megasonic system with dual nozzle transducers at 5 MHz and 3 MHz was evaluated. The aim of the study is to better understand the acoustic properties from different types of transducers for process development and monitoring in the quest to correlate with photomask cleaning.

Characterization of Cavitation in a Single Wafer or Photomask Cleaning Tool

A novel transducer for megasonic cleaning of photomasks presents an approach that differs from previous configurations, and appears to have unique features for cleaning while minimizing damage. As the cleaning and damage processes are determined by the presence of cavitation, a thorough acoustic analysis was performed on the device, by using a calibrated hydrophone scanned at the photomask location, and a quartz photomask with embedded sensors.

Acoustic characterization of two megasonic devices for photomask cleaning

Wet photomask cleaning relies on megasonic agitation to enhance the process, but there are many challenges to reliably maximize particle removal efficiency (PRE) and minimize damage. With the shift to pellicle-free EUV masks, photomask processes are more vulnerable to contamination, increasing the urgency to improve the cleaning process. This difficulty is largely due to the unavailability of appropriate measurement of the acoustic field. Typically all that is known about the acoustic output is the driving frequency and the electric power delivered to a transducer, both global parameters that tell little about the field distribution over the substrate, the actual amplitude of the sound at the substrate, or the levels of cavitation (stable and transient) present at the substrate.

Device for detection of cavitation in by spectral analysis in megasonic cleaning

The authors present a device and a method to detect cavitation in megasonic cleaning environments. The device is small enough to fit between wafers in the semiconductor industry, allowing the monitoring of the cleaning process.